Microbial Enzymes Involved in Polyurethane Biodegradation: A Review

Plastics are present in a lot of aspects of everyday life. They are very versatile and resistant to microbial attack. Polyurethanes are used in several industries and are divided in polyester and polyether polyurethanes and there are different types among them. Despite their microbial resistance, they are susceptible to the attack of fungi and bacteria but the mechanism to elucidate its biodegradation are unknown. There are reports from bacteria and fungi that are capable of degrading polyurethane but the studies about the enzymes that attack the plastic are focused on bacterial enzymes only. The enzymes reported are of type esterase and protease mainly since these enzymes are very unspecific and can recognize some regions in the polyurethane molecule and hydrolyze it. Fungal enzymes have been studied prior the 1990s decade but recently, some authors report the use of filamentous fungi to degrade polyurethane and also report some characteristics of the enzymes involved in it. This review approaches polyurethane biodegradation by focusing on the enzymes reported to date.     

Biodegradation of Compostable Plastics in Green Yard-Waste Compost Environment

Compostable plastic materials, produced from polylactic acid (PLA), corn starch, or sugarcane, degraded in a green yard-waste compost environment. The compostable plastics claim to meet ASTM D6400 standards for biodegradation, sustainable plant growth, and eco-toxicity. Biodegradation was measured by disintegration studies over 20 weeks. The commercially available compostable products, made from PLA, sugarcane, or corn starch, biodegraded while in a commercial compost facility with other common yard waste compostable items. The PLA container, cup, and knife completely degraded in 7 weeks at a rate similar to the Avicell micro-cellulose control. The corn starch-based trash bag and sugarcane plate degraded at a similar rate as the Kraft paper control. The three materials degraded between 80% and 90% after 20 weeks.     

Comparative Biodegradation Studies of Non-poronized and Poronized LDPE Using Indigenous Microbial Consortium

Seventeen bacterial isolates were screened for the utilization of low density polyethylene (LDPE) as the sole carbon source, out of which five potential strains were selected for the development of a consortium. In vitro biodegradation efficiency of the consortium was studied for two differently textured forms of LDPE viz. non-poronized and poronized. Although, both the forms were acted-upon well by the consortium, but the degradation was found to be better in the poronized form. This was substantiated by λ-max shift, FTIR spectra and simultaneous TG-DTG-DTA. The analysis revealed the breakage and formation of chemical bonds in the polymer backbone, as a result of microbial activity. The biodegraded samples of non-poronized and poronized LDPE exhibited similar weight losses at 400 °C (24.12% and 24.48%, respectively) as compared to their controls (4% and 4.5% respectively), but the latter could achieve it with greater ease as reveled by its lower heat of reactions (ΔH values). The study signifies the influence of poronization of polyethylene on its rate of biodegradation.     

Microbial Communities in Composite Biofilms Participating in the Degradation of PCB

A moorland soil site polluted with PCB showed a high diversity ofmetabolically active bacteria. Beside frequent types of 16S rRNAsequences similar to those of the species ofSphingomonasand the Acidobacterium phylum an unusual high number ofsequences from the genus Burkholderia were found. Burkholderia was also the main genus in isolates enriched onbiphenyl or various chlorobenzoates. In microcosm experimentssterilized surfaces exposed to PCB polluted soil always showed thepresence of clay aggregates formed by bacteria attached to thesubstratum. The bacteria use the PCB loaded clay colloids astransport medium for the water insoluble substrate to get accessto the carbon source. This is a novel mechanism of how bacteria dealwith hydrophobic substrates.     

Polymer Mineralization in Soils: Effects of Cold Storage on Microbial Populations and Biodegradation Potential

Soil retrieval, processing and storage procedures can have a profound effect on soil microorganisms. In particular, changes in soil microbial populations may adversely affect the biological activity of a soil and drastically alter the soil's potential to mineralize added substrates. The effects of cold storage on the biodegradation of a series of test polymers was investigated using two soils—a synthetic soil mix (SM-L8) and a field soil (Bridgehampton silt loam) from Rhode Island (RI-1). Biodegradation tests were conducted using freshly prepared/collected soil and again following storage at 4°C for 3 to 8 months. Prior to each biodegradation test, the soils were incubated at 60% water-holding capacity (WHC) and 25°C to rejuvenate the microbial populations; the soils were incubated for periods of 48 h (freshly collected soil) or 25 days (soils stored at 4°C). Soil microbial populations were assessed by enumerating different segments of the population on agar plates containing different selective media. Mineralization of the test polymers (cellulose, poly-3-hydroxybutyrate, and starch acetate, d.s. 1.5) was monitored using standard respirometric techniques. Our results demonstrated that cold storage had a generally negative effect on the soil microbial populations themselves but that its effect on the capacity of the soil microorganisms to degrade the test polymers varied between soils and polymer type. Whereas cold storage resulted in dramatic shifts in the community structure of the soil microbial populations, substantial restoration of these populations was possible by first conditioning the soils at 60% WHC and ambient temperatures for 25 days. Likewise, although the effects of cold storage on polymer mineralization varied with the test polymer and soil, these effects could be largely offset by including an initial 25-day stabilization period in the test.     

