Dye wastewater from the printing and dyeing industry is a major source of environmental contamination. Dyestuff, slurries, dyeing aids, acids and alkalis, and fibers and inorganic compounds are present in dye wastewater ^[1]. Generally, dye wastewater is characterized by a strong color, high pH, high chemical oxygen demand (COD), and low biodegradability ^[2]. Such wastewater is difficult to treat, especially with color removal, using conventional wastewater processes ^[3].

In a brief search in Scopus, many hits were obtained for the term “dye oxidation” (Fig. 1). The major peak for works on the wet air oxidation (WAO) process was in the 21st century. The present review first introduces the technology and process, and the research history and research hotspots of WAO. Its application in dye wastewater treatment is then described in detail. Major conclusions were obtained from the different procedures during the past decades, which strongly influenced the new trends and economic aspects of each time period. All these confirmed the sustainability of this process.

Recently, advanced oxidation processes (AOPs) have received attention as effective pretreatment processes for less biodegradable wastewater ^[4]. AOPs were first proposed by Glaze ^[5] in 1987. They were defined as near ambient temperature and pressure water treatment processes based on the generation of hydroxyl radicals (•OH) to initiate the oxidative destruction of organics ^[5]. •OH is a powerful, non-selective chemical oxidant, which reacts a million to a billion times faster than either ozone (O₃) or H₂O₂ (Table 1) ^[6].

Fig. 1. Works published with the “dye oxidation” keyword (data search in Scopus).

Fig. 1. Works published with the “dye oxidation” keyword (data search in Scopus).

Table 1 Relative oxidation power of some oxidizing species with chlorine as the reference species [6] Oxidation species Oxidation power Hydroxyl radical 2.05 Atomic oxygen 1.78 Ozone 1.52 Hydrogen peroxide 1.31 Permanganate 1.24 Chlorine 1.00	Table 1 Relative oxidation power of some oxidizing species with chlorine as the reference species [6]

Fig. 2. Basic wet air oxidation plant flow sheet.

Fig. 2. Basic wet air oxidation plant flow sheet.

AOPs can achieve the oxidative destruction of refractory compounds that conventional oxidation cannot. In addition, they have the potential to completely oxidize organic contaminants to CO₂, H₂O, and mineral salts. AOPs can also oxidize inorganic contaminants such as cyanides, sulfides, and nitrites ^[6]. Their advantages for the treatment of dye wastewater include fast reaction, complete treatment, and being pollution-free and of wide application.

WAO, which was established and developed by Zimmermann ^{[7, 8 9 10]}, is one of the most economical and technologically viable AOPs for wastewater treatment, particularly toxic and high organic content wastewater. The application range of each AOP is different and depends on the flow rate and organic content of the effluent ^[11]. Unlike other AOPs, WAO is suitable for high organic loads at high flow rates. Especially, WAO has great potential for treating effluents containing a high content of organic matter (about 10–100 g/L of COD) and/or toxic contaminants, for which a direct biological treatment is not feasible ^[12]. WAO is not only eco-friendly, but also economical compared to other AOPs that use harmful and expensive oxidants like O₃ and H₂O₂.

WAO is the liquid phase oxidation of organics or oxidizable inorganic components at elevated temperatures (125–320 °C) and pressures (0.5–20 MPa) using a gaseous source of oxygen (O₂ or air) as oxidant. The solubility of O₂ in aqueous solutions is greatly enhanced at elevated temperatures and pressures, which provide the strong driving force for oxidation. WAO has been demonstrated to oxidize organic compounds to CO₂ and other end products (carbon is oxidized to CO₂, nitrogen is converted to NH₃ and nitrate ions (NO₃^–), and halogen and sulfur are converted to inorganic forms). WAO is useful for the treatment of hazardous, toxic, and non-biodegradable waste streams. The process becomes self-sustaining when the feed COD is about 20000 mg/L ^[13].

