Degradation of artificial sweetener saccharin in aqueous medium by electrochemically generated hydroxyl radicals

Exploring the viability of peracetic acid-mediated antibiotic degradation in wastewater through activation with electrogenerated HClO

Electrochemical advanced oxidation processes (EAOPs) face challenging conditions in chloride media, owing to the co-generation of undesirable Cl-disinfection byproducts (Cl-DBPs). Herein, the synergistic activation between in-situ electrogenerated HClO and peracetic acid (PAA)-based reactive species in actual wastewater is discussed. A metal-free graphene-modified graphite felt (graphene/GF) cathode is used for the first time to achieve the electrochemically-mediated activation of PAA. The PAA/Cl- system allowed a near-complete sulfamethoxazole (SMX) degradation (kobs =0.49 min-1) in only 5 min in a model solution, inducing 32.7- and 8.2-fold rise in kobs as compared to single PAA and Cl- systems, respectively. Such enhancement is attributed to the occurrence of 1O2 (25.5 μmol L-1 after 5 min of electrolysis) from the thermodynamically favored reaction between HClO and PAA-based reactive species. The antibiotic degradation in a complex water matrix was further considered. The SMX removal is slightly susceptible to the coexisting natural organic matter, with both the acute cytotoxicity (ACT) and the yield of 12 DBPs decreasing by 29.4 % and 37.3 %, respectively. According to calculations, HClO accumulation and organic Cl-addition reactions are thermodynamically unfavored. This study provides a scenario-oriented paradigm for PAA-based electrochemical treatment technology, being particularly appealing for treating wastewater rich in Cl- ion, which may derive in toxic Cl-DBPs.

Oxidative treatment of micropollutants present in wastewater: A special emphasis on transformation products, their toxicity, detection, and field-scale investigations

Micropollutants have become ubiquitous in aqueous environments due to the increased use of pharmaceuticals, personal care products, pesticides, and other compounds. In this review, the removal of micropollutants from aqueous matrices using various advanced oxidation processes (AOPs), such as photocatalysis, electrocatalysis, sulfate radical-based AOPs, ozonation, and Fenton-based processes has been comprehensively discussed. Most of the compounds were successfully degraded with an efficiency of more than 90%, resulting in the formation of transformation products (TPs). In this respect, degradation pathways with multiple mechanisms, including decarboxylation, hydroxylation, and halogenation, have been illustrated. Various techniques for the analysis of micropollutants and their TPs have been discussed. Additionally, the ecotoxicity posed by these TPs was determined using the toxicity estimation software tool (T.E.S.T.). Finally, the performance and cost-effectiveness of the AOPs at the pilot scale have been reviewed. The current review will help in understanding the treatment efficacy of different AOPs, degradation pathways, and ecotoxicity of TPs so formed.

Advanced degradation of refractory organic compounds in electroplating wastewater by an in-situ electro-catalytic biological coupling reactor: Removal performance, microbial community and possible mechanism

A high-efficiency treatment system for advanced degradation of refractory organic compounds such as saccharin sodium (SS) and polyethylene glycol 6000 (PEG 6000) in electroplating wastewater was proposed, which coupled ion exchange, electrocatalysis, and microbial interactions through ion exchange particle electrode (IEPE) in a reactor, named in-situ electro-catalytic biological coupling reactor (i-SECBCR). A small-scale experimental test system was established and a feasibility investigation was conducted under the condition of 1.248 L/h continuous flow. The results revealed that (1) the i-SECBCR showed higher average removal rates of SS, PEG 6000, COD and NH4+-N, i.e. 88.48 %, 41.26 %, 66.81 % and 51.61 %,which meant an increase by 5.04 %, 12.05 %, 0.46 %, and 34.50 %, respectively, compared with BAF; (2) the optimal current intensity (CI) of i-SECBCR for simultaneous removal of SS, PEG 6000, COD and NH4+-N was 0.40 mA cm-2; (3) Rhodobacter, Defluviimonas, unclassified_f__Microscillaceae, Pseudoxanthomonas, Novosphingobium, and unclassified_f__Xanthobacteraccae accounted for the main bacterial community in i-SECBCR; (4) the possible degradation mechanism was attributed mainly to the synergistic effect of ion exchange, electrocatalytic oxidation and biology. Therefore, the i-SECBCR was suitable to simultaneously advanced remove SS, PEG 6000, COD and NH4+-N in electroplating wastewater.

