Spontaneous Particle Separation and Salt Rejection by Hydrophilic Membranes
Zhang Y, Takizawa S*, and Lohwacharin J
Department of Urban Engineering, Graduate School of Engineering, the University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
*Correspondence E-mail: takizawa@env.t.u-tokyo.ac.jp
Key Words: Exclusion-zone (EZ); Hydrophilic surface; Nafion®; Particle separation; Salt rejection
Received March 29th 2015; Revised May 21st; Accepted June 15th; Published July 1st; Available online July 7th, 2015
Abstract
The interactions between a hydrophilic surface and water can produce a particle-free zone of several hundred micrometers referred to as the exclusion zone (EZ). We examined the effects of particle concentration and surface hydrophilicity on the EZ using a vertical surface–suspension interface. According to the literature, the EZ develops within a period of several minutes and then stably persists at low particle concentrations. However, we observed that, at high particle concentrations, the flow of water from the vertical EZ region formed a distinctive supernatant from which both particles and chloride ions were rejected.
This spontaneous phase separation was affected by the hydrophilicity and area of the surface. Among the various EZ formation mechanisms proposed in the literature, our results supported the long-range water ordering hypothesis, which states that EZ water has an ordered structure that can reject both particles and ions and can move upward due to the density-driven buoyancy force. The spontaneous phase separation observed in this study has never been previously reported and may lead to breakthroughs in the applications of hydrophilic-surface-induced solid–liquid separation processes.