Efficient degradation of TCE in groundwater using Pd and electro-generated H2 and O2: a shift in pathway from hydrodechlorination to oxidation in the presence of ferrous ions

Advances in three-dimensional electrochemical degradation: A comprehensive review on pharmaceutical pollutants removal from aqueous solution

Water pollution, stemming from various contaminants including organic and pharmaceutical pollutants, poses a significant global challenge. Amidst the array of methods available for pollutant mitigation, the three-dimensional electrochemical approach emerges as a standout solution due to its environmental compatibility, cost-effectiveness, and rapid efficiency. This study delves into the efficacy of three-dimensional electrochemical processes in purging organic and pharmaceutical pollutants from aqueous media. Existing research indicates that the three-dimensional electrochemical process, particularly when employing particle electrodes, exhibits notable success in degrading organic and pharmaceutical pollutants. This achievement is largely attributed to the ample specific surface area of particle electrodes and the shortened mass transfer distance, which collectively enhance efficiency in comparison to traditional two-dimensional electrochemical methods. Moreover, this approach is lauded for its environmental friendliness and cost-effectiveness. However, it is imperative to note that the efficacy of the process is subject to various factors including temperature, pH levels, and current intensity. While the addition of oxidants can augment process efficiency, it also carries the risk of generating intermediate compounds that impede the reaction. In conclusion, the three-dimensional electrochemical method proves to be a viable and practical approach, provided that process conditions are meticulously considered and adhered to. Offering advantages from both environmental and economic perspectives, this method presents a promising alternative to conventional water and wastewater treatment techniques.

A Critical Review of Groundwater Table Fluctuation: Formation, Effects on Multifields, and Contaminant Behaviors in a Soil and Aquifer System

The groundwater table fluctuation (GTF) zone is an important medium for the hydrologic cycle between unsaturated soil and saturated aquifers, which accelerates the migration, transformation, and redistribution of contaminants and further poses a potential environmental risk to humans. In this review, we clarify the key processes in the generation of the GTF zone and examine its links with the variation of the hydrodynamic and hydrochemistry field, colloid mobilization, and contaminant migration and transformation. Driven by groundwater recharge and discharge, GTF regulates water flow and the movement of the capillary fringe, which further control the advection and dispersion of contaminants in soil and groundwater. In addition, the formation and variation of the reactive oxygen species (ROS) waterfall are impacted by GTF. The changing ROS components partially determine the characteristic transformation of solutes and the dynamic redistribution of the microbial population. GTF facilitates the migration and transformation of contaminants (such as nitrogen, heavy metals, non-aqueous phase liquids, and volatile organic compounds) through colloid mobilization, the co-migration effect, and variation of the hydrodynamic and hydrochemistry fields. In conclusion, this review illustrates the limitations of the current literature on GTF, and the significance of GTF zones in the underground environment is underscored by expounding on the future directions and prospects.

Preparation of SnO2-Sb/attapulgite (AP) clay particulate electrode for efficient phenol electrochemical oxidation

A novel SnO2-Sb/AP (attapulgite) particle electrode was prepared for three-dimensional electrocatalytic oxidation (3D/EO) of organic pollutants using a co-sintering method. The electrochemical properties and micromorphology were determined using polarization, cyclic voltammetry (CV), and field emission scanning electron microscope (FE-SEM), and compared with activated carbon (AC), AP, and TiO2/AP particle electrodes. Besides, their potential application in the electrochemical degradation of phenol was investigated. The SnO2-Sb/AP particle electrode exhibited higher electrochemical activity than other particle electrodes due to its large number of active sites, low transfer coefficient (α, 0.12), and high-volt ampere charge (q*, 1.18 C·cm-2). The electrochemical CODCr degradation efficiency (100%) of phenol on SnO2-Sb/AP particle electrodes is much higher than for other particle electrodes. Moreover, an excellent stability of the SnO2-Sb/AP particle electrode is also verified by repeated experiments. These results indicate that the SnO2-Sb/AP particle electrodes broaden the application area of clays and are expected to be a promising method for 3D/EO.

Construction of a Novel Cascade Electrolysis-Heterocatalysis System by Using Zeolite-Encaged Ultrasmall Palladium Catalysts for H2 O2 Generation

In situ generation of hydrogen peroxide (H₂ O₂ ) has attracted extensive attention, especially in water treatment. However, traditional anthraquinones can only produce high-concentration H₂ O₂ and its transportation and storage are not convenient and dangerous. Herein, an in situ and on-demand strategy to produce H₂ O₂ by using a cascade water electrolysis together with a heterocatalysis system is provided. Beginning with water, H_2, and O₂ can be generated via electrolysis and then react with each other to produce H₂ O₂ immediately on efficient zeolite-encaged ultrasmall Pd catalysts. Significantly, the H₂ O₂ generation rate in the optimized cascade system reaches up to 0.85 mol L^-1 h^-1 g_Pd ^-1 , overcoming most of the state-of-the-art catalysts in previous literature. The confinement effect of zeolites is not only beneficial to the formation of highly dispersed metal species, promoting the H₂ O₂ generation, but also inhibits the H₂ O₂ decomposition, enhancing the production yield of H₂ O₂ . In addition, the effect of electrolytes, sizes of Pd species, as well as zeolite acidity are also systematically studied. This work provides a new avenue for H₂ O₂ generation via a highly efficient cascade electrolysis-heterocatalysis system by using zeolite-supported metal catalysts. The high catalytic efficiency and green process for H₂ O₂ generation make it very promising for further practical applications.

Removal of trichloroethene by glucose oxidase immobilized on magnetite nanoparticles

To overcome the safety risks and low utilization efficiency of H₂O₂ in traditional Fenton processes, in situ production of H₂O₂ by enzymatic reactions has attracted increasing attention recently. In this study, magnetite-immobilized glucose oxidase (MIG) was prepared to catalyze the heterogeneous Fenton reaction for the removal of trichloroethene from water. The successful immobilization of glucose oxidase on magnetite was achieved with a loading efficiency of 70.54%. When combined with substrate glucose, MIG could efficiently remove 5-50 mg L^-1 trichloroethene from water with a final removal efficiency of 76.2% to 94.1% by 192 h. This system remained effective in the temperature range of 15-45 °C and pH range of 3.6-9.0. The removal was slightly inhibited by different cations and anions (influencing degree Ca²⁺ > Mg²⁺ > Cu²⁺ and H₂PO₄ ^- > Cl^- > SO₄ ^2-) and humic acid. Meanwhile, the MIG could be recycled for 4 cycles and was applicable to other chlorinated hydrocarbons. The results of reactive oxidative species generation monitoring and quenching experiments indicated that H₂O₂ generated by the enzymatic reaction was almost completely decomposed by magnetite to produce ·OH with a final cumulative concentration of 129 μM, which played a predominant role in trichloroethene degradation. Trichloroethene was almost completely dechlorinated into Cl^-, CO₂ and H₂O without production of any detectable organic chlorinated intermediates. This work reveals the potential of immobilized enzymes for in situ generation of ROS and remediation of organic chlorinated contaminants.

