*Article* **Colour Changes during the Carbamazepine Oxidation by Photo-Fenton**

**Natalia Villota 1,\*, Cristian Ferreiro <sup>2</sup> , Hussein Ahmad Qulatein <sup>3</sup> , Jose María Lomas <sup>1</sup> , Luis Miguel Camarero <sup>1</sup> and José Ignacio Lombraña <sup>2</sup>**


**Abstract:** The oxidation of aqueous solutions of carbamazepine is conducted using the Fenton reagent, combined with the photolytic action of a 150 W medium pressure UV lamp, operating at T = 40 ◦C. The effect of acidity is analysed at an interval pH = 2.0–5.0, verifying that operating at pH = 5.0 promotes colour formation (Colour = 0.15 AU). The effect of iron is studied, finding that the colour of the water increases in a linear way, Colour = 0.05 + 0.0075 [Fe]<sup>0</sup> . The oxidising action of hydrogen peroxide is tested, confirming that when operating with [H2O<sup>2</sup> ]<sup>0</sup> = 2.0 mM, the maximum colour is generated (Colourmax = 0.381 AU). The tint would be generated by the degradation of by-products of carbamazepine, which have chromophoric groups in their internal structure, such as oxo and dioxocarbazepines, which would produce tint along the first minutes of oxidation, while the formation of acridones would slowly induce colour in the water.

**Keywords:** acridone; carbamazepine; colour; oxo-carbamazepine; photo-Fenton

### **1. Introduction**

The study of emerging pollutants in wastewater, as well as its treatment and elimination, are receiving great attention in recent times due to their presence in many kinds of waters and their possible repercussions on the environment [1]. In almost all wastewater of both urban and industrial origin, different emerging pollutants have been detected in variable concentrations, depending on the activities conducted in the original areas of such waters. Recently, several governments are beginning to limit the presence of some of them, based on the Directive 2013/39/EU of the European Parliament, as well as the Council of 12 August 2013 Amending Directives 2000/60/EC and 2008/105/EC [2], although the effects that they cause or their content in the environment are largely unknown.

The main source of entry into the environment for these pollutants is through unprocessed wastewater and effluents from wastewater treatment plants (WWTPs). Conventional plants are not designed for the elimination of this type of micro-pollutants, so their removal in many cases is not complete. Based on this approach, a need arises for these studies, which seek to know the behaviour of emerging pollutants, which are selected based on European guidelines to be analysed in WWTPs. In this way, the aim of this work is to establish indicators of contamination throughout the different phases that form the treatment systems of these plants, being a key aspect to consider the degree of elimination of these contaminants in the different treatment processes currently used.

Among these priority substances, pharmaceutical products, being active biological substances, can affect living organisms in water even in small concentration. Pharmaceu-

**Citation:** Villota, N.; Ferreiro, C.; Qulatein, H.A.; Lomas, J.M.; Camarero, L.M.; Lombraña, J.I. Colour Changes during the Carbamazepine Oxidation by Photo-Fenton. *Catalysts* **2021**, *11*, 386. https://doi.org/10.3390/catal11030386

Academic Editors: Gassan Hodaifa and Rafael Borja

Received: 24 February 2021 Accepted: 16 March 2021 Published: 18 March 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

ticals such as hormones, pain relievers, and antidepressants can have adverse influence on fish, crustaceans, and algae, because they have a similar kind of receptors as humans. The consequences on animals and plants can be very different from the pharmacological effects expected in humans. For this reason, there is a current need to develop new analysis methods that ensure the effectiveness of the AOPs, in order to conduct a correct design of the new processes [3].

Following the indications of Directive 2013/39/EU of the European Parliament, this work is part of a central line of research that is focussed on the development of techniques that allow the degradation of drugs, because there are resistant micro-pollutants contained in wastewater. The purpose is to prevent their transmission to water distribution networks based on the Commission Implementing Rule (EU) 2018/840 of 5 June 2018 [4].

This work focusses on the study of the degradation of the drug carbamazepine. This drug has been selected as a model pollutant of the study, due to its persistence in conventional treatment plants, as well as its wide presence in urban water [5]. Carbamazepine (CBZ) is a medicine utilised to treat neurological conditions such as epilepsy, depression, or bipolar disorder. In humans, around 72% is absorbed and metabolised in the liver, and 28% is excreted in feces. CBZ is one of the most frequently detected pharmaceutical compounds in urban aqueous systems [6,7]. On the other hand, the main metabolites detected in urine are BBZ-epoxide, CBZ-diol, CBZ-acridan, 2-OH-CBZ, and 3-OH-CBZ [5,8,9]. CBZ is a recalcitrant pollutant identified in the effluents of sewage treatment plants and in superficial waters, which has a potential impact on the environment due to its physico-chemical properties, since it is seldom eliminated in conventional water treatments [10].

Due to its potential effect on aquatic microorganisms and human health, there is a notable concern about its removal from water. Studies performed in the presence of CBZ in relevant concentrations show that it can induce disorders in lipid metabolism, as well as damage to mitochondria and DNA in fish [11,12]. Moreover, research published by Faisal et al. [5] shows that CBZ residues in drinking water could cause congenital malformations and/or neurological development problems after long-term intrauterine exposure or breastfeeding. On the other hand, analysis of UV-irradiated aqueous CBZ solutions reveals that acridine, a compound known to be carcinogenic, is one of the byproducts formed [13].

Within this context, Advanced Oxidation Processes (AOPs) are presented as an alternative with great potential to effectively eliminate emerging pollutants. To perform the industrial implementation of AOPs, it is necessary to evaluate the different technologies to minimise toxic risks to human health [14], and to solve problems regarding technical feasibility, cost-effectiveness, and their own sustainability [15]. On the other hand, the low concentration levels in which these micro-pollutants are found in the waters limit the effectiveness of these treatments [16]. Assessing the effects induced by the discharge of these wastewaters into natural channels is a challenge, since it presents the difficulty of identifying numerous pollutants, metabolites, and transformation products in very low concentrations.

Among these technologies, this work tries to test the use of hydrogen peroxide combined with iron salts and ultraviolet (UV) light, called photo-Fenton Technology, in order to study the degradation of carbamazepine in aqueous solution. Ultraviolet light is a germicide emission that does not present any residual or secondary effects. Therefore, this technique has a great potential to become a useful tool with high viability. Nevertheless, it is necessary to develop a solid foundation of knowledge in the design of feasible processes for the degradation of emerging pollutants, which requires exhaustive research on the laboratory scale and in pilot plants.

### **2. Results**

### *2.1. Colour Changes during Carbamazepine Oxidation*

Figure 1 displays the colour changes that occur in the aqueous solution during the degradation of carbamazepine using the photo-Fenton process. The operating conditions in

the tests shown in Figure 1a lead to the formation of a tinted aqueous residue recalcitrant to oxidation. For this reason, it is chosen as a representative essay to analyse this phenomenon. The degradation of carbamazepine occurs during the first two hours of reaction following second-order kinetic guidelines. The generation of tint in the water occurs during the first 40 min of reaction until it reaches a maximum value that remains stable over time. taminant load. This leads to partial oxidation of carbamazepine towards the formation of colour precursor intermediates. By conducting the reaction with a shortage of oxidant, it causes the generated radical load to be consumed through the processes of oxidation of organic matter and iron regeneration. As a result, the interradical reactions producing oxygen release in the water are relegated.

ions are completely regenerated to ferrous. This result could be attributed to oxygen evolution reactions, where free radicals participate. The conditions that facilitate the formation of tint in the water are related to the use of scarce oxidant with respect to the con-

*Catalysts* **2021**, *11*, x FOR PEER REVIEW 4 of 13

**Figure 1.** Water quality parameters analysed during carbamazepine oxidation by photo-Fenton: (**a**) Carbamazepine concentration, colour and redox potential. (**b**) Dissolved oxygen and ferrous ion. Experimental conditions: [CBZ]0 = 50.0 mg/L; pH = 3.0; [H2O2]0 = 2.0 mM; [Fe]0 = 10.0 mg/L; [UV] = 150 W; T = 40 °C. **Figure 1.** Water quality parameters analysed during carbamazepine oxidation by photo-Fenton: (**a**) Carbamazepine concentration, colour and redox potential. (**b**) Dissolved oxygen and ferrous ion. Experimental conditions: [CBZ]<sup>0</sup> = 50.0 mg/L; pH = 3.0; [H2O<sup>2</sup> ]<sup>0</sup> = 2.0 mM; [Fe]<sup>0</sup> = 10.0 mg/L; [UV] = 150 W; T = 40 ◦C.

*2.2. pH Effect*  Figure 2 presents the effect of pH on water colour changes during the oxidation of aqueous carbamazepine solutions, operating between pH = 2.0 and 5.0. It should be noted that the acidity has remained stable throughout the reaction at the initial established value. In the tests conducted, it was found that during the first 5 min of the oxidation, tint was generated in the water until it reached a maximum value and then decreases to a Analysing the redox potential values, an intense increase is observed during the first 5 min of the reaction until reaching a maximum value that decreases, arriving to a steady state after 40 min. This similar evolution between the colour and the redox potential changes makes it possible to associate the species that produce the hue changes in the water with the degradation intermediates of carbamazepine, which cause the redox potential values considered in the solution.

stable value, around 30 min of reaction time. PH determines the value of the colour area as well as the residual hue of the oxidised water. On the other hand, it is observed that operating between pH = 2.0 and 3.5, the maximum colour formation occurs at around 5 min of reaction. However, at pH = 4.0 and 5.0 the maximun colour formation occurs between 10 and 15 min. To analyse this result in more detail, Figure 3a represents the colour of the treated It should be noted that the increase in the redox potential during the first minutes of the reaction may be due to the oxidation of the ferrous ions to ferric, which is presented in Figure 1b. This allows verifying that approximately 70% of iron added to the reaction mixture in the form of ferrous ions is oxidised through the Fenton mechanism to ferric ions. Furthermore, during the course of the reaction, it is found that under the conditions tested, complete regeneration of the catalyst to ferrous ions occurs.

water once it has reached a steady state, together with the redox potential values. It is observed that both variables show a similar evolution regarding pH effect. By increasing the value from pH = 2.0 to 5.0, the intensity of the colour and the redox potential increases, showing a maximum when carrying out the tests at pH = 5.0. As this pH increases, the colour and redox potential of the water decrease. To explain this effect, the speciation diagram of Fe (III) species as pH function in a photo-Fenton system [19] has been analysed. Then, it is found that the formation of the Fe(OH)2 species in a photo-Fenton system potentially increases from pH = 2.0 until reaching its maximum at pH = 5.5. Thus, the effect of pH on colour formation could be associated with the presence of ferric hydroxide in the aqueous medium. The colour reduction operating at values higher than pH = 5.5 may be due to the fact that from this value, the These results allow proposing a direct relationship between the redox potential and the reaction intermediates generated in the different stages of the carbamazepine oxidation mechanism. The substitution of groups of different nature (hydroxyl, oxo) in the aromatic rings affect the redox potential of the molecule, enlarging or reducing its value depending on the inducing effect of the substituent groups to accept or reject electrons in such a way that if the substitution in the ring is favored, they decrease the redox potential. In the case of hydroxylated carbamazepine molecules, when the aromatic ring loses the proton of the substituted hydroxyl group, electron delocalisation increases, thereby enlarging stability and causing the redox potential to decline [17]. Based on this hypothesis, it could be considered that the diminishment in redox potential would be related to the maximum concentration of dihydroxylated carbamazepines in the reaction medium, which would be contemplated as the precursor species of colour formation in water.

formation of ferric hydroxide takes place, which would precipitate. This could cause a decrease in the concentration of iron dissolved, diminishing the aqueous tint. On the other hand, Figure 1b shows the evolution of dissolved oxygen (DO, mg O2/L). During the first 10 min of the reaction, there is a high consumption of oxygen dissolved in water, until reaching levels around (DO = 0.1 mg O2/L). This utilisation can be related to the oxidation process through the formation of strongly oxidising radical species. In this way, a highly oxidising environment is created that requires a large consumption of oxidising species. In addition, it is found that the moment when almost all the DO is

= 150 W; T = 40 °C.

