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Article

Soil Amendment with Biochar Reduces the Uptake and Translocation of Perfluoroalkyl Substances by Horticultural Plants Grown in a Polluted Area

by
Ilaria Battisti
*,
Anna Rita Trentin
,
Andrea Sabia
,
Antonio Masi
and
Giancarlo Renella
*
Department of Agronomy, Food, Natural Resources, Animals and Environment (DAFNAE), University of Padova, Viale dell’Università 16, 35020 Legnaro, Italy
*
Authors to whom correspondence should be addressed.
Soil Syst. 2025, 9(3), 100; https://doi.org/10.3390/soilsystems9030100
Submission received: 28 June 2025 / Revised: 6 September 2025 / Accepted: 11 September 2025 / Published: 13 September 2025
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Abstract

Environmental pollution by poly- and perfluoroalkyl substances (PFAS) can impact human health through drinking water and the ingestion of contaminated agri-food. Plants can take up PFAS from polluted soils or irrigation waters, and soil amended with biochar has been proposed as a practical and sustainable option to effectively reduce the PFAS transfer from soils to plants. We evaluated the potential of biochar, the byproduct of biomass pyrolysis, to reduce or prevent PFAS uptake from contaminated soil and water in a field trial conducted in a PFAS-contaminated area, where tomato and red chicory plants were grown in succession. The PFAS content in irrigation water, soil, and tomato and red chicory plants was determined by liquid chromatography coupled to mass spectrometry before and after each cultivation trial. Compared to those grown in unamended soil, tomato plants grown in the biochar-amended soil showed a significantly lower uptake of perfluorobutane sulfonic acid (PFBS), perfluoroheptanoic acid (PFHpA), and perfluorooctanoic acid (PFOA) in the leaves (−70%, −45%, and −84%, respectively), and significantly less (−61%) perfluorobutanoic acid (PFBA) in the fruits. Compared to unamended soils, leaves of red chicory plants grown in biochar-amended soil accumulated less PFBS (−74%) in the early growth stage and less PFBA (−34%) at plant maturity. The presented results confirmed previous reports on the potential soil amendment with biochar as a sustainable and effective measure for reducing PFAS uptake by horticultural crops cultivated in PFAS-polluted areas and PFAS concentration in their edible parts. Implications of this approach are also discussed.

Graphical Abstract

1. Introduction

Poly- and perfluoroalkyl substances (PFAS) are a large family of ca. 10,000 compounds formed by a partially or fully fluorinated carbon (C) chain containing one or more functional groups. PFAS have water- and oil-repellent properties, high thermal and chemical stability, and no or negligible biodegradability, which confer notable properties to products, essential also for high-tech applications and high-performance industrial processes [1,2]. Environmental pollution by PFAS is raising concern upon the discovery of their adverse effects on human health [3,4], as an ever-increasing number of PFAS-contaminated areas worldwide are being identified by the Environmental Protection Authorities, e.g., [5,6], which are generally located close to PFAS manufacturers and users. PFAS chemical stability and repellency make them highly persistent and mobile in the environment, posing risks of contamination of groundwater and surface water, accumulation in soil, and bioaccumulation in plants and agri-food products [7], thus threatening human health and biota. Evidence of the link between PFAS production/utilization plants, levels of environmental contamination, and accumulation by resident populations has been reported for the major known PFAS pollution clusters in Europe [8,9,10,11], and adverse effects of PFAS on human health have been widely described [12,13,14].
One of these PFAS pollution clusters was discovered in 2013 in the Veneto Region (northeast Italy) [15], and it is currently impacting ca. 200,000 inhabitants spread over a ca. 180 km2 area [16]. The main cause of the pollution is ascribed to the intentional release of contaminated waste in the groundwater plume by a chemical plant, which in turn caused the contamination of surface and drinking water. Since then, surface and ground waters are regularly monitored by the Veneto Region Environmental Protection Agency (ARPAV), showing that, although the primary source was closed and remediated, PFAS concentrations in the monitored areas fluctuated around characteristic values, and the contamination plume is still enlarging following the hydrology of the watershed [17].
The main route of human exposure is contaminated drinking water [18,19], and based on epidemiological evidence, the European Food Safety Agency has indicated a Tolerable Weekly Intake (TWI) of 4.4 ng kg−1 body weight (BW) per week for the sum of perfluorooctanoic acid (PFOA), perfluorooctane sulfonic acid (PFOS), perfluorononanoic acid (PFNA), and perfluorohexane sulfonic acid (PFHxS) [20]. The rationale for choosing these four PFAS is their long production and frequent detection in food products. For example, relatively high concentrations of PFOA, PFOS, PFNA, and PFHxS in bovine muscles, seafood, and eggs produced in polluted environments have been reported [21,22].
Plants take up PFAS from soil and irrigation water [23,24,25,26], accumulating them with specific concentration patterns in roots, leaves, fruits, and other organs, as confirmed by several studies and review articles. Consolidated patterns are that long-chain molecules (C ≥ 6) are mainly retained in the roots, whereas PFAS with a shorter chain (C < 6) are more easily translocated to leaves and fruits [7,25]. Furthermore, for PFAS with an equal carbon chain length, the presence of a sulfonated functional group reduces plant uptake and translocation from roots to shoots [27]. However, it has been reported that PFAS uptake and translocation also depend on the plant species and source of PFAS pollution [28].
Biochar, the byproduct of biomass pyrolysis processes, is a carbon-rich material that can be used as an amendment to increase the fertility of agricultural soils [29]. Biochar is also known to reduce the uptake of heavy metals and organic pollutants by crop plants [30], and it has been recently reported that the biochar amendment of PFAS-polluted soils can significantly reduce their uptake by cultivated plants [7] and their leaching into groundwater [31]. In a laboratory experiment, Sørmo et al. demonstrated that biochar, unactivated or after activation by water steam or CO2, significantly reduced the leaching from a PFAS-polluted soil to various extents, depending on the degree of activation and rate [32], and Wu et al. reported that Fe or C nanotube-doped biochar was effective in the sorption of PFOA from wastewater [33]. Moreover, coconut shell-activated biochar was proven to effectively reduce PFAS leachate in soils with different total organic carbon contents [34]. Krahn et al. reported that sewage sludge-derived biochar acted as an effective PFAS sorbent [35], and the stabilization of PFOS in polluted soil by the amendment with pyrolyzed wood and sludge was confirmed by column leaching experiments [36,37].
Based on previous reports, we hypothesized that biochar amendment could reduce the PFAS uptake by plants cultivated in polluted environments by stabilizing them in the soil. To test this hypothesis, we amended the soil of a domestic orchard located in the abovementioned PFAS-contaminated area of the Veneto Region with biochar and determined the uptake and translocation of 13 PFAS in tomato and red chicory cultivated in succession. The results of this work could provide useful indications on the use of biochar as an operational safety measure for reducing the PFAS content of agri-food and residual plant biomass produced in polluted areas, thus protecting human health.

