**3. Materials and Methods**

*3.1. Preparation of Simulated Sorbent for Soil Remediation and Cr-Contaminated Simulated Sorbent for Soil Remediation*

In order to synthesize the simulated sorbent for soil remediation, 1.5 g of HA (humic acid sodium salt, Sigma-Aldrich, St. Louis, MO, USA), 6 g of vermiculite (Aldrich, St. Louis, MO, USA), and 50 mL of H2O were stirred in a 150 mL beaker for 1 d, filtered, dried at 343 K, and ground. To prepare the Cr(VI)-contaminated HA, vermiculite, and simulated sorbent for soil remediation, 7.5 g of either HA, vermiculite, or simulated soil were incubated with 10 mL of 1000 mg/L of the Cr(VI) solution for 1 h at 298 K. The 1000 mg/L Cr(VI) solution was prepared from 0.25 g of K2Cr2O7 (99%, Sigma-Aldrich, St. Louis, MO, USA) in 250 mL of H2O at 298 K. To calculate the adsorption efficiencies, the adsorbed Cr(VI) on humic acid, vermiculite, and simulated sorbent for soil remediation was digested and the chromium concentrations were measured by AA (Hitachi Z-5000, Hitachi Instruments Co., Tokyo, Japan).

#### *3.2. Adsorption Isotherm*

For the adsorption isotherm experiments, 2000, 3000, 4000, 5000, 6000, 7000, and 8000 mg/L Cr(VI) solutions were prepared using methods that were similar to those used for the preparation of the 1000 mg/L Cr(VI) solution was. Samples of 10 mL of each of these different concentration solutions was mixed with 7.5 g of each simulated sorbent for soil remediation in test tubes; the test tubes were then shaken at 298 K for 4 h. All of the experiments were run in five replicates.

The Langmuir and Freundlich adsorption equations were used to explain the adsorption isotherms:

Langmuir model:

$$q\_{\varepsilon} = (q\_{m}K\_{L}C\_{\varepsilon})/(1 + K\_{L}C\_{\varepsilon})$$

where *qe* (mg/g) is the equilibrium concentration of the absorbed chromium in the simulated sorbent for soil remediation, *Ce* (mg/L) is the equilibrium concentration of the chromium in the solution, and *KL* (1/mg) and *qm* (mg/g) are the constants.

$$R\_L = 1/(1 + K\_L C\_o)$$

where *RL* is the equilibrium parameter, and *Co* is the initial chromium concentration in the solution (mg/L). The value of *RL* suggests the tendency of the isotherm to be irreversible (*RL* = 0), favorable (0 < *RL* < 1), linear (*RL* = 1), or unfavorable (*RL* > 1).

Freundlich model:

*qe* = *KFCe* 1/*n*

where *qe* (mg/g) is the equilibrium concentration of the absorbed chromium in the simulated sorbent for soil remediation, *Ce* (mg/L) is the equilibrium concentration of the chromium in the solution, and *KF* and *n* are the constants that are associated with the adsorption capacity ((mg/g)(L/mg)1/*n*) and the adsorption intensity, respectively.
