Single and Coupled Electrochemical Processes and Reactors for the Abatement of Organic Water Pollutants: A Critical Review

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.

A comparative study of microcystin-LR degradation by electrogenerated oxidants at BDD and MMO anodes

This study investigated the electrochemical degradation of microcystin-LR (MC-LR) using boron-doped diamond (BDD) anode and mixed metal oxides (MMO, IrO₂Ta₂O₅/Ti) anode in different medium. In-situ electrogenerated oxidants including hydroxyl radical, active chlorine, and persulfate were confirmed in phosphate, chloride, and sulfate medium, respectively. Different from MMO anode, hydroxyl radical was observed to play a significant role in chlorine generation at BDD anode in chloride medium. Besides, BDD anode could activate sulfate electrochemically due to its high oxygen evolution potential, and MC-LR degradation rate increased with the decrease of solution pH. The effects of natural organic matters (NOM) and algal organic matters (AOM) on MC-LR degradation were evaluated and NOM presented stronger inhibition ability than AOM. Furthermore, the intermediates generated in MC-LR degradation in chloride and sulfate medium were identified by LC/MS/MS and possible degradation pathways were proposed based on the experiments results. Benzene ring and conjugated diene bonds of Adda group and double bonds of Mhda group were found to be the reactive sites of MC-LR. Overall, this study broadens the knowledge of electrochemical oxidation in removing microcystins in algae-laden water.

Sub-stoichiometric titanium oxide (Ti4O7) as a suitable ceramic anode for electrooxidation of organic pollutants: A case study of kinetics, mineralization and toxicity assessment of amoxicillin

Electrochemical degradation of aqueous solutions containing antibiotic amoxicillin (AMX) has been extensively studied in an undivided electrolytic cell using a sub-stoichiometric titanium oxide (Ti₄O₇) anode, elaborated by plasma deposition. Oxidative degradation of AMX by hydroxyl radicals was assessed as a function of applied current and was found to follow pseudo-first order kinetics. The use of carbon-felt cathode enhanced oxidation capacity of the process due to the generation of H₂O₂. Comparative studies at low current intensity using dimensional stable anode (DSA) and Pt anodes led to the lower mineralization efficiencies compared to Ti₄O₇ anode: 36 and 41% TOC removal for DSA and Pt respectively compared to 69% for Ti₄O₇ anode. Besides, the use of boron doped diamond (BDD) anode under similar operating conditions allowed reaching higher mineralization (94%) efficiency. Although Ti₄O₇ anode provides a lesser mineralization rate compared to BDD, it exhibits better performance compared to the classical anodes Pt and DSA and can constitutes an alternative to BDD anode for a cost effective electro-oxidation process. Moreover several aromatic and aliphatic oxidation reaction intermediates and inorganic end-products were identified and a plausible mineralization pathway of AMX involving these intermediates was proposed.

Emerging trends in photodegradation of petrochemical wastes: a review

Various human activities like mining and extraction of mineral oils have been used for the modernization of society and well-beings. However, the by-products such as petrochemical wastes generated from such industries are carcinogenic and toxic, which had increased environmental pollution and risks to human health several folds. Various methods such as physical, chemical and biological methods have been used to degrade these pollutants from wastewater. Advance oxidation processes (AOPs) are evolving techniques for efficient sequestration of chemically stable and less biodegradable organic pollutants. In the present review, photocatalytic degradation of petrochemical wastes containing monoaromatic and poly-aromatic hydrocarbons has been studied using various heterogeneous photocatalysts (such as TiO₂, ZnO and CdS. The present article seeks to offer a scientific and technical overview of the current trend in the use of the photocatalyst for remediation and degradation of petrochemical waste depending upon the recent advances in photodegradation of petrochemical research using bibliometric analysis. We further outlined the effect of various heterogeneous catalysts and their ecotoxicity, various degradation pathways of petrochemical wastes, the key regulatory parameters and the reactors used. A critical analysis of the available literature revealed that TiO₂ is widely reported in the degradation processes along with other semiconductors/nanomaterials in visible and UV light irradiation. Further, various degradation studies have been carried out at laboratory scale in the presence of UV light. However, further elaborative research is needed for successful application of the laboratory scale techniques to pilot-scale operation and to develop environmental friendly catalysts which support the sustainable treatment technology with the "zero concept" of industrial wastewater. Nevertheless, there is a need to develop more effective methods which consume less energy and are more efficient in pilot scale for the demineralization of pollutant.

