Oxalate Modulates Cadmium Mobility during Ferrihydrite Transformation: Roles of Dissolution Kinetics and Phase Selection

Abstract Cadmium (Cd) mobility is strongly influenced by ferrihydrite (Fh) transformation, and low-molecular-weight organic acids (LMWOAs) can modulate this process and thereby alter the Cd2+ environmental fate. However, the mechanisms by which organic ligands control Fh transformation and consequently influence Cd2+ behavior remain poorly understood. Here, we investigated Fh transformation and the associated fate of Cd2+ in the presence of oxalic acid (Ox), an organic ligand ubiquitous in natural environments. Results showed that Ox significantly increased Cd2+ release kinetics by accelerating Fh dissolution through coupled surface inner-sphere complexation and aqueous Fe3+-oxalate complexation. Ox also reduced the surface energy of hematite (Hm) from ∼340 to ∼183 mJ/m2, which increased the nucleation rate ratio of Hm to goethite (Gt) from ∼1.70 to ∼90.92. This shift promoted the preferential formation of Hm, which reduced the availability of reactive surface sites for Cd2+ readsorption and thereby increased the overall release extent. At growth stage, Ox altered Hm growth pathways from classical ion-by-ion growth to particle-mediated aggregation, facilitating the physical encapsulation of solid-associated Cd2+ and enhancing its stabilization in nonextractable forms. These findings demonstrated that Ox exerted a dual control on Cd2+ environmental behavior by promoting both its release and subsequent sequestration, highlighting the importance of coupling transformation kinetics, phase selection, and growth mechanisms in predicting the heavy metal fate in organic-rich environments.