Influence of the chemical nature of grafted β-cyclodextrins functional groups on the silica gels sorption specificity

The aim of this work is to study the influence of the chemical nature of grafted β-cyclodextrins side functional groups on chemically modified silica gels sorption affinity for cadmium, lead, and copper cations as the most typical water pollutants. Two organo-inorganic sorbents that differ only in the chemical nature of side functional groups of grafted β-cyclodextrin molecules wide edge were synthesized by chemical immobilization of mono-(6-O-(toluenesulfonyl))-β-cyclodextrins on the granular mesoporous silica gel surface. The obtained functionalized silica gels were characterized by IR and UV spectroscopy, chemical, elemental and derivatographic analysis, pH-metry, low-temperature nitrogen adsorption-desorption as well as sorption measurements. The results of studying the sorption of cadmium, lead, and copper cations from nitrate buffer solutions with pH = 2.0 were analyzed using the Lagergren kinetic model for pseudo-first and pseudo-second order processes, and the equilibrium sorption results were analyzed using the Langmuir and Freindlich models for homogeneous and heterogeneous surfaces. It has been found that only β-cyclodextrin-containing surface centers of chemically modified silica gels participate in the sorption of metal cations. The simultaneous formation of inclusion complexes of grafted β-cyclodextrin molecules with nitrate anions of the solution and mixed complexes of metal cations with side functional groups of the wide edge of immobilized cyclic oligosaccharides occurs during the interaction of synthesized silica gels with buffer solutions of cadmium, lead and copper nitrates. The sorption affinity of β-cyclodextrin-containing silica gel with secondary alcohol groups in the structure increases in the order Cd 2+  < Pb 2+  < Cu 2+ . The cation sorption sequence is opposite – Cu 2+  < Pb 2+  < Cd 2+ – for β-cyclodextrin-containing silica gel with side thiosemicarbazidoethyl groups. The change in the sorption specificity of functionalized silica gels to metal cations is interpreted from the standpoint of the theory of “soft” and “hard” acids and bases. The formation of “β-cyclodextrin ‒ nitrate-anion” inclusion complexes with a composition of 1:1 on the surface of chemically modified silica gels promotes metal cations sorption. The results obtained make it possible to predict the sorption specificity of β-cyclodextrin-containing silica materials and its directed change using various functional derivatives of β-cyclodextrin.