PFAS occurrence and distribution in yard waste compost indicate potential volatile loss, downward migration, and transformation †

We discovered high concentrations of PFAS (18.53 ± 1.5 m g kg − 1 ) in yard waste compost, a compost type widely acceptable to the public. Seventeen out of forty targeted PFAS, belonging to six PFAS classes were detected in yard waste compost, with PFCAs (13.51 ± 0.99 m g kg − 1 ) and PFSAs (4.13 ± 0.19 m g kg − 1 ) being the dominant classes, comprising approximately 72.5% and 22.1% of the total measured PFAS. Both short-chain PFAS, such as PFBA, PFHxA, and PFBS, and long-chain PFAS, such as PFOA and PFOS, were prevalent in all the tested yard waste compost samples. We also discovered the co-occurrence of PFAS with low-density polyethylene (LDPE) and poly-ethylene terephthalate (PET) plastics. Total PFAS concentrations in LDPE and PET separated from incoming yard waste were 7.41 ± 0.41 m g kg − 1 and 1.35 ± 0.1 m g kg − 1 , which increased to 8.66 ± 0.81 m g kg − 1 in LDPE and 5.44 ± 0.56 m g kg − 1 in PET separated from compost. An idle mature compost pile revealed a clear vertical distribution of PFAS, with the total PFAS concentrations at the surface level approximately 58.9 – 63.2% lower than the 2 ft level. This di ﬀ erence might be attributed to the volatile loss of short-chain PFCAs, PFAS's downward movement with moisture, and aerobic transformations of precursor PFAS at the surface.


Introduction
2][3][4][5] Because of the widespread and unrestricted historical use of PFAS in daily life products and injudicious disposal, PFAS have become pervasive in the environment. 6,7A recent study identied 160 different PFAS in biosolids and compost with a total PFAS concentration of 580-1300 mg kg −1 . 8ne pathway of PFAS transport to agricultural lands is via biosolids and compost application. 9,10For instance, approximately 320-920 mg kg −1 of peruoroalkyl acids (PFAAs) were identied in agricultural land that received municipal biosolids for more than ten years. 11Approximately 50% of sewage sludge in Europe and North America is processed for agricultural use, 12 and 35% of US households utilize compost for gardening. 13,140][21][22][23][24] Over time, PFAS can leach from compost into surface runoff, contaminating natural water bodies, and can also percolate through soil, polluting groundwater. 8,9This led many US states, such as Maine and Vermont, to ban PFAScontaining compostable materials (e.g., sewage sludge) for agricultural use. 25,26e critically summarized the PFAS concentration in compost from different studies (Table S1 †) and identied no single studies on pure yard waste compost, possibly because of the minimal chance of PFAS contamination of yard waste.Studies reported 0.03-133.30mg kg −1 PFAS in biosolid and manure compost, 8,27 31.5-70.8mg kg −1 in municipal organic compost (i.e., food waste, leaves, grass, and horse manure), 28 8.6-16.3mg kg −1 in green waste compost (i.e., kitchen waste, yard trimmings), 29 and 6.8-11.84mg kg −1 in commercially available compost (i.e., organic feed unknown). 30Though these studies investigated PFAS in different types of compost, exploring PFAS in pure yard waste compost remains limited.Typically perceived as the purest type of compost, yard waste compost is widely used in many households for gardening.Thus, we investigated forty PFAS in mature yard waste compost, investigated incoming and composted plastics to understand plastics-PFAS co-occurrence, and investigated an idle mature compost pile to elucidate the vertical distribution prole of PFAS.The ndings of this study will elucidate the extent of PFAS contamination in yard waste compost and inform the development of global remediation strategies for PFAS in solid materials.

