Catalytic Wet Air Oxidation Research Articles

Influence of the Active Phase (Fe, Ni, and Ni–Fe) of Mixed Oxides in CWAO of Crystal Violet

The catalytic oxidation of aqueous crystal violet (CV) solutions was investigated using Ni and Fe catalysts supported over Mg–Al oxides synthesized by the autocombustion method. The influence of temperature, loading, and selectivity were studied in the catalytic wet air oxidation (CWAO) of CV. The kind of metal had an important contribution in the redox process as significant differences were observed between Fe, Ni, and their mixtures. The catalysts with only Fe as active phase were more efficient for the oxidation of CV under normal conditions (T = 25 °C and atmospheric pressure) compared to those containing Ni, revealing the influence of the transition metal on catalytic properties. It was found that iron-containing materials displayed enhanced textural properties. The synthesis of Fe/MgAl catalysts by the autocombustion method led to solids with excellent catalytic behavior, 100% CV degradation in eight hours of reaction, 68% selectivity to CO2, and significant reduction of chemical oxygen demand (COD).

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...

Aqueous phase reforming coupled to catalytic wet air oxidation for the removal and valorisation of phenolic compounds in wastewater

Aqueous phase reforming (APR) coupled to catalytic wet air oxidation (CWAO) has been investigated as an approach to remove phenolic compounds from wastewater, converting them into valuable gases. Partial oxidation of phenol was achieved in the first CWAO stage trying to minimize mineralization so to allow a high yield to valuable gases in the second APR stage. APR runs were carried out with different mixtures of compounds corresponding to phenol oxidation pathway (phenol, quinones, long and short chain acids) and representing different degrees of oxidation in CWAO stage. A range of TOC and COD removal (74–90%) was observed in APR stage for the single compounds, with higher removal for long chain acids. Likewise, long chain acids provided with the highest conversion to gases. APR of mixtures rich in acids gave the highest yield to CH4 (11.0 mmol CH4/g TOCinitial). H2 production was low in all cases, due to competing direct conversion of long and short chain acids into CH4. TOC and COD removal from wastewater was similar in APR-CWAO and APR, however the conversion to gases and the yield to CH4 were markedly higher for APR-CWAO, thus overcoming the difficulties previously observed in the direct APR of phenol.

Synergistic effect between Mn and Ce for active and stable catalytic wet air oxidation of phenol over MnCeOx

A series of MnCeOx catalysts for the catalytic wet air oxidation (CWAO) of phenol is developed by a redox-precipitation preparation method. Among them, Mn9Ce1Ox (Mn/Ce = 9:1) shows excellent activity and stability which can achieve TOC conversion of 88.1 % in 60 min at 70 °C and retain stable for five consecutive runs. Even at 50 °C, Mn9Ce1Ox can reach a phenol conversion of 84. 2% and a TOC conversion of 73.8 %. The roles of Mn valence state and Ce modification in the catalytic performance of Mn9Ce1Ox are extensively investigated by XRD, BET, Raman, H2-TPR and XPS. It is found that high Mn4+ concentration favors phenol adsorption and oxidation while high Mn3+ concentration favors oxygen activation. Ce optimizes the Mn4+/Mn3+ ratio and thus enhances catalytic activity. Additionally, Ce retains the structure of the catalyst and prevents Mn4+ and Mn3+ from being reduced to Mn2+, which contributes the good stability.

Catalytic wet air oxidation of phenol by cordierite honeycomb washcoated with Al/Zr pillared bentonite in a plug flow reactor

Cordierite monoliths coated with Al/Zr pillared bentonite clay powder were synthesized and tested for catalytic wet air oxidation (CWAO) of phenol aqueous solutions in a plug flow reactor. The catalysts were characterized by using various techniques (EDS, SEM, BET and XRD), confirming the effective deposition of Al/Zr pillared bentonite clay on the channel walls of cordierite monolith. The composite catalyst showed good mechanical stability after ultrasound treatment. The performance of the composite catalyst was evaluated in a catalytic plug flow reactor for continuous oxidation of phenol in terms of catalytic activity as well as their stability. The composite catalyst achieved high catalytic performance (100 % phenol conversion and 91 % total organic carbon reduction) with high stability after 140 h long experiment. During the CWAO of phenol reaction, aromatic ring intermediates (p-benzoquinone, catechol and hydroquinone) and short-chain carboxylic acids (oxalic acid, maleic acid, formic acid, acetic acid and malonic acid) were identified. Kinetic parameters of the model that describes the formation and oxidation of intermediate aromatic compounds and short-chain carboxylic acids were determined. The obtained experimental data results were described sufficiently with respect to phenol conversion. The proposed CWAO of the phenol kinetic model accounts for internal and external diffusion limitations, assuming the pseudo-first-order reaction. Therefore, synthesized monolithic catalysts have great potential for application in CWAO of phenol, since the materials used in the process are inexpensive, abundant and require minimal modifications.

