Catalytic Degradation of 4-Ethylpyridine in Water by Heterogeneous Photo-Fenton Process

Abstract

In this work, the degradation of 4-ethylpyridine (4EP) in water by a heterogeneous photo-Fenton process (H₂O₂/Fe₃O₄/ultraviolet irradiation (UV)) was investigated. More rapid and effective 4EP degradation was obtained with H₂O₂/Fe₃O₄/UV than Fenton-like (H₂O₂/Fe₃O₄) and UV/H₂O_2, which is due to the larger production of hydroxyl radicals from the chemical and photolytic decomposition of H₂O₂. The operational conditions were varied during 4EP degradation experiments to evaluate the effects of pH, catalyst, concentration, and temperature on the kinetics and efficiency of H₂O₂/Fe₃O₄/UV oxidation. Under optimal conditions (100 mg/L 4EP, [H₂O₂] = 1000 mg/L, Fe₃O₄ = 40 mg/L, pH = 3 and room temperature, 300 rpm), 4EP was totally declined and more than 93% of the total organic carbon (TOC) was eliminated. Liquid chromatography analysis confirmed the formation of aromatic and aliphatic intermediates (4-hydroxypyridine, 4-pyridone, malonic, oxalic, and formic acids) that resulted in being mineralized. Ion chromatography analysis demonstrated the stoichiometric release of NH4⁺ ions during 4EP degradation by heterogeneous photo-Fenton oxidation. The reuse of the heterogeneous catalyst was evaluated after chemical and heat treatment at different temperatures. The heat-treated catalyst at 500 °C presented similar activity than the pristine Fe₃O₄. Accordingly, heterogeneous photo-Fenton oxidation can be an alternative method to treat wastewaters and groundwater contaminated with pyridine derivatives and other organic micropollutants. The combination of heterogeneous photo-Fenton oxidation with classical biological methods can be proposed to reduce the overall cost of the treatment in large-scale water treatment plants.

1. Introduction

Pyridine derivatives are synthetic compounds that are commonly used in the production of commercial products such as food aromas, paints, dyes, rubber products, waterproof fabrics, coatings, pesticides, fungicides, and antioxidants [1,2,3]. Pyridine derivatives include mainly alkyl pyridines, halogenated pyridines, and amino-pyridines, which have been largely used as the basic raw materials for the manufacturing of industrial solvents, agriculture herbicides, and pesticides. 4-ethylpyridine (4EP) is among the pyridine derivatives utilized in coking, chemical synthesis, pharmaceutical, and pesticide industries [4,5,6,7,8]. It is characterized by its high toxicity and risk of carcinogenicity, and can pose human health and environment concerns [5,6,7,8,9]. In addition, 4EP is soluble in water, very stable and difficult to biodegrade [10,11,12,13,14]. It is largely spread across the environment due to its extensive use and can contaminate groundwater and surface waters [8,9,10,11,12,13,14]. The conventional biological remediation process treatment methods are not able to completely remove 4EP from contaminated water [15,16,17]. The search for a treatment method to eliminate it from water, transform it to a biocompatible form, or lessen its toxicity is urgently desired.

Particularly, advanced oxidation processes (AOPs) showed high potential to degrade various bioresistant and hazardous organic pollutants in water [18,19,20]. These processes are capable of mineralizing several types of organic pollutants, including phenols, polyphenols, pesticides, pharmaceuticals, etc. [19,21]. The great efficacy of AOPs is mainly due to the generation of powerful oxidizing radicals, mainly hydroxyl radicals (HO^•) [22,23].

These unstable radical species with a very short lifetime and a high standard potential (E^◦ = 2.80 V/SHE) react immediately and non-selectively with organic pollutant molecules leading to total mineralization into CO₂, H₂O, and other inorganic ions, or partial transformation to substances easy to biodegrade [22,23,24]. In particular, photo-Fenton oxidation (UV/H₂O₂/Fe²⁺) is a good option to reduce and eliminate many pollutants during wastewater treatment. It is considered as the most effective H₂O₂-based process in the treatment of various effluents containing highly toxic and bio-resistant organic compounds [20,25,26].