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16

**Colour (AU)**

exhausted corresponds to the highest redox potential. This aspect can be associated to the maximum concentration of ferric ions generated in the Fenton reaction.

Next, the DO concentration begins to increase slightly until reaching levels of about 0.4 mg O2/L after two hours of reaction. This behaviour is similar to that observed in studies reported in the bibliography during the oxidation of other organic pollutants [18], where this second stage of DO production presents a clear dependence on the nature of the oxidised species. In general, it is found that DO release is higher during the oxidation of organic matter that does not form organometallic complexes with iron, due to their molecular structure configuration. When the release of DO in the water is very slow, it is attributed to the fact that the degradation intermediates can form supramolecular structures with the ferric ions, preventing the iron regeneration.

In the case of the oxidation of carbamazepine shown in Figure 1b, it is observed that the DO release rate in water is very low (kDO = 0.0017 mg O2/L min), although the ferric ions are completely regenerated to ferrous. This result could be attributed to oxygen evolution reactions, where free radicals participate. The conditions that facilitate the formation of tint in the water are related to the use of scarce oxidant with respect to the contaminant load. This leads to partial oxidation of carbamazepine towards the formation of colour precursor intermediates. By conducting the reaction with a shortage of oxidant, it causes the generated radical load to be consumed through the processes of oxidation of organic matter and iron regeneration. As a result, the interradical reactions producing oxygen release in the water are relegated. *Catalysts* **2021**, *11*, x FOR PEER REVIEW 5 of 13

#### *2.2. pH Effect* Figure 3b displays the effect of pH on the concentration of DO in the water, which

Figure 2 presents the effect of pH on water colour changes during the oxidation of aqueous carbamazepine solutions, operating between pH = 2.0 and 5.0. It should be noted that the acidity has remained stable throughout the reaction at the initial established value. In the tests conducted, it was found that during the first 5 min of the oxidation, tint was generated in the water until it reached a maximum value and then decreases to a stable value, around 30 min of reaction time. PH determines the value of the colour area as well as the residual hue of the oxidised water. On the other hand, it is observed that operating between pH = 2.0 and 3.5, the maximum colour formation occurs at around 5 min of reaction. However, at pH = 4.0 and 5.0 the maximun colour formation occurs between 10 and 15 min. leads to verify a strong increase from pH = 2.0 to pH = 4.0, where the maximum concentration of DO occurs ([DO] = 7.9 mg O2/L), and then, it decreases from pH = 4.0 to 6.0. This effect could be explained with the Pourbaix diagram for iron, which presents the predominance of the various chemical species in water for an element. Analysing the redox potential diagram of the medium as a function of pH, it can be verified that the experimental redox potential values measured for each pH (see Figure 3a) indicate that within the interval between pH = 2.0 and 4.0, the iron would be in the Fe3+ form. Meanwhile, the values analysed at pH = 5.0 would indicate that iron would be in the FeO42- form and at pH = 6.0 in the Fe2O3 form. This change in the nature of the iron species that would coexist in the system could be related with the reactions of oxygen release.

**Figure 2.** pH effect on colour changes in a photo-Fenton system during the carbamazepine oxidation. Experimental conditions: [CBZ]0 = 50.0 mg/L; [H2O2]0 = 15.0 mM; [Fe]0 = 10.0 mg/L; [UV] = 150 W; T = 40 °C. **Figure 2.** pH effect on colour changes in a photo-Fenton system during the carbamazepine oxidation. Experimental conditions: [CBZ]<sup>0</sup> = 50.0 mg/L; [H2O<sup>2</sup> ]<sup>0</sup> = 15.0 mM; [Fe]<sup>0</sup> = 10.0 mg/L; [UV] = 150 W; T = 40 ◦C.

**Redox potential (V)**1.4 1.6 1.8 7 8 9 **Dissolved oxygen (mV)** 600 700 800 **Total dissolved solids (mg/L)** To analyse this result in more detail, Figure 3a represents the colour of the treated water once it has reached a steady state, together with the redox potential values. It is observed that both variables show a similar evolution regarding pH effect. By increasing the value from pH = 2.0 to 5.0, the intensity of the colour and the redox potential increases,

During the oxidation treatment of aqueous carbamazepine samples, it is found that the water acquires colour during the first 20 min of reaction (Figure 4a). It is verified that the intensity of the tint depends on the dose of oxidant used. The results present two clear trends in the kinetics of colour formation. On the one hand, operating with low concentrations of oxidant, around [H2O2]0 = 2.0 mM, corresponding to stoichiometric ratios of 1

2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 **pH**

TDS

Dissolved oxygen

(**a**) (**b**) **Figure 3.** Indicator parameters of water quality analysed at the steady state: (**a**) Colour and redox potential. (**b**) Dissolved oxygen and total dissolved solids. Experimental conditions: [CBZ]0 = 50.0 mg/L; [H2O2]0 = 15.0 mM; [Fe]0 = 10.0 mg/L; [UV]

0.0 0.2 0.4 0.6 0.8 1.0 1.2

*2.3. Effect of Hydrogen Peroxide Dosage* 

2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 **pH**

Colour Redox potential 0.00

0.05

0.10

0.15

**Colour (AU)**

0.20

0.25

0 10 20 30 40 50 60 70 80 90 100 110 120 **Time (min)**

*Catalysts* **2021**, *11*, x FOR PEER REVIEW 5 of 13

system could be related with the reactions of oxygen release.

pH=2.0 pH=2.5 pH=3.0 pH=3.5 pH=4.0 pH=5.0

showing a maximum when carrying out the tests at pH = 5.0. As this pH increases, the colour and redox potential of the water decrease. **Figure 2.** pH effect on colour changes in a photo-Fenton system during the carbamazepine oxidation. Experimental conditions: [CBZ]0 = 50.0 mg/L; [H2O2]0 = 15.0 mM; [Fe]0 = 10.0 mg/L; [UV] = 150 W; T = 40 °C.

Figure 3b displays the effect of pH on the concentration of DO in the water, which leads to verify a strong increase from pH = 2.0 to pH = 4.0, where the maximum concentration of DO occurs ([DO] = 7.9 mg O2/L), and then, it decreases from pH = 4.0 to 6.0. This effect could be explained with the Pourbaix diagram for iron, which presents the predominance of the various chemical species in water for an element. Analysing the redox potential diagram of the medium as a function of pH, it can be verified that the experimental redox potential values measured for each pH (see Figure 3a) indicate that within the interval between pH = 2.0 and 4.0, the iron would be in the Fe3+ form. Meanwhile, the values analysed at pH = 5.0 would indicate that iron would be in the FeO42- form and at pH = 6.0 in the Fe2O3 form. This change in the nature of the iron species that would coexist in the

**Figure 3.** Indicator parameters of water quality analysed at the steady state: (**a**) Colour and redox potential. (**b**) Dissolved oxygen and total dissolved solids. Experimental conditions: [CBZ]0 = 50.0 mg/L; [H2O2]0 = 15.0 mM; [Fe]0 = 10.0 mg/L; [UV] = 150 W; T = 40 °C. **Figure 3.** Indicator parameters of water quality analysed at the steady state: (**a**) Colour and redox potential. (**b**) Dissolved oxygen and total dissolved solids. Experimental conditions: [CBZ]<sup>0</sup> = 50.0 mg/L; [H2O<sup>2</sup> ]<sup>0</sup> = 15.0 mM; [Fe]<sup>0</sup> = 10.0 mg/L; [UV] = 150 W; T = 40 ◦C.

*2.3. Effect of Hydrogen Peroxide Dosage*  During the oxidation treatment of aqueous carbamazepine samples, it is found that the water acquires colour during the first 20 min of reaction (Figure 4a). It is verified that the intensity of the tint depends on the dose of oxidant used. The results present two clear trends in the kinetics of colour formation. On the one hand, operating with low concentrations of oxidant, around [H2O2]0 = 2.0 mM, corresponding to stoichiometric ratios of 1 To explain this effect, the speciation diagram of Fe (III) species as pH function in a photo-Fenton system [19] has been analysed. Then, it is found that the formation of the Fe(OH)<sup>2</sup> − species in a photo-Fenton system potentially increases from pH = 2.0 until reaching its maximum at pH = 5.5. Thus, the effect of pH on colour formation could be associated with the presence of ferric hydroxide in the aqueous medium. The colour reduction operating at values higher than pH = 5.5 may be due to the fact that from this value, the formation of ferric hydroxide takes place, which would precipitate. This could cause a decrease in the concentration of iron dissolved, diminishing the aqueous tint.

Figure 3b displays the effect of pH on the concentration of DO in the water, which leads to verify a strong increase from pH = 2.0 to pH = 4.0, where the maximum concentration of DO occurs ([DO] = 7.9 mg O2/L), and then, it decreases from pH = 4.0 to 6.0. This effect could be explained with the Pourbaix diagram for iron, which presents the predominance of the various chemical species in water for an element. Analysing the redox potential diagram of the medium as a function of pH, it can be verified that the experimental redox potential values measured for each pH (see Figure 3a) indicate that within the interval between pH = 2.0 and 4.0, the iron would be in the Fe3+ form. Meanwhile, the values analysed at pH = 5.0 would indicate that iron would be in the FeO<sup>4</sup> <sup>2</sup><sup>−</sup> form and at pH = 6.0 in the Fe2O<sup>3</sup> form. This change in the nature of the iron species that would coexist in the system could be related with the reactions of oxygen release.

