*3.6. Spectroscopic Analysis*

The Cr K-edge XAS (X-ray absorption spectroscopy, 16A1, National Synchrotron Radiation Research Center, Hsinchu, Taiwan) spectra of the chromium on the simulated sorbent for soil remediation was recorded on the Wiggler beam line (16A1) at the Taiwan National Synchrotron Radiation Research Center. The electron storage ring was operated at an energy of 1.5 GeV and at a current of 300 mA. A chromium foil absorption edge at 5989 eV was used to calibrate the photon energy. To measure the Cr K-edge absorption spectra, the fluorescence mode on a Lytle detector was used. The XANES (X-ray absorption near edge structure) spectra of chromium model compounds, such as CrCl3·6H2O, K2CrO7, Cr(NO3)3, Cr(OH)3, Na2CrO4, Cr2O3, CrO3, Cr(VI)-HA, Cr(III)-HA, Cr(VI)ads (by impregnation of K2CrO7 (3 wt%) on vermiculite), Cr(VI)ads (by impregnation of CrCl3·6H2O (3 wt%) on vermiculite), Cr(VI) ion (prepared by dissolution of 0.5 g K2CrO7 in 50 mL H2O), Cr(III) ion (prepared by dissolution of 0.5 g CrCl3·6H2O in 50 mL H2O), and Cr foil were also measured. Cr(VI)-HA was prepared by mixing 0.5 g of K2CrO7 and HA in 50 mL of deionized water, and then filtering and drying the mixture at 343 K. To prepare Cr(VI)-HA and Cr(III)-HA, 7.5 g of HA was incubated with 10 mL of 1000 mg/L Cr(VI) and Cr(III) solution, respectively, for 1 h at 298 K, and the mixture was then filtered and dried at 343 K.

The solid samples and KBr (99.5%, Panreac, Barcelona, Spain) were uniformly mixed and ground at a ratio of 1 to 100. The mixture was pulverized in an agate mortar, and the mixed dyes were prepared in quantities of 5–8 tons. An FTIR spectrometer (Thermo Nicolet 6700, Thermo Fisher Scientific, Waltham, MA, USA) was used to investigate the structures of the HA, vermiculite, and Cr-simulated sorbent from soil remediation before and after extraction in the wavenumber range of 4000–400 cm−1, with 32 scans, and a 4 cm−<sup>1</sup> resolution. The 1H NMR spectra of the [C4mim]Cl were also determined on a Bruker Avance 300 spectrometer (Bruker, Billerica, MA, USA) with tetramethyl silane (TSM) as an internal standard (acquisition time = 1.373 s, actual pulse repetition time = 2 s, number of scans = 32, and excitation pulse angle = 30◦).
