Journal of Environmental Management 137 (2014) 93e100
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Deterioration of organic packing materials commonly used in air biofiltration: Effect of VOC-packing interactions Raquel Lebrero, José M. Estrada, Raúl Muñoz*, Guillermo Quijano Department of Chemical Engineering and Environmental Technology, Escuela de Ingenierías Industriales, Sede Dr. Mergelina, University of Valladolid, Dr Mergelina s/n, 47011 Valladolid, Spain
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 September 2013 Accepted 4 November 2013 Available online
The abiotic deterioration of three conventional organic packing materials used in biofiltration (compost, wood bark and Macadamia nutshells) caused by their interaction with toluene (used as a model volatile organic compound) was here studied. The deterioration of the materials was evaluated in terms of structural damage, release of co-substrates and increase of the packing biodegradability. After 21 days of exposure to toluene, all packing materials released co-substrates able to support microbial growth, which were not released by the control materials not exposed to toluene. Likewise, the exposure to toluene increased the packing material biodegradability by 26% in wood bark, 20% in compost and 17% in Macadamia nutshells. Finally, scanning electron microscopy analysis confirmed the deterioration in the structure of the packing materials evaluated due to the exposure to toluene, Macadamia nutshells being the material with the highest resistance to volatile organic compound attack. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Air biofiltration Biodegradability Packing material deterioration Organic packing material Volatile organic compound
1. Introduction Biological technologies for the treatment of volatile organic compounds (VOCs) are rapidly gaining acceptance as a sustainable alternative to conventional techniques such as chemical scrubbing, incineration or adsorption (Kraakman et al., 2011). The superior performance of biological technologies over their physical chemical counterparts in terms of environmental impact, process economics and social acceptance has been recently highlighted by Estrada et al. (2011, 2012). Among the available biological technologies, biofiltration is still the most common full-scale gas treatment technology, biofilters being particularly suitable for the treatment of low VOC concentrations and high gas flow rates (HernandezMelendez et al., 2008; Maestre et al., 2007). A recent compilation of data from full-scale installations revealed that approximately 87% of the biofilters operated with organic packing materials, wood chips and compost being the most popular supports (Estrada et al., 2013a). The main advantages of organic packing materials such as compost, peat, wood chips/bark and nutshells are their low price, wide availability, microbial diversity and nutrient content (Gaudin et al., 2008; Ortiz et al., 2003). However, the lifespan of organic packing materials is often
* Corresponding author. Tel.: þ34 983186424; fax: þ34 983423013. E-mail addresses: [email protected], [email protected] (R. Muñoz). http://dx.doi.org/10.1016/j.jenvman.2013.11.052 0301-4797/Ó 2014 Elsevier Ltd. All rights reserved.
lower than that of inorganic materials such as polyurethane foam, activated carbon or lava rock (Dorado et al., 2010). The loss of structural packing material properties and the subsequent reduction of the lifespan of organic materials have been traditionally attributed to microbial biodegradation (Adler, 2001; Ortiz et al., 2003). Nevertheless, deterioration due to the continuous interaction between the packing material and the VOCs (usually strong organic solvents such as toluene, benzene, hexane, styrene, etc.) cannot be ruled out. Thus, while the microbial biodegradation of the packing material cannot be avoided since microorganisms are inherently present in the packed bed, the selection of packing materials less sensitive to deterioration due to VOC exposure will help increasing the packing lifespan. In this regard, one of the main contributions to the operating costs in biofilters is the replacement of the bed packing material, accounting for approximately 47% of the total operating costs (Estrada et al., 2012). Unfortunately, to the best of our knowledge, no study on the potential deterioration of organic packing materials exclusively mediated by the interaction with VOCs is available in the literature. This information would be valuable as a tool for the selection of organic packing materials less susceptible to such abiotic VOC-mediated deterioration. The present work aimed at studying the abiotic deterioration of organic packing materials caused by their interaction with VOCs. The deterioration of the materials was assessed in terms of structural damage by scanning electron microscopy analysis, release of co-substrates and increase of the packing biodegradability.
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Compost, wood bark and Macadamia nutshells were used as model packing materials, while toluene was used as model VOC. 2. Experimental 2.1. Chemicals and mineral salt medium All chemicals for mineral salt medium (MSM) preparation were purchased from PANREAC (Barcelona, Spain) with a purity of at least 99%. Toluene (99.0% purity) was obtained from SigmaeAldrich (Madrid, Spain). The MSM was composed of (g L1): Na2HPO4e 12H2O, 6.15; KH2PO4, 1.52; (NH4)2SO4, 1; MgSO4e7H2O, 0.2; CaCl2, 0.038; and 10 mL L1 of a trace element solution containing (g L1): EDTA, 0.5; FeSO4e7H2O, 0.2; ZnSO4e7H2O, 0.01; MnCl2e4H2O, 0.003; H3BO3, 0.03; CoCl2e6H2O, 0.02; CuCl2e2H2O, 0.001; NiCl2e 6H2O, 0.002; NaMoO4e2H2O, 0.003. The final pH of medium was 7. 2.2. Packing materials Three typical organic packing materials were investigated: compost (Compo SanaÒ Universal, Spain), Macadamia ternifolia nutshells (kindly provided by Professor Herman Van Langenhove from Ghent University, Belgium) and wood bark (Populus Alba bark kindly provided by Carlos Pérez from Ros Roca Indox Cryo Energy S.L., Spain). The packing materials were characterized in terms of density, porosity, pH and water holding capacity according to standard methods (TMECC, 2002). 2.3. Biofilter setup and operation The biofilter consisted of a cylindrical PVC column (0.08 m inner diameter, 0.45 m height) with a working packed bed volume of 0.5 L. The packing materials were sterilized (121 C, 20 min) prior to use in order to eliminate any biological activity. The toluene-laden stream was obtained by evaporation of liquid toluene using a syringe pump (Fusion 100, Chemyx, USA) in a continuous air flow of 500 mL min1 (Fig. 1). The polluted air stream was passed through a 0.22 mm filter before entering the biofilter to avoid microbial
