ON THE TEMPERATURE-DEPENDENCE OF MINERAL DISSOLUTION RATES

Experimental activation energies (E(exp)) for the dissolution rates of silicate and oxide minerals far from equilibrium vary by tens of kJ/mol with pH. Measured E(exp) range from larger than 50 kJ/mol at pH conditions much more acidic or more basic than at the Point of Zero Net Proton Charge (PZNPC) to very small values near, but not necessarily at. the PZNPC. We interpret this pH variation by combining an empirical rate law for dissolution with a conditional equilibrium constant for proton adsorption to form positive surface charge. Interactions among charged sites at the mineral surface are treated with an electrostatic potential term in the conditional equilibrium constant. Proton adsorption (desorption) to create positive (negative) surface charge is exothermic (endothermic) and contributes 15-50 kJ/mol to E(exp); this contribution depends on the mole fraction of charged surface sites and, therefore, varies slightly with pH. The electrostatic potential contribution becomes increasingly important as the mineral surface acquires significant positive or negative charge; it therefore varies considerably with pH. This model approach not only explains the pH dependence of E(exp) but also leads to the following important general predictions: 1)E(exp) will vary similarly with pH for minerals which differ by homovalent cation substitutions; 2) negative E(exp) values can conceivably occur since E(exp) is the sum of enthalpies of similar magnitude but opposite sign; 3 )E(exp) depends linearly on the partial reaction rate order with respect to the concentration of charged sites; 4) dissolution rates will become increasingly sensitive to pH at elevated temperatures; and 5 )little information about the detailed energetics of bond hydrolysis in dissolution processes can be inferred from E(exp) without correcting it for the large enthalpies of proton adsorption.