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Redox fluctuations within redox dynamic environments influence the redox state of natural organic matter (NOM) and its interaction with redox-active elements, such as iron. In this work, we investigate the changes in the molecular composition of NOM during redox fluctuations as well as the impact of these changes on the Fe-NOM interaction employing Suwannee River Dissolved Organic Matter (SRDOM) as a representative NOM. Characterization of SRDOM using X-ray photoelectron spectroscopy and Fourier transform infrared spectrometry showed that irreversible changes occurred following electrochemical reduction and reoxidation of SRDOM in air. Changes in the redox state of SRDOM impacted its interaction with iron with higher rates of Fe(III) reduction in the presence of reduced and reoxidized SRDOM than in the presence of the original SRDOM. The increased rate of Fe(III) reduction in the presence of reduced SRDOM was due to the formation of reduced organic moieties on SRDOM reduction. The Fe(II) oxidation rate also increased in the presence of reduced SRDOM due to the formation of redox-active moieties that were capable of oxidizing Fe(II). Overall, our study provides useful insights into the changes in SRDOM that may occur in redox dynamic environments and the associated impact of these changes on Fe transformations.

Pd-based electrodes are recognized to facilitate effective electrochemical hydrodechlorination (EHDC) as a result of their superior capacity for atomic hydrogen (H*) generation. However, challenges such as electrode stability, feasibility of treating complex matrices, and high cost associated with electrode synthesis hinder the application of Pd-based electrodes for EHDC. In this work, we investigated the feasibility of degrading 2,4-dichlorophenol (2,4-DCP) by EHDC employing Pd-loaded activated carbon particles, prepared via a simple wet-impregnation method, as a flow cathode (FC) suspension. Compared to other Pd-based EHDC studies, a much lower Pd loading (0.02-0.08 mg cm -2 ) was used. Because of the excellent mass transfer in the FC system, almost 100% 2,4-DCP was hydrodechlorinated to phenol within 1 h. The FC system also showed excellent performance in treating complex water matrices (including hardness ion-containing wastewater and various other chlorinated organics such as 2,4-dichlorobenzoic acid and trichloroacetic acid) with a relatively low energy consumption (0.26-1.56 kW h m -3 mg -1 of 2,4-DCP compared to 0.32-7.61 kW h m -3 mg -1 of 2,4-DCP reported by other studies). The FC synthesized here was stable over 36 h of continuous operation, indicating its potential suitability for real-world applications. Employing experimental investigations and mathematical modeling, we further show that hydrodechlorination of 2,4-DCP occurs via interaction with H*, with no role of direct electron transfer and/or HO•-mediated processes in the removal of 2,4-DCP.

An industrially viable catalyst for heterogeneous catalytic ozonation (HCO) in water purification requires the characteristics of good dispersion of active species on its surface, efficient electron transfer for ozone decay, and maximum active species utilization. While metal-organic frameworks (MOFs) represent an attractive platform for HCO, the metal nodes in the unmodified MOFs exhibit low catalytic activity. Herein, we present a perfluorinated Fe-MOF catalyst by substituting H atoms on the metalated ligands with F atoms (termed 4F-MIL-88B) to induce structure evolution. The Lewis acidity of 4F-MIL-88B was enhanced via the formation of Fe nodes, tailoring the electron distribution on the catalyst surface. As a result of catalyst modification, the rate constant for degradation of the target compounds examined increased by ∼700% compared with that observed for the unmodified catalyst. Experimental evidence and theoretical calculations showed that the modulated polarity and the enhanced electron transfer between the catalyst and ozone molecules contributed to the adsorption and transformation of O 3 to • OH on the catalyst surface. Overall, the results of this study highlight the significance of tailoring the metalated ligands to develop highly efficient and stable MOF catalysts for HCO and provide an in-depth mechanistic understanding of their structure-function evolution, which is expected to facilitate the applications of nanomaterial-based processes in water purification.

Sunlight plays a crucial role in the transformation of dissolved organic matter (DOM) and the associated carbon cycle in aquatic environments. This study demonstrates that the presence of nanomolar concentrations of copper (Cu) significantly decreases the rate of photobleaching and the rate of loss of electron-donating moieties of three selected types of DOM (including both terrestrial and microbially derived DOM) under simulated sunlight irradiation. Employing Fourier transform ion cyclotron resonance mass spectrometry, we further confirm that Cu selectively inhibits the photooxidation of lignin- and tannin-like phenolic moieties present within the DOM, in agreement with the reported inhibitory impact of Cu on the photooxidation of phenolic compounds. On the basis of the inhibitory impact of Cu on the DOM photobleaching rate, we calculate the contribution of phenolic moieties to DOM photobleaching to be at least 29-55% in the wavelength range of 220-460 nm. The inhibition of loss of electrons from DOM during irradiation in the presence of Cu is also explained quantitatively by developing a mathematical model describing hydrogen peroxide (a proxy measure of loss of electrons from DOM) formation on DOM irradiation in the absence and presence of Cu. Overall, this study advances our understanding of DOM transformation in natural sunlit waters.

Ferryl ion ([Fe IV O] 2+ ) has often been suggested to play a role in iron-based advanced oxidation processes (AOPs) with its presence commonly determined using the unique oxidation pathway from methyl phenyl sulfoxide (PMSO) to methyl phenyl sulfone (PMSO 2 ). However, we show here that the oxidation products of PMSO, formed on reaction with hydroxyl radical, enhance PMSO 2 formation as a result of their complexation with Fe(III) leading to the changes in the reactivity of Fe(III) species in the homogeneous Fenton reaction. As such, PMSO should be used with caution to investigate the role of [Fe IV O] 2+ in iron-based AOPs with these insights suggesting the need to reassess the findings of many previous studies in which this reagent was used. The other common target compounds, phthalhydrazide and hydroxybenzoic acids, were also found to modify the rate and extent of iron cycling as a result of complexation and/or redox reactions, either by the probe compound itself and/or oxidation products formed. Overall, this study highlights that these confounding effects of the aromatic probe compounds on the reactivity of iron species should be recognized if reliable mechanistic insights into iron-based AOPs are to be obtained.

Heterogeneous catalytic ozonation (HCO) has gained increasing attention as an effective process to remove refractory organic pollutants from industrial effluents. However, widespread application of HCO is still limited due to the typically low efficacy of catalysts used and matrix passivation effects. To this end, we prepared an Al 2 O 3 -supported Fe catalyst with high reactivity via a facile urea-based heterogeneous precipitation method. Due to the nonsintering nature of the preparation method, a heterogeneous catalytic layer comprised of γ-FeOOH and α-Fe 2 O 3 is formed on the Al 2 O 3 support (termed NS-Fe-Al 2 O 3 ). On treatment of a real industrial effluent by HCO, the presence of NS-Fe-Al 2 O 3 increased the removal of organics by ∼100% compared to that achieved with a control catalyst (i.e., α-Fe 2 O 3 /Al 2 O 3 or γ-FeOOH/Al 2 O 3 ) that was prepared by a conventional impregnation and calcination method. Furthermore, our results confirmed that the novel NS-Fe-Al 2 O 3 catalyst demonstrated resistance to the inhibitory effect of high concentration of chloride and sulfate ions usually present in industrial effluent. A mathematical kinetic model was developed that adequately describes the mechanism of HCO process in the presence of NS-Fe-Al 2 O 3 . Overall, the results presented here provide valuable guidance for the synthesis of effective and robust catalysts that will facilitate the wider industrial application of HCO.

