Introduction

Plastics are widely used because of their outstanding advantages, such as light weight, good ductility, durability, low cost, and ease of processing [1, 2]. The replacement of traditional manufacturing materials by plastics, the dramatic increase in world population, and accessibility to a consumer society are the main reasons for the increasing use of plastic material [3, 4]. Although plastic products have played an important role in protecting people since the outbreak of the COVID-19 epidemic, up to 1.6 million tons of plastic waste are generated every day worldwide [5, 6]. Only about 2% of world plastics are recycled, and the majority (about 92%) are disposed of in landfills or in the natural environment [3, 4]. Plastics need about 200–400 years to be completely degraded in the natural environment due to their highly inert chemical characteristics [7, 8]. Waste plastic in the environment not only causes visual "white pollution" but also destroys the ecological environment, and therefore threatens the animals, plants, microorganisms, and even humans [9, 10].

Among the methods of handling waste plastics, the most widely used is landfill disposal together with disposal of other wastes [11, 12]. Although this method is simple to operate and the initial investment cost is low, it causes secondary pollution to the land and wastes land resources, and even the CH4 and other gases produced after landfill are prone to explosion [13, 14]. Incineration is also a common method of disposing waste plastics [15, 16]. In contrast to the landfill method, incineration has the advantages of a short handling cycle, high efficiency, and small footprint. Besides, the large amount of heat energy released by incineration can be used secondarily for power generation and heating [17, 18]. However, burning plastics produces smoke, solid particles and other substances that are harmful to the environment and humans [19, 20]. In addition, the recycling and reuse of plastics has been a hot topic of research in recent years [21, 22]. With this method, the recycled plastic is processed and turned into valuable substances, effectively improving resource utilization [23, 24].

Supercritical technology to degrade plastics is attracting the attention of researchers [25, 26]. Cheng et al. [27] studied the degradation behavior of carbon fiber reinforced plastics (CFRP) in supercritical alcohols and acetone, and developed a kinetic model for the degradation reaction rate. Different supercritical fluids showed the reaction rate constants for CFRP degradation in the following order: kn-butanol>kacetone>kethanol>kn-propanol>kisopropanol>kmethanol. Degradation efficiency of CFRP increased from 78.38% to 94.45% in supercritical n-butanol at 320 °C for 30 min to 240 min. Bai et al. [28] investigated the effects of temperature (500–800 °C), time (1–60 min), pressure (22–25 MPa) and feedstock concentration (2%–10%, in weight) on gasification characteristics of high impact polystyrene (HIPS) plastic in supercritical water. Gasification efficiency of plastic could be improved by increasing the reaction temperature, time, and decreasing the feedstock concentration, but the variation in pressure had little effect. Under the optimal operating conditions (temperature of 800 °C, time of 60 min, pressure of 23 MPa, and feedstock concentration of 3% in weight), plastic carbon conversion reached 94.48%. No auxiliaries or catalysts were added in these studies, so the reaction conditions were relatively harsh, such as higher temperatures and pressures.

Adding auxiliaries or catalysts that can make the reaction or degradation conditions mild has been a hot topic of study in recent years. Onwudili and Williams [29] investigated the catalytic supercritical water gasification (SWGC) of olefin plastics (polystyrene: PS, low density polyethylene: LDPE, high density polyethylene: HDPE, and polypropylene: PP) in the presence of RuO2/Al2O3 catalysts to produce CH4. The highest amount of CH4 produced from LDPE was 37 mol/kg at 450 °C for 60 min. Catalysis mechanisms involved in the gasification of plastics include steam reforming, water–gas shift reaction (WGSR) and methanation. CH4, H2 and CO2 were produced at high temperature from the plastics, and these gases were converted to more CO and H2 by steam reforming, and RuO2 was reduced to Ru metal. Subsequently, CO was reacted with water via WGSR to produce CO2 and more H2. Steam reforming and WGSR occurred almost simultaneously. Finally, CO2 and H2 were catalyzed by Ru metal to produce CH4. Okajima and Sako [30] evaluated the effect of different catalysts (K2CO3, Na2CO3, KOH and NaOH) in subcritical water on recovered carbon fibers from plastics. All four catalysts showed significant catalytic effects, and among them, K2CO3 was the optimal catalyst with a maximum yield up to 71% of phenolic monomer at 400 °C, 20 MPa, 45 min. Carbonate catalysts had higher activity than hydroxide catalysts, and potassium catalysts had better activity than sodium catalysts.

