Mineralization of the textile dye acid yellow 42 by solar photoelectro-Fenton in a lab-pilot plant

An optimized electro-fenton pretreatment for the degradation and mineralization of a mixture of ofloxacin, norfloxacin, and ciprofloxacin

The electro-Fenton process (EFP) is a powerful advanced oxidation process beneficial to treating recalcitrant contaminants, and there has been a continuing interest in combining this technology to enhance the efficiency of conventional wastewater treatment processes. In this work, an optimized EFP process is performed as pretreatment for the degradation and mineralization of three blank fluoroquinolones (FQs) drugs: ofloxacin (OFL), norfloxacin (NOR), and ciprofloxacin (CIP). The optimization of the experiment was carried out using a Box-Behnken experimental design. Faster and complete degradation of the drugs mixture was achieved in 90 min with 61.12 ± 2.0% of mineralization in 180 min, under the optimized conditions: j = 244.0 mA cm-2, [Fe2+] = 0.31 mM, and [FQs] = 87.0 mg L-1. Furthermore, a low toxicity effluent was obtained in 90 min of the experiment, according to bioassay toxicity with Vibrio fischeri. Five short-chain carboxylic acids, including oxalic, maleic, oxamic, formic, and fumaric acids, were detected and quantified, in addition to F- and NO3- inorganic ions. The inhibition of the reactive oxygen species with scavenger proof was also evaluated in this paper.

Pilot-scale study on an advanced Fe-Cu process for refractory wastewater pretreatment

The extreme pH, high color, and poor biodegradability of refractory wastewater have severe impacts on its biological treatment. To address this issue, an advanced Fe-Cu process with redox reaction and spontaneous coagulation was investigated and applied for pilot-scale (wastewater flow rate of 2000 m³·day^-1) pretreatment of separately discharged acidic chemicals and alkaline dyeing wastewater. The advanced Fe-Cu process had five functions: (1) increasing the pH of chemical wastewater to 5.0 and above, with an influent pH of approximately 2.0; (2) transforming refractory organics of chemical wastewater with 10.0% chemical oxygen demand (COD) and 30.8% color removal, thereby enhancing the ratio of biological oxygen demand after five days (BOD₅) to COD (B/C) from 0.21 to 0.38; (3) neutralizing the pH of the pretreated chemical wastewater for coagulation application with alkaline dyeing wastewater to avoid adding alkaline chemical; (4) achieving average nascent Fe(II) concentrations of 925.6 mg∙L^-1 using Fe-Cu internal electrolysis for mixed wastewater coagulation, resulting in an average of 70.3% color removal and 49.5% COD removal; (5) providing more efficient COD removal and B/C enhancement than FeSO₄∙7 H₂O coagulation while avoiding secondary pollution. The green process offers an effective, easy-implemented solution for the pretreatment of separately discharged acidic and alkaline refractory wastewater.

Solar photoelectro-Fenton: A very effective and cost-efficient electrochemical advanced oxidation process for the removal of organic pollutants from synthetic and real wastewaters

Recalcitrant and toxic organic pollutants from wastewaters are scarcely removed in conventional wastewater treatment plants. To preserve the water quality, organics need to be removed by developing powerful oxidation technologies. Our laboratory proposed in 2007 a potent electrochemical advanced oxidation process (EAOP) for wastewater remediation, so-called solar photoelectro-Fenton (SPEF). This review summarizes the advances of this emerging technology up to 2022, making evident its effectiveness and cost-efficiency for the destruction of usual organic pollutants. The simultaneous action of generated hydroxyl radicals and the photolysis by sunlight explains the high oxidation power of SPEF respect to other EAOPs. The review is initiated by describing the fundamentals of the process to remark the role of the produced oxidants and the benefits of using solar irradiation in its performance. The photoelectrochemical systems used (bench tank reactor and solar pre-pilot flow plant) and the assessment of the operating variables are discussed. The characteristics of the most common homogeneous SPEF for the degradation and mineralization of several synthetic solutions of industrial chemicals, herbicides, pharmaceuticals, and synthetic organic dyes, as well as of some real wastewaters, are further described. The influence of the photoelectrochemical cell, electrodes, solution pH, electrolyte composition, Fe²⁺ and pollutant concentration, and current density is analyzed. The performance of a homogeneous SPEF-like process with active chlorine and heterogeneous SPEF processes with solid catalysts such as Fe₃O₄ and sodium vermiculite is also discussed. Finally, the advances of homogeneous SPEF combined with other techniques like solar photocatalysis, solar photoelectrocatalysis, anaerobic digestion, and nanofiltration are reported.

