Heavy metal uptake by lignin: Comparison of biotic ligand models with an ion-exchange process

Metal uptake by kraft lignin, hereafter referred to as lignin, occurs by displacement of protons or bound metals with equilibrium constants K-ex(H) and K-ex, respectively. Values calculated for wide ranges of initial concentrations are reasonably constant, thereby demonstrating the validity of these displacement processes and proving that uptake in these systems is not simple adsorption. It was found that the stoichiometry for Sr and Cd uptake by Ca-loaded lignin is 1 mol of metal for 1 mol of Ca released. This observation for metals of very different binding strengths is difficult to rationalize with the biotic ligand model as generally applied but is in complete agreement with an ion-exchange process. Binding strengths to lignin, which contains only oxygen ligands, follow the order Pb > Cu > Zn > Cd > Ca (strongest to weakest). For proton displacement, only more tightly bound metals such as Pb, Cu, Zn, and Cd can compete with protons for anion-binding sites at low pH, but at high pH, uptake of Ca, Sr, and Li can occur. An observed logarithmic decrease of KexH with pH can be explained by having only weaker acids available for proton displacement under more basic conditions. The advantages and disadvantages of using adsorption and biotic ligand models for an ion-exchange process are discussed.