Simple But Tricky: Investigations of Terephthalic Acid Purity Obtained from Mixed PET Waste

NMR as a Discovery Tool: Exploration of Industrial Effluents Discharged Into the Environment

NMR provides unprecedented molecular information, urgently needed by environmental researchers and policy makers. However, NMR is underutilized in environmental sciences due to the lack of available technologies, limited environmental-specific training opportunities, and easy-to-use workflows. NMR has considerable potential as a discovery tool for novel pollutants, and by-products, exemplified by the recent discovery of the degradation by-product of a rubber additive, 6PPD-quinone, now considered one of the most toxic compounds presently known. This work represents a proof-of-concept case study highlighting the use of NMR to profile effluents from 38 industries across Ontario, Canada. Wastewater effluents from various industrial sectors were analyzed using several 1D and 2D 1H/13C NMR and 19F experiments and were screened both unconcentrated and after lyophilization. Common species could be identified using human metabolic NMR databases, but environmental-specific NMR databases desperately need further development. An example of manually identifying unusual NMR signatures is included; these resulted from phosphinic and phosphonic acids originating from the electroplating industry, for which the environmental impacts are not well understood. Basic 1H NMR quantification is performed using ERETIC, while an optimized approach combining relaxation agents and steady-state-free-precession 19F NMR, to reduce detection limits (at 500 MHz) to sub-ppb (< 1 μg/L) in under 15 min, is demonstrated. The future potential of benchtop NMR (80 MHz) is also considered. This paper represents a guide to others interested in applying NMR spectroscopy to environmental media and demonstrates the potential of NMR as a complementary tool to assist MS in environmental pollutant and by-product discovery.

Electrocatalytic reforming of polyethylene terephthalate waste plastics into high-value-added chemicals with green hydrogen generation

Electrochemical reforming offers a sustainable strategy for converting plastic waste into high-value-added chemicals and hydrogen fuel. Herein, a cost-effective NiFe-filmed electrode with a Tremella-like nanostructure was developed using an ultrasonic immersion etching method. This electrode enabled the electro-reforming of ethylene glycol (EG, a monomer of polyethylene terephthalate (PET)) into valuable commodity chemicals, with coproduction of hydrogen fuel. This system demonstrated notable electrocatalytic performance, achieving a current density of 100 mA/cm2 at a low overpotential of 1.52 V vs. reversible hydrogen electrode (RHE). It also exhibits notable selectivity (94.4 %) and Faradaic efficiency (94.3 %) for formate production. Furthermore, in a proof-of-concept demonstration, real-world PET plastic waste was successfully utilized to produce potassium diformate and terephthalic acid with properties comparable to those of commercial products. This study provides a comprehensive strategy for designing cost-effective catalysts for plastic electro-upcycling and lays a foundation for advancing the circular economy of plastic waste.

Molecular Level Understanding of Polyethylene Terephthalate (PET) Depolymerization in Base/Alcohol Hybrid Systems

Polyethylene terephthalate (PET) depolymerization in base/alcohol hybrid systems represents a promising low-energy approach for chemically recycling PET waste into valuable monomers. This study investigates the mechanistic pathways of PET depolymerization in NaOH/alcohol solutions, emphasizing the competing roles of hydroxide and alkoxide species. Utilizing a combination of experimental techniques, density functional theory (DFT) calculations, and molecular dynamics (MD) simulations, we explore how factors such as base concentration, alcohol chain length, and pKa values of alcohols influence PET depolymerization efficiency and pathways. Our findings indicate that alkoxide ions (RO-) exhibit notably higher reactivity than hydroxide ions (HO-), favoring an alcoholysis pathway in the base/alcohol hybrid system. Experimental results across a series of C1 to C5 alcohols show that longer-chain alcohols, particularly 1-butanol, achieve higher PET conversion, although this does not align solely with simple nucleophilicity trends of alkoxides. While DFT calculations reveal comparable activation energies for various alkoxides in PET depolymerization, MD simulations underscore the significant role of alcohol chain length, with longer-chain alcohols forming more stable or frequent interactions with PET. Additionally, the alkoxide concentration, influenced by the alcohol's pKa, directly impacts PET conversion. These suggest that PET depolymerization is governed by a balance between alkoxide concentration and alkoxide-PET interactions, rather than activation energies or nucleophilicity alone. From a practical perspective, incorporating long-chain alcohols as cosolvents may enhance process efficiency but increases raw material costs by approximately 30%. However, long-chain alcohols present a safer and more sustainable alternative to hazardous cosolvents such as dichloromethane. This work offers a molecular-level understanding of PET depolymerization in base/alcohol systems and provides insights into optimizing these systems for more efficient and sustainable PET recycling processes.

