Development of a Highly Efficient Low-Cost Filter for Effective Dissolved Heavy-Metal and Organic Contaminant Removal

Summary

This work posits a nanomaterial filter that is low-cost, scalable, highly efficient, reusable, and high-capacity, effectively solving the problems plaguing previous methods of heavy metal removal from water. Additionally, this solution shows strong promise in removing organic pollutants in a highly efficient manner. As a result of the high electronic affinity of the active catecholamine group as well as the pi-pi interactions between PDA and other organic molecules, this BNC/PDA membrane is able to adsorb >99.8% of Pb2+, Cd2+, and Cu2+ ions, as well as Rhodamine 6G, Methyl Orange, and Methylene Blue organic dyes without regeneration from 60 ppm pollutant solutions. The capabilities of this membrane far exceed conventional methods such as activated carbon. This method of pollutant removal is reusable using common chemicals, nonspecific, and biodegradable, unlike previous efforts which relied on involved, harmful cleaning and disposal procedures. This nanoscale solution to one of the world’s most important problems promises to bring rapid relief to locations plagued with heavy metal pollution, as well as be a better solution for significant water treatment endeavors.

Question / Proposal

Is it possible to develop a solution to the crisis of heavy metal pollution that addresses the problems plaguing current removal methods, specifically efficiency, cost, scalability, and environmental friendliness? This work demonstrates a novel method for remediation of heavy metal cation and aromatic pollutants from water with high efficiency and high scalability. This method utilizes the chelation potential of deprotonated catecholamine groups in polydopamine (PDA) to remove heavy metal ions from water and utilizes strong aromatic-aromatic and intermolecular interactions to remove aromatic dyes. The high surface area of these biocompatible nanoparticles increases adsorption efficiency. When combined with a biodegradable, biocompatible substrate such as bacterial nanocellulose, the resultant membrane meets or even exceeds the requirements for safe, simple and scalable removal of many dangerous water pollutants.

Research

Water pollution is increasingly becoming a major problem that affects millions of people worldwide. The most dangerous forms of water pollution stem from dissolved heavy metals, as well as toxic organic compounds. Heavy metal pollutants enter watersheds, ecosystems, and wells in a multitude of ways, including tailings from mining activities, leaching from garbage and e-waste dumps (urban runoff), and dissolution from natural deposits. Hydrocarbons, pesticides and insecticides, detergents, and coolants source from human activities and enter waterways [1]. These pollutants cause a variety of diseases, including mental retardation, acute poisoning, cancers, and organ failure, due to these metals’ ability to interfere with cellular and developmental processes. Unfortunately, even advanced methods used to remediate polluted water, such as biological treatment, activated carbon, and silica particles are too inefficient in terms of filtration capacity and cost to be accessible to the affected populations as well as be sufficiently scalable to successfully eliminate these pollutants on a large scale [2]. Recently-developed methods such as one detailed in Algappan [3] have pushed the boundaries of scalability, simplicity, and efficiency (peak >99.4% adsorption efficiency), but still lack in areas such as biodegradability, scalability, and ease of synthesis. Based on the current state of water pollutant and environmental remediation technologies, a filtration method that addresses the problems plaguing current removal methods, specifically efficiency, cost, scalability, and environmental friendliness would be able to substantially accelerate efforts in heavily polluted rural areas and could reduce economic loss and disease caused by water pollution in metropolitan areas.

Method / Testing and Redesign

Synthesis

All chemicals were purchased from Millipore Sigma, St. Louis, USA and were used without further modification. The synthesis procedure for polydopamine (PDA) nanoparticles was modified from Ai [4]. Nanopure water (252 ml) was mixed with ethanol (112 ml) in a 1000 ml glass container and an aqueous solution of ammonia (NH4OH, 1.12 ml, 28-30%) was then added. After half an hour of stirring, dopamine hydrochloride (1.4 g) was dissolved in nanopure water (28 ml, 18.2 ) and was added to the solution. The reaction was left under moderate stirring for 24 hours with no cap on the glass container. The PDA particles were collected by centrifugation (7000 rpm, 10 min) and washed with nanopure water a few times and dispersed in water (320 ml). Size optimization was based upon tests of the nanoparticles leaching from the filter during filtration.

