Estimating the Effect of Particle Surface Coatings on the Adsorption of Orthophosphate Using Sedimentation Field-Flow Fractionation

Abstract

The adsorption behaviour of pollutants to natural colloids is affected by particle surface coatings such as hydrous iron oxides, manganese oxide and natural organic matter (NOM). Sedimentation field-flow fractionation (SdFFF) was used to determine surface adsorption density distributions (SADD) for 33P-labelled orthophosphate onto suspended colloidal samples from the Peel River (NSW, Australia). The surface adsorption density (SAD) is the amount of pollutant adsorbed per unit area of particle surface. For a homogeneous particle sample, the SAD is expected to be constant. The SAD for the Peel River sample increased significantly with particle size, which may be due to changes in particle shape, mineralogy, or the nature of the surface coatings. Scanning electron microscopy (SEM) was used to determine particle shape by examining separated fractions. The SAD was shape corrected using recently developed theory. Analysis of the sample suggested particle shape was fairly constant and, thus, did not influence the trends in the SAD significantly. In light of the SEM analysis, it was more likely the nonconstant SAD was associated with a change in mineralogy or surface coating composition. The purpose of this work was to investigate the effect of these surface coatings on orthophosphate adsorption. The particle surface coatings were selectively removed from the colloidal particles by chemical treatment and the effects on orthophosphate adsorption were investigated. Removal of the iron oxyhydroxide coatings decreased the SAD slightly but removal of NOM coatings increased orthophosphate adsorption substantially.