Technical and economical assessment of the treatment of vinasse from Pisco production using the advanced oxidation process

The removal of organic matter from Pisco production wastewater was evaluated using coagulation/flocculation, filtration as a pre-treatment, and solar photo-Fenton, with the use of two types of photoreactors: compound parabolic collectors (CPC) and flat plate (FP), with and without utilizing the ozonation process. The overall removal efficiency for chemical oxygen demand (COD) was 63% and 15% using FP and CPC, respectively. Also, for the overall removal efficiency of polyphenols, a percentage of 73% and 43% were obtained using FP and CPC, respectively. When ozone was used in the solar photoreactors, the resulting trends were similar. COD and polyphenol removal, using an FP photoreactor in the solar photo-Fenton/O₃ process, resulted in values of 98.8% and 86.2% after the process. COD and polyphenol removal, using solar photo-Fenton/O₃ process in a CPC, resulted in values of 49.5% and 72.4%, respectively. The economic indicators of annual worth and economic treatment capacity established that FP reactors represent lower costs than CPCs. These results were corroborated by the economic analyses of the evolution of costs versus COD removed as well as by the cash flow diagrams projected for 5, 10, and 15 years.

Application of solar photo-electro-Fenton technology to petroleum refinery wastewater degradation: Optimization of operational parameters

Industrial and agricultural advances have led to global issues such as contamination of water sources and lack of access to clean water. Wastewater from petroleum refineries must be subjected to treatment as it poses a significant environmental threat. The present research aimed to reduce the level of chemical oxygen demand (COD) of an effluent from Bijee petroleum refinery plant, Iraq, using solar photo-electro-Fenton (SPEF) process operated in a batch recycle model. The electrochemical reactor used in the present research was of a tubular design with an anode composed of porous graphite rod and a concentric cylindrical cathode made of the same material. The impacts of operating parameters such as current density (10-50 mA/cm²), Fe²⁺ concentration (0.2-0.8 mM), NaCl addition (0-1 g/L), and time (30-90 min) on the COD removal efficiency were explored based on the response surface methodology (RSM). Results showed that the impact of Fe²⁺ concentration was most prominent, with an effective contribution of 47.7%, followed by current density, with a contribution of 18.26%, and the addition of NaCl, with a contribution of 11.20%. COD removal was found to increase with an increase in current density, Fe²⁺ concentration, NaCl addition, and time, respectively, while energy consumption was found to increase significantly with an increase in current density and a decrease in Fe²⁺ concentration, respectively. The optimum conditions were observed to be an initial pH of 3, current density of 10 mA/cm², Fe²⁺ concentration of 0.8 mM, NaCl addition of 0.747 g/L, and a duration of 87 min, upon which 93.20% COD removal efficiency was achieved, with an energy consumption of 15.97 kWh/kg COD.

Decontamination of wastewater containing contaminants of emerging concern by electrooxidation and Fenton-based processes - A review on the relevance of materials and methods

