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Making Seawater Safe to Drink

There are 1.3 billion cubic kilometres of water on Earth.  Nevertheless, ready access to clean drinking water remains a major issue for millions of people.  A much sought-after innovation was developed by a UK-based team of researchers who created a graphene-based sieve capable of removing salt from seawater.  The new technology could aid millions around the World.

The Earth is a watery place.  About 71 % of the Earth's surface is covered with water.

The oceans hold about 96.5 % of all Earth's water.  Water also exists in the air as water vapour, above ground in rivers, lakes, icecaps and glaciers, and within the ground as soil moisture and in aquifers.  Even as part as your body.

Water never sits still.  Our planet's water supply is constantly moving from one place to another and from one form to another.

Actually, things would get pretty stale without the water cycle.

 

The relative amounts of Earth's water in comparison to the size of the Earth. The volume of the largest sphere represents all the water on, in and above our planet - about 1,386,000,000 cubic kilometres $ ($332,500,000 cubic miles$ )$.  The volume of the smaller sphere represents the amount of fresh water: 10,633,450 km3.  Source: USGS

All the Water in the World

The water you now drink has been around since the Earth was very young.  Around 3.8 billion years ago, the oceans had achieved their present volumes.

The global amount of water on Earth is about 1,386,000,000 km3 $ ($cubic kilometres$ )$.  That is all the water we are ever going to get.  Because, practically speaking, the system is closed: nothing can be added, and nothing can be subtracted.

With 1.3 billion cubic kilometres of water on Earth, it may be difficult to grasp why there is a worldwide problem with fresh water supply.

However, the amount of liquid fresh water is only 10,633,450 km3, including groundwater, swamp water, streams, rivers and lakes.  And as much of this water is located deep underground, it is not available to humans.

Additionally, each day, 1,170 km3 $ ($280 mi3$ )$ of surface water evaporates into the atmosphere.  If you really want to find freshwater, most of it, 29,200,000 km3 $ ($7,000,000 mi3$ )$, is locked into glaciers and icecaps, in the polar regions and Greenland.

 

 

Worldwide Fresh Water Resources

Water SourceWater Volume $ ($km3$ )$Global Water $ ($%$ )$Fresh Water* $ ($%$ )$
Oceans, Seas, Bays1,338,000,00096.54--
Ice caps, Glaciers, Permanent Snow24,064,0001.7468.7
Groundwater23,400,0001.69--
- Fresh10,530,0000.7630.1
- Saline12,870,0000.93--
Soil Moisture16,5000.0010.05
Ground ice, Permafrost300,0000.0220.86
Lakes176,4000.013--
- Fresh91,0000.0070.26
- Saline85,4000.006--
Atmosphere12,9000.0010.04
Swamp water11,4700.00080.03
Rivers2,1200,00020.006
Biological water1,1200.00010.003

Source: Igor Shiklomanov's chapter "World fresh water resources" in Peter H. Gleick $ ($editor$ )$, 1993, Water in Crisis: A Guide to the World's Fresh Water Resources $ ($Oxford University Press, New York$ )$.

 

Most of the water people and life on Earth requires everyday comes from surface-water sources.  This amount of water would only be enough to fill a sphere about 93,113 km3 $ ($22,339 mi3 $ )$, with a diameter of about 56.2 kilometres $ ($or 34.9 miles$ )$, which overall would look smaller than the surface of Lake Michigan, although it would be much deeper.

Not an awful lot.

 

The problem is salt.

And we need salt to live, but only in minute quantities.

 

Salt can Kill

Common table salt crystals seen under an electron microscope. Source: Wikimedia

Sea water contains way more salt than we can safely metabolise - about 70 times more.  A typical litre of seawater will contain about 2.5 teaspoons of salt $ ($the kind we sprinkle on food, that is sodium hydro-chloride$ )$, but much larger amounts of other elements, compounds and other dissolved solids, collectively known as salts.

The proportions of these salts and minerals in our living tissues are uncannily similar to those in sea water.  Essentially, we sweat and cry seawater.  Curiously though, we cannot tolerate them as an input.