The ability of electrochemical oxidation with a BDD anode to inactivate Gram-negative and Gram-positive bacteria in low conductivity sulfate medium

The disinfection of 100 mL of synthetic water containing 7 mM Na2SO4 with 10(6) CFU mL(-1) of either Gram-negative or Gram-positive bacteria has been studied by electrochemical oxidation. The electrolytic cell was a stirred tank reactor equipped with a boron-doped diamond (BDD) anode and a stainless steel cathode and the trials were performed at acidic and neutral pH, at 33.3 mA cm(-2) and 25 °C. Reactive oxygen species, pre-eminently hydroxyl radicals, were efficiently produced in both media from water oxidation at the BDD anode and the bacteria concentration was reduced by ≥ 5 log units after 60 min of electrolysis, thus constituting a good chlorine-free disinfection treatment. All the inactivation kinetics were described by a logistic model, with no significant statistical differences between acidic and neutral suspensions. The electrochemical disinfection with BDD was very effective for Gram-negative bacilli like Escherichia coli and Pseudomonas aeruginosa and Gram-positive ones like Bacillus atrophaeus, whereas the Gram-positive cocci Staphylococcus aureus and Enterococcus hirae were more resistant. Thus, the latter organisms are a better choice than E. coli as process indicators. Scanning electron microscopy highlighted a transition from initial cells with standard morphology supported on clean filters to inactivated cells with a highly altered morphology lying on dirty filters with plenty of cellular debris. Larger damage was observed for Gram-negative cells compared to Gram-positive ones. The inactivation effect could then be related to the chemical composition of the outer layers of the cell structure along with the modification of the transmembrane potentials upon current passage.

Synthesis and Stabilization of Blue-Black TiO2 Nanotube Arrays for Electrochemical Oxidant Generation and Wastewater Treatment

Efficient, inexpensive, and stable electrode materials are key components of commercially viable electrochemical wastewater treatment system. In this study, blue-black TiO₂ nanotube array (BNTA) electrodes are prepared by electrochemical self-doping. The 1-D structure, donor state density, and Fermi energy level position are critical for maintaining the semimetallic functionality of the BNTA. The structural strength of the BNTA is enhanced by surface crack minimization, reinforcement of the BNTA-Ti metal interface, and stabilized by a protective overcoating with nanoparticulate TiO₂ (Ti/EBNTA). Ti/EBNTA electrodes are employed as both anodes and cathodes with polarity switching at a set frequency. Oxidants are generated at the anode, while the doping levels are regenerated along with byproduct reduction at the cathode. The estimated maximum electrode lifetime is 16 895 h. Ti/EBNTA has comparable hydroxyl radical production activity (6.6 × 10^-14 M) with boron-doped diamond (BDD, 7.4 × 10^-14 M) electrodes. The chlorine production rate follows a trend with respective to electrode type of Ti/EBNTA > BDD > IrO₂. Ti/EBNTA electrodes operated in a bipolar mode have a minimum energy consumption of 62 kWh/kg COD, reduced foam formation due to less gas bubble production, minimum scale formation, and lower chlorate production levels (6 mM vs 18 mM for BDD) during electrolytic wastewater treatment.

Influence of the current density on the electrochemical treatment of concentrated 1-butyl-3-methylimidazolium chloride solutions on diamond electrodes

This paper focuses on the influence of the current density treatment of a concentrated 1-butyl-3-methylimidazolium chloride (BMImCl) solution on an electrochemical reactor with a boron-doped diamond (BDD) anode. The decrease in the total organic carbon (TOC) and the BMImCl concentration demonstrate the capability of BDD in oxidizing ionic liquids (ILs) and further mineralizing (to CO2 and NO3 (-)) more rapidly at higher current densities in spite of the reduced current efficiency of the process. Moreover, the presence of Cl(-) led to the formation of oxychlorinated anions (mostly ClO3 (-) and ClO4 (-)) and, in combination with the ammonia generated in the cathode from the nitrate reduction, chloramines, more intensely at higher current density. Finally, the analysis of the intermediates formed revealed no apparent influence of the current density on the BMImCl degradation mechanism. The current density presents therefore a complex influence on the IL treatment process that is discussed throughout this paper.

Multilayer Heterojunction Anodes for Saline Wastewater Treatment: Design Strategies and Reactive Species Generation Mechanisms

Multilayer heterojunction SbSn/CoTi/Ir anodes, which consist of Ir0.7Ta0.3O2 bottom layers coated onto a titanium base, Co-TiO2 interlayers, and overcoated discrete Sb-SnO2 islands, were prepared by spray pyrolysis. The Ir0.7Ta0.3O2 bottom layer serves as an Ohmic contact to facilitate electron transfer from semiconductor layers to the Ti base. The Co-TiO2 interlayer and overcoated Sb-SnO2 islands enhance the evolution of reactive chlorine. The surficial Sb-SnO2 islands also serve as the reactive sites for free radical generation. Experiments coupled with computational kinetic simulations show that while ·OH and Cl· are initially produced on the SbSn/CoTi/Ir anode surface, the dominant radical formed in solution is the dichlorine radical anion, Cl2·(-). The steady-state concentration of reactive radicals is 10 orders of magnitude lower than that of reactive chlorine. The SbSn/CoTi/Ir anode was applied to electrochemically treat human wastewater. These test results show that COD and NH4(+) can be removed after 2 h of electrolysis with minimal energy consumption (370 kWh/kg COD and 383 kWh/kg NH4(+)). Although free radical species contribute to COD removal, anodes designed to enhance reactive chlorine production are more effective than those designed to enhance free radical production.