In the presence of catalysts, it was obvious that the degradation effect of plastics was improved, but the catalysts were easily oxidized or reduced in supercritical fluids. Besides, currently, the main supercritical fluids used to recycle plastics are water, alcohols and ketones [31]. These reaction media not only require high temperature and pressure to reach the supercritical state but are also difficult to remove from the reactor after the reaction because they are liquid at room temperature and pressure [32, 33]. In comparison, supercritical CO2 (Sc-CO2) has milder conditions (critical pressure of 7.29 MPa and critical temperature of 31.26 °C), and it is easily removed with less residue, as CO2 is gas at ambient temperature and pressure. Besides, Sc-CO2 can also thermochemically reduce CO2 or fix CO2 for emission reduction [34, 35]. Applications of Sc-CO2 in different fields have been widely explored, such as the preparation of nanosheet [36], extraction of oil [37], degradation of coal [38], and recycling of power generation [39]. Wei et al. [40] prepared carbon microsphere by converting polyethylene terephthalate (PET) in Sc-CO2 for use in lithium-ion battery anode material. The yield of carbon microsphere was as high as 47.5% at 650 °C for 9 h. Sc-CO2 facilitated the decomposition of PET into aromatic hydrocarbon, which further decomposed to produce carbon microsphere. Our group [41] studied the carbon gasification process of PS in supercritical H2O/CO2. The presence of CO2 increased the carbon conversion efficiency (CE) of the PS, reaching 47.6% of CE at 20 min, 700 °C, and 12.5% of consumed CO2.

Although the application of Sc-CO2 has been very widespread, there are almost no studies on the degradation of plastics in Sc-CO2 [42, 43]. Therefore, in the current work, an approach for the degradation of PS plastics in Sc-CO2 was proposed. Supercritical CO2 not only provided a homogeneous environment for the degradation process of plastics, but also acted as a solvent that reduced the mass transfer resistance between material interfaces [40]. There is also a unique advantage of using Sc-CO2, i.e., after the reaction, CO2 can be completely released as gas at ambient temperature and pressure without affecting the separation of products. Moreover, the solubility of feedstocks in Sc-CO2 is limited and is usually improved by the addition of co-solvents [44]. Thereby, in this work, a base or acid solution was added to Sc-CO2 to assist PS degradation. Besides, the reaction models were designed using response surface methodology (RSM) to explore the effects of different factors on the experimental results and the interactions between the factors. Finally, the degradation results and products under different conditions were analyzed as a  reference for degrading plastics or recovering resources from waste plastics in the future.

Feedstocks and methods

Feedstocks

PS plastic powder was used as the experimental feedstock, and the molecular structure is shown in Fig. S1 (Supplementary Information). Detailed information about the elemental analysis, proximate analysis, and gasification characteristics of PS sample are given in Table S1 and Fig. S2 (Supplementary Information). It was found from the molecular structure and elemental analysis that PS is mainly composed of C and H elements with a content of 98.43% (in weight) and a small amount of O and N. From the gasification results of PS, whether in N2 or CO2 atmosphere, the initial gasification temperature of PS was about 350 °C, and the weight loss reached 100% at about 450 °C. In contrast to N2 atmosphere, CO2 atmosphere could advance the weight loss of PS by about 2 °C.

Methods

All experiments were carried out in a reactor (1 cm inner diameter×15 cm length) made of iron-nickel alloy that could withstand a pressure and temperature of 30 MPa and 800 °C, respectively [41]. Figure 1 shows a schematic diagram of PS plastic degradation in Sc-CO2. PS (0.15 g) and 1 mL base/acid solution were added before each experiment, and the reactor was purged with CO2. A certain amount of CO2 was added before the start of the reaction to maintain the initial pressure of the reactor at 6 MPa. Reactor was set on a heating furnace, the pressure of the reactor increased with the temperature, and the timing was started when the pressure shown on the monitor rose to 7.29 MPa (critical pressure of CO2). Reaction temperature (300–500 °C), time (30–120 min) and concentration of the base/acid solution (1%–5%, in weight) were varied, and the products were collected after reactions. Gases were collected completely at the outlet of the reactor using a 3 L aluminum foil sampling bag (E-Switch, Shanghai Shenyuan Scientific Instruments Co., Ltd., Shanghai, China), followed by opening the reactor and collecting liquids and solid residues with several washes of CH2Cl2. Both liquids and solid residues were obtained separately by centrifugation and dried in an oven for 10 h.

