From Eggshells to Quicklime: Using Carbonate Cycle as an Integrating

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

pubs.acs.org/jchemeduc

From Eggshells to Quicklime: Using Carbonate Cycle as an Integrating Concept To Introduce Students to Materials Analysis by TGA and FTIR Fernando Luna Vera,* Marcelo Guancha Chalapud, Ingrid Castillo Viveros, and Edgar Alexander Vásquez Medina Grupo de Investigación en Desarrollo de Materiales y Productos (GIDEMP), Centro Nacional de Asistencia Técnica a la Industria (ASTIN-SENA), Cali 760002, Colombia S Supporting Information *

ABSTRACT: In this paper, we report an experiment intended as a tool to introduce chemistry and materials science students to several chemical concepts through the determination of the best experimental conditions for the formation of calcium oxide (lime) from a source of high content of calcium carbonate (eggshells). Thermal gravimetric analysis (TGA), infrared spectroscopy (FTIR), and qualitative tests were used to optimize the experimental conditions of calcination temperature and time for the formation of calcium oxide.

KEYWORDS: Laboratory Instruction, Hands-On Learning/Manipulatives, Second-Year Undergraduate, Analytical Chemistry, Materials Science, Bioinorganic Chemistry, Demonstrations, Inquiry-Based/Discovery Methods

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thereby ideally eliminating waste entirely”.2,3 Emphasis is placed on the use of natural materials produced as byproducts from livestock and agriculture industries which may be converted into valuable biomaterials using simple transformation methods.4,5 A second session was used to focus the study of this practice: eggshell waste in the food and poultry industry.6,7 Eggshells are approximately 94 wt % calcium carbonate (limestone); therefore, they are a good source of calcium carbonate.8,9 Calcium carbonate can be readily transformed into calcium oxide (quicklime) by the liberation of carbon dioxide through thermal decomposition. Calcium oxide has significant, wellknown industrial applications such as in furnace lining, metal smelting, glass, fertilizer, drying agent, mortar, paper, pulp, cement, and drilling fluid,10−13 as well as experimental applications, such as a replacement for homogeneous basic catalysts, like potassium hydroxide, in the transesterification of refined oils to produce biodiesel14,15 and CO2 sequestration agent.16,17 Due to its wide use, finding new, cheaper, and environmentally friendly methods to obtain CaO is a current and important challenge. Chemistry students can use this experiment to rationalize and approach this problem in a critical

INTRODUCTION It is well-known that students exposed to problem-based learning (PBL) activities are more interested and engaged.1 The use of PBL activities ultimately aims to foster students’ inquiring skills. However, many times the problems used to drive discussions and activate the learning processes lack global context and narrative that provide the connection to the real world. Herein we propose a validated practice which uses the narrative of the transformation of waste materials, in this case, into valuable industrial materials (quicklime). Through this problem, basic concepts of qualitative and instrumental analysis, including thermal gravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR), are introduced. Through guided inquiry, students understand the use of these techniques for the evaluation of production of calcium oxide from the thermal decomposition of calcium carbonate. This practice was implemented as a short project lasting for 8 weeks and performed in groups of up to 36 students working in pairs. An initial 2 h session was used to introduce students to the general context in which the problem is framed a “zero waste economy” that uses waste as raw material for new products and applications, which follows the logic of the circularity: “waste streams of production processes are to be redeployed to serve other processes internal to the system, © XXXX American Chemical Society and Division of Chemical Education, Inc.

