In the Laboratory
Teaching Sustainable Development Concepts in the Laboratory: A Solid–Liquid Extraction Experiment Juan Carlos Parajó,* Herminia Domínguez, Valentín Santos, José Luis Alonso, and Gil Garrote Department of Chemical Engineering, University of Vigo–Campus Ourense, As Lagoas, 32004 Ourense, Spain; *
[email protected] Using Tree Bark as a Renewable Source of Phenols Biomass of lignocellulosic nature (mainly made up of inorganic components, extractives, polysaccharides, and lignin) is one of the most important resources for sustainable development, owing to its renewable character, low cost, and widespread availability. Lignocellulosic biomass can be used as raw material for the chemical industry to produce a variety of commercial products that are currently manufactured using dwindling, fossilbased resources. While the 20th century saw the emergence and establishment of an organic chemicals manufacturing industry based on petroleum refining, the 21st century is expected to see the development of a new organics industry based on biomass refining (1). Among lignocellulosic materials, pine barks—for example, Pinus radiata or Pinus pinaster barks—demonstrate particular features as feedstocks for chemical processing, particularly related to the nature and amount of their extractive fractions. Most native lignocellulosic materials have extractive contents (measured as the soluble fractions in organic solvents, water, and mild alkaline solutions) in the range 1–5% of their dry weight, although this amount increases up to near 40% of dry weight in the case of some barks (2). This difference is mainly caused by the presence of extractable mono- and polyphenols with a flavonoid structure, a type of compound suitable for replacing phenol, at least in part, in industrial applications. Figure 1 shows a generalized, generic formula for pine bark polyflavonoids. The structural units contain two aromatic rings: the phloroglucinol-type “A” ring (with –OH or –OR rings in relative positions 1–3–5), and the resorcinol-type “B” ring (with two adjacent –OH groups) (3). Under the operational conditions usually employed, the B ring is hardly reactive in comparison with ring A. This latter possess a strong nucleophilic center able to give polymerization reactions in the presence of formaldehyde by forming methylene bridges (3). This feature enables the use of pine bark polyflavonoids for making wood adhesives (e.g., for use in particleboard or plywood). Replacing chemicals obtained from petroleum (such as phenol and phenol derivatives) with alternative materials obtained from renewable resources (pine bark extracts) provides the opportunity to introduce basic principles of sustainable development to students, a topic that is being increasingly incorporated in university curricula (4). In this context, the laboratory experiment described here may be of interest for a wide audience—for example, green chemistry, environmental chemistry, and natural products courses—as well as for illustrating basic concepts, such as mass transfer and material balances. Extraction and Condensation of Pine Bark Polyphenols Many real-life, biochemical processes occur in hetereo genous media, yet, traditionally, little attention has been paid
972
to these processes (5), probably because of the complexity of the phenomena involved. Phenols can be easily extracted from pine bark using polar organic solvents, water, or aqueous solutions. For practical purposes, alkaline extraction is the preferred operational procedure. The operational conditions used in alkaline extraction of phenols from barks determine the properties of the products: for example, extraction is affected by the type of alkali employed in the processing of Pinus pinaster barks (6). In the same way, extraction of polyphenols from Pinus radiata barks under strong alkaline conditions result in high extraction yields, although the products are unsuitable for adhesive applications because of their high viscosity. Conversely, fractions extracted under mild alkaline conditions show workable viscosities (7). Using bark extracts as adhesives can be accomplished by a variety of methods, including direct reaction with formaldehyde, or polymerization with condensation products from phenol (or phenolic compounds) and formaldehyde (8–10). If necessary, the condensation kinetics can be accelerated by adding ammonia as a catalyst. In this latter case, the existence of benzyl amine bridging networks in the hardened state has been demonstrated (7). Possible Processing Schemes for Integral Use of Bark Alkaline extraction of bark yields polyphenols as major products, although other fractions (e.g., waxes) are also solubilized. These compounds possess water-repellent properties, and
n
OH
B
O
HO
OH
A OH
OH OH HO
B
O
OH
A OH
OH OH HO
B
O
OH
A OH
OH OH HO
B
O
OH
A OH OH
Figure 1. Structure of pine bark polyflavonoids.
