A review on adsorptive remediation of Cr (VI) by magnetic iron oxides and their modified forms

Abstract

Cr (VI) is highly toxic, carcinogenic and bio accumulative to living organisms. Its extensive industrial applications contaminate the natural aqueous ecosystems. Different techniques such as photocatalysis, adsorption, reverse osmosis, ion exchange, chemical precipitation, electrochemical reduction, solvent extraction etc. have been investigated to remove hexavalent chromium from contaminated water. Among these, adsorption is deliberated as the most effective and promising process because it is easy to design, simple to operate and suitable to avoid formation of secondary pollutants. Various adsorbents such as clay minerals, biosorbents, industrial wastes and magnetic nanomaterials have been used for the remediation of Cr (VI). The use of magnetic nanomaterials as adsorbents for removal of heavy metals has attracted much attention in recent years because of their large surface area and superparamagnetic characteristics. These materials not only exhibited high adsorption capacity but also could easily be recycled by applying an external magnetic field, making the separation process much easier. In this review, the adsorption behavior of different iron oxides such as maghemites, magnetites, ferrites and their modified forms towards the removal of Cr (VI) was discussed. The present review focuses on adsorption mechanism of Cr (VI) through the evaluation of adsorption capacities. Kinetics, thermodynamics and equilibrium studies for the adsorption process were depicted briefly. Optimum adsorption conditions (initial Cr (VI) concentration, pH, contact time, adsorbent dose and temperature), thermodynamic properties (ΔHo, ΔGo, ΔSo), kinetic model, fitted isotherm, adsorption mechanism and maximum adsorption capacities were listed in the table.