Abstract

Bioelectrochemical reduction of dilute nitrate (NO3-; sub-mM to low-mM range) to ammonium (NH4+) offers a promising route toward circular nitrogen management from contaminated groundwater and environmental waters. However, on-site application of bioelectrochemical systems remains challenging due to low reduction rates and poor electron transfer efficiency of naturally formed biofilm electrodes. Here, we constructed a hydrogel biocathode by applying a carbon black/riboflavin/sodium alginate/cellulose hydrogel incorporating Shewanella oneidensis MR-1 cells to a graphite felt electrode via brush coating. The hydrogel electrode achieved NH4+ production rates of 0.16-0.19 mol m-3 h-1 without NO2- accumulation, and these rates were maintained without significant performance loss across three consecutive cycles with medium exchange over 1.5 days of total operation. The hydrogel electrode increased the current density by more than 5-fold compared with a conventional S. oneidensis biofilm electrode, indicating enhanced electron transfer efficiency per unit biomass, which directly contributed to the high NH4+ production rates. The electricity consumption for NH4+ production of 1.68-2.39 x 102 kJ g-N-1 was substantially lower than that of metal catalyst systems at comparable NO3- concentrations (typically, >104 kJ g-N-1). These findings demonstrate that the hydrogel electrode design represents an energy-efficient and readily fabricated platform for bioelectrochemical NH4+ production from dilute NO3-.