Anaerobic biodegradation of the microbial copolymer poly(3-hydroxybutyrate-co-3-hydroxyhexanoate): Effects of comonomer content, processing history, and semi-crystalline morphology

Comprehensive Biodegradation Analysis of Chemically Modified Poly(3-hydroxybutyrate) Materials with Different Crystal Structures

This work presents a comprehensive analysis of the biodegradation of polyhydroxybutyrate (PHB) and chemically modified PHB with different chemical and crystal structures in a soil environment. A polymer modification reaction was performed during preparation of the chemically modified PHB films, utilizing 2,5-dimethyl-2,5-di(tert-butylperoxy)-hexane as a free-radical initiator and maleic anhydride. Films of neat PHB and chemically modified PHB were prepared by extrusion and thermocompression. The biological agent employed was natural mixed microflora in the form of garden soil. The course and extent of biodegradation of the films was investigated by applying various techniques, as follows: a respirometry test to determine the production of carbon dioxide through microbial degradation; scanning electron microscopy (SEM); optical microscopy; fluorescence microscopy; differential scanning calorimetry (DSC); and X-ray diffraction (XRD). Next-generation sequencing was carried out to study the microbial community involved in biodegradation of the films. Findings from the respirometry test indicated that biodegradation of the extruded and chemically modified PHB followed a multistage (2-3) course, which varied according to the spatial distribution of amorphous and crystalline regions and their spherulitic morphology. SEM and polarized optical microscopy (POM) confirmed that the rate of biodegradation depended on the availability of the amorphous phase in the interspherulitic region and the width of the interlamellar region in the first stage, while dependence on the size of spherulites and thickness of spherulitic lamellae was evident in the second stage. X-ray diffraction revealed that orthorhombic α-form crystals with helical chain conformation degraded concurrently with β-form crystals with planar zigzag conformation. The nucleation of PHB crystals after 90 days of biodegradation was identified by DSC and POM, a phenomenon which impeded biodegradation. Fluorescence microscopy evidenced that the crystal structure of PHB affected the physiological behavior of soil microorganisms in contact with the surfaces of the films.

Enhancement of biogas production rate from bioplastics by alkaline pretreatment

The effect of alkali-based pretreatment on the methanization of bioplastics was investigated. The tested bioplastics included PHB [poly(3-hydroxybutyrate)], PHBH [poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)], PHBV [poly(3-hydroxybutyrate-co-3-hydroxyvalerate], PLA (polylactic acid), and a PLA/PCL [poly(caprolactone)] 80/20 blend. Prior to methanization tests, the powdered polymers (500-1000 μm) at a concentration of 50 g/L were subjected to alkaline pretreatment using NaOH 1 M for PLA and PLA/PCL, and NaOH 2 M for PHB-based materials. Following 7 days of pretreatment, the amount of solubilized carbon for PLA and its blend accounted for 92-98% of the total initial carbon, while lower carbon recoveries were recorded for most PHB-based materials (80-93%), as revealed by dissolved total organic carbon analysis. The pretreated bioplastics were then tested for biogas production by means of mesophilic biochemical methane potential tests. Compared to unpretreated PHBs, methanization rates of pretreated PHBs were accelerated by a factor of 2.7 to 9.1 with comparable (430 NmL CH₄/g material feed) or slightly lower (15% in the case of PHBH) methane yields, despite featuring a 1.4-2.3 times longer lag phases. Both materials, PLA and the PLA/PCL blend, were only extensively digested when pretreated, yielding about 360-380 NmL CH₄ per gram of material fed. Unpretreated PLA-based materials showed nearly zero methanization under the timeframe and experimental conditions tested. Overall, the results suggested that alkaline pretreatment can help to enhance the methanization kinetics of bioplastics.

