Rapid Production of a Porous Cellulose Acetate Membrane for Water

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Laboratory Experiment pubs.acs.org/jchemeduc

Rapid Production of a Porous Cellulose Acetate Membrane for Water Filtration using Readily Available Chemicals Adrian Kaiser, Wendelin J. Stark, and Robert N. Grass* Institute for Chemical and Bioengineering, ETH Zurich, Vladimir-Prelog-Weg 1, CH-8093 Zurich, Switzerland S Supporting Information *

ABSTRACT: A chemistry laboratory experiment using everyday items and readily available chemicals is described to introduce advanced high school students and undergraduate college students to porous polymer membranes. In a three-step manufacturing process, a membrane is produced at room temperature. The filtration principle of the membrane is then illustrated by filtering solutions containing pigmentary watercolor or food coloring. A comparison of the filtration results shows that insoluble watercolor pigments are too large to pass the pores of the membrane and are successfully rejected by the membrane, whereas the food coloring is completely soluble in water and easily passes the membrane. The laboratory experiment can be performed in a 2 h activity and serves the purpose of (1) exposing students to a new and interesting field of material science. It (2) makes them familiar with porous membranes for the production of safe drinking water and (3) introduces them to a templateremoval technique utilizing acid/base theory. There were 52 advanced high school students and 55 high school teachers in Switzerland who already successfully performed the laboratory experiment and found the activity engaging and motivating. KEYWORDS: Membranes, Materials Science, Precipitation/Solubility, High School/Introductory Chemistry, Environmental Chemistry, Laboratory Instruction, Nanotechnology, First-Year Undergraduate/General, Interdisciplinary/Multidisciplinary, Hands-On Learning/Manipulatives

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INTRODUCTION

experiments for the removal of nanomaterials have been previously described,9 and a laboratory experiment comparing various biochemical separation technologies (including filtration) has been presented.10 The experiment presented here differs from previous approaches, as the filtration device itself (a porous membrane) is fabricated by the students from standard materials, and then tested for its separation performance.

Importance of Safe Drinking Water

Access to safe and clean drinking water is still a global problem. In developing countries, about 80% of illnesses are linked to poor water and sanitation conditions.1 Among the most common health risks of impure drinking water are waterborne diseases caused by pathogenic bacteria, viruses, and protozoan parasites. In a comprehensive study, Bain and co-workers reviewed the microbial quality of drinking water, and estimated that around 1.8 billion people globally use a source of drinking water that is contaminated with E. coli or other thermotolerant coliform (TTC) bacteria.2 Various approaches, with their inherent advantages and disadvantages, have been studied to remove microorganisms related to waterborne diseases.3,4 Portable water purification devices currently in use include systems based on ultra- and microfiltration membranes, ceramic filters, activated carbon filters, or chemical disinfection with halogens.5 The drawbacks of the best performing systems are usually their complex installation and periodic maintenance, resulting in high operational costs and thus rendering these systems unsuitable for developing countries. Additionally, many point-of-use treatment devices are not effective against all possible contaminants. Therefore, novel water treatment technologies are currently being investigated.6,7 Water quality issues are an integral part of many high school chemical courses, and several experiments have been developed as educational laboratory procedures.8 Water filtration © XXXX American Chemical Society and Division of Chemical Education, Inc.

Microfiltration Membranes

Solutions to improve the sanitary conditions in developing countries should preferentially be easy to handle, low-cost, durable, and environmentally sustainable.11 Microfiltration membrane technology represents an effective alternative to obtaining safe drinking water due to its high efficiency in removing microbiological contaminants.4,12 Moreover, the technology benefits from its ease of operation, relatively low energy consumption, and high water flux at low operating pressure.13 A procedure for the production of mesoporous microfiltration membranes by removing a nanoparticle pore template was developed by Kellenberger et al.14 In their method, calcium carbonate nanoparticles were used as pore templates to fabricate polymeric membranes with tunable pore sizes. This technique was upscaled to an industrial level and resulted in the development of the commercially available DrinkPure Received: October 26, 2016 Revised: February 24, 2017

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DOI: 10.1021/acs.jchemed.6b00776 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

water filter.15,16 The key part of this water filter is a microfiltration membrane that is produced via pore templation using calcium carbonate nanoparticles. The industrial production of this membrane is easy and cheap, but it relies heavily on sophisticated machinery for processes such as ball-milling, dispersion coating, and solvent evaporation.