Journal of Chemical Education • Vol. 85 No. 7 July 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory
can provide useful properties to the adhesives obtained from extracts. Conversely, alkaline extraction of barks under mild conditions results in a solid phase with enhanced polysaccharide content (including hemicellulose and cellulose), because these fractions remain almost untouched after treatments. Based on these ideas, an integral benefit of bark could be obtained— according to the “biomass refinery” concept—by sequential treatments of alkaline extraction and delignification (for example, with environmentally friendly oxidizing compounds) to yield several products of interest (11). Figure 2 shows a possible integrated process based on this approach. Experimental Sample Preparation and Extraction Air-dried pine bark samples are ground to particles with size ≤0.5 mm and homogenized in a single lot. Prior to the development of the extraction experiment, the moisture content of the lot should be determined by oven drying at 105 °C to constant weight. The desired amount of water and alkali are mixed in a Erlenmeyer flask with magnetic stirring and thermostated at the desired temperature. Then, the corresponding amount of ground pine bark is contacted with the alkaline solution, and samples are withdrawn at selected times in the range 0–20 min and filtered through 0.45 μ syringe filters. Filtered samples are mixed with a
pine bark
NaOH solution
extraction liquors containing phenols concentration
water
concentrated liquors
HCHO
polymerization
adhesives
solid phase
H2O2 solution
lignin oxidation
soluble solid lignin-derived phase products cellulases
cellulose hydrolysis
glucose solutions (suitable as fermentation media for producing bioethanol or chemicals)
Figure 2. Possible scheme for pine bark processing according to the “biomass refining” principle.
chromogenic reagent and analyzed spectrophotometrically as described below. The last sample is diluted at selected proportions, and the resulting solutions are subjected to spectrophotometric analysis to confirm compliance with the Beer–Lambert law. At the end of the extraction, liquors are recovered by filtration and reacted with formaldehyde under reflux to measure the content of reactive phenols gravimetrically. Optionally, this step can be carried out separately by the instructor in order to avoid student exposure to formaldehyde. The precipitate is recovered by filtration, oven-dried, and used to illustrate the properties of the final products (color, brittleness, water resistance, etc.) as an adhesive. Depending on the equation chosen for extraction modeling, the knowledge of the maximum extractable amount of condensable bark polyphenols may be necessary (see discussion). In this case, a previous solid–liquid extraction experiment should be performed for a period long enough to ensure that all the polyphenols have been removed from the solid phase (in our case, 2 hours), and the corresponding liquors must be gravimetrically assayed for reactive phenols. Again, this step can be carried out separately by the instructor. Spectrophotometric Analysis The chromogenic reagent for spectrophotometric analysis was made by mixing water (228 g), sodium molybdate dihydrate (2.5 g), 15 mL of 10 wt % acetic acid solution, and 0.5 g NaOH. The reagents were added sequentially, as indicated, using an Erlenmeyer flask with magnetic stirring. Bark extracts diluted at a suitable ratio were mixed with 2 mL of the chromogenic reagent, and the absorbance of the complex formed between the molybdate ions and the vicinal –OH groups present in the B ring of polyphenols was read at 400 nm against a blank composed of water (1 mL) and chromogenic reagent (2 mL). Compliance with the Beer–Lambert law may be checked by analyzing extract–water solutions obtained at suitable dilution ratios. Gravimetric Determination of Formadehyde-Condensable Polyphenols Determination of reactive phenols was carried out as reported by Tahir et al. (12): bark extracts obtained at the end of the experiment (50 mL) were reacted with an aqueous formaldehyde solution (37%; 10 mL) and a 10 M hydrochloric acid solution (5 mL) in a 150 mL glass flask under reflux for 30 min and filtered through a fritted glass crucible (porosity 2). The precipitate is washed with warm distilled water and oven-dried at 105 °C to constant weight. Note that the gravimetric determination of phenols must be carried out just for the last sample obtained in the extraction experiment, because this provides the information needed to convert the absorbance readings into polyphenol concentration data. Depending on the equation chosen for modeling, an additional gravimetric measurement from the sample obtained in a long-lasting experiment may be carried out to establish the maximum amount of extractable, reactive polyphenols present in bark (see below). Hazards Sodium hydroxide is corrosive and causes burns. Sodium molybdate (VI) dihydrate is a skin, eye, and lung irritant: direct contact should be avoided. Formaldehyde may cause mutagenic,
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 7 July 2008 • Journal of Chemical Education
973
In the Laboratory
mass unextracted phenols / g mass insert solid / g
0.7
Absorbance
0.6 0.5 0.4 0.3 0.2
0.0
0
5
10
15
20
ln
0.1
25
Time / min
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4
0
1
2
3
4
ln(t / min)
Figure 3. Absorbance readings measuring the kinetics of phenol extraction from pine bark.
