Abstract

The treatment of toxic and difficult-to-degrade phenolic compounds has become a key issue in the coking, pharmaceutical, and chemical industries. Considering the polymerization and oxidation of phenolic compounds in supercritical water partial oxidation/supercritical water oxidation (SCWPO/SCWO), the present study reviewed the removal efficiency and reaction pathway of phenolic compounds and phenolic waste/wastewater under different reaction conditions. Temperature is the dominant factor affecting the SCWO reaction. When the oxidizing ability is insufficient, the organics polymerize to form phenolic compounds. The gradual increase of oxidant equivalent causes the intermediate product to gradually oxidize to CO2 and H2O completely. Finally, the free radical reaction mechanism is considered to be a typical SCWO reaction mechanism.

INTRODUCTION

Phenol and its derivatives are regarded as highly toxic organic pollutants (Barrios-Martinez et al. 2006), which are placed in the priority pollutant list by the United States Environmental Protection Agency (Ren et al. 2017). At low concentrations, these compounds are teratogenic, mutagenic, and carcinogenic (Vázquez et al. 2007). They can result in damage to the central nervous system, liver, and kidney of humans and animals (Zhang et al. 2013a; De Silva et al. 2017; Ma et al. 2017a). High concentrations of refractory phenolic compounds are commonly produced by the coking, gas, dyeing, pharmaceutical, chemical, and petrochemical industries (Burmistrz et al. 2014; Seif et al. 2016). The phenol concentration of coking wastewater produced during high-temperature partial carbonization of raw coal and gas purification can reach 10,000 mg/L (Ren et al. 2018). Subject to the biological toxicity and chemical structure stability of phenol and its derivatives (Gai et al. 2017; Ma et al. 2017b; Ren et al. 2017; Zhang et al. 2018; Zhao et al. 2018), traditional physicochemical and microbial techniques are not ideal for the treatment of phenolic wastewater (Martínková & Chmátal 2016; Seif et al. 2016; Yan et al. 2016; Wei et al. 2017). During the widely used microbial method, phenolic wastewater produced in a high-temperature environment (Ma et al. 2017b; Meng et al. 2017) has great influence on the biochemical performance of the method due to the presence of a delocalized π bond in the aromatic ring. Moreover, microorganism activity is strongly inhibited by phenolic compounds (Barrios-Martinez et al. 2006). Therefore, the method is also unable to effectively guarantee a stable discharge of wastewater. In addition, handling difficulties and treatment costs of phenolic wastewater indirectly lead a certain amount of industrial wastewater to be discharged directly into surface water bodies (Méndez et al. 2015; UNESCO 2017), which has caused great pressure on water resources and aquatic environments (Sasidharan Pillai & Gupta 2016) in underdeveloped areas. Excessive emissions and illegal discharge of phenolic wastewater can cause significant long-term pollution to water, air, and soil environments (Lu et al. 2014).

The medium entering the supercritical water (SCW) state has extremely strong oxidation performance and has received extensive attention in the treatment of high concentration organic wastewater and organic wet solid waste (García-Jarana et al. 2013). As shown in Figure 1, it is well known now that SCW behaves as a special state of water between its gaseous and liquid states, at temperature and pressure above its thermodynamic critical point (TC = 374.3 °C, PC = 22.1 MPa) (Pińkowska et al. 2012; Gorbaty & Bondarenko 2017). Low density, low dielectric constant, and high ion mass are excellent properties of SCW (Anikeev et al. 2005; Weiss-Hortala et al. 2010; Kıpçak et al. 2011; Chopra & Choudhury 2017; Kıpçak & Akgün 2018). Change in dielectric constant often modifies SCW solubility, which is beneficial to dissolve low volatility matter (Nadjiba et al. 2016; Yu et al. 2016).

Figure 1

Phase diagram of water and the corresponding critical point.

Figure 1

Phase diagram of water and the corresponding critical point.

It is well known that density can be affected by changes in temperature and pressure. However, in addition to that, properties of SCW such as viscosity, dielectric constant, and ion product increase with increased density, while the diffusion coefficient decreases. By changing the temperature and pressure the density of SCW can be effectively regulated, so its related characteristics can also be adjusted. Moreover, water density and dielectric constant are also indicators of the SCW solubility characteristics. Many unique properties of water are determined by its hydrogen bonds, which can exist even under high temperature conditions. In the SCW state, there are fewer hydrogen bonds and weaker polarity; low viscosity and good mixing performance; large diffusion and high mass transfer rate (Gorbaty & Kalinichev 1995; Skarmoutsos et al. 2017); so that organic matter and gases can be dissolved at any ratio in the SCW state (Ding et al. 2014; Yu et al. 2016). In summary, SCW can be used to overcome the mass transfer limitation of the cross-phase (Kalinichev 2017; Wang et al. 2018), and for partial oxidation decomposition of organic matter under high-temperature and high-pressure conditions.

