Eco-Friendly alternative for water treatment from nanotechnology

To cite this article  use:  Roksana, M. Eco-Friendly alternative for water treatment from nanotechnology.  J. Nano Sc. Tech, 4(2016)28-34.

Nature-derived polymers in water treatment as sustainable remediation mechanism

Roksana Markiewicz

The amount of water around us has remained constant over time, while the population of the planet’s inhabitants is growing expeditiously. Each year, the competition for a clean supply of water intensifies strongly. Access to fresh water is fundamental for our well-being and is critical not only for our industrial and social development, but also for other important aspects such as nutrition, equality, education and eradication of poverty [1]. The topic of fresh water access is most often raised up in the context of countries with low natural water supplies; unfortunately it is still heavily neglected in countries where water is plentiful, but its purity fails to meet even the minimum of the standards. In today’s world, the environment is filled with various types of toxic pollutants emitted from human activities or industrial processes [2]. So far, a variety of nanotechnological tools have demonstrated exceptional efficiency on the improvement of water quality, with the biggest impact on the fields of separation/filtration, remediation and disinfection.

One of the biggest challenges of the modern world is the purification of water for human consumption purposes. Water laced with various contaminants claims millions of lives each year. Among those contaminants we can find particulates, microorganisms such as bacteria or viruses, toxins, dissolved inorganic elements and compounds – like heavy metals (e.g. lead, cadmium, mercury, iron and others), arsenic, chloride, fluoride; as well as dissolved organic substances, like pesticides, plant or animal remains and fragments, and any other natural or man-made chemicals resulting from exposure to the environment.

The existing water purification technologies include the removing of all undesirable contaminants, like suspended particles, microbes as well as organic and inorganic compounds, by means of physical, biological or chemical methods, however, in most of the cases a complex combination of all these methods is required to reach minimum acceptance values. In order to fully explain the content of this article, it is important to describe the available methods of water purification; this is because the essence of the each method will not change  regardless of the material used for its implementation.

A very commonly used physical method for water purification is filtration, which can be explained as the separation of fluids and solids by the use of an interposing medium (membranes or filters of any kind, e.g. charcoal, polymers, porous zeolites or ceramics), by which only a fluid can pass. The second method, sedimentation, uses the tendency of the suspended particles to settle out of the fluid they are suspended in, this can be enabled by means of gravity, centrifuging or even external electromagnetic field. The last method, distillation, uses evaporation in order to separate constituents of a liquid. The liquid is heated and further condensed in its pure form. In the industrial processes of water purification, a key factor that is always important to consider is the cost of any purification processes. In large scale, the biggest disadvantage of the filtration process is its speed, as well as the fouling phenomena on the surface of any interposing medium. The sedimentation method is very time consuming, when one considers gravity as the main driven force and the viscosity of the treated fluid. Finally, applying a strong external force (heat, magnetic field etc.) is expensive, time consuming and in many cases not suitable.

Several chemical methods are used in water purification, namely flocculation (called also coagulation or aggregation), chlorination/ozonation or use of other chemicals (oxidizing or reacting with the contaminants like pesticides, bacteria etc.), ion exchange (mostly used in the water softening process) or the use of radiation, for example ultraviolet light. During the flocculation process, the particles suspended or dissolved in a liquid medium are destabilized by any chelating or clarifying agents, and then precipitated from the water solution. The use of radiation, based on oxidizing organic pollutants or/and bacteria, although highly effective, unfortunately works well only on the clearly filtered water. On the other hand, purely biological methods are used to disinfect water from bacteria, fungi, viruses, algae, etc., by means of the activated sludge, aeration, biological oxidation or microorganisms [3].

When selecting an appropriate water purification method, one always needs to answer few essential questions. First of all, What can be the potential contamination of the water to be purified?, Are there any predicted variations in its quality?, and furthermore, What is the level of water quality required?. Other factors such as: costs, time, easiness in operation and maintenance, or the technical requirements and availability are also very important and cannot be left out from the consideration factors. Nevertheless, although it is very important to count with a highly efficient purification method, it might be even more critical and cost effective, to develop and invest in the prevention measures in order to eliminate various contamination sources and hence improve quality and protect all of the water sources.

