Fruit and Vegetable Peels as Efficient Renewable Adsorbents for

Laboratory Experiment Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

pubs.acs.org/jchemeduc

Fruit and Vegetable Peels as Efficient Renewable Adsorbents for Removal of Pollutants from Water: A Research Experience for General Chemistry Students Cindy Samet*,† and Suresh Valiyaveettil‡ †

Department of Chemistry, Dickinson College, Carlisle, Pennsylvania 17013, United States Department of Chemistry, National University of Singapore, 117543 Singapore

‡

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S Supporting Information *

ABSTRACT: Sustainability is emerging as a prominent curricular initiative at the undergraduate level, and as a result, involving students in real-world problems in the classroom and laboratory is an important goal. The specific problem of a dwindling supply of clean and safe drinking water is also of utmost importance and relevance. This general chemistry laboratory curriculum provides first-year students with an opportunity to design and implement their own experiments that employ fruit and vegetable peels as adsorbents to remove pollutants from water. The project is nine laboratory periods long, with the first 2 weeks devoted to providing students with the necessary tools to perform original research. In the third week, students visit the Dickinson College farm and brainstorm possible hypotheses. Working in pairs, students perform original research in the fourth through sixth weeks and investigate adsorption capacity and percent removal. In the final 3 weeks, students perform calculations and engage in peer review of their posters, which are presented at an all-college public poster session. This project introduces students to UV−vis and AA spectroscopy, making standard solutions and employing Beer’s Law, as well as literature searching and experiment design. If time allows, FTIR spectroscopy may be employed to examine the chemical makeup of the peels. This curriculum can be used in subsets with additional guidance in a standard two-semester introductory course sequence. KEYWORDS: First-Year Undergraduate/General, Second-Year Undergraduate/Upper-Division Undergraduate, Green Chemistry, Spectroscopy, Interdisciplinary/Multidisciplinary, Environmental Chemistry, Hands-On Learning/Manipulatives

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INTRODUCTION In this article, we describe a general chemistry laboratory curriculum that explores a possible renewable, low cost, and efficient method for removing dissolved heavy metals and dyes from water. The work presented here stems directly from a research publication titled “Fruit Peels as Efficient Renewable Adsorbents for Removal of Dissolved Heavy Metals and Dyes from Water”.1 In particular, students gain experience forming their own hypotheses relating to the adsorption capacity and removal efficiency of fruit and vegetable biomass, which we refer to as peels for simplicity, keeping in mind that peels may be seeds or other parts of the fruit or vegetable. Thus, students have the opportunity to connect what they are learning to the larger problem faced by countries around the world of a diminishing supply of safe and clean drinking water. To our knowledge, there are currently no laboratory experiments reported in this Journal or any other chemical education journal that use fruit peels as adsorbents for removing pollutants from water. An experiment published in 1999, prior to the appearance of fruit peel research in primary, noneducation journals, describes a laboratory experiment in © XXXX American Chemical Society and Division of Chemical Education, Inc.

which undergraduate students remediate water contaminated with an azo dye using a photocatalytic reactor.2 The only other published work that employs an adsorbent to remove pollutants from water is an environmental and analytical laboratory experiment in which students used magnetic biochar to remove salicylic acid and 4-nitroaniline from water.3 Thus, the laboratory experiment presented here is unique in that it is the first attempt to bring an original fruitpeel-based water purification experiment1 into the undergraduate curriculum. At Dickinson College, introductory chemistry students may select either of two pathways. One is the more common twosemester sequence of general chemistry with laboratory, employing the model of separate classroom and laboratory sessions (and random assignment to lab sections, so that students often have a different professor for each). Alternatively, students who achieve a certain minimum score Received: April 3, 2018 Revised: May 29, 2018

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DOI: 10.1021/acs.jchemed.8b00240 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

on the department’s placement exam may enroll in a onesemester course titled accelerated general chemistry with laboratory, which is capped at 24 first-year students who have the same professor for both classroom and laboratory sessions. This course, which has been taught since 2009, has a strong research component and, since its inception, has been closely tied to sustainability and the Dickinson College Farm. When teaching the course for the first time, one of us (CS) decided to select a different research project, and the fruit peels research1 emerged as a project with potential for undergraduate students. As questions arose during the development of the project, a collaboration of the co-authors developed that enabled the successful execution of the research module described herein. This paper presents the module as it was employed in the accelerated “honors” class and also provides recommendations for how it might be broken down into individual guided experiments in spectroscopy and kinetics for a standard introductory course. This module could also be employed in an upper-division quantitative analysis or other project-based course. Finally, we have sought to introduce the research process into the general chemistry laboratory while keeping the work appropriate for undergraduate students. The module has both group and individual discussion and guidance as needed throughout. As real-world complexity is often beyond the scope of introductory experiments, we follow the protocol of published research, which is to work at higher concentrations of pollutant than exist in wastewaters1 and to focus on organic dyes and heavy metals, which are major components of industrial and agricultural effluent.4,5 In this manner, we allude to real-world environmental challenges while making the experimental work accessible to undergraduates using instrumentation that is rugged, rapid, and commonly available.

