Removal of Hexavalent Chromium from Aqueous Solutions Using Natural Zeolite Coated with Magnetic Nanoparticles: Optimization, Kinetics, and Equilibrium Studies

Abstract

Stringent discharge limits, high costs, and low removal efficiency of the conventional treatment methods are facing challenges to handle industrial effluents containing heavy metals. The objective of this study was to use a recoverable magnetic zeolite to remove Cr(VI) from aqueous solution. The study investigated the application of nanotechnology to improve surface properties, recoverability, and adsorptive capacity of natural zeolite and the CCD-RSM-based optimization of adsorption process variables. Natural zeolites coated with various fractions of magnetic nanoparticles (25%, 33.33%, 50%, and 75%) were investigated for surface characters, adsorption capacity, removal efficiency, and recoverability. Natural zeolite coated with 33.33% (MZ33) was found a better adsorbent in terms of surface characters, adsorption capacity, and removal efficiency. Thirty batch adsorption experiments designed with CCD were carried out in order to optimize adsorption process variables using response surface methodology (RSM). It was found that $\text{adsorbent}\text{dose}=2\text{g}/\text{L}$ , $\text{contact}\text{time}=75\min$ , $\text{initial}\text{Cr}\left(\text{VI}\right)\text{concentration}=10\text{mg}/\text{L}$ , and $\text{solution}\text{pH}=1.5$ were the optimum conditions to achieve 93.57% Cr(VI) removal, which is very close to the experimental result of 94.88%. The adsorption isotherm determined from the operating parameters revealed that experimental data fit to the Langmuir isotherm model with ${\text{R}}^2=0.9966$ and $\text{maximum}\text{adsorption}\text{capacity}=43.933\text{mg}/\text{g}$ . This proved that the adsorption of Cr(VI) on magnetic zeolite involved monolayer adsorption on the active sites. The separation factor, ${\text{R}}_{\text{L}}$ , value lies between 0 and 1 indicating that adsorption of Cr(VI) on the magnetic zeolite is favorable. The adsorption kinetics study follows pseudo-first order in the removal of Cr(VI). FTIR analysis of magnetic zeolite revealed the presence of numerous functional groups participating in Cr(VI) adsorption. The current study confirmed that magnetic zeolite is a cost-effective and favorable material for the removal of Cr(VI) from aqueous solution.