Figure 4. Linearized Othmer’s plot for the data shown in Figure 3.
carcinogenic, and teratogenic effects; it is harmful by most means of exposure and must be handled with extreme caution. It is noted especially as an irritant of the eyes, nose, throat, and lungs. Hydrochloric acid is extremely corrosive: inhaling the vapor or ingesting the liquid can cause serious injury. Contact with the liquid can cause severe damage to skin and eyes. Additional information about these compounds is provided in the online supplement. Students must be instructed to follow general laboratory safety rules before conducting this experimental work.
In our case, the students’ experimental work is organized in a four-hour session in which the fundamentals of the experiment are explained, and data concerning the polyphenol content of bark and the slope of the Beer–Lambert plot are provided. Then, working in groups of two or three, students do two consecutive extraction experiments (lasting 20 min each). Filtration is done directly with syringe filters, and filtrates are separately collected in tubes. Finally, when the filtered extracts are available, students perform the dilution and analysis-related tasks. As an example, Figure 3 shows absorbance profiles determined by students.
Results and Discussion Biomass is a clean, affordable, and renewable raw material suitable for producing a broad range of products (not for human or animal consumption). The development and commercialization of bio-based products would provide new and expanded markets for low-cost feedstocks from forestry and agriculture, fostering rural and sustainable development. In this field, biobased (renewable) materials hold great promise for achieving the goals of sustainable development (13). Graduate curricula are incorporating the key principles of sustainable development. In this context, the laboratory experiment described here represents an innovative and convenient approach to address key concepts of sustainability. The experiment can also be a valuable aid for teaching fundamental chemical concepts and core mass transfer concepts using simple experimental techniques and easy models for data interpretation. Only basic laboratory equipment is needed, and students do not need special lab skills to perform the experimental tasks. This lab experiment can be gauged to different degrees of difficulty: in the simplest case, the instructor can provide students with the moisture content of bark and data coming from gravimetric analysis (bark content of reactive polyphenols, slope of the Beer–Lambert plot), and the students can determine the solute concentration profiles along the extraction process. In this case, the condensation of phenols with formaldehyde could be carried out by the instructor in the presence of students. Alternatively, the students can undertake both parts of the lab (spectrophotometric measurement of kinetic data and gravimetric analysis), following the necessary safety precautions.
974
Extraction Modeling Modeling of data is carried out in an additional session lasting about one hour, in which students use spreadsheet software for data interpretation. Kinetic data modeling can be carried out by a two-parameter model, which can be easily addressed by linear regression, or by a three-parameter model that involves using an optimization algorithm for finding the best-fit values of the parameters. Othmer’s equation (14), originally proposed for oil seed extraction, provides a convenient method for modeling the time dependence of the extraction data by linear regression. The proposed expression is:
Cs a t b
(1)
where a and b are regression parameters, Cs measures the concentration of residual phenols in solid phase (expressed as mass unextracted phenols/mass inert solid, oven-dried basis) and t is time (usually expressed in min). If the amount of extractable polyphenols in bark is known, the amount of inert solid can be calculated by difference from the ovendried weight of bark. On the other hand, the calculation of the amount of extracted phenols corresponding to a given absorbance can be performed using information from the gravimetric analysis, and the values of Cs can be immediately determined from its definition for each extraction time. Once the set of Cs/t data is available, ln(Cs) can be plotted against ln(t). The y intercept, [ln (a)], and the slope, (b), lead to the
Journal of Chemical Education • Vol. 85 No. 7 July 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory 2.5
Figure 5 presents experimental data obtained by students and the model predictions by the models described above. Note that the parameters of this model can be calculated even if the polyphenol content of the bark is unknown. On the other hand, a comparison of C0 and C * provides an estimate of the relative amount of small particles in the extracted sample.
