Influence of Iron Substitution and Solution Composition on Brucite Carbonation.

Carbon neutral or negative mining can potentially be achieved by integrating carbon mineralization processes into the mine design, operations, and closure plans. Brucite [Mg(OH)2] is a highly reactive mineral present in some ultramafic mine tailings with the potential to be rapidly carbonated and can contain significant amounts of ferrous iron [Fe(II)] substituted for Mg; however, the influence of this substitution on carbon mineralization reaction products and efficiency has not been thoroughly constrained. To better assess the efficiency of carbon storage in brucite-bearing tailings, we performed carbonation experiments using synthetic Fe(II)-substituted brucite (0, 6, 23, and 44 mol % Fe) slurries in oxic and anoxic conditions with 10% CO2. Additionally, the carbonation process was evaluated using different background electrolytes (NaCl, Na2SO4, and Na4SiO4). Our results indicate that carbonation efficiency decreases with increasing Fe(II) substitution. In oxic conditions, precipitation of ferrihydrite [Fe10IIIO14(OH)2] and layered double hydroxides {e.g., pyroaurite [Mg6Fe2III(OH)16CO3·4H2O]} limited carbonation efficiency. Carbonation in anoxic environments led to the formation of Fe(II)-substituted nesquehonite (MgCO3·3H2O) and dypingite [Mg5(CO3)4(OH)2·âˆ¼5H2O], as well as chukanovite [Fe2IICO3(OH)2] in the case of 23 and 44 mol % Fe(II)-brucite carbonation. Carbonation efficiencies were consistent between chloride- and sulfate-rich solutions but declined in the presence of dissolved Si due to the formation of amorphous SiO2·nH2O and Fe-Mg silicates. Overall, our results indicate that carbonation efficiency and the long-term fate of stored CO2 may depend on the amount of substituted Fe(II) in both feedstock minerals and carbonate products.

Simple Organic Carbon Sources and High Diversity Inocula Enhance Microbial Bioneutralization of Alkaline Bauxite Residues.

Bioneutralization of pH by microbial fermentation of added carbon substrates is a promising new method for remediation of the 1.7 GT/yr of alkaline mining tailings produced globally. Here, we present the first study to systematically compare and optimize the efficacy of microbial inocula of varying diversities, structures, and provenance and organic carbon substrates of varying complexities on the rate and extent of pH bioneutralization in alkaline bauxite residue tailings. Laboratory-scale bioreactors inoculated with soda lake sediments or with monosaccharide substrates added had a significantly lower minimum pH (<8) and a significantly higher maximum rate of pH neutralization (>0.02 µmol H+ day-1) and achieved these in significantly less time (<26 days) compared to bioreactors with other inocula or substrates. The soda lake sediment introduced a significantly higher-diversity microbial community with a distinct structure (dominated by Euryarchaeota and Bacteroidetes, rather than Acidobacteria and Actinobacteria), supporting higher acetate and formate-yielding fermentation pathways compared to other inocula. The strong performance of monosaccharides is attributed to widespread microbial capacity for efficient fermentation. Using either monosaccharide carbon substrates or soda lake sediments is recommended to maximize bioneutralization efficiency at the industrial scale.

Temperature limits to deep subseafloor life in the Nankai Trough subduction zone.

Microorganisms in marine subsurface sediments substantially contribute to global biomass. Sediments warmer than 40°C account for roughly half the marine sediment volume, but the processes mediated by microbial populations in these hard-to-access environments are poorly understood. We investigated microbial life in up to 1.2-kilometer-deep and up to 120°C hot sediments in the Nankai Trough subduction zone. Above 45°C, concentrations of vegetative cells drop two orders of magnitude and endospores become more than 6000 times more abundant than vegetative cells. Methane is biologically produced and oxidized until sediments reach 80° to 85°C. In 100° to 120°C sediments, isotopic evidence and increased cell concentrations demonstrate the activity of acetate-degrading hyperthermophiles. Above 45°C, populated zones alternate with zones up to 192 meters thick where microbes were undetectable.