Hydrothermal Treatment of E-Waste Plastics for Tertiary Recycling

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Hydrothermal treatment of e-waste plastics for tertiary recycling: product slate and decomposition mechanisms Xuyuan Zhao, Yuhan Xia, Lu Zhan, Bing Xie, Bin Gao, and Junliang Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05147 • Publication Date (Web): 21 Nov 2018 Downloaded from http://pubs.acs.org on November 24, 2018

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ACS Sustainable Chemistry & Engineering

Hydrothermal treatment of e-waste plastics for tertiary recycling: product slate and decomposition mechanisms Xuyuan Zhaoa, Yuhan Xiaa, Lu Zhana,b*, Bing Xiea,b, Bin Gaoc, Junliang Wangd a Shanghai Key Lab for Urban Ecological Processes and Eco-Restoration, School of Ecological and Environmental Sciences, East China Normal University, Shanghai 200241, China b Shanghai Institute of Pollution Control and Ecological Security, 1515 North Zhongshan Road, Shanghai 200092, PR China c Department of Agricultural and Biological Engineering, University of Florida, Gainesville, FL 32611, USA d College of Environment, Zhejiang University of Technology, Hangzhou 310032, PR China

Corresponding author: Lu Zhan (E-mail: [email protected] ; Phone: +86 21 54341064; Fax: +86 21 54341064) Mailing address: Shanghai Key Lab for Urban Ecological Processes and Eco-Restoration, School of Ecological and Environmental Sciences, East China Normal University, 500 Dongchuan Road, Shanghai 200241, China.

Emails for other co-authors as follows: Xuyuan Zhao ([email protected]); Yuhan Xia ([email protected]); Bing Xie ([email protected]); Bin Gao ([email protected]); Junliang Wang ([email protected]).

1

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Abstract Amounts of e-waste plastics have been one of the fast growing global waste streams and threaten to grow into an unmanageable problem. This phenomenon needs to be solved urgently by efficient and cost-effective ways. In this study, a novel hydrothermal treatment technology was implemented to convert the e-waste plastics into organic products which can be used as monomers of plastic production or chemical feedstock. We systematically investigated the recovery efficiencies of organic products, the product slate, the possible hydrothermal degradation mechanisms and the microstructure of solid residues. The results showed that the yields of organic products derived from four kinds of e-waste plastics ranged from 81.4 wt% to 97.6 wt% at 350℃. The recovered products contained styrene monomers, styrene derivatives, bisphenol A (BPA), caprolactam (CPL) and other valuable commodity chemicals. On the basis of the systematically analysis of hydrothermal organic products, the possible degradation mechanisms were proposed which involved a variety of reactions such as de-polymerization, hydrothermal cracking, hydrolysis, nucleophilic substitution and free radicals reaction in it. The degradation mechanisms provided the theoretical basis for the hydrothermal treatment of e-waste plastics. All the results demonstrated that hydrothermal treatment is a viable and prospective method for sustainable recycling of e-waste plastics.

Keywords Hydrothermal treatment; E-waste plastics; Tertiary recycling; Product slate; Decomposition mechanisms 2


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Introduction With the development of the times, plastics have become the ubiquitous materials of the contemporary society due to its unrivalled functional properties and low cost. Plastics production surged from 15 million tons in 1964 to above 300 million tons in 2017 globally.1, 2 At the same time, more and more plastic products are being used in electronic and electrical equipment (EEE)3 to replace metal in pursuit of lightweight and better user experience. The fast update of EEE has made e-waste a fast growing waste stream over the world

4-6

and more than 40 million metric tons

e-waste are generated each year.7 The waste plastics are significant fraction, which account for about 30% of the total amount of e-waste.8 Therefore, the growth in accumulated stocks of waste e-waste plastics has become an urgent environmental problem that needs to be solved. Recycling and disposal of waste plastics mainly includes primary recycling, secondary recycling, tertiary recycling and energy recovery.9, 10 Primary recycling and secondary recycling are the recycling of leftover materials and waste plastics which can be processed into products with similar or slightly inferior performance to new plastics.1 Both of these methods are coupled with mechanical means and apply to clean or semi-clean single-polymer plastics. Excessive contaminants and impurities make them essential to undergo additional separation, contaminant washing and drying steps to ensure the purity and quality of recycled products. Therefore, primary recycling and secondary recycling are non-priority choices for dirty plastics from e-waste. Tertiary recycling is a term referred to advanced technology processes 3



