*3.4. Extraction of Cr(VI) from Sorbents with [C4mim]Cl*

In order to test the extraction efficiency of the Cr(VI) by [C4mim]Cl, 0.4 g of Cr(VI) -contaminated HA, vermiculite, and the simulated sorbent for soil remediation were incubated with 1.5 g of [C4mim]Cl in 0.5 mL of H2O in glass tubes that were each shaken for 30 min. Afterwards, the mixture was filtered to separate the [C4mim]Cl from the solids. The solids were washed with deionized water several times to remove any residual [C4mim]Cl that was present in the HA, vermiculite, and the simulated sorbent for soil remediation. The resulting [C4mim]Cl was removed using H2O and a rotary evaporator. In order to clarify the influence of [C4mim]Cl on the extraction process, H2O was used as the extraction solution without [C4mim]Cl using similar extraction steps. All of the experiments were run with five replicates. Moreover, a blank experiment was also carried out. The extraction efficiencies of Cr(VI) from the Cr(VI)-contaminated HA, vermiculite, and the simulated sorbent for soil remediation into [C4mim]Cl were estimated by the following equation:

$$\text{Extraction efficiency (\%)} = (\text{C}\_1 - \text{C}\_2) / \text{C}\_1 \times 100\%$$

where *C*<sup>1</sup> is the initial chromium concentration in the humic acid, vermiculite, and the simulated sorbent for soil remediation (mg/g), and *C*<sup>2</sup> is the chromium concentration in the humic acid, vermiculite, and the simulated sorbent for soil remediation after extraction with [C4mim]Cl (mg/g).
