Heavy metals binding to biosorbents. Insights into Non-competitive models from a simple pH-dependent model

The possible explanation for the applicability of NCAMs (Non-Competitive Adsorption Models, i.e. the models corresponding to the Langmuir model in which the maximum uptake value is pH-dependent) for biosorption systems is proposed. Both the metal ion speciation in the bulk solution and the formation of multidentate surface complexes are taken into account. This combined effect can be responsible for the specific behaviour of experimental metal sorption isotherms, measured at different pHs and described well by NCAMs. This concept finds its support in the numerical investigations performed by using the model biosorbent and Cu, Cd and Pb as heavy metals. Acid-base properties of typical biosorbent surfaces are also considered.