Bioinspired humic acid-based membranes for water desalination: Mechanistic insights from molecular simulations

Abstract

Bioinspired membranes have received considerable interest for separation, but their design is challenging because of lack in fundamental understanding of separation mechanism involved. Herein, we explore renewable humic acid (HA)-based bioinspired membranes for water desalination via molecular simulations. Different HA membranes (monomeric, Y- and star-crosslinked) with microscopic “ridge-and-valley” patterned pore structures are constructed. With the existence of intermolecular hydrogen bonds, the HA membranes possess good mechanical strength with Young's moduli ranging from 0.44 to 1.64 GPa. Water desalination performance of the HA membranes is comprehensively examined in 12 salt solutions with different cations (Mg2+, Ca2+, Li+, Na+, K+ and NH4+) and anions (CO32−, SO42−, F−, OH−, Cl− and NO3−), respectively. Water and ion in the HA membranes are revealed to follow a dynamical “hopping-transport” mechanism. The HA membranes exhibit superior desalination performance, comparable to commercial reverse osmosis membranes and graphene oxide membranes. Interestingly, water flux in the monomeric HA membrane is enhanced by tailoring the membrane structure from monomeric to crosslinked architecture. Upon varying cation or anion in a salt solution, salt rejection drops when ion hydration level decreases. From bottom-up, the simulations provide mechanistic insights into water desalination and would facilitate the development of new membranes for high-performance water desalination.