Generating clean drinking water for everyone on this planet is one of the biggest global challenges. It’s also a particular interest of mine since there are so many ways in which chemistry can contribute.
One of the big problems with water is the presence of microbes – bacteria, viruses and parasites – which can cause a range of diseases. Gastrointestinal infections related to poor sanitation kill 2.2 million people a year.[i] However, there are also many other sources of water contamination. These can be man-made pollutants, such as fabric dyes or agricultural run-off. There is also increasing concern about drug molecules such as hormones entering the water supply from both animal and human excrement. Another important source of water contamination is from nature. The earth contains plenty of toxic elements and these can leach into water from rocks. It would be impossible to talk about all the fascinating chemistry research into combatting all of these water problems so I’m just going to focus on one: arsenic.
Arsenic is found in many different minerals in the Earth’s crust. It’s also used in a range of different industries but as I mentioned above, the main source of arsenic contamination in water actually comes from the element leaching from rocks into groundwater. This is widespread, including countries such as Bangladesh, India, China and Argentina.[ii] The actual concentrations of arsenic in groundwater are quite low – too low to cause acute arsenic poisoning. The problem comes from long-term consumption of arsenic-contaminated water, as well as use of groundwater to irrigate crops. This can cause a range of unpleasant health problems and, since arsenic is carcinogenic, it has been linked to cancers of the skin, lungs and bladder.
A particularly interesting example of tackling arsenic removal actually uses the same chemistry as I mentioned in this blog yesterday – photocatalysis. Many research groups around the world are working on ways to use sunlight to help remove arsenic from water. It’s similar chemistry to self-cleaning windows, where sunlight activates catalysts on the surface of the window to break down molecules of dirt that have accumulated. Since arsenic is an element, we are not looking at breaking it down, but we can convert it into a different form. Once it’s in that form, it’s much easier to remove.
The chemistry works by using a photocatalyst – a material that can absorb energy from the sun and use that energy to drive a chemical reaction. Arsenic in groundwater is mainly present as positively charged ions – each ion having a charge of +3 (As3+). In this form, arsenic is very mobile – it’s not absorbed very well by normal water filters and it can move easily into the body. The purpose of the new photocatalyst chemistry is to convert arsenic into a different form that is more easily absorbed onto water filters. By shining sunlight onto these photocatalysts, arsenic is converted from +3 ions to +5 ions. These are much more easily removed.
The beauty of this chemistry is that it uses sunlight as the energy source. It could also be integrated with some conventional water-filtration materials such as activated carbons. The challenge is now to get the materials (the photocatalysts) optimized. Catalysts work best with a high surface area and so a lot of current research into these photocatalysts is in structuring the material – more on this tomorrow!
Dr Zoe Schnepp is a Birmingham Fellow in the School of Chemistry at the University of Birmingham.