Effects of Alkaline Cleaning on the Conversion and Transformation of Functional Groups on Ion-Exchange Membranes in Polymer-Flooding Wastewater Treatment: Desalination Performance, Fouling Behavior, and Mechanism.

The aging effects of sodium hydroxide (NaOH) on ion-exchange membranes were systematically studied, including the membrane properties, desalination performance, and fouling behaviors. After aging in NaOH solution, there were minor changes in the cation-exchange membrane (CEM) properties; however, functional groups (i.e., quaternary amines) on the anion-exchange membranes (AEMs) were converted into benzylic alcohol, alkene, and tertiary amines, respectively, by nucleophilic substitution, Hofmann elimination, and ylide formation. These degradations rendered decreased ion-exchange capacity (IEC), increased electrical resistance, lost hydrophilicity, and weakened mechanical strength. Moreover, severe deteriorations of desalination performance were observed due to the little ion-exchange ability of the degraded AEMs. The desalination rates were restored after cultivating the aged AEMs in acid solution, mainly because the tertiary amines transformed from the hydroxide form (OH-form) to the ionic chlorine form (Cl-form). The restored desalination rates indicated that the main degradation products were tertiary amines. In addition, the antifouling performance decreased in the order of aged OH-form > aged Cl-form > original AEMs due to the reduction of foulant-membrane intermolecular interactions after aging in NaOH solution. The results contribute to establishing a more comprehensive understanding of the effects of alkaline cleaning on IEMs and provide new insights into cleaning-process optimization and membrane modification.

Performance and autopsy of nanofiltration membranes at an oil-field wastewater desalination plant.

In this study, the long-term operational performance of an on-site NF facility at a full-scale oil-field wastewater desalination plant was monitored. The NF facility with poor permeability due to membrane fouling enables efficient multivalent salt removal (rejections of Mg2+, Ca2+, Fe3+, and Al3+ were approximately 100%). Moreover, a comparison of the cleaning efficiencies of two on-site cleaning modes indicated that PL-007 cleaning helped to improve the effectiveness of subsequent acid cleaning in the removal of inorganic foulants. Furthermore, a spiral-wound NF membrane module harvested from the plant was unfolded and autopsied. The results showed that both anionic polyacrylamide (APAM) and crude oil were identified as the predominant organic matter on the membrane surface and collectively accounted for a substantial fraction (86.3%) in terms of dry weight. Additionally, dissolved organics with a high molecular weight were prone to accumulation on the membrane surface. Multivalent elements, including Mg, Ca, Al, Fe, and Si, were the primary inorganic species in the fouling layer. Among the inorganic elements, Si occupied a high proportion and existed in the form of SiO2 in the fouling layer. According to the autopsy results, organic fouling combined with inorganics was responsible for the decline in the flux.