Studies of Cu(II) in soil by X-ray absorption spectroscopy

Based on original data for copper, this paper evaluates the use and advantages of X-ray absorption fine structure (XAFS) in studies of heavy metals in soils. The structural parameters of the Cu²⁺ target were quantified for soil exposed to copper and the same soil eluted with 0.1 M HCl. Experimental data were obtained using synchrotron sources of second and third generations. X-ray absorption near edge structure (XANES) indicated that the copper in both samples is likely to be predominantly bound by oxygen-containing functional groups. Extended X-ray absorption fine structure (EXAFS) analysis showed that the equatorial Cu-O distances were similar to those seen for Cu ²⁺-humic complexes in aqueous solutions, but the axial distances for both samples were longer than in water. In the axial direction, this may indicate a type of ligation dissimilar to that in solutions. The magnitude of the mean square deviation of the axial Cu-O distance in the 0.1 M HCl eluted sample indicated a stronger bond compared with that in the sample containing both extractable and tightly bound copper. The results indicate that careful examination of structural data for sequentially eluted soils in combination with the development of new radiation sources to improve the sensitivity and quality of the data are likely to improve insight into the nature of interactions between heavy metals and soil.