activity during the experiments. The biofilter was operated at 25 1 C for 21 days at an empty bed residence time (EBRT) of 1 min and an inlet toluene gas concentration of 1 g m3 for each packing material. Periodical measurements of toluene and CO2 gas concentrations were performed at the inlet/outlet of the biofilter to confirm the absence of microbial activity. A continuous toluenefree air flow of 500 mL min1 was maintained for 24 h following the 21 days of biofilter operation in order to remove the toluene absorbed in the packing material. 2.4. Release of co-substrates from the packing material At day 21 (end of the biofilter operation), the packing materials were irrigated with 200 mL of distilled water from the top of the biofilter. The resulting leachates were analyzed by solid-phase microextraction (SPME) coupled to GCeMS in order to determine the potential release of co-substrates from the packing material. The same irrigation procedure and subsequent leachate analysis was performed with control sterilized packing materials not previously exposed to toluene. 2.5. Packing material biodegradability tests The biodegradability of the packing materials was determined in duplicate in 300-mL gas-tight glass bottles supplied with 100 mL of MSM, 1 mL of fresh activated sludge (Valladolid wastewater treatment plant, Spain), a sample of the packing material (3.1e 3.7 gdryweight of compost, 8.5e9.3 gdryweight of nutshells and 5.9e 6.0 gdryweight of wood bark) from the biofilter, and 5 mL of liquid toluene in order to stimulate microbial activity. The packing material samples used in the test were taken from the biofilter at day 21, after irrigation with distilled water. Bottles supplied with sterilized packing material not exposed to toluene were used as controls. The bottles were incubated under orbital agitation at 250 rpm and 25 C. Toluene and CO2 concentrations in the headspace were periodically monitored. The heterogeneity in size of the samples of packing materials tested hindered the precise addition of packing material, and therefore the results of CO2 production were referred to the exact amount of sample added in each bottle. A higher specific CO2 production relative to the control bottles was considered as a measure of packing material biodegradability (Quijano et al., 2010). 2.6. Scanning electron microscopy (SEM) Samples of packing material from the biofilter at day 21 and samples of sterilized packing material not exposed to toluene were analyzed in a digital scanning electron microscope (Quanta 200FEG, Eindhoven, Netherlands) using accelerating voltages of 2e 15 kV. 2.7. Analytical methods Toluene gas concentration was measured in a Varian 3900 gas chromatograph (Palo Alto, USA) equipped with a flame ionization Table 1 Packing material characterization.
Fig. 1. Biofilter setup, where GMP: gas mixing port, AF: 0.22 mm air filter, GSP: gas sampling port and CR: calibrated rotameter.
Packing material
Density (g mL1)
Wet bed density (g mL1)
Porosity (%)
Water holding capacity (gwater g1 packing)
pH
Compost Macadamia nutshells Wood bark
0.33 0.34
0.93 1.09
62.5 66.7
1.95 0.11
5.85 4.26
0.13
0.35
62.5
0.42
6.88
R. Lebrero et al. / Journal of Environmental Management 137 (2014) 93e100 Table 2 Detected co-substrates released by the toluene-exposed packing materials after biofilter operation for 21 days. These compounds were not released by the control materials. Packing material
Co-substrate
Compost
Azulene
Nutshells
Butanoic acid
Molecular formula
95
175 C, respectively. Helium was used as the carrier gas at 13.7 mL min1. The released co-substrates in the biofilter leachate were characterized as follows: 1.7-mL glass vials were filled with 1.5 mL of biofilter leachate and closed with Teflon/rubber caps. Samples were pre-concentrated by SPME by immersing an 85-mm Carboxen/PDMS fibre (Supelco, Bellefonte, USA) into the leachate for 5 min. The SPME fibre was then injected and allowed to desorb for 1 min in an Agilent 6890N GCeMS equipped with a DB-WAX column (30 m 0.250 mm 0.25 mm) (J&W ScientificÒ, CA, USA). The injector temperature was set at 200 C while the oven temperature was initially maintained at 40 C for 4 min and then increased at 10 C min1 up to 200 C. Source and MS quadrupole temperatures were set at 230 and 150 C, respectively. Only compounds with a match quality 90% were considered in the discussion.
Pentanoic acid
3. Results and discussion 3.1. Packing material characterization Hexanoic acid
Compost and Macadamia nutshells showed a similar and higher density and wet bed density compared to wood bark (Table 1). The significant increase in the density of the compost after wetting was associated to its high water holding capacity (w18 and w5 times higher than that of nutshells and bark, respectively), whereas the high wet bed density recorded for the nutshells was attributed to the high bed-void volume of this packing material. A high porosity was exhibited by the three packing materials, ranging from 62 to 67%. The pH value, related to the chemical composition of the packing material, was near neutrality for wood bark, 5.85 for compost, while the lowest pH (4.26) was recorded for nutshells, which render this packing material particularly suitable for the growth of fungi.
Furfural
Wood bark
2-octanamine
Nonyl cyclopropane
3.2. Release of co-substrates from the packing material
detector and a SupelcoWax (15 m 0.25 mm 0.25 mm) capillary column. The injector, detector and oven temperatures were set at 200 C, 200 C and 140 C, respectively. Helium was used as the carrier gas at 2 mL min1. CO2 concentration was determined in a Bruker 430 gas chromatograph (Palo Alto, USA) coupled with a thermal conductivity detector and equipped with a CP-Molsieve 5A (15 m 0.53 mm 15 mm) and a CP-PoraBOND Q (25 m 0.53 mm 10 mm) columns. The oven, injector and detector temperatures were maintained at 40 C, 150 C and
During the 21 days operational period, the monitoring of toluene and CO2 gas concentrations at the inlet and outlet of the biofilter confirmed the absence of toluene biodegradation. At the end of the experimental period, the biofilter was irrigated with distilled water in order to wash-out potential co-substrates released from the packing material as a result only of the VOC-packing material interaction. Several co-substrates capable of supporting microbial growth, which were not found in the control packing materials not exposed to toluene, were indeed detected in the biofilter leachate (Table 2).
