Extraction of copper species from the nanoporous sorbent with an ionic liquid

The extraction of copper species from the nanoporous sorbent into an ionic liquid ([C4mim][PF6], 1-butyl-3-methylimidazolium hexafluorophosphate) was studied herein by means of X-ray absorption near edge structure (XANES) and X-ray absorption fine structure (EXAFS) spectroscopies. The nanoporous sorbent is composed of adsorbed humic acid (HA) on nanoporous SiO2 to simulate copper-contaminated nanoporous soils. During extraction with [C4mim][PF6] at 298, 323 and 393 K, approximately 20, 40 and 54%, respectively, of the copper species on the nanoporous sorbent were extracted into [C4mim][PF6]. Using the least-square fitted XANES spectra, the major copper species on the nanoporous sorbent was determined to be Cu(II)-HA (Cu(II) adsorbed on HA) (86%). At 298, 323 and 393 K, the copper dissociated from the extracted Cu(II)-HA and immediately formed Cu(mim)42 + (mim = 1-methylimidazole) in [C4mim][PF6]. At 323 and 393 K, Cu(mim)42 + was oxidized with oxygen in [C4mim][PF6] to form nanosized CuO. In this study, it was found that at least three reaction paths for the extracted copper to take from the nanoporous sorbent to [C4mim][PF6] at 298-393 K: (1) extraction of Cu(II)-HA from the nanoporous sorbent to [C4mim][PF6]; (2) dissociation of copper from extracted Cu(II)-HA to form Cu(mim)42 + in [C4mim][PF6]; and (3) oxidation of Cu(mim)42 + to form nanosized CuO in [C4mim][PF6]. Combining 1H and 31P NMR spectra, nanosized CuO interacted with [C4mim][PF6] at 393 K. The fitted EXAFS spectra show the Cu(mim)42 + in [C4mim][PF6] possessed a CuN bond distance of 1.82 Å at 298 and 323 K. It is also found that the CuO bond distance was 1.93 Å at 393 K. This work demonstrates that the XANES technique can reveal structural changes of the copper species from the nanoporous sorbent into [C4mim][PF6] during extraction.