In this process, HO^• radicals generated from H₂O₂ decomposition by UV photolysis and by ferrous iron catalyst (Equations (1) and (2)):

$${\mathrm{Fe}}_{\mathrm{aq}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2{}_{\mathrm{aq}}\to {\mathrm{Fe}}_{\mathrm{aq}}^{3+}+{\mathrm{HO}}^{•}+{\mathrm{H}}^{+}{}_{\mathrm{aq}}$$

(1)

$${\mathrm{H}}_2{\mathrm{O}}_2{}_{\mathrm{aq}}+\mathrm{h}\mathsf{\nu }\to {\mathrm{HO}}^{•}+{\mathrm{HO}}^{•}$$

(2)

However, high amounts of Fe²⁺ are needed to attain complete removal of organic pollutants, which results in another environmental problem related to the discharge of treated water or sludge contaminated with iron, and it also increases the operational costs of the treatment process. Using heterogeneous iron catalysts can solve these economic and environmental problems since the catalyst can be assembled at the end the treatment and recycled many times [27,28,29,30,31,32,33,34]. Natural and synthetic magnetite, Fe₃O₄, containing both Fe²⁺ and Fe³⁺ catalytic sites, was successfully used to degrade organic pollutants [34,35,36,37,38,39]. The decomposition of H₂O₂ on the surface Fe₃O₄ produces radicals species (HO₂^• and HO^•) that are capable of degrading any type of organic pollutant according to the following equations (Equations (3) and (4)):

$$\parallel {\mathrm{Fe}}_{\mathrm{Surface}}^{3+}+{\mathrm{H}}_2{\mathrm{O}}_2\to \parallel {\mathrm{Fe}}_{\mathrm{Surface}}^{2+}+{\mathrm{HOO}}^{•}+{\mathrm{H}}^{+}$$

(3)

$$\parallel {\mathrm{Fe}}_{\mathrm{Surface}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2\to \parallel {\mathrm{Fe}}_{\mathrm{Surface}}^{3+}+{\mathrm{HO}}^{•}+{\mathrm{H}}^{+}$$

(4)

The catalyst can be easily collected by magnet or by filtration and be regenerated and recycled for additional experiments. The efficiency of natural and synthetic Fe₃O₄ heterogeneous catalysts in the production of HO^• radicals by decomposition of H₂O₂ was demonstrated by several authors [34,35,36,37,38,39]. Furthermore, it was also reported that the combination of Fenton-like oxidation using Fe₃O₄ as a catalyst with UV irradiation enhanced the efficiency of the whole process [34,35,36]. The hydroxyl radicals produced by photo-decomposition of H₂O₂ (Equation (2)) attack the molecules adsorbed on the surface of the catalyst degrading them and make more catalytic sites available. The formation of stable iron (III)-carboxylate complexes would be minimized and higher mineralization yield would be obtained.

The aim of this present work was to investigate the degradation of 4EP in water by heterogeneous photo-Fenton oxidation using Fe₃O₄ as a catalyst and to demonstrate the reuse of the heterogeneous catalyst several times in photo-Fenton oxidation. Different methods of activation of the catalyst were used, including acid treatment, drying at room temperature, and heat treatment at 100 °C, 250 °C, and 500 °C. The effects of principal experimental parameters, such as initial pH, Fe₃O₄ dose, H₂O₂, and 4EP concentrations on the performance of 4EP degradation and mineralization by heterogeneous photo-Fenton process were evaluated. 4EP and the formation of organic intermediates during degradation experiments were monitored by high-performance liquid chromatography (HPLC). The formation of inorganic nitrogen species (NO₃⁻, NO₂⁻, NH₄⁺) as mineralization end-products were monitored by ion chromatography. Accordingly, a simple 4EP degradation mechanism by the heterogeneous photo-Fenton process was proposed.

