Materials that unlock light-controlled specific separations to enable sustainable desalination (LUCENT)

Recycling urban wastewater into usable clean water is an environmental win.

Using renewable energy to power this process reduces its carbon footprint and makes this idea even better.

What about obviating waste generation from this low-carbon process by recovering waste components as resources without using chemicals that typically generate more waste?

With 380 billion cubic metres of municipal wastewater produced globally in 2020 where every litre of this wastewater contains 0.75 mg of Zn, 285,000 metric tonnes of Zn can be recovered from global municipal wastewater. This is about 2% of the world's total Zn consumption in 2021. In a UK context, about 4300 tonnes of Zn can be recovered from UK municipal wastewater per year - about 5% of the Zn imported into the UK. However, the recovery of heavy metals from municipal wastewater is not practiced currently and these valuable resources are lost to the environment as the effluents of treated wastewater are discharged into the environment. This is due to the low metal concentrations in this wastewater and the recovery of metals from such dilute mixtures with legacy technologies typically create more waste. Moving towards a circular economy, it is crucial that these valuable metals are reclaimed without creating more wastes in its own right.

To solve such a global challenge, there is a need to re-think how metal-metal separations should be achieved, where the current focus is only on recovering metals from waste streams with high enough metal content. We should also consider how this process can be achieved in-situ of existing processes as well as obviating waste generation associated with chemicals used for separating metals from each other or to regenerate separation media.

In this Fellowship I propose to design and engineer photo-responsive covalent organic frameworks, a class of microporous polymers with tailorable pore sizes, to achieve zero-waste specific metal-metal separations in-situ of desalination. I will use recent advancements in photo-modulated desalination to engineer a library of covalent organic frameworks that can specifically and reversibly complex with a target metal cation, separating various metal types from each other in complex and dilute mixtures into reusable high-purity metal streams.

Light-responsive, zwitterionic molecules can separate cations and anions from water, and monovalent cations from divalent ions, as a function of their tailorable metal compatibility via chemical functionalisation. With training in computational simulations , I will design a series of chemically-functionalised zwitterionic photo-switches that can be embedded within the pores of covalent organic frameworks to separate metals from each other via a novel separation mechanism underpinned by size selection and specific metal complexation. I will validate the concept of light-controlled specific metal-metal separation in-situ desalination using these novel materials as adsorbents and membranes in bench-scale experiments using model and complex mixtures and real-world municipal wastewater samples. I will close the desalination waste loop associated with fabrication and end-of-life of desalination media by exploring the use of additive manufacturing technologies that reduce waste generation during membrane fabrication and depolymerisation techniques to recycle spent desalination media into reusable chemical compounds, respectively. Beyond exploiting the concept of light-controlled specific separations to unlock desalination as a circular economy solution, I will work with other researchers to explore using this technology in other applications such as organic solvent nanofiltration, drug delivery, self-cleaning coatings. I will also perform life cycle assessment studies to evaluate the sustainability and feasibility of technologies developed here for metal recovery from municipal wastewater.