REACTION RATES OF NATURAL ORPIMENT OXIDATION AT 25 TO 40°C AND PH 6.8 TO 8.2 AND COMPARISON WITH AMORPHOUS AS2S3 OXIDATION

Abstract

The oxidation rate of natural orpiment from Carlin-type deposits was measured at 25 to 40°C in a mixed flow reactor as a function of pH (6.8 to 8.2) and dissolved oxygen concentration (6.4 to 17.4 ppm) with a starting ionic strength of 0.01 M NaCl. All experiments ran for approximately 30 h, where steady-state conditions were reached after 20 h at a flow rate of 10 mL/min. The stoichiometric ratio of As/S was observed once steady state was reached. The rate law of orpiment oxidation is as follows:R=10-11.77(+/-0.36)[DO]0.36(+/-0.09)[H+]- 0.47(+/-0.05), where R signifies the rate of orpiment destruction (mol m-2 s-1), [DO] is the concentration of dissolved oxygen (M), and [H+] is the concentration of proton (M). The activation energy for the orpiment oxidation by dissolved oxygen at a temperature range of 25 to 40°C is 59.1 kJ/mol. Oxidation reactions of orpiment show incomplete oxidation of arsenic and sulfide in solution. At a pH range of 6.8 to 8.2, As(III) exists as H3AsO3 and As(V) is present as HAsO42- and H2AsO4-. Sulfite, sulfate, and thiosulfate are present as small fractions of total sulfur. The possible major sulfur species are intermediate oxidation state species. The oxidation rate of natural orpiment oxidation is slightly lower by a factor of 0.002 to 0.560 than that of As2S3(am) at the considered pH 7 to 10 and DO concentrations of 1 to 10 ppm. The dependence factors on pH for natural orpiment oxidation are lower by a factor of 0.37 compared with As2S3(am). However, the calculated activation energy is much larger for natural orpiment than As2S3(am) by a factor of 3.5. As(III) and As(V) are the major products for both As2S3(am) and natural orpiment oxidation along with intermediate sulfur species. The rate of orpiment oxidation increases with pH and results in an increase in the release of As. In mining-impacted environments with alkaline waters, as may be found in carbonate-hosted ore deposits, the natural attenuation of As oxyanions by sorption to oxide/hydroxide mineral surfaces is minimized because of a negative surface charge at a higher pH range. Thus, As concentrations may increase in mining-impacted waters at higher pH values (>8).