Assessment of photo electro-Fenton and solar photo electro-Fenton processes for the efficient degradation of asulam herbicide

The degradation of asulam herbicide by photo electro-Fenton (PEF) and solar photo electro-Fenton (SPEF) processes was studied using an undivided electrochemical BDD/carbon-felt cell to generate H2O2 continuously. A central composite design combined with response surface methodology was applied to determine the optimal operating conditions of current intensity = 0.30 A, [Fe2+] = 0.3 mM, and [Na2SO4] = 0.11 M at pH 3 to achieve the complete degradation of asulam by electro-Fenton. Subsequently, the SPEF process was more efficient treatment compared to PEF, achieving a complete degradation of asulam and 98% of mineralization in 180 min. Moreover, 4-aminobenzenesulfonamide, 4-aminophenol, and 4-benzoquinone were detected as aromatic intermediates, whereas acetic acid, oxalic acid, and NO3- ions were identified as final degradation by-products. Thus, the SPEF process is an efficient alternative for the complete degradation and mineralization of herbicide asulam in an aqueous solution under natural sunlight.

Heterogeneous Electro-Fenton-Catalyzed Degradation of Rhodamine B by Nano-Calcined Pyrite

The use of natural pyrite as a catalyst for the treatment of recalcitrant organic wastewater by an electro-Fenton system (pyrite-EF) has recently received extensive attention. To improve the catalytic activity of natural pyrite (Py), magnetic pyrite (MPy), and pyrrhotite (Pyr), they were obtained by heat treatment, and the nanoparticles were obtained by ball milling. They were characterized by X-ray diffraction, X-ray electron spectroscopy, and scanning electron microscopy. The degradation performance of rhodamine B (Rhb) by heterogeneous catalysts was tested under the pyrite-EF system. The effects of optimal pH, catalyst concentration, and current density on mineralization rate and mineralization current efficiency were explored. The results showed that the heat treatment caused the phase transformation of pyrite and increased the relative content of ferrous ions. The catalytic performance was MPy > Py > Pyr, and the Rhb degradation process conformed to pseudo-first-order kinetics. Under the optimum conditions of 1 g L^-1 MPy, an initial pH of five, and a current density of 30 mA cm^-2, the degradation rate and TOC removal rate of Rhb wastewater reached 98.25% and 77.06%, respectively. After five cycles of recycling, the chemical activity of MPy was still higher than that of pretreated Py. The main contribution to Rhb degradation in the system was •OH radical, followed by SO4•-, and the possible catalytic mechanism of MPy catalyst in the pyrite-EF system was proposed.

