Proton competition in Cu(II) adsorption by fungal mycelia

Abstract

Fungal mycelia have been shown to be effective adsorbents for the removal of heavy metals from dilute solutions. Copper ions and protons become adsorbed to the fungal surface through reaction with specific surface sites including amino-, phosphate, carboxyl- or hydroxo-groups. In this study, an alkalimetric/metal titration technique was used to evaluate the competitive adsorption between protons and Cu(II) ions onto acid-washed mycelia of Aspergillus oryzae and Rhizopus oryzae. A general macroscopic mechanism has been established to describe the specific adsorption of both protons and Cu(II) ions. Using a simple graphic or a non-linear least squares technique, the adsorption parameters; i.e. adsorption constant and maximum adsorption capacity, were evaluated. The results predicted by the two-site model with a double reciprocal plot or the Scatchard plot were compared by non-linear regression. In the presence of multiple classes of sites, better fit of the experimental data was obtained by treating the fungal surface as one which possesses two major functional groups, namely: (1) high affinity of monodentate binding, (2) low affinity of bidentate binding. Because hydrogen ions compete significantly with Cu(II) ions for adsorption sites, the competitive Langmuir isotherm so derived was characterized by three parameters: two-site Cu(II) affinity, two-site Cu(II) adsorption density and apparent proton affinity. This enabled differentiation of the Cu(II) adsorption from protons. Two intrinsic equilibrium constants for Cu(II) surface complexation were estimated. With this information, it was possible to predict the adsorption of Cu(II) over a wide range of metal loading conditions at various pH values.