Biodegradation of waste PET based copolyesters in thermophilic anaerobic sludge

A comprehensive review on recent advancements in biodegradation and sustainable management of biopolymers

Recent years have seen upsurge in plastic manufacturing and its utilization in various fields, such as, packaging, household goods, medical applications, and beauty products. Due to various adverse impacts imposed by synthetic plastics on the health of living well-being and the environment, the biopolymers have been emerged out an alternative. Although, the biopolymers such as polyhydroxyalkanoates (PHA) are entirely degradable. However, the other polymers, such as poly (lactic acid) (PLA) are only partially degradable and often not biosynthesized. Biodegradation of the polymers using microorganisms is considered an effective bioremediation approach. Biodegradation can be performed in aerobic and anaerobic environments. In this context, the present review discusses the biopolymer production, their persistence in the environment, aerobic biodegradation, anaerobic biodegradation, challenges associated with biodegradation and future perspectives. In addition, this review discusses the advancement in the technologies associated with biopolymer production, biodegradation, and their biodegradation standard in different environmental settings. Furthermore, differences in the degradation condition in the laboratory as well as on-site are discussed.

The role of (bio)degradability on the management of petrochemical and bio-based plastic waste

In order to mitigate the social and ecological impacts of post-consumer plastic made of conventional petrochemical polymers, the market of (bio)degradable plastics have recently become more widespread. Although (bio)degradable plastics could be an environmentally friendly substitute of petrochemical ones, the consequences of their presence in the waste management system and in the environment (if not correctly disposed) are not always positive and plastic pollution is not automatically solved. Consequently, this work aims to review how plastic (bio)degradability affects the municipal solid waste management cycle. To this end, the state-of-the-art of the intrinsic (bio)degradability of conventional and unconventional petrochemical and bio-based polymers has been discussed, focusing on the environment related to the waste management system. Then, the focus was on strategies to improve polymer (bio)degradability: different types of eco-design and pre-treatment approach for plastics has been investigated, differently from other works that focused only on specific topics. The information gathered was used to discuss three typical disposal/treatment routes for plastic waste. Despite many of the proposed materials in eco-design have increased the plastics (bio)degradability and pre-treatments have showed interesting results, these achievements are not always positive in the current MSW management system. The effect on mechanical recycling is negative in several cases but the enhanced (bio)degradability can help the treatment with organic waste. On the other hand, the current waste treatment facility is not capable to manage this waste, leading to the incineration the most promising options. In this way, the consumption of raw materials will persist even by using (bio)degradable plastics, which strength the doubt if the solution of plastic pollution leads really on these materials. The review also highlighted the need for further research on this topic that is currently limited by the still scarce amount of (bio)degradable plastics in input to full-scale waste treatment plants.

Potential Use of Microbial Enzymes for the Conversion of Plastic Waste Into Value-Added Products: A Viable Solution

The widespread use of commercial polymers composed of a mixture of polylactic acid and polyethene terephthalate (PLA-PET) in bottles and other packaging materials has caused a massive environmental crisis. The valorization of these contaminants via cost-effective technologies is urgently needed to achieve a circular economy. The enzymatic hydrolysis of PLA-PET contaminants plays a vital role in environmentally friendly strategies for plastic waste recycling and degradation. In this review, the potential roles of microbial enzymes for solving this critical problem are highlighted. Various enzymes involved in PLA-PET recycling and bioconversion, such as PETase and MHETase produced by Ideonella sakaiensis; esterases produced by Bacillus and Nocardia; lipases produced by Thermomyces lanuginosus, Candida antarctica, Triticum aestivum, and Burkholderia spp.; and leaf-branch compost cutinases are critically discussed. Strategies for the utilization of PLA-PET's carbon content as C1 building blocks were investigated for the production of new plastic monomers and different value-added products, such as cyclic acetals, 1,3-propanediol, and vanillin. The bioconversion of PET-PLA degradation monomers to polyhydroxyalkanoate biopolymers by Pseudomonas and Halomonas strains was addressed in detail. Different solutions to the production of biodegradable plastics from food waste, agricultural residues, and polyhydroxybutyrate (PHB)-accumulating bacteria were discussed. Fuel oil production via PLA-PET thermal pyrolysis and possible hybrid integration techniques for the incorporation of thermostable plastic degradation enzymes for the conversion into fuel oil is explained in detail.

Transcriptome-Guided Insights Into Plastic Degradation by the Marine Bacterium

Polyethylene terephthalate (PET) is a common single-use plastic that accumulated in the environment because of its non-degradable characteristics. In recent years, microbes from different environments were found to degrade plastics and suggested their capability to degrade plastics under varying environmental conditions. However, complete degradation of plastics is still a void for large-scale implications using microbes because of the lack of knowledge about genes and pathways intricate in the biodegradation process. In the present study, the growth and adherence of marine Bacillus species AIIW2 on PET surface instigating structural deterioration were confirmed through weight loss and hydrophobicity reduction, as well as analyzing the change in bond indexes. The genome-wide comparative transcriptomic analysis of strain AIIW2 was completed to reveal the genes during PET utilization. The expression level of mRNA in the strain AIIW2 was indexed based on the log-fold change between the presence and absence of PET in the culture medium. The genes represent carbon metabolism, and the cell transport system was up-regulated in cells growing with PET, whereas sporulation genes expressed highly in the absence of PET. This indicates that the strain AIIW2 hydrolyzes PET and assimilated via cellular carbon metabolism. A protein-protein interaction network was built to obtain the interaction between genes during PET utilization. The genes traced to degrade PET were confirmed by detecting the hydrolytic product of PET, and genes were cloned to improve PET utilization by microbial system as an eco-friendly solution.