Received: August 5, 2017 Revised: February 10, 2018

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DOI: 10.1021/acs.jchemed.7b00597 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

Figure 1. Limestone cycle. Representation of chemical interplay of calcium carbonate with water, carbon dioxide, and heat.

water and carbon dioxide. The chemical transformations occurring within the cycle are shown in Figure 1. Since this practice is intended to be used as a guided inquiry laboratory experiment,21 processes depicted in Figure 1 are presented without in-depth explanation. However, students are challenged to present formal balanced equations for the three major reactions. Also, students should recognize the conditions and chemicals involved in the reactions as well as their source during the experiment. For example, where are water and carbon dioxide coming from in the reaction? It is expected that students realize that these compounds come from the atmosphere. Students may further speculate about the influence of factors regarding handling of samples or conditions such as relative humidity.22 After the major reactions have been studied and rationalized, the laboratory exercise was introduced and explained. The laboratory task consists of sample preparation, calcination at different temperatures, and characterization of the product by TGA, FTIR, and qualitative analysis. During weekly prelab sessions, students were faced with questions such as the following: How do we know that CaO has been produced? What is the optimal range of temperatures to obtain the purest CaO? Why is it difficult to obtain and maintain CaO with good purity? Why is TGA not a definitive method to establish the presence of CaO? After discussions were initiated, new questions from students were presented and possible answers (hypotheses) were proposed and tested in further laboratory sessions either by experimental activities or computational analysis of previously generated data.

way. Also, and most important from one point of view, providing the students with a reason behind the problem will establish a clear link between basic chemical processes and their use in real world applications. After these two initial sessions, students are introduced to the practical challenge: find the right temperature to obtain the purest calcium oxide by calcination of eggshells. In this experiment we evaluated temperature as the only independent variable due to the ease of using a conventional muffle furnace for short periods of time. However, optimization of several other variables, such as particle size, bed thickness, time of calcination, time of storage, and CO2 pressure, may also be implemented to increase complexity. Two analytical tools were used to identify conditions that produce higher yields of calcium carbonate conversion: TGA and FTIR. These two techniques were introduced to students by presenting key concepts used in the characterization of sample purity. In the case of TGA, it was pointed out that analytical signal is dynamic, produced by mass changes in the original sample as calcination temperature increases. In contrast, in FTIR no physical changes are induced, but a surveillance of molecular bonds at different wavelengths occurs during measurement. Rationalization of mass changes due to chemical reactions is critical for understanding mass losses due to production of gases which are able to leave the sample cell. It was also important to show the quantitative power of both TGA and FTIR. TGA was used to show that mass changes are stoichiometrically related to the molar mass and may be used to identify those compounds produced after heating.18 Meanwhile, FTIR peak areas are related to the amount of substance present.19,20 After students acquired a basic understanding of the two characterization techniques, the chemistry behind the experiment was detailed. This laboratory experiment is based on the limestone cycle, which describes the chemical changes occurring among calcium carbonate, calcium oxide, and calcium hydroxide upon application of heat and their reactions with

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EXPERIMENTAL DETAILS

Limestone Calcination

Eggshells were obtained by students from local restaurants and bakeries. They were washed with distilled water to remove any traces of impurities and membranes. Next, the eggshells were dried in an oven at 105 °C for 12 h. Once dry, the eggshells were reduced in size using a coffee grinder. A small portion of B




Figure 2. Thermal gravimetric analysis (TGA) of eggshells after calcination for 1 h at (A) 600 °C, (B) 800 °C, and (C) 1000 °C.

