A New Mechanism in Electrochemical Process for Arsenic Oxidation: Production of H2O2 from Anodic O2 Reduction on the Cathode under Automatically Developed Alkaline Conditions

Phosphate ligand-mediated production of reactive oxygen species during oxygenation of Fe(II)-phosphate complexes

Reactive oxygen species (ROS) production upon the oxygenation of reduced iron minerals is of critical importance to redox cycles of Fe and the fate of refractory organic contaminants. The environmental impact factors during this process, however, have been underappreciated. In this study, prominently enhanced production of hydroxyl radicals (•OH) was observed by oxygenation of Fe(II) with 5-50 mM phosphate. The results of spin trap electron spin resonance (ESR) experiment showed that Fe(II)-phosphate complexes facilitated the generation of •OH. The degradation experiment of p-nitrophenol (PNP) confirmed that •OH formation was dominated by a consecutive one-electron O2 reduction (90.2-96.9 %), and the quantification of PNP degradation products revealed that Fe(II)/phosphate molar ratios regulated the O2 activation pathways for O2•- or •OH production. The further experimental and theoretical investigation demonstrated that the coordination of phosphate with Fe(II) plays a dual role in ROS generation that facilitated O2•- formation by lowering the energy barrier for Fe(II) oxidation and altered the reaction pathway of •OH formation due to its occupation of sites for electron transfer. The present work highlights an important role of natural oxyanions in O2 activation by Fe(II) and raises the possibility of in situ degradation of contaminants in subsurface environment.

Catalytic mechanisms for As(III) oxidation by H2O2 over TiO2 surfaces, and effects of support, vacancy and photoirradiation

As(III) is much more toxic than As(V) while shows apparently lower affinity at minerals surfaces. Oxidation of As(III) to As(V) by H2O2 over anatase surface provides an attractive avenue for pollution control, and the chemocatalytic and photocatalytic mechanisms are unraveled by means of the DFT + D3 approach. Impacts of anatase as support, O2c/O3c vacancy, photoirradiation are addressed as well. As(III) oxidation under various reaction conditions leads to As(V) through dual electron transfers, while energy barriers differ substantially and decline as 1.80 (direct oxidation) > 1.35 (anatase as support) > 1.24 (O3c vacancy) > 0.50 (chemocatalysis) > 0.28 (photocatalysis) ≥ 0.26 (O2c vacancy) eV. Anatase as support promotes the reaction through bonding with H2O2/As(OH)3 and electron transfers, and its close participation during chemocatalysis produces the TiOOH active site that causes As(III) oxidation to proceed facilely under ambient circumstances. TiOOH exists in two forms (monodentate and bidentate mononuclear) and is critical for chemocatalysis, while its destruction for O3c vacancy exhibits strongly adverse effects to As(III) oxidation. Photoirradiation readily generates the OH• radicals, and corresponding mechanism is plausible while less preferred than the newly posed mechanism based on the Ti(H2O2) active site. Synergism among a number of surface atoms conduces to the superior activity for O2c vacancy and photocatalysis. Results provide a comprehensive understanding for As(III) oxidation to As(V) by H2O2, and facilitate catalysts design for As(III) oxidation that alleviates environmental pollution.

In-situ groundwater remediation of contaminant mixture of As(III), Cr(VI), and sulfanilamide via electrochemical degradation/transformation using pyrite

