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

[摘要] Galactomannans are mainly found in the endosperm of seeds from the Leguminosae family. They consist mainly of mannose and galactose in different ratios with the ratio varying with different species, crops, portions or fractions. They are used either in their native states or as derivatives. Their properties depend on their chemical structure, such as chain length, availability of cis-OH groups, steric hindrance and substituents. Any additional crosslinking via hydrogen bonds or via any other chemical reaction means less solubility, so an increase in substitution in the main chain of the polysaccharide leads to higher solubility. Galactomannans play an important role as improving agents in processes where the aqueous system has to be thickened or where hydrophilic materials need to be coated, depressed or suspended 1.Galactomannans from seeds of Leguminosae are alternative sources for other polysaccharides employed in industry such as guar gum2 and locust bean gum3, since they have the same sugar composition. The variation in the degree of substitution and the ability of complexing metal ions may lead to different chemical properties.Equilibrium studies of some metal ions and mono-saccharides (aldoses e ketoses) and oligosaccharides were recently reviewed being the mathematical model used to calculate the binding constants only suitable for simple molecules rather than polysaccharides4.In this work the complexing ability of a galactomannan (mannose to galactose ratio of 2.6:1) was investigated with respect to the metal ions Co2+, Mn2+, Ni2+ and Zn2+. Also, investigated was the complexed species with the metal ion Cu2+ 5. These metal ions have roles in the environment, in the chemistry of soils and in everyday human life6-11.The knowledge of the binding constants and their speciation as functions of pH values12 can contribute to the development of applications, such as the use of galactomannan in the elimination of metal ions during the flocculation step of contaminated water treatment, and especially in those cases where those solid complexes can be used as slow-release fertilizers 8-10.Potentiometric titration was used to evaluate the binding constants of the complexes in aqueous solutions and electron paramagnetic resonance spectroscopy (EPR) and thermal analysis by thermogravimetry - differential scanning calorimetry (TG-DSC), were used to investigate some structural aspects of the complexed species between the biopolymer and the metal ions in the solid state.  ExperimentalMaterialsAll chemicals used were of analytical-reagent grade and were used as received. Freshly boiled distilled de-ionised water and grade A glassware were used in preparation of all solutions. The galactomannan used was extracted from seeds of Leucaena leucocephala (Leu), as described elsewhere5. The identity and proportion of monosaccharides were determined by analysis of their alditol acetate derivatives13,14.The final solution of the polysaccharide used in the potentiometric titrations was 1g L-1. The molecular weight of either one of the two galactomannan monomers was used to provide the number of mols of the solution. Any monomeric sugar portion of the biopolymer is referred as the ligand (L) throughout this work.The metal ion (referred throughout this work as M) aqueous solutions were made from the appropriate mass of nitrate salts for Co(II), Ni(II) and Zn(II) (Carlo Erba - Brazil) and from a TitrisolTM solution (Merck - Brazil) for Mn(II). All the three nitrate solutions were standardized using methodology from the literature 15.A carbonate free solution of 0.1 mol L-1 KOH was prepared from pellets (Merck - Brazil) and standardized by titration with potassium acid phthalate (Carlo Erba - Brazil). KNO3 (Merck - Germany) was used as supporting electrolyte to maintain the ionic strength (m) at 0.100 mol L-1.MethodsThe alditol acetates derived from the biopolymers studied were analyzed by GLC-MS with a model 3300 Varian equipped with an OV-225 capilary column (0.25mm id x 30m) linked to a Finnigan Trap model 419 mass spectrometer unit at 70 e.V. Injections were carried out at 50oC and the column was then heated (4.0oC min-1) to 220oC.All potentiometric titrations were carried out using an Orion (USA) model 420-A research grade pH meter with an Orion (Switzerland) model 91-61 glass electrode and a double junction Ag/AgCl reference electrode Orion (USA) model 90-02, stored in