Liquid-gas phase transition enables microbial electrolysis cell to treat organic pollution and synchronously remediate nitrate-contaminated groundwater via hydrogenotrophic denitrification

Groundwater denitrification is challenged by a lack of electron donors and usually requires additional energy input or chemical agents. MEC can convert organic pollution into clean electron donor (hydrogen) through a gas-liquid phase transition. In this study, the MEC was combined with permeable bio-reactive barrier (PRB) as a hybrid system to achieved simultaneous organic waste removal, hydrogen production and groundwater remadition via long-distance hydrogenotrophic denitrification by biogas. The semi-hydrophobic PTFE-coated granular activated carbons (GAC) and hydrophilic GAC were filled as two kinds of biocarriers in PRBs, in which the hydrophobic PTFE surface coating improved hydrogen utilization. The PTFE-coated GAC maked the denitrification performance of PRB insensitive to hydrogen partial pressure within the tested range of 0.01–0.04 MPa and achieved effluent nitrate concentration of <20 mg L⁻¹ within 12 h (The Water Quality Standard for Drinking Water Sources in China). The gas production in MECs was actively pumped to PRBs and spontaneously achieve negative pressure on the MEC side and positive pressure on the PRB side. The actively pumping of biogas induced negative pressure in MEC and positive pressure in PRB. The negative pressure improved current density by 18.6 ± 0.7% in MECs and the MEC-PRB hybrid system achieved a nitrate removal of 85.0 ± 0.8% in actual groundwater with an effluent concentration of 15.0 ± 0.8 mg L⁻¹ at 72 h. This work demonstrated the feasibility of matching MEC and PRB as a novel hybrid system for organic pollution degradation and effective nitrogen removal in electron-donor-lacking groundwater.