Air Oxidation Process Research Articles

The used automobile catalytic converter as an efficient catalyst for removal of malathion through wet air oxidation process

The automobile catalytic converter (ACC) contains a huge number of precious metals as catalysts. When an ACC fails to meet standards, it is removed from the exhaust of an automobile but retains some catalytic activity. However, the recovery and/or activation of this waste is a high-cost process and includes several chemical treatments. Catalytic wet air oxidation (CWAO) has been reported as an effective wastewater treatment method. The most important disadvantage of CWAO is cost-nonefficiency. Herein, to overcome these problems, the simple recovery of catalysts from waste ACC for reuse in CWAO was investigated. The optimum conditions of reaction were investigated through response surface methodology (RSM). The optimum removal efficiency was 88% when the reaction conditions were set on the 20 bar of pressure at 111.5 °C over 77 min and using 0.41 g of recovered catalyst. In addition, toxicity testing was performed on a model of malathion-contaminated wastewater before and after CWAO treatment. Final product identification was performed which showed that CWAO eliminated the toxicity of wastewater and was determined to be malaoxon, present at acceptable concentrations, and tributyl phosphate. In conclusion, there may be important potential for the use of recovered ACC catalyst in the treatment of toxic wastewater.

Maximize methane recovery from sludge anaerobic digestion by combining an optimal wet air oxidation process

Anaerobic digestion (AD) is widely used to recover energy from sewage sludge, but much digested sludge remains. In this study, wet-air oxidation (WAO) was used to decompose digested sludge for further methanogenesis via AD. The results showed that carbohydrate and protein in digested sludge were degraded to acetic acid and other volatile fatty acids during WAO, but residual humic acids and their derivatives with molecular weights less than 300 Da became the main refractory substances. They accounted for approximately 10% of total chemical oxygen demand in raw sludge. After WAO, the treated sludge was dewatered, and the filtrate was returned to AD. In the integrated AD and WAO system, the peak removal rate of sludge organic matter was 83.3% at 240 °C, but the conversion rate to methane was only 42.7%, even when removed ammonia in advance. By contrast, the organic removal rate was only 71.0% in the integrated system with WAO at 180 °C, but the conversion rate to methane reached 53.3%. Moreover, the relatively low concentration of ammonia had no significant effect on AD. In comparison with only AD, the integrated system with WAO at 180 °C harvested 51.4% more methane.

Microbubbles intensification and mechanism of wet air oxidation process of MDEA-containing wastewater

ABSTRACT In order to the intensification of gas–liquid mass transfer of MDEA-containing wastewater during wet air oxidation (WAO) process, the microbubbles and millimetre bubbles were applied by fine-pore sparger (5 and 20–30 μm) and single pore sparger (6.35 mm), respectively. Effect of the superficial gas velocity on the average microbubble size, gas holdup and oxygen mass transfer coefficient (KLa) of MDEA-containing wastewater at the ambient conditions was studied. The results showed that the microbubbles (less than 1 mm) were beneficial to enhance mass transfer process and had a higher dissolved oxygen concentration during WAO process of MDEA-containing wastewater owing to higher gas holdup and larger oxygen mass transfer coefficient. The COD removal ratio was 66% at low superficial gas velocity (ug = 0.3 cm/s) in WAO process by microbubbles, while it achieved at high superficial gas velocity (ug = 3.0 cm/s) by millimetre bubbles. The critical oxygen mass transfer coefficient KLa was 0.183 min−1 of MDEA-containing wastewater by 20–30 and 5 μm fine pore sparger, which was 2∼5 times more than that of single pore sparger (<0.1 min−1). The microbubbles could improve dissolved oxygen concentration and enhance the formation of hydroxyl radical at short time with atmospheric pressure. During the WAO process, the MDEA would be converted into intermediates including formic acid, acetic acid, ammonium, nitrite and nitrate. The WAO process with microbubbles could significantly improve the gas–liquid mass transfer performance at low superficial gas velocity and greatly reduce air consumption for MDEA-containing wastewater.