The described methods are of outs-tanding efficiency when the aim is to make an ideal system from the already potable (or close to) water, reducing only the natural contamination with minerals and organic matter of the existing water resources. However, this is not the case in many regions or sectors around the world. What about the regions with very low water quality?

The most common cases are the regions where water is not only contaminated with naturally existing salts and minerals, but also with biological contaminants,  and there is no access to any disinfectants. What about regions where the mankind hasn’t been careful enough and the water is contaminated with variety of man-made chemicals, like pesticides, pharmaceutical and others?. Sadly, those are the most threatening cases, and in those areas there is still  a great need for novel, sustainable systems and methods for reliable water purification.

Nanotechnological tools for water treatment

Nanotechnology, the engineering and manipulation of matter at the nanoscale, is a highly inter- and multi-disciplinary research area oriented on novel solutions and applications. Not only does it find its use in nanomedicine, solar cells, sensor development and many other applications, but can also be effectively applied to the prevention and remediation of water pollution. Nanotechnology has provided plenty of solutions for surface water, groundwater, and wastewater treatment technologies, regarding their contamination by different sources, such as: toxic metal ions, organic and inorganic compounds and microorganisms. The specific physical, chemical and biological properties of nanomaterials, allow the development of novel high-tech routes for more efficient water and wastewater treatment processes [4], a comparison of their applicability, advantages and disadvantages is presented in the Table 1.

Nanoparticles can be used, regarding the stage of water purification, for the removal of variety of sediments, chemical effluents, charged particles or microorganisms, including a variety of toxic trace elements. Application of the mentioned nanoparticles is very often connected with a great investment, and even though the water purification yields are very promising, very often it is not possible to use them on a large scale.

From the practical point of view, it is hard to imagine a large water container, to which a vast amount of nanoparticles (i.e: magnetite) would be added and then it would be driven out by a powerful external magnetic field. The purification itself would be of a great energetic expenditure. Similarly, it would be hard to implement water purification system based on silver nanoparticles, or carbon nanotubes, not to mention the great investment needed in order to address and implement: separation and stability processes and protocols for the nanomaterials in order to avoid making this promising materials part of the contamination problem.

Nanotechnology is considered a key enabling technology, with vast amounts of funding for its research and development, even if the regulations concerning its implementation are still in its infancy.

The use of nanomaterials alone is unfortunately limited due to the high operational cost and technical difficulties in the preparation of those materials on a large scale. The water industry requires the production of great amounts of pure water, therefore there is a great need  for the development of cost-effective, stable materials and technological innovation in this area. In spite of the wide variety of nanomaterials presented for water treatment technologies, it seems now that a lot of them will remain confided only to scientific laboratories, never to be implemented in the real life. This shows that the efforts should focus now on delivering more effective routes for preserving water supplies and keeping them with the highest standards possible while bearing in mind the implementation, scalability and cost/efficiency of the applied methods. It must be yet emphasized, that the significance of the nanotechnological tools developed so far cannot be underestimated, since they are of great importance when it comes to innovative water treatment technologies and fundamental knowledge of the underlying mechanisms of such processes.

Recent advances in nanotechnology offer unusual opportunities in the development of the next-generation of water supply systems. The current water treatment techniques are no longer sustainable and efficient, and do not enable the needed and affordable level of performance. These solutions are expected not to rely on large infrastructures, such as the generally implemented, but on smaller and portable water purification products. All this, is needed so it is possible to use also the unconventional water resources, like the sea or wastewater.

Nevertheless, nanoparticles, mostly in the powder form, have been incorporated into existing water treatment processes mainly as absorbers, nevertheless, always an additional separation process is needed, and the recovery of the nano-adsorbents is necessary after few cycles. To this date, there are few industrial and personal small/medium scale solutions that incorporate nanoparticles, in drinking water treatment systems. Among this systems we can find ArsenXnp a commercial hybrid ion exchange medium of iron oxide nanoparticles and polymers, or ADSORBSIA^TM, a nanocrystalline titanium dioxide medium in the form of beads, both highly efficient in the area of arsenic removal [8].

Nature derived polymers as eco-friendly alternative for nanotechnology

Since the first half of the 20th century, along with the great technological progress and the advances in chemical technology, an innovative class of man-made materials, called polymers or plastics, have been introduced to the marked and have taken over our lives since then. At the time, they seemed inexpensive and safe materials, coming with different shapes and sizes. “In product after product, market after market, plastics challenged traditional materials and won, taking the place of steel in cars, paper and glass in packaging, and wood in furniture.”[9].

