In 2009 I wrote a post called Good Carbon News about graphene; an allotrope of carbon. Allotrope means an element in more than one physical form e.g. carbon can be in the form of graphite, diamond or graphene. I go into more detail in the 2009 article which is worth a read.
Graphene is in the news again this week because researchers at Manchester University have developed a system for filtering out the salt ions from sea water using a sieve made from graphene oxide. Salt (sodium chloride) is an ionic compound meaning that is splits into positively charged sodium ions and negatively charged chloride ions when dissolved in water (or when it is melted). This is why molten salt and salt solution both conduct electricity. The charged ions are able to carry the current in a similar way to electrons in piece of wire. Sodium ions and chloride ions are very small. Sodium is element number 11 on the periodic table meaning that it has 11 protons and 11 electrons. Chlorine is element number 17 so is a little larger with 17 protons and 17 electrons. When sodium and chlorine react, sodium loses one electron and gains a positive charge of +1. The chlorine gains an electron and so has the negative charge -1.
It is useful to imagine atoms and ions as spheres. A sodium ion (Na+) has a radius of about 116 pm (picometre) whereas chloride (Cl–) is about 167 pm. 1 pm is 10-12 metres thus 1000 pm is 1 nm (nanometre); 1ooo nm is 1 μm (micrometre); 1000 μm is 1 mm (millimetre); and 1000 mm makes 1 m (metre). A water molecule has a diameter of roughly 150 pm. The graphene oxide sieve has been coated in epoxy resin to make a rigid structure with holes with radii between 300 and 450 pm. By activating this material with heat it is possible to filter out the salt from the water making the possibility of cheap desalination plants a possibility for the first time.
You may be wondering how the sieve can filter water from dissolved ions if the ions are smaller than water molecules. This had me scratching my head for a while so I went to the journal Nature to read the free extract from the paper. It refers to hydrated ions – and then the penny dropped! Water molecules are polar; this means that the hydrogen end is slightly positively charged and the oxygen end is slightly negatively charged. This is why they stick to each other allowing things like surface tension and capillary action to work. When you put a positively or negatively charged ion into water the water molecules flock around the ion surrounding it and producing a hydrated ion. This is much larger than a single water molecule or an individual ion and it is this that can be trapped by the graphene oxide sieve.
If this process can be scaled up it could make a huge difference to people’s lives. Desalination plants are very expensive and the United Nations estimates that by 2025, 14% of the world’s population will be short of water. This will most likely be due to climate change. It is interesting that the elements carbon and oxygen, which are responsible for much of global warming as CO2, could be rearranged as graphene oxide filters to reduce one of its greatest consequences.
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Questions…
- Suggest a non-metal element other than carbon.
- How would I normally suggest that you separate salt from water?
- Which major subatomic particle, that adds mass to atoms, has not been mentioned in the passage above? What charge does it have?
- What is 150 metres in picometres, in standard form?
- A magnesium atom reacts with two chlorine atoms to make magnesium chloride (MgCl2). What charge would the magnesium ion have when magnesium chloride is dissolved in water?
- If sodium chloride is held together by ionic bonds, what kind of bonds holds a water molecule together?
- When chemical bonds form do they absorb energy or release energy?
- Graphene can exist as flat sheets of carbon atoms. Name any other shapes that graphene can form?
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