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Article

Electrochemical Oxidation of Pollutants in Textile Wastewaters Using BDD and Ti-Based Anode Materials

by
César Afonso
*,
Carlos Y. Sousa
,
Daliany M. Farinon
,
Ana Lopes
and
Annabel Fernandes
Fiber Materials and Environmental Technologies (FibEnTech-UBI), Department of Chemistry, Universidade da Beira Interior, 6201-001 Covilhã, Portugal
*
Author to whom correspondence should be addressed.
Textiles 2024, 4(4), 521-529; https://doi.org/10.3390/textiles4040030
Submission received: 9 September 2024 / Revised: 8 November 2024 / Accepted: 13 November 2024 / Published: 15 November 2024
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Abstract

:
This study aims to evaluate the electrochemical oxidation of real textile wastewater using boron-doped diamond (BDD) and different titanium-based mixed metal oxide (Ti/MMO) commercial anodes, namely Ti/RuO2-TiO2, Ti/IrO2-Ta2O5, Ti/IrO2-RuO2, and Ti/RuO2/IrO2-Pt. Experiments were conducted in batch mode, with stirring, at different applied current densities. The results showed that BDD attained the best results, followed by Ti/RuO2-TiO2, which achieved total color removal, a chemical oxygen removal of 61% with some mineralization of organic compounds, and a similar specific energy consumption to BDD. The worst performance was observed for Ti/IrO2-Ta2O5, with a specific energy consumption four times superior to BDD due to a negligible organic load removal.

Graphical Abstract

1. Introduction

Water usage in industry represents 16% of global water demand and is expected to increase to 22% by 2030 [1]. One of the main water-demanding industries in Europe is the textile industry, with an estimated consumption of 600 million m3 of freshwater per year [2]. From this, around 108 million tons of wastewater are produced, containing approximately 36 million tons of chemicals, which require appropriate treatment before disposal [2]. Several treatment technologies can be considered to remove these chemicals from the wastewater. However, due to the variability of textile wastewater (TW) characteristics and its recalcitrant nature, many treatment processes have shown to be ineffective, requiring complementarity with other technologies and, thus, increasing the treatment duration and costs [3].
Advanced oxidation processes have been studied for TW treatment, with an emphasis on electrochemical oxidation (EO), which yields high pollutant removal without requiring the addition of reagents or resulting in the production of sludges or concentrates [4,5]. In the EO process, the organic compounds can be either oxidized by direct (at low extension) or indirect oxidation through oxidizing species that are electrogenerated, like hydroxyl radicals (•OH) and active chlorine species [5,6]. Several factors can influence the performance of the EO process, from which the anode material and the applied current density are among the most important to consider [7]. The anodic materials can be divided into active and non-active. Active anodes, like Pt, IrO2, and RuO2, present a strong interaction between the anode surface and •OH, lessening the hydroxyl radicals’ reactivity for the organic compounds’ oxidation [7,8]. Conversely, non-active anodes, especially boron-doped diamond (BDD), present a weak interaction between the anode surface and •OH, allowing the direct reaction of the organic compounds with the hydroxyl radicals, which leads to high current efficiency and mineralization of the organic compounds [9]. According to the literature, BDD has proven to be the most effective anode material for the electrooxidation of dyes and TW, leading to the complete mineralization of the organic compounds and, thus, significantly reducing the chemical oxygen demand (COD), total organic carbon, and color [5,10]. Several studies have reported the complete removal of COD and color from real textile effluents by EO with BDD anode [11]. Despite these features that pose BDD as a reference material in the EO process, the high cost of this electrode hampers its application at an industrial scale [10,12]. Therefore, searching for lower-cost electrode materials that can successfully remove pollutants is a challenge nowadays.
Titanium-based mixed metal oxides (Ti/MMO) are promising materials for EO application due to the excellent stability of the titanium substrate and the high catalytic activity conferred by modifying the anode surface through the combination of several metal oxides [13]. When Ti/MMO anodes are used, the hydroxyl radicals formed can be either physisorbed (Ti/MMO(•OH)) or chemisorbed (Ti/MMOx+1), which confers to these anodes a strong oxidation power and reusability [14]. Ti-based RuO2 and IrO2 have been widely studied in recent years due to their low potential for oxygen evolution reaction and promising results in wastewater remediation [14]. Several Ti-based RuO2 and IrO2 electrodes are commercially available at a much lower price than BDD. However, the efficiency of these commercial MMO materials in treating real wastewater matrices has not been established. The literature identifies a gap between promising laboratory catalysts and their commercial use, mainly ascribed to insufficient testing at smaller scales and problems arising from underestimating synthesis steps and insufficient knowledge of raw material properties, synthesis chemistry, and desirable catalyst characteristics [15].
The present study aimed to evaluate the performance of different Ti/MMO anodes, namely Ti/RuO2-TiO2, Ti/IrO2-Ta2O5, Ti/IrO2-RuO2, and Ti/RuO2/IrO2-Pt, for the electrochemical oxidation of textile wastewater, compared to BDD. The effect of the applied current density on the anodes’ performance was assessed, and the performance was evaluated in terms of pollutant removal and specific energy consumption.

