Adsorption of a soil humic acid at the surface of goethite and its competitive interaction with phosphate

In order to be able to make predictions about the bioavailability and transport of contaminants and nutrients in soils, detailed knowledge is required about the adsorption of natural organic matter on the surface of oxides, as well as the processes of competition with the different ions present in the soil. With this aim, experiments investigating the adsorption of a soil humic acid (SHA) on goethite were carried out at pH 4.5, 7.0 and 10.0 and ionic strengths of 0.1 and 0.01 M, and the competition between SHA and phosphate was analysed by performing adsorption isotherms and electrophoresis experiments. Adsorption of SHA was clearly dependent on pH and ionic strength, decreasing with increasing pH or decreasing ionic strength. The results show that SHA and phosphate compete for the goethite surface, since the presence of phosphate decreases the adsorption of SHA and the presence of SHA decreases the adsorption of phosphate. Under certain conditions of pH, a decrease in phosphate adsorption of up to 45% and a decrease of SHA adsorption between 20 and 35% is produced. Furthermore, experiments performed by varying the order of addition of adsorbates to the goethite dispersion suggest that equilibration is attained more quickly when the adsorbates are added simultaneously than when one of the adsorbates is added before the other.