Enhanced prediction of cadmium partitioning in oxidized sediments: The role of Fe hydroxides and ferrihydrite

Abstract

This study enhances the prediction of cadmium (Cd) partition coefficients (K_d) in sediments by integrating the roles of total organic carbon (TOC), amorphous iron oxides (ferrihydrite), and iron hydroxides (Fe(OH)₃). Using the Windermere Humic Aqueous Model (WHAM 7) alongside experimental data, K_d values were predicted across a wide pH range and compared to measured values from 21 sediment samples. The results show that while traditional models primarily focusing on TOC provide reasonable K_d estimates under certain conditions, they may fail in samples with significant contributions from other binding phases, particularly Fe oxides. The study revealed that at pH levels above the point of zero charge (PZC) of ferrihydrite (∼7.5), Cd binding is dominated by amorphous Fe oxides, surpassing TOC in importance. This highlights the need for models to incorporate the significant role of Fe oxides in Cd partitioning, especially in oxic environments. Sensitivity analysis further emphasized the influence of pH, showing a shift in the dominant binding phase from TOC at lower pH levels to amorphous Fe oxides at higher pH levels. The comparison between predicted and measured K_d values demonstrated that most predictions were within an order of magnitude of the measured values. However, discrepancies in samples with low TOC and Fe oxide content suggest the need to account for additional binding phases, such as Mn oxides, in future models. In conclusion, this study provides a more robust framework for predicting Cd partitioning in sediments, emphasizing the need to consider multiple binding phases to improve accuracy in environmental risk assessments.