Assessing Critical Metal Incorporation in Ca-Carbonate Minerals using Cyanobacteria: Application to Mine Site Environments

Abstract

Meeting the current global market demand for critical metals will require an increase in the number of mining operations in Canada. While mining will generate mine tailings, which can be an environmental concern, tailings also present an opportunity for carbon sequestration and metal recovery. Carbon sequestration in mine tailings can be implemented by using divalent cations from the tailings to form stable carbonate minerals. Ultramafic mine tailings typically contain an abundance of divalent cations, including various transition metals, that can be incorporated into carbonate minerals. During this process, critical metal enrichments are possible, and thus tailings may become valuable sources of metals as high-grade ore deposits continually become less accessible for mining. Microorganisms, including cyanobacteria, can contribute to carbonate mineral precipitation, however, this process is understudied with respect to transition metal incorporation into biogenic carbonate minerals. This thesis explores the application of these processes to ultramafic mine sites through two laboratory experiments using a pure culture of cyanobacteria, 𝘚𝘺𝘯𝘦𝘤𝘩𝘰𝘤𝘰𝘤𝘤𝘶𝘴 𝘭𝘦𝘰𝘱𝘰𝘭𝘪𝘦𝘯𝘴𝘪𝘴. In the first experiment, a biosorption study explores the metal sorption abilities of 𝘚. 𝘭𝘦𝘰𝘱𝘰𝘭𝘪𝘦𝘯𝘴𝘪𝘴 in a nutrient limited environment. The second experiment examines the incorporation of transition metals into precipitates during microbial mineral carbonation. In Chapter 2, the ability of 𝘚. 𝘭𝘦𝘰𝘱𝘰𝘭𝘪𝘦𝘯𝘴𝘪𝘴 to remove Co²⁺ and Ni²⁺ from solution via biosorption in a nutrient limited environment what tested in a lab experiment. Comparison between nutrient enriched conditions and a nutrient deficient condition (simulating an ultramafic mine site solution) was conducted in mono-metal and di-metal systems. The results revealed that measured nickel and cobalt concentrations were lowest in the first 3 days of the experiment, which indicates a fast metal removal rate. The biosorption of nickel and cobalt was upwards of 34.2–49.4% removal of metal from solution. Imposing nutrient limitations caused increased production of extracellular polymeric substances (EPS), which can increase metal sorption, and resulted in a decrease in measured nickel concentrations in solution in the di-metal system. The findings from this experiment indicate that inducing additional stress through metal exposure and nutrient limitations can increase the metal biosorption capacity of 𝘚. 𝘭𝘦𝘰𝘱𝘰𝘭𝘪𝘦𝘯𝘴𝘪𝘴. In Chapter 3, a microbially induced carbonate precipitation experiment was conducted to test the incorporation of nickel and cobalt into biogenic calcium carbonate mineral precipitates. These results are preliminary due to an experimental failure that occurred. Nevertheless, the measured concentrations of dissolved cobalt and nickel in solution indicated metal(s) removal success of up to 89.5% and 94.5% in the first day after metal addition. Observation of the biofilms using scanning electron microscopy (SEM) revealed nanometer-scale amorphous calcium carbonate (ACC) precipitates. This preliminary result suggests that inducing calcium carbonate precipitation may remove dissolved metals solution. The results from Chapter 2 and Chapter 3 together reveal that 𝘚. 𝘭𝘦𝘰𝘱𝘰𝘭𝘪𝘦𝘯𝘴𝘪𝘴 can quickly remove metals from solution, which could be applied to both metal recovery and remediation projects. Data from this research could be applied to the development of photobioreactors at ultramafic mine sites. The outcomes suggest that inducing nutrient limitation can enhance metal removal by increasing metal binding through enhanced EPS production. The research presented in this thesis will contribute to the development of sustainable mine operations, with the aim of recovering metals from tailings, and lowering CO₂ emissions, thereby working towards net-neutral mining operations.