Oxidation ratio (OR) is defined as the ratio between the added amount of oxidant and its theoretical requirement (Qian et al. 2016). Theoretically, the OR value of 1 distinguishes oxidation from partial oxidation. Since non-phenolic organic compounds consume a large amount of oxidant, SCW technology can achieve complete oxidation when OR reaches 300% in the treatment of actual organic wastewater (Du et al. 2013; Yan et al. 2016). Supercritical water oxidation (SCWO, OR ≥1) was proposed as a promising technology for the treatment of high concentration organic wastewater in the 1980s by the American Modell (Modell & Mass 1982). SCWO technology is a new technology, based on SCW as the reaction medium, which degrades and completely oxidizes organic compounds to CO2, H2O, N2, and other harmless small molecules (De Silva et al. 2017; Gong et al. 2017; Chen et al. 2018). Although this technology can treat waste/wastewater in a harmless manner, efficiently degrading organic wastewaters that are difficult to treat by traditional methods, most of the small molecule gas produced cannot be used as an energy source (Zhang et al. 2013b). In contrast with SCWO technology, supercritical water partial oxidation (SCWPO) technology can partially oxidize organic matter to produce hydrogen-rich gas in the treatment of high concentration organic wastewater and wet organic solid waste. Guan et al. (Guan et al. 2011b) confirmed that SCWPO technology is a potential way to convert organic waste/wastewater into hydrogen, a high value-added energy source. Yoshida et al. (Yoshida & Matsumura 2001; Yoshida et al. 2004) also found that SCWPO technology is effective for depolymerization. Other researchers indicated that SCWO/SCWPO technology has a good effect on the treatment of phenolic compounds from phenolic wastewater (Kıpçak & Akgün 2012; Wang et al. 2014; Yan et al. 2016; Cengiz et al. 2017).

SCWO/SCWPO technology has processing advantages over traditional oxidation in the treatment of high concentration organic wastewater and wet organic solid waste with >30 wt.% water (DiLeo et al. 2007). The water content of the material to be treated is commonly high, and the removal of water increases the energy requirements and costs of converting organic matter to the gas or liquid phase. Therefore, the characteristics of the SCWO/SCWPO technology are adequate for this situation. In contrast with the traditional oxidation method, the SCWO process uses water as the reaction medium, which can completely oxidize the pollutants in the wastewater. It has high reaction efficiency, high gas production, and high pressure so that the generated high-pressure gas is easy to store and transport (Yan & Wei 2008). Therefore, SCWO/SCWPO technology has excellent potential and advantages for organic wastewater which is difficult to be treated and effectively purified by traditional methods (Wang et al. 2013), and is gradually being applied worldwide (Yan & Wei 2008; Guo et al. 2015; Cocero 2018).

FORMATION AND INHIBITION OF PHENOLS IN SUPERCRITICAL WATER

Hydrogen production by partial oxidation of phenols in supercritical water

The initial research on SCW technology mainly focused on the optimization of H2 production (Sınaǧ et al. 2003; Chen et al. 2013a), which can produce clean energy. Ge et al. (Ge et al. 2014) studied the partial oxidation of SCW in coal. When gaseous products were detected, it was found that with increased temperature, the proportion of H2 increased and of CO2 decreased. Nevertheless, as typical organic compounds in organic waste/wastewater, phenolics have attracted significant attention due to their high toxicity and chemical stability (Kim & Ihm 2011). Gökkaya et al. (Gökkaya et al. 2015) studied the partial oxidation of phenol under SCW conditions. They observed that gaseous products containing H2, CH4, CO, and CO2 are produced under high temperatures and additive conditions, which is consistent with the results from the partial oxidation of phenol by Guan et al. (Guan et al. 2011b). Wang et al. (Wang et al. 2016) studied the partial oxidation of a mixture of phenol, naphthalene, and acetic acid in a ratio of 1:1:1 at 560 °C, 25 MPa, and OR of 0.2. It was observed that only 10 s are needed to reach equilibrium. The H2 yield was 70.16 mmol/g, and the H2 production rate reached 240.25%. Further, it was proposed that oxygen can promote the production of ring-opening products and inhibit the production of polymers. Jia et al. (Jia et al. 2017) also concluded that SCW technology can produce H2 by treating phenol. Kou et al. (Kou et al. 2018) established that the carbon gasification efficiency (CGE) can reach 98.8% when treated with oil-containing wastewater. Casademont et al. (Casademont et al. 2018) researched olive oil mill wastewater (OMW) at 700 °C and 23 MPa in a continuous reactor, with a residence time of 40.8 s. The H2 yield was 112.5 ± 6.2 mol/kg of dry OMW. It can be concluded that high concentration organic wastewater can achieve a high H2 production rate after SCWO/SCWPO reaction. Su et al. (Su et al. 2015a) used SCW technology to treat Zhundong coal. CGE was close to 100% at 850 °C and 15 min, and the hydrogen molar moisture exceeded 50%.

