Development and characterization of a clay-HDTMABr composite for the removal of Cr(VI) from aqueous solutions with special emphasis on the electrochemical interface

Abstract

This research aims to study the removal of chromium in its toxic form, Cr (VI), from wastewater, using clay and modified clay with Hexadecyltrimethyl ammonium bromide (HDTMABr)as adsorbents. The modified clay is obtained by impregnating a known quantity of HDTMABr into the clay. The two materials were characterized by different techniques, such as X-ray diffraction, which is used to determine the inorganic composition and the phyllite structures it contains. Thermal analysis TGA isused to understand the thermal behavior as function of the sintering temperature. Nitrogen adsorption/desorption is used to get the pores' size and surface area, using the Brunauer-Emmett-Teller (BET) theory. Scanning electron microscopy (SEM) coupled with Energy Dispersive X-ray Spectroscopy (SEM/EDAX) is used to study the surface morphology of the bare clay and the HDTMABr/clay. The operating conditions, such as contact times, pH, and adsorption temperatures, were studied, and optimized to improve the efficiency of the adsorption process. The adsorption is favored in acidic medium and room temperature, giving adsorption capacities for Cr(VI) of 12 mg/g and 250 mg/g for the clay and clay/HDTMABr, respectively. The reaction kinetics were studied and found to fit a pseudo-second-order model. The standard entropy (ΔS°), enthalpy (ΔH°), and Gibbs free energy (ΔG°) changes indicate that the chromium (VI) adsorption process is a physical, spontaneous, endothermic process. Cyclic voltammetry (CV) and the electrochemical impedance spectroscopy (EIS) are used for the electrochemical characterization of the raw clay and clay/HDTMABr in [Fe(CN)6]3-/4-/ 0.1 M KCl solution. The linear scan anodic stripping voltammetry (LSASV) is used for the reduction the Cr(VI) to Cr(III), indicating that Cr(VI) accumulates better at the surface of clay/HDTMABr compared to the bare graphite and the raw clay.