Adsorption of Volatile Organic Compounds

Adsorption of Volatile Organic CompoundsADSORPTION OF VOLATILE ORGANIC COMPOUNDS ON DIFFERENT TIMBER SPECIESMarco Vasconcelos1, Nereida Cordeiro1,2* raiseDue to the health issues associated to VOC, this work aims to study the adsorption of these compounds onto timbers, in order to improve air quality. For this task, inverse gas chromatography (IGC) was utilise to creditize move up properties (surface efficiency, particular proposition unleash energy and heat content of adsorption). Dispersive component of the surface energy (SD) ranged from 39.95 to 43.81 mJ/m2, correspondent to Amburana and Courbaril respectively. yellow(a) ache shows higher values of the specific go off energy of adsorption (Gs), indicating a higher number/energy of polar participating sites. On the other hand, the enthalpy of adsorption values (Hs) dont go in agreement with Gs, probably due to its temperature-dependence. That said, the Hs values indicate a basic surface and Amburana establishes stronge r interactions with polar analyses while yellowness Pine establishes stronger interactions with cyclohexane. Additionally, the thermodynamic compensation effect was evaluated on these samples, being notice linearity in Yellow Pine, Grapia and Brazilian sepia. The slope was similar surrounded by the antithetical species, indicating a similar adsorption mechanism.Keywords Inverse Gas Chromatography, Surface Properties, Wood Fibers, Volatile Organic Compounds, thermodynamic compensation effect1. INTRODUCTIONVolatile ingrained compounds (VOC) are organic compounds produced in industries, motor vehicles, power plants, etc. They are usually distinguished by boiling temperatures under 250C, at standard atmospheric pressure (1 atm), and with high vapor pressures, surpassing 0.5 kPa at 25C (Dobre, Prvulescu, Iavorschi, Stroescu, Stoica, 2014). The aforementioned compounds afford harmful impact on human health, such(preno momental) as eye and throat irritation, damage to the liver, central nervous system and carcinogenic effects, due to prolonged exposure (Das, Gaur, Verma, 2004). Therefore, reducing the VOC concentration on the aura is an important task in order to improve air quality and avoid health issues.Wood fibers consist in both(prenominal) dead and live cells, being at a macroscopic level organized within a layer of xylene, in the wood. From the chemical point of view, consists mostly in cellulose, followed by hemicellulose and lignin. Due to the hydrogen bonds established by the hydroxyl groups of cellulose of the surface, it affects strongly on the properties of the material, such as hydrophobicity and therefore its reactivity (Hodzic, Shanks, 2014). close to 80.5% of the wood fibers are used in the paper industry and around 17% for wood fibers modification (composites Kazayawoko, Balatinecz, Romansky, 1997 Adusumali, Reifferscheid, Weber, Roeder, Sixta, Gindl, 2006 Cao, Hu, Liu, 2008).In this work, Inverse gas Chromatography (IGC) was used to c haracterize the surface properties (surface energy, specific free energy and enthalpy of adsorption) of Amburana, Yellow Pine, Grapia, Courbaril and Brazilian Mahogany to evaluate the adsorption of some VOC (Chloroform, cyclohexane, ethyl acetate, benzene and toluene). Additionally, the thermodynamic compensation effect was analyze to investigate the mechanism of thermodynamic adsorption on the different samples analyzed.2. MATERIALS AND METHODS2.1 MaterialsMethane (99.99% purity) was used an inert reference probe and the carrier gas utilized was helium (99.99% purity), both supplied by Air Liquide Company. The probe molecules were supplied by SigmaAldrich with GC grade (99% purity).2.2 iGC analysisiGC measurements were carried out, at least, in duplicate, on a commercial inverse gas chromatograph (Surface Measurements Systems, London, UK) fit out with a flame ionization (FID), thermal conductivity (TCD) and mass spectrometer (MS) detectors. Standard glass silanized (dymethyldichlo rosilane Repelcote BDH, UK) columns with 0.4 cm ID and 30 cm in length were used.About 1.5 g of timber sawdust mesh 40-60 was packed by vertical tapping for 10 min. The columns, with the samples, were pre-treated for 2h at 343 K and 10 ml/min of flow rate, to remove the impurities adsorbed on the surface and 2h at the measurement conditions. After pre-treatment, pulse