Cadmium contamination of agricultural soils and crops resulting from sphalerite weathering

•Sphalerite containing cadmium presents a hazard when present in agricultural soils.•Sphalerite dissolution was slow (0.6-1.2% y−1) but constant in contrasting soils.•Cadmium was released during dissolution and was bioavailable to wheat and rice.•Wheat grains accumulated potentially harmful cadmium concentrations.•Flooded paddy (reducing) soils reduced cadmium bioavailability to rice.

The biogeochemistry and bioavailability of cadmium, released during sphalerite weathering in soils, were investigated under contrasting agricultural scenarios to assess health risks associated with sphalerite dust transport to productive soils from mining. Laboratory experiments (365 d) on temperate and sub-tropical soils amended with sphalerite (<63 μm, 0.92 wt.% Cd) showed continuous, slow dissolution (0.6-1.2% y−1). Wheat grown in spiked temperate soil accumulated ≈38% (29 μmol kg−1) of the liberated Cd, exceeding food safety limits. In contrast, rice grown in flooded sub-tropical soil accumulated far less Cd (0.60 μmol kg−1) due to neutral soil pH and Cd bioavailability was possibly also controlled by secondary sulfide formation. The results demonstrate long-term release of Cd to soil porewaters during sphalerite weathering. Under oxic conditions, Cd may be sufficiently bioavailable to contaminate crops destined for human consumption; however flooded rice production limits the impact of sphalerite contamination.