Table 3 Comparative specific CO2 production in toluene-degrading biofilters packed with organic and inorganic materials. Packing material
Load (g m3 h1)
CO2 production (gCO2 gVOC1 degraded)
Microorganisms
Reference
Porcelite (porous ceramic)
58e114
1.91
(Sakuma et al., 2006)
Perlite
58e114
2.01
Pelletized diatomaceous earth (CeliteÒ) Vermiculite Perlite Compost Peat Mixture of composted pig manure and sawdust Compost
46.9
2.37
Mixed culture of toluene-degrading microorganisms Mixed culture of toluene-degrading microorganisms Aerobic microbial consortium
(Kim and Sorial, 2007)
220 150 40e140 65e195 6e221
1.57 1.40e1.50 2.30 2.47 2.84
Fungus Paecilomyces variotii CBS 115145 Activated sludge and Pseudomonas putida F1 Indigenous compost microflora Activated sludge Activated sludge
(Garcia-Peña et al., 2008) (Estrada et al., 2013b) (Delhomenie et al., 2002) (Alvarez-Hornos et al., 2008) (Gallastegui et al., 2013)
150
2.70e2.90
Activated sludge
(Estrada et al., 2013b)
(Sakuma et al. 2006)
R. Lebrero et al. / Journal of Environmental Management 137 (2014) 93e100
3.3. Packing material biodegradability The exposure of the packing materials to toluene resulted in a higher biodegradability of all the materials tested. Fig. 2 clearly shows that a higher CO2 production per gram of dry packing material was reached in the three packing materials exposed to toluene for 21 days compared with their corresponding controls not exposed to toluene. For the three packing materials tested, the specific CO2 production exceeded by far the maximum CO2 production expected from the full mineralization of the initial
a
8
Toluene depleted
CO2 (gCO2 gpacking-1 x 103)
7 6 5 4 3 2 1 0 0
5
10
15
20
25
Time (days)
b
6
Toluene depleted
5
CO2 (gCO2 gpacking-1 x 103)
The main co-substrate released from toluene-exposed compost was azulene, an isomer of naphthalene commonly found in nature as a constituent of pigments in mushrooms, guaiac and vetiver oils, chamomile plant and several plant roots (Gunther and Buzzetti, 1965). Toluene-exposed Macadamia nutshells released several readily biodegradable co-substrates such as butanoic acid (butyric acid), pentanoic acid (valeric acid) and hexanoic acid (caproic acid). The presence of these compounds is common in tree nuts. For instance, Macadamia nuts can have up to 0.1 ppm of furfural on dry basis (Adams et al., 1997). Likewise, Macadamia nuts can contain up to 19% of saturated fatty acids, butanoic, pentanoic and hexanoic acids being typically found in many tree nuts (Alasalvar and Shahidi, 2009). On the other hand, wood bark released 2-octanamine and nonyl-cyclopropane. 2-octanamine has been previously detected in benzene/ethanol extractions from Eucalyptus urophylla wood (Ma et al., 2009), while compounds similar to nonyl-cyclopropane such as 1-isopropyl-1-methyl-2-nonyl-cyclopropane and 1,1-dimethyl-2-nonyl-cyclopropane have been identified in the Dacryodes edulis tree bark and in Capsicum chinense peppers (Okwu and Ighodaro, 2009; Sousa et al., 2006). Interestingly, molecules containing the cyclopropane ring exhibit antibacterial activity, which may bring insights on why wood barks are especially suitable for promoting the development of fungi (Hawser et al., 2006; Okwu and Ighodaro, 2009). The results here obtained showed that the interaction between toluene and the packing materials resulted in the release of organic compounds that can act as co-substrates during the VOC biofiltration process. In the particular case of Macadamia nutshells and wood bark, some of the released co-substrates correspond to readily biodegradable compounds, whose degradation by the microbial community during biofilter operation would result in an increased CO2 production. A comprehensive literature review on toluene biofiltration (Table 3) shows that when biofilters are operated with inorganic packing materials the CO2 produced per mass of toluene degraded is consistently lower than that observed with organic supports. In this context, it is worth noting that the CO2 production in biofilters operated with organic materials is often close to the theoretical stoichiometric CO2 yield without considering biomass formation (3.3 gCO2 g1 toluene). Consequently, a low biomass production might be expected in such organic-based biofilters since practically all the VOC should have been mineralized. However, biomass clogging issues have been reported in those works, where a constant toluene load was applied and no periodical bed unpacking or homogenization was conducted (Delhomenie et al., 2002; Estrada et al., 2013b). These apparently contradictory results can be explained by the biodegradation of the packing material, including the degradation of the co-substrates released from the packing materials as a result of VOC exposure, which supports the production of additional CO2 not related to VOC oxidation. The release of such co-substrates has ambivalent consequences since they increase the robustness of the biofiltration process by stimulating microbial growth during process startup and starvation periods, but results in packing material deterioration and a subsequent reduction in packing media lifespan.
4
3
2
1
0 0
2
4
6
8
10
12
14
Time (days)
c
6
Toluene depleted
5
CO2 (gCO2 gpacking-1 x 103)
96
4
3
2
1
0 0
5
10
15
20
25
Time (days) Fig. 2. Time course of the specific CO2 production in the biodegradability tests performed with (a) compost, (b) nutshells and (c) wood bark. Horizontal dotted lines stand for the maximum theoretical CO2 production without biomass formation due to the initial toluene concentration. Grey zones indicate the elapsed time until complete toluene depletion in the test bottles.
toluene measured in the test bottles. Therefore, the additional CO2 produced was a clear indication of the packing material biodegradation mediated by the exposure to toluene. In order to quantify the increase in packing material biodegradability due to the VOC-packing interaction, the total CO2 production per gram of dry packing material was obtained by
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Fig. 3. SEM microphotographs of compost exposed to toluene for 21 days in the biofilter and control packing material not exposed to toluene, where: (a) soil particles, (b) leaves, and (c) root-like structures.
integrating the areas under the curves depicted in Fig. 2 over the entire experimental time. The specific CO2 production from compost, nutshells and wood bark exposed to toluene were 20, 17 and 26% higher than that recorded in the controls, respectively. Thus, the packing material biodegradability was enhanced by the interaction with toluene in the following order: wood bark > compost > nutshells. The information obtained from the biodegradability tests can be used as a tool for selecting organic packing materials less susceptible to be deteriorated by the interaction with VOCs, which will therefore support a longer media lifespan. It is also important to highlight that wood bark and
compost were more biodegradable than nutshells after exposure to toluene, even when more readily biodegradable co-substrates were released by the nutshells (e.g. short-chain organic acids). However, the amount of co-substrates released was not quantified due to the qualitative nature of the SPME-GC-MS technique in such a complex analytical matrix. In this regard, the hydrophobic nature of the SPME fibre used (polydimethylsiloxane-based polymer) facilitated the preferential detection of hydrophobic compounds and therefore, a potential release of additional hydrophilic co-substrates from the packing materials cannot be ruled out. These results indicated that both a characterization of the co-substrates released
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Fig. 4. SEM microphotographs of Macadamia nutshells exposed to toluene for 21 days in the biofilter and control packing material not exposed to toluene, where: (a) outer surface, (b) inner surface, and (c) internal structures (shell rupture zones).
