An effective microwave-assisted process for recycling low-density polyethylene (LDPE) waste into value-added chemicals was developed. To achieve fast and effective oxidative degradation aimed at production of dicarboxylic acids, nitric acid was utilized as an oxidizing agent. Different conditions were evaluated, where recycling time and concentration of oxidizing agent were varied and the end products were characterized by FTIR, NMR, and HPLC. After just 1 h of microwave irradiation at 180 °C in relatively dilute nitric acid solution (0.1 g/mL), LDPE powder was totally degraded. This transformation led to few well-defined water-soluble products, mainly succinic, glutaric, and adipic acids, as well as smaller amounts of longer dicarboxylic acids, acetic acid, and propionic acid. The length of the obtained dicarboxylic acids could to some extent be tuned by adjusting the reaction time, temperature, and amount of oxidizing agent. Finally, the developed process was verified by recycling LDPE freezer bags as model LDPE waste. The freezer bags were converted mainly into dicarboxylic acids with a yield of 71%, and the carbon efficiency of the process was 37%. The developed method can, thus, contribute to a circular economy and offers new possibilities to increase the value of plastic waste.
Due to the steric effects imposed by bulky polymers, the formation of catalytically competent enzyme and substrate conformations is critical in the biodegradation of plastics. In poly(ethylene terephthalate) (PET), the backbone adopts different conformations, gauche and trans, coexisting to different extents in amorphous and crystalline regions. However, which conformation is susceptible to biodegradation and the extent of enzyme and substrate conformational changes required for expedient catalysis remain poorly understood. To overcome this obstacle, we utilized molecular dynamics simulations, docking, and enzyme engineering in concert with high-resolution microscopy imaging and solid-state nuclear magnetic resonance (NMR) to demonstrate the importance of conformational selection in biocatalytic plastic hydrolysis. Our results demonstrate how single-amino acid substitutions in Ideonella sakaiensis PETase can alter its conformational landscape, significantly affecting the relative abundance of productive ground-state structures ready to bind discrete substrate conformers. We experimentally show how an enzyme binds to plastic and provide a model for key residues involved in the recognition of gauche and trans conformations supported by in silico simulations. We demonstrate how enzyme engineering can be used to create a trans-selective variant, resulting in higher activity when combined with an all-trans PET-derived oligomeric substrate, stemming from both increased accessibility and conformational preference. Our work cements the importance of matching enzyme and substrate conformations in plastic hydrolysis, and we show that also the noncanonical trans conformation in PET is conducive for degradation. Understanding the contribution of enzyme and substrate conformations to biocatalytic plastic degradation could facilitate the generation of designer enzymes with increased performance.
High-density polyethylene (HDPE) waste was successfully feedstock recycled, and the obtained chemicals were utilized for synthesis of plasticizers for polylactide (PLA). First, an effective route to recycle HDPE through a microwave-assisted hydrothermal process was established. This process led to selective degradation of HDPE to a few well-defined chemicals, namely, succinic, glutaric, and adipic acid. A model plasticizer was synthesized from the same composition of dicarboxylic acids, 1,4-butanediol, and crotonic acid. The function of crotonic acid was to produce oligomers with crotonate end groups for coupling the plasticizer to PLA main chain. The plasticizer was then blended with or coupled to PLA by a reactive extrusion process. Adding the plasticizer to PLA decreased the T g and increased the strain at break, thus reducing the brittleness of the films. The addition of 20% (w/w) grafted plasticizer increased the strain at break of PLA from 6 to 156% and decreased the T g by 15 °C compared with neat PLA. Finally, to verify the concept, a plasticizer was also synthesized from the dicarboxylic acid product mixture obtained from the feedstock recycling of HDPE. The recycled grafted plasticizer increased the strain at break of PLA to 142% and reduced the T g by 10 °C. A promising route for designing from recycled feedstock, turning HDPE waste to PLA plasticizers, was thus demonstrated .