Gluconacetobacter hansenii (ATCC®53582) was cultured in test tubes containing 16 ml of #1765 medium at 30 °C under shaking at 250 rpm. PDA particles (160 ml) were centrifuged, and the pellets were dispersed in bacterial growth solution (15 ml). This mixture was then transferred to a petri dish (6 cm diameter). After 24 hours, the excess solution was removed. Two days later, a thin uniform layer of PDA filled BNC was formed. The PDA/BNC hydrogel was collected and washed in boiling water for 2 hours, then dialyzed in nanopure water for one day. Then, the PDA/BNC hydrogels were either freeze-dried for adsorption tests or air-dried for filtration tests.

Batch Adsorption 

Batch adsorption tests were conducted at room temperature to evaluate the adsorption behavior of PDA/BNC. To investigate the effect of pH, a small piece of PDA/BNC foam was placed in a 5 ml solution of each pollutant and shaken until adsorption equilibrium was reached. For heavy metals and organic pollutants, pH values from 2 to 6 and 7 were tested. The solution pH was modified by adding HCl and NaOH. Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES PE Optima 7300DV) was utilized to evaluate the heavy metal cation concentrations.

Membrane Performance

To compare the adsorption capabilities of PDA particles against an existing method of remediation, 80mg of PDA and activated carbon was added to separate glass vials containing 15 ml of 200 ppm Pb (II) solution. The vials were shaken for 24h, and Pb (II) concentration before and after the test was measured.

Filtration

Filtration experiments were performed using a conventional vacuum filtration setup. Starting concentrations of each pollutant were prepared as analogs of highly polluted environmental water, and the pH was adjusted to the optimal pH for specific pollutant adsorption. At typical pressures of 0.6-0.8 bar, vacuum filtration was performed until the concentration of the filtered water for each pollutant was lower than the detection limit (<.1 ppm) of the instrumentation. All the membranes used in vacuum filtration had a diameter of 3cm. Membrane regeneration testing was carried out by washing the membrane with ethanol and 0.1 M sodium citrate solution for organic dyes and heavy metals, respectively.

Results

 

Multiple factors contribute to a change in the adsorption capabilities of the PDA/BNC membrane. The adsorption capabilities of the PDA/BNC membrane increase as the pH of the pollutant increases until pH 6-7, the pH of waterways, which enables the membrane to be used without modification in the environment. The presence of other positive ions such as Na2+ and Ca2+ in the polluted water also increases the heavy metal ion adsorption capacity of the PDA nanoparticles due to the ionic strength effect, up to 160% at high concentrations of Ca2+. Even without the aid of additional cations, heavy metal removal was highly effective.

        Calculation of adsorption efficiency was done via the following equation, developed by Algappan [3]:

Adsorption Efficiency(%)=[1-(Cf/Ci)] × 100

Where Ci and Cf are the initial and final concentrations of the polluted solution before and after filtration.

At concentrations thousands of times higher than the EPA limit for aqueous heavy metals, a 3cm diameter membrane was able to remove >99.8% of the tested heavy metal ions from >100ml of pollutant, reaching 200ml for Pb2+ before filtrate exceeded the detection limit of the ICP-OES. Organics removal was also highly effective due to the intermolecular interactions between the tested dyes’ aromatic rings and the PDA nanoparticles. No R6G was detected in the filtered water until >100ml of pollutant was filtered, and >25ml of methyl orange was filtered before breaching the lower detection limit of UV-Vis spectroscopy. Regeneration of the PDA/BNC membrane was highly effective, and relative uptake stayed close to 95% even after ten reuses. When compared to activated carbon, an effective and commonly used heavy metal removal material, 80mg of PDA nanoparticles removed 4.5 times more Pb2+ ions from 200ppm solution than a similar mass of activated carbon.

The filter membrane has many advantages over conventional methods. Not only does this method of remediation match or even exceed the highest levels of efficiency even for very high concentrations of pollutant, but it is also entirely waste-free. The membrane material is entirely biodegradable. Bacterial nanocellulose is already being used in the food industry for packaging and filler. The chemicals utilized for reusing the membrane and washing off the adsorbed pollutants can be purified and reused as well via methods that are optimized for ethanol and citric acid purification (i.e., ion exchange) and are scalable. The scalability of this removal method is very high due to the nature of the manufacturing process. Bacterial nanocellulose is growable in very large sheets, up to 1m2 or more while maintaining strength and shape [5]. PDA nanoparticles can be easily synthesized in large quantities, as well. In this research, the maximum batch size made at one time was 1.5 L, but the only limit to the successful formation of the nanoparticles is oxygen availability, which can be remedied with larger containers and better stirring methods. The cost of synthesis is also very low, and in-place infrastructure for nanocellulose growth would lower the cost even more. Because of this, an 1m^2 filter membrane would cost only 10 dollars in raw material. While activated carbon is slightly cheaper, it is not biodegradable and has far less absorbance capacity per unit mass. Due to these traits, this filter shows great promise in multiple applications. In small communities, these membranes can be synthesized very simply even in a domestic setting due to the simplicity, benignity, and cost efficiency of the manufacturing process. In rural settings where other methods of removal are prohibitively expensive, difficult to dispose of, and are not adequate for highly polluted watersheds, this membrane would be very useful. It can be used multiple times using readily available chemicals (conventional alcohol and acid) However, due to the scalability of membrane synthesis, large water treatment operations could also use these membranes to purify water to a higher degree while reducing operating costs. One could visualize a setting where first responders to an environmental crisis could set up large membranes and quickly make nontoxic water available.