Yard waste compost
The City of Fargo (North Dakota, USA) residents can drop off yard waste at various locations year-round for free collection by the city authority.The Fargo yard waste compost site accepts tree leaves and grass from the residents.Unfortunately, there has been a tendency to dump unwanted substances on the site (e.g., plastics, paper, packaging) alongside yard waste, mainly due to the lack of awareness and supervision.Yard waste composting typically takes 3-4 months. 31The mature compost from the 13-acre Fargo compost site is then piled together for subsequent screening.We collected mature compost in October 2020 just aer nal screening with a Komptech Trommel Screen, which ensured proper homogenization.When the compost site has a surplus of mature compost, they store it inside the locked fence (Fig. S1 †).We also collected yard waste compost from the surface and 2  depth of the compost storage pile from three locations at least six feet apart (composts A, B, and C) in July 2021.The compost storage pile, approximately 1500 cubic feet in volume, has been idle inside a locked fence (100  × 100 ) for over six months.
We also separated and collected bulk plastics from incoming fresh yard waste and composted yard waste.All the compost and plastic samples were collected in LC-MS grade methanol-cleaned plastic zip-lock bags and stored in a refrigerator at 4 °C before analysis and extraction.Aer proper homogenization, a certain mass of the composts was extracted using the ASTM D3987-12 protocol, 32 and the ammonium and nitrate concentrations in the extracts were quantied to calculate the maturity index (NH 4 + /NO 3 − ) of the compost samples. 33The extracts were ltered through 0.45 mm Whatman lter papers (Sigma Aldrich, MO, USA) and the organic carbon (as dissolved organic matter) of the extracts was measured using a Shimadzu TOC Analyzer (Text S1 and Table S2 †).All the collected compost samples were fully mature (maturity index <0.5)(Table S3 †).The total carbon, organic carbon, and total inorganic carbon concentrations of all the compost samples were quantied using a Shimadzu TOC Solid Sample Analyzer (Text S2 and Table S2 †).Monthly total precipitation and average temperature data at the compost site from Jan 2020 to Dec 2021 are provided in Table S4.†

PFAS extraction and analysis
Homogenized compost samples were sieved twice through a 1.5 mm pore-sized LC-MS grade methanol-cleaned aluminum net.Ten grams of compost from each compost were taken in triplicates in pre-cleaned 50 mL polypropylene tubes.Then, 100 mL mass-labeled PFAS internal standard (MPFAC-HIF-ES, Wellington Laboratories, Guelph, ON, Canada) was added to the sample and adequately mixed (Table S5 †).The targeted analytes, extracted internal standards, and non-extracted internal standards were similar to the EPA dra method 1633 (ref.34) (Table S6 †). 10 mL of LC-MS grade methanol (Fisher Scientic, USA) was added to the sample tubes for extraction on a shaker (Thermo Scientic, MA, USA) at 200 rpm for 24 hours, followed by room temperature centrifugation at 10 000 rpm for 2 hours.Aer centrifugation, methanol was poured into precleaned 10 mL polypropylene tubes and concentrated to 1 mL with a nitrogen blowdown evaporator.For PFAS extraction from plastics, plastics were rst dried at room temperature and dusted off loose organics using compressed air.Plastics were cut down into small pieces (∼10 mm), and a certain mass was extracted by following the same protocol for compost.A detailed characterization procedure for these plastics, which includes crack analysis on the surface, can be found in the ESI (Text S3, S4, and Table S7 †).We also prepared procedural blank samples to assess background contamination during extraction (Text S5 †).The extraction recovery for the mass-labeled PFAS internal standards was 58-126%, similar to previous studies that used a low concentration of ammonium salts with methanol for PFAS extraction from compost (Table S1 †).
We targeted forty PFAS (demonstrated in the EPA dra method 1633 (ref.34)) in compost and plastics from seven PFAS classes.These include eleven peruorocarboxylic acids (PFCAs), eight peruorosulfonic acids (PFSAs), three uorotelomer carboxylic acids (FTCAs), three uorotelomer sulfonic acids (FTSs), seven peruoroalkane sulfonyl uorides (PASFs) or sulfanomido compounds, ve peruoroether carboxylic acids (PFECAs), and three peruoroether sulfonic acids (PFESAs).The PFAS were quantied using an ultra-performance liquid chromatograph (UPLC) coupled with a Triple Quadrupole mass spectrometer (Vanqish UPLC-Altis MS, Thermo Scientic, USA) in the negative ionization mode.UPLC separation was carried out using a Phenomenex Luna Omega C18 column (2.1 × 100 mm) following an already published protocol (Text S6 and Table S8 †). 35,36The measured PFAS mass was normalized by the dry mass of compost.Statistical analysis and plotting were performed using OriginPro (version 2023b, 10.05).Two-way multivariate analysis (MANOVA) was performed to understand the impact of sampling depth and moisture on the distribution of PFAS.We also conducted a one-way analysis of variance (ANOVA, a # 0.05) for the total carbon and dissolved organic matter (DOM) at the surface and at 2  depth in composts A-C to nd the correlation with PFAS distribution.Additionally, linear regression analysis (signicant at a # 0.05) of DOM versus total PFAS at varying depths was conducted to understand the correlation between DOM and total PFAS distribution.Pearson correlation was performed to test for the co-occurrence of different PFAS at different depths.Furthermore, we calculated the ratio of PFAS concentrations at 2  to the PFAS concentrations at the surface level.Linear regression analysis was performed between these values and the water solubility and vapor pressure values of PFAS to elucidate the correlation (signicant at a # 0.05).

