Environmental Sustainability of the Removal of Alpaca Fiber Dye Using a Thermally Modified Sludge from a Drinking Water Treatment Facility

Abstract

In this work, the removal of dye using thermally modified sludge from a drinking water treatment facility (DWTS) was evaluated. This study gives value to the waste from the coagulation flocculation process (waste sludge) in order to remove an emerging organic agent (Bordeaux B). The sustainability of the process leads to a circular economy, which represents an important environmental contribution. The physicochemical characterization of the DWTS was carried out by standard methods. DRX and FTIR spectroscopy, SEM, and superficial specific area S_BET N₂ at 77 K were used. Thermal activation processes were carried out (200–600 °C) to obtain the best activated thermal conditions for dye removal (T: 500 °C). Muscovite and other minerals were found in the DWTS. Experimental conditions (batch mode) were determined: contact time (CT), pH, adsorbent dose (AD), and dye initial concentration (Co). S_BET = 54.77 and 67.90 m²/g by DWTS and TA-500. The best removal efficiency was achieved at 500 °C (R = 85.57 ± 0.76 %, q max = 37.45 ± 0.14 mg/g), which, compared to other unconventional adsorbents, is more reliable and competitive. The adsorption process was adjusted to the Langmuir mathematics model, following pseudo-second-order kinetics (R² = 0.99).

1. Introduction

The most important solid waste generated in drinking water treatment plants (DWTPs) is drinking water treatment sludge (DWTS) [1]. Its production rate is estimated to be 100,000 tons/year in a conventional DWTP [2]. On a global scale, approximately 10,000 tons of DWTS is produced daily [3]. Gomes et al. [4] showed that in countries such as Portugal, Taiwan, and Spain, around 6.42, 5.06, and 2.59 tons/inhabitant/year are produced, respectively. Therefore, it is necessary to be able to start considering DWTS as a resource rather than a waste [1,2]. In this way, the drinking-water-producing industry would contribute to the circular economy, generating a sustainable and recycling-oriented culture.

Currently, developing countries dispose of DWTS by discharging it into bodies of water, sewers, landfills or wastelands due to the minimal or lack of environmental regulations [2]. The greatest risk associated with the improper disposal of DWTS is the possible leaching of Al, Fe, and heavy metals [5,6]. Operational costs related to its disposal in sanitary landfills can be significant due to the fact that its world production rate increases at the same rate as the demand for drinking water [1,3].

Lots of researchers have been adding contributions that allow the reuse of DWTS from a scientific point of view by, for example, improving the physical and chemical characteristics of the soil, raw materials, or additives in the construction industry [4,7]; developing coagulants or adjuvants in water purification processes [8,9,10]; introducing adsorbents of various contaminants, such as phosphorus [10], heavy metals, and metalloids [11,12], emerging pollutants [13], and dyes [14,15,16].

The textile industry uses substantial amounts of water and chemicals in their processes, so the generated effluents loaded with different compounds (organic and inorganic) must be treated prior to discharge. Dyes are qualified as the most environmentally important compound due to their degree of impact at low concentrations and their toxicity in ecosystems [17,18]. These compounds reduce the concentration of oxygen in the water because they block the passage of sunlight and cause the death of many organisms through consumption or contact [17,18]. For dye elimination or removal, there are physical, chemical, and biological methods, which are characterized as being practical, highly flexible, low cost, easy to operate, and non-sensitive to toxic contaminants and physical methods, specifically, adsorption processes, and they are easy to design [19,20].

The most commonly used adsorbent worldwide, thanks to its high efficiency, is activated carbon; however, its high production costs and demand on resources represent problems for application on an industrial scale [20,21]. Therefore, using the waste generated by various local industries, such as DWTS, to remove textile dyes from wastewater is a cheap, sustainable, and efficient solution that has been developed and researched for some years now [15,16,22,23,24,25,26,27,28]. The physicochemical composition of DWTS depends on (1) the quality of the raw water withdrawn (surface or underground) and (2) the methodology used in the purification process, specifically the type and dose of coagulant applied [2,5,12,29,30]. This is why the physicochemical characteristics of DWTS vary; therefore, they condition the efficiency of the dye adsorption process. Therefore, four main criteria for sustainability supported this study: environmental (dye removal and use of the residual sludge without environmental liabilities), economic (adsorbent production cost), social (increasing employment), and technical sustainability (innovation in the thermal process).

The present work aimed to investigate the adsorption of an organic textile dye called Bordeaux B by an adsorbent generated from the thermal modification of DWTS and to study the effect of several operational parameters (initial dye concentration, adsorbent dose, pH of the solution, and contact time), as well as the kinetics of the process on the adsorption efficiency. This study adds value to the waste produced from the coagulation flocculation process (waste sludge) in order to remove an emerging organic agent (Bordeaux B). The sustainability of the process leads to a circular economy, which represents a significant environmental contribution.

2. Materials and Methods

2.1. Molecular Structure of Bordeaux

Bordeaux B dye was obtained from a local textile company in Arequipa, Perú. We obtained three-dimensional images of the molecular structure’s dimension, one-dimensional images, and a photo showing the granular form of Bordeaux B, as shown in Figure 1.

The optimized molecular structures of these dyes are presented in one- and three-dimensional planes (3D) (Gauss View Program v 6.0; Gaussian 16 Program) [31]. The visualization and quantum calculation of the dimensioning of these structures allowed us to establish differences in terms of their shapes and sizes (Programs VMD and Gaussian 19 respectively) [32]. Considering the degree of complexity according to its conformation and the calculation of its molecular weight, 1082.0968 g/mol was obtained. These molecules are azo dyes with very complex conjugation structures π ($N=N)$, naphthalenes and sulfonic groups ${SO}_3^{-}$ that interact noncovalently with atoms such as ${Na}^{+}$ and ${Cr}^{3+}$. These functional characteristics present certain similarities with less complex conventional dye structures than those used in other studies; these molecules denote resistance to natural biodegradation due to the aromatic rings present in their structure [33].

2.2. Bordeaux B Dye Analysis

Spectral conditions defined with great precision and accuracy were applied. The spectral scan was conducted using a UV-Vis spectrophotometer Thermo Scientific Genesys 150 (USA), in order to obtain a maximum absorption wavelength (λ = 519 nm).