2. Materials and Methods

2.1. Chemicals

Analytical standards of perfluorobutanoic acid (PFBA), perfluorobutane sulfonic acid (PFBS), perfluoropentanoic acid (PFPeA), perfluorohexanoic acid (PFHxA), perfluorohexane sulfonic acid (PFHxS), perfluoroheptanoic acid (PFHpA), perfluorooctanoic acid (PFOA), perfluorooctane sulfonic acid (PFOS), perfluorononanoic acid (PFNA), perfluorodecanoic acid (PFDA), perfluoroundecanoic acid (PFUnA), perfluorododecanoic acid (PFDoA), and perfluorotetradecanoic acid (PFTeDA) were purchased from Wellington Laboratories (Guelph, ON, Canada). Identification and quantification of all PFAS were performed using the external standard mix (ES) of isotopically labeled PFAS (13C4-PFBA, 13C3-PFBS, 13C5-PFPeA, 13C5-PFHxA, 13C3-PFHxS, 13C4-PFHpA, 13C8-PFOA, 13C8-PFOS, 13C9-PFNA, 13C6-PFDA, 13C7-PFUnA, 13C2-PFDoA, and 13C2-PFTeDA) from Wellington Laboratories. A 13C-labeled PFAS internal standard mixture (IS, Wellington Laboratories) containing 13C3-PFBA, 13C2-PFOA, 13C4-PFOS, and 13C2-PFDA was used to evaluate the accuracy and recovery of PFAS extraction. UHPLC-MS grade water and methanol (>99.95% purity) were obtained from Carlo Erba Reagents (Cornaredo (MI), Italy).

2.2. Experimental Site

The experimental trial was conducted at the same site, located in the PFAS-polluted area of the Veneto Region described by Battisti et al. [38]. The dominant soil type in the area is calcareous Cambisols. The experimental area consisted of six plots of 6 m2 each, three amended with biochar and three unamended, representing the control plots. The biochar produced by slow pyrolysis at 550 °C of mixed oak woody biomass was purchased from Biocharitaly Luna s.r.l. (Bastia Umbra (PG), Italy) and homogeneously incorporated at a rate equivalent to 80 t ha−1 at 0–20 cm depth using a hand-operated rototiller (Grillo 50, Grillo S.p.A., Cesena (FC), Italy). Unamended soils were tilled in the same manner for consistency. Unamended and biochar-amended plots were alternated, and 50 cm strips of non-tilled soil were left to prevent lateral mixing of amended and non-amended soil. The physical and chemical properties of biochar are reported in Table S1. Within each plot, a drip-irrigation system was set up using plastic tubes with 30 cm spaced drippers, allowing each a flow rate of 1.5 L h−1 to supply PFAS-contaminated groundwater taken from a local well in the early plant development stages and on demand during the growing season. The soil was then covered with a mulching film before seedling transplant. One week after plowing, soils from all plots were sampled for preliminary analysis of chemical properties and PFAS contamination (described in Section 2.4 and Section 2.5).

2.3. Plant Cultivation and Sample Collection

2.3.1. Tomato Plants

Tomato (Lycopersicum esculentum Mill., cv. Roma) plantlets were transplanted with a planting distance of 0.50 m between rows and 0.50 m within rows on all plots in May 2023 and harvested at full fruit maturity in August 2023. The average monthly air temperatures in the growth period were as follows: May, 17.5 °C; June, 22.6 °C; July, 24.7 °C; and August, 24.0 °C. Monthly rainfall was as follows: May, 148.6 mm; June, 77.0 mm; July, 139.6 mm; and August, 128.4 mm. These meteorological data were in line with typical seasonal patterns. Based on the soil chemical data and recommendations, fertilization with 150–80–160 kg ha−1 of N-P2O5-K2O for the unamended soil and 120–45–90 kg ha−1 of N-P2O5-K2O for the biochar-amended soil was performed at the beginning of the experiment. Tomato plants were regularly irrigated with groundwater taken from the local well. The PFAS contamination profile of the water used for irrigation was evaluated before and during the plant growth period.
Tomato leaves and fruits were sampled for PFAS analysis from the first truss of each plant of all plots at full fruit maturity. For each plant, nine ripened fruits and the two proximal fully expanded leaves from each truss were sampled and immediately transported to the laboratory in cooled boxes. Fresh (FW) and dry (DW) weights of fruits and leaves were determined by weighing them before and after drying at 60 °C until reaching a constant weight. PFAS were extracted from dried samples, as described in Section 2.5.

