Nanoscale Secondary Ion Mass Spectrometry for Quantifying Membrane Fouling: Fouling Layer Structure and Chemical Interaction

Membrane technology has garnered considerable attention for applications in wastewater treatment and resource recovery. Nevertheless, membrane fouling remains a major barrier, yet the lack of high-resolution non-destructive characterization techniques limits mechanistic understanding and model validation. Here, nanoscale secondary ion mass spectrometry (Nano SIMS) is introduced as a powerful analytical tool to visualize and quantify the fouling layers on forward osmosis (FO) membranes. Using sodium alginate as a model foulant and Ca² ⁺ as a representative multivalent cation, Nano SIMS enabled simultaneous mapping of organic matter (¹ ²C^-, ¹ ⁶O^-) and Ca-associated matter (⁴ ⁰Ca¹ ⁶O^-), revealing the formation and structural evolution of Ca² ⁺-organic networks within the fouling layer. A binarization-based image analysis method was developed to quantify the polymer volume fraction (φ₂), which increased markedly with Ca² ⁺ concentration and correlated strongly with flux decline, providing direct experimental support for the application of Flory-Huggins thermodynamic theory to the interpretation of membrane fouling. Application to real landfill leachate further demonstrated that Nano SIMS retains strong ion-recognition specificity and is capable of resolving fouling structures in complex matrices. This work establishes Nano SIMS as a versatile and robust non-destructive characterization technique for membrane fouling research, offering new opportunities for mechanistic investigation and model development in water treatment technologies.