2. Materials and Methods

2.1. Textile Wastewater

The TW sample used in the present study was collected in October 2023 from a textile plant in northern Portugal. Sample characterization is presented in Table 1.

2.2. Electrochemical Oxidation Experiments

The electrochemical oxidation experiments were conducted using an undivided electrochemical glass cell containing 200 mL of TW. During the assays, continuous magnetic stirring was applied (300 rpm) to enhance the mass transport from the bulk of the solution to the surface of the electrodes. A stainless-steel plate was used as a cathode, with an immersed area of 10 cm2. As anode, a BDD, purchased from NeoCoat SA, La Chaux-de-Fonds, Switzerland, and several Ti/MMO materials were utilized: Ti/RuO2-TiO2 and Ti/IrO2-Ta2O5, purchased from Insoluble Anode Technology BV, Oirschot, The Netherlands, Ti/IrO2-RuO2, provided by UTron Technology Co., Ltd., Shaanxi, China, and Ti/RuO2/IrO2-Pt, purchased from Baoji Qixin Titanium Co., Ltd., Shaanxi, China. The anode and the cathode were positioned in the center of the cell, parallel to each other, with a similar immersed area (10 cm2) and a 0.5 cm gap between them. A GW Laboratory DC GPS-3030D (0–30 V, 0–3 A) was the power supply for a constant and continuous current density (j) of 300 A m−2. Further testing at a lower current density, 100 A m−2, was conducted for BDD (used as reference) and the better-performing Ti/MMO anodic material. The duration of the experiments was 8 h.

2.3. Analytical Methods

COD determination followed the open reflux method described in Standard Methods, section 5220B [16]. Biochemical oxygen demand (BOD5) was determined by the respirometric method, described in detail elsewhere [17]. The BOD5/COD ratio calculated the biodegradability index. Total dissolved carbon (TDC), dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and total dissolved nitrogen (TDN) were measured in a Shimadzu TOC-VCPH analyzer combined with a TNM-1 unit (Izasa Scientific, Carnaxide, Portugal). For TDC, DOC, DIC, and TDN determinations, samples were filtered through 1.2 μm glass microfiber membranes, according to the Standard Methods, section 5310 [18]. Electrical conductivity (EC) was measured with a Mettler Toledo SevenEasy S30 conductivity meter (MTBrandão, Oporto, Portugal), pH with a Mettler Toledo SevenEasy S20 pH meter (MTBrandão, Oporto, Portugal), and turbidity with a Lovibond TB350 turbidity meter (Dias de Sousa S.A., Maia, Portugal). Ion concentration was determined by ionic chromatography using a Shimadzu 20A Prominence (Izasa Scientific, Carnaxide, Portugal) system, as described elsewhere [17].

