Elsevier

Carbohydrate Polymers

Volume 75, Issue 1, 5 January 2009, Pages 63-70
Carbohydrate Polymers

Metal complexes of crosslinked chitosans: Correlations between metal ion complexation values and thermal properties

https://doi.org/10.1016/j.carbpol.2008.06.011Get rights and content

Abstract

A series of heavy metal complexes of crosslinked chitosans were evaluated by thermogravimetric studies. The metal complexes with Cu, Cd and Hg ions exhibiting the highest complexing ability to chitosans (Hg 354–364, Cu 100–112, and Cd 121–160, in mg/g chitosan), had the lowest onset of degradation temperatures (range 194–210 °C) and the lowest final degradation temperatures (generally less than 294–304 °C for Hg, 296–338 °C for Cu, and 305–368 °C for Cd complexes). Mn ion, with the lowest binding to chitosans (Mn 5–7 mg/g), showed the reverse behavior, having onset (240–248 °C) and final degradation temperatures (range 300–368 °C). Zn (binding 74–87 mg/g) and Pb (binding 39–62 mg/g) ions have a binding ability intermediate to Cu/Cd/Hg and Mn extremes, and therefore the effects on onset and final degradation temperatures are intermediate to these values.

Introduction

A key property of functional polymers is their ability to complex with a variety of metal ions in solution. A very large number of publications have dwelt on the complexation ability of chitosan and its crosslinked derivatives with complex transition metals, organic species like dyes, and enzymes (Bassi et al., 2000, Dobetti and Delben, 1992, Domard and Piron, 2000, Juang et al., 2002, Li et al., 2003, Merrifield et al., 2004, Rhazi et al., 2002, Schmuhl et al., 2001, Taboada et al., 2003, Tan et al., 1999, Trimukhe and Varma, 2008a, Varma et al., 2004). Recently we reported (Trimukhe & Varma, 2008a) our detailed investigation into the heavy metal ion binding (Hg, Cu, Cd, Pb, Zn, Mn) to a series of crosslinked chitosans, using trimelitic anhydride, diisocyanatohexane, and dibromodecane as crosslinking agents. This is the first study on crosslinked chitosan materials, wherein the chitin was first crosslinked and then deacetylated to give crosslinked chitosans retaining all the amino groups, which are crucial functional groups for specific heavy metal ion complexation. We also investigated the morphologies of these metal complexes by a study of SEM and WAXRD (Trimukhe & Varma, 2008b). Morphological information from SEM studies indicate that metal ions like Hg which are very strongly bound to the chitosan and crosslinked chitosans, even to the extent of 364 mg/g chitosan, are not seen as distinct moieties on the surface of the polymer, whereas with decreasing extent of binding, in the order Hg > Cu > Cd > Zn > Pb > Mn, we observed increasing presence of metal on the surface. These results agreed with the morphological information obtained with WAXRD studies, as well as with the metal complexation data of these metals with these same chitosan polymer/crosslinked polymer systems. However, thermal properties of these metal complexes are not reported. Thermal stability data of metal complexes is important to confirm the presence of metal ions present either as inclusion complexes or as adsorbed species on the surface of the polymer. Metals ions present as inclusion complexes are expected to have a greater effect on the thermal properties of the polymer, in addition to specific effects of different metal ions. Therefore this paper investigates the thermal degradation of metal complexes of chitosan and crosslinked chitosan under nitrogen atmosphere in the temperature range 50–600 °C. This paper also attempts to correlate the thermal degradation behavior of metal complexes of chitosans/crosslinked chitosans with the binding abilities of metal ions with chitosans/crosslinked chitosans (expressed as binding mg (metal ion)/g (chitosan)) for a series of heavy metal complexes of chitosans. This appears to be the first such reported correlation between the thermal degradation of metal complexes of crosslinked chitosans and their saturated heavy metal ion complexation values.

Section snippets

Materials

The chitin and chitosan used in this study are commercial products of Meron Biopolymers, Cochin, Kerala, India. d-Glucosamine was obtained from Sigma Chemical Co. (St. Louis, MO). Diisocyanatohexane (HDI) was obtained from Sigma–Aldrich Chemical Co (Bangalore, India), trimellitic anhydride (TMA), and dibromodecane (DBD) was obtained from Merck (Mumbai, India). Dimethylformamide, toluene and sodium hydroxide pellets were AR grade chemicals, obtained from SD fine chemicals, Mumbai. Sodium hydride

Results and discussion

There are several reports in literature of changes occurring in the thermal degradation behavior of chitosans on crosslinking or on their chemical modification. For example, in one study (Neto et al., 2005) addition of PEO grafts led to a slight increase in thermal stability (from 297.3 to 300.7 °C), whereas crosslinking the chitosan decreased the thermal stability to a small extent (from 297.3 to 288.8 °C). In another report, crosslinking was seen to improve the heat stability of the chitosan (

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