Electrochemical advanced oxidation processes (EAOPs) are effective in removing persistent contaminants from groundwater. However, their practical applicability depends significantly on various site-specific characteristics. Therefore, the primary objective of this investigation was to study the feasibility of EAOPs and pyrite, which is a sulfide mineral, to effectively remove the mixture of arsenic (As (III)), chromium (Cr (VI)), and sulfanilamide in groundwater. We conducted a comparison of three systems: (1) EAOP alone, (2) pyrite alone, and (3) a combined EAOP and pyrite system. In EAOP alone, sulfanilamide was effectively oxidized (80%), while the electrochemical transformation of As(III)/Cr(VI) into As(V)/Cr(III) was limited. In just the pyrite system, As(III), Cr(VI), and sulfanilamide were adsorbed onto the surface of pyrite (60%, 20%, and 18%). Neither the EAOP nor the pyrite system alone could effectively treat the contaminants mixture. Nonetheless, the combined system completely removed As(III), Cr(VI), and sulfanilamide by the synergistic reaction. This could be attributed to the formation of green rust, a natural adsorbent mineral produced as a reaction of dissolved iron, generated via electrochemical pyrite oxidation, with the groundwater electrolyte (e.g., CO3 or SO4). This system harmonized the combined approach of EAOP and pyrite to effectively eliminate both organic and inorganic contaminants. ENVIRONMENTAL IMPLICATION: A paper proposed electrochemical oxidation (EO) with pyrite to remove both organic and inorganic contaminants from groundwater. The removal performance of the combined system was evaluated, and the synergistic mechanism was revealed. The combination of EO and pyrite with synergistic removal effectively removed the mixture of both contaminants. This could be attributed by the formation of green-rust by electrochemical activation for pyrite. Compared to the single system of EO and pyrite alone, the combined system with EO and pyrite improved removal performance. Results suggested that the combined system could be used for green groundwater remediation.

Inhibiting effects of humic acid on iron flocculation hindered As removal by electro-flocculation on air cathode iron anode

Activated carbon air cathode combined with iron anode oxidation-flocculation synergistic Arsenic (As) removal was a new groundwater purification technology with low energy consumption and high efficiency for groundwater with high As concentration. The presence of organic matter such as humic acid (HA) had ambiguous effects on formation of organic colloids in the system. The effects of the particle size distribution characteristics of these colloids on the formation characteristics of flocs and the efficiency of As purification was not clear. In this work, we used five different pore size alumina filter membranes to separate mixed phase solutions and studied the corresponding changes in iron and arsenic concentrations in the presence and absence of humic acid conditions. In the presence of HA, the arsenic concentration of < 0.05 µm particle size components was 1.01, 1.28, 3.07, 7.69, 2.85 and 1.24 times of that in the absence of HA. At the same time, the arsenic content in 0.05-0.1 µm and 0.1-0.45 µm particle size components was also higher than that in the system without HA, which revealed that the presence of HA hindered the flocculation behavior of As distribution to higher particle sizes in the early stage of the reaction. The presence of HA affected the flocculation rate of iron flocs from small to large particle size fractions and it had limited effect on the behavior of large-size flocs in adsorption of As. These results provide a theoretical basis for targeted, rapid, and low consumption synergistic removal of arsenic and organic compounds in high arsenic groundwater.

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.

Efficient removal of As(III) from groundwaters through self-alkalization in an asymmetric flow-electrode electrochemical separation system

It remains a great challenge to efficiently remove As(III) from groundwater using traditional technologies due to its stable electroneutral form. This study constructed an asymmetric flow-electrode electrochemical separation (AFES) system, which overcomes the drawback of H+ release from anodic carbon oxidation and achieves continuous self-alkalization function and highly efficient removal of As(III) from groundwater. At the applied voltage of 1.2 V and initial pH 7.5, the system could rapidly decrease the total As (T-As) concentration from 150.0 to 8.9 μg L-1 within 90 min, with an energy consumption of 0.04 kWh m-3. The self-alkalization was triggered by the generation of H2O2 from dissolved oxygen reduction and the adsorption of H+ on the cathode in the feed chamber, which significantly promoted the dissociation and oxidation of As(III), resulting in the removal of T-As predominantly in the form of As(V). The removal performance of T-As was slightly affected by the initial pH and coexisting ions in the feed chamber. The AFES system also exhibited considerable stability after 20 cycles of continuous experiments and superior performance in treating As-containing real groundwater. Moreover, the pH of the alkalized solution can be restored to the initial level by standing or aeration operation. This work offers a novel and efficient pathway for the detoxication of As(III)-contaminated groundwaters.

