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DEHP degradation and dechlorination of polyvinyl chloride waste in subcritical water with alkali and ethanol: A comparative study
Chemosphere 249 (2020) 126138
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DEHP degradation and dechlorination of polyvinyl chloride waste in subcritical water with alkali and ethanol: A comparative study Fu-Rong Xiu a, b, *, Yongwei Lu a, Yingying Qi a, b a b
College of Geology and Environment, Xi’an University of Science and Technology, Xi’an, 710054, People’s Republic of China Shaanxi Provincial Key Laboratory of Geological Support for Coal Green Exploitation, Xi’an, 710054, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
We report two effective processes for DEHP degradation and dechlorination of PVC. CW-NaOH and CW-C2H5OH have different DEHP degradation and dechlorination mechanism. Hydrolysis and reduction were the main degradation pathways of DEHP in CW-NaOH. Transesterification was the main degradation pathway of DEHP in CWC2H5OH. Nucleophilic substitution and dehydrochlorination co-existed in dechlorination.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 September 2019 Received in revised form 6 January 2020 Accepted 5 February 2020 Available online 6 February 2020
In this study, subcritical water-NaOH (CWeNaOH) and subcritical water-C2H5OH (CWeC2H5OH) processes were developed for diethylhexyl phthalate (DEHP) degradation and dechlorination of polyvinyl chloride (PVC) waste. The introduction of NaOH or C2H5OH in subcritical water had a noticeable influence on the mechanism of DEHP degradation and dechlorination. For both CW-NaOH and CW-C2H5OH treatments, the increase in temperature could increase dechlorination efficiency (DE) of PVC. The DE of CW-NaOH is much higher than that of CW-C2H5OH under the same conditions. The DE of CW-NaOH could exceed 95% at 300 C. Hydroxyl nucleophilic substitution was the main dechlorination mechanism in CW-NaOH, while nucleophilic substitution and direct dehydrochlorination were equally important in CW-C2H5OH. In CW-NaOH treatment, 2-ethyl-1-hexanol, benzaldehyde, and toluene were obtained by hydrolysis and reduction reactions of DEHP. Acetophenone was produced by the further cyclization, dehydrogenation and rearrangement reactions of 2-ethyl-1-hexanol. Transesterification was the main degradation pathway of DEHP in CW-C2H5OH at 300 C. The cyclization and dehydration of 2ethyl-1-hexanol resulted in producing a high level of ethyl-cyclohexane and 1-ethyl-cyclohexene in CWC2H5OH at 350 C. Furthermore, high concentration of ethyl palmitate and ethyl stearate could be prepared in CW-C2H5OH system by the strong reactivity of C2H5OH with the lubricants in PVC. © 2020 Elsevier Ltd. All rights reserved.
Handling Editor: Tsair-Fuh Keywords: PVC Phthalic acid ester DEHP Subcritical water Degradation
1. Introduction * Corresponding author. College of Geology and Environment, Xi’an University of Science and Technology, Xi’an, 710054, People’s Republic of China. E-mail address: [email protected] (F.-R. Xiu). https://doi.org/10.1016/j.chemosphere.2020.126138 0045-6535/© 2020 Elsevier Ltd. All rights reserved.
Phthalic acid esters (PAEs) are an important chemical raw
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material and are widely used as a plasticizer in the production of high-molecular resin materials and various products (e.g. construction material, paint, toy, food containers, and medical devices). Global annual production of PAEs has reached 6 million tons (Wang et al., 2013). Diethylhexyl phthalate (DEHP) is the most common kind of PAEs. The production of DEHP accounted for more than 50% of the total PAEs output (Hao et al., 2013). DEHP can easily be transferred from the end-of-life resin products into various environmental media due to that there is no chemical bond between DEHP molecules and polymer matrix (Heudorf et al., 2007). DEHP is chemically stable and difficult to degrade in the environment. The and hydrolysis half-life of DEHP can reach several decades (Julinova Slavík, 2012). Many studies revealed that DEHP has several biological toxicities such as reproductive toxicity, endocrine toxicity, embryonic development toxicity, and immunotoxicity (Barakat et al., 2017; Qi et al., 2019). Therefore, the high-efficiency degradation of DEHP in the end-of-life resin materials is very important for the safety management of DEHP plasticized resin waste. Polyvinyl chloride (PVC) is one of the most widely used plastics in the world (Yu et al., 2016). DEHP is the most frequently used plasticizer in PVC due to the good compatibility between DEHP and PVC molecule (Xu et al., 2018; Xiu et al., 2020). The quality percentage of DEHP in soft PVC plastic products can reach as high as 40%e50% (Yang et al., 2018). Another notable feature of the PVC product is its high chlorine content (Hapipi et al., 2018). Therefore, the safety management of PVC waste faces two great challenges: the high level of chlorine and high toxicity DEHP. At present, the main final disposal methods of PVC waste are landfill and incineration (Yu et al., 2016). However, landfill inevitably leads to the migration of high content of DEHP and other toxics in PVC products into various environmental media, such as soil, water, and atmo ska, 2017; Qi et al., 2020). Incinsphere (Wowkonowicz and Kijen eration is easy to produce chlorinated dioxins and other highly toxic substances (Yu et al., 2016). The treatment methods of landfill and incineration for PVC waste can bring great risks to the environment and human health. Therefore, before the PVC waste was sent to final disposal, efficient removal or degradation of DEHP and Cl in PVC waste can greatly reduce the environmental risks caused by DEHP and Cl. It is crucial for the safety management of DEHP plasticized PVC waste. In recent years, many technologies have been proposed for the removal and degradation of DEHP including adsorption separation (Zhang et al., 2014), TiO2 photocatalysis (Chung and Chen, 2009), ozonation (Hammad and Jung, 2008), UV photolysis (Huang et al., 2017; Chen, 2010), and microbial degradation (Xu et al., 2017; Singh et al., 2017). However, these studies mainly focus on the degradation of low-level DEHP in environmental media such as natural water and sediment. It is difficult for the reported methods to be effectively applied to the actual engineering treatment of PVC solid waste with DEHP content over 40%. In fact, the degradation of DEHP before DEHP plasticized PVC waste enters the environment can effectively manage and control the amount of DEHP releasing into the environment. Therefore, it is urgent to developed new and efficient processes for the degradation of DEHP contained in PVC waste. Lately, Hankett et al. tested the UV/H2O2 treatment for the DEHP degradation in plasticized PVC (Hankett et al., 2014). But the degradation efficiency was low and the UV exposure time was long. Furthermore, all of the reported methods have no effect on the chlorine remove of PVC waste. Over the last two decades, sub and supercritical water (Sub/ SCW) technology has been widely used as an efficient treatment for organic solid waste due to its unique physicochemical properties (Posmanik et al., 2017; Wang et al., 2014). Several studies found that PVC molecules could be effectively dechlorinated and decomposed
in Sub/SCW (Nagai et al., 2007; Takeshita et al., 2004) and high value-added chemical raw materials or chemical products could be obtained from PVC waste under certain conditions (Yu et al., 2016). However, the high temperature and high pressure conditions of Sub/SCW treatment technology and the acidity corrosion of reaction equipment greatly limit its further practical application (Xiu et al., 2018, Xiu et al., 2019a, 2019b). In our previous study (Qi et al., 2018) supercritical methanol with lower critical temperature and pressure was used for PVC dechlorination and plasticizer recovery. It was found that the removed chlorine eventually entered into the gas phase products, which brought difficulties to the chlorine recovery. Simultaneously, some new phthalate esters were formed in the supercritical methanol treatment system because of the transesterification between DEHP and critical methanol molecules. So, the environmental risks posed by PAEs still cannot be efficiently eliminated by using supercritical methanol treatment technology. Another strategy to overcome the shortcomings of Sub/SCW treatment technology is to reduce the critical temperature/pressure by adding alcohol to construct subcritical water-alcohol (CWalcohol) reaction system or inhibit the acidity corrosion of reaction equipment by introducing alkali to construct subcritical wateralkali (CW-alkali) reaction system. From the point of view of organic chemical reaction, the treatment of DEHP plasticized PVC waste by CW-alcohol system and CW-alkali system may change the reaction path and produce new chemical substances due to the reactivity of alcohol and alkali. The introduction of alcohol or alkali may bring significant influence on the behavior of DEHP and chlorine in subcritical water treatment. However, it is not clear about the migration and transformation behavior of high level of DEHP and chlorine in PVC waste during the treatment by using CWalcohol and CW-alkali processes. Therefore, the purposes of this work are (1) the comparative study for the removal behavior and reaction pathway of chlorine in DEHP plasticized PVC waste in CW-alcohol and CW-alkali treatment systems and (2) the comparative study for the degradation behavior and reaction pathway of DEHP in DEHP plasticized PVC waste in CW-alcohol and CW-alkali treatment systems. On this basis, two new reaction media, CW-alcohol and CW-alkali, were proposed for both dechlorination and DEHP degradation of PVC waste. This study will provide strong support for the further actual engineering application of critical water technology in PVC waste treatment. In this work, CW-NaOH and CW-C2H5OH were selected as model systems for the comparative study.
