Mercury
is one of most toxic substances for humans. Its
consumption can result in a variety of brain, skin, and
heart disorders, and can even be fatal. The Central Pollution Control
Board said in a paper last year that just 1 g of
mercury, found in a clinical thermometer, was enough to contaminate a water body of surface area nearly 20 acres to the extent that the fish in it would be unsafe
to eat.
Mercury is produced naturally, for example in volcanic eruptions, and also by a large number of industrial processes. Mercury contamination is not
widespread in India, but there are areas, in states like Maharashtra, Gujarat, Haryana and Odisha, where it has
been found to be mixed in water in dangerous concentrations. Water bodies in mining areas, and around leather
industries — which release mercury — are prone to contamination.
About four years ago, a group of
researchers from IIT-Madras developed a simple kit to help people test
whether their drinking water had traces of mercury, or similar toxic heavy
transition metal ions like cadmium or lead. The water, when tested with this
kit, takes on a colour if contaminated. Their work appeared in the journal ACS Sustainable Chemistry and Engineering.
While the kit was very useful,
the researchers soon realised that they needed to go a step further and find a
way to purify contaminated water. Thus began a search for a purification method, which, after four years of
effort, yielded a novel process that not only filters
out the mercury impurities from water but also produces clean hydrogen as a by-product that can be used as a
fuel.
This new process developed by the
team at IIT-Madras involves the creation of a mercury
nano-alloy that, apart from separating itself out of the contaminated
water, splits water molecules to produce clean hydrogen
gas. This work has been accepted for publication in the International Journal of Hydrogen Energy.
To start the process, an aluminium salt, like a nitrate or sulphate, is added
to the contaminated water sample. This salt becomes the source of Al3+ ions. Mercury is available in the sample in the
form of Hg2+ ions. One of the researchers, Dr Tiju Thomas, explained that the
key to the process is the simultaneous reduction of
these positive ions to their neutral elemental form, Al and Hg. For
this, the team added a powerful reducing agent
like sodium borohydride to the solution. Once
the aluminium and mercury are reduced, they mix with each other to form a nano-alloy.
This nano-alloy
then reacts with water instantaneously. In this reaction, aluminium gets back to Al3+ form, but this is entirely benign to human beings. Mercury, on the other hand, accelerates
the splitting of water molecules, ensuring a high rate of release of hydrogen gas, which can be collected and
stored. Mercury itself is left behind as sediment, and can be filtered out easily. The hydrogen
produced in the process can be used as a fuel.
Incidentally, the nano-alloy has
been found to be a useful catalyst in other chemical reactions. Dr Thomas said
the novelty of their process lay in the fact that it was achieving multiple objectives, all with important
benefits in their own ways. And this was not accidental, either. Right from the
beginning, he said, the effort had been to look for processes that could yield
collateral benefits.
Credit: Indian Express Explained
(http://indianexpress.com/article/explained/from-the-lab-new-way-to-clean-water-polluted-by-mercury-5185917/)
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