Efficient Decomposition of Organic Pollutants over nZVI/FeOx/FeNy-Anchored NC Layers via a Novel Dual-Reaction-Centers-Based Wet Air Oxidation Process under Natural Conditions

The wet air oxidation (WAO) process is a promising advanced oxidation process used for the degradation of organic pollutants; however, severe reaction conditions, including high temperature and high pressure, have significantly limited its application. In this report, we describe a new discovery that nZVI/FeOx/FeNy-anchored NC composites (nZVI/FeN5.5O7.2/C) exhibit an excellent catalytic performance for organic pollutant removal through natural dissolved oxygen (DO) microactivation at room temperature and atmospheric pressure. Characterization techniques reveal the key structural information: a special kind of construction of dual-reaction-centers (DRCs) with electron-rich Fe microcenters (ERCs) and electron-poor C(π)/N microcenters (EPCs) is successfully constructed on the surface of the catalyst through bonding of nZVI, FeOx, and FeNy, which is responsible for the novel DRCs-WAO process. During the reaction, DO obtains electrons and is rapidly activated into active oxygen species at the ERCs. Pollutants, as electron donors, lose electrons at EPCs and are oxidized. Electron transfer between EPCs and ERCs is achieved through the constructed Fe–O–C and Fe–N bonding bridges. This process is even free from the interference of inorganic ions, pH, or natural organic matter in the water environment. Our findings address the key limitation of the classical WAO process and are of positive significance for developing new wastewater treatment technologies with low energy consumption and high efficiency.

Photocatalytic oxidation and catalytic wet air oxidation of real pharmaceutical wastewater in the presence of Fe and LaFeO3 doped activated carbon catalysts

Real pharmaceutical wastewater was treated by photocatalytic oxidation and catalytic wet air oxidation (CWAO) processes. The catalytic performance of Fe/AC and LaFeO3/AC (AC: Commercial activated carbon) was compared in the advanced oxidation processes. The influences of the reaction parameters were determined by using Box–Behnken design. Fe/AC catalyst was more effective in both of the photocatalytic oxidation and catalytic wet air oxidation processes. In the photocatalytic oxidation process, 72.7% COD removal was achieved at pH = 4.5, 2.0 g/L, and [H2O2]o = 0.32 mM. Under the optimum conditions of the CWAO, which were determined as 3 g/L of catalyst loading, pH = 3, and 50℃, 83.1% COD removal efficiency was obtained. CWAO method was determined as more suitable since a higher COD removal efficiency was achieved in a shorter reaction time. The toxicity in terms of L. sativum growth inhibition was evaluated as 4.82% after the CWAO treatment. A two-step first-order reaction kinetics provided the best fit of the CWAO experimental data. The activation energies of the fast and the slow steps were 25,661.16 and 28,327.46 J/mole, respectively.

Biotemplated copper oxide catalysts over graphene oxide for acetaminophen removal: Reaction kinetics analysis and cost estimation

TCuOX-GO (TCuO: Tomato skin inspired copper oxide; GO: Graphene oxide; X, weight percentage of copper oxide: 10, 50, and 90) catalysts were prepared and their performances on acetaminophen removal were tested in catalytic wet air oxidation (CWAO) process. Copper oxide particles were synthesized from tomato skin biotemplates to contribute valorization of food wastes and the development of ecofriendly catalysts. The most effective catalyst was determined as TCuO50-GO. Under the optimum conditions 52.1% TOC removal and 96.2% acetaminophen degradation efficiencies were achieved. The acetaminophen degradation kinetics was found to fit Langmuir-Hinshelwood model. The heat of adsorption and adsorption entropy were calculated as −19148.8 J/mol (for oxygen), −10380.9 J/mol (for acetaminophen), −10.17 J/molK (for oxygen), and −14.5 J/molK (for acetaminophen), respectively. The use of biomimetic catalysts in CWAO process and investigation of treatment cost and reaction kinetics in the presence of the biomimetic catalysts are the main original approaches of this study.

