The adsorption of phosphate on manganese dioxide (δMnO2) was determined in seawater as 6 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 δMnO2. SO42- 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 Ca2+ and Mg2+ in 0.7 M NaCl was found to enhance the adsorption of phosphate on δMnO2 at pH > 4. This increased adsorption might be due to the changes in both the surface charge of δMnO2 and the solution speciation of phosphate in the presence of Ca2+ and Mg2+. 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.
ASJC Scopus subject areas
- Environmental Chemistry