3. Results and Discussion

The TW characterization presented in Table 1 shows that the biodegradability index is below 0.3, indicating that biodegradation processes are inhibited by the wastewater characteristics [19]. The TW’s low BOD5/COD ratio is ascribed to synthetic organic dyes, among other chemicals, that present high resistance to microbial attack [20]. These poorly biodegradable substances, which may be toxic to microbes, inhibit microbial activity [21]. According to the literature, wastewater biodegradation is relatively slow for BOD5/COD ratios lower than 0.6 and completely inhibited when this ratio is lower than 0.3 [22]. Therefore, the EO process can be a suitable strategy to remove the organic load.
Figure 1 presents the COD and DOC decay (normalized) for the EO experiments performed with different anode materials at 300 A m−2.
As expected, BDD was the most effective for removing the organic load, followed by Ti/RuO2-TiO2. Ti/IrO2-Ta2O5 and Ti/IrO2-RuO2 presented the lowest COD removals. Contrary to what was observed with BDD, DOC removal was meaningless when Ti/MMO anodes were used. In some of the experiments, an increase in DOC concentration was observed (Figure 1b), which can be explained by particles in solution containing organic compounds. Those particles are dissolved throughout the EO treatment, leading to an increase in DOC concentration. For all the anode materials studied, the DOC removal rate is lower than that of COD, especially in the first hours of the experiment. A decline in DOC content signifies that EO mineralizes pollutants and converts organic matter into CO2. A reduction in COD reflects the change in the chemical species during the process [23]. The difference between COD and DOC removal rates is explained by the incomplete oxidation of the organic compounds, being more pronounced at Ti/MMO anodes probably due to their higher porosity, which causes stronger adsorption of the •OH formed through the anodic water discharge [24]. Ti/MMO (•OH) are less prone to react than BDD (•OH), which favors the indirect oxidation of the organic compounds through active chlorine species, such as chlorine and hypochlorite, generated from the direct chloride oxidation at Ti/MMO surface [25]. Contrarywise to the oxidation through •OH that favors the organic matter mineralization, the indirect oxidation through active chlorine species favors, according to the literature, their partial oxidation, in agreement with the low DOC removal observed when using Ti/MMO anodes [18]. In addition, in the case of “active” anodes (such as Ti/MMO anodes), Ti/MMO (•OH) can be oxidized to form the Ti/MMOx+1 oxide. Ti/MMOx+1 operates as a go-between in the oxidation process, leading to the partial oxidation of the organic compounds [26]. In contrast, at “non-active” anodes (like BDD), adsorbed hydroxyl radicals react directly with the oxidizable substrate, resulting in mineralization. Thus, when using the BDD anode, known for its capability to generate large quantities of weakly adsorbed hydroxyl radicals, the organic compounds oxidation through •OH is favored and, consequently, their mineralization, although the oxidation by active chlorine species also takes place in parallel [27].
BDD and Ti/RuO2-TiO2, the most effective at organic load removal, were evaluated at a lower applied current density, 100 A m−2. Figure 2 displays the COD as a function of the passed electric charge and the DOC vs. COD plots for the experiments run at both applied current densities.
For both applied j, the highest current efficiency of the BDD anode, compared to Ti/RuO2-TiO2, is well noticed. At low applied electric charges (<1.5 kC), proximity between the current efficiencies of the experiments performed at 100 and 300 A m−2 is found. At applied electric charges above 1.5 kC, different trends are observed. For BDD, a decrease in the current efficiency with the increase in j was attained, which, according to the literature, is ascribed to the occurrence of secondary reactions that are enhanced at higher j, such as oxygen evolution and inorganic ions oxidation [28]. Conversely, for Ti/RuO2-TiO2, a decrease was observed in the current efficiency after 4 h of experiment, which could be due to the passivation of the electrode surface [29]. DOC vs. COD plots, presented in Figure 2b, indicate the mineralization degree of the organic compounds along the EO experiments. The higher slopes of these plots indicate higher mineralization rates [30]. As expected, BDD attained the highest mineralization degree compared to Ti/RuO2-TiO2.
DIC and pH variation along the EO experiments performed with BDD and Ti/RuO2-TiO2, displayed in Figure 3, showed more pronounced variations at higher j values and when using BDD.
