Using electrophoresis in modeling sulfate, selenite, and phosphate adsorption onto goethite

Abstract

The authors evaluate methods for determining two quantities generally considered necessary in modeling adsorption. First, the electrical component of net binding constants is determined by electrophoresis in conjunction with pH measurement. Second, three techniques for determining the maximum concentration of available surface sites are compared. The authors use a Stern model to make out evaluations; however, the results should be applicable to adsorption modeling in general. Electrophoretic mobility data are modeled quite successfully, but the empirical determined maximum surface-site concentration on colloidal goethite (..cap alpha..-FeOOH) varies considerably for sulfate, selenite, and phosphate despite their similar sizes and binding mechanisms. The authors suggest that aggregation processes, due to both proximity to the isoelectric point of the solid and adsorbate bridging between particles, may have significant effects on surface-site availability.