Influence of pH on mineral speciation in a bioreactor simulating acid mine drainage

Mineral precipitates formed under conditions representative of acid mine drainage were prepared by oxidizing 0.1 M FeS04 · 7H20 solutions at 24°C and pH 2.3, 2.6, 3.0, 3.3 and 3.6 using a bioreactor and a strain ofThiobacillus ferrooxidans. The oxidation of dissolved Fe2+ was monitored colorimetrically and was completed within 90 to 120 h at all pHs. Schwertmannite, Fe8O8(OH)6SO4, was a major component of the precipitates and was the only phase formed at pH 3.0. Jarosite, (H,Na,K)Fe3(OH)6(SO4)2, increased in abundance with decreasing pH whereas goethite, α-FeOOH, appeared at pH 3.3 and 3.6. A similar relationship between pH and mineralogy has been reported in natural specimens of mine drainage ochres.