Combined use of Bacillus strains and Miscanthus for accelerating biodegradation of poly(lactic acid) and poly(ethylene terephthalate)

Background

The aim of this study was to verify whether the presence of Bacillus strains and of miscanthus influence biodegradation and formed of biofilm of poly(lactic acid) (PLA) and poly(ethylene terephthalate) (PET).

Methods

The experiment conducted in compost soil showed that strains Bacillus subtilis and Bacillus cereus isolated from heavy metal contaminated environment have biochemical activity and accelerate biodegradation of both plastic materials.

Results

For PLA film it was found that the carbonyl index dropped by over 15% in the presence of B. subtilis, while the film tensile strength decreased by 35% and the oxygen to carbon O/C ratio was higher by 3% in the presence of B. cereus, and the presence of miscanthus resulted in a loss of weight. For PET film, a decrease in the carbonyl index by 16% was observed following inoculation with B. cereus. The metabolic activity of this strain contributed to the reduction of the film’s tensile strength by 17% and to the increase in the permeability to O₂ and CO₂. The most intense degradation of PET film was observed in the presence of bacteria and plants. B. subtilis strain combined with miscanthus plantings may be a promising method for accelerating PLA and PET degradation in compost soil.

Exploring microbial consortia from various environments for plastic degradation

Many complex natural and synthetic compounds are degraded by microbial assemblages rather than single strains, due to usually limited metabolic capacities of single organisms. It can therefore be assumed that plastics can be more efficiently degraded by microbial consortia, although this field has not been as widely explored as plastic degradation by individual strains. In this chapter, we present some of the current studies on this topic and methods to enrich and cultivate plastic-degrading microbial consortia from aquatic and terrestrial ecosystems, including substrate preparation and biodegradation assessment. We focus on both conventional and biodegradable plastics as potential growth substrates. Cultivation methods for both aerobic and anaerobic microorganisms are presented.

Degradation of Plastics under Anaerobic Conditions: A Short Review

Plastic waste is an issue of global concern because of the environmental impact of its accumulation in waste management systems and ecosystems. Biodegradability was proposed as a solution to overcome this problem; however, most biodegradable plastics were designed to degrade under aerobic conditions, ideally fulfilled in a composting plant. These new plastics could arrive to anaerobic environments, purposely or frequently, because of their mismanagement at the end of their useful life. This review analyzes the behavior of biodegradable and conventional plastics under anaerobic conditions, specifically in anaerobic digestion systems and landfills. A review was performed in order to identify: (a) the environmental conditions found in anaerobic digestion processes and landfills, as well as the mechanisms for degradation in those environments; (b) the experimental methods used for the assessment of biodegradation in anaerobic conditions; and (c) the extent of the biodegradation process for different plastics. Results show a remarkable variability of the biodegradation rate depending on the type of plastic and experimental conditions, with clearly better performance in anaerobic digestion systems, where temperature, water content, and inoculum are strictly controlled. The majority of the studied plastics showed that thermophilic conditions increase degradation. It should not be assumed that plastics designed to be degraded aerobically will biodegrade under anaerobic conditions, and an exact match must be done between the specific plastics and the end of life options that they will face.

Structure and thermal properties of various alcoholysis products from waste poly(ethylene terephthalate)

Waste polyethylene terephthalate (PET) has been a core member in plastic polluters due to the great amount consumption in food packaging, soft-drink bottles, fibers and films. It is essential to recycle waste PET and alcoholysis is a significant way to accomplish chemical recycling. In this work, three kinds of dihydric alcohols, including neopentyl glycol (NPG), dipropylene glycol (DPG) and poly(propylene glycol) (PPG), were employed to decompose waste PET with different temperatures, catalysts, and PET. A series of alcoholysis products with different appearance were obtained. The bulk structure and thermal properties of alcoholysis products were investigated by FTIR, ¹H NMR, MALDI-TOF, DSC and TG experiments. It is found that poly(propylene glycol) may react with waste PET to generate copolymer instead of oligomer products, dimers or trimers, etc. This product possesses excellent shelf stability and present transparent appearance, which may hold a great potential application in chemical industry. Moreover, the alcoholysis activity of DPG is the lowest comparing with NPG and EG in degradation of waste PET.

Physical and thermal properties of poly(ethylene terephthalate) fabric coated with electrospun polyimide fibers

Two analogous polyimides (PIs) containing flexible isopropylidene units were prepared. One was based on 4,4′-oxydiphthalic anhydride and 4,4′-(4,4′-isopropylidenediphenyl-1,1′-diyldioxy) dianiline and the other was based on 4,4′-(4,4′-isopropylidenediphenoxy) bis(phthalic anhydride) and bis(3-aminophenyl) methyl phosphine oxide. The ability of these two PIs to form uniform nanoscaled fibers was investigated by electrospinning technique. At optimal spinning conditions, PI fibers were electrospun onto the surface of woven poly(ethylene terephthalate) (PET) support to form a bilayer composite structure. These new fabric systems were analyzed regarding morphology, air permeability, wetting properties, and thermal stability. It was expected that the new PET/PI mats would possess enhanced properties compared with the initial woven PET fibers due to the excellent properties of PIs. Experimental results showed that PET woven substrate coated with electrospun PI fibers had improved values of air permeability, water contact angle and thermal stability when compared with the initial woven PET fibers.