Oxidative Degradation of Lurgi Coal-Gasification Wastewater with Mn

Article pubs.acs.org/IECR

Oxidative Degradation of Lurgi Coal-Gasification Wastewater with Mn2O3, Co2O3, and CuO Catalysts in Supercritical Water Yuzhen Wang, Shuzhong Wang,* Yang Guo, Donghai Xu, Yanmeng Gong, Xinying Tang, and Honghe Ma Xi’an Jiaotong University, Suzhou Academy, Suzhou, 215123, China S Supporting Information *

ABSTRACT: Lurgi coal-gasification wastewater was degraded in supercritical water using Mn2O3, Co2O3, and CuO as catalysts. The experiments were performed in a batch reactor at temperatures of 380−460 °C and oxygen ratios of 1.5−3.5. The results involved evaluation of TOC and NH3−N removal efficiencies; detection of the main products in the effluent; XRD, SEM, and BET analyses of the catalysts; and detection of metal ions leached from the catalysts. Maximum TOC and NH3−N removals were found with Co2O3 catalyst at 460 °C and OR = 3.5. The effluent quality could meet class-I criteria of the Integrated Wastewater Discharge Standard (GB 8978-1996). The catalytic effects on pollutant removal were in the order Co2O3 > Mn2O3 > CuO. The major phase of Mn2O3 transformed into MnO2 with a decreasing BET surface area at 460 °C and an oxygen ratio of 3.5. Serious Cu-ion leaching occurred during the process and intensified with increasing temperature.

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INTRODUCTION The coal gasification industry in China has been developing rapidly in recent years. Moreover, the Lurgi gasification technology is used extensively because of its technological maturity and capability of producing large amounts of methane.1 However, the wastewater discharged from the Lurgi process contains high concentrations of toxic and refractory compounds, such as phenolics, ammonia, and pyridine, posing a huge challenge to environmental safety.2 The treatment of Lurgi coal-gasification wastewater (LCGW) commonly includes physicochemical (e.g., ammonia stripping and phenol solvent) and biological methods.3,4 These techniques, although quite effective, still confront some problems, such as poor stability in the solvent extraction process and hazardous sludge production in the biological processes. Supercritical water oxidation (SCWO) is an advanced oxidation process in which water surpasses its thermodynamic critical point (T > 374 °C and P > 22.1 MPa).5 In this case, water is completely miscible with organic substances and gases, creating a homogeneous reaction medium. Thus, SCWO technology offers two advantages over similar treatment processes. First, it occurs at a much higher reaction rate, giving more complete reaction than wet air oxidation. Second, it produces less harmful byproducts than incineration because of its lower operating temperature.6 So far, few studies on the SCWO process for LCGW treatment have been reported. Similar studies on the SCWO of coking wastewater (also with high concentrations of phenolics) show that the SCWO process could give a removal efficiency of total organic carbon (TOC) higher than 99% at 540 °C. However, the necessary reaction temperature for the complete removal of ammonia nitrogen (NH3−N) would be higher than 600 °C.7−9 Given the energy consumption and corrosion stress under high temperature, application of catalysts has attracted considerable attention. Catalysts containing first-row transition metals have been reported to be active for phenolics, NH3, and other refractory compounds. For example, complete oxidation © 2012 American Chemical Society

of phenol could be achieved at a temperature as moderate as 480 °C and an oxidation ratio of 6.29 with CuO/Al2O3 catalyst,10 and 99% NH3 conversion was achieved at a relatively low temperature of 450 °C and an oxygen ratio of 2.98 using Mn−Ce−O.11 Similarly, increased catalytic oxidation rates were also observed for other compounds with different catalysts, as listed in Table 1. Table 1. Examples of Catalysts Used in SCWO Process for the Treatment of Refractory Compounds compound

catalyst(s)

ref(s)

nitrobenzene pyridine quinoline aniline para-aminophenol acetonitrile

MnO2, H4SiW12O40 Pt/γ-Al2O3, MnO2/γ-Al2O3, MnO2/CeO2 MnO2/CuO MnO2/CeO2 CuO/γ-Al2O3, MnO2/γ-Al2O3 MnO2

