Long-term evolution of Fe oxyhydroxide mineralogy and potentially toxic element partitioning in mining-impacted sediments of the Lower Doce River fluvial-estuarine system.

The collapse of the Fundão dam in southeastern Brazil released large quantities of Fe-rich tailings into the Doce River basin, generating a long-term legacy of mining-derived sediments enriched in potentially toxic elements (PTEs). Although Fe oxyhydroxides are recognized as major geochemical hosts of contaminants, their long-term mineralogical evolution and relationships with PTE partitioning in tropical fluvial-estuarine systems remain poorly understood. This study investigated the distribution, crystallographic properties, and geochemical behavior of Fe oxyhydroxides in sediments collected from reservoir and estuarine environments of the lower Doce River between 2019 and 2024. Selective dissolution procedures, X-ray diffraction, diffuse reflectance spectroscopy, magnetic susceptibility measurements, and multivariate analyses were used to characterize Fe oxyhydroxide assemblages and their relationships with the investigated PTEs. Crystalline Fe oxyhydroxides predominated throughout the study area, although temporal variations in Fe partitioning were consistent with ongoing mineralogical reorganization across contrasting depositional environments. Higher Feo/Fed ratios at specific estuarine stations indicated relative enrichment of poorly crystalline Fe phases. Goethite remained the dominant Fe oxyhydroxide, whereas hematite exhibited greater spatial and temporal variability. Variations in crystal size, specific surface area, and Al substitution further suggest that Fe oxyhydroxide properties continue to evolve under present-day environmental conditions. Principal component analysis revealed strong associations between Fe oxyhydroxides and several PTEs, including As, Cr, Ni, Co, Pb, Zn, and Mo. The results indicate that Fe oxyhydroxides in mining-impacted sediments remain major geochemical hosts of the investigated PTEs more than a decade after the disaster, with their temporal variability closely associated with trace-element partitioning and potential redistribution. These findings highlight the importance of Fe mineralogy for understanding trace-element partitioning and potential redistribution and reinforce the value of mineralogical indicators and selective extraction procedures within long-term environmental monitoring frameworks.