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A kinetic study of the thermal degradation of chitosan-metal complexes

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The thermal degradation of metal complexes formed by chitosan with Cu(II), Ni(II), Co(II), and Hg(II), at different metal concentrations, was studied by thermogravimetric analysis in a nitrogen atmosphere over the temperature range 25–800°C. The
  A Kinetic Study of the Thermal Degradation ofChitosan-Metal Complexes Edelio Taboada, 1 Gustavo Cabrera, 2 Romel Jimenez, 3 Galo Cardenas 4 1 Universidad Cato´ lica de Temuco, Facultad de Ingenierı´ a, Escuela de Ingenierı´ a Ambiental, Campus Norte, Av. Rudecindo, Ortega 02950, Casilla 15-D, Temuco, Chile 2 Universidad Adolfo Iba´ n˜ ez, Escuela de Negocios, VentureL@b, Diagonal Las Torres 2700, Santiago de Chile, Chile 3 Departamento de Ingenierı´ a Quı´ mica, Universidad de Concepcio´ n, Facultad de Ingenierı´ a, Concepcio´ n, Chile 4 Departamento de Polı´ meros, Universidad de Concepcio´ n, Facultad de Ciencias Quı´ micas, Concepcio´ n, Chile Received 11 September 2008; accepted 4 May 2009DOI 10.1002/app.30796Published online 30 June 2009 in Wiley InterScience ( ABSTRACT:  The thermal degradation of metal complexesformed by chitosan with Cu(II), Ni(II), Co(II), and Hg(II), atdifferent metal concentrations, was studied by thermogravi-metric analysis in a nitrogen atmosphere over the tempera-ture range 25–800  C. The results indicate that thermaldegradation of chitosan and chitosan-metal ion complexescould be of one or two-stage reaction. In the thermal degra-dation of chitosan with metal complexes, the temperature of initial weight loss and the temperature of maximum weightloss rate decrease. Fourier transform infrared spectroscopywas used to probe the interaction of chitosan with metalions. The bands of   A N A H,  A C ¼¼ O,  A C A O A C A  groups of chitosan are shifted or change their intensity in the presenceof metal. These changes in the characteristic bands are takenas evidence of the influence of metal ions on the thermalstability of chitosan. Broido’s method was employed toevaluate the activation energies as a function of the degreeof degradation. The presence of metal ions provoked adecrease in the thermal stability of chitosan, which becamemore marked when the concentration of metal wasincreased. The dynamic study showed that the apparentactivation energy values of the main stage of the thermaldegradation of chitosan-metal complexes decrease as thestrength of the polymer-metal interaction increases.  V V C  2009Wiley Periodicals, Inc. J Appl Polym Sci 114: 2043–2052, 2009 Key words:  chitosan; metal complex; thermal degradation;activation energy; Broido’s method INTRODUCTION Chitosan, poly- b (1-4)-2-amino-2-deoxy- D -glucopyra-nose is a biopolymer obtained by total or partialN-deacetylation of chitin, poly- b (1-4)-2-acetamide-2-deoxy- D -glucopyranose, the main constituent of crustacean shells. Partial deacetylation producesa heteropolymer consisting of glucosamine and N  -acetylglucosamine sugar residues.Chitosan is characterized by a strong affinity fortransition metals. This property makes it attractivefor use in water treatment. 1–3 Furthermore, whenchitosan is employed as a metal retention resin, itforms complexes with dangerous heavy metals fromsolution. 4,5 In addition, chitosan-metal complexescan be used in heterogeneous catalysis for applica-tions in the fields of hydrogenation, oxidation andfine chemical synthesis reactions. 