Most of the plastics are made from non-renewable resources and are resistant to natural decomposition, accumulating in the ecosystem and polluting several areas. This promoted the ongoing research to come back to the natural resources, and to develop bio-based and biodegradable polymers, called biopolymers. They can be defined as polymeric biomolecules which contain monomeric units bonded to form larger molecules. Biopolymers can be divided in variety of ways, mainly by means of their origin: nature-derived polymers, formed within the living organisms,and synthetically-derived polymers based on nature-inspired monomers. In the first group, it is most important to mention the polysaccharides, like cellulose, starch or chitin, polypeptides and polynucleotides, as presented in the Figure 1. The natural-derived polymers are renewable, non-toxic, biodegradable and in general environmental friendly materials, possessing excellent functional properties.

Figure 1. Nature-derived polymers.

Among these natural products, it is worth mentioning the great abundance of the polysaccharides, which are considered the most plentiful biomolecules in nature. The main importance of this kind of biopolymers is the fact, that a lot of them can be derived from the agricultural waste. Every year, approximately 10–50 billion of dry lignocellulosic wastes are produced worldwide. This fact makes them cost-effective and, more importantly, non-competitive with food production.

Cellulose, chitin or starch, as well as their derivatives, has been already widely explored as an attractive alternative for synthetic polymer-based adsorbents in the area of water treatment. It should be emphasized, that those biopolymers, apart from being environmental friendly are characterized by their superior stability, reactivity and feasibility of their modification, and most importantly, an excellent selectivity towards aromatic compounds and metals, which is a result of the presence of chemical reactive groups (hydroxyl, acetamido or amino functions). Biopolymers are already used in adsorption processes, which are one of the most popular methods for water decontamination applications.

Pure biopolymers yet are not as effective as their combination with nanoparticles–the bionanocomposites. The bionanocomposites derived from natural organic materials have received considerable attention because of their properties: high specific strength and modulus, low cost/high volume applications, low density compared to composites reinforced by inorganic materials, low energy consumption, easy processability, renewable origin and possibility of recycling. When combined with various nanoparticles, the nature-derived polymers gain novel properties, and seem an excellent choice for water treatment systems.

Membrane processes, like micro-filtration, ultrafiltration, nanofiltration or reverse osmosis, play an important role in water purification, exceeding the commonly used techniques. As membrane processes are considered the key components of the advanced water purification technologies, a continuous search for novel membrane materials is being done [10]. Various nanomaterials have been used for these purposes, such as activated carbon, clays, siliceous materials, zeolites or metal and metal-based nanoparticles (silver, gold, titanium oxide, iron oxide) however their applications is limited due to the associated costs, regeneration or limited cycles of application.

The use of nature-derived polymers, thanks to their biodegradability, availability or mechanical properties, have gained significant interest in water technologies. They are effectively used as adsorbers for both inorganic and organic micropollutants, as well as for the removal of negatively charged contaminants such as viruses and bacteria, at a rate faster than in case of the conventional filters. “While the current generation of nanofilters may be relatively simple, it is believed that future generations of nanotechnology-based water treatment devices will capitalize on the properties of new nanoscale materials” explain the researchers at the D.J. Sanghvi College of Engineering, in Mumbai, India[11]. According to that team, the main advantage of using nanosystems in the area of water treatment is their higher efficiency due to the large surface area of nanomaterials, when compared to conventional methods.

A few bionanocomposite water filters have already been presented to the world, showing a great potential of this group of materials. The first example, is the polypeptide-based hybrid filter membrane at Swiss Federal Institute of Technology in Zurich. The technology described by this team has an extremely simple structure, comprising low-cost raw material – protein amyloid fibrils, for example whey and activated porous carbon, both applied on a cellulose filter paper. This hybrid filter can be used to remove heavy metal ions and radioactive waste from water in just a single pass through the filter membrane [12]. As presented in the Figure 2, the contaminated water is drawn through the membrane, and the heavy metal ions, which are presented as red spheres, are bound to the protein fibres. Such approach allows for obtaining water of drinking quality.