Assessment of the Ecological Integrity of Traunsee (Austria) Via Analysis of Sediments and Benthic Microbial Communities

Since nearly one hundred years Traunsee experiences the import of tons of liquid and solid waste originating from salt and soda production. Today, the lake exhibits chloride concentrations of up to 170 mg L^-1 and 19% of the lake floor are directly or indirectly influenced by industrial deposits (ID). Based on the comparison of several microbial parameters in unaffected, directly affected and intermediate lake bottom sediments, the ecological integrity of the lake was evaluated. The highly alkaline ID, which were exclusively colonized by microorganisms, harbored a bacterial community reduced by a factor of 10 in abundance and biomass compared to undisturbed sediment areas within the lake. The bacterial community of ID was furthermore characterized by a reduced content of actively respiring cells (INT-formazan reduction), a lower frequency of dividing cells (FDC) and a significantly reduced cell and biomass production. A 80 to 90% reduction in carbon recycling is estimated for the area exclusively covered by ID. Protists, although occasionally absent from the industrial sediments, were in general found to be less sensitive to the contaminant stress. Differences in alkalinity and dissolved organic carbon (DOC) concentrations of sediment porewaters as well as the total organic content and C/N ratios of sediments partly explain the microbial pattern observed at the various sampling sites. Possible consequences of the continuous industrial tailings for the whole lake ecosystem and the validation of the ecological integrity are discussed.     

Anaerobic Biodegradation of Polymer Composites Filled with Natural Fibers

Polymer composites with natural fibers prepared by melt blending were investigated. Synthetic and natural macromolecules were used, including poly(lactic acid), polyhydroxybutyrate-co-polyhydroxyvalerate and low density polyethylene. These polymers were filled with flax fibers. Mechanical properties of the composites, biogas production and mass loss under anaerobic digestion have been presented. It has been shown that the mechanical properties sustain after 28 days of biodegradation. Such materials can be found in applications as packaging, as well as in medicine as polymeric scaffolds, and drug delivery systems etc.     

Modeling Biodegradation of Nonylphenol

Nonylphenol is the primary breakdown product of nonylphenol ethoxylates, a certain class of nonionic surfactants. Nonylphenol has been found to be toxic to aquatic organisms and has been suspected of being harmful to humans due to its xenoestrogenic properties. Although there are known releases of nonylphenol to the environment, there is a lack of data describing the extent of biodegradation. This study thus focuses on much needed information on the biodegradation kinetics of nonylphenol. Oxygen uptake, cell growth and nonylphenol removal data were collected using batch reactors in an electrolytic respirometer. Nonylphenol removal, cell growth and substrate removal rates were modeled by the Monod, Haldane, Aiba, Webb, and Yano equations. The differential equations were solved by numerical integration to simulate cell growth, substrate removal, and oxygen uptake as a function of time. All models provided similar results with the Haldane model providing the best fit. The values of the kinetic parameters and the activation energy for nonylphenol were determined. These values can be used for predicting fate and transport of nonylphenol in the environment. The validity of applying each model to the biodegradation of nonylphenol was analyzed by computing the R ² values of each equation.     

Biodegradation of Waste Cellulose

Environmental issues such as the depletion of nonrenewable energy resources and pollution are very topical and would need more scientific attention in order to be addressed in a way beneficial to life. The extent of solid waste production is a global concern and development of its bioenergy potential can simultaneously address issues such as pollution control and renewable energy development. Various wastepaper materials, a major component of solid waste, have been treated with cellulase from Trichoderma viride to bioconvert its cellulose component into fermentable sugars that could be utilized as feedstock for bioproduct development. These paper materials exhibited different susceptibilities toward the cellulase and showed different sugarreleasing patterns when increasing amounts of each paper were treated with the enzyme. Bioconversion of paper with different enzyme concentrations and during various time intervals also resulted in nonsimilar sugar-releasing patterns. With all the paper materials, a general decline in efficiency was observed with increasing amounts of sugar produced during the different bioconversion variables investigated.     