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(chemical or thermo-chemical means), which convert polymer materials into petrochemicals or monomer feedstock that can be used in industrial loops.1, 11, 12 Due to the energy sustainability and economical principle, tertiary recycling is gaining more and more recognition around the world.10, 13, 14 Energy recovery is carried out by incinerating waste plastics to achieve the purpose of decrease in volume and heat recovery. However, the incineration of waste plastics without pre-treatment will lead to the emission of air pollutants such as volatile organic compounds (VOCS), smoke, polycyclic aromatic hydrocarbons (PAHS), polychlorinated dibenzofurans (PCDFS) and dioxins.1,

15, 16

Tail gas treatment as post-treatment for eliminating the air

pollution often stifles the economic benefits brought by energy recovery. A lot of tertiary recycling techniques (pyrolysis, catalytic pyrolysis, gasification, hydrogenation and glycolysis) have been studied extensively. Pyrolysis is a quick thermo-chemical plastics treatment technique to destroy plastic structures and turn them into chemical products. Pyrolysis has been investigated as a viable route of recycling by a number of scholars for the case of single HIPS in fluidized bed reactor,17,

18

co-pyrolysis of PE/PP/PS/Br-HIPS.19 Based on the conventional

pyrolysis, catalytic pyrolysis relies on the addition of catalysts such as zeolites, acid or non-acid mesoporous materials, FCC catalyst, metallic oxide, etc. to reduce the difficulty of the reaction and increase the target products.20,

21

Gasification is a

thermolysis technology that converts waste plastics into mixed combustible gas such as CO and H2 in the presence of gasification agent (O2 or air). Hydrogenation of plastic generally occurs at about 400℃ and 2-15MPa hydrogen pressures for the 4


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conversion of heavy plastic molecules to liquid fuels of low boiling molecules.10 Glycolysis is widely used tertiary recycling technique for polyurethane through transesterification reaction of urethane bonds with glycols or glycerine.11, 22 Also the recovered polyols as glycerolysates could be used to produced cast PUS.12 Hydrothermal treatment is recognized as an appealing thermochemical technology and widely used for the conversion waste biomass,23, 24 sewage sludge 26

25,

and municipal wastes27 into added value products. Specially, upgrading waste

biomass as renewable fuels via hydrothermal treatment creates a sustainable development path. From a techno-economic perspective, although there are many key factors affecting the production cost, the hydrothermal treatment for renewable fuels or chemicals is still promising and highly cost competitive to other processes.28-30 However, the hydrothermal treatment of e-waste plastics as one feasible advanced technology of tertiary recycling is rarely focused by the scholars. Under subcritical or supercritical conditions, water acts a key ingredient with significant changes in physical and chemical properties,31 which has tremendous amounts of energy and can destroy carbon-carbon bond of organic components. Meanwhile, a series of hydrothermal cracking, hydrolysis, free radicals reaction, nucleophilic substitution and cyclization reactions occur in the reaction medium that convert the waste plastics matrix into monomers or chemical feedstock. More advantageous is that hydrothermal treatment can receive unsorted mixed and organic contaminated plastics, which eliminates the sorting step and greatly improves the recovery efficiency and economic benefit of the plastic recovery. In an airtight anaerobic atmosphere, even 5



halogen-containing e-waste plastics do not produce carcinogens such as dioxins and furans released into the air during hydrothermal treatment. Therefore, hydrothermal treatment of e-waste plastics aims at forming a green and sustainable closed-loop recycling way. In this paper, four kinds of e-waste plastics were treated under subcritical conditions by hydrothermal treatment, and the high quality liquid organic products were successfully recovered. The feasibility of hydrothermally recycling e-waste plastics as a tertiary recycling method was confirmed. Most important of all, the specific objective of this study is to analyze and deduce the pathways of hydrothermal degradation from organic products composition, based on a novel perspective. A clear understanding of the hydrothermal behavior of each single plastic can provide a theoretical cornerstone for the regulation of target chemical products in the further hydrothermal treatment of mixed plastics, as well as hydrothermal catalytic treatment of e-waste plastics.