12,13 14 15 16 17 18

The most promising active metal oxides for phenolics and NH3 degradation have been found to be MnO2 and CuO. However, the crystalline structure of MnO2 is easily transformed into Mn2O3 during the SCWO process.19,20 In addition, the catalysis of CoOx/TiO2 in wet air oxidation was reported to be fairly effective,21 although studies on the catalytic effect of cobalt oxides in SCWO process are rare. We expect that Co2O3 would also provide an excellent catalytic performance. On this basis, in this work, we chose the more stable manganese oxide, namely, Mn2O3, along with CuO and Co2O3 as catalysts for LCGW treatment. The main aim of this work was to make treated LCGW meet class-I criteria specified in the Integrated Wastewater Discharge Standard of China (GB 8978-1996). The Received: Revised: Accepted: Published: 16573

August 17, 2012 December 3, 2012 December 5, 2012 December 5, 2012 dx.doi.org/10.1021/ie302211s | Ind. Eng. Chem. Res. 2012, 51, 16573−16579

Industrial & Engineering Chemistry Research

Article

Information.) A porous frame (10-μm sintered filter, SUS316) was hung in the center of the reactor to hold the catalyst. The catalysts were confined within the frame. The reactor was heated by a 2-kW electric furnace (Wote Machinery), and the internal temperature of the reactor was controlled by a temperature indicator and controller (TIC) using Pt(100) as the temperature sensor. A cooling coil was fixed in the reactor for rapid cooling of the reactor with water at the end of each experiment. Pressure was measured with a pressure gauge and controlled by the total quantity of liquid added to the reactor. For a typical test, a certain volume of wastewater and the catalyst were initially placed in the reactor, and the weight ratio of the catalyst to the wastewater was 1:100. Then, the whole system was purged with argon for 5 min to reduce the influence of residual oxygen on the experimental results as much as possible. This was followed by the addition of a known volume of hydrogen peroxide through a high-pressure metering pump. The reactor was then heated at 15 °C/min (average value) to the desired temperature. As soon as the set temperature was reached, the reactor was held for 2 min before the heating was stopped. At the end of each experiment, the mixture of products emitted from the reactor was promptly cooled by a heat exchanger, depressurized through a back-pressure regulating valve, and separated into gaseous and liquid phases in the gas−liquid separator. The reactor was then cooled by the cooling coil to ambient temperature. The effluents were collected in a conical beaker, and the catalysts in the reactor were obtained after opening the end closure of reactor. Analysis. The TOC in the effluents was evaluated using a total organic carbon analyzer (Euro Tech, ET1020A). COD and NH3−N were determined by individual Merck cell test using a photometer (Merck, Spectroquant Nova 60). Qualitative analysis of the liquid products was performed on a gas chromatograph/mass spectrometer (Agilent, 68905793N) outfitted with an HP-5MS capillary column (50 m × 0.20 mm i.d., 0.33-μm film thickness) and using helium as the carrier gas. The composition of the catalysts was identified by powder Xray diffraction (XRD) measurements. The XRD spectrum (Rigaku, D/max-2200/PC) was obtained with Cu Kα radiation at 30 mA and 40 kV, and the XRD patterns were scanned at 10° to 80° (2θ) with a 0.2° step. Data interpretation was carried out using the software X-POW (STOE) and the database of powder diffraction files (PDFs) of the International Centre for Diffraction Data (ICDD). The crystallite morphologic micrographs were observed by scanning electron microscopy (SEM) (JEOL, JSM-6390A). Surface areas of catalysts were measured using a surface and porosity analyzer (Micromeritics, ASAP2020) based on the BET method. Metal ions in effluents were determined by atomic absorption spectrophotometry (AAS) (Purkinje General, TAS-996). Calculations of Oxygen Concentration. The oxygen ratio (OR) is defined as

concentration limits of TOC and NH3−N specified in the standard are listed in Table 2. The investigations involved Table 2. Concentration Limits of TOC and NH3−N Specified in Integrated Wastewater Discharge Standard of China (GB 8978-1996) item

class I

class II

TOC (mg/L) NH3−N (mg/L)

20 15

30 50

probing the catalytic effects on the TOC and NH3−N removal efficiencies along with the temperature and oxygen ratio, the structural stability of catalysts such as changes in crystalline and morphology, and the leaching of metal ions from catalysts during the SCWO process. The findings are also expected to be useful for selecting promising catalysts with both high activity and stability in LCGW treatment.

