Complexation reactions of phthalic acid and aluminium (III) with the surface of goethite

Abstract

Complexation reactions of phthalic acid at the goethite-solution interface have been investigated in the absence and the presence of Al(III) in order to evaluate an equilibrium model applicable in chemical modelling. Phthalic acid was chosen as a model compound for natural organic substances. The study was performed as potentiometric titrations in 0.100 M NaNO3 at 298.2 K using a glass electrode. Evaluation of experimental data was based on the constant-capacitance model (K = 1.28 Fm−2). Complexation of phthalic acid (H2L) at the surface of goethite is rather weak and results in the formation of mononuclear species, as described by the equilibria and intrinsic equilibrium constants (±3σ): FeOH + H2L FeLH + H2O; logβ0,1,0,1(int)s = 6.68 (±0.05) and FeOH + H2L FeL− + H2O + H+; logβ−1,1,0,1(int)s = 0.35 (±0.06).In the presence of Al(III), the adsorption behavior of phthalic acid indicated formation of ternary surface complexes in the region 4 ≤−log [H+] ≤7. Additional complexes occurring in the four-component system are described by the equilibria and intrinsic equilibrium constants: FeOH + Al3+ + H2L FeOAlL + 3H+; logβ−3,1,1,1(int)s = −2.7 (±0.29) and FeOH + Al3+ + H2L + H2O FeOAl(OH)L− + 4H+; logβ−4,1,1,1(int)s = −8.3 (±0.16).