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Enhanced mobility and reversibility of biodegradable compared to non-biodegradable nanoplastics: Influence of goethite-coated sand and chlortetracycline hydrochloride coexistence
Summary
"Biodegradable" plastics are often assumed to be the safer choice, but this study found that biodegradable plastic nanoparticles (from PLA, a common compostable plastic) actually move through soil more easily than traditional plastic particles and can even remobilize later when conditions shift, like after rain washes over iron-rich soil. This matters because these tiny particles could travel further into groundwater supplies than conventional plastics do, suggesting that switching to "biodegradable" plastics in farming may not reduce—and could even increase—the risk of plastic contamination reaching the water we drink.
The rapid transition from conventional to biodegradable plastics in agriculture underscores the urgent need to evaluate their comparative environmental risks. This study investigates the divergent transport and remobilization mechanisms of biodegradable polylactic acid (PLA) and non-biodegradable polystyrene (PS) nanoplastics in saturated porous media with a specific focus on the interplay between goethite (GT) coating and chlortetracycline hydrochloride (CTC) presence. Results demonstrate that PLA exhibits higher mobility than PS in single systems. PLA maintains relatively high colloidal stability and mobility under high concentrations of CTC because its surface polar functional group and stable hydration layer prevent the aggregation induced by CTC, with hydrophobic PS experiencing rapid heteroaggregation and immobilization under identical conditions. In heterogeneous media, GT coatings exerted a non-monotonic effect, where low coating ratios facilitated transport via physical separation driven by surface roughness. Thus, high coating ratios maximized retention through global electrostatic capture. Sequential elution experiments reveal that retained PLA remains susceptible to remobilization under alkaline perturbations, contrasting with the irreversible attachment of PS. These findings challenge the biodegradable safety assumption by revealing that the mobility and chemical lability of PLA challenge the presumed safety of biodegradable plastics, posing a persistent risk to deep aquifers that exceeds conventional counterparts.