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Simultaneous detoxification and bioethanol fermentation of furans-rich synthetic hydrolysate by digestate-based pyrochar
Journal of Environmental Management 183 (2016) 1026e1031
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Research article
Simultaneous detoxification and bioethanol fermentation of furans-rich synthetic hydrolysate by digestate-based pyrochar C. Sambusiti a, *, F. Monlau a, b, N. Antoniou c, A. Zabaniotou c, A. Barakat a IATE, CIRAD, Montpellier SupAgro, INRA, Universit e de Montpellier, 34060, Montpellier, France APESA, Plateau Technique, Cap Ecologia, 64230 Lescar, France c Biomass Group, Chemical Engineering Department, Aristotle University of Thessaloniki, Un. Box 455, 54124 Thessaloniki, Greece a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 May 2016 Received in revised form 25 August 2016 Accepted 18 September 2016 Available online 28 September 2016
Pyrolysis is a sustainable pathway to transform renewable biomasses into both biofuels and advanced carbonaceous materials (i.e. pyrochar) which can be used as adsorbent of furan compounds. In particular, the aim of this study was to: i) evaluate the effect of vibro-ball milling on physical characteristics of pyrochar and its consequent performance on solely detoxification of a synthetic medium, containing furans and soluble sugars; ii) study the simultaneous detoxification and bioethanol fermentation, by adding activated pyrochar into fermentation medium. Results demonstrated that, compared to untreated pyrochar, the use of milled pyrochar increased by 52% furfural removal from the synthetic medium. Furfural removal rate was also increased (adsorption kinetic constant increased from 0.015 min1 up to 0.215 min1), at a pyrochar loading of 40 g L1. Although, the simultaneous addition of pyrochar into the fermentation medium did not improve the bioethanol yield of the synthetic medium, it has significantly increased the bioethanol production rate. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Bioethanol Digestate Furans Inhibition Pretreatment Pyrolysis
1. Introduction Renewable energies and biofuels production from lignocellulosic biomasses have attracted a growing attention within the research community, in order to face the worldwide energy crisis, to reduce the fossil fuels dependency and to overcome the greenhouse gas emissions. Among biofuels, 2nd generation bioethanol presents several advantages, as it can be used as a transport fuel to replace gasoline, but also as fuel for power generation in thermal combustion or in cogeneration systems (Taherzadeh and Karimi, 2008). To produce bioethanol from lignocellulosic feedstocks, a pretreatment step, permitting to hydrolyze structural carbohydrates (i.e. cellulose and hemicelluloses) into soluble sugars, is usually applied (Taherzadeh and Karimi, 2008; Klasson et al., 2011). Nonetheless, if the pretreatment conditions are too drastic, furan compounds (i.e. furfural, 5-HMF) are also generated and released
Abbreviations: 5-HMF, Hydroxymethylfurfural; HPLC, High Pressure Liquid Chromatography; HRT, Hydraulic Retention Time; OLR, Organic Loading Rate; SA, Accessible Surface Area; TS, Total Solids; Vmi, Micropore volume; Vme, Mesopore volume; Vtot, Total pore volume; VS, Volatile Solids. * Corresponding author. E-mail address: [email protected] (C. Sambusiti). http://dx.doi.org/10.1016/j.jenvman.2016.09.062 0301-4797/© 2016 Elsevier Ltd. All rights reserved.
into the lignocellulosic hydrolysate (Klasson et al., 2013; Monlau et al., 2014). Such compounds are known to be inhibitory for bioethanol fermentation, at concentrations below 1 g L1 (Delgenes et al., 1996; Mussatto and Roberto, 2004). To avoid this detrimental effect, several detoxification methods have been reported in literature, including adsorption on commercial active charcoal (Mussatto and Roberto, 2001; Mussatto and Roberto, 2004). As suggested by Klasson et al. (2013), sustainable and renewable lignocellulose-based activated carbons (pyrochars and their derivative activated carbons) can be used for detoxification. In particular, pyrochar, obtained from solid-anaerobic digestate pyrolysis, exhibits high accessible surface area and porosity that are key parameters for chemical adsorption mechanisms, further empowering environmental remediation systems (Monlau et al., 2015; Inyang et al., 2012). In our previous study, Monlau et al. (2015) highlighted the efficacy of the digestate pyrochar for removing furans (49 mg furans g1 pyrochar) from lignocellulosic hydrolysate, without affecting the initial concentration of soluble sugars. Consequently, digestate pyrochar seems to provide a sustainable alternative for the detoxification of lignocellulosic hydrolysates, prior to bioethanol fermentation. Nonetheless, detoxification yields obtained by digestate pyrochar still remain lower than those observed by using commercial coal-based
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activated carbons (Fatehi et al., 2013; Zhang, 2011). Thus, an activation step is generally required to improve its adsorption capacity (Tay et al., 2009). Among activation processes, the most applied are physical (steam) and/or chemical (i.e. H2SO4, H3PO4) activations (Zabaniotou et al., 2014; Antoniou et al., 2014). However, steam and/or chemical activations require high energy loads and chemical reagents, strongly affecting the economic viability and the environmental impact of the whole process. To overcome these barriers, Peterson et al. (2012) investigated the use of ball-milling treatment to enhance the physical properties (i.e. accessible surface area and porosity) of pyrochars and their adsorption capacity. To date, detoxification of lignocellulosic hydrolysate by using activated carbon has been found efficient, but it was always performed prior to bioethanol fermentation (Zhang, 2011; Alves et al., 1998). Moreover, such configuration options are complex and in most cases require additional expensive equipment, ultimately leading to an uneconomical biorefinery chain (Klasson et al., 2013). Thus, the main objectives of this study were the following: (i) Evaluation of the effect of mechanical treatment (i.e. vibroball milling), used as activation process, on the physical characteristics of pyrochar and its consequent performance on solely detoxification of a synthetic medium containing furfural. (ii) Study the simultaneous detoxification and fermentation by adding milled-activated pyrochar in the fermentation medium together with the yeast strain.