Paper: Effect of aging process on steam oxidation behaviour of austenitic steels Super 304H and HR3C

The aim of this work was to evaluate effect of aging process on steam oxidation resistance of Cr rich steels with around 18 wt % (Super 304H) and around 25 wt % (HR3C). The samples are still under exposure, however the results after 100, 200, 500 and 1000 h are available (the whole test is planned for 10 000 h). The effect of aging process on steam oxidation after first 1000 h showed the formation of a thinner oxide scale in contrast to the samples exposed in steam atmosphere only. Further, it was found, fine grain structure determine lower mass gain of the sample exposed in steam oxidation process, but in air oxidation process mass gain is mainly governed by Cr content in metal matrix. The post exposed analyses in this work were carried out using Scanning Electron Microscopy (SEM) supported by Energy Dispersive X-ray Spectroscopy (EDS) using X-Ray mapping function.

(Invited) Ultrahigh Areal Capacity Holey Graphene Air Cathodes for Li-O2 and Li-CO2 Batteries

Advanced lithium (Li) batteries using gaseous cathode reactants such as oxygen (O2) and carbon dioxide (CO2) are attractive energy storage platforms because the gases are obtained externally and thus not accounted for in the total battery weight when fully charged. The discharge products at the cathode, typically Li2O2 for Li-O2 batteries and Li2CO3for Li-CO2 batteries, are insoluble in the electrolyte. Therefore, in order for such batteries to function properly, an “air cathode”, which is a conductive scaffold within the battery cell, is required as a physical location for cathode electrochemical reactions to occur. Prior research has identified many carbon nanomaterials such as carbon nanotubes and graphene as viable choices for air cathode scaffold, while various metallic and metal-free catalytic systems integrated onto carbon-based air cathodes have been developed to improve the sluggish discharge and charge reactions. For future practical applications, the air cathode must exhibit a usable capacity per unit electrode area, or areal capacity, a critical parameter that has been largely overlooked so far in this field. In order to achieve high areal capacity, the air cathode must exhibit a sufficient amount of accessible void volume per unit electrode area while maintaining the conductive scaffold integrity during the entire electrochemical process.Here we present an ultrathick, holey graphene-based air cathode platform fabricated from a facile dry compression process that exhibits remarkable areal capacity values. Holey graphene is a carbon nanomaterial derived from graphene, but with nanometer sized holes through the nanosheet thickness. The holes are generated in a controlled air oxidation process in which the defective carbon on the graphene surface are selectively oxidized and removed. The presence of these holes enhances mass transport through electrode thickness and also enables the unique dry-press fabrication process that is not achievable using other carbon scaffold materials. The dry-pressed holey graphene air cathode platform is compatible with catalyst incorporation to improve battery reaction kinetics. This platform also allows for novel engineering of electrode architectures not achievable using conventional electrode fabrication approaches. The applications of such highly versatile, ultrahigh areal capacity air cathode platforms to both Li-O2 and Li-CO2 battery chemistries will be discussed.

Highly Efficient Ru Supported on Carbon Nanosphere Nanoparticles for Ciprofloxacin Removal: Effects of Operating Parameters, Degradation Pathways, and Kinetic Study

In this work, the capability of the catalytic wet air oxidation process (CWAO) using a ruthenium-carbon nanosphere catalyst (CNS-Ru) for the degradation of the widely spread antibiotic ciprofloxaci...

3D nitrogen-doped carbon supported non-precious metals electrocatalyst for oxygen reduction reaction

Sluggish oxygen reduction reaction (ORR) is a rate-limiting step in some electrochemical devices such as fuel cells. Thus, the synthesis and development of high performance, inexpensive and durable catalysts for ORR is a necessity. Metallic nanostructures decorated heteroatoms-doped carbons have been recognized as excellent catalysts in energy applications. However, nitrogen-doped partially oxidized three-dimensional carbonaceous supports were seldom studied for ORR. Herein, the synthesis of low Fe-Co bimetallic contents deposited on three-dimensional (3D) activated carbon (AC) is reported as an efficient and cost-effective catalyst for ORR in alkaline electrolyte. In the present work, AC was thermally treated by direct heat in air at 430 °C to enhance its electrochemical activity through increasing structure deficiency. AC and nitrogen doped AC (N-AC) were used to synthesize Fe-Co/AC and Fe-Co/N-AC. In both Fe-Co/AC and Fe-Co/N-AC, low amounts of non-precious Fe and Co nanoparticles were deposited on carbonaceous substrates. The air oxidation process, for partially oxidation of carbon structure and catalyst post treatment, was used for introducing defects to the catalyst structure. The performance of synthesized samples was evaluated by ORR process in alkaline medium.

