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

ABSTRACT 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 Ca ^{2 +} as a representative multivalent cation, Nano SIMS enabled simultaneous mapping of organic matter ( ^{1 2} C ⁻ , ^{1 6} O ⁻ ) and Ca‐associated matter ( ^{4 0} Ca ^{1 6} O ⁻ ), revealing the formation and structural evolution of Ca ^{2 +} –organic networks within the fouling layer. A binarization‐based image analysis method was developed to quantify the polymer volume fraction ( φ ₂ ), which increased markedly with Ca ^{2 +} 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.