Introducing Environmental and Sustainable Chemistry Topics Using a

Laboratory Experiment pubs.acs.org/jchemeduc

Introducing Environmental and Sustainable Chemistry Topics Using a Nanotechnology Approach: Removing Hazardous Metal Ions by Means of Humic-Acid-Modified Superparamagnetic Nanoparticles Delmárcio Gomes da Silva,† Fernando Menegatti de Melo,† Alceu Totti Silveira, Jr.,† Bruno Constancio da Cruz,‡ Caio Cesar Pestana Prado,‡ Luana Cristina Pereira de Vasconcelos,‡ Vitor Amaral Sanches Lucas,‡ and Henrique Eisi Toma*,† †

Universidade de São Paulo, Departamento Química Fundamental, Instituto de Química, São Paulo, São Paulo 05508-000, Brazil Centro Estadual de Educaçaõ Tecnológica Paula Souza, Etec Osasco II, Osasco, São Paulo 06296-220, Brazil

‡

S Supporting Information *

ABSTRACT: A laboratory experiment has been developed to illustrate environmental and sustainability aspects, focusing on the wastewater treatment by means of superparamagnetic nanoparticles functionalized with humic acid. The experiment, conducted by a group of high school students, involves nanoparticle synthesis and minor characterization, followed by their interaction with typical metal ion contaminants in water. Coordination chemistry concepts were explored to help students understand the experiment, using competitive agents, such as ammonia, for recovering and recycling the nanoparticles, highlighting the great affinity of humic acid for the metal ions.

KEYWORDS: First-Year Undergraduate/General, High School/Introductory Chemistry, Environmental Chemistry, Interdisciplinary/Multidisciplinary, Laboratory Instruction, Hands-On Learning/Manipulatives, Inquiry-Based/Discovery Learning, Colloids, Magnetic Properties, Nanotechnology

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INTRODUCTION Nowadays, nanotechnology, environment, and sustainability issues are becoming quite relevant in chemistry courses. However, there is a limited number of experiments available for exploring such aspects in an interactive way. Such challenge can be rather gratifying for teaching nanotechnology concepts, exploring their possible application in environmental problems using natural resources or alternatives.1 Pursuing this task, this didactic experiment focuses on the nanoparticles chemistry and explores their association with humic acidsan important natural substancefor removing hazardous metal ions from contaminated water in the environment. The experiment also calls the attention to the water management problem, reinforcing the challenges of developing environmentally acceptable technologies and the importance of keeping improving educational strategies.2−4 In this context, it should be noticed that among the several problems of water contamination, a major concern is brought about by the heavy metals. They are particularly troublesome because, unlike most organic species, they are not biodegradable and can accumulate in living tissues. Moreover, heavy, hazardous metals are not only introduced in aquatic systems from the natural wear of soils and rocks5 but also result from human activities such as mine processing and the industrial use of substances containing © 2016 American Chemical Society and Division of Chemical Education, Inc.

Hg(II), Pb(II), Cr(III), Cr(VI), Ni(II), Co(II), Cu(II), Cd(II), Ag(I), and As(III,V) species. In order to perform environmental detoxification, techniques involving adsorption, precipitation, ion exchange, reverse osmosis, electrochemical treatments, membrane filtration, evaporation, flotation, oxidation, and biosorption processes are being used.6 With the advent of nanotechnology, such techniques can be greatly improved by incorporating versatile nanoparticles, nanomaterials and nanosensors in the process.7−9 In this regard, VanDorn et al.10 have employed magnetite nanoparticles (Fe3O4) to adsorb arsenic ions and for removing them from the contaminated solution by making use of their magnetic properties. Furlan et al.11 have impregnated magnetic nanoparticles with activated carbon, introducing a very interesting didactic experiment to illustrate the use of such materials in the purification of water. Herein, we are exploring a new concept, starting from the functionalization of Fe3O4 nanoparticles with humic acids, in Received: March 9, 2016 Revised: August 5, 2016 Published: September 23, 2016 1929

