Draw solutions for Forward Osmosis process: Osmotic pressure of binary and ternary aqueous solutions of magnesium chloride, sodium chloride, sucrose and maltose

Abstract

The objective of the present work is to investigate the behaviour of binary and ternary aqueous systems, which could be employed in the selection criteria for draw agents (DA) to be used in Forward Osmosis (FO) process applications. In this study the osmotic properties of the selected binary and ternary aqueous solutions of magnesium chloride (MgCl2), sodium chloride (NaCl), sucrose and maltose are investigated. Osmotic pressures were calculated from water activities obtained from measured relative humidity of the solutions of concentrations in the range 0.5–6.0molkg−1 at 298.15K. The osmotic behaviours of the ternary systems were compared with their binary counter parts; the results showed either positive or negative osmotic synergic effects. This could be used besides transport properties for considering the selection of favourable draw agents from those that exhibited positive synergy, i.e. the osmotic pressure of a ternary solution is greater than the sum of the pressures of the corresponding binary solutions. The results showed that the ternary aqueous solutions of MgCl2+NaCl showed significant positive synergy and therefore are possible suitable candidates as draw solutions, less so were the sugar–electrolyte systems.