Phosphate complexation at the goethite-water interface has been studied in 0.1 M Na(NO3) medium at 298.2 K in the range 3.0 <—log[H+] < 9.0. Equilibrium measurements were performed as potentiometric titrations supplemented with spectrophotometric phosphate analyses. The experimental data were evaluated on the basis of the constant capacitance model and with the aid of the computer program FITEQL, version 2.0. The acid/base properties of the goethite-water interface have been investigated earlier and are described by two intrinsic equilibrium constants logβs 1,1,0(int) = 7.47 and logβs-1,1,0(int) = −9.51 and with a specific capacitance of 1.28 F m−2. The binding between phosphate and goethite was found to be strong within the investigated -log[H+] range, and the amount of adsorbed phosphate is slowly decreasing with increasing—log[H+] value. The model describing the phosphate complexation is represented by the following equilibria and intrinsic constants:
≡FeOH + H2PO4− ⇔ ≡FePO42− + H+ + H2O; logβ−1,1,1(int)S = 2.16 ± 0.10
≡FeOH + H2PO4− ⇔ ≡FePO4H− + H2O; logβ0,1,1(int)S = 7.93 ± 0.09
≡FeOH + H2PO4− + H+ ⇔ ≡FePO4H2 + H2O; logβ1,1,1(int)S = 12.68 ± 0.05
Further support for the proposed model is given by zeta-potential measurements.
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