Metal-binding affinity of a series of ω-amino-alkylbenzimidazoles immobilized on silica

Abstract

A series of ω-amino-alkylbenzimidazoles, with a varying number ( n = 0–3) of methylene groups in the alkyl chain, have been coupled to the epoxy-containing spacer (3-glycidoxypropyl)-trim ethoxysilane (GLYMO) through a nucleophilic ring opening by the primary amine of the ligand, followed by grafting onto silica. The obtained ligand concentrations vary from 0.59 to 0.79 mmol/g ion exchanger. These novel ion exchangers were characterized by solid-state cross-polarization and magic-angle spinning nuclear magnetic resonance spectroscopy, by infrared spectroscopy and by elemental analyses. These ion-exchange materials selectively adsorb Cu 2+ with a maximum capacity ranging from 0.33 to 0.45 mmol/g ion exchanger at pH 5.6 from aqueous solutions containing a mixture of the divalent metal ions Cu 2+, Ni 2+, Cd 2+, Co 2+ and Zn 2+. An exception is n = 0, which shows very low capacity for any of the metal ions used (maximum capacity 0.07 mmol/g ion exchanger). The metal-uptake capacities of the ion exchanger correlates to the length of the alkyl bridge in the chelating ligand. Increasing the length of the alkyl bridge increases the susceptibility towards acid, thereby making the metal-uptake capacity more pH dependent. Distribution coefficient measurements show that the stability of the Cu 2+ complexes formed on the ion exchangers depend on the ringsize of the metal to ligand chelate complexes. The Cu 2+ uptake was fast with t 1 2 values of 15 to 120 s for 0.08 molar solutions, except for n = 3 with t 1 2 = 900 s. Fast and selective removal of Cu 2+ from a mixture of Cu 2+ and Zn 2+ was achieved with n = 1 and n = 2. Regeneration of Cu 2+-loaded ion exchangers with 0.5 M H 2SO 4 is also very fast with t 1 2 = 10–60 s. The capacity for Cu 2+ remains at the same level after several cycles of consecutive loading and regeneration.