Encapsulation of zero valent iron nanoparticles in biodegradable amphiphilic janus particles for groundwater remediation

Reactive Zero Valent Iron (ZVI) nanoparticles have been widely explored for in situ ground water remediation to degrade both non-aqueous phase liquid (NAPL) and water-soluble contaminants. However, they usually suffer from rapid oxidation and severe agglomerations restricting their delivery at NAPL/water interface. Aim of this study was to encapsulate the ZVI nanoparticles (50 nm) in amphiphilic bicompartmental Janus particles (711 ± 11 nm) fabricated by EHDC (electrohydrodynamic co-jetting). The dual compartments were composed of PLA (polylactic acid) and a blend of PLA, PE (poly (hexamethylene 2,3-O-isopropylidenetartarate) and PAG (photo acid generator). Upon UV irradiation, PAG releases acid to unmask hydroxyl groups present in PE to make only PE compartment hydrophilic. The entrapped ZVI nanoparticles (20 w/w%; ∼99 % encapsulation efficiency) were observed to degrade both hydrophilic (methyl orange dye) and hydrophobic (trichloro ethylene) contaminants. UV treated Janus particles provided stable dispersion (dispersed up to 3 weeks in water), prolonged reactivity (∼24 days in contaminated water), and recyclability (recyclable up to 9 times) as compared to non-treated ones. In addition, the amphiphilic Janus particles demonstrated high transportability (>95%) through porous media (sand column) with very low attachment efficiency (0.07), making them a promising candidate to target contaminants at NAPL/water interface prevailed in groundwater.

Stepwise dechlorination of chlorinated alkenes on an Fe-Ni/rGO/Ni foam cathode: Product control by one-electron-transfer reactions

Research on the stepwise hydrogenation dechlorination of chlorinated alkenes forms an important basis for eliminating toxic intermediate incomplete dechlorination products. The low-cost Fe-Ni/rGO/Ni foam cathode both supplied electrons and exhibited hydrogen conversion activity, and it was an excellent tool for the study of stepwise dechlorination. Electrochemical reduction experiments were carried out on homologous chlorinated alkenes. The conditions affecting the dechlorination efficiency and the repeatability of the catalytic electrode were analyzed. The trichloroethylene (TCE) removal rates were all above 78.0% over 8 cycles. The maximum EHDC efficiency was as high as 86.1%, and the faradaic efficiency was over 78.8%. Electrochemical methods combined with the calculation of the electron transfer number are proposed to verify the good hydrogenation ability of the electrode and the stepwise reduction ability at proper voltages. The stepwise dechlorination electroreduction characteristics of chlorinated alkenes were explained. The C-Cl bond dissociation enthalpies of chlorinated alkenes were calculated by density functional theory (DFT), and the 4-Cl and 5-Cl of TCE were expected to be removed first. The stepwise cleavage of chlorinated alkenes on Fe-Ni/rGO/Ni foam during dichlorination provided a reference for controlling the reduction products of chlorinated alkenes and preventing the pollution caused by toxic intermediate products formed during incomplete dechlorination.

Photoelectrocatalytic coupling system synergistically removal of antibiotics and antibiotic resistant bacteria from aquatic environment

Antibiotics, antibiotic resistant bacteria (ARB) and antibiotic resistance genes (ARGs) are ubiquitous in the reclaimed water, posing a potential threat to human and ecological health. Nowadays, the reuse technology of reclaimed water has been widely concerned, but the removal of antibiotics, ARB and ARGs in reclaimed water has not been sufficiently studied. This study used TiO2 nanotube arrays (TNTs) decorated with Ag/SnO2-Sb nanoparticles (TNTs-Ag/SnO2-Sb) as the anode and Ti-Pd/SnO2-Sb as the cathode to construct an efficient photoelectrocatalytic (PEC) system. In this system, 99.9% of ARB was inactivated in 20 min, meanwhile, ARGs was removed within 30 min, and antibiotics were almost completely degraded within 1 h. Furthermore, the effects of system parameters on the removals of antibiotics, ARB and ARGs were also studied. The redox performance of the system was verified by adding persulfate. Escherichia coli, as a representative microorganism in aquatic environments, was used to evaluate the ecotoxicity of PEC treated chloramphenicol (CAP) solution. The ecotoxicity of CAP solution was significantly reduced after being treated by PEC. In addition, transformation intermediates of CAP were identified using liquid chromatography-tandems mass spectrometry (LC-MS/MS) and the possible degradation pathways were proposed. This study could provide a potential alternative method for controlling antibiotic resistance and protecting the quality of reclaimed water.

Triclosan detoxification through dechlorination and oxidation via microbial Pd-NPs under aerobic conditions

The present study focuses on the successful preparation of microbial palladium nanoparticles (Pd-NPs). The even distribution of Pd in the periplasmic space of B. megaterium Y-4 cells is characterized using a transmission electronic microscopy (TEM) and scanning electron microscope (SEM). X-Ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) revealed the domination of Pd (0) in Pd-NPs. The microbial Pd-NPs were selected to detoxify triclosan (TCS). Liquid chromatography-mass spectrometry (LC-MS) was used to analyze the intermediate products of dechlorination and oxidization. Free radicals quenching and 5,5-dimethyl-1-pyrroline N-oxide (DMPO) capturing experiments confirmed the crucial contribution of atomic H• and O₂^·- to TCS degradation. Besides, TCS degradation by microbial Pd-NPs could alleviate the cytotoxicity of TCS polluted water. Meanwhile, great circulating utilization of microbial Pd-NPs was obtained in degrading TCS. Corresponding findings in the present study could provide new insight into the role of microbial Pd-NPs in detoxifying pollutants.

Reduction-oxidation series coupling degradation of chlorophenols in Pd-Catalytic Electro-Fenton system

Organochlorine pesticides are widespread in soils, sediments and even in groundwater, causing great concern to human health because of its toxicity and carcinogenic effects. The remarkable mineralization and lowered toxicity are particularly important during the removal of organochlorine pesticides. In this study, Pd/CeO₂ was prepared and employed as a bifunctional catalyst, to construct the reduction-oxidation series coupling Electro-Fenton (EF) system. The removal of chlorophenols (CPs) reached over 95% within 10 min at pH 3.0 and a current density of 25 mA/cm² in Pd/CeO₂-EF system. The second-order rate constant of CPs degradation was 10.28 L mmol^-1min^-1 in Pd/CeO₂-EF system, which was 29 times as fast as the sum of electrolysis with Pd/CeO₂ (0.24 L mmol^-1min^-1) and EF (0.11 L mmol^-1min^-1). Dehydrochlorination by Pd [H] contributed to the removal of CPs in Pd/CeO₂-EF system. The generated reactive oxygen species, mainly OH was also confirmed by ESR to contribute to the removal of CPs. The reduction-oxidation series coupling degradation of CPs in Pd/CeO₂-EF system increased the TOC removal to 70% in 360 min. The analysis of intermediate products further revealed the reductive and oxidative products in Pd/CeO₂-EF. Moreover, the system of Pd/CeO₂-EF exhibited an excellent performance treatment for CPs in actual groundwater. This study provides a new stratagem to eliminate organochlorine pesticides in groundwater environments rapidly and thoroughly.