Electrochemical reduction of nitrate is a promising method for the removal of nitrate from contaminated groundwater. However, the presence of hardness cations (Ca 2+ and Mg 2+ ) in groundwaters hampers the electroreduction of nitrate as a result of the precipitation of carbonate-containing solids of these elements on the cathode surface. Thus, some pretreatment process is required to remove unwanted hardness cations. Herein, we present a proof-of-concept of a novel three-chambered flow electrode unit, constituting a flow electrode capacitive deionization (FCDI) unit and a flow cathode (FC) unit, which achieves cation removal, nitrate capture and reduction, and ammonia generation in a single cell without the need for any additional chemicals/electrolyte. The addition of the FCDI unit not only achieves removal of hardness cations but also concentrates the nitrate ions and other anions, which facilitates nitrate reduction in the subsequent FC unit. Results show that the FCDI cell voltage influences electrode stability but has a minimal impact on the overall nitrate removal performance. The concentration of coexisting anions influences the nitrate removal due to competitive sorption of anions on the electrode surface. Our results further show that stable electrochemical performance was obtained over 26 h of operation. Overall, this study provides a scalable strategy for continuous nitrate electroreduction and ammonia generation from nitrate contaminated groundwaters containing hardness ions.

Cu and Ni complexes with ethylenediaminetetraacetic acid (Cu/Ni-EDTA), which are commonly present in metal plating industry wastewaters, pose a serious threat to both the environment and human health due to their high toxicity and low biodegradability. In this study, the treatment of solutions containing either or both Cu-EDTA and Ni-EDTA using an electrochemical process is investigated under both oxidizing and reducing electrolysis conditions. Our results indicate that Cu-EDTA is decomplexed as a result of the cathodic reduction of Cu(II) with subsequent electrodeposition of Cu(0) at the cathode when the cathode potential is more negative than the reduction potential of Cu-EDTA to Cu(0). In contrast, the very negative reduction potential of Ni-EDTA to Ni(0) renders the direct reduction of EDTA-complexed Ni(II) at the cathode unimportant. The removal of Ni during the electrolysis process mainly occurs via anodic oxidation of EDTA in Ni-EDTA, with the resulting formation of low-molecular-weight organic acids and the release of Ni 2+ , which is subsequently deposited as Ni 0 on the cathode. A kinetic model incorporating the key reactions occurring in the electrolysis process has been developed, which satisfactorily describes EDTA, Cu, Ni, and TOC removal. Overall, this study improves our understanding of the mechanism of removal of heavy metals from solution during the electrochemical advanced oxidation of metal plating wastewaters.

The performance of membrane capacitive deionization (MCDI) desalination was investigated at bench, pilot, and field scales for the removal of uranium from groundwater. It was found that up to 98.9% of the uranium can be removed using MCDI from a groundwater source containing 50 μg/L uranium, with the majority (94.5%) being retained on the anode. Uranium was found to physiochemically adsorb to the electrode without the application of a potential by displacing chloride ions, with 16.6% uranium removal at the bench scale via this non-electrochemical process. This displacement of chloride did not occur during the MCDI adsorption phase with the adsorption of all ions remaining constant during a time series analysis on the pilot unit. For the scenarios tested on the pilot unit, the flowrate of the product water ranged from 0.15 to 0.23 m 3 /h, electrode energy consumption from 0.28 to 0.51 kW h/m 3 , and water recovery from 69 to 86%. A portion (13-53% on the pilot unit) of the uranium was found to remain on the electrodes after the brine discharge phase with conventional cleaning techniques unable to release this retained uranium. MCDI was found to be a suitable means to remove uranium from groundwater systems though with the need to manage the accumulation of uranium on the electrodes over time.

The formula assignment of the Fourier transform ion cyclotron resonance mass spectrometry coupled with positive-ion electrospray ionization [ESI(+)-FT-ICR MS] is challenging because of the extensive occurrence of adducts. However, there is a paucity of automated formula assignment methods for ESI(+)-FT-ICR MS spectra. The novel automated formula assignment algorithm for ESI(+)-FT-ICR MS spectra developed herein has been applied to elucidate the composition of dissolved organic matter (DOM) in groundwater during air-induced ferrous [Fe(II)] oxidation. The ESI(+)-FT-ICR MS spectra of groundwater DOM were profoundly impacted by [M + Na] + adducts and, to a lesser extent, [M + K] + adducts. Oxygen-poor and N-containing compounds were frequently detected when the FT-ICR MS was operated in the ESI(+) mode, while the components with higher carbon oxidation states were preferentially ionized in the negative-ion electrospray ionization [ESI(-)] mode. Values for the difference between double-bond equivalents and the number of oxygen atoms from -13 to 13 are proposed for the formula assignment of the ESI(+)-FT-ICR MS spectra of aquatic DOM. Furthermore, for the first time, the Fe(II)-mediated formation of highly toxic organic iodine species was reported in groundwater rich in Fe(II), iodide, and DOM. The results of this study not only shed light on the further algorithm development for comprehensive characterization of DOM by ESI(-)-FT-ICR MS and ESI(+)-FT-ICR MS but also highlight the importance of appropriate treatment of specific groundwater prior to use.

Iron complexes of tetra-amido macrocyclic ligands (Fe-TAML) are recognized to be effective catalysts for the degradation of a wide range of organic contaminants in homogeneous conditions with the high valent Fe(IV) and Fe(V) species generated on activation of the Fe-TAML complex by hydrogen peroxide (H 2 O 2 ) recognized to be powerful oxidants. Electrochemical activation of Fe-TAML would appear an attractive alternative to H 2 O 2 activation, especially if the Fe-TAML complex could be attached to the anode, as this would enable formation of high valent iron species at the anode and, importantly, retention of the valuable Fe-TAML complex within the reaction system. In this work, we affix Fe-TAML to the surface of carbon black particles and apply this "suspension anode" process to oxidize selected target compounds via generation of high valent iron species. We show that the overpotential for Fe(IV) formation is 0.17 V lower than the potential required to generate Fe(IV) electrochemically in homogeneous solution and also show that the stability of the Fe(IV) species is enhanced considerably compared to the homogeneous Fe-TAML case. Application of the carbon black-supported Fe-TAML suspension anode reactor to degradation of oxalate and hydroquinone with an initial pH value of 3 resulted in oxidation rate constants that were up to three times higher than could be achieved by anodic oxidation in the absence of Fe-TAML and at energy consumptions per order of removal substantially lower than could be achieved by alternate technologies.

Vivianite (Fe 3 (PO 4 ) 2 ·8H 2 O) crystallization has attracted increasing attention as a promising approach for removing and recovering P from wastewaters. However, Fe II is susceptible to oxygen with its oxidation inevitably influencing the crystallization of vivianite. In this study, the profile of vivianite crystallization in the presence of dissolved oxygen (DO) was investigated at pHs 5-7 in a continuous stirred-tank reactor. It is found that the influence of DO on vivianite crystallization was highly pH-related. At pH 5, the low rate of Fe II oxidation at all of the investigated DO of 0-5 mg/L and the low degree of vivianite supersaturation resulted in slow crystallization with the product being highly crystalline vivianite, but the P removal efficiency was only 30-40%. The removal of P from the solution was substantially more effective (to >90%) in the DO-removed reactors at pH 6 and 7, whereas the efficiencies of P removal and especially recovery decreased by 10-20% when Fe II oxidation became more severe at DO concentrations >2.5 mg/L (except at pH 6 with 2.5 mg/L DO). The elevated degree of vivianite supersaturation and enhanced rate and extent of Fe II oxidation at the higher pHs led to decreases in the size and homogeneity of the products. At the same pH, amorphous ferric oxyhydroxide (AFO)─the product of Fe II oxidation and Fe III hydrolysis─interferes with vivianite crystallization with the induction of aggregation of crystal fines by AFO, leading to increases in the size of the obtained solids.

