Recycling Metals from Spent Screen-Printed ... - ACS Publications

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

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Recycling Metals from Spent Screen-Printed Electrodes While Learning the Fundamentals of Electrochemical Sensing María-Isabel González-Sánchez,† Beatriz Gómez-Monedero,† Jerónimo Agrisuelas, and Edelmira Valero* Department of Physical Chemistry,Higher Technical School of Industrial Engineers, University of CastillaLa Mancha, Campus Universitario s/n, 02071 Albacete, Spain S Supporting Information *

ABSTRACT: A laboratory experiment in which students recycle silver and platinum selectively from spent screenprinted platinum electrodes is described. The recovered silver in solution is used to show its spontaneous redox reaction with a copper sheet. The recovered platinum is electrodeposited onto a screen-printed carbon electrode to develop a sensor for hydrogen peroxide quantification in a commercially available hair lightener. The experiment is designed for a 3 h laboratory period and can be adapted for upper-division undergraduate education, graduate education, or even research students in electrochemistry, environmental chemistry, analytical chemistry, materials science, chemical engineering, or physical chemistry laboratories. It allows students to train in adequately handling strong acids, working in fume hoods, and neutralizing acid gases. It also allows one to teach the basics of metal recycling and enables students to learn the fundamentals of electrochemical sensing. The experiment also helps to raise student awareness of waste disposal problems and how recycling can help to reduce the amount of waste that we create. KEYWORDS: Upper-Division Undergraduate, Graduate Education/Research, Analytical Chemistry, Laboratory Instruction, Environmental Chemistry, Hands-On Learning/Manipulatives, Electrochemistry, Metals

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Environmental Sciences, or Engineering.7 For this reason, besides the importance of recycling metals, this lab experiment attempts to help students become familiar with some important electrochemical concepts and techniques. Particularly, students will perform the deposition of recycled silver onto copper substrates as an example of a redox spontaneous reaction. On the contrary, recycled platinum electrochemical deposition on screen-printed carbon electrodes (SPCEs) is herein presented as an example of a non spontaneous reaction. Electrodeposition of other cheaper metals, like nickel, was reported some years ago as a laboratory experiment.8 However, as far as we know, this is the first time that recycled Pt from SPPtEs is electrodeposited during laboratory practical sessions. Although Pt is expensive, the fact that recycled metal is used makes the experiment more affordable and appealing for students. The resulting Pt-modified SPCEs (Pt@SPCEs) will be used for the electroanalysis of H2O2 because Pt is a potent catalyzer of the decomposition of this compound.6,9−11 To perform this lab experiment, students need to previously learn some fundamentals of electrochemistry. Therefore, this practice could be paired with others to learn electrochemical

INTRODUCTION Nowadays, concern about environmental sciences and sustainability is growing. Accordingly, new subjects related to these topics are emerging in the curricula of different degrees in sciences and technology. Indeed, it is important to raise student awareness about environmental protection, and new laboratory experiments that deal with recycling,1 reusing components,2 and removing hazardous compounds3 are being introduced. In the lab experiment presented here, students will use spent screen-printed platinum electrodes (SPPtEs) to selectively recover silver and platinum from conductive inks by different leaching solutions. This protocol can be extrapolated to the recycling of other materials based on platinum, gold, palladium, and silver inks. Screen-printing technology allows the manufacture of lowcost, easy-to-handle disposable electrodes.4 These devices are made of conductive inks based on metals such as silver, gold, and platinum (Figure S1, Supporting Information), which are highly valuable. Screen-printed electrodes (SPEs) are being widely used in research laboratories, in industry, and even in teaching.5−8 The vast consumption of disposable electrodes after single analytical use involves storing vast amounts of solid waste which has to be properly treated.9 Therefore, recycling precious metals from SPEs is noticeably necessary. Electrochemistry is a very important subject in the curricula of students in different science degrees, such as Chemistry, © XXXX American Chemical Society and Division of Chemical Education, Inc.

