Reflection of the structural distinctions of source—different humic substances on organic fouling behaviors of SWRO membranes

• This study determines the fouling mechanism at a molecular level to some extent.
• Humic substances were isolated from seawater and characterized.
• Differences in fouling behaviors were revealed according to three different organic foulants.

Numerous efforts have been made to resolve the organic fouling issue in seawater reverse osmosis (SWRO) desalination plants. However, SWRO membranes still suffer fouling in practice. This study proposed a fundamental approach to gain insight into the actual fouling phenomenon. First, humic substances were isolated from seawater (SeaHSs) and characterized by FTIR, 1H-NMR, and MS-IT-TOF. SeaHSs demonstrated some distinctions in average molecular weight, proton distribution, and functional groups compared to humic substances originating from river water and soil which have been popularly utilized to study organic fouling. SeaHSs were then used to study, for the first time, the organic fouling behaviors of SWRO membranes in comparison with Aldrich humic acid (AHA) and Suwannee River natural organic matter (SuNOM). AHA induced the most severe fouling while SeaHSs caused the lowest level of fouling, consistent with the postulation from structural differences. In addition, to further demonstrate the fouling mechanism at a molecular level, unused membranes, fouled membranes, and cleaned membranes (after being fouled) were analyzed exclusively using contact angles and FTIR. The analysis helped to rationalize the variations in fouling behaviors and the distinctions of different types of organic foulants on those variations. The results provide valuable information for all researchers involved in the field in order that they become more cautious in selecting model organic foulants for studying organic fouling in SWRO desalination plants.