Soil Amendments to Reduce Bioavailability of Metals in Soils: Experimental Studies and Spectroscopic Verification

摘要

The overall objective of this study was to attempt to remediate metal-contaminated soils by finding an amendment or combination of amendments that could be applied and reduce chemical lability and bioavailability. We located three soils that were contaminated with at least of the metals Pb, Cd, Cr, and As. The soils were characterized for an array of chemical and physical properties including total metals. All soils had a mixture of metals requiring attention, and made the remediation challenge much greater because the chemistry of each metal was quite different from the others. Our approach to finding remediation solution using in situ amendments was to sequentially address the metals with additives known to target at least one metal. We then examined the soils for chemical lability (concentrations of metals removed from the soil by an extractant), bioaccessibility (metals available for removal from the soil by a sequence of extractants demonstrated to be correlated with availability to a given organism), and biotoxicity. Orthophosphate is a known, successful amendment for Pb, and this was our first amendment. Quite predictably, the addition of orthosphosphate decreased Pb but greatly increased As and sometimes Cr concentrations. Therefore, our challenge was to find additional amendments that could suppress the other metals without impacting the effect of phosphate on Pb. In laboratory studies, combinations of chemical amendments, including rare earth elements, Mn and P, were added to soil with low redox potential to reduce the bioaccessible fraction of As, Cr, Cd and Pb. Lanthanum and Ce were able to form low solubility precipitates with As, as determined in aqueous solutions. Spectroscopic studies confirmed that LaAsO4(s) can form under pH conditions as low as 2.2.