Adsorption of natural organic matter onto goethite

Abstract

The factors influencing the association of two natural organic matter (NOM) samples (from Redwater Creek in Australia and Suwannee River in USA) with goethite were studied. Sorption occurred very rapidly (minutes) and at very low NOM concentrations (about 0.5 mg C 1 −1), implying that mineral surfaces in the natural aquatic environment would not remain uncoated by NOM for long. The sorption behaviour of the two NOM samples followed the Langmuir adsorption equation. The maximum sorption density for the unfractionated Redwater Creek NOM was 1.1 μmol C mg −1 of goethite. The hydrophobic fractions (both acidic and neutral) separated from the Redwater Creek sample had much greater sorption affinities for the goethite surface than did the hydrophilic fraction. Calculation of the surface crowding indicates that a monolayer of sorbed NOM exists over the goethite surface. Sorption was influenced by solution pH and calcium ion concentration. Maximum sorption density occurred at around pH 3–4, and decreased at higher pH. The sorption density of NOM also appears to be influenced by the presence of non-ionic hydrophobic solutes. This component could shield the electrostatic repulsive forces between neighbouring adsorbed anionic NOM molecules on the goethite surface, thus leading to an increase in the maximum amount of NOM adsorbed. Calcium ion concentration increased sorption of NOM up to calcium concentrations of around 1 mM but higher calcium concentrations resulted in reduced sorption. Sorption of the hydrophobic acid fraction was most affected by calcium concentration. The main effect of NOM sorption on the surface characteristics of goethite is alteration of the surface charge. The isoelectric point of the NOM-coated goethite is shifted markedly to lower pH, compared to that of uncoated goethite. Thus, at the normal pH of natural waters, NOM-coated goethite has a net negative charge which would significantly influence the types of interaction occurring at the surface of the NOM-coated particles in natural aquatic systems. The model advanced for suspended particulate matter (SPM) in aquatic systems, namely a mineral core with coatings of hydrous metal oxides and NOM, appears to explain adequately many aspects of the behaviour of natural SPM.