2. Materials and methods 2.1. Materials and chemicals The DEHP plasticized PVC used in the experiments was purchased from a resin material factory in Shaanxi, China. Pure PVC sample was obtained from DAXIANG material company, Beijing. DEHP was purchased from national pharmaceutical reagent group of China. The formula of PVC resin provided by the resin factory is as follows: PVC powder 100 phr, DEHP 45 phr, fatty acid heat stabilizer 1.5 phr, lubricant 2.5 phr, and processing aid 1 phr. The C and H elemental analysis of DEHP plasticized PVC were conducted by using an elemental analyzer (Thermo Scientific FLASH 2000). We analysed the Cl contents in PVC powder and the treated PVC sample by using an oxygen combustion bomb-ion chromatography (IC, DionexICS2000, USA). The C, H, and Cl analysis results are presented in Table S1. The molar ratio of H to C was 1.70. Prior to the CW-NaOH and CW-C2H5OH experiments, the DEHP plasticized PVC is pre-treated by scissor into pieces with the size of around 1 cm2.
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2.2. CW-NaOH and CW-C2H5OH experiments Fig. 1 shows the schematic drawing of CW-NaOH and CWC2H5OH experiments of DEHP plasticized PVC. The CW-NaOH and CW-C2H5OH experiments were conducted in a 100-mL critical fluid reactor. The reactor material is 316 alloy. Treatment conditions of the experiments are shown in Table S2. The amount of PVC sample used for each experiment is about 3.0 g. The volume of water in the CW-NaOH experiment was 75 ml, and the amount of NaOH added was 2 g. In CW-C2H5OH experiment, the volume of water and ethanol is 50 and 25 ml. The volume ratio of water to ethanol is 2:1. The reaction temperature ranges from 250 to 350 C and the reaction time ranges from 30 to 90 min. In a typical CW-NaOH or CWC2H5OH experiment, about 3.0 g DEHP plasticized PVC sample was first added to the critical fluid reactor with appropriate volume of water or water-ethanol mixture. The screws of the reactor were tightened and sealed. Then the heating power switch was turned on and the temperature in the reactor began to increase. It is the starting point for the reaction time when the system temperature reached the selected value. After treatment, the power was turned off and the reactor was cooled to room temperature in a mixed bath of ice water. Finally, the reactor was opened and the mixture produced after the reaction was separated by filtration using a vacuum filter. The resulting liquid phase product was mixed with n-hexane in a 1:1 ratio (v/v ¼ 1:1). A separating funnel was applied to extract the oil product. The extracted oil phase product was dehydrated with anhydrous sodium sulfate, and then diluted by n-hexane for 5000 times. The obtained oil sample was used for the subsequent GC-MS analysis.
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water phase after all experiments. An FT-IR spectrometer (Nicolet iS50, USA) was used to analyze functional groups of the obtained solid residue from DEHP plasticized PVC samples after CW-NaOH and CW-C2H5OH reactions. 3. Results and discussion 3.1. Dechlorination mechanism of DEHP plasticized PVC in CWNaOH and CW-C2H5OH processes In order to compare the effects of NaOH and C2H5OH on the dechlorination behavior of DEHP plasticized PVC in CW, the dechlorination efficiency (DE) of DEHP plasticized PVC in CWNaOH and CW-C2H5OH at different temperatures was investigated. DE is calculated as follows:
DEð%Þ ¼ ðM2 M1 Þ 100 = M2
(1)
where M1 is the Cl content PVC residue after treatment, M2 the Cl content in DEHP plasticized PVC before treatment. Fig. 2 shows the DE of DEHP plasticized PVC in CW-NaOH and
2.3. Products analysis and characterization The obtained oil sample after extraction and dilution was analysed on a GC-MS apparatus (Agilent 7890A-5975C, USA). The carrier gas flow of GC was 20 ml/min, and the inlet temperature was 250 C. The column temperature was controlled by a temperature programming. The column temperature rose from 60 C to 220 C at 20 C/min and then to 280 C at 5 C/min. An EI ion source was used for mass spectrometry. The ion scan mode was SIM. The temperature of ion source was 230 C. The temperature of quadrupole was 150 C. The pressure of the rear injection port was 8.7 MPa, and the temperature of the rear injection port was 260 C. The Cl content in aqueous solution after reaction was analysed by ion chromatography. The analysis results showed that all of the removed Cl from DEHP plasticized PVC sample was transferred into
Fig. 2. Dechlorination efficiency of DEHP plasticized PVC in CW-NaOH and CWC2H5OH at different temperatures.
Fig. 1. The treatment of DEHP plasticized PVC waste by CW-NaOH and CW-C2H5OH process.
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CW-C2H5OH at 250, 300, and 350 C. For both CW-NaOH and CWC2H5OH treatment systems, the increase of temperature could increase the DE of DEHP plasticized PVC. Apparently, the DE of CWNaOH system is much higher than that of CW-C2H5OH system. For instance, the DE of CW-C2H5OH system was only 75.33% at 250 C, while the DE of CW-NaOH system exceeded 90%. The DE of CW-NaOH system was close to 100% at the reaction temperature of 350 C. This suggested that alkaline system is more conducive to promoting the dechlorination process of DEHP plasticized PVC. The reason could be attributed to the trapping effect of alkaline NaOH on acidic dechlorination product HCl due to the neutralization. The relevant functional group information of the treated PVC samples can be obtained by infrared spectrum analysis of solid residue after the reaction. The functional group information can reveal the dechlorination process of DEHP plasticized PVC samples and help us to understand the dechlorination mechanism. Fig. 3 shows the FT-IR of pure PVC and DEHP plasticized PVC treated by CW-NaOH and CW-C2H5OH at 250, 300, and 350 C. In the FT-IR spectrum of pure PVC before reaction (Fig. 3C), the absorption peaks at 2972.85 cm1 and 2911.42 cm1 are designated as the asymmetric and symmetric stretching vibrations of eCH2- contained in the PVC molecular chain of eCH2-CHCl-. The absorption peak at 1537.19 cm1 can be attributed to the vibration of eCHe in the structure of -CHCl-. The absorption peak at 1429.38 cm1 can be contributed to the symmetrical vibration of eCH2-CHCl-. The strong absorption peak at 1271.27 cm1 is derived from the stretching vibration of -CHCl-. The absorption peak at 1098.75 cm1 is derived from the skeleton bending vibration of -C-C-. The absorption peak at 960.32 cm1 is derived from the in-plane deformation oscillation of eCH2-. The absorption peak at 688.16 cm1 is derived from the stretching vibration of CeCl. Fig. 3A presents the infrared spectroscopy of solid product after CW-NaOH process of DEHP plasticized PVC at 250, 300, and 350 C. The IR peak of 3443 cm1 was derived from the stretching vibration of eOH group. The IR peak of 2919 and 2851 cm1 were derived from the vibration of methylene eCH2. The IR peak of 1434 cm1 was derived from the filler CaCO3 in PVC. The absorption peak at 1650 cm1 was derived from the vibration of polyene structure, and the one at 675 cm1 was derived from the vibration superposition of CeCl and CO2 3 . According to the infrared spectrum analysis results, we propose two possible pathways for the dechlorination of DEHP plasticized PVC in CW-NaOH treatment system. Fig. 4 shows the proposed dechlorination pathways. The eOH group could be derived from polyol structures, which were generated by the nucleophilic substitution of CeCl bonds in PVC molecules (pathway 1 in Fig. 4). The polyene structures could be generated after the direct dehydrochlorination of PVC (pathway 2 in Fig. 4). From Fig. 3A, it can be found that the infrared absorption peaks of hydroxyl and polyene coexist simultaneously, indicating that both the two dechlorination pathways (presented in Fig. 4) exist. However, the strength of hydroxyl peak was much stronger than polyene peak, revealing that the pathway 1 was the main dechlorination mechanism for DEHP plasticized PVC in the CW-NaOH process. In addition, it can be found from Fig. 3A that the absorption peak of CeCl becomes very weak when the reaction temperature increase to 350 C, showing that DEHP plasticized PVC can be fully dechlorinated at 350 C by CW-NaOH treatment. Fig. 3B presents the IR analysis results of DEHP plasticized PVC treated by CW-C2H5OH process at 250, 300, and 350 C. The main infrared absorption peaks of residue obtained after the reaction of CW-C2H5OH are consistent with that of CW-NaOH reaction system. One important change is that the infrared absorption peak of polyene structure was significantly enhanced, indicating that the probability of the direct dehydrochlorination and the forming of polyene (pathway 2) increased. In CW-NaOH reaction system, the
Fig. 3. FT-IR of pure PVC and DEHP plasticized PVC treated by CW-NaOH and CWC2H5OH at 250, 300, and 350 C.
introduction of sodium hydroxide can significantly enhance the acid-base catalytic activity of the system. Thus, hydroxyl nucleophilic substitution reaction becomes the dominant reaction path for PVC dechlorination. However, the introduction of alcohol can significantly reduce the critical temperature and pressure of the CW system, leading to that the ionic chain reaction of direct dehydrochlorination of PVC increases in CW-C2H5OH treatment system. 3.2. Degradation mechanism of DEHP in DEHP plasticized PVC by CW-NaOH treatment Several researches found that dechlorination was the main
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OH
OH
Cl
OH
pathway 1
Nucleophilic substitution Cl
OH
5
Cl
-HCl pathway 2 Dehydrochlorination Fig. 4. Two possible pathways for the dechlorination of DEHP plasticized PVC in CW-NaOH treatment system.