### *2.3. Effect of Hydrogen Peroxide Dosage*

During the oxidation treatment of aqueous carbamazepine samples, it is found that the water acquires colour during the first 20 min of reaction (Figure 4a). It is verified that the intensity of the tint depends on the dose of oxidant used. The results present two clear trends in the kinetics of colour formation. On the one hand, operating with low concentrations of oxidant, around [H2O2]<sup>0</sup> = 2.0 mM, corresponding to stoichiometric ratios of 1 mol CBZ: 10 mol H2O2, tint is generated in the water according to a ratio of 0.0086 AU/min, until reaching its maximum intensity (Colourmax = 0.353 AU) at 30 min of reaction. Subsequently, the hue continues increasing but much more slowly, following ratios of 0.0005 AU/min, until it arrives at the steady state (Colour<sup>∞</sup> = 0.381 AU).

the intermediates that provide tint to colourless species.

**Figure 4.** (**a**) Effect of hydrogen peroxide on colour changes in a photo-Fenton system during the carbamazepine oxidation. (**b**) Maximum colour formation (Colour max, AU) and time corresponding to the maximum colour formation (Time colour max, min) as a function of the oxidant dosage. Experimental conditions: [CBZ]0 = 50.0 mg/L; pH = 3.0; [Fe]0 = 10.0 mg/L; [UV] = 150 W; T = 40 °C **Figure 4.** (**a**) Effect of hydrogen peroxide on colour changes in a photo-Fenton system during the carbamazepine oxidation. (**b**) Maximum colour formation (Colourmax, AU) and time corresponding to the maximum colour formation (Timecolour max, min) as a function of the oxidant dosage. Experimental conditions: [CBZ]<sup>0</sup> = 50.0 mg/L; pH = 3.0; [Fe]<sup>0</sup> = 10.0 mg/L; [UV] = 150 W; T = 40 ◦C

Colour max = 0.3759 − 0.011 [H2O2]0 (r2 = 0.9988) (1) t colour max = 58.31 × [H2O2]0 <sup>−</sup>0.8813 (r2 = 0.9916) (2) The results shown indicate the existence of two stages in colour formation based on the carbamazepine degradation mechanism proposed in Figure 5. The first step takes place during the first stages of decomposition and leads to the formation of highly tinted species. This stage would involve hydroxylation reactions through the electrophilic attack of the hydroxyl radicals to the olefinic double bond in the central and lateral heterocyclic rings of carbamazepine, conducting to the formation of the corresponding hydroxylated carbamazepines. The action of hydroxyl radicals can generate a new hydroxylation of the molecule, leading to the creation of cis and trans-dihydroxy-carbamazepine [20]. The formation of the rare cis isomer appears to be less than that of trans [21]. Finally, the oxidation of these species would produce colour precursors, oxo and dioxo-carbazepines (10-OH-Performing with oxidant concentrations greater than [H2O2]<sup>0</sup> = 5.0 mM, corresponding to stoichiometric ratios greater than 1 mol CBZ: 25 mol H2O2, the colour formation follows the evolution of a reaction intermediate, with rapid colour formation during the first minutes of oxidation, until reaching a maximum value, and decreasing until obtaining a colourless solution. The oxidant dosage determines both the maximum colour generated (Equation (1)) and the time in which the formation of the highest colour intensity occurs (Equation (2)), as it is shown in Figure 4b. This result indicates that the stoichiometric ratio of oxidant utilised determines the degree of oxidation achieved—that is, the stage of the carbamazepine degradation mechanism reached and, consequently, the nature of the intermediates that coexist in solution. As a result, the higher the molar ratio of oxidant, the lower the intensity of the tint generated, so that the formation of coloured species is reduced. The fact that under these conditions, a colourless oxidised residue is obtained shows that operating in all conditions, the dose of oxidant is sufficient to degrade the intermediates that provide tint to colourless species.

mol CBZ: 10 mol H2O2, tint is generated in the water according to a ratio of 0.0086 AU / min, until reaching its maximum intensity (Colour max = 0.353 AU) at 30 min of reaction. Subsequently, the hue continues increasing but much more slowly, following ratios of

Performing with oxidant concentrations greater than [H2O2]0 = 5.0 mM, corresponding to stoichiometric ratios greater than 1 mol CBZ: 25 mol H2O2, the colour formation follows the evolution of a reaction intermediate, with rapid colour formation during the first minutes of oxidation, until reaching a maximum value, and decreasing until obtaining a colourless solution. The oxidant dosage determines both the maximum colour generated (Equation (1)) and the time in which the formation of the highest colour intensity occurs (Equation (2)), as it is shown in Figure 4b. This result indicates that the stoichiometric ratio of oxidant utilised determines the degree of oxidation achieved—that is, the stage of the carbamazepine degradation mechanism reached and, consequently, the nature of the intermediates that coexist in solution. As a result, the higher the molar ratio of oxidant, the lower the intensity of the tint generated, so that the formation of coloured species is reduced. The fact that under these conditions, a colourless oxidised residue is obtained shows that operating in all conditions, the dose of oxidant is sufficient to degrade

0.0005 AU/min, until it arrives at the steady state (Colour ∞ = 0.381 AU).

$$\text{Color}\_{\text{max}} = 0.3759 - 0.011 \, [\text{H}\_2\text{O}\_2]\_0 \quad \text{(r}^2 = 0.9988\text{)}\tag{1}$$

$$\mathbf{t}\_{\text{colour}} = \mathbf{58.31} \times \left[ \mathbf{H}\_2 \mathbf{O}\_2 \right]\_0^{-0.8813} \quad \text{(r}^2 = 0.9916\text{)}\tag{2}$$

The results shown indicate the existence of two stages in colour formation based on the carbamazepine degradation mechanism proposed in Figure 5. The first step takes place during the first stages of decomposition and leads to the formation of highly tinted species. This stage would involve hydroxylation reactions through the electrophilic attack of the hydroxyl radicals to the olefinic double bond in the central and lateral heterocyclic rings of carbamazepine, conducting to the formation of the corresponding hydroxylated carbamazepines. The action of hydroxyl radicals can generate a new hydroxylation of the molecule, leading to the creation of cis and trans-dihydroxy-carbamazepine [20]. The formation of the rare cis isomer appears to be less than that of trans [21]. Finally, the oxidation of these species would produce colour precursors, oxo and dioxo-carbazepines (10-OH-CBZ, 9-OH-CBZ, EP-CBZ, OX-CBZ), due to the presence of chromophore groups in their molecular structure.

molecular structure.

**Figure 5.** Reaction intermediates causing colour in oxidised carbamazepine solutions. **Figure 5.** Reaction intermediates causing colour in oxidised carbamazepine solutions.

Figure 6 shows the effect of the oxidant concentration used on several parameters that indicate the quality of the water once it is treated. Analysing the tint of the oxidised water, it is found that operating with concentrations [H2O2]0 = 2.0 mM, the oxidation of carbamazepine leads to the formation of highly coloured species. On the other hand, working with concentrations higher than [H2O2]0 = 5.0 mM, a colourless water is obtained. During the second stage, there would be the creation of additional species that coexist with those generated in the previous stage, which provide less intensity of tint to the water. In this case, it is possible to consider the formation of degradation by-products of the carbazepine species, generating hydroxylated molecules of acridine (9-OH-acridine) and the corresponding acridones that cause the additional contribution of colour.

CBZ, 9-OH-CBZ, EP-CBZ, OX-CBZ), due to the presence of chromophore groups in their

the corresponding acridones that cause the additional contribution of colour.

During the second stage, there would be the creation of additional species that coexist with those generated in the previous stage, which provide less intensity of tint to the water. In this case, it is possible to consider the formation of degradation by-products of the carbazepine species, generating hydroxylated molecules of acridine (9-OH-acridine) and

Simultaneously, the redox potential shows an evolution characterised by a slight decrease until reaching a minimum value ([Redox] min = −0.489 V) in [H2O2]0 = 2.0 mM, when the maximum colour formation take place (Colour max = 0.381 AU). Subsequently, it increases practically linear with respect to the concentration of oxidant applied. To explain this minimum value of redox potential, a relationship can be established between the evolution of the potential and the reaction intermediates generated in the different stages of the oxidation mechanism. Studies carried out on the effect of the substitution of groups of different nature in aromatic rings indicate that they affect the value of the redox potential of the molecule, increasing or decreasing depending on the inducing Figure 6 shows the effect of the oxidant concentration used on several parameters that indicate the quality of the water once it is treated. Analysing the tint of the oxidised water, it is found that operating with concentrations [H2O2]<sup>0</sup> = 2.0 mM, the oxidation of carbamazepine leads to the formation of highly coloured species. On the other hand, working with concentrations higher than [H2O2]<sup>0</sup> = 5.0 mM, a colourless water is obtained. Simultaneously, the redox potential shows an evolution characterised by a slight decrease until reaching a minimum value ([Redox]min = −0.489 V) in [H2O2]<sup>0</sup> = 2.0 mM, when the maximum colour formation take place (Colourmax = 0.381 AU). Subsequently, it increases practically linear with respect to the concentration of oxidant applied.

effect of the substituent groups to accept or transfer electrons [17]. Therefore, if ring substitution is favored, the redox potential value diminishes. In the case of carbamazepine, there is a small stabilisation by resonance, which is attributable to electronic delocalisation. When the ring loses the proton of the substituted hydroxyl group, electron delocalisation increases, thus favoring stability and reducing the redox potential. Therefore, based on these hypotheses, the minimum value observed To explain this minimum value of redox potential, a relationship can be established between the evolution of the potential and the reaction intermediates generated in the different stages of the oxidation mechanism. Studies carried out on the effect of the substitution of groups of different nature in aromatic rings indicate that they affect the value of the redox potential of the molecule, increasing or decreasing depending on the inducing effect of the substituent groups to accept or transfer electrons [17]. Therefore, if ring substitution is favored, the redox potential value diminishes.

In the case of carbamazepine, there is a small stabilisation by resonance, which is attributable to electronic delocalisation. When the ring loses the proton of the substituted hydroxyl group, electron delocalisation increases, thus favoring stability and reducing the redox potential. Therefore, based on these hypotheses, the minimum value observed would be related to the maximum concentration of hydroxylated and dihydroxylated carbamazepines in the reaction medium, which would be the precursors of the tint that the solution acquires. By increasing the oxidant ratio, these intermediates are degraded, increasing the degree of oxidation, and it is found that the redox potential of the system evolves to positive values, which would indicate the formation of quinones and acridines.

remains constant in all the tests performed.