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RESULTS During the experiment, students determine an optimal temperature range to obtain CaO from eggshells. The main features of TGA of calcined samples, i.e., mass loss at specific temperatures, were analyzed to evaluate the extent of the chemical transformation of limestone to quicklime. For example, Figure 2 shows the thermal behavior of samples calcined at 600, 800, and 1000 °C for 1 h. Figure 2A shows a smooth curve with a defined mass loss between 750 and 810 °C with a maximum rate of loss at 804.7 °C as shown by the first derivative of the TGA curve. This behavior illustrated that the sample obtained after calcination at 600 °C has further thermal decomposition that leaves a residue equivalent to 60% of the total mass of the sample. Students were asked to speculate about the chemical reaction that produces 40% mass loss. After discussion, students arrived at the conclusion that the material produced after initial calcination at 600 °C is almost 100% calcium carbonate (MW: 100 g/mol) which liberates CO2 (MW: 44 g/mol) and leaves behind CaO (MW: 56 g/mol). Experimentally, the residue showed a value somewhat different than the theoretical 56% as in this case (60%) but that also worked to initiate a discussion about the purity of the initial calcium carbonate found in eggshells.23 Comparisons were performed with pure CaCO3. The effects of increased calcination temperature are presented in Figure 2B,C, which shows the cases for 800 and 1000 °C, respectively. Two phenomena are observed when these TGA profiles are compared: mass loss between 750 and 850 °C is diminished while a new mass loss becomes greater between 400 and 450 °C as calcination temperature is increased from 600 to 1000 °C. Students were able to associate this evidence with the chemistry presented in the limestone cycle. During weekly meetings where new results and data were shared within working groups, students demonstrated under-

ground eggshells, approximately 5 g, was place in porcelain crucibles and calcined at different temperatures (600, 800, and 1000 °C) for 1 h. After cooling, samples were stored in closed glass containers. Material Characterization

Thermal analyses were performed using a TGA DSC 2 STAR System, Mettler Toledo. Each sample was 10 ± 0.05 mg. A nonreactive atmosphere (nitrogen) was used, as was a temperature ramp of 10 °C/min from 25 to 900 °C. Chemical characterization of samples before and after calcination was performed by infrared spectroscopy (Thermo Scientific Nicolet 380) using an ATR device in the range from 500 to 4000 cm−1. A qualitative test for observation of gas evolution was performed by adding 1 N HCl to freshly calcined samples. Observation of bubbles indicated gas evolution and therefore a reaction. Also, pH changes were monitored by a regular bench pH meter before and after calcined samples were dissolved in deionized water. All data analyses such as calculation of peak area and correlations between peak area and temperature were performed using Origin software (OriginLab Corporation).

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HAZARDS Good laboratory practices should be observed to minimize the risk of ingestion, inhalation, or contact, especially during the qualitative test when diluted acids are used. Since calcination is a significant part of the experiment, care should be used when using furnaces and/or handling hot crucibles. Pans or crucibles used in TGA experiment can reach 900 °C, and portions of the TGA can become hot. Care should be taken around the instrument when heating. When handling powdered solids, the use of dust mask is advised. Gloves and goggles are advised to be used when manipulating dilute acid solutions. C




Figure 3. FTIR spectra of eggshell samples treated with different temperatures of calcination: (A) 600 °C, (B) 800 °C, and (C) 1000 °C.

treatments and identified that moving from lower to high calcination temperatures (Figure 3A−C) produces samples with less carbonate (1445 and 877 cm−1) and increased calcium hydroxide (3642 cm−1), which is consistent with TGA observations. These trends were intuitively observed from the height and width of peaks; however, if primary data is available each peak area should be calculated and graphed so trends can be quantitatively analyzed, as shown in Supporting Information. By correlating TGA and FTIR data with the chemical transformations happening in the limestone cycle, students were able to develop a set of experimental conditions to produce the purest CaO. To supply enough evidence for the final conclusion, students were invited to perform several qualitative tests. Samples with a high content of calcium carbonate will react with acid to release carbon dioxide; this is observed by fizzing upon addition of dilute acid. Further, CaO is a stronger base than calcium carbonate; samples with higher content of CaO exhibited a higher pH, usually >10 when dissolved in water. Evaluation of the final laboratory reports of 35 students, during two different semesters, shows that most of the students could correlate FTIR and TGA data in order to evaluate the chemical composition of calcined samples, and then determine the right set of conditions for highest conversion of eggshells to calcium carbonate. However, some students struggled with collecting accurate data from TGA experiments due to the appearance of a common effect called the “buoyancy effect”, which produces an apparent gain of mass, as shown in Figure 2, due to the density of the surrounding gas decreasing on heating at the beginning of TGA.27 Nevertheless, this effect may be minimized in some instruments by setting up a TGA method that includes a background correction to subtract the signal of a blank from the following TGA runs. The blank is performed by

