Investigation of the redox behavior of biochar-based bipolar electrochemistry in porous media

Bipolar electrochemistry (BEC) refers to the redox actuation occurring at the wirelessly connected conductors, the bipolar electrodes (BPEs), but the redox behavior of biochar-based BEC in porous media remains unclear. This study evaluated the redox behavior of biochar-based BEC in porous media using the conversion rates of Cr(III) and Cr(VI). Results showed activated BEC in porous media, with bamboo biochar (B-BC) BPEs exhibiting superior redox properties compared to wood (WO-BC) and walnut biochar (W-BC). Soil texture influenced BEC performance, with sandy soils exhibiting the highest Cr conversion rates (36.22% for Cr(III) to Cr(VI) and 48.6% for Cr(VI) to Cr(III)). A linear relationship existed between Cr conversion and voltage gradient (2.2–4.2 V/cm). Excessive biochar addition (>2.5% w/w) reduced Cr conversion. Further analysis revealed exceptional electron transfer capability of B-BC and WO-BC, producing ·O2- and ·OH with catalytic activation and indirect oxidation potential. Overall, our findings suggest biochar-based BEC induction in porous media, facilitating rapid changes in heavy metal speciation, primarily driven by direct oxidation of biochar. Exceptional biochar behavior arises from intricate properties such as honeycomb structures, quinoid groups, phenolic-OH, and high crystallinity. Optimizing biochar's structure, functional groups, and crystallinity holds promise for effective implementation of biochar-based BEC in porous soil remediation.