Ion binding to natural organic matter: competition, heterogeneity, stoichiometry and thermodynamic consistency

Abstract

The general principles of cation binding to humic materials are discussed. Important aspects that need to be included in general purpose speciation models are: the extreme binding heterogeneity of natural humic materials, the variable stoichiometry of binding (monodentate, bidentate and tridentate), the competition between specifically-bound ions, especially protons and metal ions, and electrostatic effects which give rise to ionic strength effects and the non-specific binding of counterions. The NICCA–Donnan model is a semi-empirical model that addresses these issues. It is similar to the previously published NICA–Donnan model except that it introduces an additional degree of scaling that ensures thermodynamic consistency and allows for variable stoichiometry of binding. It implicitly accounts for the large degree of chemical heterogeneity of humic particles. The NICCA (consistent NICA) model also recognizes that the affinity distributions are ion specific and are not fully correlated. The model requires no assumptions to be made about the geometry of the humic particles, but the Donnan submodel does allow for shrinking and swelling. Important model parameters such as the site density and median binding constants ( log K ̃ ) are not dependent on pH, metal ion concentration, ionic strength, etc. Data are analysed for H +, Ca 2+, Cd 2+, Cu 2+, Pb 2+ and Al 3+ binding to a single purified peat humic acid. The NICCA–Donnan model captures the non-linearity of the observed isotherms even at very low free metal ion concentrations. After fitting the model to datasets containing only the proton and one metal ion, the model was able to predict Cd 2+–Ca 2+, Cu 2+–Ca 2+ and Pb 2+–Al 3+ competition reasonably well. It also gave satisfactory predictions of the H +/M z+ molar exchange ratios. These ratios varied strongly with metal ion: Ca 2+ (0.2–0.5); Cd 2+ (0.5–1.0); Pb 2+ (1.1–1.2); Cu 2+ (1.2–1.7) and Al 3+ (2.1–2.7), and also to a varying degree with pH and free metal ion concentration.