Fenton/Fenton-like processes with in-situ production of hydrogen peroxide/hydroxyl radical for degradation of emerging contaminants: Advances and prospects

Fenton processes based on the reaction between Fe²⁺ and H₂O₂ to produce hydroxyl radicals, have been widely studied and applied for the degradation of toxic organic contaminants in wastewater due to its high efficiency, mild condition and simple operation. However, H₂O₂ is usually added by bulk feeding, which suffers from the potential risks during the storage and transportation of H₂O₂ as well as its low utilization efficiency. Therefore, Fenton/Fenton-like processes with in-situ production of H₂O₂ have received increasing attention, in which H₂O₂ was in-situ produced through O₂ activation, then decomposed into hydroxyl radicals by Fenton catalysts. In this review, the in situ production of H₂O₂ for Fenton oxidation was introduced, the strategies for activation of O₂ to generate H₂O₂ were summarized, including chemical reduction, electro-catalysis and photo-catalysis, the influencing factors and the mechanisms of the in situ production and utilization of H₂O₂ in various Fenton/Fenton-like processes were analyzed and discussed, and the applications of these processes for the degradation of toxic organic contaminants were summarized. This review will deepen the understanding of the tacit cooperation between the in situ production and utilization of H₂O₂ in Fenton process, and provide the further insight into this promising process for degradation of emerging contaminants in industrial wastewater.

Role of carbon fiber electrodes and carbonate electrolytes in electrochemical phenol oxidation

In-situ chemical oxidation (ISCO) requires an injection of oxidants into a contaminated site. However, the oxidants decompose and react with contaminants during transport to the contaminated region, which causes oxidant over-consumption. In-situ oxidant generation can solve this problem, and electrochemical methods can be applied to achieve this. Electrochemical oxidation is highly dependent on electrode material type. In this study, we evaluated graphite and carbon fiber as candidates for electrochemical oxidant generation and phenol as the model compound. The carbon fiber anode oxidized the phenol more effectively than graphite, with removal proportional to the applied current. Carbonate electrolytes were more effective at oxidizing phenols than sulfate electrolytes. The faster carbon fiber anode phenol oxidation is due to its large surface area. Carbonate radicals in the carbonate electrolyte contribute to phenol oxidation as well as further intermediate oxidation. The carbon fiber cathode was not an effective phenol oxidizer even though it generated more hydrogen peroxide. This is because there was no catalyst to transform the hydrogen peroxide into hydroxyl radicals. Results indicate that electrochemical oxidation using carbon fiber is an effective method for treating phenol found in groundwater with high concentrations of (bi)carbonate.

Activating palladium nanoparticles via a Mott-Schottky heterojunction in electrocatalytic hydrodechlorination reaction

This work exploited one novel power of the Mott-Schottky heterojunction interface in activating the palladium (Pd) in electrocatalytic hydrodechlorination reaction (EHDC, one reaction targeted for the abatement of chlorinated organic pollutants from water). By forming a Mott-Schottky contact with polymer carbon nitride (Pd-PCN), the Pd nanoparticles enable a relatively complete and pseudo-first-order conversion of 2,4-dichlorophenol (2,4-DCP) to phenol and Cl^- with the reaction rate constant (k_obs) triple that of the conventional Pd-C (0.68 vs. 0.26 min^-1 mol_Pd^-1). Further comparison in k_obs of Pd-PCN and the Pd catalysts reported in literatures revealed that our Pd-PCN was among the top active catalysts for EHDC. The robust performance of Pd-PCN was attributed to the strong metal-support interactions at the Mott-Schottky heterojunction interface, which enriched the electron on Pd and improved its anti-poisoning ability against phenol. The strong support-metal interactions also endowed Pd-PCN with high activity/structure stability in EHDC. The presence of some anions in water body including NO₃^-, NO₂^- and Cl^- exerted little effect on EHDC, while the reduced sulfur compounds (S^2- and SO₃^2-), even in a very low concentration (1 mM), could significantly deactivate the catalyst. This work provides a facile and efficient strategy to activate noble metals in catalytic reactions.

A Bimetallic Fe-Mn Oxide-Activated Oxone for In Situ Chemical Oxidation (ISCO) of Trichloroethylene in Groundwater: Efficiency, Sustained Activity, and Mechanism Investigation

Bimetallic Fe-Mn oxide (BFMO) has been regarded as a promising activator of peroxysulfate (PS), the sustained activity and durability of BFMO for long-term activation of PS in situ, however, is unclear for groundwater remediation. A BFMO (i.e., Mn_1.5FeO_6.35) was prepared and explored for PS-based in situ chemical oxidation (ISCO) of trichloroethylene (TCE) in sand columns with simulated/actual groundwater (SGW/AGW). The sustained activity of BFMO, oxidant utilization efficiency, and postreaction characterization were particularly investigated. Electron spin resonance (ESR) and radical scavenging tests implied that sulfate radicals (SO₄^•-) and hydroxyl radicals (HO^•) played major roles in degrading TCE, whereas singlet oxygen (¹O₂) contributed less to TCE degradation by BFMO-activated Oxone. Fast degradation and almost complete dechlorination of TCE in AGW were obtained, with reaction stoichiometry efficiencies (RSE) of ΔTCE/ΔOxone at 3-5%, much higher than those reported RSE values in H₂O₂-based ISCO (≤0.28%). HCO₃^- did not show detrimental effect on TCE degradation, and effects of natural organic matters (NOM) were negligible at high Oxone dosage. Postreaction characterizations displayed that the BFMO was remarkably stable with sustained activity for Oxone activation after 115 days of continuous-flow test, which therefore can be promising catalyst for Oxone-based ISCO for TCE-contaminated groundwater remediation.

Transformation of tetrachloroethylene in a flow-through electrochemical reactor

Electrochemical transformation of harmful tetrachloroethylene (PCE) is evaluated as a method for management of groundwater plumes to protect the drinking water resource, its consumers and the environment. In contrast to previous work that reported transformation of trichloroethylene, a byproduct of PCE, this work focuses on transformation of PCE in a saturated porous matrix and the influence of design parameters on the removal performance. Design parameters investigated were electrode configuration, catalyst load, electrode spacing, current intensity, orientation of reactor and flow through a porous matrix. A removal of 86% was reached in the fully liquid-filled, horizontally oriented reactor at a current of 120 mA across a cathode → bipolar electrode → anode arrangement with a Darcy velocity of 0.03 cm/min (150 m/yr). The palladium load on the cathode significantly influenced the removal. Enhanced removal was observed with increased electrode spacing. Presence of an inert porous matrix improved PCE removal by 9%-point compared to a completely liquid-filled reactor. Normalization of the data indicated, that a higher charge transfer per contaminant mass is required for removal of low PCE concentrations. No chlorinated intermediates were formed. The results suggest, that PCE can be electrochemically transformed in reactor designs replicating that of a potential field-implementation. Further work is required to better understand the reduction and oxidation processes established and the parameters influencing such. This knowledge is essential for optimization towards testing in complex conditions and variations of contaminated sites.