For both anodes, at 300 A m−2, an increase in DIC concentration was observed at the first hours of treatment, with a subsequent decrease during the last hours. The opposite trend was noticed for pH, with an initial decrease accentuated by the increase in j. These findings suggest an inversely proportional relation between DIC and pH. Attending to the initial pH of the TW sample (9.6) and that, in the pH range between 8.5 and 12, inorganic carbon is in the form of HCO3− and CO32− [31], the DIC increase can be ascribed to the organic compounds mineralization (Equations (1)–(3), where M denotes the anode and R represents the organic compounds). This follows the higher mineralization degree and DIC formation observed for BDD, compared to Ti/RuO2-TiO2 anode.
M(•OH) + R → M + CO2 + H2O + H+ + e−
CO2 + HO− ⇔ HCO3−
HCO3− ⇔ CO32− + H+
The release of H+ from the reactions presented by Equations (1) and (3) and from other redox reactions that may occur, and the carboxylic acids formation from the organic matter oxidation can explain the observed decrease in pH when DIC concentration is increasing [32]. These reactions are enhanced with increased j and when using a BDD anode. The decrease in pH should, in turn, force the chemical re-equilibrium of the DIC system, which can result in the release of CO2 and, consequently, in the decrease in DIC concentration. Moreover, the reaction between HCO3− and active chlorine species (Equation (4)) can also explain the decrease in DIC concentration [33]. The suppression of some of the reactions that lead to H+ in solution and the enhancement of secondary reactions, as the reaction of hydrogen evolution, caused, for instance, by the reduction in oxidizable compounds, can explain the pH increase when DIC is decreasing [32].
HOCl + HCO3− ⇔ CO2 + OCl− + H2O
Electrochemical oxidation utilizing BDD and Ti/RuO2-TiO2 anodes generated a colorless solution after 8 h, particularly at the highest current density. The prior literature has established the effectiveness of EO in reducing TW coloration. This effectiveness is attributed to the active chlorine species generated from chloride oxidation, which promotes color removal and water disinfection [32].
Figure 4 presents the electric energy consumption (E, in W h) along the EO experiments performed with BDD and Ti/MMO anodes and the specific energy consumption (Esp, in W h gCOD−1) attained by the 8 h treatment, determined, respectively, according to Equations (5) and (6), where U (V) is the cell voltage, resulting from the applied current intensity I (A), Δt (h) is the experiment time interval considered, V (L) is the volume of TW used in each experiment, and ΔCOD (g L−1) is the COD removed during Δt [32].
E = U × I × Δt
Esp = (U × I × Δt)/(V × ΔCOD)
BDD presented the highest electric energy consumption for both applied current densities studied (100 and 300 A m−2) due to the higher potential difference attained by this material compared to the MMO materials. According to the literature, this higher potential difference observed for BDD, compared to MMO, can be ascribed to the following [34]: (i) higher voltages required for reactions initiation on BDD anode; (ii) higher overpotentials for the oxidation of organic species on BDD due to its strong diamond lattice structure, which can hinder charge transfer; (iii) facilitated direct electron transfer reactions on MMO anodes, resulting in lower cell potentials.
The lowest specific energy consumptions were observed at the lowest applied j, indicating that more energy-efficient degradation is attained when applying lower current densities. According to the literature, lower j reduces the occurrence of parasitic reactions, leading to a more efficient degradation process [34]. At 100 A m−2, BDD and Ti/RuO2-TiO2 presented similar performance, although BDD had the best result. This is explained by the lower voltage values achieved by Ti/RuO2-TiO2 compared to BDD’s. Similar performance was also observed between BDD, Ti/RuO2-TiO2, and Ti/RuO2/IrO2-Pt anodes at 300 A m−2, with the later presenting the lowest Esp value. Ti/IrO2-Ta2O5, followed by Ti/IrO2-RuO2, attained the highest Esp since the lower potential difference attained by these Ti/MMO anodes was not enough to compensate for the much lower COD removal compared to BDD.
Table 2 presents a comparative analysis of the different anode materials evaluated. Despite the much higher cost, BDD outperforms the MMO materials regarding pollutant load removal. Contrarywise, Ti/IrO2-Ta2O5 and Ti/IrO2-RuO2, with a relatively low cost, are inadequate for TW treatment at the experimental conditions studied, which, among other reasons, can be due to the gap described in the literature between promising laboratory catalysts and their industrial production [15]. From the MMO materials evaluated, Ti/RuO2-TiO2 appears to be a feasible option for replacing BDD. However, the indirect oxidation through active chlorine species promoted by this material may lead to the formation of undesirable chlorinated products [5].

