Effect of surface area of carbon nanotubes on membrane performance for effective water desalination

Abstract

Carbon nanotubes (CNTs) have emerged as promising novel membrane materials for enhancing desalination performance. So far, the salt rejection mechanism of these materials has been through the narrow diameter of these nanotubes that exclude salt and allow the transfer of water. It is challenging to control the inner diameter of all the nanotubes and there is the problem of low permeation after several measurements due to the narrow channels that water molecules have to travel through. There is also the problem of fouling after a few measurements, because even if a low percentage of ions leak through some of the channels, they can easily get stuck there and eventually block the channels. Here, the rejection mechanism of various CNTs, grown on Si hole arrays, is based on the effective surface area of the nanotubes. The rejection of NaCl and MgSO4 ions and the permeability of water were measured across CNT structures grown on micro-fabricated Si hole arrays in a gravity-driven pressure setup. The electric conductivity of the feed and permeate solutions was measured and showed acceptable values for salt rejection and high values for water permeation. A comparison between the performance of bare Si surface and those decorated with CNTs shows the effectiveness of surface modification with CNTs in salt rejection.