The presence of excessive concentrations of nitrate in industrial wastewaters, agricultural runoff, and some groundwaters constitutes a serious issue for both environmental and human health. As a result, there is considerable interest in the possibility of converting nitrate to the valuable product ammonia by electrochemical means. In this work, we demonstrate the efficacy of a novel flow cathode system coupled with ammonia stripping for effective nitrate removal and ammonia generation and recovery. A copper-loaded activated carbon slurry (Cu@AC), made by a simple, low-cost wet impregnation method, is used as the flow cathode in this novel electrochemical reactor. Use of a 3 wt % Cu@AC suspension at an applied current density of 20 mA cm -2 resulted in almost complete nitrate removal, with 97% of the nitrate reduced to ammonia and 70% of the ammonia recovered in the acid-receiving chamber. A mathematical kinetic model was developed that satisfactorily describes the kinetics and mechanism of the overall nitrate electroreduction process. Minimal loss of Cu to solution and maintenance of nitrate removal performance over extended use of Cu@AC flow electrode augers well for long-term use of this technology. Overall, this study sheds light on an efficient, low-cost water treatment technology for simultaneous nitrate removal and ammonia generation and recovery.

Boron is present in the form of boric acid (B(OH) 3 or H 3 BO 3 ) in seawater, geothermal waters, and some industrial wastewaters but is toxic at elevated concentrations to both plants and humans. Effective removal of boron from solutions at circumneutral pH by existing technologies such as reverse osmosis is constrained by high energy consumption and low removal efficiency. In this work, we present an asymmetric, membrane-containing flow-by electrosorption system for boron removal. Upon charging, the catholyte pH rapidly increases to above ∼10.7 as a result of water electrolysis and other Faradaic reactions with resultant deprotonation of boric acid to form B(OH) 4 - and subsequent removal from solution by electrosorption to the anode. Results also show that the asymmetric flow-by electrosorption system is capable of treating feed streams with high concentrations of boron and RO permeate containing multiple competing ionic species. On the basis of the experimental results obtained, a mathematical model has been developed that adequately describes the kinetics and mechanism of boron removal by the asymmetric electrosorption system. Overall, this study not only provides new insights into boron removal mechanisms by electrosorption but also opens up a new pathway to eliminate amphoteric pollutants from contaminated source waters.

Persulfate (PS)-based advanced oxidation processes (AOPs) have been promoted as alternatives to H 2 O 2 -based AOPs. To gauge the potential of this technology, the PS/Fe(II) and Fenton (H 2 O 2 /Fe(II)) processes were comparatively evaluated using formate as a simple target compound and nanofiltration concentrate from a municipal wastewater treatment plant as a complex suite of contaminants with the aid of kinetic modeling. In terms of the short-term rate and extent of mineralization of formate and the nanofiltration concentrate, PS/Fe(II) is less effective due to slow Fe(II)/Fe(III) cycling attributable to the scavenging of superoxide by PS. However, in the concentrate treatment, PS/Fe(II) provided a sustained removal of total organic carbon (TOC), with ∼81% removed after 7 days with SO 4 •- consistently produced via homolysis of the long-life PS. In comparison, H 2 O 2 /Fe(II) exhibited limited TOC removal over ∼57% after 10 h due to the futile consumption of H 2 O 2 by HO • . PS/Fe(II) also offers better performance at transforming humic-like moieties to more biodegradable compounds as a result of chlorine radicals formed by the reaction of SO 4 •- with the matrix constituents present in the concentrate. The application of PS/Fe(II) is, however, subject to the limitations of slow oxidation of organic contaminants, release of sulfate, and formation of chlorinated byproducts.

An electrochemical advanced oxidation process (EAOP) is demonstrated with a catalytic cathode capable of simultaneously catalyzing the hydrogen evolution reaction (HER) and the oxygen reduction reaction (ORR) with resultant in situ generation of atomic hydrogen (H*) and hydrogen peroxide (H 2 O 2 ). A palladium-coated carbon-PTFE gas diffusion electrode (Pd/C GDE) was used as a catalytic cathode with hydroxyl radical ( • OH) formed as a result of the reaction of electrogenerated H* with H 2 O 2 . As both the HER and ORR can be induced to occur at the same cathode, the H*/GDE process results in more effective degradation of organic contaminants than can be achieved by a conventional H*/H 2 O 2 process involving direct addition of H 2 O 2 . At circumneutral pH, 82.7% of added formate was degraded after 2 h treatment at an applied potential of -1.0 V vs Ag/AgCl with relatively low concentrations of generated H 2 O 2 remaining in the solution. We also show that H* and H 2 O 2 (and thus • OH) can be electrogenerated effectively over a wide range of pH (3.2-7.0). These results suggest that by in situ generation of H* and H 2 O 2 , the H*/GDE process is able to produce significant amounts of • OH without external chemical addition and thus offers an alternative method for abatement of aqueous organic contaminants.

The efficacy of oxidation of recalcitrant organic contaminants in municipal and industrial wastewaters by ozonation is influenced by chemical reaction kinetics and hydrodynamics within a reactor. A 3D computational fluid dynamics (CFD) model incorporating both multiphase flow and reaction kinetics describing ozone decay, hydroxyl radical ( • OH) generation, and organic oxidation was developed to simulate the performance of continuous flow ozonation reactors. Formate was selected as the target organic in this study due to its well-understood oxidation pathway. Simulation results revealed that the dissolved ozone concentration in the reactor is controlled by rates of O 3 (g) interphase transfer and ozone self-decay. Simulations of the effect of various operating conditions showed that the reaction stoichiometric constraints between formate and ozone were reached; however, complete utilization of gas phase ozone was hard to achieve due to the low ozone interphase mass transfer rate. Increasing the O 3 (g) concentration leads to an increase in the formate removal rate by ∼5% due to an enhancement in the rate of O 3 (g) interphase mass transfer. The CFD model adequately describes the mass transfer occurring in the two-phase flow system and confirms that O 3 (g) interphase mass transfer is the rate-limiting step in contaminant degradation. The model can be used to optimize the ozone reactor design for improved contaminant degradation and ozonation efficiency.

The heterogeneous Fenton process in the presence of Fe-containing minerals is ubiquitous in nature and widely deployed in wastewater treatment. While there have been extensive relevant studies, the dependence on pH of the nature and extent of oxidant generation and key reaction pathways remain unclear. Herein, the adsorption and decomposition of formate and H 2 O 2 were quantified in the presence of ferrihydrite within the pH range of 3.0-6.0, and experiments with methyl phenyl sulfoxide were conducted to distinguish between HO • and weaker oxidant(s) which react via oxygen atom transfer including ferryl ion ([Fe IV O] 2+ ) and/or ferric hydroperoxo intermediates (≡Fe III (O 2 H)). Both HO • and [Fe IV O] 2+ /≡Fe III (O 2 H) are concurrently produced on the surface over the acidic to near-neutral pH range. Despite the simultaneous formation of both oxidants, HO • is the major oxidant responsible for substrate oxidation in the interfacial boundary layer with [Fe IV O] 2+ /≡Fe III (O 2 H) exhibiting limited exposure to substrates. With an increase of pH, the yield of both oxidants is inhibited by the decreasing availability of surface sites due to ferrihydrite particle aggregation. Increasing pH also favors the nonradical decay of H 2 O 2 as evident from the consistent oxidant production rate relative to the surface area (SSA) despite an accelerated H 2 O 2 decay rate relative to SSA with pH increase.

