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Modeling and Mechanism of the Adsorption of Proton and Copper to Natural Bamboo Sawdust Using the NICA–Donnan Model

Bamboo sawdust represents an attractive low-cost adsorbent for heavy metal removal from contaminated waters. In the present study, the binding of protons and copper (Cu) ions to purified natural bamboo sawdust was investigated through acid–base titrations and Cu adsorption isotherms. The experimental data were modeled with the nonideal competitive adsorption (NICA)–Donnan equation (the NICA–Donnan model) to account for the heterogeneity of binding sites, stoichiometry of ion binding, as well as electrostatic interaction. Our results showed that the NICA–Donnan modeling of acid–base titration curves provided more accurate fitting results than the two-site surface complexation model. The stoichiometry parameters of the adsorption for Cu were both mono-dentate and binuclear, with the latter existing mainly under low ionic strength and high pH conditions. The contribution of low proton affinity and high proton affinity sites to Cu binding was dependent on pH. For high proton affinity sites, the partial inclusion of CuOH+ species adsorption led to an increase in the Cu ion affinity constant, whereas the incorporation of species adsorption decreased the affinity constant. GRAPHICAL ABSTRACT