• Mirror or glass plate (148 × 210 mm2 or larger area) • Scotch tape • Ruler with stainless-steel cutting edge or aluminum profile • Two plastic basins for diluted hydrochloric acid and water bath • Cellulose acetate (Sigma-Aldrich No. 180955) • Calcium carbonate (Sigma-Aldrich No. 21069) • Watercolor (e.g., Artists Loft Fundamentals Watercolor Pan Set from amazon.com) • Food coloring (e.g., Brilliant Blue FCF E133)

Cellulose Acetate Membrane for Water Filtration

In this paper, we describe a laboratory experiment to produce a drinking water membrane filter that, in its production stages, resembles the commercially available drinking filter, but is accomplished using everyday items such as a kitchen blender, glass plates, rulers, and readily available chemicals. The whole experiment can be performed in a 2 h laboratory exercise. The Supporting Information for this article includes a student handout and instructor notes for the performance of the experiment in a class (ca. 20−40 students). Production of the membrane is based on the addition of calcium carbonate to a solution of cellulose acetate dissolved in acetone. Glycerol is also added to the reaction mixture and acts as a pore forming spacer.17 The polymer solution is then mixed using a kitchen blender, and the resulting dispersion is spread on a glass plate using a stainless-steel ruler. After evaporation of the solvent in ambient air, the calcium carbonate particles and glycerol are washed out of the polymeric matrix in an acid bath and subsequently rinsed with a water bath (Figure 1).

Preparation of Cellulose Acetate Polymer Solution

The cellulose acetate polymer solution was prepared by mixing 20 g of cellulose acetate with 200 g of acetone in a 250 mL Schott flask. A magnet was added to the flask, the flask was closed (to limit solvent evaporation), and the polymer solution was stirred over a period of 1 h using a magnetic stirrer. The mixing of the polymer solution was finished when the solution became clear. Preparation of Cellulose Acetate Dispersion

The whole cellulose acetate polymer solution was transferred into a kitchen blender. Next, 41 g of calcium carbonate and 17.6 g of glycerol were added, and the suspension was stirred for 3 min at the highest setting of the kitchen blender. The dispersion was put into a fresh 250 mL Schott flask and closed to limit solvent evaporation. The kitchen blender was immediately filled with water to precipitate the remaining dispersion, and the polymer waste was disposed into the household refuse. Production of Membrane Sheet

Two parallel edges of a mirror or glass plate were plastered with four consecutive layers of Scotch tape, which will define the thickness of the membrane (Figure 2a). Subsequently, the Figure 1. Production of polymeric membranes using a kitchen blender. A calcium carbonate particle−polymer dispersion is cast on a glass support and the solvent evaporated in ambient air. Acid bath incubation of the membrane dissolves the calcium carbonate particles and yields a porous membrane.

Functional Testing of Membrane Using Watercolor and Food Coloring

Students test the performance of the resultant membrane with a solution of watercolor and food coloring. While, upon mixing with water, both result in colored water, the used food color is a water-soluble dye, and the watercolor is a pigmentary dispersion, with particles in the size range 40−80 μm. The pigmentbased watercolor solution is first passed through the membrane. The filtration is then repeated with the food coloring solution. In both cases the students are asked to record their observations. The experiments with both solutions are subsequently repeated with a standard filter for a Buchner funnel.

