A Nanotechnological Approach to Contaminated Water Treatment

A Nanotechnological Approach to Contaminated Water Treatment
ABSTRACT


Scarcity of water, in terms of both quantity and quality, poses a significant threat to the current and future well-being of people worldwide, but especially to people in developing countries. Sustainable water management is a critical aspect of addressing poverty, equity, and related issues. Science and technology has a role to play in contributing to the development of new methods, tools, and techniques to solve specific water quality and quantity problems. Projects that meet economic, social, and environmental criteria can contribute to sustainable management of water resources and improve access to clean water for poor people in developing countries. This paper provides an overview of water treatment devices that incorporate nanotechnology; some of these are already on the market while others are still in development. The paper then explores potential environmental and health risks, risk governance issues, and socio-economic issues regarding the potential use of nanotechnology to improve access to clean water and basic sanitation.

In exploring whether and how nanotechnology could be applied responsibly to offer new and better solutions, this paper describes a range of available products and promising research that apply nanotechnology to provide clean water. In particular, this paper describes nano-filtration membrane technologies used to clean water. It then describes socioeconomic issues and potential environmental and human health risks of using nanotechnology to clean water. Here we demonstrate the differences in access to technology, field conditions, and the types of technologies that may be appropriate in different circumstances

Water Pollution and Nanofiltration
Water that does not meet drinking water standards should be treated to ensure that the health of the consumer or community is not compromised through exposure to toxic pollutants. Polluted water is often treated by conventional or pressure-driven membrane processes to make it comply with drinking water standards. The conventional water treatment process consists of several stages. These include pre-treatment, coagulation, flocculation, sedimentation, disinfection, aeration, and filtration. The pre-treatment stage removes suspended solids. Coagulation and flocculation are carried out to precipitate dissolved impurities through sedimentation. The water is then filtered to remove any suspended particles. One of the disadvantages of the conventional water treatment method is that it cannot remove dissolved salts and some soluble inorganic and organic substances.
Pressure-driven membrane technology is an ideal method for the treatment of water to any desired quality. The integral part of the technology is the membrane. The membrane is a barrier that separates two homogenous phases. It allows some solutes to pass through but rejects the permeation of others. It achieves the separation of solutes of a fluid mixture when a driving force is applied. The force could be a pressure difference (Æp), concentration gradient (Æc), temperature difference (ÆT), or electrical potential difference (ÆE).The basic principle of operation is illustrated in Figure 1. Phases 1 and 2 are generally the feed water and the product water or permeate, respectively. The basis of separation is that each membrane has unique characteristics for the selective permeation and rejection of different solutes.



Figure 1: Schematic Representation of a Two-Phase System Separated by a Membrane

There are four pressure-driven membrane processes. These are micro filtration (MF), ultra filtration (UF), nanofiltration (NF), and reverse osmosis (RO).These processes may be distinguished by pore size, transport mechanism, applied pressure, and range of applications. The pore sizes for MF, UF, NF, and RO are respectively 0.05 10 μm, 1 – 100 nm, < href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEixXSEPSQt55mn8ugqhEzJ0WJBjsYHjVNBO-cbPo9GzgS66OCyzRStQqTtG9feHSWGH27i6PPKrE0MbSWSUuilpu6oMQHsixWCQHsYTL-uH39V4mtdLi34wtCTD9I36DLrgxIgMkS775UQ/s1600/a2.jpg">

Fig 2. Nanoporous zeolite


Fig 3.nanoporous polymer architecture

Researchers at Los Alamos National Laboratory have developed a new class of nanoporous polymeric materials that can be used to reduce the concentration of common organic contaminants in water to parts-per-trillion levels.61 There organic nanoporous polymers with narrow pore-size distribution (0.7 – 1.2 nm) have been synthesized using cyclodextrins as basic building blocks. The researchers say that the binding between organic contaminants and the nanoporous polymer is 100,000 times greater than the binding between organic contaminants and activated carbon, which is commonly used in wastewater treatment. These materials can be used for the purification of municipal water supplies or for recycling and reuse of industrial wastewater.

