A unifying model of cation binding by humic substances

Abstract

Model V describes the binding of ions by humic substances in terms of complexation at discrete sites, modified by electrostatic attraction and/or repulsion, and also takes account of nonspecific binding due to counterion accumulation. The model operates over wide ranges of pH (3–11) and ionic strength (0.001-1 M). Electrostatic effects on specific binding are described with an empirical relationship involving net humic charge and an electrostatic interaction factor. Accumulation of counterions is described by Donnan-type expressions. The model assumes the presence of eight proton-dissociating groups in the humic material, distinguished by intrinsic pK values. In general, the description of proton dissociation requires seven parameters, but for fulvic-type material only six are needed. The proton-dissociating groups may interact individually with other ions, or pairs of them may form bidentate sites. Binding at the monodentate and bidentate sites is characterized by intrinsic equilibrium constants for cation-proton exchange; there are two such constants (pKMHA and pKMHB) for each cation. Model parameters are derived from published data for fulvic-type material on proton dissociation (eight data sets) and metal binding (twenty-six data sets, eleven metals). In the case of proton dissociation, the greatest variability among samples is in site densities, while intrinsic dissociation constants and electrostatic interaction factors are relatively consistent. With parameters for proton dissociation fixed, adjustments of pKMHA and pKMHB permit reasonable fitting of metal binding data, including pH dependence. There are insufficient available data to evaluate properly ionic strength and competition effects on metal binding, but correct trends are reproduced by the model. Values of pKMHA for metals indicate that binding strength increases in the order Mg2+ < Ca2+ < Mn2+ < Cd2+ < Co2+ < Ni2+ ~ Zn2+ < Pb2+ < Cu2+ < VO2+. The strong correlation between pKMHA and the analogous constant for lactic acid may be useful for estimating values of pKMHA in cases where humic binding data are lacking.