Adsorption of Phosphate on Manganese Dioxide in Seawater
The adsorption of phosphate on manganese dioxide (δMnO 2 ) was determined in seawater as a function of pH, temperature, and salinity. The adsorption in a simple electrolyte solution (0.7 M NaCl) was adequately fitted using the triple-layer surface complexation model, assuming that phosphate formed outer-sphere complexes on the surface of δMnO 2 . SO 4 2 - and humic acid were found to suppress the adsorption at low pH, but had no effect in the pH range of seawater. The addition of Ca 2+ and Mg 2+ in 0.7 M NaCl was found to enhance the adsorption of phosphate on δMnO 2 at pH > 4. This increased adsorption might be due to the changes in both the surface charge of δMnO 2 and the solution speciation of phosphate in the presence of Ca 2+ and Mg 2+ . The results suggest that manganese oxides can act as important adsorbents of phosphate in natural waters, as well as in surface sediments, when they are present at higher concentrations than other reactive constituents, such as iron and aluminum oxides.