Dechlorination of polyvinyl chloride by hydrothermal treatment with cupric ion
Introduction
Polyvinyl chloride (PVC) is made of vinyl chloride by free radical addition polymerization, which is one of the most widely used chlorinated plastics (Özsin and Pütün, 2018). In 2016, the global production of PVC have reached 58.5 million tons and it is expected to grow at an annual rate of 3.2 % by 2021 (Yu et al., 2016), the disposal of used PVC resulting in a large amount of PVC wastes.
At present, PVC wastes is mainly treated by landfill, incineration or pyrolysis (Buekens and Cen, 2011; Sadat-Shojai and Bakhshandeh, 2011; Yu et al., 2016). However, these disposal methods exist obvious shortcomings and high risk due to the Cl in PVC. For example, PVC is difficult to decompose by landfill and the degradation products (e.g., microplastic or chlorinated organic compounds in leachate) can lead to soil and groundwater contamination (Qi et al., 2020; Takeshita et al., 2004; Wowkonowicz and Kijenska, 2017). Even though pyrolysis is an likely suitable method to convert PVC wastes into energy, one noteworthy problem in PVC pyrolysis is that the obtained liquid products normally contains quite amount of Cl. Combustion of PVC or chlorinated liquid fuels will exhaust sorts of harmful substance, such as polychlorinated dibenzo-p-dioxins (dioxins) and polychlorinated dibenzofurans (furans) (Liu et al., 2012; Suresh et al., 2017). Therefore, it is necessary to find effective methods to remove Cl from PVC before burning or pyrolysis.
Previous studies have proved that PVC could be decomposed and effectively dechlorinated by hydrothermal treatment (HTT) to prepare chlorine free solid fuels (Lu et al., 2020; Ma et al., 2019; Poerschmann et al., 2015; Shen, 2016; Soler et al., 2018; Yu et al., 2016; Zhao et al., 2020). Chlorine in PVC can be mostly removed by HTT while temperature higher than 300 ℃ which is still too energy intensive to be feasible for industrial application (Yu et al., 2016). For lower dechlorination temperature, many studies found that pine wood (Huang et al., 2019), bamboo (Lu et al., 2020), corncob (Yao and Ma, 2018) and cellulosic materials (Ma et al., 2019) etc. showed positive synergistic effect on the dechlorination of PVC. However, the defects of biomass additives were that the optimal synergistic temperature was higher than 250 ℃, and the residence time was even up to 4 h.
Except for biomass additives, Zhao found that adding Ni2+ in PVC hydrothermal process can increase the dechlorination efficiency by 6 times at 220℃ for 30 min (Zhao et al., 2017). In the past two decades, employing Cu2+ as a catalyst to participate in organic chemical reaction has been widely investigated (Kolb et al., 2001; Rostovtsev et al., 2002). However, the effect of Cu2+ on the PVC dechlorination by HTT was not focused and well discussed in the previous studies. As copper is easily available, a win-win effect could be expected during the treatment of copper-containing wastes and PVC wastes by co-HTT. For the dechlorination mechanism, the previous studies normally assisted by FT-IR characterization combining with SEM. It is limited to quantitatively identify the dechlorination routes of elimination and substitution. XPS is considered to be a powerful tool give information of the relative amount of different functional groups on the surface of the solid hydrochar.
This work proposes to enhance hydrothermal dechlorination of PVC by Cu2+, intending to reduce the hydrothermal dechlorination temperature. Further information on hydrochar was obtained by utilizing the FT-IR, XPS, SEM analysis. It is expected to provide insights about how Cu2+ catalyzes PVC degradation and Cl removal as well as to study the possible pathways and dechlorination mechanism of PVC HTT with Cu2+. The results indicate that Cu2+ can be utilized as promising catalyst for hydrothermal dechlorination of PVC.
Section snippets
Materials
PVC powder (CAS: 9002-86-2) without modifier with particle size of 0.1−0.2 mm is purchased from Aladdin Industrial Corporation, used for reaction to exclude the influence of other elements in the reaction process. The ultimate analysis of PVC is shown in supplementary material. The content of carbon, hydrogen and oxygen is closed to the theoretical value according to the chemical formula, saying the PVC powder is rational for this research. The oxygen content might be due to the error of the
Effect of Cu2+ on the solid recovery of PVC
Fig. 2 shows the effect of Cu2+ on the solid recovery as hydrochar at different hydrothermal temperature of PVC for 30 min. When the temperature was at 200 ℃, the solid recovery of PVC HTT without and with Cu2+ was above 98 wt.%, showing inconspicuous difference. When the temperature rose to 220 ℃, the solid recovery of PVC HTT with 0.1 mol/L Cu2+ was 58.1 wt.%, significantly lower than that of PVC HTT without Cu2+ (98.71 wt.%), indicating the Cu2+ provide a strong catalyst effect on the
Conclusions
In this study, the effect of Cu2+ on dechlorination of PVC in HTT has been investigated. It was confirmed that Cu2+ exhibited an extraordinary catalytic effect on the degradation and dechlorination of PVC at 220 ℃. After adding 0.1 mol/L Cu2+, the solid recovery was significantly reduced to 58.1 wt.% as compared to 98.71 wt.% in the absence of Cu2+, while the removal efficiency of Cl was remarkably enhanced from 6 wt.% to 58 wt.%. Both the elimination and substitution acted as important roles
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was co-supported by the National Key R&D Program of China (2018YFC1903201), National Natural Science Foundation of China (51762004 & 51906264), Science and Technology Department of Guangxi Zhuang Autonomous (AA17204066), Natural Science Foundation of Guangxi Zhuang Autonomous Region (2016GXNSFCA380027 & 2017GXNSFAA198205), and Fundamental Research Funds for the Central Universities (No. 19lgpy152).
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