injections were carried out with a 0.25 l gas loop. The iGC system was fully automated with purpose written authorization software (SMS iGC Controller v1.3) and data were analyzed using iGC Standard v1.3 and Advanced Analysis Software v1.21. The presented results are the average of the values obtained for each sample with a standard deviation minor than 5%.Measurements of the dispersive interaction were made with n-alkanes (n-decane, n-nonane, n-octane, n-heptane and n-hexane) at 298 K, at 0% RH. The carrier gas (Helium) flow rate was 10 ml/min. To acid-base studies cyclohexane, benzene, chloroform, toluene and ethyl acetate were used at 0% RH. Heat of sorption measurements were done with the polar probes at 298 K, 0% RH, and 10 ml/min flow rate.3. RESULTS AND DISCUSSION3.1 Dispersive component of the surface stressThe solventsorption properties of the timbers were studied at infinite dilution condition in order to remove the interactions between the probe molecules adsorbed at the surface of the samples. The methane injection allows the calculation of the dead time and subtracts it to the retention time in order to obtain the retention volume (Thielmann, 2004). The change by reversal net retention volume VN is given bywhere Vd is the mobile phase hold-up volume (called dead volume), and VR the careful retention volume. The 0 superscript indicates a correction for the column pressure drop given by jwhere where T is the column temperature, F is the exit flow rate at 1 atm and 273.15K, tR is the retention time and t0 is the mobile phase hold-up time.The dispersive component of the surface ener gy, as well as the specific free energy of sorption, was metrical with dispersive and acid-base probe molecules. The relation between the retention volume and free energy of sorption GS is given by the following equationG0S = RTln VR0 + Kwhere R is the gas constant and K is the De Boer or Kemball/Rideal constant depending on the chosen reference state (De Boer, 1953 Kemball Rideal, 1946). Moreover, G0S is related to the energy of adhesion WA (between probe molecule and solid) by the equationG0S = NAaWAwhere a is the cross sectional area of the adsorbate and NA the Avogadro constant. consort to Fowkes (1964), the dispersive contribution of the work of adhesion WA is given byWA = 2(SDLD)1/2with SD and LD as the surface tightness of the adsorbent and the adsorbate respectively. Combining the equations aboveThus, the dispersive component of the surface tension (SD) values could be compute based on the plots of RT lnV0R vs a(LD)1/2 for the adsorption of n-alkane probes, as illustrat ed in predict 1. Good linear correlations (0.9991-0.9996) were obtained for the n-alkane series for all samples analyzed. The calculated SD values are given in evade 1 and show very similar results between the different timber species. The surface energy ranged from 39.95 to 43.81 mJ/m2, corresponding to Amburana and Courbaril respectively. These values are similar to those reported by Kazayawoko, Balatinecz, Romansky (1997) and Shu, Xu, JinWei, BaoLi (2007) for wood flour (35.6 mJ/m2 at 323K) and ashtree wood meal (36.52 mJ/ m2 at 323K) respectively. Gouveia, Cordeiro and basin (2011) reported sundry(a) lignocellulosic fibres (flax, hemp, kenaf, agave, agave hybrid pineapple, and sisal), with SD values ranged from 37.17 to 51.37 mJ/ m2 (at 298K).In the same article aforementioned, its also explained that variations in the SD values can be due to the different composition, maturement conditions of the fibers and method of pedigree. The latter is justified due to the fibers bei ng usually covered by a layer of wax, and may interfere with elutant-fiber interactions. When removed during the method of extraction its expected to change drastically the surface energy.3.2 Polar probes sorption studiesA widespread approaching to the Lewis acid/base surface interactions can provide emend perceptive of the chemical-physical properties of the timber surfaces. The heading of acid and base active sites on the surfaces increases the possibility of specific intermolecular interactions with solvents and others compounds.The specific free energy of sorption, Gs was determined by the distance between the alkane line and the points corresponding to the Gs value of polar probes (Gamble, Leane, Olusanmi, Tobyn, Supuk, 2012).Gs = RTln(VN) RTln(VNref)The