Development of Iron-Phosphate Biofilms on Pyritic Mine Waste Rock Surfaces Previously Treated with Natural Phosphate Rocks

Abstract

Various treatments have been proposed to attenuate and eventually inhibit the generation of acid mine drainage (AMD) or acid-rock drainage (ARD). The addition of Natural Phosphate Rocks (NPR) to mining wastes has been shown to reduce acid generation. The biogeochemical reactions underlying the phosphate precipitation reactions are however poorly understood, even though the chemical reactions are well defined. The present study was designed to study the role of solution chemistry and bacterial activity on phosphate precipitation on waste rock surfaces. Waste rock samples (rich in sulphides) previously weathered for 989 days in the presence of NPR were submersed in 2 different phosphate-rich growth media in order to enhance the growth of acidophilic and neutrophilic bacteria. DAPI and FISH analyses revealed that most cells belonged to the bacteria domain, and that alpha- and beta-proteobacteria were the dominant neutrophiles. ESEM, SEM and TEM observations of the samples revealed the presence of a biofilm on the surface of the rocks at both pH conditions. Bacteria and fine-grained precipitates were trapped in an exopolymer matrix. At low pH, the formation of fine precipitates rich in Fe and P within the biofilm corresponded to a decline of phosphate concentrations in the growth medium. This was in agreement with the solubility calculations which indicated that the medium became over-saturated with respect to some Fe-phosphate minerals. In the pH neutral system, solubility calculations indicated that Ca- and Mg-phosphate minerals were stable, but they were not detected in the biofilm. Solubility calculations also indicated that vivianite became unstable over time, which could explain the release of soluble phosphate over time in the pH neutral system. Our results showed that precipitation reactions played an important role in the solubility of phosphate in both systems, but a series of complex nucleation reactions involving bacterial exopolymers and the presence of microenvironments within the biofilms were likely important factors as well. Our findings also imply that the reduction of acid generation in NPR-treated waste rocks could be due in part to the formation of biofilms on the rock surfaces because the biofilms would act as a physical and chemcial barrier to limit the extent of pyrite oxidation.

Keywords:

• attenuation of acid mine drainage
• iron phosphate biofilms
• natural phosphate rock (NPR)

The project was supported by a research grant from the Centre for Research in Earth and Space Technology (CRESTech) and Boojum Research Limited. We thank F. G. Ferris (University of Toronto), P. Hamilton (Canadian Museum of Nature), and L. Ling (Carleton University) for their assistance with SEM and ESEM analyses. We also thank B. Harris for his help with the transmission electron microscopy observations and K. Kato and T. Maruyama (Shizuoka University) for their help with FISH analysis.

Notes

*most PO₄ ³⁻ came from the organic substrates: (Tryptone (0.25 g/L), peptone (0.25 g/L), and yeast extract (0.50 g/L)).

*: From the database of Mineql + v.3.01b.

**: From Fytianos et al. (1998).

*: No Fe(III) was present in the medium at time 0.

*: No Fe(III) was present in the medium at time 0.

*: Fe(III) was added as a soluble species even though it was not detected after 14 days. Since it was likely below the flame atomic absorption detection limit (i.e., 1.8 × 10⁻⁶ M), a concentration of 10⁻¹⁸ M was selected as a representative value.