6 All these reactionsare favored by the increase of the temperature. Forthis reason the thermal behavior of the polymer-metal complexes submitted to these conditions must be known to prevent side reactions and/or polymerdegradation.Once the chitosan has fulfilled such purposes, itremains as a residue which must be treated as solidwaste. One treatment option is to dispose of thesolid waste in a sanitary backfill. This option is notadvisable because the high concentration of heavymetals can affect the operation of the backfill andcontaminate the groundwater. The thermal degrada-tion of these solid wastes seems to be a better dis-posal solution because the process produces energyand the metallic residues can be confined or reused.Thermogravimetric analysis is a very widespreadtechnique. For instance, it has been extensivelyemployed to study chitosan degradation and stabil-ity. 3,7–12 Nevertheless, a survey of the literatureshows that thermal studies of chitosan-metal com-plexes are rare. 13 We used this technique to observethe changes that are produced in different chitosan-metal complexes during the pyrolysis process, in anitrogen atmosphere, with different concentrationsof metallic ions. To determine the kinetic parametersof the degradation processes, the method developed by Broido 14 was utilized.  JournalofAppliedPolymerScience,Vol.114,2043–2052(2009) V V C  2009 Wiley Periodicals, Inc. Correspondence to:  E. Taboada ( grant sponsor: DGIUCT 2006-2-04.  The thermal degradation of the chitosan-metal com-plexes is a complex phenomenon. The metal ions cata-lyze the degradation and the process is affected by thekind and concentration of metal ions, the structure of complex, the strength of the polymer-metal interac-tion, etc.The aim of this study was to contribute to theknowledge of the pyrolysis of chitosan-metal com-plexes. A better understanding of this process couldhelp to encourage the industrial application of thesecomplexes and their final processing as waste. MATERIALS AND METHODSMaterials Chitosan was obtained in our laboratories with adegree of deacetylation of 96.5% and a molecularweight of 131,000 g/mol. The methodology used isan adaptation of that presented in a previous work. 4 The modifications consist in the reduction of the pe-riod of deacetylation to 90 min with the addition of 10% NaBH 4  to reduce oxidation. Chitosan was usedin the form of a powder.All chemicals used were pure grade: NaOH andHgCl from Merck (Merck S.A., Santiago de Chile,Chile); CuNO 3  3H 2 O, NiNO 3  6H 2 O and CoNO 3  H 2 Ofrom Scharlau Chemie (Equilab Ltda., Santiago deChile, Chile). Analysis of the degree of acetylation (DA) The degree of acetylation was determined by the useof RMN  1 H in the liquid state according to themethod described by Brugnerotto et al. 15,16 The RMN  1 H spectra were performed in the AC300 Bruker Spectrometer (Bruker Corporation, Ger-many) with a controller processor, coupled to an AS-PECT 3000 computer and a variable temperaturesystem. Sodium 4.4-dimethyl-4-sylapentane sulpho-nate (SDS) was used as external reference. Determination of the molecular weight The molecular weight of the chitosan was deter-mined by the viscometric technique. Using theMark-Houwink equation and [ g ] values obtained instatic experiment it was possible to determine theaverage viscometric molecular weight (  M V  ). ½ g  ¼ K     M v   a The Mark-Houwink parameters  K   ¼  0.076 (g/mL)and  a  ¼  0.76 used were determined by Brugnerottoet al. 15 for the chitosan dissolved in the solvent sys-tem CH 3 COOH 0.3  M /CH 3 COONa 0.2  M  at 25  C.The intrinsic viscosity [ g ] of chitosan samples wasmeasured with an Ostwald viscometer at 25  C   0.05  C using chitosan concentrations ranging from0.5    10  3 g/mL to 2    10  3 g/mL. Preparation of chitosan-metal complexes The chitosan-metal complexes were prepared fromsolutions of copper (pH  ¼  5.0), nickel (pH  ¼  5.5–6.5),cobalt (pH  ¼  4.0–5.0) and mercury (pH  ¼  3.0–4.0) ionswith concentrations of 50, 600, and 2000 mg/L, with-out adjustment of the pH. The complexes were syn-thesized by mixing 25 mL of solutions containing ionswith 25 mg of polymer in solid state. Contact wasmaintained for 24 h at 25  C in an orbital shaker withtemperature control. The solid polymer-metal com-plexes were filtered and the metal concentrations inthe filtered solutions were determined. The solid wasdried in a vacuum oven at 60  C for 24 h. Determination of metal concentration Elemental analyses were performed in the ChemicalAnalysis