*2.2. Adsorption Equilibrium of Chromium Species onto Simulated Sorbent for Soil Remediation*

In the adsorption experiments, the concentration of the adsorbed Cr(VI) that was present in the simulated sorbent in soil increased as *Ce* increased (see Figure 2). In Table 2, the model parameters for Cr(VI) absorption onto the simulated sorbent for soil remediation were estimated by fitting the experimental data. According to the values of the correlation coefficient (*R*2), the Cr(VI) absorption onto the simulated sorbent for soil remediation tended to follow the Langmuir and Freundlich isotherm equations in the *C*<sup>0</sup> range of 1000–8000 mg/L. The Langmuir isotherm fit the experimental data better than the Freundlich isotherm. In the Langmuir isotherm model, the monolayer saturation capacity of the Cr(VI) that was present in the simulated sorbent in soil remediation was 5.57 mg/g. The calculated *RL* value from the Langmuir isotherm model was 0.139, which indicated favorable Cr(VI) absorption onto the simulated sorbent for soil remediation. Moreover, the value of 1/*n* in the Freundlich isotherm model was 0.325, which also indicated that Cr(VI) was favorably absorbed on the simulated sorbent for soil remediation.

**Figure 2.** Experimental adsorption equilibrium results for the absorption of Cr(VI) onto the simulated sorbent for soil remediation. Error bars show the standard deviation of five replicates.

**Table 2.** Langmuir and Freundlich isotherm parameters for Cr(VI) absorption on the simulated sorbent in soil remediation.


*2.3. Extraction of Chromium Species from Cr-Contaminated Simulated Sorbent for Soil Remediation with [C4mim]Cl*

The compound [C4mim]Cl, a hydrophilic IL, can be used to extract HA and is able to intermix with Cr(VI) [28,29]. Thus, in this study, [C4mim]Cl was used to extract Cr(VI) from the Cr-simulated sorbent in soil remediation. The results show that approximately 33.48 ± 0.79% of the Cr(VI) was extracted into [C4mim]Cl (see Table 3). To understand whether different matrices affected the extraction efficiency of Cr(VI), HA and vermiculite were also tested. The results show that the extraction efficiencies of Cr(VI) compared to those of HA and vermiculite were about 82.85 ± 0.96 and 21.97 ± 1.11%, respectively, showing that HA and vermiculite affected the extraction efficiency. In a similar study, approximately 70% of Cr(VI) that had been chelated with HA in a mesoporous sorbent was able to be extracted into [C4mim]Cl [30]. Therefore, the extraction efficiency of Cr(VI) in HA was higher than that in the vermiculite and in the simulated sorbent for soil remediation.

**Table 3.** Extraction efficiencies of Cr(VI) from sorbents into [C4mim]Cl.


#### *2.4. FTIR Analysis*

To further understand the extraction mechanism of [C4mim]Cl, the structures of different matrices were tested both before and after extraction using FTIR spectroscopy. As shown in Figure 3, the band at 1088 cm−<sup>1</sup> was attributed to the C-O group stretching in the ester. The two bands found at 1032 and 1010 cm−<sup>1</sup> corresponded to the C-O group stretching in polysaccharides [31]. In Figure 3a,b, the broadened peak at 1088 cm−<sup>1</sup> and

the red shifts (ν, cm–1: 1032→1011 and 1010→960) were found because of the interaction between HA and vermiculite in the simulated sorbent for soil remediation. Moreover, the bands at 1011 and 960 cm−<sup>1</sup> were also shifted to 999 and 958 cm−1, respectively, in the Cr-simulated sorbent for soil remediation (see Figure 3b,c). It was clear that the Cr(VI) had been absorbed onto the C-O bonds of HA interacted with vermiculite. In Figure 3d, a band shift (999→1001 cm<sup>−</sup>1) was identified when [C4mim]Cl was used to extract Cr(VI) from the Cr-contaminated simulated sorbent for soil remediation, showing that the Cr(VI) that had been absorbed on HA was able to be extracted into [C4mim]Cl. Furthermore, the changes in the peak at 1088 cm−<sup>1</sup> were barely observable during extraction with [C4mim]Cl (see Figure 3b–d). A slight perturbation was observed in the vermiculite during extraction with [C4mim]Cl.

**Figure 3.** Fourier-transform infrared spectra of (**a**) humic acid, (**b**) simulated sorbent for soil remediation, and Cr–contamined simulated sorbent for soil remediation (**c**) before and (**d**) after extraction with [C4mim]Cl.
