Deciphering Mechanisms of Silica-Metal Scaling on RO Membranes via 3D Structural and Compositional Analysis

Deciphering the structure and composition of the scaling layer is crucial for understanding its formation mechanisms in the reverse osmosis (RO) process. However, conventional characterization techniques face challenges in providing high three-dimensional resolution and precise compositional analysis of mixed scales, which hinders in-depth elucidation of the underlying mechanisms. In this study, we combined the exceptional depth resolution of time-of-flight secondary ion mass spectrometry (ToF-SIMS) and the superior mixed-scale discrimination capability of thermogravimetry-infrared spectroscopy (TG-IR) to analyze Si/Al scaling, a common issue in industrial RO systems. Under acid conditions, ToF-SIMS measurements revealed Al species enrichment on the membrane surface, attributed to the strong affinity between Al³⁺ and the membrane. The preferential deposition of Al³⁺ further facilitated the heterogeneous nucleation of polymerized silica through the electrostatic shielding effect, leading to the rapid formation of a thin and dense scaling layer. In contrast, neutral and alkaline conditions produced a slower-developing, uniform, thicker, and loosely structured scaling layer through physical deposition of supersaturated Si/Al complex scales. TG-IR analysis revealed that neutral conditions favored coprecipitated adsorption-bound Si/Al species (6-coordinate Al) and Si/Al polymers (4-coordinate Al), whereas alkaline conditions primarily produced coprecipitated silica and Al(OH)₄^-. These findings advance the mechanistic understanding of Si/Al scaling and provide a foundation for targeted control strategies in silica-metal combined scaling in RO systems.