Geometry, Charge Distribution, and Surface Speciation of Phosphate on Goethite

Abstract

The surface speciation of phosphate has been evaluated with surface complexation modeling using an interfacial charge distribution (CD) approach based on ion adsorption and ordering of interfacial water. In the CD model, the charge of adsorbed ions is distributed over two electrostatic potentials in the double-layer profile. The CD is related to the structure of the surface complex. A new approach is followed in which the CD values of the various surface complexes have been calculated theoretically from the geometries of the surface complexes. Molecular orbital calculations based on density functional theory (MO/DFT) have been used to optimize the structure of a series of hydrated surface complexes of phosphate. These theoretical CD values are corrected for dipole orientation effects. Data analysis of the PO4 adsorption, applying the independently derived CD coefficients, resolves the presence of two dominant surface species. A nonprotonated bidentate (B) complex is dominant over a broad range of pH values at low loading (< or =1.5 micromol/m(2)). For low pH and high loading, a strong contribution of a singly protonated monodentate (MH or MH-Na) complex is found, which differs from earlier interpretations. For the conditions studied, the doubly protonated bidentate (BH2) and monodentate (MH2) surface complexes and the nonprotonated monodentate (M) complex are not significant contributors. These findings are discussed qualitatively and quantitatively in relation to published experimental in-situ CIR-FTIR data and theoretical MO/DFT-IR information. The relative variation in the peak intensities as a function of pH and loading approximately agrees with the surface speciation calculated with the CD model. The model correctly predicts the proton co-adsorption of phosphate binding on goethite and the shift of the IEP at low phosphate loading (< or =1.5 micromol/m(2)). At higher loading, it deviates.

Nanoparticles in natural systems I: The effective reactive surface area of the natural oxide fraction in field samples

“…For goethite, the pH dependent adsorption behavior of phosphate has been studied extensively (Rahnemaie et al, 2007b). The goethite-phosphate interaction can be described using two surface species (Table 2).…”

Adsorption and surface oxidation of Fe(II) on metal (hydr)oxides

“…The bond strength is related to the relative bond length between the central ion of a complex and the coordinating ligands (Brown and Altermatt, 1985;Brown, 2002). In principle, the variation in bond length is experimentally accessible and can also be approached with molecular orbital (MO) calculations Stachowicz et al, 2006;Hiemstra et al, 2007;Rahnemaie et al, 2007). The variation in bond strength, due to differences in the geometry of metal surface complexes, can rationalize the differences in the CD values found for a series of divalent metal cations like Hg(II), Pb(II), Cu(II) and Cd(II) (Hiemstra and van Riemsdijk, 2002).…”

“…We further simplify by setting the logK H value for both types of surface groups equal to the value of the PZC as done for TiO 2 and c-AlOOH. For the ion pair formation constants, we rely on the values found for goethite (Rahnemaie et al, 2007).…”

“…The ion pair formation constants, given in Table 7, are from Rahnemaie et al (2007). In the modeling, a capacitance value of 0.93 F/m 2 (well-structured goethite) is used.…”

“…and n 1 + n H1 = À0.03 ± 0.05 v.u. The uncertainty (±0.05) is estimated based on our previous experience that the precise value depends on the model used, the hydration and the defined FeOOH cluster (Hiemstra and van Stachowicz et al, 2006;Hiemstra et al, 2007;Rahnemaie et al, 2007). The dipole correction term is negligible, leading to a set of CD coefficients of Dz 0 = 1.03 v.u.…”