Electrospun functionalized nanofibers for the removal of anionic nitrogen species from drinking water

Abstract

Point source discharges from municipal treatment plants, compounded by nonpoint source runoff from excessive land application of fertilizer and manure, has produced nitrogen-impaired surface waters and groundwater across the Midwest. This contamination poses health risks to consumers who rely on drinking water sources with elevated nitrate, which has well-established acute and emerging chronic health impacts. Here, we develop surface-functionalized polymer nanofibers for the selective and high-capacity removal of nitrate from contaminated drinking water supplies, including decentralized sources like private wells. Using the material fabrication approach of electrospinning, we harnessed the properties of surface-segregating quaternary ammonium surfactants (QAS) for the dual purpose of reducing surface tension in the precursor solution to produce more uniformly distributed polyacrylonitrile (PAN) nanofibers and to enrich the PAN scaffold with ion exchange sites resembling those commonly used in nitrate-specific strong base anion exchange resins. Through tailored nanofiber synthesis, we determined the optimal nanofiber formulation for nitrate and nitrite uptake across various material properties and solution conditions representative of water treatment. Notably, work with various QAS (chloride, bromide and iodide salts of tetrabutylammonium; TBAC, TBAB, and TBAI, respectively) led us to propose that the size of the halide counterion impacts QAS surface segregation, which in turn influences the number and density of ion exchange sites on the polymer surface, with uptake greatest using TBAC. Materials with an optimal QAS loading of ~20 wt.% relative to PAN in the precursor sol gel resulted in relatively fast uptake (equilibrium time between 0.75-1.5 hours from kinetic uptake studies) with nitrite and nitrate capacity of 6 and ~8 mg/g, respectively, from isotherm experiments. Uptake was confirmed to occur via an ion exchange mechanism, with an established exchange stoichiometry of 1.3(± 0.5) and 1.5(± 0.2) for nitrate and nitrite, respectively. Materials could also be regenerated with concentrated brine, while also exhibiting selectivity (in more complex solutions with sulfate and carbonate) consistent with commercially available nitrate-selective resins. Overall, electrospinning produces surface-functionalized nanofibers with performance that is comparable to commercial nitrate selective resins while in a versatile membrane platform that may help aid integration into existing point-of-use and point-of-entry treatment devices.