The Impact of PFAS in Biosolids on Land Reclamation

LCGC International sat down with Linda S. Lee, Distinguished Professor of Agronomy, at Purdue University to discuss her latest research investigating whether highly degraded urban soils could be restored using biosolids.

The family of per- and polyfluoroalkyl substances (PFAS) is a hot topic in environmental analysis. Because PFAS can have deleterious effects on the environment, scientists are primarily concerned with developing and implementing new analytical techniques and instrumentation that can improve detection of these harmful substances. PFAS are known to enter waterways and soil through other industrial processes, such as wastewater treatment in agriculture, where biosolids, which are nutrient-rich solid organic matter that is recovered from wastewater treatment processes (derived from “sewage sludge”), are used (1).

Linda S. Lee and her team are exploring this topic in environmental analysis. Lee is a Distinguished Professor of Agronomy, in the Department of Agronomy at Purdue University in West Lafayette, Indiana. She also serves as a Professor of Environmental and Ecological Engineering, as a Program Head in the Ecological Science and Engineering Interdisciplinary Graduate Program, and as an Assistant Dean of Graduate Education, Research, and Faculty Development, also at Purdue.

Her group’s recent research study examined the use in agriculture of biosolids derived from wastewater treatment, focusing on how biosolids may introduce PFAS and other harmful chemicals into soils (1). The study investigated how PFAS move through soil when biosolids are applied at different rates. Over a two-year period, researchers analyzed PFAS in the leachate, finding that higher biosolids application rates led to higher PFAS concentrations (2).

LCGC International recently sat down with Lee to talk about her group’s findings, as well as future research efforts in this field.

Can you elaborate on the persistence of PFAS in biosolids and how it impacts their application in agriculture and land reclamation?

PFAS are resistant to both chemical and biological degradation to different degrees; this characteristic, along with their oil- and water-resistant and surfactant interfacial properties, has made them so useful in industrial processing, fire-fighting foams, and a myriad of products. The C-F bond is the strongest single bond known to man coupled to completely perfluorinated carbons being attached to other completely perfluorinated carbons results in a very hard to degrade molecule. Although PFAS are “forever chemicals” in nature, some PFAS do degrade to other PFAS. These partially degradable PFAS are referred to as precursors that degrade to a suite of intermediates to terminal metabolites, which are the perfluoroalkyl carboxylates and sulfonates that were noted in response to the prior questions. Fluorotelomer PFAS, which refers to PFAS that have a -CH₂CH₂- linkage between the perfluoroalkyl tail and the polar carboxylate head group, are easily degraded to multiple shorter chain perfluoroalkyl carboxylates. For example, one molecule of an 8:2 fluorotelomer PFAS can produce several perfluoroalkyl carboxylates of varying shorter-chain lengths. The electrochemically-derived PFAS, which contain an -SO₂- group linked directly to the perfluoroalkyl tail, degrade very slowly through several intermediates but only lead to one terminal metabolite—the perfluoroalkyl acid with the same perfluoroalkyl tail length. The hard-to-degrade nature of PFAS means that upon entry into our wastewater treatment systems, these PFAS or subsequent intermediates and terminal metabolites will end up in the sludge or the wastewater that is discharged to surface waters. Converting sludge to biosolids for their use as a fertilizer kills pathogens, but it does not alter the overall fluorine-containing PFAS present. PFAS present in biosolids can translocate into plants that are consumed by humans directly or via plant-fed animal consumption. PFAS present in biosolids can also leach through the soil profile and enter the groundwater as well as enter surface waters through run-off or tile-drain discharge from agricultural fields. Therefore, both biosolids and direct wastewater discharge or reuse can affect drinking water sources whether groundwater or surface waters. Therefore, although plants and soils respond beautifully to land-application of biosolids and neither are adversely affected by PFAS presence in biosolids, PFAS’ inadvertent presence in our biosolids serves as a potential direct and indirect human exposure route. The latter is challenging our sustainable and beneficial reuse of waste product that has high nutrient and sequestered carbon value and will increase our reliance on resource-consuming synthetic fertilizers, which comes with another set of challenges.

