Inorganic bioelectric system for nitrate removal with low N2O production at cold temperatures of 4 and 10 °C
Mingyi Xu, Francesco Savio, Charlotte Kjærgaard, Marlene Mark Jensen, Adam Kovalovszki, Barth F. Smets, Borja Valverde‐Pérez, Yifeng Zhang
Abstract
• Nearly complete denitrification at 4 and 10 °C driven by bioelectrodes. • Significantly reduced N 2 O emissions at 4 or 10 °C in comparison to 20 °C. • Shifts in dominant denitrifiers to Chryseobacterium at 4 °C and Dechloromonas at 10 °C. • Elevated gene levels associated with electrophilic denitrification. Groundwater, essential for ecological stability and freshwater supply, faces escalating nitrate contamination. Traditional biological methods struggle with organic carbon scarcity and low temperatures, leading to an urgent need to explore efficient approaches for groundwater remediation. In this work, we proposed an inorganic bioelectric system designed to confront these challenges. At 10 and 4 °C, the system achieved total nitrogen (TN) removal efficiencies of 95.4 ± 2.7% and 90.9 ± 1.9% at 2 h hydraulic retention time (HRT), while maximum TN removal rates were recorded as 206.0 ± 6.3 and 178.3 ± 9.4 g N·m -3 ·d -1 at 1 h HRT. The microbial analysis uncovered shifts in dominant genera across temperatures, with Dechloromonas prevalent at 10 °C and Chryseobacterium at 4 °C, highlighting adaptability to cold-tolerant species. Gene analysis on narG, napA, nirS, nirK, norB, nosZI, nosZII , and nifA examined the nitrate reduction processes, and analysis on mtrC and omcA hinted at electrotrophic processes. Additionally, we demonstrated system resilience to disruptions of power outage and short periods without flow through. These findings establish a foundational understanding of electricity-driven nitrate bioreduction in cold environments, crucial in groundwater remediation strategies and paving the way for future optimization and upscaling efforts.