Removal of hexavalent chromium from groundwater by Mg/Al-layered double hydroxides using characteristics of in-situ synthesis

Abstract

This study aimed to develop a novel in-situ method to directly remove toxic hexavalent chromium anions from groundwater. The characteristics of Mg/Al-layered double hydroxides (LDH) involving in-situ synthesis and interlayer exchangeable anions can facilitate to remove Cr(VI) from solution. Two different methods of LDH preparation were employed to explore the adsorption efficiency of (di)chromates, such as traditional coprecipitation (CO₃-LDH) and innovative in-situ synthesis (in-situ-LDH). The synthesized LDH samples were characterized using scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and zeta potential. The results demonstrated that the adsorptive amount of Cr(VI) for the in-situ synthesis process dramatically increased with an increase in initial Cr(VI) concentrations from 100 mg/L to 900 mg/L. The kinetic study indicated that the constant rate (k₂) of the pseudo-second-order equation significantly decreased when the initial concentration of Cr(VI) exceeded 500 mg/L. The removal efficiency of Cr(VI) was slightly dependent on solution pH (5.0-12) values. The in-situ-LDH absorbent (339 mg/g) exhibited the significantly higher Langmuir maximum adsorption capacity than CO₃-LDH (246 mg/g). The primary adsorption mechanism was anion exchange; meanwhile, the adsorption-coupled reduction mechanism also played an integral role. The advanced in-situ synthetic method can be developed to efficiently remove toxic hexavalent chromium anions from groundwater.

Keywords: (Di)chromate; Anion exchange; Groundwater remediation; In-situ synthesis; Layered double hydroxides.