Both high temperature and additives can promote the production of H2 and other gases by phenolic compounds to a certain extent. Therefore, phenolic wastewater can be used as a raw material to produce H2 (Blasi et al. 2007). H2 has the highest energy content (120 MJ/kg) compared with conventional fuels. Its sole combustion product is water (Seif et al. 2016), which does not pollute the environment. Hence, hydrogen has a broad development prospect as a clean energy source (Xiao et al. 2013).

Polymerization of phenols

During the study of hydrogen production optimization in SCW, it was found that the intermediate products were prone to polymerization of phenol and its derivatives during the cooling process. At first, the synthesis of phenolic compounds was observed in waste/wastewater without phenolic compounds during SCW partial oxidation (Sınaǧ et al. 2003; Susanti et al. 2012; Cao et al. 2016; Casademont et al. 2018). Chen et al. (Chen et al. 2013b) treated sewage sludge in SCW, during which phenols, nitrogen-containing and nitrogen-free aromatic compounds, such as quinazoline, were detected in the liquid phase. Xu et al. (Xu et al. 2018) observed that oil-containing wastewater is polymerized into alkanes, alkenes, and naphthenes during SCW gasification. Further, phenols and polycyclic aromatic hydrocarbons (PAHs) are formed by cyclization and condensation or the Diels-Alder reaction (Korzenski & Kolis 1997). The PAHs formation mechanism, which is currently recognized by the Diels-Alder reaction based on hydrocarbon dehydrogenation and olefin addition to form cyclic substances that precede PAHs, is shown in Figure 2 (Williams & Taylor 1993; Zhang et al. 2011). Later, many studies have found that the synthesis and degradation of phenolic compounds exist simultaneously in the SCW (Su et al. 2015b; Zhu et al. 2018), and they can also synthesize PAHs (Sharma & Hajaligol 2003). In the product of SCW treatment of phenol and protein mixture, Su et al. (Su et al. 2015b) detected a large amount of phenols and bicyclic aromatic hydrocarbon products. Lignin and complex phenols were hydrolysed into simple phenols and acids, which formed new phenols by condensation and cross-linking reactions. New phenols polymerized further into coke and tar. Many studies on lignin (Gökkaya et al. 2015) and alkali lignin (Pińkowska et al. 2012; Wang et al. 2015) have also found that a variety of organic intermediates can be produced in the process of SCW, including varieties of phenolic compounds such as guaiacol and cresol.

Figure 2

Reactions leading to the formation of PAHs in pyrolysis (Zhang et al. 2011).

Figure 2

Reactions leading to the formation of PAHs in pyrolysis (Zhang et al. 2011).

The formation of phenolic compounds is mainly due to the polymerization of cyclic substances. During the SCW reaction, phenol-free organic matter will first form alkanes, alkenes, and cycloalkanes, and then a ring by alkane dehydrogenation, olefin addition, or condensation. The substance then becomes a phenolic compound, which can further synthesize PAHs. Phenolics can form new phenolic compounds by condensation and cross-linking and can continue to polymerize into coke and tar.

