ABSTRACT

X-ray absorption fine structure (XAFS) analysis was applied to elucidate Cr(VI/III) speciation during reaction with standard humic acid materials extracted from Japanese soils. The degree of reduction of Cr(VI) by humic acids differed for different degrees of humification. Adsorption of Cr(III) to humic substances was higher for higher carboxylic humic substance contents. After reduction experiments of Cr(VI) by humic acids, Cr binding with humic acids has a trivalent state. Nevertheless, a substantial amount of Cr(VI) remains in solution. The X-ray absorption near-edge structure (XANES) spectra of the Cr(VI) experimental system were almost identical to those of Cr(III) adsorbed onto humic acids in a low-pH condition. However, during adsorption of Cr(III) by humic acids, Cr was partly precipitated as Cr(OH)₃·nH₂O. Its concentration was higher for higher pH. XANES spectra of Cr(VI/III) reacted with different humic acids showed no systematic differences, indicating that the binding form of Cr to humic substances is common, irrespective of differences in functional group concentrations. Linear combination fitting using XANES spectra of reference compounds to HA-Cr samples suggests that 50% hydrated Cr(III) was adsorbed onto HA and that the remainder of Cr(III) was bound to carboxylic acid of HA. The local structure of Cr binding to humic acids was characterized using extended X-ray absorption fine structure (EXAFS) spectroscopy. The EXAFS features of Cr binding to humic acids were similar to those of Cr³⁺ aquo ions having simple octahedral symmetry of the oxygen coordination sphere. They show no clear presence of carboxylic group of humic acids around the Cr site. Scattering from the C atom might not be visible because the backscatter amplitude of a C atom becomes weak steeply with increasing wavenumber (k) and because it is smaller than those of the O atom and the Cr atom. Considering the linear combination fitting results of XANES spectra, the structural mode of Cr binding with HA is R-COO-Cr-(OH₂)₅.