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 bioavailabil ity in the soil. We developed and tested a sequential extraction procedure (SEP) for As by choosing extraction reagents commonly used for sequential e xtraction 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 efficien cy for As, or to be insufficiently selective or specific for the phases tar geted. The final sequence obtained includes the following five extraction s teps: (1) 0.05 M (NH4)(2)SO4, 20 degreesC/4 h; (2) 0.05 M NH4H2PO4, 20 degr eesC/16 h; (3) 0.2 M NH4+-oxalate buffer in the dark, pH 3.25, 20 degreesC/ 4h; (4) 0.2 M NH4+-oxalate buffer + ascorbic acid, pH 3.25, 96 degreesC/0.5 h; (5) HNO3/H2O2 microwave digestion. Within the inherent limitations of c hemical fractionation, these As fractions appear to be primarily associated with (1) non-specifically sorbed; (2) specifically-sorbed; (3) amorphous a nd poorly-crystalline hydrous oxides of Fe and Al; (4) well-crystallized hy drous oxides of Fe and Al; and (5) residual phases. This interpretation is supported by selectivity and specificity tests on soils and pure mineral ph ases, and by energy dispersive X-ray microanalysis (EDXMA) of As in selecte d 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 adapt able in routine soil analysis, is dependable as indicated by repeatability (w greater than or equal to 0.98) and recovery tests. This SEP can be usefu l in predicting the changes in the lability of As in various solid phases a s a result of soil remediation or alteration in environmental factors. (C) 2001 Elsevier Science B.V. All rights reserved.