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Figure 2. (a) Preparation of mirror plate for dispersion-casting, (b) acid bath incubation of cellulose acetate membrane, and (c) final product, removed from the washing bath.

surface was rinsed with a towel and small amount of ethanol. Approximately 10 g of the cellulose−acetate polymer dispersion was transferred from the Schott flask to a smaller beaker glass to facilitate the pouring process. This amount of dispersion was then poured onto the mirror as an ∼1 in. wide stripe. The stripe of dispersion was then distributed from the top to the bottom of the glass plate using the edge of a stainless-steel ruler. The final membrane sheet was dried in ambient air for 5 min.

PROCEDURE

Equipment and Chemicals

The experiment utilizes infrastructure and materials available in a an undergraduate level chemistry laboratory (see Supporting Information Instructor and Student Notes) as well as some additions, which can easily be obtained: • Kitchen blender with at least 800 W power (e.g., Philips HR 2195/04)

Removal of Calcium Carbonate Particles

For the removal of the calcium carbonate particles, one plastic basin was filled with 5 L of water, and another basin was filled B




Figure 3. Scanning electron microscopy (FEI nova NanoSEM 450, 3 kV, spot size 2.5) images of cellulose acetate membrane cross sections at various magnifications. Images on the top row show the membrane cross sections prior to removal of the calcium carbonate pore template; images on the bottom row show the membrane cross sections after the calcium carbonate pore template has been removed.

surface area of about 1/2 page. Removal of the calcium carbonate particles using hydrochloric acid left behind pores with a diameter of approximately 1−5 μm as shown in the scanning electron microscope images (Figure 3). Hydrochloric acid dissolves the calcium carbonate particles, and carbon dioxide escapes as gas according to the following chemical equation:

with 5 L of hydrochloric acid (0.24 M). The mirror plate containing the membrane sheet was then submerged into the hydrochloric acid bath (Figure 2b). The membrane sheet loosened itself from the mirror plate, and gas bubbles indicated the removal of calcium carbonate particles. After 10 min, the membrane sheet was transferred into the basin containing only water and washed for another 5 min. Finally, the membrane sheet was pulled out of the water bath (Figure 2c) and placed onto a kitchen towel. Another kitchen towel was used to dry the membrane.

CaCO3(s) + 2H+(aq)

Functional Testing of Membrane

→ Ca 2 +(aq) + H 2O(aq) + CO2 (g)

A standard filter for a Buchner funnel was placed onto the dried membrane sheet and the shape of the filter copied using a permanent marker and then cut out of the membrane. The cut out membrane was placed cautiously onto a Buchner funnel on top of a suction flask. A 20 mL portion of a blue watercolor dispersion was prepared in a small beaker glass and transferred onto the same membrane. The watercolor solution was filtered by applying vacuum. Next, a 20 mL portion of an aqueous food coloring solution was prepared and also filtered by means of the membrane. The suction flask and Buchner funnel were then washed, and the two filtration experiments were repeated with a standard Buchner funnel filter.

(1)

Consequently, holes in the membrane are formed, which have the same size as the calcium carbonate particles. For achievement of a good particle size (and resulting pore size), the blending step is important. This step not only is required for the formation of a homogeneous particle dispersion in the polymer solution, but also decreases the size of the particles. The chosen blending time (3 min) ensures a good dispersion, and a significant decrease in the particle size. Longer blending times, however, do not result in significantly smaller particles (and pore sizes) because as the particles get smaller the shear stress acting on the individual particles decreases as well, resulting in a minimal achievable particle size (to achieve smaller particles more energy intensive mixing methods are required, e.g., ball-milling). Cross-section images of the membranes (Figure 3b,d,f) show highly inhomogeneous cavity sizes as well as an asymmetric membrane morphology: the size of the cavities increases from the top side of the membrane (air side) to the bottom side (side facing the glass substrate). This can be explained by the asymmetry of the drying process, where solvent evaporation only occurs on one side (the air side) of the membrane and results in a polymer solubility gradient within the material. On a similarly produced membrane based on poly(ether sulfone) (PES), Hess et al. showed that glycerol and the template (in this instance, zinc oxide nanoparticles) are the key parameters in

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HAZARDS Safety goggles must be worn at all times. Nitrile safety gloves must be worn while working with hydrochloric acid. Hydrochloric acid is corrosive. Acetone and ethanol are flammable. Handle all chemicals employed with care, and in the case of skin contact, wash immediately with water and soap, and rinse for at least 15 min. In the case of eye contact, rinse with water for at least 15 min.