Materials and methods Materials and apparatus. Dismantling and sorting out four typical kinds of e-waste plastics from discarded electronic equipment manually. The material identification of e-waste plastics on their surface confirms that they are the frequently-used polymer of high impact polystyrene (HIPS), polycarbonate (PC), acrylonitrile butadiene styrene (ABS), polyamide 6 (PA6). The hydrothermal treatment were carried out in a 1000ml reaction autoclave made of stainless steel (316L) with a quartz lining imbedded, whose designed 6


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temperature and pressure were 400℃ and 22MPa. The autoclave was obtained from ShangHai YanZheng Instruments Co., Ltd. Experimental procedures. In a typical run, the weighted e-waste plastic (about 10g) was mixed with 300ml of ultrapure water and introduced into the reaction autoclave for each experiment. The reactor was sealed without leakage, purged with continuous nitrogen for 10min to remove air to prevent the oxidation of products and the generation of contaminants. The autoclave was heated to three desired temperatures (250℃, 300℃, and 350℃) by an oven, and maintained the target temperature for one hour. On the termination of reaction, the reactor was cooled to room temperature spontaneously. The reactor was opened to sample the products. The mixture was passed through a filter funnel equipped with dried and pre-weighted filter paper by vacuum filtration, then the solid residues were separated and the liquid products were obtained. The liquid products containing both the aqueous and organic phase were extracted by dichloromethane to obtain the organic products. Meanwhile, the solid residues, the inner wall of the autoclave and the pipeline need to be washed by the extractant to reduce the loss of organic products. The dichloromethane containing organic products was dehydrated by anhydrous sodium sulfate and evaporated by a gentle stream of pure nitrogen to get the mass of concentrated liquid organic products (also called oils) for GC-MS analysis. Moreover, the solid residues were dried to a constant weight at 70℃ for 24h in an electric thermostatic drying oven and wait for further analyzed. Analysis. The organic products were qualitatively characterized by GC-MS. 7



Analysis were performed using an Agilent 7890 gas chromatograph fitted with Agilent 5975C mass spectrometry detector. High purity He was used as carrier gas with a flow rate of 1.1 mL/min and the injection amount of sample was 1μL. The capillary column was a non-polar HP-5MS (5% phenyl methyl silox: length 30m, I.D. 250mm, film thickness 0.25um). The preset heating program for column oven temperature is shown as follows: set at 50℃ and maintained for 5℃/min, then heated to 100℃ at 10℃/min for 1min, finally heated to 280℃ at 5℃/min and held for a further 20min. The ion mass spectra derived were automatically compared to spectral libraries to determine the type of compounds. It was trusty to identify compounds when the similarity indexes (SI) of ＞80%.The degradation pathways were predicted based on the different components of organic products. The solid residues were observed by using cold field emission scanning electron microscopy (SEM, S-4800, HITACHI, Japan) to investigate the morphologies and microstructure.

Results and discussion Products distribution and influence factors. Temperature plays a critical role in the thermal treatment.32-34 Hence, we investigated the effect of the reaction temperature on the yields of liquid organic products when four e-waste plastics were hydrothermally treated at 250℃~350℃. The quantitative results of products distribution analysis are shown in Figure 1, where we can clearly see the yields of liquid organic products (L), solid residues (S), and gas (G) at different temperature for four e-waste plastics. 8


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Figure 1. Hydrothermal products distribution derived from four e-waste plastics during 250℃~300℃

At 250℃, the polymers of HIPS and ABS basically did not degrade, and still existed in solid form with less than 10% oils yields. The opposite results were observed when processing PC and PA6 under the same condition. The recovery efficiency of organic products of PC and PA6 were up to 82.1% and 74% respectively. The contained groups were susceptible to hydrolysis, what made polyamide and polycarbonate were susceptible to hydrothermal treatment even at low temperature. As the temperature rose to 300℃, the yields of organic products of four plastics increased correspondingly. However, more than half of HIPS and ABS remained unchanged in solid forms which also indicated they had strong thermal stability under hydrothermal conditions. The vast majority of PC underwent degradation and was turned into organic products which accounted for 91.6%. The products distribution of plastic PA6 changed little. In the case of 350℃, the yields of organic products of HIPS and ABS had a dramatically increasing tendency reaching 9



