Adsorption of cadmium to iron and manganese oxides during estuarine mixing

The adsorption of Cd to aged hydrous oxides (FeOOH and MnO2) and to fractionated (< 63 μm) sediment has been studied along an estuarine gradient created by mixing filtered river and marine end-members from the Beaulieu, southern England. In all cases, adsorption decreased logarithmically with increasing salinity but, for a given salinity, the conditional particle–water distribution coefficient, KD, was two orders of magnitude greater on the Mn oxide than either the Fe oxide or fractionated sediment. The reversibility of Cd adsorption, evaluated by extraction of particles in NH4OAc, decreased in the order: FeOOH > sediment ≫ MnO2. The aqueous equilibrium speciation of Cd along the estuarine gradient was computed by WHAM 6, with organic complexation modelled using either the default settings for fulvic substances or measurements of the hydrophobic (C18-retained) fraction of Cd. Free ion concentrations predicted from the latter approach correlated well with conditional distribution coefficients for each particulate phase, and slopes of the relationships yielded estimates of distribution coefficients normalised with respect to the free ion activity, KD′, of about 7 × 104, 4 × 104 and 5 × 106 mL g− 1 for sediment, FeOOH and MnO2, respectively. The results and model fits suggest that (i) Mn oxides exert an impact on the retention, mobilisation and availability of Cd in estuaries that is disproportional to their abundance, and (ii) the degree of organic complexation of Cd in estuaries may be overestimated using published, generic binding constants.