Boron paradox in reverse osmosis desalination: A comprehensive review of removal mechanisms, hybrid process design, and resource recovery

Boron removal in seawater desalination presents a chemically distinct challenge from bulk salt rejection: at natural seawater pH, boron persists as neutral boric acid, rendering it largely impervious to the size-exclusion and Donnan-exclusion mechanisms that underpin conventional reverse osmosis (RO). Single-pass RO systems routinely exceed both the WHO drinking water guideline (2.4 mg/L) and, more critically, the irrigation threshold (0.5 mg/L), establishing the central engineering problem this review addresses. We critically examine the mechanistic basis and technological trajectory of established and emerging boron removal strategies, including advanced RO membrane architectures, forward osmosis, membrane distillation, electrodialysis, electrocoagulation, capacitive deionization, and boron-selective sorption, with explicit emphasis on how molecular-level transport mechanisms (steric hindrance, hydrogen bonding, electrostatic exclusion, and surface complexation) govern the performance boundaries of each technology class. Emerging hybrid configurations that exploit these mechanisms synergistically to reduce chemical dosing and energy demand while enabling resource recovery are critically assessed. Gulf Cooperation Council (GCC) operational practice, where the world's highest per-capita desalination capacity intersects with the most stringent boron constraints, is analyzed as a real-world technology-readiness benchmark. The review concludes that sub-0.5 mg/L boron cannot be reliably achieved at an industrial scale by any standalone technology, and that intelligently integrated, material-advanced hybrid systems designed from first principles of boron speciation chemistry represent the only sustainable path forward.