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Adsorption behavior of commercial biodegradable plastics towards pollutants during the biodegradation process: Taking starch-based biodegradable microplastics, oxytetracycline and Cu (II) as examples
Summary
This study found that microbial colonization on biodegradable starch-based microplastics (PBAT/PLA/TPS blend) actually increases their ability to adsorb the heavy metal copper and the antibiotic oxytetracycline, while further biodegradation reduces that capacity. When both pollutants are present together, however, even partially degraded microplastics can still carry significantly elevated contaminant loads. This reveals that biodegradable plastics are not risk-free — their breakdown in the environment changes, but does not eliminate, their role in transporting harmful chemicals.
With the widespread use of biodegradable plastic bags, their potential environmental risks need further assessment. This study focused on commercial starch-based blended biodegradable microplastics (70% Poly(butylene adipate-co-terephthalate) (PBAT)+5% Poly(lactic acid) (PLA)+20% Thermoplastic starch (TPS), PPT MPs) to investigate their adsorption behaviors towards Cu(II) and oxytetracycline (OTC) under microbial colonization and biodegradation. Post-biodegradation, the hydroxyl (-OH) peak intensity of starch in PPT significantly decreased, while carbonyl (C=O) peaks of PBAT and PLA broadened, with O/C ratio rising from 14.65% to 35.82%. The starch's degradation in PPT altered its thermal properties. Microbial colonization on PPT (B-PPT) enhanced Cu(II) and OTC adsorption, while biodegradation (D-PPT) reduced their adsorption. Reduced surface carbonyl and hydroxyl groups, alongside increased crystallinity, diminished D-PPT's Cu(II) adsorption. While OTC adsorption, driven by hydrophobic partitioning, was less affected by biodegradation. In the binary pollutant system, the Cu(II) and OTC adsorption of D-PPT increased by 20.27% and 8.63 times, respectively; B-PPT showed decreased adsorption of both. Coexisting organic matter and pH significantly affected PPT's adsorption behavior by altering Cu(II) and OTC speciation, and influencing adsorption competition, hydrogen bonding and bridging effects. This study is the first to explore biodegradation impacts of commercial starch-based microplastics on typical heavy metals and antibiotics adsorption, providing important theoretical insights for understanding their environmental risks.