reaction when PVC molecule was treated in critical water below 350 C (Yu et al., 2016; Takeshita et al., 2004). However, the behavior of plasticizer such as DEHP in critical water below 350 C is unclear. Fig. 5 shows the gas chromatography-mass spectrometry analysis results of oil produced from DEHP plasticized PVC after CW-NaOH process at 300 C and 350 C. After DEHP plasticized PVC was treated by CW-NaOH process at 300 C, four main products including 2-ethyl-1-hexanol, benzaldehyde, acetophenone, and toluene were generated. The molecular structure of DEHP is presented in Fig. 6. Apparently, 2-ethyl-1-hexanol was derived from the hydrolysis of DEHP molecule. It was reported that CW had strong acid-base catalytic performance (Wang et al., 2015). In the CW-NaOH system, the introduction of alkali in CW can further enhance the basic catalytic activity of critical water for the hydrolysis of ester groups in DEHP molecules. The catalytic hydrolysis of DEHP resulted in the generation of and phthalic acid and 2-ethyl-1hexanol. The schematic diagram of DEHP degradation pathway in CWNaOH is shown in Fig. 6. From the molecular structure point of view, benzaldehyde and toluene came from further decarboxylation and reduction of phthalic acid. Firstly, the phthalic acid was decomposed and a carboxyl group is removed to form benzoic acid. Then, benzoic acid was reduced to produce benzaldehyde or further reduced to form toluene (The reaction pathway on the left of Fig. 6). However, there is no additional reducing agent in the CW-NaOH reaction system. From Fig. 3, it can be found that a great deal of polyol and polyene are produced by the dechlorination of DEHP plasticized PVC. According to the principle of organic chemical
reaction, both polyol and polyene structures have strong reducibility, and the high level of polyol and polyene forms a reductive reaction surrounding in CW. Therefore, the reduction of benzoic acid to benzaldehyde and toluene in the reaction system could be attributed to the reduction activity of polyol and polyene generated after the dechlorination of DEHP plasticized PVC. Another chemical compound acetophenone with high concentration also could be generated by the degradation of DEHP after CW-NaOH treatment at 300 C (Fig. 5). From the microscopic molecular structure, acetophenone is a benzene ring derivative with eight carbon atoms. In the products hydrolyzed by DEHP, 2-ethyl-1hexanol has exactly eight carbon atoms. This interesting consistency prompted us to speculate that the high level of acetophenone in the products was derived from the further reactions of 2-ethyl-1hexanol. We proposed a possible pathway for the further reactions of 2-ethyl-1-hexanol to produce acetophenone (The reaction pathway on the right of Fig. 6). Firstly, 2-ethyl cyclohexanol was generated by the cyclization reaction of 2-ethyl-1-hexanol in CWNaOH system. Then, the further dehydrogenation and rearrangement reactions of 2-ethyl cyclohexanol occurred and acetophenone was produced. As the treatment temperature of CW-NaOH rose to 350 C, one important change in degradation products was that the relative peak strength of 2-ethyl-1-hexanol decreased significantly, while that of acetophenone increased significantly (Fig. 5). This indicated that as the reaction temperature went up, the reactions of cyclization, dehydrogenation, and rearrangement of 2-ethyl-1-hexanol enhanced and the produced acetophenone increased (The
Fig. 5. Gas chromatography-mass spectrometry analysis results of oil produced from DEHP plasticized PVC after CW-NaOH process at 300 C and 350 C.
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C 2H 5
O
O O
O
C 2H 5
O
NaOH/H 2O C 2H 5
OH OH
+
HO
O
Cyclization
O
C 2H 5 OH
OH
Dehydrogenation/ Rearrangement
O
O H
a
3
CH 3
Fig. 6. The schematic diagram of DEHP degradation pathway in CW-NaOH.
reaction pathway on the right of Fig. 6). Another change in degradation products of DEHP was with reaction temperature increases, the relative content of toluene increased significantly, while that of benzaldehyde decreased, revealing that more generated benzaldehyde could be further reduced to toluene at 350 C (The reaction pathway on the left of Fig. 6). 3.3. Degradation mechanism of DEHP in DEHP plasticized PVC by CW-C2H5OH treatment Fig. 7 shows the gas chromatography-mass spectrometry analysis results of oil produced from DEHP plasticized PVC by CWC2H5OH treatment at 300 C and 350 C. At 300 C, the four main products were 2-ethyl-1-hexanol, benzoic acid ethyl ester, ethyl palmitate, and ethyl stearate, respectively. Except for 2-ethyl-1hexanol, the other three products were completely different from that of CW-NaOH treatment system. Therefore, the decomposition pathways of DEHP in the two treatment systems are quite different. We proposed a possible pathway of the decomposition pathways of DEHP in CW-C2H5OH process. Fig. S1 presents the schematic diagram of DEHP degradation pathway in CW-C2H5OH. Firstly, 2-ethylhexyl benzoate was generated by the Ph-C bond
rupture of DEHP molecule. Then ethyl benzoate and 2-ethyl-1hexanol were produced by the transesterification between 2ethylhexyl benzoate and C2H5OH. In CW-NaOH system, the ester base hydrolysis of molecule was the dominant reaction in the initial stage of DEHP degradation. However, transesterification was the main pathway of DEHP degradation in CW-C2H5OH system in the initial stage of the reaction. The results indicated that C2H5OH has high reactivity for DEHP molecule in CW-C2H5OH medium. The strong reactivity of C2H5OH inhibited the hydrolysis reaction of DEHP and the further of reduction reaction of hydrolysis products (Fig. 6). In addition, acetophenone was not detected in the reaction system of CW-C2H5OH, revealing that the cyclization, dehydrogenation and rearrangement reactions of 2-ethyl-1-hexanol were significantly weakened in CW-C2H5OH when compared to CWNaOH system. Under the reaction condition of CW-C2H5OH at 300 C, we also found two other interesting products with high concentrations: ethyl palmitate and ethyl stearate. Both hexadecanoic acid and octadecanoic acid are important lubricants and are widely used in the manufacture of PVC products. Evidently, ethyl palmitate and ethyl stearate were derived from the esterification reaction between C2H5OH and hexadecanoic acid/octadecanoic acid. This result further confirmed that C2H5OH in CW-C2H5OH system had high reactivity and actively participated in the degradation reaction of DEHP molecule as reactant. When CW-C2H5OH temperature increased to 350 C, the number of peaks in GC-MS analysis result increased notably. The product composition became more complex. In comparison with the products of 300 C, one remarkable change was the disappearance of the peak of 2-ethyl-1-hexanol. Simultaneously, two new chemical compounds with high concentration were found: ethyl-cyclohexane and 1-ethyl-cyclohexene. Therefore, we can draw a conclusion that with the increase of reaction temperature, ethyl-cyclohexane is produced from the dehydration and cyclization of 2-ethyl-1-hexanol (Fig. S1), and 1-ethyl-cyclohexene is produced from the cyclization followed by dehydration of 2-ethyl1-hexanol (Fig. S1). It can be seen from the GC-MS analysis results in Fig. 7 that the two reaction pathways of ethyl-cyclohexane and 1ethyl-cyclohexene exist simultaneously. In addition, the reaction time might influence the final products of reaction. To investigate the effect of the reaction time on the final products, the products obtained from CW-NaOH treatments at 250 C, 300 C, and 350 C for 90 min were all analysed by GC-MS. In comparison with the treatments for 60 min, the results for 90 min showed that the main products (2-ethyl-1-hexanol, benzaldehyde, acetophenone, and toluene) did not change. The only change was the relative peak strength of the products. For the products obtained from CW-C2H5OH treatments at 250 C, 300 C, and 350 C for 90 min, the main products also did not change for every temperature. The main products at 250 C and 300 C for 90 min were 2-ethyl-1-hexanol, benzoic acid ethyl ester, ethyl palmitate, and ethyl stearate. The main products at 350 C for 90 min were ethyl-cyclohexane, 1-ethyl-cyclohexene, ethyl palmitate, and ethyl stearate. This indicated that the reaction time of 60 min was sufficient for the experiments of CW-NaOH and CWC2H5OH at 250 C, 300 C, and 350 C to obtain stable and final products. 3.4. Verification of dechlorination and degradation mechanisms of DEHP plasticized PVC by the simulation of molecular reactions To verify the reaction products and the proposed reactions mechanisms, pure PVC and pure DEHP were also used in the experiments of CW-NaOH and CW-C2H5OH at the same treatment conditions.
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Fig. 7. Gas chromatography-mass spectrometry analysis results of oil produced from DEHP plasticized PVC by CW-C2H5OH treatment at 300 C and 350 C.