At 25 °C, the dye solutions pH was stable (pH = 6.46 ± 0.1 and pKa =3.67 ± 0.04), this was measured using a JENWAY model M3520 pH meter (China). The Bordeaux B solution pH value was found slightly below neutrality, so it is possible to call it anionic or acid dye [33]. In addition, the pKa of the dye indicates that at a pH value below (3–4), the internal and superficial charge concentration will be older, so they are more protoned. The pKa value obtained is similar to those reported by other authors who also worked with anionic dyes [34,35,36].

The dye analysis results applying different pH values (3.34–11.25), showed that absorbance (λ₅₁₉) didn’t change considerably. It is possible that sulfonic and naphthalene azo groups are present in the dye molecular structure, affecting these conditions [37].

2.3. Adsorbent: Drinking Water Treatment Sludge (DWTS)

The DWTS was obtained from decanters in the Flocculation-Coagulation System of a Drinking Water Treatment Plant in the City of Arequipa -Peru. The DWTS was washed with distilled water and dried in an oven at 103–105 °C during 24 h.

2.4. Thermal Modification

During 2 h at different temperatures (200, 300, 400, 500 and 600) °C in an oven, the DWTS was calcined using the THERMO SCIENTIFIC model FB1310M, muffle furnace, and then it was washed with distilled water until reaching pH 7 and dried in a FDS056 BINDER model oven at 105 °C for 24 h. The DWTS samples (T-200, T-300, T-400, T-500 and T-600) were stored in the desiccator until further use. A control test was considered for the dye. Crude DWTS photographs and the activated DWTS at different temperatures were taken from the settling unit (Figure 2). The activation temperature, time and sludge calcination conditions were defined based on the results shown in a previous research by Laib et al. [26], Nageeb Rashed et al. [27] and Tony [15] and the exploratory tests developed in the laboratory.

2.5. DWTS Characterization

The DWTS samples were characterized by apparent density and pH of the material, moisture content, and ash content analysis according to ASTM D2854-96 [38], ASTM D3838-80 [39], ASTM D2867-04 [40], and ASTM D2866-99 [41], standards, respectively. An X-ray diffraction (XRD) analysis of the sample was done by using a RIGAKU Miniflex 600 model diffractometer with filtered Cu Kα radiation (n = 1.5418 Å) and operated at 40 kV and 15 mA. The XRD pattern was recorded from 3 to 90° with a 2 Theta (θ) measurement range. For the FTIR analysis, 0.05 mg DWTS samples, along with 50 mg of KBr, were crushed in an agate mortar, then, using a SPECAC Mini Pellet Press, the samples were manually compressed into 1.75 TONS to form pellets that were placed in the sample holder of the Nicolet Summit Thermal Scientific model spectrometer which registered between 500 and 4000 cm⁻¹. The surface analysis of the samples—specific surface area, average pore diameter and average pore volume were determined through the generation of adsorption isotherms with 77 K N₂, having 25 adsorption points and 25 desorption points (50 analysis points) using an ANTON PAAR Nova 600 model physical sorption sort meter equipment.

2.6. Batch Adsorptions Test: Operation Variables

The operation variables are shown in

Table 1. Operation variables applied in mode Batch experiments.

Variable	Unit	Levels
Activation temperature	°C	200, 300, 400, 500, 600 and 700
CT	Min	5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55 and 60
pH solutions	Units	3, 4, 5, 6, 7, 8 and 9
AD	g/L	1, 1.5, 2, 5 and 7.5
Co	mg/L	20, 30, 40, 50, 60, 70 and 80

.

A factor-by-factor evaluation method was used to evaluate the effect of variables such as pH (6 to 10) and DTWS activation temperature (200 to 600) °C on the dye removal efficiency. Experiments were conducted at 0.5 g/L, 50 mL absorbent concentration and 80 mg/L dye concentration. The dye removal efficient (R) was determined using the following Equation (1):

$$R (\%)={\displaystyle \frac{C_0-C_e}{C_0}} \ast 100$$

(1)

where: R = Dye removal efficient (%); C₀ and C_e = initial and equilibrium concentration respectively (mg/L).

The adsorption capacity was calculated following the Equation (2):

$$q_e={\displaystyle \frac{{(C}_0-C_e) \ast V}{m}}$$

(2)

where: q_e = adsorbate amount present in the adsorbent when it is balanced (mg/g); m = adsorbent mass (g) and V = solution volume (L).

The Freundlich and Langmuir mathematic models were applied to describe the adsorptive processes. In addition, pseudo first and second order kinetics models were applied.

3. Results

3.1. Adsorbent Characterization

The total content of moisture and flying ash in the raw DWTS were 91.93 ± 1.42% and 24.87 ± 0.21%, respectively. The obtained values were similar to the ones reported by other authors [3,42]. In addition, the FT found (FT = 24.87 ± 0.21%) is typical for Drinking Water Treatment Plants (DWTP) that capture water resources from surface sources due to the presence of organic matter [43].

3.2. Bordeaux B Removal Capacity on Activated DWTS

The removal capacity (%) results of the DWTS activated at different temperatures on Bordeaux B dye are shown in Figure 3.

T-500 (Figure 3) was the activated adsorbent with the highest Bordeaux B removal percentage (R = 53.16 ± 1.71%) under defined operating conditions (CT = 30 min, AA = 5 g/L, Co = 50 mg/L, pH solutions = 6.46, Agitation = 350 RPM and T = 25 ± 0.5 °C).

The standard deviations were similar, so it is possible to apply inferential statistical models that allow comparing the means for each level or TA [44]. In this sense, the statistical comparison for all the means, applying Fisher and Tukey’s LSD, was obtained. Both methodologies agree that the TA at 500 °C differs from others, significantly (p = 0.05). For this reason, it is possible to affirm that the T-500 was the most suitable adsorbent for the process, and it is significantly different from the others. Hence, all the subsequent experiments were carried out using the T-500 adsorbent. In Figure 4, the Dry DWTS y Activated DWTS (T-500) is shown.

