Polymer brush layer is key to new fouling-resistant membrane
A new class of fouling-resistant reverse-osmosis (RO) membranes has been announced by the University of California Los Angeles (UCLA) which are claimed to have low mineral-scaling propensity and high permeability.In a paper Polymer surface nano-structuring of reverse osmosis membranes for fouling resistance and improved flux performance in the Journal of Materials Chemistry, the researchers from the Henry Samueli School of Engineering, Nancy H Lin, Myung-man Kim, Gregory T Lewis and Yoram Cohen, write that the surface-structured RO membranes have been developed with a hydrophilic brush layer of terminally anchored polymer chains.
"Besides possessing high water permeability, the new membrane also shows high rejection characteristics and long-term stability," said Nancy H Lin, a UCLA Engineering senior researcher. "Structuring the membrane surface does not require a long reaction time, high reaction temperature or the use of a vacuum chamber. The anti-scaling property, which can increase membrane life and decrease operational costs, is superior to existing commercial membranes."
The new membrane was synthesized through a three-step process. First, researchers synthesized a polyamide (PA) thin-film composite membrane using conventional interfacial polymerization. Next, they activated the polyamide surface with atmospheric pressure plasma to create active sites on the surface.
Finally, these active sites were used to initiate a graft polymerization reaction with a monomer solution to create a polymer "brush layer" on the polyamide surface. This graft polymerization is carried out for a specific period of time at a specific temperature in order to control the brush layer thickness and topography.
These poly brush layers on the PA surface resulted in RO membranes of significantly lower mineral-scaling propensity, evaluated with respect to the mineral scalant calcium sulfate dihydrate, compared with the Hydranautics LFC-1 membrane of similar salt rejection (~ 95%) and (~ 70 nm). Direct membrane surface imaging indicated that the rate of nucleation and hence scaling were reduced due to the brush layer. Fouling resistance was also demonstrating using the protein BSA and alginic acid.
"In the early years, surface plasma treatment could only be accomplished in a vacuum chamber," says Yoram Cohen, UCLA professor of chemical and biomolecular engineering and a corresponding author of the study. "It wasn't practical for large-scale commercialization because thousands of meters of membranes could not be synthesized in a vacuum chamber. It's too costly. But now, with the advent of atmospheric pressure plasma, we don't even need to initiate the reaction chemically. It's as simple as brushing the surface with plasma, and it can be done for almost any surface."
In this new membrane, the polymer chains of the tethered brush layer are in constant motion. The chains are chemically anchored to the surface and are thus more thermally stable, relative to physically coated polymer films. Water flow also adds to the brush layer's movement, making it extremely difficult for bacteria and other colloidal matter to anchor to the surface of the membrane.
Prof Cohen says, "Protein or bacteria need to be able to anchor to multiple spots on the membrane to attach themselves to the surface -- a task which is extremely difficult to attain due to the constant motion of the brush layer. The polymer chains protect and screen the membrane surface underneath."
Another factor in preventing adhesion is the surface charge of the membrane. Cohen's team is able to choose the chemistry of the brush layer to impart the desired surface charge, enabling the membrane to repel molecules of an opposite charge.
The team's next step is to expand the membrane synthesis into a much larger, continuous process and to optimize the new membrane's performance for different water sources.
"We want to be able to narrow down and create a membrane selection system for different water sources that have different fouling tendencies," Lin says. "With such knowledge, one can optimize the membrane surface properties with different polymer brush layers to delay or prevent the onset of membrane fouling and scaling.
Cohen's team, in collaboration with the UCLA Water Technology Research (WaTeR) Center, is currently carrying out specific studies to test the performance of the new membrane's fouling properties under field conditions.