Table 2. Class Project Topic Examples Exploring Removal of Pollutants under Varying Conditions group 1 2 3 3 5 6 7

5, 6 7 8 9

Pb /MB 2+

Pb Pb2+/MB MB MB MB Cu2+

9 10

okra orange; clementine

Pb2+/MB MB

11

pumpkin rind

MB

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MB

description contact time with 3 parts of bulb (root/ straw, outer peel/paper, clove cover) effect of pH with peel vs seeds contact time with one type of seed prep method (boil vs alcohol) with naval orange contact time with peel vs leaves contact time with organic vs nonorganic contact time with people vs birds, grocery store purchase contact time with 3 levels of ripeness (green, ripe yellow, spotted) contact time with fruit vs seeds contact time with naval orange vs clementine (large difference in thickness of peel) contact time with inner pulp and outermost peel

EXPERIMENTAL

Preparation of Peel Adsorbent

All peels must be cleaned and dried in an oven. There are three possible methods for cleaning the surface of the peels: 1. Boil 0.5 g of peel in about 50 mL of water for 30 min 2. Stir 0.5 g of peel in 10 mL dilute (∼0.01M) acid or base for 1 h 3. Stir 0.5 g of peel in 20 mL of isopropanol or ethanol for 30 min All students selected method 1 except for the group whose hypothesis was to compare two different preparation methods. Students cut or tore peels into chunks that would fit easily in a 250 mL beaker. Most students estimated how much peel they thought they would need, keeping in mind that dried peels weigh less than the peel right off the fruit. Next, peels were rinsed under warm running water to remove soluble components from the surface, patted dry, and put on a watch glass cover in an oven and dried at 100 °C overnight. Peels were removed from the oven by the class TA the following morning and put in a desiccator until the following lab period. Prior to beginning experimentation, all students ground their dried peels using a mortar and pestle, as that turned out to be more efficient than using blenders or choppers.

Table 1. Module Calendar

2 3 4

lemon pumpkin seeds orange peel

2+

The laboratory procedures for the entire project focus on spectroscopy and Beer’s Law for analyzing the amount of pollutant remaining in solution after adsorption onto the peels. Students had the opportunity to study the dyes methylene blue (MB), Alcian blue, Remazol brilliant blue, and neutral red using UV−vis spectroscopy, Cu2+ or Fe3+ ions using UV−vis spectroscopy, and Pb2+ ions using atomic absorption (AA) spectroscopy. Although additional lamps would have allowed a wider variety of ions to be studied using AA, it was not feasible to change lamps during one laboratory period.

THE CURRICULUM During the 9 week module, students worked in pairs, except for one group of three students (one student withdrew at the start of the semester). Each laboratory period was 3 h, and there was one TA for the course. An overview of the experimental activities is provided in Table 1. Many alternative schedules are

1

garlic

pollutant

pineapple banana sunflower seeds banana

8

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week

peel/ adsorbent

activities introduce project; determination of methylene blue by UV−vis spectroscopy/avocado peel test determination of Pb2+ by AA spectroscopy/avocado peel test visit Dickinson College Farm; brainstorm hypotheses; obtain produce preparation of fruit and vegetable (peels) adsorbent; dry in oven; research design; begin making standard solutions grind peels; experimentation calculations workshop; complete unfinished work final drafts of posters; peer review of posters poster session

possible; for example, the project could be completed in 6 weeks without peer review and the poster session. Because all 23 students were in class and lab together with the same professor, the project was often discussed during class time, and in fact, kinetics topics were taught using the equations from the original research paper.1 Table 2 provides a full listing of student topics.

Batch Adsorption Studies

In all experiments, no matter what the hypothesis, students study the amount of pollutant adsorbed onto the peels by measuring the absorbance of the pollutant that remains in the B




according to the equation qe = (C0 − Ce)V/M. Plots of qt (mg/ L) versus time may be made from these calculations. In a similar manner, the percent removal can be determined using the equation:

solution. Thus, solutions of pollutant without peels serve as standards, whereas the solutions containing peels are the “unknown” solutions for which the absorbance is measured and concentration determined using a Beer’s Law calibration graph. It is thus always the case that the unknown solution will have a lower absorbance value than the control. Determination of MB/Dyes by UV−Vis Spectroscopy. Students were supplied with a 200 mg/L methylene blue solution for the first Beer’s Law study (week 1). We closely matched the concentrations used in the original research paper1 by having students perform serial dilutions of the 200 mg/L stock to produce solutions of 100, 50, 25, and 5 mg/L standard solutions. Students typically made 10.00 mL of each solution. The absorbance peak wavelength and absorbance values for the standards were determined by UV−vis spectroscopy. Students used either the supplied avocado peels (already ground and dried) or their selected peels (weeks 5 and 6) as the unknown. Solutions with peels added were shaken using an orbital shaker (usually at 30 °C and 200 rpm), and an aliquot was removed for the absorbance measurement. Because removing an aliquot large enough for a cuvette would change the concentration of the remaining solution, several identical solutions of known pollutant concentration and the same amount of added peel “adsorbent” were made and shaken for varying lengths of time, and the absorbance of each was measured. Three to five shaking times proved to be sufficient. Some students let their solutions sit unshaken until lab the following week to get a final equilibrium absorbance measurement. Determination of Pb2+/Heavy Metals by AA Spectroscopy. Students were supplied with a 200 mg/L Pb2+ stock solution made from solid Pb(NO3)2. Standards were made in the same manner as for the UV−vis work. An AA spectrometer fitted with a Pb lamp and employing the 357 nm emission line was used for the absorbance measurements. For the batch adsorption studies, students made duplicate solutions of a specific concentration of adsorbent and removed just one aliquot from each solution (as above with UV−vis) by aspirating a small amount into the flame. We note, however, that removing multiple aliquots from the same solution did not change the AA results. Care was taken to avoid aspirating any solid peels into the flame.

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ji C − Cf zyz removal % = jjj i 100 j C zzz i k { where Ci and Cf (mg/L) are initial and final pollutant concentrations in water, respectively. Again, Ci is known for the standard solution being used, and Cf is obtained from the Beer’s Law calibration graph and the measured absorbance for the final solution. The calibration curve for lead is nonlinear in this region, and students fit the data to a second-order polynomial in Excel. Kinetic Studies Students who obtain qt as a function of time (upon changing some experimental factor) can also study the kinetics of the adsorption process. Although the original research paper1 contains kinetic equations, students can use the standard equations they are used to seeing to test for first- or secondorder kinetics. To test for first-order kinetics, students plot ln(qt) versus time, and for second-order kinetics, they plot 1/qt versus time and observe which plot is linear or has the highest R2 value. Example Research Project A group of two students designed their project to explore the removal of both Pb2+ and MB from solution using pumpkin seeds as their adsorbent. Their original hypothesis was that there would be a difference in adsorption capacity and percent removal for the two pollutants. The students found the qe and percent removal to be 33 mg/g and 81.9% for Pb2+ compared to 5.5 mg/g and 13.7% for MB. Their results for adsorption capacity are summarized in Figure 1. The students were

ANALYSIS OF DATA

Adsorption Studies: Calculation of Pollutant Adsorbed and Removal Percentage

Most students selected one variable to change (contact time, pH) while keeping the amount of pollutant and peels constant. (Table 2 provides a summary of class projects.) Students were able to calculate the amount of pollutant after a set time of shaking (qt) as well as pollutant adsorbed at equilibrium (qe) using the equation:

Figure 1. Adsorption capacity of Pb2+ and MB for pumpkin seeds as a function of contact time.

qt = (C0 − Ct )V /M

where C0 and Ct (mg/L) are concentrations of pollutants at the initial stage (i.e., with no biopeel) and time t, respectively, V (L) is the volume of pollutant solution, and M (g) is the mass of adsorbent (i.e., peels) used. The concentration of pollutant at time t is obtained from the Beer’s Law calibration curve and the measured absorbance (“y” in equation of the line given by Excel, for example). The equilibrium amount of pollutant is simply time t when the concentration no longer changes,

surprised at this result because in the research paper they were working from, MB had a qe much higher than that of Pb2+ (average 61.1 mg/g for MB with the three fruits studied versus 5.81 mg/g for Pb2+). They concluded that perhaps the difference was related to the nitrogen content. The three fruits studied (avocado, hamimelon or muskmelon, and dragon fruit) contain very little nitrogen,1 and their adsorption capacity for C




Table 3. Comparison of Average Class Grades on Posters poster learning goals

who is assessed

visually attractive poster presentation

group

well-organized poster content

group

coherent oral presentation to professor prior to public poster session

individual

class average, % (N = 23)

criteria for assessment compelling (makes you want to read it); graphs and data are legible (axes labeled, data points readable) flow of information is sensible; all sections are present; important features are highlighted; results are clearly presented content is succinct and correct; main features are quickly summarized; delivery is confident and clearly audible; the 3 min time limit is observed

98 95 97

first-semester college students exuding confidence, poise, and excitement during a campus-wide poster session. Depending on the resources and time available, students can also obtain FTIR spectra of the peels and perhaps even study other pollutant ions via titration (chromate anions, for example). Furthermore, this work can be divided into smaller activities with more guidance provided for a standard twosemester introductory course sequence, with students doing separate spectroscopy and kinetics experiments (see the Supporting Information). Students at a variety of levels and backgrounds can gain important insights into scientific research and its relationship to important issues involving sustainability and the environment.