Figure 2 shows a typical WAO treatment system. The wastewater is brought into the system using a high pressure pump. Air (or O₂) is added to the reactor using a compressor. Preheating may be necessary to raise the temperature of the wastewater. The feed temperature is adjusted to let the exothermic heat of reaction raise the mixture temperature to the operating temperature. The treated effluent can be used to preheat fresh wastewater ^[14]. Liquid and non-condensable gases are separated in a separator. The off-gases can be expanded in a turbine to recover energy. The gasses are then treated using carbon absorption beds or afterburners to reduce the concentration of organic matters. The oxidized liquid phase can be directly disposed of or, most often, subjected to biological treatment.

WAO requires much less fuel than any other thermal oxidation process such as incineration. This is because the only energy required for WAO is that used to make up the difference in enthalpy between the incoming and outgoing streams. The capital cost of a WAO system is higher, and the reactor itself can account for a significant fraction (50%) of the total equipment cost. The capital investment can be reduced if one selects a WAO system based on O₂ instead of air. Prasad et al. ^[15] studied the technological and economic aspects of both air- and O₂-based systems and concluded that O₂-based WAO systems gave higher profitability. The operating costs are almost entirely for power consumption to compress air and to pump liquids ^[16]. Furthermore, WAO becomes self-sustaining with no additional fuel requirement when the COD is above 20000 mg/L ^[17], that is, energy recovery (thermal or mechanical) becomes possible when the feed COD is sufficiently high ^[18]. Costs can further be reduced by reducing the severity of the oxidation conditions by the use of suitable catalysts.

The first patent on wet air oxidation was 102 years ago. In 1911, Strehlenert applied for a patent for the treatment of sulfite liquor (from pulp production) by oxidation with compressed air at 180 °C ^[13]. In 1927, Hanglin and Stauf ^[19] purified a solution containing metallic salts and organic impurities using wet air oxidation. The temperature was above 130 °C, and the pressure was above 0.2 MPa. The WAO technology was firmly established and developed by Zimmermann ^{[7, 8 9 10]}. The first known WAO plant was set up in 1958 in Norway for the treatment of sulfite liquors, but this was later closed down due to uneconomical operation.

A truly successful application of the WAO technique as a commercial process for wastewater treatment came in the early 1960s with its application for the recovery of pulping chemicals from waste liquors and the complete oxidation of sewage sludge. The Zimpro Corporation built several WAO plants for the oxidation of sewage sludge. On the basis of the work with sewage sludge, new applications for WAO were discovered, such as the conditioning of sludge for easy dewatering and improving settling characteristics. Commercial applications for hazardous wastes were started in the early 1970s. Up to 1995 more than 200 full scale WAO plants were in operation for the treatment of a wide variety of effluent streams ^[13]. The major application of WAO is still for the treatment of sewage sludge, with more than 50% of the WAO plants built being used for this purpose ^[20].

Although WAO can quickly and completely treat high concentration and highly toxic pollutants, its operational conditions and equipment requirements are severe. Capital and operating costs are high compared to other treatments, such as biological treatments. Therefore, the wide application of WAO has restrictions.

During the past two decades, the application of catalysts for the catalytic WAO (CWAO) process has been a research hotspot. CWAO not only reduces the severity of the reaction conditions but also more easily decomposes even refractory pollutants, thereby reducing capital and operational cost ^{[13, 21, 22 23 24, 25, 26, 27 28, 29, 30, 31]}. Regardless of the wastewater type, the operating cost of CWAO is about half that of non-catalytic WAO due to milder operating conditions and shorter residence time ^[32]. According to the type of catalyst, CWAO can be divided into homogeneous CWAO processes and heterogeneous CWAO processes.

Homogeneous copper salts are the most active catalysts in the homogeneous CWAO process ^{[13, 33 34 35]}. As there is no additional third phase, mass transfer and reaction in a homogeneous CWAO process are therefore similar to the non-catalytic system. Reactor operation is also simpler compared to the heterogeneous CWAO process ^[25]. The main disadvantage of the homogeneous CWAO process is that an additional separation step is required to remove or recover the metal ions (homogeneous catalyst) from the treated effluent due to their toxicity, which increases operational cost ^[12]. Several heterogeneous CWAO processes have been developed in the last two decades, including the Ciba-Geigy process, LOPROX (low pressure wet oxidation) process, and WPO (wet peroxide oxidation) process ^[28].

