Evaluation of the complexes of galactomannan of Leucaena leucocephala and Co2+, Mn2+, Ni2+ and Zn2+.

The binding constants for the complexed species formed in aqueous solution between galactomannan of Leucaena leucocephala and the metal ions Co2+, Mn2 +, Ni2+ and Zn2+ were determined by potentiometric titrations. The calculat ed values showed Ni2+ as the best Lewis acid towards the Lewis base -OH gro ups of the sugar monomers, with Zn2+ being the poorest. For all systems, a higher percentage of the complexed species was present near pH=7.0, althoug h complexed species existed over a wide range of acidic and basic pH values . The isolated solid complexes were studied by TG-DSC thermal analysis and by EPR spectroscopy. The thermal profiles obtained showed higher thermal resi stance to final degradation than the biopolymer alone for the complexed spe cies ML having the smallest log K values. The EPR spectra confirmed the com plexation of the metal ionsvia the Lewis base deprotonated hydroxyl groups (-O) and showed that the distances between metal ions in the complexed biop olymer structure depend on the nature of the metal ion. The ability of galactomannans to complex a variety of metal ions in their w eb like structure and the resistance to high temperatures and a wide range of pH values of these complexes open new perspectives in possible industria l uses whenever these properties are required, such as in bioremediation of waste waters and in the application of slow-release fertilizers.