Fundamental Insights on Hydration Environment of Boric Acid and Its Role in Separation from Saline Water

Desalination and water purification are among the most promising approaches to supply clean water in the context of a rapidly growing global water shortage. Reverse osmosis (RO) is a common water purification approach that utilizes a pressure-driven solvent diffusion across a semi-permeable membrane to remove small ions or small molecules from the feed-stream. Traditional polymeric RO membranes, e.g., based on polyamide as the active layer, are inexpensive and have been shown to perform well for a variety of salts and effluents; however, these materials also have a number of limitations, including chemical and thermal sensitivity, rapid fouling, and difficulty in cleaning. Further, traditional polymer membranes can have difficulties in rejecting certain neutral solutes such as boric acid (H3BO3). Boron plays a critical dual effect on living systems on earth, and the difference between boron deficiency and boron toxicity levels is quite small. Recent work on graphene-based membranes has demonstrated that tuning the nanoporous structure and chemistry high water permeability and excellent selectivity with respect to monovalent ions (e.g., Na+) is achievable. In this study, we examine the potential for nanoporous graphene to reduce boron concentrations with a rejection rate of 95%, employing a theoretical approach based on a combination of density functional theory (DFT) and classical molecular dynamics (MD). Computer simulations are employed for the examination of the solvation properties of boric acid in water to evaluate the dimension of the hydration shell of the molecule and to estimate the membrane pore diameters required for its rejection. The relationship between the atomic structure of nanoporous graphene and its effectiveness in removing H3BO3 when employed as RO membranes in desalination is then examined, and an optimal pore size is determined for the rejection both salt and boric acid while maintaining high water fluxes.