Tiny Plastics and PFAS Forever Chemicals in Soil: Review Reveals Gaps

Few environmental storylines sound as alarming as the image of “forever chemicals” hitchhiking through the ground on fragments of plastic, and a new systematic review has now stress-tested that narrative with unusual rigor. Writing in Environmental Geochemistry and Health, researchers led by Pedro Augusto Soares at the São Carlos School of Engineering, University of São Paulo, synthesized the available evidence on how per- and polyfluoroalkyl substances (PFAS) interact with microplastics and nanoplastics in soils, and their conclusion resists the tidy vector story often repeated in headlines: plastic particles can carry PFAS, block them, or leave them essentially untouched, depending on conditions that most laboratory studies never reproduce. Following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) framework, the team searched Scopus and Web of Science, identified 430 records — 208 and 222 respectively — and filtered them down to 37 original studies organized into a two-level evidence framework of eight core soil or soil-relevant investigations and 29 complementary mechanistic studies. Their central finding is that the PFAS vector effect of microplastics and nanoplastics is conditional rather than universal, and that in many soils the mineral and organic phases, not plastic, will decide where these contaminants end up.

Both contaminant families arrive in soil through the same gateways. Biosolids and compost spread on farmland, landfill leachate, irrigation with reclaimed wastewater, degrading agricultural films, atmospheric deposition, and fibers shed by fluorinated textiles all deliver PFAS and plastic particles to the same terrestrial reservoirs. Wastewater treatment plants are especially important crossroads because they concentrate both pollutants in the sludge later applied to land: a field study of linked landfill and wastewater systems detected microplastics and PFAS throughout the facilities, with both preferentially transferred to solid residuals such as biosolids. PFAS have even been measured on plastic materials in yard-waste compost, and at Taihu Lake in China the morphology and biofilm load of microplastics tracked the distribution of specific PFAS, including the emerging compound HFPO-DA. Yet the review’s authors separate co-occurrence from causation. Showing that two pollutants share a pathway does not prove that plastic transports the chemicals; that requires measurements of particle-associated PFAS, adsorption and desorption kinetics, changes in mobility, or altered biological uptake — data that remain rare in the field. Plastics can even act as secondary PFAS sources: side-chain fluorinated polymer coatings on textiles shed precursors that transform into mobile perfluoroalkyl acids.

The chemistry explains why no two PFAS–plastic pairings behave alike. PFAS owe their persistence to carbon–fluorine bonds among the strongest in organic chemistry, but their soil behavior comes from an amphiphilic architecture: a water-repelling fluorinated chain welded to a polar, usually negatively charged head group. At typical environmental pH, perfluoroalkyl carboxylic and sulfonic acids exist as anions, so the fluorinated tail seeks hydrophobic and dispersion-driven contacts while the charged head responds to electrostatic forces. Chain length sets the baseline: short-chain compounds stay dissolved and mobile in pore water, whereas long-chain homologs partition strongly onto organic matter, proteins, air–water interfaces and plastic surfaces. Functional groups add nuance. In one cited experiment, neutral perfluorooctanesulfonamide partitioned onto polyethylene, polystyrene and PVC far more readily than anionic PFOS of similar chain length, and sulfonates and sulfonamides generally out-adsorbed many carboxylates. Polymer identity matters just as much. Nonpolar polyethylene, polypropylene and polystyrene interact mainly through hydrophobic partitioning and dispersion forces, whereas polyamide’s amide groups enable hydrogen bonding and stronger polar interactions: a comparison of 18 polymers with eight PFAS found polyamide to be the strongest sorbent, with computed interaction energies matching the experimental trend. Modeling of PFOS on polypropylene nanoplastics likewise identified dispersion forces as dominant, with PFOS occupying both external and internal polymer domains.

The trouble is that such tidy results rarely survive contact with a real soil. Much of the literature rests on pristine, monodisperse beads suspended in water or packed into homogeneous quartz-sand columns at contaminant concentrations far above environmental levels. Real plastic debris is irregular, additive-laden and mineral-filled — commercial plastics containing talc and glass fibers sorbed more pollutants than pure polymers — and plastics incubated in a lake accumulated substantially more PFAS than clean particles exposed in laboratory water. The review’s most striking counterexample comes from the only core study that compared isolated particles, soil and a soil–plastic mixture directly. Polyamide microplastics initially showed enormous affinity for PFOA, PFHxS, 6:2 fluorotelomer sulfonic acid and GenX, but mixing them into paddy soil cut their adsorption capacity roughly sevenfold to 108-fold — an inhibition of 22 to 97 percent — as scanning electron microscopy revealed mineral grains and biofilm-like material smothering the polymer surface. Equilibrium adsorption stretched from about 24 hours for the isolated polymer to roughly 96 hours in the mixture, betraying mass-transfer bottlenecks absent from simplified systems. The authors frame the resulting behavior in three regimes — plastic-dominated, matrix-dominated and coating-mediated — between which any given particle may drift as it weathers, ages and acquires coatings.