Degradation of saccharin by UV/H2O2 and UV/PS processes: A comparative study

Artificial sweeteners have raised emerging concern due to their potential threats to human health, which were frequently detected in aquatic environment with median concentrations. Although current researches have widely reported that ultraviolet light-activated persulfate process (UV/PS) was superior to UV/H₂O₂ process for the degradation of refractory organic contaminants, UV/H₂O₂ process presented a more satisfactory saccharin (SAC) removal efficiency than UV/PS process, completely degraded 20 mg/L SAC within 45 min. Hence, quenching and probe experiments were employed to investigate the difference between hydroxyl radical (OH)- and sulfate radical (SO₄^-)-mediated oxidation mechanisms, which revealed the higher reactivity of OH (1.37-1.56 × 10⁹ M^-1 s^-1) toward SAC than SO₄^- (3.84-4.13 × 10⁸ M^-1 s^-1). A combination of density functional theory calculation and transformation products identification disclosed that OH preferred to attack the benzene ring of SAC via hydrogen atom transfer pathway, whereas SO₄^- oxidation was conducive to the cleavage of -C-NH₂ bond. Increasing oxidant concentration significantly accelerated SAC degradation in both processes, while UV/H₂O₂ process consumed lower electrical energy with respect to UV/PS process. Additionally, UV/H₂O₂ system presented excellent adaptability and stability under various water matrices parameters (e.g. pH, anions and humic acid). While both UV/H₂O₂ and UV/PS processes promoted the generation of disinfection by-products (DBPs) during subsequent chlorination, and prolonging pretreatment time posed positive effect on reducing the formation of DBPs. Overall, the results clearly demonstrate the high efficiency, economy and practicality of UV/H₂O₂ process in the remediation of SAC-contaminated water.

Environmental Impact of the Presence, Distribution, and Use of Artificial Sweeteners as Emerging Sources of Pollution

Artificial sweeteners are posing a new threat to the environment. The water ecosystem is the primary recipient of these emerging contaminants. Once ingested, sufficient amount of these artificial sweeteners escape unchanged from the human body and are added to the environment. However, some are added in the form of their breakdown products through excretion. Artificial sweeteners are resistant to wastewater treatment processes and are therefore continuously introduced into the water environments. However, the environmental behavior, fate, and long-term ecotoxicological contributions of artificial sweeteners in our water resources still remain largely unknown. Some artificial sweeteners like saccharin are used as a food additive in animal feeds. It also forms the degradation product of the sulfonylurea herbicides. All artificial sweeteners enter into the wastewater treatment plants from the industries and households. From the effluents, they finally reside into the receiving environmental bodies including wastewaters, groundwaters, and surface waters. The global production of these sweeteners is several hundred tons annually and is continuously being added into the environment.

Dual radicals-enhanced wet chemical oxidation of non-biodegradable chemicals

Total organic carbon (TOC) has been suggested and utilized as an index of organic matter in aqueous phases. The overall performance of TOC is highly dependent on the method of oxidation of organic matter to carbon dioxide, such as high-temperature combustion (HTC) and wet chemical oxidation (WCO). HTC requires more energy and maintenance cost, it is a major barrier to the field application. In contrast, WCO is more suitable for the application of on-line monitoring systems due to requiring lower energy and easy maintenance. However, WCO shows lower oxidation than HTC, thus, oxidation performance should be improved for the application to the field. In this study, a dual radical system (DRS), including sulfate and hydroxyl radicals, was proposed to enhance oxidation ability. The DRS uses alkaline pH and persulfate to generate sulfate radicals, which have been used to activate hydroxyl radicals and oxidize organic matter. The oxidation mechanism for the DRS has been verified using model chemicals with different reaction rate constants. The applicability of the DRS has been confirmed using authentic wastewater with a high concentration of chloride. In this study, the DRS showed similar performance compared to the HTC within 10 % error range. The DRS shows similar oxidation performance with HTC even at a high concentration of chloride. DRS did not show interference by the presence of chloride up to 30,000 mg/L of chloride. Results of this study indicate that the DRS can enhance overall oxidation performance compared to the conventional WCO system.

