Kinetics and mechanisms of metal retention/release in geochemical processes in soil. 1998 annual progress report

Abstract

'The long-term fate of toxic metals in soils cannot be precisely predicted, and often remediation recommendations and techniques may be ineffective or unnecessary. This work will generate basic knowledge on the kinetics and mechanism(s) of heavy metal retention/release by soil mineral colloids. The information should assist in improving remediation strategies for toxic heavy metal contaminated soils. The objectives are: (1) To determine the effects of residence time on the mechanisms of Cr(VI), Cu(II), Co(II), Cd(II), Pb(II), and Ni(II) sorption/release on Fe and Al oxide and clay mineral surfaces using kinetic studies coupled to extended x-ray absorption fine structure (EXAFS) spectroscopy and fourier transform infrared (FTIR) spectroscopy. (2) To study the effect of temperature, pH, and phosphate on metal sorption by oxides, and derive thermodynamic parameters to describe the sorption process. As of June, 16, 1997 several clay minerals were tested for their efficiency of removing Cr from aqueous systems. The materials tested--smectite, vermiculites, illites, and kaolinite--represent the natural clay minerals that are abundant in soils and sediments. The clays were used in either their original or reduced (reduced with sodium dithionite) forms. The experimental result indicate that the reduced clays acted as an efficient remover of Cr(VI) from an aqueous system. The XANES spectra of Cr-treated clays provided evidence that the clays reduced Cr(VI) to Cr(III) and immobilized Cr in the clays at the same time. Sodium dithionite applied directly into aqueous systems reduced Cr(VI) to Cr(III), but could not immobilize Cr even in the presence of the clays. The Cr(VI) removal capacity varied with the clay mineral type and the structural Fe content. For the clays used in this study, the removal capacity follows the orders of smectites > vermiculites and illites > kaolinite. Within the same type of clay minerals, reduction of Cr(VI) is highly related to the ferrous iron content of the clays, which indicates that the reduction of Cr(VI) to Cr(III) results from the oxidation of structural Fe(II) to Fe(III). The results of this study can be used to develop a method for remediation of chromium in waste water. The mechanisms and conditions for coupled reduction-sorption of chromate in dithionite-reduced smectites was investigated in this study by both equilibrium and spectroscopic methods. The batch study showed that the maximum removal of Cr(VI) by the reduced smectite was highly related to its structural Fe(II) content. The surface cation type and ion strength which affected the expanding of smectite also influenced the electron transfer, and thus the coupled reduction-sorption processes. The pH of the aqueous system was a determining factor for both the amount of Cr(VI) reduced to Cr(III) and the partition of Cr(III) between aqueous and adsorbed species. The environments of chromium in the smectite were studied with various spectroscopies (IR, XRD, XANES, and EXAFS).'