Linking boron-polyol complexation mechanisms to selective transport in ligand-functionalized polyether membranes

Conventional water purification membranes exhibit poor rejection of small, neutral solutes such as boric acid, the primary species of boron at circumneutral pH. The development of membranes that effectively remove boron could expand our portfolio of energy-efficient water reuse technologies. Incorporation of chelating ligands in membranes to selectively sorb boric acid is a promising approach for boron removal. We demonstrate a polyether membrane platform functionalized with N-methyl-D-glucamine (NMDG), a polyol known to interact selectively with boron. The NMDG-functionalized membranes exhibit dual mode sorption and diffusion behavior due to specific interactions between boron and membrane sites. High sorption capacity of the NMDG membranes for boric acid is attributed to the buffering capacity of the amino group in NMDG, which promotes borate-NMDG complexation at neutral solution pH. Theory-guided X-ray absorption spectroscopy at the boron K-edge reveals molecular-level distinctions between specific boron-NMDG complexes (i.e., monochelate, bischelate) and informs macroscopic transport models. This research establishes fundamental structure/property rules for boron selectivity that could lead to new material designs for water purification membranes.