Nitrite (NO2 –) is an abundant contaminant in nature that threatens human health. A catalytic process that converts NO2 – to less harmful products has been proven to be an effective strategy for NO2 – removal. Most previous studies, however, targeted selectivity toward N2 using Pd catalysts, which severely limits the potential for the recovery of value-added byproducts from the catalytic process. Here, we report experimental and theoretical evidence that both Ir and Cu x Ir(100–x) nanoparticles possess near 100% NH3 selectivity for NO2 – reduction compared to the <1% NH3 selectivity achieved by nano-Pd. These NH3-selective catalysts could be useful for both water purification and ammonia production.

Nitrate (NO3 –) has impacted more groundwater supplies than any other pollutant in the world. It is currently removed at water treatment plants by ion exchange, which is effective but comes at a steep financial and environmental cost. (Electro)­catalytic treatment of nitrate has emerged as a promising alternative technology, which relies on reducing nitrate to dinitrogen gas or ammonium via reduction on a bimetal catalyst with atomic hydrogen oxidation. The bimetal catalyst contains a platinum group metal, and atomic hydrogen is either generated from supplied hydrogen gas (catalytic) or an applied current (electrocatalytic). However, (electro)­catalytic treatment of nitrate is not being implemented at water treatment plants. This perspective addresses the most important technical challenges limiting widespread adoption of (electro)­catalytic nitrate removal in drinking water treatment. These challenges affect precious metal amounts and cost, the efficiency and safety of hydrogen use, and end-product selectivity. This perspective is concluded by a prioritization of technology challenges, and their implications for attracting industry investment and achieving regulatory acceptance.

The influence of solution chemistry on the adsorption of human serum albumin (HSA) proteins on graphene oxide (GO) was investigated through batch adsorption experiments and the use of a quartz crystal microbalance with dissipation (QCM-D). The conformation of HSA layers on GO was also examined with the QCM-D. Our results show that an increase in ionic strength under neutral pH conditions resulted in stronger binding between HSA and GO, as well as more compact HSA layers on GO, emphasizing the key role of electrostatic interactions in controlling HSA-GO interactions. Calcium ions also facilitated HSA adsorption likely through charge neutralization and bridging effect. At physiological ionic strength conditions (150 mM), maximum HSA adsorption was observed at the isoelectric point of HSA (4.7). Under acidic conditions, the adsorption of HSA on GO led to the formation of protein layers with a high degree of fluidity due to the extended conformation of HSA. Finally, the attachment of GO to a supported lipid bilayer that was composed of zwitterionic 1,2-dioleoyl-sn-glycero-3-phosphocholine, a model for cell membranes, was reduced in the presence of protein coronas. This reduction in GO attachment was influenced by the conformation of the protein coronas on GO.

A parallel-plate thin-layer (PPTL) flow reactor with potential control and custom-made cathode of Pd−In modified activated carbon cloth was developed for electrocatalytic removal of nitrite from water; the effect of applied potential and flow rate were investigated. Compared to other reactors in the literature, rapid nitrite reduction (first-order rate constant is 0.38 L g Pd −1 min −1 ), high current efficiency (CE, 51%), and low ammonium selectivity (5.4%) were observed at an applied potential of −0.60 V vs the Ag/ AgCl reference electrode (RE) and a flow rate of 40 mL min −1 in a phosphate buffer solution of pH 6.5. Slightly faster kinetics were observed at more negative potentials (0.57 L g Pd −1 min −1 at −1.0 V/RE), but then ammonium production (88%), H 2 gas evolution (E 0 = −0.61 V/RE), and current loss (CE < 10%) became problematic. Nitrite reduction was measured in the PPTL flow reactor for almost 50 2 h cycles over six months, with little apparent loss (30%) in activity. A reactive transport model was developed and used to simulate the kinetic data. The fitted intrinsic rate constant (k w ) was 5.2 × 10 −6 m s −1 , and ratios of dimensionless Nusselt numbers to reaction rate constant values support the reactor being more reaction than mass transfer limited. Application of the parametrized model demonstrated how the PPTL reactor could be scaled (e.g., cathode dimensions, flow channel thickness), operated (i.e., flow rate), or modified (i.e., greater intrinsic catalyst activity) to most efficiently remove nitrite from larger flow streams.

Novel nanomaterials, such as carbon nanofibers (CNFs), present a unique opportunity to advance photoelectrochemical drinking water treatment by integrating a photocatalyst to improve material properties and performance. To this end, we have fabricated electrospun CNF-TiO2 composite photoelectrodes with high surface area, conductivity, chemical stability, and mechanical strength for use in photoelectrochemical water treatment applications. The CNF-TiO2 physical, chemical, and electrical properties can be tailored to influence drinking water pollutant transformation pathways by selectively varying fabrication parameters (e.g. TiO2 content, carbonization temperature). Integrating the photocatalyst into the nonwoven CNF framework provides a material that transforms emerging organic pollutants with diverse chemical properties via photochemical (UV/Vis radiation), electrochemical (applied potential) and photoelectrochemical (UV/Vis radiation + applied potential) processes at circumneutral pH. The favored transformation pathway is primarily influenced by carbonization temperature, which controls the CNF-TiO2 electrical conductivity and crystal structures. Electrical resistance decreases almost logarithmically with increasing carbonization temperatures (450 to 1000 °C) to produce composites with more orderly carbon structure (graphitic) and more conductive photocatalyst (rutile TiO2). At higher carbonization temperatures (≥ 1000 °C), pollutants with diverse chemical properties (e.g. log Kow) rapidly sorb to the composite electrode surface. The effect of CNF-TiO2 composite structure on pollutant sorption dramatically overwhelms sorption effects due to pollutant chemical properties. At lower carbonization temperatures (< 1000 °C), composite properties provide minimal pollutant sorption. While composites processed at higher carbonization temperatures encourage direct electrochemical pollutant transformation at photoelectrode surface, lower carbonization temperatures suggest composites are well-suited for indirect photochemical pollutant transformation. Herein we present photoelectrodes carbonized at different temperatures and their performance in transforming model organic pollutants. Outcomes of this work will help identify the types and properties of next-generation photoelectrode materials that are most promising for improving photoelectrochemical cells purposed for drinking water treatment.