standing about which chemical species were responsible for the changes observed. For example, after some discussion and a quick literature search, students arrived at the conclusion that mass loss around 420 °C corresponded with the dehydration of calcium hydroxide.24 Therefore, it may be concluded that the calcium hydroxide becomes more prevalent at higher temperatures of calcination. Further, as previously indicated, at high temperatures (1000 °C) very little calcium carbonate remained, and eggshells were most likely converted directly into calcium oxide, which is thermally stable (melting point of 2572 °C); therefore, Figure 2C shows a stable line after 600 °C. However, calcium hydroxide appearance was related to the exposure of calcined sample to atmospheric humidity. CaO is highly sensitive to moisture and can readily transform into its hydroxide form which is responsible for the dehydration observed in the TGA profile at 400−450 °C, a process depicted in Figure 1 between quicklime and slacklime. Students concluded that best conditions for maximum production of CaO include calcination temperatures above 800 °C. However, pure CaO is very hard to obtain due to its propensity to form its hydroxide form.25 Conclusions addressed by TGA about the chemical identity of each sample after calcination should be supported by corresponding FTIR spectra, and they should clearly show how they are complementary techniques. In Figure 3, FTIR spectra of samples are shown. As established by TGA, samples treated at 1000 °C showed evidence of calcium carbonate and calcium hydroxide. The signal at 3643 cm−1 is due specifically to O−H stretching in Ca(OH)2. Bands associated with carbonate ions due to vibration modes of C−O are observed around 1060 and 713 cm−1. However, signals around 1445 and 877 cm−1 are used as fingerprints for carbonate; the latter, for example, can be used to determine the kind of cation that goes alone with carbonate ion.26 Students looked for trends among different D



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running a TGA test under the same conditions as those for samples using an empty crucible. As part of the final report of the experiment, students were asked to propose a real application for the quicklime produced in the laboratory. By asking students to devise real applications, they could connect the results of the experiment with the initial topic of sustainability and use of waste materials. Student proposals consisted of ideas such as replacing HCl and other acids as neutralizers of acid waste produced during regular lab sessions or even as soil modifier for our schoolś gardens. An evaluation of the fitness of the experiment with student expectations demonstrated the importance of giving a context for the whole experience. Students felt encouraged to participate more actively in part due to the specific link established between the general and real world challenge of reuse of industrial waste into valuable chemicals used as new raw materials.

CONCLUSION The determination of the optimal conditions for the transformation of eggshells into calcium oxide is a suitable inquirybased laboratory exercise for an instrumental analytical chemistry course at the introductory level at higher education institutions. To develop the inquiry exercise, students are required to use their knowledge of the chemical properties and reactions regarding the limestone cycle and the interaction of reagents with heat. Understanding of the limestone cycle provides a tool to further research the optimal temperature to produce the highest yield of transformation from eggshells (calcium carbonate) to calcium oxide. An approximation of the reaction yield is provided by the detailed observation and interpretation of complementary techniques like FTIR and TGA. This combination of characterization techniques may be used to exemplify the fact that one instrumental method does not always tell the full story and that chemical behavior exposed in classic qualitative tests is important to consolidate information from instrumental methods. Furthermore, this experiment is an example of the high interest sparked in students when chemical inquiry is linked with a real, relevant challenge such as the development of a sustainable society. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.7b00597. Student handout and instructor notes (PDF, DOCX)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: fl[email protected]. ORCID

Fernando Luna Vera: 0000-0003-0022-7283 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the collaboration by SENA, Centro ASTIN and Tecnoparque Nodo Cali, and ICESI’s chemistry department. We also acknowledge the economic support from COLCIENCIAS through the grant FP44842-258-2015. E




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From Eggshells to Quicklime: Using Carbonate Cycle as an Integrating

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