The Effect of Synthetic Brine Constituents on the Rate of Arsenic Release from Arsenopyrite

Geologic carbon dioxide storage (GCS) in deep saline sedimentary formations is a potential method for mitigating increased levels of atmospheric CO2. Injection of CO2 in those formations may induce dissolution of reservoir minerals. Leakage of CO2-saturated brines and native brines could impact overlying drinking water aquifers by contaminating them with toxic constituents. Of particular concern is the effect of CO2 on the rates of dissolution of arsenic and other toxic metals from reservoir minerals. The most common pure phase arsenic mineral in sedimentary geologic formations is arsenopyrite (FeAsS). Natural brines have high salinities (up to 7M), with high concentrations of Na+, Ca2+, Mg2+ and K+. The focus of this study is to examine the effect of brine components on the dissolution rate of arsenic from arsenopyrite. A small-scale flow-through column system was constructed for this purpose and is being used to measure arsenic release rates from arsenopyrite. Influent solutions of NaCl, CaCl2, and MgCl2 at equal ionic strengths were used to examine the effect of the cationic species. A cleaning procedure to remove prior surface oxidation on the surface of the arsenopyrite particles was also developed. Preliminary results with NaCl and CaCl2 at an ionic strength of 0.011M indicate that the rate of dissolution of arsenic is dependent on the cationic species but independent of ionic strength.