Valorization of Waste Orange Peel to Produce Shear-Thinning Gels

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Valorization of Waste Orange Peel to Produce Shear-Thinning Gels Lucy S. Mackenzie,† Helen Tyrrell,† Robert Thomas,† Avtar S. Matharu,‡ James H. Clark,‡ and Glenn A. Hurst*,†,‡ †

Department of Chemistry, University of York, Heslington, York YO10 5DD, England, United Kingdom Green Chemistry Centre of Excellence, Department of Chemistry, University of York, Heslington, York YO10 5DD, England, United Kingdom

‡

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S Supporting Information *

ABSTRACT: A laboratory experiment was developed to introduce students to waste valorization. This is the process of reusing, recycling, or composting from wastes, useful products, or sources of energy. In this laboratory experiment, waste valorization is demonstrated through transforming waste orange peel (WOP) into a marmalade-type gel by extracting a pectin-based mixture (or sol) and forming a gel in combination with an acidified sugar solution. Upon isolating the pectin sol, students examined how the rheological properties varied as a function of temperature using capillary viscometry. Gelation was followed via rotational viscometry, and non-Newtonian, shear-thinning properties were demonstrated by monitoring the viscosity change as a function of spindle RPM. In addition to providing a safe and green alternative to the traditional borax-cross-linked poly(vinyl alcohol) gel for students to study, this experiment demonstrates that valuable products for everyday use can be created from household waste such as WOP. Students making the transition from a first to second year undergraduate chemistry program within a natural-sciences degree have successfully conducted this laboratory experiment. KEYWORDS: Second-Year Undergraduate, Polymer Chemistry, Hands-On Learning/Manipulatives, Colloids, Green Chemistry, Materials Science, Public Understanding/Outreach, Physical Properties, Systems Thinking, Sustainability

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INTRODUCTION During materials-based laboratory experiments, it is common for students to query what will become of the product they have prepared, to which a typical response would describe its disposal. This is often disheartening for students and does not represent an adequate appreciation of green-chemistry principles by instructors, translating to a perception that sustainability is of minimal importance.1 To mitigate against this, there is a significant body of published laboratory experiments embedding green chemistry throughout.2,3 More specifically, there is an expanding portfolio of experiments embracing green principles to prepare and characterize polymeric materials.4−9 However, this can be taken further to show students that we can not only employ renewable feedstocks and sustainable processes to teach green chemistry in the laboratory but also get value from waste. Given waste from organic materials, including food and agricultural waste, produces a great deal of methane in landfills,10 can we show students that we can transform organic-waste materials into useful products for consumer and industrial use, thus substituting petroleum-derived and other nonsustainable products as well as reducing greenhouse gas emissions? Citrus peel is an excellent candidate to demonstrate valorization of waste to students given it is one of the most underutilized and geographically diverse biowaste residues on the planet.11 Following citrus-juice extraction, the residual peel accounts for about 50 wt % of the fruit, presenting significant global challenges in the utilization of this resource, with 15.6 © XXXX American Chemical Society and Division of Chemical Education, Inc.

million metric tons of waste produced from 31.2 million metric tons of processed citrus fruit annually.12 Waste orange peel (WOP) comprises 20% dry matter (sugars, cellulose, hemicellulose, pectin, and D-limonene (essential oil)) and 80% water and is frequently dumped on land adjacent to production sites or directly in landfills.13,14 Such disposal results in large tracts of land containing significant amounts of putrefying waste, leading to a high risk to local water courses and uncontrolled greenhouse-gas production.15 Citrus waste has a low pH (3− 4), high water content (80−90%), and high organic-matter content (95% of total solids), and according to European regulations,16 such properties forbid disposal in landfills. As such, disposal of fresh peels is a major problem for many factories because of the pollution produced and loss of valuable resources for subsequent biorefinery processes. Popular wastetreatment methods (e.g., anaerobic digestion) and use in animal feed are problematic because of the inherently high acidity and low protein content. Therefore, because of the magnitude of this global issue, student awareness of this problem and potential solutions to it are highly desirable. This can be achieved through developing a Special Issue: Reimagining Chemistry Education: Systems Thinking, and Green and Sustainable Chemistry Received: December 7, 2018 Revised: March 15, 2019

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DOI: 10.1021/acs.jchemed.8b01009 J. Chem. Educ. XXXX, XXX, XXX−XXX

