1. Introduction
Plastics have cemented their status as indispensable materials in contemporary society, with their utilization undergoing a remarkable surge over the past few decades. Recent data indicate that global plastic production has more than doubled since the year 2000, reaching approximately 368 million metric tons in 2019 [
1]. This upward trajectory has been accompanied by a proportional increase in plastic waste generation, posing significant environmental challenges. Notably, plastic waste generation has outpaced population growth in recent years, highlighting the urgency of addressing this issue [
2].
While plastics constitute a relatively small proportion of urban solid waste by weight (approximately 7%), their low density allows them to occupy a disproportionately large volume, contributing to visual pollution and environmental degradation [
3]. Of particular concern is the persistence of plastics in the environment, with estimates suggesting that certain plastic items can take hundreds to thousands of years to decompose fully [
4]. This persistence, coupled with an inadequate waste management infrastructure and practices, has led to the accumulation of plastic debris in terrestrial and marine ecosystems worldwide, with profound implications for biodiversity and human health [
5].
Among the various types of plastics, poly(ethylene terephthalate) (PET) has emerged as a widely used thermoplastic polymer, valued for its versatility, durability, and transparency. PET finds extensive applications in packaging, including bottles for beverages, food containers, and textiles [
6]. However, the widespread adoption of PET has also contributed to its prevalence in the waste stream, necessitating effective strategies for its management and recycling.
In response to the mounting challenges posed by plastic waste, there has been a growing emphasis on sustainable waste management practices, with recycling playing a pivotal role. Recycling offers multiple benefits, including resource conservation, energy savings, and waste diversion from landfills and incinerators. However, despite the potential advantages of recycling, current recycling rates for plastics remain relatively low, with significant room for improvement [
7].
Mechanical recycling represents the most common method of recycling PET currently employed. This process involves shredding and reprocessing PET waste into pellets or flakes, which can then be used to manufacture new products. While mechanical recycling offers a viable solution for certain PET applications, its effectiveness is limited by factors such as contamination, degradation of material properties, and technological constraints [
8].
In addition to mechanical recycling, there is growing interest in advanced recycling technologies, including chemical recycling, which enables the conversion of plastic waste back into its constituent monomers or other value-added products. Chemical recycling holds promise for achieving higher-quality recycled materials and closing the loop in the plastics value chain. However, challenges related to scalability, cost-effectiveness, and environmental impact must be addressed to realize the full potential of this approach [
9].
In recent years, several methods of depolymerization of PET have been developed with the aim of obtaining terephthalic acid (TPA) [
10], dimethyl terephthalate (DMT) [
11] or bis(2-hydroxyethyl) terephthalate (BHET) [
12], all of which are possible monomers to produce new polyesters. The exact monomer obtained from such depolymerization will depend on the chemical agent used for the breakdown of the chain, according to which the different reactions can be classified as follows: methanol, hydrolysis, glycolysis, ammonolysis, and aminolysis. In hydrolysis, the reaction of PET with water allows the breakdown of the chain into TPA and ethylene glycol (EG), being able to develop in an acidic, basic or neutral medium. Hydrolysis in an acidic medium is usually performed with sulfuric acid, and the large amount required makes the process substantially more expensive [
13]. Furthermore, the use of this acid makes it necessary to supply the equipment with an extra resistance to corrosion [
14]. On the other hand, the main disadvantage of neutral hydrolysis is the use of large excess reagents, as well as the need for high temperatures and pressures [
15]. In addition, this process produces co-products such as oligomers, TPA derivatives or even cyclic trimers [
16]. The alkaline hydrolysis reaction involves the treatment of polyester with an aqueous NaOH solution (between 4% and 20% by weight) [
17]. The literature suggests that the most recent studies on the alkaline hydrolysis of PET studies were conducted at temperatures ranging from 80 °C to 220 °C [
18]. The latest patent for alkaline hydrolysis reported the application of sodium hydroxide to produce TPA from PET, and the reaction temperature range was 130–150 °C [
19]. Under appropriate reaction conditions, a disodium salt of TPA is formed and, by acidification, the TPA is recovered from the solution as a precipitate.
Although previous research has identified hydrolysis as one of the most suitable chemical recycling processes for PET recycling, applications developed from these findings mainly focus on relatively clean PET materials with a known composition and rely on the use of phase transfer catalysts (PTC) that can be difficult to recover and also expensive. Furthermore, the present work deals with real PET waste that is currently being generated and landfilled or incinerated in large quantities, so this paper aims to present a technically feasible solution based on the principles of the circular economy, thus allowing the manufacture of new value-added products and closing the cycle of PET materials through the application of a chemical recycling process. Nevertheless, the consulted literature suggests that no complex PET waste streams have been studied for the pressurized and alkaline hydrolysis of PET until now. Therefore, this work aims to explore the feasibility of the pressurized hydrolysis of complex PET waste, without the use of a catalyst, and study the progression of the reaction by means of the TPA yield.
In view of the need to improve and optimize chemical recycling processes to make them as effective and feasible as possible, the present study tries to explore the optimization of the process with a lower amount of NaOH and a lower temperature compared to previous results, in order to reduce operating costs and to make the process more profitable and sustainable in terms of material and energy consumption, always maintaining TPA yield values higher than 90%. Therefore, this study aims to investigate the hydrolysis of PET as a key step in the chemical depolymerization process for PET recycling. By optimizing the hydrolysis process, we seek to enhance the efficiency and sustainability of PET recycling, thereby contributing to the transition towards a circular economy.
4. Conclusions
Plastics, particularly PET, have become indispensable in modern society, leading to a significant increase in plastic waste production. PET’s desirable properties make it widely used in packaging for various industries, but its environmental impact is a concern due to the high volume of production and the reliance on non-renewable resources like oil. To address these concerns, efforts have been made to recycle PET, with mechanical recycling being the most common method. However, mechanical recycling has limitations in producing high-quality recycled PET.
Chemical recycling, particularly depolymerization, shows promise in overcoming the limitations of mechanical recycling by breaking down PET into its original monomers for reuse. Hydrolysis, especially alkaline hydrolysis, is a prominent method for PET depolymerization, but challenges such as high cost, co-product formation, and energy consumption exist. This research focused on optimizing the pressurized alkaline hydrolysis process for post-consumer PET packaging waste, considering variables like temperature and the NaOH/PET ratio.
The results showed that higher temperatures significantly increased TPA yield, while the NaOH/PET ratio had a minimal effect. The study identified the optimal conditions (T = 150 °C, NaOH/PET = 2) for PET depolymerization, considering both yield and cost factors. The purity of the obtained TPA met expectations, with an average purity of 98.5%. Comparison with virgin PET showed slightly lower yields from the sample, possibly due to the presence of impurities affecting the reaction kinetics. Despite challenges, the optimized pressurized alkaline hydrolysis process offers a viable solution for recycling post-consumer PET packaging waste, contributing to environmental sustainability.