Laboratory of the Chemical Science Facultyat the University of Concepcio´n. The metal concen-trations were determined from solutions in a UnicanM Series Atomic Absorption equipment (ThermoFisher Scientific) with hollow cathode lamp. Thermogravimetric analysis Thermogravimetric experiments were carried out ina Cahn VersaTherm equipment (Thermo FisherScientific) with a temperature control microprocessorand a data station for thermal analysis. The mass of the samples fluctuated between 3 and 5 mg.The pyrolysis was carried out over a temperaturerange from 25 to 820  C with a heating rate of 10  C/min under nitrogen flow. The mass of the samplewas registered continuously as a function of temperature. Fourier transform infrared (FTIR) analysis Fourier transform infrared (FTIR) spectra weremeasured using FTIR Nicolet Magna 5PC spectro-photometer (Thermo Fisher Scientific) coupled to aPC with software (‘‘OMNIC’’) to data analysis. TheKBr disks were prepared by thorough blending of the KBr with dried polymer at 2% concentration.Spectra were recorded at a resolution of 2 cm  1 andwith an accumulation of 128 scans. THEORETICAL APPROACH The kinetic parameters of the thermal decompositionreaction can be evaluated by dynamic and isother-mal experiments. In the former case, the sample isheated from room temperature to complete decom-position at a linearly programmed rate, while in the 2044 TABOADA ET AL.  Journal of Applied Polymer Science  DOI 10.1002/app  latter case several isothermal experiments are carriedout for different periods of time at a temperatureclose to the degradation temperature.Various methods exist for studying the thermaldecomposition of polymers. In this work Broido’smethod was utilized. 14 The development of this method begins by sup-posing that a pure solid substance, when it is heatedin a vacuum, suffers pyrolysis by means of a reac-tion in which some of the products are volatile.The progress of the reaction can be determined bycontinuously weighing the samples. The weight  W  t at any time  t , is related to the fraction of the initialnumber of molecules which have not yet decom-posed,  y , by means of the equation:  y ¼  N N  0 ¼ ð W  t  W  1 Þð W  0  W  1 Þ  (1)If the pyrolysis is carried out isothermically, thevelocity of the reaction is given by: dydt  ¼ ky n (2)where  n  is the reaction order. The velocity constantk changes with absolute temperature according tothe equation of Arrhenius. k ¼  A  e  E = RT  (3)If the temperature is a linear function of   t , then T   ¼ T  0 þ u  t  (4) u  is the heating rate. The first derivative of thisequation is: dT   ¼ u  dt  (5)This rearranges to: dy y n  ¼  Au    e  ERT  dT   (6)The thermogravimetric analysis curve for thisreaction represents the last equation integrated froma temperature  T  0 , where  y  ¼  1, to a temperature foranother value of   y . Z  1  y dy y n  ¼  Au Z  T  0 T  e  ERT  dT   ¼  Au Z  T T  0 e  ERT  dT   (7)The main consideration of this method is that thereaction is of the first order. With this suppositionthe left side of the reaction can be resolved. Z  1  y dy y n  ¼ Z  1  y dy y  ¼ ln  y ¼ ln 1  y    (8)There are various methods for resolving the rightside of the equation. Broido’s method is based onapproximations done by other authors. We have uti-lized the approximations introduced by Horowitzand Metsger. 16 e  ERT    T  m T    2 e   ERT m  (9) T  m  is the temperature at which the maximumreaction velocity occurs. Introducing this approxima-tion has Ln 1  y ¼  Au Z   T  m T    2 e  ERT  dT   (10)Changing variable has x ¼  1 T   )  dx ¼  1 T  2  dT   )  dT   ¼  1 x 2   dx ln1  y ¼  Au T  2 m Z   x 2 e  ER X    1 x 2   dx  (11)ln1  y ¼  A  R  T  2 m E  u   e  ER X  ln1  y ¼  A  R  T  2 m E  u   e  ER 1 T  ð Þ ln ln1  y ¼ ER 1 T    þ ln  A  R  T  2 m E  u    (12)This equation can be represented by:ln ln1  y ¼ ER 1 T    þ const : Equation 12 is a straight line. The gradient of thegraph lnln  1  y  vs :  1 T     is the activation energy  E a  andthe intercept is frequency