Association of heavy metals with secondary iron oxide minerals

Abstract

Retention of heavy metals by secondary iron oxide minerals is an important process in the soil environment. Therefore, knowledge of the mechanisms of heavy metals sorption on iron oxide minerals surfaces is necessary to predict the fate and mobility of heavy metals. This study examines the interactions of heavy metals (Pb, Zn, Cu, and Cd), with Fe oxide minerals in contaminated soil using selective extraction, EMP, MINTEQA2, and TEM with ED and EDX. The soil samples were collected from a contaminated area near Leadville, CO. Total elemental concentration analysis of soil samples showed them to be contaminated with respect to Pb, Zn, Cu and Cd. Goethite was identified in the chemically untreated clay fraction for the 15-20 cm sample by using XRD. Amorphous and crystalline iron oxide minerals were identified by using TEM/ED and selective extractions. They ranged from 0.61 - 4.7% and 1.4 - 3.9% respectively. Electron microprobe analysis demonstrated that the association of Pb with Fe is more pronounced than is Zn with Fe. Electron microprobe also revealed separate particles of Pb, Zn and Fe. These particles of Pb and Zn could be oxide, carbonate, sulfate or sulfide forms. According to the results of MINTEQA2, the sequence of the adsorption of heavy metals to iron oxides is Pb> Cu> Zn> Cd. The modeled concentration of Pb associated with iron oxides was 38,100 mg kg-1, for amorphous iron oxides. The analysis of five-selected iron oxide particles by using TEM/EDX gave an average Pb concentration of 42,900 mg kg-1 (Fe-oxides). Acidic ammonium oxalate in the darkness extraction gave 70,000 mg kg-1 (Fe-oxides). The AAOD reagent may over-estimate Pb associated with amorphous Fe-oxides, due to stripping of Pb adsorbed to crystalline Fe-oxides and also partial dissolution of Pb-rich particles not associated with Fe. Our conclusions were that; the surface complexation models provide a fairly accurate prediction of the concentration of Pb bound to iron oxides. Also, surface adsorption was the most important mechanism of interaction of Pb with iron oxides in this soil. Selective extraction overestimated the concentration of Zn associated with Fe-oxides.