Temporal insights into electromagnetic field-tuned scaling pathways of CaCO3 and CaSO4•2H2O during reverse osmosis desalination of real brackish water

Electromagnetic field (EMF) treatment is a promising nonchemical strategy for scale control in reverse osmosis (RO) desalination; however, the mechanisms by which it influences polymorph selection and competitive precipitation in multi-ionic waters remain poorly understood. Here, we present the first detailed temporal analysis of scale formation, elucidating how EMF alters CaCO₃ and gypsum scaling in multi-ionic, Mg-bearing brackish groundwater (Mg/Ca = 0.67). Without EMF, aragonite and Mg-calcite co-precipitated, consistent with Mg²⁺-mediated inhibition of calcite growth. EMF accelerated bulk crystallization, amplified Mg²⁺ suppression of calcite, and drove an almost complete shift toward aragonite precipitation while maintaining elevated Mg/Ca ratios. EMF also mitigated gypsum scaling by promoting early, uniform CaCO₃ precipitation, reducing free Ca²⁺ availability, and delaying gypsum nucleation during initial concentration stages. At high gypsum supersaturation, precipitation occurred under both EMF and control conditions; however, EMF produced more porous and loosely adherent gypsum deposits. This dual effect—favoring aragonite formation and delaying or modifying gypsum deposition—resulted in more uniform, easily removable scaling layers. Although short-term flux improvements were modest, these mechanistic shifts enhance long-term membrane performance and cleanability, particularly when combined with hydraulic flushing.