Gs values are given in Table 2 and represented in jut out 2. Chloroform was used as an acid probe on this work. The values ranged from 0.53 to 2.50 kJ/mol, corresponding to Brazilian Mahogany and Yellow Pine respectively. Based on the results obtained, the Gs values of Chloroform decrease in the following order Pine Grapia Courbaril Amburana Mahogany. This observation indicates that Yellow Pine may have a higher quantity/energy of active sites with basic character compared to the other fibers.A similar order was detect on Cyclohexane, indicating that Yellow Pine might also have a higher quantity/energy of active sites with apolar character. Although, the same order is not observed in SD, since that for the surface energy were used n-alkanes and due to the shape of cyclohexane and consequent different steric hindrance, it will have different interactions with the surface. On the other hand, Brazilian Mahogany shows the lowest Gs values for the overall probes, indicating a spurn quantity/energy of active sites.Ethyl acetate was used as a basic probe in Amburana, Yellow Pine and Courbaril. The interaction of this probe, alongside chloroform, with the timber surface indicates the presence of both a cidic and basic sites on the solid surface.3.3 Heat of sorption measurementsIn the infinite dilution region, where the isotherm is linear, VN should be measured at a range of column temperatures and ln(VN/T) plan vs. 1/T, which yields the heat of sorption Hs according to (Conder Young, 1979)To determine the enthalpies of sorption (HS) of the VOCs in the timber species, the specific retention volume (VN) was measured at different temperatures (293 to 298K) and the retention diagrams ln VN vs 1/T were plotted (Fig. 3-7).Some of VOCs couldnt be used in some of the timber species because the retention time was too low to mark accurate calculations due to the peak overlap.The Hs values are given in Table 3, determined at three different temperatures. Good linear correlations (0.99) were obtained for the probes used, as shown in Figures 3-7. In this work, the Hs values for the basic probes (Ethyl acetate Higher DN/AN*) were lower than the Hs values for the acid probes (Chloroform Lowe r DN/AN*), indicating a basic character of the solid surface. Cordeiro, Mendona, Pothan and Varma (2012) and Gouveia, Cordeiro and John (2011) observed the same basic character in macro and nanobanana fibers, flax, hemp, kenaf, agave, agave hybrid pineapple, and sisal.For chloroform, the Hs values decreased in the following order Amburana Brazilian Mahogany Grapia Courbaril Yellow Pine. At first sight it indicates that Amburana establishes stronger interactions with the surface, compared with the other timbers. On the other hand, Yellow Pine establishes the weakest interactions with chloroform. A similar order was also observed for cyclohexane, for the exception of Yellow Pine, that shows the highest Hs value for this probe.The results dont go in agreement with the ones obtained from the specific free energy of adsorption study, probably due to the detail that the temperature might influence the Gs values and therefore it might mislead to incorrect conclusions. Thus, the Hs val ues will give a better sagacity of which timber is advised (or not) to remove VOC from the atmosphere since it already have in account the temperature. Being said, the results indicate that for polar probes Amburana have stronger interactions while for apolar probes Yellow Pine have stronger interactions.3.4 Entropy of Sorption calculationThe linear dependence between HS and SS is called Thermodynamic Compensation Effect. Normally, stronger intermolecular interactions (related to HS) result in less degrees of freedom of the elutant, confidential information into a greater order of the system, decreasing the entropy of the system (Liu L., Guo Q-X., 2001). The entropy of sorption was directly calculated according toFigure 8 represents the entropy-enthalpy correlation, being only observed on Yellow Pine, Grapia and Brazilian Mahogany. All three dependences are linear and almost parallel, with a slope between 2.7910-3 and 3.7310-3 K-1. Therefore, these three samples follow the previo us equation. According to Korolev A. et al., (2011), a similar slope indicates similar adsorption mechanism on the samples studied on this work. Since all the samples