DOI: 10.1021/acs.jchemed.6b00172 J. Chem. Educ. 2016, 93, 1929−1934

Journal of Chemical Education

Laboratory Experiment

order to generate a water-soluble nanomaterial exhibiting high capacity to sequester metal ions. Magnetite is a rather common raw mineral, however in the nanometric scale it is better prepared by synthetic routes. Its great advantage is associated with the easy recovery and processing by using a magnetic field or a commercial magnet. However, it should be noticed that magnetite nanoparticles are prone to air oxidation and aggregation in aqueous media, requiring their surface modification with charged polymers, polyelectrolytes or surfactants.12,13 In this sense, the use of humic acid (HA) can be particularly rewarding, because it as a natural antioxidizing compound exhibiting a great affinity for metal ions,14,15 as well as for Fe3O4 nanoparticles (MagNP), because of its rich phenolic content (Figure 1). The sorption of HA on the Fe3O4 nanoparticles (MagNP/HA) also enhances the stability of the nanodispersions, preventing their aggregation.16

As major components of natural organic matter, humic substances are involved in chemical interactions in soil and aqueous systems. They are widely present in natural waters and soils, and allow plants to extract metal elements from their environments.17 Humic substances are usually divided into two fractions: humic acid and fulvic acid. The former is soluble in alkaline solutions but insoluble in acidic solutions, whereas the latter, of a lower molecular weight, is soluble in both alkaline and acidic solutions. HA has a skeleton of alkyl and aromatic units supporting carboxylic acid, phenolic hydroxyl, and quinone functional groups, as shown in Figure 1, and it is an abundant organic compound in natural aqueous systems.16 These functional groups are suitable for binding metal ion contaminants from water.18,19 In this work, we describe a 3 h laboratory experiment which was successfully applied to high school students, focusing on the preparation of magnetic nanoparticles modified with humic acid, and their application as absorbers for metal ion contaminants in water. Before the laboratory class, the students attended a 2 h lecture/discussion on environmental chemistry, sustainability and nanotechnology issues. As illustrated in Figure 2, the students could see that humic acid has a natural potential to capture ions in a plant root acting as a natural soil conditioner. Exploring this concept, the ability of HA to form complexes with metal ions was used in a synergic association with magnetic nanoparticles for the treatment of water containing some most common contaminants, such as Ni2+, Co 2+, Cu 2+ , Hg 2+, and Pb 2+ ions. Operational issues, encompassing the synthesis of MagNP and its functionalization with HA were also dealt with in this preliminary lecture. During the laboratory activity, the students performed the synthesis of nanoparticles and they also conducted remediation tests on contaminated water samples provided by the instructor. After the experiment, the students discussed the results and a brief scientific report was requested, including

Figure 1. Structure of humic acid showing the aromatic units with carboxylic acid, phenolic hydroxyl, and quinone functional groups.

Figure 2. Capturing sustainable chemistry concepts by observing a natural phenomenon and developing new ideas with a mimicking nanomaterial for removing heavy metals from water. 1930




Figure 3. (A) Students preparing the reagents for the synthesis of MagNP/HA. (B), (C) Students conducting the process of purification to remove the excess of humic acid that is not bound to nanoparticles.

(brown color washings, as shown in Figure 3) and then dried in a vacuum oven. Typically, 5.0 g of MagNP/HA were dispersed in 100 mL of water, in order to obtain a 5.0% (w/w) suspension.

their own evaluation of the subject and the pedagogical strategy employed.

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EQUIPMENT AND REAGENTS In the laboratory work, the following facilities have been employed: a semianalytical balance, a hot plate with mechanical or magnetic stirrer, a three-neck round-bottom flask equipped with a reflux condenser, 0−100 °C thermometer, and conventional laboratory glassware. Depending on the existing support, the experiment can be further extended in order to include specific measurements for the characterization of the nanoparticles, such as DLS (dynamic light scattering) for size evaluation and EDS (energy dispersive spectra) for metal ion analysis. Standard colorimetric methods can also be employed for this purpose as an alternative method for qualitative determination.20

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Preparation of Heavy Metal Solution

The students received 50 mL of a previously prepared heavy metal (chlorides or sulfates) solution containing Ni2+ (50 ppm), Co2+ (50 ppm), Cu2+ (50 ppm), Pb2+ (35 ppm), and Hg2+ (5 ppm). Adsorption of Heavy MetalsWater Purification

