Water purification: No-filter filtration

Everyone needs an abundant supply of clean water, but not everyone has it. Constant efforts are being made to improve water purification processes to meet the growing global demand for clean water. One of those efforts, described by Howard Stone and colleagues at Nature Communications, provided a technique that uses CO2, not a filter, to produce pure water.

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Normally, particles suspended in water are removed by filtration or sedimentation. Although filtration has shown promise for water purification, it is expensive because the membranes must be replaced periodically and efficient pumps are required to generate the necessary pressure. Sedimentation does not require a membrane, but uses an external force to induce agglomeration. In a static suspension this could be the applied force, for example in a centrifuge. However, in the stream, Stone and his collaborators recognized that a chemical gradient could provide the necessary resistance. Dissolving CO2 in water produces H + and HCO3−, which can be used to produce a chemical gradient and a diffusion potential of approximately 92 mV, much greater than the potential generated by dissolving common salts, such as NaCl or KCl. “This high diffusion potential can control the movement of charged particles suspended in a liquid, so we realized that this approach would create opportunities to clean particulates from water in ways that did not involve traditional filtration,” says Stone.
The device designed and tested by the Stone group includes a channel through which an aqueous suspension flows and is exposed to a cross flow of CO2. The resulting ion concentration gradient induces a redistribution of charge on the surface of the suspended particles, so that they move in a direction perpendicular to the water flow, the so-called diffusiophoretic motion. Particles accumulate on one side of the pipe and can therefore be isolated by simply dividing the flow at the end of the channel. “Diffusiophoresis, the direct movement of particles exposed to a concentration gradient, is an imbalance effect that is not well known to those outside of this specific field,” says Stone. “However, it appears to be an important process for controlling and manipulating colloidal materials.”

“No one has suggested using these controlled chemical gradients to remove particles in a continuous flow process, which is the key to thinking about removing particles without filtration.”

“Our approach, which uses a cross field in a continuous flow process to manipulate the position and transport of particles, is a modern variant of the traditionally known field flow fractionation. To our knowledge, no one has suggested using these controlled chemical gradients to remove particles in a continuous flow process, which is the key to thinking about particle removal without filtration, “explains Stone. This proposed membraneless purification technique would require much less energy than the filtration methods currently in use. The effective use of CO2, an abundant pollutant, would avoid the use of more aggressive chemicals for the removal of bioparticles, such as viruses and bacteria, generally laden and therefore easily influenced by potential for diffusion. However, there are still some unanswered questions and time will tell how scalable this method will be and how well it will work for more complex solutions. One indisputable thing is how attractive such a process would be as part of a sustainable and effective approach to water purification.

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