Figure 2.  Bionanocomposite-based filters for water purification: a) amyloid-carbon system [12]; b) a portable $16 bionanocomposite-based water filter [13].

An excellent model system based on bionanocomposites has been also presented by an another team from India [13]. They developed a portable two-fold system of separate components for microbes (the upper filter unit and microbial membrane), and a multilayer filter block, which can be customized in dependence on the occurring contaminants. The construction of the filter is presented in the Figure 2b. The microbe filter is built of silver nanoparticles embedded in aluminium hydroxide nanoparticles/chitosan composite, designed in a way to block the macroscale water contaminants and protect the nanoparticles from the contaminants, which otherwise would accumulate on their surfaces, thus reducing their microbe-killing power. The membrane filter at the top kills variety of microorganisms, and the multi-layered filter block at the bottom can be custom-fitted for lead, mercury or arsenic. Production of such filter requires no electricity (filters are made at room temperature), and every litre of water used to make the material goes to filtering 500 litres of water. According to the designers of this nanofilter, every six months, the filter must be boiled for about four hours to remove the deposits that accumulate in the filters, reducing the filters’ activity.

The last example that gained a lot of attention lately, is a cheap, bionanocomposite-based filter proto-type developed by researchers at the Lulea University of Technology (Sweden) in collaboration with the Imperial College (UK) [14]. They have combined a cheap residue from the cellulose industry, with functional nano-cellulose to prepare adsorbent sheets with high filtration capacity towards environmentally hazardous contaminants from industrial effluents (heavy metal ions from industrial waters, dyes residues from the printing industry and nitrates from municipal water). The prepared material is placed inside a cartridge, which was already tested in two Spanish factories, and a Spanish water company (Acondaqua Ingeniería del Aqua SL in Valencia Spain), and are found to be highly effective. Filters have been scaled up to the size needed to purify both municipal and industrial effluents, providing positive health and environmental effects[15].

CONCLUSIONS

For the last few decades, there has been a long debate, about the scarcity of water resources, and the threats that it can bring for the mankind. For many people, this topic is still disregarded, but the fact is, that the time to worry about the availability of pure water has already come. Water has no other alternatives, and it cannot be replaced with anything else.

There are many investments made in the area of water purification, and it seems now, that the nanotechnological tools, can be helpful to solve current water problems. Mostly yet, it is extremely important, that the water crisis will not be only the next meaningless cliché, to which people will be unresponsive, but for the mankind to really know, this is the last call in changing the world.

References

[1] World Health Organization and UNICEF Joint Monitoring Programme, (2015) Progress on Drinking Water and Sanitation, 2015 Update and MDG Assessment.

[2] Yunus, I. S. et al. Environmental Technology Reviews 1(1), 136-148 (2012).

[3] http://www.enviroalternatives.com/watermethods.html

[4] Gehrke, I. et al. Nanotechnology, Science and Applications  8, 1-17 (2015)

[5] Amin, M. T. et al. Advances in Materials Science and Engineering ID 825910 (2014).

[6] Mintova, S. et al. Nanoscale, 5, 6693–6703 (2013).

[7] Lee, S.-Y. et al. Journal of Industrial and Engineering Chemistry 19(6), 1761–1769 (2013).

[8] http://www.systematixusa.com; http://www.dow.com/en-us/markets-and-solutions/products/ADSORBSIATitaniumBasedMedia/ADSORBSIAAs600

[9] Freinkel, S., Plastics: A Toxic Love Story (New York: Henry Holt, 2011),

[10] Eugene Cloete, T. et al. Nanotechnology in Water Treatment Applications (Caister Academic Press, UK 2010)

[11]www.sciencedaily.com/releases/2010/07/100728111711.htm

[12] Bolisetty, S., Mezzenga R., Nature Nanotechnology, doi:10.1038/nnano.2015.310,(2016)

[13] http://www.scientificamerican.com/article/cheap-nanotech-filter-water/

[14] http://www.nanowerk.com/nanotechnology-news/newsid=38556.php

[15] http://phys.org/news/2016-02-nano-cellulose-filters-highly-effective.html

______________________

Roksana Markiewicz Ph.D.

NanoBioMedical Centre, Adam Mickiewicz University, Umultowska 85, 61-614 Poznan,  Poland  E-mail: roksana.markiewicz@amu.edu.pl