Kinetics of the Dissolution of Scheelite in Groundwater: Implications for Environmental and Economic Geology

Tungsten, an emerging contaminant, has no EPA standard for its permissible levels in drinking water. At sites in California, Nevada, and Arizona there may be a correlation between elevated levels of tungsten in drinking water and clusters of childhood acute lymphocytic leukemia (ALL). Developing a better understanding of how tungsten is released from rocks into surface and groundwater is therefore of growing environmental interest. Knowledge of tungstate ore mineral weathering processes, particularly the rates of dissolution of scheelite (CaWO4) in groundwater, could improve models of how tungsten is released and transported in natural waters.Our research focused on the experimental determination of the rates and products of scheelite dissolution in 0.01 M NaCl (a proxy for groundwater), as a function of temperature, pH, and mineral surface area. Batch reactor experiments were conducted within constant temperature circulation baths over a pH range of 3-10.5. Cleaned scheelite powder with grain diameters of 106-150µm were brought in contact with 0.01 M, NaCl in a Teflon vessel. Aliquots of solution were taken periodically for product analysis by ICP-OES. Changes in mineral surface characteristics were monitored using SEM and EDS methods.The integral method was used to interpret data from experiments and to develop a rate law. The specific rate for the dissolution of scheelite in groundwater is expressed as: (dM)/(sch overall)/dt = kr(Ksp - IAP)where kr is the reverse rate constant in units of mol/L-sec, Ksp is the solubility product in mol/L , and IAP is the ion activity product for scheelite in mol/L. At pH 6.2 and 20oC, the specific rate law for the dissolution of scheelite in a 0.01M NaCl groundwater proxy is:(dM)/(sch overall)/dt = (9.09x10-8) (2.63x10-10-IAP)The dissolution of scheelite shows variable behavior depending on the pH and temperature. At low pH, the release of Ca and W is disrupted by the formation and adsorption of tungstic acid onto grain surfaces. In groundwater with progressively higher alkalinity, W releases at a higher rate. The reaction also follows typical Arrhenius behavior. These results suggest that the warmer and more basic the groundwater that scheelite is in contact with, the greater the propensity for an elevated level of tungsten within it