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EXPERIMENTAL SECTION Materials. The LCGW sample, with a dark brown color and strong phenolic odor, was collected from a coal-gasification plant located in North China. The initial chemical oxygen demand (COD), TOC, and NH3−N of the LCGW were about 25500, 8520, and 7600 mg/L, respectively. The organic components analyzed by gas chromatography/mass spectrometry (GC/MS) are listed in Table S1 (Supporting Information). Bulk Mn2O3, CuO, and Co2O3 (99.999% pure) were purchased from China National Medicines Co., Ltd., and had Brunauer−Emmett−Teller (BET) surface areas of 30.2, 25.8, and 8.4 m2/g, respectively. Hydrogen peroxide (H2O2, 30 wt % purity, China National Medicines Co., Ltd.) was used as an oxidant. Apparatus and Procedures. The experiments were conducted in an unstirred high-pressure isothermal batch reactor. A schematic diagram of the apparatus is shown in Figure 1. The reactor was made of SUS316 stainless steel and

Figure 1. Schematic diagram of the apparatus: (1) argon cylinder, (2) high-pressure pump, (3) hydrogen peroxide tank, (4) pressure gauge, (5) inlet of cooling water, (6) outlet of cooling water, (7) temperature indicator and controller, (8) temperature indicator, (9) catalyst frame, (10) cooling coil, (11) reactor, (12) electric furnace, (13) backpressure valve, (14) heat exchanger, (15) gas/liquid separator, (16) conical beaker.

OR =

[O2 ]0 [COD]0 + 1.71[NH3−N]0

(1)

where [O2]0 is the concentration of O2 fed into the reactor (mg/L) (calculated by eq 2) and [COD]0 and [NH3−N]0 are the initial COD and NH3−N concentrations, respectively, of the LCGW (mg/L). The value of 1.71[NH3−N]0 is the oxygen demand of NH3−N oxidized to N2 and H2O (calculated by eq 3). As the initial NH3−N concentration is as high as 7600 mg/

was designed to withstand a maximum temperature and pressure up to 480 °C and 30 MPa, respectively. The volume capacity of the reactor was 450 cm3, and the total liquid that can be treated (mixture of LCGW and hydrogen peroxide) was in the range of 45−60 mL when operated at 380−460 °C and 25 MPa. (Detailed calculations are presented in the Supporting 16574

dx.doi.org/10.1021/ie302211s | Ind. Eng. Chem. Res. 2012, 51, 16573−16579


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Figure 2. Effects of temperature on TOC and NH3−N removals at OR of 2.5: (■) without catalyst, (●) with Mn2O3 catalyst, (Δ) with Co2O3 catalyst, (▽) with CuO catalyst.

Figure 3. Effects of OR on TOC and NH3−N removals at 460 °C: (■) without catalyst, (●) with Mn2O3 catalyst, (Δ) with Co2O3 catalyst, (▽) with CuO catalyst.

pressures was not easy to control accurately at several seconds compared with the continuous operation adopted in industry.9,23 We did not study the effects of reaction time on pollutant removal in this study. The reaction time was set as 2 min after the final temperature had been reached. The experimental conditions are summarized in Table S2 (Supporting Information). Effects of Temperature on TOC and NH3−N Removals. Figure 2 shows the effects of temperature on the TOC and NH3−N removals. The results clearly show that increasing the temperature rapidly improved the removals of TOC and NH3− N. Aside from the positive effect of a higher reaction temperature,15 the increasing heating time might also contribute. This is mainly because the reactor was heated at 15 °C/min (average rate) to the desired temperature. The higher the final temperature is, the longer the heating time will

L, its oxygen demand for complete oxidation must be included in the total oxygen demand.

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2H 2O2 → 2H 2O + O2 ↑

(2)

4NH3 + 3O2 → 2N2 + 6H 2O

(3)

RESULTS AND DISCUSSION In this work, we mainly investigated the effects of temperature and OR on pollutant removal. This was mainly based on the following considerations: For the pressure, many related studies reported that the effects of pressure on pollutant removal are minimal at the pressure range of 23−30 MPa.7,8,22 In the following study, the pressure was kept at 25 MPa. For the reaction time, because the experiments were performed in a batch reactor, the reaction time at the desired temperature and 16575