Degradation of sulfamethazine by biochar-supported bimetallic oxide/persulfate system in natural water: Performance and reaction mechanism

The rapid development of aquaculture results in the increased concentrations and kinds of antibiotics in water environment, and the sharply growing antibiotic contamination has caused increasing concerns. Herein, an innovative sulfamethazine (SMT) removal approach was developed by activation of persulfate (PS) using biochar-based materials prepared by co-precipitation and pyrolysis: Fe-Mg oxide/biochar (FeMgO/BC). Experiments on the activation of PS by FeMgO/BC under different factors were carried out. The involved mechanism and degradation pathway were also studied. Notably, the SMT removal rate reached 99 % under the optimum reaction condition, while the TOC removal efficiency reached 77.9 %. PS was activated by FeMgO/BC and the dominated active radical was SO_4•^-. Fe²⁺ from FeMgO and the hydroxyl and carboxyl groups on the surface of biochar contributed to the production of SO_4•^-. The dehydrogenation, bond cracking and unsaturated bond addition process occurred in the degradation of SMT. Furthermore, FeMgO/BC exhibits excellent reusability and stability. Considering the outstanding actual water application performances and the weak biotoxicity, FeMgO/BC shows a promising potential in the removal of antibiotics under actual water conditions.

Highly efficient and stable FeIIFeIII LDH carbon felt cathode for removal of pharmaceutical ofloxacin at neutral pH

The traditional electro-Fenton (EF) has been facing major challenges including narrow suitable range of pH and non-reusability of catalyst. To overcome these drawbacks we synthesized Fe^IIFe^III-layered double hydroxide modified carbon felt (Fe^IIFe^III LDH-CF) cathode via in situ solvo-thermal process. Chemical composition and electrochemical characterization of Fe^IIFe^III LDH-CF were tested and analyzed. The apparent rate constant of decay kinetics of ofloxacin (OFC) with Fe^IIFe^III LDH-CF (0.18 min^-1) at pH 7 was more than 3 times higher than that of homogeneous EF (0.05 min^-1) at pH 3 with 0.1 mM Fe²⁺ under same current density (9.37 mA cm^-2). Also, a series of experiments including evolution of solution pH, iron leaching, OFC removal with trapping agent and quantitative detection of hydroxyl radicals (OH) were conducted, demonstrating the dominant role of OH generated by surface catalyst via ≡ Fe^II/Fe^III on LDH cathode for degradation of organics as well contributing to high efficiency and good stability at neutral pH. Besides, formation and evolution of aromatic intermediates, carboxylic acids and inorganic ions (F^-, NH₄⁺ and NO₃^-) were identified by High-Performance Liquid chromatography, Gas Chromatography-Mass Spectrometry and ionic chromatography analyses. These findings allowed proposing a plausible degradation pathway of OFC by OH generated in the heterogeneous EF process.

Electro-Fenton oxidation of para-aminosalicylic acid: degradation kinetics and mineralization pathway using Pt/carbon-felt and BDD/carbon-felt cells

Degradation of a widely used antibiotic, the para-aminosalicylic acid (PAS), and mineralization of its aqueous solution was investigated by electro-Fenton process using Pt/carbon-felt and boron-doped diamond (BDD)/carbon-felt cells with applied currents in the range of 50-1000 mA. This process produces the highly oxidizing species, the hydroxyl radical (^•OH), which is mainly responsible for the oxidative degradation of PAS. An absolute rate constant of 4.17 × 10⁹ M^-1 s^-1 for the oxidation of PAS by ^●OH was determined from the competition kinetics method. Degradation rate of PAS increased with current reaching an optimal value of 500 mA with complete disappearance of 0.1 mM PAS at 7 min using Pt/carbon-felt cell. The optimum degradation rate was reached at 300 mA for BDD/carbon-felt. The latter cell was found more efficient in total organic carbon (TOC) removal where a complete mineralization was achieved within 240 min. A multi-step mineralization process was observed with the formation of a number of aromatic intermediates, short-chain carboxylic acids, and inorganic ions. Eight aromatic intermediate products were identified using both LC-Q-ToF-MS and GC-MS techniques. These products were the result of hydroxylation of PAS followed by multiple additions of hydroxyl radicals to form polyhydroxylated derivatives. HPLC and GC/MS analyses demonstrated that extended oxidation of these intermediate products conducted to the formation of various short-chain carboxylic acids. Prolonged electrolysis resulted in a complete mineralization of PAS with the evolution of inorganic ions such as NO₃^- and NH₄⁺. Based on the identified intermediates, carboxylic acids and inorganic ions, a plausible mineralization pathway is also deduced. The remarkably high degree of mineralization (100%) achieved by the present EF process highlights the potential application of this technique to the complete removal of salicylic acid-based pharmaceuticals from contaminated water.