Inhibition of phenols

In order to inhibit the formation of phenolic compounds, it is necessary to study and analyse the synthesis mechanism of phenolic compounds in the reaction of phenolic and phenol-free waste/wastewater, where the reaction direction and path are affected by the reactants themselves and other factors. High temperature (Weiss-Hortala et al. 2010) is beneficial to the degradation and conversion of phenolic compounds. In order to reduce energy consumption and equipment requirements, organic matter conversion can be promoted by adding an appropriate amount of oxidant (Matsumura et al. 2000). Gong et al. (Gong et al. 2016) conducted a SCWO/SCWPO study in a tubular reactor using quinazoline as the test material. The results showed that the total organic carbon (TOC) removal rate was 97.2% at 600 °C and OR of 4.0. Pérez et al. (Pérez et al. 2004) performed SCWPO of phenol at 393–505 °C, 25 MPa, 30–60 s, and OR of 0–39%. It was found that the removal rates of phenol and TOC can be increased to 99.98% and 99.77%, respectively. Additionally, in all experimental schemes, the conversion of TOC was consistently lower than the conversion of phenol, indicating the presence of non-phenolic organic compounds in the liquid phase product. Castello et al. (Castello et al. 2015) observed that phenol interferes with the dehydration pathway that impacts the degradation of glucose, mainly affecting the composition of the liquid phase product. In addition to ring-opening products, phenolic compounds also condensate into undesired polymers that are not as easily degraded into small molecules as ring-opening products. Guan et al. (Guan et al. 2011b) also observed dimers such as dibenzofuran, 2-phenoxyphenol, 4-phenoxyphenol, and 2,20-phenoxyphenol in the liquid phase product of partial oxidation of phenol. The removal rate of TOC was 96%, under the same conditions of coking wastewater treatment. Matsumura et al. (Matsumura et al. 2000) oxidized phenol (2 wt.%) in SCW at 400 °C and 25 MPa. As the reaction progressed, the polymer content constantly changed, including 1,1-biphenyl-2,2-diol and dibenzofuran. DiLeo et al. (DiLeo et al. 2007) observed that phenol and guaiacol produced dibenzofuran and biphenyl at 600 °C. Wang et al. (Wang et al. 2018) also discovered that phenol condensates in the presence of amino acids in SCW, and phenol could be polymerized into various intermediate products. The SCW reaction can rapidly degrade non-phenolic organic matter to produce hydrogen (Susanti et al. 2012), and the SCWO reaction can fully oxidize the chemically stable phenol and its derivatives (Qian et al. 2015).

SCWO technology requires oxygen in excess of 300% to remove phenolic pollutants because phenolic-free organic matter consumes large amounts of oxidants. Table 1 shows published literature on supercritical oxidation and partial oxidation of phenolic compounds. Dong et al. (Dong et al. 2015) studied the treatment of p-nitrophenol wastewater by using compressed air and H2O2 as oxidants under supercritical conditions. They observed that the effect of temperature on the removal efficiency of p-nitrophenol was clearer than that of pressure, residence time, and oxygen excess. Theoretically, H2O2 is more suitable as an oxidant for SCWO of organic compounds than compressed air. Shin et al. (Shin et al. 2009) used SCWPO technology to treat acrylonitrile wastewater. The removal rate of TOC reached 97% in 15 s at 552 °C. Chen et al. (Chen et al. 2017) studied SCWPO of oil-based cuttings. The results showed that the removal rate of TOC can reach 89.2% at 500 °C, 10 min, and OR of 2.5. Du et al. (Du et al. 2013) studied supercritical oxidation of two coking wastewaters containing high concentrations of phenol. The results showed that phenol, COD, and NH3-H were converted completely within 24 s at 650 °C, 25 MPa, and 300% excess oxygen. In addition, it was observed that phenol can be decomposed under almost all testing conditions. Zhang et al. (Zhang et al. 2013b) studied the SCWO desizing wastewater and found that the TOC removal rate was 98.25% at 550 °C, 26 MPa, and OR of 2.5. Zhang et al. (Dong et al. 2015) studied the p-nitrophenol wastewater treatment by SCWO technology at 400 °C, 24 MPa, 36 s, and OR of 2. The removal efficiency of p-nitrophenol and TOC were close to 100% and 97%, respectively.

Table 1

Summary of investigation of SCWO or SCWPO of phenolic compounds in the published literature