2. Materials and Methods

2.1. Chemicals

4EP (C₆H₅NO₂) and H₂O₂, 30% (w/w) solution were purchased from Fluka. Other chemicals such as Fe₃O₄ (powder, particles size < 5 μm, 95%), H₂SO₄, NaOH and Na₂SO₃ were of analytical grade Sigma-Aldrich chemicals. The preparation of synthetic solutions was performed using deionized water with resistivity > 18 MΩ cm⁻¹ at 25 °C from a Millipore Milli-Q system. The eluents (CH₃CN and CH₂O) used in HPLC analysis were of HPLC grade and received from Merck.

2.2. Analytical Methods

Total organic carbon (TOC) was measured by an Analytik Jena TOC analyzer to follow up the mineralization of 4EP in aqueous solutions. An ion chromatograph (Shimadzu LC-20A) equipped with a Shodex IC YK-421 column was used to analyze the inorganic ions (nitrate, nitrite, and ammonium) formed during the mineralization. A mixture of 5.0 mM tartaric acid, 1.0 mM dipicolinic acid, and 24.3 mM boric acid was used as the eluent in isocratic mode (1.0 mL min⁻¹). The analysis of 4EP and its degradation intermediates was carried out using liquid chromatography (HPLC–UV), with an Agilent 1100 unit at constant temperature (25 °C) equipped with a Phenomenex Gemini 5 μm C18 column. The wavelength of the UV detector was performed at 260 nm. A lineal gradient chromatographic elution was used by initially running 10% of Solvent B ascending to 100% in 40 min. Solvent A was composed of 25 mM of formic acid water solution, and Solvent B was acetonitrile. All the samples withdrawn from the treated solutions underwent a filtration aver 0.20 μm membranes before analysis. Absorbance measurements and absorption spectra of 4EP aqueous solutions were recorded using a Shimadzu UV-1603 spectrophotometer in quartz cells (1 cm). The pH was determined using an InoLab WTW pH-meter.

2.3. Photo-Fenton Experiments

The photochemical experiments were carried out in the reactor equipped with a mercury vapor UV lamp (Heraeus Noblelight) (YNN 15/32) with a power input of 125 W, a magnetic stirrer, and a thermometer. The photo-reactor (Pyrex) of 2 L capacity was placed in the first part. The lamp was located in an axial position submerged in a vertical immersion tube contained in a vertical cooling tube and immersed in the solution. Water was circulated between the lamp and glass vessel. The experiment was mounted on a magnetic stirrer. In this work, mercury vapor lamps emit UV irradiation at 254 nm, which is mainly absorbed by H₂O₂. The volume of 4EP wastewater was 1 L. The pH of the solution was adjusted to the desired values by addition of sodium hydroxide or sulfuric acid. After pH adjustment, a given weight of Fe₃O₄ was added. The magnetite was mixed very well with the solution of 4EP wastewater to form a homogeneous suspension. After the light of the lamp, a precise amount of 30% hydrogen peroxide was mixed with the suspension formed. At certain time intervals, samples of 10 mL volume were taken from the solution. The reaction was quenched with Na₂SO₃ and then analyzed immediately to determined pH. The samples were filtered through 0.20 μm nylon filters and analyzed for inorganic ions, TOC, target compounds and intermediates, and UV–VIS absorbance at a wavelength of λ = 260 nm.