Electrosorption of cadmium and arsenic from wastewaters using nitrogen-doped biochar: Mechanism and application

Biochar has been widely applied as an adsorbent, whose electrochemical capacity and heavy metal adsorption performance can be improved by nitrogen doping. In this work, nitrogen-doped biochar (NBC) was synthesized by calcinating sodium humate with sodium bicarbonate (NaHCO₃) and urea as the activation agent and nitrogen source, respectively. The NBC was then used to electrochemically adsorb Cd(II) and As(III,V) from simulated and actual wastewaters, respectively. The results indicated that NaHCO₃ activation and nitrogen doping could increase the surface area and nitrogen content of the biochar, contributing to the enhancement of adsorption performance for Cd(II) and As(III,V). The electrosorption capacities for Cd(II) and total arsenic (As(T)) increased first and then reached equilibrium with increasing nitrogen content, increased first and then decreased with increasing calcination temperature, and consistently increased with increasing voltage. The Cd(II) electrosorption capacity (79.0 mg g^-1) and As(T) removal ratio (94.0%) at 1.2 V in actual As-contaminated wastewater (1.16 mg L^-1) were about 4 and 2.6 folds of their inorganic adsorption capacities, respectively. After five cycles of reuse, the Cd(II) and As(T) removal ratio could be maintained at 65.8% and 51.7% of the initial electrosorption capacity. This work expands the application of NBC for heavy metal removal.

Removal of As(III) by Electrically Conducting Ultrafiltration Membranes

As(III) species are the predominant form of arsenic found in groundwater. However, nanofiltration (NF) and reverse osmosis (RO) membranes are often unable to effectively reject As(III). In this study, we fabricate highly conducting ultrafiltration (UF) membranes for effective As(III) rejection. These membranes consist of a hydrophilic nickel-carbon nanotubes layer deposited on a UF support, and used as cathodes. Applying cathodic potentials significantly increased As(III) rejection in synthetic/real tap water, a result of locally elevated pH that is brought upon through water electrolysis at the membrane/water interface. The elevated pH conditions convert H₃ASO₃ to H₂AsO₃^-/HAsO₃^2- that are rejected by the negatively charged membranes. In addition, it was found that Mg(OH)₂ that precipitates on the membrane can further trap arsenic. Importantly, almost all As(III) passing through the membranes is oxidized to As(V) by hydrogen peroxide produced on the cathode, which significantly decreased its overall toxicity and mobility. Although the high pH along the membrane surface led to mineral scaling, this scale could be partially removed by backwashing the membrane. To the best of our knowledge, this is the first report of effective As(III) removal using low-pressure membranes, with As(III) rejection higher than that achieved by NF and RO, and high water permeance.

Abiotic oxidation of arsenite in natural and engineered systems: Mechanisms and related controversies over the last two decades (1999-2020)

Abiotic oxidation of toxic As(III) to As(V) is being deemed as a necessary step for the overall arsenic decontamination in both natural and engineered systems. Direct oxidation of As(III) by chemical oxidants, such as ozone, permanganate, ferrate, chlorine and chloramine, or naturally occurring minerals like Mn, Fe oxides, seems straightforward. Both O₂ and H₂O₂ are ineffective for arsenite oxidation, but they can be activated by reducing substances like Fe²⁺, Fe⁰ to increase the oxidation rates. Photo-induced oxidation of As(III) has been demonstrated effective in Fe complexes or minerals, NO₃^-/NO₂^-, dissolved organic matter (DOM), peroxygens and TiO₂ systems. Although a variety of oxidation methods have been developed over the past two decades, there remain many scientific and technical challenges that must be overcome before the rapid progress in basic knowledge can be translated into environmental benefits. To better understand the trends in the existing data and to identify the knowledge gaps, this review describes in detail the complicated mechanisms for As(III) oxidation by various methods and emphasizes on the conflicting data and explanation. Some prevailing concerns and challenges in the sphere of As(III) oxidation are also pointed out so as to appeal to researchers for further investigations.