and quantitative biodegradability tests must be simultaneously performed to determine the deterioration of packing materials due to a continuous exposure to VOCs. 3.4. Scanning electron microscopy observations The release of co-substrates and the increase in packing material biodegradability after the 21-days exposure to toluene strongly suggested that the packing materials suffered structural damages, which might progressively result in a severe bed compaction, gas
flow channelling and a pressure drop build-up in the long-term biofilter operation (Kennes and Veiga, 2002). Therefore, SEM microphotographs of each packing material before and after exposure to toluene were taken at several magnifications in order to visualize the deterioration degree caused by the VOC. The deterioration extent of compost due to its exposure to toluene was difficult to assess since compost is a very heterogeneous matrix. Thus, in order to perform a fair comparison between the compost exposed to toluene and the control, the SEM analysis was performed on similar macroscopic structures present in both
R. Lebrero et al. / Journal of Environmental Management 137 (2014) 93e100
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Fig. 5. SEM microphotographs of wood bark exposed to toluene for 21 days in the biofilter and control packing material not exposed to toluene, where: (a) 400 magnification, (b) 1000 magnification and (c) 2000 magnification.
samples. Despite the complexity of the compost matrix, a clear deterioration of several compost elements such as soil particles, leaves and root-like structures was observed due to the exposure to toluene (Fig. 3). The relatively fast decomposition of compost as a packing material in biofilters and the subsequent deterioration of the bed structure have been widely reported in the literature (Iranpour et al., 2005; Ortiz et al., 2003). In fact, compost together with peat, coconut fibre and pine leaves have been classified among the supports with the shorter lifespan, supporting an estimated durability of two years (Dorado et al., 2010).
The SEM analysis of nutshells was performed in both the outer and inner surfaces as well as in the shell rupture zones in order to get a comprehensive evaluation of the packing material deterioration (Fig. 4). A clear deterioration of the outer surface was observed, which likely entailed the exposure of the internal material of the nutshell to the polluted air stream. Interestingly, the inner surface of the nutshells showed a less visible damage compared to that observed in the outer surface, the internal structure of the nutshell being still protected from the inner surface. These results clearly indicate that the material covering the
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outer surface of the nutshells was more susceptible to deterioration compared with the inner surface material. On the other hand, further analysis of the internal material visible from the shell rupture zones revealed that exposure to toluene did not damaged the internal structure of the material. These results reveal the potential of Macadamia nutshells as packing materials for biofiltration as a result of its high structural strength. Even when the outer surface suffered a visible damage, the internal material and structure remained unchanged after 21 days of exposure to toluene. This empirical finding is also in agreement with the biodegradability tests, which showed that nutshells were the less biodegradable material tested. The good structural characteristics of Macadamia nutshells have been recently highlighted by Volckaert et al. (2013). These authors used nutshells as a packing material in a biofilter treating ethylbenzene for 5 months with a maximum pressure drop of 0.85 kPa m1, which was significantly low compared with the pressure drop values of 1.4e20 kPa m1 typically recorded in biofilters operated with other organic packing materials during long periods of time (Estrada et al., 2013b). In the case of wood bark, it was difficult to assess the damage produced by the exposure to toluene with SEM at 400 magnifications or lower. However, the sharp edges observed at 1000 and 2000 magnifications in the control material contrasted with the polished structures observed in the bark exposed to toluene (Fig. 5). Particularly, at 2000 magnification it was observed that the bark surface was significantly deteriorated by the exposure to toluene. Wood bark is mainly composed of cellulose and lignin, and despite cellulose is the main component, lignin provides the hardness and rigidity to the material (Byrne and Nagle, 1997). According to the biodegradability tests, wood bark underwent the highest increase in biodegradability after its exposure to toluene, this deterioration being likely related to the solubility of lignin in toluene. It is well known that lignin dissolves better in solvents holding a Hildebrand solubility parameter of approximately 11 (cal/cc)1/2 (Sun et al., 2000), and toluene exhibits a very close value of 9.56 (cal/cc)1/2 (Belmares et al., 2004). Therefore, the deterioration of the wood bark could be attributed to a progressive lignin solubilization in the toluene absorbed in the bed, resulting in both material damage and increase of the packing material biodegradability. 4. Conclusions The VOC-packing material interaction noticeably contributed to the deterioration of the organic packing materials evaluated. The materials released co-substrates able to support microbial growth after exposure to toluene, which were not released by the controls. The exposure to toluene also resulted in an increase in the material biodegradability in the following order: wood bark > compost > nutshells. The SEM analysis confirmed the packing deterioration, Macadamia nutshell being the material with a higher resistance to VOC contact. The experimental protocol here proposed was successful for selecting organic packing materials less susceptible to deterioration by exposure to the target VOCs. Acknowledgements This research was supported by the Spanish Ministry of Economy and Competitiveness (Contracts JCI-2011-11009 and BES2010-030994 and Projects CTQ2012-34949 and CONSOLIDER-CSD 2007-00055). References Adams, T.B., Doull, J., Goodman, J.I., Munro, I.C., Newberne, P., Portoghese, P.S., Smith, R.L., Wagner, B.M., Weil, C.S., Woods, L.A., Ford, R.A., 1997. The FEMA
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Microbiol. Biotechnol. 91, 873e886. Ma, Q.Z., Peng, W.X., Zhang, D.Q., Wu, Y.Q., Qi, H.C., 2009. Study on bioenergy utilization benzene/ethanol extractives of Eucalyptus urophylla wood by Py-GC/ MS. In: Proceedings of the Power and Energy Engineering Conference, APPEEC 2009 Asia-Pacific. Wuhan, China. Maestre, J.P., Gamisans, X., Gabriel, D., Lafuente, J., 2007. Fungal biofilters for toluene biofiltration: evaluation of the performance with four packing materials under different operating conditions. Chemosphere 67, 684e692. Okwu, D.E., Ighodaro, B.U., 2009. GC-MS evaluation of the bioactive compounds and antibacterial activity of the oil fraction from the stem barks of Dacryodes edulis G Don Lam. Int. J. Drug. Dev. Res. 1, 117e125. Ortiz, I., Revah, S., Auria, R., 2003. Effects of packing material on the biofiltration of benzene, toluene and xylene vapours. Environ. Technol. 24, 265e275. Quijano, G., Hernandez, H., Villaverde, S., Thalasso, F., Muñoz, R., 2010. A step-forward in the characterization and potential applications of solid and liquid oxygen transfer vectors. Appl. Microbiol. Biotechnol. 85, 543e551. Sakuma, T., Hattori, T., Deshusses, M.A., 2006. Comparison of different packing materials for the biofiltration of air toxics. J. Air Waste Manage. Assoc. 56, 1567e1575. Sousa, E.T., Rodrigues, F.M., Martins, C.C., Oliveira, F.S., Pereira, P.A.P., Andrade, J.B., 2006. Multivariate optimization and HS-SPME/GC-MS analysis of VOCs in red, yellow and purple varieties of Capsicum chinense sp. peppers. Microchem. J. 82, 142e149. Sun, R.C., Tomkinson, J., Jones, G.L., 2000. Fractional characterization of ash-AQ lignin by successive extraction with organic solvents from oil palm EFB fibre. Polym. Degrad. Stabil. 68, 111e119. TMECC (Test Methods for the Examination of Composting and Compost), June 2002. The US Composting Council Research and Education Foundation & The US Department of Agriculture. Volckaert, D., Alvarez-Hornos, F.J., Heynderickx, P.M., Kittikoon, C., Van Langenhove, H., 2013. Ethylbenzene removal under mesophilic conditions in a biofilter with Macadamia ternifolia nutshells as a carrier material. J. Chem. Technol. Biotechnol. 88, 81e87.
