Extraction behavior comparison of hydrophilic ionic liquids for chromium on humic acid-microporous system

In soil, humic acids (HAs) are found on the external surface of micropores. HAs supply a sorption site for metals. In the present work, the fate of hexavalent Cr (Cr(VI)) without effect of other metals in microporous system was observed. By component-fitted X-ray absorption near the edge structure (XANES) spectra, approximately 84% of hexavalent Cr (Cr(VI)) was adsorbed on HA-zeolite Y (simulated HA on external surface of microporous system in soil). The reduction of Cr(VI)-HA (Cr(VI) adsorbed on HA) occurred to form Cr(III)-HA (Cr(III) adsorbed on HA) in HA-zeolite Y. The hydrophilic ionic liquids (ILs) (1-butyl-3-methylimidazolium chloride ([C(4)mim][Cl]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([C(4)mim][BF4])) were used to extract the chromium compounds in HA-zeolite Y. By XANES, Cr(VI)-HA desorbed from the external surface of zeolite Y and was substantially reduced to Cr(III)-HA in [C(4)mim][Cl] and [C(4)mim][BF4]. The extraction and reduction efficiencies of chromium species were less in [C(4)mim][BF4] than in [C(4)mim][Cl] due to high hydrogen bond basicity of [C(4)mim][Cl]. The H-1 NMR and FTIR spectra also show that the interaction between extracted chromium species and cations of [C(4)mim][Cl] is associated with the feasibilities of extraction and reduction. The extracted and non-extractable Cr(VI)/Cr(III) mole ratios in ILs and HA-zeolite Y were decreased, respectively, which diminished the toxicity of the chromium compounds in HA-zeolite Y. Appropriate ILs, e.g., [C(4)mim][Cl], used as extracting solutions, can enhance extraction and reduce toxicity for Cr(VI) compounds.