Electrochemical transformation of an aged tetrachloroethylene contamination in realistic aquifer settings

Electrochemical removal of chlorinated ethenes in groundwater plumes may potentially overcome some of the challenges faced by current remediation technologies. So far, studies have been conducted in simplified settings of synthetic groundwater and inert porous matrices. This study is a stepwise investigation of the influence of field-extracted groundwater, sandy sediment and groundwater aquifer temperatures on the removal of an aged partially degraded contamination of tetrachloroethylene (PCE) at a typical groundwater flow rate. The aim is to assess the potential for applying electrochemistry at contaminated sites. At a constant current of 120 mA, pH and conductivity were unaffected downgradient the electrochemical zone. Major groundwater species were reduced and oxidized. Some minerals deposited, others dissolved. Hydrogen peroxide, a strong oxidant, was formed in levels up to 5 mg L^-1 with a limited distribution into the sandy sediment. Trichloromethane was formed, supposedly by oxidation of organic matter in the sandy sediment in the presence of chloride. The more realistic the settings, the higher the PCE removal, bringing concentrations down to 7.8 ± 2.3 μg L^-1. A complete removal of trichloroethylene and cis-1,2-dichloroethylene was obtained. The results suggest that competing reactions related to the natural complex hydrogeochemistry are insignificant in terms of affecting the electrochemical degradation of PCE and chlorinated intermediates.

Challenges in electrochemical remediation of chlorinated solvents in natural groundwater aquifer settings

Establishment of electrochemical zones for remediation of dissolved chlorinated solvents in natural settings was studied. An undivided 1D-experimental column set-up was designed for the assessment of the influence of site-extracted contaminated groundwater flowing through a sandy aquifer material, on the execution of laboratory testing. A three-electrode system composed of palladium coated pure iron cathodes and a cast iron anode was operated at 12 mA under varying flow rates. The natural settings added complexity through a diverse groundwater chemistry and resistance in the sand. In addition, significant precipitation of iron released through anode corrosion was observed. Nevertheless, the complex system was successfully modelled with a simple geochemical model using PHREEQC. A ranking of the significances of system parameters on the laboratory execution of electrochemical remediation in natural settings was proposed: Geological properties > anode corrosion > site-extracted contaminated groundwater > the carbonate system > sulphate > hydrology > less significant unidentified parameters. This study provides insight in actual challenges that need to be overcome for in situ electrochemical remediation.

Activated carbon as effective cathode material in iron-free Electro-Fenton process: Integrated H2O2 electrogeneration, activation, and pollutants adsorption

Major challenges for effective implementation of the Electro-Fenton (EF) water treatment process are that conventional efficient cathodes are relatively expensive, and H2O2 activation by Fe2+ may cause secondary pollution. Herein, we propose a low-cost activated carbon/stainless steel mesh (ACSS) composite cathode, where the SS mesh distributes the current and the AC simultaneously supports H2O2 electrogeneration, H2O2 activation, and organic compounds (OCs) adsorption. The oxygen-containing groups on the AC function as oxygen reduction reaction (ORR) sites for H2O2 electrogeneration; while the porous configuration supply sufficient reactive surface area for ORR. 8.9 mg/L H2O2 was obtained with 1.5 g AC at 100 mA under neutral pH without external O2 supply. The ACSS electrode is also effective for H2O2 activation to generate ‧OH, especially under neutral pH. Adsorption shows limited influence on both H2O2 electrogeneration and activation. The iron-free EF process enabled by the ACSS cathode is effective for reactive blue 19 (RB19) degradation. 61.5% RB19 was removed after 90 min and 74.3% TOC was removed after 720 min. Moreover, long-term stability test proved its relatively stable performance. Thus, the ACSS electrode configuration is promising for practical and cost-effective EF process for transformation of OCs in water.

Immobilized palladium-catalyzed electro-Fenton's degradation of chlorobenzene in groundwater

This study investigates the effect of palladium (Pd) form on the electrochemical degradation of chlorobenzene in groundwater by palladium-catalyzed electro-Fenton (EF) reaction. In batch and flow-through column reactors, EF was initiated via in-situ electrochemical formation of hydrogen peroxide (H2O2) supported by Pd on alumina powder or by palladized polyacrylic acid (PAA) in a polyvinylidene fluoride (PVDF) membrane (Pd-PVDF/PAA). In a mixed batch reactor containing 10 mg L-1 Fe2+, 2 g L-1 of catalyst in powder form (1% Pd, 20 mg L-1 of Pd) and an initial pH of 3, chlorobenzene was degraded under 120 mA current following a first-order decay rate showing 96% removal within 60 min. Under the same conditions, a rotating Pd-PVDF/PAA disk produced 88% of chlorobenzene degradation. In the column experiment with automatic pH adjustment, 71% of chlorobenzene was removed within 120 min with 10 mg L-1 Fe2+, and 2 g L-1 catalyst in pellet form (0.5% Pd, 10 mg L-1 of Pd) under 60 mA. The EF reaction can be achieved under flow, without external pH adjustment and H2O2 addition, and can be applied for in-situ groundwater treatment. Furthermore, the rotating PVDF-PAA membrane with immobilized Pd-catalyst showed an effective and low maintenance option for employing Pd catalyst for water treatment.

Mechanisms for Abiotic Dechlorination of Trichloroethene by Ferrous Minerals under Oxic and Anoxic Conditions in Natural Sediments

Bench-scale experiments were performed on natural sediments to assess abiotic dechlorination of trichloroethene (TCE) under both aerobic and anaerobic conditions. In the absence of oxygen (<26 μM), TCE dechlorination proceeded via a reductive pathway generating acetylene and/or ethene. Reductive dechlorination rate constants up to 3.1 × 10^-5 d^-1 were measured, after scaling to in situ solid:water ratios. In the presence of oxygen greater than 120 μM, TCE dechlorination proceeded via an oxidative pathway generating formic/glyoxylic and glycolic/acetic acids, and oxidative dechlorination rate constants (again scaled to in situ conditions) up to 7.4 × 10^-3 d^-1 were measured. These rates correspond to half-lives of 60 and 0.25 years for abiotic TCE dechlorination under anaerobic and aerobic conditions, respectively, indicating the potentially large impact of aerobic TCE oxidation in the field. For both reductive and oxidative TCE dechlorination pathways, measured first-order rate constants increased with increasing ferrous iron content, suggesting the role of iron oxidation. Hydroxyl radical formation was measured and increased with increasing oxygen and ferrous iron content. Rate constants associated with TCE oxidation products increased with increasing hydroxyl radical generation rates, and were zero in the presence of a hydroxyl radical scavenger, suggesting that oxidative TCE dechlorination is a hydroxyl radical driven process.

