Biodegradable plastics were supposed to be agriculture’s answer to the microplastic problem, but a new study reveals that the very methods scientists use to hunt for these materials in soil may be destroying them before they can be counted. In research published in the journal Microplastics and Nanoplastics, a team led by Grace Davies of the University of Birmingham, working with colleagues at the University of Bayreuth, exposed thin films of two of the most widely used biodegradable polymers — polylactic acid (PLA) and polyhydroxybutyrate (PHB) — to a standard laboratory protocol for extracting microplastics from soil. The results were striking: both polymers showed measurable degradation, and PLA suffered severe damage, with one replicate film disappearing entirely during a single enzymatic step. The findings cast serious doubt on the reliability of current environmental monitoring for biodegradable plastics and carry consequences for how regulators and researchers assess whether these materials truly vanish from the environment.

The context is a rapidly growing one. Global plastic production reached 413.8 million tonnes in 2023, and agriculture alone consumes an estimated 12.5 million tonnes annually in mulching films, polytunnels, irrigation systems, seed coatings, and the polymer encapsulation of fertilisers and pesticides. Much of this plastic is single-use and poorly recoverable from fields, leaving persistent fragments in soil. Conventional microplastics — particles smaller than five millimetres, and nanoplastics smaller than one micrometre — have been linked to altered soil properties, effects on plant performance, and broader concerns about human exposure. Regulators have responded: the European Chemicals Agency recently amended the REACH regulation to restrict intentionally produced synthetic polymer microparticles across all sectors, but deliberately exempted biodegradable polymers on the premise that complete environmental biodegradation prevents persistent microplastic pollution. That exemption rests on the assumption that biodegradable plastics fully mineralise into microbial biomass, water, and carbon dioxide — an assumption that recent field studies have begun to question, with fragments of biodegradable polymers now being identified in the environment.

To verify complete biodegradation, scientists need reliable ways to detect and quantify biodegradable polymer fragments in soil, including their number, size, and shape — characteristics central to risk assessment. But soil is a formidable analytical matrix, a dense mixture of organic and inorganic particles that overwhelms spectroscopic techniques such as Fourier transform infrared (FTIR) and Raman spectroscopy. Mass-based approaches such as pyrolysis gas chromatography-mass spectrometry can quantify polymer content but destroy the solid particles in the process, forfeiting size and shape information. The standard workaround is a multi-step extraction: sieving, density separation to float off mineral particles, and digestion to strip away organic matter. One widely adopted approach, described by Möller and colleagues in 2022, combines density separation in zinc chloride solution with Fenton’s reagent oxidation and a sequence of enzymatic digestions — protease, pectinase, viscozyme, and cellulase — plus a sodium dodecyl sulfate (SDS) surfactant treatment. The method achieves high organic matrix removal while remaining compatible with conventional polymers, but no ISO standards for microplastic extraction currently account for biodegradable polymers, which are chemically and structurally designed to fall apart.

The Birmingham-Bayreuth team pressed powdered PLA and PHB into films roughly 100 micrometres thick, cut them into 10-millimetre squares, and ran them through the full extraction protocol in triplicate, alongside separate exposures to each individual reagent. They then interrogated the films with an unusually comprehensive analytical battery. Stereomicroscopy and mass and thickness measurements tracked physical integrity. Differential scanning calorimetry (DSC) probed thermal transitions — glass transition temperature, melting temperature, and percent crystallinity, the latter calculated from melting enthalpy normalised against the enthalpy of a fully crystalline polymer (93.6 J/g for PLA, 146 J/g for PHB). Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) monitored surface chemistry, and gel permeation chromatography (GPC) measured PLA molecular weight distributions — though PHB, with a molecular weight of 550 kilodaltons, proved insoluble and could not be analysed this way. Control films incubated in ultrapure water anchored the comparison.

For PLA, the full treatment sequence was devastating. Films turned opaque, lost significant mass, and thinned from 100 micrometres to just 41 — a reduction of more than half. Crystallinity climbed from 13 to 20 percent, a classic fingerprint of degradation in which the vulnerable amorphous regions of the polymer are attacked first, leaving behind a relatively enriched crystalline skeleton. The number-average molecular weight collapsed from 98,500 to 30,400 grams per mole, while dispersity — the breadth of the molecular weight distribution — widened from 1.72 to 2.67, the signature of random chain scission cutting long polymer chains into shorter, more variable fragments. ATR-FTIR revealed new peaks at 1652 and 1532 wavenumbers in the carbonyl region, corresponding to carboxylic acid bonds, a known degradation product of PLA hydrolysis. One replicate film was lost entirely during the protease step; others fragmented to varying degrees. In a polymer engineered to biodegrade, these are precisely the changes that laboratory handling should never inflict.