88.4% and 98.4%, due to the temperature in the autoclave were higher than their degradation temperature. ABS basically achieved complete conversion into oils. Although PC and PA6 held high yields of organic products at 300℃, the difference were not significant by contrasting other conditions. Moreover, no gas or only tiny gas was produced, which proved the environmental advantage of hydrothermal treatment. From the foregoing results, it is indicated that not only temperature but also material properties play vital role in hydrothermal treatment. In general, the yields of liquid organic products increase with increasing temperature for each plastic. But with regard to PC and PA6, the trace growth of organic products is not enough to compensate for the cost of raising temperature. Therefore, the temperature range (250℃-300℃) can be used as the optimum conditions in hydrothermal treatment for PC and PA6. 350℃ is considered as the best temperature for HIPS and ABS converted into amounts of oils. Compositional analysis and hydrothermal decomposition mechanism. (1) HIPS. Organic product slate. GC/MS results (Figure 2) revealed a series of peaks that emerged for the organic products harvested at 350℃, which were mainly attributed to aromatic structure compounds with the number of carbons atoms varying from 7 to 24. The detailed information of main products was listed in Table 1. The (%) referred to the relative peak area in GC-MS results, and did not express the mass percent of the oils. 10


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Figure 2. GC-MS results of the analysis of organic products derived from HIPS.

From Figure 2 and Table 1, it was clear that the organic products were composed of three kinds of constituents including single-ringed aromatic compounds, diphenyl compounds and muti-benzene compounds. The first category of compounds were identified as containing styrene (5.14%), which could be used as raw monomers of plastics polystyrene. Toluene, ethylbenzene, (1-methylethyl)-benzene and α-methylstyrene as styrene derivatives accounted for 1.62% ， 6.71%, 1.45% and 2.10% respectively. These substances occupied an important position because of their simplicity and similarity in structure. This characteristic enabled them easier to purity and made them be used as commodity chemicals more directly. The biggest portion of all the compounds was the range of diphenyl compounds which stemmed from degradation intermediates in the oils. These all diphenyl-skeletons constituted above 30%,

including

1,3-diphenylpropane,

(1-methylpropane-1,3-diyl)dibenzene,

1,2-diphenylcyclopropane, 2-phenyl-naphthalene and anthracene. In addition, relatively low concentration of muti-benzene compounds like phenyl-terphenyl and 11



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quaterphenyl presented in the organic products. Table 1 The detailed information for the identified main organic products derived from HIPS NO. 1 2 3 4 5 6 7 8 9 10 11 12

Name of compound Toluene Ethylbenzene Styrene (1-methylethyl)-Benzene α-Methylstyrene 1,3-Diphenylpropane (1-methylpropane-1,3-diyl)Dibenzene 1,2-Diphenylcyclopropane 2-Phenyl-Naphthalene Anthracene 4'-phenyl-1,1':2',1''-Terphenyl 1,1':4',1'':4'',1'''-Quaterphenyl

RT( min ) 10.530 13.094 13.833 14.886 16.438 34.235 34.911 35.823 40.829 49.185 53.976 60.796

MF C7H8 C8H10 C8H8 C9H12 C9H10 C15H16 C16H18 C15H14 C16H12 C14H10 C24H18 C24H18

MW PRA(%) 92.14 1.62 106.17 6.71 104.15 5.14 120.19 1.45 118.18 2.10 196.29 18.98 210.31 6.28 194.27 3.01 204.27 2.58 178.23 6.94 306.40 3.49 306.40 4.99

RT: retention; MF: molecular formula; MW: molecular weight; PRA: relative peak area Hydrothermal decomposition mechanism. High impact polystyrene (HIPS) is a copolymer by blending two monomers of butadiene and styrene through conjunct polymerization.35 The essence of hydrothermal treatment of HIPS was the mixed degradation of polystyrene phase and polybutadiene phase. On the basis of the aforementioned analyses of organic products, we proposed a possible decomposition pathway of HIPS during hydrothermal treatment, as shown in Figure 3.