**Figure 6.** Indicator parameters of water quality analysed at the steady state: (**a**) Colour and redox potential. (**b**) Dissolved oxygen and total dissolved solids. Experimental conditions: [CBZ]0 = 50.0 mg/L; pH = 3.0; [Fe]0 = 10.0 mg/L; [UV] = 150 W; T = 40 °C. **Figure 6.** Indicator parameters of water quality analysed at the steady state: (**a**) Colour and redox potential. (**b**) Dissolved oxygen and total dissolved solids. Experimental conditions: [CBZ]<sup>0</sup> = 50.0 mg/L; pH = 3.0; [Fe]<sup>0</sup> = 10.0 mg/L; [UV] = 150 W; T = 40 ◦C.

*2.4. Effect of Iron Dosage*  Figures 7 and 8 show the effect of catalyst concentration on the colour acquired by oxidised carbamazepine solutions. Operating with different iron concentrations (Figure 7a), it is observed that adding the iron dose established for each experiment increases tint to the initial carbamazepine solution (Colour 0, AU). The colour that the water gains shows a second degree polynomial increase (Equation (3)) with respect to the concentration of total iron supplied ([Fe]0, mg/L). The initial iron added to the solution in the form of ferrous sulfate undergoes a series of equilibrium reactions between species, because the pH of the sample is adjusted to pH = 3.0 (Figure 7b). For this reason, one part of the iron ions The dissolved oxygen analysed in treated samples is consistent with their redox potential values. It is observed that the DO concentration in water increases as the treatment is conducted with higher concentrations of oxidant, up to a maximum operating point, which corresponds to [H2O2]<sup>0</sup> = 11.0 mM, with a DO = 8.4 mg O2/L. However, in the test carried out using [H2O2]<sup>0</sup> = 15.0 mM, the DO experienced a big decrease until values of DO = 4.2 mg O2/L. These lower levels of DO are observed throughout the course of the reaction, which could be due to operating with excess of oxidant with respect to the iron concentration. On the other hand, the concentration of Total Dissolved Solids (TDS, mg/L) remains constant in all the tests performed.

would be related to the maximum concentration of hydroxylated and dihydroxylated carbamazepines in the reaction medium, which would be the precursors of the tint that the solution acquires. By increasing the oxidant ratio, these intermediates are degraded, increasing the degree of oxidation, and it is found that the redox potential of the system evolves to positive values, which would indicate the formation of quinones and acridines. The dissolved oxygen analysed in treated samples is consistent with their redox potential values. It is observed that the DO concentration in water increases as the treatment is conducted with higher concentrations of oxidant, up to a maximum operating point, which corresponds to [H2O2]0 = 11.0 mM, with a DO = 8.4 mg O2/L. However, in the test carried out using [H2O2]0 = 15.0 mM, the DO experienced a big decrease until values of DO = 4.2 mg O2/L. These lower levels of DO are observed throughout the course of the reaction, which could be due to operating with excess of oxidant with respect to the iron concentration. On the other hand, the concentration of Total Dissolved Solids (TDS, mg/L)

#### is in a reduced state and the other is oxidised, being the ferric ions the providers of the tint to the water. *2.4. Effect of Iron Dosage*

When the oxidant is added and the oxidation of the carbamazepine begins, the hue generated in the water increases until reaching a maximum value (Colour max, AU) at 5 min after oxidation in all the tests conducted. This fact indicates that when using the same concentration of oxidant, the degradation intermediates of carbamazepine formed in water are similar species. Therefore, the colour peaks occur simultaneously, and following identical kinetics, they are displaced in parallel. This linear displacement is established by the iron concentration (Equation (4)). Figures 7 and 8 show the effect of catalyst concentration on the colour acquired by oxidised carbamazepine solutions. Operating with different iron concentrations (Figure 7a), it is observed that adding the iron dose established for each experiment increases tint to the initial carbamazepine solution (Colour0, AU). The colour that the water gains shows a second degree polynomial increase (Equation (3)) with respect to the concentration of total iron supplied ([Fe]0, mg/L). The initial iron added to the solution in the form of ferrous sulfate undergoes a series of equilibrium reactions between species, because the pH of the sample is adjusted to pH = 3.0 (Figure 7b). For this reason, one part of the iron ions is in a reduced state and the other is oxidised, being the ferric ions the providers of the tint to the water.

When the oxidant is added and the oxidation of the carbamazepine begins, the hue generated in the water increases until reaching a maximum value (Colourmax, AU) at 5 min after oxidation in all the tests conducted. This fact indicates that when using the same concentration of oxidant, the degradation intermediates of carbamazepine formed in water are similar species. Therefore, the colour peaks occur simultaneously, and following identical kinetics, they are displaced in parallel. This linear displacement is established by the iron concentration (Equation (4)).

*Catalysts* **2021**, *11*, x FOR PEER REVIEW 9 of 13

**Figure 7.** (**a**) Effect of iron on colour changes in a photo-Fenton system during the carbamazepine oxidation. (**b**) Ferrous ions concentration in water solution during carbamazepine oxidation. Experimental conditions: [CBZ]0 = 50.0 mg/L; pH = 3.0; [H2O2]0 = 15.0 mM; [UV] = 150 W; T = 40 °C. **Figure 7.** (**a**) Effect of iron on colour changes in a photo-Fenton system during the carbamazepine oxidation. (**b**) Ferrous ions concentration in water solution during carbamazepine oxidation. Experimental conditions: [CBZ]<sup>0</sup> = 50.0 mg/L; pH = 3.0; [H2O<sup>2</sup> ]<sup>0</sup> = 15.0 mM; [UV] = 150 W; T = 40 ◦C. **Figure 7.** (**a**) Effect of iron on colour changes in a photo-Fenton system during the carbamazepine oxidation. (**b**) Ferrous ions concentration in water solution during carbamazepine oxidation. Experimental conditions: [CBZ]0 = 50.0 mg/L; pH = 3.0; [H2O2]0 = 15.0 mM; [UV] = 150 W; T = 40 °C.

**Figure 8.** (**a**) Effect of iron dosage on water colour levels observed during the carbamazepine oxidation. (**b**) Relation-ship between total dissolved solids and the residual colour of water oxidized. Experimental conditions: [CBZ]0 = 50.0 mg/L; pH = 3.0; [H2O2]0 = 15.0 mM; [UV] = 150 W; T = 40 °C. **Figure 8.** (**a**) Effect of iron dosage on water colour levels observed during the carbamazepine oxidation. (**b**) Relation-ship between total dissolved solids and the residual colour of water oxidized. Experimental conditions: [CBZ]0 = 50.0 mg/L; pH = 3.0; [H2O2]0 = 15.0 mM; [UV] = 150 W; T = 40 °C. **Figure 8.** (**a**) Effect of iron dosage on water colour levels observed during the carbamazepine oxidation. (**b**) Relation-ship between total dissolved solids and the residual colour of water oxidized. Experimental conditions: [CBZ]<sup>0</sup> = 50.0 mg/L; pH = 3.0; [H2O<sup>2</sup> ]<sup>0</sup> = 15.0 mM; [UV] = 150 W; T = 40 ◦C.

On the other hand, the persistant colour that lasts in the oxidised sample (Colour ∞, AU) increases linearly with the iron concentration (Equation (5)). It is observed that both the maximum colour and the residual increase linearly with the total iron concentration, according to an average ratio of k Fe = 0.0075 AU L/mg Fe. Furthermore, it is found that they remain constant in all the tests: a difference between the maximum colour and the residual of 0.0843 AU. This tint value is explained by the contribution of iron species that can interact with the organic load of the water, forming metallic complexes, which are degraded during oxidation. As shown in Figure 8b, the lasting residual colour is provided by the iron species in suspension, which contribute linearly (Equation (7)) to the total suspended solids (TDS, mg/L). On the other hand, the persistant colour that lasts in the oxidised sample (Colour ∞, AU) increases linearly with the iron concentration (Equation (5)). It is observed that both the maximum colour and the residual increase linearly with the total iron concentration, according to an average ratio of k Fe = 0.0075 AU L/mg Fe. Furthermore, it is found that they remain constant in all the tests: a difference between the maximum colour and the residual of 0.0843 AU. This tint value is explained by the contribution of iron species that can interact with the organic load of the water, forming metallic complexes, which are degraded during oxidation. As shown in Figure 8b, the lasting residual colour is provided by the iron species in suspension, which contribute linearly (Equation (7)) to the total suspended solids (TDS, mg/L). On the other hand, the persistant colour that lasts in the oxidised sample (Colour∞, AU) increases linearly with the iron concentration (Equation (5)). It is observed that both the maximum colour and the residual increase linearly with the total iron concentration, according to an average ratio of kFe = 0.0075 AU L/mg Fe. Furthermore, it is found that they remain constant in all the tests: a difference between the maximum colour and the residual of 0.0843 AU. This tint value is explained by the contribution of iron species that can interact with the organic load of the water, forming metallic complexes, which are degraded during oxidation. As shown in Figure 8b, the lasting residual colour is provided by the iron species in suspension, which contribute linearly (Equation (7)) to the total suspended solids (TDS, mg/L).

$$\text{Color}\_0 = 0.0117 \,\text{[Fe]}\_0 - 0.0002 \,\text{[Fe]}\_0 \,^2 \quad \text{(r}^2 = 0.9901\text{)}\tag{3}$$

Colour max = 0.132 + 0.0074 [Fe]0 (r2 = 0.9946) (4) Colour max = 0.132 + 0.0074 [Fe]0 (r2 = 0.9946) (4) Colourmax = 0.132 + 0.0074 [Fe]<sup>0</sup> (r<sup>2</sup> = 0.9946) (4)

$$\text{Color}\_{\infty} = 0.0477 + 0.0076 \text{ [Fe]}\_{0} \quad \text{(r}^{2} = 0.9961\text{)}\tag{5}$$

$$\text{[TDS]} = 72.982 + 20.211 \text{ [Fe]}\_0 \quad \text{(r}^2 = 0.9974\text{)}\tag{6}$$

$$\text{Color}\_{\infty} = 0.0004 \text{ [TDS]} \quad \text{(r}^2 = 0.9826\text{)}\tag{7}$$

### **3. Materials and Methods**

### *3.1. Experimental Methods*

Samples of carbamazepine aqueous solutions ([CBZ]<sup>0</sup> = 50.0 mg/L, Fagron 99.1%) were studied in a photocatalytic 1.0 L reactor provided with an UV-150 W mercury lamp of medium pressure (Heraeus, 95%, transmission between 300 and 570 nm). Reactions started adding the iron catalyst as ferrous ion ([Fe]0, mg/L), operating between [Fe]<sup>0</sup> = 5.0–40.0 mg/L (FeSO<sup>4</sup> 7 H2O, Panreac 99.0%) and the oxidant dosage for each set of experiments, which varied between [H2O2]<sup>0</sup> = 0–15.0 mM (Panreac, 30% *w*/*v*). All the experiments were conducted at around 40 ◦C in order to simulate real working conditions, considering the heat absorbed by the water that is in direct contact with the UV lamp. Assays were performed under different initial pH conditions (pH between 2.0 and 5.0) in order to assess the effect of this parameter on colour formation during the oxidation of carbamazepine aqueous solutions. Acidity was kept constant adding NaOH and HCl.