After the treatments of pure PVC by CW-NaOH and CW-C2H5OH at 300 C, the infrared spectrum analysis of solid residue after the reactions was performed. The FT-IR of pure PVC treated by CWNaOH and CW-C2H5OH at 300 C is shown in Fig. S2. In the FT-IR of pure PVC treated by CW-NaOH at 300 C, a strong absorption peak at 3445.75 cm1 and an absorption peak at 1660.77 cm1 appeared. The two absorption peak can be attributed to the stretching vibration of eOH group and the vibration of polyene structure, indicating that the nucleophilic substitution of CeCl bonds and the direct dehydrochlorination of PVC coexisted in the treatment process. Also it can be found that the strength of hydroxyl peak was stronger than polyene peak, revealing that the pathway of nucleophilic substitution was the main dechlorination mechanism for pure PVC in the CW-NaOH process. The results were consistent with that of DEHP plasticized PVC at the same treatment conditions of CW-NaOH. In the FT-IR of pure PVC treated by CW-C2H5OH at 300 C, the absorption peak of hydroxyl structure was significantly decreased in comparison with the CW-NaOH system, indicating that the probability of the nucleophilic substitution decreased and the direct dehydrochlorination increased. The results also were consistent with that of DEHP plasticized PVC. Therefore, the
dechlorination mechanism of pure PVC was consistent with that of DEHP plasticized PVC at the same treatment conditions of CWC2H5OH. To verify the degradation mechanisms of DEHP in CW-NaOH process, the products obtained from pure DEHP after the treatment of CW-NaOH at 300 and 350 C were analysed by GC-MS. The results are shown in Fig. S3. In comparison with the results of DEHP plasticized PVC, the main products of pure DEHP after the treatment of CW-NaOH at 300 and 350 C were consistent with that of DEHP plasticized PVC (Fig. 5). The main products of pure DEHP were 2-ethyl-1-hexanol, benzaldehyde, acetophenone, and toluene. With the increase of treatment temperature (from 300 to 350 C), the relative peak strength of 2-ethyl-1-hexanol and benzaldehyde decreased, while the relative peak strength of acetophenone and toluene increased. The change features were consistent with that of DEHP plasticized PVC. Therefore, the degradation mechanisms of DEHP in CW-NaOH proposed in this study are justified. To verify the degradation mechanisms of DEHP in CW-C2H5OH process, the products obtained from pure DEHP after the treatment of CW-C2H5OH at 300 and 350 C were analysed by GC-MS. The results are shown in Fig. S4. It can be found from Fig. S4 that all of
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the main products of pure DEHP after the treatment of CW-C2H5OH at 300 and 350 C are consistent with that of DEHP plasticized PVC (Fig. 7), indicating that the proposed degradation mechanisms of DEHP in CW-C2H5OH also are justified. In comparison with DEHP plasticized PVC, ethyl palmitate and ethyl stearate can not be found in Fig. S4 because of pure DEHP contains no lubricants. According to the experimental results and the reactions pathways analysis of dechlorination and DEHP degradation for DEHP plasticized PVC in CW-NaOH and CW-C2H5OH processes, a complete reactions mechanisms of the DEHP plasticized PVC degradation are presented in Fig. S5. 4. Conclusions The reaction behaviors of DEHP and chlorine of DEHP plasticized PVC in CW-NaOH and CW-C2H5OH treatments were compared in this study. The mechanism of DEHP degradation and dechlorination in CW-NaOH and CW-C2H5OH systems was verified to be different from each other. With the elucidation of mechanism, CW-NaOH and CW-C2H5OH processes were developed as two promising safety management strategies for DEHP plasticized PVC waste. This study provided theoretical basis for the application of CW-NaOH and CW-C2H5OH processes in the actual engineering treatment of DEHP plasticized PVC. (1) The results of this work showed that the DEHP degradation and dechlorination have different mechanisms and pathways in CW-NaOH and CW-C2H5OH processes. Hydroxyl nucleophilic substitution and direct dehydrochlorination coexisted in the dechlorination process of DEHP plasticized PVC by using CW-NaOH and CW-C2H5OH. The alkalinity of CW-NaOH was beneficial to enhance the dechlorination efficiency of DEHP plasticized PVC. (2) Nucleophilic substitution was dominant in the dechlorination process of PVC in CW-NaOH and polyol was the main dechlorination product. The acid-base catalysis of NaOH resulted in the DEHP hydrolysis. The reducibility of polyol in CW-NaOH resulted in the reduction of hydrolysis product benzoic acid. The final reduction products were benzaldehyde and toluene. Acetophenone was produced by the further reaction of another hydrolysis product 2-ethyl-1hexanol. (3) Plenty of polyol and polyene were formed after CW-C2H5OH treatment due to the equal reactions of hydroxyl nucleophilic substitution and direct dehydrochlorination of DEHP plasticized PVC. C2H5OH was high reactivity in CW-C2H5OH system and the esterification of C2H5OH was dominant in the degradation of DEHP. The products of the esterification reaction mainly included benzoic acid ethyl ester, ethyl palmitate, ethyl stearate.
CRediT authorship contribution statement Fu-Rong Xiu: Conceptualization, Methodology, Investigation, Writing - original draft, Validation. Yongwei Lu: Investigation, Data curation, Visualization, Software. Yingying Qi: Supervision, Visualization, Writing - review & editing. Acknowledgements This work was supported financially by the National Natural Science Foundation of China (No.21605018), the Natural Science Basic Research Project of Shaanxi Province of China (No.2018JM5149), the Foundation Research Project of Shaanxi
Provincial Key Laboratory of Geological Support for Coal Green Exploitation (No.MTy2019-05), and the Natural Science Foundation of Fujian Province of China (No.2017J01472). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.chemosphere.2020.126138. References Barakat, R., Lin, P.C.P., Rattan, S., Brehm, E., Canisso, I.F., Abosalum, M.E., Flaws, J.A., Hess, R., Ko, C.M., 2017. Prenatal exposure to DEHP induces premature reproductive senescence in male mice. Toxicol. Sci. 156, 96e108. Chen, C.Y., 2010. The oxidation of di-(2-ethylhexyl) phthalate (DEHP) in aqueous solution by UV/H2O2 photolysis. Water, Air, Soil Pollut. 209, 411e417. Chung, Y.C., Chen, C.Y., 2009. Degradation of di-(2-ethylhexyl) phthalate (DEHP) by TiO2 photocatalysis. Water, Air, Soil Pollut. 200, 191e198. Hammad, K.M., Jung, J.Y., 2008. Ozonation catalyzed by homogeneous and heterogeneous catalysts for degradation of DEHP in aqueous phase. Chemosphere 72, 690e696. Hankett, J.M., Welle, A., Lahann, J., Chen, Z., 2014. Evaluating UV/H2H2 exposure as a DEHP degradation treatment for plasticized PVC. J. Appl. Polym. Sci. 131, 318e323. Hao, Y., Zheng, G., Li, Q., Xu, H., Zhang, Y., Yan, L., 2013. Combined toxic effects of DBP and DEHP on sperm in male mice. In: London: Informatics and Management Science I, 204. Springer, pp. 729e734. Hapipi, B.M.Y.A.M., Hiroki, S., Azhar, U.M., Yoshiei, K., 2018. Dechlorination of polyvinyl chloride under superheated steam with catalysts and adsorbents. Energy Fuel. 32, 7792e7799. Heudorf, U., Mersch-Sundermann, V., Angerer, J., 2007. Phthalates: toxicology and exposure. Int. J. Hyg Environ. Health 210, 623e634. Huang, J., Li, X., Ma, M., Li, D., 2017. Removal of di-(2-ethylhexyl) phthalate from aqueous solution by UV/peroxymonosulfate: influencing factors and reaction pathways. Chem. Eng. J. 314, 182e191. , M., Slavík, R., 2012. 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J. 147, 789e796. Qi, Y., Ma, J., Chen, X., Xiu, F.R., Chen, Y., Lu, Y., 2020. Practical aptamer-based assay of heavy metal mercury ion in contaminated environmental samples: convenience and sensitivity. Anal. Bioanal. Chem. 412, 439e448. Singh, N., Dalal, V., Mahto, J.K., Kumar, P., 2017. Biodegradation of phthalic acid esters (PAEs) and in silico structural characterization of mono-2-ethylhexyl phthalate (MEHP) hydrolase on the basis of close structural homolog. J. Hazard Mater. 338, 11e22. Takeshita, Y., Kato, K., Takahashi, K., Sato, Y., Nishi, S., 2004. Basic study on treatment of waste polyvinyl chloride plastics by hydrothermal decomposition in subcritical and supercritical regions. J. Supercrit. Fluids 31, 185e193. Wang, B.F., Huang, Y., Zhang, J., 2014. Hydrothermal liquefaction of lignite, wheat straw and plastic waste in sub-critical water for oil: product distribution. J. Anal. Appl. Pyrol. 110, 382e389. Wang, J., Luo, Y., Teng, Y., Ma, W., Christie, P., Li, Z., 2013. Soil contamination byphthalate esters in Chinese intensive vegetable production systems with different modes of use of plastic film. Environ. Pollut. 180, 265e273. Wang, R., Chen, Y., Xu, Z., 2015. Recycling acetic acid from polarizing film of waste liquid crystal display panels by sub/supercritical water treatments. Environ. Sci. Technol. 49, 5999e6008. ska, M., 2017. Phthalate release in leachate from municipal Wowkonowicz, P., Kijen landfills of central Poland. PloS One 12, e0174986. Xiu, F.R., Li, Y., Qi, Y., Yu, X., He, J., Lu, Y., Gao, X., Deng, Y., Song, Z., 2019. A novel treatment of waste printed circuit boards by low-temperature near-critical aqueous ammonia: debromination and preparation of nitrogen-containing fine chemicals. Waste Manag. 84, 355e363. Xiu, F.R., Qi, Y., Wang, S., Nie, W., Weng, H., Chen, M., 2018. Application of critical water-alcohol composite medium to treat waste printed circuit boards: oil phase products characteristic and debromination. J. Hazard Mater. 344, 333e342. Xiu, F.R., Wang, Y.X., Yu, X., Li, Y.F., Lu, Y.W., Zhou, K., He, J.H., Song, Z.Q., Gao, X., 2020. A novel safety treatment strategy of DEHP-rich flexible polyvinyl chloride
F.-R. Xiu et al. / Chemosphere 249 (2020) 126138 waste through low-temperature critical aqueous ammonia treatment. Sci. Total Environ. 708, 134532. Xiu, F.R., Yu, X., Qi, Y., Li, Y., Lu, Y., Wang, Y., He, J., Zhou, K., Song, Z., Gao, X., 2019. A novel management strategy for removal and degradation of polybrominated diphenyl ethers (PBDEs) in waste printed circuit boards. Waste Manag. 100, 191e198. Xu, J., Lu, Q., Toledo, R.A., Shim, H., 2017. Degradation of di-2-ethylhexyl phthalate (DEHP) by an indigenous isolate Acinetobacter sp. SN13. Int. Biodeter. Biodegr. 117, 205e214. Xu, J., Tazawa, N., Kumagai, S., Kameda, T., Saito, Y., Yoshioka, T., 2018. Simultaneous recovery of high-purity copper and polyvinyl chloride from thin electric cables