As shown in Table 1, 1.0 mL samples of the metal containing solution were transferred into four test tubes. Each student was responsible for a test tube. After adding different volumes of MagNP/HA (5.0% w/w), the pH was adjusted to 9 using NH4OH (25%), and the final volume of the mixture was adjusted to 5.0 mL. The use of the ammonia solution provided a complexing media capable of preventing the precipitation of

EXPERIMENTAL PROCEDURE

Synthesis of MagNP/HA

Table 1. Sample Components for the Evaluation of the Metal Removal Efficiency Using Variable Amounts of MagNP/HA

The synthesis of MagNP/HA was based on a modified chemical coprecipitation method,13 starting from 1.5 g of humic acid (Sigma-Aldrich), dissolved in 300 mL of water and 25 mL of NH4OH (25%). This mixture was heated to 90 °C in a 500 mL three-neck round-bottom flask equipped with a reflux condenser (solution A). In the other flask, 4.28 g of FeSO4· 7H2O and 8.56 g of FeCl3·6H2O were previously dissolved in 100 mL of water (solution B). This solution was rapidly added to solution A. The mixture was mechanically (600 rpm) stirred during the process and the temperature was maintained at 90 ± 5 °C for 30 min and then cooled to room temperature. The black MagNP/HA precipitate was confined at the bottom, using an external magnet. The wet paste was rinsed several times with water in order to remove the excess of humic acid

Sample Tubes Parameters Solution Heavy Metals, mL MagNP/HA suspension, mL NH4OHconc, mL pH, Adjusted Water, mL Final Volume, mL 1931

1

2

3

4

1.0

1.0

1.0

1.0

1.0

0.5

0.25

0.1

0.15

0.15

0.15

0.15

9−10 2.85 5.0

9−10 3.35 5.0

9−10 3.60 5.0

9−10 3.75 5.0

Control 1.0

4.0 5.0




Figure 4. Left: Flasks containing different amounts of nanoparticles MagNP/HA (% w/w). Right: After the magnetic separation.

The zeta potential of the MagNP/HA measured at pH 9 was −52.1 mV, indicating that the magnetic nanoparticles were negatively charged, as shown in Figure 5A. Such a large zeta potential is important to prevent the coalescence of the nanoparticles, facilitating the sorption of the positively charged metal ions. The size distribution of MagNP/HA was probed by

the metal hydroxides, by forming amine complexes with the metal ions. It should be noticed that humic acid is a strong complexing agent, and even under diluted conditions, it is able to displace the NH3 ligands from the metal ion coordination sphere. The mixture was stirred for 30 min and the magnetic nanoparticles (MagNP/HA) containing the sequestered metal ions were separated from the mixture using a neodymium (Nd2Fe14B) permanent magnet. Then each student separated an aliquot of this supernatant (Figure 4) and delivered to the instructor, in order to perform the analysis of the residual concentration of the metals by EDS (X-ray fluorescence). As a control, the instructor also prepared standard samples, by transferring 1.0 mL of solution containing the metals into another test tube and adjusting the final volume to 5.0 mL with water.

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HAZARDS

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RESULTS AND DISCUSSION

Heavy metals are toxic substances. The instructor should prepare the hazardous metals solution beforehand in order to reduce the student exposure, with the special care required for handling mercury and lead salts. Standard procedures for safe handling of chemicals should be followed. Students must wear lab coats, nitrile gloves, and goggles at all times. The neodymium magnets used for iron removal are very strong and should be handled with care to avoid pinching.

Characterization of MagNP/HA

For the purposes of this experiment, it is not necessary to characterize the MagNP/HA nanoparticles. For this reason, these results were presented as a Supporting Information for the students, to be discussed in the proper occasion. The characterization was performed by the team of supervisors of this project, but every procedure and result was discussed with the students along the laboratory experiments. In particular, the evaluation of the zeta potential was highly rewarding, allowing to understand the role of HA in the suspension stability and the changes resulting from the binding of the metal ions.