2.3.2. Red Chicory Plants

After the tomato harvest, the soil was surface-plowed (0–15 cm) in preparation for the transplant of red chicory (Cichorium intybus L., cv. late Treviso). Red chicory seedlings were transplanted with the same planting distance as tomato on all plots in late September 2023 and harvested at full fruit maturity in January 2024. The average monthly air temperatures in the growing period were as follows: September, 20.8 °C; October, 16.3 °C; November, 7.5 °C; and December, 4.8 °C. Monthly rainfall was as follows: September, 52 mm; October, 173.8 mm; November, 144.4 mm; and December, 62.6 mm. These meteorological data were in line with typical seasonal patterns. Unamended and biochar-amended soils were fertilized with 65–30–80 kg ha−1 of N-P2O5-K2O, and red chicory plants were irrigated only for the first two weeks after transplant with groundwater taken from the local well.
Red chicory leaves were sampled after two months from transplant (November 2023) for an intermediate analysis of PFAS uptake and harvested at full maturity. In both cases, one biological replicate consisted of 4 pooled plants. Samples were immediately transported to the laboratory in cooled boxes, and fresh and dry weights were determined. As for tomato samples, PFAS were extracted from dried leaves, as reported in Section 2.5.

2.4. Soil Chemical Analyses

At the beginning of the experimental trial, composite samples from control and biochar-amended soils (three biological replicates for each condition, one per plot) were taken before seedling transplant for chemical analyses. The composite sample representative of each plot consisted of four individual samples taken at the four cardinal points at 20 cm depth with a soil auger, and each subsample consisted of a single borehole of approximately 1 kg of soil. For each sample, the soil texture was determined by the hydrometer method [39], the pH value was measured potentiometrically in ultrapure water (soil:water 1:2.5 w/v), Total Carbon (TC) and Total Nitrogen (TN) contents were determined by dry combustion using a CNS Vario Macro elemental analyzer (Elementar, Langenselbold, Germany), and the Total Organic Carbon (TOC) content was determined using the method of Walkley and Black [40]. Quantification of total phosphorus (P) was performed by microwave-assisted mineralization into modified polytetrafluoroethylene (TFM™) closed vessels (Ethos 1600, Milestone, Sorisole (BG), Italy) using a mixture of 6 mL of concentrated hydrochloric acid (30%) and 3 mL of concentrated nitric acid (65%) of Suprapur® quality (Merck, Darmstadt, Germany), followed by quantification of elemental concentrations by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES Spectro Arcos, Ametek, Weiterstadt, Germany), whereas the plant-available P was evaluated using the extraction method of Olsen et al. [41], followed by P quantification by ICP-OES. The content of exchangeable base cations (Na+, K+, Mg2+, and Ca2+) was determined by using ammonium acetate as an extractant [42] prior to quantification by ICP-OES. Concentrations of nitrate, ammonium, and sulfates were determined by extraction with 1 M potassium chloride and quantification by ion chromatography (930 Compact IC Flex, Metrohm, Herisau, Switzerland), according to Page et al. [43]. The physical and chemical properties of the studied soil are reported in Table 1.

2.5. PFAS Extraction and Quantification by LC-MS/MS Analysis

PFAS were extracted with methanol from 1.0 g of the dried sample (soil, leaves, and fruits) using an accelerated solvent extraction (ASE) system (Dionex ASE 350, Thermo Fisher Scientific, Sunnyvale, CA, USA), as previously reported [38]. Samples were finely ground and spiked with a 13C-labeled PFAS internal standard mixture (IS, Wellington Laboratories) before the extraction to assess the recovery rate. Extracts (~17 mL) were filtered with 0.22 µm cellulose acetate (CA) syringe membranes and stored at 4 °C until liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis. Irrigation water samples were spiked with the same 13C-labeled PFAS IS mixture and filtered before the LC-MS/MS analysis. The accuracy and recovery rates of the PFAS extraction by ASE were evaluated, and data are reported in Table S2.
PFAS quantification was conducted using a triple quadrupole mass spectrometer (TSQ Quantiva, Thermo Fisher Scientific, Sunnyvale, CA, USA) coupled with ultra-high performance liquid chromatography (Ultimate 3000 UHPLC, Dionex, Thermo Fisher Scientific, Sunnyvale, CA, USA). The mass spectrometer conditions and the chromatographic method were previously described in detail by Sharma et al. [44]. The instrument operated in selected reaction monitoring (SRM) mode (optimized parameters for SRM transitions are reported in Table S3). Samples were spiked with a 13C-labeled PFAS external standard mixture (ES, Wellington Laboratories) before injection for quantification purposes. Raw data were processed with Skyline MS software v. 21.2.0.425 [45] to quantify the PFAS content of all tested samples.

2.6. Quality Control and Quality Assurance

The potential PFAS loss by volatilization during the biomass drying at 60 °C was independently assessed in previous tests in our laboratory, showing no significant losses of the studied C4–C14 compounds. We ascribe this outcome to the fact that the PFAS present in the plant biomass are dissolved in an aqueous yet chemically complex solution at a pH value close to neutrality, which causes them to behave as moderate to strong acids. Since the PFAS volatilization depends on their ionic state, which minimizes the volatilization as compared to their nonionic counterparts, even the long-chain PFAS were predominantly present in nonvolatile forms [46].
All materials potentially containing fluorinated polymers were avoided to prevent background contamination, and the instrumental performance and possible carry-over were monitored with blank injections (methanol spiked with the 13C-labeled PFAS ES mixture (Wellington Laboratories) after every three sample injections. The linearity of the standard calibration curves for all the analyzed PFAS was assessed between 0 and 20 µg L−1, and all molecules showed correlation coefficients R2 > 0.99 (Table S4). PFAS contents were quantified using the corresponding matrix-matched calibration curves, which were made using tomato and red chicory plants of the same varieties grown in uncontaminated soil of the same site and irrigated with rainwater. These matrices were tested for PFAS before selecting them for the calibration curves. The limit of detection (LOD) and the limit of quantification (LOQ) values were calculated for each target PFAS as the analyte peak with a signal-to-noise ratio of 3 and 10, respectively, and are reported in Table S4.