Received: May 18, 2017 Revised: March 20, 2018

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DOI: 10.1021/acs.jchemed.7b00345 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

Screen-Printed Electrodes Treatment

techniques which, at the same time, will generate the waste electrodes to be recycled. For example:

First, in order to remove their plastic cover, three spent SPPtEs per student group were immersed in 5 mL of commercial H2SO4 solution for 20 min. Afterward, each electrode was washed under tap water until the plastic cover was eliminated and was then rinsed with deionized water. Once dried, electrodes were weighed and placed in 10 mL of a ∼30% (v/v) HNO3 solution (leaching solution no. 1, LS1) to dissolve the silver ink. After 5 min, the electrodes were removed from the solution and rinsed with deionized water, dried, and reweighed. LS1, which contained the dissolved silver as AgNO3, was immediately used to observe Ag deposition onto a Cu sheet (see the next section). Next, the electrodes were immersed in 20 mL of a freshly prepared diluted aqua regia solution (1:4 ratio in deionized water) and heated at 90−95 °C (near boiling) until the platinum ink completely dissolved. During this process, toxic gases such as NOx and Cl2 are produced. Therefore, to minimize gas emissions to the atmosphere and for safety reasons, two gas traps, the first was empty and the second contained 1 M NaOH, were used to collect and neutralize such gases (Figure S2). The resultant solution (leaching solution no. 2, LS2), which contained the dissolved platinum as H2PtCl6,9,16 was cooled down to room temperature and kept for further use. Electrodes were then rinsed with water, dried, and reweighed.

• Students could learn several electrochemical techniques (cyclic voltammetry, differential pulse voltammetry, square wave voltammetry, etc.) 12 in a previous experiment. • They could study hydrogen evolution and the overpotential.13 • Lab practices could include the modification of electrodes using conducting polymers.9,14 • Lab experiments could include determination of any other metabolite like glucose.15 Therefore, the tasks proposed herein provide students with five main learning outcomes: (1) learning how to recover precious metals from electronic waste through selectively retrieving silver and platinum from SPPtEs, (2) learning how to safely handle strong acids in the laboratory, (3) understanding the difference between spontaneous and nonspontaneous redox reactions, (4) learning basic strategies to modify electrodes using recycled metals as well as the use of Faraday’s equation to evaluate the efficiency of the process, and (5) learning how to prepare a calibration straight line, calculate the corresponding analytical parameters, and evaluate the concentration of a compound in a real sample.

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Spontaneous Silver Deposition on a Copper Wire

A copper sheet was used to observe the spontaneous deposition of silver from LS1, as an example of a galvanic cell. The latter was diluted with deionized water (1:4), and the Cu sheet was placed into the solution and kept until the reaction occurred.

EXPERIMENTAL SECTION

Reagents and Instrumentation

H2SO4 (98%), HNO3 (65%), and HCl (37%) were acquired from Panreac. NaOH (99%), KH2PO4 (99.5%), and K2HPO4 (99%) were purchased from Merck. H2O2 (35%) came from Sigma-Aldrich. The hair lightener (stated composition: water, alcohol denat, Chamomilla recutita flower extract, hydrogen peroxide, parfum, phosphoric acid, amyl cinnamal, coumarin, linalool) was purchased from a local supermarket. All the reagents were used as received without further purification. Potassium-phosphate-buffered solutions (PB, 0.1 M, pH 7) were prepared from 0.1 M K2HPO4 and KH2PO4 solutions, and were used as the supporting electrolyte for the electrochemical measurements. Electrochemical experiments were carried out at room temperature. PB was prepared in deionized water, and pH values were determined by a Consort C830 multiparameter analyzer (Consort, Belgium). SPPtEs (DRP-550, DropSens) were the waste electrodes chosen for recycling. In these electrodes, the working and counter electrodes are made of platinum, and the reference electrode is made of silver. New SPCEs (DRP-150, DropSens) with a carbon working electrode, a platinum counter electrode, and a silver reference electrode were used for Pt deposition. All the experiments were performed at room temperature and without solution agitation, unless otherwise specified. The Pt deposition process was controlled by an ac/dc power supply (GRELCO, GE0122DVAV, 0−12Vcc/0−2A, Gubar S.A., Spain), which worked in the dc mode and also allowed monitoring of the resultant current. Amperometric analyses of H2O2 were carried out with an AUTOLAB potentiostat− galvanostat setup (PGSTAT204, Metrohm Autolab B.V., The Netherlands) with the NOVA 2.0 software package. For Pt deposition and H2O2 sensing, the power supply/potentiostat was connected to the electrode by an SPE adapter (DRP-CAC, DropSens, Spain).