2. Materials and methods 2.1. Feedstocks materials Solid digestate, used for pyrochar production, was collected from a mesophilic full-scale biogas plant located in Southern Italy (Puglia region). The characteristic of the anaerobic digester was: digester volume of 3720 m3, Organic Loading Rate (OLR) of 56 t FM d1and Hydraulic Retention Time (HRT) of 62 days. Once collected, digestate was dried overnight at 105 C, prior to pyrolysis. Pyrochars were produced through a pyrolysis process at 500 C, according to the process previously described by Monlau et al. (2015). Pyrochar was dried overnight at 105 C prior to mechanical treatment (i.e. vibro-ball milling). 2.2. Mechanical activation During this process, 1 g of raw pyrochar was milled by a vibratory ball mill “Retsch MM400, Haan, Germany”, composed of two steel balls (i.e., 62.5:1 ball-to-raw pyrochar mass ratio) and operated at ambient temperature, at a frequency of 20 s1, for 5 min (VBM-5) and 20 min (VBM-20), respectively. 2.3. Analytical determination Untreated and milled pyrochars were analyzed in terms of Total solids (TS), Volatile Solids (VS) and ash content, according to APHA standard methods (APHA, 2005). Ultimate analysis (C, H, N) was accomplished with an elemental analyzer (ELEMENTAR VARIOMACROCUBE CHNS, France). Accessible surface area (SA) was determined by N2 adsorptiondesorption method that was performed with a 3Flex Surface characterization analyzer at 77 K. Prior to adsorption measurements, samples were outgassed at 373 K under vacuum overnight. Specific surface area of the samples was evaluated using the BrunauerEmmettTeller (BET) method (Brunauer et al.,1938). Pore size distributions were calculated from
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the N2 adsorption isotherms with the ‘‘classic theoretical model’’ of Barret, Joyney and Halenda (BJH). 2.4. Adsorption experiments Detoxification performances of untreated and milled pyrochars were tested on a synthetic medium, containing 5 g L1 of glucose, 5 g L1 of xylose, 3 g L1 of furfural. It is noteworthy that only furfural was considered as potential fermentation inhibitor during adsorption experiments, since it is generally present in major quantity into real lignocellulosic hydrolysates. Adsorption tests were firstly realized during 6 h without agitation, at pyrochar concentrations varying from 20 g L1 to 60 g L1, according to previous results reported by Monlau et al. (2015). Then, the effect of contact time (varying from 0 to 360 min) on the adsorption capacity of the pyrochar, at a fixed concentration of 40 g L1, was studied. It is noteworthy that adsorption tests were realized in duplicate for all conditions tested. Soluble sugars (i.e. glucose, xylose) and furfural concentrations were analyzed by high-pressure liquid chromatography (HPLC) system (Waters corporation, Milford, USA), equipped with an Aminex HPX-87H (Biorad, Marnes-la-Coquette, France) column at 40 C, a refractive index detector at 40 C and a 0.005 M H2SO4 solvent at 0.3 mL min1. Glucose, xylose and furfural standards were provided by Sigma Aldrich®. To quantify and compare the adsorption kinetics of furfural onto the pyrochar, a first order kinetic model was plotted by using leastsquares fit of furans removal data during time (t), according to the following equation:
RðtÞ ¼ Rmax 1 ekt
(1)
Where: R(t) (%) is the furans removal at time t(d), Rmax (%) is the maximum furans removal and k (min1) is the adsorption kinetic constant. 2.5. Bioethanol fermentation Bioethanol fermentation tests were performed by using glass flasks (working volume of 5.5 mL) closed with rubber septa and equipped with an air vent system, constituted of sterilized needle and filter, in order to evacuate the CO2 produced during the bioconversion. A synthetic medium (8 g L1 of glucose, 2.5 g L1 of furfural, 1 g L1 of acetic acid and 0.5 g L1 of 5-HMF), which simulates the composition of a lignocellulosic hydrolysate, was chosen as substrate (Monlau et al., 2014). It is noteworthy that only glucose was considered as carbon source for bioethanol fermentation, as xylose is not converted into bioethanol by the S. cerevisae yeast strain and it is not adsorbed onto activated carbon during the detoxification step. Two lyophilized Saccharomyces cerevisiae strains (“Lalvin ICV-K1”, INRA Montpellier, France and “Ethanol Red®”, FERMENTIS, a division of S. I. LESAFFRE, Lille, France) were used as inoculums. Lyophilized yeast cells were previously washed with sterilized distilled water and then suspended in sterilized distilled water to a concentration of 30 g TS L1. Ethanol tests were realized in the presence or not of mechanically treated pyrochar (VBM-20) at an initial solid loading of 40 g L1. In addition (or not) of pyrochar, each flask contained 4.75 mL of synthetic medium, 0.25 mL of yeast solution and 0.5 mL of nutrients solution (composed of 50 gTS L1 yeast extract (Difco), 4 g TS L1 urea and 0.5 g TS L1 chloramphenicol). Finally, Methyl Ester Sulfonates buffer (MES) was added to obtain a final concentration of 50 mM MES buffer and the pH was adjusted to 5 by adding HCl 2N. It is noteworthy that fermentation tests were realized in duplicate for all conditions tested. Flasks were incubated at 37 C for 72 h with
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stirring at 500 rpm. Samples (200 mL each) were withdrawn at 0, 2, 6, 24, 48 and 72 h and the cell free supernatant was evaluated for ethanol, glucose, furfural, 5-HMF and acetic acid concentrations by a HPLC system (Waters corporation, Milford, USA), equipped with a BioRad HPX-87H column at 40 C, a refractive index detector at 40 C and a 0.005 M H2SO4 solvent at 0.3 mL min1. Glucose, acetic acid, 5-HMF, furfural and ethanol standards were provided by Sigma Aldrich®. 3. Results and discussion 3.1. Physico-chemical characteristics of pyrochars Table 1 shows the main physico-chemical characteristics of untreated and milled pyrochars. Untreated pyrochar had TS, VS and ash contents of 93, 67 and 21 g 100 g1TS, respectively. Pyrochars were mainly composed of carbon with a content of 58.8 g 100 g1TS for untreated pyrochar. Similar carbon contents have been previously reported in literature, with values ranging from 63.5 to 73.5 g 100 g1TS (Inyang et al., 2010, 2012; Sun et al., 2013). In general, the carbon content is strongly dependent on the composition of precursor material, along with the operational parameters of the pyrolysis process (Pituello et al., 2015; Stefaniuk and Oleszczuk, 2015; Enders et al., 2012). The concentrations of C, H and N were poorly affected by the mechanical treatment, while this process affected primarily the physical parameters like the accessible surface area Table 1 Main physico-chemical characteristics of untreated and mechanically-treated pyrochars. Values correspond to mean ± SD of measurement performed in duplicate.
TS (g 100 g1FM) VS (g 100 g1TS) Ash (g 100 g1TS) C (g 100 g1TS) H (g 100 g1TS) N (g 100 g1TS) SA (m2 g1) Vtot (cm3 g1) Vmi (cm3 g1) Vme (cm3 g1) n.d. ¼ not determined.
Pyrochar
Pyrochar VBM-5
Pyrochar VBM-20
93(±0.11) 67(±0.25) 21(±0.52) 58.73 (±0.06) 1.62 (±0.01) 1.60 (±0.03) 38 0.007 0.004 0.003
n.d. n.d. n.d. 57.74 (±0.10) 1.94 (±0.17) 1.53 (±0.04) 49 0.011 0.009 0.002
n.d. n.d. n.d. 57.32 (±0.07) 2.08 (±0.01) 1.57 (±0.03) 64 0.052 0.016 0.036
and the porosity, as previously reported by Barakat et al. (2013). In particular, vibro ball-milling treatment increased the accessible surface area of pyrochars up to 49 m2g1 and 64 m2g1 for VBM-5 and VBM- 20, respectively. The improvement of the accessible surface area and porosity after a ball milling pretreatment have been previously reported in literature (Peterson et al., 2012; Barakat et al., 2013). For instance, Peterson et al. (2013) investigated both the accessible surface area and micropores of two ball-milled pyrochars, obtained after the pyrolysis of switchgrass and corn stover. They reported accessible surface areas of 46 m2 g1 and 74 m2 g1 for switchgrass and corn stover pyrochars, respectively. The micropore surface areas were 14 and 23 m2 g1 for switchgrass and corn stover pyrochars, respectively. Similarly, Peterson et al. (2012) investigated the impact of planetary ball milling on pyrochar (i.e. from corn stover feedstock) characteristics. They reported accessible surface areas of 3 and 40e70 m2 g1 for untreated and ball milled pyrochars, respectively. As reported in Table 1 both untreated and VBM-pyrochar samples were composed of micropores and mesopores, while macropores were absent, as previously observed by Monlau et al. (2015). Interestingly, both micropore and mesopore volumes increased after VBM, and especially at VBM-20. These results are in accordance with those reported by Peterson et al. (2012), who found that untreated pyrochar did not exhibit any measurable micropore surface area, whereas ball milled pyrochars exhibited micropore surface areas from 8 to 53 m2 g1. 3.2. Detoxification of the synthetic medium Different pyrochar concentrations and adsorption times were investigated on soluble sugars (data not shown) and furfural (Figs. 1 and 2) removal efficiency. Fig. 1 clearly displays that the furfural removal efficiency increased with the increase of the pyrochar concentration (from 20 to 60 g L1). Such phenomenon was noticed for both untreated and ball-milled (VBM-5, VBM-20) pyrochars. Interestingly, milled pyrochars (i.e. VBM-5; VBM-20) showed better performance than untreated pyrochar for furfural removal in the synthetic medium for all concentrations tested. Indeed, lower furfural removal efficiencies were noticed for untreated pyrochar, compared to milled pyrochar, with values ranging from 35% to 66%, at solid loading of 20 g L1 and 60 g L1, respectively. In particular,
Fig. 1. Effect of untreated and milled pyrochar concentrations (20, 40, 60 g L1) on furfural removal efficiency at fixed adsorption time (6 h). Values correspond to mean ± SD of measurement performed in duplicate.