Fig. 1
figure 1

Schematic diagram of PS plastic degradation in Sc-CO2. PS polystyrene, Sc-CO2 supercritical CO2

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To investigate the effects of single or multiple factors on the experimental results, the effects of reaction temperature, reaction time and base/acid solution concentration on the results of PS degradation in Sc-CO2 were studied using the RSM. Box-Behnken design (BBD) gave 17 experiments (Table S2 in Supplementary Information), evaluated the experimental data, and generated response surface. The experimental results were fitted to a second-order polynomial equation, the accuracy of the model was predicted using statistical analysis of variance (ANOVA), and single- and multi-factor interactions were given to optimize the response conditions.

Product analysis

Plastic degradation efficiency (Y) was calculated using Eq. 1. Products obtained from the degradation of PS in Sc-CO2 included gases, liquids and solid residues, and these products were analyzed using different methods. The components of gases products were analyzed using gas chromatograph (GC, Agilent 7890, Agilent Technologies, Santa Clara, CA, USA). Solid residues were analyzed by scanning electron microscope (SEM, Superscan SSX-550, Kyoto, Japan) and energy dispersive X-ray spectroscope (EDS) for their morphologies and elements. Liquid products obtained at the end of reactions were analyzed using a gas chromatograph-mass spectrometer (GC–MS, Agilent 7890, Agilent Technologies, Santa Clara, CA, USA).

$$Y = \frac{{m_{0} }}{m} \times 100\%$$
(1)

where m is the mass of the initial PS, i.e., 0.15 g; m0 is the mass of the solid residue after degradation.

Results and discussion

RSM-BBD results

Experiments were designed with the reaction temperature (A), reaction time (B) and base/acid concentration (C) as independent variables and degradation efficiency (Y) of PS as the response value, and results are shown in Table S2 (Supplementary Information). Quadratic response surface regression analysis was performed on the experimental results to obtain the simulation equations for the degradation efficiency of PS and each independent variable.

$$\begin{gathered} Y_{{{\text{NaOH}}}} = { 142}.{18 }{-} \, 0.{3821}A{-} \, 0.0{31}0{2}B + \, 0.{6814}C + \, 0.000{436}AB{-} \, 0.00{1625}AC + \, 0.00{6}0{3}BC \hfill \\ + \, 0.000{512}A^{2} {-} \, 0.000{398}B^{2} + \, 0.00{994}C^{2} \hfill \\ \end{gathered}$$
$$\begin{gathered} Y_{{{\text{HCl}}}} = { 119}.{18 }{-} \, 0.{2682}A + \, 0.00{412}B + { 1}.{5}0{29}C + \, 0.000{1}0{2}AB{-} \, 0.00{4575}AC + \, 0.0{14}BC \hfill \\ + \, 0.000{411}A^{2} + \, 0.0000{73}B^{2} + \, 0.0{38688}C^{2} \hfill \\ \end{gathered}$$

Table 1 gives the significance tests and ANOVAs for the two regression models. F value means the significance of fitted equation and p-value is used to measure the difference between the predicted and experimental groups. Generally, large F value and low p value (p<0.05) indicate that regression model is significant. For both models, F values were 26.48 and 16.20, respectively, and p values were less than 0.05, which indicated that the model was highly significant. Coefficients R2 of the two models were 0.9715 and 0.9542, showing that the actual and predicted values were well fitted [45]. Accordingly, the models were significantly regressed and the equations could accurately represent the relationship between PS degradation efficiency and each factor. It could be concluded from the F-values that the main order of each factor influencing the degradation efficiencies of PS was A>B>C. This indicated that temperature was the main factor affecting the degradation of plastics and that the base/acid concentration had the least effect on the degradation of plastics.

