Rational design of reverse osmosis membranes for boron removal: A counter-intuitive relationship between boron rejection and pore size

Owing to the toxicity of excess boron, developing reverse osmosis membranes that are capable of effectively removing boric acid, a boron specie commonly exists in seawater, is critical to advance water purification technology. To date, design principles of reverse osmosis membranes for boron removal have not yet been systematically explored. Using nanoporous graphene as a model system and by employing state-of-the-art molecular simulation techniques, this study discovers the critical role of the adsorptive competition of boric acid against water in the nanopores of membranes in their design. Specifically, the preferential adsorption of water onto the pore rims can be exploited to impede the permeation of boric acid, facilitating the size-exclusion effect for much improved boron rejection. Moreover, a well-balanced adsorptive competition is desired; water adsorption should not be too strong so that efficient water permeation can still be preserved. This results in membranes that are not only highly permeable but also capable of effectively removing boric acid. Detailed investigations on energetic, dynamics, and spatial distribution of water and boric acid as well as their free energy landscapes are also conducted for better understandings. Moreover, bilayer nanoporous graphene membranes with a heterogeneous design are identified to effectively block both boric acid and salt with a decent water permeation. Overall, the outcomes achieved herein can fundamentally guide the rational design of RO membranes for more efficient and effective boron removal.