desalination
Desalination is the removal of dissolved salts from raw or untreated water by either thermal or membrane processes. A thermal process uses heat to evaporate water, which is then collected by condensation. In a membrane process, pressure is applied to force the raw water through a membrane that retains the dissolved salts. Reverse osmosis (RO) membranes can retain all the salt, whereas other membrane processes, such as nanofiltration (NF), selectively retain some salts. Desalination is carried out for various reasons, including limited freshwater, increasing demand, global warming, regulation, cost effectiveness, and politics. A reverse osmosis (RO) desalination plant consists of the following sequence of stages: feed water intake system, pre-treatment facility, high-pressure feed pumps, RO membrane, and desalinated water conditioning system. A pressure of 40 – 80 bars is required for the permeation of water through the RO membrane for the desalination of seawater. Two membrane sheets are glued together and spirally wound around a perforated central tube. The product water exits through this tube. Nanotechnology is used in Israel for the desalination of saline waters. The Grand Water Research Institute of the Israel Institute of Technology is working with corporate and other partners to treat salt water and create fresh sources for drinking water and irrigation.

Fig 4. Desalination process
They are using reverse osmosis whereby pressure is applied to salt water, forcing the fluid through a very fine membrane resulting in (virtually) pure water.

The major setback of desalination is that production costs are very high. … It is expected that Nanotechnology… will drive down the costs of desalination.

A major impediment to wider adoption of desalination technology is high production costs. A third of the costs are required for supplying the energy that forces the water through the membrane. Although significant advancements in technology have extended membrane life while lowering energy requirements, overall energy consumption remains extremely high. Desalination costs about USD 1 per m3 of salt water and USD 0.60 per m3 of brackish water. It is expected that nanotechnology will contribute to improvements in membrane technology that will drive down the costs of desalination. For instance, the Long Beach Water Department has reduced the overall energy requirement (by 20 to 30 percent) of seawater desalination using a relatively low-pressure two-staged nanofiltration process. This unique process is now being tested on a larger scale

Suitability of the Nanomembrane Technologies
There are merits and demerits of nanomembrane technologies (e.g., nanofiltration and reverse osmosis) over conventional filtration technologies. The conventional sand filter does not retain some microbes and dissolved salts (e.g., arsenate). Nanofiltration (NF) and reverse osmosis (RO) membranes remove all multivalent ions and bacteria. The conventional carbon filter, biological sand, and biological carbon filters do not remove some bacteria and dissolved salts (e.g., calcium). Calcium is readily removed by the nanomembrane processes. However, NF membranes have a low rejection coefficient (R) for monovalent ions. Thus, they are not well suited for the removal of nitrate and fluoride ions from water, which could be an advantage in cases where fluoride ion levels are suitable for the healthy development of teeth.
The only additional equipment required for NF membrane filtration, compared to conventional filtration, is cartridge filters. These serve as a pre-treatment for the removal of particulate matter before membrane filtration. Their cost is insignificant. One of the setbacks of nanomembrane technology is cost. The cost of a full-scale conventional filtration plant is about 70% of a nanomembrane plant. The durability of a nanomembrane plant is comparable to that of a conventional filter plant and is determined by the nanomembrane whose life span ranges between five and six years. The life span can be further prolonged by using an effective pre-treatment.
Nanomembrane filtration technologies are suitable for developing countries. Nanomembrane plants can be built as portable units, which can be assembled in the major urban centers and then transported to the outlying areas (i.e., rural and peri-urban) where they are needed. By building the plants as portable units, the initial capital required for the construction can be lowered.

The paper demonstrates that nanotechnology research is being conducted in a broad spectrum of areas relevant to water treatment – filters, catalysts, magnetic nanoparticles, and sensors. However, the maturity of research and development efforts is uneven across these areas, with nanofiltration currently appearing as the most mature. Interest in the application of nanotechnology to water treatment devices appears to be driven by several factors including, but not limited to, reduced costs, improved ability to selectively remove contaminants, durability, and size of device. While the current generation of nanofilters may be relatively simple, many researchers believe that future generations of nano-based water treatment devices will capitalize on the new properties of nanoscale materials. Advances through nanotechnology, therefore, may prove to be of significant interest to both developed and developing countries.


References:
www.freepatentsonline.com
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www.wikipedia.org