This result differs from that reported by Nageeb Rashed et al. [27], who obtained a greater removal with 700 °C adsorbent activation, using “Methylene Blue” dye (both pyrolyzed for 1 h). Tony [15] found that at a 400 °C activation temperature, the adsorption of the “Protion Blue” dye was much higher than at (600 and 800) °C (all calcined for 2 h). Laib et al. [26] concluded that at TA = 300 °C, it was possible to obtain a greater removal of “Reactive Blue 19” and “Methylene Blue” dyes. The results of this investigation and those of all the aforementioned authors agree that by subjecting the DWTS to a temperature higher than 105 °C, the R of the dye increases considerably. In this case, it went from R = 25.54 ± 1.35% (DWTS) to R = 53.16 ± 1.71% (T-500). This may be due to the elimination of certain organic compounds present in the sludge and/or to the formation of functional groups with a higher chemical and/or electromagnetic affinity to the adsorbate molecules [15,24,26,27,28].

However, it is inferred that the adsorptive capacity of the DWTS was the result of favorable operational conditions for the Al-O and Fe-O (+) functional groups, present in the minerals that make up the adsorbent, to achieve removing the anionic dye from the solution. The minerals that favored the adsorption of the anionic dye, both in DWTS and in T-500 were Magnetite, Albite and Anorthoclase, since these minerals have a positively charged net surface and could interact with the dye and retain it through electrostatic or ionic mechanisms [45,46,47]; Possibly, the increased presence of the alkaline feldspar called Anorthoclase in T-500 was responsible for increasing the R of the dye, having a predominantly positive net surface charge, thank the amount of cations in its composition [43].

It is unlikely that the high presence of Muscovite in DWTS and Illite in T-500 influenced the adsorption of the dye, because, in media close to neutrality, these 2:1 non-expandable clays have a low anion adsorption capacity since the charges of the DWTS structure have a negatively charged net surface [36,48,49]; even more when the pH does not favor the isomorphic substitution of their interlaminar charges [50,51]. Similarly, Dolomite, Lime and Calcite did not intervene in the adsorption of the dye because these minerals, in media with neutral pH, possess a negative net surface charge [46,52]; so, as those minerals possess the same charge as the dye, they repel its molecules [51].

It is worth mentioning that the minerals involved in the removal of “Bordeaux B” in a near-neutral medium do not behave in the same way as in an alkaline or acidic pH medium, as seen in the following sections; this is mainly due to the particular and pH-dependent ion exchange capacity of each mineral, especially those of the silicate group—Kaolinite, Ilite, Muscovite—and those of the feldspar group—Anorthoclase and Albite [45,46].

In addition, the decrease in the percentage of R removal at TA > 500 °C, may be due to a mineralogical and structural re-composition of particles present in the DWTS. Gadekar & Ahammed [14], Tong et al. [28] and Tony [15] reported the synthesis of hydroxides and zeolites by subjecting the DWTS to TA between (550–650) °C in non-inert atmospheres. This would indicate that having a high concentration of negative surface charges [45,46] decreases the adsorption of dye molecules charged with the same sign (-) on the surface of the adsorbent.

3.3. DWTS and T-500 Physicochemical and Surface Characterization

Dry DWTS and TA-500 photos are shown in Figure 4. For every 1.5 L of raw wet DWTS, approximately 30 g of dry DWTS was generated—that is, a 20 g yield of dry DWTS was obtained for every 1 L of crude DWTS (17.28% mass loss).

The two adsorbents differ in color, the first one having a darker shade of brown when compared to the second one (T-500). This color change is characteristic, due to the presence of clay minerals that are transformed into ceramic minerals with temperature increase [53].

The percentage of mass lost is low due to thermal activation (17.68%), if compared to what is reported in the literature [30,42,54]. This mass loss may be due to the elimination of organic material and water remaining in the dry DWTS, both inside the particles and in the interstitial spaces, at temperatures higher than 400 °C [27,30,53,54,55]. The apparent density (AD) corresponding to both (Dry DWTS and TA-500) was 0.83 ± 0.01 g/mL. In the meantime, the pH here was around 6 units.

The Da for both adsorbents remained constant. This would indicate their potential use in adsorption processes in continuous flow columns forming part of a fixed bed, (Da > 0.3 g/mL), since they would not offer significant mechanical resistance [43]. Besides, this characteristic allows them to be used in different applications as an additive for construction materials [53,54]. The apparent density value obtained for both samples was lower than that reported by Yang et al. [42].

The pH value obtained for both adsorbents presents a slight variation from 6.20 for the Dry DWTS to 6.88 for the T-500. This may be due to the transformation of minerals and functional groups (iron or aluminum oxides) during the thermal activation process [42,54]. The pH values obtained are similar to those reported in the literature [3,7,30,56].

Regarding their mineralogical composition, as shown in Figure 5, Muscovite, Albite, Kaolinite and Illite were the main crystalline components found in the dry DWTS. Their presence is very common in the sludge produced by DWTP that captures raw water from surface sources [7,42,53,54,55]. The high silicate clay percentage of “Muscuvite or Muscovite” in the DWTS sample is characteristic of areas with a history of volcanic activity, since this mineral of igneous origin is the product of the cooling and crystallization process of magma upon reaching the earth’s surface [46,52]. In that sense, through a study carried out by INGEMMENT, Díaz & Ramírez [57] reported that the Cerro Colorado District and its surroundings have a substantial percentage of “Muscovite” in their lithological formations. This would indicate that the igneous rocks presenting this mineral in their composition, are eroded and transported, by abrasive agents such as rain, towards the riverbed from which the water that the DWTP “La Tomilla” treats, is extracted.

The main mineral components found in T-500 were Illite, Quartz, Anorthoclase and Muscovite (Figure 5). As seen in Figure 5, the reduction in the amount of Muscovite, Albite and Kaolinite in the T-500 with respect to the dry DWTS, may be due to the dihydroxylation of its mineral structure thank the increase in temperature (dehydration of the layer basal) which caused its crystalline network to reorganize in a new phase (recrystallization), possibly synthesizing it in Illite, Anorthoclase or Metakaolin (amorphous), the latter being the product of the transformation of Kaolinite [7,15,42,54,55]. Moreover, the increased presence of Quartz in the T-500 compared to the dry DWTS, may be due to the recrystallization of the amorphous silicon oxides present in the dry DWTS sample [7,42,53,55].

In general, the DWTS usually contains a high content of Al(OH)₃ due to the coagulating agent used by the DWTP (as it happens in this case), however, the diffraction peak for this compound cannot be detected due to its amorphous colloidal nature [42].

