Simultaneous Oxidation Rates of Iron and Arsenic in Acid Mine Drainage (Rio Tinto, Spain) and of Arsenic and Antimony in Neutral Mine Drainage (Giant Mine, Canada)

Oxidation rates of inorganic redox species in natural waters are sometimes controlled by the growth kinetics of chemoautotrophic microorganisms and sometimes by abiotic processes. To better quantify actual oxidation rates of iron, arsenic, and antimony and their mechanisms, samples of acid mine drainage containing iron-oxidizers and neutral mine drainage containing arsenite-oxidizers were obtained from mine drainage in the Rio Tinto Basin, Spain and underground at the Giant Mine, Northwest Territories, Canada, respectively. Samples were kept chilled and in the dark and in appropriate culture media (9K medium at pH 2.3; and arsenite medium at pH 7) until oxidation experiments were run. At low pH, ferrous iron and arsenite oxidize simultaneously suggesting either that the Fe(III) formed by microbial oxidation of ferrous iron was abiotically oxidizing the As(III) or that the same microbe is oxidizing both Fe(II) and As(III). Abiotic experiments indicate that Fe(III) oxidizes As(III) much more slowly than in the biotic experiments, indicating microbial catalysis of As(III) oxidation. The simultaneous oxidation curves of iron and arsenic suggests the same microbes oxidize both iron and arsenic. Research is underway to identify these microbes. At circumneutral pH, As(III) and Sb(III) oxidize simultaneously but the oxidation rate for arsenic is about 3 orders of magnitude faster than the oxidation rate for antimony. Oxidation rates for Fe(II) are typically 500 micromoles/L/h, for As(III) typically 10-60 micromoles/L/h at circumneutral pH and about 0.03 micromoles/L/h at low pH, for Sb(III) typically 0.025 micromoles/L/h at circumneutral pH. These differences in rate at circumneutral pH may explain differences in the redox speciation and geochemical behavior of arsenic and antimony in the aquatic environment.