Nanotechnology in water treatment

The terms 'stress' and 'scarcity' do not take into account physical access to water sources, or the quality of the water, or the irregularity of availability due to droughts and storms, or seasonal change. Instead, the terms give an indication of the close relation between population dynamics and renewable freshwater availability.

Only 30% of all freshwater on the planet is not locked up in ice caps or glaciers (not for much longer, though). Of that, some 20% is in areas too remote for humans to access and of the remaining 80% about three-quarters comes at the wrong time and place - in monsoons and floods - and is not always captured for use by people. The remainder is less than 0.08 of 1% of the total water on the planet (Source: World Water Council). Expressed another way, if all the earth's freshwater were stored in a 5-liter container, available fresh water would not quite fill a teaspoon. The problem is that we don't manage this teaspoon very well. Currently, 600 million people face water scarcity. Depending on future rates of population growth, between 2.7 billion and 3.2 billion people may be living in either water-scarce or water-stressed conditions by 2025:

Freshwater looks like it will become the oil of the 21st century - scarce, expensive and the reason for armed conflicts. While in our previous article we have only talked about nanotechnology and water in general terms, a new paper gives us the opportunity to look in more detail at the role that nanotechnology could play in resolving issues relating to water shortage and water quality. This review highlights the uses of nanotechnology in areas relevant to water purification, including separation and reactive media for water filtration, as well as nanomaterials and nanoparticles for use in water bioremediation and disinfection.
The potential impact areas for nanotechnology in water applications are divided into three categories, i.e., treatment and remediation, sensing and detection, and pollution prevention. Within the category of treatment and remediation, nanotechnology has the potential to contribute to long-term water quality, availability, and viability of water resources, such as through the use of advanced filtration materials that enable greater water reuse, recycling, and desalinization. Within the category of sensing and detection, of particular interest is the development of new and enhanced sensors to detect biological and chemical contaminants at very low concentration levels in the environment, including water. 

Detection of microbial pathogens

An adequate supply of safe drinking water is one of the major prerequisites for a healthy life, but waterborne diseases is still a major cause of death in many parts of the world, particularly in young children, the elderly, or those with compromised immune systems. As the epidemiology of waterborne diseases is changing, there is a growing global public health concern about new and reemerging infectious diseases that are occurring through a complex interaction of social, economic, evolutionary, and ecological factors. An important challenge is therefore the rapid, specific and sensitive detection of waterborne pathogens. Presently, microbial tests are based essentially on time-consuming culture methods. However, newer enzymatic, immunological and genetic methods are being developed to replace and/or support classical approaches to microbial detection. Moreover, innovations in nanotechnology and nanosciences are having a significant impact in biodiagnostics, where a number of nanoparticle-based assays and nanodevices have been introduced for biomolecular detection.

Bioactive nanoparticles for water disinfections

There is a growing threat of water-borne infectious diseases, especially in the developing world. This threat is rapidly being exacerbated by demographic explosion, a global trend towards urbanization without adequate infrastructure to provide safe drinking water, increased water demand by agriculture that draws more and more of the potable water supply, and emerging pollutants and antibiotic-resistant pathogens that contaminate our water resources. No country is immune. Even in OECD countries, the number of outbreaks reported in the last decade demonstrates that transmission of pathogens by drinking water remains a significant problem. It is estimated that water-borne pathogens cause between 10 and 20 million deaths a year worldwide.
According to Cloete, nanotechnology may present a reasonable alternative for development of new chlorine-free biocides. Among the most promising antimicrobial nanomaterials are metallic and metal-oxide nanoparticles, especially silver, and titanium dioxide catalysts for photocatalytic disinfections.

Nanofibers and nanobiocides

Electrospun nanofibers and nanobiocides show potential in the improvement of water filtration membranes. Biofouling of membranes caused by the bacterial load in water reduces the quality of drinking water and has become a major problem. Several studies showed inhibition of these bacteria after exposure to nanofibers with functionalized surfaces. Nanobiocides such as metal nanoparticles and engineered nanomaterials are successfully incorporated into nanofibers showing high antimicrobial activity and stability in water.