Table 1 Significance tests and ANOVAs of the regression models
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In order to explore the interaction between the factors, F values of interaction between the factors are given in Table S3 (Supplementary Information). Under the condition of adding NaOH solution, the interaction between factors A and B was the most pronounced and the interaction between factor A and C was the weakest. However, under the condition of adding HCl solution, the interaction between factors B and C was the most pronounced, and the interaction between factors A and B was the weakest. The factor interaction response surface plots and contour plots are given in Fig. 2. As shown in Fig. 2, the degradation efficiencies of PS increased with each factor regardless of adding NaOH or HCl solution, which was also consistent with the optimized results of RSM-BBD. Optimal conditions for PS degradation were a temperature of 500 °C, time of 120 min, and NaOH/HCl concentration of 5% (in weight), and PS degradation efficiencies could reach almost 100%±0.05%. Experiments were conducted under optimal condition. Degradation efficiency of PS was 99.25%±0.05% with addition of NaOH solution, which differed from the predicted value by 0.25%±0.05%. Degradation efficiency of PS was 99.88%±0.05% with addition of HCl solution, which differed from the predicted value by 0.12%±0.05%. Accordingly, the experimental results and the predicted values were similar, indicating that the model was reliable.

Fig. 2
figure 2

a, c Response surface plots and b, d contour plots of interaction between factors of reaction temperature (A), reaction time (B), and NaOH/HCl concentration (C): a, b A–B interaction, under the condition of adding NaOH solution with a concentration of 5% (in weight); c, d B–C interaction, under the condition of adding HCl solution at a reaction temperature of 500 °C

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Effect of different factors on PS degradation

From the RSM-BBD model, it was found that temperature had the greatest effect on plastic degradation, and the degradation efficiencies of PS at different temperatures are given in Fig. 3. Under either condition, more PS was degraded as the temperature increased from 300 °C to 500 °C, and PS was almost completely degraded when the temperature was 500 °C. At 300 °C, 400 °C and 500 °C, the degradation efficiencies of PS compared with those only at high temperature (only PS groups in Fig. 3) were significantly increased from 52.84%±0.05%, 75.38%±0.05% and 97.81%±0.05% to 53.19%±0.05%, 80.58%±0.05% and 97.43% ±0.05% (or 60.58%±0.05%, 81.72%±0.05% and 97.89%±0.05%) by adding NaOH (or HCl) solution, respectively. The introduction of Sc-CO2 created a homogeneous environment for PS to be highly dispersed, and coupled with the solubilization ability of Sc-CO2, the degradation efficiencies of PS were increased to 58.35%±0.05%, 79.86%±0.05% and 98.63%±0.05%, respectively. When NaOH (or HCl) solution and Sc-CO2 acted together, the degradation efficiencies of PS were obviously increased again, up to 82.53%±0.05%, 84.96%±0.05% and 99.25%±0.05% (or 85.42%±0.05%, 90.34%±0.05% and 99.88%±0.05%) at 300 °C, 400 °C and 500 °C, respectively. It was shown that NaOH/HCl solution and Sc-CO2 have a synergistic effect on PS degradation, which was attributed to NaOH/HCl solution acting as co-solvent, which improved the solubility of PS in Sc-CO2 and caused the plastic polymer to swell [46]. In addition, NaOH/HCl also provided OH−/H+, which created a base/acid environment and reduced the reaction activation energy, accelerating the depolymerization of PS [47].

Fig. 3
figure 3

Effect of reaction temperatures on the degradation efficiencies of PS in different environments (reaction time of 120 min, NaOH/HCl concentration of 5% in weight)

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The effect of reaction time and NaOH/HCl concentration on the PS degradation efficiencies is given in Fig. 4. It was found that the degradation efficiencies of PS increased over time in both the Sc-CO2/NaOH and Sc-CO2/HCl systems, from 77.35%±0.05% and 81.46%±0.05% to 86.09%±0.05% and 92.78%±0.05%, respectively, as the reaction time increased from 30 min to 120 min. Likewise, the degradation efficiencies of PS improved from 82.55%±0.05% and 84.62%±0.05% to 86.09%±0.05% and 92.78%±0.05%, respectively, as the concentration of NaOH/HCl increased from 1% to 5% (in weight). However, different from the results of the temperature effect, the degradation efficiencies of PS showed a moderate increase with time from 30 min to 120 min and NaOH/HCl concentration from 1% to 5% (in weight). It was observed that the degradation of PS was more influenced by the reaction time than by the NaOH/HCl concentration, as shown in Fig. 4a and b. Thus, in combination with Figs. 3 and 4, it could also be found that the reaction temperature had the greatest effect on the degradation of PS, followed by the reaction time and then the NaOH/HCl concentration, which was consistent with the RSM-BBD result.