Electrocoagulation: Simply a Phase Separation Technology? The Case of Bronopol Compared to Its Treatment by EAOPs

Electrocoagulation (EC) has long been considered a phase separation process, well suited for industrial wastewater treatment since it causes a quick, drastic decay of organic matter content. This research demonstrates that EC also behaves, at least for some molecules like the industrial preservative bronopol, as an effective transformation technology able to yield several breakdown products. This finding has relevant environmental implications, pointing to EC as a greener process than described in literature. A thorough optimization of EC was performed with solutions of bronopol in a simulated water matrix, yielding the complete disappearance of the parent molecule within 20 min at 200 mA (∼20 mA/cm(2)), using Fe as the anode and cathode. A 25% of total organic carbon (TOC) abatement was attained as maximum, with bronopol being converted into bromonitromethane, bromochloromethane, formaldehyde and formic acid. N atoms were accumulated as NO3(-), whereas Br(-) was stable once released. This suggests that mediated oxidation by active chlorine, as well as by hydroxyl radicals resulting from its reaction with iron ions, is the main transformation mechanism. Aiming to enhance the mineralization, a sequential combination of EC with electro-Fenton (EF) as post-treatment process was proposed. EF with boron-doped diamond (BDD) anode ensured the gradual TOC removal under the action of (•)OH and BDD((•)OH), also transforming Br(-) into BrO3(-).

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.

Comparative electrochemical degradation of salicylic and aminosalicylic acids: Influence of functional groups on decay kinetics and mineralization

Solutions of 100 mL with 1.20 mM of salicylic acid (SA), 4-aminosalicylic acid (4-ASA) or 5-aminosalicylic acid (5-ASA) have been comparatively degraded by anodic oxidation with electrogenerated H2O2 (AO-H2O2), electro-Fenton (EF) and photoelectro-Fenton (PEF). Trials were carried out with a stirred tank reactor with a BDD anode and an air-diffusion cathode for continuous H2O2 production. A marked influence of the functional groups of the drugs was observed in their decay kinetics, increasing in the order SA < 5-ASA < 4-ASA in AO-H2O2 and 5-ASA < SA < 4-ASA in EF and PEF, due to the different attack of OH generated at the BDD surface and in the bulk from Fenton's reaction, respectively. This effect was clearly observed when varying the current density between 16.7 and 100 mA cm(-2). The relative mineralization power of the processes always followed the sequence: AO-H2O2 < EF < PEF. The three drugs underwent analogous mineralization abatement up to 88% by AO-H2O2 at 100 mA cm(-2). The mineralization rate in EF and PEF grew in the order: 4-ASA < 5-ASA < SA. The most powerful process was PEF, attaining >98% mineralization for all the drugs at 100 mA cm(-2). Oxalic and oxamic acids were detected as final short-linear aliphatic carboxylic acids by ion-exclusion HPLC, allowing the fast photolysis of their Fe(III) complexes by UVA light to justify the high power of PEF.

Removal of oxyfluorfen from ex-situ soil washing fluids using electrolysis with diamond anodes

In this research, firstly, the treatment of soil spiked with oxyfluorfen was studied using a surfactant-aided soil-washing (SASW) process. After that, the electrochemical treatment of the washing liquid using boron doped diamond (BDD) anodes was performed. Results clearly demonstrate that SASW is a very efficient approach in the treatment of soil, removing the pesticide completely by using dosages below 5 g of sodium dodecyl sulfate (SDS) per Kg of soil. After that, complete mineralization of organic matter (oxyflourfen, SDS and by-products) was attained (100% of total organic carbon and chemical oxygen demand removals) when the washing liquids were electrolyzed using BDD anodes, but the removal rate depends on the size of the particles in solution. Electrolysis of soil washing fluids occurs via the reduction in size of micelles until their complete depletion. Lower concentrations of intermediates are produced (sulfate, chlorine, 4-(trifluoromethyl)-phenol and ortho-nitrophenol) during BDD-electrolyzes. Finally, it is important to indicate that, sulfate (coming from SDS) and chlorine (coming from oxyfluorfen) ions play an important role during the electrochemical organic matter removal.

Depollution of syringic acid aqueous solutions by electrochemical oxidation using high oxidation power anodes

High oxidation power anodes (PbO₂ or BDD), lead to complete electrochemical removal of syringic acid under particularly experimental conditions.

A hydrophobic three-dimensionally networked boron-doped diamond electrode towards electrochemical oxidation

Three-dimensionally networked boron-doped diamond with a microstructure and enhanced mass transfer showed excellent electro-oxidation ability towards contaminants.

The peculiar roles of chloride electrolytes in BDD anode cells

UV-vis spectra changes of phenol in NaCl media (of different initial concentrations) during the electrocatalytic oxidation on a BDD anode.