Molecular-Level Investigation of Humic Acid Interactions with Ionic-Liquid-Processed Polyaniline/Polysulfone Composite Membranes

Abstract Organic foulants, such as humic acid (HA), remain a critical challenge in membrane-based water treatment processes. In such processes, the presence of multivalent ions in the aqueous solution significantly influences the foulant-membrane interactions. This work presents molecular modeling and an understanding of a polyaniline (PANI)-polysulfone (PSF) IL-mediated membrane compared with an NMP-mediated membrane, as well as the underlying interactions with HA in aqueous solution in the presence of monovalent Na+ and divalent Ca2+ salts at fixed salt concentrations. Several steps were systematically followed to model the PANI–PSF composite membrane. The root-mean-square deviation (RMSD) profiles exhibited relatively small fluctuations within the final analysis window, indicating temporal stationarity of the IL-mediated membrane after the introduction of HA into the aqueous salt solution. 2-D radial distribution function (RDF) showed weak HA-membrane interactions and strong HA-HA interactions. The molecular atomic density profile provides insight into the spatial distribution and conformational arrangement of the species after HA loading, revealing a comparatively uniform distribution of water within the polymer-rich interfacial region. For hydrogen binding, increased HA-HA interactions with HA loading indicate stronger aggregation, thereby decreasing membrane-HA interactions. H-bond autocorrelation indicated faster relaxation of HA-water interactions compared to membrane-water interactions. This highlights the dynamic nature of HA association. The HA-membrane association competes with favorable HA-water interactions. Collectively, the observed interaction pattern suggests that HA interactions may not be dominated by strong membrane-HA interaction but may instead be influenced by a dynamically hydrated interfacial environment under the investigated mixed-salt conditions, providing molecular-level insights into fouling behavior.