In recent years, there has been an increasingly growing interest regarding the use of electrochemical advanced oxidation processes (EAOPs) which are considered highly promising alternative treatment techniques for addressing environmental issues related to pollutants of emerging concern. In EAOPs, electrogenerated oxidizing agents, such as hydroxyl radical (HO^•), can react non-selectively with a wide range of organic compounds, degrading and mineralizing their structures to unharmful molecules like CO₂, H₂O, and inorganic ions. To this date, a broad spectrum of advanced electrocatalysts have been developed and applied for the treatment of compounds of interest in different matrices, specifically aiming at enhancing the degradation performance. New combined methods have also been employed as alternative treatment techniques targeted at circumventing the major obstacles encountered in Fenton-based processes, such as high costs and energy consumption, which still contribute significantly toward inhibiting the large-scale application of these processes. First, some fundamental aspects of EAOPs will be presented. Further, we will provide an overview of electrode materials which have been recently developed and reported in the literature, highlighting different anode and cathode structures employed in EAOPs, their main advantages and disadvantages, as well as their contribution to the performance of the treatment processes. The influence of operating parameters, such as initial concentrations, pH effect, temperature, supporting electrolyte, and radiation source, on the treatment processes were also studied. Finally, hybrid techniques which have been reported in the literature and critically assess the most recent techniques used for evaluating the degradation efficiency of the treatment processes.

Comparison of homogeneous and heterogeneous electrochemical advanced oxidation processes for treatment of textile industry wastewater

This study aimed at understanding the influence of the generation of oxidants in a heterogeneous way at boron-doped diamond (BDD) anode (anodic oxidation (AO)) or homogeneously in the bulk (electro-Fenton (EF)) during treatment of a textile industry wastewater. Both processes achieved high TOC removal. A yield of 95 % was obtained by combining EF with BDD anode during 6 h of treatment. The EF process was found to be faster and more efficient for discoloration of the effluent, whereas AO was more effective to limit the formation of degradation by-products in the bulk. An advantage of AO was to treat this alkaline effluent without any pH adjustment. Operating these processes under current limitation allowed optimizing energy consumption in both cases. However, using BDD anode led to the formation of very high concentration of ClO₃^-/ClO₄^- from Cl^- oxidation (even at low current density), which appears as a key challenge for treatment of such effluent by AO. By comparison, EF with Pt anode strongly reduced the formation of ClO₃^-/ClO₄^-. Operating EF at low current density even maintained these concentrations below 0.5 % of the initial Cl^- concentration. A trade-off should be considered between TOC removal and formation of toxic chlorinated by-products.

Electro-Fenton-Based Technologies for Selectively Degrading Antibiotics in Aqueous Media

The viability of the Electro-Fenton (EF) process in the selective degradation of penicillin G (PenG) in complex solutions has been studied. The role of the anode material (boron-doped diamond (BDD) or mixed metal oxide (MMO)) and the cathode 3D support (foam or mesh), as well as the synergistic effect of UVC light irradiation (photoelectron-Fenton, PEF), have been evaluated. The results show that Pen G can be efficiently and selectively removed by EF, obtaining higher PenG removal rates when using the BDD anode (100%) than when using the MMO anode (75.5%). Additionally, mineralization is not favored under the experimental conditions tested (pH 3, 5 mA cm−2), since both aromatic and carboxylic acids accumulate in the reaction system as final products. In this regard, the EF-treated solution presents a high biological oxygen demand and a low percentage of Vibrio fischeri inhibition, which leads to high biodegradability and low toxicity of this final effluent. Furthermore, the combination with UVC radiation in the PEF process shows a clear synergistic effect on the degradation of penicillin G: 166.67% and 83.18% using MMO and BBD anodes, respectively. The specific energy required to attain the complete removal of PenG and high inhibition of the antibiotic effect is less than 0.05 Ah dm−3. This confirms that PEF can be efficiently used as a pretreatment of conventional wastewater treatment plants to decrease the chemical risk of complex solutions polluted with antibiotics.

Research trends in the development of anodes for electrochemical oxidation of wastewater

The review focuses on the recent development in anode materials and their synthesis approach, focusing on their compatibility for treating actual industrial wastewater, improving selectivity, electrocatalytic activity, stability at higher concentration, and thereby reducing the mineralization cost for organic pollutant degradation. The advancement in sol–gel technique, including the Pechini method, is discussed in the first section. A separate discussion related to the selection of the electrodeposition method and its deciding parameters is also included. Furthermore, the effect of using advanced heating approaches, including microwave and laser deposition synthesis, is also discussed. Next, a separate discussion is provided on using different types of anode materials and their effect on active •OH radical generation, activity, and electrode stability in direct and indirect oxidation and future aspects. The effect of using different synthesis approaches, additives, and doping is discussed separately for each anode. Graphene, carbon nanotubes (CNTs), and metal doping enhance the number of active sites, electrochemical activity, and mineralization current efficiency (MCE) of the anode. While, microwave or laser heating approaches were proved to be an effective, cheaper, and fast alternative to conventional heating. The electrodeposition and nonaqueous solvent synthesis were convenient and environment-friendly techniques for conductive metallic and polymeric film deposition.