A review on poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) [P(3HB-co-3HHx)] and genetic modifications that affect its production

Polyhydroxyalkanoates (PHAs) have garnered global attention to replace petroleum-based plastics in certain applications due to their biodegradability and sustainability. Among the different types of PHAs, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) [P(3HB-co-3HHx)] copolymer has similar properties to commodity plastics, making them a suitable candidate to replace certain types of single-use plastics, medical devices, and packaging materials. The degradation rate of P(3HB-co-3HHx) is faster than the commercial petroleum-based plastics which take a very long time to be degraded, causing harmful pollution to both land and marine ecosystem. The biodegradability of the P(3HB-co-3HHx) is also dependent on its 3HHx molar composition which in turn influences the crystallinity of the material. Various metabolic pathways like the common PHA biosynthesis pathway, which involves phaA, phaB, and phaC, β-oxidation, and fatty acids de novo synthesis are used by bacteria to produce PHA from different carbon sources like fatty acids and sugars, respectively. There are various factors affecting the 3HHx molar composition of P(3HB-co-3HHx), like PhaCs, the engineering of PhaCs, and the metabolic engineering of strains. It is crucial to control the 3HHx molar composition in the P(3HB-co-3HHx) as it will affect its properties and applications in different fields.

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.

A Review of the Applications and Biodegradation of Polyhydroxyalkanoates and Poly(lactic acid) and Its Composites

Overconsumption of plastic goods and improper handling of petroleum-derived plastic waste have brought a plethora of negative impacts to the environment, ecosystem and human health due to its recalcitrance to degradation. These drawbacks become the main driving force behind finding biopolymers with the degradable properties. With the advancement in biopolymer research, polyhydroxyalkanoate (PHA) and poly(lacyic acid) (PLA) and its composites have been alluded to as a potential alternative to replace the petrochemical counterpart. This review highlights the current synthesis process and application of PHAs and PLA and its composites for food packaging materials and coatings. These biopolymers can be further ameliorated to enhance their applicability and are discussed by including the current commercially available packaging products. Factors influencing biodegradation are outlined in the latter part of this review. The main aim of this review article is to organize the scattered available information on various aspects of PHAs and PLA, and its composites for packaging application purposes. It is evident from a literature survey of about 140 recently published papers from the past 15 years that PLA and PHA show excellent physical properties as potential food packaging materials.

In vivo and Post-synthesis Strategies to Enhance the Properties of PHB-Based Materials: A Review

The transition toward "green" alternatives to petroleum-based plastics is driven by the need for "drop-in" replacement materials able to combine characteristics of existing plastics with biodegradability and renewability features. Promising alternatives are the polyhydroxyalkanoates (PHAs), microbial biodegradable polyesters produced by a wide range of microorganisms as carbon, energy, and redox storage material, displaying properties very close to fossil-fuel-derived polyolefins. Among PHAs, polyhydroxybutyrate (PHB) is by far the most well-studied polymer. PHB is a thermoplastic polyester, with very narrow processability window, due to very low resistance to thermal degradation. Since the melting temperature of PHB is around 170-180°C, the processing temperature should be at least 180-190°C. The thermal degradation of PHB at these temperatures proceeds very quickly, causing a rapid decrease in its molecular weight. Moreover, due to its high crystallinity, PHB is stiff and brittle resulting in very poor mechanical properties with low extension at break, which limits its range of application. A further limit to the effective exploitation of these polymers is related to their production costs, which is mostly affected by the costs of the starting feedstocks. Since the first identification of PHB, researchers have faced these issues, and several strategies to improve the processability and reduce brittleness of this polymer have been developed. These approaches range from the in vivo synthesis of PHA copolymers, to the enhancement of post-synthesis PHB-based material performances, thus the addition of additives and plasticizers, acting on the crystallization process as well as on polymer glass transition temperature. In addition, reactive polymer blending with other bio-based polymers represents a versatile approach to modulate polymer properties while preserving its biodegradability. This review examines the state of the art of PHA processing, shedding light on the green and cost-effective tailored strategies aimed at modulating and optimizing polymer performances. Pioneering examples in this field will be examined, and prospects and challenges for their exploitation will be presented. Furthermore, since the establishment of a PHA-based industry passes through the designing of cost-competitive production processes, this review will inspect reported examples assessing this economic aspect, examining the most recent progresses toward process sustainability.

