Behaviours and mechanisms of nanofiltration membrane fouling by anionic polyacrylamide with different molecular weights in brackish wastewater desalination

Abstract

The effect of the molecular weight (MW) of anionic polyacrylamide (APAM) on the fouling behaviours of nanofiltration membranes in brackish wastewater desalination was investigated, and its implications were elucidated using the interfacial free energy and intermolecular interactions. The foulant accumulation (mirr) was closely related to the intermolecular maximum adhesion force (Fad, max). For APAM with a larger MW of 5,000,000 Da (500APAM), the longer molecular chains were more susceptible to entanglement and thus caused a larger Fad, max between the APAM chains and membrane, leading to a larger mirr than for APAM with a MW of 500,000 Da (50APAM). In contrast, the irreversible fouling resistance (Rirr) strongly correlated with the interfacial free energy (ΔG). The ΔG for the 500APAM–membrane was higher than that for the 50APAM–membrane, which indicates a more stable thermodynamic system of the membrane surface and 500APAM molecules and thus resulted in a higher Rirr. The higher ΔG and larger Fad, max for 500APAM intensified mirr and Rirr and resulted in more severe flux declines. In addition, the strong coordination between Ca2+, when present, and carboxylates led to the intense attraction between APAM and the membrane, resulting in further flux declines of both APAMs.