This filter could also be used as an easily replaceable tap-based unit. In fact, little modification would be needed due to the vacuum filtration testing method used having the same approximate pressure as tap water. Water flux would need to be optimized, which can be done by tuning the size of the BNC membrane pores. Because chelation is nonspecific, the concentration breach limit for all heavy metals are the same. Since the copper and cadmium detection limits are under the potable water threshold (5ppb), they can be used to extrapolate the filtration abilities for this filter for any heavy metal. In Flint, Michigan, for example, the median concentration of lead ions was 27ppb. 230 liters of 27 ppb lead-polluted water can be filtered by a 3cm diameter membrane filter before the safe level of 5 ppb in the filtrate is breached. The average US family indoor water use is 138 gallons of water per day = 522.4 liters of water. Thus, just three membranes of a 3 cm diameter would purify enough water for an average American family for one day without regenerating the membrane. A membrane that is 1m^2, without regeneration, would last such a family for more than a year. With regeneration, the only limit for the use of this membrane is an eventual mechanical failure since adsorption capacity is maintained. Even then, with the excellent mechanical properties of bacterial nanocellulose, such failure would not happen for an extended period. People who have been suffering from heavy metal pollution now have a solution that is not cost-prohibitive and could be used in the interim before government efforts.

Conclusion

The issue of water pollution by heavy metals and organic wastes has been important for a long time due to the lack of a solution to the multiple problems associated with these pollutants. In this work, it has been demonstrated that bacterial nanocellulose is a viable substrate for environmental remediation efforts in locations where other methods prove to be too expensive.

This method of removal has successfully fulfilled several areas of interest:

1. A small-diameter membrane is able to remove more than 99.8% of heavy metal and organic pollutants from water, which is better than any existing method.

2. The cost of synthesizing this filter in both small and large scales is very low due to the easily attainable reagents and already-existing infrastructure.

3. Adsorption efficiency is conserved even when washed and reused, and the filter is biodegradable, resulting in greatly reduced waste compared to other methods of pollutant removal.

It has also been demonstrated that catecholamine-based nanoparticles are an effective removal method of transition metal ions due to their high chelating ability and are effective at filtering out aromatic organics due to ubiquitous intermolecular interaction. Because these characteristics are highly nonspecific, various heavy metal cations and organic molecules can be removed by one filter, eliminating the need for multistep processes and reducing cost. The facile and risk-averse synthesis of non-toxic PDA nanoparticles coupled with the highly scalable growth ability of nanocellulose membranes and accessible regeneration chemicals results in a filtration method that is more efficient, effective, and cost-effective than currently used methods.

 

About me

I am a 12th grader who attends the Davidson Academy of Nevada. I have been conducting scientific research since the age of 7, either through science fairs, school-based research, or through university laboratories. I am very interested in materials science and aerospace engineering, and I hope to one day be able to use the knowledge I gain to push humanity forward to a better world where all people are healthy. I originally became interested in these fields when I learned about how the advanced materials in solar panels enable them to turn photons from the sun into electricity. I knew then that I wanted to work with new materials technologies that can push humanity forward. My pursuit of STEM has enabled me to participate in science competitions and symposiums, as well as conduct research in futuristic areas of science, like nanotechnology. In my free time, I like to build and fly drones and learn about technology that augments humanity. In that vein, my role models in science and technology are Elon Musk and my grandfather, who was the first urologist in India and has invented new kinds of medical equipment that are currently used in surgery today. I have been inspired by these people to continue exploring science and creating engineering solutions that can benefit us all. If I was fortunate enough o earn recognition at this fair, it proves to me that my work is visible and valuable. By receiving a prize from this competition, I would be able to continue my research and possibly expand, as well as reduce the expense of my college tuition. My future educational career would involve education in aerospace engineering and materials science, and I hope to pursue a Ph.D. in a related field.