3. Results and Discussions

3.1. Comparative Performance of H₂O₂/Fe₃O₄, UV/H₂O₂, and UV/H₂O₂/Fe₃O₄ Processes

In order to highlight the importance of combining the Fenton-like process and UV irradiation, the performance of single and combined processes, including UV irradiation, H₂O₂ oxidation (H₂O₂ = 1000 mg/L), Fe₃O₄ adsorption (Fe₃O₄ = 40 mg/L), H₂O₂/Fe₃O₄ (Fe₃O₄ = 40 mg/L and H₂O₂ = 1000 mg/L), UV/H₂O_2, and UV/H₂O₂/Fe₃O₄ (Fe₃O₄ = 40 mg/L and H₂O₂ = 1000 mg/L) in treating 4EP aqueous solutions were compared. Figure 1 presents 4EP removals from 100 mg/L 4EP aqueous solutions at pH 3 using the single and combined processes over 60 min. For the single processes (UV irradiation, H₂O₂ oxidation, Fe₃O₄ adsorption), 4EP removals did not exceed more than 6% indicating the slowness of the reactions involved in the elimination of 4EP from water. UV irradiation alone does not have enough energy to rupture all the chemical bonds in 4EP molecules. Fe₃O₄ active adsorption surface sites adsorb only a few 4EP molecules because of its low specific area. Even H₂O₂ oxidation showed a higher 4EP removal compared to the two other single processes (UV irradiation and Fe₃O₄ adsorption), its power is not sufficient to oxidize large 4EP molecules in water.

However, combined processes showed higher 4EP removals than single processes. The heterogeneous photo-Fenton (UV/H₂O₂/Fe₃O₄) process achieved the highest 4EP removal of 95%, however, the Fenton-like (H₂O₂/Fe₃O₄) and UV/H₂O₂ processes removed 65% and 43% 4EP from water within 60 min. For further investigations of 4EP degradation kinetics using the combined processes, the changes of 4EP concentration with time are plotted and presented in Figure 2 for each of the combined processes. This figure shows that the 4EP concentration declined with time for all combined processes indicating the degradation of 4EP molecules. The highest 4EP removals were measured after 120 min, 180 min, and 360 min for the heterogeneous photo-Fenton (99%), Fenton-like (95%), and UV/H₂O₂ (94%) processes, respectively. The degradation of 4EP in water by each of the combined processes follows pseudo-first-order kinetics as confirmed by the linear plots of Ln[4EP]₀/[4EP]_t vs. time (inset of Figure 2). Pseudo-first-order rate constants of 0.007 min⁻¹, 0.02 min⁻¹, 0.05 min⁻¹ are calculated for UV/H₂O₂, Fenton-like, and heterogeneous photo-Fenton processes, respectively. These results demonstrate that heterogeneous photo-Fenton process is more rapid and more efficient than UV/H₂O₂ and Fenton-like processes in degrading 4EP in water. The higher effectiveness of the heterogeneous photo-Fenton (UV/H₂O₂/Fe₃O₄) oxidation can be due to the larger production of hydroxyl radicals from both the catalytic decomposition of H₂O₂ by Fe₃O₄ catalyst, and the photo-decomposition of hydrogen peroxide by irradiation UV.

3.2. Detailed Onvestigations of the Degradation of 4EP by the Heterogeneous Ohoto-Fenton Process

The comparative experiments demonstrated the effectiveness of the heterogeneous photo-Fenton process in degrading 4EP in water. However, taking into account the fact that the treatment of 4EP by the heterogeneous photo-Fenton process aims to mineralize it into CO₂, H₂O, and NO₃⁻ (or NH₄⁺) or to transform it into easily biodegradable compounds, further experiments are needed to optimize the operating conditions targeting the integration of this method in larger applications than the lab scale. For that purpose, a series of experiments were conducted to examine the effects of some experimental parameters on the kinetic and the efficiency of the treatment of 4EP synthetic wastewaters by UV/H₂O₂/ Fe₃O₄ process. Initial pH, Fe₃O₄ dose, and H₂O₂ and 4EP concentrations are the experimental parameters selected to be optimized to reach the highest yields of 4EP degradation and mineralization.

