Sorption and fractionation of natural organic matter on kaolinite and goethite

High pressure size exclusion chromatography (HPSEC) was used to determine directly the molecular-weight fractionation that occurred upon sorption of a muck fulvic acid (I;A) to kaolinite and goethite, at pH 3.7, 22 degrees C, 24hrs. equilibration. FA sorption affinity was found to be much greater for goethite than for kaolinite, with sorption maxima, A(max) = 0.22 and 0.05 mgC m(-2), respectively. Fractionation measurements agreed well with theoretical predictions. At concentrations < A(max), for both sorbents, the average molecular weight (M-w) of FA remaining in solution decreased by as much as several hundred daltons, indicating preferential sorption of the higher molecular weight components. At concentrations > A(max) solution M-w did not change, most probably because preferential sorption was overwhelmed by excess FA. Polydispersity decreased slightly upon sorption to kaolinite, but there was no significant change for goethite. Our results demonstrate quantitatively the preferential sorption of higher molecular weight components, and show that effects on solution M-w vary as a function of [FA].