4. Conclusions

Compared to BDD, the Ti/MMO anodes under study attained a lower organic load removal rate and mineralization degree during the electrochemical oxidation of textile wastewater. Ti/MMO anodes’ performance regarding COD removal followed the order Ti/IrO2-Ta2O5 < Ti/IrO2-RuO2 < Ti/RuO2/IrO2-Pt < Ti/RuO2-TiO2. Ti/RuO2-TiO2, the best among the Ti/MMO anodes studied, achieved COD removals of approximately 60% after 8 h at 300 A m−2. Experiments run at a lower applied current density (100 A m−2) using this anode material showed similar current efficiency to that observed at 300 A m−2. However, a decrease in the current efficiency was noticed after 4 h of experiment, probably due to the passivation of the electrode surface. Still, the complete color removal of TW was observed, particularly at the highest applied current density, due to the active chlorine species generated from chloride oxidation, which promotes color removal and water disinfection. Ti/RuO2-TiO2 led to lower DIC concentrations than BDD, although the pH of the treated TW was higher. Moreover, Ti/RuO2-TiO2 presented energy consumptions similar to BDD due to the lower voltage values attained by this anode material. In sum, from the Ti/MMO anodes studied, Ti/RuO2-TiO2 showed promising features for TW treatment by EO. Still, further optimization of the experimental conditions is required.

Author Contributions

Conceptualization, A.F.; methodology, A.F.; validation, A.F. and A.L.; formal analysis, C.A.; investigation, C.A., C.Y.S. and D.M.F.; data curation, C.A.; writing—original draft preparation, C.A.; writing—review and editing, A.F. and A.L.; visualization, C.A.; supervision, A.F.; project administration, A.L.; funding acquisition, A.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Fundação para a Ciência e a Tecnologia, FCT, project UIDB/00195/2020 and research contract CEECINST/00016/2021/CP2828/CT0006 awarded to Annabel Fernandes under the scope of the CEEC Institutional 2021, and by the Lusitano project 01/C05-i09/2024.PC644933224-00000043, financed by the Recovery and Resilience Plan.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding authors.

