Release of dissolved cadmium and sulfur nanoparticles from oxidizing sulfide minerals

Abstract

Cadmium enrichment (relative to Fe and Zn) in paddy rice (Oryza sativa L.) grain occurs during the preharvest drainage of flooded soil, which causes oxidative dissolution of sulfide minerals present in reduced soil. We investigated this process over a range of environmentally realistic Cd-containing sulfide minerals/mixtures, comparing Cd and Fe-co-precipitated sphalerites (0.6−1.2 mol % Cd, 24 mol % Fe, 74.8−75.4 mol % Zn) with mixtures of the pure sulfides at concentration ratios equal to co-precipitated sphalerites, as well as comparing freeze- and oven-dried sulfide minerals. Solutions were analyzed for dissolved and nanoparticulate elements to determine potential phytoavailability. Cadmium and Fe release from co-precipitated (Cd, Fe, Zn) sphalerites and mixtures of pure sulfides was mostly congruent; however, Zn was released in preference to Cd. The Zn/Cd ratio in solution of a mixture of pure sulfides was higher than that of a co-precipitated (Cd, Fe, Zn) sulfide (Zn/Cd ratio 126 in both starting solids), which may have been due to galvanic interactions. The greater crystallinity of the oven-dried (Cd, Fe, Zn) co-precipitated sphalerite resulted in lower Cd/Fe and Cd/Zn ratios in solution and preferential release of Cd. Nanoparticles comprised as much as 50% of dissolved S, but did not contribute to dissolved metals. Under the environmentally realistic scenario of co-precipitated sphalerite or pure sulfide mixtures, oxidation is unlikely to give rise to the increased availability of Cd relative to Fe and Zn to rice during preharvest drainage.