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Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
Deterioration of organic packing materials commonly used in air biofiltration: Effect of VOC-packing interactions Raquel Lebrero, José M. Estrada, Raúl Muñoz*, Guillermo Quijano Department of Chemical Engineering and Environmental Technology, Escuela de Ingenierías Industriales, Sede Dr. Mergelina, University of Valladolid, Dr Mergelina s/n, 47011 Valladolid, Spain
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 September 2013 Accepted 4 November 2013 Available online
The abiotic deterioration of three conventional organic packing materials used in biofiltration (compost, wood bark and Macadamia nutshells) caused by their interaction with toluene (used as a model volatile organic compound) was here studied. The deterioration of the materials was evaluated in terms of structural damage, release of co-substrates and increase of the packing biodegradability. After 21 days of exposure to toluene, all packing materials released co-substrates able to support microbial growth, which were not released by the control materials not exposed to toluene. Likewise, the exposure to toluene increased the packing material biodegradability by 26% in wood bark, 20% in compost and 17% in Macadamia nutshells. Finally, scanning electron microscopy analysis confirmed the deterioration in the structure of the packing materials evaluated due to the exposure to toluene, Macadamia nutshells being the material with the highest resistance to volatile organic compound attack. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Air biofiltration Biodegradability Packing material deterioration Organic packing material Volatile organic compound
1. Introduction Biological technologies for the treatment of volatile organic compounds (VOCs) are rapidly gaining acceptance as a sustainable alternative to conventional techniques such as chemical scrubbing, incineration or adsorption (Kraakman et al., 2011). The superior performance of biological technologies over their physical chemical counterparts in terms of environmental impact, process economics and social acceptance has been recently highlighted by Estrada et al. (2011, 2012). Among the available biological technologies, biofiltration is still the most common full-scale gas treatment technology, biofilters being particularly suitable for the treatment of low VOC concentrations and high gas flow rates (HernandezMelendez et al., 2008; Maestre et al., 2007). A recent compilation of data from full-scale installations revealed that approximately 87% of the biofilters operated with organic packing materials, wood chips and compost being the most popular supports (Estrada et al., 2013a). The main advantages of organic packing materials such as compost, peat, wood chips/bark and nutshells are their low price, wide availability, microbial diversity and nutrient content (Gaudin et al., 2008; Ortiz et al., 2003). However, the lifespan of organic packing materials is often
* Corresponding author. Tel.: þ34 983186424; fax: þ34 983423013. E-mail addresses: [email protected], [email protected] (R. Muñoz). http://dx.doi.org/10.1016/j.jenvman.2013.11.052 0301-4797/Ó 2014 Elsevier Ltd. All rights reserved.
lower than that of inorganic materials such as polyurethane foam, activated carbon or lava rock (Dorado et al., 2010). The loss of structural packing material properties and the subsequent reduction of the lifespan of organic materials have been traditionally attributed to microbial biodegradation (Adler, 2001; Ortiz et al., 2003). Nevertheless, deterioration due to the continuous interaction between the packing material and the VOCs (usually strong organic solvents such as toluene, benzene, hexane, styrene, etc.) cannot be ruled out. Thus, while the microbial biodegradation of the packing material cannot be avoided since microorganisms are inherently present in the packed bed, the selection of packing materials less sensitive to deterioration due to VOC exposure will help increasing the packing lifespan. In this regard, one of the main contributions to the operating costs in biofilters is the replacement of the bed packing material, accounting for approximately 47% of the total operating costs (Estrada et al., 2012). Unfortunately, to the best of our knowledge, no study on the potential deterioration of organic packing materials exclusively mediated by the interaction with VOCs is available in the literature. This information would be valuable as a tool for the selection of organic packing materials less susceptible to such abiotic VOC-mediated deterioration. The present work aimed at studying the abiotic deterioration of organic packing materials caused by their interaction with VOCs. The deterioration of the materials was assessed in terms of structural damage by scanning electron microscopy analysis, release of co-substrates and increase of the packing biodegradability.