The heterogeneous Fenton process has been widely applied though some aspects of this process are still poorly understood. In this study, we simultaneously quantify the adsorption and decomposition of formate and H 2 O 2 at pH 4.0 in the presence of freshly formed ferrihydrite and provide new insights into the ferrihydrite-induced heterogeneous Fenton mechanism with the aid of kinetic and reactive-transport modeling. Our results show that the decomposition of H 2 O 2 and formate is controlled by surface-initiated reactions. Adsorbed formate occupies the surface sites otherwise available for reaction with H 2 O 2 and therefore hampers the surface Fenton reactions despite the negligible accumulation of H 2 O 2 on the surface. The minimal impact of methanol (an effective HO • scavenger) on formate oxidation as well as the poor oxidation of fully adsorbed oxalate compared with the ready oxidation of partially adsorbed formate demonstrates that oxidation mainly occurs in the solid-liquid boundary layer, rather than in bulk or on the surface. This is suggested to be due to the diffusion of surface-generated HO • , rather than surface Fe(II), to the boundary layer with the results of kinetic and reactive-transport modeling supporting this conclusion. The new findings are critical to our understanding of the removal behavior of more complex organic target species and to the design of more effective heterogeneous Fenton technologies.

In this study, copper aluminum layered hydroxides (Cu-Al LDHs) and copper oxide (CuO) were utilized as catalysts for heterogeneous catalytic ozonation (HCO). Target compounds oxalate and formate were used with removal by adsorption and oxidation quantified to elucidate the role of the catalyst in contaminant removal. Oxidation of oxalate mostly occurred on the catalyst surface via interaction of surface oxalate complexes with surface-located oxidants. In contrast, the oxidation of formate occurred in the bulk solution as well as on the surface of the catalyst. Measurement of O 3 decay kinetics coupled with fluorescence microscopy image analysis corresponding to 7-hydroxycoumarin formation indicates that while surface hydroxyl groups in Cu-Al LDHs facilitate slow decay of O 3 resulting in the formation of hydroxyl radicals on the surface, CuO rapidly transforms O 3 into surface-located hydroxyl radicals and/or other oxidants. Futile consumption of surface-located oxidants via interaction with the catalyst surface was minimal for Cu-Al-LDHs; however, it becomes significant in the presence of higher CuO dosages. A mechanistic kinetic model has been developed which adequately describes the experimental results obtained and can be used to optimize the process conditions for the application of HCO.

Iron oxychloride (FeOCl) has been reported to be a highly efficient heterogeneous Fenton catalyst over a wide pH range. In order to determine the true catalytic performance of FeOCl, we simultaneously quantified the adsorptive and oxidative removal of formate, oxalate, and rhodamine-B (RhB) and the formation of RhB oxidation products at both pH 4.0 and 7.0. FeOCl was found to be a poor Fenton catalyst at either pH, as gauged by the oxidation of formate, oxalate, and rhodamine B and the decomposition of H 2 O 2 , in comparison with ferrihydrite (Fhy), one of the most common Fe-containing Fenton catalysts. The adsorption of target contaminants to FeOCl and homogeneous Fenton processes, induced by dissolved iron, resulted in overevaluation of the catalytic performance of FeOCl, especially for (i) the use of strongly adsorbing target compounds, without consideration of the role of adsorption in their removal and (ii) exceedingly high concentrations of H 2 O 2 to remove trace quantities of target contaminants. Overall, this study highlights that the systematic quantification of H 2 O 2 decomposition, target compound adsorption, and oxidation as well as the concentrations of oxidized products formed are prerequisites for unequivocal elucidation of the catalytic nature and reaction mechanism of solid Fenton catalysts.

The contamination of water resources by nitrate is a major problem. Herein, we report a mechanically flexible 2D-MXene (Ti 3 C 2 T x ) membrane with multilayered nanofluidic channels for a selective electrochemical reduction of nitrate to nitrogen gas (N 2 ). At a low applied potential of -0.8 V (vs Ag/AgCl), the MXene electrochemical membrane was found to exhibit high selectivity for NO 3 - reduction to N 2 (82.8%) due to a relatively low desorption energy barrier for the release of adsorbed N 2 (*N 2 ) compared to that for the adsorbed NH 3 (*NH 3 ) based on density functional theory (DFT) calculations. Long-term use of the MXene membrane for treating 10 mg-NO 3 -N L -1 in water was found to have a high faradic efficiency of 72.6% for NO 3 - reduction to N 2 at a very low electrical cost of 0.28 kWh m -3 . Results of theoretical calculations and experimental results showed that defects on the MXene nanosheet surfaces played an important role in achieving high activity, primarily at the low-coordinated Ti sites. Water flowing through the MXene nanosheets facilitated the mass transfer of nitrate onto the low-coordinated Ti sites with this enhancement of particular importance under cathodic polarization of the MXene membrane. This study provides insight into the tailoring of nanoengineered materials for practical application in water treatment and environmental remediation.

Results of investigations into factors influencing contaminant mobility in a replica trench located adjacent to a legacy radioactive waste site are presented in this study. The trench was filled with nonhazardous iron- and organic matter (OM)-rich components, as well as three contaminant analogues strontium, cesium, and neodymium to examine contaminant behavior. Imposed redox/water-level oscillations, where oxygen-laden rainwater was added to the anoxic trench, resulted in marked biogeochemical changes including the removal of aqueous Fe(II) and circulation of dissolved carbon, along with shifts to microbial communities involved in cycling iron ( Gallionella , Sideroxydans ) and methane generation ( Methylomonas , Methylococcaceae ). Contaminant mobility depended upon element speciation and rainfall event intensity. Strontium remained mobile, being readily translocated under hydrological perturbations. Strong ion-exchange reactions and structural incorporation into double-layer clay minerals were likely responsible for greater retention of Cs, which, along with Sr, was unaffected by redox oscillations. Neodymium was initially immobilized within the anoxic trenches, due to either secondary mineral (phosphate) precipitation or via the chemisorption of organic- and carbonate-Nd complexes onto variably charged solid phases. Oxic rainwater intrusions altered Nd mobility via competing effects. Oxidation of Fe(II) led to partial retention of Nd within highly sorbing Fe(III)/OM phases, whereas pH decreases associated with rainwater influxes resulted in a release of adsorbed Nd to solution with both pH and OM presumed to be the key factors controlling Nd attenuation. Collectively, the behavior of simulated contaminants within this replica trench provided unique insights into trench water biogeochemistry and contaminant cycling in a redox oscillatory environment.

With the increasing severity of global water scarcity, a myriad of scientific activities is directed toward advancing brackish water desalination and wastewater remediation technologies. Flow-electrode capacitive deionization (FCDI), a newly developed electrochemically driven ion removal approach combining ion-exchange membranes and flowable particle electrodes, has been actively explored over the past seven years, driven by the possibility of energy-efficient, sustainable, and fully continuous production of high-quality fresh water, as well as flexible management of the particle electrodes and concentrate stream. Here, we provide a comprehensive overview of current advances of this interesting technology with particular attention given to FCDI principles, designs (including cell architecture and electrode and separator options), operational modes (including approaches to management of the flowable electrodes), characterizations and modeling, and environmental applications (including water desalination, resource recovery, and contaminant abatement). Furthermore, we introduce the definitions and performance metrics that should be used so that fair assessments and comparisons can be made between different systems and separation conditions. We then highlight the most pressing challenges (i.e., operation and capital cost, scale-up, and commercialization) in the full-scale application of this technology. We conclude this state-of-the-art review by considering the overall outlook of the technology and discussing areas requiring particular attention in the future.