Furfural removal efficiency (%)
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100 80 60 Untreated
40
VBM-5 20
VBM-20
0 0
60
120
180 240 Time (min)
300
360
Fig. 2. Effect of adsorption time on furfural removal efficiency for untreated and pretreated pyrochars. Values correspond to mean ± SD of measurement performed in duplicate.
at pyrochar concentration of 40 g L1, furfural removal efficiency was improved by 31% and 52%, for pyrochar VBM-5 and VBM-20, respectively, compared to untreated pyrochar. These improved
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efficiencies can be attributed to the increase of the surface areas and also to the degrees of microporosity previously observed, as a direct result of the vibro-ball milling treatment, thus empowering their adsorption potentials towards environmental remediation applications (Peterson et al., 2012). At untreated pyrochar concentration of 40 g L1, an adsorption capacity of 42 mg furfural g1 pyrochar was observed (data not shown), which is in accordance with that reported by Monlau et al. (2015) who found an adsorption capacity of 49 mg furfural g1 pyrochar, by using similar pyrochar. The slight difference between the two values, can be attributed to both the presence of agitation during adsorption tests, and to the higher pyrolysis temperature applied (600 C) in the previous study, responsible for higher surface area, compared to this study (49 m2g1) (Monlau et al., 2015). Furthermore, the physico-chemical structure of pyrochar, which is considered dependent of pyrolysis conditions, could also affect furans removal (Wang et al., 2015; Enders et al., 2012). Interestingly, adsorption capacities were improved by using mechanically treated pyrochars, with 55 mg furfural g1 pyrochar and 64 mg furfural g1 pyrochar for pyrochar VBM-5 and VBM-20,
Fig. 3. Simultaneous detoxification and bioethanol fermentation performed in presence or not of VBM-20 pyrochar, with “ Ethanol Red®” (S1) and “Lalvin ICV-K1” (S2) yeast strains, respectively. Values correspond to mean ± SD of measurement performed in duplicate.
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respectively. However, despite vibro-ball milling treatment improved the adsorption capacities of furfural compounds, values still remained lower than those observed in literature (120 mg furfural g1AC) using a steam activated carbon from biomass-based precursors (Klasson et al., 2011). Interestingly, for all assays, no soluble sugars (i.e glucose, xylose) removal was observed, as reported in our previous study (Monlau et al., 2015). The mechanism of selectivity towards furan compounds was already reported and detailed in literature (Li et al., 2013; Zhang et al., 2011; Monlau et al., 2015; Miyafuji et al., 2003). The effect of contact time (studied in the range from 0 to 360 min) on furfural removal from the synthetic medium was investigated for both untreated and mechanically treated (VBM-20, VBM-5) pyrochars at a solid loading of 40 g L¡1 (Fig. 2). It was observed that furfural removal increased with the increase of contact time in accordance with the results reported by Monlau et al. (2015). The furfural removal efficiency of pyrochars in the synthetic medium was simulated by a first order kinetic model. For all assays, the model was found efficient (R2 > 0.97) in simulating the removal efficiency, according to the adsorption time. A kinetic constant of 0.015 (min1) was observed for the test with untreated pyrochar. Interestingly, the kinetic constants of the obtained mechanical treated pyrochars were significantly increased (0.092 and 0.215 min1 for pyrochars “VBM-5” and “VBM-20”, respectively). Such increase is attributed to the increase of both accessible surface area (from 38 to 64 m2 g1) and micropores volume (from 0.004 to 0.016 cm3 g1) caused by the vibro-ball milling treatment. This would improve the adsorption sites of the pyrochar and its further adsorption capacity (Peterson et al., 2012). According to these results, VBM-20 pyrochar was chosen for the prosecution of the experimentation. 3.3. Simultaneous detoxification and bioethanol fermentation Results of bioethanol fermentation from the synthetic medium in presence or not of VBM-20 pyrochar and with “Ethanol Red®” (S1) and “Lalvin ICV-K1” (S2) yeast strains are presented in Fig. 3. Results revealed that, for both un-detoxified and detoxified synthetic media, the yeasts quickly consumed glucose after inoculation and the totality of bioethanol was produced during the first 24 h in all fermentations. Acetic acid concentrations remained constant during bioethanol fermentation tests (data not shown), confirming the low adsorption efficiencies (less than 35%) obtained by using pyrochar (Monlau et al., 2015). Furfural and 5-HMF adsorptions occurred quickly in presence of VBM-20 pyrochar. However, it was observed that in absence of VBM-20 pyrochar, a reduction of furfural and 5-HMF concentration occurred, suggesting that furans were converted by the yeast strains into less toxic compounds, such as furfuryl and 5-hydroxymethylfurfuryl alcohols (data not shown), as previously observed by other authors (Liu et al., 2005; Wallace-Salinas and Gorwa-Grauslund, 2013). The furans conversion into less toxic alcohols could explain similar bioethanol yields at time 24 h, from both un-detoxified and detoxified synthetic media. Fig. 4 shows the bioethanol production rate in function of the time, in presence or not of VBM-20 pyrochar and with “Ethanol Red®” (S1) and “Lalvin ICV-K1” (S2) yeast strains, respectively. Interestingly, the addition of VBM-20 pyrochar favored the bioethanol production rate, which was more pronounced in presence of the “Ethanol Red®” yeast strain. Such improvement could be partially explained by the fact that this yeast strain is known to be developed for its high tolerance towards the inhibitors released during biomass pretreatment (Wallace-Salinas and GorwaGrauslund, 2013). Additionally, this could be justified by the instant adsorption (in few minutes) of furfural onto the VBM-20
3h Eth-S2 pyrochar
24h
62
Eth-S2
Eth-S1 pyrochar
7h
37
52
0
20
24
40
60
1
25
80
97
Eth-S1
3
36
0
20
100
64
40
60
80
100
Bioethanol production rate (%) Fig. 4. Bioethanol production rate (%) according to the fermentation time (3h, 7h and 24 h), in presence or not of VBM-20 pyrochar, with “Ethanol Red®” (S1) and “Lalvin ICV-K1” (S2) yeast strains, respectively.
pyrochar, thus permitting to reduce the toxicity of the medium. Similarly, Klasson et al. (2013) reported an increase of the bioethanol production rate, during simultaneous detoxification and fermentation of a weak acid hydrolysate, by using steam-activated pyrochars produced from pyrolysis of flax shaves at 700 C for 1 h. The results of this study appear promising because although the addition of pyrochar did not enhance the final bioethanol yield, it improved the bioethanol production rate which can probably decrease both the hydraulic retention time and the reactor volume in a commercial full-scale fermentation plant. Nevertheless, further research, especially with real lignocellulosic hydrolysates at a continuous reactor scale would be needed to draw final conclusions on the feasibility and the economic advantage of the process. 4. Conclusion Vibro-ball milling increased both the accessible surface area and the porosity of pyrochar, leading to a better adsorption capacity of furfural and 5-HMF present in the synthetic medium. No increase in bioethanol yield from the synthetic medium was observed during simultaneous detoxification and bioethanol fermentation, as the yeast strains were able to convert furfural and 5-HMF into less toxic alcohols. Conversely, the simultaneous addition of pyrochar into the fermentation medium improved the bioethanol production rate, more pronounced in presence of the “Ethanol Red®” yeast strain, partially due to the instant adsorption of furfural and 5-HMF onto pyrochars. Acknowledgment Authors are grateful to Dr. Abderrahim Solhy (Mohammed VI Polytechnic University, Morocco) for his help in performing accessible surface area and porosity analyses. Authors are also grateful to the owner of the anaerobic digestion plant for providing digestate samples.
