OXIDATION KINETICS OF ARSENIC-SULFIDE MINERALS

Arsenic-sulfide minerals, orpiment (As2S3) and realgar (AsS), are important contributors to arsenic in natural waters. Stable within reducing conditions, these minerals dissolve when exposed to oxidizing agents and may release arsenic in concentrations to nearly 20 ppm based on geochemical modeling simulations. The occurrence of orpiment and realgar is becoming increasingly observed in a variety of environments. Orpiment is commonly observed (1) in hydrothermal disseminated gold deposits, (2) precipitating in hot springs and pools associated with active geothermal systems, and (3) as a weathering product in tailings impoundments. Realgar has been observed in abundance in sediment-hosted gold deposits. We have studied As2S3(am), orpiment, AsS(am), and realgar to determine the dependence of the oxidation rates of these arsenic-sulfide solids on dissolved oxygen, pH, and temperature. Oxidation was evaluated using mixed flow reactors over a range of pH from 6.8 to 8.8, temperature range of 25 to 40ºC, and dissolved oxygen range of 5.9 to 17.4 ppm. Major findings of our experimental work are: 1) the rate of arsenic-sulfide oxidation increases with pH; 2) the oxidation of arsenic-sulfides is acid generating; and 3) the rate of orpiment and realgar are slower, and the rate of As2S3(am) and AsS(am) are faster than the oxidation of pyrite at pH ~ 8 based on published pyrite oxidation rates. Rate laws derived from our experiments have been used to model arsenic (As) concentrations in a mining environment. Major results from our modeling are: 1) simulated release of As from both orpiment and realgar exceeds the current U.S.Drinking Water Standards (0.05 ppm) in approximately 200-300 days and decreases pH up to 4 units after 2000 days; and (2) limestone may not be useful in remediating acid mine drainage resulting from weathering of arsenic-sulfides because As release from these minerals increases at higher pH values. Further, mobility of arsenic oxyanions at higher pH values may not be limited by sorption processes because surfaces of oxide/hydroxide minerals are negatively charged. Thus, mining impacted waters treated with limestone to increase pH may be affected by high As concentrations if arsenic-sulfides are present.