Microplastic biodegradation on land

Polymers divide on a single feature: whether the backbone contains a heteroatom. Esters, amides, urethanes and glycosidic linkages contain a bond that can be hydrolysed, and a class of enzymes exists for it. Polyethylene, polypropylene, polystyrene and PVC are built from carbon–carbon bonds, and there is nothing there to hydrolyse. Everything known about microplastic biodegradation in soil falls along that line.

What genuinely works#

In 2016 a hydrolase was isolated from the bacterium Ideonella sakaiensis that cleaves the ester bond of polyethylene terephthalate to mono(2-hydroxyethyl) terephthalate, which a second enzyme takes apart to terephthalic acid and ethylene glycol. The mechanism is the ordinary serine hydrolase one and is related to cutinases, which evolved on the plant polyester cutin; engineered variants of leaf-branch compost cutinase are now used in industrial enzymatic recycling.

The constraint on that route is hard and physical. The enzyme is a large molecule working at a solid interface, and it attacks only amorphous regions of the polymer. Crystalline domains are inaccessible: chains there are tightly packed and gain mobility above the glass transition only in the amorphous fraction. So the industrial process begins not with the enzyme but with feedstock preparation — micronisation and amorphisation by quenching from the melt — and runs near 70 °C, at the upper edge of PET’s glass transition region. Rate scales with accessible surface area: this is surface erosion, not a bulk reaction.

None of that exists in soil. Extracellular enzymes are present at minute concentrations, sorb to clay minerals and humus and lose activity, temperatures sit two or three tens of degrees below the optimum, and bottle-grade PET carries high crystallinity after stretch orientation during moulding. The one factor working in favour is the dispersity of the particle itself, which gives a high surface-to-mass ratio.

What does not work#

For a carbon–carbon backbone, no characterised enzyme is known that depolymerises polyethylene, polypropylene or polystyrene at any environmentally meaningful rate. The theoretical route — oxidation of an unactivated bond by an oxygenase or laccase — requires a pre-existing oxygen-bearing handle, and in every report where mass loss was observed it followed abiotic photo-oxidation. Reports of a few percent loss over months, in insect guts or in culture, sit near the limit of gravimetric method and are routinely confounded by plasticiser leaching and accumulated biofilm mass.

Telling degradation from disappearance#

The discipline of evidence here is the substance of the question. Mass loss from a specimen, its disappearance from a soil litter bag, or a fall in visible size is fragmentation: the particle went below the detection threshold rather than away. The only defensible criterion is mineralisation — polymer carbon evolved as carbon dioxide, most reliably tracked with a carbon-13 label in the polymer itself.

The standards already encode the distinction. EN 13432 describes industrial composting at 58 °C and requires 90 percent mineralisation within 180 days; it is not a soil claim. Its soil counterpart, EN 17033 for biodegradable mulch films, requires 90 percent relative to cellulose at ambient temperature within two years. PET, polyethylene and polypropylene satisfy neither.