Iron, Sulfur, and Carbon Dynamics Collectively Regulate the Fate of Cadmium over the Sulfidation-Reoxidation Cycle
Qian Sun, Edward D. Burton, Zhenghong Yu, Lina Chen, Lidong Bi, Peixin Cui, Yujun Wang
Abstract
Cadmium bioavailability is sensitive to redox fluctuations, with its fate linked to the coupled dynamics of Fe, S, and C. This study examines the behavior of Cd-loaded ferrihydrite (Fh) with/without organic matter (OM) undergoing S(-II)-induced reduction followed by O 2 -induced reoxidation. During sulfidation, S(-II) was fully consumed, and Fh was partially reduced to Fe(II) species, with some OM released from the Fh surface. Meanwhile, Cd initially adsorbed on Fh was completely converted to CdS, regardless of Cd loading or the presence of OM. Upon reoxidation, Fe(II) species were reoxidized to Fe(III) oxides, which recaptured OM, while solid-phase S(-II) was oxidized to S 0 and sulfate. Concurrently, partial oxidation of CdS occurred, mainly driven by H 2 O 2 generated during Fe(II) oxidation, with minor contributions from • OH and O 2, but OM inhibited CdS oxidation, primarily by scavenging H 2 O 2 . Released Cd from CdS oxidation was predominantly readsorbed on Fe(III) oxides. Additionally, released Cd was partially structurally incorporated into newly formed Fe(III) oxides while some CdS was encapsulated within Fe(III) oxide aggregates. However, OM interactions with Fe(III) oxides reduced the formation of these Cd species. These findings provide insights into the molecular-scale mechanisms governing Cd dynamics in redox-dynamic environments.