Vertical distribution of PFAS in a compost storage pile
The compost PFAS concentration at the surface level was signicantly lower compared to the PFAS level at 2  depth, Similarly, the total PFAS concentration at 1  depth was 57.65% greater than the surface level in compost A (Fig. S5 and S6 †).Additionally, the concentration of two dominant PFAS classes, such as PFCAs and PFSAs, at 2  depths were 2.8-3.6 and 1.4-1.7 times of the surface level concentrations (Fig. 3).In compost A, at 2  depth, PFBA had the highest concentration (2.95 ± 0.08 mg kg −1 ), trailed by PFBS (1.88 ± 0.1 mg kg −1 ); a trend mirrored in composts B and C (Fig. S7-S9 †).Of the short chain PFCAs, PFBA and PFPeA concentrations at 2  depth were approximately 5.9-9.3 times of the surface level concentrations in composts A-C (Fig. 3).Downward movement of short-chain PFAS was reported in soil columns because of the high water solubility from their terminal functional groups' water affinity. 49,50Thus, the precipitation (e.g., rainfall, snow) at the compost site will directly affect the downward movement of water-soluble PFAS in the compost piles.The compost moisture content ranged from 28.1 to 29.9% at the surface and 41.3 to 44.8% at 2  depths, similar to the PFAS distribution trend with depth (Table S3 †).The ratio of PFAS concentration at 2  depths to the surface was positively correlated with the water solubility of PFAS (Fig. 4A and Table S10 †), which suggests that the water solubility of PFAS impacted their downward movement in the compost pile.
Besides downward movement with moisture, volatile loss and transformation might explain PFAS reduction at the pile's surface.Literature reveals that short-chain PFAS (e.g., PFBA, PFPeA, PFHxA) with high vapor pressure are more volatile than their longchain counterparts (Fig. S10 †).2][53] A strong positive correlation between the vapor pressure of PFAS and the ratio of PFAS concentration at 2  depth to the surface in the compost samples was observed (Fig. 4B and Table S11 †).This suggests the possibility of volatile loss of PFAS with high vapor pressure from the compost surface, leading to reduced PFAS levels at the surface.PFAA precursors such as uorotelomer sulfonates and sulfonamides are semi-volatile due Fig. 3 The ratio of PFAS concentrations at 2 ft depths to the surface in the yard waste compost.To calculate the ratio, PFAS concentrations at 2 ft depths were divided by the PFAS concentrations at the surface for composts A, B, and C. Nineteen out of the forty analyzed PFAS were detected in these compost samples.
to their large molecular size and strong intermolecular forces, limiting evaporation. 51,54These precursors have low water affinity due to their longer carbon chains, making them more likely to sorb on solids. 55,56However, we found that semi-volatile PFAA precursors such as FOSA-1's concentrations at the surface level were 40-60% less compared to the 2  level, EtFOSAA was 45-60% less, and 6 : 2 FTS was 25-80% less at the surface than at 2  (Fig. 3 and S7-S9 †).Besides the downward movement with moisture and volatile loss, this high percent loss from the surface could be attributed to the biological transformation of PFAA precursors into short-chain PFAAs at the top compost layer (0-10 cm) under oxygen-rich conditions. 57[59] Long-chain PFAS were less mobile in the compost pile than short-chain PFAS and PFAS precursors.The change of longchain PFAS concentrations at the compost surface was notable, but less than the change in short-chain and precursor PFAS concentrations (Fig. 3).2][63][64][65][66][67] Interestingly, DOM at 2  depth (7.82-14.21g kg −1 ) was signicantly (a # 0.05) greater than the DOM at the compost surface (3.57-4.78g kg −1 ), showing a strong correlation with the total PFAS at varying depths (Tables S12, S13 and Fig. S5 †).However, there was no signicant variation in the total carbon content across depths in composts A-C (Tables S2, S14 and Fig. S6 †).The fate of PFAS is also inuenced by their structural variations, such as isomers.Research indicates that linear PFAS isomers tend to adhere to soil and sediments, whereas branched isomers are more prone to movement. 60This difference is linked to the greater polarity of branched isomers relative to linear ones.Our ndings revealed that branched PFOS levels were 2.5 times greater at 2  depth than at the surface, suggesting water-assisted downward movement (Fig. 3).Conversely, linear PFOS levels were similar at both depths, indicating sorption by compost.][70] Furthermore, biogeochemical elements like iron deposits on compost can also inuence PFAS adsorption, especially with iron oxides binding short-chain PFAS via surface complexation or ion exchange. 50,713][74] Although theoretically, stronger surface adsorption of PFAS might occur by biogeochemical factors that could slow down PFAS's downward movement, this study did not show such evidence.
Composts at greater depths had notably more moisture than surface layers (Table S3 †).Depth and moisture content were interdependent, signicantly impacting the concentration of PFAS species at 0 and 2  depths (Table S15 †).Statistical analyses showed signicant correlations among certain short-chain PFAS (i.e., PFBA, PFBS, PFPeA, PFHxA, and PFHpA) and between them and compounds like 6 : 2 FTS, 8 : 2 FTS, and FOSA-1, suggesting a shared origin (Fig. 5).Likewise, long-chain PFAS (i.e., PFNA, PFDA, and PFUdA) were signicantly correlated with one another and with certain short-chain compounds and precursors (i.e., PFHpA, 8 : 2 FTS, and EtFOSAA), hinting at a common origin (Fig. 5).However, PFSAs showed no distinct correlation in yard waste compost, indicating different sources.6][77][78] A leading contributor of PFAS in yard waste is plastics, which oen end up in yard waste due to the lack of public awareness and monitoring.Paper, paper products, and packaging materials can also act as sources of PFAS in yard waste compost.Moreover, the water used to maintain the moisture content of yard waste can act as a source of PFAS in compost.The proximity of the compost site near the Fargo landll increases the likelihood of PFAS entering the compost through atmospheric deposition, particularly due to the Fig. 4 Linear regression analysis between the ratio of PFAS concentrations at 2 feet depth to surface level and their water solubility (A) and vapor pressure (B).To calculate the ratio, PFAS concentrations at 2 ft depth were divided by the concentrations at the surface for composts A, B, and C. The median value of water solubility and vapor pressure of PFAS were used for the analysis and plots.Median value for each PFAS were derived by consolidating data from 82 peer-reviewed sources for water solubility and 77 peer-reviewed sources for vapor pressure (Fig. S10 †).potential presence of volatile PFAS in emissions from landll materials.Given these varied sources, it is essential to conduct an in-depth investigation to pinpoint the precise origins of PFAS in yard waste compost.

