Authors: Yi Han, Zhen Xu , and Chao Gao
By: Ehsan Moaseri, Behzad Changalvaie
It is predicted that in near future, clean water may become a scarce resource due to rapid population growth, climate change, and growing demands. Therefore, many investigators have focused their efforts on developing solutions for the emerging water crisis. Among these solutions, nanofiltration of water might be the most promising method because of its low energy cost, simple operational process, and high efficiency.
Nanofiltration refers to the use of membranes which filter out pollutants and unwanted material from the passing fluid. Two classes of materials have widely been used in creating such membranes: polymeric membranes, which have the advantages of flexibility, simple preparation process, and low cost; and inorganic ceramic membranes, which offer thermal stability, certain solvent resistance, and long lifetime. However, each class of membranes face some problems. For example, polymeric membranes might have poor chemical resistance, limited lifetime, and membrane fouling; and inorganic ceramic membranes might have a complex fabrication process, be brittle, and expensive. Thus, there is a dire need for a new class of membranes which offers the combined advantages of polymeric and inorganic membranes, and accounts for their shortcomings.
Carbon nano-structures are attracting more and more attention in the past decade to be used as nanofiltration platforms. The atoms of carbon can bond together in different ways, termed allotropes of carbon. The best known are graphite, diamond, fullerene, nanotube and graphene as shown in figure below.
Carbon nanotubes (CNTs) are great candidates for water nanofiltration. They have ceramic-like stability and polymer-like flexibility and processiblity. In addition, theoretical calculations predict that the rate of permeation of water through CNTs is extremely fast. However, 1D CNT-based membranes have not reached the practical application stage because of the high cost of CNTs, time-consuming and complex process of obtaining high density of vertically-aligned CNTs, and difficulties for achieving large-scale production.
Recently, 2D graphene has been used for nanofiltration applications. Chemically converted graphene (CCG) or graphene oxide can form highly ordered films with 2D nanochannels between two graphene sheets. In this publication, ultrathin graphene membranes (20-50-nm thick) are designed and fabricated. These membranes are used for high flux water purification (22 L.m-2.h-1.bar-1) and it is shown that these membranes show good retention rates for organic dyes and salts.
The study uses chemically converted graphene (CCG) as membrane material. The use of CCG has been controversial because of CCG’s inherent defects, such as holes that would blunt its electrical properties and limit its application in many fields. However, in this case, these holes result in more permeation “gates” and higher water reflux. The high flux of water through small-diameter carbon nanotubes can also be explained by two factors: 1) low friction between water and hydrophobic carbon wall and 2) ordered hydrogen bonds formed by the single file of water molecules.
It is important to note that the oxygen-contained groups on graphene sheets block water molecules because of strong interactions between them. Thus, only the graphene regions without functional groups are responsible for the fast transport of water. The transport of liquids with different polarities was tested for these filters, and it was found that more hydrophobic liquids showed a lower flux under the same pressure, because of greater interaction between more hydrophobic liquids and graphene walls. Thus, the hydrophobic carbon nanochannels in graphene oxide sheets favor high water flux.
The graphene oxide sheets stack with each other to form sub-1-nm sized 2D nanocapillaries. These membranes show excellent performance in retaining of organic dyes. Compared with the membranes made of aligned CNTs, the stacked graphene membranes have the advantages of low cost and simple fabrication process. The graphene membranes also combine the merits of graphene (high thermal stability, fine chemical resistance, excellent mechanical properties) and polymer materials (high flexibility, and more importantly, scalability). These membranes could be further improved by optimizing the size and density of holes on the graphene sheets. Finally, this new class of graphene membranes is a promising candidate for nanofiltration of water.