Heavy metals in industrial wastewaters are typically present as stable metal-organic complexes with their cost-effective treatment remaining a significant challenge. Herein, a self-enhanced decomplexation scenario is developed using an electrochemical membrane filtration (EMF) system for efficient decomplexation and Cu recovery. Using Cu-EDTA as a model pollutant, the EMF system achieved 81.5% decomplexation of the Cu-EDTA complex and 72.4% recovery of Cu at a cell voltage of 3 V. The • OH produced at the anode first attacked Cu-EDTA to produce intermediate Cu-organic complexes that reacted catalytically with the H 2 O 2 generated from the reduction of dissolved oxygen at the cathode to initiate chainlike self-enhanced decomplexation in the EMF system. The decomplexed Cu products were further reduced or precipitated at the cathodic membrane surface thereby achieving efficient Cu recovery. By scavenging H 2 O 2 (excluding self-enhanced decomplexation), the rate of decomplexation decreased from 8.8 × 10 -1 to 4.1 × 10 -1 h -1 , confirming the important role of self-enhanced decomplexation in this system. The energy efficiency of this system is 93.5 g kWh -1 for Cu-EDTA decomplexation and 15.0 g kWh -1 for Cu recovery, which is much higher than that reported in the previous literature (i.e., 7.5 g kWh -1 for decomplexation and 1.2 g kWh -1 for recovery). Our results highlight the potential of using EMF for the cost-effective treatment of industrial wastewaters containing heavy metals.

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 - , SO 4 2- , and NO 3 - 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 O 2 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 .

The formation of vivianite (Fe 3 (PO 4 ) 2 ·8H 2 O) in iron (Fe)-dosed wastewater treatment facilities has the potential to develop into an economically feasible method of phosphorus (P) recovery. In this work, a long-term steady Fe III -dosed University of Cape Town process-membrane bioreactor (UCT-MBR) system was investigated to evaluate the role of Fe transformations in immobilizing P via vivianite crystallization. The highest fraction of Fe II , to total Fe (Fe tot ), was observed in the anaerobic chamber, revealing that a redox condition suitable for Fe III reduction was established by improving operational and configurational conditions. The supersaturation index for vivianite in the anaerobic chamber varied but averaged ∼4, which is within the metastable zone and appropriate for its crystallization. Vivianite accounted for over 50% of the Fe tot in the anaerobic chamber, and its oxidation as it passed through the aerobic chambers was slow, even in the presence of high dissolved oxygen concentrations at circumneutral pH. This study has shown that the high stability and growth of vivianite crystals in oxygenated activated sludge can allow for the subsequent separation of vivianite as a P recovery product.

The recent discovery of magnetic nanoparticles (NPs) in human brain tissue has raised concerns regarding their source and neurotoxicity. As previous studies have suggested that magnetite in urban dust may be the source, we collected urban magnetic dust and thoroughly characterized the nature of ambient urban magnetic dust particles prior to investigating their neurotoxic potential. In addition to magnetite, magnetic dust contained an abundance (∼40%) of elemental iron (Fe 0 ). The coexistence of magnetite and elemental iron was found in magnetic dust particles of inhalable (<10 μm) and nanoscale (<200 nm) size ranges with these particles small enough to enter the human brain via the respiratory tract and olfactory bulbs. The magnetic dust also contained nonferrous water-soluble metals (particularly Cu) that can induce formation of reactive oxygen species (ROS). Previous studies used engineered pure-magnetite for in vitro ROS studies. However, while magnetite was present in all magnetic dust particles collected, engineered pure-magnetite was relatively unreactive and contributed minimally to the generation of ROS. We fill a critical knowledge gap between exposure to heterogeneous ambient iron-particles and in vitro experiments with engineered versus ambient, incidental iron-bearing nanoscale minerals. Our work points to the need to further investigate the presence and properties of magnetic NPs in respirable dust with respect to their potential role in neurodegeneration.

Excited triplet states of natural organic matter ( 3 NOM*) are important reactive intermediates in phototransformation of organic contaminants in sunlit waters. The main goal of this study was to explore the influence of Cu on triplet-sensitized transformation rates of 20 selected phenolic and amine contaminants. Fourteen of the compounds examined exhibited a marked decrease in their 4-carboxybenzophenone (CBBP)-mediated phototransformation rate in the presence of trace amounts of Cu(II) (25-500 nM). Both mathematical modeling of these rate data and transient absorption spectroscopy measurements support the hypothesis that the decrease in the rate and extent of phototransformation of organic contaminants is due to the reduction of radical intermediates of the contaminants by photochemically formed Cu(I). The Cu-induced inhibition of oxidation of organic contaminants photosensitized by Suwannee River NOM (SRNOM) could also take place in the presence of nanomolar concentrations of Cu. The inhibitory effect of Cu on the oxidation rates of amine contaminants in SRNOM solutions was found to be significantly weaker compared to that in CBBP solutions, but little difference was observed on depletion of phenols. This behavior was attributed to the intrinsic inhibitory effect of the antioxidant moieties present in NOM on phototransformation of amine compounds, partially neutralizing the potential for further Cu inhibition.

In this study, we investigate the influence of pH on the kinetics and mechanism of photoreductive dissolution of amorphous iron oxyhydroxide (AFO) in view of the recognition that the light-mediated dissolution of iron oxides controls Fe availability in many natural waters. Our results show that both ligand-to-metal charge transfer (LMCT) and photogenerated superoxide (O 2 •- ) play an important role in AFO photoreductive dissolution in the presence of the chosen surrogate of natural organic matter, Suwannee river fulvic acid (SRFA). The pH dependence of LMCT-mediated AFO photoreductive dissolution is mainly controlled by the influence of pH on AFO solubility. A decrease in pH increases the concentration of the dissolved and more photolabile Fe(III)-SRFA complex present in equilibrium with AFO, a complex in which Fe(III) is readily reduced by LMCT. The pH dependence of superoxide-mediated Fe(III) reduction (SMIR) is also controlled by the influence of pH on AFO solubility with an increase in the dissolved inorganic Fe(III) concentration with the decrease in pH resulting in an increased rate of SMIR. No influence of pH was observed on the steady-state O 2 •- concentration generated on SRFA irradiation as well as the O 2 •- decay rate in the presence of SRFA, suggesting that the concentration and lifetime of O 2 •- are not important factors in controlling the pH dependence of O 2 •- -mediated AFO dissolution. Overall, the results of this study show that the impact of acidification of natural waters on Fe availability will be much more pronounced when Fe is present as iron oxyhydroxide compared to that observed when organically bound Fe dominates with this effect because of the strong dependency of iron oxyhydroxide solubility on pH. The increased rate and extent of dissolution of iron oxyhydroxides on the acidification of natural waters will also have implications to the fate of other contaminants (such as heavy metals and organic compounds) that may be present on the iron oxyhydroxide surface.

Magnéli phase titanium suboxide, Ti 4 O 7 , has attracted increasing attention as a potential electrode material in anodic oxidation as a result of its high efficiency and (electro)chemical stability. Although carbon materials have been amended to Ti 4 O 7 electrodes to enhance the electrochemical performance or are present as an unwanted residual during the electrode fabrication, there has been no comprehensive investigation of how these carbon materials affect the electrochemical performance of the resultant Ti 4 O 7 electrodes. As such, we investigated the electrochemical properties of Ti 4 O 7 electrodes impregnated with carbon materials at different contents (and chemical states). Results of this study showed that while pure Ti 4 O 7 electrodes exhibited an extremely low rate of interfacial electron transfer, the introduction of minor amounts of carbon materials (at values as low as 0.1 wt %) significantly facilitated the electron transfer process and decreased the oxygen evolution reaction potential. The oxygen-containing functional groups have been shown to play an important role in interfacial electron transfer with moderate oxidation of the carbon groups aiding electron uptake at the electrode surface (and consequently organic oxidation) while the generation of carboxyl groups-a process that is likely to occur in long-term operation-increased the interfacial resistance and thus retarded the oxidation process. Results of this study provide a better understanding of the relationship between the nature of the electrode surface and anodic oxidation performance with these insights likely to facilitate improved electrode design and optimization of operation of anodic oxidation reactors.

