Biomineralisation of metals in soil – effect of metal toxicity and precipitation as a protective mechanism

Biomineralisation offers the potential for in-situ sequestration and subsequent reduction in the bioavailability of heavy metals and radionuclides in the subsurface environment. Calcium carbonate minerals are known to sorb and form solid solutions with a range of target elements, and are readily produced by the actions of common microorganisms on simple chemical precursors. The ability of a commonly used urea-degrading, calcium carbonate-precipitating bacterium, Sporosarcina pasteurii, to tolerate the presence of a model contaminant, strontium, was determined in aqueous solution, with reduction in growth only seen at concentrations of 10 mM. Its ability to remove strontium from solution via calcium carbonate precipitation was then determined, and here S. pasteurii was shown to be able to remove 99% (+/1%) strontium from solution at concentrations up to 30 mM. This suggests that biomineralisation of metallic elements may afford a protective mechanism for the bacteria through providing a means to reduce the overall concentrations to tolerable levels. Finally, we explored the effects of ground conditions on mineralisation and strontium sequestration in different sand fractions (fine, medium and coarse), in a series of batch experiments. Almost all (97-99%) strontium present was removed from aqueous solution after three days, whereas no precipitation was observed in control samples over the same period. The amount of strontium removed increased with coarseness of sand grains under these static conditions, although over a very small range.