Influence of tryptophan on formation of biogenic secondary iron hydroxysulfate minerals

Secondary iron hydroxysulfate minerals were synthesized by A. ferrooxidans in solution with different L⁃tryptophan concentrations. Trends in pH, oxidation reduction potential (ORP), Fe2+ oxidation efficiency and total Fe precipitation efficiency, and the amount, elemental composition and mineral phase of secondary minerals in incubation systems were investigated. The results indicated that the higher the concentration of L⁃tryptophan in systems, the smaller the decreasing range of pH and the increasing range of ORP. The effect of L⁃tryptophan on secondary iron hydroxysulfate minerals formation is related to the concentration of L⁃tryptophan. When the concentration of L⁃tryptophan was lower than 1.67 g·L-1, the more L⁃tryptophan added in systems, the higher total precipitation efficiency and the more amount of biogenic minerals formed, indicating that L⁃tryptophan could stimulate iron hydroxysulfate mineral formation under a certain concentration. While the L⁃tryptophan addition was up to 6.67 g·L-1, Fe2+ oxidation efficiency, total precipitation efficiency and the amount of biogenic minerals formed in this system were far below those of the control system, suggesting that high concentration of L⁃ tryptophan would restrain the formation of iron hydroxysulfate mineral. The Fe / S ratios of secondary minerals were between the theoretical value of jarosite and schwertmannite, suggesting that the secondary minerals obtained from the five systems were all mixtures of jarosite and schwertmannite. Mineralogical analyses showed that A. ferrooxidans tended to mediate the formation of schwertmannite from jarosite with the increasing of L⁃tryptophan concentration.