The pH-dependent soil-water partitioning of six perfluoroalkyl substances (PFASs) of environmental concern (PFOA, PFDA, PFUnDA, PFHxS, PFOS and FOSA), was investigated for 11 temperate mineral soils and related to soil properties such as organic carbon content (0.2-3%), concentrations of Fe and Al (hydr)oxides, and texture. PFAS sorption was positively related to the perfluorocarbon chain length of the molecule, and inversely related to solution pH for all substances. The negative slope between log K_d and pH became steeper with increasing perfluorocarbon chain length of the PFAS (r² = 0.75, p ≤ 0.05). Organic carbon (OC) alone was a poor predictor of the partitioning for all PFASs, except for FOSA (r² = 0.71), and the OC-normalized PFAS partitioning, as derived from organic soil materials, underestimated PFAS sorption to the soils. Multiple linear regression suggested sorption contributions (p ≤ 0.05) from OC for perfluorooctane sulfonate (PFOS) and FOSA, and Fe/Al (hydr)oxides for PFOS, FOSA, and perfluorodecanoate (PFDA). FOSA was the only substance under study for which there was a statistically significant correlation between its binding and soil texture (silt + clay). To predict PFAS sorption, the surface net charge of the soil organic matter fraction of all soils was calculated using the Stockholm Humic Model. When calibrated against charge-dependent PFAS sorption to a peat (Oe) material, the derived model significantly underestimated the measured K_d values for 10 out of 11 soils. To conclude, additional sorbents, possibly including silicate minerals, contribute to the binding of PFASs in soil. More research is needed to develop geochemical models that can accurately predict PFAS sorption in soils.

The spread of per- and polyfluoroalkyl substances (PFAS) in the environment poses a severe threat to soil organisms, aquatic life, and human health. Many PFAS compounds are mobile and easily transported from soils to groundwater and further to surface waters. Leaching tests are valuable tools for assessing the site-specific leaching behaviour of contaminants. Here, we report the results of an evaluation of two standardized leaching tests for PFAS-contaminated soil materials: the batch test (ISO 21268-2:2019) using either demineralized water or 1 mM CaCl₂ as leachants (liquid-to-solid (L/S) ratio of 10) and the up-flow percolation test (ISO 21268-3:2019) using 1 mM CaCl₂ as leachant. One field-contaminated soil and three spiked (12 PFAS compounds) soils (aged 5 months) were included in the study. Desorption kinetics in the batch test were fast and equilibrium was obtained for all PFAS compounds within 24 h, the prescribed equilibration time. The same solubility was obtained for short-chain PFAS (PFBA, PFHxA, PFHpA, PFBS) in demineralized water and 1 mM CaCl₂, whereas significantly lower solubility was often observed for long-chain PFAS in CaCl₂ than in water, probably due to decreased charge repulsion between soil surfaces and PFAS compounds. In the up-flow percolation test, concentrations of short-chain PFAS in leachates decreased rapidly with increasing L/S, in contrast to long-chain PFAS, where concentrations decreased gradually or remained constant. Solid–solution partitioning coefficients (K_d), calculated from the data of the batch and percolation tests (1 mM CaCl₂), were generally in agreement, although differing by more than three orders of magnitude between different PFAS compounds. Uncertainties and pitfalls when calculating K_d values from leaching test data are also explored.

Environmental context: The widespread use of the insecticide DDT has left a legacy of pollution that still threatens ecosystems today. This study presents a method to accurately measure the bioavailability of DDT and its breakdown products in contaminated soils. This will improve risk assessments and guide sustainable land management practices, helping to protect both the environment and human health. Rationale: The insecticide dichlorodiphenyltrichloroethane (DDT) and its degradation products (collectively DDX) are persistent organic pollutants that pose significant environmental risks due to their persistence and bioaccumulation in ecosystems. Accurate quantification of DDX bioavailability in soil systems is crucial for effective land management and risk assessment. Methodology: This study utilised equilibrium passive sampling with polyoxymethylene (POM) to determine the bioavailability of DDX in soil. The sorption dynamics of 10 DDX compounds were investigated (p,p′-DDT, o,p′-DDT, p,p′-dichlorodiphenyldichloroethane (p,p′-DDD), o,p′-DDD, p,p′-dichlorodiphenyldichloroethene (p,p′-DDE), o,p′-DDE, p,p′-dichlorodiphenylmethane (p,p′-DDM), p,p′-dichlorobenzophenone (p,p′-DBP), 1-chloro-2,2-bis(4-chlorophenyl)ethylene (p,p′-DDMU) and dicofol) and their POM–water partition coefficients (KPOM) were determined. The study involved interlaboratory comparisons, using soils from nine historically contaminated sites and ecotoxicology assessments (mortality, reproduction and bioaccumulation in earthworms, Eisenia fetida) to validate the POM method. Results: K POM values for 9 of the 10 DDX compounds were successfully determined, allowing for accurate quantification of freely dissolved pore water concentrations of DDX in historically contaminated soils. The interlaboratory study highlighted important considerations in extraction and gas chromatography–mass spectrometry analysis, and the ecotoxicology study demonstrated the potential of POM passive sampling as a reliable tool for assessing DDX bioavailability (bioaccumulation in Eisenia fetida). Discussion: The POM method proved to be a robust and reliable approach for quantifying freely dissolved DDX, with implications for improving the accuracy of risk assessments and guiding sustainable land management practices. The study also highlighted the need for careful consideration of analytical challenges, such as the potential degradation of DDX compounds during gas chromatography analysis, to ensure accurate quantification.

