Modelling the sorption of Mn(II), Co(II), Ni(II), Zn(II), Cd(II), Eu(III), Am(III), Sn(IV), Th(IV), Np(V) and U(VI) on montmorillonite: Linear free energy relationships and estimates of surface binding constants for some selected heavy metals and actinides

Abstract

In solution thermodynamics, and more recently in surface chemistry, it is well established that relationships can be found between the free energies of formation of aqueous or surface metal complexes and thermodynamic properties of the metal ions or ligands. Such systematic dependencies are commonly termed linear free energy relationships (LFER). A 2 site protolysis non-electrostatic surface complexation and cation exchange (2SPNE SC/CE) model has been used to model “in house” and literature sorption edge data for eleven elements: Mn(II), Co(II), Ni(II), Zn(II), Cd(II), Eu(III), Am(III), Sn(IV), Th(IV), Np(V) and U(VI) to provide surface complexation constants for the strong sites on montmorillonite. Modelling a further 4 sets of sorption isotherms for Ni(II), Zn(II), Eu(III) and U(VI) provided complexation constants for the weak sites. The protolysis constants and site capacities derived for the 2SPNE SC/CE model in previous work were fixed in all of the calculations. Cation exchange was modelled simultaneously to provide selectivity coefficients. Good correlations between the logarithms of strong S K x−1 and weak W1 K x−1 site binding constants on montmorillonite and the logarithm of the aqueous hydrolysis constants OH K x were found which could be described by the following equations: Strong (≡S SOH) sites: log S K X − 1 = 8.1 ± 0.3 + ( 0.90 ± 0.02 ) log OH K X Weak (≡S W1OH) sites: log W 1 K X − 1 = 6.2 ± 0.8 + ( 0.98 ± 0.09 ) log OH K X where x is an integer. Sorption data for heavy metals and actinides such as Pd(II), Pb(II), Pu(III), Zr(IV), U(IV), Np(IV), Pu(IV) and Pa(V), are important in safety studies for radioactive waste repositories and are either very poorly known or not available at all. The LFER approach was used to estimate surface complexation constants for these radionuclides on both site types. The surface protolysis constants, site capacity values, selectivity coefficients and surface complexation constants given in this work, coupled with the LFERs established for the strong and weak sites on montmorillonite, are considered to form a sound basis for a thermodynamic sorption database.