In situ transformation of ferrihydrite to goethite: mechanisms of trace metal(oid) redistribution and potential release in acid mine drainage sediments.

The transformation of iron(III) minerals and the associated solid-phase partitioning of trace metal(oid)s are crucial for controlling the fate and transport of hazardous metal(oid)s in acid mine drainage (AMD) environments. This study investigated the mineralogical changes and the redistribution of trace metal(oid)s (As, Cd, Co, Cr, Cu, Ni, Pb, and Zn) during the in situ transformation of ferrihydrite to goethite in sediment cores from a coal mine acid mine drainage (AMD) wetland. X-ray diffraction (XRD) and scanning electron microscopy (SEM), supported by specific surface area measurements, confirmed the gradual transformation of ferrihydrite to goethite with increasing core depth, accompanied by a reduction in surface area. Total concentration data and Fe-normalized ratios showed As, Co, and Zn decreased relative to Fe with depth, indicating potential release as ferrihydrite transforms into more crystalline phases with lower surface areas. Conversely, Cr exhibited an opposite trend, suggesting an apparent preferential association with goethite-rich layers, which maintain a higher positive charge at the near-neutral pH of the drainage. Sequential extraction revealed that while As, Pb, and Zn became more strongly bound (primarily in the crystalline fraction) during the transformation, Cr and Cd remained predominantly in the exchangeable fraction, indicating high sensitivity to environmental changes. These results provide field-based insights into how ferrihydrite-to-goethite transformation under natural AMD conditions influences solid-phase partitioning of trace metal(oid)s, which may affect their long-term stability and remobilization risk in AMD-affected areas.