Fenton and photo-Fenton reactions are strongly pH-dependent as reported in the literature [40,41,42,43]. Especially, it is well reported that the initial pH conditions significantly influence the production of hydroxyl radicals [20,43,44,45,46]. To examine the effect of initial pH on the treatment of aqueous solutions containing 100 mg/L of 4EP by the heterogeneous photo-Fenton process, the initial pH values was varied in the range between 3 and 9 using 1000 mg/L H₂O₂ and 40 mg/L Fe₃O₄ at room temperature and a stirring rate under 300 rpm. The effect of the initial pH on TOC removal is given in Figure 3. This figure shows that TOC decreased from the beginning of the treatment for all initial pH values. The highest TOC removal was obtained at pH 3. Increasing pH from 3 to 5, to 7, and then to 9 decreased TOC removal. During the treatment of 4EP aqueous solutions by the heterogeneous photo-Fenton process, TOC declined from 56 mg C/L to 2.1 mg C/L at pH 3.0, to 12.3 mg C/L at pH 5.0 and 17.5 mg C/L at pH 7.0 and 9.0 after 360 min. This indicates that more than 96% of TOC removal was achieved at pH 3.0, while TOC removals between 68% and 78% were calculated for the other initial pH values. The TOC removal during the treatment of 100 mg/L 4EP aqueous solution using the homogeneous photo-Fenton process (UV/H₂O₂/Fe²⁺: 1000 mg/L H₂O₂, 10 mg/L Fe²⁺ (similar amount of iron used with Fe₃O₄)) achieved 98% at pH 3.0, 72% at pH 5.0, 56% at pH 7.0, and 37% at pH 9.0 after 360 min (data not shown here). These results demonstrated that the initial pH affects the efficiency of the treatment for both heterogeneous and homogeneous photo-Fenton processes; however, the lower effect on TOC removal was observed with the heterogeneous photo-Fenton. The difference between the heterogeneous and homogeneous photo-Fenton processes can be related to the slower regeneration of the homogeneous catalyst (Fe²⁺) at pH > 5 because of the precipitation of Fe(OH)₃. In addition, the decrease in the TOC removal in both the homogeneous and heterogeneous photo-Fenton processes could be due to the acceleration of auto-decomposition of H₂O₂ leading to the lower production of hydroxyl radicals [20,42,43,44,45]. The analysis of iron leached from the Fe₃O₄ solid catalyst is needed in this case to have an idea about the effect of the pH on the dissolution of iron during the treatment at different initial pH values.

Table 1. Analysis of dissolved iron during the treatment of 100 mg/L EP aqueous solutions by the heterogeneous photo-Fenton using 40 mg/L Fe₃O₄ and 1000 mg/L H₂O₂ at pH 3 under 300 rpm stirring and 25 °C.

Time	pH = 3.0	pH = 5.0	pH = 7.0	pH = 9.0
0 min	0.08 mg/L Fe	0.05 mg/L Fe	ND	ND
60 min	0.58 mg/L Fe	0.19 mg/L Fe	0.05 mg/L Fe	0.01 mg/L Fe
120 min	1.96 mg/L Fe	1.43 mg/L Fe	0.11 mg/L Fe	0.02 mg/L Fe
180 min	2.13 mg/L Fe	1.51 mg/L Fe	0.45 mg/L Fe	0.05 mg/L Fe
360 min	2.37 mg/L Fe	1.62 mg/L Fe	0.63 mg/L Fe	0.14 mg/L Fe

illustrates the results of the analysis of iron by ICP–OES during the heterogeneous photo-Fenton treatment in the same conditions as Figure 3. The results show that small amounts of iron were measured in solution at neutral and alkaline media (0.63 and 0.14 mg/L at pH 7 and pH 9); however, 25% and 17% iron was dissolved at pH 3 and pH 5 (9.65 mg/L iron are contained in 40 mg/L Fe₃O₄) at the end of the treatment. Based on the data obtained in the present study, an initial pH between 3.0 and 5.0 can achieve higher TOC removal than 75% using the heterogeneous photo-Fenton process with less than 20% of the iron being leached from the Fe₃O₄ solid catalyst.

