Dataset for Fh-As&Cd

Soil contamination by heavy metals and metalloids poses significant environmental challenges, and iron oxide transformation processes in soils and sediments play a pivotal role in controlling contaminant mobility. While individual metal-ion interactions with Fe oxides have been widely studied, the simultaneous nanoscale effects of coexisting oxyanions and cations during Fe(II)-induced transformations remain poorly understood. Here, we conducted batch extraction experiments to quantify the aqueous, NaOH-extractable arsenic (As(V)), and HCl-extractable cadmium (Cd(II)) during Fe oxide transformation, complemented by nanoscale analyses to resolve the sequestration mechanisms of As(V) and Cd(II). Results showed that Cd(II) slightly increased the extractability of As(V) (from 53.7% to 58.6% at 24 h). In contrast, As(V) strongly promoted lepidocrocite formation (92.7%), which reduced Cd(II) retention and resulted in most Cd(II) being partitioned into the aqueous (79.3%) and extractable (18.5%) fractions at 168 h. Nanoscale observations revealed that As(V) exhibited stronger retention on residual ferrihydrite domains, consistent with its stronger binding affinity with iron oxides, while Cd(II) showed weaker and more evenly distributed associations, often residing on lepidocrocite surfaces. Despite this stronger nanoscale association with ferrihydrite, bulk As(V) partitioning was dominated by lepidocrocite due to its phase abundance (~90%), suggesting the dual control of binding affinity and mineralogical composition. These findings demonstrate that Fe(II)-induced ferrihydrite transformation governs contaminant sequestration pathways and provide mechanistic insights into the mobility of coexisting As and Cd in contaminated systems.