A team led by a University of Limerick researcher has achieved a major breakthrough in water filtration that lets it use 1,000 times less energy than conventional methods.
With climate change expected to make vast tracts of land unsuitable for human habitation – whether that be through coastal flooding or drought – the need to find cheap and easy ways to filter water is now essential.
Last month, a team of researchers from the University of Manchester (UM) demonstrated a system that would use the wonder material graphene to sieve out salt in seawater, making it potable.
Now, a group of researchers – including the University of Limerick’s Dr Orest Shardt and Dr Sangwoo Shin from the University of Hawaii – have found a similar and novel way to filter water using CO2.
Currently, water filtration technologies, such as microfiltration or ultrafiltration, use porous membranes to remove suspended particles and solutes.These processes trap and remove suspended particles, such as fine silt, by forcing the suspension through a porous material with gaps that are smaller than the particles.
However, energy must be wasted to overcome the friction of pushing the water through these small passages, making it expensive for industries to pump and maintain these filtration systems.
Publishing their findings in the journal Nature Communications, the researchers demonstrated their own method, which is an alternative, membraneless system that works by exposing the colloidal suspension (a materials-heavy solution) to CO2.
Now to make it affordable
The device is developed from a standard silicone polymer that is commonly used in microfluidics research and has similar traits to sealants used in people’s homes.
“While we have not yet analysed the capital and operating costs of a scaled-up process based on our device, the low pumping energy it requires – just 0.1pc that of conventional filtration methods – suggests that the process deserves further research,” said Shardt.
“What we need to do now is to study the effects of various compounds, such as salts and dissolved organic matter that are present in natural and industrial water, to understand what impact they will have on the process.”
This research was conducted last year by Shardt and Shin when they were post-doctoral researchers at Princeton University.
Water purification technologies such as microfiltration/ultrafiltration and reverse osmosis utilize porous membranes to remove suspended particles and solutes. These membranes, however, cause many drawbacks such as a high pumping cost and a need for periodic replacement due to fouling. Here we show an alternative membraneless method for separating suspended particles by exposing the colloidal suspension to CO2. Dissolution of CO2 into the suspension creates solute gradients that drive phoretic motion of particles. Due to the large diffusion potential generated by the dissociation of carbonic acid, colloidal particles move either away from or towards the gas–liquid interface depending on their surface charge. Using the directed motion of particles induced by exposure to CO2, we demonstrate a scalable, continuous flow, membraneless particle filtration process that exhibits low energy consumption, three orders of magnitude lower than conventional microfiltration/ultrafiltration processes, and is essentially free from fouling.
With global demand for clean water increasing, there is a continuing need to improve the performance of water treatment processes1,2. Membraneless separation of suspended particles is conventionally achieved by sedimentation, which relies on the gravitational force3. When particle suspensions are stable, for example, due to the particles being small, neutrally buoyant and highly charged, unassisted sedimentation is ineffective and additives are needed to induce aggregation (coagulation or flocculation4) and accelerate sedimentation. Therefore, inducing directed motion of colloidal particles in stable suspensions is critical to membraneless separation processes.
There are several ways to induce directed motion of colloidal particles such as using an external force, for example, electrostatic5, dielectric6, magnetic7, acoustic8, or optical9, inertial effects10 and so on. One less known but significant driving force is a chemical gradient, which causes diffusiophoresis11. Typically, diffusiophoresis is observed in liquid environments where chemical gradients are generated by contact between solutions with different solute concentrations12,13,14,15.
Here we show that diffusiophoresis of colloidal particles can be achieved by dissolution of a gas into a liquid, namely CO2 dissolution into water and its subsequent dissociation that produces ion concentration gradients. Using this principle, we show that the directed motion driven by dissolution of CO2 can be exploited to separate particles with very low energy consumption. Moreover, CO2 is abundant, biologically benign when dissolved in water, and can be easily separated. These features make CO2 dissolution a very promising means to clean water especially for the developing world, which requires water purification technologies that have low energy demands and are not chemically intense2.