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The reduction of environmental pollutants by Fe2+bound to iron oxides is an important process that determines pollutant toxicities and mobilities. Recently, we showed that pollutant reduction rates depend on the thermodynamic driving force of the reaction in a linear free energy relationship that was a function of the solution pH value and the reduction potential, EH, of the interfacial Fe3+/Fe2+redox couple. In this work, we studied how carbonate affected the free energy relationship by examining the effect that carbonate has on nitrobenzene reduction rates by Fe2+bound to goethite (α-FeOOH). Carbonate slowed nitrobenzene reduction rates by inducing goethite particle aggregation, as evidenced by surface charge and particle size measurements. We observed no evidence for carbonate affecting Fe3+/Fe2+reduction potentials or the mechanism of nitrobenzene reduction. The linear free energy relationship accurately described the data collected in the presence of carbonate when we accounted for the effect it had on the reactive surface area of goethite. The findings from this work provide a framework for determining why common groundwater constituents affect the EH-dependence of reaction rates involving oxide-bound Fe2+as a reductant.

Vanadium(V) is an emerging contaminant in the most recent Environmental Protection Agency’s candidate contaminant list (CCL4). The redox chemistry of vanadium controls its occurrence in the aquatic environment, but the impact of vanadium(V) speciation on the redox properties remains largely unknown. This study utilized the rotating ring-disk electrode technique to examine the reduction kinetics of four pH- and concentration-dependent vanadium(V) species in the presence and the absence of phosphate. Results showed that the reduction of VO2+, HxV4O12+x(4+x)–(V4), and HVO42–proceeded via a one-electron transfer, while that of NaxHyV10O28(6–x–y)–(V10) underwent a two-electron transfer. Koutecky–Levich and Tafel analyses showed that the intrinsic reduction rate constants followed the order of V10> VO2+> V4> HVO42–. Ring-electrode collection efficiency indicated that the reduction product of V10was stable, while those of VO2+, HVO42–, and V4had short half-lives that ranged from milliseconds to seconds. With molar ratios of phosphate to vanadium(V) varying from 0 to 1, phosphate accelerated the reduction kinetics of V10and V4and enhanced the stability of the reduction products of VO2+, V4, and HVO42–. This study suggests that phosphate complexation could enhance the reductive removal of vanadium(V) and inhibit the reoxidation of its reduction product in water treatment.

Hydrogen-tuned 185 nm vacuum ultraviolet (VUV/H2) photolysis is an emerging technology to destroy per- and polyfluoroalkyl substance (PFAS) in brine. This study discovered the promotive effects of two major brine anions, i.e., chloride and sulfate in VUV/H2photolysis on the hydrated electron (eaq–) generation and perfluorocarboxylates (PFCAs) destruction and established a kinetics model to elucidate the promotive effects on the steady-state concentration of eaq–([eaq–]ss). Results showed that VUV/H2achieved near-complete defluorination of perfluorooctanoic acid (PFOA) in the presence of up to 1000 mM chloride or sulfate at pH 12. The defluorination rate constant (kdeF) of PFOA peaked with a chloride concentration at 100 mM and with a sulfate concentration at 500 mM. The promotive effects of chloride and sulfate were attributed to an enhanced generation of eaq–via their direct VUV photolysis and conversion of additionally generated hydroxyl radical to eaq–by H2, which was supported by a linear correlation between the predicted [eaq–]ssand experimentally observed kdeF. The kdeFvalue increased from pH 9 to 12, which was attributed to the speciation of the H·/eaq–pair. Furthermore, the VUV system achieved >95% defluorination and ≥99% parent compound degradation of a concentrated PFCAs mixture in a synthetic brine, without generating any toxic perchlorate or chlorate.

Removal of excess nitrate is critical to balance the nitrogen cycle in aquatic systems. This study investigated a novel denitrification process by tailoring photochemistry of nitrate with formate. Under UV light irradiation, short-lived radicals (i.e., HO•, NO2•, and CO3•–) generated from nitrate photolysis partially oxidized formate to highly reductive formate radical (CO2•–). CO2•–further reduced nitrogen intermediates generated during photochemical denitrification (mainly NO•, HNO, and N2O) to gas-phase nitrogen (i.e., N2O and N2). The degradation kinetics of total dissolved nitrogen was mainly controlled by the photolysis rates of nitrate and nitrite. The distribution of final products was controlled by the reaction between CO2•–and N2O. To achieve a simultaneous and complete removal of dissolved nitrogen (i.e., nitrate, nitrite, and ammonia) and organic carbon, the formate-to-nitrate stoichiometry was determined as 3.1 ± 0.2 at neutral pH in deionized water. Solution pH impacted the removal rates of nitrate and nitrite but not that of total dissolved nitrogen or formate. The presence of dissolved organic matter at levels similar to those in groundwater had a negligible impact on the photochemical denitrification process. A high denitrification efficiency was also achieved in a synthetic groundwater matrix. Outcome from this study provides a potential denitrification technology for decentralized water treatment and reuse facilities to abate nitrate in local water resources.

Chlorate (ClO3–) is a toxic oxyanion pollutant from industrial wastes, agricultural applications, drinking water disinfection, and wastewater treatment. Catalytic reduction of ClO3–using palladium (Pd) nanoparticle catalysts exhibited sluggish kinetics. This work demonstrates an 18-fold activity enhancement by integrating earth-abundant vanadium (V) into the common Pd/C catalyst. X-ray photoelectron spectroscopy and electrochemical studies indicated that VVand VIVprecursors are reduced to VIIIin the aqueous phase (rather than immobilized on the carbon support) by Pd-activated H2. The VIII/IVredox cycle is the predominant mechanism for the ClO3–reduction. Further reduction of chlorine intermediates to Cl–could proceed via VIII/IVand VIV/Vredox cycles or direct reduction by Pd/C. To capture the potentially toxic V metal from the treated solution, we adjusted the pH from 3 to 8 after the reaction, which completely immobilized VIIIonto Pd/C for catalyst recycling. The enhanced performance of reductive catalysis using a Group 5 metal adds to the diversity of transition metals (e.g., Cr, Mo, Re, Fe, and Ru in Groups 6–8) for water pollutant treatment via various unique mechanisms.