12


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Figure 3. Hydrothermal degradation mechanism of HIPS in subcritical water. The degradation of HIPS contained zip depolymerization and random chain breaking. First of all, the resin depolymerized to form two different polymers. The polystyrene cracked into the monomer of styrene through zip depolymerization. A 13



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part of styrene eliminated double bonds by addition reaction, and then hydrothermal energy broken the C-C bond to produce phenyl radicals, phenylmethyl radicals and alkyl radicals. Meanwhile, the energy provided by the subcritical water could break C-C bonds dissociation energy36 of the polybutadiene phase through random chain fracture to form linear alkyl radicals with different carbon atoms. Generally, the decomposition of polymers often involves three processes including the chain initiation, free radicals reaction and termination reaction. Some organic products such as the diphenyl compounds with lengthened carbon chain between phenyl radicals were profited from the combined reactions of different free radicals. Some possible radical reactions mechanisms were illustrated in the Figure 3 and a series of reactions involved in it. For example, the formation of toluene was derived from the combination of phenylmethyl radicals and hydrogen radicals, or the combination of phenyl and alkyl radicals. A phenylmethyl radical was bonded to two alkyl radicals with individual carbon atom to form (1-methylethyl)-Benzene, then the elimination reaction occurred in the autoclave made it α-methylstyrene. Also the linear alkyl radical with three carbon atoms which acted as bridging to fill the gap between individual phenyl radical, forming 1,3-diphenyl-Propane. Similarly, pentyl radicals could also link the aromatic radicals to lead the chain termination, and the chance of cyclization reaction increased with increasing carbon chain37 was the reason of the formation 1,2-diphenylcyclopropane. Moreover, the intermolecular rearrangement between

dissociative

phenyl

radicals

resulted

2-phenyl-naphthalene and muti-benzene compounds. 14


in

the

emergence

of

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(2) ABS. Organic product slate. Figure S1 and Table S1 showed the composition of the organic products obtained during the degradation of acrylonitrile butadiene styrene (ABS) in detail. As we expected, the compounds derived from ABS had a very similar affinity with the oils of HIPS on the constitution. They had the same thing in common that the categories of oils can be divided into three classes. The difference was that the content of organic products varied greatly. The mono-benzene ring compounds accounted for 27.09% of all substances, which became the largest content of the three categories. Ethylbenzene had the overriding proportion (13.52%), followed by toluene (5.39%), styrene (4.95%) and (1-methylethyl)-benzene (1.81%) in the first category of styrene derivatives. The products slate of substituted aromatics found in the organic products including 1-Methylnaphthalene (1.42%), 1,3-DiphenylPropane (11.92%), 1,1'-(1-methyl-1,3-propanediyl)-bisbenzene (5.14%)

and 1,5-Diphenylhex-3-ene

(7.81%). The m-terphenyl represented all the third type of polycyclic substance, whose relative peak area was only 0.77%. Hydrothermal decomposition mechanism. Similar to HIPS, ABS is the co-polymer obtained from three polymerized monomers of polyacrylontrile, polybutadiene and polystyrene. Due to the existence of the same polystyrene phase and polybutadiene phase, the product slate had great similarity. Depolymerization reaction, cracking reaction and free radical reaction ran through the whole process of hydrothermal treatment of ABS. Styrene, toluene and other styrene derivatives are intermediate products that derived from the continuous degradation of polystyrene 15



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and free radicals combination. The free radicals produced by the cracking of polystyrene and polybutadiene combined with each other through end-to-end reaction to scavenge the reaction space, forming the diphenyl compounds with alkyl bridging. These mechanisms are in full accordance with the degradation mechanisms of HIPS. Thus, we could explain these pathways with reference to Figure 3. Differently, the amount of gas generated by hydrothermal treatment of ABS was higher than that of other samples. This was due to the reaction of acrylonitrile contained in the resin. Acrylonitrile produced two intermediates containing C-N bonds by Michael-type reaction under hydrothermal conditions.38 These intermediates could be hydrolyzed to their corresponding counterparts of carboxylic acids. The carboxylic has been reported to participate in decarboxylation reaction to produce carbon dioxide easily in superheated water.39 (3) PC. Organic product slate. There are carbonic ester bonds that are relatively sensitive to water and heat existed in the molecular chain of polycarbonate (PC).40 Based on this characteristic, a majority of PC resins were recovered as organic products under hydrothermal environment. The chromatograms of organic products derived from PC revealed the presence of bisphenol A (BPA) and phenolic homologues which were in accordance with expectation. Figure 4 and Table S2 showed the composition and the detailed information of oils produced by polycarbonate,

where

it

could

be

seen

the

largest

component

is

3-(1-methylethyl)-Phenol up to 43.41%. Phenol and BPA also made up a significant 16