In recent years, heterogeneous CWAO has received much attention because a separation step is not necessary. Many solid catalysts including noble metals, metal oxides, and mixed oxides have been widely studied for the CWAO process ^[12]. However, in addition to being cost-effective, the solid catalyst must also show satisfactory stability and durability under the severe operating conditions and acidic environments in WAO systems. Catalyst deactivation can occur due to leaching, sintering, poisoning of active sites or fouling of the catalyst surface by deposition of organic or inorganic compounds ^[25]. To overcome these problems, several solid catalysts based on supported precious metals were studied ^{[36, 37, 38 39 40 41 42 43]}. Supported precious metal catalysts not only are less prone to deactivation by the leaching of the active phase, but they also have higher overall activity for the oxidation of various pollutants. Several commercial heterogeneous CWAO processes have been developed, such as the NS-LC (Nippon Shokubai liquid chromatography) process, Osaka Gas process, and Kurita Process ^[28].

Synthetic dyes are widely used in many industries such as the textile, cosmetic, printing, pharmacy, and food processing industries ^[44]. These dyes, which exist in large volumes of wastewater generated by the factories, threaten human health by their toxicity and potential carcinogenicity ^[45]. The removal of these dyes in the wastewater has attracted much interest and is an important practical problem ^[46]. There were many studies for dye removal with WAO in the past two decades. According to the treated agents, these studies can be divided into those on pure compounds (important model pollutants) and wastewater containing toxic and hazardous compounds. From their findings, a better understanding of the reactions taking place during WAO (reaction mechanism and kinetics) was obtained, which is important as it leads to the reliable design of oxidation reactors and also to cost reduction by optimization of the operating conditions.

5.1. Model pollutants

Table 2 Summary of WAO studies with model dyes. Year Dye Reactor Catalyst Conditions Effect Ref. 2001 Direct dye Autoclave CoAIPO4-5 135 or 145 °C, 1 MPa O2 Activation energy of color removal 75 kJ/mol [47] 2002 H-acid Autoclave Ce3Cu1 135 or 145 °C, 1 MPa O2 Removal of 90% COD in 30 min [49] 2002 Acid dye Orange II Stainless steel unions Non-catalytic; H4SiW12O40 and Na2HPW12O40 160–290 °C, 0.6–3.0 MPa O2 Arrhenius parameters Ea = 84 kJ/mol and A = 5.6 x 107/min [50] 2003 Reactive dye Autoclave CoAlPO4-5 & CeO2 135 °C, 1.0 MPa O2 Color and COD removal were 95% and 90%, respectively, after 2 h [48] 2004 Phenol Trickle bed reactor Active Pt and Ru 393 and 473 K, 5 and 8 MPa O2 58–107 kJ/mol of activation energy [51] 2005 Azo dyes Stainless-steel high-pressure reactor Pt/MWNT 150 °C, 6.9 MPa O2 Almost 100% removal of dyes in 2 h [52] 2007 Methyl orange Glass reactor Fe2O3-CeO2-TiO2/γ-Al2O3 25 °C, atm. pressure air Removal of 98.09% color and 96.08% TOC in 2.5 h [53] 2007 Cationic red X-GRL Autoclave Non-catalytic 60–180 °C, 0–1.2 MPa O2 Removal of 92% dye at 180 °C in 1 h [54] 2009 Safranin-T Three-neck glass flask Zn1.5PMo12O40 nanotube 25 °C, atm. pressure air Removal of 98% color and 95% COD within 40 min [55] 2011 Basic Yellow 11 Stainless-steel high-pressure reactor Ni- and Fe-doped mixed oxides 120 °C, 5 MPa O2 High TOC and toxicity removal efficiency [56] 2011 CI Basic Yellow 11 Autoclave Pt nanotube 120–180 °C, 5 MPa O2 Effective to destroy the dye structure [57] 2011 Basic Yellow 11, Crystal violet, Naphtol Blue Black Autoclave Ni catalysts supported on hydrotalcites 120 °C, 5 MPa O2 Removal of 95% dye [58] 2012 Crystal violet Autoclave Hi/HT 220 °C, 5 MPa O2 11.15 kJ/mol of activation energy [59] 2012 Cationic red GTL Glass reactor Mo-Zn-Al-O catalyst 25 °C, atm. pressure air Removal of 80.1% color and 50.9% TOC [60] 2012 Crystal violet Trickle-bed reactor Ni/MgAlO 120–180 °C, 2.5–6 MPa O2 Removal of 64% TOC in 350 h [61] 2012 Crystal violet Autoclave Ni/MgAlO 140–200 °C, 5 MPa O2 Removal of 98% dye [62] 2013 Azo dyes Autoclave CuO/γ-Al2O3 80 °C, atm. pressure O2 Removal of 99% color and 70% of TOC in 2 h [63] 2013 Chromotrope 2R Trickle-bed reactor Ni/MgAlO 100–180 °C, 2.5 MPa air Conversion of 82% TOC at 150 °C [64] 2013 Basic Yellow 11 Autoclave Ni/MgAlO 100–200 °C, 3–6 MPa O2 Improvement of 49.1% for the CWAO process [65]

