**4. Conclusions**

The absorption efficiencies of Cr(VI) onto HA, vermiculite, and the simulated sorbent for soil remediation were 81.32 ± 1.05%, 64.47 ± 1.62%, and 91.78 ± 1.82%, respectively. The simulated sorbent for soil remediation that contained HA and vermiculite had a higher Cr(VI) adsorption efficiency. The experimental absorption data were fitted better by the Langmuir isotherm model than by the Freundlich isotherm model. It was also found that Cr(VI) was favorably absorbed onto the simulated sorbent for soil remediation. During the extraction of Cr(VI) from the Cr-contaminated simulated sorbent for soil remediation with [C4mim]Cl, about 33.48 ± 0.79% of the Cr(VI) was extracted into [C4mim]Cl. The results of the FTIR spectra showed that Cr(VI) could adsorb onto HA, which interacted with vermiculite in the simulated sorbent for soil remediation. After extraction, the absorbed Cr(VI) could be extracted from the simulated sorbent for soil remediation into [C4mim]Cl. An interaction between the 1-methylimidazole of the [C4mim]Cl and Cr(VI) was also observed during extraction by 1H NMR. The [C4mim]Cl and vermiculite had weaker interaction, which effected the Cr(VI) extraction efficiency. This work exemplifies that the 1H NMR technique can reveal the changes that take place in [C4mim]Cl during extraction of chromium species from Cr-contaminated simulated sorbent into [C4mim]Cl.

**Author Contributions:** Conceptualization, H.-L.H.; methodology, H.-L.H., P.C.L. and H.-H.H.; software, H.-L.H.; validation, H.-L.H. and H.-H.H.; formal analysis, H.-L.H., P.C.L., H.T.W. and H.-H.H.; investigation, H.-L.H. and H.-H.H.; resources, H.-L.H.; data curation, H.-L.H., P.C.L., H.T.W. and C.-H.W.; writing—original draft preparation, H.-L.H.; writing—review and editing, H.-L.H. and H.-H.H.; visualization, H.-L.H.; supervision, H.-L.H.; project administration, H.-L.H.; funding acquisition, H.-L.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Ministry of Science and Technology ROC, grant number MOST 109-2622-E-239-004-CC3, and National United University, grant number 109-NUUPRJ-13.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors thank Ling-Yun Jang of the Taiwan National Synchrotron Radiation Research Center for their XAS experimental assistance. The authors gratefully acknowledge the use of the NMR 000700 equipment belonging to the Core Facility Center of National Cheng Kung University.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Samples of the compounds are available from the authors.
