Metal removal by soil washing for an iron oxide coated sandy soil

A contaminated, iron oxide coated (15.5% Fe₂O₃), sandy soil was studied to evaluate the effectiveness of conventional soil-washing extractants for metal removal. Metals of interest included As, Cu, Pb, Hg, and Zn. The extractant solutions were HCl, HNO₃, H₂SO₄, EDTA and NH₂OH · HCl. Experiments assessed effects of extractant strength and contact time. Individual soil size fractions were studied for the various extractants. Metal-binding mechanisms were evaluated using a sequential extraction procedure. Metals in all size classes were bound strongly, presumably by an Fe oxide coating observed on the sandy soil. From an analysis of particle size fractions, contaminated soil metal concentration correlated approximately with the surface area/volume ratio of soil particles, suggesting that the metals were associated with soil surfaces. During soil washing, a typical rapid metal release in HCl occurred initially, followed by a much slower step. The fraction associated with the slow metal release correlated reasonably well with the residual metal fraction. Although sandy soils often are good candidates for soil washing, surface coatings may make metal extraction for even sandy soils difficult.