Influence of surface-water irrigation on the distribution of organophosphorus pesticides in soil-water systems, Jianghan Plain, central China

Surface-water irrigation is one of the most important irrigation methods in areas with abundant surface water. Although this method of irrigation is both economical and convenient, many contaminants are also introduced into the soil-water systems such as organophosphorus pesticides (OPPs). To study the influence of surface-water irrigation on the distribution of OPPs in soil-water systems, 42 water samples (38 groundwater and four surface water) and 85 soil samples (78 profile soil samples and seven topsoil samples) were taken from Shahu in the Jianghan Plain, China. Shahu is a typical Chinese surface-water irrigation district. During sampling, three types of areas were considered: surface-water irrigated areas, groundwater-irrigated areas away from rivers, and non-irrigated areas adjacent to rivers. The results showed that the concentrations of OPPs in the groundwater and soil in the surface-water irrigated farmland were higher than those in groundwater-irrigated farmland. The groundwater flow field and surface-water irrigation were responsible for the OPPs. Thus, it is clear that the surface-water irrigation had a strong influence on the distribution of OPPs in soil-water systems. Principal component analysis for OPPs content in groundwater showed that the key influencing factors on the distribution of OPPs in groundwater were the groundwater flow field and current pesticide use.

Experimental and theoretical insights into the RCS-Involved electro-catalytic transformation of 4-nitrophenol

The important role of reactive chlorine species (RCS) in electrochemical system has been widely concerned for water disinfection recently. In this study, we built an electrochemical system using carbon nanotube as cathode and oxide precursor (Ti/SnO₂-Sb₂O₅-IrO₂) as anode, where RCS was produced from Cl^-. This system was used to degrade nitrogen contaminants, i.e. NO₃^- and 4-nitrophenol. Optimization of the reaction conditions was carried out by a treatment of inorganic nitrogen contaminant NO₃^- and the optimal condition of the electrochemical system was determined at U = 5.5 V, and pH = 10 with a Cl^- concentration of 2000 mg L^-1, and the removal efficiency of NO₃^- can reach up to 60.6% in 150 min. Under the optimal condition, a common nitrogenous organic pollutant, 4-nitrophenol was treated and a removal efficiency of nearly 100% in 90 min. To investigate the detailed degradation mechanism in the applied electrochemical system, a combined method of products identification and density functional theory (DFT) calculation was employed. It concluded that Cl radicals' generation was stimulated was stimulated by the OH radicals after adding Cl^- into the electrochemical system. These two radicals jointly promoted the transformation of 4-nitrophenol resulting in the formation of more toxic organic and inorganic substances. In addition, a conversion of organic nitro group to amino group leading to the formation of 4-aminophenol was found and explained by the indirect reduction theory.

Simultaneous Sulfite Electrolysis and Hydrogen Production Using Ni Foam-Based Three-Dimensional Electrodes

The electrochemical oxidation of sulfite ions offers encouraging advantages for large-scale hydrogen production, while sulfur dioxide emissions can be effectively used to obtain value-added byproducts. Herein, the performance and stability during sulfite electrolysis under alkaline conditions are evaluated. Nickel foam (NF) substrates were functionalized as the anode and cathode through electrochemical deposition of palladium and chemical oxidation to carry out the sulfite electro-oxidation and hydrogen evolution reactions, respectively. A combined analytical approach in which a robust electrochemical flow cell was coupled to different in situ and ex situ measurements was successfully implemented to monitor the activity and stability during electrolysis. Overall, satisfactory sulfite conversion and hydrogen production efficiencies (>90%) at 10 mA·cm^-2 were mainly attributed to the use of NF in three-dimensional electrodes with a large surface area and enhanced mass transfer. Furthermore, stabilization processes associated with electrochemical dissolution and sulfur crossover through the membrane induced specific changes in the chemical and physical properties of the electrodes after electrolysis. This study demonstrates that NF-based electrocatalysts can be incorporated in an efficient electrochemical flow cell system for sulfite electrolysis and hydrogen production, with potential applications at a large scale.

Selective Arsenic Removal from Groundwaters Using Redox-Active Polyvinylferrocene-Functionalized Electrodes: Role of Oxygen