Catalytic behaviors of manganese oxides in electro-assisted catalytic air oxidation reaction: Influence of structural properties

The electro-assisted catalytic wet air oxidation (ECWAO) process is effective for pollutants removal, by taking advantage of an anodic electric field to activate O2 on a catalyst surface. Herein, three different manganese oxides, including MnO2, Mn3O4 and MnO, are fabricated on graphite felt (GF) support to serve as the catalysts of ECWAO reaction. Structural properties of these manganese oxides are analyzed, and their catalytic behaviors in ECWAO reaction are investigated with bisphenol A as a probe pollutant. The manganese centers on the surface of manganese oxides are found to be present in multi-valence states, favoring the formation of chemisorbed oxygen species which are responsible for pollutants oxidation in the ECWAO process. Therefore, the manganese oxides demonstrate good and stable activity for catalytic oxidation of bisphenol A under room condition. The sequence of their catalytic activities is as follows: Mn3O4 > MnO > MnO2, depending on their low-temperature reducibility and oxygen mobility in them. These results illustrate a guideline to the design of highly active manganese oxide catalysts for the ECWAO reaction.

Kinetic study of glyphosate degradation in wet air oxidation conditions

Glyphosate is one of the most widely used herbicides in the world against perennial and annual weeds. It has been reported to be a micro pollutant, and its degradation in different wastewater treatment processes must be studied. For that purpose, the kinetics of wet air oxidation of glyphosate was studied in an autoclave reactor at a temperature range of 423–523 K and under a total pressure of 15 MPa. Oxidation reactions obeyed the first-order kinetics with respect to glyphosate concentration. The activation energy for glyphosate oxidation was found to be equal to 68.4 kJ mol−1. Furthermore, the possible reaction intermediates and main end products of glyphosate degradation in the wet air oxidation process were identified and quantified using UV-spectrophotometry and liquid chromatography coupled to high resolution mass spectrometry. A degradation pathway for glyphosate oxidation was proposed.

A novel isophorone wastewater treatment technology-wet electrocatalytic oxidation and its degradation mechanism study

How to solve the poisoning and loss of catalysts in catalytic wet air oxidation (CWAO) process remains a great challenge. In this work, an electric field was introduced into wet air oxidation (WAO) process for the efficient degradation of isophorone (IP) wastewater for the first time, named as wet electrocatalytic oxidation (WEO) process. Different composite electrodes including Ti/PbO2, Ti/Pt, Ti/Ru-Ir and Ti/Ir-Ta electrode were selected as the anodes of WEO technique and the results showed that the total organic carbon (TOC) removal via WEO process with PbO2 anode (89.56 %) was much higher than CWAO equipped with noble metal catalyst (Ru/TiZrO4, 75.0 %). Additionally, the current efficiency of WEO process was 85.6 %, which was significantly better than that of EO process (12.1 %). A response surface methodology was applied to elucidate the effects of reaction conditions on IP degradation. Analysis of response surface model showed TOC removal were markedly affected (p ≤ 0.01) by the reaction time (t), temperature (T), current density (ID), T2 and ID2, and also determined (p ≤ 0.05) by the interactions of T with t and ID respectively. In addition, a synergistic effect was proved to take place in WEO process with synergistic effect factor f of 1.2 at optimized conditions. As an advanced wastewater treatment technology, WEO integrates the advantages of both electro-catalytic oxidation (EO) and WAO.

