Size fractionation upon adsorption of fulvic acid on goethite: equilibrium and kinetic studies

We examined adsorption equilibrium and kinetics of an aquatic fulvic acid (XAD-8 resin extract) onto goethite (α-FeOOH). Molecular weight distributions were determined using high-pressure size exclusion chromatography (HPSEC). Overall adsorption isotherms and those of the most abundant intermediate molecular weight (IMW) fraction (1250–3750 Da) fit the Langmuir adsorption equation, as is commonly observed for humic substances. However, this overall fit masked the non-Langmuir isotherm shape of high and low molecular weight (HMW, LMW, respectively) fractions. We observed preferential adsorption of HMW fractions at low pH and of IMW fractions at higher pH. We also observed preferential adsorption of components with higher absorbance normalized to moles C (ε 280), probably reflecting greater aromaticity. Over the first 6 h of adsorption experiments, adsorbed organic carbon increased and weight average molecular weight ( M w ) of the organic matter remaining in solution decreased, consistent with slower adsorption of higher molecular weight components. Observations of fractionation upon adsorption agreed well with a field study showing lower M w and lower ε 280 organic matter in deeper ground water relative to surface and shallow ground water.