Stimulating the Biodegradation of Crude Oil with Biodiesel Preliminary Results

Experiments using biodiesel derived from vegetable oils have demonstrated the considerable potential for removing crude oil from contaminated beaches. During laboratory studies in small boxes, contaminated sand treated with biodiesel also demonstrated the rapid biodegradation of the crude oil. Water soluble components were washed through the sand columns and these components subsequently precipitated with cold storage. This solid fraction was not soluble in organic solvents but could be re-dissolved in dilute acid. The sediments after four weeks were black in colour due to the precipitation of metal sulphides although no H₂S was generated because the pH of the seawater kept the sulphides in solution. Further work is investigating which components of the oil were degraded and what products were formed.     

芳香族化合物的生物降解途径

分析了以苯、甲苯、萘和联苯为代表的芳香族化合物的生物降解途径,发现其共同点都是经过两步加氧酶作用生成二醇,最后再开环;介绍了苯、甲苯、萘和联苯这几种芳香族化合物分解酶的组分、相对分子质量及其辅基。     

Biodegradation Study of Starch-graft-Acrylonitrile Copolymer

In this study the biodegradability of starch-graft-acrylonitrile (St-g-AN) copolymer has been investigated using some microorganisms including Aspergillus niger. The fungus A. niger was isolated from the soil and from the wastewater of an acrylic fiber company. The effects of four factors including environment temperature, primary inoculum concentration, pH and weight of copolymer film, on the biomass generation as a measure of biodegradation rate of copolymer, were studied using Taguchi experimental design. The statistical analysis of the results showed that the primary inoculum concentration and temperature were the most important factors affecting the biodegradation of St-g-AN copolymer. The optimum levels of temperature, pH, inoculum concentration, and weight of films to attain the maximum biodegradation (as much as 8.59 % by weight percentage during 28 days) were obtained as 30 °C, 4.75, 10⁸ spore/mL, and 1.1 g, respectively. The changes in the structure and morphological properties of the copolymer before and after degradation were determined using transform infrared spectroscopy and scanning electron microscopy.     

Biodegradation of thermally synthesized polyaspartate

Polyaspartate synthesized using thermal methods (thermal polyaspartate; TPA) has been shown to have dispersant and crystallization inhibition activities. These activities suggest that the polymer may be used in water treatment and paper processing and as a detergent and paint additive. The commercial potential for TPA is enhanced by the fact that it may be synthesized on a large scale. Therefore, a study of the biodegradation of the polymer was undertaken. TPA was produced by hydrolysis of a polysuccinimide synthesized by dry thermal polymerization of aspartic acid. The resulting polymer was a poly(,-dl-aspartate) having a 70% structure and containing a racemic mixture of aspartic acid. TPA was incubated with both dilute effluent and activated sludge from a wastewater treatment plant. Low-biomass effluent experiments showed changes in molecular size of TPA concomitant with oxygen demand induced by the polymer, suggesting susceptibility of TPA to at least partial biodegradation. Low-biomass sludge experiments (SCAS, modified Sturm) yielded approximately 70% mineralization of 20 mg L^–1 TPA by 28 days, suggesting that a significant portion of the polymer was labile. High-biomass sludge experiments using¹⁴C-TPA at 1 mg L^–1 revealed approximately 30% mineralization and 95% total removal of TPA carbon from solution in 23 days, with most of the mineralization and removal taking place in less than 5 days. Additional short-term studies using a variety of particulate substrates, including activated sludge, confirmed that TPA is subject to removal from solution by adsorption. From these studies with labeled TPA, it was concluded that TPA is subject to rapid removal and at least partial degradation in a wastewaster treatment plant. Using gel and thin-layer chromatography, it was determined that at least part of the unmineralized residue from the high biomass assays was polyaspartate. It is speculated that the unusual structure of TPA compared to natural proteins may limit the rate of proteolysis of the polymer and thus its overall degradation rate.     

Biodegradability and Biodegradation of Polyesters

A variety of biodegradable plastics have been developed in order to obtain useful materials that do not cause harm to the environment. Among the biodegradable plastics, aliphatic polyesters such as: poly(3-hydroxybutyrate) (PHB), poly(ε-caprolactone) (PCL), poly(butylene succinate) (PBS), and poly(l-lactide) (PLA) have become the focus of interest because of their inherent biodegradability. However, before their widespread applications, comprehensive studies on the biodegradability and biodegradation mechanisms of these polyesters are necessary. Thus, this paper describes the degradation mechanisms and the effects of various factors on the degradation of polyesters. The distribution of polymer-degrading microorganisms in the environment, different microorganisms and enzymes involved in the degradation of various polyesters are also discussed.     

