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Technical Article / Peer-reviewed ArticleO PAPEL vol. 73, num. 4, pp. 59 - 64 APR 2012For the quantification of HexA, the UV-Visible spectrophotometerUnicam 5625 with Aurora scan software and the HUT procedurewere used. UV-Vis reflectance spectra were recorded using a PerkinElmer Lambda 900 spectrometer. Then, the Kubelka-Munk equationwas used and the absorbance spectra were obtained. The changesin hexenuronic acid content can be seen at 240 nm and in lignincontent at 285 nm.The UV resonance Raman spectra were collected with a Renishaw(Cloucestershire, UK) 1000 UV Raman spectrometer equipped witha Leica (Wetzlar, Germany) DMLM microscope and an Innova 300CFreD frequency-doubled Ar + ion laser (Coherent Inc., Santa Clara, CA,USA). The excitation wavelength applied was 244 nm. The outputpower of the laser was adjusted to 10 mW and the spectral collectiontime was 20 seconds per sample. Spectra were processed usingGrams/32 AI (6.00) software. The spectrum was scaled according to1095 cm -1 cellulose band.RESULTS AND DISCUSSIONThe kappa number of kraft pulps correlates with the total contentof lignin and hexenuronic acid groups. Then, the kappa number afterthe selective hydrolysis of hexenuronic acid groups gives a bettermeasure of lignin content. The time dependent change in the kappanumber was modeled by a first order rate equation (Vuorinen et al.,1999)(Vuorinen et al., 1996).Where k 0is the initial kappa number (at t=0), ∆k maxis the maximumchange in kappa number and k 1is the rate constant.Kappa number exponentially decreased with the longer hydrolysistime at all the temperatures as it could be expected (Figure 1).It can be noted that when a higher temperature was used,(1)the ∆k maxwas increased; nevertheless this parameter was nearlythe same at the highest temperatures used. This behavior canbe explained because at 80ºC and 100ºC there are degradationreactions that could not take place due to their high activationenergy. Those reactions are faster at 140ºC than at 120ºC, but ifthe time is enough, at both temperatures the kappa number thatcan achieve has almost the same value.The data fitted well to the first order kinetic although theexperimental data were few to validate the model. The validity ofthe first order kinetic model for the kappa number reduction hadbeen proved in previous works (Vuorinen et al., 1999) (Vuorinenet al., 1997). In Table 1 the parameters of the model for eachtemperature are shown.Table 1. First order kinetic model parameters for kappa number reductionTemp. (ºC) 80 100 120 140∆k max5.12 6.89 8.59 8.65k 1(min -1 ) 0.0075 0.0237 0.0396 0.101Assuming the hydrolysis follows a first order kinetics, theconcentration of HexA was plotted against the reaction time andan exponential model was fitted, according to equation (2), as canbe seen in Figure 2 (Vuorinen et al., 1996) (Tenkanen et al., 1999).∆HexA∞: is the maximum 2-Furoic acid concentration that can betheoretically achieved.As it could be expected, the hexenuronic acids removal increasedwith temperature and reaction time. At 80ºC and 100ºC and for the(2)Figure 1. Kappa number reduction versus reaction time at different temperatures60 Revista O Papel - abril/April 2012
Technical Article / Peer-reviewed ArticleO PAPEL vol. 73, num. 4, pp. 59 - 64 APR 2012Figure 2. HexA content of filtrates versus reaction time at different temperaturestimes used in this work, the HexA removal was not complete. InTable 2 the parameters of the model for the concentration of HexAversus time are presented.As it can be seen, the maximum amount of HexA that can beremoved (DHexA∞) increased with temperature. Despite of that,the DHexA∞ obtained at 120ºC and 140ºC were very similar,which means that almost all the hexenuronic acids present inthe pulp were removed. Therefore, it is better to work at 120ºCTable 2. Model parameters of HexA content versus timeTemp. (ºC) 80 100 120 140DHexA∞ (meq/kg) 36,76 52,70 68,87 70,32rather than 140ºC, due to the pulp is less degraded and the HexAremoval is almost the same.Combining the equation models (1) and (2) for kappa numberreduction and hexenuronic acid content, the equation (3) was obtained.The correlation between kappa number and 2-furoic acid contentof the filtrates was significant (R 2 =0.9969) for the dataset fromall the experiments at various temperatures, as it can be seen inFigure 3. Additionally, the reduction of one kappa number unitcorresponded to a reduction in 9.2 meq/kg pulpof hexenuronic acid.(3)Figure 3. 2-furoic acid content of the filtrates versus kappa numberabril/April 2012 - Revista O Papel61
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