UiO-66 MOF/Zr-di-terephthalate/cellulose hybrid composite synthesized via sol-gel approach for the efficient removal of methylene blue dye

This research addresses the issue of organic dye pollution in water by developing eco-friendly, cost-effective adsorbents. Zr-di-terephthalate organic-inorganic hybrid materials, with and without cellulose, were synthesized using a one-pot sol-gel method with zirconium butoxide and terephthalic acid. The materials were characterized by XRD, FTIR, XPS, SEM, EDS, TEM, and BET analysis and showed a crystalline structure with some amorphous content, high surface area, and small pores, indicative of a composite form containing UiO-66 metal-organic frameworks (MOFs). Adsorption studies on methylene blue dye showed that both materials followed Langmuir isotherms, with maximum adsorption capacities of 147.54 mg/g for Zr-(TPA)2-U and 151.38 mg/g for Zr-(TPA)2-UC. The materials exhibited rapid adsorption within a short period of 30 min and adhered to pseudo-second order kinetics. The adsorption process was spontaneous and endothermic, driven by electrostatic interactions, π-π stacking, pore filling, van der Waals forces, and hydrogen bonding. The fact that the composite materials contain the UiO-66 structure contributed to their effective adsorption by increasing the crystalline properties of the material. The hybrids also showed excellent reusability, maintaining high efficiency over multiple cycles. These findings highlight the potential of Zr-di-terephthalate hybrids as sustainable, high-performance adsorbents for water purification, addressing dye contamination effectively.

Optimization of PET depolymerization for enhanced terephthalic acid recovery from commercial PET and post consumer PET-bottles via low-temperature alkaline hydrolysis

The increasing demand for plastic has resulted in a surge in plastic waste production. Polyethylene terephthalate (PET), commonly used in beverage bottle manufacturing, is only partially recycled, with an estimated recycling rate of just 28.4% in 2019. This accumulation of plastic waste is harmful to the environment and living organisms, necessitating effective recycling methods for PET waste. One promising method is alkaline hydrolysis using NaOH, which can break down PET into its monomer components, terephthalic acid (TPA) and ethylene glycol (EG). This process not only recycles PET efficiently but also manages contaminants effectively, producing high-quality TPA, supporting the development of a circular economy. This study looks into PET depolymerization via alkaline hydrolysis at low temperature by investigating effects of various factors: pH levels, water to ethanol ratio, NaOH concentration, NaOH to PET ratio, reaction time, PET size, reusability of unreacted PET, air plasma pretreatment of PET, and different kinds of PET. Promisingly, PET conversion rates of over 90% and a TPA purity of 99.6% were achieved in this study highlighting the efficacy of alkaline hydrolysis in depolymerizing post-consumer PET waste. Ultimately, this research advances sustainable plastic waste management and supports the integration of PET into a circular economy framework.

Development of Taste Sensor with Lipid/Polymer Membranes for Detection of Umami Substances Using Surface Modification

A taste sensor employs various lipid/polymer membranes with specific physicochemical properties for taste classification and evaluation. However, phosphoric acid di(2-ethylhexyl) ester (PAEE), employed as one of the lipids for the taste sensors, exhibits insufficient selectivity for umami substances. The pH of sample solutions impacts the dissociation of lipids to influence the membrane potential, and the response to astringent substances makes accurate measurement of umami taste difficult. This study aims to develop a novel taste sensor for detecting umami substances like monosodium L-glutamate (MSG) through surface modification, i.e., a methodology previously applied to taste sensors for non-charged bitter substance measurement. Four kinds of modifiers were tested as membrane-modifying materials. By comparing the results obtained from these modifiers, the modifier structure suitable for measuring umami substances was identified. The findings revealed that the presence of carboxyl groups at para-position of the benzene ring, as well as intramolecular H-bonds between the carboxyl group and hydroxyl group, significantly affect the effectiveness of a modifier in the umami substance measurement. The taste sensor treated with this type of modifier showed excellent selectivity for umami substances.

