STUDY OF BIOLOGICAL RECYCLING PROCESS OF POLY (DL-LACTIC ACID) USING ACTINOMYCETES ENZYMES

Abstract

Nowadays, plastics are used to produce many kinds of products which make our lives easier. Since the 1950s, plastic usage has steadily grown every year because of its properties, such as versatility, light weight, flexibility, strong and inexpensive. Meanwhile, plastic pollution has become a serious problem for the environment because it takes many years to degrade. Moreover, it has an effect on wildlife, marine life, and human life, as well as leading to global environmental problems. Therefore, biodegradable plastic an interesting replacement for petrochemical plastic. Poly (lactic acid) or PLA is a biodegradable plastic derived from agricultural products and can be degraded by a variety of enzyme produced by microorganisms. The aim of this study is to complete the biological process consisting of PLA degradation and polymerization using the enzyme produced by Actinomycetes. The production of PDLLA degrading enzyme production by an immobilized Actinomadura keratinilytica strain T16-1 on a scrub pad as an immobilizer studied with the maximum enzyme activity of 942.67 U/mL at an aeration rate of 0.25 vvm, an agitation speed of 170 rpm, at a temperature of 45°C for forty eight hours under repeated batch fermentation. Afterwards, the PDLLA degrading enzyme was used for PDLLA degradation by the simultaneous usage of the PDLLA degradation and dialysis method. The results showed the highest percent weight loss in reactions containing PDLLA powder with lactic acid concentration of 18,018 mg/L. Moreover, PDLLA re-polymerization using thermo-solvent tolerant lipase produced by the Streptomyces sp. strain A3301 was investigated. The optimum conditions for lipase production by Streptomyces sp. strain A3301 was performed in production medium containing 1.5% xylose, 2% yeast extract, 0.01% NH2HPO4, pH 7.0 incubated at a temperature of 30°C for three days with the highest enzyme activity of 321 U/mL. Subsequently, PDLLA degraded product (lactic acid) was used as substrate for PDLLA re-polymerization, which was performed using the thermo-solvent tolerant lipase produced by the Streptomyces sp. strain A3301 as bio-catalyst. The PDLLA re-polymerization was achieved with a molecular weight of PDLLA product of 577 DA (n=8) under optimized conditions at 10% of freeze dried crude enzyme, at 60°C for eight hours under nitrogen atmosphere. The results indicated that this thesis could offer a solution to the biological recycling of PLA, including the PDLLA degradation and PDLLA polymerization. Therefore, this research might be adopted as a model to develop the technology to reduce the plastic waste in the future.

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