Nitrate (NO3-) is one of the most widespread groundwater contaminants. Reducing NO3- in water and wastewater to ammonia or nitrogen gas is of great interest for pollution control and resource recovery. Compared to biological processes, electrocatalytic reduction of NO3- has the advantages of faster kinetics, simple operation, and no further treatments for wastes. The transformation of NO3- to NO2- is the first and rate-limiting step of the reaction chain, during which NO3- adsorption onto the electrode surface is an essential initiation. However, electric repulsion between electrode and NO3- hindered surface adsorption and thus decreased reduction efficiency. Here we designed a modulated polarity operation in electrocatalytic reduction of NO3- using Fe nanoparticle catalysts, which overcame the mass transfer limitation by enhancing surface adsorption of NO3- to improve reduction efficiency. Reduction of NO3- was conducted by a three-electrode system in an H cell. For polarity modulation operation, two potential sections (adsorption, reduction) were designed. In the adsorption section, the potential was +0.1 V (vs. RHE) for 30 s, the electrode was charged as anode for NO3- adsorption. In the following reduction section, the potential was -0.7 V for 270 s, and the electrode was cathode for NO3- reduction/repulsion. Both sections performed in turns for 24 cycles. For the constant polarity operation, only one potential is set as -0.7 V constantly. 3.98 mmol L-1 of NO3- was removed by carbon electrode with constant polarity. In comparison, the Fe catalyst achieved 12.88 and 16.22 mmol L-1 removal at constant and modulated polarity, respectively. The faradic efficiency performed the same trend that 74% for Fe in polarity modulation, 52% for Fe in constant polarity, and 31% for carbon. These results confirmed that Fe catalyst possessed higher catalytic activity than carbon, and the polarity modulation could further improve the NO3- reduction. For the kinetic study, adsorption and reduction time length would also influence the reduction rate. Optimized time lengths (30 s adsorption +270 s reduction) demonstrated sufficient surface adsorption of NO3- with minimum re-oxidation effect, thus resulting in better reduction performance. Modulated polarity provides a feasible and effective way to improve electrocatalytic NO3- removal, and due to the same mechanisms, this method can also be applied in other transfer-limited electrochemical processes.

Polarity modulation enhances electrocatalytic reduction of nitrate by iron nanoparticle catalysts

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.