Relating Ion Binding by Fulvic and Humic Acids to Chemical Composition and Molecular Size. 1. Proton Binding

Abstract

Proton binding by a soil fulvic acid, humic acid, and a set of size fractions of the humic acid was studied as a function of pH and ionic strength by potentiometric titrations. The negative charge of the humic substances resulting from deprotonation of acidic functional groups generally increased with increasing pH and increasing ionic strength. At any given pH and ionic strength, the fulvic acid fraction exhibited much higher negative charge than the humic acid fraction. For the size-fractionated humic acids, negative charge decreased steadily with increasing apparent molecular weight, as determined by size exclusion chromatography. Observed differences in proton binding by the various humic substances corresponded well to differences in functional group composition, which has been extensively characterized in a previous study using a combination of analytical techniques. The proton binding behavior of the humic substances was described very well by the consistent NICA-Donnan model. However, when all adjustable model parameters were determined using a least-squares minimization technique without introducing parameter constraints, the values of some parameters turned out physically and chemically unreasonable. Therefore, we propose to derive some model parameters from chemical characterization results obtained by size exclusion chromatography and solid-state 13C NMR spectroscopy. Using this approach, we obtained excellent descriptions of all titration data, and the model parameter values were more consistent and chemically reasonable. Our results demonstrate that characterization results of humic substances can be used in NICA-Donnan modeling to reduce the number of free fitting parameters without arbitrary constraints and, thereby, obtaining a more reliable database for environmental modeling.

Humic matter and contaminants. General aspects and modeling metal ion binding

“…After the description of the PPHA data set by the NICA-Donnan model, the model has been applied successfully to the description of proton and metal ion binding data to various other humic and fulvic acids [75][76][77][78][79]. In their work, Christl et al [78,79] have shown that it was possible to reduce the number of fitting parameters if results from size exclusion chromatography and solid-state C-13 NMR spectroscopy were used to estimate two of the parameters.…”

“…In their work, Christl et al [78,79] have shown that it was possible to reduce the number of fitting parameters if results from size exclusion chromatography and solid-state C-13 NMR spectroscopy were used to estimate two of the parameters. Moreover, this procedure resulted in a more consistent parameter set than by fitting alone.…”

“…colloid is impenetrable for ions (surface charge models) [61,65,[68][69][70][71][72], the other considers the colloid as a gel phase in which all charges of the organic matter are compensated within the gel phase (Donnan models) [59][60][61][62]68,[73][74][75][76][77][78][79]. Mostly, it is assumed that the humic particles are spherical.…”

“…In the Donnan models [59][60][61][62]68,[73][74][75][76][77][78][79], the charge due to the dissociation of the functional groups of a particle is smeared out over the volume of the (mostly) spherical particle, and a volume charge density is obtained. The volume charge density is now converted into the Donnan potential, a potential that is assumed to be the same everywhere inside the volume of the particle and zero outside the particle boundary [68].…”

A direct potentiometric titration study of the dissociation of humic acid with selectively blocked functional groups

“…Humic polyelectrolytic macromolecule possess the potential to carry predominantly negative charge, at naturaly occurring pH values, due to the dissociation of acidic functional groups, mainly carboxylic and phenolic [3]. Even though there are many papers dealing with the charge development [2,[4][5][6] there is still some uncertity about how humic functional group heterogeneity affects it [7].…”