Biodegradation of LDPE/Modified Starch Blends Sterilized with Gamma Radiation

Blends of LDPE/modified starch were prepared, sterilized by gamma radiation and investigated with respect to their microbial degradation by a mixture of fungal strains in liquid medium after 90 days, was analyzed by carbon dioxide (CO₂) production (Sturm test). Biodegradation of blends was evaluated by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction; mechanical testing, scanning electron microscopy (SEM). The biodegradation of LDPE/modified starch blends was attributed to microbiological attack, with alterations in the chemical structure of the blend with an increase in the carbonyl and vinyl indices and the appearance of new crystalline symmetry generating a crystalline domain not existing before in the blend and decrease in the mechanical properties.     

废塑料的回收利用

介绍了国外废塑料再生利用、热分散回收低分子化合物和焚烧回收热能的技术与方法。     

Biodegradation of Polycaprolactone Powders Proposed as Reference Test Materials for International Standard of Biodegradation Evaluation Method

Polycaprolactone (PCL) powders were prepared from PCL pellets using a rotation mechanical mixer. PCL powders were separated by sieves with 60 and 120 meshes into four classes; 0–125 μm, 125–250 μm, 0–250 μm and 250–500 μm. Biodegradation tests of PCL powders and cellulose powders in an aqueous solution at 25°C were performed using the coulometer according to ISO 14851. Biodegradation tests of PCL powders and cellulose powders in controlled compost at 58°C were performed by the Mitsui Chemical Analysis and Consulting Service, Inc. according to ISO 14855-1 and by using the Microbial Oxidative Degradation Analyzer (MODA) instrument according to ISO/DIS 14855-2. PCL powders were faster biodegraded than cellulose powders. The reproducibility of biodegradation of PCL powders is excellent. Differences in the biodegradation of PCL powders with different class were not observed by the ISO 14851 and ISO/DIS 14855-2. An enzymatic degradation test of PCL powders with different class was studied using an enzyme of Amano Lipase PS. PCL with smaller particle size was faster degraded by the enzyme. PCL powders with regulated sizes from 125 μm to 250 μm are proposed as a reference material for the biodegradation test.     

Biodegradation Behavior of Composite Films with Poly (Vinyl Alcohol) Matrix

The main objective of this study was to develop biodegradable, composite materials, based on poly (vinyl alcohol), bacterial cellulose and chitosan for possible application in packaging industry. Two composite materials were prepared, one containing poly (vinyl alcohol) (PVA) and bacterial cellulose (BC), named PVA/BC, and the other containing PVA, BC but also chitosan (CTS), named PVA/BC/CTS. The biodegradation behavior was studied in a fed-batch bioreactor, in aerobic and anaerobic conditions, using activated sludge. Biodegradation tests were based on weight loss measurements. Structural changes were confirmed by Fourier transform infrared spectroscopy (FTIR) and the morphological ones by scanning electron microscopy (SEM). After 4?weeks, the biodegradation experiments have shown a relative high degradation of the PVA/BC/CTS film compared with the PVA/BC one. These results were confirmed by spectral analysis and also by SEM images. Besides, the SEM images revealed that biodegradation occurs also inside the composite materials, not only on the surface.     

Biodegradation of Whey Protein-Based Edible Films

The biodegradability of the edible films made of whey proteins by disulfide cross-linking was investigated. Whey protein concentrate (WPC) and whey protein isolate (WPI) films were subjected to microbial degradation using Pseudomonas aeruginosa and composting burial degradation. Results from the microbial degradation showed that whey protein films could support the growth of P. aeruginosa. The bacterial growth characteristics were well described using the Gompertz model. WPC films degraded faster than WPI films, suggesting that the biodegradability of protein films is associated with the film composition and the extent of covalent cross-linking. WPI films buried in a compost pile began to degrade in two days and became darker over time. More than 80% of total solids were lost in 7 days.     

Biodegradation and flushing of MBT wastes

Mechanical–biological treatment (MBT) processes are increasingly being adopted as a means of diverting biodegradable municipal waste (BMW) from landfill, for example to comply with the EU Landfill Directive. However, there is considerable uncertainty concerning the residual pollution potential of such wastes. This paper presents the results of laboratory experiments on two different MBT waste residues, carried out to investigate the remaining potential for the generation of greenhouse gases and the flushing of contaminants from these materials when landfilled. The potential for gas generation was found to be between 8% and 20% of that for raw MSW. Pretreatment of the waste reduced the potential for the release of organic carbon, ammoniacal nitrogen, and heavy metal contents into the leachate; and reduced the residual carbon remaining in the waste after final degradation from ～320 g/kg dry matter for raw MSW to between 183 and 195 g/kg dry matter for the MBT wastes.