Fast microwave-assisted hydrolysis of unsaturated polyester resin into column packing for rapid purifying of dye wastewater
Graphical abstract
Introduction
Thermosetting unsaturated polyester resins (TUPR) have been widely applied in many fields for its excellent properties (He et al., 2017; Okajima et al., 2017; Zhao et al., 2018). With the increasing number of waste TUPR (WTUPR) produced within end-of-lifecycle products, scrap and by-products generated, environmental and resource issues are becoming much more intense (Hanan et al., 2017; Ribeiro et al., 2011). The simplest and most popular recycling approaches, such as landfill and incineration, can improve environmental compliance in some degree, but cause secondary pollution and huge waste of resources (Iwaya et al., 2008; Pickering et al., 2000). Instead, some resource-saving approaches, including mechanical, thermal and chemical recycling, have been widely explored (Grause et al., 2013; Meira Castro et al., 2014; Shi and Bao, 2011). Among the approaches, the chemical depolymerization of the polymer materials has recently attracted significant interest because it offers a prospect of closed-loop recycling (Arturi et al., 2018; Kamimura et al., 2011) and transforms WTUPR into valuable organic raw materials including styrene-fumaric acid copolymer (Nakagawa and Goto, 2015a, b; Wang et al., 2015), resin monomers (Arturi et al., 2018; Oliveux et al., 2012), or fuel gas and fuel oil (Arturi et al., 2018; Grause et al., 2013). However, different from thermoplastic polymers, recycling of thermosetting polymers has been more challenging because of their insoluble and infusible properties (Fidanovski et al., 2017; Sokoli and Søgaard, 2015; Wang et al., 2015). Harsh conditions are usually required during degradation process, such as high temperature (>200°C) and pressure (Oliveux et al., 2013), as well as strong corrosive reaction media (Vallee et al., 2004). Accordingly, chemical bonds in TUPR are randomly destroyed resulting in low-molecular-weight and complex degraded products, which in turn leads to energy-consuming separation process and low-valued reutilization of the products. Therefore, it is extremely demanding for feasible recycling and cost-effective reusing of WTUPR.
In our very recent work, we successfully realized the transform of WTUPR into gel material (GM) with high swelling via selectively and controllably cleaving the partial ester groups while retaining most of its network structure. The resultant GM showed distinguished adsorption performance and can serve as a high-efficiency absorbent (An et al., 2019; Wang et al., 2019). So it provided a new insight to realize maximum utilization of degradation products by minimizing the degradation degree of WTUPR. It inspired us to explore more green and efficient approach on preparation of new functional materials for high-efficiency wastewater treatment and remediation.
Hydrolysis reaction is an effective method for cleaving ester bond (Theodorou et al., 2007) and has been used to degrade waste polymers (Geyer et al., 2016; Wang et al., 2018). However, the hydrolysis for WTUPR is usually carried out in super/subcritical fluids to ensure the degradation of TUPR for obtaining small molecule liquid products (Nakagawa and Goto, 2015b; Oliveux et al., 2012; Suyama et al., 2007). Compared with the traditional heating, microwave heating, as an energy-efficient, fast and environment-friendly technology (Wang et al., 2016), can show selective heating specialty and provide another degree of process flexibility (de Melo et al., 2017d; Luo et al., 2017; Zhou et al., 2017). Hence, microwave heating is one of the most promising techniques to accelerate chemical transformations at relatively low temperature with less energy consumption. It can also accelerate the hydrolysis. However, related research mainly focuses on the waste thermoplastic polyesters (Khalaf and Hasan, 2012; Liu et al., 2005). Recently, we successfully realized high-efficiency and mild degradation of anhydride-cured epoxy resin (MER) through microwave heating. The microwave heating improved both mass transfer and heat transfer and thus accelerated the degradation of MER (Zhao et al., 2019). However, its application on the degradation of TUPR is limited since both TUPR and the exciting degradation solvents are not susceptive to the microwave. Herein, we present a low temperature microwave-assisted hydrolysis method to transform the TUPR into a low-swelling GM. H2O was used as solvents in this work since H2O was more susceptive to microwave and can facilitate the heat transfer. A high-efficiency conversion was successfully realized at 100°C for only 1 h. The resultant GM was suitable as packing of adsorption column for purifying sewage with a super high filtering rate. The work provides a practical approach to energy-efficient valued reutilization of waste polymers and rapid purifying of organic pollutants.
Section snippets
Materials
TUPR was synthesized at our laboratory according to the procedure described by Korbar (Korbar et al., 1993). Dichloromethane (CH2Cl2), potassium hydroxide (KOH), and methylene blue (MB) were purchased from Tianjin Kermel Chemical Reagent Co., Ltd. (China). All the materials were used without purification.
Preparation of GM
Typically, TUPR (15 g) was immersed in CH2Cl2 (50 mL) at room temperature for 24 h to obtain a pretreated resin (PTUPR) and then CH2Cl2 was recycled by filtration. The PTUPR (5 g, 20–40 mesh)
Preparation of GM
The smooth surface microstructure of the TUPR became coarse, porous and inhomogeneous after pretreatment (Fig. 1a), which was expected to improve the reaction rate by facilitating mass transfer. Meanwhile, at the assist of microwave heating in the present work, alkaline hydrolysis of partial ester groups in PTUPR was further accelerated in water. It took only 1 h to obtain GM at 100°C, more than 10 times faster than the one in our very recent work (Wang et al., 2019). The alkaline concentration
Conclusions
A green and mild method was developed for transform of TUPR into high value-added GM via microwave heating. The rich carboxylate groups and porous structure were introduced in the final GM via partial cleavage of the ester groups in TUPR under microwave heating at a low temperature (100°C) for only 1 h using water as solvents. Compared with traditional heating, microwave heating made it possible to achieve an energy efficient conversion of TUPR along with high yield but low swelling of GM. It
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work is supported by the Program for the Science Fund for Creative Research Groups of the National Natural Science Foundation of China (No. 51721091), Changjiang Scholars, Innovative Research Team in University (IRT. 1026), and Sichuan Province Youth Science and Technology Innovation Team (No. 2017TD0006).
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