Degradation of bisphenol A by electrocatalytic wet air oxidation process: Kinetic modeling, degradation pathway and performance assessment

Pharmaceuticals and personal care products (PPCPs) are becoming ubiquitous in environment, and effective removal of PPCPs from contaminated water bodies is urgently needed to avoid their potential hazards to human health. Electrocatalytic wet air oxidation (ECWAO) process is a highly efficient technology for the rapid and deep mineralization of PPCPs under room condition. In this work, the degradation behavior of the bisphenol A (BPA), a typical PPCP compounds, in the ECWAO process is comprehensively investigated. A simple kinetic model is developed to describe the removal rate of BPA as a response to the initial concentration of BPA and intensity of electric field. BPA removal in the ECWAO process is accompanied by its partial mineralization, with p-isopropenyl phenol, benzoquinone and propanedioic acid as intermediate products. At last, the performances of the ECWAO process in treating BPA and other two PPCP compounds, i.e. triclosan (TCS) and sulfamethoxazole (SMX), are assessed under a condition optimized by the kinetic model. The results demonstrate the ECWAO process is able to totally eliminate the toxicity of PPCPs-contaminated waters at low energy consumption. Notably, the treated-waters show good aerobic biodegradabilities, enabling such a process a prospective pretreatment for coupling with aerobic biological process.

Membrane filtration coupled with wet air oxidation for intensified treatment of biorefractory effluents.

This work aims to analyse the performances of a new hybrid process: membrane filtration to concentrate biorefractory wastewater before treatment by a hydrothermal process such as wet air oxidation. The aim is to obtain a complete discharge of the effluent in the environment. The three different synthetic wastewaters under study were pharmaceutical wastewater, grey wastewater and bilge wastewater. The results of the membrane filtration showed high retention rates as it could reach between 75% and 100% of total organic carbon retention, more than 99% of turbidity removal and more than 70% of hydrocarbon retention. Moreover, it was possible to achieve high concentration factors comprised between 17 and 40 times. Membrane fouling was chemically reversible regardless of the type of pollution. Then, the treatment of the membrane retentates by wet air oxidation process (300 °C, 15 MPa) could eliminate more than 83% of organic pollution for all the tested effluents. In summary, the hybrid intensified process could finally decrease the volume and the waste load of wastewater before possibly discharging it into the environment.

The effect of different inorganic anions on mineralization of acrylic acid in wet air oxidation.

Wet Air Oxidation (WAO) process is being developed as a very promising technique for efficient removal of high-concentration organic pollutants. However, because of the technical constraints, many chemical wastewater contain inorganic anions (e.g., Cl-, SO42-, PO43- and NO3-). These inorganic salts can cause equipment corrosion and also affect WAO reactivity. But very limited studies of the effect of inorganic anions on WAO in chemical wastewater have been performed. Here we for the first time intensively investigate the effect of different inorganic anions on kinetics and pathways of WAO reaction, acrylic acid was selected as substrate. We used a nonlinear least-squares curve fitting method using Matlab 2014® to obtain the kinetic model. Importantly, it was demonstrated that this proposed kinetic model represented the experimental data well, and acetic acid was the only residual short-chain carboxylic acid. Moreover, the higher concentration anions the higher selectivity of WAO of CO2 by acrylic acid. And the selectivity order of WAO to acetic acid is PO43->NO3->Cl->SO42- at low concentration (100mmolL-1). Finally, the theory calculation disclosed the feasibility of the reactions between these anions and acrylic acid, calculation results revealed that atoms 1#, 7# and 8# have the strongest chargeability and are more vulnerable to oxidant attack because of their high charge density. And the Total Organic Carbon (TOC) removal was positively correlated with the electronegativity of the central atom of oxidizing acid.

Investigations on Cationic Dye Degradation Using Iron-Doped Carbon Xerogel

Iron-doped carbon xerogels were prepared using sol-gel synthesis, with potassium-2,4-dihydroxybenzoate and formaldehyde as starting materials, followed by an ion exchange step. The obtained samples were characterized (XRD, FTIR, SED-EDX, TEM) and investigated as catalysts in heterogeneous Fenton and catalytic wet air oxidation (CWAO) processes. Experiments were conducted in the same conditions (0.1 g catalysts, 25 mL of 100 mg/L dye solution, 25 °C, initial solution pH, 3 h) in thermostated batch reaction tubes (shaking water bath, 50 rpm) at atmospheric pressure. A series of three cationic dyes were considered: Brilliant green (BG), crystal violet (CV), and methyl green (MG). Dyes and TOC removal efficiencies up to 99% and 92%, respectively, were obtained, in strong correlation with the iron content of the catalyst. Iron content measured in solution at the end of the reaction, indicated that its amount was less than 2 ppm for all tested catalysts.