Table 2 Summary of WAO studies with model dyes.

Studies on the WAO of model dyes are listed in Table 2. Lin et al. ^[47] studied the WAO of a prepared direct dye solution using a CoAIPO₄-5 catalyst. The results showed that CoAIPO₄-5 effectively improved the rate of color removal, and the activation energy of the color removal reaction was decreased from 110 to 75 kJ/mol as the catalyst loading was increased from 0 to 3.0 g/L. In their subsequent work ^[48], the performance of CoAlPO₄-5 and CeO₂ as catalysts in the WAO process was studied. By adsorption and oxidation, CoAlPO₄-5 effectively decreased the American Dye Manufacturers Institute color and COD values in thedyesolution. At a reaction temperature of 135 °C and a pressure of 1.0 MPa, color and COD removal after 2 h reached 95% and 90%, respectively.

Zhu et al. ^[49] used four types of powder catalysts whose main active components were Cu, Ce, Cd, and Co-Bi in the CWAO treatment of a H-acid solution. The comparison of the efficiency of the different catalysts showed that the Ce3Cu1 (3:1) catalyst was the best. At the conditions of 200 °C, 3 MPa of O₂ partial pressure, pH = 12, and 30 min of reaction time, the COD removal rate was over 90%.

To get further insight into the dominant reaction pathway of the catalytic and non-catalytic WAO processes, Arslan-Alaton et al. ^[50] added isopropyl alcohol and bromide as •OH scavengers into the reaction solution. The catalytic processes were appreciably less sensitive to the presence of •OH scavengers than non-catalyzed WAO, implying that CWAO did not mainly use a free radical type reaction mechanism. The results supported the free radical chain mechanism only for non-catalyzed WAO ^[50].

Cybulski et al. ^[51] developed active Pt and Ru catalysts for the CWAO of phenol solution. A kinetic analysis of phenol conversion and COD changes was given based on a lumped reaction network. Most of the activation energy obtained ranged from 58 to 107 kJ/mol. The reaction orders for phenol ranged from 0.94 to 2.0 and those for O₂ from 0.22 to 0.85. They later use supported Pt catalysts to treat wastewater from a fine chemicals plant and also obtained good results ^[66].

Garcia and co-workers ^[52] have done much work on the CWAO of model dyes. They first used multi-walled carbon nanotubes (MWNT) to prepare supported Pt catalysts and applied them in the CWAO of azo dye solutions ^{[52, 57, 67]}. The CWAO of a textile wastewater was also performed using the same catalyst. Complete color removal and 51.2% TOC abatement were obtained. Later, they used Ni catalysts supported on hydrotalcites ^[58] and mixed oxides ^{[56, 59, 61, 62, 64, 65]}, and especially for the Ni catalyst supported on Mg-Al mixed oxides (Ni/MgAlO), much work was done to apply it in different reactors ^{[61, 64]} and with different dyes ^{[64, 65]}.

