High-Performance Graphene Oxide Nanofiltration Membranes for Black Liquor Concentration

Black liquor (BL) concentration by multieffect evaporation is an extremely energy-intensive operation in the kraft pulping cycle. Membranes can significantly save energy in this process, but conventional membranes are strongly challenged by low solute rejections and poor stability in BL, which is a complex mixture containing dissolved lignin, other nonlignin organics, multiple inorganic salts at highly alkaline pH, and process temperatures of 70–85 °C. Here we describe in detail the fabrication, modification, and characterization of robust and high-performance graphene oxide (GO) nanofiltration membranes for BL concentration. We show that poly(ether sulfone) (PES)-supported GO membranes prepared from chemically reduced GO, and then subjected to high-pressure hydraulic compaction, show excellent chemical and mechanical stability under real BL conditions in comparison to conventional GO membranes. These membranes (referred to as “GO-3” in this work) show near-perfect (>99%) lignin rejection, high total organic carbon (TOC) rejection (up to 93%), and greatly improved inorganic rejections especially for divalent anions that are predominant in BL. Finally, the GO-3 membranes are scaled up on larger PES sheets (∼660 cm2 in size) and are operated under realistic cross-flow conditions with real BL feed flow rates as high as 10 L/min at 70 °C. The GO-3 membranes show robust performance over more than 1500 h (60 days) of continuous operation in multiple cycles of 10–50 bar transmembrane pressures, attaining stable and sustained permeate fluxes as high as 25 LMH and excellent rejection performance equal to that obtained at smaller scale. The main results of this work have strong implications on the development of membrane processes for BL dewatering and more generally for processing of complex biorefinery feed streams.