20-04-2011, 09:29 AM
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INTRODUCTION
Water has always played a prominent role in human civilization. When people first began settling in one place and growing crops for sustenance, it was invariably near water sources like rivers, lakes or ground water springs. Water was needed for drinking, preparing food, bathing, cleaning, irrigating crops and a variety of other tasks, so it was important to have ready access to this resource. The water sources used for supplying water were not always clean and water was treated in one way or other to improve smell, taste and clarity or to remove disease causing pathogens, throughout the recorded history.
Water treatment describes those processes used to make water more acceptable for a desired end use. It may be for domestic, industrial processes, medical and many other uses. The goal of all water treatment processes is to remove existing contaminants so that the water becomes fit to use.
Water treatment includes treating water before its use and treating waste water generated after use. The latter includes sewage or domestic, agriculture or industrial waste water treatment.
Many methods are available for the above types of water treatment processes. All have their own merits and demerits, ease of use, economics, efficiency and end use which drive their selection.
In recent years nanotechnology has entered the sphere of water treatment processes. Many different types of nanomaterials are being evaluated and also being used in water treatment processes. Nanotechnology holds great promise in desalination, purification and waste water treatment. Desalination is a key market area. Vast majority of the world’s water is salt water, and though technology has existed for years that enables the desalination of ocean water, it is often a very energy intensive procedure and therefore expensive. However, as discussed below, nanotechnology applications can greatly reduce the costs of desalination. Nanotechnology is also being applied to the purification of water used for human consumption and in the production of ultrapure water required for certain manufacturing processes. Finally, as will be discussed below, nanotechnology has provided several applications that can be used to treat wastewater, making the recycling of water a viable and cost efficient way to address water supply shortages.
Chapter 2
NANOTECHNOLOGY AND ITS APPLICATIONS IN WATER TREATMENT
2.1 Nanotechnology:
Nanotechnology is the study of manipulating matter at atomic and molecular scale. It deals with structures sized between 1 to 100nm and involves developing materials or devices with this size. Substances or materials generated having nanoscale dimensions are referred to as ‘nanomaterials’. There are generally two categories of nanomaterials: fullerenes and nanoparticles. [1]
A fullerene is any molecule composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube. Spherical fullerenes are called buckyballs. Cylindrical ones are called carbon nanotubes or buckytubes. The carbon nanotubes have a lenghth to diameter ratio of 132,000,000:1. Have diameter of order 1nm and length upto 18cm. They exhibit extraordinary strength and unique electrical properties. They are categorized as single walled and multiwalled nanotubes. [2]
A nanoparticle is a metallic, semiconductor or oxide particle having dimensions between 1 to100nm. They are having a large surface area, very reactive and exhibit different properties compared to the bulk material of the same substance. They exhibit very interesting mechanical, magnetic, optical, chemical and other properties.
2.2 Nanotechnology in water treatment:
Recent advances in nanoscale science and engineering are providing unprecedented opportunities to develop more cost effective and environmentally acceptable water purification processes. It is suggested that many of the issues involving water quality could be resolved using the products resulting from the developments in nanotechnology. Development of affordable novel technologies to desalinate water are among the most exciting and promising features of nanotechnology. Utilization of specific nanoparticles either embedded in membranes or on other structural media that can effectively, inexpensively and rapidly render unusable water is being explored at a variety of institutions. Innovative use of nanoparticles for the treatment of industrial waste water is another potentially useful application. Many factories generate large amounts of wastewater. Removal of contaminants and recycling of the water would provide significant reductions in cost, time and labour to industry. Aquifier and groundwater remediation are also critical issues becoming more important as water supplies steadily decrease and demand continues to increase. Here too nanotechnology is showing promising results.
Many types of nanomaterials are being evaluated and used in purification of water contaminated with toxic metal ions, radionuclides, organic and inorganic solutes, bacteria and viruses.
The classes of materials being evaluated as functional materials for water purification are metal containing nanoparticles, fullerenes, zeolites and dendrimers. These have a broad range of physicochemical properties that make them particularly attractive as separation and reactive media for water treatment. The nanoparticles and fullerenes have been discussed above.
Zeolites are microporous, alluminosilicate minerals commonly used as commercial adsorbents. Zeolites have a porous structure that can accommodate a wide variety of cations such as Na+, K+, Ca2+, Mg2+ and others. These positive ions are rather loosely held and can be exchanged for others in a contact solution. They can be acquired from natural sources or fabricated in laboratories. Synthetic zeoloites are usually made from silicon-alluminium solutions or coal fly ash and are used as sorbents or ion exchange media in cartridge or column filters. Nanoparticles of zeolites are being evaluated for the water treatment processes. [3]
Dendrimers are repeatedly branched, roughly spherical large molecules. It is symmetric around the core and often adapts spherical three-dimension morphology. A Dendron usually contains a single chemically addressable group called the focal point. The properties of dendrimers are dominated by the functional groups on the molecular surface. Dendrimers can be considered to have three portions a core, an inner shell and an outer shell. Ideally a dendrimer can be synthesized to have different functionality in each of these portions to control properties such as solubility, thermal stability and attachment of compounds for particular applications. Dendrimer based nanomaterials are being evaluated and are discussed later. [4]
Chapter 3
NANOMATERIALS IN WATER TREATMENT SYSTEMS
3.1 Nanofilters:
Membrane processes play an important role in water purification, since conventional water treatment techniques such as coagulation, flocculation, sedimentation, activated carbon adsorption are not always able to remove organic pollutants to prescribed specifications. Because membrane components are considered key components of advanced water purification and desalination technologies, there is a continuous search for new material and technology for membrane fabrication. In this regard nanomaterials (e.g. carbon nanotubes, dendrimers) are contributing to the development of more efficient and cost effective water filtration processes. Nanofiltration is one of the four membrane technologies, which utilize pressure to effect separation of contaminants from water streams. The other three are microfiltration, ultrafiltration and reverse osmosis. All of these technologies utilize semi-permeable membrane that have the ability to hold back (reject) dissolved and/or suspended solids from a water stream containing these contaminants. The characteristics of the four technologies are summarized in the table 2.1.
