Biosorption of metals onto plant biomass: exchange adsorption or surface precipitation?

Heavy metal ions readily adsorb onto the nonliving biomass of many aquaphytes. Further, in many cases the metal ions can be readily desorbed from the biomass by use of a suitable eluting agent, It has been shown in certain cases, at least, that the biomass can be subjected to a number of loading and elution cycles without the biomass losing its adsorption capacity. It has been widely reported that the adsorption is through a specific ion exchange mechanism and a number of researchers have shown experimental evidence supporting such a mechanism. However, there is also evidence that the adsorption is through simple surface precipitation of metal hydroxide species, The present study examines some of the existing data on adsorption of metal ions onto aquaphyte biomass and attempts to evaluate which mechanism is the more likely. (C) 2001 Elsevier Science B.V. All rights reserved.