Redirecting gypsum scaling in pervaporation desalination via activated alumina granules

Membrane scaling, particularly CaSO4 (gypsum) crystallisation, severely limits the practical application of pervaporation (PV) desalination for high-salinity wastewater treatment. Herein, an online interception strategy using nitric acid-activated alumina granules (AAG) is proposed to redirect gypsum scaling from the membrane surface to a sacrificial medium. By tuning nitric acid activation, the physicochemical properties of AAG were optimised. AAG activated with 7.5% nitric acid exhibits an optimal pore structure (specific surface area of 295.4 m2/g, pore volume of 0.361 cm3/g) and surface positive charge, significantly reducing the heterogeneous nucleation energy barrier for gypsum crystallisation. This enables the transfer of scaling from the membrane surface to AAG, reducing the membrane surface scaling amount by approximately 45%. Combined with the extended XDLVO theory analysis, the dominant mechanism of surface-induced heterogeneous nucleation by AAG was elucidated. Moreover, ultrasonic–EDTA cleaning enables efficient regeneration of AAG, and the strategy is validated using real reverse osmosis concentrate. This work establishes a scaling control paradigm based on crystallisation relocation rather than inhibition, providing mechanistic insight and a scalable approach for fouling mitigation in PV desalination.