Being able to remove micropollutants from water is one of the greatest global challenges, as micropollutants are often endocrine disrupters that affect basic human function such as behavior development and fertility. In a time of rapid decline in sperm counts, the exposure to pollutants must be limited, both for humans and the environment where such pollutants accumulate and are known to, for example, turn male fish female. Steroid hormones have been our pollutant of choice since about 2000 when the topic ‘toilet to tap’ sparked a huge media outcry as a result of proposals to potentially reuse wastewater for drinking. Notably, wastewater is always reused for drinking, it just depends how long the cycle is before we drink the water again. Hormones are everywhere where human beings are as we excrete them naturally, as well as using them in agriculture and medical therapies.
Working a lot on nanofiltration, our work focused initially on removing micropollutants such as steroid hormones. This was partially successful, but it soon turned out that these micropollutants partition into polymeric membrane materials and gradually make their way through to the clean filtrated water. Also, even if removal is successful, a concentrate stream is produced that requires further treatment. Nanofiltration, by nature, needs quite a lot of energy as it operates with pressures in the order of 10 bar. Thus, a classic engineering solution just creates new problems!
The idea of photocatalytic membranes to destroy these micropollutants was born in the early 2000s, when my (now) husband, Bryce Richards, was cursing about the quality of his TiO2 coatings for solar cells. I had a quick look at his SEM images and said – wow, great membranes – given the amount of holes in his (hoped-for-dense) thin films. We submitted a grant – our first one - to the Australian Research Council at the time, but were not funded and had no other means to carry out the work. So, we forgot all about this, even though we always knew it was a great idea – and got busy with other things.
In 2014, I was invited to join a DFG consortium to work on a similar topic. I was invited primarily to add a female investigator to a group of 15 men. With a lot of engagement, I contributed great ideas and concepts, the consortium was funded, except for the project of the only woman! I declined the offer to be part of the consortium without funding and I decided to find ways and means to do the work anyhow. Instead, I found a number of great collaborators to develop the right analytical and membrane reactor tools. Clearly, it was the right time and the earlier set-backs have only served to fuel determination. The technical challenges to be overcome we could probably not have solved earlier.
Firstly, the analytical tool that we developed for steroid hormone analysis to the required sub-ng/L concentrations was liquid scintillation counting for radiolabeled steroid hormones. For photocatalytic degradation, this method is not suitable as the tritium is detected in both intact and degraded molecules. A separation step was required and it was hard work to make for this to succeed.
Secondly, photocatalytic membranes require light in a flow through cell to make use of the enhanced mass transfer in a micro- or nano-reactor (the pores of a microfiltration membrane are a few hundred nanometers in size, while for nanofiltration this is in the order of 1 nm or less). Small scale, small volumes, meaningful hydrodynamics and needing to get light into the system required a lot of collaborative efforts, in particular with Bryce Richards whose optics and solar energy harvesting knowledge once again came in handy.
Thirdly, choosing good membranes was critical and we are lucky to be able to produce or modify our own membranes and work with a number of great collaborators. In this case the membranes are produced at the Leibniz Institute of Surface Engineering (IOM) in Leipzig by Agnes Schulze and Kristina Fischer. Highly photocatalytic active TiO2 nanoparticles are synthesized via hydrothermal synthesis and immobilized on microfiltration membranes. The material is simple and abundant.
In essence, we have been able to attain a steroid hormone removal such that the new drinking guidelines of 1 ng/L can nearly be achieved - the analytical detection limited of 4 ng/L was reached. Naturally, we are very proud of what we have achieved and this work is a huge milestone as well as a solid foundation for future developments.
Photocatalytic membrane filtration system with solar simulator, micro-crossflow cell operated in dead end mode membranes coated with TiO2 nanoparticles at IOM, sampling through switching valve and analysis via
There is a myriad of possibilities to now create other materials that achieve the degradation to lower concentrations, even faster, at lower energy requirements at different wavelengths and using natural light, and perhaps most importantly, degrade other pollutants such as per- and polyfluoroalkyl substances (PFASs) or pesticides like glyphosate. Interference of real water contaminants, effectiveness in mixtures, potential generation of by-products especially when treating real waters, are topics that will keep researchers busy in the coming decade. The challenge to upscale photocatalysis considering the long term (5-10 year) stability of polymeric membranes and good engineering to guide the light into membrane modules require creative engineering minds. No doubt, ‘water catalysts’ are coming!
Read our paper in Nature Nanotechnology: https://www.nature.com/articles/s41565-022-01074-8