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.

An experimental and theoretical study of the erosion of semi-crystalline polymers and the subsequent generation of microparticles

The increase of plastics and microplastics in the environment is a major environmental challenge. Still, little is known about the degradation kinetics of macroplastics into smaller particles, under the joint actions of micro-organisms and physico-chemical factors, like UV or mechanical constraints. In order to gain insight into (bio)-degradation in various media, we perform accelerated erosion experiments by using a well-known enzymatic system. We show that the microstructure of semi-crystalline polymers plays a crucial role in the pattern formation at their surface. For the first time, the release of fragments of micrometric size is evidenced, through a mechanism that does not involve fracture propagation. A geometric erosion model allows a quantitative understanding of erosion rates and surface patterns, and provides a critical heterogeneity size, parting two types of behavior: spherulites either released, or eroded in situ. This new geometric approach could constitute a useful tool to predict the erosion kinetics and micro-particle generation in various media.

Polyhydroxyalkanoates based copolymers

Polyhydroxyalkanoates (PHAs) belong to a family of natural polyesters and are produced under unbalanced growth conditions as intracellular carbon and energy reserves by a wide variety of microorganisms. Being biodegradable, biocompatible and environmental friendly thermoplastics, the PHAs are considered as future polymers to replace petrochemicals based plastics. In this review, the introduction section deals with the brief discussion on PHA nature, availability, raw materials for production, processing etc. This is followed by the discussions on modifications. The copolymer syntheses by bacterial and chemical methods have been discussed. Under chemical methods, unsaturated side chains and their derivatives, oligomer, coupling, macro-initiating, trans-esterification, radiation grafting, click chemistry, ring opening and several miscellaneous polymerization methods have been elaborated. A brief discussion on applications has been incorporated. The last section includes conclusion and future perspectives.

Microbial Degradation of Polyhydroxyalkanoates with Different Chemical Compositions and Their Biodegradability

The study addresses degradation of polyhydroxyalkanoates (PHA) with different chemical compositions-the polymer of 3-hydroxybutyric acid [P(3HB)] and copolymers of P(3HB) with 3-hydroxyvalerate [P(3HB/3HV)], 4-hydroxybutyrate [P(3HB/4HB)], and 3-hydroxyhexanoate [P(3HB/3HHx)] (10-12 mol%)-in the agro-transformed field soil of the temperate zone. Based on their degradation rates at 21 and 28 °C, polymers can be ranked as follows: P(3HB/4HB) > P(3HB/3HHx) > P(3HB/3HV) > P(3HB). The microbial community on the surface of the polymers differs from the microbial community of the soil with PHA specimens in the composition and percentages of species. Thirty-five isolates of bacteria of 16 genera were identified as PHA degraders by the clear zone technique, and each of the PHA had both specific and common degraders. P(3HB) was degraded by bacteria of the genera Mitsuaria, Chitinophaga, and Acidovorax, which were not among the degraders of the three other PHA types. Roseateles depolymerans, Streptomyces gardneri, and Cupriavidus sp. were specific degraders of P(3HB/4HB). Roseomonas massiliae and Delftia acidovorans degraded P(3HB/3HV), and Pseudoxanthomonas sp., Pseudomonas fluorescens, Ensifer adhaerens, and Bacillus pumilus were specific P(3HB/3HHx) degraders. All four PHA types were degraded by Streptomyces.

Formation of banded spherulites and the temperature dependence of the band space in olefin block copolymer

The banded spherulites for olefin block copolymer result from continuous lamellar twisting with an intriguing temperature tendency of the band space.