The results of the superficial functional group analysis can be seen in

Table 2. Results of the DWTS and T-500 superficial functional group analysis.

Sample	Wave Number (cm−1)	Associated Functional Group	References
Dry DWTS	3439 and 3621	O-H	Dash et al. [49], Laib et al. [26], Likus et al. [30], Pigatto et al. [54], Tantawy & Mohamed [55] and Yang et al. [42].
	1636	O-H-O
	954	Al-OH
	468	Si-O-Si/Si-O-Al
	536	Fe-O/Al-O
T-500	3431	O-H
	1630	O-H-O
	955	Al-OH
	472 and 795	Si-O-Si/Si-O-Al

.

The analysis of the functional groups present on the surface of the dry DWTS and the T-500, showed that the bands 3439, 3431 and 3621 cm⁻¹ were associated with the O-H functional group, belonging to the water molecules that conform the hydrated mineralogical structures of both samples (structural water); the bands 1636 and 1630 cm-1 were associated with the H-O-H functional group which is characteristic for the vibration of water molecules present in the interlaminar zone of both samples; the bands 954 and 955 cm⁻¹ were associated with the Al-OH functional group, present in the feldspathic minerals of both samples; bands 472, 468 and 795 cm⁻¹ were associated with the Si-O-Si and Si-O-Al functional groups, belonging to the structure of the silicate minerals of both samples; the 536 cm⁻¹ band observed only in the DWTS, were associated with both the Fe-O group, present in the magnetite, and the Al-O group, present in the mineral oxides and hydroxides of the sample. This would indicate that the thermal activation at 500 °C of DWTS caused the signal associated with this functional group to disappear, possibly due to the transformation of the magnetite, aluminum oxides and hydroxides. The results of the FTIR analysis (Table 2) coincide with the results of the XRD (Figure 5 and Figure 6).

Additionally, qualitative infrared spectroscopy analysis performed on the T-500 saturated with “Bordeaux B” dye confirmed the presence of the organic functional groups of the dye on the adsorbent: the 1039 cm⁻¹ band was associated with the CO group [58], the 1553 cm⁻¹ band was associated with the C=O group [37], the 1444 cm⁻¹ band was associated with the aromatic C=C group [37,58], the 1487 cm⁻¹ band was associated with the azo group N=N- [59] and the 1712 cm⁻¹ band was associated with the amide NH₂ group [59]. Most of the groups found in the saturated adsorbent are present in the “Bordeaux B” dye which was studied and characterized by Colina et al. [37]. This indicates that the dye was actually adsorbed by the adsorbent and that it was not the operational conditions that caused its disappearance of the dye in the synthetic sample. Figure 7 shows the spectra resulting from the FTIR analysis for the sample of (A) DWTS, (B) T-500 and (C) T-500 sat-urated with the “Bordeaux B” dye.

Besides, the results of the surface analysis can be seen in

Table 3. DWTS and T-500 N₂ 77 K surface analysis results.

Sample	Parameter	Units	Results
DWTS	Specific surface area	m2/g	54.77
	Average pore volumen	cm3/g	0.01
	Average pore diameter	nm	3.14
T-500	Specific surface area	m2/g	67.90
	Average pore volume	cm3/g	0.01
	Average pore diameter	nm	3.00

.

The specific surface area increased from 54.77 m²/g for the dry DWTS to 67.90 m²/g for the T-500 (Table 3). This may be due to the removal of organic material and/or remaining water in the interstitial spaces, meso and micropores of the DWTS particles which would be modifying their morphology, and consequently, their specific surface area [27,30,54]. The specific surface area of the DWTS found, using the S_BET adsorption model (54.77 m²/g), is within the average range reported by other authors who also studied DWTS from surface waters [7,11,13,14,26,42,60,61,62], however, this value is much lower compared to DWTS from groundwater [30,56]. This difference is possibly caused by the mineralogical characteristics of the sediments present in groundwater [43]. Likewise, the increase in the specific surface area of T-500, compared to the inactivated one found, using the S_BET adsorption model (67.90 m²/g), coincides with what was reported by Abo-El-Enein et al. [11] and Pigatto et al. [54], however, Yang et al. [42] reported the reverse effect in their research. This may be due to both the morphology of the DWTS studied and its mineralogical composition [7].

In addition to this, the adsorbents did not present any difference regarding the average pore volume (Table 3), however, the average pore diameter presented a meaningless difference. The values found for both adsorbents and both parameters are well below those reported in the literature [11,54]. Likewise, the adsorption and desorption isotherms of N₂ at 77 K for both adsorbents presented in Figure 8, indicated little presence of the hysteresis phenomenon (DWTS > T-500). This would demonstrate that both samples do not have micro and/or mesopores of considerable volume in their structure, that is, they are not very porous materials.

3.4. Effect of Adsorbent Variables (CT, pH, AD and Co) on the Removal of Bordeaux R in the T-500

The results obtained to evaluate the effect of TC, pH, AD and Co on the R removal efficiency of the T-500 over the Bordeaux B dye are shown in Figure 9.

Figure 9 shows the best performance conditions of the adsorptive variables, considering the R removal efficiency of the T-500 over the Bordeaux B dye, were the following: CT = 50 min (R = 65.13 ± 0.28%), pH = 3.48 units (R = 83.78 ± 0.65%), Da = 5 g/L (R = 84.29 ± 0.63%) and Co = 40 mg/L (R = 85.57 ± 0.76%). Likewise, it can be observed that the standard deviations are similar, so it is possible to apply inferential statistical models which allow comparing the means for each data group [44].

Considering the statistical comparison for all the means analyzed, these differ in other values studied for each variable (p = 0.05). The mean interval that is repeated for both is between CT = (35 and 60) min. This may be due to the fact that the adsorption processes generally reach equilibrium at a certain time (CT), after which the removal of the adsorbate is negligible or null [63,64], the treatments that obtained a significantly equal mean, due to the adsorbent, was at the saturation point. Before this, it is possible to state that, for practical purposes, the Contact Time that allows efficient removal of the dye can be anything between (35 and 60) min. Therefore, all subsequent experiments were worked with a 45 min (R = 64.75 ± 0.29%) Contact Time.