2.7. Health Risk Assessment

Assessment of the health risks associated with consumption of PFAS-contaminated tomato and red chicory was evaluated by calculating the target hazard quotient (THQ) for individual PFAS detected in the edible plant parts at the end of the experiment with the following equation: THQi = Ci × DI/RfDi, where Ci is the i-PFAS concentration (mg g−1 FW), DI is the average dietary daily intake per unit of body weight (BW) (g kg−1 BW day−1), and RfDi is the oral reference dose (mg kg−1 BW day−1) of the i-PFAS [47]. The DI of raw tomato fruits (DI = 1.16 g kg−1 BW day−1) for the Italian adult population was obtained by the EFSA Comprehensive European Food Consumption Database (data.europa.eu website: https://data.europa.eu/data/datasets/the-efsa-comprehensive-european-food-consumption-database?locale (accessed on 21 March 2024)). Since the DI of red chicory leaves was not available in the EFSA database, we estimated the DI (DI = 0.71 g kg−1 BW day−1) by assuming 50 g as the consumed daily portion per person and 70 kg as the average body weight of an adult individual, according to Baldi et al. [48]. The chronic RfD value for PFBA (0.001 mg kg−1 BW day−1) was obtained by the US EPA’s IRIS Toxicological Review for PFBA and related salts (https://www.epa.gov/iris (accessed on 21 March 2024)). Since in this study PFBA was the only PFAS detected in tomato fruits and red chicory leaves, the calculated hazard index (HI) coincides with the THQ value, and values of HI < 1 indicate unlikely adverse effects due to PFAS exposure [49].

2.8. Data Analysis

Statistical analysis was conducted with RStudio software v. 2023.06.0 [50], and differences were considered significant at p ≤ 0.05. Potential outliers were tested by Grubbs’s test and removed from the dataset prior to further statistical analyses. Soil physico-chemical properties were analyzed by Student’s t-test after checking for normality (Shapiro-Wilk test) and variance equality (Levene’s test). Left-censored data (PFAS concentrations) were analyzed by Wilcoxon test using the functions available in the NADA package for R, according to Helsel [51].

3. Results

3.1. Soil Properties and PFAS Contamination of the Irrigation Water

Physical and chemical analyses of unamended and biochar-amended soils were carried out at the beginning of the experimental trial, and results are reported in Table 1. The biochar amendment did not significantly alter the measured soil properties with respect to the control soil, except for total and organic C and total N, which were higher in biochar-amended soil as expected.
The PFAS content of the irrigation water was measured several times during the experimental trial, and the results are reported in Table 2. Short-chain PFAS up to PFOA were detected, and the contamination profile was consistent with that reported by the ARPAV during the institutional water pollution survey [16].

3.2. PFAS Concentration in Soil and Tomato Samples

PFAS contamination of the soil was measured at the beginning and at the end of tomato cultivation (Figure 1; Table S5). Before the tomato transplant, control and biochar-amended soils showed a similar content of PFBS and PFOA and traces of PFPeA, whereas all the other analyzed PFAS were below the LOD value, except for PFBA, PFHxA, and PFOS, which showed values below the LOQ (Figure 1A). More PFAS were detected in the soil at the end of the tomato cultivation period (Table S5). The biochar-amended soil had higher concentrations of PFBA and PFPeA, similar concentrations of PFBS and PFHxA, and lower contents of PFOA and PFOS, compared to the control soil (Figure 1B). However, such differences were not statistically significant (Table S5).
Regarding tomato leaves, only PFBS, PFHxA, PFHpA, and PFOA were found, whereas PFBA was below the LOQ value (Figure 1C; Table S5). Leaves of plants grown in biochar-amended soil had a lower content of all detected compounds, with significant reductions ranging between −45% (PFHpA) to −84% (PFOA). Tomato fruits only accumulated PFBA, and the fruits of plants grown in biochar-amended soil showed a statistically significant reduction in PFBA concentration of −61%, compared to plants grown in unamended soil (Figure 1D; Table S5).

3.3. PFAS Concentration in Soil and Red Chicory Samples

As for tomato, the PFAS content in the soil was measured at the beginning and at the end of the red chicory cultivation (Figure 2; Table S6). Before the red chicory seedling transplant, control and biochar-amended soils showed similar concentrations of PFOA and PFOS, and lower concentrations of PFHxA were detected in the biochar-amended soil (Figure 2A). The PFBS and PFPeA were present in higher concentrations in the unamended soil, whereas all the other analyzed PFAS were below the LOD value (Figure 2A). At the end of the red chicory growth period, the biochar-amended soil showed higher concentrations of all detected PFAS (PFBA, PFBS, PFPeA, PFOA, and PFOS), although differences in concentrations between unamended and biochar-amended soils were not statistically significant (Figure 2B).
Analysis of red chicory leaves at the intermediate growth stage showed the uptake of PFBA, PFBS, and PFOA, with PFBS showing a significantly lower accumulation in the plants grown in the biochar-amended soil (−74%), with respect to those grown in the control soil. PFBA and PFOA showed similar concentrations in plants cultivated in both amended and unamended soil, whereas concentrations of all the other analyzed PFAS were below the LOD (Figure 2C; Table S6). In leaves collected at full maturity, concentrations of all the analyzed PFAS were below the LOD, except for PFBA, whose concentration was 34% lower (but not statistically significant) in plants cultivated in the biochar-amended soil than those grown in the unamended soil (Figure 2D; Table S6).

