Reverse osmosis membranes prepared by interfacial polymerization in n-heptane containing different co-solvents

Abstract

The objective of this work was to develop a suitable and highly efficient reverse osmosis membrane incorporating a co-solvent system. A polyamide thin-film composite membrane has been prepared by interfacial polymerization in n-heptane using diethyl ether and ethyl acetate as co-solvents at various concentrations. Heptane has a molecule of larger C–C chain than that of traditionally used hexane. Heptane was selected in order to study its effect on the morphology of the prepared membrane as well as on salt rejection ability and flux volume. Heptane appeared to produce an improved morphology, salt rejection ability, and flux volume, compared to hexane. To the best of our knowledge, this is the first attempt to prepare and characterize TFC RO membrane in n-heptane mixed with co-solvent. The membranes were characterized using microscopy, spectroscopy, and contact angle measurement techniques. From surface spectroscopic analyses, the addition of co-solvents led to a decrease in the roughness properties of the membranes. The synthesized polyamide membranes consist of large and ordinal ridge-and-valley formations. The salt rejection and water permeate flux were well controlled by the categories and amounts of co-solvents that were added. Thermal gravimetric analysis results indicated that all of the membranes exhibited high thermal stability with degradation temperatures of about 481 ± 2°C. The high stability of the membranes was attributed to the sulfonic groups on the polymer chains. The contact angle also increased with an increasing co-solvent concentration. The performance characteristics of the membranes were evaluated by measuring the flux and rejection of isopropyl alcohol, NaCl, and MgCl₂ solutions. The rate of flux was proportional to the co-solvent concentration (i.e. flux increases with an increasing concentration), whereas the salt rejection properties were constantly high. Membranes formed with diethyl ether showed the highest salt rejection. Based on these data, membranes prepared using diethyl ether or ethyl acetate as a co-solvent in a non-polar heptane medium performed efficiently and exhibited high salt rejection characteristics with large flux values. These membranes represent promising candidates for freshwater desalination and removal of organic impurities.

Keywords:

• Interfacial polymerization
• Modified reverse osmosis membrane
• Desalination
• Co-solvent

Acknowledgement

Authors are grateful to King Abdulaziz City for Science and Technology, Riyadh, Saudi Arabia, for financially supporting this work and providing the use of the facilities in its labs.