Thermodynamic and kinetic coupling model of Cd(II) and Pb(II) adsorption and desorption on goethite.

The adsorption and desorption behaviors of Cd(II) and Pb(II) on goethite have important effects on the mobility and bioavailability of these compounds in soils. However, while the respective adsorption isotherms have been extensively investigated, the kinetic process has often been overlooked. Thus, in batch adsorption and stirred-flow experiments this study investigated the thermodynamic and kinetic behaviors of Cd(II) and Pb(II) on goethite as a function of the pH and the loading concentration. CD-MUSIC and integrated kinetic models were proposed to predict metal adsorption behavior, after careful consideration of the surface properties and site heterogeneity of goethite. The results showed that both the surface species and the binding ability of the two metals differed. While Cd(II) formed two corner-sharing species, (≡FeOH)2Cd+ and (≡FeOH)2CdOH, Pb(II) formed two weak corner-sharing species, (≡FeOHa)2Pb+ and (≡FeOHa)2PbOH, and one strong edge-sharing species, (≡FeOHb)2Pb+. Optimized binding constants (logK) and desorption rate coefficients (kdi) were calculated to quantify the contribution of each surface species to equilibrium adsorption and time-dependent adsorption/desorption under various conditions. Compared with Cd(II), the adsorption of Pb(II) on goethite has two kinds of reaction processes with different affinities, which was attributed to the site heterogeneity of goethite. The proposed model has the potential to be applied to various chemical conditions and goethite samples.