MODELS FOR ASSOCIATION OF METAL-IONS WITH HETEROGENEOUS ENVIRONMENTAL SORBENTS .1. COMPLEXATION OF CO(II) BY LEONARDITE HUMIC-ACID AS A FUNCTION OF PH AND NACLO4 CONCENTRATION

A discrete log K spectrum model has been developed to represent the binding of protons and Co(II) to leonardite humic acid (LHA) over a wide variation of solution composition: pH 4.5-9.5, [NaClO4] 0.01-0.1 M, T-Co 200 nM-500 mu M. The model is internally self-consistent over the range indicated without an explicit electrostatic term. The LHA was represented by four acid sites (HL(i) = H+ + L(i)(-)) with a fixed pK(a) spectrum: pK(a) = 4, 6, 8, and 10. From the acid-base titration data, total concentrations of these sites and Na+ binding constants (L(i)(-) + Na+ = NaL(i)) were obtained. From Co2+ binding as a function of pH, constants for the reaction L(i)(-) + Co2+ = CoL(i)(+) were obtained. Total concentrations of sites and binding constants were similar to those expected from other studies. This discrete log K approach has been selected as the easiest way to parameterize multidimensional data (i.e., data with variations in many solution chemistry parameters) for subsequent application in transport models.