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References Alves, L.A., Felipe, M.G.A., Silva, J.B.A.E., Silva, S.S., Prata, A.M.R., 1998. Pretreatment of sugarcane bagasse hemicellulose hydrolysate for xylitol production by Candida guilliermondii. Appl. Biochem. Biotechnol. 70e72, 89e98. Antoniou, N., Stavropoulos, G., Zabaniotou, A., 2014. Activation of end of life tyres pyrolytic char for enhancing viability of pyrolysis e critical review, analysis and recommendations for a hybrid dual system. Renew. Sust. Energy. Rev. 39, 1053e1073. APHA, 2005. Standard Methods for the Examination of Water and Wastewater, 21th ed. American Public Health Association, Washington DC, USA. Brunauer, S., Emmett, P.H., Teller, E., 1938. Adsorption of gases in multimolecular layers. J. Am. Chem. Soc. 60, 301e319. Barakat, A., De Vries, H., Rouau, X., 2013. Dry fractionation process as an important step in current and future lignocellulose biorefineries: a review. Bioresour. Technol. 134, 362e373. Delgenes, J.P., Moletta, R., Navarro, J.M., 1996. Effects of lignocellulose degradation products on ethanol fermentations of glucose and xylose by Saccharomyces cerevisiae, Zymomonas mobilis, Pichiastipitis, and Candida shehatae. Enzyme. Microb. Tech. 19, 220e225. Enders, A., Hanley, K., Whitman, T., Joseph, S., Lehmann, J., 2012. Characterization of biochars to evaluate recalcitrance and agronomic performance. Bioresour. Technol. 114 (0), 644e653. Fatehi, P., Ryan, J., Ni, Y., 2013. Adsorption of lignocelluloses of model pre-hydrolysis liquor on activated carbon. Bioresour. Technol. 131, 308e314. Inyang, M., Gao, B., Pullammanappallil, P., Ding, W., Zimmerman, A.R., 2010. Biochar from anaerobically digested sugarcane bagasse. Bioresour. Technol. 101, 8868e8872. Inyang, M., Gao, B., Yao, Y., Xue, Y., Zimmerman, A.R., Pullammanappallil, P., Cao, X., 2012. Removal of heavy metals from aqueous solution by biochars derived from anaerobically digested biomass. Bioresour. Technol. 110, 50e56. Klasson, K.T., Dien, B.S., Hector, R.E., 2013. Simultaneous detoxification, saccharification, and ethanol fermentation of weak-acid hydrolyzates. Ind. Crop. Prod. 49, 292e298. Klasson, K.T., Uchimiya, M., Lima, I.M., Boihem, L.L., 2011. Feasibility of removing furfurals from sugar solutions using activated biochars made from agricultural residues. Bioresour. Com. 6 (3), 3242e3251. Li, Y., Shao, J., Wang, X., Yang, H., Chen, Y., Deng, Y., Zhang, S., Chen, H., 2013. Upgrading of bio-oil: removal of the fermentation inhibitor (furfural) from the model compounds of bio-oil using pyrolytic char. Energy. Fuels 27 (10), 5975e5981. Liu, Z.L., Slininger, P.J., Gorsich, S.W., 2005. Enhanced biotransformation of furfural and hydroxymethylfurfural by newly developed ethanologenic yeast strains. Appl. Biochem. Biotechnol. 121e124, 451e460. Miyafuji, H., Danner, H., Neureiter, M., Thomasser, C., Bvochora, J., Szolar, O., Braun, R., 2003. Detoxification of wood hydrolysates with wood charcoal for increasing the fermentability of hydrolysates. Enzyme. Microbiol. Technol. 32,
1031
396e400. Monlau, F., Sambusiti, C., Antoniou, N., Zabaniotou, A., Solhy, A., Barakat, A., 2015. Pyrochars from bioenergy residue as novel bio-adsorbents for lignocellulosic hydrolysate detoxification. Bioresour. Technol. 187, 379e386. me neur, M., Trably, E., Steyer, J.P., Monlau, F., Sambusiti, C., Barakat, A., Que Carrere, H., 2014. Do furanic and phenolic compounds of lignocellulosic and algae biomass hydrolysate inhibit anaerobic mixed cultures? A comprehensive review. Biotechnol. Adv. 32, 934e951. Mussatto, S.I., Roberto, I.C., 2001. Hydrolysate detoxification with activated charcoal for xylitol production by Candida guilliermondii. Biotechnol. Lett. 23 (20), 1681e1684. Mussatto, S.I., Roberto, I.C., 2004. Alternatives for detoxification of diluted-acid lignocellulosic hydrolyzates for use in fermentative processes: a review. Bioresour. Technol. 93 (1), 1e10. Peterson, S.C., Appell, M., Jackson, M.A., Boateng, A.A., 2013. Comparing corn stover and switchgrass biochar: characterization and sorption properties. J. Agric. Sci. 5 (1). Peterson, S.C., Jackson, M.A., Kim, S., Palmquist, D.E., 2012. Increasing biochar surface area: optimization of ball milling parameters. Powder. Technol. 228, 115e120. Pituello, C., Francioso, O., Simonetti, G., Pisi, A., Torreggiani, A., Berti, A., Morari, F., 2015. Characterization of chemicalephysical, structural and morphological properties of biochars from biowastes produced at different temperatures. J. Soils. Sediments 15 (4), 792e804. Stefaniuk, M., Oleszczuk, P., 2015. Characterization of biochars produced from residues from biogas production. J. Anal. Appl. Pyrol 115, 157e165. Sun, L., Wan, S.G., Luo, W.S., 2013. Biochars prepared from anaerobic digestion residue, palm bark, and eucalyptus for adsorption of cationic methylene blue dye: characterization, equilibrium, and kinetic studies. Bioresour. Technol. 140, 406e413. Taherzadeh, M.J., Karimi, K., 2008. Pretreatment of lignocellulosic wastes to improve ethanol and biogas production: a review. Int. J. Mol. Sci. 9 (9), 1621e1651. €z, S., 2009. Preparation and characterization of activated Tay, T., Uçar, S., Karago carbon from waste biomass. J. Hazard. Mater 165, 481e485. Wallace-Salinas, V., Gorwa-Grauslund, M.F., 2013. Adaptive evolution of an industrial strain of Saccharomyces cerevisiae for combined tolerance to inhibitors and temperature. Biotechnol. Biofuels 6, 151. Wang, S., Gao, B., Zimmerman, A.R., Li, Y., Ma, L., Harris, W.G., Migliaccio, K.W., 2015. Physicochemical and sorptive properties of biochars derived from woody and herbaceous biomass. Chemosphere 134 (0), 257e262. Zabaniotou, A.A., Antoniou, N., Stavropoulos, G., 2014. Novel sorbent materials for environmental remediation via depolymerization of used tyres. Desalin. Water. Treat. 1e10. Zhang, K., 2011. Removal of the Fermentation Inhibitor, Furfural, Using Activated Carbon in Cellulosic Ethanol Production [PhD Thesis]. Georgia Institute of Technology.