### *3.2. Analytical Methods*

Carbamazepine concentration (CBZ, mg/L) was assessed along the reaction at λ = 210 nm by a High-Performance Liquid Chromatograph attached to a spectrophotometer UV/Vis (HPLC Agilent 1200). Analysis was performed by injecting manually 20.0 µL samples, which were dragged by a carrier of 1.0 mL/min flow, consisting of a mixture of methanol and distilled water MeOH/H2O: 80/20, through a Column C18, XBridge Phenyl 5 µm 4.6 × 250 mm (Bridge Waters), with limit of detection 0.1 mg/L.

Colour expressed in Absorbance Units (AU) was quantified by the absorbance of the aqueous solution analysed at λ = 455 nm and ferrous ion ([Fe2+], mg/L) at λ = 510 nm by the phenanthroline method using an UV/Vis Spectrophotometer 930-Uvikon [22]. Dissolved oxygen (DO, mg/L) was measured by a DO-meter HI9142. Total dissolved solids (TDS, mg/L) were analysed by a TDS Metter Digital and Redox potential (V) by a conductimeter (Basic 20 Crison).

### *3.3. Liquid Chromatography-Mass Spectrometry to Elucidate the Intermediates of Carbamazepine Degradation*

Samples were analysed by Liquid Chromatography-Mass Spectrometry to find the carbamazepine degradation pathways that induce high levels of colour in the water during the oxidation process. Analysis was performed with an LC/Q-TOF provided with an ionisation source ESI + Agilent Jet Stream, with the following conditions: Kinetex column EVO C18 (100 × 3 mm) 2.6 µm. Moving phase 0.1% Formic Acid (A): Acetonitrile 0.1% Formic Acid (B). Gradient, %B: time (min): 20:0; 20:2; 100:24; 100:28; 20:30. Flow 0.3 mL/min. Column Temperature 35 ◦C. Injection volume 5 µL. Ionisation: Gas temperature 300 ◦C, drying gas 10 L/min, nebuliser 20 psig, shelf gas temperature 350 ◦C, shelf gas flow 11 L/min, frag 125 V. Vcap 3500 V.

A screening method was developed, allowing the elution and ionisation of the majority of compounds in the sample. Before starting the analysis, the stabilisation of the system, the reproduction in the signals, and the correction of the exact masses were checked. With the aforementioned conditions, the chronogram shown in Figure 9 was attained.

9 **Figure 9.** Chromatographic profile of a methanol blank (grey line) and of the sample (red line).

The search for compounds was performed using the Find deconvolution algorithm by molecular features and a subsequent screening of the proposed compounds, based on compounds detected in the blank, background noise, and minimum abundance of the compound (Figure 10). Appendix A summarises the major ions (*m*/*z*) and the experimental masses calculated for each of the compounds. 9

treatment systems of these plants, being a key aspect to consider the degree of elimination

of these contaminants in the different treatment processes currently used.

**Figure 10.** Chromatographic profile of the major compounds in the oxidised carbamazepine sample.

### **4. Conclusions**

Among these priority substances, pharmaceutical products, being active biological substances, can affect living organisms in water even in small concentration. Pharmaceuticals such as hormones, pain relievers, and antidepressants can have adverse influence on fish, crustaceans, and algae, because they have a similar kind of receptors as humans. The consequences on animals and plants can be very different from the pharmacological Among these priority substances, pharmaceutical products, being active biological The stoichiometric ratio of oxidant determines the degree of oxidation achieved, that is, the nature of the intermediates that coexist in solution. Performing with low concentrations of oxidant, corresponding to stoichiometric ratios of 1 mol CBZ: 10 mol H2O2, colour is generated in the water until it reaches its maximum intensity (oxo and dioxo-carbazepines). Subsequently, the tint continues to increase more slowly, until arriving at the steady state, remaining a coloured aqueous residue that would contain hydroxylated acridines and acridones. Applying concentrations higher than 1 mol CBZ: 25 mol H2O2, the colour formation follows the evolution of a reaction intermediate, obtaining a colourless solution.

effects expected in humans. For this reason, there is a current need to develop new analysis methods that ensure the effectiveness of the AOPs, in order to conduct a correct design of the new processes [3]. Following the indications of Directive 2013/39/EU of the European Parliament, this work is part of a central line of research that is focussed on the development of techniques that allow the degradation of drugs, because there are resistant micro-pollutants contained in wastewater. The purpose is to prevent their transmission to water distribution networks based on the Commission Implementing Rule (EU) 2018/840 of 5 June 2018 [4]. This work focusses on the study of the degradation of the drug carbamazepine. This substances, can affect living organisms in water even in small concentration. Pharmaceuticals such as hormones, pain relievers, and antidepressants can have adverse influence on fish, crustaceans, and algae, because they have a similar kind of receptors as humans. The consequences on animals and plants can be very different from the pharmacological effects expected in humans. For this reason, there is a current need to develop new analysis methods that ensure the effectiveness of the AOPs, in order to conduct a correct design of the new processes [3]. Following the indications of Directive 2013/39/EU of the European Parliament, this work is part of a central line of research that is focussed on the development of techniques The initial iron added to the solution, in the form of ferrous sulfate, undergoes a series of equilibrium reactions between species. This is due to the fact that the acidity of the sample is adjusted to pH = 3.0 Therefore, a part of the iron ions are found in a reduced state and the another in its oxidised, being the ferric ions that provide tint to the water. Both the maximum colour and the persistent colour increase with the concentration of iron used in the treatment, according to an average ratio of kFe = 0.0075 AU L/mg Fe. The maximum tint would be generated by the iron species that interact with the organic load, forming metallic complexes, while the lasting colour would be generated by the iron species in suspension.

drug has been selected as a model pollutant of the study, due to its persistence in conventional treatment plants, as well as its wide presence in urban water [5]. Carbamazepine that allow the degradation of drugs, because there are resistant micro-pollutants contained in wastewater. The purpose is to prevent their transmission to water distribution networks based on the Commission Implementing Rule (EU) 2018/840 of 5 June 2018 [4]. This work focusses on the study of the degradation of the drug carbamazepine. This drug has been selected as a model pollutant of the study, due to its persistence in conventional treatment plants, as well as its wide presence in urban water [5]. Carbamazepine **Author Contributions:** Conceptualization, N.V. and J.M.L.; methodology, L.M.C. and H.A.Q.; software, J.M.L; validation, C.F., J.I.L. and N.V.; formal analysis, J.I.L.; investigation, N.V., H.A.Q. and C.F.; resources, C.F.; data curation, N.V.; writing—original draft preparation, N.V., C.F. and H.A.Q.; writing—review and editing, J.M.L., L.M.C. and J.I.L.; visualization, N.V. and C.F.; supervision, L.M.C.; project administration, N.V. and J.I.L. and J.I.L. acquired the funding. All authors have read and agreed to the published version of the manuscript.

**Funding:** Authors are grateful to the University of the Basque Country UPV/EHU the financial support to carry out this research study through the scholarship Student Movility for Traineeships in the Erasmus + Programme between the Anadolu University in Eskisehir (Turkey) and the Faculty of Engineering Vitoria-Gasteiz (Spain), and the research Project PPGA20/33. *Catalysts* **2021**, *11*, x FOR PEER REVIEW 12 of 13

> **Acknowledgments:** The authors thank for technical and human support provided by Central Service of Analysis from Álava—SGIker—UPV/EHU. **Appendix A**

**Conflicts of Interest:** The authors declare no conflict of interest.

**Figure A1.** Major ions (m/z) and experimental masses calculated for each of the intermediate compounds detected in a

#### **Appendix A** sample of carbamazepine oxidized by photo-Fenton treatment under operating conditions that lead to the formation of coloured solution.


**References**  1. Adedara, I.A.; Ajayi, B.O.; Afolabi, B.A.; Awogbindin, I.O.; Rocha, J.B.T.; Farombi, E.O. Toxicological Outcome of Exposure to Psychoactive Drugs Carbamazepine and Diazepam on Non-Target Insect Nauphoeta Cinerea. *Chemosphere* **2021**, *264*, 128449, **Figure A1.** Major ions (*m*/*z*) and experimental masses calculated for each of the intermediate compounds detected in a sample of carbamazepine oxidized by photo-Fenton treatment under operating conditions that lead to the formation of coloured solution.

#### doi:10.1016/j.chemosphere.2020.128449. **References**


*Chemosphere* **2011**, *82*, 244–252, doi:10.1016/j.chemosphere.2010.09.062.

*Environ.* **2012**, *438*, 15–25, doi:10.1016/j.scitotenv.2012.08.061.

7. Al Aukidy, M.; Verlicchi, P.; Jelic, A.; Petrovic, M.; Barcelò, D. Monitoring Release of Pharmaceutical Compounds: Occurrence and Environmental Risk Assessment of Two WWTP Effluents and Their Receiving Bodies in the Po Valley, Italy. *Sci. Total* 

Microbial Community Assessment. *Appl. Microbiol. Biotechnol.* **2016**, *100*, 2401–2415, doi:10.1007/s00253-015-7105-0.


### *Article* **Quarry Residue: Treatment of Industrial Effluent Containing Dye**

**Lariana Negrão Beraldo de Almeida 1,\*, Tatiana Gulminie Josué 2 , Othavio Henrique Lupepsa Nogueira <sup>2</sup> , Daniele Toniolo Dias <sup>3</sup> , Angelo Marcelo Tusset <sup>4</sup> , Onélia Aparecida Andreo dos Santos <sup>1</sup> and Giane Gonçalves Lenzi <sup>2</sup>**


**Abstract:** This work is devoted to the investigation of the discoloration of the synthetic and industrial effluent, using a quarry residue (MbP), which is a material naturally composed of mixed oxides, compared to zinc oxide (ZnO), acting as photocatalysts and adsorbents. The optimization of the pH and catalyst concentration parameters was carried out, and the industrial effluent was then treated by photocatalytic reactions, adsorption, and photolysis. Industrial effluent was supplied by a packaging company and was collected for a period of seven consecutive days, showing the oscillation of the parameters in the process. The material characterizations were obtained by scanning electron microscopy (SEM-EDS), X-Ray diffraction (XRD), and photoacoustic spectroscopy (PAS). The results indicated that the composition of the quarry waste is mainly silica and has Egap 2.16 eV. The quarry residue as photocatalyst was active for the artificial effluent (synthetic dye solution), with a maximum of 98% discoloration, and as an adsorbent for industrial effluent, with a maximum of 57% of discoloration. Although the quarry residue has shown results lower than ZnO, it is considered a promising material in adsorption processes and photocatalytic reactions for discoloration of aqueous solutions.