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Newtonian properties of the system before finishing the experiment having prepared a useable product. Production of such a marmalade-based product from waste also serves as a good opportunity to demonstrate to students the role green chemistry can play in the provision of humanitarian aid. Similarly, this laboratory experiment to valorize WOP can be utilized to stimulate students to appreciate the life cycle of materials in a more holistic fashion. In doing so, this can help students consider where materials come from, how they are transformed and used, and what happens at the end of their life spans. More globally, this laboratory experiment allows students to appreciate that industrialized nations are resource-intensive societies heavily reliant on crude oil (a finite resource that, through its use, represents a major environmental burden) for energy, chemical, and material needs on the basis of traditional manufacturing processes. Therefore, development of new manufacturing processes and technologies using biobased feedstocks ideally produced as waste within the confines of a sustainable economy is of global importance. Within the food and drink sector, WOP is an example of such a bioresource that is readily available, comprising underexploited chemicals with a range of potential commercial applications. Hence, this laboratory experiment enables students to see how to achieve whole-systems-thinking “closed-loop” manufacturing of food products with all input materials fully utilized. Instructors can align this resource with the UN Sustainable Development Goals, specifically Goal 12, which aims to reduce the environmental impact of wastes through better utilization and management within the context of circular economy principles.26 Therefore, through such a systems-thinking approach, by highlighting the interdependence of components, students can be challenged to apply scientific principles to solve real-world problems with due consideration of ethics and demonstrate the essential role that green chemistry has to play in finding solutions to global challenges.27,28 The viscosity of a Newtonian fluid can be measured by recording the time of flow of a given volume through a vertical capillary under the influence of gravity. This flow is described by Poiseuille’s law (eq 1), where η is the viscosity of the fluid, dV/dt is the rate of liquid flow through a cylindrical tube of radius r and length L, and (p1 − p2) is the pressure difference between the two ends of the tube. A schematic of a capillary viscometer and operating instructions are provided for students and instructors on page 7 of the student lab manuscript as part of the Supporting Information. Students can investigate the viscosity as a function of temperature of the native pectin sol from the WOP using this technique.

laboratory experiment on biowaste valorization that will also enhance the practical skills of the students together with their knowledge of related scientific concepts. Indeed, examples of transforming waste materials such as unwanted office paper (cellulose), corncobs, and waste razor blades into useful products such as poly(lactic acid) plastics, sunscreen, and iron oxide nanoparticles, respectively, have been reported.17−19 A laboratory experiment has even been developed to use fruit and vegetable peels as adsorbents for the removal of pollutants from water.20 This work, as opposed to the latter, in which students used peels as a resource to complete a process, valorizes WOP to produce a marmalade-based product that students can identify with as part of their own lives. In this experiment, students are able to enjoy eating oranges at the beginning of the session while retaining the WOP. Through working in groups, students can perform an aqueous extraction in acidic conditions to form a pectin-based mixture in the form of a sol from the WOP. Hot distilled water and ethanol are used to enhance the purity of the isolated pectin by facilitating the removal of ethanol-solvent components, including sugars, and inactivating enzymes without leaching soluble pectin fractions.21 Ethanol also serves to precipitate the pectin to form a sol, presenting an opportunity to study its rheological properties through capillary viscometry, a subject that has been identified as a troubling threshold concept within the biochemistry curriculum.22 Pectin (Figure 1) is a structural heteropolysac-

Figure 1. Structure of pectin.

charide consisting of a chain of galacturonic acid units that are linked by α-1,4 glycosidic bonds with the galacturonic acid chain partially esterified as methyl esters. In combination with a sugar solution, students can use their pectin to form a gel and in doing so, follow the gelation as a function of sugar addition. An acidic environment is required (e.g., via addition of lemon juice) to minimize electrostatic repulsion between negatively charged pectin side chains. Sugar molecules facilitate the formation of junction zones between neighboring pectin polymer chains by promoting hydrophobic interactions between ester methyl groups and hydrogen bonding between undissociated carboxyl and secondary alcohol groups.23,24 The contribution from hydrophobic interactions to the free energy of formation of junction zones is half that of hydrogen bonding. However, hydrophobic interactions are essential to overcoming the entropic barrier to gelation.25 After formation of a gel, students can subsequently study the non-