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Compost, wood bark and Macadamia nutshells were used as model packing materials, while toluene was used as model VOC. 2. Experimental 2.1. Chemicals and mineral salt medium All chemicals for mineral salt medium (MSM) preparation were purchased from PANREAC (Barcelona, Spain) with a purity of at least 99%. Toluene (99.0% purity) was obtained from SigmaeAldrich (Madrid, Spain). The MSM was composed of (g L1): Na2HPO4e 12H2O, 6.15; KH2PO4, 1.52; (NH4)2SO4, 1; MgSO4e7H2O, 0.2; CaCl2, 0.038; and 10 mL L1 of a trace element solution containing (g L1): EDTA, 0.5; FeSO4e7H2O, 0.2; ZnSO4e7H2O, 0.01; MnCl2e4H2O, 0.003; H3BO3, 0.03; CoCl2e6H2O, 0.02; CuCl2e2H2O, 0.001; NiCl2e 6H2O, 0.002; NaMoO4e2H2O, 0.003. The final pH of medium was 7. 2.2. Packing materials Three typical organic packing materials were investigated: compost (Compo SanaÒ Universal, Spain), Macadamia ternifolia nutshells (kindly provided by Professor Herman Van Langenhove from Ghent University, Belgium) and wood bark (Populus Alba bark kindly provided by Carlos Pérez from Ros Roca Indox Cryo Energy S.L., Spain). The packing materials were characterized in terms of density, porosity, pH and water holding capacity according to standard methods (TMECC, 2002). 2.3. Biofilter setup and operation The biofilter consisted of a cylindrical PVC column (0.08 m inner diameter, 0.45 m height) with a working packed bed volume of 0.5 L. The packing materials were sterilized (121 C, 20 min) prior to use in order to eliminate any biological activity. The toluene-laden stream was obtained by evaporation of liquid toluene using a syringe pump (Fusion 100, Chemyx, USA) in a continuous air flow of 500 mL min1 (Fig. 1). The polluted air stream was passed through a 0.22 mm filter before entering the biofilter to avoid microbial
activity during the experiments. The biofilter was operated at 25 1 C for 21 days at an empty bed residence time (EBRT) of 1 min and an inlet toluene gas concentration of 1 g m3 for each packing material. Periodical measurements of toluene and CO2 gas concentrations were performed at the inlet/outlet of the biofilter to confirm the absence of microbial activity. A continuous toluenefree air flow of 500 mL min1 was maintained for 24 h following the 21 days of biofilter operation in order to remove the toluene absorbed in the packing material. 2.4. Release of co-substrates from the packing material At day 21 (end of the biofilter operation), the packing materials were irrigated with 200 mL of distilled water from the top of the biofilter. The resulting leachates were analyzed by solid-phase microextraction (SPME) coupled to GCeMS in order to determine the potential release of co-substrates from the packing material. The same irrigation procedure and subsequent leachate analysis was performed with control sterilized packing materials not previously exposed to toluene. 2.5. Packing material biodegradability tests The biodegradability of the packing materials was determined in duplicate in 300-mL gas-tight glass bottles supplied with 100 mL of MSM, 1 mL of fresh activated sludge (Valladolid wastewater treatment plant, Spain), a sample of the packing material (3.1e 3.7 gdryweight of compost, 8.5e9.3 gdryweight of nutshells and 5.9e 6.0 gdryweight of wood bark) from the biofilter, and 5 mL of liquid toluene in order to stimulate microbial activity. The packing material samples used in the test were taken from the biofilter at day 21, after irrigation with distilled water. Bottles supplied with sterilized packing material not exposed to toluene were used as controls. The bottles were incubated under orbital agitation at 250 rpm and 25 C. Toluene and CO2 concentrations in the headspace were periodically monitored. The heterogeneity in size of the samples of packing materials tested hindered the precise addition of packing material, and therefore the results of CO2 production were referred to the exact amount of sample added in each bottle. A higher specific CO2 production relative to the control bottles was considered as a measure of packing material biodegradability (Quijano et al., 2010). 2.6. Scanning electron microscopy (SEM) Samples of packing material from the biofilter at day 21 and samples of sterilized packing material not exposed to toluene were analyzed in a digital scanning electron microscope (Quanta 200FEG, Eindhoven, Netherlands) using accelerating voltages of 2e 15 kV. 2.7. Analytical methods Toluene gas concentration was measured in a Varian 3900 gas chromatograph (Palo Alto, USA) equipped with a flame ionization Table 1 Packing material characterization.
Fig. 1. Biofilter setup, where GMP: gas mixing port, AF: 0.22 mm air filter, GSP: gas sampling port and CR: calibrated rotameter.
Packing material
Density (g mL1)
Wet bed density (g mL1)
Porosity (%)
Water holding capacity (gwater g1 packing)
pH
Compost Macadamia nutshells Wood bark
0.33 0.34
0.93 1.09
62.5 66.7
1.95 0.11
5.85 4.26
0.13
0.35
62.5
0.42
6.88
R. Lebrero et al. / Journal of Environmental Management 137 (2014) 93e100 Table 2 Detected co-substrates released by the toluene-exposed packing materials after biofilter operation for 21 days. These compounds were not released by the control materials. Packing material
Co-substrate
Compost
Azulene
Nutshells
Butanoic acid
Molecular formula
95
175 C, respectively. Helium was used as the carrier gas at 13.7 mL min1. The released co-substrates in the biofilter leachate were characterized as follows: 1.7-mL glass vials were filled with 1.5 mL of biofilter leachate and closed with Teflon/rubber caps. Samples were pre-concentrated by SPME by immersing an 85-mm Carboxen/PDMS fibre (Supelco, Bellefonte, USA) into the leachate for 5 min. The SPME fibre was then injected and allowed to desorb for 1 min in an Agilent 6890N GCeMS equipped with a DB-WAX column (30 m 0.250 mm 0.25 mm) (J&W ScientificÒ, CA, USA). The injector temperature was set at 200 C while the oven temperature was initially maintained at 40 C for 4 min and then increased at 10 C min1 up to 200 C. Source and MS quadrupole temperatures were set at 230 and 150 C, respectively. Only compounds with a match quality 90% were considered in the discussion.
Pentanoic acid
3. Results and discussion 3.1. Packing material characterization Hexanoic acid
Compost and Macadamia nutshells showed a similar and higher density and wet bed density compared to wood bark (Table 1). The significant increase in the density of the compost after wetting was associated to its high water holding capacity (w18 and w5 times higher than that of nutshells and bark, respectively), whereas the high wet bed density recorded for the nutshells was attributed to the high bed-void volume of this packing material. A high porosity was exhibited by the three packing materials, ranging from 62 to 67%. The pH value, related to the chemical composition of the packing material, was near neutrality for wood bark, 5.85 for compost, while the lowest pH (4.26) was recorded for nutshells, which render this packing material particularly suitable for the growth of fungi.
Furfural
Wood bark
2-octanamine
Nonyl cyclopropane
3.2. Release of co-substrates from the packing material
detector and a SupelcoWax (15 m 0.25 mm 0.25 mm) capillary column. The injector, detector and oven temperatures were set at 200 C, 200 C and 140 C, respectively. Helium was used as the carrier gas at 2 mL min1. CO2 concentration was determined in a Bruker 430 gas chromatograph (Palo Alto, USA) coupled with a thermal conductivity detector and equipped with a CP-Molsieve 5A (15 m 0.53 mm 15 mm) and a CP-PoraBOND Q (25 m 0.53 mm 10 mm) columns. The oven, injector and detector temperatures were maintained at 40 C, 150 C and
During the 21 days operational period, the monitoring of toluene and CO2 gas concentrations at the inlet and outlet of the biofilter confirmed the absence of toluene biodegradation. At the end of the experimental period, the biofilter was irrigated with distilled water in order to wash-out potential co-substrates released from the packing material as a result only of the VOC-packing material interaction. Several co-substrates capable of supporting microbial growth, which were not found in the control packing materials not exposed to toluene, were indeed detected in the biofilter leachate (Table 2).