Pb2+ is very low. Future work would have included elemental analysis.

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HAZARDS Students should wear goggles and gloves throughout the experiment and prevent skin contact with all chemicals. All of the liquid chemical waste must be disposed in the hazardous waste containers supplied.

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DISCUSSION The published fruit peel research1 provided the students with a firm foundation from which to develop original research projects. Most important is that this research has the potential to provide unique student projects for years to come. The combination of a wide variety of peels and pollutants, as well as many variables to change, ensures that such a project can be done year after year without repletion of hypotheses. Students can study factors such as pH, temperature, surface area, shaking parameters, and even regenerate the adsorbent and repeat the study. In addition, the adsorption phenomenon seems to be universal in that all peels studied were successful at removing pollutants. Percentage removal values ranged from 11% for sunflower seeds with Cu2+ to 100% for okra with Pb2+. The learning objectives for this module included (i) introducing Beer’s Law and the use of AA and UV−vis spectroscopy to students in general chemistry and (ii) introducing the topic of water pollution and purification concerns to undergraduates. The 23 students in the class scored a grade of 88% or better on exam questions related to UV−vis (90% average) and AA (91% average). The poster was used as a further assessment in a similar manner as a lab report. Table 3 shows the average class grade for the specific learning goals. None of the students in the class had ever made a research poster. Students were given a template and specific instructions for formatting from the college print center. Lab notebooks were carefully graded and discussed, and this served as practice for the posters. In terms of retention, 22 of the students have remained in a science track, 15 students are doing independent research as sophomores, and 14 have declared a chemistry major (9 biochemistry and 5 chemistry).

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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.8b00240. Notes for the instructor (PDF, DOCX) Instructions for the students (PDF, DOCX)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Cindy Samet: 0000-0002-9398-8104 Suresh Valiyaveettil: 0000-0001-6990-660X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge Dickinson College for its curricular support and for the outstanding facilities at the Rector Science Complex where this work was carried out. Special thanks to Jenn Halpin and everyone at the Dickinson College Farm and to the students whose experiments were discussed in the article.

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REFERENCES

(1) Mallampati, R.; Xuanjun, L.; Adin, A.; Valiyaveettil, S. Fruit Peels as Efficient Renewable Adsorbents for Removal of Dissolved Heavy Metals and Dyes from Water. ACS Sustainable Chem. Eng. 2015, 3 (6), 1117−1124. (2) Bumpus, J.; Tricker, J.; Andrzejewski, K.; Rhoads, H.; Tatarko, M. Remediation of Water Contaminated with an Azo Dye: An Undergraduate Laboratory Experiment Utilizing an Inexpensive Photocatalytic Reactor. J. Chem. Educ. 1999, 76 (12), 1680−1683. (3) Karunanayake, A. G.; Bombuwala Dewage, N.; Todd, O. A.; Essandoh, M.; Anderson, R.; Mlsna, T.; Mlsna, D. Salicylic Acid and 4 Nitroaniline Removal from Water Using Magnetic Biochar: An Environmental and Analytical Experiment for the Undergraduate Laboratory. J. Chem. Educ. 2016, 93 (11), 1935−1938.

CONCLUSIONS This 9 week experience provides students in an honors type “accelerated” general chemistry course with an opportunity to perform original research, working from a published research article. Students gained experience making solutions, performing UV−vis and AA spectroscopy, and using standard Beer’s Law curves to study adsorption. In a stepwise fashion, students were able to learn basics and practice laboratory techniques, do literature reviews, construct a hypothesis and carry out their experimental plan, and finally present their results in the form of a poster. It is difficult to quantify how awesome it is to see D




(4) Sarkheil, H.; Noormohammadi, F.; Rezaei, A. R.; Borujeni, M. K. Dye Pollution Removal from Mining and Industrial Wastewaters Using Chitson Nanoparticles. International Conference on Agriculture, Environment and Biological Sciences (ICFAE’14); Antalya, Turkey, 2014. (5) Sivakumar, P.; Palanisamy, P. N. Low-Cost Non-Conventional Activated Carbon for the Removal of Reactive Red 4: Kinetic and Isotherm Studies. Rasayan J. Chem. 2008, 1 (4), 871−883.

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Fruit and Vegetable Peels as Efficient Renewable Adsorbents for

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