Relating ion binding by fulvic and humic acids to chemical composition andmolecular size. 1. Proton binding

Proton binding by a soil fulvic acid, humic acid, and a set of size fractio ns of the humic acid was studied as a function of pH acid ionic strength by potentiometric titrations. The negative charge of the humic substances res ulting from deprotonation of acidic functional groups generally increased w ith increasing pH and increasing ionic strength. At any given pH acid ionic strength, the fulvic acid fraction exhibited much higher negative charge t han the humic acid fraction. For the size-fractionated humic acids, negativ e charge decreased steadily with increasing apparent molecular weight, as d etermined 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 b inding behavior of the humic substances was described very well by the cons istent NICA-Donnan model. However, when all adjustable model parameters wer e determined using a least-squares minimization technique without introduci ng parameter constraints, the values of some parameters turned out physical ly and chemically unreasonable. Therefore, we propose to derive some model parameters from chemical characterization results obtained by size exclusio n chromatography and solid-state C-13 NMR spectroscopy. Using this approach , we obtained excellent descriptions of all titration data, and the model p arameter 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 wit hout arbitrary constraints and, thereby, obtaining a more reliable database for environmental modeling.