Adsorption and fractionation of a muck fulvic acid on kaolinite and goethite at pH 3.7, 6, and 8

Molecular weight (MW) of humic materials is a key factor controlling proton and metal binding and organic pollutant partitioning. Several studies have suggested preferential adsorption of higher MLW, more aromatic moieties to mineral surfaces; quantification of such processes is fundamental to devel opment of predictive models, We used high pressure size exclusion chromatog raphy (HPSEC) to quantify MW changes upon adsorption of a muck fulvic acid (MFA) extracted from a peat deposit to kaolinite and goethite, at pH 3.7, 6 , and 8 at 22 degrees C, I = 0.01 (NaCl), 24-h reaction time. MFA adsorptio n affinity was greater for goethite than for kaolinite, At concentrations l ess than the adsorption maximum (A(max)) for both adsorbents, the weight-av erage MW (M-w) of MFA remaining in solution decreased by as much as several hundred Daltons relative to control samples, indicating preferential adsor ption of the higher MW components, At concentrations more than A(max), M-w of MFA in solution did not change appreciably, Although total adsorption de creased significantly as pH increased, fractionation as measured by change in M-w remained similar, perhaps indicating greater selectivity for higher MW components at higher pH, Absorptivities at lambda = 280 nm normalized to mg C L-1 (epsilon) suggeste d preferential adsorption of more aromatic moieties to kaolinite, epsilon c ould not be used for goethite-reacted samples because high Fe concentration s in the aqueous phase brought about by goethite dissolution interfered wit h the spectroscopic analysis, Preliminary kinetic experiments suggested tha t smaller molecules adsorbed first and were replaced by larger molecules wh ose adsorption was thermodynamically favored.