Scientists have converted waste mussel shells from the food industry into a mineral found in bones and teeth that may also prove valuable for cleaning wastewater. In the presence of UV radiation, the researchers found hydroxyapatite (HAP) generated chemical radicals that degraded methylene blue, similar to compounds found in textile wastewater, in both oxygen poor and rich conditions.
Usually, semiconductors such as titanium catalyst (TiO2) are useful as photocatalysts to break down difficult to treat chemicals in the final stage of a wastewater treatment plant. However, titanium is expensive and there is limited availability.
‘We found that HAP is pretty good as a photocatalyst, though not as good as titanium dioxide. But it comes from waste and we think there is also an advantage in that it comes from atoms that are not limited in the world – calcium and phosphate,’ lead scientist Darrell Patterson at the University of Bath, UK, explains. The HAP used was generated by crushing and then heating mussel shells to 800°C to convert the calcium carbonate (CaCO3) in shells to lime (CaO); water and phosphate were then added, under controlled conditions with no oxygen, to generate HAP after a few hours. ‘I believe will be easily scalable,’ says Patterson.
The next step is to scale up to see if the costs and benefits balance out and to see how effective it is against recalcitrant wastewater contaminants.
‘Our priority chemicals are endocrine disrupting compounds, so things like hormones, some pesticides and pharmaceuticals, which are hard to break down conventionally in wastewater treatment plants, but can be broken down by technologies like photocatalysts,’ he says. The HAP – produced in a collaboration between Bath and the University of Auckland in New Zealand – was comparable to commercial HAP. Around 39% of the methylene blue was degraded after 6 hours under oxygen limited conditions; under oxygen rich conditions, degradation during the initial 6 hours was around 54%, increasing to 62% after 24 hours.
However, Darren Sun at Nanjang Technological University in Singapore, questions whether the process will be cost-effective. ‘To convert [calcium carbonate] together with organic materials to [lime] and release CO2 is also an energy intensive process,’ he says.