Efficient conversion of bicarbonate (HCO3−) to acetate and simultaneous heavy metal Cr(VI) removal in photo-assisted microbial electrosynthesis systems combining WO3/MoO3/g-C3N4 heterojunctions and Serratia marcescens electrotroph

Abstract

The removal of the hazardous heavy metal Cr(VI) in water and the simultaneous production of acetate from the reduction of inorganic carbon (HCO3−) is demonstrated in a photo-assisted microbial electrosynthesis (MES) system incorporating WO3/MoO3/g-C3N4 Z-scheme heterojunctions and Serratia marcescens Q1 electrotroph cathode. The rates of acetate production (6.1 ± 0.3 mg/L/h) and Cr(VI) removal (4.5 ± 0.1 mg/L/h) recorded at a circuital current of 2.8 ± 0.1 A/m2 were 2.4-fold (acetate production), 1.7-time (Cr(VI) removal) and 1.6-fold (circuital current) of those in the controls recorded in the absence of WO3/MoO3/g-C3N4, and 1.6-fold (acetate production) and 1.8-time (circuital current) of those in the absence of both Cr(VI) and WO3/MoO3/g-C3N4. Photogenerated WO3/MoO3/g-C3N4 conduction bands electrons favored both direct or indirect (via S. marcescens) reductions of Cr(VI) and H+, with the latter producing H2 which was further metabolized by S. marcescens with HCO3− to yield acetate. The higher circuital current drawn under photoirradiation conditions refilled the photo-generated valence band holes in the semiconductor and provided the driving force for the reduction reactions. This study provides an alternative and feasible approach for achieving complete removal of toxic heavy metal from water and industrial waters with simultaneous conversion of inorganic carbon to key block chemicals.