Kinetics for adsorptive removal of chromium(VI) from aqueous solutions by ferri hydroxide/oxohydroxides

Adsorption behaviors of chromate(VI) on two different iron oxides were investigated through batch experiments under different operational conditions. The operational variables studied included sorbent doses, sorbent species, solution pH, contact time, and initial metal ion concentrations. Both ferri hydroxide and ferri oxohydroxides were capable of removing Cr(VI) from aqueous solution effectively. The extent of metal ion removed from aqueous solution by both sorbents increased with increasing contact time and initial metal ion concentrations, while decreased with elevating solution pH initially. Sorption equilibrium time was observed in 150 min. Equilibrium isotherm data were analyzed using the Langmuir and Freundlich isotherms. The Freundlich model yields a better fit than the Langmuir model. Although kinetic measurements with both sorbents had a similar pattern, ferri hydroxide always showed higher sorptive potential for Cr(VI) than ferri oxohydroxides. Results also indicated that sorptive removal of Cr(VI) by both sorbents was observed to be negative in response to solution pH, due to changes in speciation of Cr(VI) in aqueous solution. At pH lower than 3.0, both iron oxides exhibited much greater capacities of removing Cr(VI) from aqueous solution than other treatments. Therefore, it is to conclude that adsorptive removal of Cr(VI) by both iron oxides is effective. Adsorption capacity is mainly determined by speciation of Cr(VI) in solutions, which is controlled and driven by solution pH.