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This research was performed to elucidate the adsorption mechanisms of two monoester phosphates, glucose‐1‐phosphate (G1P) and myo‐inositol hexaphosphate (I6P), on a synthetic goethite with reference to orthophosphate (Pi). Adsorption isotherms were built and electrophoretic and pH measurements were carried out in order to determine the nature of the adsorbent‐adsorbate bonds. The quantity of adsorbed compound reached 5.41 µmol Pi m−2, 5.06 µmol G1P m−2, and 1.37 µmol I6P m−2. The G1P and I6P were adsorbed on the surface hydroxyls of goethite through their phosphate groups. The proposed adsorption mechanism, notwithstanding the nature of the organic moiety, is a specific adsorption by a ligand exchange between the phosphate groups and the surface reactive hydroxyls. Inner‐sphere surface complexes are formed between phosphate groups and Fe. Very little dissolution of the goethite occurred during the adsorption of either of the two ligands (G1P and I6P). Tentative configurations of the formed organophosphate‐goethite complexes are proposed.

A quantitative theoretical analysis of the enthaplic effects accompanying ion adsorption at the oxide/electrolyte interface, based on a model of energetically heterogeneous surface oxygens, is presented. The triple layer complexation model is accepted, along with the 2-pK charging mechanism. For the purpose of illustration a set of experimental data is subjected to that quantitative analysis including titration curves, radiometrically measured individual iostherms of ions, and calorimetric titration data for the alumina/NaCl electrolyte system. Two models of energetic heterogeneity were taken into consideration. One of them assumes that the binding-to-oxygen energies of the surface complexes vary but are highly correlated when going from one to another surface oxygen. The other model of surface heterogeneity assumes that these correlations are very small. Our numerical simultaneous analysis of the titration data, of the individual isotherms of Na+ and Cl- adsorption, and of the accompanying heat effects advocates strongly for the model of surface heterogeneity assuming small correlations to exist. A good simultaneous fit of all three kinds of experimental data is obtained, with a small uncertainty as for the values of the estimated adsorption parameters. A simultaneous fit of the measured enthalpic effects appears to be an especially strong criterion for a proper choice of adsorption parameters.

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