Sunlight-Driven Direct/Mediated Electron Transfer for Cr(VI) Reductive Sequestration on Dissolved Black Carbon–Ferrihydrite Coprecipitates
Leiye Sun, Tianming Wang, Bo Li, Meiqing Chen, Jiayan Wu, Zhongbo Shang, Pingxiao Wu, Zhi Dang, Nengwu Zhu
Abstract
Surface runoff horizontally distributed chromium (Cr) pollution into various surface environments. Sunlight is a vital factor for the Cr cycle in the surface environment, which may be affected by photoactive substances such as ferrihydrite (Fh) and dissolved black carbon (DBC). Herein, sunlight-driven transformation dynamics of Cr species on DBC–Fh coprecipitates were studied. Under sunlight, the removal of aqueous Cr(VI) by DBC–Fh coprecipitates occurred through sunlight-driven reductive sequestration including adsorption, followed by surface reduction (pathway 1) and aqueous reduction, followed by precipitation (pathway 2). Additionally, coprecipitates with a higher DBC content exhibited a more effective reduction of both adsorbed ( k app,S_red ) and aqueous Cr(VI) ( k app,A_red ). Photoelectrons facilitated Cr(VI) reduction through direct electron transfer; notably, electron donating DBC promoted the production of photoelectrons by consuming photogenerated holes. Photogenerated Fe(II) species (mineral-phase and aqueous Fe(II)) mediated electron transfer for Cr(VI) reduction, which was reinforced via a ligand-to-metal charge transfer (LMCT) process between DBC–organic ligands and mineral Fe(III). Furthermore, ·O 2 – also mediated Cr(VI) reduction, although this impact was limited. Overall, this study demonstrates that photoelectrons and photogenerated electron mediators play a crucial role in Cr(VI) reductive sequestration on DBC–Fh coprecipitates, providing new insights into the geochemical cycle of Cr pollution in sunlight-influenced surface environments.