Transport experiments deliver the same conditional verdict. For a particle to act as a PFAS vector, the chemical must stay bound while the particle itself migrates; adsorption onto an immobile particle does the opposite, retarding movement. In saturated quartz-sand columns, PFOA adsorption dampened the negative surface charge of negatively charged polystyrene microplastics, increasing their retention, while it softened the positive charge of aminated particles, boosting their mobility — yet both particle types reduced PFOA transport, because the plastics were less mobile than the dissolved acid. Ionic strength and cation type can flip the outcome: divalent calcium retained polystyrene particles far more effectively than sodium through charge neutralization and possible cation bridging, and at pH 5, competition between PFOA and microplastics for attachment sites freed the chemical to break through faster. Natural sediments complicate matters further. There, tiny 0.13-micrometer polystyrene particles sailed through while larger fragments were strained out, and the sediment buffered acidic influent toward neutrality, erasing pH effects engineered at the inlet. In another sediment study, highly mobile 0.1-micrometer nanoplastics left PFOA transport essentially unchanged, with more than 91 percent of the chemical remaining unbound, while larger 5-micrometer microplastics produced only a small increase in retention. The same polymer–PFAS pair can therefore facilitate, retard, or ignore transport.

Biology obeys the same law of contingency, and the terrestrial evidence remains startlingly thin. The sole core soil-organism study found that PVC microplastics raised the bioaccumulation factors of PFOA and PFOS in earthworms by up to 200 percent and, at doses of 500 and 1,000 milligrams per kilogram, suppressed juvenile production — direct evidence that ingested plastic can open an additional exposure route. But the particles were pristine, doses high, and only one soil and species were tested. Plant studies point in both directions. In hydroponic soybean sprouts, PFOS adsorbed onto polystyrene particles lowered the freely dissolved fraction and blunted acute phytotoxicity, even as the PFOS coating encouraged plant tissues to internalize the nanoplastics; in water hyacinth, polystyrene microplastics raised whole-plant bioconcentration of PFOA, PFOS, GenX and F-53B, yet the combined toxicity was antagonistic for the legacy compounds and synergistic for the alternatives. Aquatic models fill mechanistic gaps with warnings about particle design: smaller 20-nanometer particles drove roughly 3.2-fold increases in PFOA uptake by Pacific oysters versus 2.3-fold for 500-nanometer particles, positively charged nanoplastics caused greater bioaccumulation of the alternative PFAS F-53B in aquatic insect larvae than negative ones, and natural organic matter coating nanoplastics intensified chronic liver, gut and microbiome damage in adult zebrafish.

Environmental aging cuts both ways, which is precisely why the review refuses to generalize. Naturally weathered polystyrene showed higher partition coefficients for legacy and emerging PFAS than virgin particles, with some compounds detectable only on aged surfaces, and biofilm-covered polyethylene, polypropylene, polyester and polyamide microfibers adsorbed more PFOS while releasing it more slowly — evidence that extracellular polymeric substances create new binding domains that prolong retention. Yet mineral coatings and competitive adsorption can mask those same domains, as the polyamide experiment showed, and biodegradation of polylactic acid increased PFOA uptake by decorating its surface with hydroxyl, carbonyl and carboxyl groups. The review also flags an underappreciated twist: some PFAS found clinging to environmental plastics may never have been adsorbed at all. Fluorinated textile coatings, water-repellent fabrics and multilayer materials shed fibers that already contain residual or transformation-derived PFAS, and decades-scale degradation studies show side-chain fluorotelomer polymers slowly releasing perfluoroalkyl acids in soils and water. A plastic particle can therefore be a sorbent, a temporary reservoir, or a source, and telling these roles apart requires polymer analysis, precursor measurements and controlled desorption tests rather than simple detection.

Behind these conclusions lies a sobering map of where the science has and has not looked. Of the 148 publications that survived full-text screening, 45.9 percent fell into the Ecosystem domain and 33.1 percent into Water, while Health accounted for 10.8 percent and Air for 2.7 percent — leaving Soil with just 7.4 percent of the literature. Scientific output, which surged after 2020, concentrates heavily in China and the United States, limiting how well findings transfer to regions with different soils, climates and waste practices. The eight core studies reduce to five saturated column experiments, one batch soil adsorption test, one earthworm bioaccumulation study and one compost survey; polystyrene and PFOA dominate the particle–chemical matrix far beyond their real-world prevalence, and quartz sand, pristine spheres, short exposures and elevated concentrations remain the default. Analytical obstacles compound the problem: extracting microplastics from organic-rich soils without altering them, distinguishing submicron particles, and quantifying PFAS amid laboratory background contamination — much of it from PTFE labware — and severe LC–MS/MS matrix effects all push current methods to their limits.

The road forward, the authors argue, runs through field reality. Priority work includes monitoring PFAS and particle-associated fractions together across soil profiles and land uses — biosolid-amended fields, firefighting-foam sites, landfill surroundings and reclaimed-water-irrigated cropland — alongside intact soil cores that preserve macropores, aggregates and preferential flow, unsaturated and transient moisture conditions in which PFAS gather at air–water interfaces, particles conditioned in real soils before testing, environmentally realistic concentration gradients, and complete mass balances that track dissolved, soil-bound, plastic-associated and lost PFAS rather than nominal doses. Desorption deserves the attention adsorption has hoarded, because a particle only vectors contamination if the chemical stays aboard during transport and disembarks somewhere exposure occurs — including inside a gut. Broader panels of polymers and PFAS chemistries, direct tests of trophic transfer through terrestrial food webs, and models that treat PFAS and plastics as interacting but distinct phases round out the agenda. Until such data exist, the team advises treating the plastic vector role as a site-specific hypothesis rather than an inherent property of plastic pollution — because in most soils, organic matter and minerals, not plastic, still rule the fate of forever chemicals.