Transcript Use of Nano-scale materials in Water Purification

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
• Chemical
The methods used
for this are:
• Mechanical
– Activated Carbon
– Chlorination
– UV light
• Biological
– Bacteria to decompose
waste
– Oxidation of chemicals
– 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
99%
Sodium Sulfate, Na2SO4
99%
Calcium Chloride, CaCl2
99%
Magnesium Sulfate, MgSO4
Even at the present
stage these filters are
shown to be very
efficient
Fructose, MW 180
>99% Even better values can
98% be obtained by
90% connecting various
filters in series
>99%
Sucrose, MW 360
>99%
Humic Acid
>99%
Sulfuric Acid, H2SO4
Hydrochloric Acid, HCl
Viruses
99.99%
Proteins
99.99%
Bacteria
99.99%
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