3.4. Dietary Exposure Assessment

When grown in unamended soil, the calculated THQ values were < 1 for both plant species (0.36 for tomato fruits and 0.13 for red chicory leaves), indicating no potential risk associated with their consumption. To note, the THQ values were reduced by 61% (from 0.36 to 0.14) for tomato fruits and by 37% (from 0.13 to 0.08) for red chicory leaves when grown on biochar-amended soil.

4. Discussion

The PFAS pollution of the studied soil was caused by the historical use of contaminated groundwater for the irrigation of horticultural crops and by the advection of the water table occurring in this area during the winter period. The PFAS profile of the water used for irrigation at the experimental site was comparable to the contamination profile of the local groundwater and surface water [16,17], and it was consistent with the results reported by Battisti et al. from previous trials conducted on the same experimental site [38].

4.1. PFAS Accumulation in Plant Biomass

Irrigation of the two consecutive crops with contaminated water led to the accumulation of various PFAS in the unamended and biochar-amended soils and in the cultivated tomato and red chicory plants. Except for PFHxA and PFPeA, at the end of each of the two cultivation trials, the contamination profile of the soil mirrored that of the irrigation water (Figure 1B and Figure 2B; Table 2). PFBA and PFOA accumulation in soil could be explained by their high concentrations in the irrigation water and their relatively high environmental mobility [52], whereas the accumulation of PFBS and PFOS could be explained by their relatively low environmental mobility and high affinity for the soil solid phases [53].
Except for PFOA in red chicory leaves at the intermediate growth stage, plants grown on biochar-amended soil displayed significantly lower PFAS concentrations, regardless of plant organ, plant species, development stage, and PFAS molecule. In our opinion, the reduction in PFAS concentrations in tomato leaves and red chicory at intermediate growth sampling could be due to the retentive biochar capacity for two main reasons: (i) the PFAS content of the tomato leaves well reflected the PFAS pollution profile of irrigation water and soil, and (ii) red chicory intermediate leaves were irrigated with contaminated water only during the first two weeks of growth. This resulted in relatively high concentrations of PFBA, PFBS, and PFOA, i.e., the most represented PFAS in the irrigation water, compared to the leaves of the mature plants, in which only PFBA was detected (Table 2 and Table S6). Similarities between the PFAS contamination of tomato leaves and the PFAS profile of the irrigation water, together with the fact that PFBA was the only compound detected in tomato fruits, confirmed previous results obtained from the same experimental site [38]. However, the fruits of tomato plants grown in biochar-amended soils accumulated 61% less PFBA than those of plants grown in the control soil. Interestingly, the degree of reduction in PFAS accumulation by tomato leaves at harvest and red chicory at the intermediate growth stage was consistent, with the magnitude of the reduction in PFAS leaching from polluted soils amended with non-activated biochar [32] or with other pyrolyzed woody materials and sludge [37].
Overall, these trends confirmed that PFAS uptake in plants decreases with the increase in the carbon chain length and that sulfonic PFAS are less mobile than carboxylic ones [54]. Higher water solubility and mobility in soil of short-chain and carboxylic over sulfonic PFAS also explained why PFBA, and not PFBS, was translocated into tomato fruits and accumulated into red chicory leaves and why PFOA, and not PFOS, was absorbed by both plants (Figure 1 and Figure 2). The observed reduction in PFAS concentration in red chicory leaves from the intermediate to the maturity stages, up to undetectable concentrations of PFBS and PFOA, could be due to a growth-related dilution effect, previously reported for other persistent contaminants such as heavy metals in the tissues of leafy vegetables, e.g., [55,56]. However, the physiological and molecular mechanisms whereby this ‘biological’ dilution occurs are not fully elucidated yet, and their in-depth study does not fall within the aims of this work. Whether this phenomenon is due to a decrease in PFAS uptake or PFAS bioavailability, a reduction in PFAS root-to-shoot translocation, or remobilization from leaves to roots at plant maturity deserves further dedicated research. To our knowledge, no such growth-related dilution effect has been reported for PFAS to date.
PFAS concentrations in tomato fruits and red chicory leaves were low, and the values of HI were <1, indicating no risks associated with their consumption [49]. However, the observed reduction in PFBA in tomato fruits and red chicory leaves grown on biochar-amended soils increased their safety levels, which is an important mitigation effect when considering that these products are only part of the daily diet in the contaminated areas. An additional mitigation effect of the biochar-induced stabilization of PFAS in cultivated soils concerns the residual biomass of plants cultivated in contaminated areas, for which the lower PFAS concentration can prevent their spread in the environment during the ordinary plant biomass recycling routes, such as composting or digesting, which could reduce the occupational exposure of farmers and waste management chain workers.