**Keywords:** adsorption; photolysis; photocatalysis; quarry residue; discoloration; packaging industry

### **1. Introduction**

Dyes are chemical compounds that have the ability to color the surface of materials such as fabric fibers, packaging, and food. Industrial processes that use water and dyes in the production stages probably will present dye residual concentrations in effluent. Effluents, before being released into the environment or even reused, will need to undergo treatments to remove, among other compounds, their coloring.

Different techniques are performed to promote discoloration, including ultrafiltration, coagulation, flocculation, sonochemical decomposition, adsorption, biological treatments, heterogeneous photocatalysis, Fenton and photo-Fenton processes, advanced oxidation electrochemical processes, etc. [1–4].

Heterogeneous photocatalysis is classified as an advanced oxidative process [5] and has been widely studied due to its diversified application, such as the reduction of chromium VI to chromium III [6], mercury reduction [7], degradation of emerging pollutants such as caffeine [8], drugs [9,10], dyes [11], and reduction of bromate in water intended for human consumption [12].

**Citation:** Almeida, L.N.B.d.; Josué, T.G.; Nogueira, O.H.L.; Dias, D.T.; Tusset, A.M.; Santos, O.A.A.d.; Lenzi, G.G. Quarry Residue: Treatment of Industrial Effluent Containing Dye. *Catalysts* **2021**, *11*, 852. https:// doi.org/10.3390/catal11070852

Academic Editors: Gassan Hodaifa and Rafael Borja

Received: 10 June 2021 Accepted: 12 July 2021 Published: 16 July 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

In photocatalytic reactions, there are many variables that must be considered in order, such as the radiation source, pH of the reaction medium, temperature, and material used as a semiconductor material. The main materials used in the photoreactions are TiO<sup>2</sup> and ZnO as they are nontoxic and have good photochemical properties [13–16]; however, researchers commonly describe improvements in the catalysts' properties through new syntheses [17], the addition of metals [18,19], mixed oxides, etc.

In this direction, the search for new catalytic materials is also a very interesting and encouraging subject, particularly if this new material is a by-product in large quantities or a waste product without the exact destination of any industrial process.

The rice husk can be mentioned as an example; as agro-industrial residue, it has already been used as a source of silica for the synthesis of catalysts applied in the degradation of terephthalate acid [20] and discoloration of methyl violet dye [21].

Some quarries generate, in excess volumes, a by-product called stone powder or rock powder. This material has characteristics and chemical composition that varies and depends on the type of rock to be explored. In intrusive igneous rocks, silicon dioxide (SiO2) is one of the most significant components and the classification of the rock occurs according to the SiO<sup>2</sup> present amount: above 63% are acid igneous rocks, 52–63% are intermediate or neutral igneous rocks, 45–52% are mafic or basic igneous rocks, and <45% are ultramafic or ultrabasic igneous rocks [22]. Using this type of in-water treatment waste is a recent and promising proposal.

In this context, the present work contributes with a new material catalytic/adsorbent material, giving value a residue, as an alternative to traditional catalysts/adsorbent (mineral by-product). In addition, tests with industrial effluent, from the packaging industry, samples were collected for a period of seven consecutive days, considering the industrial process dynamics. The photocatalytic and adsorptive capacity of the stone powder was evaluated in the discoloration, applied both to the synthetic effluent and to the industrial effluent.

### **2. Results and Discussion**

### *2.1. Catalysts Characterizations*

Figure 1 shows the results for SEM-EDS, PAS, and XRD for ZnO and the mineral by-product (MbP). *Catalysts* **2021**, *11*, x FOR PEER REVIEW 3 of 13

(**b1**) (**b2**)

2.0 2.5 3.0 3.5 4.0

0 10 20 30 40 50 60 70 80 90

2Theta

E (eV)

3.16 eV

(**c1**)

(**d1**)

0

5

10

15

Intensity (u.a)

20

25

(Abs x E)2

1.0 1.5 2.0 2.5 3.0 3.5 4.0

2.16 eV

E (eV)

0 10 20 30 40 50 60 70 80 90

2Theta

(**c2**)

(**d2**)

**Figure 1.** Characterizations of ZnO and mineral by-product (MbP): (**a1**,**b1**,**c1**) and (**d1**) correspond to SEM, EDS, PAS and

XRD to ZnO, respectively. And (**a2**,**b2**,**c2**) and (**d2**) correspond to SEM, EDS, PAS and XRD to MbP, respectively.

0.00 0.01 0.02 0.03 0.04 0.05 0.06

0

5

10

Itensity (u.a)

15

20

25

(Abs x E)

2

(**a1**) (**a2**)

ZnO MbP

**Figure 1.** Characterizations of ZnO and mineral by-product (MbP): (**a1**,**b1**,**c1**) and (**d1**) correspond to SEM, EDS, PAS and XRD to ZnO, respectively. And (**a2**,**b2**,**c2**) and (**d2**) correspond to SEM, EDS, PAS and XRD to MbP, respectively. **Figure 1.** Characterizations of ZnO and mineral by-product (MbP): (**a1**,**b1**,**c1**) and (**d1**) correspond to SEM, EDS, PAS and XRD to ZnO, respectively. And (**a2**,**b2**,**c2**) and (**d2**) correspond to SEM, EDS, PAS and XRD to MbP, respectively.

The ZnO composition is defined by approximately 80% zinc and 20% oxygen (Figure 1), the material bandgap energy is 3.16 eV, and a crystalline definition in hexagonal wurtzite, which is the most common for this material. Such characteristics are similar to those already described in the literature [23–25].

For the MbP, there is a large number of elements in its structure; however, the elements O, Si, and Al correspond to approximately 90% of the material. In this way, is possible to compare the quarry residue used with quartz sand, which is also naturally composed of silica and other impurities such as Fe2O3, Al2O3, TiO2, CaO, MgO, and K2O [24]. Materials containing silica (SiO2) and alumina (Al2O3) can be found in the literature as supporting materials for catalysts [26,27]. Regarding the MbP crystallinity, the peaks identified in 2θ between 20 and 30◦ is characteristic of the hexagonal mesoporous silicate [28]; in addition, the MbP Egap value was close to 2.16 eV, being very close to Egap of Si nanoparticles from quartz sand, defined at 2.22 eV [29].

0.0

0.2

0.4

0.6

C/Co

0.8

1.0

**(a)**

#### *2.2. Experimental Tests 2.2. Experimental Tests*

#### PH and Catalyst Concentration PH and Catalyst Concentration

The experimental tests first carried out were aimed at identifying the optimal parameters, pH, and catalyst/adsorbent concentration for both industrial and synthetic effluents. Figures 2 and 3 indicate the results obtained for synthetic and industrial effluent and material used. All graphs referring to the experimental tests are presented as C/Co versus time (min), where C (mg L−<sup>1</sup> ) is the concentration at any time t (min), and Co is the initial concentration (t = 0). The experimental tests first carried out were aimed at identifying the optimal parameters, pH, and catalyst/adsorbent concentration for both industrial and synthetic effluents. Figures 2 and 3 indicate the results obtained for synthetic and industrial effluent and material used. All graphs referring to the experimental tests are presented as C/Co versus time (min), where C (mg L−1) is the concentration at any time t (min), and Co is the initial concentration (t = 0).

**Figure 2.** Catalyst concentration (**a,b**) and pH (**c,d**) tests for ZnO with synthetic and industrial effluent. **Figure 2.** Catalyst concentration (**a**,**b**) and pH (**c**,**d**) tests for ZnO with synthetic and industrial effluent.

**MbP Synthetic Effluent Industrial Effluent**  0.5 g L-1 1.0 g L-1 3.0 g L-1 Synthetic solution unadjusted pH 0.8 1.0 ~12% ~7% **(b)** Figures 2 and 3 have some aspects in common that can be highlighted; for example, when the industrial effluent was treated, there was a greater and faster solution discoloration. This fact is related to the effluent composition, that is, while the synthetic effluent contained only dye and distilled water, the industrial effluent had a large organic load. Figure 7 indicated that the presence of organic compounds can impair photocatalytic reactions since such compounds will compete for surface adsorption with the material's active sites.

 6.0 g L-1 9.0 g L-1 64% 1.0 gL-1 0.4 0.6 C/Co 1.0 g L-1 3.0 g L-1 Effluent ~54% ~38% 6.0 g L-1 Another aspect that can be highlighted is that the catalyst concentration has also been changed independently of the used material. When the photocatalytic process was applied in the synthetic effluent, 1.0 g L−<sup>1</sup> was the solution was satisfactorily discolored; on the other hand, for the industrial effluent, 6.0 g L−<sup>1</sup> was needed so that greater percentages of discoloration were achieved.

0 30 60 90 120 150 180 Time (min) 83% 91% 0 30 60 90 120 150 180 0.0 0.2 Time (min) 6.0 g L-1 9.0 g L-1 unadjusted pH Although with MbP in the 9.0 g L−<sup>1</sup> concentration, at the beginning of the reaction, it had a quick discoloration, the difference for when 6.0 g L−<sup>1</sup> was used was small; thus, this would not justify an increase of 50% in catalyst concentration for the reaction.

0.2

0.4

0.6

C/Co

0.8

1.0

**(c)**

0.0

0.2

0.4

0.6

C/Co

0.8

1.0

**(a)**

**Figure 2.** Catalyst concentration (**a,b**) and pH (**c,d**) tests for ZnO with synthetic and industrial effluent.

*2.2. Experimental Tests* 

PH and Catalyst Concentration

initial concentration (t = 0).

 0.5 gL-1 1.0 gL-1 3.0 gL-1 6.0 gL-1 9.0 gL-1

> ~91% ~96%

0 30 60 90 120 150 180

Synthetic solution ZnO 1 g L-1

1.0 gL-1

Time (min)

Time (min)

unadjusted pH (~4.2)

 pH 2 pH 6 pH 8

unadjusted pH (~4.2)

Synthetic solution unadjusted pH

**ZnO Synthetic Effluent Industrial Effluent** 

0.0

**(d)**

0.0

0.2

0.4

0.6

C/Co

0.8

1.0

0.2

0.4

Effluent unadjusted pH

Effluent ZnO 6 gL-1

**(b)**

0.6

C/Co

0.8

1.0

The experimental tests first carried out were aimed at identifying the optimal parameters, pH, and catalyst/adsorbent concentration for both industrial and synthetic effluents. Figures 2 and 3 indicate the results obtained for synthetic and industrial effluent and material used. All graphs referring to the experimental tests are presented as C/Co versus time (min), where C (mg L−1) is the concentration at any time t (min), and Co is the

0 30 60 90 120 150 180

Time (min)

0 30 60 90 120 150 180

 pH 2 natural pH (~6) pH 10 pH 4

unadjusted pH (~6)

Time (min)

 1.0 g L-1 3.0 g L-1 6.0 g L-1 9.0 g L-1

~55%

~55% ~45% ~40%

~24%

6.0 g L-1

~22% ~18%

**Figure 3.** Catalyst concentration (**a,b**) and pH (**c,d**) tests for MbP with synthetic and industrial effluent. **Figure 3.** Catalyst concentration (**a**,**b**) and pH (**c**,**d**) tests for MbP with synthetic and industrial effluent.