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proportion accounting for 29.98% and 8.24% severally. In a smaller extent, the production of p-isopropenylphenol (4.45%) and 4-methyl-2-phenylphenol (2.59%) were recovered.

Figure 4. GC-MS results of the analysis of organic products derived from PC.

Hydrothermal decomposition mechanism. According to the products analysis, the organic compounds contained hydroxyl terminal groups were the main products of hydrothermal treatment of PC. Therefore, the hydrolysis of carbonic ester bonds actually was the main degradation pathway in the early stage. With the increasing of temperature, the cracking reaction was also involved in it as assistances. As shown in Figure 5, the proposed degradation pathways clearly elucidated the formation mechanism of various substances and were significant for tertiary recycling of PC.

17



Figure 5. Hydrothermal degradation mechanism of PC in subcritical water

First, the depolymerization of polymers of PC was accomplished by hydrolysis. Water attacked the carbonic ester bond and reacted with them to release carbon dioxide. As a liquid above 200℃, water possesses prosperity that the native hydronium and hydroxyl ion concentrations in it are higher than room temperature.41, 42

Therefore, a large number of hydrated protons participated in the reaction and

combined with the groups of missing electrons to generate bisphenol A (BPA). Based on the bond-energy theory, the bond energy of C-C, C=C, C-H and O-H are 332 kJ/mol, 837 kJ/mol, 414 kJ/mol and 464 kJ/mol respectively. The C-C bond on the main chain of BPA was broken by the energy provided by the reaction system, and 18


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the intermediate product was combined with hydrogen proton to form phenol. On the other side, the 3-(1-methylethyl)-phenol was also produced by the same cleavage mechanism of BPA. Meanwhile, the intermediate products produced by the cracking of BPA could form phenol and p-isopropenylphenol (IPP) through intermolecular hydrogen transfer. The hydrogenation and cracking of IPP extended the way to generate 3-(1-methylethyl)-phenol and phenol. Deeper and intense cracking would eliminate hydroxyl groups to produce phenyl substances. The formation of small amount of 4-Methyl-2-phenylphenol might come from the combination of phenols and phenyl. (4) PA6. Organic product slate. Hydrothermal processing of polyamide 6 (PA6) primarily produced solid residues and organic products, which approximately 80% of the oils were recovered from the degradation of PA6. Like PC, PA6 showed to be extremely susceptible to hydrolysis as a polymer with acyl and amido linkages. However, the yields of oils and solid residues basically maintained balance due to the glass fiber additive supplemented in the PA6, which also demonstrated that the hydrothermal treatment of PA6 are successfully for tertiary recycling. From Figure S2 and Table 2, it was evident that the product slate was simple. The caprolactam (CPL) were the principal and important organic products which held 42.39% of the whole components. The CPL is usually used to synthetize nylon 6 through the anionic ring-opening polymerization. The other identified products were found to be: 4-hexanolide (1.41%), 3-methyl-N-allyl-but-2-enoic acid-amide (16.64%) and 19



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1-cyclohexyl-1-propanone (14.20%). Table 2 The detailed information for the identified main organic products derived from PA6 NO. 1 2 3

Name of compound RT( min ) MF 4-Hexanolide 17.382 C6H10O2 Caprolactam 23.530 C6H11NO But-2-enoic acid, amide, 53.698 C8H15N 3-methyl-N-allyl4 3,6-dimethyl-5-Octen-2-one 56.712 C9H16O RT: retention; MF: molecular formula; MW: molecular weight; area