Anodic Oxidation of Industrial Winery Wastewater Using Different Anodes

Winery wastewater represents the largest waste stream in the wine industry. This deals with the mineralization of the organic matter present in winery wastewater using anodic oxidation and two types of anodes—namely, a boron-doped diamond electrode (BDD) and two mixed metal oxides (MMO), one with the nominal composition Ti/Ru0.3Ti0.7O2 and the other with Ti/Ir0.45Ta0.55O2. To conduct the study, the variability of different quality parameters for winery wastewater from the Chilean industry was measured during eight months. A composite sample was treated using anodic oxidation without the addition of supporting electrolyte, and the experiments were conducted at the natural pH of the industrial wastewater. The results show that this effluent has a high content of organic matter (up to 3025 ± 19 mg/L of total organic carbon (TOC)), which depends on the time of the year and the level of wine production. With MMO electrodes, TOC decreased by 2.52% on average after 540 min, which may be attributed to the presence of intermediate species that could not be mineralized. However, when using a BDD electrode, 85% mineralization was achieved due to the higher generation of hydroxyl radicals. The electrolyzed sample contained oxamic, acetic, and propionic acid as well as different ions such as sulfate, chloride, nitrate, and phosphate. These ions can contribute to the formation of different species such as active species of chlorine, persulfate, and perphosphate, which can improve the oxidative power of the system.

Degradation of oxamic acid using dimensionally stable anodes (DSA) based on a mixture of RuO2 and IrO2 nanoparticles

Dimensionally stable anodes (DSA) have been widely used to degrade organic compounds because these surfaces promote the electrogeneration of active chlorine species in the bulk of the solution, as well as in the vicinity of the anode when NaCl is used as supporting electrolyte. In this work, the nanoparticles synthesis of IrO₂ and RuO₂ was performed to obtain two types of DSA electrodes named Class I and II to degrade oxamic acid. For Class I and II DSA, the nanoparticles used were synthesized separately and in the same reaction medium, respectively. Electrolysis were carried out in an open cylindrical cell without division at 25 °C, DSAs were used as anodes and a stainless-steel electrode as cathode, both elements have a geometric area of 2.8 cm² immersed in 0.05 mol L^-1 of NaCl or Na₂SO₄ and a current density of 3 mA cm^-2 was applied for 6 h. Active chlorine species generated in the absence of oxamic acid in NaCl were also detected and quantified through ion chromatography. In Na₂SO₄ there was no degradation of the compound, but in NaCl the oxamic acid concentration reaching 85% with Class I DSA. The same tendency is observed in mineralization, in which Class I DSA allowed reaching a CO₂ transformation close to 73%. The difference in the results occurs because with Class I DSA, more hypochlorite is generated than with Class II and therefore there is a larger amount of oxidizing species in the solution that enables the degradation and mineralization of oxamic acid.

A review on the photoelectro-Fenton process as efficient electrochemical advanced oxidation for wastewater remediation. Treatment with UV light, sunlight, and coupling with conventional and other photo-assisted advanced technologies