πr 4(p1 − p2 ) dV = dt 8ηL

(1)

As the pressure difference is proportional to the density of the liquid, ρ, for a given total volume of liquid, eq 2 can be formed where t is the time required for the meniscus to fall from the upper to the lower mark on the capillary viscometer, and C is the viscometer constant that is determined via calibration with a liquid of known viscosity (such as water). η = Ct ρ (2) A rotational viscometer measures the torque required to rotate a spindle in a fluid, which is driven by a motor through a B

DOI: 10.1021/acs.jchemed.8b01009 J. Chem. Educ. XXXX, XXX, XXX−XXX

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Should a laboratory test sieve be unavailable, use of a kitchen sieve with a fine mesh is sufficient. Approximately 240 g of a pectin sol was retrieved.

calibrated spring. The deflection of the spring results in a torque, whereby a value for the viscosity can be calculated. An introduction and accompanying equations and diagram are provided for students and instructors on page 4 of the student lab manuscript in the Supporting Information. The rotational viscometry of traditional borax-cross-linked poly(vinyl alcohol) (PVA) solutions has been previously utilized as an undergraduate laboratory experiment to explore rheology.29 This laboratory experiment not only offers a green and safe alternative to a traditional PVA−borax system7 but also demonstrates waste valorization to students. In completing the session, students will learn how to extract a pectin sol from WOP that will be used to make a valuable product and, in doing so, learn how to determine the rheological properties of a shear-thinning solution using two key techniques: capillary and rotational viscometry. Students will have to demonstrate advanced practical skills in order to extract as much of the pectin sol as possible in order to be able to form a successful gel. As such, this experiment is particularly suited to application with second-year undergraduates studying materials-science or polymer-chemistry modules.

Variation in Viscosity as a Function of Pectin-Based-Solution Temperature

The capillary viscometer was thoroughly cleaned and dried using copious amounts of hot water, followed by deionized water, ethanol, and finally acetone. The capillary viscometer was then immersed in a transparent water bath maintained at 25 °C. Deionized water was used to calibrate the capillary viscometer, in which the time for the meniscus of the solution to fall between the highest and lowest marks was recorded. This was repeated twice, and an average time recorded was determined to calculate the viscometer constant, C. The pectin sol (1 g) was diluted with 10 mL of distilled water so that the time measurements were on the order of 2−3 min. The solution was then transferred to the capillary viscometer, and three time measurements were recorded in which the density of each of the solutions was determined using a 2.5 mL pycnometer. This was repeated by immersing the capillary viscometer into water baths at 35 and 45 °C. Time permitting, more measurements can be taken at other temperatures.

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MATERIALS Oranges, lemon juice, and white granulated beet sugar were purchased from Sainsbury’s as their own make. Ethanol (CAS 64-17-5) was purchased from Sigma-Aldrich. Jif lemon juice was purchased from Nisa Local. An Ostwald U-tube capillary viscometer (Size B, Poulton Self) was used to investigate the viscosity of the pectin sols as a function of temperature. The viscometer was immersed in a transparent water bath (in the form of a 5 L beaker) at the required temperature. A rotational viscometer (in this case a Brookfield DV-E RV Viscometer) was employed to analyze the rheological behavior of the gel. A spirit level can be used to ensure the instrument is level. The experiments can also be conducted using other viscometers available on the market (e.g., Cole-Parmer, Thermo Scientific, or Rheosys). An RV4 spindle was used with a diameter of 27.3 mm and height of 49.21 mm. Hot water should be used to clean the guard leg and spindle between appropriate sets of measurements. RPM values between 0 and 100 were available to select, and accurate torque values could be obtained between 10 and 100%.

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Variation in Viscosity as a Function of Gelation

Sugar (100 g) was dissolved in 250 mL of distilled water in a 400 mL beaker, and a glass rod was used to aid dissolution. The sugar−water mixture can also be gently heated to help to form the solution. Once the sugar dissolved, the solution was cooled to room temperature, and 5 mL of lemon juice was added. The viscosity of the sugar solution was measured using the RV4 spindle in which the pectin sol was added in 10 g additions to the sugar solution. The pectin sol was added to the sugar solution in this case because of the constraints of the rotational viscometer in that a minimum volume is required in order to record a measurement. The amount of the pectin sol added between each recording can be varied depending upon the time available in the session. Depending on the instrument, the experiment could be adapted for the sugar solution to be added to the pectin sol. Following each addition, the mixture was stirred for 30 s with a glass rod in order to assist the formation of a uniform gel, and a viscosity measurement was recorded. The spindle was cleaned between each measurement. This was repeated until a total of 150 g of the pectin sol was added. Viscosity is reported in centipoise, where 1 mPa·s is equal to 1 cP. This is aligned with the centimeter−gram−second (CGS) system of units displayed by the Brookfield DV-E Viscometer.

