Brackish water desalination via stimuli-responsive polymeric hydrogels

Abstract

Although current water desalination technologies are mature enough and advanced, the shortage of freshwater is still considered as one of the most pressing global issues. Therefore, there is a strong incentive to explore and investigate new potential methods with low energy consumption. It is well-known that polymer hydrogel network has the ability to absorb water via swelling. In the case of polyelectrolyte hydrogels, the charges localized on the polymer chains, which mainly drive the swelling pressure inside the hydrogel, can also separate added salt via charge-based selectivity (Donnan exclusion). When combining this material with a temperature-sensitive polymer, the heat generated by solar energy can trigger the desorption process via conformational change of polymer chains. Hence, hydrogels designed from both materials, polyelectrolyte and thermo-responsive polymer can reduce the salinity of water, such as brackish water by means of reversible thermally-induced absorption and desorption desalination processes. In addition, the desorption process can also be achieved based solely on a polyelectrolyte hydrogel system by altering the ionization of charges within the hydrogel via pH. In this thesis, hydrogel-based water desalination process were developed using acrylic acid (AAc)/sodium acrylate (SA)-based polyelectrolytes as the charge-based separation function, alone or with a combination of N-isopropylacrylamide (NIPAAm) as thermo-responsive comonomer. In the latter case, a series of chemically cross-linked polymeric hydrogels were synthesized via either free radical-initiated copolymerization or reversible addition-fragmentation chain transfer (RAFT) polymerization, thus realizing different macromolecular architecture. According to the nature of hydrogels, the reversible sorption/desorption state were triggered by either chemical stimulus (pH), or physical stimulus (heat) as the thermo-responsive polymer introduced into the hydrogels. In detail, the effect of hydrogel composition as well as the influence of the macromolecular architecture on the swelling/deswelling behavior for the synthesized hydrogels were studied. For this, their properties including their responses to external stimuli were investigated, and their ability to desalinate brackish water as well as the effciency of such desalination process were evaluated. Generally, the results demonstrated correlations between macromolecular architecture of the network structure and their performance in the proposed desalination process, such as salt rejection and desalination capacity. Moreover, the potential of the best performance materials for applications was also discussed.