The effectiveness of biochar amendment for remediation purposes depends on many factors related to the biochar and the contaminated site. Therefore, each application must be evaluated site-specifically. To facilitate full-scale implementation, more information from field studies on biochar-amended contaminated sites, as well as cost-effective approaches to evaluate the remediation efficacy of specific biochar materials are needed. We studied the effects of biochar and peat on metal solubility and bioavailability in a contaminated soil in a fully factorial field trial. The biochar was produced from wood via gasification in a floating fixed-bed reactor at 750 °C. Soil solutions from field-installed lysimeters, grass (Lolium perenne L), and earthworms (Eisenia fetida) were analyzed. In addition, a standardized batch leaching test (ISO 21268-2:2019) was performed to evaluate its feasibility to mimic soil solution concentrations. The results showed that biochar generally reduced the solubility and uptake of cationic metals. In situ solubility of Cu and Hg was reduced more than 80%, and Zn up to 70%. Soil solution concentrations of Cr increased in biochar-amended soils, but this effect was reduced by peat. Peat had small effects on in situ solubility of other metals. For cations, the batch test showed the same trends as the soil solution, with biochar decreasing solubility. However, mobilization of colloids during shaking in the batch test induced artefacts, leading to an overestimation of the solubility of some metals, especially Pb and Hg, an effect that was enhanced by peat applications.

Per- and polyfluoroalkyl substances (PFAS) is a chemical family that contains many mobile compounds, including through groundwater transport. As much of the world's water supply originates from groundwater, it is crucial to predict PFAS transport through these systems, governed by their solid-liquid partitioning coefficient (K_d). In this study, K_d of PFAS in low (<0.2%) organic carbon (OC) sandy aquifers from PFAS-contaminated sites were determined using i) batch leaching tests ii) field-derived K_d values based on analyses of drill cores and groundwater, and iii) the saturated paste extraction method (SPEM). A strong correlation was observed between SPEM-derived K_d values and PFAS perfluoroalkyl chain length, demonstrating that SPEM is sufficiently sensitive to resolve small differences in PFAS K_d even at low absolute values. Extraction time of SPEM was critical, with K_d values increasing during prolonged extraction, likely due to air entering the soil column, increasing the air-water interface area. A suitable extraction time was identified as 10-15 s, corresponding to the extraction of ∼40% of the total liquid volume. The mean K_d values from the 10-15 s SPEM extraction ranged from 0.0034 (perfluoropentanoic acid (PFPeA)) to 1.4 (perfluorooctanoic sulfonic acid (PFOS)), with an increase in K_d of ∼0.41 log units per perfluoroalkyl carbon. 6:2 fluorotelomer sulfonamido betaine (FTAB) exhibited a higher K_d of 6.3, likely due to its bulky and zwitterionic head group. The SPEM-derived K_d agreed with the field-derived values. This study highlights SPEM as a promising method for determining PFAS K_d in low sorption environments and the high mobility of PFAS in low-OC sand aquifers.

Microbial communities are potentially useful indicators of the status of soil functioning in response to anthropogenic disturbances. However, the effect of compounded disturbances, particularly of different disturbance types, is of increasing interest given the multiple threats to soils. We hypothesized that microbial communities under continuous exposure to soil contaminants (’press’ disturbance) will be more affected by a second stressor, exemplified by drought (’pulse’ disturbance), than those in uncontaminated soil. To test this, we performed a two-phase experiment in which we first established outdoor soil mesocosms to assess the impact of separate copper (Cu) and polycyclic aromatic hydrocarbon (PAH) contamination gradients on total prokaryotic and ammonia oxidizing archaea (AOA) and bacterial (AOB) communities in two soils with contrasting properties over a 16-month period. We then subjected contaminated and uncontaminated soils to drought conditions in laboratory microcosms to test whether contamination legacy altered microbial resistance to drying and rewetting stress and if they could recover. We show that Cu and PAH contamination decreased the diversity and altered the composition of all communities as well as decreased AOA abundance and nitrification activity in both soils. Ammonia oxidizing archaeal and bacterial communities responded differently to drought stress across both soil and contamination types, though a consistent negative effect of Cu legacy was observed for nitrification activity. However, the composition and diversity of total microbial communities in contaminated soils were overall less resilient to drought stress, with stronger legacy effects of Cu contamination. These results underscore increased vulnerability of microbial communities in contaminated soils to climate change, with implications for soil functioning.