Wastewaters containing recalcitrant and toxic organic pollutants are scarcely decontaminated in conventional wastewater facilities. Then, there is an urgent challenge the development of powerful oxidation processes to ensure their organic removal in order to preserve the water quality in the environment. This review presents the recent development of an electrochemical advanced oxidation process (EAOP) like the photoelectro-Fenton (PEF) process, covering the period 2010-2019, as an effective treatment for wastewater remediation. The high oxidation ability of this photo-assisted Fenton-based EAOP is due to the combination of in situ generated hydroxyl radicals and the photolytic action of UV or sunlight irradiation over the treated wastewater. Firstly, the fundamentals and characteristics of the PEF process are described to understand the role of oxidizing agents. Further, the properties of the homogeneous PEF process with iron catalyst and UV irradiation and the benefit of sunlight in the homogeneous solar PEF one (SPEF) are discussed, supported with examples over their application to the degradation and mineralization of synthetic solutions of industrial chemicals, herbicides, dyes and pharmaceuticals, as well as real wastewaters. Novel heterogeneous PEF processes involving solid iron catalysts or iron-modified cathodes are subsequently detailed. Finally, the oxidation power of hybrid processes including photocatalysis/PEF, solar photocatalysis/SPEF, photoelectrocatalysis/PEF and solar photoelectrocatalysis/SPEF, followed by that of sequential processes like electrocoagulation/PEF and biological oxidation coupled to SPEF, are analyzed.

Simultaneous Electrochemical Generation of Ferrate and Oxygen Radicals to Blue BR Dye Degradation

In this study, electro-oxidation (EOx) and in situ generation of ferrate ions [Fe(VI)] were tested to treat water contaminated with Blue BR dye (BBR) using a boron-doped diamond (BDD) anode. Two electrolytic media (0.1 M HClO4 and 0.05 M Na2SO4) were evaluated for the BDD, which simultaneously produced oxygen radicals (•OH) and [Fe(VI)]. The generation of [Fe(VI)] was characterized by cyclic voltammetry (CV) and the effect of different current intensity values (e.g., 7 mA cm−2, 15 mA cm−2, and 30 mA cm−2) was assessed during BBR degradation tests. The discoloration of BBR was followed by UV-Vis spectrophotometry. When the EOx process was used alone, only 78% BBR discoloration was achieved. The best electrochemical discoloration conditions were found using 0.05 M Na2SO4 and 30 mA cm−2. Using these conditions, overall BBR discoloration values up to 98%, 95%, and 87% with 12 mM, 6 mM, and 1 mM of FeSO4, respectively, were achieved. In the case of chemical oxygen demand (COD) reduction, the EOx process showed only a 37% COD reduction, whereas combining [Fe(VI)] generation using 12 mM of FeSO4 achieved an up to 61% COD reduction after 90 min. The evolution of reaction byproducts (oxalic acid) was performed using liquid chromatography analysis.

Study on the efficient removal of azo dyes by heterogeneous photo-Fenton process with 3D flower-like layered double hydroxide

As organic dyes are the main pollutants in water pollution, seeking effective removal solutions is urgent for humans and the environment. A novel environmentally friendly three-dimensional CoFe-LDHs (3D CoFe-LDHs) catalyst was synthesized by one-step hydrothermal method. Scanning electron microscopy, energy dispersive spectroscopy, Fourier transform infrared spectra, X-ray diffraction, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller technique as well as UV-Vis diffuse reflectance spectra were used to characterize the prepared samples. The experimental results revealed that 3D CoFe-LDHs exhibited a rapid decolorization of methyl orange and Rhodamine B by heterogeneous photo-Fenton process after reaching the adsorption equilibrium, and the final decolorization efficiency reached 91.18% and 93.56%, respectively. On the contrary, the decolorizing effect of 3D CoFe-LDHs on neutral blue was relatively weak. The initial concentrations of azo dyes, pH and H₂O₂ concentration affected the decolorization of dyes and the catalyst maintained excellent reusability and stability after reuse over five cycles. The quenching experiments found that •OH, •O₂ ^- and h⁺ were the main active substances and reaction mechanisms were further proposed. The study suggests that the synergistic effect of photocatalysis and Fenton oxidation process significantly improved the removal of azo dyes and the synthesized catalyst had potentially promising applications for difficult-to-biodegrade organic pollutants in wastewater.

