Complexation of methylphosphonic acid with the surface of goethite particles in aqueous solution

Adsorption of methylphosphonic acid (MPA) onto goethite (alpha-FeOOH) was s tudied as a function of pH and the concentration of MPA in solution. In sit u attenuated total reflectance-FTIR (ATR-FTIR) techniques were used to iden tify the type of surface complexes formed under each of the solution condit ions used in the adsorption studies. Additionally, electrophoretic mobility measurements were performed in each of these systems. At low pH and high G amma, MPA was bound to the surface of goethite predominantly as a monodenta te protonated species, while at high pH and low Gamma the prevailing struct ure was that of a bridging bidentate complex. The change in denticity of th e surface MPA under different solution conditions may explain the charge re versals shown in the electrophoretic mobility curves.