Use of Nano-scale Use of Nano-scale materials in Water materials in Water

PurificationPurificationRobert MeservyRobert Meservy

Dept. PhysicsDept. 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 Water, water everywhere but nary a drop to drink!nary a drop to drink!• Over 75% of the Earths surface is covered Over 75% of the Earths surface is covered

in waterin water• 97.5% of this water is salt water, leaving 97.5% of this water is salt water, leaving

only 2.5% as fresh water only 2.5% as fresh water • Nearly 70% of that fresh water is frozen in Nearly 70% of that fresh water is frozen in

the icecaps of Antarctica and Greenland; the icecaps of Antarctica and Greenland; most of the remainder is present as soil most of the remainder is present as soil moisture, or lies in deep underground moisture, or lies in deep underground aquifers as groundwater not accessible to aquifers as groundwater not accessible to human use. human use. 

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

Encyclopedia Britannica

Current Purification MethodsCurrent Purification Methods

• ChemicalChemical– Activated CarbonActivated Carbon– ChlorinationChlorination– UV lightUV light

• BiologicalBiological– Bacteria to decompose Bacteria to decompose

wastewaste– Oxidation of chemicalsOxidation of chemicals

• MechanicalMechanical– SettlingSettling– Sand or similar Sand or similar

screening materialscreening material

This water must of This water must of course be first course be first purified to be fit purified to be fit for human for human consumptionconsumption

The methods used The methods used for this are:for this are:

Advanced types of Mechanical Advanced types of Mechanical FiltrationFiltration

• Some methods of Some methods of mechanical filtering mechanical filtering are actually capable are actually capable of doing so on the of doing so on the nano-metre scale: nano-metre scale:

i.e. Diatom filtrationi.e. Diatom filtration

Reverse OsmosisReverse Osmosis

WikipediaWikipedia

Diatom FiltrationDiatom FiltrationSEM 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 impuritiesretrieved from http://www.osen.orgretrieved from http://www.osen.org

Reverse OsmosisReverse Osmosis

Pressure is applied across a membrane, driving pure water across while leaving concentrate behindDrawbacks:

• Most of the water wasted ~87%• High pressures are needed to maintain flow• Membrane rapidly loses efficacy

Nanotech SolutionsNanotech Solutions

• Use of nano-tubes as filtering devicesUse of nano-tubes as filtering devices

• Use of nano-particles as treatment Use of nano-particles as treatment agentsagents

Nanotube filtersThe 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).

Nature Materials 3, 610–614 (2004) Carbon nanotube filters A. SRIVASTAVA,O. N. SRIVASTAVA, S. TALAPATRA, R. VAJTAI and P. M. AJAYANNature Materials AOP Published online: 1 August 2004 | doi:10.1038/nmat1192

Views of the FilterViews of the Filter

a. SEM picture of filter cartridge

b. SEM of wall of cartridge (scale 100µm)

c. Same (scale 10µm)

d. Lattice of Carbon Nanotubes can be seen (5µm)

How the Filter WorksHow 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 Removal of bacteria using nanotube filternanotube 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

AdvantagesAdvantages• Much less pressure required to move water across filterMuch less pressure required to move water across filter

• Much more efficientMuch more efficient

• Filter easily cleaned by back flushingFilter easily cleaned by back flushing

• Selective adsorption properties of nanotube surfacesSelective adsorption properties of nanotube surfaces

• Incredibly large surface areaIncredibly large surface area

• Manmade nanotube membranes allow fluid flow 10,000 to 100,000 times faster Manmade nanotube membranes allow fluid flow 10,000 to 100,000 times faster than conventional fluid flow theory would predictthan conventional fluid flow theory would predict

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

University of Kentucky

Rejection ValuesRejection ValuesSpecies

Sodium Chloride, NaCl 99%

Sodium Sulfate, Na2SO4 99%

Calcium Chloride, CaCl2 99%

Magnesium Sulfate, MgSO4 >99%

Sulfuric Acid, H2SO4 98%

Hydrochloric Acid, HCl 90%

Fructose, MW 180 >99%

Sucrose, MW 360 >99%

Humic Acid >99%

Viruses 99.99%

Proteins 99.99%

Bacteria 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-Nano-particlesparticles

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

ConclusionConclusion

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