Pd based in situ AOPs with heterogeneous catalyst of FeMgAl layered double hydrotalcite for the degradation of bisphenol A and landfill leachate through multiple pathways

In situ degradation of organic contaminants by Pd and electro-generated H₂ and O₂ overcomes the drawbacks to traditional Fenton process, and conducting heterogeneous catalyst of FeMgAl layered double hydrotalcite (LDH) further improved the efficiency and stability. Using bisphenol A (BPA) as the model contaminants, 90% removal can be achieved with 1200 mg/L Pd/Al₂O₃ and FeMgAl-2. The reusability was satisfying due to the very limited leaching of Fe ions at 0.1 ppm level. FeMgAl also amplified the window of pH for Pd-catalyzed in situ advanced oxidation processes (AOPs) from 3 by homogenous Fe(II) to 3-7 by FeMgAl LDH. The COD of landfill leachate effluent of the MBR system removed by about 52.3% by this system by the initial pH was 5. Characterizations revealed the distinguishing features associated with LDH structure such as large surface area, good stability, basic character, and strong linage among active sites were accounted for the remarkable performances over a wide pH window. Five reactive intermediates were observed and multiple degradation pathways were proposed in Pd-catalyzed in situ AOP for the first time. Interestingly, because of the unique role of Pd catalyst, these degradation pathways were clearly distinguished from traditional Fenton or Fenton-like AOPs and may provide a new approach of in situ heterogeneous AOPs for refractory contaminants in future.

Transient reactive transport model for physico-chemical transformation by electrochemical reactive barriers

A comprehensive model that integrates coupled effects of chemical, physical, and electrochemical processes, is necessary for design, analysis, and implementation of the electro-remediation of groundwater under flow conditions. A coupled system of equations to solve for transport and multiple reactions in an electrochemical reactor is numerically intensive due to highly stiff nature of reaction model formulation. In this study, the focus is to develop an efficient model for reactions associated with the transport and physico-chemical transformation in an electrochemical reactor. The model incorporates effects of transport mechanisms as well as chemical and electrochemical reactions. Model verification is provided for pH profiles under different electrolyte compositions in two sets of reactors; a batch and a flow-through reactor. The model is able to predict the concentration of species during the electrochemical remediation process with a close correlation to experimental data (R² = 0.99 for batch and R² = 0.78 for flow-through reactor.) Imposing polarity reversal to the system will cause fluctuation of pH, however, the trend stays the same as if no polarity were applied. Ultimately, volumetric charge flow is introduced as a unique parameter characterizing the electroremediation reactor for operating purposes.

A CuNi bimetallic cathode with nanostructured copper array for enhanced hydrodechlorination of trichloroethylene (TCE)

To address the challenges of low hydrodechlorination efficiency by non-noble metals, a CuNi bimetallic cathode with nanostructured copper array film was fabricated for effective electrochemical dechlorination of trichloroethylene (TCE) in aqueous solution. The CuNi bimetallic cathodes were prepared by a simple one-step electrodeposition of copper onto the Ni foam substrate, with various electrodeposition time of 5/10/15/20 min. The optimum electrodeposition time was 10 min when copper was coated as a uniform nanosheet array on the nickel foam substrate surface. This cathode exhibited the highest TCE removal, which was twice higher compared to that of the nickel foam cathode. At the same passed charge of 1080C, TCE removal increased from 33.9 ± 3.3% to 99.7 ± 0.1% with the increasing operation current from 5 to 20 mA cm^-2, while the normalized energy consumption decreased from 15.1 ± 1.0 to 2.6 ± 0.01 kWh log^-1 m^-3. The decreased normalized energy consumption at a higher current density was due to the much higher removal efficiency at a higher current. These results suggest that CuNi cathodes prepared by simple electrodeposition method represent a promising and cost-effective approach for enhanced electrochemical dechlorination.

Carbon-nanotube-doped Pd-Ni bimetallic three-dimensional electrode for electrocatalytic hydrodechlorination of 4-chlorophenol: Enhanced activity and stability

A novel composite bimetallic electrode, palladium-nickel/multi-walled carbon nanotubes/graphite felt (Pd-Ni/MWCNTs/GF), was synthesized for the electrocatalytic hydrodechlorination of 4-chlorophenol (4-CP). GF with a three-dimensional structure was used as the electrode substrate, and doped with MWCNTs, which can improve the GF conductivity and serve as a skeleton for metal loading. Ni and Pd were deposited on the electrode surface stepwise to obtain a well-aligned, highly active and stable Pd-Ni/MWCNTs/GF electrode. The Pd-Ni/MWCNTs/GF cathode showed a high reactivity for the electrocatalytic hydrodechlorination of 4-CP; up to 100% removal of 4-CP was achieved within 30 min, and followed pseudo-first-order kinetics with a rate constant of 0.162 min^-1. Compared with other cathodes, the Pd-Ni/MWCNTs/GF electrode showed superior performance in 4-CP reduction. Excessive current will lower the reaction efficiency and current efficiency because of hydrogen evolution, and acidic solution conditions are more conducive to electrocatalytic reactions. Experiments confirmed that the Ni had a small amount of loss under acidic conditions but remained stable under neutral and alkaline conditions, whereas the loss of Pd for different pH values was constantly low. In cycle tests, the bimetallic electrode exhibits a better reactivity and stability than the single-metal Pd electrode in the long-term.

Degradation of 4-Chlorophenol in Aqueous Solution by Sono-Electro-Fenton Process

Electro-Fenton (EF) and ultrasound radiation (US) have been of interest for the removal of chlorinated compounds from water. This study evaluates the effects of different parameters on sono-electro-Fenton (SEF) for degradation of 4-chlorophenol (4-CP) in an aqueous solution. This study uses pulsing US waves along with Pd-catalyzed EF to degrade contaminants in water while maintaining temperature. The usage of pulsing US waves along with Pd catalyzed EF to remove contaminants while maintaining temperature has not been reported previously. SEF ability to degrade 4-CP was compared with the performance of each process (EF and sonolysis) alone. Initial pH, current density, background electrolyte, Fe²⁺ concentration, Pd/Al₂O₃ catalyst concentration, US waves, and sonifier amplitude were optimized in a two electrode (Ti/mixed metal oxide or Ti/MMO) batch system. The degradation of 4-CP increased from 1.85% by US to 83% by EF to nearly >99.9% by coupled SEF. With US radiation under 70% amplitude and 1:10 ON/OFF ratio, the removal rate of 4-CP increased to 98% compared to 62% under EF alone within the first 120 min in the presence of 80 mg L^-1 Fe²⁺, 16.94 mA cm^-2 of current density, 1 g L^-1 Pd/Al₂O₃ catalyst (10 mg Pd), and initial pH of 3. However, the degradation rate decreased after 120 min of treatment, and complete 4-CP removal was observed after 300 minutes. The sonolysis impacted the 4-CP removal under coupled SEF, mostly due to the contribution of mass transfer (micromixing), while radical formation was found to be absent under the conditions tested (20kHz). The pulsed US was found to increase the temperature by only 8.7°C, which was found not to impact the 4-CP volatilization or degradation. These results imply that low-level US frequency through pulses is a practical and efficient approach to support electro-Fenton reaction, improving reaction rates without the need for electrolyte cooling.

