ABSTRACT

The retention mechanisms of metal ions during interaction of clay with metal-rich aqueous solutions is usually investigated by sorption isotherms. Although classical isotherm models may provide sufficient information about the characteristics of the solid–liquid system, they do not distinguish among the various retention mechanisms. This study presents a methodological approach of combining batch experiment data and geochemical modelling for the characterization of the interaction of Mg-Fe-rich clay materials with monometallic solutions of Pb and Cu. For this purpose, a palygorskite clay (PCM), an Fe-smectite clay (SCM) and a natural palygorskite-Fe-smectite mixed clay (MCM) were assessed for their effectiveness as metal ion sorbents. The sorption capacity of the materials follows the order MCM > SCM > PCM and ranges between 27.6–52.1 mg g–1 for Pb and 7.7–17.6 mg g–1 for Cu. Based on the experimental results that allowed the speciation calculations, fitting of sorption isotherms and the investigation of relationships between protons, Mg and the metals studied we suggest that a combination of sorption mechanisms occurs during the interaction of clay materials with metal solutions. These involve surface complexation, ion exchange and precipitation of solid compounds onto the solid surface. A three-term isotherm model was employed to quantify the role of each of the above mechanisms in the overall retention process. The superior performance of mixed clay among the materials tested is attributed to the synergetic effect of exchange in the interlayer and specific sorption on the clay edges.

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