Comparative efficiency of three advanced oxidation processes for thiosalts oxidation in mine-impacted water

Abstract

Grinding and flotation of sulfide-containing ores produce metastable, partially oxidized sulfur compounds known as thiosalts, which are notorious contributors to receiving water bodies’ toxicity by producing delayed acidity. Thus, the continuous optimization of thiosalts oxidation into sulfate is required before treated mine water is discharged into natural streams, as thiosalts oxidation slows down at low temperatures and in the absence of UV. With this aim, the present study evaluated the treatment of thiosalts at initial concentrations of 100 mg/L, 290 mg/L and 630 mg/L using three advanced oxidation processes (AOPs): (1)wet ferrates Fe(VI), 65 mg/L; (2)ozone microbubbles (O3-microbubbles), 75 g/h sparging rate; and (3)hydrogen peroxide (H2O2), 2:1 (H2O2:thiosalts) molar ratio, at 8 °C and 22 °C. All treatability tests were performed on synthetic effluents, while H2O2 efficiency was also assessed with a real effluent, a leachate of non-desulfurized tailings collected at a flotation plant. Results showed that O3-microbubbles gave better efficiency (99%) than wet Fe(VI) (82%) after 2 h of thiosulfate treatment. Similar trends were observed with synthetic and real effluents for treatment with H2O2. Efficient removal of thiosalts in the real effluent required several days of reaction at 8 °C (98%), whereas efficient removal at 22 °C (90%) was reached after 1 h of treatment. All three AOPs tested proved promising for thiosalts removal. The O3-microbubbles showed the best efficiency in terms of thiosalts removal and treatment time. Moreover, H2O2 allowed for better thiosulfate removal than wet Fe(VI), but proved inefficient at oxidizing other intermediate S species and required longer treatment time.