Enhanced hydrolysis of polyethylene terephthalate (PET) plastics by ozone and ultrasound pretreatment
The rapidly accumulating post-consumer polyethylene terephthalate (PET) plastics pose a great threat to our environment as they constitute one of the most used products in our day-to-day life. As a result, degradation of PET and recycling has become the focus of considerable interest in the last decade. Hydrolysis of PET is very challenging as they are extremely resistant to both biotic and abiotic degradation. A technically and economically feasible approach to degrade PET waste from the environment is highly desirable. Physicochemical pre-treatment can play an important role in making PET more degradable by changing their surface properties. Direct recycling of segregated PET has problems of contamination of additives and components used in various PET products. PET hydrolysis however can lead to recovery of the monomers terephthalic acid (TPA) and ethylene glycol (EG) as well as the dimers bis(2-hydroxyethyl) terephthalate (BHET) and mono(2- hydroxyethyl) terephthalate (MHET) which can be reused for making new PETs. This can potentially solve the difficulties associated with PET recycling and lead to a circular economy. The present study reports the effect of ozone and ultrasound pretreatment on both enzymatic and chemical hydrolysis of PET. The results showed that combination of ozone pretreatment followed by ultra-sonication during enzymatic hydrolysis using HiC cutinase enzyme resulted in almost 9-fold increase in TPA and EG recovery compared to enzymatic hydrolysis of untreated PET. However, the long reaction time in enzymatic hydrolysis prompted us to investigate chemical hydrolysis. Although, chemical hydrolysis of pretreated PET films using methanolic sodium hydroxide as solvent resulted in 80% weight loss (at 50C, atmospheric pressure), the recovery of monomers was relatively not as efficient as enzymatic hydrolysis. Size reduction of the PET films followed by chemical hydrolysis gave the highest (90%) breakdown, but it is a very energy intensive process.