When you hear the term “carbon dioxide,” the first thing that comes to mind is most likely the various environmental problems attributed to its presence, most notably its role as a greenhouse gas and its contribution to global warming. Yet carbon dioxide also has tremendous potential to serve as a purifier, instead of a pollutant. Professor Howard A. Stone, the Donald R. Dixon ’69 and Elizabeth W. Dixon Professor in Mechanical and Aerospace Engineering at Princeton University, is working to do just this. Along with a small team of researchers, Stone has recently discovered a way to utilize what we generally perceive to be a pollutant to address another equally prevalent environmental issue that affects over a billion people worldwide: contaminated water.

The energy consumed is less than a thousandth of what typical filters use...and the apparatus is easily mass-producible.

The race to develop an efficient, inexpensive, and mass-producible method of purifying water has been going strong in recent decades. Perhaps the most explored method is filtration, which utilizes a pressure source to pump water through porous membranes, separating the liquid from its contaminants. But the cost of using pressure pumps as well as constantly replacing dirty filters is high. Disinfection, another common purification method, is most often performed with chlorine, but chlorine can form carcinogenic by-products when reacted with water. Sedimentation, which uses gravity to separate unwanted particles from the main body of water, is another possible method of purification, but is inefficient and doesn’t guarantee removal of microscopic-level impurities. Filtration, disinfection, and sedimentation, then, all pose undesirable disadvantages.

Stone and his team have been working on a method of purification that does not require a porous filter. Sangwoo Shin, now an assistant professor at the University of Hawaii, Orest Shardt, now a professor at the University of Limerick in Ireland, and Patrick B. Warren, now a staff scientist at Unilever’s R&D center in the UK, worked with Stone on the research. Their method involves a chemical phenomenon known as diffusiophoresis to shed light on a new method of water purification that doesn’t require a porous filter. Diffusiophoresis describes the motion of colloids – particles suspended in a fluid – when exposed to a chemical gradient. In a fluid, particles in areas of higher concentration move to areas of lower concentration via diffusion. This concentration difference is described as a gradient and is analogous to the chemical gradient that causes diffusiophoresis. Diffusiophoresis, however, differs in that it describes the movement of charged particles in response to a chemical solute in a solution. It can be produced in response to differences in ion concentration throughout a fluid, and this ionic difference, in turn, has the potential to freely move charged colloids.

“We induced a chemical gradient by dissolving carbon dioxide, a passive, ordinary gas in water perpendicular to the fluid flow,” Stone said. “This moves charged particles to one side of the flow, and then the flow can be split to separate the purified water from the waste stream.”

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Schematic_of_particle_illustrating_diffusiophoresis
This diagram illustrates the process of diffusiophoresis, in which a particle (blue) moves with diffusiophoretic velocity v_dp in a solution of a solute (red) in a solvent (green), where there are concentration gradients of the solute and solvent.

Graphic by: Wikimedia Commons

When carbon dioxide is dissolved in water, it produces negatively charged bicarbonate ions and positively charged hydrogen ions. This induces a chemical gradient as a result of the ionic concentration difference. Since many water impurities exhibit a net charge, these impurities can be controlled via diffusiophoresis and potentially be separated from water. To design a system that would utilize these features of diffusiophoresis, Stone and his team arranged three channels parallel to each other: the middle one consisting of water filled with particles or impurities, one of the outside channels consisting of air, and the other containing a flow of carbon dioxide. To mimic impurities in water, they used polystyrene particles, the same material that composes styrofoam. Separating the outside channels from the central flow of impure water were membranes that permitted the carbon dioxide and air to freely pass through, which allowed the carbon dioxide to dissolve into the water stream. The air channel served to prevent the carbon dioxide from saturating the solution. The impurities were split from the water by directing the charged particles to one channel and the purified water to another.

The results were nothing short of remarkable. The separated purified water in Stone and his team’s apparatus had, for every 22 million particles, removed all but about 104 particles. This corresponds to approximately 99.999% of impurities removed, a removal rate that is on par with the standard many filtration systems in common use. Not only is it effective, but it is also energy-efficient. Because the system doesn’t require pressure pumps to propel water through small porous membranes, but instead allows for a wider channel, the energy consumed is less than a thousandth of what typical filters use. In addition to saving energy, the apparatus is easily mass-producible due to its simple design. Stone and his research team hope, with more research, to show that their model can be scaled up dramatically.

With the numerous technological advances our society has experienced in the past few decades alone, it’s difficult to imagine that a billion people still lack access to clean drinking water. Current methods of water purification are still largely inefficient, expensive, or ineffective. Stone and his team’s innovative diffusiophoresis apparatus not only purifies water while addressing these drawbacks, but can also utilize the carbon dioxide produced in factory operations. In the search for dependable solutions to today’s most pressing sustainability issues, Stone and his colleagues offer a novel solution by taking what we typically call waste and revolutionizing it into a resource that can make a global impact.

About The Author

Victor Hua