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which would consequently lead to greater formation of light organic compounds and, further, CO2.15,31 Incomplete Oxidation Products of Pollutants. The organic components found in the effluent treated at 380 °C and OR = 2.5 are reported in Table S3 (Supporting Information). The results show that the main compounds in the noncatalytic experiment can be classified as cyclohexanone, aniline, benzoheterocyclic compounds, and phenoxy compounds. Normally, cyclohexanones are formed mainly by the oxidation of cyclohexane or cyclohexanol or the hydrogenation of benzene rings.32 However, the reactions in our experiments occurred in a strong oxidation environment, so the latter means of generating cyclohexanone can be excluded. Therefore, we speculate that cyclohexanones might be the oxidation products of the cyclic hydrocarbons or cyclitol compounds present in LCGW. During catalysis with Mn2O3 and Co2O3, the 3methylcyclopentanone was found in the effluents. This can be explained by the isomerization of cyclohexanone, which further undergoes oxidation to CO2. Aniline was found in both noncatalytic and catalytic cases. This can be explained as the undegraded aniline in LCGW, as aniline is widely present in LCGW and difficult to degrade at lower tempratures.33,34 The heterocyclic compounds found in the noncatalytic case were mainly 3-methylbenzo[b]thiophene and benzothiazole. In contrast, in the catalytic cases, the ringopening products, such as tetramethylthiourea and 2,5diisobutylthiophene were found. This reflects the fact that the catalysts promoted the ring-opening reaction. Moreover, the similar intermediates found in the catalytic cases indicate similar degradation pathways. For the phenoxy compounds, the types of compounds detected in all cases were similar. In the SCWO process, derivatives containing the phenoxy group have been demonstrated to be the main intermediates in phenol oxidation. As more than 70% of the organic compounds in LCGW are phenolics, the degradation pathway of phenol oxidation in SCWO is representative. Studies of phenol oxidation in the catalytic SCWO process showed that the reactions between adsorbed oxygen species and adsorbed phenol are the initial step in the generation of phenoxy radicals.35,36 Then, radical− radical reactions and the combination of two phenoxy radicals followed by tautomerization form the single-ring products and dimers. Phenoxy compounds such as 1-(2-methoxyphenyl)ethanone and acetic acid 2-acetoxy-phenyl ester detected in our experiments might be the combination products of the phenoxy radicals. Krajnc and Levec concluded that the mechanism of phenol oxidation is similar in both the noncatalytic and catalytic SCWO processes after the phenoxy radical is formed.29 Based on our experimental results, we speculate that the role of the catalysts in phenolics oxidation is to generate the phenoxy radicals more quickly. In addition, the diverse activities of catalysts can be explained by the differences in the production rates of phenoxy radicals.37 XRD Analysis of the Catalysts. The XRD patterns of Mn2O3 samples used at different temperatures and ORs are shown in Figure 4. The results show that Mn2O3 was still the dominant phase after SCWO at 380, 420, and 460 °C when the OR was kept at 2.5 (Figure 4, patterns a−c). However, some hints for the existence of MnO2 were found. This indicates that part of the Mn(III) was oxidized to Mn(IV). When the OR was lower than 3, the major crystalline phase remained as Mn2O3 (Figure 4, patterns c and d), and the patterns showed little difference. However, a more dramatic

