Competitive adsorption of negatively charged ligands on oxide surfaces

The competitive adsorption of phosphate + arsenate and phosphate + selenite on goethite and gibbsite has been measured. The oxide surface contains sites common to both anions on which competitive adsorption takes place and sites on which only one or the other anion is able to adsorb. The maximum amount of anions adsorbed from a mixture is approximately equal to the sum of the maximum adsorption for each anion in the absence of competition. The number of sites at which competition takes place is almost equal to the number of sites available to the anion with the lower adsorption in the absence of a competitor. Competition is described by a Langmuir-type exchange equation.