COMPETITIVE SURFACE COMPLEXATION OF O-PHTHALATE AND PHOSPHATE ON GOETHITE (ALPHA-FEOOH) PARTICLES

Complexation of o-phthalate (1,2-benzenedicarboxylate) and competitive complexation of phosphate and phthalate at the goethite-water interfa ce have been studied in 0.1 M Na(NO3) media at 298.2 K within the rang e 3.0 < -log [H+] < 8.5. Equilibrium measurements were performed as po tentiometric titrations supplemented with spectrophotometric phosphate and phthalate analyses. The binary and ternary chemical subsystems H-goethite and H+-goethite-H2PO4- have been investigated earlier and de scribed according to the constant capacitance model. The adsorption of phthalate showed a strong ionic strength dependence which indicated t hat phthalate is adsorbed as outer-sphere complexes. The experimental data in the subsystem H+-goethite-phthalate were evaluated on the basi s of an extended constant capacitance model with the aid of the comput er program FITEQL, version 2.0. One plane for inner sphere complexatio n and one plane for outer-sphere complexation, each with an associated constant capacitance, were included in the extended constant capacita nce model. Surface complexation of phthalate is described by two outer -sphere complexes, =FeOH(2)(+)L(2-) and =FeOH(2)(+)HL(-). In the exper iments with simultaneous complexation of phosphate and phthalate, the complexation of phosphate was not influenced by the presence of phthal ate. On the other hand, the complexation of phthalate was decreased ev en at low phosphate concentrations. The equilibrium models determined for the subsystems were used to predict the adsorption of phosphate an d phthalate in the quaternary system. It was found that these predicti ons were in good agreement with experimental titration and phosphate/p hthalate adsorption data. Diffuse reflectance IR-spectra were recorded to obtain structural information of the phthalate complexes. The spec troscopic data did not contradict the outer-sphere model. However, bec ause of the complexity of the phthalate molecule conclusive structural assignment could not be made.