distilled water for short period of times, and in its filling solution (10% KNO3 - Merck - Germany) for longer period of times. The standard procedure to standardize the pH meter followed strictly the procedures described in literature 16, where the slope was set by several trial titrations of standard HCl (Merck - Brazil) 5x10-3 mol L-1 [m = 0.100 mol L-1 (KNO3)] and KOH 0.1 mol L-1 up to the third pH decimal digit until the experimental values fitted the calculated ones by < 0.005 pH units in buffer and at low pH values, and < 0.015 pH units at high pH values 16. The pH studied range was from 2.000 to 11.000. The standardization at low pH was made with a standard HCl solution, around 5 x 10-3 mol L-1 [m = 0.100mol L-1 (KNO3)] whenever a new experiment was to be performed.All titrations were made in triplicate under a stream of purified N2 (White-Martins, Brazil) using three aqueous solutions, the first one of pyrogallol (Merck-Germany) in KOH, the second of KOH 1 mol L-1, and the third of KOH 0.1 mol L-1. The temperature was maintained at 25.0 ± 0.1oC (MQBTC 99-20, Microquímica - Brazil).A Sigma Techware Digitrate manual piston buret was used to deliver the 0.1 mol L-1, 0.02 ± 0.01 mL KOH CO2 - free solution.The solid complexes of the biopolymers and the metal ions were obtained as described earlier 5 and were submitted to the analytical techniques described below.The electronic paramagnetic resonance (EPR) first derivative spectra of solid native and complexed polysaccharides of powdered samples were recorded using quartz tubes at room controlled temperature of 25oC in a Bruker EPR ESP 300 E spectrometer, 9.7 Ghz, 100KHz field modulation, Germany.The simultaneous Thermogravimetry - Differential Scanning Calorimetry (TG - DSC) analyses were recorded in a Netzsch simultaneous Thermal Analyzer STA 409 EP, under air, from 21 to 520oC, 2oC min-1, using opened cylindrical aluminum opened sample pans, 4mm diameter, 2mm high.ComputationsThe mathematical model that best described the results for the formation of the equilibrium complexes was the one where one hydroxyl group of each complexing sugar unit was depleted of its proton, generating a basic site. The protonation constant for the biopolymer was taken from the literature for the monomer galactose (using UV-Vis spectroscopy20), as in the following equations. The hydrolysis constants for the metal ions reported by Baes and Mesmer21 were fully used in the calculations. The dissociation constant of water (pKw) at 25.0oC and m = 0.100 mol L-1 used was 13.7816 . All these constants were kept fixed during refinement of the binding constants of the metal ions and the biopolymer with the aid of the Best7 program 16. This mathematical model was adjusted in Best7 in order to represent the formation of the complexed species as in the equations below: where M= metal ions Co2+, Mn2+, Ni2+ and Zn2+ and L= monomer sugar unit of galactomannan, either mannose or galactose.The species distribution was calculated with the program SPE16 that uses as input data the output data of the Best7 program. The species considered in the equilibria were those which are most likely to be formed and also for which the other analytical techniques employed in this work showed some consistency. These species were ML, ML2, M2L, M2L2, M2L4, ML3 and their protonated counterparts.All other mathematical aspects of the microcomputer programs employed are described elsewhere 5,16,22 .  Results and DiscussionThe potentiometric equilibrium profiles of 0.4 mmole of galactomannan from L. leucocephala (Leu) in the absence of Mn2+ (10 points) and in the presence of 0.4 mmole (15 points) and 0.2 mmole of Mn2+ (11 points) are depicted in Figure 1. The curve of the galactomannan alone starts at pH 4.5, followed by a small break until pH 6.0 and continues until precipitation near pH 9.0. In the alditol acetate assay the presence of galacturonic acid was detected in this galactomannan and it is this acid that imparts the shape of the buffer around pH values of 8.0 and causes the early precipitation in the system. The curves with Mn2+ show a displacement in the x axis ending in pH values near 7.0 due to formation of insoluble products. As a result of the ability of Mn2+ to form external sphere complexes, mainly with water, the initial pH of those titrations done in the presence of Mn2+ started at slightly higher values than was the

[学科分类] 化学（综合）

[关键词] potentiometric titrations;EPR spectroscopy;TG-DSC;metal complexes