New insight into Na+-promoted microbial electrolysis cell towards hydrogen energy recovery: Feasibility and dual mechanism in salt-containing wastewater treatment

Abstract

The salt-containing wastewater treatment has become serious challenge, while conductivity and micro-ecosystem disorder severely limited microbial electrolysis cell (MEC) efficiency. In order to synchronously overcome the above bottlenecks, this study proposed an innovative Na+-promoted MEC system for efficient salt-containing wastewater treatment. The Na+ presence greatly improved conductivity to impair electric resistance, resulting in rapid electron transfer pattern towards organic degradation in anode and hydrogen synthesis in cathode. Furthermore, the selective Na+ stress triggered microbial community evolution and improved electroactive bacteria activity after salinity cultivation in MEC (0.09–0.17 mol Na+/L), which played “bacteria screening” role for salt-tolerant “electrochemical hydrogen-producing” bacteria community formation. The organic-degrading and hydrogen-producing bacteria with their assistors were tolerant to Na+ stress, which were considerably enriched into dominant microorganisms, while the unfavorable bacteria responsible for hydrogen consumption and competition were inhibited due to salinity-sensitivity. Correspondingly, the metabolic pathways for metabolism, genetic information processing and environmental information processing were improved, positively associating with organic degradation and hydrogen production. Such dual driving contributions of Na+-improved electron transfer-utilization pattern and Na+-modified microbial metabolisms boosted the organic pollutant degradation at anode to 91.34 % while improving the performance and speeds of hydrogen production by 1.57–1.70 times. The hydrogen abundance was also increased to 71.57 % in biogas. The energy efficiencies relative to external electricity and substrates were elevated by 8.80–28.44 %, the coulombic efficiency raised by 14.05 % and reached 109.72 %, suggesting the positive net energy benefits. The findings provided inspirations to the feasible application of MEC in salt-containing wastewater treatment with efficiency benefits.