The fate of cadmium during the hydrolysis and solid-state transformation of iron

The ubiquitous hydrolysis of Fe(III) ions in nature leads to the formation of ferrihydrite (Fh), which then undergoes solid-state transformation into crystalline mineral phases. However, the impact of this process on the geochemical fate of cadmium (Cd) in aquatic environments is not yet fully understood. This investigation systematically examined the partitioning mechanisms of Cd during the aforementioned processes through extraction techniques and multiple characterization methodologies. The experimental results demonstrated that Fe(III) ions first undergo the diffusion-limited aggregation (DLA) stage, forming loosely bound clusters with high Cd co-precipitation capacity. Upon transitioning to the reaction-limited aggregation (RLA) stage, Fe clusters exhibit increased structural complexity, wherein the expansion of radius of gyration predominantly governs Cd retention. As colloidal Fh develops, surface adsorption becomes the predominant mechanism for Cd sequestration. Throughout solid-state transformation in systems containing 1 mol% Cd, dehydroxylation processes induced acidification, facilitating the progressive liberation of Cd. Conversely, in systems with 10 mol% Cd, significant Cd incorporation induces distortion in the crystalline lattice structure, promoting system alkalinization and, consequently, enhancing Cd immobilization. pH gradient solid-state transformation experiments demonstrated that when initial pH values were below 8, final pH measurements were significantly lower than initial values, and vice versa. pH conditions potentially regulate the speciation of Fe within Fh, ultimately exerting substantial influence on Cd partitioning behavior. In summary, Fh formation beneficially stabilizes Cd, while its subsequent solid-state transformation triggers Cd redistribution.