Photodriven chemical processes – the environmental detection and degradation of water-borne pollutants
With the support of the Cambridge-Africa ALBORADA Research Fund, Melisew Alula was able to visit Cambridge to develop expertise in photochemical research, with the aim of translating these skills to a new photochemistry laboratory being established at BIUST.
Pollution is a global problem. But how to deal with it can create a whole new set of issues, particularly if the means of mitigating it involve complex or expensive measures and have their own environmental footprint. This can be exacerbated when the technologies for addressing the root problem are not available locally, complicating the balance between effectiveness on the one hand and cost and practicality on the other. One project that has recently been initiated between Professor Andrew Wheatley from Cambridge and Dr. Melisew Alula from BIUST involves attempting to harness the copper-iron sulfide mineral chalcopyrite to drive the photodetection and photocatalytic degradation of pollutants in watercourses. This approach offers a range of potential advantages. Using sunlight to drive chemical processes can make it possible to achieve high efficiencies using only an abundant, natural source of energy. Meanwhile, chalcopyrite, a narrow band gap semiconductor capable of photoactivity in response to visible light exposure, is a locally abundant material easily mined in Botswana itself.
Melisew was able to prepare nanoscaled samples of the photoactive material chalcopyrite at Cambridge. This enabled comparisons with mined chalcopyrite, a readily available resource in Botswana.
The first steps in this emergent collaboration have involved Melisew conducting research at Cambridge for 6 months. This has enabled him to explore a range of photocatalytic processes. In working with chalcopyrite, he has compared and contrasted the photocatalytic properties of both mined and laboratory synthesised samples. The aim has been to investigate the effects of particle size, shape and purity on performance and stability; top-down preparations of micrometre-scaled material, based on the ball-milling of both raw and processed mined samples, have been complemented by the bottom-up synthesis of nanometre-scaled particles in the laboratory. Mineral samples have been coated with small, readily available acid molecules – an approach that contrasts with the more typical use of more complex, expensive and/or corrosive additives. The resulting photocatalytically active species have been explored in the quenching of a range of chemically resilient dye molecules – typical water-borne pollutants.
Melisew using a solar simulator at Cambridge to explore the photodegradation of organic dyes. The persistence of these in watercourses is a significant environmental problem in sub-Saharan Africa.
Copper is one of a small number of metals that exhibit a phenomenon called localised surface plasmon resonance. This involves light interacting with electrons at the surface of the metal and causing them to resonate. This may produce heat that can be used to drive chemical processes. Alternatively, resonance alters the spectroscopic properties of the metal – meaning that we can monitor which wavelengths of light are absorbed by it. Interest in electronic properties like this led Melisew to explore further applications of chalcopyrite and also the chemistry of another metal that demonstrates resonance: silver. He and Andrew are jointly developing spectroscopic techniques for monitoring organic molecules like antibiotics and also metal ions, both of which are frequently encountered as watercourse pollutants. To do this, they are creating different colourimetric tests based on enzyme-mimicking behaviour by chalcopyrite and the resonance activity of nanoscaled silver.
These efforts in the fields of light-driven detection and chemical degradation of pollutants are now transferring back to Botswana. There, Melisew is harnessing his experience gained in the UK to help establish his own photochemistry laboratory at BIUST, where he is currently seeking to scale up the chemistry that underwent preliminary exploration at Cambridge. The ability to decontaminate or detect pollutants in real-world samples of water underpins applications that will have implications for a range of UN Sustainable Development Goals in Sanitation and Heath & Wellbeing.
Melisew (second left) with members of the Cambridge group and also with another co-worker of Andrew’s – Dr. Gift Mehlana of Midlands State University, Zimbabwe (middle).
By Professor Andrew Wheatley, University of Cambridge, and Dr. Melisew Alula, Botswana International University of Science and Technology (BIUST).