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RESULTS AND DISCUSSION The method outlined above usually produced enough material for the production of 25 membrane sheets, each having the C



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controlling the pore size and pore gradient along the cross section of the membrane.17 The performance of the membrane was tested with a filter setup consisting out of a suction flask and Buchner funnel. A watercolor dispersion and food coloring solution were filtered, and both filtration results were compared. Watercolor paints are pigments (10−100 μm particle size) held together by a water-soluble binder. The pigments are insoluble, colored particles and are rejected by the membrane. Food coloring (e.g., Brilliant Blue FCF E133) is completely soluble in water and therefore passes through the pores of the membrane. The pores of standard filter paper used in school laboratories usually have a wider diameter (>20 μm), and therefore, the pigments can pass the membrane relatively easily. This laboratory experiment was performed by 52 advanced high school students during a 2 h laboratory experiment at a high school in Winterthur, Switzerland, and by 55 high school teachers at the “Future of Chemical Education” workshop during the fall meeting of the Swiss Chemical Society (SCS). The experiment was performed in two different variants. In one the cellulose acetate membrane was prepared in advance, and the filtration experiments were presented as an in-class demonstration. The other students and teachers followed the whole procedure for the fabrication of the cellulose acetate membrane and produced everything themselves. The feedback from the students and teachers was very favorable. From the survey conducted after the laboratory experiment, it was found that most people liked the hands-on approach of using readily available chemicals and everyday items to produce a porous membrane. Many participants found it attractive that the experiment has a direct link to the solution of the ever-present problem of producing safe drinking water. Additionally, many teachers welcomed the fact that the costs for the experiment are relatively low. For a class of 25 students the cost for chemicals accounts for approximately 10 USD.

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CONCLUSION

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ASSOCIATED CONTENT


AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Robert N. Grass: 0000-0001-6968-0823 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank ETH Zurich and the Swiss Federal Department of Economic Affairs, Education and Research kindly for their financial support. We gratefully acknowledge the generous assistance and valuable information provided to us by Christoph Kellenberger and Michael Loepfe from Novamem Ltd. Special thanks also to Amadeus Baertsch, Raphael Sigrist, and Jonas Halter for their tremendous help in reviewing the manuscript and verifying the school laboratory suitability of the laboratory experiment.

