Clean drinking water is a basic human need. The pity is after so much of advancement in science and technology, 780 million people around the world do not have access to clean drinking water even today. To put it in to a context, this number is slightly higher than the population of all European countries combined. In other words, one in every nine people in the earth suffering from limited accessibility to safe drinking water. Also, around 2.5 billion people in to world do not have adequate access to sanitation due to limitations in clean water supply.
In many parts of the world, poverty and clean drinking water go hand in hand. Ironically, it is mostly the poorest of the world who are severely affected by the limited supply of clean water. This is further aggravated by the fact that majority of this population depend directly on water for their main livelihood which is usually agriculture. The effects of this situation is clearly evident in these societies. Diseases such as cholera, E. coli Infection, parasitic infections and Typhoid fever are particularly prevalent among these societies and amounts to higher percentage of death especially in children. Therefore, access to clean drinking water would dramatically transform the way of life of many people, especially in the developing world.
There are six major classes of water pollutants.
Biodegradable plant debris and animal waste
Organic chemicals such as oil, plastics, pesticides, solvents, gasoline
Nitrates and phosphates
Heavy metals such as Lead, Mercury and Arsenic
Bacteria, viruses and parasites
Clean drinking water may not be completely free from these components. However, concentration levels of these contaminants should be adequately low so that they wouldn’t cause a short term or long term health issues.
Conventional water treatment strategies such as filteration, uv water purification, chemical treatment (water treatment tablet) and desalination remain largely limited to industrial processes due to inherent complexities associated with these processes. Among others, high cost and complexity of these systems are major factors that inhibited their use in affected areas.
Over the years scientists have looked at nano-enabled technologies to find a solution to this long prevailing crisis. Many organizations and companies have adopted these techniques to solve technical challenges associated with water purification using conventional systems. Large body of these efforts are concentrated on developing easy to use, less complicated, durable home water purification systems or portable water purification systems. Nanotechnology has also given inspiration number of completely new water purification techniques as well. Some of these nano-enabled technologies are already on the market or in advance stages of development.
In this review we will highlight five most popular water purification techniques inspired by nanotechnology.
Adsorption in to nanomaterials
Water treatment using adsorption commonly referred to the process of removing of the harmful pollutant from the water by absorbing it in to the surface of a nanomaterial. Adsorption technique is heavily used for removal of organic pollutants and heavy metal irons. Nanomaterials have extremely high specific surface area, high concentration of sorption sites, tunable pore sizes and surface chemistry which gives them a large boost in adsorption rate and capacity. Carbon nanomaterials, metal oxide nanomaterials and nanofibers are among the mostly researched materials for water purification by adsorption. One other great feature of most of these nano adsorbers is their ability to recharge/regenerate with simple treatments, making it usable for long duration.
Photocatalytic water treatment
Photocatalysts are a special class of nanomaterial that can absorb light and divert the absorbed energy to drive a chemical reaction. Nanoscale Titanium dioxide and Zinc oxide are good examples of photocatalytic nanoparticles and are among most widely used materials in both research and practical applications. Upon excitation by a photon (light particle) these nanomaterials can produce a free electron and a hole in the nanoparticle. These charged particles will slowly migrate to the surface and will interact with nearby organic molecules. Electron can drive a reduction reaction while hole will give rise to an oxidation reaction. If a continuous supply of light is present, photogenerated electrons and holes can completely reduce and oxidize a nearby organic molecule to most elementary, carbon dioxide and water.
Same strategy can be used for the removal of trace organic containments and microbial pathogens from contaminated water. This technology has been adopted as an initial polishing step to remove intractable toxic organic compounds and as a pretreatment for hazardous and non-biodegradable contaminants. Current research focus is primarily on preparation of bed reactors with securely bound photocatalytic nanomaterials. However, current limitations such as limited photocatalytic activity and robustness has prevented this methodology from gaining wide acceptance as a practical method of water purification.
Disinfection of water with nanomaterials
Disinfection of water involves treatment for pathogenic microorganisms using a chemical or/and physical approach. Two of the major drawbacks of common conventional disinfectants such as chlorine treatment, ozone and UV water purification is the formation of disinfection by products and high dosage requirements. This greatly limits their applicability in home water purification techniques.
Some of the nanomaterials show broad spectrum antimicrobial activity at very low dosages making them great candidates for water disinfection procedures. Nanomaterials such as nano silver, nano Zinc oxide, nano titanium dioxide, nano cerium oxide, carbon nanotubes and fullerenes show strong disinfection abilities without mechanisms involving strong oxidation. Therefore, these nanomaterials show lower tendencies to form toxic byproducts. There are number of commercial nanoenabled disinfectant products developed for home water purification systems and portable water purification systems. Current research is mainly focused on developing materials that are stable and durable for long term use.
Filtration membranes provides a physical barrier for materials based on their size and widely used in both conventional and modern water purification systems to remove harmful or undesirable constituents from water. One of the major challenge in membrane technology in water purification is to find the balance between filtration selectivity and permeability. Current advancements in nanomaterial preparation, nanocomposites technology and surface modification has given rise to novel class of membranes call nanomembranes. Most of the current products and developments in these membranes are primarily focused on reducing the power consumption for filtration by hitting the correct balance, giving rise to interesting home water purification techniques such as water purification straw or bottle.
Most of the commercially available nanomembranes consist of either polymer nanofibers, nano pore membranes or polymer nanolayers. These nanomembranes typically have pore sizes that range from 1 nanometers to 50 nanometers filtering out almost all the bacteria and many harmful substances.
Nanoparticles of Zero valent iron
Zero valent iron is the iron in the elemental metallic form. Nanoparticles of zero valent iron has emerged as an interesting water treatment/purification strategy because of its ability to reduce harmful organic molecules, disinfect and lower heavy metal contaminants in the water.
When released to a water body, elemental metallic iron rapidly oxidizes to water soluble Fe(II) and slowly to Fe(III) ions. During its oxidation, it can reduce a nearby contaminant such as an inorganic iron or a harmful organic molecule. Since Fe(II) and Fe(III) ions are nontoxic and soluble in water, zero valent iron provides an effective and a reliable pathway for treating waterborne contaminants.
Nanoparticles of zero valent iron is heavily researched for the use of treatment of contaminated groundwater reservoirs. Work in the recent years have shown that zero valent iron has the capability to remove contaminants related to drinking water such as viruses and bacteria without the formation of disinfection by products. Nano zero valet iron chemistry is currently adopted in municipal water treatment plants and portable water purification systems.
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