Table 3 Comparative specific CO2 production in toluene-degrading biofilters packed with organic and inorganic materials. Packing material
Load (g m3 h1)
CO2 production (gCO2 gVOC1 degraded)
Microorganisms
Reference
Porcelite (porous ceramic)
58e114
1.91
(Sakuma et al., 2006)
Perlite
58e114
2.01
Pelletized diatomaceous earth (CeliteÒ) Vermiculite Perlite Compost Peat Mixture of composted pig manure and sawdust Compost
46.9
2.37
Mixed culture of toluene-degrading microorganisms Mixed culture of toluene-degrading microorganisms Aerobic microbial consortium
(Kim and Sorial, 2007)
220 150 40e140 65e195 6e221
1.57 1.40e1.50 2.30 2.47 2.84
Fungus Paecilomyces variotii CBS 115145 Activated sludge and Pseudomonas putida F1 Indigenous compost microflora Activated sludge Activated sludge
(Garcia-Peña et al., 2008) (Estrada et al., 2013b) (Delhomenie et al., 2002) (Alvarez-Hornos et al., 2008) (Gallastegui et al., 2013)
150
2.70e2.90
Activated sludge
(Estrada et al., 2013b)
(Sakuma et al. 2006)
R. Lebrero et al. / Journal of Environmental Management 137 (2014) 93e100
3.3. Packing material biodegradability The exposure of the packing materials to toluene resulted in a higher biodegradability of all the materials tested. Fig. 2 clearly shows that a higher CO2 production per gram of dry packing material was reached in the three packing materials exposed to toluene for 21 days compared with their corresponding controls not exposed to toluene. For the three packing materials tested, the specific CO2 production exceeded by far the maximum CO2 production expected from the full mineralization of the initial
a
8
Toluene depleted
CO2 (gCO2 gpacking-1 x 103)
7 6 5 4 3 2 1 0 0
5
10
15
20
25
Time (days)
b
6
Toluene depleted
5
CO2 (gCO2 gpacking-1 x 103)
The main co-substrate released from toluene-exposed compost was azulene, an isomer of naphthalene commonly found in nature as a constituent of pigments in mushrooms, guaiac and vetiver oils, chamomile plant and several plant roots (Gunther and Buzzetti, 1965). Toluene-exposed Macadamia nutshells released several readily biodegradable co-substrates such as butanoic acid (butyric acid), pentanoic acid (valeric acid) and hexanoic acid (caproic acid). The presence of these compounds is common in tree nuts. For instance, Macadamia nuts can have up to 0.1 ppm of furfural on dry basis (Adams et al., 1997). Likewise, Macadamia nuts can contain up to 19% of saturated fatty acids, butanoic, pentanoic and hexanoic acids being typically found in many tree nuts (Alasalvar and Shahidi, 2009). On the other hand, wood bark released 2-octanamine and nonyl-cyclopropane. 2-octanamine has been previously detected in benzene/ethanol extractions from Eucalyptus urophylla wood (Ma et al., 2009), while compounds similar to nonyl-cyclopropane such as 1-isopropyl-1-methyl-2-nonyl-cyclopropane and 1,1-dimethyl-2-nonyl-cyclopropane have been identified in the Dacryodes edulis tree bark and in Capsicum chinense peppers (Okwu and Ighodaro, 2009; Sousa et al., 2006). Interestingly, molecules containing the cyclopropane ring exhibit antibacterial activity, which may bring insights on why wood barks are especially suitable for promoting the development of fungi (Hawser et al., 2006; Okwu and Ighodaro, 2009). The results here obtained showed that the interaction between toluene and the packing materials resulted in the release of organic compounds that can act as co-substrates during the VOC biofiltration process. In the particular case of Macadamia nutshells and wood bark, some of the released co-substrates correspond to readily biodegradable compounds, whose degradation by the microbial community during biofilter operation would result in an increased CO2 production. A comprehensive literature review on toluene biofiltration (Table 3) shows that when biofilters are operated with inorganic packing materials the CO2 produced per mass of toluene degraded is consistently lower than that observed with organic supports. In this context, it is worth noting that the CO2 production in biofilters operated with organic materials is often close to the theoretical stoichiometric CO2 yield without considering biomass formation (3.3 gCO2 g1 toluene). Consequently, a low biomass production might be expected in such organic-based biofilters since practically all the VOC should have been mineralized. However, biomass clogging issues have been reported in those works, where a constant toluene load was applied and no periodical bed unpacking or homogenization was conducted (Delhomenie et al., 2002; Estrada et al., 2013b). These apparently contradictory results can be explained by the biodegradation of the packing material, including the degradation of the co-substrates released from the packing materials as a result of VOC exposure, which supports the production of additional CO2 not related to VOC oxidation. The release of such co-substrates has ambivalent consequences since they increase the robustness of the biofiltration process by stimulating microbial growth during process startup and starvation periods, but results in packing material deterioration and a subsequent reduction in packing media lifespan.
4
3
2
1
0 0
2
4
6
8
10
12
14
Time (days)
c
6
Toluene depleted
5
CO2 (gCO2 gpacking-1 x 103)
96
4
3
2
1
0 0
5
10
15
20
25
Time (days) Fig. 2. Time course of the specific CO2 production in the biodegradability tests performed with (a) compost, (b) nutshells and (c) wood bark. Horizontal dotted lines stand for the maximum theoretical CO2 production without biomass formation due to the initial toluene concentration. Grey zones indicate the elapsed time until complete toluene depletion in the test bottles.
toluene measured in the test bottles. Therefore, the additional CO2 produced was a clear indication of the packing material biodegradation mediated by the exposure to toluene. In order to quantify the increase in packing material biodegradability due to the VOC-packing interaction, the total CO2 production per gram of dry packing material was obtained by
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Fig. 3. SEM microphotographs of compost exposed to toluene for 21 days in the biofilter and control packing material not exposed to toluene, where: (a) soil particles, (b) leaves, and (c) root-like structures.
