Competitive Adsorption of Molybdenum in the Presence of Phosphorus or Sulfur on Gibbsite

Anion adsorption on the aluminum oxide, gibbsite, was investigated as a function of solution pH (3Y11) and equilibrium solution Mo (3.13, 31.3, or 313 mu mol/L), P (96.9 mu mol/L), or S (156 mu mol/L) concentration. Adsorption of all three anions decreased with increasing pH. Electrophoretic mobility measurements indicated a downward shift in point of zero charge, indicative of an inner-sphere adsorption mechanism for all three anions. The constant capacitance model, having an inner-sphere adsorption mechanism, was able to describe Mo and P adsorption; whereas the triple-layer model with an outer-sphere adsorption mechanism was used to describe S adsorption. Competitive adsorption experiments showed a reduction of Mo adsorption at a Mo/P ratio of 1:30 and 1:300 but no reduction at a Mo/S ratio of 1:52 and 1:520. These concentrations are realistic of natural systems where Mo is found in much lesser concentrations than P or S. Using surface complexation constants from single-ion systems, the triple-layer model predicted that even elevated S concentrations did not affect Mo adsorption. The constant capacitance model was able to predict the competitive effect of P on Mo adsorption semiquantitatively.