Electrochemical Deposition of Platinum

SPCEs were used as a support for the electrodeposition of platinum from LS2, as an example of an electrolytic cell. To that end, this solution was diluted with deionized water to a final volume of 20 mL. The SPCE was weighed, connected to the power supply, and immersed in the diluted leaching (Figure S3). A potential of 12.4 V was applied for 12 min, and the current that resulted during the process was recorded. Next, the power supply was disconnected, and the electrode was rinsed with deionized water and reserved. This procedure should be carried out in a fume hood due to the highly acidic nature of the aqua regia. Nonenzymatic Hydrogen Peroxide Sensor

The Pt-modified SPCE was tested as a sensor to measure the H2O2 concentration in a commercial hair lightener. Amperometric measurements were taken at 0.7 V in PB with constant magnetic stirring.9 H2O2 content was determined by additions of the sample to PB. A calibration plot within the 0−6.5 mM range was previously prepared by successive additions of a standard H2O2 solution and by measuring the differences in the obtained current intensity. Lab Duration

Students who work in pairs are expected to complete this laboratory experiment in roughly 3 h. The SPPtEs treatment accounts for up to 60 min, including the preparation of material for spontaneous silver deposition on the copper sheet. The step that involves Pt electrochemical deposition can be performed in nearly 30 min. The electroanalysis of H2O2 content should be completed within 90 min, which includes calibration plot preparation. B




Spontaneous Redox Reaction: Ag Deposition on a Cu Sheet

Supporting Information (Instructor notes and Student Handout) shows a detailed description of the procedure that needs to be followed by students, the items that they should include in their lab report, and some considerations to be taken into account by the instructor.

As shown in Figure 2, the spontaneous Ag deposition on a Cu sheet from LS1 was easily observed. The AgNO3 in the solution

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HAZARDS Hydrogen peroxide can irritate skin and eyes. Hydrochloric, sulfuric, and nitric acids are strong corrosive mineral acids. Aqua regia, composed of strong acids, is unstable, corrosive, and fuming. Never store a stoppered bottle of aqua regia because it may explode. The used glassware must be clean and free of organic matter. Avoid breathing vapors from all the aforementioned chemicals, and handle with proper personal protection items, including a lab coat, safety goggles, and gloves. If skin or eye exposure occurs, flush with plenty of water. All the operations must be carried out in a fume hood. The liquid waste generated in this experiment cannot be tipped down the drain because it is acid and contains heavy metals, which are very harmful to health. Therefore, it must be neutralized and kept in a decanter to be recycled by a specific company.

Figure 2. Pictures by a student group of the Ag deposition on a Cu sheet at different times: 2 min (A), 20 min (B), and 2 h 40 min (C).

was reduced to form Ag, which was spontaneously deposited onto the Cu sheet. At the same time, Cu from the sheet was oxidized to Cu(NO3)2, which passed to the aqueous media to confer it the bluish color observed in Figure 2C. The reactions involved in this process are shown below (eqs 1−3):

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RESULTS AND DISCUSSION The results here presented correspond to those obtained by 8 master students in industrial engineering, which worked in pairs, and 2 students in their last year of the industrial engineer degree (undergraduates) who were about to do their final year dissertation in our research laboratory, who worked individually.

Ag +(aq) + e− → Ag(s)

(E° = + 0.80 V)

(1)

Cu(s) → Cu 2 +(aq) + 2e−

(E° = − 0.34 V)

(2)

The overall reaction is 2AgNO3(aq) + Cu(s) → 2Ag(s) + Cu(NO3)2 (aq) (E° = +0.46 V)

Removing Precious Metals from Spent SPPtEs

(3)

The progress of the reaction is observed from the initial state of Cu and the AgNO3 solution (Figure 2A) to the early formation of a silver film on the Cu sheet (Figure 2B), finally followed by the formation of larger Ag deposits and the change in the aqueous solution to a bluish hue due to the presence of Cu2+ in solution (Figure 2C).

Spent SPPtEs were treated according to the above-described procedure (see Supporting Information for details). Figure 1

Nonspontaneous Redox Reaction: Pt Electrochemical Deposition

The Pt from LS2 was electrodeposited onto an SPCE. It was possible to observe the deposition of a Pt-film on the working electrode of the SPCE at first glance (Figure 3). The Pt deposition on the working electrode follows reaction 4. After considering this, students were asked to calculate the Figure 1. SPPtEs: (A) spent, (B) after H2SO4 treatment, (C) after HNO3 treatment, and (D) after “aqua regia” treatment.

shows a spent SPPtE before treatment (Figure 1A) and after all the procedure steps. H2SO4 treatment clearly removed the plastic cover without affecting the metallic inks of the electrode (Figure 1B). The HNO3 treatment removed both the reference electrode and electrical connections made of silver without dissolving platinum ink (Figure 1C). The aqua regia treatment was able to dissolve the Pt-ink from the working and counter electrodes (Figure 1D).9 Students weighed three electrodes before and after each step. The plastic cover was previously removed by the instructor to shorten times (loss of 6.8 ± 1.3 mg per electrode was observed after the H2SO4 treatment). The removal of Ag-ink and Pt-ink involved a loss of 4.3 ± 0.9 mg and 3.9 ± 0.5 mg (n = 6 groups of students), respectively, per electrode in every step.

