Simultaneous oxidation of arsenic and antimony at low and circumneutral pH, with and without microbial catalysis.

Abstract

Arsenic and antimony are common mine-water pollutants and their toxicity and fate are strongly influenced by redox processes. In this study, simultaneous Fe(II), As(III), and Sb(III) oxidation experiments were conducted to obtain rates under laboratory conditions similar to those found in the field for mine waters of both low and circumneutral pH. Additional experiments were performed under abiotic sterile conditions to determine the biotic and abiotic contributions to the oxidation processes. Our results showed that under abiotic conditions in aerated Fe(III)-sulfuric acid solutions, Sb(III) oxidizes slightly faster than As(III). The oxidation rates of both elements were accelerated by increasing As(III), Sb(III), Fe(III), and chloride concentrations in the presence of light. For unfiltered circumneutral water from the Giant Mine (Yellowknife, NWT, Canada), As(III) oxidized at 15-78 µmol/L/h whereas Sb(III) oxidized at 0.03-0.05 µmol/L/h during microbial exponential growth. In contrast, As(III) and Sb(III) oxidation rates of 0.01-0.03 and 0.01-0.02 µmol/L/h, respectively, were obtained in experiments performed with acid unfiltered mine waters from the Iberian Pyritic Belt (SW Spain). These results suggest that the Fe(III) formed from microbial oxidation abiotically oxidized As(III) and Sb(III). After sterile filtration of both mine water samples, neither As(III), Sb(III), nor Fe(II) oxidation was observed. Hence, under the experimental conditions, bacteria were catalyzing arsenic and antimony oxidation in the Giant Mine waters and iron oxidation in the acid waters of the Iberian Pyrite Belt.