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

A series of omega-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)-trimethoxysilane (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 Cu2+ 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 Cu2+, Ni2+, Cd2+, Co2+ and Zn2+. 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 Cu2+ complexes formed on the ion exchangers depend on the ringsize of the metal to ligand chelate complexes. The Cu2+ 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 Cu2+ from a mixture of Cu2+ and Zn2+ was achieved with n = 1 and n = 2. Regeneration of Cu2+-loaded ion exchangers with 0.5 M H2SO4 is also very fast with t(1/2) = 10-60 s. The capacity for Cu2+ remains at the same level after several cycles of consecutive loading and regeneration.