Modifying Water-Purification Membranes with Nanomaterials

Research scientists at Howard University are applying the tools of nanotechnology to the age-old problem of water treatment.

Finding water on Mars is a big deal; not so much on Earth, whose ocean covers more than two-thirds of its surface.

And yet, drinkable water is scarce—and becoming increasingly so—on the blue planet. We take for granted the technologies developed in recent decades that have provided developed countries with a seemingly abundant supply of drinking water. But our water sources are becoming increasingly contaminated with bacteria, viruses, chemicals, and particulates from industrial waste and consumer products. Those contaminants have a particularly pernicious effect on water-treatment systems that use membranes as filters.

Reverse osmosis (RO) systems, for example, contain membranes that allow only water molecules to pass—and only under high applied pressure—blocking virtually anything larger, from salts to sand. The video below shows how reverse osmosis works.

To conserve energy, and to reduce the concentration of contaminants impacting the RO membrane surface, dirty water is first passed through filtration systems that use membranes with larger pores. The table below shows the range and application of membrane filtration systems (“DOC” is dissolved organic carbon).

For more than a decade now, research on water-filtration membranes has been carried out in the Physico-chemical Processes Lab at Howard University. The lab is led by Kimberly Jones, chair of the university’s civil and environmental engineering department. She is also recipient of the 2015 Woman in STEM Researcher of the Year Award from the NSF-funded HU-ADVANCE Institutional Transformation program to advance the careers of women faculty in STEM.

Jones’s connection to membrane research took off with the establishment of the Keck Center for the Design of Nanoscale Materials for Molecular Recognition in 2002. Her contribution—to that center and to various other multi-institutional collaborations—involves research that examines the attachment of materials (ions, electrons and molecular recognition sites) onto membranes and other polymeric surfaces, transport of charged species across the membrane, and the nature of the chemical interactions between the membrane material and the molecules will be studied.

For example, in 2012 she and collaborators at Howard and NIST published a paper in Environmental Science & Technology that discussed successful results to reduce fouling of microfiltration membranes (see image above) by bacteria and organic solutes. A “fouled” membrane is one that has been irreversibly coated with a thin film of solute—dental plaque is an example of a biofilm—that reduces the membrane’s overall effectiveness, especially the flux, or rate, of water flow through it. By incorporating negatively charged silver nanoparticles into the membrane’s surface, the research team made the membrane more “attractive” to water (more hydrophilic) and more repellent to negatively charged foulants. (See image below.)

Recently, one of Jones’s former postdocs, and a coauthor on that paper, was chosen as lead guest editor by the open-access Journal of Chemistry for its upcoming special issue, “Nano-Enabled Membranes for Water Treatment.” Howard research scientist Malaisamy Ramamoorthy will lead a team of three editors who will evaluate submissions that discuss “how different types of membranes are developed and modified with nanomaterials, their characterization, durability (stability), and application into drinking water treatment.”

The following excerpt from the journal’s call for manuscripts captures the essence of this research effort :

Membrane separation process for drinking water treatment has evolved and transformed significantly over the past decade when scientists discovered that nanomaterials can change the dimensions of the membranes from just being sieving filters to reactive sites. The nanomaterials incorporated on the membrane surface react with feed components either in presence or in absence of a catalyst. Thus filtration becomes a synergistic process between the membrane material (or the functional groups of it) and the nanomaterial.