Copper is a critical trace nutrient and, at higher concentrations, a toxicant in natural waters, with the relative rates of transformation between the Cu(I) and Cu(II) oxidation states being key to its speciation, bioavailability, and toxicity. While the influence of chloride (Cl - ) and natural organic matter on Cu speciation and associated redox transformations has been studied separately, their combined influence on Cu speciation and Cu redox transformations has not been examined. As such, in this study, we investigate the impact of Cl - and Suwannee River fulvic acid (SRFA) on Cu(II) reduction and Cu(I) oxidation kinetics at pH 8.2. SRFA plays a dual role in providing Cu(II) reducing moieties as well as Cu ligating sites. Our results indicate that the SRFA-bound Cu(II) is less reactive than the inorganic Cu(II), and the SRFA-bound Cu(I) being much more rapidly oxidized than the inorganic Cu(I). The presence of Cl - weakens Cu(II) binding by SRFA, thereby increasing the reactivity of Cu(II). Similarly, weakening of Cu(I) binding by SRFA and concomitant binding of Cu(I) by Cl - stabilizes Cu(I). Our results further show that continuous formation of hydrogen peroxide occurs in the presence of Cu(II), SRFA, and Cl - in air-saturated solution with the presence of H 2 O 2 enhancing the dynamic nature of the system.

The oxygenation of mackinawite (FeS) frequently occurs at anoxic-oxic interfaces in both natural and engineered systems such as intertidal sediment, in activated sludge in water treatment processes, and during sulfidized zero-valent iron particle corrosion. During reoxygenation events, FeS may drive a Fenton-like process leading to the production of strong oxidants though the details of this process are poorly understood. In this study, benzoic acid (BA) has been used to probe both the magnitude and identity of these strong oxidants under circumneutral pH conditions. The major product of BA oxidation during FeS oxygenation was found to be 2,5-dihydroxybenzoic acid (2,5-DHBA) rather than monohydroxybenzoic acids identified to be the major products in a range of hydroxyl radical (HO · )-dominated systems. Based upon relative reactivity with other competitive probes and nature of the hydroxybenzoate product distribution, it is hypothesized that the strong oxidant must be a surface-localized entity such as high-valent iron or surface-associated hydroxyl or sulfur-based radicals with reactivity differing from those formed in free solution. The importance of both the reactivity of the oxidant and adsorption of the substrate to the surface is demonstrated.

While flow-electrode CDI is a promising desalination technology that has major advantages when the electrodes are operated in the short-circuited closed-cycle (SCC) mode, little attention has been paid to the water recovery rate, which, in the SCC mode, is determined by the need for partial replacement of the saline electrolyte of the flow electrodes. Results of this study show that an extremely high water recovery rate of ∼95% can be achieved when desalting a 1000 mg NaCl L -1 brackish influent to a potable level of 150 mg L -1 . The improved performance with regard to the electrical cost is related, at least in part, to the alleviated concentration polarization at the membrane/electrolyte interface during electrosorption. In effect, the current efficiency decreases with an increase in the water recovery rate. This finding is ascribed to inevitable co-ion leakage since the flow electrodes reject ions with the same charge. In addition, water transport across the ion exchange membranes also influences the water recovery rate. The effect of partial replacement of the saline electrolyte during (semi-)continuous operation requires particular consideration because the associated dilution of the carbon content in the flow electrodes results in a decrease in process performance.

Two major pathways are reported to account for photochemical reduction of Fe(III) in sunlit surface waters, namely, ligand-to-metal charge transfer (LMCT) and superoxide-mediated iron reduction (SMIR). In this study, we investigate the impact of Fe(III) speciation (organically complexed (Fe(III)L versus iron oxyhydroxide (AFO)) on Fe(III) reducibility by photogenerated superoxide (O 2 • - ) and LMCT. To simulate conditions typical of fresh, estuarine, and coastal waters, we have used Suwannee River Fulvic Acid (SRFA) as a representative of the natural organic matter likely to associate with Fe(III). Our results show that the photolabile Fe(III)SRFA complex is reduced rapidly by LMCT, while O 2 • - does not play a role in reduction of these entities. In contrast, the relatively less photolabile AFO is reduced by both O 2 • - and LMCT. The reduction of AFO by O 2 • - occurs following the dissolution of AFO, and hence, the contribution of O 2 • - to reductive dissolution of AFO is dependent on conditions such as the age of the AFO and initial AFO concentration affecting the rate of dissolution of AFO. Our results further show that while colloidal Fe(III) (in this work, particles >0.025 μm) is reduced by O 2 • - , there is no involvement of O 2 • - in dissolved Fe(III) reduction. Overall, our results show that superoxide-mediated iron reduction will be important only in natural waters containing limited concentrations of Fe binding ligands.

Flow-electrode capacitive deionization (FCDI) is an emerging electrochemically driven technology for brackish and/or sea water desalination with merits of large salt adsorption capacity, high flow efficiency, and easy electrode management. While FCDI holds promise for continuous operation, there are very few investigations with regard to the regeneration/reuse of flowable electrodes and the separation of brine from electrodes with these operation prerequisites for real nonintermittent water desalination. In this study, we propose a novel module design to achieve these critical steps involving integration of an FCDI cell and a ceramic microfiltration (MF) contactor. Our investigations reveal that the brine discharge rate is the dominant factor for stable and efficient operation of the integrated module. Results obtained show that the integrated FCDI/MF system can be used to successfully separate brackish water (of salinities 1, 2 and 5 g L -1 ) into both a potable stream (<0.5 g L -1 ) and a brine stream (concentrated by 2-20 times) in a continuous manner with extremely high water recovery rates (up to 97%) and reasonable energy consumption. Another notable characteristic of the integrated system is the high thermodynamic energy efficiency (∼30%) with such efficiencies 4-5 times larger than those of conventional capacitive deionization units and comparable to reverse osmosis and electrodialysis systems achieving similar separation efficiencies. In brief, the results of studies described here indicate that continuous and efficient operation of FCDI is a real possibility and pave the way for scale-up of this emerging technology.

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 cm 2 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.

By mimicking iron(Fe)-based phosphorus (P) immobilization in natural environments, an Fe-retrofitted UCT-MBR involving in situ vivianite crystallization for removing and recovering P from sewage was developed, and its performance was examined in this work. We show that dosing of ferrihydrite, once biological P uptake reached its limit, enabled effective ongoing P removal; whereas conventional conditions in the anaerobic chamber of the University of Cape Town (UCT) system (i.e., a sludge retention time of hours and a completely mixed sludge phase) was insufficient for a satisfactory Fe(III) bioreduction, with the overaccumulation of Fe(III) as fine particles finally resulting in severe membrane fouling and collapse in P removal. The enhancement of reductive conditions in the anaerobic chamber by lowering agitation and adding biocarriers to favor Fe(III) reduction was found to be effective in enabling ongoing P removal and recovery. The average level of effluent P was as low as 0.18 mg/L for a period of 258 d under this condition. Using chemical and spectroscopic methods, the P product was identified as primarily vivianite: Fe 3 (PO 4 ) 2 ·8H 2 O. The in situ crystallization of vivianite as a sink for P enabled the UCT-MBR to continuously remove and recover sewage P with no need for sludge discharge.