factor  A . RESULTS AND DISCUSSIONFTIR analysis of chitosan-metal complexes Spectroscopy in the infrared region was used tomonitor structural changes and the principal interac-tions between chitosan and the metal ions beforethermal degradation. The spectra of chitosan andchitosan with Cu(II), Hg(II), Co(II) and Ni(II) areshown in Figure 1. The characteristic FTIR bands of the samples are listed in Table I.FTIR analysis of chitosanIn the region of 3000–3700 cm  1 of the spectrum,chitosan exhibits a strong, broad band due to the THERMAL DEGRADATION OF CHITOSAN-METAL COMPLEXES 2045  Journal of Applied Polymer Science  DOI 10.1002/app  axial stretching of OH and N A H bonds centredat 3425 cm  1 . This band is broad because of thehydrogen bonds. The OH band overlaps the N A Hstretching band. The bands observed at 2915 and2865 cm  1 correspond to the axial stretching of C A H bonds.The amide I band (C ¼¼ O) characteristic of chitosanwith acetylated units appears at 1653 cm  1 . The band at 1590 cm  1 is produced by the overlappingof in-plane bending (scissoring) of N A H of aminechitosan groups and the amide II band.The bands at 1420 cm  1 result from C A N axialstretching and the bands corresponding to the poly-saccharide skeleton, including the vibrations of theglycosidic bonds (C A O and C A O A C stretching)were observed in the range 1153–897 cm  1 .FTIR analysis of chitosan-Cu(II) complexesThe main bands observed in the chitosan spectrumare also present in the chitosan Cu(II) spectrum. Theprincipal changes produced by the interaction between chitosan and Cu(II) is the overlapping of the amide I band and the band due to angular defor-mation of   A N A H bonds observed at 1630 cm  1 . Thisoverlapping is due to the interaction of Cu(II) withamine and C ¼¼ O groups. These changes are the prin-cipal evidence of the influence of the Cu(II) on thethermal stability of chitosan. The strong band at1380 cm  1 correspond to the symmetrical stretchingof the N ¼¼ O group from the salt CuNO 3 .FTIR analysis of chitosan-Ni(II) complexesWhen chitosan interacts with Ni(II) ions the band becomes unfolded in the region of 3000 to3700 cm  1 , and the stretching of O A H is observed at3440 cm  1 . Chitosan has N A H amide bonds andN A H amine bonds. The asymmetrical and symmetri-cal N A H stretching vibrations band becomes unfoldedinthepresenceofNi(II)at3270and3108cm  1 .When chitosan is charged with Ni(II) ions the amideI band appears at a high wave number (1660 cm  1 ).Electron-attracting groups attached to the nitrogenincrease the frequency of absorption since they effec-tively compete with the carbonyl oxygen for theelectrons of the nitrogen, thus increasing the forceconstant of the C ¼¼ O bond. Another band is unfoldedat 1633 cm  1 corresponding to the N A H bonds.FTIR analysis of chitosan-Co(II) complexesThe main bands observed in the chitosan spectrumare also present in the chitosan Co(II) spectrum.However, due to the interaction of Co(II) with thenitrogen, when chitosan is charged with Co(II) ionsthe amide I band (C ¼¼ O) and the band correspond-ing to the N A H bonds appears at high wave num- bers (1660 and 1605 cm  1, respectively). Changes arealso observed in the intensities of the bands TABLE ICharacteristic FTIR Bands of Chitosan and Chitosan-Metal Complexes Sample FTIR (Wave number, cm  1 )Chitosan 3440 cm  1 ( A OH,  A NH); 2915 cm  1 and 2865 cm  1 ( A C A H); 1653 cm  1 ( A C ¼¼ O, amide I); 1590 cm  1 ( A N A H); 1420 cm  1 ( A C A N A ) coupled with 1380 cm  1 ( A N A H); 1155-875 cm  1 (skeleton: C A O and  A C A O A C A )Ch-Cu 3540 cm  1 ( A OH), shoulder 3132 ( A NH); 2915 cm  1 and 2875 cm  1 ( A C A H); 1630 cm  1 ( A C ¼¼ O and  A N A H overlaps); 1420 cm  1 ( A C A N A ) coupled with 1380 cm  1 ( A N A H); 1155-875 cm  1 (skeleton: C A O and  A C A O A C A )Ch-Co 3440 cm  1 ( A OH,  A NH); 2915 cm  1 and 2875 cm  1 ( A C A H); 1660 cm  1 ( A C ¼¼ O, amide I); 1605 cm  1 ( A N A H); 1420 cm  1 ( A C A N A ) coupled with 1380 cm  1 ( A N A H); 1155-875 