be. In addition, the heating time of each experiment (>25 min) is very long compared with the reaction time (2 min). Therefore, we speculate that the reactions during the heating time are rather more important than those at the final temperature, particularly at the higher final temperatures. Figure 2 also reveals that the addition of catalysts dramatically improved the degradation of NH3−N. For example, at a temperature of 460 °C, the TOC and NH3−N removal efficiencies were 98.03% and 81.52%, respectively, without catalyst, whereas the values reached 99.45% and 98.13%, respectively, with Mn2O3; 99.94% and 98.89%, respectively, with Co2O3; and 98.8% and 94.67%, respectively, with CuO. In the temperature range of 380−460 °C, the catalytic effects were in the order Co2O3 > Mn2O3 > CuO. In our experiments, the NH3−N removal efficiency reached 81% at 460 °C without catalyst, which is relatively higher than in studies under similar conditions.24,25 This might be due to the presence of more easily oxidized organic compounds in LCGW. Mizuno et al.26 reported that the activation energy in the destruction of NH3 in a waste was slightly lower than that studied by Goto et al. using both the same oxidant and a similar experimental facility.25 They attributed this discrepancy to differences in composition. In addition, the catalytic effects of the reactor wall might also represent a favorable factor. The reactor we used was made of SUS316 stainless steel, whose main components of Fe, Ni, and Cr might form transitionmetal oxide compounds during SCWO and play the role of catalysts.27 A similar study showed that, when inconel beads were packed in the same reactor, the ammonia conversion increased 4-fold.28 Effects of OR on TOC and NH3−N Removals. Figure 3 clearly shows that the order of catalytic effects at 460 °C was the same, namely, Co2O3> Mn2O3> CuO, at ORs from 1.5 to 3.5. However, the catalytic effect of Mn2O3 was almost as high as that of the Co2O3 when the OR reached 3.0. Aside from the promoting effect of the oxygen concentration, changes in the catalyst structure might also contribute. The detailed reasons are discussed in a later section. Under the test conditions, maximum TOC and NH3−N removal efficiencies of 99.97% and 99.83%, respectively, were found with Co2O3 catalyst at 460 °C and OR = 3.5. Moreover, the residual concentrations of TOC and NH3−N in the effluent were 3 and 13 mg/L, respectively, which can meet class-I criteria specified in the Integrated Wastewater Discharge Standard of China (GB 89781996). Figure 3 also reveals that an increase in OR dramatically enhanced the TOC and NH3−N removal efficiencies, especially for NH3−N. The reasons could be as follows: For the noncatalytic experiments, an increase in oxidant would increase the oxidation agent of OH• radicals, which are extremely powerful oxidizing species.29 For the catalytic experiments, the oxidation is most probably initiated by the adsorption of reactant species on the active sites of the catalyst. Adsorbed oxygen is present mainly as the superoxide ion O2−, which can decompose further with the formation of O− ion.29 All three forms of activated oxygen, namely, neutral O2 and ionic O2− and O− species, are strongly electrophilic reactants and much more active than OH• and HO2• radicals.24,30 An increase in oxygen content would increase the regeneration rate of active sites on the catalyst, thereby enhancing the reaction rate in SCWO.24 In addition, an increase in oxygen content would also inhibit the dimerization reactions of aromatic compounds, 16576



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the morphology of MnO2 crystal.39 These findings are in good agreement with the XRD analysis results. In addition, the BET surface area of Mn2O3 decreased from 30.17 to 19.95 m2/g upon use. However, because the catalytic effect at 460 °C and OR = 3.5 was more active, we assume that the negative effect of the decreasing BET surface area was inferior to the positive effect of the crystalline changes from Mn2O3 to MnO2. Figure 5c,d shows that the phase of fresh Co2O3 was amorphous whereas the structure became crystalline with rectangular-shaped crystals after SCWO. Relevant studies also found that accelerate crystal of cobalt oxide can be synthesized in supercritical water environment.40 The crystalline structure of CuO was spherical in shape, and differences between the fresh and used catalysts were hard to be identified (Figure 5c,f). The BET surface areas of the used Co2O3 and CuO were 22.62 and 8.0 m2/g, respectively, which showed minimal changes from the fresh catalysts. Metal Ions Leached from the Catalysts. Figure 6 shows the concentrations of leached metal ions in effluents as

Figure 4. XRD patterns of used Mn2O3 catalyst: (a) 380 °C, OR = 2.5; (b) 420 °C, OR = 2.5; (c) 460 °C, OR = 2.5; (d) 460 °C, OR = 1.5; (e) 460 °C, OR = 3.5.

alteration was observed when the OR was increased to 3.5 (Figure 4, pattern e), in which case the major phase turned into MnO2. This can be explained by the oxidation of Mn2O3 in the presence of excess oxygen. In manganese oxides, the cations exist in mainly three oxidation states (Mn2+, Mn3+, and Mn4+), and the oxides of MnO, Mn3O4, Mn2O3, and MnO2 can be converted from one to another by suitable adjustments of temperature and oxygen partial pressure.38 In this study, OR values higher than 3.5 were efficient for the generation of MnO2 at 460 °C. In addition, changes in the crystalline structure from Mn2O3 to MnO2 might be a factor in the enhanced activity at OR = 3.5. This might be caused by differences in the bond energy of Mn−O and the adsorption rate of oxygen. The crystalline structures of Co2O3 and CuO remained the same as that of the fresh catalysts. Because of space limitations, we did not include the XRD patterns here. Morphology Changes of the Catalysts. To further indentify the crystalline forms of the catalysts, the surface morphologies of fresh Mn2O3, Co2O3, and CuO and that after SCWO at 460 °C and OR = 3.5 were characterized by SEM, as shown in Figure 5. The fresh Mn2O3 particles (Figure 5a) were mainly of spherical shape. However, needlelike crystallites were obtained after SCWO (Figure 5b), which were documented as

