Chlorine-resistant polymeric membranes for desalination

Water resources are becoming increasingly scarce in many areas of the world and an urgent need for access to secure, sustainable sources of fresh water raises membrane filtration based on reverse osmosis (RO) as one of the most promising ways to desalinate seawater. Commercially available RO membranes are derived from two basic classes of polymers: cellulose acetate (CA) and polyamides (PA). One of the most outstanding features of CA membranes is their claimed resistance to Cl2. However, their performance can be deteriorated by hydrolysis. CA membranes are vulnerable to degradation by bio-fouling, undergo compaction at higher temperatures and pressures, and are limited to a relatively narrow pH range. On the other hand, PA membranes exhibit better transport properties at a given applied pressure and are more stable over a wider range of pH values than CA membranes. Despite all the advantages, such membranes are easily biofouled, limiting their applications in water purification. An effective approach to mitigate this problem is pretreatment by chlorine which can cause the degradation of polyamide RO membranes at the same time. In this work, chlorine-resistant RO membranes have been made using sulfonated poly(ether ether ketone) (S-PEEK) and have been used successfully for water desalination. S-PEEK with different degree of sulfonation (DOS) was directly synthesized from sulfonated monomers. The direct synthesis of the S-PEEK from a sulfonated monomer was found to be more advantageous than that of post-sulfonation. Indeed, the polymer chain cleavage and side reactions, which can make it difficult to control the DOS and molecular weight and consequently drop the mechanical and the thermal properties of S-PEEK, can occur in the post-sulfonation method. S-PEEK with different DOS (0.4, 0.8, 1.0 and 1.2) was dissolved in dimethylformamide (DMF) and spin-coated on alumina supports (α-alumina disk with a very thin γ-alumina top layer) to form chlorine-resistant membranes. The membranes showed increasing anion retentions and water permeability with increasing DOS up to 1.0. For DOS higher than 1.0, performance of membranes dropped which might be due to rougher film structures and accordingly formation of defects in the prepared membranes. The order of salt retention in a membrane with DOS of 1.0 was as follows: Na3PO4 (99.9 %) > Na2SO4 (97.1 %) >NaCl (96.0%). The membranes were also used for desalination of solutions with different concentration of ClO− (10, 100, 200 and 500 ppm) and showed retentions higher than 90%.status: publishe