Landfill leachate tests reveal divergent fates of biodegradable plastics

Contrary to common assumptions about biodegradable plastics, a collaborative research effort led by the Korea Advanced Institute of Science and Technology (KAIST) and Ulsan National Institute of Science and Technology (UNIST) has unveiled significant disparities in how these materials degrade in landfill leachate under anaerobic conditions.

The study, published in the journal Nature Communications, aimed to shed light on the varying fates of biodegradable plastics and identify potential microorganisms and enzymes involved in the process.

The researchers examined three biodegradable plastics—polyhydroxybutyrate (PHB), poly(lactic acid) (PLA), and poly(butylene adipate-co-terephthalate) (PBAT)—alongside conventional low-density polyethylene (LDPE) as a control. Using landfill leachate, a water mixture containing dissolved substances and microorganisms that have interacted with waste, the team incubated the plastics at 35°C for up to 320 days.

Landfill leachate is distinct from other disposal environments as it contains a complex mixture of dissolved substances and microorganisms, making it an ideal scenario to explore the effects of biodegradable plastics in real-world conditions.

The results showcased starkly different degradation outcomes among the plastics. PHB reached 99.9% mineralization within just 17 days, indicating rapid degradation. PLA, on the other hand, experienced an initial lag of approximately 20 days before degrading gradually, ultimately achieving 83.6% mineralization after 320 days. In stark contrast, PBAT only reached 2.7% mineralization after the same period, and conventional LDPE reached only 0.4%.

These findings underscore the significant variability in degradation rates among biodegradable plastics even when subjected to the same experimental conditions.

Notably, PLA's behavior was particularly intriguing. Despite its generally known propensity for faster degradation under high-temperature composting conditions, the study revealed a substantial mineralization rate at a relatively moderate temperature of 35°C. Electron microscopy revealed biofilms—thin communities of microorganisms attached to the plastic surfaces—associated with cracks and damaged areas on PLA films. These biofilms likely played a role in facilitating the degradation process.

To gain deeper insights, the research team delved into the gene activity of microorganisms associated with the plastics. Through metagenomic and metatranscriptomic analyses, they identified candidate genes encoding enzymes potentially involved in breaking down PHB and PLA. Out of 238 reconstructed microbial genomes, 118 contained these candidate genes.

However, only 49 of these genomes showed evidence of transcription, indicating that the mere presence of a degradation-related gene does not necessarily mean it is actively being expressed. Many of the transcription-supported candidates belonged to poorly characterized microbial lineages, suggesting that there are underexplored microbial communities with potential plastic-degrading capabilities.

The study also highlighted the importance of considering the conditions biodegradable plastics will experience after disposal. The researchers emphasized that biodegradability is not solely an inherent property of the plastic material but also depends on the environmental conditions and microbial communities it encounters. This finding suggests that future assessments of biodegradable plastics should consider the specific disposal environments and microbial factors rather than relying solely on material properties or biodegradability labels.

Myung remarked, "The substantial degradation of PLA under mesophilic anaerobic conditions was particularly surprising." These findings highlight the need for a more nuanced understanding of biodegradable plastics and their degradation pathways in various disposal scenarios, ultimately aiding in the development of more effective strategies for managing plastic waste.