Table 3 Summary of studies on dye wastewater WAO. Year Wastewater Reactor Catalyst Conditions Effect Ref. 1996 Desizing wastewater High pressure Parr reactor CuSO4 and Cu(NO3)2 150–250 °C, 7 MPa air 80% COD removal in 1 h [69] 1997 Desizing wastewater High pressure Parr reactor CuSO4 and Cu(NO3)2 175–260 °C, 4–8 MPa air 70% COD removal in 1 h [70] 2000 Textile wastewater Autoclave CuSO4, FeSO4, Fe2(SO4)3, MnSO4, Cu(NO3)2, Fe(NO3)3, Mn(NO3)2 240 °C, 1.5 MPa O2 61%–77% COD removal in 30 min [71] 2001 Textile wastewater Autoclave CuO and Cu(NO3)2 150–300 °C, 2.65 MPa O2 1st order rate constant: 0.0492–0.0559/min [72] 2003 Textile wastewater Autoclave batch reactor Non-catalytic 423–563 K, 1.69 MPa O2 activation energy: 37.5 kJ/(mol·K) [73] 2005 Textile wastewater Stainless-steel high pressure reactor Pt/MWNT 150 °C, 6.9 MPa O2 51.2% TOC abatement [52] 2006 Chemical plant wastewater Fixed-bed reactor CBC-supported Pt 393–473 K, 5.0 MPa O2 High COD removal rates [66] 2011 Ink-plant wastewater Autoclave Pt nanotube 120–180 °C, 5 MPa O2 Effective to remove TOC and toxicity [57] 2011 Textile wastewater Autoclave Ni catalysts (supp. Hydrotalcites) 120 °C, 5 MPa O2 Effective to remove TOC [58]

Table 3 Summary of studies on dye wastewater WAO.

In order to develop a catalyst with high activity and stability for the CWAO process at room temperature and atmospheric pressure, Liu et al. ^[53] prepared Fe₂O₃-CeO₂-TiO₂/γ-Al₂O₃ by sequential impregnation. The degradation of an azo dye (methyl orange) in the CWAO process with this catalyst at room temperature and atmospheric pressure was investigated, and it was shown that the catalyst had excellent catalytic activity in treating synthetic wastewater containing 500 mg/L methyl orange. The 98.09% of the color and 96.08% of TOC were removed in 2.5 h. In their recent work ^[60], a new Mo-Zn-Al-O catalyst was developed for the CWAO of cationic red GTL at room temperature and atmospheric pressure. The optimal conditions of the pH value, initial concentration of dye, and catalyst dosage were 4.0, 85 mg/L, and 2.72 g/L, respectively, for the maximum decolorization of 80.1% and TOC removal of 50.9%. Zhang et al. ^[55] have also developed a catalyst with high activity and stability for the CWAO of pollutant dyes at room condition. Using the new polyoxometalate Zn_1.5PMo₁₂O₄₀ with a nanotube structure as the catalyst, the CWAO of Safranin-T (10 mg/L), a hazardous textile dye, using air at room temperature and atmospheric pressure, the removal of 98% color and 95% COD within 40 min was achieve. Hua et al. ^[63] also prepared a catalyst (CuO/γ-Al₂O₃) that was used under atmospheric pressure. At 80 °C, almost 99% of color and 70% of TOC of azo dye solution were removed in 2 h.

Studies on the WOA of dye wastewater began from the middle 1990s. The early studies focused on the treatment of wastewater. In the last decade, research focus changed to model dye studies. Most WOA studies are on CWOA, especially heterogeneous catalytic WOA. The use of a solid catalyst not only improved treatment efficiency but also reduced the severity of the reaction conditions, e.g., a Mo-Zn-Al-O catalyst can be active under room conditions. Many present studies are fundamental research focusing on the development of new catalysts for the WAO process. Process technology and modeling techniques should also be developed to optimize the process and solve problems in the practical application of WAO for the treatment of dye wastewater.