Enhancement in biodegradation of thermoplastics using UV-pretreatment and biosurfactant

Abstract

Non-degradable plastics such as polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyethylene terephthalate (PET) are among the most generated plastic wastes in municipal and industrial waste streams. The mismanagement of abandoned plastics and toxic plastic additives have threatened marine and land fauna as well as human beings for several decades. The available thermal processes can degrade plastic at pilot- and commercial-scale. However, they are energyintensive and can generate toxic gases. Degradation of plastic waste with the help of live microorganisms (biodegradation) is an eco- and environmentally friendly method for plastic degradation, although the slow processing time and low degradation rate still hinder its applications at pilot- and large-scale. The capability of different strains derived from New Zealand’s soil, activated sludge, farm sludge, and worms’ excreta were investigated for biodegradation of the above-mentioned plastics in unstimulated and stimulated conditions. Among several microorganisms Penicillium raperi, Aspergillus flavus, Penicillium glaucoroseum and Pseudomonas spp. were isolated as the most plastic degrading microbes. Maximum weight loss was seen by incubation of polyethylene with A. flavus (5.5 %) in unstimulated mix condition. To enhance the biodegradation efficiency, UV-pretreatment (ageing) of plastics was conducted at various conditions. The findings of this research indicated that UV-pretreatment at optimum condition (UV dose of 7.02 × 1012 μW.cm-2 .s) (t2 d2 condition) resulted in a higher roughness, hydrophilicity, microbial viability, biofilm formation, surface degradation, and more significant physical and molecular weight reduction. The highest biodegradation happened for PE and PS with respective 7.8 and 5.13 % physical weight loss and 4.71 and 2.1 fold molecular weight reduction compared to the “un-pretreated & strain-free bio-treated” (control-3). The hydrophilicity of PS and PE were increased to the “UV-pretreated & bio-treated” samples with a reduction in water contact angle from 105° to 5° in PS and 102° to 60° in PE. Microscopic analysis indicated significant surface property changes or degradation (cracks and holes) on “UV-pretreated & bio-treated” samples. Chemical transformation with Fourier Transform Infrared Spectroscopy (FT-IR) also revealed new peaks in “UV-pretreated & bio-treated” samples, indicating the positive role of UV ii in biodegradation efficiency. Statistics analysis showed between 45 and 90 days of biodegradation, 45 days was adequate to obtain optimum biodegradation efficiency (p< 0.05). The identification of short-chain by-products by gas chromatography - mass spectrometry (GC-MS) also confirmed that UV-pretreatment at optimum condition in presence of the identified strains was achievable. The step further and increase the biodegradation rate, the capability of the identified strains was also evaluated by integration of the optimum-UV-pretreatment condition in presence of 160 mg/L rhamnolipid biosurfactant. The study found that the “UV-pretreated & bio-treated + biosurfactant” was the most effective condition for increasing the biodegradation rate of PS samples. The highest physical weight loss (7.47 %), surface degradation which was about 2.3 % more than condition with no biosurfactant. The higher wettability (< 5˚) and biofilm formation were also observed in this condition. In contrast, PE and PET had a higher biodegradation efficiency only in “UVpretreated & bio-treated” condition. The utilisation of biosurfactant had negative effects on biodegradation and wettability of PE and PET, due to the consumption of rhamnolipid as food source rather than the plastic itself. Chemical transformation indicated new peak (C-O) in PS at both “UV-pretreated & bio-treated” and UV-pretreated & bio-treated + biosurfactant) conditions. However, the chemical transformation of PE and PET remained unchanged in all condition except “UV-pretreated & bio-treated”. Thermogravimetric analysis showed 20 ˚C lower thermal stability of PS incubated at “UV-pretreated & bio-treated + biosurfactant” than other conditions. However, the pre-treated PE and PET had lower thermal stability where biosurfactant was not used. This research proved that despite the main limitations in biodegradation and its low technology readiness levels (TRL), a path toward increasing the efficiency of plastic degradation was feasible upon using UV-pretreatment, biosurfactant and the combination of microbial strains.