The adaptation to new technologies for water purification has been very slow historically. However, the last decade or two has seen a massive growth in the development of new technologies for water treatment. These technologies include but not limited to membrane filtration, UV irradiation, advanced oxidation, ion exchange, and biological filtration.Past research and developments in the field of CNTs for water treatment has thought that CNT membranes and readily available commercial membranes have similar water permeance.
However, in the past primary focus has been on water treatment rather than the speed of water treatment- which is of paramount importance if CNT membranes have to be used for commercial water treatment and to meet the demands of future. In this paper they have tried to demonstrate that the water permeability of a CNT membrane can be improved tenfold to that previously reported for any CNT membrane by developing a special milli-metre thic ulrafiltration membrane consisting of a vertically alligned carbon nanotube wall that provide 6nm wide 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 is decreased 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 decreasing pore size or increasing densification of the CNT array. In this study they have used mechanical densification method to obtain better membrane performance to alter the pore dimensions, pore density and 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 densified CNTs 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 the effect of the entrance resistance of the CNT on the water permeability. For this purpose, an oxygen plasma treatment in the form of oxygen reactive-ion etching (RIE) was performed on the surface of the outer-wall CNT membrane.
Oxygen RIE was chosen to open the CNT fullerene caps to make the inner pores of the CNTs available for water passage during the process of making the entrances and exits of the pores hydrophilic.The dual effects resulting from the oxygen RIE, that is, opening of sealed CNT tubes and making the entrances hydrophilic, led to a significant increase in the water permeability. The water permeability increased more than twofold compared with that obtained after thermal treatment. This attained water permeability of the CNT wall membrane, which is close to 30,000 LMH bar?1 nearly ten times the highest 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, desalination capacity, solute selectivity, robustness, antifouling, energy savings and scalability. CNT-membranes could 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; making them more favourable at industrial scale.