Oxides, hydroxides and oxyhydroxides (oxides s.l.) are major components of soils, especially for Al, Fe and Mn. In addition, oxides are major host phases for many trace elements, such as Ni, Co, Zn, Pb etc. Trace elements are either adsorbed on the surfaces or substitute for major elements in the lattices. The thermodynamic properties of those mixed phases are different from the properties of pure phases, which are thus not relevant for soils. In addition, sorption of trace metals on aluminosilicates is often a first step towards co-precipitation of trace metals, e.g. Ni, Zn with Al hydroxide as layered double hydroxides (LDHs), which lower the trace metal solubilities. LDHs consist of a brucitic layer in which substitutions of M(III) for M(III) give an excess positive charge compensated in the interlayer by different anions. Another group of LDHs is made of « green rusts » (GRs), i.e. mixed Fe(II)-Fe(III) double hydroxides. Fougerite is the natural mineral of the group of GRs and can incorporate Mg in its lattice. LDHs and GRs are easily formed and dissolved and likely to control the solubilities of major and trace metals in soils and sediments. Thermodynamic data on those complex minerals are scarce. Substitutions of metals of different electric charge and /or size results in LDHs being non ideal solid solutions. Regular solid solution models have been successfully fitted to experimental data on GRs (Bourrié et al., 2004) and LDHs (Peltier et al., 2006). In those minerals, due to both structural and geochemical constraints, the extent of substitutions in the brucitic layer is rather limited, while the nature of the compensating anion can vary largely. Thermochemical data show that small changes on the composition of the interlayer influence very largely the solubility of LDHs (Allada et al., 2002). The chemical potentials of those components depend both on the composition of the brucitic layer and on the nature of the interlayered anion. The regular solid solution model leads to a parabolic dependance of the chemical potential with mole fractions, while at quasi-constant composition of the layer, a linear relation has been obtained between the chemical potential of the solid solution and the electronegativity of the compensating anion, taken on the Allred-Rochow scale. It is shown here that thermodynamic properties, Gibbs free energies of formation and enthalpies of formation of oxides (s.l.) are related with the electronegativities of the elements, both divalent and trivalent cations and of the solids.

References :

Allada, R.K., Navrotsky, A., Berbeco, H.T. and Casey, W.H. 2002. – Thermochemistry and aqueous solubilities of hydrotalcite-like solids. Science, 296, 721-723.

Bourrié, G., Trolard, F., Refait, Ph. and Feder, F. 2004. – A solid-solution model for Fe(II) – Fe(III) – Mg(II) green rusts and fougerite and estimation of their Gibbs free energies of formation. Clays and Clay Minerals, 52, 382-394.

Peltier, E., Allada, R.K., Navrotsky, A. and Sparks, D.L. 2006. – Nickel solubility and precipitation in soils : a thermodynamic study. Clays and Clay Minerals (in press).