Consideration of the methods for evaluating the Cr(VI)-removing capacity of biomaterial

Over the last few decades, many researchers have tested various biomaterials for the removal of toxic Cr(VI) from aquatic systems. It is now widely accepted that the mechanism of Cr(VI) biosorption is not 'anionic adsorption' but 'adsorption-coupled reduction'. Unfortunately, however, many researchers have still used common equilibrium isotherm models, such as Langmuir and Freundlich ones, based on 'anionic adsorption' mechanism in order to evaluate the Cr(VI)-removing capacity of biomaterial tested. In this study, a fermentation waste of Corynebacterium glutamicum, capable of removing Cr(VI) efficiently, was used as a model biomaterial to show why equilibrium isotherm models cannot be used to evaluate the Cr(VI)-removing capacity of biomaterial. Meanwhile, some alternative methods considering the mechanism of Cr(VI) biosorption were suggested; the maximum Cr(VI)-removing capacity of the biomaterial could be evaluated by a Cr(VI)-biosorption experiment under biomaterial-limited condition as well as by a simplified kinetic model based on the reduction mechanism of Cr(VI).