Design of Nanoporous Materials as Reverse Osmosis Membranes for Boron Removal: Pore Shape Matters

Water shortage has been a pressing issue of our time, and reverse osmosis (RO) desalination has been employed as a vital technique to produce clean water at a large scale. While salt ions are the major solutes in seawater, boron species, particularly in the form of boric acid, also prevalently exist and are harmful. Owing to the size being comparable between boric acid and water, removing boric acid using conventional RO membranes has been a great challenge. To this end, it is crucial to develop more effective membranes for boron removal. Graphyne, an emerging class of two-dimensional materials with diverse pore shapes (e.g., hexagon and triangle), has been demonstrated to be promising membrane materials for water desalination. It is therefore of interest and importance to explore their potential in boron removal as well as to fundamentally understand the effect of pore shapes on their boron removal performance. In this study, by employing molecular simulation techniques, we demonstrate that graphyne can provide opportunities as an RO membrane for boron removal. In particular, both alpha- and beta-graphyne membranes, which feature hexagonal pores, can completely remove boric acid. By contrast, the gamma-graphyne membrane with triangular pores of a similar size (i.e., pore limiting diameter (PLD)) interestingly demonstrates a poor ability to remove boric acid. Detailed investigations on the permeation process of boric acid molecules, including their distribution and orientation in the nanopores, reveal that the rotation motion of plane-like boric acid molecules can allow them to permeate through the triangular pores relatively easily, thus resulting in a dramatically low boron rejection. These results suggest the difficulty in geometrically blocking boric acid and the effectiveness of the hexagonal pore as compared to the triangular pore. Provided the PLD alone may not sufficiently inform the boron removal capability, the free energy barrier of boron transport is computed using a newly developed algorithm and demonstrated to be capable of better probing the performance of membranes in boron removal. Overall, the outcomes of this work are anticipated to offer insights into the rational design of RO membranes for efficient and effective boron removal.