REACTION-KINETICS AND THERMODYNAMICS OF AQUEOUS DISSOLUTION AND GROWTH OF ANALCIME AND NA-CLINOPTILOLITE AT 25-DEGREES-C

Low-temperature (25 degrees C), long-term (up to 2.6 yr) batch-type di ssolution and precipitation experiments have been conducted to examine reaction kinetics and thermodynamics of aqueous dissolution and growt h of analcime and Na-clinoptilolite at pH close to 9. Experimental dat a show that analcime dissolution and growth are approximately stoichio metric. Mass transfer data indicate that no other solid phases influen ced analcime dissolution and growth experiments at aqueous SiO2 concen trations less than approx 4 ppm. Dissolution rate data for analcime ca nnot be rationalized using a fixed preaffinity rate constant. Two hypo thetical dissolution mechanisms provide reasonable interpretations of the mass transfer rate data. One hypothesis invokes the presence of a mass of surface material that reacts rapidly and dissolves completely. The bulk material is hypothesized to react in parallel with the extra -reactive material according to a smaller rate constant. In an alterna te hypothesis also yielding a good data regression, the reaction rate is assumed to be inversely proportional to the square of the aqueous A I(OH)4 activity as well as reaction affinity. existing data do not per mit a selection among these or other possible mechanisms, the decrease in reaction rate with reaction progress under conditions that are far from equilibrium offers a likely explanation for reported discrepanci es between laboratory and field weathering rate measurements. The equi librium solubility of analcime was closely approached from both unders aturation and supersaturation. Interpretation of the aqueous solution data permits extraction of the equilibrium constant and standard state Gibbs free energy for the analcime dissolution reaction and the stand ard state Gibbs free energy of formation of analcime. The equilibrium solubility of analcime obtained in this study differs significantly fr om values based on thermodynamic data reported in the literature. Rele ase of Si in Na-clinoptilolite dissolution studies follows a systemati c pattern of decreasing rate with increasing solution concentration an d approach to equilibrium. The time-dependent data can be rationalized using a conventional dissolution rate equation. Net release of Al is much less than that of Si, indicating incongruent dissolution. Attempt s to determine the solubility of Na-clinoptilolite were complicated I precipitation of an unidentified, low-solubility, aluminous JY phase. Nevertheless, a provisional equilibrium constant and corresponding sta ndard Gibbs free energy for the dissolution reaction and standard stat e Gibbs free energy of formation of Na-clinoptilolite were extracted f rom the data, adopting plausible assumptions about the composition of the equilibrated clinoptilolite and the saturation state of the experi mental solutions. Groundwater chemical data from the tuffaceous aquife r in the vicinity of Yucca Mountain and from Oasis Valley, Nevada, hav e aqueous silica activities that vary over a small range at a value gr eater than that corresponding to mutual equilibration of analcime and Na-clinoptilolite. Reported chemical analyses including data for aqueo us aluminum indicate that the natural waters appear to be at equilibri um with analcime using thermodynamic data derived in this study. Howev er the natural waters are supersaturated with respect to the provision al Na-clinoptilolite solubility.