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 NH4OAc, CaCl2-DTPA, 0.01 mol/L CaCl2, 0.1 mol/L CaCl2, 0.5 mol/L NaH2PO4, 0.05 mol/L NaHCO3, 0.1 mol/L NaNO3, 0.1 mol/L HNO3), 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, CaCl2, HNO3, and CaCl2-DTPA solutions exhibited high extraction efficiency for Cd (42.2–88.4 %); for As, NaH2PO4, HCl, and HNO3 have the extraction efficiency (0.85–23.4 %). The concentration of Cd extracted by 0.01 mol/L CaCl2 was significantly positively correlated with Cd levels in rice. The potential risk extraction threshold of CaCl2 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 CaCl2-DTPA and 0.05 mol/L NaHCO3 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 CaCl2, CaCl2-DTPA, and 0.05 mol/L NaHCO3 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. [Display omitted] •The content of Cd & As in rice was positively correlated with the available Cd & As abundance in soil.•HCl, DTPA, CaCl2, and HNO3 showed high extraction efficiency for Cd (>60 %).•NaH2PO4 showed high extraction efficiency for As (13.4–23.4 %).•The positive correlation between CaCl2 or NaHCO3 extracted Cd-As and in rice content.•The critical risk values of Cd-As in sandy and clay soil are studied and calculated.