Comparing CaCl2, EDTA and DGT methods to predict Cd and Ni accumulation in rice grains from contaminated soils

Abstract

Urbanization and industrialization have elevated metal concentrations in soils. However, systematic investigation on their availability in regional soils under industrial impacts is lacking. In this study, 230 paired soil-rice samples were collected from two areas in Southeast China, with low and high industrial impacts. Classic equilibrium-based CaCl2 and EDTA extraction methods, and dynamic-based diffusive gradients in thin-films (DGT) technique were used to study metal availability in soils, with the results being compared with metal concentrations in soils and rice grains. Generally, Cd, Ni, Cu, Zn, Cr and Pb concentrations in soils exceeded the Chinese Soil Quality Standard (GB15618-2018), whereas only Cd and Ni in some rice grains exceeded the Chinese Safety Guidelines. CaCl2 and EDTA extractions, DGT method and soil total metal concentrations provided good predication of grain Cd (R = 0.51–0.66, p < 0.01), whereas only CaCl2 and DGT tests provided good predication of grain Ni (R = 0.36–0.47, p < 0.01). Overall, CaCl2 extraction best predicted Cd and Ni accumulation in rice grains, explaining 66% of grain Cd and 47% of grain Ni. The extraction rate of available Cd was higher than that of Ni, indicating higher Cd availability than Ni, consistent with the parameters (response time, Tc, and desorption rate, k–1) from DIFS (DGT-induced flux in soils) model and bioconcentration factor values. This study showed that, at regional scale, CaCl2 extraction method is efficient in predicting Cd and Ni accumulation in rice grains from contaminated soils.