[B-CG07] Geo-Bio Interactions and its Applications

[BCG07-P03] Remediation of acid mine drainage using microbially induced mineralization of calcium phosphate

*Toshio Sudo¹, Yohey Suzuki¹, Mariko Kouduka¹ (1.Department of Earth and Planetary Science, The University of Tokyo)

Keywords:Acid Mine Drainage, Remediation, Calcium phosphate

Acid mine drainage (AMD) is a global environmental problem which is occurred mainly due to the metal toxicity to most of living organisms. Cost-effective remediation technology is needed for the subsurface environment where the remediation of AMD-contaminated groundwater is technically difficult. In general, the high cost of subsurface remediation is resulted from excavation of soils or pumping of groundwater from wells. Therefore, in-situ remediation using microorganisms is expected to be a cost-effective alternative. Since calcium phosphate is known to remove metals from solution, incubation experiments were performed to testify if microbial formation of calcium phosphate results in metal removal from naturally occurring AMD solution from the Yoshino Mine in Yokote City, Akita Prefecture, Japan. For incubations, an AMD-amended culture medium was inoculated in 6 glass vials with sediments from a pond in the Koishikawa Botanical Garden. For a negative control, the culture medium was incubated with autoclaved sediments in one glass vial. After two-week incubation, the total cell count and pH increased from ~2 × 10⁶ cells/mL to ~0.2-3 × 10⁸ cells/mL and from 5.5 to ~8, respectively. In contrast, the increase in total cell count and pH were not evident in one of six incubation vial and the negative control. The decreases in Ca and phosphate concentrations in solution were confirmed by ion chromatography for the five vials accompanied with the pH increase to 8. In addition, the formation of calcium phosphate was demonstrated by solid phase analyses using an attenuated total reflection Fourier transform infrared spectrophotometer (ATR-FT-IR) and a field-emission scanning electron microscope (FE-SEM) equipped with energy dispersive X-ray spectrometer (EDS). From these results, it becomes clear that the increase in pH and the formation of calcium phosphate are attributed to microbial activity. In terms of the removal of metals, changes in metal concentrations in solution were analyzes by inductively coupled plasma mass spectrometer (ICP-MS). As a result, Cu, Zn, Cd, and Pb concentrations were decreased from several ppm to sub-ppm levels in the five vials accompanied with the pH increase to 8, while in one of the five vials, the decrease in Cu was limited. From this exceptional result, it was clarified that the decrease in Cu concentration cannot be explained by the adsorption to the container. To investigate the processes involved in metal removal, the pH was increased to 8 by the manual addition of sodium hydroxide solution. Although Zn, Cd, and Pb were removed with the formation of calcium phosphate, Cu was not removed from the solution. This result indicates that the behavior of Cu in the culture medium is different from those of the other metals. By SEM-EDS analysis, Cu was detected with S, suggesting that Cu may be removed from the solution as a metal sulfide. Although this study is not conclusive about the metal removal processes such as the adsorption on containers (except for Cu), the sorption on microbial cells and/or calcium phosphate, or metal phosphate formation, the metal removal from the AMD was successfully mediated by the stimulation of microbial activity. Future research will be needed to clarify the removal processes in order to apply this microbial process to the remediation of the subsurface environment.