LABORATORY TESTING OF THE POTENTIAL FOR THE INFLUENCE OF SUSPENDED SEDIMENTS ON THE ELECTROCHEMICAL REMEDIATION OF KARST GROUNDWATER

Due to the complicated nature of karst aquifers, many groundwater treatment technologies are difficult to implement successfully. A particular challenge arises because sediments are ubiquitous and mobile in karst systems and may either facilitate contaminant transport or act as long-term substrates for storage via sorption. However, electrochemical remediation is a promising technology to be optimized for karst aquifers due to easy manipulation and control of groundwater chemistry as well as low cost, ability for in situ application, and performance under alternative power sources. This study investigates the effects of suspended karst sediments on the electrochemical remediation of groundwater via electro-Fenton (EF) mechanism. The EF mechanism relies on direct electrolysis (i.e., water electrolysis and ferrous iron release) and indirect, electrochemically-induced processes (i.e., Pd catalyzed H₂O₂ production). These processes can be optimized for H₂O₂ generation and support of its activation to hydroxyl radicals - a powerful oxidant capable of degrading and transforming a wide range of contaminants (e.g., chlorinated solvents). In this study, we tested sediments varying in concentrations of Fe, Mn and buffering capacities. When the sediments were introduced into the EF experiments, there were adverse effects on the H₂O₂ content: at steady state (120 min), Pd catalyzed formation of H₂O₂ decreased by 60%, 57%, and 75% in the presence of suspended sediment collected from three separate karst locations. Presented results imply that sediments' presence influences EF mechanism in electrochemical systems, but given the flexibility of the technology, it can be optimized in terms of electrode materials, current intensities and current regimes to address these challenges.

TCE degradation in groundwater by chelators-assisted Fenton-like reaction of magnetite: Sand columns demonstration

Trichloroethylene (TCE) degradation in sand columns has been investigated to evaluate the potential of chelates-enhanced Fenton-like reaction with magnetite as iron source for in situ treatment of TCE-contaminated groundwater. The results showed that successful degradation of TCE in sand columns was obtained by nitrilotriacetic acid (NTA)-assisted Fenton-like reaction of magnetite. Addition of ethylenediaminedisuccinic acid (EDDS) resulted in an inhibitory effect on TCE degradation in sand columns. Similar to EDDS, addition of ethylenediaminetetraacetic acid (EDTA) also led to an inhibition of TCE degradation in sand column with small content of magnetite (0.5 w.t.%), but enhanced TCE degradation in sand column with high content of magnetite (7.0 w.t.%). Additionally, the presence of NTA, EDDS and EDTA greatly decreased H₂O₂ uptake in sand columns due to the competition between chelates and H₂O₂ for surface sites on magnetite (and sand). Furthermore, the presented results show that magnetite in sand columns remained stable in a long period operation of 230 days without significant loss of performance in terms of TCE degradation and H₂O₂ uptake. Moreover, it was found that TCE was degraded mainly to formic acid and chloride ion, and the formation of chlorinated organic intermediates was minimal by this process.

Efficient degradation of tetrabromobisphenol A via electrochemical sequential reduction-oxidation: Degradation efficiency, intermediates, and pathway

Tetrabromobisphenol A (TBBPA), a toxic persistent pollutant, should be effectively removed from the environment. In this study, an electrochemical sequential reduction-oxidation system was proposed by controlling reaction atmosphere with Pd-Fe nanoparticles modified Ni foam (Pd-Fe/Ni) electrode as cathode for TBBPA degradation. To obtain an efficient Pd-Fe/Ni electrode for TBBPA degradation, various factors, like Pd loading, Fe²⁺ adding amounts, were examined. The Pd-Fe/Ni electrode exhibited higher TBBPA conversion and debromination than the counterparts, due to the synergism of Fe⁰ and electrochemical reduction. Similar TBBPA conversions and debromination ratios were observed for the cases of sparging N₂ only and sparging N₂ followed by air, which were higher than those of aeration. Reductive debromination occurred while first bubbling N₂, forming tri-BBPA, di-BBPA, mono-BBPA and BPA; and these intermediates were likely to be further oxidized by OH generated from H₂O₂ together with Pd-Fe/Ni electrode under aeration. Reductive and oxidative intermediates (including aromatic ring-opened product) were identified by HPLC and UPLC-QTOF-MS. Based on the intermediates, the possible TBBPA degradation mechanism and pathway were proposed. This study demonstrates that sequential reduction-oxidation process tuned by N₂ and air bubbling was favored for TBBPA degradation, thus, it should be a promising process for HOCs degradation.

Kinetic and thermodynamic studies of chlorinated organic compound degradation by siderite-activated peroxide and persulfate

The efficacy of two oxidant systems, iron-activated hydrogen peroxide (H₂O₂) and iron-activated hydrogen peroxide coupled with persulfate (S₂O₈^2-), was investigated for treatment of two chlorinated organic compounds, trichloroethene (TCE) and 1,2-dichloroethane (DCA). Batch tests were conducted at multiple temperatures (10-50 °C) to investigate degradation kinetics and reaction thermodynamics. The influence of an inorganic salt, dihydrogen phosphate ion (H₂PO₄^-), on oxidative degradation was also examined. The degradation of TCE was promoted in both systems, with greater degradation observed for higher temperatures. The inhibition effect of H₂PO₄^- on the degradation of TCE increased with increasing temperature for the iron-activated H₂O₂ system but decreased for the iron-activated hydrogen peroxide-persulfate system. DCA degradation was limited in the iron-activated hydrogen peroxide system. Conversely, significant DCA degradation (87% in 48 hours at 20 °C) occurred in the iron-activated hydrogen peroxide-persulfate system, indicating the crucial role of sulfate radical (SO₄^-·) from persulfate on the oxidative degradation of DCA. The activation energy values varied from 37.7 to 72.9 kJ/mol, depending on the different reactants. Overall, the binary hydrogen peroxide-persulfate oxidant system exhibited better performance than hydrogen peroxide alone for TCE and DCA degradation.

Surface-bound sulfate radical-dominated degradation of 1,4-dioxane by alumina-supported palladium (Pd/Al2O3) catalyzed peroxymonosulfate

Sulfate radicals have been demonstrated as an alternative to hydroxyl radicals in advanced oxidation processes. Unfortunately, the efficient activation of peroxymonosulfate (PMS), one of the most commonly used oxidants for the generation of sulfate radicals, still relies heavily on cobalt-bearing materials that are potential carcinogens. Although copper-iron bimetallic materials are promising activators, stoichiometric amounts of metals are required to achieve satisfactory performance. In this study, we propose a real catalytic process that is capable of degrading extremely recalcitrant 1,4-dioxane using a combination of alumina-supported metallic palladium (Pd/Al₂O₃) with PMS. The metal loading-normalized pseudo-first-order constant for 1,4-dioxane degradation with Pd/Al₂O₃ was more than 16,800 times that with copper-iron bimetallic materials. Complementary to Fenton reagents, Pd/Al₂O₃-PMS had a wide effective pH range from 4.0 to 8.5. In the absence of a substrate, PMS underwent more rapid decomposition under all conditions investigated, which suggests that its activation did not likely proceed via the previously proposed non-radical mechanism. On the basis of the strong inhibitory effects of common scavengers, we instead propose that surface-bound sulfate radicals were probably the dominant active species. A near-100% conversion rate of PMS to radicals was achieved with the Pd/Al₂O₃ catalyst.