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Research article
Simultaneous detoxification and bioethanol fermentation of furans-rich synthetic hydrolysate by digestate-based pyrochar C. Sambusiti a, *, F. Monlau a, b, N. Antoniou c, A. Zabaniotou c, A. Barakat a IATE, CIRAD, Montpellier SupAgro, INRA, Universit e de Montpellier, 34060, Montpellier, France APESA, Plateau Technique, Cap Ecologia, 64230 Lescar, France c Biomass Group, Chemical Engineering Department, Aristotle University of Thessaloniki, Un. Box 455, 54124 Thessaloniki, Greece a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 May 2016 Received in revised form 25 August 2016 Accepted 18 September 2016 Available online 28 September 2016
Pyrolysis is a sustainable pathway to transform renewable biomasses into both biofuels and advanced carbonaceous materials (i.e. pyrochar) which can be used as adsorbent of furan compounds. In particular, the aim of this study was to: i) evaluate the effect of vibro-ball milling on physical characteristics of pyrochar and its consequent performance on solely detoxification of a synthetic medium, containing furans and soluble sugars; ii) study the simultaneous detoxification and bioethanol fermentation, by adding activated pyrochar into fermentation medium. Results demonstrated that, compared to untreated pyrochar, the use of milled pyrochar increased by 52% furfural removal from the synthetic medium. Furfural removal rate was also increased (adsorption kinetic constant increased from 0.015 min1 up to 0.215 min1), at a pyrochar loading of 40 g L1. Although, the simultaneous addition of pyrochar into the fermentation medium did not improve the bioethanol yield of the synthetic medium, it has significantly increased the bioethanol production rate. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Bioethanol Digestate Furans Inhibition Pretreatment Pyrolysis
1. Introduction Renewable energies and biofuels production from lignocellulosic biomasses have attracted a growing attention within the research community, in order to face the worldwide energy crisis, to reduce the fossil fuels dependency and to overcome the greenhouse gas emissions. Among biofuels, 2nd generation bioethanol presents several advantages, as it can be used as a transport fuel to replace gasoline, but also as fuel for power generation in thermal combustion or in cogeneration systems (Taherzadeh and Karimi, 2008). To produce bioethanol from lignocellulosic feedstocks, a pretreatment step, permitting to hydrolyze structural carbohydrates (i.e. cellulose and hemicelluloses) into soluble sugars, is usually applied (Taherzadeh and Karimi, 2008; Klasson et al., 2011). Nonetheless, if the pretreatment conditions are too drastic, furan compounds (i.e. furfural, 5-HMF) are also generated and released
Abbreviations: 5-HMF, Hydroxymethylfurfural; HPLC, High Pressure Liquid Chromatography; HRT, Hydraulic Retention Time; OLR, Organic Loading Rate; SA, Accessible Surface Area; TS, Total Solids; Vmi, Micropore volume; Vme, Mesopore volume; Vtot, Total pore volume; VS, Volatile Solids. * Corresponding author. E-mail address: [email protected] (C. Sambusiti). http://dx.doi.org/10.1016/j.jenvman.2016.09.062 0301-4797/© 2016 Elsevier Ltd. All rights reserved.
into the lignocellulosic hydrolysate (Klasson et al., 2013; Monlau et al., 2014). Such compounds are known to be inhibitory for bioethanol fermentation, at concentrations below 1 g L1 (Delgenes et al., 1996; Mussatto and Roberto, 2004). To avoid this detrimental effect, several detoxification methods have been reported in literature, including adsorption on commercial active charcoal (Mussatto and Roberto, 2001; Mussatto and Roberto, 2004). As suggested by Klasson et al. (2013), sustainable and renewable lignocellulose-based activated carbons (pyrochars and their derivative activated carbons) can be used for detoxification. In particular, pyrochar, obtained from solid-anaerobic digestate pyrolysis, exhibits high accessible surface area and porosity that are key parameters for chemical adsorption mechanisms, further empowering environmental remediation systems (Monlau et al., 2015; Inyang et al., 2012). In our previous study, Monlau et al. (2015) highlighted the efficacy of the digestate pyrochar for removing furans (49 mg furans g1 pyrochar) from lignocellulosic hydrolysate, without affecting the initial concentration of soluble sugars. Consequently, digestate pyrochar seems to provide a sustainable alternative for the detoxification of lignocellulosic hydrolysates, prior to bioethanol fermentation. Nonetheless, detoxification yields obtained by digestate pyrochar still remain lower than those observed by using commercial coal-based
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activated carbons (Fatehi et al., 2013; Zhang, 2011). Thus, an activation step is generally required to improve its adsorption capacity (Tay et al., 2009). Among activation processes, the most applied are physical (steam) and/or chemical (i.e. H2SO4, H3PO4) activations (Zabaniotou et al., 2014; Antoniou et al., 2014). However, steam and/or chemical activations require high energy loads and chemical reagents, strongly affecting the economic viability and the environmental impact of the whole process. To overcome these barriers, Peterson et al. (2012) investigated the use of ball-milling treatment to enhance the physical properties (i.e. accessible surface area and porosity) of pyrochars and their adsorption capacity. To date, detoxification of lignocellulosic hydrolysate by using activated carbon has been found efficient, but it was always performed prior to bioethanol fermentation (Zhang, 2011; Alves et al., 1998). Moreover, such configuration options are complex and in most cases require additional expensive equipment, ultimately leading to an uneconomical biorefinery chain (Klasson et al., 2013). Thus, the main objectives of this study were the following: (i) Evaluation of the effect of mechanical treatment (i.e. vibroball milling), used as activation process, on the physical characteristics of pyrochar and its consequent performance on solely detoxification of a synthetic medium containing furfural. (ii) Study the simultaneous detoxification and fermentation by adding milled-activated pyrochar in the fermentation medium together with the yeast strain.