The catalyst dose is considered as a determining parameter for the production of radical oxidant species from the H₂O₂ decomposition (Equations (1) and (3)) [44,45,46]. Iron(II)/Iron(III) in solid form or in solution are excellent catalysts to decompose H₂O₂ efficiently into hydroxyl radicals HO^•. The dose of the catalyst should be carefully adjusted to achieve high degradation yields and avoid the formation of stable iron(II)/(III) organic complexes. To evaluate the effect of the Fe₃O₄ catalyst, some experiments of heterogeneous photo-Fenton treatment of 100 mg/L 4EP synthetic wastewater were carried using a fixed H₂O₂ concentration of 1000 mg/L at pH 3, and varying the dose of Fe₃O₄ in the range from 0 to 100 mg/L at room temperature. The changes of TOC with time at different doses of Fe₃O₄ are presented in Figure 4. These results display that the TOC decreased with time for all Fe₃O₄ doses from the beginning of the treatment. The addition of Fe₃O₄ catalyst accelerated the TOC decay compared to the experiment in its absence. The increase of Fe₃O₄ concentration up to 40 mg/L improved the kinetics and the efficiency of the heterogeneous photo-Fenton process (87% and 96% TOC removals were calculated for 20 and 40 mg/L Fe₃O₄, respectively). However, higher doses of Fe₃O₄ than 40 mg/L decreased the efficiency of the degradation of 4EP by heterogeneous photo-Fenton process in terms of TOC removal. Using 80 and 100 mg/L Fe₃O₄ resulted in 85% and 74% TOC removals, respectively. It can be concluded that the most rapid TOC decay and the highest TOC removal were obtained with 40 mg/L Fe₃O₄.

These results can be explained by the acceleration of the production of radical species (mainly hydroxyl radicals), the latter can reduce preferably Fe(III) sites on the catalyst surface when higher Fe₃O₄ doses than 40 mg/L are added instead of degrading 4EP and its intermediates. The reuse of the catalyst was tested by the collection of the solids by magnet and washing them several times with deionized water. The collected solids were heat treated at temperatures of 105, 180, 250, and 500 °C.

Table 2. Results of the reuse of the catalyst during the treatment of 100 mg/L EP aqueous solutions by the heterogeneous photo-Fenton using 40 mg/L treated catalyst and 1000 mg/L H₂O₂ at pH 3 under 300 rpm stirring and 25 °C.

Heat Treatment (240 min)	Mass Collected (mg/L)	% TOC Removal (360 min)
T = 105 °C	36.2	64
T = 180 °C	33.6	70
T = 250 °C	32.4	76
T = 500 °C	30.3	89 (85/87/84/86)

presents the results of heat treatment and the reuse of the catalyst in treating 100 mg/L 4EP aqueous solutions using 1000 mg/L H₂O₂, 40 mg/L reused catalyst at pH 3 and under 300 rpm stirring and room temperature. The increase of the temperature of the heat treatment decreased the mass of the catalyst, which can be explained by the dehydration and the release of volatile organics adsorbed on the surface of the catalyst with possible transformation of the catalyst by oxidation/reduction reactions in presence of air. In contrast, the increase of the heat treatment temperature enhanced TOC removal, which keep values higher than 64% for the treated catalyst. The same catalyst was collected, then treated at 500 °C, and reused in treating 4EP aqueous solutions in the same conditions. TOC removals in the range of 83%–89% were measured indicating the good efficacy of the reused catalyst. These results confirm the possible reuse of the solid catalyst for several times in heterogeneous photo-Fenton treatment of 4EP in water.