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by plasticizer extraction and ball milling. RSC Adv. 8, 6893e6903. Yang, T., Ren, L., Jia, Y., Fan, S., Wang, J., Wang, J., Nahurira, R., Wang, H., Yan, Y., 2018. Biodegradation of di-(2-ethylhexyl) phthalate by rhodococcus ruber yc-yt1 in contaminated water and soil. Int. J. Environ. Res. Publ. Health 15, 964. Yu, J., Sun, L., Ma, C., Qiao, Y., Yao, H., 2016. Thermal degradation of PVC: a review. Waste Manag. 48, 300e314. Zhang, C., Zeng, G., Huang, D., Lai, C., Huang, C., Li, N., Xu, P., Cheng, M., Zhou, Y., Tang, W., He, X., 2014. Combined removal of di(2-ethylhexyl)phthalate (DEHP) and Pb (ii) by using a cutinase loaded nanoporous gold-polyethyleneimine adsorbent. RSC Adv. 4, 55511e55518.
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DEHP degradation and dechlorination of polyvinyl chloride waste in subcritical water with alkali and ethanol: A comparative study Fu-Rong Xiu a, b, *, Yongwei Lu a, Yingying Qi a, b a b
College of Geology and Environment, Xi’an University of Science and Technology, Xi’an, 710054, People’s Republic of China Shaanxi Provincial Key Laboratory of Geological Support for Coal Green Exploitation, Xi’an, 710054, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
We report two effective processes for DEHP degradation and dechlorination of PVC. CW-NaOH and CW-C2H5OH have different DEHP degradation and dechlorination mechanism. Hydrolysis and reduction were the main degradation pathways of DEHP in CW-NaOH. Transesterification was the main degradation pathway of DEHP in CWC2H5OH. Nucleophilic substitution and dehydrochlorination co-existed in dechlorination.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 September 2019 Received in revised form 6 January 2020 Accepted 5 February 2020 Available online 6 February 2020
In this study, subcritical water-NaOH (CWeNaOH) and subcritical water-C2H5OH (CWeC2H5OH) processes were developed for diethylhexyl phthalate (DEHP) degradation and dechlorination of polyvinyl chloride (PVC) waste. The introduction of NaOH or C2H5OH in subcritical water had a noticeable influence on the mechanism of DEHP degradation and dechlorination. For both CW-NaOH and CW-C2H5OH treatments, the increase in temperature could increase dechlorination efficiency (DE) of PVC. The DE of CW-NaOH is much higher than that of CW-C2H5OH under the same conditions. The DE of CW-NaOH could exceed 95% at 300 C. Hydroxyl nucleophilic substitution was the main dechlorination mechanism in CW-NaOH, while nucleophilic substitution and direct dehydrochlorination were equally important in CW-C2H5OH. In CW-NaOH treatment, 2-ethyl-1-hexanol, benzaldehyde, and toluene were obtained by hydrolysis and reduction reactions of DEHP. Acetophenone was produced by the further cyclization, dehydrogenation and rearrangement reactions of 2-ethyl-1-hexanol. Transesterification was the main degradation pathway of DEHP in CW-C2H5OH at 300 C. The cyclization and dehydration of 2ethyl-1-hexanol resulted in producing a high level of ethyl-cyclohexane and 1-ethyl-cyclohexene in CWC2H5OH at 350 C. Furthermore, high concentration of ethyl palmitate and ethyl stearate could be prepared in CW-C2H5OH system by the strong reactivity of C2H5OH with the lubricants in PVC. © 2020 Elsevier Ltd. All rights reserved.
Handling Editor: Tsair-Fuh Keywords: PVC Phthalic acid ester DEHP Subcritical water Degradation
1. Introduction * Corresponding author. College of Geology and Environment, Xi’an University of Science and Technology, Xi’an, 710054, People’s Republic of China. E-mail address: [email protected] (F.-R. Xiu). https://doi.org/10.1016/j.chemosphere.2020.126138 0045-6535/© 2020 Elsevier Ltd. All rights reserved.
Phthalic acid esters (PAEs) are an important chemical raw
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material and are widely used as a plasticizer in the production of high-molecular resin materials and various products (e.g. construction material, paint, toy, food containers, and medical devices). Global annual production of PAEs has reached 6 million tons (Wang et al., 2013). Diethylhexyl phthalate (DEHP) is the most common kind of PAEs. The production of DEHP accounted for more than 50% of the total PAEs output (Hao et al., 2013). DEHP can easily be transferred from the end-of-life resin products into various environmental media due to that there is no chemical bond between DEHP molecules and polymer matrix (Heudorf et al., 2007). DEHP is chemically stable and difficult to degrade in the environment. The and hydrolysis half-life of DEHP can reach several decades (Julinova Slavík, 2012). Many studies revealed that DEHP has several biological toxicities such as reproductive toxicity, endocrine toxicity, embryonic development toxicity, and immunotoxicity (Barakat et al., 2017; Qi et al., 2019). Therefore, the high-efficiency degradation of DEHP in the end-of-life resin materials is very important for the safety management of DEHP plasticized resin waste. Polyvinyl chloride (PVC) is one of the most widely used plastics in the world (Yu et al., 2016). DEHP is the most frequently used plasticizer in PVC due to the good compatibility between DEHP and PVC molecule (Xu et al., 2018; Xiu et al., 2020). The quality percentage of DEHP in soft PVC plastic products can reach as high as 40%e50% (Yang et al., 2018). Another notable feature of the PVC product is its high chlorine content (Hapipi et al., 2018). Therefore, the safety management of PVC waste faces two great challenges: the high level of chlorine and high toxicity DEHP. At present, the main final disposal methods of PVC waste are landfill and incineration (Yu et al., 2016). However, landfill inevitably leads to the migration of high content of DEHP and other toxics in PVC products into various environmental media, such as soil, water, and atmo ska, 2017; Qi et al., 2020). Incinsphere (Wowkonowicz and Kijen eration is easy to produce chlorinated dioxins and other highly toxic substances (Yu et al., 2016). The treatment methods of landfill and incineration for PVC waste can bring great risks to the environment and human health. Therefore, before the PVC waste was sent to final disposal, efficient removal or degradation of DEHP and Cl in PVC waste can greatly reduce the environmental risks caused by DEHP and Cl. It is crucial for the safety management of DEHP plasticized PVC waste. In recent years, many technologies have been proposed for the removal and degradation of DEHP including adsorption separation (Zhang et al., 2014), TiO2 photocatalysis (Chung and Chen, 2009), ozonation (Hammad and Jung, 2008), UV photolysis (Huang et al., 2017; Chen, 2010), and microbial degradation (Xu et al., 2017; Singh et al., 2017). However, these studies mainly focus on the degradation of low-level DEHP in environmental media such as natural water and sediment. It is difficult for the reported methods to be effectively applied to the actual engineering treatment of PVC solid waste with DEHP content over 40%. In fact, the degradation of DEHP before DEHP plasticized PVC waste enters the environment can effectively manage and control the amount of DEHP releasing into the environment. Therefore, it is urgent to developed new and efficient processes for the degradation of DEHP contained in PVC waste. Lately, Hankett et al. tested the UV/H2O2 treatment for the DEHP degradation in plasticized PVC (Hankett et al., 2014). But the degradation efficiency was low and the UV exposure time was long. Furthermore, all of the reported methods have no effect on the chlorine remove of PVC waste. Over the last two decades, sub and supercritical water (Sub/ SCW) technology has been widely used as an efficient treatment for organic solid waste due to its unique physicochemical properties (Posmanik et al., 2017; Wang et al., 2014). Several studies found that PVC molecules could be effectively dechlorinated and decomposed
in Sub/SCW (Nagai et al., 2007; Takeshita et al., 2004) and high value-added chemical raw materials or chemical products could be obtained from PVC waste under certain conditions (Yu et al., 2016). However, the high temperature and high pressure conditions of Sub/SCW treatment technology and the acidity corrosion of reaction equipment greatly limit its further practical application (Xiu et al., 2018, Xiu et al., 2019a, 2019b). In our previous study (Qi et al., 2018) supercritical methanol with lower critical temperature and pressure was used for PVC dechlorination and plasticizer recovery. It was found that the removed chlorine eventually entered into the gas phase products, which brought difficulties to the chlorine recovery. Simultaneously, some new phthalate esters were formed in the supercritical methanol treatment system because of the transesterification between DEHP and critical methanol molecules. So, the environmental risks posed by PAEs still cannot be efficiently eliminated by using supercritical methanol treatment technology. Another strategy to overcome the shortcomings of Sub/SCW treatment technology is to reduce the critical temperature/pressure by adding alcohol to construct subcritical water-alcohol (CWalcohol) reaction system or inhibit the acidity corrosion of reaction equipment by introducing alkali to construct subcritical wateralkali (CW-alkali) reaction system. From the point of view of organic chemical reaction, the treatment of DEHP plasticized PVC waste by CW-alcohol system and CW-alkali system may change the reaction path and produce new chemical substances due to the reactivity of alcohol and alkali. The introduction of alcohol or alkali may bring significant influence on the behavior of DEHP and chlorine in subcritical water treatment. However, it is not clear about the migration and transformation behavior of high level of DEHP and chlorine in PVC waste during the treatment by using CWalcohol and CW-alkali processes. Therefore, the purposes of this work are (1) the comparative study for the removal behavior and reaction pathway of chlorine in DEHP plasticized PVC waste in CW-alcohol and CW-alkali treatment systems and (2) the comparative study for the degradation behavior and reaction pathway of DEHP in DEHP plasticized PVC waste in CW-alcohol and CW-alkali treatment systems. On this basis, two new reaction media, CW-alcohol and CW-alkali, were proposed for both dechlorination and DEHP degradation of PVC waste. This study will provide strong support for the further actual engineering application of critical water technology in PVC waste treatment. In this work, CW-NaOH and CW-C2H5OH were selected as model systems for the comparative study.
