IR and XANES spectroscopic studies of humic acids reacting with Cr(III) and Cr(VI)

Abstract

    To elucidate processes of Cr(VI) reduction by humic acids in soils, humic acids reacting with Cr(VI) were characterized using IR and X-ray absorption near edge structure (XANES) spectroscopy. It is expected that oxidation of humic acid by Cr(VI) gives rise to the formation of aldehyde, ketone, and carboxyl species. However, no significant increase of IR-bands assignable to these functional groups has been reported. Instead, IR absorption spectra of humic acid oxidized by Cr(VI) show similar features to those of humic acid reacting with Cr(III). Consequently, the types and proportions of functional groups of HA substances did not differ significantly among experimental conditions. The IR spectra of humic acid reacting with Cr(III) and Cr(VI) show increased intensity of IR bands at 3400 cm⁻¹, 1608 cm⁻¹, and 1384 cm⁻¹, with decreased intensity of peaks at 1707 cm⁻¹ and 1236-1250 cm⁻¹. Those changes suggest the existence of bonding structures of two kinds: hydrated Cr forming an outer-sphere complex with humic acid, and Cr forming an inner-sphere complex with the carboxylate ligand of humic acid.     The Cr-K edge XANES spectra of humic acid reacting with Cr(VI) suggest that Cr(III), reduced from Cr(VI), binds with humic acid. No systematic difference of XANES spectra was found with increased amounts of Cr(VI) in experimental solutions. These features are consistent with IR absorption spectra. The fractions of two kinds of binding forms suggested by IR spectra were determined from linear combination fitting using XANES spectra of reference compounds: 50% hydrated Cr(III) adsorbed onto HA electrostatically and the remainder of Cr(III) binding to carboxylic acid of HA. However, with increasing pH of experimental solutions, some Cr precipitates as Cr(OH)₃•nH₂O in the solid phase. The pH level or Cr concentration must be reduced to prevent production of a Cr(OH)₃•nH₂O precipitate.