Revealing the Role of Molecular Functionality in Regulating Antiscalant Efficiency for Mitigating Silica Scaling during Membrane Distillation

Silica scaling is a major constraint to membrane desalination due to its unique formation mechanism and the lack of antiscalants. Despite existing efforts being invested to develop antiscalants to inhibit silicic acid polymerization, antiscalants that mitigate silica scaling during membrane desalination have been rarely reported in the literature. Further, key factors governing the performance of antiscalants remain unclear for silica scaling. In this study, we employed a combination of experimental and simulation approaches to investigate the role of molecular functionality in regulating the efficiency of the antiscalant to mitigate silica scaling in membrane distillation (MD). Five antiscalants, which share a poly(ethylene glycol) (PEG) backbone but possess different functional terminal groups or molecular structures, were tested and compared for their performance in hindering silicic acid polymerization and mitigating water vapor flux decline due to silica scaling in static experiments and dynamic MD desalination, respectively. Specifically, PEG-based antiscalants with an ether or amine terminal group significantly extended water recovery of MD from ∼40% to over 70% when a feedwater containing supersaturated silica with a saturation index of 0.37 was used. We demonstrated that structural variations in terminal functional groups regulated the molecular properties of PEG-based antiscalants and greatly altered their inhibition efficiency in MD, which contrasted with results from static experiments on silicic acid polymerization. Scanning electron microscopy and molecular simulations reveal that the efficiency of antiscalants is determined collectively by the morphology of the silica scale, the molecular conformation of antiscalants in aqueous solution, and the tendency of antiscalants to reach the membrane surface. Our study sheds light on the mechanisms and design principles of antiscalants tailored to controlling silica scaling in membrane desalination.