cm  1 (skeleton: C A O and  A C A O A C A )Ch-Ni 3440 cm  1 ( A OH); 3270 cm  1 and 3108 cm  1 ( A NH); 2915 cm  1 and 2875 cm  1 ( A C A H); 1660 cm  1 ( A C ¼¼ O, amide I); 1630 cm  1 ( A N A H, amide II); 1420 cm  1 ( A C A N A ) coupledwith 1380 cm  1 ( A N A H); 1155-875 cm  1 (skeleton: C A O and  A C A O A C A )Ch-Hg 3430 cm  1 ( A OH); 3250 cm  1 and 3132 cm  1 ( A NH); 2915 cm  1 and 2875 cm  1 ( A C A H); 1630 cm  1 ( A C ¼¼ O, amide I); 1590 cm  1 ( A N A H); 1420 cm  1 ( A C A N A ) coupled with 1380 cm  1 ( A N A H); 1155-875 cm  1 (skeleton: C A O and  A C A O A C A ) Figure 1  FTIR spectra of chitosan and chitosan metalcomplexes.2046 TABOADA ET AL.  Journal of Applied Polymer Science  DOI 10.1002/app  corresponding to C A N axial stretching, N A H angu-lar deformation and the bands corresponding to thepolysaccharide skeleton. These changes are taken asevidence of the influence of the cobalt on the ther-mal stability of chitosan. The band at 1380 cm  1 cor-respond to the symmetrical stretching of the N ¼¼ Ogroup from the salt CoNO 3 .FTIR analysis of chitosan-Hg(II) complexesWhen chitosan interacts with Hg(II) ions the bands,in the region of 3000–3700 cm  1 , become unfoldedand the stretching of O A H is observed at 3430 cm  1 .The asymmetrical and symmetrical N A H stretchingvibrations band becomes unfolded at 3250 and 3132cm  1 . Another significant change is produce by theinteraction of Hg with C ¼¼ O groups. The amide I band corresponding to the C ¼¼ O bond is notobserved and another band appears at 1630 cm  1 due to angular deformation of   A N A H. Thermogravimetric analysis of chitosan—Cu(II)complexes The TG and the corresponding DTG curves of chito-san Cu(II) complexes are shown in Figures 2 and 3respectively. The temperatures for the occurrence of the main thermal events and corresponding masslosses are given in Table II. In Figures 2 and 3 it isobserved that the first thermal event occurs in thetemperature range 25–140  C, where all samples pres-ent a mass loss of between 4 and 7%. This is attrib-uted to the evaporation of water, the content of which is a function of the morphology, crystallinityand hydrophilicity of the polymers. 11,17,18 Accordingto the results presented in Figures 2 and 3, the masslosses corresponding to the evaporation of waterappear to depend on the presence and number of ions in the polymer chains.The second thermal event occurs in the tempera-ture range 190–410  C for chitosan. In the presence of Cu(II) ions a displacement toward lower degrada-tion temperatures is observed when the metallic ion Figure 2  Thermogravimetric analysis de chitosan Cu(II)complexes. Figure 3  DTG curves of the chitosan Cu(II) complexes. TABLE IICharacteristic Temperatures and Weight Loss (%) for theThermal Degradation of Chitosan and Chitosan-MetalComplexes Temperature(  C) a Peakmax RangeWeightloss (%)First stageChitosan – 25–140 6.6chit-Cu (0.14 meq/g) – 25–140 5.0chit-Cu (0.74 meq/g) – 25–140 7.0chit-Cu (0.97 meq/g) – 25–140 6.2chit-Ni (0.05 meq/g) – 25–140 7.2chit-Ni (0.34 meq/g) – 25–140 6.3chit-Ni (0.67 meq/g) – 25–140 6.3chit-Co (0.02 meq/g) – 25–140 4.0chit-Co (0.16 meq/g) – 25–140 4.0chit-Co (0.36 meq/g) – 25–140 6.3a-chit-Hg ( < 0.05 meq/g) – 25–140 3.0 b-chit-Hg ( < 0.05 meq/g) – 25–140 5.2chit-Hg (1.17 meq/g) – 25–140 5.2Second stageChitosan 295 190–410 51chit-Cu (0.14 meq/g) 277 180–360 45chit-Cu (0.74 meq/g) 225 150–400 47chit-Cu (0.97 meq/g) 208 130–400 50chit-Ni (0.05 meq/g) 285 200–360 47chit-Ni (0.34 meq/g) 260 180–380 46chit-Ni (0.67 meq/g) 257 180–380 45chit-Co (0.02 meq/g) 284 200–360 48chit-Co (0.16 meq/g) 269 180–400 48chit-Co (0.36 meq/g) 260 180–400 46a-chit-Hg ( < 0.05 meq/g) 275 180–360 47 b-chit-Hg ( < 0.05 meq/g) 240 140–320 49chit-Hg (1.17 meq/g) 235 140–355 54 a This temperature was obtained from the differentialthermogravimetric analysis.THERMAL DEGRADATION OF CHITOSAN-METAL COMPLEXES 2047  Journal of Applied Polymer Science  DOI 10.1002/app
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