Microplastics: A potential booster for PFAS in biosolids

Abstract

Imagine enjoying a refreshing glass of water, only to discover tiny plastic particles swirling within. This unsettling reality is becoming increasingly common as microplastics (MPs), plastic fragments smaller than a grain of rice (<5 mm diam.), infiltrate our environment at an alarming rate. From the deepest trenches of the ocean to the peaks of mountains, these invisible invaders pose a significant potential threat to wildlife and even human health (Li et al., 2023; Zolotova et al., 2022). Microplastics are now recognized as a major contemporary global problem (Mitrano & Wagner, 2021; Sendra et al., 2021), with a current estimate of 1.5 million tons of MP waste in the waterways globally (Boucher & Friot, 2017).

Per- and polyfluoroalkyl substances (PFAS), often referred to as “forever chemicals” due to their persistence in the environment, present a hidden threat to human health (Fenton et al., 2021). These man-made chemicals, lauded for their water and stain-repelling properties, lurk unseen in a vast array of consumer products. However, their presence comes at a cost. Most recently (January 2024) method 1633, which created a stable and uniform approach for the analytical identification of PFAS, was approved by USEPA to identify 40 PFAS compounds. On 10 April 2024, the USEPA announced the final National Primary Drinking Water Regulation (NPDWR) for six PFAS (PFOA, PFOS, PFHxS, PFNA, PFBS, and HFPO-DA). This enables USEPA to establish legally enforceable levels, called Maximum Contaminant Levels, for six PFAS in drinking water.

In addition to being a primary source of pollution, MPs can also act as a carrier (via sorption and desorption) for other contaminants including PFAS. Some of the plastic types, including polytetrafluoroethylene and polyvinyl fluoride, can contribute PFAS directly to the environment. However, this is a very small contribution compared with the potential adsorption pathway via widespread MP pollution globally. This does not disregard PFAS concerns, as some authors have suggested (Lohmann et al., 2020). Rather, MPs might also increase the overall availability of PFAS in biosolids. As MPs degrade, they could release any absorbed PFAS, making them more bioavailable (available for uptake by organisms).

There are also concerns that MPs can be more efficient in adsorbing PFAS in the presence of other organic and inorganic matter, when compared with controlled environments, due to their large surface area and strong hydrophobic nature (Scott et al., 2021). The adsorption of PFAS to MPs was identified as thermodynamically spontaneous due to the increased entropy at 25 °C, based on Gibb's free energy (ΔG = −16 to −23 kJ/mol), reaching equilibrium within 7–9 h (Salawu et al., 2024). This suggests that PFAS may partition to the MP surface within a few hours in fresh and marine water. Biofilms, composed of algae, bacteria, and other microorganisms in the environment, can further influence the sorption properties of MPs (Lagarde et al., 2016). This is particularly concerning because both PFAS and MPs tend to accumulate together in wastewater, with a higher proportion ending up in solid waste. These concentrated conditions create favorable environments for increased PFAS sorption by MPs. This becomes a point of concern when the biosolid waste is further used in land application (56%), comprising agricultural land (31%), reclamation sites (e.g., mining sites) (1%), and other (24%); landfilling (27%), comprising municipal solid waste (24%) and monofil (3%); incineration (16%); and other (1%) within the continental USA (USEPA, 2023). Application of biosolids with MP-sorbed PFAS may enhance bioavailability to crops, which can bioconcentrate in the longer run. Recently, a study reported that 23 years of biosolids application resulted in MP concentrations of 360 to 500 particles per kg of soil located on the Waterville plateau in Douglas County, north-central Washington state (Adhikari et al., 2024). This highlights the potential for biosolids to be a source of MPs and PFAS in the environment.

Currently, there are no regulations that can provide guidance on most of the contaminants of emerging concerns (CECs), including MPs and PFAS in wastewater. Regulating MPs and PFAS, as well, most likely, as other persistent emerging contaminants, in biosolid waste is crucial. This could not only protect public health but also drive research efforts toward developing effective removal technologies. Due to the low mass of MPs, they cannot easily be separated in the wastewater removal process. There is thus a need for cost-effective wastewater treatment technologies that can help MP removal from biosolids before their application to fields. This commentary calls on researchers to learn more about MPs–PFAS interactions and help create effective solutions. To safeguard our health and environment, we need a multipronged approach, including regulation, research, and technological innovations. Implementing clear regulations for MPs and PFAS levels in biosolids is crucial. This will guide responsible management practices and protect public health. Continued research is vital to understanding the long-term consequences of MPs and PFAS exposure, as well as developing cost-effective treatment technologies for wastewater to remove MPs before biosolid creation. Exploring and developing innovative technologies for biosolids treatment that effectively remove CECs, including MPs and PFAS, is essential. This is a collective responsibility, demanding action from researchers, policymakers, and the public alike.