Fig. 4
figure 4

Effect of a reaction time and b NaOH/HCl concentration on PS degradation efficiency at a reaction temperature of 400 °C: a with an NaOH/HCl concentration of 5 wt%; b with a reaction time of 120 min

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Products analysis

The products of PS degradation included mixtures of gases, solids (plastic residues) and liquid oils. First, several blank experiments were conducted, i.e., NaOH or HCl solution (concentration of 5%, in weight) interacted with Sc-CO2 (temperature of 400 °C) without the addition of PS for different times. The results (Table S4 in Supplementary Information) showed that NaOH or HCl solution in Sc-CO2 produced different amounts of H2, and corresponding amount of H2 was subtracted in the subsequent calculation of the gases products after PS degradation. The gas products of PS degradation in Sc-CO2 were H2, CO, CH4 and C2Hx (x=4, 6), and their distributions are given in Fig. 5. As could be seen, PS was converted into more gas with a longer reaction time. With a reaction time of 120 min and with the assistance of NaOH/HCl solution, the total gas volume produced by PS degradation reached 126.88/116.99±5 mL, in which the amounts of H2 and CH4 were 74.18/62.78±5 mL and 31.68/45.29±5 mL, respectively. The molecular structure and elemental analysis showed that PS was mainly composed of C and H elements. CO2 not only promoted the degradation of PS in Sc-CO2, but also reacted with the gas products to produce new CO and more CH4 and C2Hx. It was also found that CO2 consumption increased with the reaction time. With a reaction time of 120 min and with the assistance of NaOH/HCl solution, the consumption of CO2 was 81.2/71.5±5 mL. More CO2 was consumed by the reaction with the aid of NaOH solution than HCl solution because a small amount of CO2 reacted with NaOH to form Na2CO3 (Fig. S3 in Supplementary Information), which was verified by the control experiments.

Fig. 5
figure 5

Gas products of PS degradation assisted by a NaOH and, b HCl solution (NaOH/HCl concentration of 5% in weight, reaction temperature of 400 °C)

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SEM (Everhart–Thornley detector, ETD) was used to analyze the pristine PS plastic and solid residues after PS degradation, as shown in Fig. S4 (Supplementary Information) and Fig. 6, respectively. Also, the elements of the PS residues were analyzed by EDS, and the morphology and corresponding atomic percentages of the residues obtained in different reaction environments were different. Surface of the pristine PS plastic was full of streaks, and with the assistance of NaOH, the residues were mainly microspheres and some scattered flocs. In contrast, with the assistance of HCl, the residues were mainly irregular stripes and trace amounts of microspheres. It was observed that the residues after PS degradation contained both C and O elements. The Na element was from NaOH solution, and the Cl element was from HCl solution. In addition, Al, Fe and Ni elements were also observed in the residues after HCl treatment, which were not present in the residues after NaOH treatment. It was indicated that the HCl corroded the reactor and made the metal species leaked from the inside of the reactor. Based on the fact that the addition of HCl solution-assisted degradation of PS was more effective, it was speculated that these leaked metal species might play a catalytic role in addition to the effect of HCl itself. To avoid uncertainties caused by the reactor, it can be treated with anticorrosion treatment inside the reactor or use ceramic materials for its better improvement. Compared with HCl, the PS solid residue obtained by NaOH solution-assisted degradation contains more O element. Inferred from the results of the control experiments, this phenomenon should be caused by the production of Na2CO3, which also verified that NaOH reacted with CO2.

Fig. 6
figure 6

a, b SEM images and c, d EDS results of solid residues from PS degradation assisted by (a, c) NaOH and (b, d) HCl solution with a concentration of 5% in weight (reaction temperature of 400 °C, reaction time of 120 min). Scale bar: 1 μm