2. Materials and methods 2.1. Feedstocks materials Solid digestate, used for pyrochar production, was collected from a mesophilic full-scale biogas plant located in Southern Italy (Puglia region). The characteristic of the anaerobic digester was: digester volume of 3720 m3, Organic Loading Rate (OLR) of 56 t FM d1and Hydraulic Retention Time (HRT) of 62 days. Once collected, digestate was dried overnight at 105 C, prior to pyrolysis. Pyrochars were produced through a pyrolysis process at 500 C, according to the process previously described by Monlau et al. (2015). Pyrochar was dried overnight at 105 C prior to mechanical treatment (i.e. vibro-ball milling). 2.2. Mechanical activation During this process, 1 g of raw pyrochar was milled by a vibratory ball mill “Retsch MM400, Haan, Germany”, composed of two steel balls (i.e., 62.5:1 ball-to-raw pyrochar mass ratio) and operated at ambient temperature, at a frequency of 20 s1, for 5 min (VBM-5) and 20 min (VBM-20), respectively. 2.3. Analytical determination Untreated and milled pyrochars were analyzed in terms of Total solids (TS), Volatile Solids (VS) and ash content, according to APHA standard methods (APHA, 2005). Ultimate analysis (C, H, N) was accomplished with an elemental analyzer (ELEMENTAR VARIOMACROCUBE CHNS, France). Accessible surface area (SA) was determined by N2 adsorptiondesorption method that was performed with a 3Flex Surface characterization analyzer at 77 K. Prior to adsorption measurements, samples were outgassed at 373 K under vacuum overnight. Specific surface area of the samples was evaluated using the BrunauerEmmettTeller (BET) method (Brunauer et al.,1938). Pore size distributions were calculated from
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the N2 adsorption isotherms with the ‘‘classic theoretical model’’ of Barret, Joyney and Halenda (BJH). 2.4. Adsorption experiments Detoxification performances of untreated and milled pyrochars were tested on a synthetic medium, containing 5 g L1 of glucose, 5 g L1 of xylose, 3 g L1 of furfural. It is noteworthy that only furfural was considered as potential fermentation inhibitor during adsorption experiments, since it is generally present in major quantity into real lignocellulosic hydrolysates. Adsorption tests were firstly realized during 6 h without agitation, at pyrochar concentrations varying from 20 g L1 to 60 g L1, according to previous results reported by Monlau et al. (2015). Then, the effect of contact time (varying from 0 to 360 min) on the adsorption capacity of the pyrochar, at a fixed concentration of 40 g L1, was studied. It is noteworthy that adsorption tests were realized in duplicate for all conditions tested. Soluble sugars (i.e. glucose, xylose) and furfural concentrations were analyzed by high-pressure liquid chromatography (HPLC) system (Waters corporation, Milford, USA), equipped with an Aminex HPX-87H (Biorad, Marnes-la-Coquette, France) column at 40 C, a refractive index detector at 40 C and a 0.005 M H2SO4 solvent at 0.3 mL min1. Glucose, xylose and furfural standards were provided by Sigma Aldrich®. To quantify and compare the adsorption kinetics of furfural onto the pyrochar, a first order kinetic model was plotted by using leastsquares fit of furans removal data during time (t), according to the following equation:
RðtÞ ¼ Rmax 1 ekt
(1)
Where: R(t) (%) is the furans removal at time t(d), Rmax (%) is the maximum furans removal and k (min1) is the adsorption kinetic constant. 2.5. Bioethanol fermentation Bioethanol fermentation tests were performed by using glass flasks (working volume of 5.5 mL) closed with rubber septa and equipped with an air vent system, constituted of sterilized needle and filter, in order to evacuate the CO2 produced during the bioconversion. A synthetic medium (8 g L1 of glucose, 2.5 g L1 of furfural, 1 g L1 of acetic acid and 0.5 g L1 of 5-HMF), which simulates the composition of a lignocellulosic hydrolysate, was chosen as substrate (Monlau et al., 2014). It is noteworthy that only glucose was considered as carbon source for bioethanol fermentation, as xylose is not converted into bioethanol by the S. cerevisae yeast strain and it is not adsorbed onto activated carbon during the detoxification step. Two lyophilized Saccharomyces cerevisiae strains (“Lalvin ICV-K1”, INRA Montpellier, France and “Ethanol Red®”, FERMENTIS, a division of S. I. LESAFFRE, Lille, France) were used as inoculums. Lyophilized yeast cells were previously washed with sterilized distilled water and then suspended in sterilized distilled water to a concentration of 30 g TS L1. Ethanol tests were realized in the presence or not of mechanically treated pyrochar (VBM-20) at an initial solid loading of 40 g L1. In addition (or not) of pyrochar, each flask contained 4.75 mL of synthetic medium, 0.25 mL of yeast solution and 0.5 mL of nutrients solution (composed of 50 gTS L1 yeast extract (Difco), 4 g TS L1 urea and 0.5 g TS L1 chloramphenicol). Finally, Methyl Ester Sulfonates buffer (MES) was added to obtain a final concentration of 50 mM MES buffer and the pH was adjusted to 5 by adding HCl 2N. It is noteworthy that fermentation tests were realized in duplicate for all conditions tested. Flasks were incubated at 37 C for 72 h with
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stirring at 500 rpm. Samples (200 mL each) were withdrawn at 0, 2, 6, 24, 48 and 72 h and the cell free supernatant was evaluated for ethanol, glucose, furfural, 5-HMF and acetic acid concentrations by a HPLC system (Waters corporation, Milford, USA), equipped with a BioRad HPX-87H column at 40 C, a refractive index detector at 40 C and a 0.005 M H2SO4 solvent at 0.3 mL min1. Glucose, acetic acid, 5-HMF, furfural and ethanol standards were provided by Sigma Aldrich®. 3. Results and discussion 3.1. Physico-chemical characteristics of pyrochars Table 1 shows the main physico-chemical characteristics of untreated and milled pyrochars. Untreated pyrochar had TS, VS and ash contents of 93, 67 and 21 g 100 g1TS, respectively. Pyrochars were mainly composed of carbon with a content of 58.8 g 100 g1TS for untreated pyrochar. Similar carbon contents have been previously reported in literature, with values ranging from 63.5 to 73.5 g 100 g1TS (Inyang et al., 2010, 2012; Sun et al., 2013). In general, the carbon content is strongly dependent on the composition of precursor material, along with the operational parameters of the pyrolysis process (Pituello et al., 2015; Stefaniuk and Oleszczuk, 2015; Enders et al., 2012). The concentrations of C, H and N were poorly affected by the mechanical treatment, while this process affected primarily the physical parameters like the accessible surface area Table 1 Main physico-chemical characteristics of untreated and mechanically-treated pyrochars. Values correspond to mean ± SD of measurement performed in duplicate.
TS (g 100 g1FM) VS (g 100 g1TS) Ash (g 100 g1TS) C (g 100 g1TS) H (g 100 g1TS) N (g 100 g1TS) SA (m2 g1) Vtot (cm3 g1) Vmi (cm3 g1) Vme (cm3 g1) n.d. ¼ not determined.
Pyrochar
Pyrochar VBM-5
Pyrochar VBM-20
93(±0.11) 67(±0.25) 21(±0.52) 58.73 (±0.06) 1.62 (±0.01) 1.60 (±0.03) 38 0.007 0.004 0.003
n.d. n.d. n.d. 57.74 (±0.10) 1.94 (±0.17) 1.53 (±0.04) 49 0.011 0.009 0.002
n.d. n.d. n.d. 57.32 (±0.07) 2.08 (±0.01) 1.57 (±0.03) 64 0.052 0.016 0.036
and the porosity, as previously reported by Barakat et al. (2013). In particular, vibro ball-milling treatment increased the accessible surface area of pyrochars up to 49 m2g1 and 64 m2g1 for VBM-5 and VBM- 20, respectively. The improvement of the accessible surface area and porosity after a ball milling pretreatment have been previously reported in literature (Peterson et al., 2012; Barakat et al., 2013). For instance, Peterson et al. (2013) investigated both the accessible surface area and micropores of two ball-milled pyrochars, obtained after the pyrolysis of switchgrass and corn stover. They reported accessible surface areas of 46 m2 g1 and 74 m2 g1 for switchgrass and corn stover pyrochars, respectively. The micropore surface areas were 14 and 23 m2 g1 for switchgrass and corn stover pyrochars, respectively. Similarly, Peterson et al. (2012) investigated the impact of planetary ball milling on pyrochar (i.e. from corn stover feedstock) characteristics. They reported accessible surface areas of 3 and 40e70 m2 g1 for untreated and ball milled pyrochars, respectively. As reported in Table 1 both untreated and VBM-pyrochar samples were composed of micropores and mesopores, while macropores were absent, as previously observed by Monlau et al. (2015). Interestingly, both micropore and mesopore volumes increased after VBM, and especially at VBM-20. These results are in accordance with those reported by Peterson et al. (2012), who found that untreated pyrochar did not exhibit any measurable micropore surface area, whereas ball milled pyrochars exhibited micropore surface areas from 8 to 53 m2 g1. 3.2. Detoxification of the synthetic medium Different pyrochar concentrations and adsorption times were investigated on soluble sugars (data not shown) and furfural (Figs. 1 and 2) removal efficiency. Fig. 1 clearly displays that the furfural removal efficiency increased with the increase of the pyrochar concentration (from 20 to 60 g L1). Such phenomenon was noticed for both untreated and ball-milled (VBM-5, VBM-20) pyrochars. Interestingly, milled pyrochars (i.e. VBM-5; VBM-20) showed better performance than untreated pyrochar for furfural removal in the synthetic medium for all concentrations tested. Indeed, lower furfural removal efficiencies were noticed for untreated pyrochar, compared to milled pyrochar, with values ranging from 35% to 66%, at solid loading of 20 g L1 and 60 g L1, respectively. In particular,
Fig. 1. Effect of untreated and milled pyrochar concentrations (20, 40, 60 g L1) on furfural removal efficiency at fixed adsorption time (6 h). Values correspond to mean ± SD of measurement performed in duplicate.
