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NANOPOROUS GRAPHENE DESALINATION TO SOLVE THE CLEAN
WATER CRISIS
Abigail Doyle ([email protected])
INTRODUCTION: A GLOBAL NECESSITY
FOR CLEAN WATER
Water is one of the most important necessities of life. It
is also unavailable to one out of every six people in our
world according to the National Academy of Engineering
[1]. Ironically, the United States Geological Survey
estimates that 71 percent of Earth is covered in water [2]. A
source of clean water is needed not only for drinking, but
also for sanitary uses. The World Health Organization
reports that each day, “easily preventable causes claim the
lives of approximately 5,000 people, most of them young
children” [3]. It truly overwhelms me that our world
contains so much water, a basic necessity, and yet almost a
sixth of our population cannot find clean water to use. A
report from the United Nations claimed, “Overcoming the
crisis in water and sanitation is one of the greatest human
development challenges of the early 21st century.”
Engineers are coming together to solve the water crisis, one
of the greatest challenges in engineering to date. Some of the
most common reasons for a lack of clean water can usually
be traced back to the simple fact that only three percent of
the Earth’s water is fresh. To solve this problem,
desalination is a process that can be used to remove the salt
from saltwater, which makes up most of our current water
source [1]. More specifically, engineers have found that
graphene is a material that has proven to have good
absorption qualities and can therefore be a way of filtering
the salt during desalination [4]. Saudi Arabia is already
using the desalination process and actually accounts for
almost a tenth of the world’s desalination [1]. Though
graphene desalination can be expensive and not yet practical
in most areas of the world, advancement in this technology
could solve our water crisis and enhance hundreds of
millions of lives.
REVERSE OSMOSIS DESALINATION: A
CURRENT TECHNOLOGY
Since most of the world is covered in saltwater, a
logical solution to the clean water crisis would involve
taking the salt out of that water to enable it to be used as
drinking water. An article written by E. Wang and R. Karnik
UniversityofPittsburgh,SwansonSchoolofEngineering
11.01.2016
says that desalination, the process of removing salt, uses
high pressures to push saltwater through a semi-permeable
membrane. The semi-permeable membrane has small
openings to allow only water molecules to be pushed
through. Since the salt ions do not fit through the openings,
they are left on the other side of the membrane and the
saltwater has therefore been filtered as shown in Figure 1
[4]. However, J. Gerhart identifies that this current method
of reverse osmosis desalination has a few disadvantages.
While removing salt ions, this process may also be removing
molecules with a size comparable to that of salt such as
calcium, fluoride, iron, and manganese. All of these
elements are essential to a human’s health and removing
these particles to produce “clean” water would actually be
creating a new dietary issue within itself [5]. In addition to
this concern, L. Zyga believes that reverse osmosis is
actually a very inefficient process. For every gallon of
purified water produced, there are actually two to three
gallons of water that cannot be used, which ultimately
wastes a large amount of energy [6]. Reverse osmosis is a
current method of desalination that is not quite practical
because it can create more dietary issues and is not cost
efficient.
FIGURE 1 [4]
This figure shows the process of reverse osmosis where
the membrane allows the water molecules to pass through
while keeping the salt ions retained on the other side.
NANOPOROUS GRAPHENE : A MORE
EFFICIENT PROCESS
While reverse osmosis may to be impractical, the
general idea of desalination seems to be realistic. However,
AbigailDoyle
it is difficult to find membranes that are suited for this job.
Most selectively permeable membranes that would even be
considered for this type of process have pores that are too
large to exclude the salt ions from passing through. To
combat this issue, materials scientists David Cohen-Tanugi
and Jeffrey Grossman from the Massachusetts Institute of
Technology have researched and proposed that graphene
would be a suitable membrane due to its structural stability,
atomic thickness, and subnanometre size pores. Graphene is
essentially carbon in in a single layer sheet that is only one
atom thick. It also has characteristics that allow its
permeability to be altered to allow different molecules to
pass through. Since graphene has this feature, scientists are
able to manipulate its pores to be larger than the size of a
water molecule, but smaller than the size of a salt molecule
to allow the water to flow through without letting the salt
through. This method would be far more effective than
reverse osmosis because this process operates without high
pressure and therefore does not require nearly as much
energy. In addition, this process saves time because it allows
the water to be filtered at a higher rate. In reverse osmosis,
the rate that water flows through these membranes is often
too slow to be worthwhile. To compensate for this issue,
membranes have been manufactured to have better defined
pores. Still, these membranes are expensive to produce and
difficult to scale to size. Graphene, however, can naturally
transport up to 66 L per (cm2)(day)(MPa) with a salt
rejection higher than 99%. Meanwhile a membrane typically
used in reverse osmosis will only transport up to 0.05 L per
(cm2)(day)(MPa) with that same salt rejection [6]. Graphene
shows these signs of having much greater transport
properties because it is only one carbon atom thick [7].
It does not stop there though. Researchers such as
Cohen-Tanugi and Grossman are finding ways to advance
desalination even further by changing the edges of the
graphene membrane pores to adapt its chemical
functionality. Recall that the pores must be large enough for
the water to pass through, but small enough to prevent the
salt from passing. However, larger pores are also known for
having a smaller salt rejection. In response, these researchers
have bonded hydrogen groups (H-) and hydroxyl groups
(OH-) to the edges of the carbon atoms to reject the salt
more effectively. The H-pores are hydrophobic, meaning
that they do not like water and can therefore restrict the
number of water molecules that pass through. The OH-pores
on the other hand, have hydrophilic properties that cause the
water to flow faster because the pores like water. With this
alternation pattern as shown in figure 2, these hydrogen and
hydroxyl groups are more likely to create bonds with the salt
ions and keep then restricted more effectively. Because
graphene can allow more water to flow through while
retaining its salt retention properties, it is much more
resourceful than reverse osmosis desalination [8].