2. Materials and methods 2.1. Materials and chemicals The DEHP plasticized PVC used in the experiments was purchased from a resin material factory in Shaanxi, China. Pure PVC sample was obtained from DAXIANG material company, Beijing. DEHP was purchased from national pharmaceutical reagent group of China. The formula of PVC resin provided by the resin factory is as follows: PVC powder 100 phr, DEHP 45 phr, fatty acid heat stabilizer 1.5 phr, lubricant 2.5 phr, and processing aid 1 phr. The C and H elemental analysis of DEHP plasticized PVC were conducted by using an elemental analyzer (Thermo Scientific FLASH 2000). We analysed the Cl contents in PVC powder and the treated PVC sample by using an oxygen combustion bomb-ion chromatography (IC, DionexICS2000, USA). The C, H, and Cl analysis results are presented in Table S1. The molar ratio of H to C was 1.70. Prior to the CW-NaOH and CW-C2H5OH experiments, the DEHP plasticized PVC is pre-treated by scissor into pieces with the size of around 1 cm2.
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2.2. CW-NaOH and CW-C2H5OH experiments Fig. 1 shows the schematic drawing of CW-NaOH and CWC2H5OH experiments of DEHP plasticized PVC. The CW-NaOH and CW-C2H5OH experiments were conducted in a 100-mL critical fluid reactor. The reactor material is 316 alloy. Treatment conditions of the experiments are shown in Table S2. The amount of PVC sample used for each experiment is about 3.0 g. The volume of water in the CW-NaOH experiment was 75 ml, and the amount of NaOH added was 2 g. In CW-C2H5OH experiment, the volume of water and ethanol is 50 and 25 ml. The volume ratio of water to ethanol is 2:1. The reaction temperature ranges from 250 to 350 C and the reaction time ranges from 30 to 90 min. In a typical CW-NaOH or CWC2H5OH experiment, about 3.0 g DEHP plasticized PVC sample was first added to the critical fluid reactor with appropriate volume of water or water-ethanol mixture. The screws of the reactor were tightened and sealed. Then the heating power switch was turned on and the temperature in the reactor began to increase. It is the starting point for the reaction time when the system temperature reached the selected value. After treatment, the power was turned off and the reactor was cooled to room temperature in a mixed bath of ice water. Finally, the reactor was opened and the mixture produced after the reaction was separated by filtration using a vacuum filter. The resulting liquid phase product was mixed with n-hexane in a 1:1 ratio (v/v ¼ 1:1). A separating funnel was applied to extract the oil product. The extracted oil phase product was dehydrated with anhydrous sodium sulfate, and then diluted by n-hexane for 5000 times. The obtained oil sample was used for the subsequent GC-MS analysis.
3
water phase after all experiments. An FT-IR spectrometer (Nicolet iS50, USA) was used to analyze functional groups of the obtained solid residue from DEHP plasticized PVC samples after CW-NaOH and CW-C2H5OH reactions. 3. Results and discussion 3.1. Dechlorination mechanism of DEHP plasticized PVC in CWNaOH and CW-C2H5OH processes In order to compare the effects of NaOH and C2H5OH on the dechlorination behavior of DEHP plasticized PVC in CW, the dechlorination efficiency (DE) of DEHP plasticized PVC in CWNaOH and CW-C2H5OH at different temperatures was investigated. DE is calculated as follows:
DEð%Þ ¼ ðM2 M1 Þ 100 = M2
(1)
where M1 is the Cl content PVC residue after treatment, M2 the Cl content in DEHP plasticized PVC before treatment. Fig. 2 shows the DE of DEHP plasticized PVC in CW-NaOH and
2.3. Products analysis and characterization The obtained oil sample after extraction and dilution was analysed on a GC-MS apparatus (Agilent 7890A-5975C, USA). The carrier gas flow of GC was 20 ml/min, and the inlet temperature was 250 C. The column temperature was controlled by a temperature programming. The column temperature rose from 60 C to 220 C at 20 C/min and then to 280 C at 5 C/min. An EI ion source was used for mass spectrometry. The ion scan mode was SIM. The temperature of ion source was 230 C. The temperature of quadrupole was 150 C. The pressure of the rear injection port was 8.7 MPa, and the temperature of the rear injection port was 260 C. The Cl content in aqueous solution after reaction was analysed by ion chromatography. The analysis results showed that all of the removed Cl from DEHP plasticized PVC sample was transferred into
Fig. 2. Dechlorination efficiency of DEHP plasticized PVC in CW-NaOH and CWC2H5OH at different temperatures.
Fig. 1. The treatment of DEHP plasticized PVC waste by CW-NaOH and CW-C2H5OH process.
