BRONSTED REACTIONS ON OXIDE MINERAL SURFACES AND THE TEMPERATURE-DEPENDENCE OF THEIR DISSOLUTION RATES

The pH-dependence of oxide dissolution rates is controlled by Bronsted acid-base reactions at the mineral surface. These reactions are rapid but depend explicitly on temperature, as do the subsequent slow rates of bond hydrolysis. The net result is that dissolution rates vary in a complicated fashion with temperature and solution pH. The enthalpy c hanges of acid-base reactions on oxide materials are sufficiently simi lar, however, that general statements can be made about their contribu tion. The enthalpy changes from proton adsorption to a hydroxyl functi onal group (solid right-pointing triangle SOH), or to a deprotonated f unctional group (solid right-pointing triangle SO-), are generally exo thermic. The enthalpy changes become increasingly endothermic, however , as charge accumulates on the mineral surface and the charged species interact electrostatically. The result is that mineral dissolution ra tes are least sensitive to temperature, as measured with an Arrhenius- like rate law, at pH conditions near the Point of Zero Net Proton Char ge.