Sequentially extracted arsenic from different size fractions of contaminated soils

Five soils from Austria that have high contents of arsenic (As) due to anthropogenic and/or geogenic sources were separated into sand, silt, and clay fractions. The distribution of As among the particle-size fractions was clay > > silt > sand. The particle-size fractions were analysed according to an 8-step sequential extraction procedure (SEP) designed to assess the primary reservoirs and extractability of As. Steps 1 and 2, (i.e. NH4NO3 and (NH4)(2)SO4) extracted the least As, while considerably greater amounts were extracted in steps 3, 4 and 5 (i.e. NH4H2PO4, NH2OH-HCl, and NH4F). The greatest amounts of As were extracted in steps 6 and 7, both NH4-oxalate extractants targeting the amorphous and crystalline oxide fractions. The residual fraction (step 8) was typically low. The extraction of the clays contained generally more As in steps 3 to 8, while As release in the first two steps was typically highest in the sand and silt fractions. These findings are in accord with factor analysis indicating that the extractants of the SEP are separated into weak (steps 1 and 2) and strong (steps 3 to 8) extractants. SEP data along with microprobe analyses (X-ray microanalyses) indicate that most As is associated with Fe and is therefore not readily mobile. Anyhow, As could be released upon changes in redox potential or pH. The greater extractability of As from sand fractions using weak extractants may explain the higher toxicity of As in coarse-textured soils, as found in previous studies.