have similar composition, it was expected the same adsorption mechanism between them.4. CONCLUSIONIGC was used to evaluate different surface properties, namely surface energy, specific free energy and enthalpy of adsorption. The SD values were measured at 298K and ranged from 39.95 to 43.81 mJ/m2. These variations between results are due to different composition, growing conditions and method of extraction.The results dont go in agreement between enthalpy of adsorption and the specific free energy of adsorption probably due to the influence of the temperature on the Gs. In Yellow Pine were observed higher Gs values for chloroform and cyclohexane compared to the other fibers, indicating a higher quantity/energy of active sites with basic and apolar character. In Amburana, Yellow Pine and Courbaril, the basic probes show ed lower Hs values compared to acid Hs values, indicating a basic character. The same was observed on other fibers found in literature. Amburana establishes stronger interactions with Chloroform while Yellow Pine establishes stronger interaction with cyclohexane, compared to the other timber species.The Thermodynamic Compensation Effect was only observed in Yellow Pine, Grapia and Brazilian Mahogany, with linear fits almost parallel. The slope ranged between 2.7910-3 and 3.7310-3 K-1. similar slopes indicate similar adsorption mechanism, which makes sense due to similar composition.ACKNOWLEDGEMENTReferencesAdusumali R-B, Reifferscheid M, Weber H, Roeder T, Sixta H, Gindl W. Mechanical properties of regenerated cellulose fibres for composites. Macromolecular Symposia 2006, 244 11925.Cao S., Hu B., Liu H. Synthesis of pH-responsive crosslinked polystyrene-co-(maleic sodium anhydride) and cellulose composite hydrogel nanofibers by electrospinning. Polymer outside(a) 2009, 58 545551.C ordeiro N., Gouveia C., John M. J. Investigation of surface properties of physico-chemically modified natural fibers using inverse gas chromatography. Industrial Crops and Products 2011, 33 108115.Cordeiro, N., Mendona, C., Pothan, L. A., Varma, A. Monitoring surface properties evolution of thermochemically modified cellulose nanofibres from banana pseudo-stem. Carbohydrate Polymers 2012, 88, 125131.De Boer, J.H., 1953. The Dynamic Character of Chemisorption, 2nd Ed., Clarendon Press, Oxford.Gamble J., Leane M., Olusanmi D., Tobyn M., Supuk E., Khoo J., Naderi M., 2012, Surface energy analysis as a tool to probe the surface energy characteristics of micronized materials A comparison with inverse gas chromatography International Journal of Pharmaceutics 422 238-244Kemball C., Rideal, E.K. The Adsorption of Vapours on Mercury. I. Non-Polar Substances. Proceedings of the Royal Society A 1946, 187 5373.Korolev A., Shiryaeva V., Popova T., Kurganov A. 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Chichester John Wiley Sons Ltd.TABLE CAPTIONSTable 1 Dispersive component of the surface tension (SD).Table 2 Specific free energy of sorption (GS).Table 3 Enthalpy of sorption (HS).Table 4 Entropy of sorption (SS).Table 1Table 2Table 3Table 4FIGURE CAPTIONSFigure 1 Energy of adsorption vs a(DL)1/2 for n-alkanes on the timbers (Amburana, Grapia, Courbaril, Brazilian Mahogany and Yellow Pine) surface at 298K and 0%RH.Figure 2 Specific free energy of the adsorption (Gs) of Chloroform and Cyclohexane onto samples analyzed at 298K.Figure 3 Enthalpy of sorption plot and linear fits for cyclohexane, chloroform and ethyl acetate measurement on Amburana timber surface 293-298K, at 0 % RH and 10ml/min.Figure 4 Enthalpy of sorption plot and linear fits for cyclohexane, chloroform and ethyl acetate measurement on Yellow Pine timber surface 293-298K, at 0 % RH and 10ml/min.Figure 5 Enthalpy of sorption plot and linear fits for cyclohexane, chloroform and benzene measurement on Grapia timber surface 293-298K, at 0 % RH and 10ml/min.Figure 6 Enthalpy of sorption plot and linear fits for cyclohexane, chloroform and ethyl acetate measurement on Courbaril timber surface 293-298K, at 0 % RH and 10ml/min.Figure 7 Enthalpy of sorption plot and linear fits for cyclohexane, chloroform and toluene measurement on Brazilian Mahogany timber surface 293-298K, at 0 % RH and 10ml/min.Figure 8 Entropy-enthalpy compensation effect on Yellow Pine, Grapia and Brazilian Mahogany.Figure 1Figure 2Figure 3Figure 4Figure 5Figure 6Figure 7Figure 8