Temperature-responsive cellulose-based hydrogels as draw agents in Forward Osmosis

Abstract

Forward Osmosis is a powerful tool to produce clean water at a low cost if appropriate draw agents and regeneration methods are employed (1). In this frame, stimuli-responsive hydrogels are interesting candidates due to their ability to uptake water and release it under physical stimuli, such as hydrostatic pressure and temperature variations. In particular, temperature-responsive hydrogels that are able to absorb large amount of water at room temperature and release it upon a slight temperature increase are very appealing candidates because they can be regenerated easily and with very low energy consumption (2). Chemical stability and biocompatibility are two further requirements that hydrogels can also exhibit. Hydroxypropyl cellulose (HPC) is a temperature-responsive cellulose derivative, its hydrogels displaying a transition temperature around 42°C (3). This means that HPC-based hydrogels are able to absorb water at room temperature and release it when they are conditioned at temperature higher than 42°C for a few minutes. Besides, this biopolymer has several advantages if compared with synthetic, temperature-responsive polymers: it is non toxic, biodegradable, abundantly present in nature and available in large quantities at a relatively low cost. In this work, chemically crosslinked hydroxypropyl cellulose hydrogels with various crosslinking degrees are produced and evaluated as draw agents in Forward Osmosis. In particular, they are characterized for their swelling/deswelling behavior (in water and NaCl aqueous solutions), morphology, rheological and thermal properties. Their performance in a discontinuous Forward Osmosis unit is assessed.