4.2. Biochar-Induced PFAS Stabilization in Soil: Is That Good or Bad?

By comparing the PFAS concentrations in the control and amended soils at the end of the experiment, the content of the detected PFAS in both the control and biochar-amended soils increased from the beginning until the end of the experimental period (Figure 3), except for PFHxA in the control soil, which was mostly unchanged. Although concentrations of control and biochar-amended soils did not differ significantly at the beginning of the experiment, they leveled off in the control soil at the end of the experimental period (Figure 3A), whereas an accumulation trend emerged in the biochar-amended soil for all PFAS, particularly for PFBS, PFOA, and PFOS (Figure 3B). The accumulation increased with longer carbon chain lengths and was more pronounced for sulfonates (PFBS, PFOS) compared to carboxylates (PFBA, PFOA) with the same chain length.
Accumulation of PFBA at similar levels in both the control and amended soils could be explained by its relatively high and constant concentration in the irrigation water throughout the experimental period (Table 2) and by its accumulation at the level of the plant roots and its release into the soil after plant harvest. The release of PFBA and other short-chain carboxylic PFAS resulting from the degradation of various sources and precursor PFAS with longer chains has been previously reported [57,58,59]. This has been observed in laboratory experiments by making use of thermal or chemical processes [60], and it has also been hypothesized, but not demonstrated, to occur in soils or plants. The primary source of PFAS contamination in the area surrounding the experimental site was found to release mainly PFOA, hexafluoropropylene oxide dimer acid (HFPO-DA), and C6O4 (trade name of ammonium and potassium salts of perfluoro ([5-methoxy-1,3-dioxolan-4-yl]oxy) acetic acid), but short-chain PFAS resulting from the byproducts of industrial production processes were also released. These PFAS are still the main contaminants identified by the ARPAV surveys in the surface waters and groundwaters of the pollution cluster of the Veneto Region [16,17].
Accumulation of long-chain PFAS, owing to their high affinity for the soil solid phases and sorption dynamics onto inorganic and organic soil colloids, confirmed previous results [7,31,32]. Moreover, PFAS stabilization by pyrolyzed materials and soil particles was reported by Krahn et al. in batch experiments [35].
In our experimental trial, PFAS accumulation in biochar-amended soil (Figure 3B) could be ascribed to the adsorption capacity of biochar towards PFAS, except towards PFHxA and PFOA, in this case, as reported by Zhao et al. [61]. PFAS adsorb from water onto a variety of solid phases present in soil, including organic matter, clay minerals, and metal oxides [62]. The observed PFAS trends were in good agreement with the organic C-to-PFAS partition coefficients and PFAS molecular structure reported by Bhhatarai and Gramatica [63]. Burkhardt et al. predicted the sorption behavior of 428 PFAS (of which 7 were experimentally confirmed) onto granular activated carbon using the Polanyi Potential Adsorption Theory and reported that the Freundlich isotherm parameters describing the effectiveness of PFAS sorption depended on the PFAS length, molecular conformation, and head functional group [64]. In a study on sites polluted by aqueous film-forming fire-fighting foams containing PFAS, Anderson et al. showed that the solid-phase organic carbon content significantly influenced PFAS release from the soil to groundwater and that PFAS sorption increased with the increasing chain length [65]. The increase in PFAS concentration in biochar-amended soils over time could be due to their accumulation into tomato and red chicory roots, which could be released into the soil by the fine root mass turnover and the root decomposition into the soil after harvest. Gredelj et al. reported that red chicory mainly accumulated PFAS in its roots, particularly PFBA and long-chain PFAS [66], and Felizeter et al. reported that cultivated plants accumulate large proportions of PFAS in their root apparatus [67]. This PFAS dynamic could also explain the detection of PFBA in the soil at measurable levels after the harvest of both tomato and red chicory, whereas its concentration in the soil was below the LOD at the beginning of the cultivation period.
Therefore, while the amendment of soil with native or functionalized biochar might effectively reduce PFAS accumulation by cultivated plants in polluted environments, we believe that this practice should mainly be considered as an operational safety option and not a remediation option. This strategy could reduce the environmental mobility and phytoavailability of PFAS, especially for long-chain compounds, which are the most bioaccumulative and dangerous ones, and allow for safer crop production and the safer management of crop residues. However, saturation of biochar sorption sites or remobilization of PFAS-enriched biochar particles due to anthropogenic or natural factors could spread out PFAS from biochar-amended soils with a ‘chemical time bomb’ effect [68], unless PFAS biodegradation is triggered in the amended soils. Soil amendment with biochar, along with continuous cultivation, could lead to the development of microbial populations with a PFAS degradation potential sustained by the energy substrates released by plant rhizodepositions, under the principle of metabolic infallibility [69]. Tang et al. reported the partial biodegradation of trifluoroacetate (TFA), PFOA, and hexafluoropropylene oxide dimer acid (HFPO-DA) by a microbial community in a 10-month laboratory incubation experiment, with the formation of shorter carbon chain biodegradation intermediate products and the generation of fluoride anions [70]. In another PFAS biodegradation study based on the use of ligno-cellulosic nanomaterials, Li et al. showed that microbial immobilization facilitated the degradation of PFOA and PFOS by creating an amphiphilic environment capable of stabilizing the PFAS and supporting the proliferation of fungi with a PFAS degradation potential [71]. Therefore, to answer the question, biochar-induced PFAS stabilization in soil should be considered as a good option if future research demonstrates an increase in the microbial biodegradation potential in the amended soils. If proven, soil management could be optimized to support microbial proliferation by maximizing the inputs of energy-rich substrates.

