Ion polarization at heart of new desalination research
A new desalination method using ion-selective membranes has been reported in Nature Nanotechnology by four researchers from the Massachusetts Institute of Technology (MIT) and the Pohang University of Science and Technology (PUST), Korea.The MIT/PUST technology is a process for converting seawater (salinity 30,000 mg/L) to fresh water (salinity <600 mg/L) in which a continuous raw-water stream is divided into desalted and concentrated streams by ion concentration polarization, a phenomenon that occurs when an ion current is passed through ion-selective membranes.
The researchers are Sung Jae Kim and Jongyoon Han (MIT) and Sung Hee Ko and Kwan Hyoung Kang (PUST). During operation, says their paper, Direct seawater desalination by ion concentration polarization, both salts and larger particles (cells, viruses and microorganisms) are pushed away from the membrane (a nanochannel or nanoporous membrane), which significantly reduces the possibility of membrane fouling and salt accumulation, thus avoiding two problems that plague other membrane filtration methods.
To implement this approach, a simple microfluidic device was fabricated and shown to be capable of continuous desalination of sea water (~99% salt rejection at 50% recovery rate) at a power consumption of less than 3.5 Wh/L, which is comparable to current state-of-the-art systems. Rather than competing with larger desalination plants, the method could be used to make small- or medium-scale systems, with the possibility of battery-powered operation.