Figure 3. An SPCE before (A) and after (B) being modified with Pt by electrodeposition. C




Figure 4. Example of an amperometric response to H2O2 additions of one of the student groups (A) and the calibration plot (B) for the Pt@SPCE at 0.7 V. The red numerals in part A indicate the concentration change in H2O2 in the media (10 mL) as a result of these additions. Error bars in part B represent the standard deviation from the mean, corresponding to the results obtained by six student groups.

theoretical amount of Pt that should have been electrodeposited onto the SPCE according to Faraday’s eq (Equation 5), where m is the mass of substance deposited on the electrode, F is Faraday’s constant, Q is the total electric charge, M is the molar mass of the substance being deposited and z is the number of electrons transferred per ion. PtCl 6 2 −(aq) + 4e− → Pt(s) + 6Cl−(aq) m=

1 QM F z

(4)

(5)

When comparing the weight of SPCEs before and after Pt electrodeposition, students found that the mass of the deposited Pt (1.2 ± 0.4 mg) was significantly lower than the expected value according to Faraday’s equation (∼5.5 mg, by taking into account the measured current intensities, an average value of ∼0.015 A for 12 min). They gave satisfactory explanations of this issue, in relation to the actual efficiency of the Pt electrodeposition from H2PtCl6 (reported to account for ∼20%, i.e., ∼1.1 mg).16 The poor efficiency of the process (low Pt deposition) was connected to the evolution from the protons of the acidic media to H2 (in their reports students indicated that they had observed bubbles forming during the electrodeposition process). A high proportion of the charge applied to the system was consumed during this process.

Figure 5. Example of an amperometric response (raw data) of a Pt@ SPCE at 0.7 V to three 10 μL additions of a commercial hair lightener to a volume of 10 mL of PB.

10 mL of PB. Students used the average of the current variation to calculate the H2O2 concentration in the lotion. The obtained results follow: 1.3 ± 0.4 M, with a variation coefficient of 29.1% (n = 6). This result agrees well with the data previously obtained by the spectrophotometric method (1.07 ± 0.04 M), which was considered to be the “true” value. The statistical analysis yielded a recovery of 118.7%. The noise in the signal (Figures 4A and 5) increased with successive additions of H2O2. This phenomenon is connected with the formation of oxygen bubbles on the working electrode surface due to the Pt-catalyzed decomposition of H2O2 into water and oxygen gas.

Electrochemical Determination of H2O2 Using Pt-Modified SPCEs

The Pt@SPCE prepared in the previous step was tested as a H2O2 sensor. The amperometry technique was employed by using a potentiostat. Figure 4A shows a typical amperometric response of a Pt@SPCE to the addition of consecutive volumes of H2O2 in 10 mL of PB at 0.7 V (vs the pseudo-Ag-reference). The studied calibration range fell between 0 and 6.5 mM of H2O2. Upon the addition of H2O2 the Pt@SPCE response to the analyte was fast and allowed stable steps to be obtained (Figure 4A). Students obtained linear calibration plots of current versus concentration (Figure 4B) with good R2 values. The different student groups obtained similar results, with sensitivities of 71 ± 20 mA μM−1 and limits of detection based on an S/N ratio = 3 of 80 ± 62 μM (n = 6). The obtained calibration plot was used to determine the H2O2 concentration in a commercial hair lightener. Figure 5 shows an example of the amperometric response of a modified electrode to three 10 μL additions of lotion in a total volume of

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EVALUATION The students’ achievement of the learning outcomes was evaluated both by the successful completion of the laboratory experiment and by the assessment of their laboratory reports. The items that such a report should include are listed in the Student Handout (Supporting Information). The most common misconceptions that they displayed were connected with the identification of anode/cathode, their polarity, and, consequently, the sense in which electrons/ions flowed in the system. Considering the characteristics of our students (they have achieved an engineering degree with a minor program in chemistry), it was necessary to revise/explain some basic concepts in electrochemistry related to the experiment. We used the spare time during Pt dissolution in aqua regia to do so, D