MW 114.14 113.16 125.21

PRA(%) 1.41 42.39 16.64

140.22 14.20 PRA: relative peak

Hydrothermal decomposition mechanism. A degradation mechanism proposed by the analysis of organic products in subcritical water was shown in Figure 6. The caprolactam (CPL) were produced by a series of consecutive reactions at the reaction initial stage. First, the hydrogen bonds in the cross-linked polymers were broken by the superheated water and transformed into the linear polymers. Subsequently, the hydrolysis of the linear polymers occurred in the subcritical water to form aminocaproic acid. Water acted as a nucleophile and substituted for the carbonyl groups through the nucleophilic substitution reaction. Followed by the dehydration and cyclization reactions, converted the aminocaproic acid into the CPL, which were the primary products in the oils. Additionally, the structures of PA6 were also vulnerable to the effects of temperature. When the energy provide by the increasing temperature were higher than the activation energy, the chemical bonds of some intermediates would be destroyed. For instance, the heteroatoms connected to the carbon atoms like N-C were first interrupted and followed by the irregular C-C bonds were broken in the monomer. 20


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Therefore, many free radicals fragments formed through the cracking and re-cracking of intermediate products. These free radicals bound together and came into being secondary products like 4-Hexanolide, 3-methyl-N-allyl-But-2-enoic-acid-amide and 1-cyclohexyl-1-propanone through possible elimination reaction and cyclization reactions.

Figure 6. Hydrothermal degradation mechanism of PA6 in subcritical water

The microstructure of solid residues. The morphology and microstructure of these hydrothermal residues were observed by SEM and shown in Figure 7. It can be clearly observed that the solid residues surfaces of HIPS and ABS are oily, and some hollow pits and bubbles exist on the surface, but no pores exist. For the residues of 21



PC, there are some holes penetrated the surface, but it is not same to the developed pore structure of activated carbon. Moreover, it is found that the residues of PA6 exhibited clear dispersive filamentous material, which testified that the solid residues were glass fiber added in the matrix.

Figure 7. The SEM micrographs: a. the solid residues of HIPS; b. the solid residues of ABS; c. the solid residues of PC; d. the solid residues of PA6

As aforesaid, the microstructure of hydrothermal residues of HIPS, ABS and PC do not have well-developed pore structure，which excludes the possibility of being used as adsorbent materials like hydrochar derived from biomass.43 The greatly reduced solid residues can be directly incinerated. Furthermore, the feasibility of recovering glass fiber from hydrothermal residues of PA6 is confirmed.

Conclusion 22


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In this study, four types of e-waste plastics were processed in subcritical water to recovered organic chemical products. The results showed that the recovery efficiencies of organic products increased with the increasing temperature. The yields of organic products were ranged from 81.4 wt% to 97.6wt% for HIPS, ABS, PC and PA6 at 350℃. A large number of recovered organic chemicals contained styrene, styrene derivatives, bisphenol A (BPA), caprolactam (CPL) and other valuable commodity chemicals, which could be used as raw materials for plastic production or chemical feedstock. Moreover, the microstructure of the solid residues eliminated the possibility of being absorbent. Different possible hydrothermal degradation mechanisms were proposed. In the processes of hydrothermal degradation of HIPS and ABS, hydrothermal cracking and free radicals reaction played a predominant role. But hydrolysis was the preferential reaction during the hydrothermal treatment of PC and PA6 in subcritical water. A variety of reaction processes such as bridging, cyclization, intermolecular rearrangement, nucleophilic substitution were involved. The different mechanisms provided the basis for optimizing the follow-up research, such as increasing the yield of the polymer monomers products and reducing the yield of by-products under catalytic conditions.

Supporting information: the supporting information is available free of charge and contains: Table S1-S2 and Figure S1-S2.

Conflict of interest 23



The authors have no conflict of interest.

Acknowledgements This work is partly supported by the National Natural Science Foundation of China (21677050), Shanghai Pujiang Program (17PJD013) and the Science & Technology Innovation Action Plan of Shanghai under the Belt and Road Initiative (18230742800). The authors are grateful to the reviewers who help us improve the paper by many pertinent comments and suggestions.

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Graphical abstract

Synopsis: the clarity of product slate is a prerequisite for sustainable tertiary recycling and the proposed degradation mechanisms elucidate the process of hydrothermal treatment of e-waste plastics.

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Hydrothermal Treatment of E-Waste Plastics for Tertiary Recycling

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