mass phenols / g mass solvent / kg
2.0
1.5
1.0
Literature Cited
0.5
0.0
0
5
10
15
20
25
t / min Figure 5. Experimental phenol concentrations and data calculated according to eq 3.
values of the regression parameters. As an example, Figure 4 shows the linearized plot corresponding to the absorbance data presented in Figure 3. Alternatively, a suitable three-parameter model could be justified on the basis of fast-extractable fraction of bark (small particles, from which the phenols are solubilized just at the beginning of the extraction), and a slow-extractable fraction (for which the film mass transfer is the rate-controlling step). Under these hypotheses,
dC kL a C * C
dt
(2)
where C is the phenol concentration in liquid phase, kLa is the product of the mass transfer coefficient and the specific surface area, and C * is the phenol concentration in liquid phase corresponding to the equilibrium conditions. If the easily extractable fraction leads to a phenol concentration in the medium C0 at t near zero, integration of eq 2 leads to the expression:
C C * C * C0 exp kL a t
(3)
The values of C can be calculated for each contact time from the corresponding absorbances, and the series of data C/t can be fitted to eq 3 by non-linear regression (for example, using the Solver routine in Microsoft Excel software) to yield the values of C *, C0, and kLa. As an example, Figure 4 shows the experimental and calculated values of the variable C (expressed in terms of g/kg) corresponding to the absorbance data presented in Figure 3.
1. Clark, J. H.; Budarin, V.; Deswarte, Fabien, E. I.; Hardy, J. J. E.; Kerton, F. M.; Hunt, A. J.; Luque, R.; Macquarrie, D. J.; Milkowski, K.; Rodriguez, A.; Samuel, O.; Tavener, S. J.; White, R. J.; Wilson, A. J. Green Chem. 2006, 8, 853–860. 2. Vázquez, G.; Antorrena, G.; Parajó, J. C. Wood Sci. Technol. 1987, 21, 65–74. 3. Pizzi, A. J. Macromol. Sci. Polymer Rev. 1980, C 18, 247–315. 4. Mulder, K. F. Eur. J. Eng. Educ. 2006, 31, 133–144. 5. Rodríguez, M. F.; Ríos, M. C.; Mosquera, M.; Ríos, A. M.; Mejuto, J. C. J. Chem. Educ. 1995, 72, 662–663. 6. Vázquez, G.; Antorrena, G.; Parajó, J. C. Wood Sci. Technol. 1987, 21, 155–166. 7. Panamgama, L. A. J. Appl. Polym. Sci. 2007, 103, 2487–2493. 8. Jorge, F. C.; Pascoal Neto, C.; Irle, M. A.; Gil, M. H. Holz als Roh- und Werkstoff 2002, 60, 303–310. 9. Vázquez, G.; Antorrena, G.; Parajó, J. C.; Francisco, J. L. Holz als Roh- und Werkstoff 1989, 47, 491–494. 10. Grigsby, W.; Warnes, J. Holz als Roh- und Werkstoff 2004, 62, 433–438. 11. Parajó, J. C.; Vázquez, G.; Antorrena, G.; Francisco, J. L. Holzforsch. Holzverwert. 1989, 41, 106–108. 12. Tahir, P. M.; Musgrave, O. C.; Ashaari, Z. Holzforsch. Holzverwert. 2002, 56, 267–272. 13. Narayan, R. Proceedings, 227th ACS National Meeting, March 28–April 1, 2004, Anaheim, CA, USA. 14. Othmer, D. F.; Agarwal, J. C. Chem. Eng. Progr. 1955, 51, 372–378.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2008/Jul/abs972.html Abstract and keywords Full text (PDF)
Links to cited JCE articles
Supplement
Student handouts
Instructor notes
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 7 July 2008 • Journal of Chemical Education
975