Figures 2 and 3 have some aspects in common that can be highlighted; for example, when the industrial effluent was treated, there was a greater and faster solution discoloration. This fact is related to the effluent composition, that is, while the synthetic effluent contained only dye and distilled water, the industrial effluent had a large organic load. Figure 7 indicated that the presence of organic compounds can impair photocatalytic re-The reaction speed is related to the photocatalyst concentration. As the concentration of catalysts increases, reaction speed is increased, as more active sites will be available. However, when this concentration reaches an optimum point, if more photocatalyst is added, the reaction may be impaired, and therefore, radiation penetration tends to decrease with the light screening effect, a factor often observed by different authors with different pollutants [30–32].

actions since such compounds will compete for surface adsorption with the material's active sites. Another aspect that can be highlighted is that the catalyst concentration has also Regarding the pH of the reaction medium, for both cases, it appears that the pH without adjustment, that is, the unadjusted pH proper to the effluent (synthetic ~4.2 or industrial ~6.2) were identified as ideal for photoreaction.

been changed independently of the used material. When the photocatalytic process was applied in the synthetic effluent, 1.0 g L−1 was the solution was satisfactorily discolored; on the other hand, for the industrial effluent, 6.0 g L−1 was needed so that greater per-The working advantage with the solution itself, that is, without adjustments, is the time that is saved in the adjustment steps before and after reactions. Figure 4 shows the results obtained for photolysis and adsorption tests (synthetic solution).

centages of discoloration were achieved. Both photolysis and adsorption results demonstrate the need for a photocatalystradiation combination for greater discoloration of the synthetic solution.

Although with MbP in the 9.0 g L−1concentration, at the beginning of the reaction, it had a quick discoloration, the difference for when 6.0 g L−1 was used was small; thus, this would not justify an increase of 50% in catalyst concentration for the reaction. The reaction speed is related to the photocatalyst concentration. As the concentra-After defining the pH and catalyst concentration conditions, i.e., pH without adjustment (unadjusted pH) and 6.0 g L−<sup>1</sup> of ZnO or MbP, the experimental tests were realized for samples collected from the industrial effluent during seven consecutive days.

tion of catalysts increases, reaction speed is increased, as more active sites will be available. However, when this concentration reaches an optimum point, if more photocatalyst is added, the reaction may be impaired, and therefore, radiation penetration tends to decrease with the light screening effect, a factor often observed by different authors with dif-

Regarding the pH of the reaction medium, for both cases, it appears that the pH without adjustment, that is, the unadjusted pH proper to the effluent (synthetic ~4.2 or

The working advantage with the solution itself, that is, without adjustments, is the time that is saved in the adjustment steps before and after reactions. Figure 4 shows the

results obtained for photolysis and adsorption tests (synthetic solution).

industrial ~6.2) were identified as ideal for photoreaction.

ferent pollutants [30–32].

*Catalysts* **2021**, *11*, x FOR PEER REVIEW 6 of 13

**Figure 4.** Photolysis and adsorption with synthetic solution, unadjusted pH, and concentration of 1 gL<sup>−</sup>1 of ZnO or MbP. **Figure 4.** Photolysis and adsorption with synthetic solution, unadjusted pH, and concentration of 1 gL−<sup>1</sup> of ZnO or MbP. justment (unadjusted pH) and 6.0 g L−1 of ZnO or MbP, the experimental tests were realized for samples collected from the industrial effluent during seven consecutive days.

~2% ~16%

#### *2.3. Industrial Effluent 2.3. Industrial Effluent*

0.8

1.0

Both photolysis and adsorption results demonstrate the need for a photocatalyst-radiation combination for greater discoloration of the synthetic solution. After defining the pH and catalyst concentration conditions, i.e., pH without ad-Molded pulp packaging, which is the product manufactured in the company where the industrial effluent was collected, is basically made of paper, water, and additives, and follows a process, as described in Figure 5. Molded pulp packaging, which is the product manufactured in the company where the industrial effluent was collected, is basically made of paper, water, and additives, and follows a process, as described in Figure 5.

**Figure 5.** The manufacturing process of molded pulp packaging. **Figure 5.** The manufacturing process of molded pulp packaging.

The first stage of the process is the formation of the pulp, which consists of water, chemical additives, and paper of different types; then, this pulp undergoes a process in which the gross impurities are removed. Subsequently, the mass proceeds to a storage tank that feeds the equipment used for molding, which basically consists of pressure and suction operations of excess water. Then, the ready packages are processed, by means of mats, for drying which, depending on the moisture conditions of the material, are heated from 180 to 240 °C for approximately 10 to 15 min and, finally, before advancing to the printing (the step in which it adds images and writings according to what was established by the client), the packages undergo a pressing process in which the objective is to provide a better finish and resistance [33]. The first stage of the process is the formation of the pulp, which consists of water, chemical additives, and paper of different types; then, this pulp undergoes a process inwhich the gross impurities are removed. Subsequently, the mass proceeds to a storage tank that feeds the equipment used for molding, which basically consists of pressure and suction operations of excess water. Then, the ready packages are processed, by means of mats, for drying which, depending on the moisture conditions of the material, are heated from 180 to 240 ◦C for approximately 10 to 15 min and, finally, before advancing to the printing (the step in which it adds images and writings according to what was established by the client), the packages undergo a pressing process in which the objective is to provide a better finish and resistance [33].

The first stage of the process is the formation of the pulp, which consists of water, chemical additives, and paper of different types; then, this pulp undergoes a process in

tank that feeds the equipment used for molding, which basically consists of pressure and suction operations of excess water. Then, the ready packages are processed, by means of mats, for drying which, depending on the moisture conditions of the material, are heated from 180 to 240 °C for approximately 10 to 15 min and, finally, before advancing to the printing (the step in which it adds images and writings according to what was established by the client), the packages undergo a pressing process in which the objective is to

provide a better finish and resistance [33].

**Figure 5.** The manufacturing process of molded pulp packaging.

Currently, the effluent that leaves the molding stage is reused without being subjected to any treatment at the inputs entrance for the pulp manufacture. However, it is very difficult to adjust the color of the next batch of packaging, since in some cases, other dyes are also used. This situation causes dissatisfaction among customers, as there is no color standardization. Therefore, based on the results we had in item 2.2, the effluent treatment was realized, aiming to identify the parameter variation and how the reactions would be conducted. Figure 6 shows the results. Currently, the effluent that leaves the molding stage is reused without being subjected to any treatment at the inputs entrance for the pulp manufacture. However, it is very difficult to adjust the color of the next batch of packaging, since in some cases, other dyes are also used. This situation causes dissatisfaction among customers, as there is no color standardization. Therefore, based on the results we had in item 2.2, the effluent treatment was realized, aiming to identify the parameter variation and how the reactions would be conducted. Figure 6 shows the results.

**Figure 6.** Tests of heterogeneous photocatalysis (**a**,**b**), adsorption (**c**,**d**), and photolysis (**e**) of industrial effluents. **Figure 6.** Tests of heterogeneous photocatalysis (**a**,**b**), adsorption (**c**,**d**), and photolysis (**e**) of industrial effluents.

Figure 7 shows the initial composition of each collected effluent sample and its subsequent composition after the reactions used. The variables total soluble solids and N (NO3); surfactants are not quantified in the graph since they always presented the same values of <100, <10, and <10, respectively. Figure 7 shows the initial composition of each collected effluent sample and its subsequent composition after the reactions used. The variables total soluble solids and N (NO3); surfactants are not quantified in the graph since they always presented the same values of <100, <10, and <10, respectively.

**Figure 7.** Organic load (**a**) initially and (**b**) after reactions with industrial effluents. The dashed lines indicate the initial average of COD, BOD, and TOC (mg L<sup>−</sup>1) of the 7 days from the collected effluent. **Figure 7.** Organic load (**a**) initially and (**b**) after reactions with industrial effluents. The dashed lines indicate the initial average of COD, BOD, and TOC (mg L−<sup>1</sup> ) of the 7 days from the collected effluent.

There was great variation in the efficiency of the tests performed, beginning with the photocatalytic tests that showed an average of 26% and 3% discoloration when used ZnO and MbP, respectively, which is a significant drop when compared to the results shown in Figures 3 and 4. Photolysis had unfavorable performance, both in the discoloration of the dye (Figure 6e) and in the organic load reduction (Figure 7b). Regarding adsorption, it appears that MbP was, generally, a good adsorbent of the present dye in the industrial effluent, presenting on average 22%. There was great variation in the efficiency of the tests performed, beginning with the photocatalytic tests that showed an average of 26% and 3% discoloration when used ZnO and MbP, respectively, which is a significant drop when compared to the results shown in Figures 3 and 4. Photolysis had unfavorable performance, both in the discoloration of the dye (Figure 6e) and in the organic load reduction (Figure 7b). Regarding adsorption, it appears that MbP was, generally, a good adsorbent of the present dye in the industrial effluent, presenting on average 22%.