Conversion of PET Bottle Waste into a Terephthalic Acid-Based Metal-Organic Framework for Removing Plastic Nanoparticles from Water

Micro- and nanoparticles of plastic waste are considered emerging pollutants with significant environmental and health impacts at high concentrations or prolonged exposure time. Here we report the synthesis and characterization of a known metal-organic framework (MOF) using terephthalic acid (TPA) recovered from the hydrolysis of polyethylene terephthalate (PET) bottle waste. This approach adds value to the existing large amounts of bottle waste in the environment. Fully characterized zinc-TPA MOF (MOF-5) was used for the extraction and removal of engineered polyvinyl chloride (PVC) and polymethylmethacrylate (PMMA) nanoparticles from water with a high efficiency of 97% and 95%, respectively. Kinetic and isotherm models for the adsorption of polymer nanoparticles (PNPs) on the MOF surface were investigated to understand the mechanism. The Qmax for PVC and PMMA NPs were recorded as 56.65 mg/g and 33.32 mg/g, respectively. MOF-5 was characterized before and after adsorption of PNPs on the surface of MOF-5 using a range of techniques. After adsorption, the MOF-5 was successfully regenerated and reused for the adsorption and removal of PNPs, showing consistent results for five adsorption cycles with a removal rate of 83-85%. MOF-5 was characterized before and after adsorption of PNPs on the surface using a range of techniques. The MOF-5 with PNPs on the surface was successfully regenerated and reused for the adsorption and removal of polymer nanoparticles, showing consistent results for five extraction cycles. As a proof of concept, MOF-5 was also used to remove plastic particles from commercially available body scrub gel solutions. Such methods and materials are needed to mitigate the health hazards caused by emerging micro- and nanoplastic pollutants in the environment.

Solid-State Enzymatic Hydrolysis of Mixed PET/Cotton Textiles

Waste polyester textiles are not recycled due to separation challenges and partial structural degradation during use and recycling. Chemical recycling of polyethylene terephthalate (PET) textiles through depolymerization can provide a feedstock of recycled monomers to make "as-new" polymers. While enzymatic PET recycling is a more selective and more sustainable approach, methods in development, however, have thus far been limited to clean, high-quality PET feedstocks, and require an energy-intensive melt-amorphization step ahead of enzymatic treatment. Here, high-crystallinity PET in mixed PET/cotton textiles could be directly and selectively depolymerized to terephthalic acid (TPA) by using a commercial cutinase from Humicola insolens under moist-solid reaction conditions, affording up to 30±2 % yield of TPA. The process was readily combined with cotton depolymerization through simultaneous or sequential application of the cellulase enzymes CTec2®, providing up to 83±4 % yield of glucose without any negative influence on the TPA yield.

Kevlar-like Aramid Polymers from Mixed PET Waste

This work describes the synthetic approaches, spectroscopic and thermal characterization of aramid polymers prepared from waste polyethylene terephthalate (PET) via sustainable and scalable processes. Direct depolymerization of PET with aliphatic diamines under melt conditions resulted in decomposition without substantial formation of any aramid polymer. The Higashi-Ogata methodology or direct polycondensation of terephthalic acid (TPA) derived from PET waste and p-phenylenediamine, resulted in oligomerization and formation of aramids with a low degree of polymerization. The highest molecular weight polymers were obtained via the acid chloride of TPA, the traditional method. A proprietary solvent enabled the dissolution of most polymers and subsequent size exclusion chromatography analysis in the same solvent. We emphasize that although the soluble polymer compounds are prepared via the traditional route, they are novel. The apparent molecular weights of the soluble polymers ranged between 10-35 kDa (M _n ) and 28-81 kDa (M _w ). All analogues were prepared with commercially available diamines and diamine combinations. The obtained solid powders were dissolved in D₂SO₄ and analyzed spectroscopically to qualitatively evaluate the degrees of polymerization, while the solids were characterized via thermogravimetric analysis and differential scanning calorimetry. Many reaction conditions were employed to improve the solution polycondensation reaction, and it was found that addition of pyridine (2 eq) to the NMP reaction medium was crucial in preventing the precipitation of the polymer. Contrary to conventional wisdom, CaCl₂ did not play a crucial role in the molecular weight increase of the polymer when oxydianiline was used. Our data indicated that the temperature and absence of CaCl₂ provided a boost in molecular weight. Both room temperature and 0 °C reactions generated similar polymers as suggested by nuclear magnetic resonance; however, the cold conditions enhanced gel formation, an important attribute in the future processing of these materials to obtain fibers. All analogues had a high degradation temperature at 5 and 10% weight loss (_5% and T_10%), above 400 °C, along with high percent char values. A glass transition (T _g) was not detected in any of the analogues prepared.