EXPERIMENTAL METHOD

Extracting Pectin from WOP

Shear-Rate Dependence of the Viscosity of the Gel

After students enjoyed 10 oranges outside of the laboratory, all peels were retained and, using scissors, were cut into small pieces (approximately 0.5 × 0.5 cm) and placed into a 1 L roundbottom flask with a heating mantle. Distilled water (600 mL) was added to the flask followed by lemon juice (50 mL) to obtain a solution with a pH in the range of 2−3 (universal litmus pH paper). A condenser was attached to the flask, and the mixture was heated under reflux for 75 min and allowed to cool to room temperature (the flask can be immersed in an ice−water bath to minimize waiting time). The resultant slurry was filtered using a kitchen sieve to remove the WOP. The WOP was then washed with 100 mL of boiling distilled water; 600 mL of ethanol was added to the filtrate, which was left to stand at room temperature for 30−40 min. Pectin was then precipitated to form a sol, which was isolated through filtration using a laboratory test sieve with a 75 μm aperture (Endecotts Ltd.).

The shear-rate dependence of the viscosity of the gel between 0 and 100 rpm was determined (150 g of the pectin sol had been added to 250 mL of sugar solution). The number of air bubbles around the spindle was kept at a minimum by stirring carefully to allow accurate viscosity measurements. A student lab manuscript and an instructor guide are available as part of the Supporting Information. An adapted student lab manuscript is also available for application with students who are nonchemistry majors.

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HAZARDS Oranges, lemon juice, and sugar are not classified as hazardous substances, although normal lab practice should be followed. Ethanol is a highly flammable liquid and vapor that can cause C

DOI: 10.1021/acs.jchemed.8b01009 J. Chem. Educ. XXXX, XXX, XXX−XXX

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serious eye irritation; if ethanol gets in the eyes, they should be rinsed cautiously with water for several minutes.

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RESULTS This experiment has been completed by students as part of a transition course from first to second year in undergraduate chemistry within natural sciences. The course serves to prepare students from an interdisciplinary background to make the transition to specialize in chemistry. Participating students had diverse backgrounds after studying natural sciences for one year, including (1) archeology, biology, chemistry, and environment; (2) biology, chemistry, and physics; (3) chemistry, mathematics, and physics; (4) nanoscience; and (5) neuroscience. As such, the experiment was designed to be interdisciplinary, allowing multiple departments (such as chemistry and physics) to collaborate in order to facilitate the session. Twenty-four students worked in groups of four and completed the experiment within 6 h (one iteration). The timeline could be adapted into two sessions of 3 h each. In the first session, students can isolate the pectin sol from WOP and record measurements with a capillary viscometer. In the second session, students can follow the gelation of the pectin sol and examine the rheological properties through rotational viscometry. Following the experiment, students presented their work as part of an oral presentation in their groups for 5 min each. Representative data for all parts of the experiment are presented as follows. Following extraction of the pectin sol from WOP, students studied the effect of varying the temperature on the viscosity of the diluted pectin-based solution using a capillary viscometer. Figure 2 shows the viscosity decreases upon an increase in temperature owing to an increase in the thermal velocity of the pectin molecules in solution.

Figure 3. Variation in viscosity of a 255 mL solution of lemon juice and water with 100 g of dissolved sugar upon addition of the pectin sol. This was recorded using a rotational viscometer at an angular velocity of 8.75 s−1. Gelation occurred upon addition of 20 g of pectin.

Figure 4. Variation in viscosity of the pectin-based gel upon variation in shear rate. The gel studied was obtained upon completion of the experiment detailed in Figure 3.

thinning properties. Instructors can compare this result to the behavior of ketchup.