Efficient activation of peroxymonosulfate by magnetic Mn-MGO for degradation of bisphenol A

A heterogeneous manganese/magnetite/graphene oxide (Mn-MGO) hybrid catalyst was fabricated through the reduction of KMnO₄ by ethylene glycol in the presence of magnetite/GO (MGO) particles. The Mn-MGO catalyst exhibited high efficacy and long-term stability in activating peroxymonosulfate (PMS) to generate sulfate radicals for the removal of bisphenol A (BPA) from water. The results of the batch experiments indicated that an increase in the catalyst dose and solution pH could enhance BPA degradation in the coupled Mn-MGO/PMS system. Regardless of the initial pH, the solution pH significantly dropped after the reaction, which was caused by catalytic PMS activation. The production of sulfate radicals and hydroxyl radicals was validated through radical quenching and electron paramagnetic resonances (EPR) tests. BPA degradation pathways were proposed on the basis of LC-MS and GC-MS analyses. Finally, a possible mechanism of catalytic PMS activation was proposed that involved electron transfer from MnO or Mn₂O₃ to PMS with the generation of sulfate radicals, protons and MnO₂, as well as the simultaneous reduction of MnO₂ by PMS.

Total petroleum hydrocarbon degradation by hybrid electrobiochemical reactor in oilfield produced water

The crude oil drilling and extraction operations are aimed to maximize the production may be counterbalanced by the huge production of contaminated produced water (PW). PW is conventionally treated through different physical, chemical, and biological technologies. The efficiency of suggested hybrid electrobiochemical (EBC) methods for the simultaneous removal of total petroleum hydrocarbon (TPH) and sulfate from PW generated by petroleum industry is studied. Also, the factors that affect the stability of PW quality are investigated. The results indicated that the effect of biological treatment is very important to keep control of the electrochemical by-products and more TPH removal in the EBC system. The maximum TPH and sulfate removal efficiency was achieved 75% and 25.3%, respectively when the detention time was about 5.1min and the energy consumption was 32.6mA/cm(2). However, a slight increasing in total bacterial count was observed when the EBC compact unit worked at a flow rate of average 20L/h. Pseudo steady state was achieved after 30min of current application in the solution. Also, the results of the study indicate that when the current intensity was increased above optimum level, no significant results occurred due to the release of gases.

Accelerated Catalytic Fenton Reaction with Traces of Iron: An Fe-Pd-Multicatalysis Approach

An accelerated catalytic Fenton (ACF) reaction was developed based upon a multicatalysis approach, facilitating efficient contaminant oxidation at trace levels of dissolved iron. Beside the Fe(II)/H2O2 catalyst/oxidant pair for production of OH-radicals, the ACF system contains Pd/H2 as catalyst/reductant pair for fast reduction of Fe(III) back to Fe(II) which accelerates the Fenton cycle and leads to faster contaminant degradation. By this means, the concentration of the dissolved iron catalyst can be reduced to trace levels (1 mg L(-1)) below common discharge limits, thus eliminating the need for iron sludge removal, which is one of the major drawbacks of conventional Fenton processes. ACF provides fast degradation of the model contaminant methyl tert-butyl ether (MTBE, C0 = 0.17 mM) with a half-life of 11 min with 1 mg L(-1) dissolved iron, 500 mg L(-1) H2O2, 5 mg L(-1) Pd (as suspended Pd/Al2O3 catalyst) and 0.1 MPa H2, pH 3. The effects of pH, H2 partial pressure and H2O2 concentration on MTBE degradation rates were studied. Results on kinetic deuterium isotope effect and quenching studies are in conformity with OH-radicals as main oxidant. The heterogeneous Pd/Al2O3 catalyst was reused within six cycles without significant loss in activity.

Electrochemical degradation of trichloroethylene in aqueous solution by bipolar graphite electrodes

In this study, we tested the use of the bipolar electrodes to enhance electrochemical degradation of trichloroethylene (TCE) in an undivided, flow-through electrochemical reactor. The bipolar electrode forms when an electrically conductive material polarizes between feeder electrodes that are connected to a direct current source and, therefore, creates an additional anode/cathode pair in the system. We hypothesize that bipolar electrodes will generate additional oxidation/reduction zones to enhance TCE degradation. The graphite cathode followed by graphite anode sequence were operated without a bipolar electrode as well as with one and two bipolar graphite electrodes. The system without bipolar electrodes degraded 29% of TCE while the system with one and two bipolar electrodes degraded 38% and 66% of TCE, respectively. It was found that the removal mechanism for TCE in bipolar mode includes hydrodechlorination at the feeder cathode, and oxidation through reaction with peroxide. The results show that the bipolar electrodes presence enhance TCE removal efficiency and rate and imply that they can be used to improve electrochemical treatment of contaminated groundwater.

The influence of cathode material on electrochemical degradation of trichloroethylene in aqueous solution

In this study, different cathode materials were evaluated for electrochemical degradation of aqueous phase trichloroethylene (TCE). A cathode followed by an anode electrode sequence was used to support reduction of TCE at the cathode via hydrodechlorination (HDC). The performance of iron (Fe), copper (Cu), nickel (Ni), aluminum (Al) and carbon (C) foam cathodes was evaluated. We tested commercially available foam materials, which provide large electrode surface area and important properties for field application of the technology. Ni foam cathode produced the highest TCE removal (68.4%) due to its high electrocatalytic activity for hydrogen generation and promotion of HDC. Different performances of the cathode materials originate from differences in the bond strength between atomic hydrogen and the material. With a higher electrocatalytic activity than Ni, Pd catalyst (used as cathode coating) increased TCE removal from 43.5% to 99.8% for Fe, from 56.2% to 79.6% for Cu, from 68.4% to 78.4% for Ni, from 42.0% to 63.6% for Al and from 64.9% to 86.2% for C cathode. The performance of the palladized Fe foam cathode was tested for degradation of TCE in the presence of nitrates, as another commonly found groundwater species. TCE removal decreased from 99% to 41.2% in presence of 100 mg L(-1) of nitrates due to the competition with TCE for HDC at the cathode. The results indicate that the cathode material affects TCE removal rate while the Pd catalyst significantly enhances cathode activity to degrade TCE via HDC.

Hydrodechlorination of TCE in a circulated electrolytic column at high flow rate

Palladium-catalytic hydrodechlorination of trichloroethylene (TCE) by cathodic H2 produced from water electrolysis has been tested. For a field in-well application, the flow rate is generally high. In this study, the performance of Pd-catalytic hydrodechlorination of TCE using cathodic H2 is evaluated under high flow rate (1 L min(-1)) in a circulated column system, as expected to occur in practice. An iron anode supports reduction conditions and it is used to enhance TCE hydrodechlorination. However, the precipitation occurs and high flow rate was evaluated to minimize its adverse effects on the process (electrode coverage, clogging, etc.). Under the conditions of 1 L min(-1) flow, 500 mA current, and 5 mg L(-1) initial TCE concentration, removal efficacy using iron anodes (96%) is significantly higher than by mixed metal oxide (MMO) anodes (66%). Two types of cathodes (MMO and copper foam) in the presence of Pd/Al2O3 catalyst under various currents (250, 125, and 62 mA) were used to evaluate the effect of cathode materials on TCE removal efficacy. The similar removal efficiencies were achieved for both cathodes, but more precipitation generated with copper foam cathode (based on the experiments done by authors). In addition to the well-known parameters such as current density, electrode materials, and initial TCE concentration, the high velocities of groundwater flow can have important implications, practically in relation to the flush out of precipitates. For potential field application, a cost-effective and sustainable in situ electrochemical process using a solar panel as power supply is being evaluated.