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The compositions of liquid products after PS degradation mainly consisted of phenanthrene (C14H10), anthracene (C14H10), 1-phenyl-naphthalene (C16H12), 2-phenyl-naphthalene (C16H12), 1-phenylmethyl-naphthalene (C17H14), fluoranthene (C16H10) triphenylene (C18H12) and 1,3,5-triphenylbenzene (C24H18). Figure 7 shows the compositions and relative contents of the liquid products at different reaction temperatures. Detailed retention times and corresponding structures of these substances are given in Table S6 (Supplementary Information). The amount of material peaks in the liquid products decreased as the temperature increased from 300 °C to 500 °C. Combined with the change in the total amount of gas products with reaction temperature, it was indicated that more liquid was converted to gas at higher temperature. When the temperature was 300 °C, the liquid products contained more biphenyl, fluorene and benzene, which are commonly used as raw materials for the synthesis of plastics. However, these substances were not observed in the liquid products obtained at 400 and 500 °C. It was indicated that the degradation of PS was more complete at 400 °C and 500 °C than at 300 °C. At higher reaction temperatures, the carbon chain structure of PS undergoes condensation, generating more phenanthrene, 1-phenyl-naphthalene, m-terphenyl, triphenylene and other condensed-ring aromatic substances, which are commonly contained in tar. Moreover, the carbon chain structure of PS was changed, which also promoted the production of more H2. This result was consistent with the increase in H2 volume (Table S5 in Supplementary Information) with increasing reaction temperature.

Fig. 7
figure 7

GC–MS of liquid products from PS degradation at different reaction temperatures assisted by a NaOH and, b HCl solution with a concentration of 5% in weight (reaction time of 120 min). The spectra in (a) were shifted upward by 0, 10×107 and 20×107 amu, and those in (b) were shifted upward by 0, 10×107 and 25×107 amu. GC–MS gas chromatograph-mass spectrometer, Amu atomic mass unit

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It was thus inferred that a series of reactions occurred during the degradation of PS. PS first decomposed into oligomers and then into monomers, H2 and CH4. Oligomers from plastic decomposition and CO2 reacted to form monomers, H2, CH4 and CO. In short, Sc-CO2 fluid provided a unique degradation environment to effectively improve the degradation efficiencies of PS. More importantly, CO2 could also react with the products and change the compositions of the products.

$$\mathrm{PS}\to {\mathrm{C}}_{n}{\mathrm{H}}_{m} \, (\mathrm{oligomers})$$
$${\mathrm{C}}_{n}{\mathrm{H}}_{m}\to {\mathrm{C}}_{x}{\mathrm{H}}_{y} \, \left(\mathrm{monomers}\right)+{\mathrm{H}}_{2}+{\mathrm{CH}}_{4}$$
$${\mathrm{C}}_{n}{\mathrm{H}}_{m}+{\mathrm{CO}}_{2}\to {\mathrm{C}}_{x}{\mathrm{H}}_{y} \, \left(\mathrm{monomers}\right)+\mathrm{CO}{+\mathrm{H}}_{2}+{\mathrm{CH}}_{4}$$

Conclusions

In this work, an approach was proposed for the degradation of PS plastics in Sc-CO2 assisted by NaOH or HCl solution. Reaction models were designed using RSM-BBD to investigate the effects of reaction temperature, reaction time and NaOH/HCl concentration on the degradation of PS, which were verified through experiments. The main conclusions of this work are as follows.

  1. 1.

    RSM-BBD results showed that the effects of a single factor on PS degradation efficiencies were A (reaction temperature) >B (reaction time) >C (NaOH/HCl concentration) in order.

  2. 2.

    At a reaction temperature of 300 °C, reaction time of 120 min, NaOH/HCl concentration of 5% in weight, degradation efficiencies of PS increased significantly from 52.84%±0.05% to 58.35%±0.05%, 82.35%±0.05% and 85.42%±0.05% with the introduction of Sc-CO2, Sc-CO2/NaOH solution and Sc-CO2/HCl solution, respectively.

  3. 3.

    At a temperature of 400 °C, time of 120 min, and NaOH/HCl concentration of 5% in weight, the PS degradation with an efficiency of 84.96%±0.05%/90.34%±0.05%, produced 126.88/116.99±5 mL of gases with 74.18/62.78±5 mL of H2, and consumed 81.2/71.5±5 mL of CO2.

  4. 4.

    With the assistance of NaOH solution, the solid residues after PS degradation were mainly microspheres and some scattered flocs. In contrast, with the assistance of HCl solution, the solid residues were mainly irregular stripes and trace amounts of microspheres.

In the process of PS degradation, Sc-CO2/NaOH or Sc-CO2/HCl was introduced to make PS highly dispersed and uniformly heated, and enhanced the solubilization of PS in Sc-CO2, which promoted more complete degradation of PS. More importantly, CO2 could react with the products to fix CO2 and contribute to emission reduction.