Acid/Base, Copper Binding, and Cu²⁺/H⁺ Exchange Properties of a Soil Humic Acid, an Experimental and Modeling Study

Acid/base, copper binding and proton exchange properties of a soil humic acid were studied over a range of pHs and ionic strengths. Results indicate that multiple classes of copper binding sites exist. A number of model formulations were used to simulate the data; none were able to reasonably match all aspects of the observed behavior. Model results and analyses suggest that successful replication of humic behavior over a wide range of conditions requires a model formulation which incorporates electrostatic effects for a variable geometry entity, multiple site classes, and probably both uni and multi dentate complexation. Additionally site heterogeneity estimates from acid/base titration data are not likely to reasonably assess the presence of low concentration, high (metal) affinity sites that can be relevant in the study of trace metal complexation.