The enhanced effect of Acidithiobacillus ferrooxidans on pyrite based Cr(VI) reduction

Cr(VI) pollution attracted increasing attention in these years due to its toxic and carcinogenic effect on living body. Pyrite based remediation was an economical and efficient way in Cr(VI) reduction. However, the bulk body of pyrite cannot be effectively utilized due to the passivation effect. In this study, Fe/S oxidizing bacteria Acidithiobacillus ferrooxidans was introduced to the reduction system to relieve the inhibition effect, and resultly accelerated pyrite dissolution and increased the corresponding Cr(VI) reduction efficiency. The results showed that pyrite based Cr(VI) reduction was a proton consumption process. Pyrite dissolution in Cr(VI) reduction is inhibited due to the passivation effect when A. ferrooxidans were absent. Reduction efficiency of A. ferrooxidans system is 4.42 times higher than that of chemical system. The highest reduction efficiency was achieved under S/FeS2 ratio 1:1 condition. Cr(VI) were reduced in two ways, fast free Fe(II) reduction and the slowly pyrite surface active sites reduction. The activity of A. ferrooxidans promoted pyrite disulfide bond cleavage and Cr(III) release. In addition, it oxidized intermediate state sulfur S-n(2-) and S-0 to sulfate, which contributed to pyrite dissolution. A. ferrooxidans adhesion and biofilm formation on pyrite surface facilitated passivation layer removal, which was the indicator of the deep interaction between bacteria and mineral. Furthermore, acidophilic bacteria mediated secondary iron mineral formation and Cr(III) coprecipitation. A. ferrooxidans play the roles as the decomposer of pyrite and template of the secondary minerals formation.