Electrodes in the Determination of Quinine in Tonic Water. J. Chem. Educ. 2013, 90 (12), 1681−1684. (6) Popa, A.; Abenojar, E. C.; Vianna, A.; Buenviaje, C. Y. A.; Yang, J. H.; Pascual, C. B.; Samia, A. C. S. Fabrication of Metal NanoparticleModified Screen Printed Carbon Electrodes for the Evaluation of Hydrogen Peroxide Content in Teeth Whitening Strips. J. Chem. Educ. 2015, 92 (11), 1913−1917. (7) Martin-Yerga, D.; Costa Rama, E.; Costa García, A. Electrochemical Study and Determination of Electroactive Species with Screen-Printed Electrodes. J. Chem. Educ. 2016, 93 (7), 1270−1276. (8) Chyan, Y.; Chyan, O. Metal electrodeposition on an integrated, screen-printed electrode assembly. J. Chem. Educ. 2008, 85 (4), 565− 567. (9) Agrisuelas, J.; González-Sánchez, M. I.; Valero, E. Hydrogen peroxide sensor based on in situ grown Pt nanoparticles from waste screen-printed electrodes. Sensors Actuators B Chem. 2017, 249, 499− 505. (10) González-Sánchez, M. I.; González-Maciá, L.; Pérez-Prior, M. T.; Valero, E.; Hancock, J.; Killard, A. J. Electrochemical detection of extracellular hydrogen peroxide in Arabidopsis thaliana: a real-time marker of oxidative stress. Plant, Cell Environ. 2013, 36 (4), 869−878. (11) Chen, W.; Cai, S.; Ren, Q. Q.; Wen, W.; Zhao, Y. D. Recent advances in electrochemical sensing for hydrogen peroxide: a review. Analyst 2012, 137 (1), 49−58. (12) Hendel, S. J.; Young, E. R. Introduction to Electrochemistry and the Use of Electrochemistry to Synthesize and Evaluate Catalysts for Water Oxidation and Reduction. J. Chem. Educ. 2016, 93 (11), 1951− 1956. (13) Lyon, Y. A.; Roberts, A. A.; McMillin, D. R. Exploring Hydrogen Evolution and the Overpotential. J. Chem. Educ. 2015, 92 (12), 2130− 2133. (14) Lunsford, S. K.; Speelman, N.; Stinson, J.; Yeary, A.; Choi, H.; Widera, J.; Dionysiou, D. D. Electroanalytical and spectroscopic studies of poly(2,2′-bithiophene)-modified platinum electrode to detect catechol in the presence of ascorbic acid. J. Chem. Educ. 2008, 85 (1), 128−129. (15) Amor-Gutierrez, O.; Rama, E. C.; Fernandez-Abedul, M. T.; Costa-Garcia, A. Bioelectroanalysis in a Drop: Construction of a Glucose Biosensor. J. Chem. Educ. 2017, 94 (6), 806−812. (16) Rao, C. R. Q.; Trivedi, D. C. Chemical and electrochemical depositions of platinum group metals and their applications. Coord. Chem. Rev. 2005, 249, 613−631.

although a previous lesson covering these points could also be suitable.

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CONCLUSIONS This experiment provides students with the chance to experience first-hand some important concepts in electrochemistry. Seeing the differences between spontaneous and nonspontaneous redox reactions helped them to understand the theory behind these processes. Evaluations made from lab reports showed that students enjoyed the real-world nature of the experiment, thanks to the recycling of electronic waste (ewaste) and the determination of an analyte in a real commercial sample. Students became aware of the importance of recycling metals from e-waste and felt more engaged in protecting the environment.

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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.7b00345. Notes for instructors and a complete student handout with theoretical introduction (PDF, DOCX)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jerónimo Agrisuelas: 0000-0003-0193-2857 Edelmira Valero: 0000-0001-8636-4574 Author Contributions †

M.-I.G.-S. and B.G.-M. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the Spanish Ministry of Economy and Competitiveness (MINECO), Projects BFU2013-44095-P and BFU2016-75609-P (cofounded with FEDER funds, EU). B.G.-M. is a postdoctoral research fellow of the Youth Employment Initiative (JCCM, Spain, cofounded with ESF funds, EU).

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REFERENCES

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