Biosolubilization of low-grade rock phosphate by mixed thermophilic iron-oxidizing bacteria

A bacterial culture, which was mixed by three thermophilic iron-oxidizing bacteria, namely Sulfobacillus thermosulfidooxidans, Leptospirillum ferriphilum and Acidithiobacillus caldus, was used to solubilize low grade rock phosphate (RP) in 9K basal salts medium containing pyrite as an energy substrate. Culture of a single mesophilic iron-oxidizing bacterium Acidithiobacillus ferrooxidans was used as control. Experimental results show that the phosphate solubilizing capacity of the mixed thermophilic iron-oxidizing bacteria was more effective than that of the single bacterium, and such positive effect was mainly attributed to the bioaugmentation of pyrite oxidation with coinoculation of these thermophilic iron-oxidizing bacteria. Results also show that the biosolubilization of low-grade RP by the mixed thermophilic iron-oxidizing bacteria was influenced markedly by environmental conditions. The highest phosphate releasing rate was achieved at 45-50 degrees C. The rate of phosphate released was highest when the pH was at range from 2.0 to 2.5. The increase in pulp density generates a decrease in the phosphate releasing rate, if the pulp density exceeded 3% w/v. The culture led to the highest phosphate releasing rate when the mass ratio of pyrite to RP was at 2:1 or 3:1. However, volume ratio between thermophilic iron-oxidizing bacteria had no significant effect on the rate of phosphate released.