Binding of copper(II) and zinc(II) ions on various lignins

Lignins formed by the decay of woods are present in rivers, lakes, at estuaries and in swampy coastal waters. Their interactions with copper(II) and zinc(II) ions were studied and were shown to depend on the provenience of lignin. Copper is bound more strongly than zinc. The pH-dependence indicates that COO^- may be involved in the binding of Cu(II), whereas phenolate groups in that of Zn(II). The binding is predominantly irreversible indicating that ion-exchange is not the prevailing mechanism. Some specimens of lignin show simple adsorption isotherms indicating the presence of a single type of a binding site, some others show complex adsorption isotherms, with two different binding sites or states of accessibility. The effect of neutral salts on adsorption isotherms may be due to changes in lignin structure, revealing or concealing different adsorption sites and will affect the adsorption ability of lignin in water of varying salinity.