The efficacy of the Fenton and photo-Fenton processes in treating wastewaters containing organic pollutants depends largely on the production of hydroxyl radicals by chemical and photochemical decomposition of hydrogen peroxide [40,41,42]. H₂O₂ is the main source of hydroxyl radicals even if little amount of OH^• can be formed by water photolysis. Consequently, the optimization of H₂O₂ initial concentration ([H₂O₂]₀) is fundamental to achieve a high production of hydroxyl radicals [18,19,20]. To examine the effect of H₂O₂ initial concentration on the performance of the heterogeneous photo-Fenton process, aqueous solutions containing 100 mg/L 4EP at pH 3.0 and 25 °C were treated by H₂O₂ initial concentration in the range of 500–2000 mg/L. The dose of Fe₃O₄ was calculated for each H₂O₂ initial concentration from the mass ratio H₂O₂/Fe₃O₄ fixed to 25. The results illustrated in

Table 3. Influence of H₂O₂ and EP concentrations on TOC removal during the treatment by the heterogeneous photo-Fenton process (UV/H₂O₂/Fe₃O₄) during 120 UV irradiation at pH 3 and under 300 rpm stirring and 25 °C.

[H2O2]0 (mg/L)	[EP]0 (mg/L)	% TOC Removal
500	100	86
1000	100	93
2000	100	81
1000	50	92
1000	100	93
1000	200	84

confirm that the efficiency of the heterogeneous photo-Fenton process depends on the H₂O₂ initial concentration. The results of Table 1 showed that the TOC removal increased from 86% to 93%, then it decreased to 81% after 120 min of adding 500, 1000, and 2000 mg/L of H₂O₂, respectively.

The fact that the highest TOC removal was achieved with 1000 mg/L H₂O₂ initial concentration is essentially due to the production of satisfactory amounts of HO^• radicals capable to mineralize most of the organic matter. Although, the increase of H₂O₂ dose to higher than 1000 mg/L would theoretically result in a larger production of HO^• radicals, it did not result in an enhancement of the TOC removal. This may be due to the consumption of HO^• radicals by excessive amounts of H₂O₂ (Equation 5 and 6) and the acceleration of H₂O₂ disproportionation into O₂ and H₂O (Equation (7)) [42,43,44,45,46,47,48]:

$${\mathrm{HO}}^{•}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{H}}_2\mathrm{O}+{{\mathrm{HO}}_2}^{•}$$

(5)

$${\mathrm{HO}}^{•}+{{\mathrm{HO}}_2}^{•}\to {\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2$$

(6)

$$2{\mathrm{H}}_2{\mathrm{O}}_2\to 2{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2$$

(7)

To evaluate the effect of 4EP initial concentration on the efficiency of the heterogeneous photo-Fenton process, pH and temperature were fixed at 3 and 25 °C, and the mass ratio H₂O₂/Fe₃O₄ and H₂O₂]/4EP were maintained constant and equal to 25 and 10, respectively. The 4EP initial concentration was varied in the range of 50–200 mg/L (see Table 3). Increasing 4EP initial concentration from 50 to 100 mg/L increased TOC removal from 87% to 93% after 120 min; while increasing the 4EP initial concentration from 100 to 200 mg/L decreased TOC removal to 84%. This is mainly due to the smaller production of radical species limited by H₂O₂ concentration and Fe₃O₄ dose for low 4EP initial concentrations and to the absorption of UV light by 4EP molecules when high 4EP initial concentrations are treated.

