Benzenecarboxylate surface complexation at the goethite (alpha-FeOOH)/water interface: II. Linking IR spectroscopic observations to mechanistic surface complexation models for phthalate, trimellitate, and pyromellitate

A study combining information from infrared spectroscopy and adsorption exp eriments was carried out to investigate phthalate, trimellitate, and pyrome llitate complexes at the goethite (alpha-FeOOH)/water interface. Infrared s pectra showed evidence for inner-sphere complexes below pH 6 and outer-sphe re complexes in the pH range 3 to 9. Normalized infrared peak areas were us ed as a semi-quantitative tool to devise diagrams showing the molecular lev el surface speciation as a function of pH. Surface complexation models that simultaneously predict these diagrams, the proton balance data and the lig and adsorption data were developed with surface complexation theory. Surfac e complexation modeling was carried out with a Charge Distribution Multisit e Complexation Model (CD-MUSIC), assuming goethite particles with surfaces represented by the {110} plane (90% of total particle surface area) and by the {001} plane (10% of total particle surface area). Inner-sphere complexe s were described as mononuclear chelates at the {001} plane, whereas outer- sphere complexes were described as binuclear complexes with singly coordina ted sites on the {110} plane. The Three-Plane Model (TPM) was used to descr ibed surface electrostatics and to distribute the charges of the inner- and the outer-sphere complexes on different planes of adsorption. Copyright (C ) 2000 Elsevier Science Ltd.