Self-Activated Ni Cathode for Electrocatalytic Nitrate Reduction to Ammonia: From Fundamentals to Scale-Up for Treatment of Industrial Wastewater.

Electrocatalytic reduction has recently received increasing attention as a method of converting waste nitrate into value-added ammonia, but most studies have focused on complex strategies of catalyst preparation and little has been done in the way of large-scale demonstrations. Herein, we report that in situ activation of a pristine Ni electrode, either on a lab scale or a pilot scale, is effective in facilitating nitrate reduction to ammonia, exhibiting extraordinarily high activity, selectivity, and stability. The self-activated Ni cathode has a robust capacity to reduce nitrate over a wide range of concentrations and achieves great conversion yield, NH4+-N selectivity, and Faradaic efficiency, respectively, 95.3, 95.5, and 64.4% at 200 mg L-1 NO3--N and 97.8, 97.1, and 90.4% at 2000 mg L-1 NO3--N, for example. Fundamental research indicates that Ni(OH)2 nanoparticles are formed on the Ni electrode surface upon self-activation, which play crucial roles in governing nitrate reduction reaction (NO3RR) through the atomic H*-mediated pathway and accordingly suppressing hydrogen evolution reaction. More importantly, the self-activated Ni(OH)2@Ni cathode can be easily scaled up to allow large volumes of real industrial wastewater to be processed, successfully transferring nitrate into ammonia with high yields and Faradaic efficiency. This study demonstrates a new, mild, and promising method of cleaning nitrate-laden wastewater that produces ammonia as a valuable byproduct.

Discovering the Importance of ClO• in a Coupled Electrochemical System for the Simultaneous Removal of Carbon and Nitrogen from Secondary Coking Wastewater Effluent.

Inorganic constituents in real wastewater, such as halides and carbonates/bicarbonates, may have negative effects on the performance of electrochemical systems because of their capability of quenching HO•. However, we discovered that the presence of Cl- and HCO3- in an electrochemical system is conducive to the formation of ClO•, which plays an important role in promoting the simultaneous elimination of biorefractory organics and nitrogen in secondary coking wastewater effluent. The 6-h operation of the coupled electrochemical system (an undivided electrolytic cell with a PbO2/Ti anode and a Cu/Zn cathode) at a current density of 37.5 mA cm-2 allowed the removal of 87.8% of chemical oxygen demand (COD) and 86.5% of total nitrogen. The electron paramagnetic resonance results suggested the formation of ClO• in the system, and the probe experiments confirmed the predominance of ClO•, whose steady-state concentrations (8.08 × 10-13 M) were 16.4, 26.5, and 1609.5 times those of Cl2•- (4.92 × 10-14 M), HO• (3.05 × 10-14 M), and Cl• (5.02 × 10-16 M), respectively. The rate constant of COD removal and the Faradaic efficiency of anodic oxidation obtained with Cl- and HCO3- was linearly proportional to the natural logarithm of the ClO• concentration, and the specific energy consumption was inversely correlated to it, demonstrating the crucial role of ClO• in pollutant removal.

Fe2+/HClO Reaction Produces FeIVO2+: An Enhanced Advanced Oxidation Process.

The reaction between Fe2+ and HClO constitutes a promising advanced oxidation process (AOP) for removing pollutants from wastewater, and •OH has been considered the dominant reactive oxidant despite limited evidence for this. Herein, we demonstrate that the Fe2+/HClO reaction enables the production of FeIVO2+ rather than •OH in acid medium, a finding that is strongly supported by multiple lines of evidence. Both X-ray absorption near-edge structure spectroscopic tests and Mössbauer spectroscopic tests confirmed the appearance of FeIVO2+ as the reactive intermediate in the reaction between Fe2+ and HClO. The determination of FeIVO2+ generation was also derived from the methyl phenyl sulfoxide (PMSO)-based probe experiments with respect to the formation of PMSO2 without •OH adducts and the density functional theory studies according to the lower energy barrier for producing FeIVO2+ compared with •OH. A dual-anode electrolytic system was established for the in situ generation of Fe2+ and HClO that allows the production of FeIVO2+. The system exhibits an enhanced capacity for oxidizing a model pollutant (e.g., phosphite) from industrial wastewater, making it an attractive and promising AOP for the abatement of aqueous contaminants.

One-Step Treatment of Phosphite-Laden Wastewater: A Single Electrochemical Reactor Integrating Superoxide Radical-Induced Oxidation and Electrocoagulation.

Elimination of phosphite from water is more difficult than elimination of phosphate owing to its higher solubility and resistance to biotransformation. Herein, we report an efficient, facile, electrochemical method integrating electrooxdiation (EO) and electrocoagulation (EC) to treat phosphite-laden wastewater. The mechanistic studies demonstrate that in-situ-generated Fe2+ at an Fe anode can react with in-situ-generated O2 at a mixed metal oxide (MMO) anode, leading to formation of •O2-, a reactive species predominantly responsible for oxidation of phosphite to phosphate. The phosphate is immediately coagulated by Fe hydroxides that are formed due to the production of OH- at a stainless-steel cathode. The integrated EO/EC system enables a phosphite removal efficiency of 74.25% (MMO anode, 100 mA; Fe anode, 100 mA; reaction time, 60 min), a significantly higher efficiency rate than the rate obtained in the control experiments in the absence of an MMO anode (<23.41%) and the rate obtained with the chemical coagulation process (<5.03%). The quenching experiments with scavengers and electron spin resonance tests verify the pivotal role of •O2- in transformation of phosphite. Tests carried out with nickel-plating wastewater further demonstrate the superiority of this integrated system, as evidenced by efficient removal of phosphite and nickel from the solution.