One-step Cross-linking and Tannic Acid Modification of Polyacrylonitrile Hollow Fibers for Organic Solvent Nanofiltration

Abstract

Simple and versatile modification techniques are highly sought after in the field of membrane science. However, common procedures such as interfacial polymerization require multiple solutions and several steps, particularly when dealing with hollow fiber membranes. In line with strategies laid out for fabricating greener membranes, we report a single-step cross-linking and modification process involving only an aqueous solution of the naturally-occurring tannic acid (TA) and hydrazine for polyacrylonitrile-based hollow fibers. Hydrazine plays a dual role of cross-linking the polyacrylonitrile while forming bonds with TA to yield a robust modification. By simply changing the amount of TA used, hollow fibers with different filtration properties can be obtained, as demonstrated by standard pore size tests using aqueous feeds. TA may even penetrate into the membrane bulk and further improve its mechanical properties. Solvent-related tests such as immersion tests and pure solvent permeation are first conducted for a preliminary understanding of membrane performance. To further demonstrate the efficacy of the one-step modification, organic solvent feeds, such as dyes dissolved in methanol, are employed in nanofiltration tests using these hollow fibers. Long-term tests reveal a stable methanol permeance of 1.2 L m-2 h-1 bar-1 and sustained 100% rejection of Evans Blue (Mw = 960.81 g mol-1). Thus, with each reagent playing several roles in this one-step process, a greener yet simple modification method is established for polyacrylonitrile-based membranes and materials.