Ka Ming NG
The Hong Kong University of Science and Technology
Environmental challenges ranging from keeping the air, water and soil clean to ameliorating the ominous climate change have to be met without delay with innovations by the global technical community. Nanotechnology, the engineering and manipulation of physicochemical phenomena and materials at a length scale on the order of 1-100 nm, promises to contribute to environmental sustainability through pollution prevention and remediation. The term 'pollutant' is used here to mean a substance which, above a certain concentration, can directly or indirectly cause deleterious effects to various forms of life.
The first priority is prevention of pollutants at the outset. Pollutants are often generated in manufacturing. For example, reaction by-products are expected in the production of chemicals such as pharmaceuticals and polymers that are used in our everyday lives. However, there often exist a number of reaction routes to produce the same product. The use of green manufacturing might completely avoid the generation of undesired by-products. In some cases, the undesired by-product can be converted back to the desired product or other useful products. For example, 2,4'-BPA can be converted to 4,4'-BPA, the desired product, in an isomerization reactor. Industrial plants, particularly power plants, emit huge amounts of CO2 which is known to cause greenhouse effect. One approach for reducing CO2 emission is sequestration where CO2 is captured by absorption, transported to an injection site and stored in a suitable geological formation. A more attractive alternative is to use CO2 as a raw material for producing useful end-products such as polycarbonates, CO2 polymers and synthesis gas by CO2 reformation of methane. Nanocatalysts play an important role in most of these technologies.
Another strategy is minimization of pollutants. The use of an excessive amount of plastic shopping bags has been a point of contention as they do not decompose readily and take up a significant amount of landfill space. Nano-CaCO3 particles which are cheap and inert can be used as fillers for plastic bags, thereby reducing the amount of plastics such as polyethylene in the same plastic bag by 10 to 20% by weight. Fertilizers can cause serious problems to aquatic life in streams and lakes. The use of controlled release can reduce the amount of runoff.
Manufacturing industries such as paper-making, electroplating, brewery and dyeing consume a humongous amount of water, depleting a valuable resource required for everyday human activities. These wastewater effluents loaded with pollutants require treatment before release to the environment. For example, a typical dyeing plant uses around six thousand tonnes of fresh water per day and the Chemical Oxygen Demand of the effluent stream laden with organic dyes, salts and surfactants can be as high as 10,000. Thus, there is a high demand for high-performance adsorbents, ion-exchange resins and filters for water treatment. For example, nanoparticles have been designed to remove pollutants by adsorption, followed up regeneration. Similar needs exist for treating NOx and SO2 in flue gas. Innovations such as aligned carbon nanotube cylindrical filters and fabrics made up of electrospun nanofibers may lead to enhanced performance. The development of innovative air and water treatment technologies is a necessity for persistent organic compounds that are resistant to environmental degradation through chemical and biological processes.
Treatment is neither limited to industry nor to organics and inorganics. Nanocatalysts are deployed in air purifiers to remove Volatile Organic Compounds in home and office. Nano TiO2 catalysts can potentially be used in paints on the walls to achieve similar effect. Ozone can kill germs in water but the mass transfer and its efficiency can be greatly enhanced by generating exceedingly small ozone bubbles. Pathogens can be filtered out by nanofiber filters or killed by silver nanoparticles.
Rather than releasing the treated wastewater from the wastewater treatment plant into the environment, the reuse of the treatment wastewater in the manufacturing plant is the key to sustainability. In fact, the ever-tightening regulations typically demand that over 50% of the treated water be reused, with the ultimate goal of zero emission. Given the severe water shortage in many parts of the world, this goal should be achieved as soon as possible. Ultrafiltration and reverse osmosis membranes make water reuse possible by filtering out large molecules. Surface treatment of the membrane surface can reduce the probability of fouling and prolong membrane life. Plastics can also be reused if it can be converted back into their constituent molecules. Indeed, polyesters have been decomposed back to terephthalic acid and ethylene glycol for years.
Monitoring the amount and location of pollutants in the environment is essential. The development of nanosensors has been progressing at a good pace to detect chemical and biological contaminants. For example, nanosensors based on Carbon Nanotube (CNT) / Anodized Aluminum Oxide thin films can detect changes in pH and humidity. Biofunctional nanomagnets which possess mobility and a large surface area per unit volume can be used to detect a specific biomolecule.
Pollutant degradation and sequestration are the aim of environment remediation, which can deploy the same techniques used for pollutant treatment. Iron nanoparticles are capable of decomposing chlorinated compounds in groundwater. Another option is using UV-assisted TiO2 nanocatalyst which is particularly popular because of its inertness towards other chemicals.
While nanotechnology is a promising means for meeting environmental challenges, the environment, health and safety aspects of nanomaterials themselves also need scrutiny. It is important to understand the absorption, transport and accumulation of nanomaterials in living things. For example, ZnO nanoparticles are a major ingredient in sun-screen lotions. Preliminary tests indicated that they did not enter the body through the skin. Yet it is desirable to know where these nanoparticles end up in the environment and their long-term impact, if any. Similar questions should be asked for fullerenes which are now used in organic solar cells and CNTs in sporting goods. In addition to consumers, potential risks to the researchers and workers handling these nanomaterials should also be assessed and risk management plans be implemented.
There are a number of grand environmental challenges that await solutions. One is energy production without the concomitant generation of pollutants. Wind and solar energy seem to be serious contenders in this regard, and nanotechnology plays an important role in improved turbine and solar cell materials. Another is the reduction of carbon dioxide from the atmosphere. The third challenge is the availability of a low-energy and low-cost seawater desalination technology. Collaboration among the various economies around the globe should be encouraged in order to achieve tangible results as soon as possible.