Treatment technologies for wastewaters often fail due to the presence of emulsified oils and suspended solids. Wastewaters of interest to the Navy include graywater and bilge water. Bilge water often consists of an oil-water emulsion which may contain a number of suspended solids and chemicals, such as detergents. Several investigators have considered the use of membranes for treatment of oily waters. The Navy currently uses membrane filtration as part of its water treatment operations to ensure water discharged from ships meets effluent discharge requirements. However, membrane fouling, due to the deposition of particulate matter on the surface of the membrane and concentration polarization, limits the use of membranes for water treatment. The Navy is seeking more robust alternative technologies, which could better handle wastewaters. One alternative involves mixing at the membrane interface to suppress deposition of particulate matter and break up the concentration boundary layer.
The objectives of this project were to determine if magnetic nanoparticles could be attached to the surface of membranes via polymeric nanobrushes and if these nanobrushes induce local mixing at the surface of attachment. In addition, this project investigated how the induced mixing is impacted by the design of the magnetic nanobrushes and the applied magnetic field, how the coatings used protect the nanoparticles, and how the strength and oscillation frequency of the external field impact performance. Finally, the project studied how induced mixing impacts the performance of the nanofiltration membrane.
Researchers attached commercially available amine-coated superparamagentic magnetite particles to hydrophilic nanobrushes that are grown from the surface of commercially available nanofiltration membranes. These hydrophilic brushes suppress fouling of the base polyamide membrane. The nanobrushes rotate in an oscillating magnetic field. This in turn creates mixing at the solid fluid interface, which will suppress adsorption of oils and deposition of suspended solids. Thus the magnetically responsive nanobrushes will clean the membrane during filtration. Previous studies indicated the potential value of grafting external stimuli responsive nanobrushes for controlling membrane pore size. Magnetically responsive nanobrushes have the potential to lead to a new generation of antifouling membranes that may be ideal for shipboard applications.
The results of this Limited Scope project successfully answered the questions posed by the objectives. Superparamagnetic nanoparticles may be attached to commercially available nanofiltration membranes. In an oscillating magnetic field, it was shown that the magnetically active polymer brushes do induce movement of water at the membrane surface. Thus, the attachment protocols developed for growing polymer brushes from the membrane surface and attaching the nanoparticles to the polymer brushes results in brushes that are able to move in an oscillating magnetic field. Theoretical calculations support these experimental findings.
Dead-end filtration experiments indicate that movement of the magnetically responsive nanobrushes leads to enhanced permeate fluxes and improved rejection by the membrane due to reduced deposition of insoluble particulate matter and suppression of concentration polarization. The results of this project are significant for a number of reasons and provide a strong foundation for further investigation. The initial hypothesis, that magnetically responsive membranes are fouling resistant, is validated.
This project has provided important data to support the practical viability of magnetically activated self-cleaning membranes. In particular, the modified nanofiltration membranes to be developed have a pore size ranging from 1-100 Å, so little change to current operating practices will be required. Key features also include no increase in footprint compared to current filtration systems, no additional unit operations to pretreat waste streams, and no new chemicals and supplies that require storage. The new self-cleaning membranes represent a non-mechanical system that will be fully automated and may be cleaned in place. They are lightweight, robust, and will be more efficient than current filtration membranes.