5. Conclusions

Though major Environmental Agencies are more strictly regulating the safety threshold levels and increasingly restricting the use of PFAS, owing to their massive production, global diffusion, and scarce biodegradability, they will continue to accumulate in water, soils, and biota. Agricultural soils can accumulate PFAS upon amendment with contaminated matrices or using contaminated irrigation water, and cultivated plants can take up and concentrate PFAS into biomass and transfer them to the edible parts, thus threatening human and environmental health. While contaminated water can be effectively treated with various physico-chemical technologies, the remediation of PFAS-contaminated soils is still in its infancy.
The results of the field trial conducted in the PFAS-polluted area of the Veneto Region indicated that the amendment of soil with biochar can significantly reduce the uptake of PFAS by tomato and red chicory plants caused by irrigation with contaminated water or PFAS accumulated in the soil. Biochar significantly reduced the PFBA accumulation in tomato fruits and significantly decreased the concentration of various PFAS in tomato and red chicory leaves. We ascribed these results to the PFAS sorption by biochar, as a trend of increasing PFAS concentrations in soils was observed at the end of the experimental trial. Overall, the results indicated the potential of soil amendment with biochar to mitigate PFAS accumulation in edible plants, thus contributing to lowering the risks associated with the consumption of food derived from plants cultivated in a polluted environment. In our opinion, the biochar amendment of PFAS-polluted soils could represent a sustainable operational safety temporary measure, with the potential of reducing risks for human health related to contaminated plant produce and to the management of contaminated plant biomass. However, since the biochar-amended soil could be saturated with PFAS over time, such soil amendment should be considered acceptable only if this practice is capable of triggering PFAS biodegradation processes in soil.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/soilsystems9030100/s1, Table S1: Physical and chemical properties of the biochar; Table S2: Recovery (%) of PFAS extraction by ASE and coefficient of variation (CV %); Table S3: LC-MS/MS optimized parameters for PFAS compound detection; Table S4: Limit of detection (LOD), limit of quantification (LOQ), and correlation coefficients (R2) of matrix-matched PFAS calibration curves; Table S5: Concentration of PFAS in control and biochar-amended soils, and tomato leaves and fruits at harvest; and Table S6: Concentration of PFAS in control and biochar-amended soils, and red chicory leaves at intermediate growth and full maturity stages.

Author Contributions

Conceptualization, A.M. and G.R.; methodology, A.M. and G.R.; validation, I.B. and A.R.T.; formal analysis, I.B.; investigation, I.B., A.R.T., and A.S.; resources, A.R.T.; writing—original draft, I.B. and G.R.; writing—review and editing, I.B., A.M., and G.R.; visualization, I.B.; supervision, A.M. and G.R.; project administration, A.M. and G.R.; funding acquisition, A.M. and G.R. All authors have read and agreed to the published version of the manuscript.

Funding

This research work was funded by the University of Padova, grant number BIRD223343.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Acknowledgments

The authors wish to thank the University of Padova for supporting the acquisition of the TSQ Quantiva mass spectrometer through 2015/CPDB15489 funding. The LaChi laboratory of the DAFNAE Department of the University of Padova is gratefully acknowledged for the physico-chemical analyses of soil samples. The authors wish to thank Elisabetta Donadello and Andrea Ferrari for lending the experimental area and for their active support during the experimental design and development.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
ARPAVVeneto Region Environmental Protection Agency
ASEaccelerated solvent extraction
BWbody weight
C6O4ammonium and potassium salts of perfluoro ([5-methoxy-1,3-dioxolan-4-yl]oxy) acetic acid
CAcellulose acetate
DIdaily intake
DWdry weight
ESexternal standard
FWfresh weight
HFPO-DAhexafluoropropylene oxide dimer acid
HIhazard index
ICP-OESinductively coupled plasma optical emission spectrometry
ISinternal standard
LC-MS/MSliquid chromatography-tandem mass spectrometry
LODlimit of detection
LOQlimit of quantification
PFASpoly- and perfluoroalkyl substances
PFBAperfluorobutanoic acid
PFBSperfluorobutane sulfonic acid
PFDAperfluorodecanoic acid
PFDoAperfluorododecanoic acid
PFHpAperfluoroheptanoic acid
PFHxAperfluorohexanoic acid
PFHxSperfluorohexane sulfonic acid
PFNAperfluorononanoic acid
PFOAperfluorooctanoic acid
PFOSperfluorooctane sulfonic acid
PFPeAperfluoropentanoic acid
PFTeDAperfluorotetradecanoic acid
PFUnAperfluoroundecanoic acid
RfDoral reference dose
SRMselected reaction monitoring
TCtotal carbon
TFAtrifluoroacetate
THQtarget hazard quotient
TOCtotal organic carbon
TWItolerable weekly intake
UHPLCultra-high performance liquid chromatography