Mineralization of Acid Red 1 azo dye by solar photoelectro-Fenton-like process using electrogenerated HClO and photoregenerated Fe(II)

The degradation of Acid Red 1 (AR1) azo dye by solar photoelectro-Fenton-like (SPEF-like) process involving continuously electrogenerated hypochlorous acid (HClO) and photoregenerated Fe(II) to yield hydroxyl radicals, has been studied. The assays were made in a flow plant that included a filter-press cell equipped with a Ti|Ir-Sn-Sb oxide anode, to oxidize Cl^- ion to HClO, and a stainless-steel cathode. The cell was coupled to a compound parabolic collector (CPC) photoreactor, in series with a reservoir containing 6 L of solution. The influence of the added Fe²⁺ concentration, current density and initial AR1 content over the performance of the SPEF-like process was systematically studied. The best treatment for 0.196 mM AR1 solutions in 35 mM NaCl and 25 mM Na₂SO₄ at pH 3.0 was achieved in the presence of 0.40 mM Fe²⁺ under a current density of 15 mA cm^-2, which yielded total color removal at 120 min and 74% COD decay at 480 min, with 25% of average current efficiency and 0.076 kW h (g COD)^-1 of energy consumption. The SPEF-like process was compared with anodic oxidation with electrogenerated active chlorine (AO-HClO), electro Fenton-like (EF-like) and photoelectro-Fenton-like (PEF-like) processes, and it was found that the oxidation power decreased in the sequence: SFEF-like > PEF-like > EF-like > AO-HClO. Ion-exclusion HPLC analysis of electrolyzed solutions revealed the formation of maleic and oxalic acid as final short-chain linear carboxylic acids.

Chloride or sulfate? Consequences for ozonation of textile wastewater

Ozonation of chloride-rich textile wastewater is a common pretreatment practice in order to increase biodegradability and therefore meet the discharge limits. This study is the first to investigate ozone-chloride/bromide interactions and formation of hazardous adsorbable organic halogens (AOX) in real textile wastewater. Initially effect of ozonation on chloride-rich real textile wastewater samples were investigated for adsorbable organic halogens (AOX) formation, biodegradability and toxicity. After 15 min of ozonation, maximum levels of chlorine/bromine generation (0.3 mg/l) and AOX formation (399 mg/l) were reached. OUR and SOUR levels both increased by approximately 58%. Daphnia magna toxicity peaked at 100% for 10 min ozonated sample. Considering adverse effects of ozonation on chloride-rich textile industry effluents, we proposed replacement of NaCl with Na₂SO₄. Comparative ozonation experiments were carried out for both chloride and sulfate containing synthetic dyeing wastewater samples. Results showed that use of sulfate in reactive dyeing increased biodegradability and decreased acute toxicity. Although sulfate is preferred over chloride for more effective dyeing performance, the switch has been hampered due to sodium sulfate's higher unit cost. However, consideration of indirect costs such as contributions to biodegradability, toxicity, water and salt recovery shall facilitate textile industry's switch from chloride to sulfate.

Degradation of ampicillin antibiotic by electrochemical processes: evaluation of antimicrobial activity of treated water

Ampicillin (AMP) is an antibiotic widely used in hospitals and veterinary clinics around the world for treating infections caused by bacteria. Therefore, it is common to find traces of this antibiotic in wastewater from these entities. In this work, we studied the mineralization of this antibiotic in solution as well as the elimination of its antimicrobial activity by comparing different electrochemical advanced oxidation processes (EAOPs), namely electro-oxidation with hydrogen peroxide (EO-H₂O₂), electro-Fenton (EF), and photo electro-Fenton (PEF). With PEF process, a high degradation, mineralization, and complete elimination of antimicrobial activity were achieved in 120-min electrolysis with high efficiency. In the PEF process, fast mineralization rate is caused by hydroxyl radicals (·OH) that are generated in the bulk, on the anode surface, by UV radiation, and most importantly, by the direct photolysis of complexes formed between Fe³⁺ and some organic intermediates. Moreover, some products and intermediates formed during the degradation of the antibiotic Ampicillin, such as inorganic ions, carboxylic acids, and aromatic compounds, were determined by photometric and chromatographic methods. An oxidation pathway is proposed for the complete conversion to CO₂.