Kayranli [25] reported that for all the dyes in his research, the best contact time was 100 min. Tong et al. [28] determined that at 150 min, the adsorbent lost all its capacity for removing “Acid Red G” dye from the solution. Nageeb Rashed et al. [27] and Elmontassir et al. [23] reported that at 60 min contact time, the adsorption process reached equilibrium for the “Methylene Blue” dye. Similarly, Gomonsirisuk et al. [24] indicated the same results for the “Brilliant Green” dye with respect to contact time. Also, Tony [15] reported that to reach removal equilibrium of the “Protion Blue” dye, an exposure time of 120 min. was necessary. On the other hand, Laib et al. [26] reported in their research that both the “Reactive Blue 19” dye and the “Methylene Blue”, reached equilibrium at 30 min. Although, the various contact times reported by the literature differ from the one found in this research (50 min), its value is found within the reported ranges mentioned (30–150) min.

Nevertheless, in most cases, it was observed that the highest removal percentage for all the dyes analyzed is between 2 and 30 min. This can be explained because at the beginning of the reaction; as time goes by, these interactions lose intensity due to the decrease of active sites (micropores and saturated functional groups) [15,22,23,24,25,26,27,28].

With respect to the effect of the pH solutions on the T-500 removal efficiency an R = 83.78 ± 0.65% at a pH value = 3.48, in the operational conditions previously defined (CT = 45 min, Da = 5 g/L, Co = 50 mg/L, Agitation = 350 RPM y T = 25 ± 0.5 °C), was obtained. To get to that pH value, 250 ± 10 µL sulfuric acid (H₂SO₄) at 0.1 N was added to 300 mL of dye. The means test reported statistically significant differences (p = 0.05) between the pH value pH = 3.48 and the rest of the analyzed values.

From the literature consulted on anionic dye removal it was observed that: Chu [22] reported that for the “Dianix Blue” and “Ciba-corn Yellow” the optimum pH was 9.13; Kayranli [25] found that for the dyes “Direct Blue 71”, “Acid Blue 40” and “Reactive Blue 29” dyes, the optimum pH was 5; Tong et al. [28] showed that in order to efficiently adsorb the “Acid Red G” dye, a 7.5 pH should be used; Laib et al. [26] reported that the “Reactive Blue 19” dye requires a pH equal to 3 to be removed as efficiently as possible.

Although some pH values reported by the literature differ from the one found by the present research (pH = 3.48), this can be due to the interaction between the adsorbent and the dye depending on the physical chemical characteristics of both, as well as the operational conditions of the process [19,61]. However, in general, an acid pH value favors anionic dye adsorption in cationic adsorbents, but a more alkaline pH favors adsorption of cationic dyes in anionic adsorbents [34,50].

In that sense the pH value of the solution is one of the most important key factors for the adsorption process, being the electrostatic charge of the adsorbent and the ionization state of molecules in the dye solution a condition for both [15,22,23,24,25,26,27,28]. It is possible to infer that the increase of R for the T-500 at low pH values (pH < 4) can be caused by the amino and sulphonic groups present in the organic dye “Bordeaux B” [37] being increasingly protoned (+), according to the pH of the solution approaching the value of its pKa (3.67 ± 0.04) (Figure 10). In addition to this, the effect of an acid pH on the T-500 could cause the fixed and variable charges on silicate clays present in the adsorbent (especially Illite) (Figure 10), to obtain a predominantly negative charge thank their amphoteric nature [45,46]. The adsorption process was favored due to the attraction between the negative charges of clays and the predominantly positive charges of the dye [50,51]. This phenomenon was clearly observed in other investigations where the optimum pH for anionic dye was obtained. Abidi et al. [35] reported the same phenomenon when studying the removal of an anionic dye by silicate clays such as Kaolinite and Illite, they found that the best removal results were obtained at a pH below 4.5. This value was very close to the pKa of the “Reactive Red 120” dye, favoring its protonation and therefore, providing a greater attraction to the surface of the negatively charged clays. Kouda et al. [65] found that at a more acidic pH, the removal of the “Crystal Violet” cationic dye by the Illite decreased considerably because the dye was deprotonated and became negatively charged [36] demonstrating that the removal of the “Congo Red” (anionic) dye by an Illite-based adsorbent was much higher at very acidic pH values (<2.5) compared to the one of the “Methyl Blue” (cationic), due to the increase of positive charges on the anionic dye and negative charges on the adsorbent.

It is important to mention that some investigations report that at acid pH values, the DWTS can release aluminum atoms onto the solution, so these are active or exchangeable with negative charge sites where they would be released, thus favoring the adsorption of molecules with opposite charge [5,6,10,65,66].

The adsorbent Dose (AD) applied with a higher Bordeaux B removal (R = 84.29 ± 0.63%) was 5 g/L. Chu [22] reported that the highest Removal Percentages for the studied dyes in his research were obtained at a 0,75 g/L adsorbent dose. Kayranli [25] indicates that for most dyes analyzed in his research (all of them at a 25 mg/L concentration), the most efficient AD was 2 g/L, except for the “Acid Blue 40” dye which was 7 g/L. Contrary to these findings, Tong et al. [28] reported that for a 100 mg/L concentration from the dye researched, the best AD was 1 g/L. Nageeb Rashed et al. [27] and Elmontassir et al. [23] also report that for the same dye (“Methylene Blue”) at a 100 mg/L and 20 mg/L concentration, respectively, the best adsorbent dose was 0.25 g/L and 3 g/L. Gadekar & Ahammed [14] reported that for a 75 mg/L concentration of the dye researched, 30 g/L of AD were required. Tony [15] found that for a 1,8 mg/L concentration of the dye studied, the most efficient AD was 2 g/L. Laib et al. [26] determined that for a 50 mg/L concentration for the “Methylene Blue” and “Reactive Blue 19” dyes, the AD was 1.5 g/L y 2 g/L, respectively.

Although the value determined in this research (5 g/L) differs from those reported in the literature, the value is within the range of values reported (0.75–30) g/L.

Nevertheless, the decrease in the Removal Percentage of the dye at a constant dye concentration as the Adsorbent Dose increases from a threshold dose or limit is a phenomenon that was observed in this and other investigations [15,22,25]. This can be because of an excessive presence of adsorbent material in the solution interfering with the capture capacity or the interaction of the active sites of the adsorbent with the molecules of the dye. Opposite surface charged particles interact with each other, generating competition for active sites and reducing the superficial area exposed to the adsorbent [18,65].