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CONCLUSIONS This laboratory experiment enables students to enjoy eating oranges while collecting the WOP. In retaining this waste, students extract a pectin sol to valorize the WOP into a marmalade-based product. In doing so, students explore the rheological properties of such pectin sols and resultant gels through determining the temperature effects on viscosity, monitoring gelation, and subsequently studying the nonNewtonian properties of the product via rotational viscometry. Beyond providing instructors with a green alternative to traditional borax-cross-linked PVA, this experiment allows students to utilize waste in a productive fashion to create and characterize a useful product that is familiar to them. In doing so, students can adopt a systems-thinking approach in the context of the life cycle of WOP. Considering the entire system can assist learners to transition from a fragmented and reductionist knowledge of content to a more integrated and lateral understanding of this area.

Figure 2. Variation in viscosity of the pectin solution as a function of temperature.

Students were then able to combine an acidic aqueous sugar solution with the pectin sol from the WOP to form a gel. In doing so, students studied how the viscosity of the sugar solution varied as a function of the addition of the pectin sol via rotational viscometry (Figure 3). The viscosity rises at the gelation point, after which the fluid becomes non-Newtonian to form a gel product. Finally, upon forming a gel, students determined how the viscosity changed upon variation in the spindle RPM (and hence shear rate, Figure 4). From this, they were able ascertain as to whether the observed behavior was Newtonian or nonNewtonian in nature and infer that the gel exhibits shear-

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b01009. Student lab manuscript for chemistry majors (PDF, DOCX) D

DOI: 10.1021/acs.jchemed.8b01009 J. Chem. Educ. XXXX, XXX, XXX−XXX

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Laboratory Experiment

(15) Á ngel Siles López, J.; Li, Q.; Thompson, I. P. Biorefinery of Waste Orange Peel. Crit. Rev. Biotechnol. 2010, 30 (1), 63−69. (16) Directive 2009/28/EC of the European Parliament and of the Council of 19th November 2008 on waste and repealing certain Directives. Off. J. Eur. Union 2008, L312, 3. https://eur-lex.europa.eu/ legal-content/EN/TXT/?uri=celex%3A32008L0098. (17) Tamburini, F.; Kelly, T.; Weerapana, E.; Byers, J. A. Paper to Plastics: An Interdisciplinary Summer Outreach Project in Sustainability. J. Chem. Educ. 2014, 91 (10), 1574−1579. (18) Zhou, H.; Zhan, W.; Wang, L.; Guo, L.; Liu, Y. Making Sustainable Biofuels and Sunscreen from Corncobs to Introduce Students to Integrated Biorefinery Concepts and Techniques. J. Chem. Educ. 2018, 95 (8), 1376−1380. (19) Divya, D.; Raj, K. G. From Scrap to Functional Materials: Exploring Green and Sustainable Chemistry Approach in the Undergraduate Laboratory. J. Chem. Educ. 2019, 96, 535. (20) Samet, C.; Valiyaveettil, S. Fruit and Vegetable Peels as Efficient Renewable Adsorbents for Removal of Pollutants from Water: A Research Experience for General Chemistry Students. J. Chem. Educ. 2018, 95 (8), 1354−1358. (21) Abebe Alamineh, E. Extraction of Pectin from Orange Peels and Characterizing Its Physical and Chemical Properties. AJAC 2018, 6 (2), 51−56. (22) Loertscher, T.; Green, D.; Lewis, J. E.; Lin, S.; Minderhout, V. Identification of Threshold Concepts for Biochemistry. CBE Life Sci. Educ. 2014, 13 (3), 516−528. (23) Walkinshaw, M. D.; Arnott, S. Conformations and interactions of pectins: II. Models for junction zones in pectinic acid and calcium pectate gels. J. Mol. Biol. 1981, 153 (4), 1075−1085. (24) Thakur, B. R.; Singh, R. K.; Handa, A. K.; Rao, M. A. Chemistry and Uses of Pectin − A Review. Crit. Rev. Food Sci. Nutr. 1997, 37 (1), 47−73. (25) Oakenfull, D.; Scott, A. Hydrophobic interaction in the gelation of high methoxyl pectins. J. Food Sci. 1984, 49 (4), 1093−1098. (26) Sustainable Development Goals. United Nations. https:// sustainabledevelopment.un.org/?menu=1300 (accessed March 15, 2019). (27) Mahaffy, P. G.; Krief, A.; Hopf, H.; Matlin, S. A.; Mehta, G. Reorienting Chemistry Education through Systems Thinking. Nature Reviews Chemistry 2018, 2 (4), 0126. (28) Holme, T. A.; Hutchison, J. E. A Central Learning Outcome for the Central Science. J. Chem. Educ. 2018, 95 (4), 499−501. (29) Hurst, G. A.; Bella, M.; Salzmann, C. G. The Rheological Properties of Poly(vinyl alcohol) Gels from Rotational Viscometry. J. Chem. Educ. 2015, 92 (5), 940−945.