Figure 5. (A) Zeta potential found (−52.1 mV) indicates that the magnetic nanoparticles are negatively charged. (B) Size distribution of MagNP/HA reveals an average diameter of 72.5 nm. 1932




Figure 6. Metal ions removal observed for the several samples (T1−T4) in relation to the control experiment (IC).

particles are present in small amounts, the capture of Hg2+ is not efficient because of the presence of ammonia in excess, generating [Hg(NH3)2]2+ species. The same observation applies to Cu2+ ions. For this reason, the reaction involved would be better expressed as

dynamic light scattering (DLS) as shown in Figure 5B revealing a maximum distribution rate at 72.5 nm (22.5%). Removal of ContaminantsApplication in Environmental Remediation

After each student received the results, the residual concentrations of metal ions in the supernatant were organized in a table and graphics and discussed at the end of the laboratory session. Typical results from the analysis of the supernatant after treatment with MagNP/HA by EDS can be seen in Figure 6. The results indicate a substantial removal of the metal ions from all the samples; however, the correct interpretation requires the understanding of the chemical equilibrium involved. The quantitative data collected in Table 2 allows to discuss in more detail the metal ion removal efficiency by MagNP/HA.

M2 +(aq) + n NH3 ⇌ [M(NH3)n ]2 + MagNP MagNP + [M(NH3)n ]2 + ⇌ + n NH3 HA M(HA)x Recycling the Particles

Recycling experiments can be carried out by confining magnetically the nanoparticles at the bottom of the test tube, and treating the residue with one drop of concentrated ammonia solution. After completing the volume with water, the metal ions released in the process can be easily demonstrated using classical spot tests, for example, with dithiooxamide and sodium rhodizonate in a qualitative way,20 or by means of the EDS technique. Colorimetric analysis can also be performed;20 however, in this case, we recommend dealing with just one element, separately, in order to overcome the interference problems. It should be noted that the magnetic procedure for confining and recycling the nanoparticles is very simple and quite convenient from the point of view of green chemistry and sustainability.21,22 In the experiment, the students were also stimulated to think about the fate of the heavy metal ions in natural water resources and on the essential role of the humic acids as sequestering agents for metal ions in the environment. As a final remark, the group of students were invited to participate in a schoolroom seminar program after preparing a short monograph on sustainable technologies for the treatment of water resources. This activity reinforced the very positive evaluation of their learning from this particular experiment.

Table 2. Average Analytical Data Obtained by the Students for the Metal Removal Rates Average Removal of Metal, % Sample Tube

Ni

Co

Pb

Cu

Hg

1 2 3 4

95 93 86 80

91 88 91 91

98 97 95 92

94 81 52 26

62 44 12 0

Although the removal efficiency was nearly comparable for Ni2+, Co2+, Cu2+, and Pb2+ ions in Tubes 1 and 2, the results indicated a substantial decrease in the case of the Hg2+ ions. This observation might look rather puzzling because Hg2+ ions are known to form very strong complexes with most common ligands. Actually, this point requires the understanding of soft− hard acid−base concepts because mercury ions are typically soft, and the dominant complexing groups in humic acids are hard or of intermediate nature. In this way, when MagNP/HA 1933



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(10) VanDorn, D.; Ravalli, M. T.; Small, M. M.; Hillery, B.; Andreescu, S. Adsorption of Arsenic by Iron Oxide Nanoparticles: A Versatile, Inquiry-Based Laboratory for a High School or College Science Course. J. Chem. Educ. 2011, 88 (8), 1119−1122. (11) Furlan, P. Y.; Melcer, M. E. Removal of Aromatic Pollutant Surrogate from Water by Recyclable Magnetite-Activated Carbon Nanocomposite: An Experiment for General Chemistry. J. Chem. Educ. 2014, 91 (11), 1966−1970. (12) Maity, D.; Agrawal, D. Synthesis of Iron Oxide Nanoparticles under Oxidizing Environment and Their Stabilization in Aqueous and Non-Aqueous Media. J. Magn. Magn. Mater. 2007, 308 (1), 46−55. (13) da Silva, D. G.; Toma, S. H.; de Melo, F. M.; Carvalho, L. V. C.; Magalhães, A.; Sabadini, E.; dos Santos, A. D.; Araki, K.; Toma, H. E. Direct Synthesis of Magnetite Nanoparticles from Iron (II) Carboxymethylcellulose and Their Performance as NMR Contrast Agents. J. Magn. Magn. Mater. 2016, 397, 28−32. (14) Kerndorff, H.; Schnitzer, M. Sorption of Metals on Humic Acid. Geochim. Cosmochim. Acta 1980, 44 (11), 1701−1708. (15) Tang, W.-W.; Zeng, G.-M.; Gong, J.-L.; Liang, J.; Xu, P.; Zhang, C.; Huang, B.-B. Impact of Humic/Fulvic Acid on the Removal of Heavy Metals from Aqueous Solutions Using Nanomaterials: A Review. Sci. Total Environ. 2014, 468, 1014−1027. (16) Sparks, D. L. Environmental Soil Chemistry; Academic Press: Newark, NJ, 2003. (17) Aiken, G. R.; McKnight, D. M.; Wershaw, R. L.; MacCarthy, P. Humic Substances in Soil, Sediment, and Water: Geochemistry, Isolation and Characterization; John Wiley & Sons: New York, 1985. (18) Pandey, A. K.; Pandey, S. D.; Misra, V. Stability Constants of Metal−Humic Acid Complexes and Its Role in Environmental Detoxification. Ecotoxicol. Environ. Saf. 2000, 47 (2), 195−200. (19) Hankins, N. P.; Lu, N.; Hilal, N. Enhanced Removal of Heavy Metal Ions Bound to Humic Acid by Polyelectrolyte Flocculation. Sep. Purif. Technol. 2006, 51 (1), 48−56. (20) Vogel, A. I. Quantitative Inorganic Analysis; Longmans: London, 1968. (21) Toma, H. E. Developing Nanotechnological Strategies for Green Industrial Processes. Pure Appl. Chem. 2013, 85 (8), 1655− 1669. (22) Toma, H. E. Magnetic Nanohydrometallurgy: A Nanotechnological Approach to Elemental Sustainability. Green Chem. 2015, 17, 2027−2041.

