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HIGH SCHOOL NANOSCIENCE PROGRAM

Water Purification

 

Background

Clean water is fundamental for human survival and is a critical limiting resource that powers the global economy through its use in agricultural irrigation, electricity production, and industrial processes. [i] Worldwide, 1.2 billion people lack access to clean water.[ii] The two most pressing concerns for fresh water are: water quality and water quantity. Water quality involves removing contaminants of a variety of sizes and compositions from water, from the macroscale (dirt, etc.) to the microscale (bacteria and viruses) to the nanoscale (metallic ions and small organic molecules). Depending on what contaminants are being targeted for removal, different purification processes may be used (filtration, coagulation, distillation, disinfection). Water quantity issues surround the limited supply of fresh water in the world (97% of the water in the world is saline and 2% of the fresh water is frozen in glaciers) and the demands stressing this system from the increasing world population. California has less fresh water than it needs to support the population and infrastructure, so water is imported from other states, and new alternative sources of water including wastewater recycling and desalination from the ocean are becoming more widespread.

Activated carbon, also called activated charcoal or activated coal, has been processed to create high internal surface area.[iii] Each gram of activated carbon has a total pore volume of around 1 mL. Due to its highly porous structure, it has a very large internal surface area. One gram of activated carbon can have a surface area of around 1000 m2 (approximately a fifth of an American football field). Yet powdered activated carbon consists of particles approximately 1-150 μm in size, and granular activated carbon consists of particles 0.5 – 4 mm in size. When selecting an activated carbon, both pore and particle size are considered as well as the size and structure of the molecule being removed. Activated carbon is often used in water purification applications: for example, it is found in home water-purification filters such as Brita. The contaminant particles to be removed are attracted to the carbon’s surface through Van der Waals interactions, which are non-covalent attractions and between atoms, molecules, and surfaces. The physical adhesion on the surface is called adsorption. If the contaminants are large, they will clog the pores of the carbon and thus affect the quality of the purification.

Zeolites, or molecular sieves, are crystalline aluminosilicate framework structures that create a network of porous channels with diameters smaller than 1.2 nm. Due to these unique pore structures, zeolites are capable of excluding molecular species based on size. Zeolites are also very hydrophilic. More than 200 synthetic zeolite types and 50 natural zeolites are known. Zeolites were first recognized as a new type of mineral in 1756 by the Swedish mineralogist A. F. Cronstedt. In this experiment we are using the synthetic zeolite Linde Type A (LTA). To balance the negative charge of the aluminosilicate framework, positively charged cations are associated with the framework – sitting in the channels and voids, bonding in an ionic fashion. The chemical formula of a fully hydrated LTA molecule is Na12[(AlO2)12 (SiO2)12] · 27H2O. When in an aqueous solution, ions can exchange into and out of the zeolite framework structure. This ion-exchange process is used in applications for detergents: zeolite A is able to remove calcium from water, "softening" the water. Zeolite A is also largely used as a building block for adsorbents. Zeolites are used to remove not only carbon dioxide from gas streams but also hydrogen sulfide (H2S) and organic sulfur compounds, mainly from natural gas. This ability to remove sulfurous compounds makes zeolites effective for control of some odors, and natural zeolites are often used in pet litter [iv].

A. Wire aluminosilicate LTA framework
B. Internal surface of LTA showing 3-D pore structure



[i] Elimelech, M., The global challenge for adequate and safe water. Journal of Water Supply Research and Technology-Aqua, 2006. 55(1): p. 3-10.

[ii] Shannon, M.A. and Semiat, R., Advancing materials and technologies for water purification. MRS Bulletin, 2008. 33(1): p. 9-12 

[iii] "Activated Carbon." Norit. 12 May 2009 <http://www.norit-americas.com/activated-carbon-basics.html>.

[iv] Roland, E. and Kleinschmit, P., "Zeolites," Ullmann’s Encyclopedia of Industrial Chemistry. Wiley-VCH Verlag GmbH & Co. KGaA, 2002.

5 Harris, P. J. F, Zheng, L., and Suenaga K., Imaging the atomic structure of activated carbon. Journal of Physics: Condensed Matter, 2008. 20(1): p. 362201.