Furfural removal efficiency (%)
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100 80 60 Untreated
40
VBM-5 20
VBM-20
0 0
60
120
180 240 Time (min)
300
360
Fig. 2. Effect of adsorption time on furfural removal efficiency for untreated and pretreated pyrochars. Values correspond to mean ± SD of measurement performed in duplicate.
at pyrochar concentration of 40 g L1, furfural removal efficiency was improved by 31% and 52%, for pyrochar VBM-5 and VBM-20, respectively, compared to untreated pyrochar. These improved
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efficiencies can be attributed to the increase of the surface areas and also to the degrees of microporosity previously observed, as a direct result of the vibro-ball milling treatment, thus empowering their adsorption potentials towards environmental remediation applications (Peterson et al., 2012). At untreated pyrochar concentration of 40 g L1, an adsorption capacity of 42 mg furfural g1 pyrochar was observed (data not shown), which is in accordance with that reported by Monlau et al. (2015) who found an adsorption capacity of 49 mg furfural g1 pyrochar, by using similar pyrochar. The slight difference between the two values, can be attributed to both the presence of agitation during adsorption tests, and to the higher pyrolysis temperature applied (600 C) in the previous study, responsible for higher surface area, compared to this study (49 m2g1) (Monlau et al., 2015). Furthermore, the physico-chemical structure of pyrochar, which is considered dependent of pyrolysis conditions, could also affect furans removal (Wang et al., 2015; Enders et al., 2012). Interestingly, adsorption capacities were improved by using mechanically treated pyrochars, with 55 mg furfural g1 pyrochar and 64 mg furfural g1 pyrochar for pyrochar VBM-5 and VBM-20,
Fig. 3. Simultaneous detoxification and bioethanol fermentation performed in presence or not of VBM-20 pyrochar, with “ Ethanol Red®” (S1) and “Lalvin ICV-K1” (S2) yeast strains, respectively. Values correspond to mean ± SD of measurement performed in duplicate.
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respectively. However, despite vibro-ball milling treatment improved the adsorption capacities of furfural compounds, values still remained lower than those observed in literature (120 mg furfural g1AC) using a steam activated carbon from biomass-based precursors (Klasson et al., 2011). Interestingly, for all assays, no soluble sugars (i.e glucose, xylose) removal was observed, as reported in our previous study (Monlau et al., 2015). The mechanism of selectivity towards furan compounds was already reported and detailed in literature (Li et al., 2013; Zhang et al., 2011; Monlau et al., 2015; Miyafuji et al., 2003). The effect of contact time (studied in the range from 0 to 360 min) on furfural removal from the synthetic medium was investigated for both untreated and mechanically treated (VBM-20, VBM-5) pyrochars at a solid loading of 40 g L¡1 (Fig. 2). It was observed that furfural removal increased with the increase of contact time in accordance with the results reported by Monlau et al. (2015). The furfural removal efficiency of pyrochars in the synthetic medium was simulated by a first order kinetic model. For all assays, the model was found efficient (R2 > 0.97) in simulating the removal efficiency, according to the adsorption time. A kinetic constant of 0.015 (min1) was observed for the test with untreated pyrochar. Interestingly, the kinetic constants of the obtained mechanical treated pyrochars were significantly increased (0.092 and 0.215 min1 for pyrochars “VBM-5” and “VBM-20”, respectively). Such increase is attributed to the increase of both accessible surface area (from 38 to 64 m2 g1) and micropores volume (from 0.004 to 0.016 cm3 g1) caused by the vibro-ball milling treatment. This would improve the adsorption sites of the pyrochar and its further adsorption capacity (Peterson et al., 2012). According to these results, VBM-20 pyrochar was chosen for the prosecution of the experimentation. 3.3. Simultaneous detoxification and bioethanol fermentation Results of bioethanol fermentation from the synthetic medium in presence or not of VBM-20 pyrochar and with “Ethanol Red®” (S1) and “Lalvin ICV-K1” (S2) yeast strains are presented in Fig. 3. Results revealed that, for both un-detoxified and detoxified synthetic media, the yeasts quickly consumed glucose after inoculation and the totality of bioethanol was produced during the first 24 h in all fermentations. Acetic acid concentrations remained constant during bioethanol fermentation tests (data not shown), confirming the low adsorption efficiencies (less than 35%) obtained by using pyrochar (Monlau et al., 2015). Furfural and 5-HMF adsorptions occurred quickly in presence of VBM-20 pyrochar. However, it was observed that in absence of VBM-20 pyrochar, a reduction of furfural and 5-HMF concentration occurred, suggesting that furans were converted by the yeast strains into less toxic compounds, such as furfuryl and 5-hydroxymethylfurfuryl alcohols (data not shown), as previously observed by other authors (Liu et al., 2005; Wallace-Salinas and Gorwa-Grauslund, 2013). The furans conversion into less toxic alcohols could explain similar bioethanol yields at time 24 h, from both un-detoxified and detoxified synthetic media. Fig. 4 shows the bioethanol production rate in function of the time, in presence or not of VBM-20 pyrochar and with “Ethanol Red®” (S1) and “Lalvin ICV-K1” (S2) yeast strains, respectively. Interestingly, the addition of VBM-20 pyrochar favored the bioethanol production rate, which was more pronounced in presence of the “Ethanol Red®” yeast strain. Such improvement could be partially explained by the fact that this yeast strain is known to be developed for its high tolerance towards the inhibitors released during biomass pretreatment (Wallace-Salinas and GorwaGrauslund, 2013). Additionally, this could be justified by the instant adsorption (in few minutes) of furfural onto the VBM-20
3h Eth-S2 pyrochar
24h
62
Eth-S2
Eth-S1 pyrochar
7h
37
52
0
20
24
40
60
1
25
80
97
Eth-S1
3
36
0
20
100
64
40
60
80
100
Bioethanol production rate (%) Fig. 4. Bioethanol production rate (%) according to the fermentation time (3h, 7h and 24 h), in presence or not of VBM-20 pyrochar, with “Ethanol Red®” (S1) and “Lalvin ICV-K1” (S2) yeast strains, respectively.
pyrochar, thus permitting to reduce the toxicity of the medium. Similarly, Klasson et al. (2013) reported an increase of the bioethanol production rate, during simultaneous detoxification and fermentation of a weak acid hydrolysate, by using steam-activated pyrochars produced from pyrolysis of flax shaves at 700 C for 1 h. The results of this study appear promising because although the addition of pyrochar did not enhance the final bioethanol yield, it improved the bioethanol production rate which can probably decrease both the hydraulic retention time and the reactor volume in a commercial full-scale fermentation plant. Nevertheless, further research, especially with real lignocellulosic hydrolysates at a continuous reactor scale would be needed to draw final conclusions on the feasibility and the economic advantage of the process. 4. Conclusion Vibro-ball milling increased both the accessible surface area and the porosity of pyrochar, leading to a better adsorption capacity of furfural and 5-HMF present in the synthetic medium. No increase in bioethanol yield from the synthetic medium was observed during simultaneous detoxification and bioethanol fermentation, as the yeast strains were able to convert furfural and 5-HMF into less toxic alcohols. Conversely, the simultaneous addition of pyrochar into the fermentation medium improved the bioethanol production rate, more pronounced in presence of the “Ethanol Red®” yeast strain, partially due to the instant adsorption of furfural and 5-HMF onto pyrochars. Acknowledgment Authors are grateful to Dr. Abderrahim Solhy (Mohammed VI Polytechnic University, Morocco) for his help in performing accessible surface area and porosity analyses. Authors are also grateful to the owner of the anaerobic digestion plant for providing digestate samples.