Contaminants of emerging concern: a review of new approach in AOP technologies

The presence of contaminants of emerging concern (CECs) such as pharmaceuticals and personal care products (PPCPs), endocrine-disrupting compounds (EDCs), flame retardants (FRs), pesticides, and artificial sweeteners (ASWs) in the aquatic environments remains a major challenge to the environment and human health. In this review, the classification and occurrence of emerging contaminants in aquatic environments were discussed in detail. It is well documented that CECs are susceptible to poor removal during the conventional wastewater treatment plants, which introduce them back to the environment ranging from nanogram per liter (e.g., carbamazepine) up to milligram per liter (e.g., acesulfame) concentration level. Meanwhile, a deep insight into the application of advanced oxidation processes (AOPs) on mitigation of the CECs from aquatic environment was presented. In this regard, the utilization of various treatment technologies based on AOPs including ozonation, Fenton processes, sonochemical, and TiO₂ heterogeneous photocatalysis was reviewed. Additionally, some innovations (e.g., visible light heterogeneous photocatalysis, electro-Fenton) concerning the AOPs and the combined utilization of AOPs (e.g., sono-Fenton) were documented.

Nanostructured ZnO-TiO2 thin film oxide as anode material in electrooxidation of organic pollutants. Application to the removal of dye Amido black 10B from water

Electrochemical oxidative degradation of diazo dye Amido black 10B (AB10B) as model pollutant in water has been studied using nanostructured ZnO-TiO₂ thin films deposited on graphite felt (GrF) substrate as anode. The influence of various operating parameters, namely the current intensity, the nature and concentration of catalyst, the nature of electrode materials (anode/cathode), and the adsorption of dye and ambient light were investigated. It was found that the oxidative degradation of AB10B followed pseudo first-order kinetics. The optimal operating conditions for the degradation of 0.12 mM (74 mg L^-1) dye concentration and mineralization of its aqueous solution were determined as GrF-ZnO-TiO₂ thin film anode, 100 mA current intensity, and 0.1 mM Fe²⁺ (catalyst) concentration. Under these operating conditions, discoloration of AB10B solution was reached at 60 min while 6 h treatment needed for a mineralization degree of 91 %. Therefore, this study confirmed that the electrochemical process is effective for the degradation of AB10B in water using nanostructured ZnO-TiO₂ thin film anodes.

Removal of artificial sweetener aspartame from aqueous media by electrochemical advanced oxidation processes

The degradation and mineralization of aspartame (ASP) in aqueous solution were investigated, for the first time, by electrochemical advanced oxidation processes (EAOPs) in which hydroxyl radicals were formed concomitantly in the bulk from Fenton reaction via in situ electrogenerated Fenton's reagent and at the anode surface from the water oxidation. Experiments were performed in an undivided cylindrical glass cell with a carbon-felt cathode and a Pt or boron-doped diamond (BDD) anode. The effect of Fe²⁺ concentration and applied current on the degradation and mineralization kinetics of ASP was evaluated. The absolute rate constant for the reaction between ASP and OH was determined as (5.23 ± 0.02) × 10⁹ M^-1 s^-1 by using the competition kinetic method. Almost complete mineralization of ASP was achieved with BDD anode at 200 mA constant current electrolysis. The formation and generation of the formed carboxylic acids (as ultimate end products before complete mineralization) and released inorganic ion were monitored by ion-exclusion high performance liquid chromatography (HPLC) and ion chromatography techniques, respectively. The global toxicity of the treated ASP solution during treatment was assessed by the Microtox^® method using V. fischeri bacteria luminescence inhibition.

