KINETICS OF QUARTZ DISSOLUTION AT LOW-TEMPERATURES

Quartz dissolution rates have been measured as a function of pH and ionic strength at 25° and 60°C and can be modeled by a rate law which takes into account speciation at the quartz-solution interface. Dissolution rates far from equilibrium in basic solutions are directly proportional to negative silica surface charge, changing with ionic strength and pH as surface charge changes. At pH 7.5 at 25°C and pH 7.0 at 60°C the logarithm of quart dissolution rates are -16.1 and -15.2 mol cm-2 s-1, respectively. Dissolution rates decrease slightly from pH 7 as pH is lowered, to pH 3, where they begin to increase with decreasing pH. The calculated activation enthalpy changes as a function of pH, being ∼11 kcal. mol-1 at near-neutral pH but increasing from 13 kcal. mol-1 at pH 8 to 23 kcal. mol-1 at pH 11. © 1990.