Purifying Water using CO2 may Remove the Need for Membrane Water Filtration

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Estimates suggest that 1 in 10 people do not have access to safe drinking water. As the world population continues to increase, so does the demand for clean drinking water. Current water purification technologies typically involve micro- or ultrafiltration processes to remove undesired particles.

However, such processes require membranes, which can be expensive, energy consuming and experience problems due to fouling, a process by which particles become deposited on or in the membrane pores.

Membraneless separation of clean water can typically be achieved using sedimentation, the process when gravity causes waste particles to ‘settle out’ of a solution. However, stable suspensions consisting of small, buoyant and charged particles cannot be effectively separated in this way. In such cases, sedimentation can be accelerated using directed motion of colloidal particles caused by external forces such as electrostatic, dielectric, magnetic, acoustic, optical, or inertial forces.

                     CO₂+ H₂O ⇌ H⁺ + HCO₃^-                                                    (1)

An international group of researchers have reported a novel idea for membraneless water filtration using directed particle motion caused by the chemical gradient that results from simply dissolving CO₂ in water. The idea has been disclosed recently in a Nature Communications article and is based on the fact that when CO₂ dissolves, an equilibrium is established (Equation 1).

The two ions generated by CO₂ dissolution diffuse at very different rates,  resulting in a large diffusion potential. The transient ion concentration gradient produced during CO₂ dissolution drives spontaneous particle motion in a process known as diffusiophoresis. The directed particle motion caused by diffusiophoresis results in aggregation of the colloidal particles, and a separate area of clean water.

Figure 1. Schematic diagram of continuous-flow membraneless water filtration using dissolution of CO₂.

After testing their theory using a microfluidic system, the team produced a lab-scale prototype of a continuous flow particle filtration device based on the idea (Figure 1). The device consists of a straight channel made from a gas permeable material (polydimethylsiloxane).

CO₂ passes through the wall of the device and dissolves in the water, inducing particle motion. The air channel on the opposite side of the device removes CO₂, preventing saturation and maintaining a concentration gradient across the channel.The flowing water can then be split into a stream of clean, filtered water and a waste stream.

The team tested the device using polystyrene particles and were able to achieve particle removal rates that are comparable with micro- and ultrafiltration processes. The technique also required less energy than conventional filtration systems, and furthermore, the device was free from fouling by cake formation and pore plugging.

The team aims to scale up the device by using a number of parallel channels and optimizing channel dimensions, flow rates, and split ratios. Furthermore, they aim to test the device using more representative water samples from industrial and natural water sources.

They suggest that as most bioparticles are charged, their method will be able to remove bacteria and viruses without the need for chlorination or ultraviolet treatment. If true, this may be a particularly attractive attribute of the device, as it could reduce the amount of chemicals added to drinking water.

By exploiting the dissolution properties of CO₂, the team has successfully been able to produce a device that provides membraneless water filtration. Although the device has been shown to be promising and energy efficient, it is a long way from real-world application. Despite this, the team is confident that the device can be scaled-up and become a viable option for water filtration. They are currently in the process of filing a patent covering their water filtration device.

References

http://water.org/water-crisis/water-sanitation-facts/ Accessed May 3^rd, 2017.

Shin S., Shardt O., Warren P.B., Stone H.A.,  Membraneless water filtration using CO₂. Nature Communications 8, 2017, 15181.

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