MODELING THE COMPETITION BETWEEN ALKALINE-EARTH CATIONS AND TRACE-METAL SPECIES FOR BINDING BY HUMIC SUBSTANCES

Humic ion-binding model V is used to interpret competition effects in the binding of trace metal species and alkaline earth cations (Mg2+, Ca2+) by fulvic-type humic substances. Intrinsic equilibrium constants for the alkaline earths are derived from literature data, and the values estimated from direct binding data are shown to be compatible with the results of competition studies involving copper. Within the model, three mechanisms of competition are possible, these being direct competition at discrete sites, competition for counterion accumulation, and reduction in net electrical charge on the humic molecule by alkaline earth cation binding to certain sites, which diminishes the electrostatic contribution to trace metal binding at other sites. The first mechanism is most significant for divalent trace metals having relatively high affinities for humic substances. Features of model V that permit compatibility between competition data and direct binding data are (a) sites with different relative affinities for different metals, (b) the presence of both monodentate and bidentate sites, and (c) the contribution of nonspecific counterion accumulation to alkaline earth binding.