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

We examined adsorption equilibrium and kinetics of an aquatic fulvic acid ( XAD-8 resin extract) onto goethite (alpha -FeOOH). Molecular weight distrib utions were determined using high-pressure size exclusion chromatography (H PSEC). Overall adsorption isotherms and those of the most abundant intermed iate molecular weight (IMW) fraction (1250-3750 Da) fit the Langmuir adsorp tion equation, as is commonly observed for humic substances. However, this overall fit masked the non-langmuir isotherm shape of high and low molecula r weight (HMW, LMW, respectively) fractions. We observed preferential adsor ption of HMW fractions at low pH and of IMW fractions at higher pH. We also observed preferential adsorption of components with higher absorbance norm alized to moles C (epsilon (280)), probably reflecting greater aromaticity. Over the first 6 h of adsorption experiments, adsorbed organic carbon incr eased and weight average molecular weight (M-w) of the organic matter remai ning in solution decreased, consistent with slower adsorption of higher mol ecular weight components. Observations of fractionation upon adsorption agr eed well with a field study showing lower M-w and lower epsilon (280) organ ic matter in deeper ground water relative to surface and shallow ground wat er. Copyright (C) 2001 Elsevier Science Ltd.