MODELING ION-BINDING BY HUMIC ACIDS

Humic ion-binding Model V is a discrete site/electrostatic model of io n binding by humic matter that describes competition amongst interacti ng species (protons and metal species) and the effects on binding of i onic strength. Six adjustable parameters are required to describe prot on binding. These are n(A) (equivalent acid groups per gram), pK(A) an d pK(B) (midrange intrinsic proton dissociation constants), DELTApK(A) and DELTApK(B) (ranges of pK values), and P (empirical constant for e lectrostatics). For each metal species, two further parameters are req uired; these are intrinsic equilibrium constants for metal-proton exch anges (pK(MHA) and pK(MHB)). Both monodentate and bidentate binding si tes for metal species are included, and binding by non-specific accumu lation of counterions is taken into account. In this study, Model V pa rameter values are derived from published results for humic acids (aci d-base titrations and binding studies with 15 metals). It is found tha t an approximate relationship Between PK(MHA) and pK(MHB) can be deriv ed, which eliminates an adjustable parameter and allows useful rationa lization of the different data sets. Electrostatic effects with humic acids are greater than those with fulvic acids, and the two types of h umic material also differ in the affinities of their discrete binding sites for protons and metal ions.