Preparation and characterization of Cu–rare earth/Al2O3 catalysts and their application in the electrochemical removal of p-nitrophenol

Cu and rare earth composite catalysts (Cu–rare earth/Al₂O₃) were prepared on Al₂O₃ particles using an impregnation method for the electro-catalytic oxidation of p-nitrophenol in this study.

Influence of humic substances on electrochemical degradation of trichloroethylene in limestone aquifers

In this study we investigate the influence of humic substances (HS) on electrochemical transformation of trichloroethylene (TCE) in groundwater from limestone aquifers. A laboratory flow-through column with an electrochemical reactor that consists of a palladized iron foam cathode followed by a MMO anode was used to induce TCE electro-reduction in groundwater. Up to 82.9% TCE removal was achieved in the absence of HS. Presence of 1, 2, 5, and 10 mgTOC L^-1 reduced TCE removal to 70.9%, 61.4%, 51.8% and 19.5%, respectively. The inverse correlation between HS content and TCE removal was linear. Total organic carbon (TOC), dissolved organic carbon (DOC) and absorption properties (A=254 nm, 365 nm and 436 nm) normalized to DOC, were monitored during treatment to understand the behavior and impacts of HS under electrochemical processes. Changes in all parameters occurred mainly after contact with the cathode, which implies that the HS are reacting either directly with electrons from the cathode or with H₂ formed at the cathode surface. Since hydrodechlorination is the primary TCE reduction mechanism in this setup, reactions of the HS with the cathode limit transformation of TCE. The presence of limestone gravel reduced the impact of HS on TCE removal. The study concludes that presence of humic substances adversely affects TCE removal from contaminated groundwater by electrochemical reduction using palladized cathodes.

Magnetic core-shell CuFe2O4@C3N4 hybrids for visible light photocatalysis of Orange II

Novel CuFe2O4@C3N4 core-shell photocatalysts were fabricated through a self-assembly method and characterized by X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy, transmission electron microscopy and Uv-vis diffuse reflection spectroscopy. The photocatalytic performances of the CuFe2O4@C3N4 catalysts were evaluated in photo Fenton-like discoloration of Orange II dye using H2O2 as an oxidant under visible-light irradiation (λ>420 nm). It was found the CuFe2O4@C3N4 hybrid (mass ratio of CuFe2O4/g-C3N4 at 2:1) exhibits a superior activity as compared with single component of CuFe2O4 or g-C3N4 and the mixture of g-C3N4 and CuFe2O4, due to the elevation of the separation efficiency of photoinduced electron-hole pairs, resulted from the heterojunction between the interfaces of g-C3N4 and CuFe2O4. The quenching tests of different scavengers displayed that O2(•-), OH and h(+) are responsible for the Orange II decolorization. In addition, the effects of initial concentration of the dye contaminant (0.014-0.140 mM), different anions (Cl(-), SO4(2-), NO3(-), CH3COO(-) and HCO3(-)) and temperature (15-65 °C) in photoreaction were also investigated. The CuFe2O4@C3N4 sample exhibited stable performance without obvious loss of catalytic activity after five successive runs, showing a promising application for the photo-oxidative degradation of environmental contaminants.

Impact of electrode sequence on electrochemical removal of trichloroethylene from aqueous solution

The electrode sequence in a mixed flow-through electrochemical cell is evaluated to improve the hydrodechlorination (HDC) of trichloroethylene (TCE) in aqueous solutions. In a mixed (undivided) electrochemical cell, oxygen generated at the anode competes with the transformation of target contaminants at the cathode. In this study, we evaluate the effect of placing the anode downstream from the cathode and using multiple electrodes to promote TCE reduction. Experiments with a cathode followed by an anode (C→A) and an anode followed by a cathode (A→C) were conducted using mixed metal oxide (MMO) and iron as electrode materials. The TCE removal rates when the anode is placed downstream of the cathode (C→A) were 54% by MMO→MMO, 64% by MMO→Fe and 87% by Fe→MMO sequence. Removal rates when the anode is placed upstream of the cathode (A→C) were 38% by MMO→MMO, 58% by Fe→MMO and 69% by MMO→Fe sequence. Placing the anode downstream of the cathode positively improves (by 26%) the degradation of aqueous TCE in a mixed flow-through cell as it minimizes the influence of oxygen generated at the MMO anode on TCE reduction at the cathode. Furthermore, placing the MMO anode downstream of the cathode neutralizes pH and redox potential of the treated solution. Higher flow velocity under the C→A setup increases TCE mass flux reduction rate. Using multiple cathodes and an iron foam cathode up stream of the anode increase the removal rate by 1.6 and 2.4 times, respectively. More than 99% of TCE was removed in the presence of Pd catalyst on carbon and as an iron foam coating. Enhanced reaction rates found in this study imply that a mixed flow-through electrochemical cell with multiple cathodes up stream of an anode is an effective method to promote the reduction of TCE in groundwater.

Degradation of trichloroethene by siderite-catalyzed hydrogen peroxide and persulfate: Investigation of reaction mechanisms and degradation products

A binary catalytic system, siderite-catalyzed hydrogen peroxide (H₂O₂) coupled with persulfate (S₂O₈^2-), was investigated for the remediation of trichloroethene (TCE) contamination. Batch experiments were conducted to investigate reaction mechanisms, oxidant decomposition rates, and degradation products. By using high performance liquid chromatography (HPLC) coupled with electron paramagnetic resonance (EPR), we identified four radicals (hydroxyl (HO·), sulfate (SO₄^-·), hydroperoxyl (HO₂·), and superoxide (O₂^-·)) in the siderite-catalyzed H₂O₂-S₂O₈^2- system. In the absence of S₂O₈^2- (i.e., siderite-catalyzed H₂O₂), a majority of H₂O₂ was decomposed in the first hour of the experiment, resulting in the waste of HO·. The addition of S₂O₈^2- moderated the H₂O₂ decomposition rate, producing a more sustainable release of hydroxyl radicals that improved the treatment efficiency. Furthermore, the heat released by H₂O₂ decomposition accelerated the activation of S₂O₈^2-, and the resultant SO₄^-· was the primary oxidative agent during the first two hours of the reaction. Dichloroacetic acid was firstly detected by ion chromatography (IC). The results of this study indicate a new insight to the reaction mechanism for the catalytic binary H₂O₂-S₂O₈^2- oxidant system, and the delineation of radicals and the discovery of the chlorinated byproduct provide useful information for efficient treatment of chlorinated-solvent contamination in groundwater.