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.