Approaching ratio as a guideline for substrate design of forward osmosis membranes

Abstract

Forward osmosis (FO) has been proposed as an ecologically friendly and low carbon footprint technology for concentrating enriched lithium brine from the salt lake using freely available MgCl2 onsite as the draw. In this process, Li+ concentration is raised from a few hundred mg/L to 20-30 g/L (Li+) before precipitation of Li2CO3. To allow FO technology a wide margin in competing with other concentration technologies, it is highly desirable to develop a simple, low cost method to fabricate FO membrane. Besides, due to both feed and draw solutions being of high salinity, FO membranes should also have a defect-free polyamide (PA) separation layer and a low structural parameter (S) substrate. This paper presents a simple guideline called approaching ratio (α) to adjust the substrate pore morphology based on a ternary polysulfone (PSf)/N,N-dimethylacetamide (DMAc)/diethylene glycol (DEG) solution. The α ranging between 0 and 100 % is a metric measuring how close the solution is to the cloud point. The increase of α endows instant phase separation, resulting in the substrate with increased surface porosity and sponge-like structure. Pore connectivity is improved following an increase of α below a threshold value of 50 %. Water permeability dramatically increases from α = 0 % to α = 50 % and declines at above 50 % for forming the closed cellular pores at α = 80 %. The FO water flux shows a similar trend which was found to be closely related to the substrate pore interconnectivity. The FO membrane was applied in the lithium concentration process using saturated MgCl2 as the draw, the experimental results fit well with the FO model. The results demonstrate a simple, low-cost generic method to prepare FO membranes for applications with very high salinity feed and draw.