Dissolution of Fluorapatite by Pseudomonas fluorescens P35 Resulting in Fluorine Release

ABSTRACT

Chemical weathering of fluorine-bearing minerals is widely accepted as the main mechanism for the release of fluorine (F) to groundwater. Here, we propose a potential mechanism of F release via microbial dissolution of fluorapatite (Ca₅(PO₄)₃F), which has been neglected previously. Batch culture experiments were conducted at 30°C with a phosphate-solubilizing bacteria strain, Pseudomonas fluorescens P35, and rock phosphates as the sole source of phosphate for microbial growth in parallel with abiotic controls. Rock phosphates consisted of 55–91% of fluorapatite and 5–10% of dolomite before microbial dissolution as indicated by X-ray diffraction (XRD). Mineral composition and morphology changed after microbial dissolution characterized by the disappearance of dolomite and the development of etched cavities on rock phosphate surfaces. The pH of media used was approximately 7.4 at the beginning and increased gradually to 7.7 in abiotic controls; with the inoculum, the pH decreased to acidic values of 3.7–3.8 after 27 h. Phosphate, calcium, and fluoride were released from the rock phosphate to the acidified medium. At 42 h, the concentration of F reached 8.1–10.3 mg L⁻¹. The elevated F concentration was two times higher than the F levels in groundwater in regions diagnosed with fluorosis, and was toxic to the bacteria, as demonstrated by a precipitous decrease in live cells. Geochemical modeling demonstrated that the oxidation of glucose (the carbon source for microbial growth in the medium) to gluconic acid could decrease the pH to 3.7–3.8 and result in the dissolution of fluorapatite and dolomite. Dolomite and fluorapatite remained unsaturated, while concentrations of dissolved phosphorus (P), calcium (Ca), and F increased throughout the time course Fluorite reached saturation [saturation index (SI) 0.22–0.42] after 42 h in rock phosphate–amended biotic systems. However, fluorite was not detected in XRD patterns of the final residue from microcosms. Given that phosphate-solubilizing bacteria are ubiquitous in soil and groundwater ecosystems, they could play an important role in fluorapatite dissolution and the release of F to groundwater.

KEYWORDS:

• Fluorapatite
• high-fluoride groundwater
• microbial dissolution of minerals
• phosphate-solubilizing bacteria

Acknowledgments

The XRD and XRF analyses were performed at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan. All other analytical work was carried out in the Geomicrobiology Laboratory and SEM Laboratory, State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan. The authors thank Deng Liu for his help in XRD data analysis. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Funding

This research was jointly supported by the Natural Science Foundation of China project (41130207, 41572325) and the 111 Project (grant no. B08030).