Electrical stimulation accelerated phenanthrene biodegradation coupling with nitrate reduction in groundwater

Phenanthrene (PHE) and nitrate co-contamination widely exists in the groundwater. Anaerobic bioremediation of the two suffered from the slow removal rate and limited electron transfer within microbes. Herein electrical stimulation (ES) was proved to stimulate the simultaneous removal of PHE and nitrate anaerobically. High removal of PHE and nitrate (90.94 % and 100 %, respectively) was obtained within 24 h in the ES-bioreactor, two times higher than the control anaerobic bioreactor (32.16 %). ES-bioreactor exhibited a good PHE removal performance within initial PHE ranging from 5 to 25 mg/L. In ES-bioreactor, extracellular polymeric substances increased PHE bioavailability and acted as protective capsules. Nicotinamide adenine dinucleotide (NADH) and cytochrome C (cyt-C) facilitated the extracellular and intracellular electron exchange during PHE biotransformation. ES significantly selected Acetobacterium, PHE-degraders (f_Bacteroidetes_vadin HA17, Limnobacter and Stenotrophomonas) and electrogens (Clostrium_sensu_stricto_13) to participate in PHE decomposition. Acetobacterium decomposed complex organics to acetate, proving accessible substrates for electrogens to maintain an efficient electron flow. Enzymes regulating bacterial chemotaxis, flagellar assembly and carbohydrate metabolism were up-regulated with ES, suggesting the indispensable role of such enzymatic processes in ES-bioreactor. Overall, this work demonstrates ES can promote anaerobic bioremediation of PHE and nitrate in groundwater and, thus, opens up a new bioelectrochemistry application scenario.