Evaluation of Different Extractants to Estimate Bioavailable Arsenic in Soil

Abstract

Owing to the similar chemistry of phosphorus (P) and arsenic (As), sodium bicarbonate (0.5 N NaHCO3) is commonly used to extract plant accessible As in soil. This extractant has neither been tested widely in relation to plant As, nor is this extractant compatible with inductively coupled plasma mass spectrometry (ICP-MS) due to the high concentration of dissolved solid. Subsequently, it is of utmost important to design a suitable chemical extraction method in order to estimate plant available As compatibility with ICP-MS. For this purpose, paired soil and plant samples were collected from paddy fields located in Nadia, West Bengal, India. Soil was extracted with 0.5 M NaHCO3, 0.1 N and 0.5 N phosphoric acid (H3PO4), 0.1 N and 0.5 N sulfuric acid (H2SO4), 0.1 N, 0.5 N, 1.0 N, 1.5 N HNO3 and 0.01 M calcium chloride (CaCl2) solution. Arsenic extracted with NaHCO3, H3PO4 and H2SO4 was determined in hydride generation-atomic absorption spectrophotometer (HG-AAS), while ICP-MS served to determine As extracted from soil with HNO3. Olsen-extractable As in soils ranged from 0.48 to 3.57 mg kg−1 with a mean value of 1.45 mg kg−1. The extractable As content in soil varied from 0.01 to 10.1 mg kg−1 across the extractants. In the case of grain As, 0.1 N H3PO4, 0.5 N NaHCO3 and 1.5 N HNO3 extractable As had distinctly higher correlation coefficients (r = 0.49**, r = 0.47**, r = 0.45**) when compared to other extractants. More or less similar relationships of extractable As were obtained with straw As content like that of rice grain. In view of rapidity of the soil test method for As, 1.5 N HNO3 can be recommended for assessing available As in soil.