Speciation of Chromium Compounds from Zsm-5 into an Ionic Liquid

The speciation of extracted chromium from ZSM-5 into an ionic liquid (IL) was studied using X-ray absorption near-edge structure (XANES) spectroscopy. The main adsorbed chromium species in ZSM-5 were Cr(VI)–HA [Cr(VI) chelated with humic acids (HAs)] (57%), Cr(VI)ads (Cr(VI) adsorbed on ZSM-5) (33%), and Cr(III)–HA (Cr(III) chelated with HA) (10%). In this work, 1-butyl-3-methylimidazolium chloride ([C4mim]Cl), was used as the IL to extract the chromium compounds from ZSM-5. Experimentally, approximately 75% of the chromium compounds were extracted within 30 min at 343 K. Combining the chromium extraction efficiency and component fitting results of the XANES spectra, almost all of Cr(VI)–HA was extracted into [C4mim]Cl. Following extraction, 34.5% of the Cr(VI) compounds were reduced to form Cr(III)–HA and Cr(III) ions. The Cr–O bond distance of Cr compounds was 1.69 A in [C4mim]Cl as shown by X-ray absorption fine structure (EXAFS) spectroscopy. 1H nuclear magnetic resonance (NMR) showed that the reduction and extraction of Cr(VI) compounds were affected by [C4mim]+. The nonextractable chromium species in ZSM-5 were Cr(VI)ads (9%), Cr(III)–HA (10.8%), and Cr(III)ads (5.2%). The fraction of Cr(VI) was decreased greatly because of the use of [C4mim]Cl as the extractant.