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  • 1.
    Boily, J F
    et al.
    Umeå Universitet.
    Nilsson, Nils
    Umeå Universitet.
    Persson, Per
    Umeå Universitet.
    Sjöberg, Staffan
    Umeå Universitet.
    Benzenecarboxylate surface complexation at the goethite (alpha-FeOOH)/water interface: I. A mechanistic description of pyromellitate surface complexes from the combined evidence of infrared spectroscopy, potentiometry, adsorption data, and surface complexation modeling2000In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 16, no 13, p. 5719-5729Article in journal (Refereed)
    Abstract [en]

    An investigation combining IR spectroscopy, potentiometric titrations, and adsorption experiments was carried out to study pyromellitate (1,2,4,5-benzenetetracarboxylate) sorption at the goethite (α-FeOOH)/water interface. The IR spectra show evidence of outer-sphere complexation throughout the pH range from 3 to 9. Below pH 6 additional IR spectroscopic features appear, which are tentatively assigned to inner-sphere complexes. A normalized IR peak area plot for each peak indicative of inner- and of outer-sphere complexes as a function of pH provided a semiquantitative surface speciation scheme. This scheme was successfully reproduced using surface complexation theory with a multisite complexation model calibrated on potentiometric titration and on adsorption data. The surface speciation was described with a binuclear outer-sphere complex on the {110} plane of goethite and a mononuclear inner-sphere complex on the {001} plane. Furthermore, as the IR spectra also indicated partial protonation of pyromellitate complexes at low pH, a partially protonated outer-sphere species on the {110} plane was included in the model.

  • 2.
    Nilsson, Nils
    et al.
    Umeå Universitet.
    Persson, Per
    Umeå Universitet.
    Lövgren, Lars
    Umeå Universitet.
    Sjöberg, Staffan
    Umeå Universitet.
    Competitive surface complexation of o-phthalate and phosphate on goethite (alpha-FeOOH) particles1996In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 60, no 22, p. 4385-4395Article in journal (Refereed)
    Abstract [en]

    Complexation of o-phthalate (1,2-benzenedicarboxylate) and competitive complexation of phosphate and phthalate at the goethite-water interface have been studied in 0.1 M Na (NO 3) media at 298.2 K within the range 3.0 < -log [H +] < 8.5. Equilibrium measurements were performed as potentiometric titrations supplemented with spectrophotometric phosphate and phthalate analyses.

    The binary and ternary chemical subsystems H +-goethite and H +-goethite-H 2PO 4−have been investigated earlier and described according to the constant capacitance model. The adsorption of phthalate showed a strong ionic strength dependence which indicated that phthalate is adsorbed as outer-sphere complexes. The experimental data in the subsystem H +-goethite-phthalate were evaluated on the basis of an extended constant capacitance model with the aid of the computer program FITEQL, version 2.0. One plane for inner sphere complexation and one plane for outer-sphere complexation, each with an associated constant capacitance, were included in the extended constant capacitance model. Surface complexation of phthalate is described by two outer-sphere complexes, ≡FeOH 2+L 2− and ≡FeOH 2+ HL −.

    In the experiments with simultaneous complexation of phosphate and phthalate, the complexation of phosphate was not influenced by the presence of phthalate. On the other hand, the complexation of phthalate was decreased even at low phosphate concentrations. The equilibrium models determined for the subsystems were used to predict the adsorption of phosphate and phthalate in the quaternary system. It was found that these predictions were in good agreement with experimental titration and phosphate/ phthalate adsorption data.

    Diffuse reflectance IR-spectra were recorded to obtain structural information of the phthalate complexes. The spectroscopic data did not contradict the outer-sphere model. However, because of the complexity of the phthalate molecule conclusive structural assignment could not be made.

  • 3.
    Persson, Per
    et al.
    Umeå Universitet.
    Nilsson, Nils
    Umeå Universitet.
    Sjöberg, Staffan
    Umeå Universitet.
    Structure and bonding of orthophosphate ions at the iron oxide aqueous interface1996In: Journal of Colloid and Interface Science, ISSN 0021-9797, E-ISSN 1095-7103, Vol. 177, no 1, p. 263-275Article in journal (Refereed)
    Abstract [en]

    The surface speciation of orthophosphate ions on goethite has been studied as a function of pH, time, total phosphate concentration, and ionic medium by means of diffuse reflectance FTIR spectroscopy. The samples were prepared in accordance with a distribution diagram of surface species as calculated from thermodynamic data. In agreement with the thermodynamic model three dominating surface complexes could be distinguished with IR spectroscopy, and the relative distribution of the species was shown to be primarily a function of pH. The IR characteristics of the individual surface complexes were indicative of molecular symmetries of the PO4unit of C3v, C2v, and C3v, respectively. This was concluded to be incompatible with the bidentate, bridging structural model previously suggested. Instead, the IR data are in good agreement with a monodentate coordination of phosphate to the surfaces, where the three surface complexes only differ in the degree of protonation. A comparison between the adsorption behavior of phosphate on goethite and hematite was also made. Here the importance of the aqueous stability of the adsorbent on the adsorption mechanism was shown.

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