Figure 6. Concentrations of leached metal ions in effluents as functions of temperature and OR.

functions of temperature and OR. The results reveal that the Cu ions leached seriously from the catalyst during the SCWO process, and the concentration was several orders higher than those of the Mn and Co ions. As the temperature increased, the concentration of Cu ions increased significantly, whereas he

Figure 5. SEM images of fresh and used catalysts at 460 °C and OR = 3.5: (a) fresh Mn2O3, (b) used Mn2O3, (c) fresh Co2O3, (d) used Co2O3, (e) fresh CuO, (f) used CuO. 16577



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concentrations of Mn and Co ions showed only a slight increase. In the testing temperature scale, the concentration ranges of Cu, Mn, and Co ions were 0.11−1.28, 0.01−0.08, and 0.01−0.04 mg/L, respectively. The solubility of CuO in supercritical water at 380 °C is about 4 mmol/kg of H2O,10 which is equivalent to about 0.3 mg/L Cu. Moreover, the concentration of Cu ions detected in the effluent treated at 380 °C was 0.18 mg/L, indicating that dissolution of CuO was the main means for Cu to enter into the aqueous phase. However, as the temperature increased, concentration of Cu ions in effluent exceeded orders of magnitude higher than the solubility of CuO. This phenomenon indicates that dissolution is not the exclusive way for the leaching of Cu. Other mechanisms, such as erosion, might also contribute. The concentrations of Mn and Co ions were kept at a relatively low level and showed little change with increasing OR. However, the concentration of Cu ions was as high as 1.75 mg/L at an OR of 1.5 and then dramatically decreased with increasing OR. These results indicate that the increasing oxygen content inhibited the leaching of Cu ions. This was mainly due to the oxidation of Cu ions into CuO in the presence of excess oxygen.41

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ASSOCIATED CONTENT

S Supporting Information *

Organic components in LCGW analyzed by GC/MS, detailed calculations of the total liquid volume that can be treated in the reactor, summary of experimental conditions, and organic components in effluent treated at 380 °C and an OR of 2.5. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Part of the study was supported by the Applied Fundamental Research Program of Suzhou (SYG201136).

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REFERENCES

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CONCLUSIONS Catalytic and noncatalytic oxidations of LCGW in supercritical water were conducted in a batch reactor at temperatures from 380 to 460 °C and ORs from 1.5 to 3.5 using Mn2O3, Co2O3, and CuO as catalysts. The following conclusions can be drawn from the results: (1) Increasing the temperature and OR both gave positive effects on the TOC and NH3−N removals during both noncatalytic and catalytic experiments. The catalytic effect on NH3−N was more significant than that on TOC, following the order Co2O3 > Mn2O3 > CuO. The catalysts promote the ring-opening reaction and the production rate of phenoxy radicals. Moreover, the degradation pathways of organic compounds with different catalysts are similar. (2) Maximum TOC and NH3−N removal efficiencies of 99.97% and 99.83%, respectively, were found with Co2O3 catalyst at 460 °C and OR = 3.5. The residual concentrations of TOC and NH3−N in the effluent were 3 and 13 mg/L, respectively, which can meet class-I criteria specified in the Integrated Wastewater Discharge Standard of China (GB 8978-1996). (3) Exposure of the Mn2O3 catalyst to the SCWO environment resulted in crystalline and morphology changes. The major phase of Mn2O3 turned to MnO2 with a decrease in the BET surface area from 30.17 to 19.95 m2/g when the process was operated at 460 °C and OR = 3.5. Cu ions were found to be seriously leached during the SCWO process. In the temperature range of 380− 460 °C and for an OR of 2.5, the concentration range of Cu ions was 0.11−1.28 mg/L. The leaching of Cu ions was inhibited by the increasing OR. As illustrated by the results, Co2O3 showed the highest catalytic effect, and its structure was relatively stable under the tested SCWO conditions. However, given the continuous operation, future investigations should focus on the performances in continuous-type experimental equipment. 16578



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