Biofilm removal

Three phases of biofilm life cycle

Sessile communities of bacteria encased in extracellular polymeric substances (EPS) are known as biofilms and causes serious problems in various areas, amongst other, the medical industry, industrial water settings, paper industry and food processing industry.[4] Although various methods of biofilm control exist, these methods are not without limitations and often fail to remove biofilms from surfaces. Biofilms often show reduced susceptibility to antimicrobials or chemicals and chemical by-products may be toxic to the environment, whereas mechanical methods may be labour intensive and expensive due to down-time required to clean the system. This has led to a great interest in the enzymatic degradation of biofilms. Enzymes are highly selective and disrupt the structural stability of the biofilm EPS matrix. Various studies have focused on the enzymatic degradation of polysaccharides and proteins for biofilm detachment since these are the two dominant components of the EPS. Due to the structural role of proteins and polysaccharides in the EPS matrix, a combination of various proteases and polysaccharases may be successful in biofilm removal. The biodegradability and low toxicity of enzymes also make them attractive biofilm control agents. Regardless of all the advantages associated with enzymes, they also suffer from various drawbacks given that they are relatively expensive, show insufficient stability or activity under certain conditions, and cannot be reused. Various approaches are being used to increase the stability of enzymes, including enzyme modification, enzyme immobilization, protein engineering and medium engineering. Although these conventional methods have been used frequently to improve the stability of enzymes, various new techniques, such as self-immobilization of enzymes, the immobilization of enzymes on nano-scale structures and the production of single-enzyme nanoparticles, have been developed. Self-immobilization of enzymes entails the cross-linking of enzyme molecules with each other and yields final preparations consisting of essentially pure proteins and high concentrations of enzyme per unit volume. The activity, stability and efficiency of immobilized enzymes can be improved by reducing the size of the enzyme-carrier. Nano-scale carrier materials allow for high enzyme loading per unit mass, catalytic recycling and a reduced loss of enzyme activity. Furthermore, enzymes can be stabilized by producing single-enzyme nanoparticles consisting of single-enzyme molecules surrounded by a porous organic-inorganic network of less than a few nanometers thick. All these new technologies of enzyme stabilization make enzymes even more attractive alternatives to other biofilm removal and control agents

Nanofiltration

Nanofiltration is a new type of pressure driven membrane process and used between reverse osmosis and ultrafiltration membranes. The most different speciality of nanofiltration membranes is the higher rejection of multivalent ions than monovalent ions. Nanofiltration membranes are used in softening water, brackish water treatment, industrial wastewater treatment and reuse, product separation in the industry, salt recovery and recently desalination as two pass nanofiltration system.Membrane processes are considered key components of advanced water purification and desalination technologies and nanomaterials such as carbon nanotubes, nanoparticles, and dendrimers are contributing to the development of more efficient and cost-effective water filtration processes.
There are two types of nanotechnology membranes that could be effective: nanostructured filters, where either carbon nanotubes or nanocapillary arrays provide the basis for nanofiltration; and nanoreactive membranes, where functionalized nanoparticles aid the filtration process.
The researchers also note that advances in macromolecular chemistry such as the synthesis of dendritic polymers have provided opportunities to refine, as well as to develop effective filtration processes for purification of water contaminated by different organic solutes and inorganic anions.

Reverse Osmosis

The membrane separation technologies of reverse osmosis (hyperfiltration) and nanofiltration are important in water treatment applications. Reverse osmosis is based on the basic principle of osmotic pressure, while nanofiltration makes use of molecule size for separation. Recent advances in the field of nanotechnology are opening a range of possibilities in membrane technologies. These include: new membrane preparation and cleaning methods, new surface and interior modification possibilities, the use of new nanostructured materials, and new characterization techniques.

Electrospinning

Electrospinning is a highly versatile technique that can be used to create ultrafine fibres of various polymers and other materials, with diameters ranging from a few micrometers down to tens of nanometres. The nonwoven webs of fibers formed through this process typically have high specific surface areas, nano-scale pore sizes, high and controllable porosity and extreme flexibility with regard to the materials used and modification of the surface chemistry of the fibres. A combination of these features is utilized in the application of electrospun nanofibres to a variety of water treatment applications, including filtration, solid phase extraction and reactive membranes.

Potential risks on human health

As with any other nanotechnology application where there is a possibility that engineered nanoparticles could eventually appear in various environments, the potential human and ecological risk factors associated with this are largely unknown and subject to much debate. Cloete and co-authors discuss various toxicity studies of nanomaterials and also point out several recent studies of the toxicological impact of nanoparticles on different aquatic organisms. As with any other nanotechnology application where there is a possibility that engineered nanoparticles could eventually appear in various environments, the potential human and ecological risk factors associated with this are largely unknown and subject to much debate. Cloete and co-authors discuss various toxicity studies of nanomaterials and also point out several recent studies of the toxicological impact of nanoparticles on different aquatic organisms.
The bottomline seems to be that it might be advisable to come to some definite conclusions regarding nanoparticle ecotoxicology before we embark an large-scale use of engineered nanoparticles in water applications. Nevertheless, there is a growing body of research and development that will lead to nanomaterials playing a key role in future water and wastewater treatment.
The bottomline seems to be that it might be advisable to come to some definite conclusions regarding nanoparticle ecotoxicology before we embark an large-scale use of engineered nanoparticles in water applications. Nevertheless, there is a growing body of research and development that will lead to nanomaterials playing a key role in future water and wastewater treatment.