The Initial Concentration of the dye, (Co) for which a higher R of “Bordeaux B” (R = 85.57 ± 0.76%) obtained at the operational conditions previously defined (AD = 5 g/L, CT = 45 min, pH = 3.48, Agitation = 350 RPM y T = 25 ± 0.5 °C), was 40 mg/L. Gadekar & Ahammed [14] indicated that the best combination of variables for more efficient removal of the dye was: pH of solution = 3, AD = 30 g/L and a Co = 75 mg/L. Nageeb Rashed et al. [27] reported that the best operational conditions for their research were: CT = 60 min, pH of solution = 7, AD = 0.25 g/L and a Co = 100 mg/L. Tony [15] reported that the best operational conditions for his treatment were: CT = 120 min, pH of solution = 7.5, AD = 2 g/L and a Co = 11.8 mg/L. Laib et al. [26] demonstrated that for the “Reactive Blue 19” dye, the best operational conditions were: CT = 30 min, pH of solution = 3, AD = 2 g/L and a Co = 50 mg/L, and for the “Methylene Blue” dye, they were: CT = 30 min, pH of solution = 6.5, AD = 1.5 g/L and a Co = 50 mg/L.

The value determined by this research (Co = 40 mg/L) was similar to those reported in the literature.

Nevertheless, the increase in the Removal Percentage as the Co increased from 20 to 40 mg/L, at a determined constant Adsorbent Dose, agrees with the one reported by Tony [15]; this can be because the concentration of dye-molecules in the solution was not significant enough to fill all the active sites of the adsorbent. In addition, the gradual decrease in the Removal Percentage R as the Co of the dye increased from 40 to 80 mg/L, coincides with those reported by other authors [14,26,27]; This can occur due to the decrease of active sites in the adsorbent surface (competition among dye molecules), when the adsorbent has reached a maximum saturation level at a given concentration of adsorbate. This indicates that the Co dye plays a significant role in the adsorption capacity of the adsorbent.

Kinetics evaluation of the removal process of the “Bordeaux B” dye must be carried out following its optimal operational conditions.

The results of evaluating the kinetics of the “Bordeaux B” dye removal process from a synthetic sample at optimal operational conditions previously determined (AD = 5 g/L, CT = 45 min, pH = 3.48, Co = 40 mg/L, Agitation = 350 RPM and T = 25 ± 0.5 °C), are showed in Figure 11.

Considering the performance on the removal efficiency of the T-500, an equilibrium from Interval CT = (35 and 45) min (R = 84.23 ± 0.16% and R = 84.11 ± 0.16%) is observed, coinciding with what was previously reported where a CT = 45 min was determined. The particularity about Figure 11 is observed in the first-time interval (2 min) where the value of R is already considerable (R = 70 ± 0.48%). Such a situation would be reflecting a very good affinity of the adsorbent by the adsorbate under operational conditions.

In addition to this, in Figure 11, the adsorption of the dye can be divided into three stages (separated by a black strip): the first stage (0–2) min could be related to the diffusion of dye molecules through the solution from the outer surface of the liquid film that surrounds the adsorbent where most of the active sites are available; the second stage (2–45) min could be related to the intraparticle diffusion of dye molecules in the micropores, mesopores and active sites that are still available in the adsorbent particles after the surface was saturated, so the removal of the dye was slowed down; and the third stage, (45–180) min, could be related to adsorption and desorption of the dye due to mechanical agitation and friction among saturated particles [19,26,27].

The results of the kinetic evaluation of the process through the Pseudo First Order Model (proposed by Lagergren) and Pseudo Second Order Model can be seen in

Table 4. Results of the kinetic evaluation of the process through the Pseudo First Order and Pseudo Second Order Models.

Kinetic Model	Parameter	Unit	Numerical Value
Pseudo First Order	qe (Estimated)	mg/g	1.22
	K1	min−1	0.04
	Linear correlation coefficient simple (R2)	-	0.90
Pseudo Second Order	qe (Estimated)	mg/g	8.08
	K2	g/(mg·min)	0.1
	Linear correlation coefficient simple (R2)	-	0.99
Experimental value	qe (Experimental)	mg/g	6.96

.

As shown in Table 4 and the Figure 12, the Pseudo Second Order Kinetic Model is not fully adjusted to the plotted data. As observed, the Simple Linear Correlation Coefficient for the Pseudo Second Order Model has a value close to one (R² = 0.99) and an estimated adsorption capacity similar to the experimental one, so it is possible to affirm that the adsorption process occurred from chemical and electrostatic interactions between the “Bordeaux B” dye and the active sites of the adsorbent (T-500). The results of the present investigation coincide with those reported in a previous research [15,23,24,25,26,27], being the Pseudo Second Order Kinetic Model, the best one to demonstrate the adsorption of dyes through the DWTS because the process involves chemical and electrostatic interactions between the adsorbent and the dye.

3.5. Adsorption Isotherms

The results of the evaluation process by means of Adsorption Isotherms (Langmuir and Freundlich), in the best operational conditions previously determined (AD <T-500> = 5 g/L, CT = 45 min, pH of solution = 3.48, Agitation = 350 RPM and Temperature = 25 ± 0.5 °C), a different Co (20, 30, 40, 50, 60, 70, 80, 120 y 150) mg/L, can be seen in Figure 13.

As observed in Figure 13, the mathematical model that best fits the experimental data (discrete blue line), is the one proposed by Langmuir (green continuous line). This allows us to deduce that the adsorption process is determined by physicochemical forces, however the Freundlich model (continuous yellow line) shows to be very similar to the first 6 experimental values, but not to the subsequent ones. Likewise, according to Giles et al. [67] classification, the adsorption isotherm that most closely resembles the experimental results was “L” subgroup 2 (inversed L). This would indicate that the higher the solute concentration, the greater the adsorption capacity until reaching the maximum saturation point of the adsorbent [19].

Although Figure 13 allows us to infer the model that could best fit the experimental data analytically and graphically, it is not enough to evaluate the process correctly. Through this, the constants for each model were determined, as well as the Simple Linear Correlation Coefficient and the Mean Squared Error between the experimental and modeled adsorption capacity.

