Can earthworm-secreted calcium carbonate immobilise Zn in contaminated soils?

• Earthworms secrete granules of calcium carbonate.
• The granules are mainly calcite with small amounts of vaterite.
• In Zn-bearing soils some Zn is incorporated into granules during their formation.
• The majority of the Zn is either adsorbed or incorporated into the calcite lattice.
• The granules contain insufficient Zn to significantly impact Zn mobility in soils.

We investigated the interaction of soil Zn with calcium carbonate granules secreted by the earthworm Lumbricus terrestris. Earthworms were kept in agricultural soil amended with ZnSO4 to give Zn concentrations up to 750 mg kg−1 and in two former mine site soils containing 500 and 120 mg Zn kg−1. After 21–42 days the earthworms accumulated 260–470 mg Zn kg−1. Granule production was 0.027–2.11 mg CaCO3 earthworm−1 day−1 and was positively correlated with soil and soil solution pH (r = 0.66 and 0.85 respectively, p ≤ 0.01). Granule Zn concentration was 34–163 mg kg−1. Granules collected from Zn-free control soil and left in Zn-bearing soil for 28 days contained 49–60 mg Zn kg−1 suggesting that the majority of Zn associates with granules after their secretion. However, synchrotron X-ray fluorescence indicates some incorporation of Zn into granules during their formation. X-ray diffraction and X-ray absorption spectroscopy indicate that the granules are predominantly calcite and the spectroscopy suggests that the Zn is either adsorbed to, or incorporated into, the calcite lattice. X-ray diffraction of the outer c. 35 μm of the granules supports incorporation of Zn into the calcite lattice. The low granule production rates in the mine site soils and the granule Zn concentrations suggest that earthworm secreted calcium carbonate is unlikely to impact significantly on Zn mobility in soils.