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REFERENCES

(1) World Health Organization. Progress on Sanitation and Drinking Water2015 Update and MDG Assessment; World Health Organization, WHO Press: Geneva, Switzerland, 2015. http://files.unicef. org/publications/files/Progress_on_Sanitation_and_Drinking_ Water_2015_Update_.pdf (accessed Jan 2017). (2) Bain, R.; Cronk, R.; Hossain, R.; Bonjour, S.; Onda, K.; Wright, J.; Yang, H.; Slaymaker, T.; Hunter, P.; Prüss-Ustün, A.; Bartram, J. Global Assessment of Exposure to Faecal Contamination through Drinking Water Based on a Systematic Review. Trop. Med. Int. Health 2014, 19 (8), 917−927. (3) Gadgil, A. Drinking Water in Developing Countries. Annu. Rev. Energy Environ 1998, 23 (1), 253−286. (4) 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. (5) Sobsey, M. D.; Stauber, C. E.; Casanova, L. M.; Brown, J. M.; Elliott, M. A. Point of Use Household Drinking Water Filtration: A Practical, Effective Solution for Providing Sustained Access to Safe Drinking Water in the Developing World. Environ. Sci. Technol. 2008, 42 (12), 4261−4267. (6) Pendergast, M. M.; Hoek, E. M. V. A Review of Water Treatment Membrane Nanotechnologies. Energy Environ. Sci. 2011, 4 (6), 1946− 1971. (7) Upadhyayula, V. K. K.; Deng, S.; Mitchell, M. C.; Smith, G. B. Application of Carbon Nanotube Technology for Removal of Contaminants in Drinking Water: A Review. Sci. Total Environ. 2009, 408 (1), 1−13. (8) Mandler, D.; Blonder, R.; Yayon, M.; Mamlok-Naaman, R.; Hofstein, A. Developing and Implementing Inquiry-Based, Water Quality Laboratory Experiments for High School Students To Explore Real Environmental Issues Using Analytical Chemistry. J. Chem. Educ. 2014, 91 (4), 492−496. (9) Dorney, K. M.; Baker, J. D.; Edwards, M. L.; Kanel, S. R.; O’Malley, M.; Sizemore, I. E. P. Tangential Flow Filtration of Colloidal Silver Nanoparticles: A ″Green″ Laboratory Experiment for Chemistry and Engineering Students. J. Chem. Educ. 2014, 91 (7), 1044−1049. (10) Nilsson, M. R. Survey of Biochemical Separation Techniques. J. Chem. Educ. 2007, 84 (1), 112−114. (11) Mara, D. D. Water, Sanitation and Hygiene for the Health of Developing Nations. Public Health 2003, 117 (6), 452−456. (12) Geise, G. M.; Lee, H.-S.; Miller, D. J.; Freeman, B. D.; McGrath, J. E.; Paul, D. R. Water Purification by Membranes: The Role of Polymer Science. J. Polym. Sci., Part B: Polym. Phys. 2010, 48 (15), 1685−1718.

In this paper we present a new laboratory experiment that introduces advanced high school students to the production of porous membrane materials for water purification. In a threestep process, membranes are produced at room temperature and tested with a solution of watercolor and food coloring. The experiment was initially designed for the “Future of Chemical Education” workshop during the fall meeting of the Swiss Chemical Society (SCS). Furthermore, the experiment was performed with 52 advanced high school students. Both groups (teachers and students) found the activity engaging and motivating, because it allowed them to gain exposure to the modern field of functional materials for the production of safe drinking water. Production of the membrane is feasible at a relatively low material cost (10 USD for 25 students). The activity can be performed within 2 h and is also suitable for first year undergraduate college students. S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.6b00776. Student handout (PDF, DOCX) Instructor notes (PDF, DOCX) D




(13) Orecki, A.; Tomaszewska, M.; Karakulski, K.; Morawski, A. W. Surface Water Treatment by the Nanofiltration Method. Desalination 2004, 162, 47−54. (14) Kellenberger, C. R.; Luechinger, N. A.; Lamprou, A.; Rossier, M.; Grass, R. N.; Stark, W. J. Soluble Nanoparticles as Removable Pore Templates for the Preparation of Polymer Ultrafiltration Membranes. J. Membr. Sci. 2012, 387−388, 76−82. (15) Luchinger, N. A.; Stark, W. J.; Kellenberger, C. R. Porous Polymer Membranes. U.S. Patent 20130299417 A1, 2013. (16) Kellenberger, C. R.; Hess, S. C.; Schumacher, C. M.; Loepfe, M.; Nussbaumer, J. E.; Grass, R. N.; Stark, W. J. Roll-to-Roll Preparation of Mesoporous Membranes by Nanoparticle Template Removal. Ind. Eng. Chem. Res. 2014, 53 (22), 9214−9220. (17) Hess, S. C.; Kohll, A. X.; Raso, R. A.; Schumacher, C. M.; Grass, R. N.; Stark, W. J. Template-Particle Stabilized Bicontinuous Emulsion Yielding Controlled Assembly of Hierarchical High-Flux Filtration Membranes. ACS Appl. Mater. Interfaces 2015, 7 (1), 611−617.

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Rapid Production of a Porous Cellulose Acetate Membrane for Water

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