Membrane Scaling and Wetting in Membrane Distillation: Mitigation Roles Played by Humic Substances

Abstract

Membrane scaling and wetting severely hinder practical applications of membrane distillation (MD) for hypersaline water/wastewater treatment. In this regard, the effects of feedwater constituents are still not well understood. Herein, we investigated how humic acid (HA) influenced gypsum-induced membrane scaling and wetting during MD desalination. At low concentrations (5–20 mg L–1), HA notably mitigated membrane scaling and wetting. The morphological characterization of scaled membranes revealed that the antiwetting behavior could be ascribed to the formation of a compact and protective gypsum/HA scale layer, which blocked the flow channel of scaling ions and suppressed the intrusion of scale particles into membrane pores. Based on the comprehensive analysis of the scaling process, the formation of the scale layer was related to the heterogeneous crystallization of gypsum on the membrane surface. Moreover, deprotonated HA interfered with the heterogeneous crystallization process by inhibiting the formation of gypsum nuclei and altering the orientation of crystal growth, thus delaying membrane scaling and altering the morphology of the scale layer. Thermodynamic and kinetic analyses further demonstrated the mitigation mechanism of HA. Furthermore, improved fouling reversibility and antiwetting ability in synthetic seawater treatment endowed by HA were observed. This study provides new insight into the roles played by the organic constituents of water/wastewater during membrane desalination, providing a valuable reference for developing novel strategies to improve the performance of MD.

A Tale of Two Foulants: The Coupling of Organic Fouling and Mineral Scaling in Membrane Desalination

“…The adsorbed SRHA molecules can also suppress gypsum crystallization by blocking the active growth sites and distorting the crystal morphology . In a study of combined fouling and scaling in MD, Wang et al observed a similar inhibitory effect of HA on the nucleation and crystallization of gypsum crystals. The authors measured the nucleation rate of gypsum in the bulk solution and observed that increasing HA concentration from 0 to 20 mg/L resulted in a gradual decrease in gypsum nucleation rate (Figure B).…”

“…The authors demonstrated that all the foulants delayed and slowed water flux decline (Figure C), likely because the adsorption of foulants onto membrane surface obstructs gypsum crystallization . Moreover, Wang et al added various concentrations of HA to gypsum-supersaturated feedwaters for combined fouling and scaling experiments using direct contact MD, where the addition of HA mitigated gypsum scaling and enhanced water recovery. The authors explained that a HA foulant layer on the membrane surface enhanced the free energy barrier to gypsum nucleation .…”

“…Moreover, Wang et al added various concentrations of HA to gypsum-supersaturated feedwaters for combined fouling and scaling experiments using direct contact MD, where the addition of HA mitigated gypsum scaling and enhanced water recovery. The authors explained that a HA foulant layer on the membrane surface enhanced the free energy barrier to gypsum nucleation . Such an explanation is supported by Curcio et al, who showed that HA mitigated calcite scaling during MD by increasing the free energy barrier for calcium carbonate nucleation.…”

“…(F) Measured nucleation rate of calcite on self-assembled monolayer surfaces with different terminating functional groups. Figures B–F are adapted with permission from ref (B and C, copyright 2022 American Chemical Society), ref (D, copyright 2014 Elsevier), ref (E, copyright 2021 American Chemical Society), and ref (F, copyright 2014 United States National Academy of Sciences).…”

“…Recently, researchers have started to investigate combined fouling and scaling as well as the foulant–scalant interactions. The presence of organic foulants has been shown to regulate homogeneous nucleation of mineral formation by altering the free energy barrier and/or mineral particle aggregation. − When a layer of organic foulants forms on the membrane surface, the thermodynamics of heterogeneous mineral nucleation or the deposition kinetics of mineral scales could be potentially changed. , Despite the importance of understanding the coupling of organic fouling and mineral scaling in membrane desalination, the existing literature has only seen review articles that exclusively focus on individual fouling or scaling (i.e., feedwaters contain only organic foulants or mineral scalants). ,,,,, To the best of our knowledge, there is still a lack of review articles that summarize current knowledge regarding the combined behaviors of organic fouling and mineral scaling, as well as the interactions between foulants and scalants at the water–membrane interface. We acknowledge that combined fouling involving more categories of foulants, including biological and colloidal foulants, is also common in membrane processes.…”