Tunable Graphene Oxide Membranes for Ionic and Molecular Separations

Abstract

Membrane-based dewatering of weak black liquor (BL) is an attractive alternative for significantly reducing energy usage compared with the conventional evaporation process. The overall aim of the thesis is to develop a technically feasible and economically profitable graphene oxide (GO) membranes-based BL concentration process for energy savings, process usable water generation (<0.1 wt% Solids), and resources recovery. Tuning the separation capabilities of the graphene oxide (GO) membranes via controlling the interlayer d-spacing of GO is promising towards the further high-resolution separation of the individual components in BL. I first started by GO reduction to rGO and followed by physical compaction. The compacted rGO membranes (GO-3) showed >99.5% lignin rejections with superior stability under 60 days of continuous operation. However, the rejection of GO-3 toward salts is still insufficient. Thus, I screened through a variety of chemical modification methods and find out that the intercalation of polycyclic cationic dyes into the GO-layer will significantly improve the salt rejections to ~ 80% for divalent ions, and ~ 50% for monovalent ions. The cationic dyes self-assembles with GO via π- π and electrostatic interactions, which formed unique mass transport interlayer galleries with both the reduction of d-spacing as well as the creation of diffusion barriers within the interlayers which limits the diffusion of ions. Finally, based on the above results, I developed six GO membrane based 8000 kg/min BL concentration processes and assessed their profitability by custom-developed Aspen Plus simulations and techno-economic analysis. All processes are potentially economically profitable with 1-5 years to break even and a positive net present value relative to the conventional evaporation process.