Nanotechnology and Water Purification

The last two decades have seen membrane-based separation established as a family of broadly accepted technologies that complement and often replace traditional water treatment unit processes such as granular media filtration, chemical precipitation and softening. Membranes processes such as microfiltration, ultrafiltration, nanofiltration, and reverse osmosis, can be used to remove a diverse range of pollutants from a variety of source waters. As of 2008, the membrane industry in U.S. alone is $2.9 billion and growing.

The commercial success of membrane technologies is grounded in the continuous innovation in the areas of membrane materials and processes. Recent developments in the materials science of membranes have been fueled in large part by advances in nanotechnology. Membranes with improved permeability, selectivity, and resistance to fouling have been developed using newly available nanomaterials.

Functional nanomaterials have been used to synergistically combine separation and additional functions and prepare more efficient membranes with a smaller environmental footprint. Examples of nanomaterial-enabled membranes include:

i) membranes prepared from nanomaterials (e.g., ceramic membranes that have been traditionally prepared from inorganic materials such as TiO₂, ZrO₂, Al₂O₃, etc.¹ but also the novel class of membranes prepared from carbon nanomaterials such as carbon nanotubes^2-4);
ii) membranes prepared by nanomaterial templating⁵;
iii) polymer nanocomposite membranes (e.g., using TiO₂^6-8, Ag0^9-11, Al₂O₃^12,13, and SiO₂^14-16, NaA zeolite¹⁷ as inorganic fillers);
iv) membrane reactors with functional nanoparticles (e.g., Fe/Ni^18-20, Fe/Pd^20,21, Ag0²², gold²³, zero-valent iron²⁴).

Our NSF-sponsored research project “Partnership for International Research and Education: New generation synthetic membranes - Nanotechnology for drinking water safety” is an example of a large interdisciplinary of effort focused on the development of new nanotechnology-enabled membrane processes and technologies. The Partnership is on the design of nanostructured membranes and addresses fundamental nanomaterials chemistry and materials science as applied to water quality technologies. The project is a joint effort between a number of research groups in the United States and abroad. An example PIRE project is our recent study on the design of biofouling resistant silver-polysulfone composite membranes. In this work, the nanocomposite membranes were synthesized by incorporating silver nanoparticles into the polymer matrix of a membrane.

The particles were either synthesized ex-situ and then added to the casting solution as organosol or produced in the casting solution via in-situ reduction of ionic silver by the polymer solvent. We have shown that the antibacterial capacity due to the gradual release of ionic silver by the prepared nanocomposites can be effective in reducing intrapore biofouling in nanocomposite membranes of a wide range of porosities. Such nanocomposites could also be used as materials for macroporous membrane spacers to inhibit the biofilm growth on downstream membrane surfaces¹¹.

In the hybrid system, ozonation is effective in mitigating membrane fouling due to the oxidation of foulants by ozone and/or hydroxyl radicals. By introducing nanoparticles such as Fe₂O₃ and MnO₂ at the membrane surface, the efficiency of the hybrid process can be significantly enhanced due to both the catalytic effect of the nanoparticles and more targeted oxidation of the NOM portion that is concentrated at or near the membrane surface contributing to membrane fouling.Another example of how functional nanoparticle can be used to improve membrane performance is the hybrid ozonation-ultrafiltration hybrid process. This process is at the focus of the NSF-funded research project at Michigan State University. The combination of nanoparticle-based functionalities with membrane separation in one hybrid unit improves the overall process efficiency and remove excessive redundancy^25-31.

A unique aspect of this hybrid process is that due to the effect of catalytic ozonation at the membrane surface, the surface remains relatively foulant-free; therefore, in the absence of the fouling layer, foulant-membrane interactions remain important for extended periods of membrane operation. This, in turn, increases the relevance of membrane surface engineering for longer term membrane operation.

References

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