Could you describe the artificial site constructed to mimic degraded soils and explain why this setup was chosen for the study?

The original purpose of the study was to see if highly degraded urban soils could be restored using biosolids with evidence of a successful garden. PFAS inadvertent presence in biosolids was not on the mind of those who developed the study. Having said that, the work had to be done in a place where the student and supporting staff had easy access to it. Therefore, soil was obtained from a degraded urban soil site where the topsoil was no longer present and because of heavy vehicle traffic, both common to construction activities. A 250-m² urban garden was constructed using the transported degraded soils from an urban construction site. The area was excavated to allow for placement of the urban soil to a 45-cm depth.

What are the advantages of using isotope-dilution, solid-phase extraction (SPE), and liquid chromatography tandem mass spectrometry (LC–MS) for quantifying PFAS in this study?

Isotope dilution is a technique where mass-labeled isotopes exactly or closely match the compounds of interest, which allows for correcting for a multitude of variations in instrument response, extraction efficiencies, and user preparation. In this study, isotopes were added at the start of the solvent extraction process to the solids being extracted by organic solvents or to the leachate processed through solid-phase extraction. After the various processing steps and transfer of the final solution to an injection vial, the same mass of each surrogate is added to the injection vials containing the standards. The ratio of the PFAS relative to its mass-labeled surrogate is used to prepare the standard curve. In this way, variations in instrument response (matrix effects in the mass spectrometer) and all sample preparation processes are accounted for resulting in a more accurate measurement of what was in the original sample. In some cases, both mass-labeled surrogates that track variations in the entire process are used in combination with a smaller subset of compounds with different mass-labels (for example, 13C-labeled atoms in the molecule), which enables a separate calculation to differentiate detector matrix effects from extraction recoveries.

How were the leachates collected over the two-year period, and what were the key findings regarding PFAS concentration trends in the leachates?

Leachate was collected into zero-tension lysimeters, which provide leachate collection after a rainfall event at a low-cost and with good reproducibility. We found that PFAS concentrations in leachate induced by rainfall events were highly correlated to biosolids application rates and application frequency. We also observed that blending biosolids with a woody mulch substantially reduced leaching of the longer-chain PFAS, particularly the longer chain sulfonic acids such as perfluorooctanesulfonic acid (PFOS) and perfluorohexanesulphonic acid (PFHxS), but also somewhat for the long-chain carboxylate PFOA. All three of these PFAS are now regulated by USEPA effective early this year because of their potential adverse health effects and bioaccumulation potential.

What mechanisms drive the differential mobility and retention of short-chain versus long-chain PFAS in soil and leachate?

PFAS chain length is highly correlated to retention, thus mobility, in the soil profile. As the perfluoroalkyl chain length increases, hydrophobicity increases, causing higher retention of PFAS, and thus lower mobility. Also, the longer-chain PFAS have a higher propensity to sorb to the air-water interface in unsaturated soils, which are the conditions optimal for growing most plants. Therefore, short-chain PFAS have higher mobility than the long-chain PFAS. Between classes within the PFAS family, the carboxylates are more mobile than the sulfonates for two reasons. When we refer to overall carbon chain length, one of the carbons in the carboxylates is not perfluorinated; however, PFOA has only seven perfluorocarbons whereas PFOS has eight. Also, the trigonal sulfonate group that is highly electronegative being bound directly to a highly electronegative perfluorocarbon appears to play a significant role in enhancing both affinity to soils and air-water interfaces as well as enhancing bioaccumulation.

How did varying biosolids application rates influence PFAS leachate concentrations, and what implications does this have for agricultural practices?