Constant Determination of Isotherm Models

The results of the evaluation process through the mathematical models proposed by Freundlich and Langmuir, are observed in

Table 5. Results of the evaluation process by Freundlich and Langmuir Isotherms.

Isotherm Model	Parameter	Unit	Numerical Value
Freundlich	KF	mg/g	1.13 ± 0.01
	n	-	1.09 ± 0.01
	1/n	-	0.91
	Mean Square Error (MSE)	-	2.29
	Simple Linear Correlation Coefficient	-	0.96
Langmuir	KL	L/mg	0.04 ± 0.00
	qmax	mg/g	37.45 ± 0.14
	RL (for all Initial Dye Concentrations)	-	0.14–0.56
	Mean Square Error (MSE)	-	1.02
	Simple Linear Correlation Coefficient	-	0.99

.

As observed in Table 5 and Figure 14, Langmuir’s model is the one that best fits the experimental data, having a Simple Linear Correlation Coefficient very close to the unit (R ≈ 1) and a low Mean Square Error (ECM < 2), compared to the determined values using Freundlich’s model.

Since the constant “n” value of Freundlich’s model is higher than the unit, it is possible to state that the removal process of the “Bordeaux B” dye by means of the adsorbent (T-500) is governed by chemical forces (1/n = 0.91). Additionally, being the value of RL greater than 0, but less than 1 for all Dye Initial Concentrations analyzed (0 < RL < 1) as seen in Table 4, it is possible to affirm that the chemical adsorption is favorable and reversible.

Nevertheless, although the parameters obtained from the Langmuir’s model are more suitable for describing the process, their closeness to those obtained by the Freundlich’s model indicates that both are almost equally fulfilled. This could be because the sludge surface contains some heterogeneous fractions that are uniformly distributed on the surface, so the interaction between the adsorbent and the adsorbate could be determined by both chemical end electrostatic forces [15,25,26,28].

The results of the present investigation agree with those reported in previous investigations [14,15,24,25,26,28], Langmuir’s (best fit) along with Freundlich’s model being the most suitable for evaluating the dye adsorption process through DWTS, since the interaction between the dye and the adsorbent is governed by chemical and electrostatic forces.

3.6. SEM Images

The sludge sample from the Drinking Water Treatment Plant was subjected to an SEM analysis in order to identify the external structure of the material before and after the applied thermal activation process, thus, observing certain differences, as shown in Figure 15.

Figure 15A shows a more compact, poorly disperse and more laminated-shaped material (before thermal treatment), while in Figure 15B,C, the material shows a more dispersed external structure with a more extended appearance (sponge type) which suggests an expansion of the structure. According to this, a greater number of pores and a greater surface area are referred which coincides with the S_BET analysis carried out on the sludge samples generating a greater surface area in order to capture the contaminating species. The reference image in Figure 15D–F [42], shows a similar appearance to that reported in this research.

3.7. Sustainability of the DWTS and T-500

The comparison between the Maximum Adsorption Capacity (qmax) and Removal Efficiency (R) of the T-500 in its Best Operation Conditions (BOC) to remove Bordeaux B, with other unconventional adsorbents, are shown in

Table 6. Comparison between qmax and R on their BOC to remove Bordeaux B from its aqueous medium on the T-500.

Adsorbent	Dye	qmax (mg/g)	R on Their BOC (%)	Reference
T-500	Bordeaux B	37.45 ± 0.14	85.57	This study
DWTS Chemically Activated	Methyl Blue	65.1	93.5	Elmontassir et al. [23]
DWTS Thermally & Chemically Activated	Methyl Blue	70.4	93.55	Nageeb Rashed et al. [27]
DWTS Thermally Activated	Protion Blue	6.5	>80	Tony [15]
DWTS enriched with Fe and thermally activated	Methyl Blue (MB) and Reactive Blue 19 (AR19)	46.73 for AM and 40.65 for AR19	99.19% for AM and 90.31% for AR19	Laib et al. [26]

.

Likewise, the comparative analysis obtained by using other unconventional adsorbents is shown in

Table 7. Comparison between qmax and R in their different BOC from various unconventional adsorbents used for dye removal in aqueous solution.

Adsorbent	Dye	qmax (mg/g)	R in Their BOC (%)	Reference
T-500	Bordeaux B	37.45 ± 0.14	85.57	This study
Coconut shell activated carbon	Basic Purple 3	2.8	93	Wakkel et al. [68]
Metallurgical residue (jarosite)	Methyl Blue	61.7	92.5	Kushwaha & Agarwal [69]
Pistachio shell	Basic Blue 41	21.84	70	Şentürk & Alzein [70]
Solid tannery waste	Methylene Blue	99.87	88.8	Sitab et al. [71]

.

As observed, both in Table 7, the results of the Maximum Adsorption Capacity (qmax) and the Removal Percentage in the BOC for the adsorbent produced in this research, show reliable performance compared to other unconventional adsorbents.

Wakkel et al. [68] researched about the ability of date seeds to absorb basic violet 3 (BV3) dyes through a simple drying and spraying process where they managed to eliminate 98% BV3. Kushwaha & Agarwal [69] investigated the potential use of a waste generated by the metal industry called jarosite which has metals, metal oxides and silica in its composition; Therefore, when crushed and dried, it was able to remove up to 92.5% of the methylene blue (MB) dye under its optimal conditions. Sitab et al. [71] also investigated the removal capacity of the MB dye by using a thermally modified waste from the leather tanning industry, obtaining a 99.9 mg/g maximum adsorption capacity with an 88.8%. removal efficiency at pH 12 Şentürk & Alzein [70] researched about the use of pistachio shell as an adsorbent to remove the Basic blue 41 (BB 41) dye from textile wastewater effluents, obtaining a maximum adsorption capacity in batch mode of 21.8. mg/g and 41.77 mg/g in an adsorption column. By comparing the maximum adsorption capacity and the removal percentage of T-500 with other adsorbents, it is possible to infer that raw materials of inorganic composition offer a significant advantage compared to those of organic composition. This may be due to the fact that the large number of electrically charged particles is much greater in inorganic compounds than in organic ones, generating a better interaction with the dyes and a greater number of active sites.

