METAL-BINDING AFFINITY OF A SERIES OF BIS-BENZIMIDAZOLES IMMOBILIZED ON SILICA

Three potentially tridentate bis(benzimidazol-2-ylalkyl) amines have b een immobilised on silica via the bifunctional spacer (3-glycidoxyprop yl)trimethoxysilane (GLYMO). The obtained ligand concentrations are in the range 0.53-0.66 mmol/g ion exchanger. These ion exchangers select ively 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 d ivalent 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 th at coordination via a five-membered chelate is favoured over a six-mem bered 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+ upt ake and distribution coefficients show a high similarity with Alfa-BBA H, suggesting strong chelation via the stable five-membered chelate. T he coordination chemistry of the ligand BBPAH is further illustrated b y the presentation of the crystal structures with the reference compou nds CuCl2 and Ni(NO3)(2). Fast uptake and regeneration kinetics were f ound 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 seve ral cycles of consecutive loading and regeneration which was shown to be caused by hydrolysis of the siloxane bond of the silica surface.