In this work, we investigate the impact of natural organic matter (NOM) and light on silver nanoparticle (AgNP) dissolution kinetics with particular emphasis on determining the (i) mechanism via which NOM affects the oxidative dissolution of AgNPs, (ii) the role of photogenerated organic radicals and reactive oxygen species (ROS) in oxidative dissolution of AgNPs, and (iii) the mechanism of formation of AgNPs in NOM solution under dark and irradiated conditions. We measured the oxidation of citrate stabilized AgNPs by O 2 and hydrogen peroxide (H 2 O 2 ) in the dark and in irradiated Suwannee River fulvic acid (SRFA) solutions at pH 8.0. Results show that the reactivity of AgNPs toward O 2 and H 2 O 2 in the dark decreased in the presence of SRFA as a result of blocking of AgNP surface sites through either steric or electrostatic effects. Irradiation promoted dissolution of AgNPs by O 2 and H 2 O 2 in the presence of low concentrations (≤20 mg·L -1 ) of SRFA as a result of contribution from photogenerated H 2 O 2 formed on irradiation of SRFA as well as photofragmentation of AgNPs. Furthermore, our results show that photogenerated superoxide can induce formation of AgNPs by reducing Ag(I) ions. Based on our experimental results, we have developed a kinetic model to explain AgNP transformation by O 2 and H 2 O 2 in the dark and in irradiated SRFA solutions with this model of use in predicting the transformation and fate of AgNPs in natural waters.

Unacceptably high uranium concentrations in decentralized and remote potable groundwater resources, especially those of high hardness (e.g ., high Ca 2+ , Mg 2+ , and CO 3 2- concentrations), are a common worldwide problem. The complexation of alkali earth metals, carbonate, and uranium(VI) results in the formation of thermodynamically stable ternary aqueous species that are predominantly neutrally charged (e.g ., Ca 2 (UO 2 )(CO 3 ) 3 0 ). The removal of the uncharged (nonadsorbing) complexes is a problematic issue for many water treatment technologies. As such, we have evaluated the efficacy of a recently developed electrochemical technology, termed flow-electrode capacitive deionization (FCDI), to treat a synthetic groundwater, the composition of which is comparable to groundwater resources in the Northern Territory, Australia (and elsewhere worldwide). Theoretical calculations and time-resolved laser fluorescence spectroscopy analyses confirmed that Ca 2 (UO 2 )(CO 3 ) 3 0 was the primary aqueous species followed by Ca(UO 2 )(CO 3 ) 3 2- (at circumneutral pH values). Results under different operating conditions demonstrated that FCDI is versatile in reducing uranium concentrations to <10 μg L -1 with low electrical consumption (e.g ., ∼0.1 kWh m -3 ). It is concluded that the capability of FCDI to remove uranium under these common conditions depends on the dissociation kinetics of the Ca 2 (UO 2 )(CO 3 ) 3 0 complex in the electrical field. The subsequent formation of the negatively charged Ca(UO 2 )(CO 3 ) 3 2- species results in the efficient transport of uranium across the anion exchange membrane followed by immobilization on the positively charged flow (anode) electrode.

We have previously described a novel flow-electrode capacitive deionization (FCDI) unit combined with a hydrophobic gas-permeable hollow fiber membrane contactor (designated "CapAmm") and presented results showing efficient recovery of ammonia from dilute synthetic wastewaters (Zhang et al., Environ. Sci. Technol. Lett. 2018, 5, 43-49). We extend this earlier study here with description of an FCDI system with integrated flat sheet gas permeable membrane with comprehensive assessment of ammonia recovery performance from both dilute and concentrated wastewaters. The integrated CapAmm cell exhibited excellent ammonia removal and recovery efficiencies (up to ∼90% and ∼80% respectively). The energy consumptions for ammonia recovery from low-strength (i.e., domestic) and high-strength (i.e., synthetic urine) wastewaters were 20.4 kWh kg -1 N and 7.8 kWh kg -1 N, respectively, with these values comparable to those of more conventional alternatives. Stable ammonia recovery and salt removal performance was achieved over more than two days of continuous operation with ammonia concentrated by ∼80 times that of the feed stream. These results demonstrate that the integrated CapAmm system described here could be a cost-effective technology capable of treating wastewaters and realizing both nutrient recovery and water reclamation in a sustainable manner.

The impact of the pH of natural waters on the various pathways contributing to the formation and decay of Fe(II) in the presence of Suwannee River Fulvic Acid (SRFA) is investigated in this study. Our results show that thermal Fe(III) reduction occurs as a result of the presence of hydroquinone-like moieties in SRFA with the rate of Fe(III) reduction by these entities relatively invariant with change in pH in the range 6.8-8.7. The Fe(II) oxidation rate in the dark is controlled by its interaction with O 2 and increases with increase in pH with the overall outcome that the steady-state Fe(II) concentration in the dark is strongly affected by solution pH. On irradiation, a portion of the hydroquinone-like moieties present are oxidized to form semiquinones that are capable of reducing Fe(III) and/or oxidizing Fe(II) under circumneutral pH conditions. The extent of photogeneration of semiquinones on irradiation of SRFA and the persistence of these radicals increases significantly with decrease in pH. Due to the higher concentration and longevity of these organic moieties under low pH conditions, the impact of pH on steady-state Fe(II) concentration is less pronounced in previously irradiated SRFA solution compared to that observed in dark SRFA solution. Under irradiated conditions, the rates of Fe transformation (including both Fe(II) oxidation and Fe(III) reduction) are nearly independent of pH. While ligand-to-metal charge transfer (LMCT) is the dominant pathway for photochemical Fe(III) reduction, Fe(II) oxidation under irradiated conditions mainly occurs as a result of interaction with O 2 , semiquinones and other short-lived oxidants. Overall, our data supports the conclusion that, as a result of the contribution from photogenerated organic moieties to Fe redox transformations, the steady-state Fe(II) concentration in irradiated surface waters containing natural organic matter may not be impacted significantly by changes in pH.

Nanoscale zerovalent iron (NZVI) is one of the most extensively studied nanomaterials in the fields of wastewater treatment and remediation of soil and groundwater. However, rapid oxidative transformations of NZVI can result in reduced NZVI reactivity. Indeed, the surface passivation of NZVI is considered one of the most challenging aspects in successfully applying NZVI to contaminant degradation. The oxidation of NZVI can lead to the formation of Fe II -bearing phases (e.g., Fe II O, Fe II (OH) 2 , Fe II Fe III 2 O 4 ) on the NZVI surface or complete oxidation to ferric (oxyhydr)oxides (e.g., Fe III OOH). This corrosion phenomenon is dependent upon various factors including the composition of NZVI itself, the type and concentration of aqueous species, reaction time and oxic/anoxic environments. As such, the coexistence of different Fe oxidation states on NZVI surfaces may also, in some instances, provide a unique reactive microenvironment to promote the adsorption of contaminants and their subsequent transformation via redox reactions. Thus, an understanding of passivation chemistry, and its related mechanisms, is essential not only for effective NZVI application but also for accurately assessing the positive and negative effects of NZVI surface passivation. The aim of this review is to discuss the nature of the passivation processes that occur and the passivation byproducts that form in various environments. In particular, the review presents: (i) the strengths and limitations of state-of-the-art techniques (e.g., electron microscopies and X-ray-based spectroscopies) to identify passivation byproducts; (ii) the passivation mechanisms proposed to occur in anoxic and oxic environments; and (iii) the effects arising from synthesis procedures and the presence of inorganics/organics on the nature of the passivation byproducts that form. In addition, several depassivation strategies that may assist in increasing and/or maintaining the reactivity of NZVI are considered, thereby enhancing the effectiveness of NZVI in contaminant degradation.