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CW-C2H5OH at 250, 300, and 350 C. For both CW-NaOH and CWC2H5OH treatment systems, the increase of temperature could increase the DE of DEHP plasticized PVC. Apparently, the DE of CWNaOH system is much higher than that of CW-C2H5OH system. For instance, the DE of CW-C2H5OH system was only 75.33% at 250 C, while the DE of CW-NaOH system exceeded 90%. The DE of CW-NaOH system was close to 100% at the reaction temperature of 350 C. This suggested that alkaline system is more conducive to promoting the dechlorination process of DEHP plasticized PVC. The reason could be attributed to the trapping effect of alkaline NaOH on acidic dechlorination product HCl due to the neutralization. The relevant functional group information of the treated PVC samples can be obtained by infrared spectrum analysis of solid residue after the reaction. The functional group information can reveal the dechlorination process of DEHP plasticized PVC samples and help us to understand the dechlorination mechanism. Fig. 3 shows the FT-IR of pure PVC and DEHP plasticized PVC treated by CW-NaOH and CW-C2H5OH at 250, 300, and 350 C. In the FT-IR spectrum of pure PVC before reaction (Fig. 3C), the absorption peaks at 2972.85 cm1 and 2911.42 cm1 are designated as the asymmetric and symmetric stretching vibrations of eCH2- contained in the PVC molecular chain of eCH2-CHCl-. The absorption peak at 1537.19 cm1 can be attributed to the vibration of eCHe in the structure of -CHCl-. The absorption peak at 1429.38 cm1 can be contributed to the symmetrical vibration of eCH2-CHCl-. The strong absorption peak at 1271.27 cm1 is derived from the stretching vibration of -CHCl-. The absorption peak at 1098.75 cm1 is derived from the skeleton bending vibration of -C-C-. The absorption peak at 960.32 cm1 is derived from the in-plane deformation oscillation of eCH2-. The absorption peak at 688.16 cm1 is derived from the stretching vibration of CeCl. Fig. 3A presents the infrared spectroscopy of solid product after CW-NaOH process of DEHP plasticized PVC at 250, 300, and 350 C. The IR peak of 3443 cm1 was derived from the stretching vibration of eOH group. The IR peak of 2919 and 2851 cm1 were derived from the vibration of methylene eCH2. The IR peak of 1434 cm1 was derived from the filler CaCO3 in PVC. The absorption peak at 1650 cm1 was derived from the vibration of polyene structure, and the one at 675 cm1 was derived from the vibration superposition of CeCl and CO2 3 . According to the infrared spectrum analysis results, we propose two possible pathways for the dechlorination of DEHP plasticized PVC in CW-NaOH treatment system. Fig. 4 shows the proposed dechlorination pathways. The eOH group could be derived from polyol structures, which were generated by the nucleophilic substitution of CeCl bonds in PVC molecules (pathway 1 in Fig. 4). The polyene structures could be generated after the direct dehydrochlorination of PVC (pathway 2 in Fig. 4). From Fig. 3A, it can be found that the infrared absorption peaks of hydroxyl and polyene coexist simultaneously, indicating that both the two dechlorination pathways (presented in Fig. 4) exist. However, the strength of hydroxyl peak was much stronger than polyene peak, revealing that the pathway 1 was the main dechlorination mechanism for DEHP plasticized PVC in the CW-NaOH process. In addition, it can be found from Fig. 3A that the absorption peak of CeCl becomes very weak when the reaction temperature increase to 350 C, showing that DEHP plasticized PVC can be fully dechlorinated at 350 C by CW-NaOH treatment. Fig. 3B presents the IR analysis results of DEHP plasticized PVC treated by CW-C2H5OH process at 250, 300, and 350 C. The main infrared absorption peaks of residue obtained after the reaction of CW-C2H5OH are consistent with that of CW-NaOH reaction system. One important change is that the infrared absorption peak of polyene structure was significantly enhanced, indicating that the probability of the direct dehydrochlorination and the forming of polyene (pathway 2) increased. In CW-NaOH reaction system, the
Fig. 3. FT-IR of pure PVC and DEHP plasticized PVC treated by CW-NaOH and CWC2H5OH at 250, 300, and 350 C.
introduction of sodium hydroxide can significantly enhance the acid-base catalytic activity of the system. Thus, hydroxyl nucleophilic substitution reaction becomes the dominant reaction path for PVC dechlorination. However, the introduction of alcohol can significantly reduce the critical temperature and pressure of the CW system, leading to that the ionic chain reaction of direct dehydrochlorination of PVC increases in CW-C2H5OH treatment system. 3.2. Degradation mechanism of DEHP in DEHP plasticized PVC by CW-NaOH treatment Several researches found that dechlorination was the main
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OH
OH
Cl
OH
pathway 1
Nucleophilic substitution Cl
OH
5
Cl
-HCl pathway 2 Dehydrochlorination Fig. 4. Two possible pathways for the dechlorination of DEHP plasticized PVC in CW-NaOH treatment system.
reaction when PVC molecule was treated in critical water below 350 C (Yu et al., 2016; Takeshita et al., 2004). However, the behavior of plasticizer such as DEHP in critical water below 350 C is unclear. Fig. 5 shows the gas chromatography-mass spectrometry analysis results of oil produced from DEHP plasticized PVC after CW-NaOH process at 300 C and 350 C. After DEHP plasticized PVC was treated by CW-NaOH process at 300 C, four main products including 2-ethyl-1-hexanol, benzaldehyde, acetophenone, and toluene were generated. The molecular structure of DEHP is presented in Fig. 6. Apparently, 2-ethyl-1-hexanol was derived from the hydrolysis of DEHP molecule. It was reported that CW had strong acid-base catalytic performance (Wang et al., 2015). In the CW-NaOH system, the introduction of alkali in CW can further enhance the basic catalytic activity of critical water for the hydrolysis of ester groups in DEHP molecules. The catalytic hydrolysis of DEHP resulted in the generation of and phthalic acid and 2-ethyl-1hexanol. The schematic diagram of DEHP degradation pathway in CWNaOH is shown in Fig. 6. From the molecular structure point of view, benzaldehyde and toluene came from further decarboxylation and reduction of phthalic acid. Firstly, the phthalic acid was decomposed and a carboxyl group is removed to form benzoic acid. Then, benzoic acid was reduced to produce benzaldehyde or further reduced to form toluene (The reaction pathway on the left of Fig. 6). However, there is no additional reducing agent in the CW-NaOH reaction system. From Fig. 3, it can be found that a great deal of polyol and polyene are produced by the dechlorination of DEHP plasticized PVC. According to the principle of organic chemical
reaction, both polyol and polyene structures have strong reducibility, and the high level of polyol and polyene forms a reductive reaction surrounding in CW. Therefore, the reduction of benzoic acid to benzaldehyde and toluene in the reaction system could be attributed to the reduction activity of polyol and polyene generated after the dechlorination of DEHP plasticized PVC. Another chemical compound acetophenone with high concentration also could be generated by the degradation of DEHP after CW-NaOH treatment at 300 C (Fig. 5). From the microscopic molecular structure, acetophenone is a benzene ring derivative with eight carbon atoms. In the products hydrolyzed by DEHP, 2-ethyl-1hexanol has exactly eight carbon atoms. This interesting consistency prompted us to speculate that the high level of acetophenone in the products was derived from the further reactions of 2-ethyl-1hexanol. We proposed a possible pathway for the further reactions of 2-ethyl-1-hexanol to produce acetophenone (The reaction pathway on the right of Fig. 6). Firstly, 2-ethyl cyclohexanol was generated by the cyclization reaction of 2-ethyl-1-hexanol in CWNaOH system. Then, the further dehydrogenation and rearrangement reactions of 2-ethyl cyclohexanol occurred and acetophenone was produced. As the treatment temperature of CW-NaOH rose to 350 C, one important change in degradation products was that the relative peak strength of 2-ethyl-1-hexanol decreased significantly, while that of acetophenone increased significantly (Fig. 5). This indicated that as the reaction temperature went up, the reactions of cyclization, dehydrogenation, and rearrangement of 2-ethyl-1-hexanol enhanced and the produced acetophenone increased (The
Fig. 5. Gas chromatography-mass spectrometry analysis results of oil produced from DEHP plasticized PVC after CW-NaOH process at 300 C and 350 C.
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C 2H 5
O
O O
O
C 2H 5
O
NaOH/H 2O C 2H 5
OH OH
+
HO
O
Cyclization
O
C 2H 5 OH
OH
Dehydrogenation/ Rearrangement
O
O H
a
3
CH 3
Fig. 6. The schematic diagram of DEHP degradation pathway in CW-NaOH.
reaction pathway on the right of Fig. 6). Another change in degradation products of DEHP was with reaction temperature increases, the relative content of toluene increased significantly, while that of benzaldehyde decreased, revealing that more generated benzaldehyde could be further reduced to toluene at 350 C (The reaction pathway on the left of Fig. 6). 3.3. Degradation mechanism of DEHP in DEHP plasticized PVC by CW-C2H5OH treatment Fig. 7 shows the gas chromatography-mass spectrometry analysis results of oil produced from DEHP plasticized PVC by CWC2H5OH treatment at 300 C and 350 C. At 300 C, the four main products were 2-ethyl-1-hexanol, benzoic acid ethyl ester, ethyl palmitate, and ethyl stearate, respectively. Except for 2-ethyl-1hexanol, the other three products were completely different from that of CW-NaOH treatment system. Therefore, the decomposition pathways of DEHP in the two treatment systems are quite different. We proposed a possible pathway of the decomposition pathways of DEHP in CW-C2H5OH process. Fig. S1 presents the schematic diagram of DEHP degradation pathway in CW-C2H5OH. Firstly, 2-ethylhexyl benzoate was generated by the Ph-C bond
rupture of DEHP molecule. Then ethyl benzoate and 2-ethyl-1hexanol were produced by the transesterification between 2ethylhexyl benzoate and C2H5OH. In CW-NaOH system, the ester base hydrolysis of molecule was the dominant reaction in the initial stage of DEHP degradation. However, transesterification was the main pathway of DEHP degradation in CW-C2H5OH system in the initial stage of the reaction. The results indicated that C2H5OH has high reactivity for DEHP molecule in CW-C2H5OH medium. The strong reactivity of C2H5OH inhibited the hydrolysis reaction of DEHP and the further of reduction reaction of hydrolysis products (Fig. 6). In addition, acetophenone was not detected in the reaction system of CW-C2H5OH, revealing that the cyclization, dehydrogenation and rearrangement reactions of 2-ethyl-1-hexanol were significantly weakened in CW-C2H5OH when compared to CWNaOH system. Under the reaction condition of CW-C2H5OH at 300 C, we also found two other interesting products with high concentrations: ethyl palmitate and ethyl stearate. Both hexadecanoic acid and octadecanoic acid are important lubricants and are widely used in the manufacture of PVC products. Evidently, ethyl palmitate and ethyl stearate were derived from the esterification reaction between C2H5OH and hexadecanoic acid/octadecanoic acid. This result further confirmed that C2H5OH in CW-C2H5OH system had high reactivity and actively participated in the degradation reaction of DEHP molecule as reactant. When CW-C2H5OH temperature increased to 350 C, the number of peaks in GC-MS analysis result increased notably. The product composition became more complex. In comparison with the products of 300 C, one remarkable change was the disappearance of the peak of 2-ethyl-1-hexanol. Simultaneously, two new chemical compounds with high concentration were found: ethyl-cyclohexane and 1-ethyl-cyclohexene. Therefore, we can draw a conclusion that with the increase of reaction temperature, ethyl-cyclohexane is produced from the dehydration and cyclization of 2-ethyl-1-hexanol (Fig. S1), and 1-ethyl-cyclohexene is produced from the cyclization followed by dehydration of 2-ethyl1-hexanol (Fig. S1). It can be seen from the GC-MS analysis results in Fig. 7 that the two reaction pathways of ethyl-cyclohexane and 1ethyl-cyclohexene exist simultaneously. In addition, the reaction time might influence the final products of reaction. To investigate the effect of the reaction time on the final products, the products obtained from CW-NaOH treatments at 250 C, 300 C, and 350 C for 90 min were all analysed by GC-MS. In comparison with the treatments for 60 min, the results for 90 min showed that the main products (2-ethyl-1-hexanol, benzaldehyde, acetophenone, and toluene) did not change. The only change was the relative peak strength of the products. For the products obtained from CW-C2H5OH treatments at 250 C, 300 C, and 350 C for 90 min, the main products also did not change for every temperature. The main products at 250 C and 300 C for 90 min were 2-ethyl-1-hexanol, benzoic acid ethyl ester, ethyl palmitate, and ethyl stearate. The main products at 350 C for 90 min were ethyl-cyclohexane, 1-ethyl-cyclohexene, ethyl palmitate, and ethyl stearate. This indicated that the reaction time of 60 min was sufficient for the experiments of CW-NaOH and CWC2H5OH at 250 C, 300 C, and 350 C to obtain stable and final products. 3.4. Verification of dechlorination and degradation mechanisms of DEHP plasticized PVC by the simulation of molecular reactions To verify the reaction products and the proposed reactions mechanisms, pure PVC and pure DEHP were also used in the experiments of CW-NaOH and CW-C2H5OH at the same treatment conditions.
