PROTON BINDING TO HUMIC SUBSTANCES .1. ELECTROSTATIC EFFECTS

Ion binding to humic substances is influenced by the chemical heterogeneity and by the variable charge behavior of the humics. In this paper, we concentrate on the variable charge effects. To study these effects, acid/base titration data of 11 humic substances measured at a series of salt levels are analyzed. In a first-order approach, the organic matter molecules are treated as an ensemble of small rigid and impermeable cylindrical or spherical particles of a certain size and a variable surface charge density depending on pH and salt concentration. With double-layer models for such particles, the electrostatic effects on the proton binding can be described reasonably well. When the density of the humic material is used as a constraint, the radius of the spheres or the cylinders is the only adjustable parameter in the model. The particle radii of the humics studied range from 0.6 to 4.4 nm, with 0.85 nm as the median for the spherical double-layer model. For the cylindrical double-layer model, the assessed radii are smaller and range from 0.19 to 2.5 nm, with 0.32 nm as the median. The obtained molecular dimensions correspond reasonably well to data reported in the literature. Apparently in the samples under consideration, the polydispersity effects and conformational changes due to electrostatics are of second order.