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Process considerations affecting plastic biodegradation
The plastic biodegradation process is governed by several considerations which may affect the overall biodegradation performance of microorganisms and should not be overlooked when conducting biodegradation studies on plastics. One of them includes the purity of plastic samples used in plastic biodegradation studies. The presence of impurities in the polymer chain may act as alternative carbon sources for the microorganisms and prevent the utilization of the polymer backbone chain as the main carbon source for microorganisms . Therefore, it is recommended to conduct multiple quantification techniques (e.g., evaluating the formation of functional groups, changes in surface morphology, release of degradation products, and changes in plastic properties) to confirm the true plastic-degrading ability of microorganisms.
Although the adoption of pure plastic samples (without the presence of impurities) in biodegradation studies could confirm the action of microorganisms in breaking down the main polymer chain, the degrading performance of microbial strains may not provide an exact representation of the biodegradation of consumer-end plastics, which are the main contributors toward plastic wastes pollution. In the effort to improve the biodegradation of commercial plastics, benzene and alcohol were successfully used as solvents to remove the synthetic additives such as plasticizers, coloring agents, and fillers from the plastics .
Factors such as shapes and sizes of plastic samples also contribute toward the efficiency of biodegradation process. Most biodegradation studies involve the preparation of plastic samples into smaller pieces, usually in the form of films, pellets, or powder, for easier adherence and consumption by microorganisms . An experiment conducted on the biodegradation of LDPE, HDPE, and PP plastics has shown a higher weight loss of plastics in the form of films as compared to the pelleted plastics .The greater surface area available in plastic films enhances the rate of plastic biodegradation. Another important consideration in biodegradation studies is the total surface area of plastic samples exposed toward the action of microorganisms. Since there is no guideline on the recommended size and thickness of plastic samples for the biodegradation process, varying sizes of plastic samples have been utilized in biodegradation experiments. Due to this reason, it is difficult to perform a critical comparison to evaluate the plastic biodegradation performances of various microbial strains because the total surface area available for microbial actions varied from experiment to experiment.
The adoption of purified enzymes in the biodegradation of plastics has been explored in a recent study.
Fungal peroxidases were purified from Phanerochaete chrysosporium using gel filtration chromatography and were used in the biodegradation of PVC. The study has concluded that the application of purified fungal peroxidases in the biodegradation of PVC was more effective compared to a traditional whole cell approach due to the higher rate of enzymatic reaction .
Moreover, the comparative study showed that the conventional whole cell approach took a longer processing time.
However, the cost of enzyme purification remains an unattractive factor in the large-scale enzymatic degradation of plastics, which led to the adoption of crude enzymes in plastic biodegradation, with the aim of reducing process complexity and the processing cost required. Several attempts have been made for the application of crude enzymes in plastic biodegradation studies, and the positive results observed have confirmed the viability of using crude enzymes in plastic biodegradation.
However, the daily supplementation of crude enzymes is required for the biodegradation of plastics, which may also restrain its application in the large-scale biodegradation of plastics. Nonetheless, the adoption of crude enzymes in plastic biodegradation still requires further studies because the performance and stability of crude enzymes in the biodegradation process are yet to be fully understood. To alleviate the long culture period needed for the preparation of a microbial culture, it is desirable to make use of the prepared cultures for multiple runs of biodegradation. A past study demonstrated the improvement in PE biodegradation through the incubation of acclimated biofilm communities with the naturally weathered PE samples. In that study, the biofilm formed on PE samples was harvested and incubated again with the new PE samples. Results showed that the acclimated biofilm communities were able to colonize and degrade PE samples faster, resulting in a significantly higher weight loss of polymer .
This study unlocked opportunities for reusing biofilms in subsequent biodegradation processes and reducing the need for sustaining massive volume of microbial culture, which can be advantageous especially in the large-scale biodegradation of plastics. Further research on the reusability of enzymes in plastic biodegradation will also be helpful to achieve an economical outcome in the industrialization of plastic biodegradation.
In addition, temperature is another process condition affecting the performances of microorganisms in plastic biodegradation. The biodegradation of LDPE, HDPE, and PP films by Brevibacillus sp. and Aneurinibacillus sp. was conducted at temperatures ranging from 5 to 55 °C; the highest weight loss of plastics was achieved at 50 °C, which is the optimum temperature for bacterial growth of the respective species . A further increase in the working temperature reduced the weight loss of plastics, suggesting the inhibitive effect of a temperature beyond optimal level on the catalytic activities. Furthermore, a significant increase in the biodegradation of PET plastics was observed at 70 °C, which is the glass transition temperature of PET plastics [Citation
At process temperatures near the glass transition temperature, the mobility of PET chains is enhanced and the polymers become more susceptible toward the action of enzymes in the process of plastic biodegradation.
This shows that the thermostability of enzymes is also critical for the effective biodegradation of plastics at temperatures near the glass transition temperature. A critical review on the approaches to enhance the thermostability of enzymes has been reported elsewhere by other researchers.
source: Xue Er Crystal,Sewn Cen Lo
