Sunlight-Driven\nDirect/Mediated Electron Transfer\nfor Cr(VI) Reductive Sequestration on Dissolved Black Carbon–Ferrihydrite\nCoprecipitates

Surface runoff horizontally distributed chromium (Cr)\npollution\ninto various surface environments. Sunlight is a vital factor for\nthe Cr cycle in the surface environment, which may be affected by\nphotoactive substances such as ferrihydrite (Fh) and dissolved black\ncarbon (DBC). Herein, sunlight-driven transformation dynamics of Cr\nspecies on DBC–Fh coprecipitates were studied. Under sunlight,\nthe removal of aqueous Cr(VI) by DBC–Fh coprecipitates occurred\nthrough sunlight-driven reductive sequestration including adsorption,\nfollowed by surface reduction (pathway 1) and aqueous reduction, followed\nby precipitation (pathway 2). Additionally, coprecipitates with a\nhigher DBC content exhibited a more effective reduction of both adsorbed\n(<i>k</i><sub><i>app,S_red</i></sub>) and aqueous\nCr(VI) (<i>k</i><sub><i>app,A_red</i></sub>).\nPhotoelectrons facilitated Cr(VI) reduction through direct electron\ntransfer; notably, electron donating DBC promoted the production of\nphotoelectrons by consuming photogenerated holes. Photogenerated Fe(II)\nspecies (mineral-phase and aqueous Fe(II)) mediated electron transfer\nfor Cr(VI) reduction, which was reinforced via a ligand-to-metal charge\ntransfer (LMCT) process between DBC–organic ligands and mineral\nFe(III). Furthermore, ·O<sub>2</sub><sup>–</sup> also\nmediated Cr(VI) reduction, although this impact was limited. Overall,\nthis study demonstrates that photoelectrons and photogenerated electron\nmediators play a crucial role in Cr(VI) reductive sequestration on\nDBC–Fh coprecipitates, providing new insights into the geochemical\ncycle of Cr pollution in sunlight-influenced surface environments.