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Figure 1. (A) PFAS content quantified in the soil at the beginning of the tomato cultivation. (B) PFAS detected in the soil at the end of the tomato cultivation experiment. (C) PFAS concentrations measured in tomato leaves. (D) PFAS amount detected in tomato fruits. In all plots, values are expressed as ng g-1 DW and indicate mean ± standard error (n = 3 for soil; n = 6 for leaves and fruits); * indicates p ≤ 0.05 for statistical tests on amounts detected in biochar-amended samples vs. control samples.
Figure 1. (A) PFAS content quantified in the soil at the beginning of the tomato cultivation. (B) PFAS detected in the soil at the end of the tomato cultivation experiment. (C) PFAS concentrations measured in tomato leaves. (D) PFAS amount detected in tomato fruits. In all plots, values are expressed as ng g-1 DW and indicate mean ± standard error (n = 3 for soil; n = 6 for leaves and fruits); * indicates p ≤ 0.05 for statistical tests on amounts detected in biochar-amended samples vs. control samples.
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Figure 2. (A) PFAS concentrations measured in the soil at the beginning of the red chicory experiment. (B) PFAS amounts detected in the soil at the end of the red chicory cultivation. (C) PFAS content measured in red chicory leaves collected at the intermediate growth stage. (D) PFAS amounts detected in red chicory leaves collected at full maturity. In all plots, values are expressed as ng g-1 DW and indicate mean ± standard error (n = 3 for soil; n = 6 for leaves); * indicates p ≤ 0.05 for statistical tests on amounts detected in biochar-amended samples vs. control samples.
Figure 2. (A) PFAS concentrations measured in the soil at the beginning of the red chicory experiment. (B) PFAS amounts detected in the soil at the end of the red chicory cultivation. (C) PFAS content measured in red chicory leaves collected at the intermediate growth stage. (D) PFAS amounts detected in red chicory leaves collected at full maturity. In all plots, values are expressed as ng g-1 DW and indicate mean ± standard error (n = 3 for soil; n = 6 for leaves); * indicates p ≤ 0.05 for statistical tests on amounts detected in biochar-amended samples vs. control samples.
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Figure 3. Trends of PFAS accumulation in control soil (A) and biochar-amended soil (B) from the beginning to the end of the experimental period. LOD and LOQ indicate concentrations below the detection limit and the quantification limit, respectively. Error bars represent the standard error associated with the mean values (n = 3). Percentage variations are calculated as January 2024 vs. May 2023 (except for PFOS, which is calculated as January 2024 vs. August 2023).
Figure 3. Trends of PFAS accumulation in control soil (A) and biochar-amended soil (B) from the beginning to the end of the experimental period. LOD and LOQ indicate concentrations below the detection limit and the quantification limit, respectively. Error bars represent the standard error associated with the mean values (n = 3). Percentage variations are calculated as January 2024 vs. May 2023 (except for PFOS, which is calculated as January 2024 vs. August 2023).
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Table 1. Physical and chemical properties of the control and biochar-amended soils. Values are expressed as mean ± standard error (n = 3). Symbol * indicates a significant difference at p ≤ 0.05 between control and biochar-amended soils.
Table 1. Physical and chemical properties of the control and biochar-amended soils. Values are expressed as mean ± standard error (n = 3). Symbol * indicates a significant difference at p ≤ 0.05 between control and biochar-amended soils.
Property (Unit of Measure)Control SoilBiochar-Amended Soil
Sand (%)43.845.8
Silt (%)35.834.4
Clay (%)20.419.8
pH (H2O)7.58 ± 0.087.7 ± 0.2
CTOT (g kg−1)1.56 ± 0.024.93 * ± 0.16
CORG (%)1.49 ± 0.024.77 * ± 0.17
NTOT (%)0.176 ± 0.0010.195 * ± 0.004
NORG (%)0.176 ± 0.0020.181 ± 0.005
PTOT (%)0.119 ± 0.0020.122 ± 0.002
Pavailable (%)0.026 ± 0.0010.027 ± 0.002
NO3−-N (mg kg−1)20.5 ± 3.829.5 ± 2.6
NH4+-N (mg kg−1)10.1 ± 1.215.6 ± 1.2
SO42− (mg kg−1)40.8 ± 4.244.6 ± 11.2
Caexchangeable (mg kg−1)2747 ± 422884 ± 23
Kexchangeable (mg kg−1)433 ± 9649 ± 82
Mgexchangeable (mg kg−1)308 ± 7346 ± 14
Naexchangeable (mg kg−1)40 ± 725 ± 1
Table 2. PFAS contamination levels in the irrigation water expressed in µg L−1. Samples were collected at the beginning (May 2023) and at the end (August 2023) of the tomato cultivation and at the start of the red chicory cultivation (October 2023). Water sampling at the end of the red chicory cultivation was not possible due to the well closure.
Table 2. PFAS contamination levels in the irrigation water expressed in µg L−1. Samples were collected at the beginning (May 2023) and at the end (August 2023) of the tomato cultivation and at the start of the red chicory cultivation (October 2023). Water sampling at the end of the red chicory cultivation was not possible due to the well closure.
Tomato ExperimentRed Chicory Experiment
MoleculeMay 2023August 2023October 2023
PFBA1.831.092.17
PFBS0.872.220.85
PFPeA1.150.721.24
PFHxA1.260.690.55
PFHxS<LOQ0.13<LOQ
PFHpA0.390.190.20
PFOA7.044.633.01
PFOS<LOQ<LOQ<LOQ
PFNA<LOD<LOQ<LOD
PFDA<LOD<LOD<LOD
PFUnA<LOD<LOD<LOD
PFDoA<LOD<LOD<LOD
PFTeDA<LOD<LOD<LOD
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MDPI and ACS Style

Battisti, I.; Trentin, A.R.; Sabia, A.; Masi, A.; Renella, G. Soil Amendment with Biochar Reduces the Uptake and Translocation of Perfluoroalkyl Substances by Horticultural Plants Grown in a Polluted Area. Soil Syst. 2025, 9, 100. https://doi.org/10.3390/soilsystems9030100

AMA Style

Battisti I, Trentin AR, Sabia A, Masi A, Renella G. Soil Amendment with Biochar Reduces the Uptake and Translocation of Perfluoroalkyl Substances by Horticultural Plants Grown in a Polluted Area. Soil Systems. 2025; 9(3):100. https://doi.org/10.3390/soilsystems9030100

Chicago/Turabian Style

Battisti, Ilaria, Anna Rita Trentin, Andrea Sabia, Antonio Masi, and Giancarlo Renella. 2025. "Soil Amendment with Biochar Reduces the Uptake and Translocation of Perfluoroalkyl Substances by Horticultural Plants Grown in a Polluted Area" Soil Systems 9, no. 3: 100. https://doi.org/10.3390/soilsystems9030100

APA Style

Battisti, I., Trentin, A. R., Sabia, A., Masi, A., & Renella, G. (2025). Soil Amendment with Biochar Reduces the Uptake and Translocation of Perfluoroalkyl Substances by Horticultural Plants Grown in a Polluted Area. Soil Systems, 9(3), 100. https://doi.org/10.3390/soilsystems9030100

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