Student lab manuscript for nonchemistry majors (PDF, DOCX) Instructor guide (PDF, DOCX)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Avtar S. Matharu: 0000-0002-9488-565X James H. Clark: 0000-0002-5860-2480 Glenn A. Hurst: 0000-0002-0786-312X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank the students who participated in this experiment as well as Charlotte Elkington for technical support. REFERENCES

(1) Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice; Oxford University Press: New York, 1998. (2) Fahey, J. T.; Maella, L. E. Green Chemistry Experiments in Undergraduate Laboratories; ACS Symposium Series 1233; American Chemical Society: Washington, DC, 2016. (3) Dicks, A. P. Green Organic Chemistry in Lecture and Laboratory; CRC Press: Boca Raton, FL, 2011. (4) Schneiderman, D. K.; Gilmer, C.; Wentzel, M. T.; Martello, M. T.; Kubo, T.; Wissinger, J. E. Sustainable Polymers in the Organic Chemistry Laboratory: Synthesis and Characterization of a Renewable Polymer from δ-Decalactone and L-Lactide. J. Chem. Educ. 2014, 91 (1), 131−135. (5) Hudson, R.; Glaisher, S.; Bishop, A.; Katz, J. L. From Lobster Shells to Plastic Objects: A Bioplastics Activity. J. Chem. Educ. 2015, 92 (1a), 1882−1885. (6) Fahnhorst, G. W.; Swingen, Z. J.; Schneiderman, D. K.; Blaquiere, C. S.; Wentzel, M. T.; Wissinger, J. E. Synthesis and Study of Sustainable Polymers in the Organic Chemistry Laboratory: An Inquiry-Based Experiment Exploring the Effects of Size and Composition on the Properties on Renewable Block Polymers. ACS Symp. Ser. 2016, 1233 (8), 123−147. (7) Garrett, B.; Matharu, A. S.; Hurst, G. A. Using Greener Gels to Explore Rheology. J. Chem. Educ. 2017, 94 (4), 500−504. (8) Knutson, C. M.; Schneiderman, D. K.; Yu, M.; Javner, C. H.; Distefano, M. D.; Wissinger, J. E. Polymeric Medical Structures: An Exploration of Polymers and Green Chemistry. J. Chem. Educ. 2017, 94 (11), 1761−1765. (9) Hurst, G. A. Green and Smart: Hydrogels to Facilitate Independent Practical Learning. J. Chem. Educ. 2017, 94 (11), 1766− 1771. (10) Themelis, N. J.; Ulloa, P. A. Methane Generation in Landfills. Renewable Energy 2007, 32 (7), 1243−1257. (11) Balu, A. M.; Budarin, V.; Shuttleworth, P. S.; Pfaltzgraff, L. A.; Waldron, K.; Luque, R.; Clark, J. H. Valorisation of Orange Peel Residues: Waste to Biochemicals and Nanoporous Materials. ChemSusChem 2012, 5 (9), 1694−1697. (12) Djilas, S.; Canadanovic-Brunet, J.; Cetkovic, G. By-products of Fruit Processing as a Source of Phytochemicals. Chem. Ind. Chem. Eng. Q. 2009, 15 (4), 191−202. (13) Bampidis, V. A.; Robinson, P. H. Citrus by-products as ruminant feeds: A review. Anim. Feed Sci. Technol. 2006, 128 (3−4), 175−217. (14) Siles, J. A.; Vargas, F.; Gutierrez, M. C.; Chica, A. F.; Martin, M. A. Integral valorization of waste orangel peel usinfg combustion, biomethanisation and co-composting technologies. Bioresour. Technol. 2016, 211, 173−182. E

DOI: 10.1021/acs.jchemed.8b01009 J. Chem. Educ. XXXX, XXX, XXX−XXX