In this work, we investigate selective sorption of arsenic from simulated groundwaters at pH 8 by a redox-active polyvinylferrocene (PVF)-functionalized electrode using a modified double potential step chronoamperometry (DPSC) method. Our results show that effective and sustainable As(III) removal can be achieved at 0 V once the electrode is activated via anodic polarization. During activation, ferrocene (Fc) in PVF is oxidized to the ferrocenium ion (Fc⁺) with the latter facilitating As(III) sorption and subsequent oxidation as well as As(V) sorption. The high affinity of Fc⁺ to As and weak attraction to competing anions at 0 V ensure high selectivity of As over Cl^-, SO₄^2-, and NO₃^- at concentrations typical of groundwaters. Following the removal process, efficient regeneration of the electrode is achieved at -1.2 V wherein Fc⁺ is reduced to Fc thereby facilitating As desorption from the electrode surface. Our results further show that O₂ and associated generation of hydrogen peroxide during the regeneration step drive the oxidation of Fc to Fc⁺, thereby maintaining the constant generation of Fc⁺ required to achieve As(III) removal in subsequent cycles. Our results show that 91.8 ± 0.6% of As(III) could be selectively removed from simulated groundwater over 10 cycles at an ultralow energy consumption of 0.12 kWh m^-3.

Contrasting abiotic As(III) immobilization by undissolved and dissolved fractions of biochar in Ca2+-rich groundwater under anoxic conditions

Engineered black carbon (biochar) can be introduced into groundwater through its extensive engineered applications (e.g., in-situ remediation of groundwater/soils), which can participate in geochemical processes that may alter the fate of trace contaminants such as arsenic (As(III)). Here we examined the impacts of the undissolved and dissolved fractions of reduced biochar (hereafter denoted as rUBC and rDBC, respectively) on the As(III) immobilization in the absence/presence of Ca²⁺ (50 mM) at pH 11.5 under anoxic conditions. While neither rUBC nor rDBC alone was capable of immobilizing As(III), the apparent As(III) immobilization by rUBC and rDBC synergistically occurred in the presence of Ca²⁺, with an efficiency of 73.1% and 89.6% within 24 h, respectively. In the rUBC/Ca²⁺/As(III) system, rUBC enabled full oxidation of As(III) to As(V) by its residual redox-active moieties such as quinoid CO and persistent free radicals, thereby facilitating precipitation of the newly generated As(V) with Ca²⁺ adsorbed onto the rUBC's surface. In contrast, rDBC induced in-situ local enrichment of Ca²⁺ in the nascent rDBC-derived flocs with predominant non-oxidative and slight oxidative precipitation of As(III) via ternary rDBC-Ca-As complexation. This ternary complex was created by Ca²⁺-bridging interactions between As species and oxygen-containing functional groups of rDBC, as evidenced by the FTIR results and the Ca²⁺-impeded As(III) oxidation. The generation of the flocs physically trapped a small amount of As species particularly As(III). Both the increases in Ca²⁺ concentration (0-100 mM) and solution pH (10.0-12.5) enhanced the apparent As(III) immobilization. This study provides new insights into the environmental impacts of two reduced biochar fractions released into typical Ca²⁺-rich aquifers on the fate and transport of As species.

Formation of hydroperoxo (-OOH) species on the surface of self-doped Bi2.15WO6: reactivity towards As(iii) oxidation

Bi2+xWO6 is a cost-effective and environmentally friendly photocatalyst that shows high reactivity in the oxidation of various contaminants under visible light. However, under alkaline conditions, the reactive oxidative species in the Bi2+xWO6 system are still not clear yet. In this study, it is observed that the oxidation rates of As(iii) increase with increasing pH values in the Bi2.15WO6 system. Photoluminescence and the Mott-Schottky analyses confirm that OH- promotes the separation and transfer of photogenerated electron-hole pairs over Bi2.15WO6, thus facilitating the oxidation of As(iii). Electron spin resonance spectra analysis and quenching experiments rule out contributions of •OH, O2˙-, 1O2 and superoxo species to As(iii) oxidation and indicate that surface -OOH and/or H2O2 are indeed the predominant species under alkaline conditions. The improved production of H2O2 by H-donors such as glucose and phenol, as well as the UV-vis diffuse reflectance and Raman analyses, further confirms the formation of surface -OOH on Bi2.15WO6 under alkaline conditions. In the dark, the significant higher oxidation rate of As(iii) by H2O2-Bi2.15WO6 than that by H2O2 alone reveals that surface -OOH, instead of H2O2, plays an important role in As(iii) oxidation. This study enriches our understanding of the diversity of reactive oxygen species (ROS) in the Bi2.15WO6 system and gives new insight into the mechanism involved in the oxidation of As(iii) under alkaline conditions.

