Application of Poly(diallyl dimethylammonium chloride) as a Forward Osmosis Draw Agent: The Quantified Effect of Osmotic Pressure and Viscosity

In this study, four poly(diallyl dimethylammonium chloride) (PDDA) with different molecular weights are applied for forward osmosis concentration of high-salinity wastewater; meanwhile, the quantified effect of osmotic pressure and viscosity on the FO water flux is studied systematically. The 35 wt % PDDA (<100 kDa) solution could provide a water flux of 8.8 LMH with DI water as feed, and a water flux of 8.0 LMH with 15,400 ppm synthetic high-salinity wastewater as feed. Osmotic pressure plays a major and positive role in the FO water flux, while viscosity plays a secondary and negative role. Specifically, the FO water flux exhibited a variation of 115% using four PDDA draw solutions with the same viscosity and different osmotic pressures, while it showed a variation of 11.5% with the same osmotic pressure and different viscosities. The relationship of solute diffusion resistivity K and viscosity η was formularized for detailed discussion. The data of 28 polyelectrolyte solutions are summarized and demonstrate similar inference.