Effects of pH and electrolytes on inositol hexaphosphate interaction with goethite

The pH and the nature of electrolytes are important factors affecting the s orption of anions by soil components. The effects of pH, K, and Ca on the i nteraction of inositol hexaphosphate (IHP) and inorganic phosphate (Pi) wit h goethite were investigated by sorption experiments. Laser Doppler Velocim etry-Photon Correlation Spectroscopy was employed to determine zeta potenti al (zeta) and particle size before and after sorption, In the presence of K CI, the amount of adsorbed P decreased, with increasing pH, from 4.5 to 0.1 8 mu mol P m(-2) for IHP, and from 2.5 to 0.67 mu mol P m(-2) for Pi. A mor e pronounced decrease was observed in the amount of IHP adsorbed compared w ith Pi, because of the higher negative charge of IHP and to the lower tende ncy of phosphate groups of IHP to neutralize the OH- released from the surf ace during adsorption. In CaCl2, sorption increased from 4.5 to 4.9 mu mol P m(-2) for IHP, and from 2.7 to 4.8 mu mol P m(-2) for Pi with increasing pH, For both P compounds, however, the sorption increased even beyond the m aximum adsorption capacity of the mineral. At the used concentrations of an ions and Ca2+ and in a pure system, the reaction with goethite may involve adsorption at low pH, but precipitation of Ca salts at pH >5 must be taken into account. The sorption of IHP caused a high change of the surface charg e that became negative at all pH values in K+, causing dispersion of partic les, while in Ca2+ the slightly negative phosphated surface determined part icle aggregation.