CONCLUSION The experiment provided an interesting approach for teaching advanced high school or first year undergraduate students the chemistry of nanoparticles and humic acids, highlighting relevant concepts of coordination chemistry and nanotechnology, and introducing possible strategies for dealing with metal ions in the environment. Sustainability was also valorized in the experiment, from the choice of recyclable, environmentally correct, humic acid and magnetite nanoparticles systems to perform the treatment of heavy metal contaminated water.

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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.6b00172. Safety information, detailed characterization of MagNP/ HA, experimental procedures and notes for instructors. (PDF) Safety information, detailed characterization of MagNP/ HA, experimental procedures and notes for instructors. (DOCX)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank all the teachers and supervisors of Centro Estadual de Educaçaõ Tecnológica Paula Souza and ETEC OSASCO II, and FAPESP Grant 2013/24725-4 for support.

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REFERENCES

(1) Fleischer, T.; Grunwald, A. Making Nanotechnology Developments Sustainable. A Role for Technology Assessment? J. Cleaner Prod. 2008, 16 (8), 889−898. (2) Bernal, P. J. Water and Life in the International Year of Chemistry. J. Chem. Educ. 2011, 88 (5), 526−529. (3) Nash, J. J.; Meyer, J. A.; Nurrenbern, S. C. Waste Treatment in the Undergraduate Laboratory: Let the Students Do It! J. Chem. Educ. 1996, 73 (12), 1183. (4) Dunnivant, F. M.; Kettel, J. An Environmental Chemistry Laboratory for the Determination of a Distribution Coefficient. J. Chem. Educ. 2002, 79 (6), 715. (5) Leštan, D.; Luo, C.-l.; Li, X.-d. The Use of Chelating Agents in the Remediation of Metal-Contaminated Soils: A Review. Environ. Pollut. 2008, 153 (1), 3−13. (6) Fu, F.; Wang, Q. Removal of Heavy Metal Ions from Wastewaters: A Review. J. Environ. Manage. 2011, 92 (3), 407−418. (7) Shannon, M. A.; Bohn, P. W.; Elimelech, M.; Georgiadis, J. G.; Mariñas, B. J.; Mayes, A. M. Science and Technology for Water Purification in the Coming Decades. Nature 2008, 452 (7185), 301− 310. (8) Kim, J.; Van der Bruggen, B. The Use of Nanoparticles in Polymeric and Ceramic Membrane Structures: Review of Manufacturing Procedures and Performance Improvement for Water Treatment. Environ. Pollut. 2010, 158 (7), 2335−2349. (9) Toma, H. E. O Mundo Nanométrico: A Dimensão do Novo Século; Oficina de textos: São Paulo, Brazil, 2009. 1934


Introducing Environmental and Sustainable Chemistry Topics Using a

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