Nanomaterials in Wastewater: Emerging Contaminant or Promising Treatment?
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"We have reached a defining moment in human history. The new agenda is a promise by leaders to all people everywhere. It is a universal, integrated and transformative vision for a better world.”
These were the words spoken by the then United Nations Secretary-General, Ban Ki-moon, back in 2015 as he announced1 that all 193 United Nations (UN) member states had agreed to adopt the 2030 Agenda for Sustainable Development2 — an ambitious framework made up of 17 official goals, and nearly 170 other targets, with the ultimate aim of wiping out poverty, fixing inequality, and tackling climate change within the next 15 years.
Under Sustainable Development Goal Number 6, titled “Ensure availability and sustainable management of water and sanitation for all”, the UN member states acknowledged a commitment to halving the proportion of wastewater that was currently going untreated3. According to a 2018 report from the International Water Association4, around 80 % of all wastewater produced today is discharged directly into the world’s waterways without any form of treatment or purification. This wastewater, some of which being untreated industrial effluent, is then free to mix into our rivers and our oceans, where it can be hazardous to the environment5, agriculture6, and the health of those who rely on those natural water sources7.
As a result of these potential risks, and out of an obligation to fulfill the Sustainable Development Goal, researchers around the world are making efforts to identify any new or unforeseen hazards to our waterways, as well as researching new and effective methods for treating contaminated wastewater and industrial effluents. One of the most exciting fields opening up as a result of these efforts is the study of nanomaterials and nanotechnology; both as an emerging contaminant in industrial effluents, but also as a potentially useful tool for water filtration and purification.
Nanomaterials as an emerging contaminant in wastewater
Strictly speaking, a nanomaterial is defined as a “material with any external dimension in the nanoscale [approx. 1-100 nm] or having internal structure or surface structure in the nanoscale” by the International Standards Organization8. Due to their small size and/or structures, nanomaterials often exhibit unusual or exotic properties, making them of particular interest to industry — nanomaterials such as carbon nanotubes (CNTs) and graphene have already been investigated for application in the electronics, medical, and energy sectors9,10.
There are already a number of products that make use of CNTs; the production of these nanomaterials is estimated to be in the order of hundreds of tons11 per year. Unfortunately, the growth of CNT production is also resulting in increasing amounts of CNT-containing industrial wastewater, which is notoriously difficult to treat and purify using conventional methods. CNTs are very chemically stable and normally can only be degraded using strong acids or through oxidation treatment at high temperatures — neither of which are suitable methods for purifying and recycling water.
A new paper on this topic, borne out of a joint research effort between the National Institute of Advanced Industrial Science and Technology (AIST) and Meijo University, Japan, has recently been published in the journal Scientific Reports12. The paper describes a novel and simple way of removing CNTs from contaminated industrial wastewater using environmentally friendly chemicals.
Specifically, the researchers investigated the effect of directly mixing sodium hypochlorite, a common disinfectant in domestic cleaning and an environmentally friendly oxidizing agent, into aqueous dispersions containing single-walled or multi-walled CNTs. As concentrated dispersions of CNTs in water have a distinct black color, any degradation of the CNTs would be easily indicated by the water returning to its natural transparence over time. Degradation was also tracked using transmission electron microscopy (TEM) and Raman spectroscopy at various time steps following the addition of the sodium hypochlorite oxidant.
The research team reported that after 120 hours, all of the observable CNTs were completely degraded and removed from the water after treatment with sodium hypochlorite. Further experiments on other types of carbon nanomaterials containing carbon nanohorns, as well as single- and multi-walled nanotubes, were also successful in completely removing the materials from the water.
Dr. Minfang Zhang, a Senior Researcher at the AIST, commented, “We are now trying to establish an appropriate industrial process for the practical application of this research. We hope we can establish a standard method … for advancing the wider application of carbon nanomaterial treatment using environmentally friendly methods.”
“We have also found in this research that the degradation rate of carbon nanotubes is dependent on their [structure] type, the details of which we will be publishing in a future paper.”
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Remediation of wastewater using nanotechnology
While some research groups focus on removing nanomaterials from wastewater, others are beginning to investigate the possibility that nanotechnology and specially designed nanomaterials might be useful for novel water treatment technologies.
A need for better water filtration systems13 has led to a particular interest in the application of engineered nanomaterials in water purification. Recent work14 from the College of Engineering at Majmaah University, Saudi Arabia, discusses the real-world application of nanomaterials in wastewater treatment.
Electrospun nanofibrous membranes (ENMs) are made from polymers that have been subject to electrostatic repulsive forces, in a process known as electrospinning15. In brief, a polymer solution or melt is fed through a capillary tube, which is then drawn out and shaped by the electrostatic forces applied, resulting in the formation of nanofibres. The randomly oriented nanofibers often fuse together as solvent evaporates or as the polymer melt cools, leaving an ideal, low density, open pore structured membrane that retains a good amount of structural integrity — ideal for filtration applications14.
The work from Majmaah University reviews research into the potential of ENMs, against the current conventional filtration membranes used for industrial wastewater treatment. The researchers observed that ENMs have a far higher porosity than the conventional membranes (approximately 80 %, compared to 5-35 %), as well as generally being a more cost-effective and lower energy consumption process than the making of conventional membranes. The researchers also found evidence in several cases that chemically modifying the surface of ENMs can help protect the membranes against fouling, unwanted accumulations and blockages. Interestingly, ENMs that were engineered to contain silver nanoparticles and to have extremely small pore sizes of around 10 nm were even able to remove viruses and bacteria efficiently under laboratory conditions — meaning ENMs could have the potential to purify water as a filtration system, but also as a disinfectant.
