OXIDATION OF As(III) AND PRECIPITATION OF SCORODITE IN BIOLEACHING SOLUTIONS AT 30 deg C AND 70 deg C

摘要

One strategy to stabilize dissolved arsenic in bioleaching solutions is oxidation of As(III) to As(V) in order to allow its precipitation as ferric arsenate (FeAsO_4), preferably crystallized like scorodite (FeAsO_4.2H_2O), which is studied in this paper. Studies of As(III)oxidation in bioleaching solutions with Acidithiobacillus ferrooxidans at 30 deg C and with Sulfolobus metallicus at 70 deg C, demonstrated that As(III) oxidation happens by chemical and microbial oxidation only at high oxidation potential (Eh>450 mV vs Ag/AgCl). The chemical oxidation of As(III) by Fe(III) is not spontaneous and it only happens in the presence of pyrite and micro-organisms, because pyrite acting as electrochemical and surface catalyst, whereas the micro-organisms play the roll to maintain high potentials. In addition, A. ferrooxidans as S. metallicus would be contributing to the total As(III) because they would have oxidative activity of As(III). The kinetic analysis of As(III) oxidation in bioleaching solutions demonstrated that to 70 deg C with S. metallicus is six times faster than to 30 deg C with A. ferrooxidans, and in both cases the As(III) oxidation responds to a kinetic of first order. Scorodite precipitation was observed in all the experiments of As(III) oxidation in bioleaching solutions to 70 deg C with S. metallicus and 1 percent of pyrite, while amorphous and complex ferric arsenate compounds like tooleite among others were formed to 30 deg C with A. ferrooxidans and 1 percent of pyrite, according to X-ray diffraction analysis. The mass balance of the studies to 70 deg C with S. metallicus demonstrates that 100 percent of As(III) was oxidized and 55 percent of this was precipitated like scorodite.