Metal-binding affinity of two benzimidazol-2-ylalkylthioethers immobilised on silica

Two benzimidazol-2-ylalkylthiols, one with one and one with two methylene groups in the alkyl chain, have been coupled to the epoxy-containing spacer (3-glycidoxypropyl)-trimethoxysilane (GLYMO). The coupling, performed through nucleophilic ring opening by the thiol group of the ligand, followed by grafting onto silica, results in silica modified with potentially didentate benzimidazol-2-alkylthioethers. The obtained ligand concentrations are 0.62 and 0.75 mmol/g ion exchanger, respectively. Immobilised benzimidazol-2-ylmethylthioether selectively absorbs Cu2+, with a maximum capacity of 0.27 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+. Immobilised 2-(benzimidazol-2-yl)ethylthioether shows lower binding affinity and only moderate selectivity towards Cu2+ (maximum capacity of 0.09 mmol/g ion exchanger); also significant amounts of Cd2+ and Zn2+ were found to be absorbed. The metal-uptake behaviour and distribution coefficients established at different pH, correlate with the length of the alkyl chain in the chelating ligand. A longer alkyl chain results in a lower selectivity towards Cu2+ in combination with a lower complex stability and ligand occupation.