The initial purpose of this study was to see if biosolids could help restore function of highly degraded soils; therefore, biosolids were tested at high rates, frequent applications, or both. High rates were five times greater than what would normally be applied in home garden or typical agricultural soil. At the normal application rates to an agricultural soil, which also has a higher organic carbon content, PFAS concentrations in the leachate would be reduced. When biosolids are used as fertilizers, application rates are dependent on crop need and nutrient content; therefore, if a biosolids amendment is high in nutrients then less is needed, which translates to a lower inadvertent application of unwanted contaminants like PFAS.

How does the mulch in biosolid blends reduce PFAS leachate concentrations, particularly for long-chain perfluoroalkyl sulfonic acids?

The mulch is hypothesized to increase retention in a few ways. The mulch is hypothesized to increase the pore space and air-water interfacial area in the media, thus enhancing retention particularly of the longer chain perfluoroalkyl sulfonic acids like PFHxS (C₆) and PFOS (C₈). The mulch may also retain downward water movement as it holds water well. Lastly, although the carbon in mulch is not as stable as the carbon in biosolids, the added carbon may help to increase organic chemical retention as well.

Can you discuss the findings related to the blending of biosolids with mulch and its impact on reducing PFAS leachate concentrations?

Reducing the PFAS concentrations that leach downward into the soil profile translates to lowering the risk of contaminated groundwater, which could be a source of drinking water. What was not assessed was if the blend also reduced uptake into the plants, which was a shortcoming of this study because of the PFAS assessment not being part of the original study design. Plants tend to uptake the shorter chain PFAS more than the longer chain; however, we know that the longer-chain PFAS are taken up to some level depending on plant type and species.

Based on the study’s results, how can blending biosolids with mulch serve as a pragmatic strategy for reducing PFAS leachate concentrations and promote sustainable use of biosolids in agriculture?

Blending biosolids with other low PFAS-containing media, such as mulch and biochar, can reduce leaching to groundwater of many of the PFAS of greatest concern. Also, producing biosolids that are high in nutrients by blending biosolids with other high nutrient materials, such as some biochars as well as other materials that are farmed from our sewage in the wastewater treatment process, means we can lower the biosolids application rates. This lowers the addition of PFAS and other chemicals that inadvertently entered our waste stream.

What are the next steps in this work?

We have laboratory-based column studies, greenhouse studies, and field studies at multiple sites underway that include adding amendments to biosolids during application or adding amendments to soils that already have high PFAS loads because of historically applied industrially impacted biosolids. In the greenhouse and field studies, we are measuring PFAS in soils cores, plants of different varieties and species, and in some cases, the leachate using porous cup suction lysimeters at 2-ft and 4-ft depth. These studies will provide a strong data set for assessing pragmatic mitigation options. Additional work that will increase being able to use biosolids sustainably includes treatment technologies that reduce PFAS loads in biosolids without destroying the nutrients and carbon within the biosolids as well as treatment of streams coming into and within the water resource recovery facilities. This type of research coupled to minimizing PFAS use, which requires innovation for non-PFAS alternatives, will reduce PFAS loads in biosolids and the environment overall.

References

(1) United States Environmental Protection Agency, Basic Information about Biosolids. U.S. Environmental Protection Agency. Available at: https://www.epa.gov/biosolids/basic-information-about-biosolids (accessed 2024-08-13).

(2) Peter, L.; Modiri-Gharehveran, M.; Alvarez-Campos, O.; et al. PFAS Fate Using Lysimeters During Degraded Soil Reclamation Using Biosolids. J. Environ. Quality 2024, ASAP. DOI: 10.1002/jeq2.20576

About the Interviewee

Linda S. Lee is a Distinguished Professor of Agronomy, in the Department of Agronomy at Purdue University in West Lafayette, Indiana. She also serves as a Professor of Environmental and Ecological Engineering, as a Program Head in the Ecological Science and Engineering Interdisciplinary Graduate Program, and as an Assistant Dean of Graduate Education, Research, and Faculty Development, also at Purdue.