Acknowledgments

The authors are very grateful for the support granted by the Research Unit of Fiber Materials and Environmental Technologies (FibEnTech-UBI), through the Project reference UIDB/00195/2020, funded by FCT through national funds (PIDDAC) (https://doi.org/10.54499/UIDB/00195/2020, accessed on 2 September 2024). Annabel Fernandes acknowledges FCT and the University of Beira Interior for the research contract CEECINST/00016/2021/CP2828/CT0006 under the scope of the CEEC Institutional 2021, funded by FCT (https://doi.org/10.54499/CEECINST/00016/2021/CP2828/CT0006, accessed on 2 September 2024). César Afonso, Carlos Y. Sousa, and Daliany M. Farinon acknowledge the Lusitano project 01/C05-i09/2024.PC644933224-00000043 for the research grants awarded.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Textiles 04 00030 g001
Figure 1. Normalized (a) COD and (b) DOC decay with time for the EO experiments performed with different anode materials at the applied current density of 300 A m−2.
Figure 1. Normalized (a) COD and (b) DOC decay with time for the EO experiments performed with different anode materials at the applied current density of 300 A m−2.
Textiles 04 00030 g001
Textiles 04 00030 g002
Figure 2. (a) Normalized COD decay with the electric charge for the EO experiments performed with BDD and Ti/RuO2-TiO2 at different applied current densities. (b) DOC vs. COD evolution along the experiments.
Figure 2. (a) Normalized COD decay with the electric charge for the EO experiments performed with BDD and Ti/RuO2-TiO2 at different applied current densities. (b) DOC vs. COD evolution along the experiments.
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Figure 3. (a) Normalized DIC and (b) pH variation along the EO experiments performed with BDD and Ti/RuO2-TiO2 at different applied current densities.
Figure 3. (a) Normalized DIC and (b) pH variation along the EO experiments performed with BDD and Ti/RuO2-TiO2 at different applied current densities.
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Figure 4. (a) Electric energy consumption in the EO experiments and (b) specific energy consumption attained after 8 h of treatment with BDD and Ti/MMO anodes.
Figure 4. (a) Electric energy consumption in the EO experiments and (b) specific energy consumption attained after 8 h of treatment with BDD and Ti/MMO anodes.
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Table 1. Characterization of the textile wastewater sample utilized in the study.
Table 1. Characterization of the textile wastewater sample utilized in the study.
ParameterMean Value (±SD)
Chemical oxygen demand (mg L−1)738 ± 7
Biochemical oxygen demand (mg L−1)214 ± 1
Biodegradability index0.29
Total dissolved carbon (mg L−1)323 ± 5
Dissolved organic carbon (mg L−1)233 ± 8
Dissolved inorganic carbon (mg L−1)90 ± 5
Total dissolved nitrogen (mg L−1)12 ± 3
Na+ (mg L−1)829 ± 7
Cl− (mg L−1)756 ± 1
SO42− (mg L−1)107 ± 3
pH9.6 ± 0.4
Electrical conductivity (mS cm−1)4.9 ± 0.1
Turbidity (NTU)18.6 ± 0.4
Table 2. Comparative analysis of the different materials evaluated. (−) negative and (+) positive score.
Table 2. Comparative analysis of the different materials evaluated. (−) negative and (+) positive score.
ParameterBDDTi/RuO2-TiO2Ti/RuO2/IrO2-PtTi/IrO2-RuO2 Ti/IrO2-Ta2O5
Electrode cost−+ +++ + ++ +
COD removal+ + + ++ ++ +−−
Mineralization degree+ + + ++−−−
Color removal+ + + ++ + + ++−−
Esp+ ++ ++ ++−
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MDPI and ACS Style

Afonso, C.; Sousa, C.Y.; Farinon, D.M.; Lopes, A.; Fernandes, A. Electrochemical Oxidation of Pollutants in Textile Wastewaters Using BDD and Ti-Based Anode Materials. Textiles 2024, 4, 521-529. https://doi.org/10.3390/textiles4040030

AMA Style

Afonso C, Sousa CY, Farinon DM, Lopes A, Fernandes A. Electrochemical Oxidation of Pollutants in Textile Wastewaters Using BDD and Ti-Based Anode Materials. Textiles. 2024; 4(4):521-529. https://doi.org/10.3390/textiles4040030

Chicago/Turabian Style

Afonso, César, Carlos Y. Sousa, Daliany M. Farinon, Ana Lopes, and Annabel Fernandes. 2024. "Electrochemical Oxidation of Pollutants in Textile Wastewaters Using BDD and Ti-Based Anode Materials" Textiles 4, no. 4: 521-529. https://doi.org/10.3390/textiles4040030

APA Style

Afonso, C., Sousa, C. Y., Farinon, D. M., Lopes, A., & Fernandes, A. (2024). Electrochemical Oxidation of Pollutants in Textile Wastewaters Using BDD and Ti-Based Anode Materials. Textiles, 4(4), 521-529. https://doi.org/10.3390/textiles4040030

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