KINETICS OF SCHEELITE DISSOLUTION 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 groundwaters is therefore of growing environmental interest.

The United States imports 70% of its tungsten supplies and the development of improved domestic exploration models could make the U.S. less reliant on foreign imports. Knowledge of tungstate ore mineral weathering processes, particularly the rates of dissolution of scheelite (CaWO₄) in groundwater, could improve models of how tungsten anomalies in natural waters can reveal the presence of concealed ore deposits.

Our research is focusing on experimental determination of the rates and products of tungstate mineral dissolution in synthetic groundwater, as a function of temperature, pH and mineral surface area. The initial rate method is being used to develop rate laws. Batch reactor experiments are conducted within constant temperature circulation baths over a groundwater pH range of 2-9. Cleaned scheelite powder with grain diameters of 106-150um is placed between two screens in a sample platform and then placed inside a two liter Teflon vessel filled with synthetic groundwater. Ports on the vessel allow sample extraction, temperature and pH measurement, gas inflow, and water circulation. Aliquots of solution are taken periodically for product analysis by ICP-MS. Changes in mineral surface characteristics are monitored using SEM and EDS methods.

Results so far reveal that the dissolution of scheelite is incongruent at both neutral and low pH. Solid tungstic acid forms on scheelite mineral surfaces under acidic conditions, implying that this phase controls the dissolution rate in acidic environments. The influence of dissolved CO₂ and resultant calcium carbonate precipitation on the dissolution of scheelite at higher pH is also being investigated.