FeedOxygen ratioReactor typeReactor sizeExperiment conditionsReferences
Phenol (2 wt.%) 100% H2O2 Stainless steel batch reactor 2.17 mm i.d. × 3.17 mm o.d. 350–450 °C, 25 MPa, 6.5–26 s Matsumura et al. (2000)  
Phenol and 2,4-dinitrophenol (2.7–4 wt.%) 0–39% excess O2 Inconel 625 continuous apparatus in a pilot-scale plant 6.22 mm i.d. 393–505 °C, 25 MPa, 30–60 s, 0.78–0.8 L/min Pérez et al. (2004)  
Acrylonitrile wastewater (0.27–2.10 mol/L) 0.5–2.5 H2O2 Continuous-flow system  299–552 °C, 25 MPa, 3–30 s Shin et al. (2009)  
Phenol (150 mg/L) 0–1.25 H2O2 Flow-type reactor  300–480 °C, 24 MPa, 0–180 s Guan et al. (2011b)  
Coking wastewater (Phenol: FG51,490.38 mg/L, FG12,613.43 mg/L) 0–300% H2O2 310S Stainless steel continuous-flow reactor system 8 mm o.d. × 4 mm i.d. × 6 m l. 500–650°C, 20–27.5 MPa, 10–50 s, 5–20 ml/min Du et al. (2013)  
Polyvinyl alcohol and desizing wastewater (0.2–0.5 wt.%) 100–300% H2O2 Hastelloy C − 276 tubular-flow reactor system 12.3 mm i.d. × 1.7 m l. 320–550°C, 25 ± 1 MPa, 0–45 min, 0–0.2 wt.% NaOH, pH 4.12–13.00 Zhang et al. (2013b)  
Sewage sludge (87 wt.%) 0–4 H2O2 361 L stainless steel batch reactor 572 cm3 350–550 °C, 25 MPa, 20 min Qian et al. (2015)  
p-Nitrophenol wastewater (1,000 mg/L) 1–5 H2O2 Continuous apparatus  400–480°C, 23–27 MPa, 10–60 s, Dong et al. (2015)  
Phenol, naphthalene, and acetic acid (0.5–3 wt.%) 0.0–1.0 H2O2 Hastelloy C276 continuous flow reactor  400–640°C, 25 MPa, 5–40 s Wang et al. (2016)  
Quinazoline (38.4 mmol*L−10–4.0 H2O2 316 stainless steel batch coiled tube reactors 8 mm i.d. × 14 mm o.d. × 2.5 m l. 400–600 °C, 70.79–166.28 kg/m3, 0–400 s Gong et al. (2016)  
FeedOxygen ratioReactor typeReactor sizeExperiment conditionsReferences
Phenol (2 wt.%) 100% H2O2 Stainless steel batch reactor 2.17 mm i.d. × 3.17 mm o.d. 350–450 °C, 25 MPa, 6.5–26 s Matsumura et al. (2000)  
Phenol and 2,4-dinitrophenol (2.7–4 wt.%) 0–39% excess O2 Inconel 625 continuous apparatus in a pilot-scale plant 6.22 mm i.d. 393–505 °C, 25 MPa, 30–60 s, 0.78–0.8 L/min Pérez et al. (2004)  
Acrylonitrile wastewater (0.27–2.10 mol/L) 0.5–2.5 H2O2 Continuous-flow system  299–552 °C, 25 MPa, 3–30 s Shin et al. (2009)  
Phenol (150 mg/L) 0–1.25 H2O2 Flow-type reactor  300–480 °C, 24 MPa, 0–180 s Guan et al. (2011b)  
Coking wastewater (Phenol: FG51,490.38 mg/L, FG12,613.43 mg/L) 0–300% H2O2 310S Stainless steel continuous-flow reactor system 8 mm o.d. × 4 mm i.d. × 6 m l. 500–650°C, 20–27.5 MPa, 10–50 s, 5–20 ml/min Du et al. (2013)  
Polyvinyl alcohol and desizing wastewater (0.2–0.5 wt.%) 100–300% H2O2 Hastelloy C − 276 tubular-flow reactor system 12.3 mm i.d. × 1.7 m l. 320–550°C, 25 ± 1 MPa, 0–45 min, 0–0.2 wt.% NaOH, pH 4.12–13.00 Zhang et al. (2013b)  
Sewage sludge (87 wt.%) 0–4 H2O2 361 L stainless steel batch reactor 572 cm3 350–550 °C, 25 MPa, 20 min Qian et al. (2015)  
p-Nitrophenol wastewater (1,000 mg/L) 1–5 H2O2 Continuous apparatus  400–480°C, 23–27 MPa, 10–60 s, Dong et al. (2015)  
Phenol, naphthalene, and acetic acid (0.5–3 wt.%) 0.0–1.0 H2O2 Hastelloy C276 continuous flow reactor  400–640°C, 25 MPa, 5–40 s Wang et al. (2016)  
Quinazoline (38.4 mmol*L−10–4.0 H2O2 316 stainless steel batch coiled tube reactors 8 mm i.d. × 14 mm o.d. × 2.5 m l. 400–600 °C, 70.79–166.28 kg/m3, 0–400 s Gong et al. (2016)  

The difficulty in the conversion of phenolic compounds lies in the cleavage of the aromatic rings. The hydroxyl group in the phenol affects the ring-opening action, producing many undesired polymerizations, including unstable ring-opening products and stable dimers. Increasing temperature and OR can effectively inhibit the synthesis of phenolic compounds, and the phenolic compounds in the product can be completely degraded. Phenolic compounds in phenolic and phenol-free waste/wastewater SCWO/SCWPO reactions can be effectively inhibited.

INFLUENCE OF SCWO PARAMETERS

Theoretically, phenolic compounds can produce incompletely oxidized intermediates during SCWO (Xu et al. 2011), such as dimers, ring-opening products containing organic acids such as acetic acid, and monocyclic compounds. Since incompletely oxidized intermediates are more harmful than the original contaminants (Araújo et al. 2015), it is essential to control the parameters that can avoid their formation. Previous studies revealed that changes in the condition parameters to optimize the progress of the reaction can lead to better treatment effects, increased production of hydrogen and other gases, and inhibition of undesirable intermediate product formation.