Hydrothermal processing of polyethylene-terephthalate and nylon-6 mixture as a plastic waste upcycling treatment: A comprehensive multi-phase analysis

Accumulation of plastic waste is harming eco-systems and it is time to move towards a circular plastic economy. Sustainable production and recycling processes for plastics are challenged mostly by the lack of renewable building blocks. This study investigates hydrothermal processing (HTP) as a platform for depolymerization of two commonly used plastic polymers. Subcritical water (300 °C; 10 MPa) was tested as a solvent to treat polyethylene terephthalate (PET) and nylon-6 individually and in a mixture for a short reaction time of 90 min. Monomer recovery, gaseous emissions, and the effect of polymer mixture were evaluated by comprehensive analyses of all reaction products. Terephthalic acid (TPA), one of two monomers of PET was recovered as a solid product with a mass yield of 75%. ε-caprolactam (CPL), the single monomer of nylon-6 was recovered as a liquid product with a mass yield of 92.5%. Following PET + nylon-6 co-processing, TPA recovery decreased by 20%, whereas CPL recovery was not affected. Since TPA and CPL were recovered in different phases, an easy separation can likely be created for co-processing of PET and nylon-6. While most HTP studies neglect analysis of the gas phase, acetaldehyde and cyclopentene emissions were detected during HTP of PET and nylon-6, respectively. As shown here, gaseous emissions, which may be toxic, should be addressed in future developments of HTP for plastics. The results presented here can contribute to developing HTP processes for plastic recycling, that will be part of a circular plastic economy and a more sustainable future.

Green Synthesis of Magnetite-Based Catalysts for Solar-Assisted Catalytic Wet Peroxide Oxidation

A novel synthesis method under green philosophy for the preparation of some magnetite-based catalysts (MBCs) is presented. The synthesis was carried out in aqueous media (i.e., absence of organic solvents) at room temperature with recovery of excess reactants. Terephthalic acid (H2BDC) was used to drive the synthesis route towards magnetite. Accordingly, bare magnetite (Fe3O4) and some hybrid magnetite-carbon composites were prepared (Fe3O4-G, Fe3O4-GO, and Fe3O4-AC). Graphene (G), graphene oxide (GO), and activated carbon (AC) were used as starting carbon materials. The recovered H2BDC and the as-synthetized MBCs were fully characterized by XRD, FTIR, Raman spectroscopy, XPS, SQUID magnetometry, TGA-DTA-MS, elemental analysis, and N2-adsorption-desorption isotherms. The recovered H2BDC was of purity high enough to be reused in the synthesis of MBCs. All the catalysts obtained presented the typical crystalline phase of magnetite nanoparticles, moderate surface area (63–337 m2 g−1), and magnetic properties that allowed their easy separation from aqueous media by an external magnet (magnetization saturation = 25–80 emu g−1). The MBCs were tested in catalytic wet peroxide oxidation (CWPO) of an aqueous solution of metoprolol tartrate (MTP) under simulated solar radiation. The Fe3O4-AC materials showed the best catalytic performance among the prepared MBCs, with MTP and total organic carbon (TOC) removals higher than 90% and 20%, respectively, after 3 h of treatment. This catalyst was fairly successfully reused in nine consecutive runs, though minor loss of activity was observed, likely due to the accumulation of organic compounds on the porous structure of the activated carbon and/or partial oxidation of surface Fe2+ sites.

Recycling of Plastics in the United States: Plastic Material Flows and Polyethylene Terephthalate (PET) Recycling Processes

As efforts are made toward establishing a circular economy that engages in activities that maintain resources at their highest values for as long as possible, an important aspect is understanding the systems which allow recycling to occur. In this article a common plastic, polyethylene terephthalate, i.e., PET or plastic #1, has been studied because it is recycled at relatively high rates in the U.S. as compared to other plastics. A material flow analysis is described for PET resin showing materials collected, reclaimed for flake, and converted into items with recycled content. Imports/exports, reclaimer residue, and disposal with mismanaged waste are all shown for U.S. flows of PET. Barriers to recycling PET exist in the collecting, sorting, reclaiming, and converting steps, and this article describes them, offers some solutions, and suggests some research that chemists and engineers could focus on to improve the systems. This effort also models sorting at material recovery facilities (MRF) and reclaimers, with detailed descriptions of the material streams involved, to characterize the resource use and emissions from these operations that are key processes in the recycling system. Example results include greenhouse gas intensities of 8.58 kg CO₂ equiv per ton of MRF feed and 103.7 kg CO₂ equiv per ton of reclaimer PET bale feed. The results can be used in system analyses for various scenarios and as inputs in economic input-output and life cycle assessments.