Use of Nano-scale materials in Water Purification

Robert Meservy Dept. Physics

I chose this subject because I’m a reefkeeper and as such have to use distilled water in order to not poison the corals and anemones that I keep. I was curious as to whether or not nanotechnology could provide a cheaper and more viable alternative. These principles could also be applied to providing drinking water from saltwater or contaminated sources.

Water, water everywhere but nary a drop to drink!

• • • • Over 75% of the Earths surface is covered in water 97.5% of this water is salt water, leaving only 2.5% as fresh water Nearly 70% of that fresh water is frozen in the icecaps of Antarctica and Greenland; most of the remainder is present as soil moisture, or lies in deep underground aquifers as groundwater not accessible to human use.

< 1% of the world's fresh water (~0.007% of all water on earth) is accessible for direct human uses. This is the water found in lakes, rivers, reservoirs and those underground sources that are shallow enough to be tapped at an affordable cost. Encyclopedia Britannica

Current Purification Methods

This water must of course be first purified to be fit for human consumption The methods used for this are: • • • Chemical – Activated Carbon – Chlorination – UV light Biological – Bacteria to decompose waste – Oxidation of chemicals Mechanical – Settling – Sand or similar screening material

Advanced types of Mechanical Filtration

• Some methods of mechanical filtering are actually capable of doing so on the nano-metre scale: i.e. Diatom filtration Reverse Osmosis

Wikipedia

Diatom Filtration

SEM micrographs of diatoms Diatoms are small single-celled marine algae that use silica to form hard shells. They have small pores that allow the flow of nutrients.

a-d Examples of diatom morphologies (scale 10μm) e Valve openings (scale 1μm) Due to their small size and hard shells they can be packed together to form compact filters capable of filtering objects on the micron scale Unfortunately due to the relatively large size of their pores they are incapable of removing chemical impurities

retrieved from http://www.osen.org

Reverse Osmosis

Pressure is applied across a membrane, driving pure water across while leaving concentrate behind Drawbacks: • Most of the water wasted ~87% • High pressures are needed to maintain flow • Membrane rapidly loses efficacy

Nanotech Solutions

• • Use of nano-tubes as filtering devices Use of nano-particles as treatment agents

Nanotube filters

The Use of Carbon Nano-tubes as filtering devices a. Schematic of the process b. Photograph of the bulk tube. c. SEM image of the aligned tubes with radial symmetry resulting in hollow cylindrical structure (scale 1 mm).

Views of the Filter

1. SEM picture of filter cartridge a. SEM of wall of cartridge (scale 100µm ) b. Same (scale 10µm) c. Lattice of Carbon Nanotubes can be seen (5µm)

How the Filter Works

The nano-tubes act as a kind of molecular filter, allowing smaller molecules (such as water) to pass through the tubes, while contaminants are too large to pass through.

Due to their electronic configuration smaller ions that would otherwise pass through are also blocked

Removal of bacteria using nanotube filter

a, The unfiltered water containing E. coli bacteria b, The E. coli bacteria (marked by arrows) grown by the culture of the polluted water c, The filtration experiment d, The water filtered through nanotube filter e, The filtrate after culture showing the absence of the bacterial

Advantages

• • Much less pressure required to move water across filter Much more efficient • • • • Filter easily cleaned by back flushing Selective adsorption properties of nanotube surfaces Incredibly large surface area Manmade nanotube membranes allow fluid flow 10,000 to 100,000 times faster than conventional fluid flow theory would predict

Problems to Overcome

• • Processes need to be designed to mass produce them By using a continuous spray pyrolysis method it has been possible to synthesise hollow carbon cylinders various centimetres in diameter and several centimetres long. Larger cylinders needed if this is to become practical University of Kentucky

Rejection Values

Species Sodium Chloride, NaCl Sodium Sulfate, Na 2 SO 4 Calcium Chloride, CaCl 2 Magnesium Sulfate, MgSO 4 Sulfuric Acid, H 2 SO 4 Hydrochloric Acid, HCl Fructose, MW 180 Sucrose, MW 360 Humic Acid Viruses Proteins Bacteria 99% 99% 99% >99% 98% 90% >99% >99% >99% 99.99% 99.99% 99.99% Even at the present stage these filters are shown to be very efficient Even better values can be obtained by connecting various filters in series

Nano-particles

Formation of nanoparticles suitable for the adsorption of arsenic and other large ions in the treatment of drinking water

A schematic of how iron nano-particles can be used for the selective removal of groundwater contaminants.

Field tests have shown that they can remove up to 98% of contaminants

Conclusion

Nano-technology could potentially lead to more effective means of filtration that not only remove more impurities than current methods but do so faster, more economically and more selectively