Researcher Silvia Román also predicted that the use of
graphene for desalination would take off in the year 2015,
but would require a few years to be in full use because large
investments will be necessary to start mass production of
these graphene membranes [8]. Ultimately, graphene
membranes do not require high pressures and large amounts
of energy so they save time and money while transporting
saltwater with a higher rate and a more efficient salt
rejection that the commonly used method of reverse osmosis
desalination. With these research and advancements,
nanoporous graphene could change desalination and make
great strides towards solving the water crisis.
FIGURE 2 [7]
As shown in figure 2, hydrogen groups and hydroxyl
groups alternate to create a more efficient salt retention
system.
MY OPINION ON THE SCARCITY OF
CLEAN WATER
My research on the water crisis led to so many
disheartening realizations. I was amazed to find that “twice
the population of the United States lives without access to
safe water,” according to Water.org [9]. In a world that is so
advanced in education and technology, this fact is
unacceptable. Some people are concerned about material
things while others are worried that they will not be able to
find a source of clean water. It is a global responsibility to
improve the availability of water. Engineers, especially have
a responsibility to use their knowledge and tools to conduct
more research and help society. While nanoporous graphene
has covered a lot of ground towards solving the water crisis,
there is still room for improvement. With the technology that
we have thus far, it is still only realistic in seaside cities
where they have access to a large supply of saltwater and in
countries that are wealthy enough to purify the water. And
unfortunately, most countries that are experiencing this
water crisis are underdeveloped countries that do no have the
funds for this. While there are other technologies that are
working towards providing clean water, it would be most
logical to use desalination to allow the biggest supply of
water to be converted into a useable source. The water crisis
will be solved if the technology of nanoporous graphene
desalination is improved to be more efficient and more
accessible throughout the world.
CONCLUSION: WATER IN THE FUTURE
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AbigailDoyle
Water availability is expected to decrease by almost
half by the year 2050 [9]. To put this in perspective, we
could be part of the one out of three people without access to
clean water. As a future engineer, I recognize that engineers
and society need to work together to help close the gap
between supply and demand for clean water. With the
research and technology so far, nanoporous graphene has
characteristics that have proven that it could be a great
alternative to reverse osmosis desalination, especially
because it uses saltwater, the greatest natural supply of water
on Earth. Though there are still some obstacles, engineers
will hopefully continue making revolutionary steps towards
providing the world with clean water using nanoporous
graphene desalination.
Africa”. The World Bank. 2015. Accessed 10.24.16.
http://go.worldbank.org/WDPAMJ5290
ACKNOWLEDGEMENTS
I would like to thank my writing instructor Ms. Barbara
Edelman, and Ms. Judith Brink and Ms. Ashley Sowa from
the Bevier Engineering Library for guidance on the content
and topic of my paper. I would also like to thank my
roommate Hailey Honeycutt for helping me to reflect on the
engineering topics and problems that I value and
encouraging me throughout my research.
SOURCES
[1] “National Academy Engineering Grand Challenges for
Engineering: Provide Access to Clean Water.” Grand
Challenges.
2016.
Accessed
10.24.16.
http://www.engineeringchallenges.org/9142.aspx
[2] H. Perlman. "How Much Water Is There On, In, and
above the Earth?" How Much Water Is There on Earth, from
the USGS Water Science School. 10.24.16. Accessed
10.24.16. http://water.usgs.gov/edu/earthhowmuch.html
[3] WHO/UNICEF Joint Monitoring Programme. "Water for
Life: Making It Happen." World Health Organization. 2012.
Accessed
10.24.16.
http://www.who.int/water_sanitation_health/monitoring/jmp
2005/en/
[4] E. Wang, R. Karnik. "Water desalination: Graphene
cleans up water." Nature Nanotechnology. 9-2012. Accessed
10.24.16.
http://www.nature.com/nnano/journal/v7/n9/full/nnano.2012
.153.html
[5] J. Gerhart. "Dr. Jacqueline Gerhart: There's good and bad
to using reverse osmosis systems." UW Health. 10.25.2011.
Accessed
10.24.16.
http://www.uwhealth.org/news/drjacqueline-gerhart-theres-good-and-bad-to-using-reverseosmosis-water-systems/36710
[6] "The Advantages and Disadvantages of Reverse
Osmosis." Technology Today. 2011. Accessed 10.24.16.
http://moderntechnologyoftoday.blogspot.com/2011/06/adva
ntages-and-disadvantages-of-reverse.html
[7] L. Zyga. "Nanoporous graphene could outperform best
commercial water desalination techniques." PhysOrg.
6.22.12. Accessed 10.24.16. http://phys.org/news/2012-06nanoporous-graphene-outperform-commercialdesalination.html
[8] S. Román. "Sieving at the nanoscale: desalination of
seawater through nanoporous graphene." Mapping
Ignorance.
7.28.2014.
Accessed
10.24.16
http://mappingignorance.org/2014/07/28/sieving-nanoscaledesalination-seawater-nanoporous-graphene/
[9] “Making the Most of Scarcity: Accountability for Better
Water Management Results in the Middle East and North
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