Graphene Oxide Membranes in Extreme Operating Environments: Concentration of Kraft Black Liquor by Lignin Retention

We report a detailed study of graphene oxide (GO) membranes for concentration of Kraft black liquor (BL), which is a caustic (pH similar to 12), hot (80-95 degrees C), and high volume (similar to 500 gal/min in a typical pulp mill) byproduct of the papermaking process. Membrane-based concentration of BL is attractive as an energy-efficient alternative to thermally driven evaporation processes but challenging due to the harsh operating conditions and high fouling potential of BL (15-18 wt % solids). We fabricate thin (<300 nm) GO membranes supported on macroporous poly(ethersulfone) (PES) supports by vacuum filtration techniques and discuss in detail their morphology, structure, thermomechanical stability, and chemical stability as characterized by several techniques. Furthermore, detailed permeation measurements at transmembrane pressures (TMPs) up to SO bar and temperatures up to 85 degrees C show that the membranes have high performance in concentrating BL feeds containing high and low TS (total solids): high flux (in the range of 5-50 kg m(-2) h(-1)), high lignin rejection (up to 98%), low fouling, and high stability throughout extended exposure (30 days) to BL at realistic operation conditions. The molecular weight cutoff (MWCO) of the membranes was determined to be similar to 625 Da by means of dye rejection experiments. The present membranes are also expected to have low cost due to the use of relatively inexpensive functional membrane and substrate materials (GO and PES).