Treatment of crude oil contaminated wastewater via an electrochemical reaction

A cost-effective and catalyst-free approach for the treatment of oil field formation water has been extensively explored in this work. ZnO NPs were synthesized via an electrochemical reaction using hydrogen peroxide as the electrolyte. The XRD and TEM analysis depicted the high purity and wurtzite hexagonal structure of ZnO NPs with an average size of 35 ± 5 nm. TGA data showed the thermal stability of the synthesized material up to 750 °C. The efficiency of the removal of hydrocarbons from formation water by the combination of electrochemical reaction and synthesized ZnO NPs was monitored by GC-MS and FTIR. GC-MS analysis revealed that out of 214 compounds (present in the untreated sample), a total of 131 compounds were adsorbed by ZnO NPs. Further, the absence of any compound in the chromatogram of the treated sample attests that the rest of the compounds were completely or partially degraded by electrochemical degradation reaction. Moreover, this technique overcomes some of the important drawbacks of the existing techniques in the area of electrochemical research, such as the generation of toxic byproducts, unwanted side reactions, and involvement of hazardous chemicals.

A novel electrochemical reaction was developed for the treatment of formation water and subsequently ZnO nanoparticles were synthesised in the in situ condition and further used as an adsorbing material for petroleum hydrocarbons.

Modified Double Potential Step Chronoamperometry (DPSC) Method for As(III) Electro-oxidation and Concomitant As(V) Adsorption from Groundwaters

Constrained by low energy efficiency and ineffectiveness in As(III) removal under circumneutral pH conditions by many exsiting technologies, As(III) removal has become an issue. In this work we present proof of concept of a modified double potential step chronoamperometry (DPSC) method of As(III) oxidation and concomitant As(V) electro-sorption from aqueous solution. Results show that in situ anodic As(III) oxidation, As(V) electro-sorption, and As(V) electro-desorption are affected by aqueous pH with high oxidation and sorption/desorption rates observed at the elevated pH. We particularly show that effective As(III) oxidation and concomitant As(V) adsorption are related to (i) the rapid oxidation of the deprotonated species compared to the protonated species and (ii) stronger electrochemical interaction between the multicharged As(V) species and the electrodes. At 1.2 V and an electric energy consumption of 0.06 kWh m^-3, the total As concentration can be reduced from 150 to 15 μg L^-1 using an electrochemical cell with electrode area of 10 × 8 cm² and electro-sorption time of 120 min. On the basis of the experimental results, we have developed a mathematical model to describe the kinetics and mechanism of arsenic removal by the modified DPSC method with this model of use in predicting, and potentially optimizing, process performance under various conditions.

A Three-dimensional Floating Air Cathode with Dual Oxygen Supplies for Energy-efficient Production of Hydrogen Peroxide

The in situ and cleaner electrochemical production of hydrogen peroxide (H2O2) through two-electron oxygen reduction reaction has drawn increasing attentions in environmental applications as an alterantive to traditional anthraquinone process. Air cathodes avoid the need of aeration, but face the challenges of declined performance during scale-up due to non-uniform water infiltration or even water leakage, which is resulted from changing water pressures and immature cathode fabrication at a large scale. To address these challenges, a three-dimensional (3-D) floating air cathode (FAC) was built around the commercial sponge, by coating with carbon black/poly(tetrafluoroethylene) using a simple dipping-drying method. The FAC floated on the water-air interface without extensive water-proof measures, and could utilize oxygen both from passive diffusion and anodic oxygen evolution to produce H2O2. The FAC with six times of dipping treatment produced a maximum H2O2 concentration of 177.9 ± 26.1 mg L-1 at 90 min, with low energy consumption of 7.1 ± 0.003 Wh g-1 and stable performance during 10 cycles of operation. Our results showed that this 3-D FAC is a promising approach for in situ H2O2 production for both environmental remediation and industrial applications.