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References Alves, L.A., Felipe, M.G.A., Silva, J.B.A.E., Silva, S.S., Prata, A.M.R., 1998. Pretreatment of sugarcane bagasse hemicellulose hydrolysate for xylitol production by Candida guilliermondii. Appl. Biochem. Biotechnol. 70e72, 89e98. Antoniou, N., Stavropoulos, G., Zabaniotou, A., 2014. Activation of end of life tyres pyrolytic char for enhancing viability of pyrolysis e critical review, analysis and recommendations for a hybrid dual system. Renew. Sust. Energy. Rev. 39, 1053e1073. APHA, 2005. Standard Methods for the Examination of Water and Wastewater, 21th ed. American Public Health Association, Washington DC, USA. Brunauer, S., Emmett, P.H., Teller, E., 1938. Adsorption of gases in multimolecular layers. J. Am. Chem. Soc. 60, 301e319. Barakat, A., De Vries, H., Rouau, X., 2013. Dry fractionation process as an important step in current and future lignocellulose biorefineries: a review. Bioresour. Technol. 134, 362e373. Delgenes, J.P., Moletta, R., Navarro, J.M., 1996. Effects of lignocellulose degradation products on ethanol fermentations of glucose and xylose by Saccharomyces cerevisiae, Zymomonas mobilis, Pichiastipitis, and Candida shehatae. Enzyme. Microb. Tech. 19, 220e225. Enders, A., Hanley, K., Whitman, T., Joseph, S., Lehmann, J., 2012. Characterization of biochars to evaluate recalcitrance and agronomic performance. Bioresour. Technol. 114 (0), 644e653. Fatehi, P., Ryan, J., Ni, Y., 2013. Adsorption of lignocelluloses of model pre-hydrolysis liquor on activated carbon. Bioresour. Technol. 131, 308e314. Inyang, M., Gao, B., Pullammanappallil, P., Ding, W., Zimmerman, A.R., 2010. Biochar from anaerobically digested sugarcane bagasse. Bioresour. Technol. 101, 8868e8872. Inyang, M., Gao, B., Yao, Y., Xue, Y., Zimmerman, A.R., Pullammanappallil, P., Cao, X., 2012. Removal of heavy metals from aqueous solution by biochars derived from anaerobically digested biomass. Bioresour. Technol. 110, 50e56. Klasson, K.T., Dien, B.S., Hector, R.E., 2013. Simultaneous detoxification, saccharification, and ethanol fermentation of weak-acid hydrolyzates. Ind. Crop. Prod. 49, 292e298. Klasson, K.T., Uchimiya, M., Lima, I.M., Boihem, L.L., 2011. Feasibility of removing furfurals from sugar solutions using activated biochars made from agricultural residues. Bioresour. Com. 6 (3), 3242e3251. Li, Y., Shao, J., Wang, X., Yang, H., Chen, Y., Deng, Y., Zhang, S., Chen, H., 2013. Upgrading of bio-oil: removal of the fermentation inhibitor (furfural) from the model compounds of bio-oil using pyrolytic char. Energy. Fuels 27 (10), 5975e5981. Liu, Z.L., Slininger, P.J., Gorsich, S.W., 2005. Enhanced biotransformation of furfural and hydroxymethylfurfural by newly developed ethanologenic yeast strains. Appl. Biochem. Biotechnol. 121e124, 451e460. Miyafuji, H., Danner, H., Neureiter, M., Thomasser, C., Bvochora, J., Szolar, O., Braun, R., 2003. Detoxification of wood hydrolysates with wood charcoal for increasing the fermentability of hydrolysates. Enzyme. Microbiol. Technol. 32,
1031
396e400. Monlau, F., Sambusiti, C., Antoniou, N., Zabaniotou, A., Solhy, A., Barakat, A., 2015. Pyrochars from bioenergy residue as novel bio-adsorbents for lignocellulosic hydrolysate detoxification. Bioresour. Technol. 187, 379e386. me neur, M., Trably, E., Steyer, J.P., Monlau, F., Sambusiti, C., Barakat, A., Que Carrere, H., 2014. Do furanic and phenolic compounds of lignocellulosic and algae biomass hydrolysate inhibit anaerobic mixed cultures? A comprehensive review. Biotechnol. Adv. 32, 934e951. Mussatto, S.I., Roberto, I.C., 2001. Hydrolysate detoxification with activated charcoal for xylitol production by Candida guilliermondii. Biotechnol. Lett. 23 (20), 1681e1684. Mussatto, S.I., Roberto, I.C., 2004. Alternatives for detoxification of diluted-acid lignocellulosic hydrolyzates for use in fermentative processes: a review. Bioresour. Technol. 93 (1), 1e10. Peterson, S.C., Appell, M., Jackson, M.A., Boateng, A.A., 2013. Comparing corn stover and switchgrass biochar: characterization and sorption properties. J. Agric. Sci. 5 (1). Peterson, S.C., Jackson, M.A., Kim, S., Palmquist, D.E., 2012. Increasing biochar surface area: optimization of ball milling parameters. Powder. Technol. 228, 115e120. Pituello, C., Francioso, O., Simonetti, G., Pisi, A., Torreggiani, A., Berti, A., Morari, F., 2015. Characterization of chemicalephysical, structural and morphological properties of biochars from biowastes produced at different temperatures. J. Soils. Sediments 15 (4), 792e804. Stefaniuk, M., Oleszczuk, P., 2015. Characterization of biochars produced from residues from biogas production. J. Anal. Appl. Pyrol 115, 157e165. Sun, L., Wan, S.G., Luo, W.S., 2013. Biochars prepared from anaerobic digestion residue, palm bark, and eucalyptus for adsorption of cationic methylene blue dye: characterization, equilibrium, and kinetic studies. Bioresour. Technol. 140, 406e413. Taherzadeh, M.J., Karimi, K., 2008. Pretreatment of lignocellulosic wastes to improve ethanol and biogas production: a review. Int. J. Mol. Sci. 9 (9), 1621e1651. €z, S., 2009. Preparation and characterization of activated Tay, T., Uçar, S., Karago carbon from waste biomass. J. Hazard. Mater 165, 481e485. Wallace-Salinas, V., Gorwa-Grauslund, M.F., 2013. Adaptive evolution of an industrial strain of Saccharomyces cerevisiae for combined tolerance to inhibitors and temperature. Biotechnol. Biofuels 6, 151. Wang, S., Gao, B., Zimmerman, A.R., Li, Y., Ma, L., Harris, W.G., Migliaccio, K.W., 2015. Physicochemical and sorptive properties of biochars derived from woody and herbaceous biomass. Chemosphere 134 (0), 257e262. Zabaniotou, A.A., Antoniou, N., Stavropoulos, G., 2014. Novel sorbent materials for environmental remediation via depolymerization of used tyres. Desalin. Water. Treat. 1e10. Zhang, K., 2011. Removal of the Fermentation Inhibitor, Furfural, Using Activated Carbon in Cellulosic Ethanol Production [PhD Thesis]. Georgia Institute of Technology.