Surface chemical heterogeneity of polyamide RO membranes:Measurements and implications

• Chemical heterogeneity of RO membranes was first investigated using AFM. • AFM probes were designed by employing negative CML and positive AAL colloids. • Chemical heterogeneity was more pronounced with AAL probe than CML probe. • RO membranes showed different charge distribution despite similar zeta potential. • RO membrane with greater chemical heterogeneity was more prone to fouling.

In this study, the chemical heterogeneity of typical polyamide RO membrane surfaces was systematically investigated by measuring the adhesive interaction force distribution using AFM colloidal probes with negatively charged CML particles and positively charged AAL particles. The membrane fouling tendency of two RO membranes with different chemical heterogeneities was also examined to further elucidate the mechanism of RO membrane fouling. The primary findings drawn from this study are summarized briefly as follows: • Adhesive interaction forces were significantly stronger with the positively charged AAL colloidal probe than the negatively charged CML colloidal probe since typical RO membranes were negatively charged. • The charge distribution measured by AFM colloidal probes indicated that RO membranes had chemically heterogeneous surfaces. • Standard deviation was used for the quantification of chemical heterogeneity and it could be related to membrane fouling propensity. • The membrane with high surface chemical heterogeneity was more prone to fouling. • The chemical heterogeneity provides a new insight for understanding RO fouling mechanisms, suggesting that membrane charge distribution is one of the key factors governing membrane fouling.