Role of natural organic matter in mediating the adsorption of Mo(VI) onto ferrihydrite.

Fe (hydr)oxides have significant impacts on the fate of molybdenum (Mo) via the adsorption of molybdate (MoO42-) onto their surfaces. However, these materials always interact with natural organic matter (NOM) in the natural environment, and the role of NOM in mediating the geochemical behavior of Mo(VI) at mineral surfaces remains poorly understood. Using humic acid (HA) as a model organic compound, the adsorption behavior, influencing factors, and underlying mechanism of Mo(VI) onto ferrihydrite (Fh)-HA co-precipitates were investigated via batch experiments, combined with XPS and FTIR analyses. The results revealed that HA exhibited dual effects on Mo(VI) retention, with a pronounced dependence on the C/Fe molar ratio. It inhibited Mo(VI) uptake at a higher C/Fe molar ratio (0.5), while promoting adsorption at a lower C/Fe molar ratio (0.1) under acidic-to-neutral conditions (pH 4.0-7.5). The presence of various coexisting anions exerted distinct effects on Mo(VI) adsorption. Specifically, H2PO4-/SO42- inhibited Mo(VI) adsorption, whereas Cl-/NO3- promoted its uptake. The results of XPS, FTIR characterization, and chemical extraction consistently indicated that the binding modes of Mo consisted of a mixture of outer-sphere complexes, inner-sphere complexes with both ferric hydroxyl and carboxyl groups, and ferric molybdate-like precipitates in the presence of HA, with no significant reduction of Mo(VI) occurring. These findings highlight the critical role of NOM in governing Mo speciation and mobility at Fe (hydr)oxide-water interfaces, providing essential insights for predicting Mo fate and developing remediation strategies for contaminated soils.