Influence of temperature

Figures 3 and 4 summarize several studies involving the influence of temperature on phenolic and TOC removal efficiency through SCWO/SCWPO of phenolic wastewater. With increased temperature, phenol and TOC removal efficiency gradually increased. The phenol removal efficiency can reach 99% at high temperatures, which indicates that the increase in temperature is beneficial to degradation and transformation of phenolic compounds. On the one hand, the reaction rate constant will increase with increased temperature, thus speeding up the reaction rate. On the other hand, when the water is in the supercritical state, its density is extremely sensitive to changes in temperature and pressure. If pressure and other conditions remain unchanged, the temperature increase will cause the density of SCW to decrease, thus reducing the concentration of reactants, so that the reaction rate is slowed down. In the SCWO reaction, the effect of temperature is the result of the combination of the two opposite effects described above. Yong et al. (Yong & Matsumura 2013) studied the partial oxidation of benzene and phenol mixture in SCW. It was found that the phenol content decreased gradually as the temperature increased from 370 °C to 450 °C. As the reaction time increased, the degradation effect of phenol at high temperature was more evident. Guan et al. (Guan et al. 2011b) observed that from 300 °C to 480 °C, the phenol removal efficiency stabilized and increased from 39.1% to 74.7%, respectively, indicating that when the temperature is in that range, its control is essential for a positive effect. However, when the temperature exceeded 480 °C, since the concentration of unreacted phenol in the system was already relatively small, there was only a slight effect on the phenol removal efficiency and stabilization. Yan et al. (Yan et al. 2016) and Wang et al. (Wang et al. 2016) observed that elevated temperature is the dominant factor to improve TOC removal efficiency.

Figure 3

Influence of temperature on phenol removal efficiency through SCWO/SCWPO of phenolic wastewater in the published literature.

Figure 3

Influence of temperature on phenol removal efficiency through SCWO/SCWPO of phenolic wastewater in the published literature.

Figure 4

Influence of temperature on TOC removal efficiency through SCWO/SCWPO of phenolic wastewater in the published literature.

Figure 4

Influence of temperature on TOC removal efficiency through SCWO/SCWPO of phenolic wastewater in the published literature.

The temperature increase will gradually increase phenol removal efficiency. At low temperatures, some organic acids and dimers are produced after phenol ring opening. With temperature increase, phenol is kept in a stable state and is mainly degraded into organic acid-based intermediates, then further degraded into CO2, etc.

Influence of pressure and water density

Under SCW conditions, the effects of pressure on the reaction are not as noticeable compared with temperature; it even slightly inhibited the removal of TOC (Erkonak et al. 2008). The results from pressure change are affected by changes in density. Minok et al. (Minok et al. 1997) distinguished, for the first time, the effects from pressure and from water density by injecting helium (He) into the reactor. The addition of an inert gas did not change the pressure but increased the density. Fujii et al. (Fujii et al. 2011) observed that there was no linear relationship between water density and pressure in SCW, as shown in Figure 5. With pressure increase, the rate of water density increase gradually decreases. Akizuki et al. (Akizuki & Oshima 2013) observed that water density affects the performance of TiO2 and WO3/TiO2 acidic catalysts in SCW. Sato et al. (Sato et al. 2015) also carried out a similar study and concluded that if water would cover the active site, the adsorption material on the catalyst would be stable. There is a reduction on the vacancy ratio of the catalyst active site, which enables the high water density region to inhibit the water-gas conversion reaction. When the pressure is constant, an increase in water density will accelerate the reaction rate. However, when the density is constant, the effect of pressure on the reaction rate is minimal. Du et al. (Du et al. 2013) observed that the COD and phenol removal efficiency in coking wastewater remained nearly unchanged at 600 °C and 20–27 MPa; it even slightly decreased. Studies on phenol (Gökkaya et al. 2015) also mention that increased pressure reduces the production of hydrogen and other gases, and the removal efficiency of phenol and TOC slowly increased.

Figure 5

Influence of pressure on TOC removal efficiency through SCWO/SCWPO of phenolic wastewater in the published literature.

Figure 5

Influence of pressure on TOC removal efficiency through SCWO/SCWPO of phenolic wastewater in the published literature.

The effects of pressure change are less pronounced than the effects of temperature change. The influence of pressure near the critical point is clear, but it is not evident when the pressure exceeds 22 MPa. The degradation rate of TOC has no clear change when the pressure exceeds 25 MPa. High-pressure processes require a large amount of equipment. Therefore, low pressure is preferred in partial oxidation for the production of hydrogen and other gases under the guaranteed supercritical state (Sato et al. 2012).

