Metal-binding affinity of a series of bis-benzimidazoles immobilised on silica

Abstract

Three potentially tridentate bis(benzimidazol-2-ylalkyl)amines have been immobilised on silicavia the bifunctional spacer (3-glycidoxypropyl)trimethoxysilane (GLYMO). The obtained ligand concentrations are in the range 0.53–0.66 mmol/g ion exchanger. These ion exchangers selectively adsorb Cu²⁺ , with a maximum capacity in the range 0.29–0.47 mmol g^-1 at pH 5.6 from aqueous solutions, containing a mixture of the divalent metal ions Cu²⁺ , Ni²⁺ , Co²⁺ , Cd²⁺ and Zn²⁺ . Strong binding of Cu²⁺ , even at a pH as low as 1, was observed with Alfa-BBAH showing that coordination via a five-membered chelate is favoured over a six-membered chelate as in the case of Alfa-BBEAH. Although, potentially, two different didentate coordination modes are possible with Alfa-BBPAH (i.e. through five- and seven-membered chelate structures), the Cu²⁺ uptake and distribution coefficients show a high similarity with Alfa-BBAH, suggesting strong chelation via the stable five-membered chelate. The coordination chemistry of the ligand BBPAH is further illustrated by the presentation of the crystal structures with the reference compounds CuCl₂ and Ni(NO₃)₂ . Fast uptake and regeneration kinetics were found with these ion exchangers, but complete regeneration of Alfa-BBAH takes significantly longer than for the other two. With Alfa-BBAH and Alfa-BBEAH a significant reduction of the capacity is seen after several cycles of consecutive loading and regeneration which was shown to be caused by hydrolysis of the siloxane bond of the silica surface.