All this unstable behavior observed during the reactions is related to the variation in the industrial effluent composition observed in Figure 7a. Changes in the reactions kinetic behavior when using a synthetic dye solution and when using an industrial effluent have already been observed in the literature [34] in which the authors evaluated the discoloration of synthetic effluent containing blue dye and industrial textile effluent and verified a decrease in the efficiency of the process when industrial textile effluent was used. In the literature [35], they also verified superior results when they treated a synthetic effluent containing dye rather than real industrial effluent. Both authors justified this behavior due to the effluent's composition, such as the presence of interfering ions All this unstable behavior observed during the reactions is related to the variation in the industrial effluent composition observed in Figure 7a. Changes in the reactions kinetic behavior when using a synthetic dye solution and when using an industrial effluent have already been observed in the literature [34] in which the authors evaluated the discoloration of synthetic effluent containing blue dye and industrial textile effluent and verified a decrease in the efficiency of the process when industrial textile effluent was used. In the literature [35], they also verified superior results when they treated a synthetic effluent containing dye rather than real industrial effluent. Both authors justified this behavior due to the effluent's composition, such as the presence of interfering ions and organic load.

and organic load. Organic compounds, when present in the solution, can compete for the photogenerated hydroxyl radical and the active sites available on the catalyst surface with the Organic compounds, when present in the solution, can compete for the photogenerated hydroxyl radical and the active sites available on the catalyst surface with the target pollutant, which, in the present research, was the dye CI basic yellow 96, which can decrease the efficiency of photocatalysis [36].

target pollutant, which, in the present research, was the dye CI basic yellow 96, which can decrease the efficiency of photocatalysis [36]. When comparing the initial composition of this effluent with other effluents already studied, it can be highlighted that the effluent from the molded pulp packaging industry When comparing the initial composition of this effluent with other effluents already studied, it can be highlighted that the effluent from the molded pulp packaging industry has a high organic load with high values of COD, BOD, and TOC when compared with other effluents, as reported in the literature [37,38].

has a high organic load with high values of COD, BOD, and TOC when compared with other effluents, as reported in the literature [37,38]. In addition to the concentration reduction of dye present in the effluent, it is necessary to evaluate the organic load decrease. It is noted that for all reactions, except for the photolysis reactions, a decrease in the organic load occurs, mainly in terms of TOC. This parameter (TOC) is related to the pollutant mineralization; in the present study, it is possible to notice that the dye degradation and the mineralization of the same occurred simultaneously [39,40]. It is important to note that adsorption has also been shown to be In addition to the concentration reduction of dye present in the effluent, it is necessary to evaluate the organic load decrease. It is noted that for all reactions, except for the photolysis reactions, a decrease in the organic load occurs, mainly in terms of TOC. This parameter (TOC) is related to the pollutant mineralization; in the present study, it is possible to notice that the dye degradation and the mineralization of the same occurred simultaneously [39,40]. It is important to note that adsorption has also been shown to be effective in reducing organic matter. The decrease in organic load in the industrial effluent brings benefits to the water quality since the organic load has less capacity to pollute the effluent.

effective in reducing organic matter. The decrease in organic load in the industrial effluent brings benefits to the water quality since the organic load has less capacity to pollute the effluent. In the present work, it is possible to affirm that MbP has catalytic properties; however, it also presented an adsorbent behavior in some experimental tests. MbP is a In the present work, it is possible to affirm that MbP has catalytic properties; however, it also presented an adsorbent behavior in some experimental tests. MbP is a low-cost material available in large quantities, which can bring financial benefits to the companies involved. Quartz sand, which is a material very similar to the by-product studied in the present work, has also been applied as an adsorbent and has shown promising results such

as the methylene blue dye adsorption [41], together with graphene in the adsorption of Hg2+ e Pb2+ [42] and the adsorption of Ciprofloxacin [43].

Using waste or by-products as catalysts is a recent proposal and involves numerous challenges. Some authors have already described the performance of residues as catalysts and adsorbents in the most diverse degradations and adsorption, such as, for example, the study [44], in which they studied the use of depleted acacia bark (agro-industrial waste) as support for photocatalysts used in the degradation of organic phenolic pollutants, and the study [45], in which they used bauxite residue from the aluminum industry as a new photocatalyst for hydrogen generation. However, in most research studies, the photocatalytic reactions use the residues only as a support for catalysts such as TiO<sup>2</sup> or else as a source of some metal to be impregnated in the semiconductor. Studies are carried out with pollutants present in synthetic solutions, which facilitate the reproducibility of the results and applicability favor the new catalyst.

### **3. Materials and Methods**

### *3.1. Chemicals*

The used dye in the synthetic solution reactions was supplied by the same company that supplied the real effluent; this dye is known as CI Basic Yellow 96 (BASF–Solenis), is used to dye the molded pulp packaging in industrial processes, has a yellow color, and liquid form.

The materials used as catalysts and adsorbents were (i) zinc oxide (ZnO) as a reference material, supplied by Dinâmica Química Ltd.a (Sao Paulo, Brazil) and (ii) quarry residue, in this work defined as a mineral by-product (MbP).

The MbP was supplied by a company located in the region of Campos Gerais in the state of Paraná—Brazil. This company operates in the sale of stones and gravel, asphalt, providing services in the area of earthworks and paving. The rock explored is classified as intrusive igneous granite. Both ZnO and MbP were used without previous treatment; however, their grain size was standardized. Both were sieved in order to obtain particles smaller than 0.3 mm. This characteristic is observed in the ZnO (used as a reference).

### *3.2. Catalysts Characterizations*

3.2.1. Scanning Electron Microscopy (SEM) Associated with Dispersive Energy Spectroscopy (EDS)

Samples were metalized with gold using IC-50 ION COATER (Shimadzu) for 10 min. The topographic surface images were obtained using a scanning electron microscope model VEGA 3 LMU brand TESCAN, complete with, 30 kV W filament, 3.0 nm resolution, retractable SE and BSE detectors, low-vacuum mode (500 Pa) chamber with an internal diameter of 230 mm, and a CCD camera for viewing the sample chamber. The microscope is also equipped with EDS Detector, model AZTec Energy X-Act, resolution 130 eV, brand Oxford.

### 3.2.2. X-ray Diffraction (XRD)

A Bruker D8 Advance X-ray diffractometer, 2 from 5 to 80◦ , with 2◦/min in the scan, 40 kV and 35 mA was used. The result obtained was then analyzed using standards published by the International Center for Diffraction Data (ICDD).

### 3.2.3. Photoacoustic Spectroscopy (PAS)

A source with Xenon lamp emitted a light that passed through a monochromator, (Oriel, model 66,936 (1/4 m), with inlet and outlet slits adjusted to 3.00 mm. The frequency of light modulation was controlled by a mechanical modulator (Stanford Research Systems, model SR 540) which, with a photodiode, provided a reference signal for the amplifier (lockin). The microphone attached to the photoacoustic cell (Brüel & Kjaer, model BK 4953) was connected to a power source and a preamplifier. The microphone signal was transferred to a synchronized amplifier (EG & G Instruments, model 5110), and the amplifier provided intensity, and the phase of the photoacoustic signal was transferred to a personal computer; the spectra (modulation frequency 23 Hz) were normalized with respect to the carbon signal. Direct bandgap energy (Egap) was acquired from a linear fit in the graph obtained from the square of the absorption coefficient ((Abs <sup>×</sup> (1240/λ))<sup>2</sup> ) as a function of photon energy (E) (1240/λ) [25,41].

### *3.3. Experimental Tests*

The synthetic effluents were prepared with distilled water with a dye concentration of 10 µL L−<sup>1</sup> , and industrial effluents were supplied by the molded pulp packaging.

The experimental tests were carried out in a jacketed borosilicate reactor (cooling water temperature 13 ◦C) to maintain the effluent temperature at approximately 25 ◦C; the nominal volume was 600 mL, and 500 mL of effluent (artificial or industrial) was used for the reactions. The system contained a mercury vapor lamp (250 W), coupled just above the reactor, that was open to the environment. Magnetic stirring and air bubbling (0.5 L min−<sup>1</sup> ) into the reaction medium were used. During the reaction, samples were collected, centrifuged, and the dye residual concentration was analyzed in a UV–Vis spectrophotometer (Femto-800 XI) with a wavelength of 429 nm, dye characteristic.

The photolysis tests followed the same reaction system, but without the presence of airflow and catalyst, to verify only the radiation action under the discoloration reaction. The adsorption tests were realized without radiation and airflow to verify the adsorptive capacity of the studied materials.

### 3.3.1. PH and Catalyst Concentration Influence

For the pH tests, NaOH and HCl solutions were used for the adjustments, and the values were adjusted between 2 and 10; in addition, tests were also carried out without pH adjustment (unadjusted pH): synthetic effluent ~4.2 and industrial effluent ~6.2. For the catalyst concentration optimization, values between 0.5 and 9 g L−<sup>1</sup> were evaluated. Both tests were carried out with artificial and industrial effluent preceding the tests carried out for one week.

### 3.3.2. Industrial Effluent—Treatment for One Week

After the conditions (pH and catalyst concentration) were defined, adsorption, photolysis, and photocatalytic tests were carried out for one week (seven days) to treat the industrial effluent provided by the molded pulp packaging industry. Every day, the effluent was collected immediately at the molded outlet and then transported to the laboratory where the tests were carried out in parallel, following the same conditions described in item 2.3. Industrial effluents were subjected to experimental tests as they were received, without previous treatment.

### *3.4. Characterization of Industrial Effluent*

The effluent was characterized quantitatively, identifying and comparing the parameters before and after each reaction. The analyzed parameters were biological oxygen demand (BOD), chemical oxygen demand (COD), total organic carbon (TOC), total soluble solids (TSS), surfactants, and nitrogen–nitrate N(NO3). The characterization was performed on the Pastel UV–Secomam equipment, with all analyses performed in duplicates.

This equipment determines the quality of the water and effluents. The method is based on spectral deconvolution where the ultraviolet spectrum hypothesis of effluents can be mostly modeled by a limited number of stable spectrums called the reference spectrum. Many studies have already used it to determine the quality of the effluents [42–45].

### **4. Conclusions**

The use of photocatalysts resulting from industrial waste in the effluents' treatment is favorable. However, factors such as the composition variation of the material and the pollutant should be considered, mainly because when it comes to industrial effluents, it is difficult to identify a pattern. In this research, it was found that a quarry by-product

has similar characteristics quartz sand, which is a material already used as a catalyst support and also as an adsorbent for organic pollutants. It was noted that the material has catalytic activity when used as a photocatalyst, showing behavior favorable for synthetic dye solutions. However, when applied to the industrial effluent discoloration from the packaging industry, with a high organic load, it presented better behavior, in most tests, as an adsorbent. Although it has shown lower results than ZnO, MbP can be considered a promising material in the treatment of aqueous solutions.

**Author Contributions:** Conceptualization, L.N.B.d.A.; T.G.J. and G.G.L., methodology, L.N.B.d.A.; T.G.J.; O.H.L.N. and G.G.L. validation, L.N.B.d.A.; T.G.J.; A.M.T. and G.G.L., formal analysis, G.G.L.; A.M.T.; O.A.A.d.S. and D.T.D., investigation L.N.B.d.A.; T.G.J.; O.H.L.N.; A.M.T. and G.G.L., data curation, L.N.B.d.A.; G.G.L. and A.M.T., writing—original draft preparation, L.N.B.d.A.; T.G.J. and G.G.L., writing—review and editing, L.N.B.d.A.; T.G.J., D.T.D. and G.G.L., visualization, L.N.B.d.A.; T.G.J. and G.G.L., supervision, G.G.L. and O.A.A.d.S., project administration, L.N.B.d.A.; G.G.L. and O.A.A.d.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors would like to thank the CNPq and CAPES for the financial support, UEM, and Multi-User Characterization Center in Materials Research and Development (C2MMa) for analyzes carried. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**

