Speciation of chromium compounds from humic acid-zeolite Y to an ionic liquid during extraction

By synchrotron X-ray absorption spectroscopy, chemical structures of hexavalent chromium (Cr(VI))/trivalent chromium (Cr(III)) adsorbed on humic acid (HA)-zeolite Y and extracted in an ionic liquid (1-butyl-3-methylimidazolium tetrafluoroborate ([C₄mim][BF₄])) have been studied. By combining the competitive adsorption results and reduction of Cr(VI)-HA with the carboxyl groups of HA, Cr(III)-HA (58%) was shown to be the major compound in HA-zeolite Y using synchrotron X-ray absorption near-edge structure (XANES) spectroscopy. In an ionic liquid phase, the reduction of Cr(VI)-HA to Cr(III)-HA and the desorption of Cr(III) from HA were caused by [C₄mim][BF₄]. The ⁹F nuclear magnetic resonance (NMR) spectra show that the perturbation of the [C₄mim][BF₄] anion was affected by the extractable chromium species. The formation of a Cr(III) ion affected the increase in the bond distance for the 1st shell CrO of the chromium species in [C₄mim][BF₄] using extended X-ray absorption fine structure (EXAFS) spectroscopy. The changes in the non-extractable chromium species remaining in HA-zeolite Y were also caused by [C₄mim][BF₄] during extraction. The desorption of the absorbed Cr(III) on HA and zeolite Y was observed to form Cr(III) ions. As the percentage of Cr(III) ions remaining in HA-zeolite Y increased, a slightly greater bond distance for CrO was found at 2.01 Å. The enhanced reduction of Cr(VI)-HA and desorption of Cr(III) adsorbed on the HA and zeolite Y to form Cr(III) ions were affected by [C₄mim][BF₄]. Increased mobility of Cr(III) in the simulated soil can promote the migration of Cr(III) ions into the H₂O during soil washing for remediation.