Direct and Indirect Electron Transfer Routes of Chromium(VI) Reduction with Different Crystalline Ferric Oxyhydroxides in the Presence of Pyrogenic Carbon

Electron transfer mediated by iron minerals is considered as a critical redox step for the dynamics of pollutants in soil. Herein, we explored the reduction process of Cr(VI) with different crystalline ferric oxyhydroxides in the presence of pyrogenic carbon (biochar). Both low- and high-crystallinity ferric oxyhydroxides induced Cr(VI) immobilization mainly via the sorption process, with a limited reduction process. However, the Cr(VI) reduction immobilization was inspired by the copresence of biochar. Low-crystallinity ferric oxyhydroxide had an intense chemical combination with biochar and strong sorption for Cr(VI) via inner-sphere complexation, leading to the indirect electron transfer route for Cr(VI) reduction, that is, the electron first transferred from biochar to iron mineral through C–O–Fe binding and then to Cr(VI) with Fe(III)/Fe(II) transformation on ferric oxyhydroxides. With increasing crystallinity of ferric oxyhydroxides, the direct electron transfer between biochar and Cr(VI) became the main electron transfer avenue for Cr(VI) reduction. The indirect electron transfer was suppressed in the high-crystallinity ferric oxyhydroxides due to less sorption of Cr(VI), limited combination with biochar, and higher iron stability. This study demonstrates that electron transfer mechanisms involving iron minerals change with the mineral crystallization process, which would affect the geochemical process of contaminants with pyrogenic carbon.