The step-by-step exposures pinpointed the culprits. Zinc chloride density separation and Fenton’s reagent oxidation — the harsh-sounding chemical workhorses of the protocol — proved benign; neither caused any detectable degradation in either polymer. SDS, however, rendered PLA films opaque, reduced their number-average molecular weight to 62,400 grams per mole, and increased polydispersity, consistent with hydrolytic chain scission in the amorphous regions: surfactants reduce polymer surface tension and increase wettability, opening the material to hydrolysis even without measurable mass loss. The enzymatic steps proved even more damaging. Protease treatment fragmented PLA films, cut their thickness to 46 micrometres, and produced significant mass loss. Pectinase and viscozyme each increased PLA crystallinity to roughly 30 percent, and cellulase also chipped away at molecular weight. The protease effect is particularly instructive: unlike the other enzymes, which operate at pH 5, protease runs at pH 9, an alkaline environment known to hydrolyse ester bonds. Since PLA’s glass transition temperature is 55 to 60 degrees Celsius and the treatment ran at 50, thermal effects can be ruled out — the alkaline chemistry itself is the likely driver.

PHB fared better, but not untouched. The films showed no visual degradation and only a small relative mass reduction after the full sequence, yet DSC revealed a melting temperature drop from 167.41 to 136.54 degrees Celsius and the appearance of a second melting peak at 148.46 degrees — a phenomenon previously observed when PHB films degrade in soil incubations and suggestive of multiple crystalline populations forming as the polymer breaks apart. Crystallinity rose from 42 to 51 percent, again indicating preferential attack on amorphous regions. The second melting peak appeared only after the full sequential treatment, never after any individual step, implying the damage accumulates across the cascade. ATR-FTIR detected a shoulder at 1623 wavenumbers — carboxylic acid, the degradation product — which also appeared after isolated pectinase or viscozyme exposures. Pectinase alone produced a statistically significant mass loss. The authors note that PHB’s relative stability likely owes to its high starting molecular weight of 550 kilodaltons; lower-molecular-weight PHB is known to hydrolyse far more readily, meaning real-world PHB products could be considerably more vulnerable than the material tested here.

The results also explain earlier contradictions in the literature. Pfohl and colleagues found in 2021 that Fenton’s reagent shrank PLA, PBAT, and PBS particles by 11.7 percent, with sub-100-micrometre PLA particles disappearing entirely, while Möller’s 2022 study saw no Fenton effects on larger PLA particles. The new work highlights that surface-area-to-volume ratio governs degradation kinetics — thin films and small particles present vastly more exposed surface for chemical attack than millimetre-scale fragments. This means extraction protocols validated on one polymer geometry cannot be assumed safe for another, and the problem compounds in the field: mulching films weathered by sunlight are already fragmented and destabilised before they meet the laboratory, making them still more susceptible to extraction-induced damage. Polymer blends and additives, the commercial norm, will complicate matters further.

The team’s conclusion is sobering for the field. Enzymatic-oxidative digestion does its intended job — stripping soil organic matter efficiently without strong acids or bases — yet the process is lengthy and demonstrably degrades both PLA and PHB, particularly at the enzymatic steps. No single extraction method is likely to suit all biodegradable polymers, which undermines the very idea of non-targeted screening in soils. If the extraction itself fragments and chemically alters the particles it is meant to census, the resulting measurements may tell us more about the laboratory protocol than about the environment. The authors call for polymer-specific methods designed around known degradation chemistries — avoiding alkaline protease steps for PLA, for instance — and for rigorous pre-validation of any protocol against the specific polymers it will target. As biodegradable plastics multiply across agriculture under regulatory exemptions premised on their clean disappearance, ensuring that monitoring methods preserve rather than destroy the evidence may prove essential to knowing whether the biodegradability promise is actually being kept.

Subject of Research: The impact of common microplastic extraction methods from soil matrices on the biodegradable polymers polylactic acid (PLA) and polyhydroxybutyrate (PHB)

Subject of Research: Technology and Engineering

Article Title: Determining the impact of common microplastic extraction methods from soil matrices on the biodegradable polymers polylactic acid and polyhydroxybutyrate

Article References: Davies, G., Kernchen, S., Löder, M. G. J., Brenninkmeijer, L., Laforsch, C., Krause, S., & Lynch, I. (2026). Determining the impact of common microplastic extraction methods from soil matrices on the biodegradable polymers polylactic acid and polyhydroxybutyrate. Microplastics and Nanoplastics, 6(1), Article 18. https://doi.org/10.1186/s43591-025-00167-0

Image Credits: AI Generated

DOI: 10.1186/s43591-025-00167-0

Keywords: biodegradable plastics, PLA, PHB, microplastic extraction, soil analysis, polymer degradation, enzymatic digestion, Fenton’s reagent, density separation, protease, GPC, ATR-FTIR

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Neil Sanderson. (September 4, 2026). Microplastic extraction methods alter biodegradable polymer detection in soils. Scienmag. https://scienmag.com/microplastic-extraction-methods-alter-biodegradable-polymer-detection-in-soils/

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