F.-R. Xiu et al. / Chemosphere 249 (2020) 126138
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Fig. 7. Gas chromatography-mass spectrometry analysis results of oil produced from DEHP plasticized PVC by CW-C2H5OH treatment at 300 C and 350 C.
After the treatments of pure PVC by CW-NaOH and CW-C2H5OH at 300 C, the infrared spectrum analysis of solid residue after the reactions was performed. The FT-IR of pure PVC treated by CWNaOH and CW-C2H5OH at 300 C is shown in Fig. S2. In the FT-IR of pure PVC treated by CW-NaOH at 300 C, a strong absorption peak at 3445.75 cm1 and an absorption peak at 1660.77 cm1 appeared. The two absorption peak can be attributed to the stretching vibration of eOH group and the vibration of polyene structure, indicating that the nucleophilic substitution of CeCl bonds and the direct dehydrochlorination of PVC coexisted in the treatment process. Also it can be found that the strength of hydroxyl peak was stronger than polyene peak, revealing that the pathway of nucleophilic substitution was the main dechlorination mechanism for pure PVC in the CW-NaOH process. The results were consistent with that of DEHP plasticized PVC at the same treatment conditions of CW-NaOH. In the FT-IR of pure PVC treated by CW-C2H5OH at 300 C, the absorption peak of hydroxyl structure was significantly decreased in comparison with the CW-NaOH system, indicating that the probability of the nucleophilic substitution decreased and the direct dehydrochlorination increased. The results also were consistent with that of DEHP plasticized PVC. Therefore, the
dechlorination mechanism of pure PVC was consistent with that of DEHP plasticized PVC at the same treatment conditions of CWC2H5OH. To verify the degradation mechanisms of DEHP in CW-NaOH process, the products obtained from pure DEHP after the treatment of CW-NaOH at 300 and 350 C were analysed by GC-MS. The results are shown in Fig. S3. In comparison with the results of DEHP plasticized PVC, the main products of pure DEHP after the treatment of CW-NaOH at 300 and 350 C were consistent with that of DEHP plasticized PVC (Fig. 5). The main products of pure DEHP were 2-ethyl-1-hexanol, benzaldehyde, acetophenone, and toluene. With the increase of treatment temperature (from 300 to 350 C), the relative peak strength of 2-ethyl-1-hexanol and benzaldehyde decreased, while the relative peak strength of acetophenone and toluene increased. The change features were consistent with that of DEHP plasticized PVC. Therefore, the degradation mechanisms of DEHP in CW-NaOH proposed in this study are justified. To verify the degradation mechanisms of DEHP in CW-C2H5OH process, the products obtained from pure DEHP after the treatment of CW-C2H5OH at 300 and 350 C were analysed by GC-MS. The results are shown in Fig. S4. It can be found from Fig. S4 that all of
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the main products of pure DEHP after the treatment of CW-C2H5OH at 300 and 350 C are consistent with that of DEHP plasticized PVC (Fig. 7), indicating that the proposed degradation mechanisms of DEHP in CW-C2H5OH also are justified. In comparison with DEHP plasticized PVC, ethyl palmitate and ethyl stearate can not be found in Fig. S4 because of pure DEHP contains no lubricants. According to the experimental results and the reactions pathways analysis of dechlorination and DEHP degradation for DEHP plasticized PVC in CW-NaOH and CW-C2H5OH processes, a complete reactions mechanisms of the DEHP plasticized PVC degradation are presented in Fig. S5. 4. Conclusions The reaction behaviors of DEHP and chlorine of DEHP plasticized PVC in CW-NaOH and CW-C2H5OH treatments were compared in this study. The mechanism of DEHP degradation and dechlorination in CW-NaOH and CW-C2H5OH systems was verified to be different from each other. With the elucidation of mechanism, CW-NaOH and CW-C2H5OH processes were developed as two promising safety management strategies for DEHP plasticized PVC waste. This study provided theoretical basis for the application of CW-NaOH and CW-C2H5OH processes in the actual engineering treatment of DEHP plasticized PVC. (1) The results of this work showed that the DEHP degradation and dechlorination have different mechanisms and pathways in CW-NaOH and CW-C2H5OH processes. Hydroxyl nucleophilic substitution and direct dehydrochlorination coexisted in the dechlorination process of DEHP plasticized PVC by using CW-NaOH and CW-C2H5OH. The alkalinity of CW-NaOH was beneficial to enhance the dechlorination efficiency of DEHP plasticized PVC. (2) Nucleophilic substitution was dominant in the dechlorination process of PVC in CW-NaOH and polyol was the main dechlorination product. The acid-base catalysis of NaOH resulted in the DEHP hydrolysis. The reducibility of polyol in CW-NaOH resulted in the reduction of hydrolysis product benzoic acid. The final reduction products were benzaldehyde and toluene. Acetophenone was produced by the further reaction of another hydrolysis product 2-ethyl-1hexanol. (3) Plenty of polyol and polyene were formed after CW-C2H5OH treatment due to the equal reactions of hydroxyl nucleophilic substitution and direct dehydrochlorination of DEHP plasticized PVC. C2H5OH was high reactivity in CW-C2H5OH system and the esterification of C2H5OH was dominant in the degradation of DEHP. The products of the esterification reaction mainly included benzoic acid ethyl ester, ethyl palmitate, ethyl stearate.
CRediT authorship contribution statement Fu-Rong Xiu: Conceptualization, Methodology, Investigation, Writing - original draft, Validation. Yongwei Lu: Investigation, Data curation, Visualization, Software. Yingying Qi: Supervision, Visualization, Writing - review & editing. Acknowledgements This work was supported financially by the National Natural Science Foundation of China (No.21605018), the Natural Science Basic Research Project of Shaanxi Province of China (No.2018JM5149), the Foundation Research Project of Shaanxi
Provincial Key Laboratory of Geological Support for Coal Green Exploitation (No.MTy2019-05), and the Natural Science Foundation of Fujian Province of China (No.2017J01472). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.chemosphere.2020.126138. References Barakat, R., Lin, P.C.P., Rattan, S., Brehm, E., Canisso, I.F., Abosalum, M.E., Flaws, J.A., Hess, R., Ko, C.M., 2017. Prenatal exposure to DEHP induces premature reproductive senescence in male mice. Toxicol. Sci. 156, 96e108. Chen, C.Y., 2010. The oxidation of di-(2-ethylhexyl) phthalate (DEHP) in aqueous solution by UV/H2O2 photolysis. Water, Air, Soil Pollut. 209, 411e417. Chung, Y.C., Chen, C.Y., 2009. Degradation of di-(2-ethylhexyl) phthalate (DEHP) by TiO2 photocatalysis. Water, Air, Soil Pollut. 200, 191e198. Hammad, K.M., Jung, J.Y., 2008. Ozonation catalyzed by homogeneous and heterogeneous catalysts for degradation of DEHP in aqueous phase. 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