Exploration of the bio-availability and the risk thresholds of cadmium and arsenic in contaminated paddy soils.

Cadmium (Cd) and arsenic (As) contamination risk in paddy soils has raised global concern. In order to scientifically and objectively evaluate the bioavailability of soil Cd, As and the risk of Cd or As threshold in contaminated farmland, this study was conducted to investigate different types of extractants for their potential extraction efficiency of Cd and As. Soils from two different parent materials in Hunan, Yueyang and Yiyang, typical double-cropping rice production areas in the south of China, were used as test soils. The extraction capabilities of 10 extractants (ultrapure water, 0.1 mol/L HCl, 1.0 mol/L NHOAc, CaCl-DTPA, 0.01 mol/L CaCl, 0.1 mol/L CaCl, 0.5 mol/L NaHPO, 0.05 mol/L NaHCO, 0.1 mol/L NaNO, 0.1 mol/L HNO), were compared for their extraction capabilities of soil available Cd and As. Meanwhile, the content of Cd and As in plants issues and grains of rice was monitored during harvest, and the Cd, As content correlation between extracted forms and rice was analyzed. The results showed that the HCl, CaCl, HNO, and CaCl-DTPA solutions exhibited high extraction efficiency for Cd (42.2-88.4 %); for As, NaHPO, HCl, and HNO have the extraction efficiency (0.85-23.4 %). The concentration of Cd extracted by 0.01 mol/L CaCl was significantly positively correlated with Cd levels in rice. The potential risk extraction threshold of CaCl in sandy loam soil was 0.178 mg/kg, while it was 0.312 mg/kg in clay soil. The concentration of As extracted by CaCl-DTPA and 0.05 mol/L NaHCO in clay soil was significantly positively correlated with As levels in rice, the potential risk extraction thresholds were 0.115 mol/L and 0.106 mg/kg, respectively. These investigations indicated that the heavy metals extraction methods by 0.01 mol/L CaCl, CaCl-DTPA, and 0.05 mol/L NaHCO could reflect the Cd and As pollution degree in farmland and suggest their potential to serve as methods for assessing the risk of Cd and As pollution in sandy loam and clay paddy soil.