Hypersaline Potash Mine Tailings and Brine: Microbial Communities and Metal Biosorption Applications

Abstract

Brine and tailings produced by potash mining operations create hypersaline environments where only highly salt-tolerant organisms are capable of living – generally microbes. Microbial communities within analogous hypersaline environments such as salterns, evaporite deposits, and salt lakes have been characterized in the peer-reviewed literature and individual organisms have been used for various applications in biotechnology such as in cosmetics or pharmaceuticals. Bacterial biomass has also been broadly investigated as a metal biosorbent. However, microbial communities in potash mine tailings and brine and their potential applications in environmental technology has not been extensively studied. These unique microbial communities and biomaterials may offer new ways to manage industrial wastes or remediate contaminated sites under highly saline conditions. In this thesis, the microbial communities within brine, coarse tailings, and fine tailings from a Saskatchewan potash mine were examined. Culture-independent high-throughput amplicon sequencing of the 16S rRNA gene (V4 region) and culture-dependent plating techniques were employed to examine community compositions and salinity tolerance. The brine and tailings materials were all pH neutral, sodium-dominated, and highly saline (370 g/l for brine and > 835 g/kg for tailings). High-throughput sequencing results (206164 total reads) identified a mixed community of archaea and bacteria within the brine pond sample, and bacterially dominated communities in the coarse and fine tailings. Proteobacteria were the most predominant phylum for all samples, making up 41-89% of subsampled sequences, and included high read counts in both classes Gammaproteobacteria and Betaproteobacteria. Twenty-two unique isolates that were relatives of genera observed in the high-throughput sequencing results were identified from spread plates. Isolates included known halophilic and halotolerant Archaea (Haloferax and Halorubrum species) and Bacteria (including Halomonas, Marinobacter, and Dietzia species). Salt tolerance of 0-25% (w/v) NaCl was demonstrated by 13 of the isolates, while all isolates were capable of growth in the presence of at least 15% (w/v) NaCl. The halotolerant bacterial isolate Croceicoccus sp. FTI14, selected for its fast growth in 3% (w/v) NaCl amended media, was evaluated as a potential biosorbent for the removal of dissolved Cu(II) and Cr(VI) from saline groundwater (0.55 M ionic strength). Biosorption performance of the oven-dried and finely ground material was evaluated using batch biosorption experiments at varied ionic strengths, coupled with synchrotron-based scanning X-ray transmission microscopy (STXM) and Fourier Transform Infrared (FTIR) spectroscopy. Cu(II) uptake by dried FTI14 was 1.7-7.8 times higher than Cr(VI) uptake and metal uptake decreased when ionic strength of the solution was increased. At pH 4-5 and with 40 mg/l initial metal concentrations, FTI14 removed 40.3 ± 0.7% of the dissolved Cu(II) from deionized water and 19.3 ± 0.1% from saline groundwater solutions. Biosorption isotherms for Cu(II) fit both Langmuir (R2 values of at least 0.80) and Freundlich models (R2 values of at least 0.86), while the Cr(VI) isotherm fit the Freundlich model only (R2 value = 0.97). STXM images showed that the adsorbent was a mixture of whole cells and indistinct biomass as well as demonstrated a spatial association between metal and biomass. FTIR spectra data suggested a change in amide functional groups, potentially the proteins on the biomass surface, after metal exposure. Findings suggest that the removal of metals from salt-impacted water is possible using biosorbents derived from salt-tolerant bacteria. This is the first study to utilize high throughput sequencing to investigate the membership and diversity of microbial communities in potash tailings and brine. It contributes to the broader understanding of halophilic and halotolerant microbes in natural and engineered environments, as well as investigates a potential environmental engineering application of biomaterials derived from them.