Electrochemical treatment of humic acid using particle electrodes ensembled by ordered mesoporous carbon

In order to degrade the macromolecular pollutant of humic acid, the powder ordered mesoporous carbon (POMC, average pore diameter 4.29 nm) was first applied for preparing the granular OMC (GOMC, Φ × H = 4 × 3-6 mm) as electrodes in a continuous three-dimensional (3D) electrochemical system. The POMC was synthesized by hard-templating method and characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), particle size distribution, N₂ adsorption/desorption technology, and Fourier-transform infrared (FT-IR). The effects of electrochemical degradation parameters, such as current and hydraulic retention time (HRT), were investigated, and the degradation mechanism of HA was explored as well. The results indicated that the degradation efficiency of HA, chemical oxygen demand (COD), and total organic carbon (TOC) reached 95.3, 86.2, and 62.7%, respectively, under initial HA of 100 mg/L, current of 0.2 A, and HRT of 130 min. The detection of electron paramagnetic resonance (EPR) showed that plenty of ˙OH was generated on GOMC electrodes, which made the 3D system more effective than the conventional two-dimensional (2D) system. The cyclic voltammetry curves indicated that the reactions of HA on the OMC materials surface included both direct oxidation and direct reduction.

Rates of H2O2 Electrogeneration by Reduction of Anodic O2 at RVC Foam Cathodes in Batch and Flow-through Cells

The Electro-Fenton process for in-situ H₂O₂ electrogeneration is impacted by low O₂ utilization efficiency (<0.1%) and the need of acid for pH adjustment. An electrochemical flow-through cell can develop localized acidic conditions, coupled with simultaneous formation and utilization of O₂ to enhance H₂O₂ formation. Multiple electrode configurations using reticulated vitreous carbon (RVC) foam and Ti/mixed metal oxides (MMO) are proposed to identify the optimum conditions for H₂O₂ formation in batch and flow-through cells. A pH of 2.75±0.25 is developed locally in the flow-through cell that supports effective O₂ reduction. Up to 9.66 mg/L H₂O₂ is generated in a 180 mL batch cell under 100 mA, at pH 2, and mixing at 350 rpm. In flow-through conditions, both flow rate and current significantly influence H₂O₂ production. A current of 120 mA produced 2.27 mg/L H₂O₂ under a flow rate of 3 mL/min in a 3-electrode cell with one RVC foam cathode at 60 min. The low current of 60 mA does not enable effective H₂O₂ production, while the high current of 250 mA produced less H₂O₂ due to parasitic reactions competing with O₂ reduction. Higher flow rates decrease the retention time, but also increase the O₂ mass transfer. Furthermore, 3-electrode flow-through cell with two RVC foam cathodes was not effective for H₂O₂ production due to the limited O₂ supply for the secondary cathode. Finally, a coupled process that uses both O₂ and H₂ from water electrolysis is proposed to improve the H₂O₂ yield further.

Electro-catalytic oxidation of phenacetin with a three-dimensional reactor: Degradation pathway and removal mechanism

Phenacetin is a common analgesic, anti-arthritic and anti-rheumatic drug. This study dealt with the degradation of phenacetin in alkaline media using a three-dimensional reactor with particle electrodes. Particular attention was paid to the degradation pathway and the reaction mechanism in the system. Liquid chromatography coupled with time-of-flight mass spectrometry was used to identify the intermediates. The phenacetin was observed to be firstly cut off the branch chains main by direct oxidation, and then the intermediates further degraded to ring opening products by hydroxyl radical resulting from indirect oxidation and finally mineralized to CO2, H2O. A possible removal mechanism was proposed that direct and indirect oxidation together did effect on the pollutants with oxygen.

Spontaneous arsenic (III) oxidation with bioelectricity generation in single-chamber microbial fuel cells

Arsenic is one of the most toxic elements commonly found in groundwater. With initial concentration of 200μgL(-1), spontaneous As(III) oxidation is realized completely during 7 days operation in single-chamber microbial fuel cells (MFCs) in the present study, with the maximum power density of 752.6±17mWm(-2). The product is less toxic and mobile As(V), which can be removed from aqueous solution more easily. High-throughput 16S rRNA gene pyrosequencing analysis indicates the existence of arsenic-resistant bacteria as Actinobacteria, Comamonas, Pseudomonas and arsenic-oxidizing bacteria as Enterobacter, with electrochemically active bacteria as Lactococcus, Enterobacter. They interact together and are responsible for As(III) oxidation and bioelectricity generation in MFCs. This study offers a potential attractive method for remediation of arsenic-polluted groundwater.