Influence of reaction time

In Figure 6, all factors remain stable, except reaction time. Under extended reaction time, the intermediate products will be oxidized, and the organic matter removal rate will gradually increase. Accompanied by a gradual reduction of reactant concentration, the organic compounds' removal efficiency gradually increases at a low rate. Guan et al. (Guan et al. 2011b) observed that the removal efficiencies of TOC and phenol gradually increased with reaction time, at 450 °C and 24 MPa with O/phenol ratio of 7. In 40 s, 75.6% of phenol was removed, and in 180 s, 69.3% of TOC was removed. At the beginning of the reaction, phenol is converted into oxalic acid and maleic acid, and a large amount of CO is produced. After that, H2 is produced along with the water-gas shift reaction, and CO is gradually reduced. Therefore, there is a gradual increase of H2 and CO2, and a decrease of CO. Jin et al. (Jin et al. 2015, 2018) also observed that with increased residence time during coal production, H2, CH4, and CO2 gradually increased, and CO first increased and then decreased. Yong et al. (Yong & Matsumura 2014) observed that the phenol removal efficiency increased with increased reaction time, and the gas production increased upon adjustment of phenol and benzene concentrations. Therefore, reaction time increase is beneficial to remove organic compounds and produce hydrogen and other gases.

Figure 6

Influence of reaction time on TOC removal efficiency through SCWO/SCWPO of phenolic wastewater in the published literature.

Figure 6

Influence of reaction time on TOC removal efficiency through SCWO/SCWPO of phenolic wastewater in the published literature.

Phenol takes a certain time to decompose through oxidation in SCW, and its removal efficiency increases with increased reaction time. With the increase of reaction time, while other conditions remain unchanged, phenol removal efficiency gradually increases, and the inflection point appears. The increase rate of phenol removal efficiency is high before the inflection point, but after, the phenol removal efficiency tends to be stable.

REACTION PATHWAY ANALYSIS FOR SCWO OF PHENOLS

Polymerization and oxidation of phenol in SCW are complex reaction processes. The degradation of phenol in SCW can be explained by analysing the reaction mechanism and identifying the intermediates and the by-products that may occur during the degradation process. In addition, the reaction pathway can be used to inhibit phenol production in SCW, providing a necessary theoretical basis for industrial applications. Phenol, as a monomer and hydrolysis product of lignin in biomass (Sınağ et al. 2012), is toxic and harmful and difficult to biodegrade (Portela et al. 2001; Araújo et al. 2015). The research on SCW with phenol as a biomass model compound is representative.

Phenol as model compound

As shown in Figure 7, Guan et al. (Guan et al. 2011a) investigated the effects of OR on SCWPO of phenol at 450 °C and 24 MPa. They observed that phenol was first converted into dibenzofuran, 2-phenoxyphenol, and other dimers, and into ring-opening products, such as oxalic acid and maleic acid. After that, the ring-opening products were converted into short-chain fatty acids, which were further converted into small molecules such as CO, CO2, and H2. At lower oxygen concentration, there is a predominance of acid gasification and water-gas shift reaction. At a high OR, further oxidation of acids and CO predominated. Matsumura et al. (Matsumura et al. 2000) compared the distribution of reaction products and tar substances after high and low concentration in SCW at 350–450 °C and 25 MPa, with residence time of 6.5–26 s. The initial degradation pathway of phenol was the same regardless of the phenol concentration. The pathway of subsequent degradation was affected by the concentration of free radicals. High concentration favoured the condensation of aromatic compounds. Huelsman & Savage (Huelsman & Savage 2013) investigated the effects of partial oxidation on SCW of phenol. Phenol dimerization and dehydroxylation produce the primary products dibenzofuran and benzene, respectively. Benzene was mainly formed during the dimerization reaction under 600 °C, generating dimers, trimers, and H2. Dibenzofuran decomposition regenerates 2-phenylphenol, biphenyl, benzene, PAHs, and coke, which indicates that both decomposition and polymerization exist simultaneously.

Figure 7

Pathways for partial oxidation of phenol (Guan et al. 2011a).

Figure 7

Pathways for partial oxidation of phenol (Guan et al. 2011a).

Figure 8

The oxidation reaction pathways of organics in SCW (Guan et al. 2011a).

Figure 8

The oxidation reaction pathways of organics in SCW (Guan et al. 2011a).

Phenol model compound is incompletely degraded to form dimers in SCW, and the partial oxidation is effective in depolymerization. The initial degradation pathway of phenol is the same, it is first degraded into dimers and organic acids, and further degraded into H2O and CO2.

Co-reaction of phenol and other model compounds

Su et al. (Su et al. 2015b) treated mixtures of phenol and alanine. Phenol tends to condense rather than gasify in SCW of two compounds mixed together. Phenol promotes the degradation of alanine, which promotes the condensation of the phenol. That is, in the reaction process, the phenol is first depolymerized into small molecules and then repolymerized due to the presence of long chain compounds. This is similar to the results reported by Huelsman & Savage (Huelsman & Savage 2013), in which phenol could produce dimers through condensation.

