A new NEWT publication develops materials making this possible
You may be familiar with reverse osmosis. This is the most common membrane process used in water treatment. The driving force is pressure. Applying pressure to push on the feed (contaminated) side of the membrane, forcing water through the membrane to the permeate (clean) side. However, reverse osmosis is limited because the higher the salt concentration of the wastewater the more difficult it is to push the water through the membrane.
A less common membrane process is pervaporation. This process is used to separate different substances in the liquid phase. The driving force is a vacuum or sweeping gas on the permeate side that pulls the water across the membrane. Pervaporation is a unique process that can be tailored for different separations by making the membrane either hydrophilic (having an affinity for water), or hydrophobic (repelling water). Traditionally, pervaporation is used to separate water and organic substances, such as a mixture of water and methanol.
A NEWT collaboration has developed a new pervaporation membrane that performs well as a desalination membrane. The article involves students and professors from three university partners: Elisabeth R. Thomas (ASU), Amit Jain (Rice), Stewart C. Mann (ASU), Yi Yang (ASU), Matthew D. Green (ASU), W. Shane Walker (UTEP), François Perreault (ASU), Mary Laura Lind (ASU), and Rafael Verduzco (Rice). This paper addresses adapting pervaporation for desalination situations. Pervaporation can handle concentrations of contaminants in the feed (>100 g L-1), that are unattainable for conventional reverse osmosis. Processes such as shale fracking and desalination produce wastewater streams with high contamination levels. Pervaporation could fill this gap in water treatment by further purifying these waste streams, serving the dual purpose of reducing waste and increasing water reclamation from these processes. However, no commercial pervaporation membranes exist that are tailored for desalination, and there is a need to identify materials that can serve as efficient pervaporation desalination membranes. This study uses a polymer called Nexar™ as a freestanding pervaporation desalination membrane.
Caption: Elisabeth R. Thomas (ASU), Amit Jain (Rice), Stewart C. Mann (ASU), Yi Yang (ASU), Matthew D. Green (ASU), W. Shane Walker (UTEP), François Perreault (ASU), Mary Laura Lind (ASU), and Rafael Verduzco (Rice).
Nexar™ is an attractive membrane material because it is easy to cast, has excellent water transport properties, and is mechanically robust. The membranes were freestanding (no support structure was needed) and they were able to withstand the pressure differentials in the lab-scale system. During the membrane production process, the researchers varied two factors of the membrane (degree of sulfonation and casting solvent), in order to study how those changes might impact the final membrane performance. The results revealed that these factors did not have a statistically significant impact on the membrane’s ability to perform the separation of water and salt, although it did impact the water uptake. Membranes with higher concentrations of sulfonated groups were extremely hydrophilic. However, every Nexar™ membrane outperformed commercially available pervaporation membranes (which are tailored for removal of water from organic solutions) when run in a desalination system. The new membranes also have excellent salt removal parameters, similar to commercial membranes. Therefore, Nexar™ membranes are highly effective for the water/salt separation process. NEWT researchers intend to continue their investigation into these materials, making modifications to the membrane casting process and the system configuration that can increase the rate of water passage through the membrane, while maintaining excellent salt removal.
As an associated project of NEWT’s Thrust 2 this effort helps the team learn more about how materials development for membranes can reduce the energy requirements for water treatment. Reduced energy requirements result in water treatment processes that are easier to deploy in off-grid settings.