Abstract:
A study combining information from infrared spectroscopy and adsorption experiments was carried out to investigate phthalate, trimellitate, and pyromellitate complexes at the goethite (α-FeOOH)/water interface. Infrared spectra showed evidence for inner-sphere complexes below pH 6 and outer-sphere complexes in the pH range 3 to 9. Normalized infrared peak areas were used as a semi-quantitative tool to devise diagrams showing the molecular level surface speciation as a function of pH. Surface complexation models that simultaneously predict these diagrams, the proton balance data and the ligand adsorption data were developed with surface complexation theory. Surface complexation modeling was carried out with a Charge Distribution Multisite 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 complexes were described as mononuclear chelates at the {001} plane, whereas outer-sphere complexes were described as binuclear complexes with singly coordinated sites on the {110} plane. The Three-Plane Model (TPM) was used to described surface electrostatics and to distribute the charges of the inner- and the outer-sphere complexes on different planes of adsorption.