Wang et al. (Wang et al. 2016, 2017) proposed a slow ring-opening pathway for polymer. The main purposes of oxygen in the gasification process are to promote the concentrations of HO• and HO2•, which are very strong electrophilic groups capable of breaking the C-C and C-O bonds; promote the generation of ring-opening products; increase the steam reforming and decomposition rates; and ultimately improve the production of H2.

Phenol and other model compounds can be directly degraded into ring-opening products in SCW, and dimers can be easily produced by condensation. Slow ring opening of the polymer can also be further oxidized into H2O and CO2 after the formation of organic acids.

Reaction pathway of phenols in actual waste/wastewater

Dong et al. (Dong et al. 2015) utilized the SCWO technology to treat p-nitrophenol wastewater. The liquid phase contained phenol, p-benzoquinone, o-nitrophenol, 4-phenoxyphenol, dibenzofuran, and other components, which can be produced by coupling among intermediate products or between intermediate products and free radicals. Jiang et al. (Jiang et al. 2017) studied the degradation of benzo[a]pyrene (BaP) from coking wastewater in SCW. In addition to the pathway initiated by HO•, oxygen molecules interact with BaP to form C-O bonds, breaking conjugate structures and distorting aromatic rings. Various free radicals and oxygen molecules attacked aromatic rings in the SCWPO system, and HO• combined with O2 and H2O2 molecules attacked aromatic rings to promote aromatic ring opening and BaP degradation. It is worth mentioning that the ring-opening rate of a supercritical water gasification (SCWG) system is lower than that of SCWPO and SCWO systems. The main reaction path of aromatic ring opening depends on the oxygen excess ratio. Jian et al. (Jiang et al. 2017) conducted a simulation study on SCWO/SCWPO using coking wastewater, and found that reaction pathways were different under different OR.

The oxidation reaction pathways of organics in SCW have been studied by many scholars worldwide. A free radical mechanism has been proposed, in which HO• is an important free radical. The mechanism is shown in Figure 8:  
formula
 
formula
 
formula
Among the free radicals, R• and HO2• are generated by oxidizing organics in wastewater. HO• free radicals are generated by pyrolysis of H2O2. M is the reaction medium. HO• can react with all hydrogen-containing compounds due to its high reactivity:  
formula
The organic free radical R• is oxidized to form peroxide free radicals, which take hydrogen to generate peroxides:  
formula
 
formula

Unstable peroxides quickly degrade into small molecules, and the end result is the formation of formic acid, acetic acid, and other acids. Eventually, these small molecular organic compounds will form CO2 and H2O through free radical oxidation reactions. The reactions in which R• and HO2 participate are essentially de-H reactions, while the removal of H is the first step to control the rate of oxidation.

In the process of SCWO, organic compounds are decomposed into various intermediate products. Some intermediate products cannot be directly degraded by phenol, but small molecules of aldehydes and acids formed by phenol during the ring-opening process are formed by substitution reaction with phenol in the special environment of SCW. According to the qualitative analysis of the product distribution and intermediate products, the oxidative degradation pathway of the phenols can be analysed.

CONCLUSIONS

In this work, SCWO/SCWPO technology was discussed as a potential method to treat phenolic waste/wastewater. When the oxidizing power is insufficient, organics compounds will polymerize to form phenolic compounds, and the gradual increase of the equivalent oxidizing agent causes intermediate products to gradually be completely oxidized to CO2 and H2O. For SCW technology, increasing the temperature and the residence time promotes the oxidative decomposition of phenols. For SCWO/SCWPO technology, increasing the amount of oxidant can effectively inhibit the synthesis of phenolic compounds from phenolic wastewater and promote the degradation of phenolic compounds. The application of SCWO technology to phenolic wastewater mainly focuses on monohydric phenols and their derivatives. Research reports on dihydric phenols and polyphenols are limited. However, the addition of 300% oxidant can lead to complete oxidation, but the high oxidant incorporation is a costly problem. The generation of H2 was observed during the catalyst reaction alone. Whether introducing catalysts in the process of SCWO can reduce the amount of oxidant added is worth further study. In SCWO, catalytic oxidation technology may be an effective way to address the large amount of oxidant needed, achieve hydrogen production optimization, and control phenol.

ACKNOWLEDGEMENTS

The authors are grateful for financial support from the Natural Science Foundation (51808003), Natural Science Foundation of Anhui Province (1808085QE175) and (1808085QE142) of P.R. China, Ministry of Housing and Urban-Rural Development of the People's Republic of China research and development project (2016-K4-031) for providing commercial adsorbents.

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