Health & Safety

The contact information for my supervisor is:

Srikanth Singamaneni

singamaneni@wustl.edu

Bibliography, references, and acknowledgements

REFERENCES

1. Tchounwou, P. B., Yedjou, C. G., Patlolla, A. K., & Sutton, D. J. (2012). Heavy Metal Toxicity and the Environment. Experientia Supplementum Molecular, Clinical and Environmental Toxicology,133-164. doi:10.1007/978-3-7643-8340-4_6

2. Zou, Y., Wang, X., Khan, A., Wang, P., Liu, Y., Alsaedi, A., . . . Wang, X. (2016). Environmental Remediation and Application of Nanoscale Zero-Valent Iron and Its Composites for the Removal of Heavy Metal Ions: A Review. Environmental Science & Technology,50(14), 7290-7304. doi: 10.1021/acs.est.6b01897

3. Alagappan, P. N., Heimann, J., Morrow, L., Andreoli, E., & Barron, A. R. (2017). Easily Regenerated Readily Deployable Absorbent for Heavy Metal Removal from Contaminated Water. Scientific Reports,7(1). doi:10.1038/s41598-017-06734-7

4. Ai, K., Liu, Y., Ruan, C., Lu, L., & Lu, G. M. (2012). Sp2C-Dominant N-Doped Carbon Submicrometer Spheres with a Tunable Size: A Versatile Platform for Highly Efficient OxygenReduction Catalysts. Advanced Materials,25(7), 998-1003. doi:10.1002/adma.201203923

5. Chen, G., Wu, G., Alriksson, B., Wang, W., Hong, F., & Jönsson, L. (2017). Bioconversion of Waste Fiber Sludge to Bacterial Nanocellulose and Use for Reinforcement of CTMP Paper Sheets. Polymers, 9(12), 458. http://doi.org/10.3390/polym9090458 13

BIBLIOGRAPHY

1. Shahbazi, A., Younesi, H., & Badiei, A. (2012). Batch and fixed-bed column adsorption of Cu(II), Pb(II) and Cd(II) from aqueous solution onto functionalised SBA-15 mesoporous silica. The Canadian Journal of Chemical Engineering,91(4), 739-750. doi:10.1002/cjce.21691

2. Aguila, B., Sun, Q., Perman, J. A., Earl, L. D., Abney, C. W., Elzein, R., . . . Ma, S. (2017). Efficient Mercury Capture Using Functionalized Porous Organic Polymer. Advanced Materials,29(31), 1700665. doi:10.1002/adma.201700665

3. Dong, Z., Zhang, F., Wang, D., Liu, X., & Jin, J. (2015). Polydopamine-mediated surfacefunctionalization of graphene oxide for heavy metal ions removal. Journal of Solid State Chemistry,224, 88-93. doi:10.1016/j.jssc.2014.06.030

4. Gao, H., Sun, Y., Zhou, J., Xu, R., & Duan, H. (2013). Mussel-Inspired Synthesis of Polydopamine-Functionalized Graphene Hydrogel as Reusable Adsorbents for Water Purification. ACS Applied Materials & Interfaces,5(2), 425-432. doi:10.1021/am302500v

5. Lo, S., Wang, S., Tsai, M., & Lin, L. (2012). Adsorption capacity and removal efficiency of heavy metal ions by Moso and Ma bamboo activated carbons. Chemical Engineering Research and Design,90(9), 1397-1406. doi:10.1016/j.cherd.2011.11.020

6. Zhang, Q., Yang, Q., Phanlavong, P., Li, Y., Wang, Z., Jiao, T., & Peng, Q. (2017). Highly Efficient Lead(II) Sequestration Using Size-Controllable Polydopamine Microspheres with Superior Application Capability and Rapid Capture. ACS Sustainable Chemistry & Engineering,5(5), 4161-4170. doi:10.1021/acssuschemeng.7b00129

LABORATORY ENVIRONMENT

I conducted my research at the Washington University Nano Research facility located in St. Louis, MO. The laboratory I worked at dealt with metal and organic nanostructures, and was structured like a chemistry laboratory. I learned how to use an ultraviolet-visual light spectrometer to assess the concentrations of the organic dyes before and after filtration, and I learned how to use an inductively coupled plasma optical emission spectrometer to evaluate the very low concentrations of heavy metals after filtration. 

ACKNOWLEDGEMENTS

I would like to thank Professor Srikanth Singamaneni for providing laboratory resources and space at the Washington University Nano Research Facility for me to work on this project.