The engineered silver nanoparticles (AgNPs) used in consumer products are ultimately released to the environment either as Ag(0), silver sulfide (Ag 2 S(s)), silver chloride (AgCl(s)), and/or dissolved Ag(I) complexes. Of these, AgCl(s) and Ag 2 S(s) exhibit semiconducting properties and hence may have significant implications to oxidant generation and subsequent redox transformations in natural waters. In this work, we investigate the transformation and photoreactivity of AgCl(s) under simulated natural water conditions with the photoreactivity probed by measuring the oxidation of formate (HCOO - ), a simple compound with a well-defined oxidation pathway. Our results show that AgCl(s) undergoes rapid dissolution in the presence of chloride concentrations representative of seawater (ca. 0.5 M NaCl) forming dissolved Ag(I) complexes but is stable in fresh waters and slightly brackish waters (≤200 mM NaCl). We further show that under these lower salinity conditions in which AgCl(s) is stable, pH has a significant impact on the reactivity of semiconducting AgCl(s). The photoreactivity (measured as initial HCOO - oxidation rate) of AgCl(s) is relatively constant at pH 4.0 for periods of 24 h or more; however, it decreases rapidly under alkaline conditions. The rapid transformation (or "aging") of AgCl(s) under alkaline conditions suggests that AgCl(s), potentially transported through wastewater effluent to fresh or brackish water environments, may not have a significant impact in such environments. In comparison, in situ formed AgCl(s), potentially formed as a result of the oxidation of high concentrations (≥60 μg Ag·L -1 ) of Ag(0) and/or Ag 2 S(s), may have significant implications to oxidant generation in natural waters. Our results further show that rapid cycling of Ag between the 0 and +I redox states in sunlit surface waters as a result of the presence of AgNP oxidants (such as H 2 O 2 and organic radicals) will further enhance the rate and extent of oxidant generation by AgCl(s).

While flow-electrode capacitive deionization (FCDI) is an emerging desalination technology, reduction in water hardness using this technology has so far received minimal attention. In this study, treatment of influents containing both monovalent and divalent cations using FCDI was carried out with flow-electrodes operated in short-circuited closed-cycle (SCC) configuration. Divalent Ca 2+ cations were selectively removed compared to monovalent Na + with the selectivity becoming dominant when the FCDI unit was operated at lower current densities and hydraulic retention times. Results showed that SCC FCDI operation was much more energy-efficient for brackish water softening compared to operation in isolated closed-cycle (ICC) mode, particularly with implementation of energy recovery. This finding was largely ascribed to (i) charge neutralization of the flow-electrodes in SCC configuration and (ii) regeneration of the active materials to maintain pseudo "infinite" capacity during electrosorption. In addition, mixing of the flow-electrodes in SCC operation significantly inhibited pH excursion in the flow-electrode with resultant alleviation of calcium precipitation on the carbon surface.

The triplet excited state of natural organic matter ( 3 NOM*) is an important reactive intermediate in sensitizing transformation of a wide range of environmentally relevant organic compounds, but the impact of trace metals on the fate and reactivity of 3 NOM* is poorly understood. In this study, we investigate the effect of low concentrations of copper on 3 NOM*-mediated oxidation (electron transfer) and energy transfer reactions. The oxidative efficiency of 3 NOM* from Suwannee River NOM (SRNOM) and the widely used model triplet sensitizer 4-carboxybenzophenone were determined by measuring the photooxidation of 2,4,6-trimethylphenol (TMP). The pseudo-first-order photooxidation rate constants of TMP decreased markedly in the presence of trace amounts of Cu(II) (25-500 nM) with the decrease associated with the continuous reduction of the oxidation intermediates of TMP (i.e., TMP • (-H) ) by the photochemically produced Cu(I). A kinetic model is developed that adequately describes the Cu inhibition effect in TMP photooxidation in irradiated SRNOM solutions. The 3 NOM* energy transfer ability was assessed by measuring the isomerization of sorbic acid with the rate of this process markedly retarded in the presence of significantly higher (micromolar) concentrations of Cu(II) than previously used. This result is attributed to (i) decreased formation of high energy 3 NOM* due to formation of Cu-NOM complexes and (ii) increased loss of 3 NOM* as a result of quenching by Cu. Since 3 NOM* is the precursor to singlet oxygen ( 1 O 2 ) formation, the steady-state concentrations of 1 O 2 also decreased in the presence of micromolar concentrations of Cu(II) with the quenching rate constant of 3 NOM* by Cu calculated to be 1.08 × 10 10 M -1 s -1 .

The presence of Good's buffers caused rapid ZVI corrosion and a dramatic release of Fe(II) leading to the Fe(II)-catalyzed transformation of ferrihydrite to lepidocrocite and/or the direct formation of lepidocrocite from the oxidation of Fe(II) in the pH range 4.0-6.2. In comparison, in the absence of Good's buffers, elution of Fe(II) was insignificant with ferrihydrite being the only Fe(III) oxyhydroxide detected following the oxidative transformation of ZVI. The rapid ZVI corrosion in the presence of Good's buffer is possibly due to either (i) disruption of the Fe oxide surface layer as a result of attack by Good's buffers and/or (ii) interaction of Good's buffer with the outer Fe oxide surface and surface-associated Fe(II)/Fe(III) causing the Fe oxide surface layers to be more porous with both these processes facilitating continuous O 2 access to the Fe(0) core and allowing the diffusion of Fe atoms outward. Our results further show that the deprotonated forms of Good's buffers and the surface charge of the Fe oxides formed at the ZVI surface strongly affect the sorption of the target compound (i.e., formate) and hence the oxidation of these compounds via surface-associated Fe(II)-mediated heterogeneous Fenton processes.

Iron (oxyhydr)oxides are widespread in natural and engineered systems, potent adsorbents of contaminants and a source of energy for iron-reducing bacteria. Microbial reduction of iron (oxyhydr)oxides results in the formation of Fe(II) which can induce the transformation of these iron minerals, typically from less crystalline to more crystalline forms, affecting the biogeochemical cycling of iron and the behavior of any species adsorbed to the iron (oxyhydr)oxides. Factors influencing the transformation rate of the poorly crystalline iron (oxyhydr)oxide, ferrihydrite, to more crystalline forms in the presence of the iron reducing bacterium Shewanella oneidensis MR-1 are investigated under controlled laboratory conditions in this work. In particular, the amount of Fe(II) produced increased the transformation rate while increasing concentrations of the electron donor, lactate, decreased the rate. Using kinetic parameters determined from abiotic controls, the results of transformation experiments in the presence of Shewanella oneidensis were modeled with this exercise revealing that less goethite and more lepidocrocite formed than expected. Conversely, studies using the Shewanella exudate only, containing biogenic Fe(II), displayed rates of transformation that were satisfactorily modeled using these abiotic control kinetic parameters. This result suggests that the physical presence of the microbes is pivotal to the reduction in ferrihydrite transformation rate observed in the biotic experiments relative to the analogous abiotic controls.

In this study, temporal changes in the redox properties of three 0.5 g/L smectite suspensions were investigated-a montmorillonite (MAu-1) and two nontronites (NAu-1 and NAu-2) in the presence of 1 mM aqueous Fe(II) at pH 7.8. X-ray absorption spectroscopy revealed that the amount of Fe(II) added quantitatively transformed into chloride-green rust (Cl-GR) within 5 min and persisted over 18 days. Over the same time, the reduction potential of all three suspensions increased by 50 to 150 mV to equilibrate at approximately -100 mV vs SHE. The reduction of a model organic contaminant, 4-chloronitrobenzene (4-CINB), also became increasingly slower over time with virtually no 4-CINB reduction being observed after 18 days. There was a strong correlation between reduction potential and the quantity of 4-ClNB reduced by MAu-1, although other factors were likely involved in the decreased redox reactivity observed in the nontronites. It is hypothesized that the temporal increase in reduction potential results from clay mineral dissolution resulting in increased Fe(III) contents in the Cl-GR. These results demonstrate that long-term studies are recommended to accurately predict contaminant management strategies.