A fully starch-derived bioactive 3D porous scaffold is developed. The bioactivity is introduced through nanosized graphene oxide (nGO) derived from starch by microwave-assisted degradation to carbon spheres and further oxidation to GO nanodots. nGO is covalently attached to starch to prepare functionalized starch (SNGO) via an esterification reaction. nGO and SNGO exhibit no cytotoxicity to MG63 at least up to 1000 µg mL under (3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay. Porous scaffolds consisting of starch and SNGO (S/SNGO) or nGO (S/nGO) are prepared by freeze drying. The porosity and water uptake ability of the scaffolds depend on the concentration of nGO. Moreover, nGO, as a bioactive nanofiller, functions as an effective anchoring site for inducing CaP recrystallization in simulated body fluid. Among all modified starch-based scaffolds, the S/SNGO scaffold containing the highest concentration of covalently attached SNGO (50%) induces the largest amount of hydroxyapatite, a type of CaP crystal that is closest to bone. The prepared 3D porous nGO functionalized scaffold, thus, exhibits potential promise for bone/cartilage tissue engineering.
against landfills, and incineration does not retain the intrinsic material value of the polymeric materials which is a waste of resources. [6] Synthetic carpets usually consist of a multicomponent system, where several different polymeric materials constitute the fibers and backing, respectively. The advantage of the multicomponent system is the specific properties and performances of the different polymers. This is, however, a disadvantage when it comes to recycling, as sorting and separation become challenging and polymers are seldom miscible. They also have different characteristic behaviors such as solubility and processability. [7,8] This calls for new techniques to recycle carpet waste to respond to environmental and economic concerns and to generate valuable products from the waste. [6,[9][10][11][12] Many synthetic multicomponent carpets comprise of polyamide (PA) fibers (polyamide-6 (PA-6) and/or polyamide-66 (PA-66)) with a polypropylene (PP) backing, as well as an adhesive to ensure that the fibers are well connected to the backing. Additives such as dyes, repellents, and inorganic fillers (e.g., calcium carbonate, CaCO 3 ) are also commonly added. [8,13] PA fibers are often chosen, because of their high melting points and high abrasion resistance. [6] Typically, the PAs make up around 50% of the carpet mass, making recycling of the PA section a viable option. [14] The recycling can take place through, for example, selective hydrolysis of the PA component present in the synthetic carpet. [15] Chemical recycling converts waste into new building blocks such as monomers, oligomers or functional chemicals, that in turn can be utilized to synthesize new polymers or plasticizers. [16,17] In this way, the value of the material can be retained or in some cases, it can even increase. [18] This is a potent option for PAs, since they can be depolymerized by hydrolysis of the amide bond, resulting in the cyclic monomer ε-caprolactam [19] or the linear monomer aminocaproic acid from polyamide-6 and adipic acid and hexamethylenediamine from PA-66. These studies show that high temperatures and pressures are effective tools for depolymerization. [3,5,6,20] Successful depolymerization of PA-6 was also achieved by microwave assisted recycling under high temperatures and preasures. Concentrated phosphoric acid (C = 0.50 g mL −1 ) was for example used as a catalyst, leading to a product mixture containing 90% aminocaproic acid, the linear form of ε-caprolactam during 12 minutes at 240 °C. This catalyst was chosen due the the high dipole Selective hydrolysis of polyamide-6 (PA-6) and polyamide-66 (PA-66) from commercial multicomponent PA-6/PA-66/polypropylene (PP) carpet is demonstrated by a microwave-assisted acid catalyzed hydrothermal process, yielding monomeric products and solid polypropylene residue. First, an effective method is established to chemically recycle neat PA-6 and PA-66 granules using microwave irradiation. The optimized, hydrochloric acid (HCl) catalyzed process leads to selective production of monomers...
Front Cover: A production route to novel bioactive fully starch derived porous scaffolds is demonstrated. The bioactivity is enabled by starch derived nano‐graphene oxide, which induced mineralization of calcium phosphate on the surface of starch scaffolds. This is reported by Duo Wu, Eva Bäckström and Minna Hakkarainen, article number https://doi.org/10.1002/mabi.201600397.
Recycling plastics is the key to reaching a sustainable materials economy. Biocatalytic degradation of plastics shows great promise by allowing selective depolymerization of man‐made materials into constituent building blocks under mild aqueous conditions. However, insoluble plastics have polymer chains that can reside in different conformations and show compact secondary structures that offer low accessibility for initiating the depolymerization reaction by enzymes. In this work, we overcome these shortcomings by microwave irradiation as a pre‐treatment process to deliver powders of polyethylene terephthalate (PET) particles suitable for subsequent biotechnology‐assisted plastic degradation by previously generated engineered enzymes. An optimized microwave step resulted in 1400 times higher integral of released terephthalic acid (TPA) from high‐performance liquid chromatography (HPLC), compared to original untreated PET bottle. Biocatalytic plastic hydrolysis of substrates originating from PET bottles responded to 78 % yield conversion from 2 h microwave pretreatment and 1 h enzymatic reaction at 30 °C. The increase in activity stems from enhanced substrate accessibility from the microwave step, followed by the administration of designer enzymes capable of accommodating oligomers and shorter chains released in a productive conformation.
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