Surface complexation modeling of the sorption of Zn(II) by montmorillonite

Adsorption of Zn(II) to montmorillonite samples K+-Wyoming (SWy-K), Ca2+-Texas (STx-Ca) and K+-Texas (STx-K) was studied at 25 °C in the presence of 10 mM KNO3 for SWy-K and STx-K or 3.3 mM Ca(NO3)2 for STx-Ca. X-ray diffraction measurements showed no change in the interlayer spacing of the montmorillonite samples on adsorption of zinc. Data from adsorption edges, adsorption isotherms, and potentiometric titrations were fitted by an extended constant capacitance surface complexation model (ECCM) which included two dominant sorption reactions: cation exchange at permanent negatively charged sites on the siloxane faces including interlayer regions of montmorillonite, and inner-sphere surface complex formation at variable-charge surface hydroxyl groups on the edges. At pH values above 9 the fit of the model to the data was less satisfactory, most probably because of surface precipitation. Generally the best fit parameters of the ECCM for the three samples agreed well. The exception was the concentration of permanent-charge sites, which was about four times greater for the Wyoming sample.