integrating the areas under the curves depicted in Fig. 2 over the entire experimental time. The specific CO2 production from compost, nutshells and wood bark exposed to toluene were 20, 17 and 26% higher than that recorded in the controls, respectively. Thus, the packing material biodegradability was enhanced by the interaction with toluene in the following order: wood bark > compost > nutshells. The information obtained from the biodegradability tests can be used as a tool for selecting organic packing materials less susceptible to be deteriorated by the interaction with VOCs, which will therefore support a longer media lifespan. It is also important to highlight that wood bark and
compost were more biodegradable than nutshells after exposure to toluene, even when more readily biodegradable co-substrates were released by the nutshells (e.g. short-chain organic acids). However, the amount of co-substrates released was not quantified due to the qualitative nature of the SPME-GC-MS technique in such a complex analytical matrix. In this regard, the hydrophobic nature of the SPME fibre used (polydimethylsiloxane-based polymer) facilitated the preferential detection of hydrophobic compounds and therefore, a potential release of additional hydrophilic co-substrates from the packing materials cannot be ruled out. These results indicated that both a characterization of the co-substrates released
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Fig. 4. SEM microphotographs of Macadamia nutshells exposed to toluene for 21 days in the biofilter and control packing material not exposed to toluene, where: (a) outer surface, (b) inner surface, and (c) internal structures (shell rupture zones).
and quantitative biodegradability tests must be simultaneously performed to determine the deterioration of packing materials due to a continuous exposure to VOCs. 3.4. Scanning electron microscopy observations The release of co-substrates and the increase in packing material biodegradability after the 21-days exposure to toluene strongly suggested that the packing materials suffered structural damages, which might progressively result in a severe bed compaction, gas
flow channelling and a pressure drop build-up in the long-term biofilter operation (Kennes and Veiga, 2002). Therefore, SEM microphotographs of each packing material before and after exposure to toluene were taken at several magnifications in order to visualize the deterioration degree caused by the VOC. The deterioration extent of compost due to its exposure to toluene was difficult to assess since compost is a very heterogeneous matrix. Thus, in order to perform a fair comparison between the compost exposed to toluene and the control, the SEM analysis was performed on similar macroscopic structures present in both
R. Lebrero et al. / Journal of Environmental Management 137 (2014) 93e100
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Fig. 5. SEM microphotographs of wood bark exposed to toluene for 21 days in the biofilter and control packing material not exposed to toluene, where: (a) 400 magnification, (b) 1000 magnification and (c) 2000 magnification.
samples. Despite the complexity of the compost matrix, a clear deterioration of several compost elements such as soil particles, leaves and root-like structures was observed due to the exposure to toluene (Fig. 3). The relatively fast decomposition of compost as a packing material in biofilters and the subsequent deterioration of the bed structure have been widely reported in the literature (Iranpour et al., 2005; Ortiz et al., 2003). In fact, compost together with peat, coconut fibre and pine leaves have been classified among the supports with the shorter lifespan, supporting an estimated durability of two years (Dorado et al., 2010).
The SEM analysis of nutshells was performed in both the outer and inner surfaces as well as in the shell rupture zones in order to get a comprehensive evaluation of the packing material deterioration (Fig. 4). A clear deterioration of the outer surface was observed, which likely entailed the exposure of the internal material of the nutshell to the polluted air stream. Interestingly, the inner surface of the nutshells showed a less visible damage compared to that observed in the outer surface, the internal structure of the nutshell being still protected from the inner surface. These results clearly indicate that the material covering the
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outer surface of the nutshells was more susceptible to deterioration compared with the inner surface material. On the other hand, further analysis of the internal material visible from the shell rupture zones revealed that exposure to toluene did not damaged the internal structure of the material. These results reveal the potential of Macadamia nutshells as packing materials for biofiltration as a result of its high structural strength. Even when the outer surface suffered a visible damage, the internal material and structure remained unchanged after 21 days of exposure to toluene. This empirical finding is also in agreement with the biodegradability tests, which showed that nutshells were the less biodegradable material tested. The good structural characteristics of Macadamia nutshells have been recently highlighted by Volckaert et al. (2013). These authors used nutshells as a packing material in a biofilter treating ethylbenzene for 5 months with a maximum pressure drop of 0.85 kPa m1, which was significantly low compared with the pressure drop values of 1.4e20 kPa m1 typically recorded in biofilters operated with other organic packing materials during long periods of time (Estrada et al., 2013b). In the case of wood bark, it was difficult to assess the damage produced by the exposure to toluene with SEM at 400 magnifications or lower. However, the sharp edges observed at 1000 and 2000 magnifications in the control material contrasted with the polished structures observed in the bark exposed to toluene (Fig. 5). Particularly, at 2000 magnification it was observed that the bark surface was significantly deteriorated by the exposure to toluene. Wood bark is mainly composed of cellulose and lignin, and despite cellulose is the main component, lignin provides the hardness and rigidity to the material (Byrne and Nagle, 1997). According to the biodegradability tests, wood bark underwent the highest increase in biodegradability after its exposure to toluene, this deterioration being likely related to the solubility of lignin in toluene. It is well known that lignin dissolves better in solvents holding a Hildebrand solubility parameter of approximately 11 (cal/cc)1/2 (Sun et al., 2000), and toluene exhibits a very close value of 9.56 (cal/cc)1/2 (Belmares et al., 2004). Therefore, the deterioration of the wood bark could be attributed to a progressive lignin solubilization in the toluene absorbed in the bed, resulting in both material damage and increase of the packing material biodegradability. 4. Conclusions The VOC-packing material interaction noticeably contributed to the deterioration of the organic packing materials evaluated. The materials released co-substrates able to support microbial growth after exposure to toluene, which were not released by the controls. The exposure to toluene also resulted in an increase in the material biodegradability in the following order: wood bark > compost > nutshells. The SEM analysis confirmed the packing deterioration, Macadamia nutshell being the material with a higher resistance to VOC contact. The experimental protocol here proposed was successful for selecting organic packing materials less susceptible to deterioration by exposure to the target VOCs. Acknowledgements This research was supported by the Spanish Ministry of Economy and Competitiveness (Contracts JCI-2011-11009 and BES2010-030994 and Projects CTQ2012-34949 and CONSOLIDER-CSD 2007-00055). References Adams, T.B., Doull, J., Goodman, J.I., Munro, I.C., Newberne, P., Portoghese, P.S., Smith, R.L., Wagner, B.M., Weil, C.S., Woods, L.A., Ford, R.A., 1997. The FEMA
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