Portable Iron Filtration Made Possible: Use of High, Flux Low Pressure Ion-Exchange Membranes in Series

Iron and other heavy metals are harmful contaminants that can be found in many sources of drinking water, especially well water which is used by around 43 million Americans as a source of potable drinking water (EPA, 2024). As the commercial desire for portable water treatment systems increases, the scientific advancements for these systems must also increase. Microfiltration membranes allow for portable filtration due to their low-pressure requirements, but typically cannot remove metal ions via size exclusion. This work examines adding ion exchange functionality to microfiltration membranes to allow for the capture of metal ions without sacrificing low operating pressures that are necessary for portable use. The ion-exchange membranes are synthesized via ultraviolet grafting of glycidyl methacrylate intermediate onto microfiltration membranes made of polyethersulfone (PES). These intermediates are modified using sodium sulfite to establish positive fixed charges on the membrane surface. Permeability testing of a variety of ion-exchange microfiltration PES membranes with variable grafted weights of the intermediate, depict that increased grafted weight on the membrane surface leads to increased operating pressure and decreased flux. When stacked in series, these low-grafted-weight filters maintain a practical operating pressure. Membranes with grafted weight as low as 0.4 mg performed up to 69% static removal whereas membranes with 3.7 mg of grafted weight show up to 71% static removal. Operating with these membranes in series increases removal beyond additive functions of the grafted weight of individual membranes, shown by a comparison of a single membrane versus a membrane stack at similar total grafted weights. When a 1.7 mg and 1.2 mg grafted weight membrane are stacked in series the capacity increases to 19.3 mg/g compared to the capacity of a single membrane of 3.8 mg of grafted weight which has a capacity of 8.25 mg/g. The stacked series operates below 2 psi whereas the single membrane operates at 4 psi. Use of these ion-exchange microfiltration membranes with low grafted weight in portable water systems can allow for increased removal capacity of iron water filtration and removal of additional multivalent ionic species from water in a portable system.