Arsenic fractionation in soils using an improved sequential extraction procedure

Risk assessment of contaminants requires simple, meaningful tools to obtain information on contaminant pools of differential lability and bioavailability in the soil. We developed and tested a sequential extraction procedure (SEP) for As by choosing extraction reagents commonly used for sequential extraction of metals, Se and P. Tests with alternative extractants that have been used in SEPs for P and metals, including NH4NO3, NaOAc, NH2OH·HCl, EDTA, NH4OH and NH4F, were shown to either have only low extraction efficiency for As, or to be insufficiently selective or specific for the phases targeted. The final sequence obtained includes the following five extraction steps: (1) 0.05 M (NH4)2SO4, 20°C/4 h; (2) 0.05 M NH4H2PO4, 20°C/16 h; (3) 0.2 M NH4+-oxalate buffer in the dark, pH 3.25, 20°C/4 h; (4) 0.2 M NH4+-oxalate buffer+ascorbic acid, pH 3.25, 96°C/0.5 h; (5) HNO3/H2O2 microwave digestion. Within the inherent limitations of chemical fractionation, these As fractions appear to be primarily associated with (1) non-specifically sorbed; (2) specifically-sorbed; (3) amorphous and poorly-crystalline hydrous oxides of Fe and Al; (4) well-crystallized hydrous oxides of Fe and Al; and (5) residual phases. This interpretation is supported by selectivity and specificity tests on soils and pure mineral phases, and by energy dispersive X-ray microanalysis (EDXMA) of As in selected soils. Partitioning of As among these five fractions in 20 soils was (%, medians and ranges): (1) 0.24 (0.02–3.8); (2) 9.5 (2.6–25); (3) 42.3 (12–73); (4) 29.2 (13–39); and (5) 17.5 (1.1–38). The modified SEP is easily adaptable in routine soil analysis, is dependable as indicated by repeatability (w≥0.98) and recovery tests. This SEP can be useful in predicting the changes in the lability of As in various solid phases as a result of soil remediation or alteration in environmental factors.