Reductive water treatment using hydrated electrons (eaq−) is a promising technology to destruct perfluoroalkyl substances; however, it faces challenges of slow reaction kinetics, undesirable chemical addition, and high energy consumption. Herein, we developed a hydrogen (H2)-polarized water photolysis system using vacuum UV (VUV) light at 185 nm for reductive destruction of perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS). The 185-nm photons directly photolyzed H2O and OH-into HO·, H·, and eaq−. H2elevated the quasisteady-state concentration of eaq−18 times in untuned VUV systems through eliminating the scavenging effect of dissolved oxygen and converting hydroxyl radicals (HO·/O·-) into eaq−. The polarization effect of H2increased the degradation of PFOA from 10 % to 95 % and the defluorination from 17 % to 94 % and led to 87 % of defluorination for PFOS. The pH impacted VUV photon adsorption between H2O and OH-and shifted the equilibrium between H· and eaq−, which led to an optimal pH of 10.3 for PFOA destruction. The presence of chloride and sulfate enhanced the production of eaq−and promoted PFOA destruction. H2-polarized VUV water photolysis systems produced high levels of eaq−from clean water constituents and significantly reduced energy consumption for PFAS treatment under mild alkaline conditions.

Chromium can exist as both trivalent Cr(iii) and hexavalent Cr(vi) in typical drinking water conditions, with Cr(vi) being of particular concern due to its high toxicity and new regulatory perspectives. This study critically reviewed the occurrence and speciation of chromium in drinking water distribution systems in the U.S., based on a detailed analysis of the up-to-date EPA Unregulated Contaminant Monitoring Rule (UCMR) database. Data show that the nationwide Cr(vi) levels in drinking water ranged from non-detectable to 97 μg L−1at the entry point to drinking water distribution systems, and between non-detectable and 67 μg L−1at the maximal residence time of distribution systems. Geographic distributions show that higher drinking water Cr(vi) concentrations were observed in Southwest U.S. and were closely associated with the dependence on groundwater as a drinking water source. While Cr(vi) and total chromium concentrations showed either an increase or a decrease between the entry point and the maximal residence time of drinking water distribution systems, they suggested complex physicochemical processes that can act as both sources and sinks of chromium in distribution systems, including the oxidation of Cr(iii)-containing solids by residual disinfectants and the adsorption of Cr(vi) by corrosion scales. Time-dependent data analysis revealed that a considerable fraction of Cr(iii) at the entry point to distribution systems was inadvertently converted to Cr(vi) by the time the drinking water reached the maximal residence time of distribution systems, and this conversion was positively correlated with the presence of residual disinfectants. Data analyses suggest that the reactivity of downstream water distribution should be managed to control chromium in tap water.

For the ever-growing demand of advanced lithium-ion batteries, it is highly desirable to grow self-supported micro-/nanostructured arrays on metal substrates as electrodes directly. Thein-situgrowth of electrode materials on the conducting substrates greatly simplifies the electrode fabrication process without using any binders or conductive additives. Moreover, the well-ordered arrays closely connected to the current collectors can provide direct electron transport pathways and enhanced accommodation of strains arisen from lithium ion lithiation/delithiation. This article summarizes our recent work on design and construction of lithium-ion battery electrodes on metal substrates. An aqueous solution-based process and a microemulsion-mediated process have been respectively presented to control the kinetic and thermodynamic processes for the micro-/nanostructured array growth on metal substrates, with particular attention to CuO nanorod arrays and micro-cog arrays successfully prepared on Cu foil substrates. They can be directly used as binder-free electrodes to build advanced lithium-ion batteries with high energy, high safety and high stability.

Assembling complex nanostructures on functional substrates such as electrodes promises new multi‐functional interfaces with synergetic properties capable of integration into larger‐scale devices. Here, we report a microemulsion‐mediated process for the preparation of CuO/Cu electrodes comprising a surface layer of a densely packed array of unusual cog‐shaped CuO microparticles with hierarchical nanofilament‐based superstructure and enhanced electrochemical performance in lithium‐ion batteries. The CuO particles are produced by thermolysis of Cu(OH)2micro‐cog precursors that spontaneously assemble on the copper substrate when the metal foil is treated with a reactive oil‐based microemulsion containing nanometer‐scale aqueous droplets. The formation of the hierarchical superstructure improves the coulombic efficiency, specific capacity, and cycling performance compared with anodes based on CuO nanorods or polymer‐blended commercial CuO/C black powders, and the values for the initial discharge capacity (1052 mA h g−1) and reversible capacity (810 m A h g−1) are higher than most copper oxide materials used in lithium‐ion batteries. The results indicate that a fabrication strategy based on self‐assembly within confined reaction media, rather than direct synthesis in bulk solution, offers a new approach to the design of electrode surface structures for potential development in a wide range of materials applications.

CuO/Cu electrodes comprising a surface layer of a densely packed array of unusual cog‐shaped CuO microparticles with hierarchical nanofilament‐based superstructures are presented by Stephen Mann and co‐workers on page 3516. The electrodes demonstrate an improvement in coulombic efficiency, specific capacity, and cycling performance as anodes in lithium‐ion batteries. The initial discharge capacity and reversible capacity are higher than for most copper oxide materials used.