Decoupling Energy Demand from Boron Compliance in Seawater Desalination via a Hybrid Single-Pass Multistage (HSPM) System.

Achieving stringent boron compliance in seawater desalination has long depended on energy-intensive multipass reverse osmosis (RO) with alkaline dosing, creating a trade-off between energy consumption and water quality. This work breaks this bottleneck by developing a hybrid single-pass multistage (HSPM) system with selectivity-optimized loose RO membranes. Tailoring commercial RO membranes into high-flux variants via a 5 h controlled chlorination achieves an optimal balance, whereas an extended 20-h modification causes an impractical 24.1-fold increase in boron passage. Mechanistic analysis reveals that the 4-fold increase in apparent activation energy (Ea) for B(OH)4- transport at pH 11 fails to fully offset the intrinsic loss of steric selectivity under neutral conditions. To overcome this material-level limitation, a multidimensional performance map identifies the operational sweet spot of the HSPM system, enabling membranes with moderate boron rejection and high water permeance to meet the 1.0 mg L-1 threshold via multistage blending and energy recovery. Coupling the 5 h chlorinated membrane with an optimized 3-stage configuration theoretically yields a ∼42.5% energy saving versus conventional two-pass RO. This research establishes a novel paradigm for application-specific membrane selection and process synergy, offering a scalable, low-carbon blueprint for desalination and water reuse.