Parabens abatement from surface waters by electrochemical advanced oxidation with boron doped diamond anodes

The removal efficiency of four commonly-used parabens by electrochemical advanced oxidation with boron-doped diamond anodes in two different aqueous matrices, namely ultrapure water and surface water from the Guadiana River, has been analyzed. Response surface methodology and a factorial, composite, central, orthogonal, and rotatable (FCCOR) statistical design of experiments have been used to optimize the process. The experimental results clearly show that the initial concentration of pollutants is the factor that influences the removal efficiency in a more remarkable manner in both aqueous matrices. As a rule, as the initial concentration of parabens increases, the removal efficiency decreases. The current density also affects the removal efficiency in a statistically significant manner in both aqueous matrices. In the water river aqueous matrix, a noticeable synergistic effect on the removal efficiency has been observed, probably due to the presence of chloride ions that increase the conductivity of the solution and contribute to the generation of strong secondary oxidant species such as chlorine or HClO/ClO ^-. The use of a statistical design of experiments made it possible to determine the optimal conditions necessary to achieve total removal of the four parabens in ultrapure and river water aqueous matrices.

A coupled Bio-EF process for mineralization of the pharmaceuticals furosemide and ranitidine: Feasibility assessment

A coupled Bio-EF treatment has been applied as a reliable process for the degradation of the pharmaceuticals furosemide (FRSM) and ranitidine (RNTD) in aqueous medium, in order to reduce the high energy consumption related to electrochemical technology. In the first stage of this study, electrochemical degradation of the drugs was assessed by the electro-Fenton process (EF) using a BDD/carbon-felt cell. Biodegradability of the drugs solutions was enhanced reaching BOD5/COD ratios close to the biodegradability threshold of 0.4, evidencing the formation of bio-compatible by-products (mainly short-chain carboxylic acids) which are suitable for biological post-treatment. Moreover, toxicity evaluation by the Microtox(®) method revealed that EF pre-treatment was able of detoxifying both, FRSM and RNTD solutions, constituting another indicator of biodegradability of EF treated solutions. In the second stage, electrolyzed solutions were treated by means of an aerobic biological process. A significant part of the short-chain carboxylic acids formed during the electrochemical phase was satisfactorily removed by the used selected microorganisms. The results obtained demonstrate the efficiency and feasibility of the integrated Bio-EF process.

Pyrite as a sustainable catalyst in electro-Fenton process for improving oxidation of sulfamethazine. Kinetics, mechanism and toxicity assessment

The degradation of 0.20 mM sulfamethazine (SMT) solutions was investigated by heterogeneous electro-Fenton (EF) process using pyrite as source of Fe(2+) (catalyst) and pH regulator in an undivided electrochemical cell equipped either with a Pt or a BDD anode and carbon-felt as cathode. Effect of pyrite concentration and applied current on the oxidative degradation kinetics and mineralization efficiency has been studied. The higher oxidation power of the process, named "Pyrite-EF″ using BDD anode was demonstrated. Pyrite-EF showed a better performance for the oxidation/mineralization of the drug SMT in comparison to the classic EF process: 95% and 87% TOC removal by Pyrite-EF with BDD and Pt anodes, respectively, versus 90% and 83% by classical EF with BDD and Pt anodes, respectively. The rate constant of the oxidation of SMT by OH was determined by the competition kinetics method and found to be 1.87 × 10(9) mol(-1) L s(-1). Based on the identified reaction intermediates by HPLC and GS-MS, as well as released SO4(2-), NH4(+) and NO3(-) ions, a plausible reaction pathway was proposed for the mineralization of SMT during Pyrite-EF process. Toxicity assessment by means of Microtox method revealed the formation of some toxic intermediates during the treatment. However, toxicity of the solution was removed at the end of treatment.