However, when comparing these results with other investigations in which DWTS was used as a precursor material for an adsorbent capable of removing dyes in aqueous state (Table 6), it is observed that these results are not so similar and are close to average. Nevertheless, the difference in the Maximum Adsorption Capacity (qmax) value is notable when such residues are enriched/or activated thermally and chemically. This can be because chemical activation improves both, the number of active sites available on the surface of the particles and the porosity of the material [23,27,69,72]. Similarly, enriching DWTS with some substance such as iron oxide, could increase the concentration of functional groups on the adsorbent surface, improving its adsorption capacity [26].

Removing a contaminant through a simple modification and without the use of chemical additives (thermal activation) represents a great advance for the circular economy in water treatment, contributing to the environmental sustainability of industries in the sanitation sector.

The cost of producing approximately 30 g of T-500 adsorbent from 1.5 L of raw DWTS, can be seen in

Table 8. Production Cost of 30 g of T-500 adsorbent.

Stage	Equipment	Electrical Consumption	Use Time	Price (USD)
Obtaining the precursor material (DWTS)	Hand sampler	-	-	0
Pretreatment (drying)	BINDER Brand stove	Approx. 1.10 kW	24 h	2.45
Thermal activation up to 500 °C	THERMO SCIENTIFIC muffle furnace	Approx. 1.06 kW	2 h	0.20
Total Cost				2.65
Cost/gran				0.09 USD/g

. The electrical consumption of the equipment was determined from its technical data sheet. The Price of electricity in Arequipa by the year 2023 (0.3481 soles/kWh) was obtained from the official website of the “Organismo Supervisor de la Inversión en Energía y Minería” (OSINERGMIN). The currency Exchange from soles to US dollars (S/1 = 0.266 USD) was obtained from the information consulted on the official website of the “Banco Central de Reserva del Perú” (BCRP) on 24 March 2023. It is estimated that the total DWTS produced is activated at 500 °C in a single stage, and that the loss of mass, which is a product from Thermal Activation, is negligible.

As seen in Table 8, the cost of producing 1 g of the T-500 adsorbent is less than 1 sol or 0.1 USD. This value is within the average range reported in the literature, regarding the production cost of adsorbents from waste used to removed textile dyes, therefore, it can be classified as “cheap” [34,73]. Nevertheless, the production costs of T-500 adsorbent could be significantly reduced if the DWTP that generates the residue implemented a drying or thickening system for the DWTS [43,56]. This would contribute to the reduction of the moisture content in the waste and, therefore, when pre-treating it, a greater amount of DWTS would be obtained.

4. Discussion

DWTS showed a high moisture content and moderate ash content. The pH does not differ significantly between both materials (T-500 > DWTS). The predominant presence of muscovite and other minerals in the DWTS varied after thermal activation at 500 °C (dehydroxylation), except for Illite which increased considerably. Both adsorbents have a considerable specific superficial area (S_BET) (T-500 > DWTS). They are slightly porous and also have similar functional groups (O-H y Si-O), except for the T-500, where the band associated with the Fe-O and Al-O bonds (536 cm⁻¹) possibly disappeared due to recrystallization. Activation temperature also played a significant role in the mineralogical and surface characteristics for both materials. Langmuir was the mathematical model that best fit the experimental data (R² = 0.99 y ECM = 1.02). These findings suggest that the T-500 is capable of removing the “Bordeaux B” dye, efficiently and that its production could become even more economical if drying or thickening systems were implemented in the DWTP. This section is not mandatory, but it can be added to the manuscript if the discussion is unusually long or complex.

4.1. Limitations

The main limitation of the research is found in obtaining the raw material for the adsorbent (drinking water treatment sludge). The waste generated in the raw water purification process is conditioned by the characteristics of the environmental quality of the water body captured and the methodology used by the treatment plant, as well as the conditions of the hydrological regime, geological characteristics, plant cover. and type of soil of the hydrographic basin to which the natural source of surface or groundwater belongs, resulting in a high variability in the physicochemical and microbiological composition in the waste generated by the treatment plants. This represents a problem when planning an industrial scale-up of the adsorbent, since the raw material obtained from the treatment plants may be very different depending on its hydrographic location, which may cause the waste thermal activation, generating various colloidal particles, which have no affinity with the contaminant that is intended to be removed. Likewise, the industrial scaling of this non-conventional adsorbent will be conditioned by the quantity and treatment of the residual sludge that drinking water treatment plants generate due to their population demands. Regarding the economic limitations, it is possible to indicate that, since the raw material (adsorbent) is a waste without any economic value for companies in the sanitation sector, its collection, transfer and treatment could imply a considerable investment because treatment plants must add to their process a system that allows them to obtain their waste efficiently and easily. If this is not the case, the industry will have to assume the costs derived from the pretreatment of the waste sludge, increasing its production cost.

4.2. Future Directions

Based on the results shown in this paper, it is possible to provide new information inside the circular economy in water post-stabilization companies; Likewise, the treatment of wastewater from the textile industry has minimum considerations to be able to decide the use of new adsorbent materials that purify their wastewater. The results shown give rise to new research focused on the use of T-500 in a column adsorption system under continuous conditions, estimating the efficiency of the adsorbent through the analysis of desorption cycles, backwashing and the use of real textile water waste.

The results obtained here allow to establish some parameters for future research concerning the use of these sludges as a catalyzing support in the adsorption and degradation of emerging contaminants (TiO₂ for example). These also can be used in the area of civil engineering (brick, asphalt and cement manufacture, among others).

5. Conclusions

The pH does not differ significantly between both materials (T-500 > DWTS). The predominant presence of muscovite and other minerals in the DWTS varied after thermal activation at 500 °C (dihydroxylation). Both adsorbents have a moderate specific superficial area. These materials have similar functional groups (O-H y Si-O). The activation temperature played a significant role in the mineralogical and surface characteristics for both materials. The best removal efficiency was at 500 °C (R = 85.57 ± 0.76%, q max = 37.45 ± 0.14 mg/g) which compared to other unconventional adsorbents are more reliable and competitive. These findings suggest that the T-500 can remove the “Bordeaux B” dye, efficiently and that its production could become even more economical if drying or thickening systems were implemented in the DWTP. These results showed that the implement of the sludge as adsorbent in optimal conditions would contribute to the circular economy in water treatment processes (without environmental liabilities).