Effects of pH and Electrolytes on Inositol Hexaphosphate Interaction with Goethite

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Abstract
The pH and the nature of electrolytes are important factors affecting the sorption of anions by soil components. The effects of pH, K, and Ca on the interaction of inositol hexaphosphate (IHP) and inorganic phosphate (Pi) with goethite were investigated by sorption experiments. Laser Doppler Velocimetry‐Photon Correlation Spectroscopy was employed to determine zeta potential (ζ) and particle size before and after sorption. In the presence of KCl, the amount of adsorbed P decreased, with increasing pH, from 4.5 to 0.18 μmol P m−2 for IHP, and from 2.5 to 0.67 μmol P m−2 for Pi. A more pronounced decrease was observed in the amount of IHP adsorbed compared with Pi, because of the higher negative charge of IHP and to the lower tendency of phosphate groups of IHP to neutralize the OH− released from the surface during adsorption. In CaCl2, sorption increased from 4.5 to 4.9 μmol P m−2 for IHP, and from 2.7 to 4.8 μmol P m−2 for Pi with increasing pH. For both P compounds, however, the sorption increased even beyond the maximum adsorption capacity of the mineral. At the used concentrations of anions 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 charge that became negative at all pH values in K+, causing dispersion of particles, while in Ca2+ the slightly negative phosphated surface determined particle aggregation.
Funding Information
  • Italian Ministry of University and Scientific Research