Current advances and trends in electro-Fenton process using heterogeneous catalysts - A review

Over the last decades, advanced oxidation processes have often been used alone, or combined with other techniques, for remediation of ground and surface water pollutants. The application of heterogeneous catalysis to electrochemical advanced oxidation processes is especially useful due to its efficiency and environmental safety. Among those processes, electro-Fenton stands out as the one in which heterogeneous catalysis has been broadly applied. Thus, this review has introduced an up-to-date collation of the current knowledge of the heterogeneous electro-Fenton process, highlighting recent advances in the use of different catalysts such as iron minerals (pyrite, magnetite or goethite), prepared catalysts by the load of metals in inorganic and organic materials, nanoparticles, and the inclusion of catalysts on the cathode. The effects of physical-chemical parameters as well as the mechanisms involved are critically assessed. Finally, although the utilization of this process to remediation of wastewater overwhelmingly outnumber other utilities, several applications have been described in the context of regeneration of adsorbent or the remediation of soils as clear examples of the feasibility of the electro-Fenton process to solve different environmental problems.

Effective removal of the antibiotic Nafcillin from water by combining the Photoelectro-Fenton process and Anaerobic Biological Digestion

The elimination of the antibiotic Nafcillin (NAF), which is usually used in hospitals and veterinary clinics around the world, was assessed through a combination of three advanced electrochemical oxidation processes followed by anaerobic digestion process. In the first stage different electrochemical advanced oxidation processes (EAOPs) were used: electro-oxidation with hydrogen peroxide (EO-H₂O₂), electro-Fenton (EF) and Photo electro-Fenton (PEF). After PEF, almost complete and highly efficient degradation and elimination of NAF was achieved, with the concomitant elimination of the associated antimicrobial activity. The fast degradation rate produced by PEF is explained by the oxidative action of hydroxyl radicals (•OH) together with the direct UV photolysis of complexes formed between Fe³⁺ and some organic intermediates. Total removal of NAF occurs after 90min of electrolysis by PEF, with the generation of organic intermediates that remain in solution. However, when this post PEF process solution was treated with an anaerobic biological process, the intermediates generated in the electrochemical degradation of NAF were completely eliminated after 24h. The kinetic degradation of NAF as well as the identification/quantification of products and intermediates formed during the degradation of antibiotic, such as inorganic ions, carboxylic acids and aromatic compounds, were determined by chromatographic and photometric methods. Finally, an oxidation pathway is proposed for the complete conversion to CO₂.

Role of sulfate, chloride, and nitrate anions on the degradation of fluoroquinolone antibiotics by photoelectro-Fenton

Taking ciprofloxacin (CIP) as a fluoroquinolone antibiotic model, this work explores the role of common anions (sulfate, nitrate, and chloride) during the application of photoelectro-Fenton (PEF) at natural pH to degrade this type of compound in water. The system was composed of an IrO₂ anode, Ti, or gas diffusion electrode (GDE) as cathode, Fe²⁺, and UV (254 nm). To determine the implications of these anions, the degradation pathway and efficiency of the PEF sub-processes (UV photolysis, anodic oxidation, and electro-Fenton at natural pH) were studied in the individual presence of the anions. The results highlight that degradation routes and kinetics are strongly dependent on electrolytes. When chloride and nitrate ions were present, indirect electro-chemical oxidation was identified by electro-generated HOCl and nitrogenated oxidative species, respectively. Additionally, direct photolysis and direct oxidation at the anode surface were identified as degradation routes. As a consequence of the different pathways, six primary CIP by-products were identified. Therefore, a scheme was proposed representing the pathways involved in the degradation of CIP when submitted to PEF in water with chloride, nitrate, and sulfate ions, showing the complexity of this process. Promoted by individual and synergistic actions of this process, the PEF system leads to a complete elimination of CIP with total removal of antibiotic activity against Staphylococcus aureus and Escherichia coli, and significant mineralization. Finally, the role of the anions was tested in seawater containing CIP, in which the positive contributions of the anions were partially suppressed by its OH radical scavenger action. The findings are of interest for the understanding of the degradation of antibiotics via the PEF process in different matrices containing sulfate, nitrate, and chloride ions.