Chromium (VI) removal from water using cetylpyridinium chloride (CPC)-modified montmorillonite

Montmorillonite (Mt) has been used widely for metal removal from water and wastewater due to its various advantages including low cost, large surface area, high structure stability, and high ion exchange capacity. However, the removal of anionic hexavalent chromium (Cr) using Mt is inhibited by the negative charges on the adsorbent. To enhance Cr (VI) adsorption on Mt, a cationic surfactant - cetylpyridinium chloride (CPC) - was utilized to modify the interlayer surface of Mt. Fourier-transform infrared spectroscopy and X-ray diffraction were performed to characterize the CPC modified Mt (CPC-Mt) and the structure change of Mt. Studies have shown that CPC intercalated into Mt interlayers via electrostatic interaction between Mt and CPC, as well as the hydrophobic interaction among CPC molecules. After modification, the CPC-Mt showed a positive zeta potential at pH 2-11; while the specific surface area decreased, CPC effectively increased the interlayer distance of Ca-Mt, with a maximum d(001) value of 4.37 nm, and provided more exchange sites for Cr (VI) adsorption. Cr (VI) was efficiently removed using CPC-Mt at low pH values, but the removal was influenced adversely by the increase of pH and ionic strength. The adsorption process was described by a Langmuir isotherm model with the constant of 0.342 L/mg and the maximum adsorption capacity of 43.84 mg/g at 298 K, and by a pseudo-second order kinetic model with a kinetic coefficient of 6.62 g/(mg.min). The adsorption mechanism analysis has shown that electrostatic attraction is the main mechanism for Cr (VI) removal; at the same time, the reduction of Cr (VI) to Cr (III) by Fe (II) in Mt cannot be neglected at low pH values, which increased Cr removal and was confirmed by the X-ray photoelectron spectroscopic analysis.