Membrane Filtration of Microplastic Fibers: Assessment of Membrane, Microplastic, and Solution Properties

Abstract

Microplastics (MPs), plastic particles less than 5 mm in size, have become ubiquitous in the environment due to increasing plastic production and pollution. MPs pose a significant threat to both aquatic life and human health due to their small size and heterogeneity in shape (e.g. fragments, fibers, films etc.) and chemistry (e.g. polyethylene, polystyrene, polypropylene etc.) A significant source from which MPs can enter the environment is the effluents of wastewater treatment plants (WWTPs). Membrane filtration systems, used in some WWTPs, have been shown to be effective at removing MPs, as most MPs are larger than the membrane pores. Understanding the impacts MPs have on membrane filtration is a growing area of interest due to their potential adverse effects, though all studies to date have been focused specifically on MP fragments. As MP fibers (MPFs) are common in both the influents and effluents of membrane systems in WWTPs, it is important to understand their impact on membranes and how they can be removed. In this work, polystyrene was electrospun into a fiber mat, which was cut using a cryostat to yield MPFs. Fluorescent MPFs were created by doping the electrospinning solution with meso-tetraphenylporphyrin, enabling the detection of MPFs in suspension. These MPFs were used to investigate the impacts of membrane pore size, solution chemistry, and MPF length on the removal of MPFs and membrane fouling. MPFs were found in the permeate of a 5 µm membrane, with some MPFs exceeding the nominal membrane pore size. Feed suspensions with shorter MPFs were found to result in more MPFs passing through the membranes compared to feed suspensions with wider variation in MPF lengths. When MPFs were suspended in real WW, both 0.22 µm and 5 µm membranes experienced more fouling compared to when MPFs were suspended in Milli-Q water. This work highlights the importance of working with solutions and particles realistic to those found in WWTPs and the environment to better understand the impact MPFs can have on membrane filtration.