The permeability of a CNT membrane can beimproved

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Last updated: May 27, 2019

The adaptation to new technologies for water purification has been very slow historically. However, thelast decade or two has seen a massive growth in the development of new technologies for watertreatment.

These technologies include but not limited to membrane filtration, UV irradiation, advancedoxidation, ion exchange, and biological filtration.Past research and developments in the field of CNTs for water treatment has thought that CNTmembranes and readily available commercial membranes have similar water permeance. However, inthe past primary focus has been on water treatment rather than the speed of water treatment- which is ofparamount importance if CNT membranes have to be used for commercial water treatment and to meetthe demands of future.

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In this paper they have tried to demonstrate that the water permeability of a CNT membrane can beimproved tenfold to that previously reported for any CNT membrane by developing a special milli-metrethic ulrafiltration membrane consisting of a vertically alligned carbon nanotube wall that provide 6nmwide inner pores and 7nm wide outer pores formed between the interstices of the walls of the nanotubes.From past research water permeability is well known to decrease as the pore size of the membrane isdecreased to remove finer impurities. However, the outer-wall CNT membrane does not follow this well-known behaviour and water permeability for the outer-wall CNT membrane increases with decreasingpore size or increasing densification of the CNT array. In this study they have used mechanicaldensification method to obtain better membrane performance to alter the pore dimensions, pore densityand pore tortuosity of the outer wall CNT membrane.Some aspects of this CNT membrane were optimized to obtain better membrane performance.Amorphous carbons and defects that can retard the flow, particularly in the case of highly densifiedCNTs because of a reduction in available area of smooth CNT surface that enhances the velocity.

Thermal treatment was used to clean the CNT walls resulting in doubling the flow of water.Another aspect of the CNT membrane that could be exploited to improve membrane performance is theeffect of the entrance resistance of the CNT on the water permeability. For this purpose, an oxygenplasma treatment in the form of oxygen reactive-ion etching (RIE) was performed on the surface of theouter-wall CNT membrane. Oxygen RIE was chosen to open the CNT fullerene caps to make the innerpores of the CNTs available for water passage during the process of making the entrances and exits ofthe pores hydrophilic.The dual effects resulting from the oxygen RIE, that is, opening of sealed CNT tubes and making theentrances hydrophilic, led to a significant increase in the water permeability. The water permeabilityincreased more than twofold compared with that obtained after thermal treatment.

This attained waterpermeability of the CNT wall membrane, which is close to 30,000 LMH bar?1 nearly ten times thehighest water permeability reported for CNT membranes (?2,400 LMH bar?1).Although RO, NF, MF, UF and other membranes have been used to mitigate fresh water crisis, CNT-based membranes have remarkable accomplishments in terms of water permeability, desalinationcapacity, solute selectivity, robustness, antifouling, energy savings and scalability. CNT-membranescould be used at all levels from the point of generation (POG) to the point of use (POU) treatments.Unlike other membranes, CNT membranes do not require high pressure and consequent energy; makingthem more favourable at industrial scale.Microorganisms induced membrane fouling followed by coagulation and pore blocking is a major concernin separation technologies. This challenge can be overcome by using CNT membranes which haveantimicrobial activity. CNTs rupture bacterial cell through the production of reactive oxygen species(ROS), disruption of their metabolic pathway and oxidative stress. This has brought a new revolution inmembrane technology with self-cleaning power.Many critical issues still need to be effectively addressed for CNT membranes to be commercially viable.Size controlled pores and uniform distribution is a challenge due to complicated methods in synthesis ofCNTs making it unsuitable for industrial production at the moment..

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