Sea level rise means fresh groundwater will increasingly become salty. Yat Li explains how his novel 3D-printed desalination tool can offer a solution.
By Rita Aksenfeld, UC Santa Cruz
In many areas of the world, including California, the demand for freshwater exceeds supply, and the effects of climate change are exacerbating the issue. More frequent and longer periods of drought mean communities that depend on rainfall for freshwater, such as Santa Cruz, are increasingly at risk of completely or severely depleting their freshwater supply. Additionally, as sea levels continue to rise in the coming decades, salt water is expected to intrude into fresh groundwater sources along the coasts, rendering the water undrinkable.
In response to this pending crisis, UC Santa Cruz chemistry and biochemistry professor Yat Li is working on a logical solution for creating a sustainable freshwater supply: desalinization.
There are a few ways to remove salt from seawater, but Li and his colleagues are focused on a method called capacitive deionization, which uses electrochemical technology. This method is particularly efficient for removing low amounts of salt from slightly salty, or brackish water, such as would be the case with contaminated groundwater supplies in coastal communities. With support from UCSC’s Center for Coastal Climate Resilience, Li’s team is applying 3D printing techniques to fabricate a scalable apparatus with high desalination efficiency.
The project is just one of several Li and his students are working on with the goal of designing and building functional solutions for addressing the challenges of climate change. We recently spoke with Li to learn more about this effort. This interview has been edited for length and clarity.
Can you explain how capacitive deionization works, and how it differs from more commonly used methods of water desalination?
The most conventional way to remove salts from water is via reverse osmosis, which uses a semi-permeable membrane to filter out all the salts to get fresh water. This process requires a lot of energy to push the water through those membranes, and at the same time those membranes are typically very expensive.
Capacitive deionization, by contrast, removes salts using charged electrodes instead of membranes.
The way it works is we have two electrodes which we apply voltage to. One side is positive, and the other side is negative. When salts dissolve in water, they separate into ions that carry a positive or negative charge. So, when we let seawater or brackish water flow between these two electrodes, the salt ions that carry a positive charge will move to the negative electrode and the ones that carry a negative charge will move to the positive electrode. We absorb those salts on the electrical surface and let the water pass through. To reduce the amount of salt, we can repeat this process until we achieve fresh water.
The process is reversible, so we can absorb the salts and then release them elsewhere to reuse the material.
The conventional method is more suitable for seawater desalination in terms of efficiency. For the electrochemical method, it is more suitable for lower salt concentrations. This is because the absorption takes a longer time and needs a really large surface area to accomplish that. This method is particularly efficient for brackish water because it consumes less energy at those levels of salt concentration.
What kind of material do you need to do this?
The idea requires high absorption efficiency, which is really related to surface area. The materials we developed in our lab are made of a carbon material that’s conductive. They have a hierarchical, porous structure, and their surface area is around 3,000 meters square per gram. That’s equivalent to a surface area of six or seven basketball courts per gram. If we can pack a large amount of the material together in a small volume to make an electrode, that can absorb a lot of salt.
In addition to the large surface area, the distance between the two electrodes is also critical. Closer will be better – it takes less time for the salt ions to diffuse. We’re trying to use what’s called an interpenetrating electrode structure, in which the positive and negative electrodes interlock in the same space to create this internal woven or interlock structure to reduce the diffusion length for those ions.
One of the issues with large-scale desalination is what to do with the salt. In the electrochemical process you described, where does the salt go once it’s taken out of the water?
If we want to make this process sustainable and continuous, we need to release all these salt ions that were absorbed. To do this, we will flush the system with a secondary solution while turning off the voltage, which causes the ions to desorb – instead of collecting the fresh water, we can use this loop to collect the highly concentrated salt solution. So we have two systems: in one we remove salt from the brackish water, and we collect fresh water and in the other we concentrate the salt.
In our system, we’re concentrating on treating brackish water so the starting concentration will have a lower level of salt than normal seawater. Once we’ve concentrated the salt to a level that is comparable to seawater, then we can discharge it into the ocean and it should not affect the environment significantly.
What was the current state of this technology before you started working on this?
Capacitive deionization is not a new idea – using this electrochemical method to separate salt from water. But scalability and performance have remained key challenges. What we are trying to do is to couple the idea with the material we develop in our lab that offers a large surface area. We are also developing a 3D printing technique to try to improve the design and make production more scalable and flexible.
How far along is the project and what is the end goal?
So far, we have demonstrated the 3D-printed prototype structure using polymer materials. Currently, we’re trying to convert the 3D-printed polymer into our carbon material while trying to maintain the high surface area feature.
The device architecture is the challenging part. We want to be able to print the full device rather than assembling electrodes, because that will make it easier to scale and create a more modularized system. We also want to balance performance and material cost. In other words, how much carbon do we need to balance the rate of desalination and the cost. By the end of this project, our goal is to create a scalable, 3D-printed laboratory prototype that desalinates brackish water collected directly from the field.
At its core, my lab is focused on material design, which is a research area full of opportunity for innovative ideas. My motivation and hope with this project and many others in my lab is that we can make an impact on developing solutions to climate related challenges.
