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 MW, more aromatic moieties to mineral surfaces; quantification of such processes is fundamental to development of predictive models. We used high pressure size exclusion chromatography (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 °C, I = 0.01 (NaCl), 24-h reaction time. MFA adsorption affinity was greater for goethite than for kaolinite. At concentrations less than the adsorption maximum (Amax) for both adsorbents, the weight-average MW (Mw) of MFA remaining in solution decreased by as much as several hundred Daltons relative to control samples, indicating preferential adsorption of the higher MW components. At concentrations more than Amax, Mwof MFA in solution did not change appreciably. Although total adsorption decreased significantly as pH increased, fractionation as measured by change in Mwremained similar, perhaps indicating greater selectivity for higher MW components at higher pH.