Bioinspired metal oxide particles as efficient wet air oxidation and photocatalytic oxidation catalysts for the degradation of acetaminophen in aqueous phase

The catalytic performances of the biomimetic metal oxides were tested in photo Fenton-like oxidation and catalytic wet air oxidation processes. Biomimetic copper oxide, iron oxide, and cobalt oxide catalysts were prepared by using pollen grains as biotemplate. The surface characteristics of the biomimetic metal oxides were characterized. SEM micrographs of the biomimetic catalysts demonstrated that pollen grains were successfully mimicked by metal oxide structures. The influences of UV light intensity, catalyst loading, and the initial hydrogen peroxide concentration on acetaminophen degradation were investigated in the photo Fenton-like oxidation process whereas the effects of reaction temperature and catalyst loading were investigated in catalytic wet air oxidation process. The biomimetic copper oxide was the most effective catalyst for the removal of acetaminophen in both of the advanced oxidation processes. The highest acetaminophen degradation efficiency was 86.9% in photo Fenton-like oxidation process when the initial acetaminophen concentration, catalyst loading, and the initial H2O2 concentrations were 10 mg/L, 0.1 g/L and 1 mM, respectively, at room temperature. In the catalytic wet air oxidation process, 98.3% degradation was achieved for the treatment of 100 mg/L acetaminophen solutions at 180 °C and 10 bar by using 1 g/L of catalyst loading at the same reaction time as photo Fenton-like oxidation. Mineralization analysis and the toxicity tests indicated that the biomimetic copper oxide catalysts were promising for the acetaminophen removal in catalytic wet air oxidation processes.

Wet air oxidation of leachate containing emulsified and solubilized hydrocarbons from crude oil-contaminated soil

The present research work is focussed on the treatment of leachate generated from crude oil-contaminated soil sites using wet air oxidation as an advanced oxidation process. The factors affecting the wet air oxidation (WAO) process, viz. temperature, pressure and time of treatment were optimized using central composite design and response surface methodology. The significant factors were optimized to maximize % COD removal from the leachate. The linear effects of pressure and temperature (p = 0.000); the square effects of pressure (p = 0.019) and time (p = 0.007) and the interaction effect of temperature–pressure (p = 0.002) were found to be significantly governing the % COD removal. The maximum COD removal of 76% was obtained at temperature = 244 °C, time = 30 min and pressure = 5 bar. Further, the biodegradability index (BOD5/COD) increased from 0.14 ± 0.007 of the untreated leachate to 0.48 ± 0.02 of the wetox-treated leachate. Moreover, the degradation of recalcitrant hydrocarbons in initial leachate by WAO treatment was confirmed using GC–MS analysis.

Electro-activation of O2 on MnO2/graphite felt for efficient oxidation of water contaminants under room condition

The activation of oxygen (O2) under room condition is highly desirable for oxidative removal of organic pollutants in water. Herein, we report a graphite felt (GF)-supported α-MnO2 catalyst which is active for activating O2 with assistance of an anodic electric field. The electro-assisted catalytic wet air oxidation (ECWAO) process on MnO2/GF is able to rapidly degrade a variety of dyes, pharmaceutics and personal care products (PPCPs) under room condition. The congo red, basic fuchsin, neutral red and methylene blue are completely mineralized in 160 min, and the bisphenol A, triclosan and ciprofloxacin are mineralized by 89.9%, 81.5% and 65.4%, respectively, in 300 min. Mechanistic study indicates a surface-catalyzed non-free radical pathway for the oxidation of organic pollutants by O2 in the ECWAO process. The oxygen vacancies on MnO2 are identified as the catalytically active sites, at which oxygen atom is transferred from O2 to organic molecule through chemisorbed oxygen species. The anodic electric field assists such an oxygen transfer pathway by activating the complex of chemisorbed oxygen species and organic molecule through electro-oxidation reaction. The ECWAO process on MnO2/GF electrode exhibits a great potential for practical wastewater treatment under room condition.