Take a lot of salt into your body and your metabolism will very quickly go into crisis.  From every cell, water molecules rush off to try to dilute the sudden intake and carry off the excess salt.  This leaves cells dangerously short of the water they need to carry out their normal functions: they become dehydrated.

In extreme cases, dehydration will lead to seizures, unconsciousness and brain damage.

Meanwhile, the overworked blood cells carry the salt to the kidneys, which eventually become overwhelmed and shut down.  Without functioning kidneys, you die.

Which is why we don't drink sea water.

 

Most of the water people and life on Earth requires everyday comes from surface-water sources.  This amount of water would only be enough to fill a sphere about 93,113 km3 $ ($22,339 mi3 $ )$, with a diameter of about 56.2 kilometres $ ($or 34.9 miles$ )$, which overall would look smaller than the surface of Lake Michigan, although it would be much deeper.

Not an awful lot.

 

When water molecules $ ($red and white$ )$ and sodium and chlorine ions $ ($green and purple$ )$ in saltwater, on the right, encounter a sheet of graphene $ ($pale blue grid, at the centre$ )$ perforated by holes of the right size, the water passes through $ ($left side$ )$, but the sodium and chlorine of the salt are blocked. Source: MIT

A Graphene-Based Sieve to Remove Salt from Sea Water

A UK-based team of researchers has created a graphene-based sieve capable of removing salt from seawater.

Reporting their results in Nature Nanotechnology, the scientists from the University of Manchester, led by Dr Rahul Nair, show how they solved some of the challenges by using a chemical derivative called graphene oxide.

The difficulty has been to produce large quantities of single-layer graphene using existing methods, such as chemical vapour deposition $ ($CVD$ )$.  Currently, the production routes are quite costly.

To make a one-atom thick graphene permeable, you need to drill small holes in the membrane.  However, if the hole size is larger than one nanometre, the salts will pass through that hole.  And the graphene membrane needs also to be have a very uniform less-than-one-nanometre hole size to make it useful for desalination.  All in all, this is a real technological challenge.

 

The famous Buckminsterfullerene carbon molecule Source: Moxan.com

Making the Most of Graphene

Graphene comprises a single layer of carbon atoms arranged in a hexagonal lattice.  Its unusual properties, such as extraordinary tensile strength and electrical conductivity, have earmarked it as one of the most promising materials for future applications - a wonder material first isolated and characterised by a University of Manchester-led team in 2004.

In terms of scalability and material cost, graphene oxide has a potential advantage over single-layered graphene.

Graphene oxide membranes have already proven their worth in sieving out small nano-particles, organic molecules and even large salts.  Until now, they could not be used to filter out common salts, which require even smaller sieves.

 

Inexpensive Desalination Solution

Previous experiments have demonstrated that graphene oxide membranes became slightly swollen when immersed in water, allowing smaller salts to flow through the pores with the water molecules.  Nair and his team demonstrated that using layers of epoxy resin $ ($commonly used in strong glues$ )$ on either side of the membrane was sufficient to stop the expansion.

When common salts are dissolved in water, they form a "shell" of water molecules around the salt molecules, which allows the tiny capillaries of the graphene-oxide membranes to block the salt from flowing through along with the water.

 

 

Although water molecules can go through individually, sodium chloride cannot get through the graphene-oxide membrane.  It always depends on the help of water molecules.  As the size of the shell of water around the salt is larger than the channel size, the sodium chloride cannot get through.

By 2025, the United Nations expect that 14% of the World's population will encounter water scarcity. This technology has the potential to revolutionise water filtration across the World, in particular in countries which cannot afford large scale desalination plants.

The selective separation of water molecules from ions by physical restriction of interlayer spacing opens the door to the synthesis of inexpensive graphene membranes for desalination.

With graphene-oxide membrane systems, it is anticipated that desalination can be undertaken on a smaller scale to make the technology accessible to countries unable to fund large plants and without compromising the yield of fresh water produced.

 
 
 
 

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