Monika Kulak, a graduate student working with ENMs at the University of Waterloo, Canada, explains “ENMs allow researchers to develop a vast array of filtration systems for solving water treatment challenges. Nanomembrane stability, permeability, and porosity can all be tuned and tailored based on the starting polymeric material used. Nanofibers can also be selected based on green characteristics, such as biodegradability, to create an overall environmentally positive system.”
“Outstanding examples of ENM versatility include polyacrylonitrile combining with cellulose for simultaneous removal of many toxic metals, polyethersulfones demonstrating excellent thermal and chemical stability with minimal modification, and chitosan-containing ENMs that are cost-effective and environmentally friendly. As the technology for ENMs advances, so does the possibility for clean, accessible water across the globe.”
However, the researchers at Majmaah University did also note that there are still several barriers that need to be addressed before ENMs can be considered ready for large-scale commercialization. Firstly, they say that steps need to be taken to make ENMs more compatible with the existing infrastructure in water treatment facilities. Additionally, they stress the importance of further research into how potentially commercial ENMs degrade with time, and any potential environmental risks that may come with this.
Ensuring sustainability
Characterizing the sustainability and environmental effects of any new nanotechnology may be key to future research of this kind. One recent study published in Environment International16 polled a total of 50 experts from 19 different countries on a number of different sustainability criteria relating to nanomaterials as a water and wastewater treatment technology, with the intention of evaluating what parameters will be the most important to this industry as it develops. In general, criteria associated with efficiency and safety won out as the most important factors for making sustainable choices with nanotechnology treatments, but there was also a strong base of support for these technologies to be scrutinized for performance under unexpected or extreme environmental conditions, and additionally, whether the types of nanomaterial being used had the potential for recovery and reuse.
Whether nanomaterials are cast as a worrisome emerging contaminant, or the novel water treatment technology of the future, it is clear that sustainability and potential environmental impact will be of paramount importance to the researchers tackling wastewater contamination. The continued advocacy of organizations such as the UN on this issue speaks to just how important effective wastewater management can be in preserving our water and our wider environment.
References
[1] un.org, “Secretary-General’s Remarks at Summit for the Adoption of the Post-2015 Development Agenda.” https://www.un.org/sg/en/content/sg/statement/2015-09-25/secretary-generals-remarks-summit-adoption-post-2015-development.
[2] sustainabledevelopment.un.org, “SDGs - Sustainable Development Knowledge Platform.” https://sustainabledevelopment.un.org/sdgs.
[3] sustainabledevelopment.un.org, “Goal 6 - Sustainable Development Knowledge Platform.” https://sustainabledevelopment.un.org/sdg6.
[4] International Water Association. 2018. “Wastewater Report 2018 - The Reuse Opportunity.” https://doi.org/10.5194/acp-2016-176.
[5] The US Geological Survey Water Science School, “Water Use: Wastewater Treatment.” https://water.usgs.gov/edu/wuww.html.
[6] UN Environment. 2017. “Untreated Wastewater - a Growing Danger.” https://doi.org/10.1088/1748-9326/aa75d1.
[7] National Small Flows Clearinghouse adapted by Purdue University. 1996. “On-Site Wastewater Disposal and Public Health.” https://engineering.purdue.edu/~frankenb/NU-prowd/disease.htm.
[8] International Standards Organisation. (2015). TS 80004-1:2015: Nanotechnologies — Vocabulary — Part 1: Core terms. Retrieved from https://www.iso.org/obp/ui/#iso:std:iso:ts:80004:-1:ed-2:v1:en.
[9] The University of Manchester. “Applications - Graphene.” https://www.graphene.manchester.ac.uk/learn/applications/.
[10] Peng, Huisheng., Qingwen. Li, and Tao. Chen. 2017. Industrial Applications of Carbon Nanotubes. Elsevier.
[11] Zion Market Research. “Carbon Nanotubes Market, By Type (Single-Walled Nanotubes (SWNT), Multi-Walled Nanotubes (MWNT)) for Polymers, Electrical and Electronics, Energy, Composites and Other Applications: Global Industry Perspective, Comprehensive Analysis, and Forecast, 2016–2021”. https://www.zionmarketresearch.com/report/carbon-nanotubes-market.
[12] Zhang, Minfang, Yinmei Deng, Mei Yang, Hideaki Nakajima, Masako Yudasaka, Sumio Iijima, and Toshiya Okazaki. 2019. “A Simple Method for Removal of Carbon Nanotubes from Wastewater Using Hypochlorite.” Scientific Reports 9 (1): 1284. https://doi.org/10.1038/s41598-018-38307-7.
[13] Cuffari, Benedette. 2018. “Nanotechnology and Water Purification.” AZoNano. 2018. https://www.azonano.com/article.aspx?ArticleID=4918.
[14] Tlili, I., and Tawfeeq Abdullah Alkanhal. 2019. “Nanotechnology for Water Purification: Electrospun Nanofibrous Membrane in Water and Wastewater Treatment.” Journal of Water Reuse and Desalination, January. https://doi.org/10.2166/wrd.2019.057.
[15] nanoScience Instruments. n.d. “Electrospinning - Nanoscience Instruments.” https://www.nanoscience.com/techniques/electrospin/.
[16] Kamali, Mohammadreza, Kenneth M. Persson, Maria Elisabete Costa, and Isabel Capela. 2019. “Sustainability Criteria for Assessing Nanotechnology Applicability in Industrial Wastewater Treatment: Current Status and Future Outlook.” Environment International 125 (April): 261–76. https://doi.org/10.1016/J.ENVINT.2019.01.055.
