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

Abstract

As constantes de complexação em solução aquosa para soluções de galactomanana da Leucaena leucocephala e íons metálicos Co 2+ , Mn 2+ , Ni 2+ e Zn 2+ foram obtidas por titulações potenciométricas. Os valores obtidos mostraram que o íon metálico Ni 2+ é o ácido de Lewis que se complexa melhor com os grupos-OH, bases de Lewis, dos açúcares monoméricos do biopolímero e o Zn 2+ , o ácido de Lewis que se complexa mais fracamente. A maior percentagem das espécies complexadas destes equilíbrios se concentraram no pH=7,0, existindo porém quantidades significativas em valores de pH tanto na região ácida quanto básica. Os complexos sólidos isolados das soluções aquosas foram estudados por TG-DSC e espectroscopia de EPR. As curvas de comportamento térmico mostraram que para os complexos ML de menores valores de log K (Zn 2+ e Mn 2+) as temperaturas de degradação final são maiores quando comparadas à do biopolímero. Os espectros de EPR confirmaram a complexação entre os íons metálicos e os sítios básicos do polissacarídeo, grupamentos hidroxilas desprotonados-O-, e mostraram a dependência da natureza do íon metálico na distância em que se encontram complexados dentro da estrutura do biopolímero. A habilidade da galactomanana em complexar íons metálicos variados em sua estrutura entrincada, a resistência que esses complexos exibiram frente a altas temperaturas e a faixa larga de pH em que as espécies complexadas mostraram-se presentes abrem novas perspectivas de utilização destes materiais em processos industriais onde essas propriedades são desejáveis, em bioremediação de rejeitos aquosos e na química de solos, como fertilizantes "slow-release". The binding constants for the complexed species formed in aqueous solution between galactomannan of Leucaena leucocephala and the metal ions Co 2+ , Mn 2+ , Ni 2+ and Zn 2+ were determined by potentiometric titrations. The calculated values showed Ni 2+ as the best Lewis acid towards the Lewis base-OH groups of the sugar monomers, with Zn 2+ being the poorest. For all systems, a higher percentage of the complexed species was present near pH=7.0, although 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 resistance to final degradation than the biopolymer alone for the complexed species ML having the smallest log K values. The EPR spectra confirmed the complexation of the metal ions via the Lewis base deprotonated hydroxyl groups (-O) and showed that the distances between metal ions in the complexed biopolymer structure depend on the nature of the metal ion. The ability of galactomannans to complex a variety of metal ions in their web like structure and the resistance to high temperatures and a wide range of pH values of these complexes open new perspectives in possible industrial uses whenever these properties are required, such as in bioremediation of waste waters and in the application of slow-release fertilizers.

Fe (III) - Galactomannan Solid and Aqueous Complexes: Potentiometric, EPR Spectroscopy and Thermal Data

“…Although biopolymers are not uniform compounds, the viability of using potentiometric titrations in the determination of binding constants of complexes between metal ions and biopolymers has recently been demonstrated [5][6][7]11,[20][21][22] .…”

“…The galactomannans were obtained by aqueous extraction of seeds from C. fastuosa, L. leucocephala and S. macranthera, and arabinogalactan by aqueous extraction of the leaves of P. aculeata [5][6]25 . Alditol acetate derivatives of both polysaccharides were prepared and analyzed by GLC-MS (Varian 3300) with an OV-225 capillary column (0.25 mm id x 30 m) linked to a Finnigan Trap (model 419) mass spectrometer at 70 eV.…”

“…The ratio between the galactose and mannose monomers varies according to the species and is also affected by climate, thus providing different substitution degrees (SD) in the biopolymer. Galactomannans are used in many industrial processes as an alternative to guar gum, locust beam gum and other gums, taking advantage of the fact that their rheological properties and their solubility are dependent upon their SD [1][2][3][4][5][6][7] . J. Braz.…”

“…The discovery by Angyal 17 that the conformations of sugars in the solid state differ from those in solution prompted many studies of equilibria between sugars and metal ions in the solid state [5][6][7][10][11][18][19][20][21][22][23] . Solid complexes of Fe(III) with various monosaccharides, including galactose and mannose, have been reported to have varying stoichiometries 24 .…”

New Slime

“…Bei einem pKs-Wert der Hydroxylgruppen der Galactose von ca. 12 [14] denkt man natürlich nicht unmittelbar an die Mçglichkeit einer deprotonierten Zucker-OH-Gruppe und damit eines guten Komplexbildungsliganden. Die Wirkung eines erhçhten pH-Werts (durch Zugabe von Natriumhydroxid) wird allerdings sofort deutlich, wenn man das folgende Gleichgewicht betrachtet:…”