Incorporation of strontium in earthworm-secreted calcium carbonate granules produced in strontium-amended and strontium-bearing soil

This paper investigates the incorporation of Sr into biomineralized calcium carbonate granules secreted by the earthworm Lumbricus terrestris. Experiments were conducted using an agricultural soil amended with Sr(NO3)2 to give concentrations in the range 50 - 500 mg kg-1 Sr and a naturally Sr-rich, Celestine-bearing soil containing up to 11 000 mg kg-1 Sr. Granule production rates were in the range 0.26 – 2.3 mgCaCO3 earthworm-1 day-1; they showed no relationship with soil or soil solution Sr concentration but decreased with decreasing pH. Strong relationships exist (r2 > 0.8, p < 0.01) between the Sr concentrations and Sr / Ca ratios of the granules and those of the soil, soil solution and earthworms. The highest bulk Sr concentration we recorded in the calcium carbonate granules was 5.1 wt% Sr whilst electron microprobe analysis recorded spot concentrations of up to 4.3 wt % Sr. X-ray diffraction and X-ray absorption spectroscopy indicate that the majority of the calcium carbonate is present as Sr-bearing calcite with trace amounts of Sr-bearing vaterite also being present. The granules produced in the Sr-amended soils concentrated Sr relative to Ca from the bulk soil and the earthworms. This suggests that earthworm secreted calcium carbonate may be significant in the cycling of 90Sr released into soils via nuclear accidents or leakage from nuclear waste storage facilities.