A newly developed nanoporous carbon composite membrane provides 3–20 times higher freshwater flux than existing polymeric membranes, report researchers in Saudi Arabia. Moreover, the membrane is applicable to reverse osmosis (RO), membrane distillation (MD) and forward osmosis (FO), the technologies upon which most desalination processes rely, notes the team, which is led by Zhiping Lai, associate professor of chemical and biological engineering at King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia.
The membrane consists of a layer of carbon fibers on a ceramic support that has minimum pore sizes of 30 nm (Figure 1). Tested in RO, MD and FO desalination processes, it achieved 100% salt rejection and as well as the high freshwater flux.
Figure 1. Structure consists of a network of carbon fibers deposited on a porous, hollow ceramic tube. Source: Zhiping Lai, KAUST.
Detailed investigations revealed the existence of a novel interfacial salt sieving effect, one that fundamentally differs from the solution-diffusion mechanism that occurs in polymeric membranes. This is the first time that the interfacial salt sieving effect has been observed in polymer membranes, according to Lai, who also is a member of KAUST’s Advanced Membranes and Porous Materials Center. “It is really an extraordinary process that is associated with the unique properties of a graphitized carbon surface,” he says.
As with other processes, desalination involves immersing one side of the new membrane in salt water while the other side contacts freshwater. Here, water evaporates from the salt water, quickly passing through the carbon gaps before condensing at the freshwater side. The high thermal conductivity of the carbon fibers enables recovery of most of the energy involved in the process, reducing consumption by up to 80%.
Membrane preparation is just the first step; structures can be tuned for specific applications. “The carbon membranes can be used in any industrial processes that these three membrane processes can be applied to, for instance seawater and brackish water desalination, enrichment of fruit juice, concentration and purification of pharmaceuticals, wastewater treatment and more,” Lai explains.
However, several fundamental issues — including process reproducibility, structural stability of the membrane itself and fouling control — require resolving before proceeding to pilot trials, he notes.
Even so, Lai is confident in the membranes’ ultimate commercialization. “Their performance will be hard to surpass because of the unique properties associated with the carbon materials used in their construction.”