A study on obtaining nonwovens using polyhydroxyalkanoates and the melt-blown technique

Poly(3-hydroxybutyrate) [P(3HB)] nonwovens were obtained from polymers enriched with nucleants using the melt-blown technique. The most important physico-mechanical parameters, susceptibility to hydrolytic degradation (in neutral and alkaline medium) and to biodegradation, were analysed for the obtained nonwovens. It was determined that P(3HB) nonwovens, compared to popular polypropylene (PP) nonwovens, are characterized by elementary fibres with several times greater average diameter, greater mass per unit area and greater air permeability value. P(3HB) nonwovens are, on average, seven times more susceptible to breakage, and their elongation at maximum force is more than 50 times smaller than that for PP nonwovens. Hydrolysis of P(3HB) nonwovens is faster in an alkaline than in a neutral medium, and the observed relationships led to the conclusion that, at the start, short chains are subject to hydrolysis. Analysis of the weight loss associated with the degradation in bioreactors showed that P(3HB) nonwovens are more susceptible to biodegradation under anaerobic than under aerobic conditions.

Biodegradable Polyesters from Renewable Resources

Environmental concerns have led to the development of biorenewable polymers with the ambition to utilize them at an industrial scale. Poly(lactic acid) and poly(hydroxyalkanoates) are semicrystalline, biorenewable polymers that have been identified as the most promising alternatives to conventional plastics. However, both are inherently susceptible to brittleness and degradation during thermal processing; we discuss several approaches to overcome these problems to create a balance between durability and biodegradability. For example, copolymers and blends can increase ductility and the thermal-processing window. Furthermore, chain modifications (e.g., branching/crosslinking), processing techniques (fiber drawing/annealing), or additives (plasticizers/nucleating agents) can improve mechanical properties and prevent thermal degradation during processing. Finally, we examine the impacts of morphology on end-of-life degradation to complete the picture for the most common renewable polymers.

Microbial production of poly(hydroxybutyrate) from C₁ carbon sources

Polyhydroxybutyrate (PHB) is an attractive substitute for petrochemical plastic due to its similar properties, biocompatibility, and biodegradability. The cost of scaled-up PHB production inhibits its widespread usage. Intensive researches are growing to reduce costs and improve thermomechanical, physical, and processing properties of this green biopolymer. Among cheap substrates which are used for reducing total cost of PHB production, some C₁ carbon sources, e.g., methane, methanol, and CO₂ have received a great deal of attention due to their serious role in greenhouse problem. This article reviews the fundamentals of strategies for reducing PHA production and moves on to the applications of several cheap substrates with a special emphasis on methane, methanol, and CO₂. Also, some explanation for involved microorganisms including the hydrogen-oxidizing bacteria and methanotrophs, their history, culture condition, and nutritional requirements are given. After description of some important strains among the hydrogen-oxidizing and methanotrophic producers of PHB, the article is focused on limitations, threats, and opportunities for application and their future trends.

Distribution and selection of poly-3-hydroxybutyrate production capacity in methanotrophic proteobacteria

Methanotrophs are known to produce poly-3-hydroxybutyrate (PHB), but there is conflicting evidence in the literature as to which genera produce the polymer. We screened type I and II proteobacterial methanotrophs that use the ribulose monophosphate and serine pathways for carbon assimilation, respectively, for both phaC, which encodes for PHB synthase, and the ability to produce PHB under nitrogen-limited conditions. Twelve strains from six different genera were evaluated. All type I strains tested negative for phaC and PHB production; all Type II strains tested positive for phaC and PHB production. In order to identify conditions that favor PHB production, we also evaluated a range of selection conditions using a diverse activated sludge inoculum. Use of medium typically recommended for methanotroph enrichment led to enrichments dominated by type I methanotrophs. Conditions that were selected for enrichments dominated by PHB-producing Type II methanotrophs were: (1) use of nitrogen gas as the sole nitrogen source in the absence of copper, (2) use of a dilute mineral salts media in the absence of copper, and (3) use of media prepared at pH values of 4-5.