Benzimidazole adsorption on the external and interlayer surfaces of raw and treated montmorillonite

The purpose of this study is to evaluate the efficiency of benzimidazole removal using raw, thermally and mechanically treated montmorillonite surfaces. The comparison of benzimidazole adsorption curves on these montmorillonite surfaces was made. A two step sorption mechanism that describes the data was corroborated by X-ray diffraction (XRD) results.The removal efficiency for the different sorbents at pH 6 follows the sequence raw montmorillonite > thermally treated montmorillonite > mechanically treated montmorillonite.The external and the interlayer surfaces were proposed as adsorption sites. On the latter, benzimidazole adsorbs in a planar monolayer arrangement with a consequent reduced water uptake.Two different foms of benzimidazole nitrogen surface coordination were indicated by X-ray photoelectron spectroscopy (XPS) results and isoelectric point (IEP) values. Protonated and deprotonated nitrogen were equally coordinated to the surface for raw and thermally treated samples at low temperature (350 °C). Stronger coordination of deprotonated nitrogen was found for samples treated at higher temperature (550 °C), while only a cationic exchange and no coordination to the surface was observed for the mechanically treated sample.

Research Highlights
► * Two step sorption mechanisms, at external and interlayer surfaces, were postulated for the adsorption of benzimidazole on raw, thermally and mechanically treated montmorillonite.
► * Different N coordination of the benzimidazole molecule to the surface of raw and thermally treated samples, were determined by XPS in agreement with IEP values.
► * The mechanical treated sample showed only cationic exchange mechanism, without N coordination to the surface.
► * Modified montmorillonites by thermal and mechanical treatments showed a decrease in the benzimidazole adsorption capacity respect, to the raw sample.