Figure 5a shows 4EP and TOC (in mg C/L) decay with time during the treatment of 100 mg/L 4EP aqueous solution by the heterogeneous photo-Fenton process using 1000 mg/L H₂O₂, 40 mg/L Fe₃O₄ at pH 3 under 300 rpm stirring and room temperature. As it can be seen, 4EP and TOC concentrations decline with time with different rates. As it can be observed, 4EP decay is more rapid than TOC indicating 4EP degradation into intermediates without CO₂ release, which undergoes several oxidation steps to mineralize at the end of the treatment. The UV–VIS absorption spectra plotted in Figure 5b confirm the opening of the pyridine ring by the disappearance of the band located at 260 nm after 60 min. HPLC analysis detected the formation of 4-pyridone and 4-hydroxypyridine as aromatic intermediates and maleic and oxalic acids as aliphatic intermediates. Figure 5c presents the changes of the peak area of each intermediate with time during the heterogeneous photo-Fenton treatment of 100 mg/L 4EP aqueous solution. 4-hydroxypyridine and 4-pyridone were formed at the beginning of the treatment and disappeared within 30 min. Maleic acid was detected during the first 60 min, while oxalic acid was accumulated during the treatment. These results confirm the formation of aromatic intermediates, which undergo an opening of the pyridine ring to form aliphatic carboxylic acids (maleic acid and formic acid). The mineralization of organic pollutants containing heteroatoms involves the release of inorganic ions. To obtain more details about the degradation of 4EP by the heterogeneous photo-Fenton oxidation, additional chemical analysis (ion chromatography) was conducted. Figure 5d shows the change of the concentration of nitrates, nitrites, and ammonium ions with time during the treatment of 100 mg/L 4EP by the heterogeneous photo-Fenton process under optimal conditions. The concentration of ammonium increased linearly from the beginning of the treatment to reach a plateau after 60 min. Only small amounts of nitrate and nitrite ions (<0.2 mg/L) were detected. This result indicates that the organic nitrogen contained in 4EP was released stoichiometrically as ammonium ions.

Based on HPLC, ion chromatographic, and TOC analyses, a simple mechanistic scheme can be proposed for the degradation of 4EP by the heterogeneous photo-Fenton process (see Figure 6). 4EP degradation begins by a rapid release of ethyl group as formic acid and its substitution by hydroxyl radical to form 4-pyridone and 4-hydroxypyridine (tautomeric forms). These intermediates undergo oxidative opening of pyridine ring by hydroxyl radicals to form aliphatic carboxylic acids (oxalic and formic acids) and release ammonium ions. Finally, the carboxylic acids end to mineralize into CO₂ and H₂O in the final stages of the treatment.

4. Conclusions

The results show that 4EP degradation by the heterogeneous photo-Fenton process (UV/H₂O₂/Fe₃O₄) was the most rapid and most efficient among the single processes (UV irradiation, Fe₃O₄ adsorption, and H₂O₂ oxidation) and the combined processes (Fenton-like (H₂O₂/Fe₃O₄) and UV/H₂O₂). This can be explained by higher production of radical species mainly HO^• radicals from both the catalytic decomposition of H₂O₂ on the surface of Fe₃O₄ catalyst, and the photodecomposition of H₂O₂ by irradiation UV. The highest TOC removal achieved from 100 mg/L 4EP aqueous solutions by the heterogeneous photo-Fenton process was obtained using 1000 mg/L H₂O₂, 40 mg/L Fe₃O₄ at pH 3.0 under 300 rpm and room temperature. However, 25% of the total iron contained initially in Fe₃O₄ solid catalyst was dissolved in water. The catalyst was collected and heat treated at 105 °C, 180 °C, 250 °C, and 500 °C; then it was reused in the heterogeneous photo-Fenton oxidation of 4EP under the same conditions. The heat-treated catalyst reuse resulted in TOC removals greater than 64%, but the highest TOC removals (>83% after being reused five times) were obtained with the catalyst heat-treated at 500 °C. The detailed mechanistic and kinetic studies demonstrated a pseudo-first order degradation. 4EP degradation involves several stages starting with formation of aromatic intermediates by release of carboxylic group followed by oxidative opening of pyridine ring to form aliphatic carboxylic acids that end to mineralize into CO₂, H₂O, and NH₄⁺. These results confirm that the heterogeneous photo-Fenton process could be effectively used to treat wastewaters contaminated with food additives including vitamins. Additionally, treated wastewaters can be recycled in several agricultural and industrial applications and/or reused in landscapes/gardens irrigation systems.