Conclusions
This novel investigation revealing substantial PFAS contamination and plastic-PFAS co-occurrence in yard waste compost calls for attention to mitigate risks from land application.With total PFAS detected at 18.53 ± 1.5 mg kg −1 (Fig. 1), concentrations comparable to those in biosolids-derived composts, demanding rigorous testing and regulations for this popular soil amendment.The prevalence of short-chain PFAS, such as PFBA, and the downward movement of PFAS with moisture through the 2  compost prole demonstrate the potential for groundwater leaching and surface water pollution via runoff (Fig. S11 †) when PFAS-laden composts are spread in gardens and on agricultural elds.Enriching PFAS on plastics during composting also identies an overlooked exposure pathway to humans and wildlife.The adsorption of PFAS on plastics in compost can lead to two outcomes.Removing these plastics before processing reduces PFAS contamination risks.Yet, due to the low protability of composting, this step is skipped, leading to the fragmentation of PFAS-laden plastics into microand nanoplastics during compost grinding.This results in PFAS being distributed along with these tiny plastic particles, which can elevate the environmental hazards associated with PFAS.While surface volatilization may reduce short-chain PFAS levels, continuous environmental recirculation limits this as a permanent solution.The precise quantication of PFAS downward migration, volatile loss, and transformation in a compost pile is outside the scope of this study.With the mounting evidence on PFAS uptake in crops, implementing science-based testing, treatment, and regulatory guidance is imperative to sustain the circular benets of composting without perpetuating toxic PFAS pollution.Deriving innovative solutions for PFAS removal (e.g., cost-effective engineered adsorbents) will transform compost into a driver of healthy soils and sustainable food systems rather than a source of persistent organic pollutants.

Fig. 2
Fig.2Ten out of forty analyzed PFAS were detected in low-density polyethylene (LDPE) and polyethylene terephthalate (PET) separated from incoming and composted yard waste.Extended forms of the PFAS compounds are available in TableS9.†