A filter for water purification that can replace, with a single step, the complex, time-consuming and multi-stage processes currently needed, has been developed by a team from CSIRO Manufacturing (Sydney, Australia; www.csiro.au), led by Dong Han Seo. The team has demonstrated water desalination via a membrane distillation (MD) process using a graphene membrane, called graphair. Water permeation is enabled by nanochannels of multilayer, mismatched, partially overlapping graphene grains.
As described in a recent issue of Nature Communications, Graphair is made by a chemical-vapor-deposition (CVD) process, and the film is then transferred onto a conventional MD membrane, such as polytetrafluoroethylene (PTFE). The CVD process takes place at ambient-air conditions using renewable soybean oil as the source for the graphene growth. This graphene film allows water to permeate through, but rejects salts and pollutants, such as surfactants and oils. Moreover, it prevents one of the great problems associated with desalination and filtration methods: fouling.
The team tested the pristine PTFE and permeable graphene-based membranes using saline solutions containing surfactants such as sodium dodecyl sulfate (SDS). As expected, significant fouling was evident for the pristine PTFE-based membrane. This membrane is able to process saline/SDS feed solution, but over 32 h there was a significant reduction in water flux from 40 L/m 2 /h to 14.2L/m 2 /h. Salt rejection also decreased, from 100 to 97.1%.
In contrast, the permeable graphene-based membrane demonstrated stable and high water-vapor flux (50 L/m 2 /h) and stable salt rejection (100%) over 72 h of operation under similar operating conditions. The superior antifouling performance of the graphair-coated PTFE was observed when a high concentration of oil compounds was included.