Catalytic wet air oxidation of phenol: Review of the reaction mechanism, kinetics, and CFD modeling

Advanced oxidation processes, specifically catalytic wet air oxidation (CWAO), are considered as useful and robust methods for the treatment of refractory organic compounds such as phenol in wastewater. This paper reviews reaction mechanisms, kinetics and computational fluid dynamics (CFD) modeling of CWAO of phenol. Different reaction mechanisms have been proposed by various researchers to account for possible intermediates during phenol oxidation. These mechanisms can be either direct or indirect; with the indirect path resulting in the formation of intermediates, while with the direct mechanism, no intermediates are formed, the reaction proceeds straight to carbon dioxide and water. Acetic acid is considered as the most stable intermediate of phenol oxidation. Power law and Langmuir–Hinshelwood models were used to account for the adsorption/desorption of the reactants on the surface of the catalyst. The reviewed studies show that phenol conversion increases with increased temperature, pressure, gas velocity and decreases with increasing liquid space velocity. The formation of hot spots next to the walls of the reactor leads to safety issues and may cause catalyst deactivation and reactor thermal run away. This prompted a review of liquid mal-distribution and the formation of hot spots inside the reactor using the CFD modeling technique. In most cases, liquid channeling was observed, resulting in the formation of hot spots and catalyst deactivation.

Catalytic wet air oxidation of high BPA concentration over iron-based catalyst supported on orthophosphate.

The catalytic performance of Fe supported on nickel phosphate (NiP) was evaluated for the removal of bisphenol A (BPA) by catalytic wet air oxidation (CWAO) at 140°C and 25bar of pure oxygen pressure. The prepared NiP and Fe/NiP materials were fully characterized by XRD, N2-physisorption, H2-TPR, TEM, and ICP analysis. Iron (Fe/NiP) impregnation of NiP support enhanced the BPA removal efficiency from 37.0 to 99.6% when CWAO was performed. This catalyst was highly stable given the operating conditions of acidic medium, high temperature, and high pressure. The Fe/NiP catalyst showed an outstanding catalytic activity for oxidation of BPA, achieving almost complete removal of BPA in 180min at a concentration of 300mg/L, using 4g/L of Fe/NiP. No iron leaching was detected after the CWAO of BPA. The stability of Fe/NiP was performed over three consecutive cycles, noting that BPA conversion was not affected and iron leaching was negligible. Therefore, this catalyst (Fe/NiP) could be considered as an innocuous and effective long-lasting catalyst for the oxidation of harmful organic molecules.

Synthesis and characterization of nanomesoporous lanthanum aluminate (LaAlO3) for catalytic wet air oxidation of phenol

Nanomesoporous LaAlO3 synthesized by reverse microemulsion method were tested in the catalytic wet air oxidation (CWAO) of phenol in a fixed bed reactor. Synthesized LaAlO3 was characterized in depth by TG-DSC, XRD, FTIR, BET, TEM, TPD-CO2, TPD-NH3, and H2-TPR in order to relate its performances to the chemical and textural characteristics. Nanomesoporous LaAlO3 showed high activity towards phenol and in total organic carbon (TOC) removal with greater selectivity to non-organic compounds, a clear increase in the removal efficiency and selectivity relatively to non-catalytic wet air oxidation being observed. Effect of temperature, catalyst dosage, air flow, and phenol pumping rate in CWAO of phenol was investigated. LaAlO3 displayed high catalytic stability even after 6 h of continued reaction. Structural and textural stability of LaAlO3 was confirmed by XRD and BET surface area analysis of regenerated catalyst. Lewis basic sites and mesoporous nature of nano-LaAlO3 may play an important role in phenol degradation and TOC removal efficiency with high selectivity to non-organic compounds.

Life cycle assessment of wastewater reclamation in a petroleum refinery in Turkey

Wastewater reclamation in a petroleum refinery in Turkey was evaluated with life cycle assessment (LCA). The goal of the study was to determine whether or not refinery wastewater reclamation for different industrial purposes, namely boiler feedwater, cooling water and fire water, leads to an overall benefit across different environmental aspects, besides alleviating freshwater resources, when compared to current wastewater disposal practices. The basis for the assessment was the hypothetical scale-up of a demonstration plant tested with real wastewaters from November 2018 to May 2019 at the Izmit petroleum refinery operated by Tüpraş. This demonstration plant consisted of different treatment modules, including dissolved air flotation, ceramic membrane bioreactor, catalytic wet-air oxidation, advanced oxidation with ozone and hydrogen peroxide, and reverse osmosis. The LCA was conducted following consequential modelling principles, and six environmental indicators were analysed in detail at midpoint level: global warming, respiratory inorganics, marine ecotoxicity, aquatic eutrophication, freshwater consumption and non-renewable energy demand. All three reclamation scenarios (boiler, cooling, fire water) succeeded in achieving a life-cycle freshwater saving, of around 1 m3 freshwater saved per m3 refinery wastewater. Beneficial results were also obtained in marine ecotoxicity and aquatic eutrophication, where impact is reduced up to 90% and 84%, respectively. With regard to global warming and non-renewable energy demand, only the boiler feedwater application appeared to involve an improvement over wastewater disposal, showing a net reduction of 2.2 kg CO2-eq and 40 MJ per m3 wastewater, respectively, thanks to potential thermal energy savings. For cooling makeup water and fire water, impacts were between 2 and 2.5 times higher in these two indicators when compared to wastewater disposal. Finally, the indicator on respiratory inorganic effects, showed higher impact, by a factor 2 to 7, for all reuse scenarios, due to electricity demand, which is linked to the Turkish electricity production mix with a substantial contribution from coal power plants. Thus, the results reflect that achieving a product water of very high quality comes at the price of a high energy demand. Nevertheless, A sensitivity analysis shows that the environmental performance of all scenarios would improve to a great extent when shifting to an electricity mix with a higher share of renewables, as is the current trend in most European countries.

Study on Heterogeneous Catalyst Classification Based on CWAO Process of Organic Waste-Water

Catalytic wet air oxidation (CWAO) technology has the characteristics of wide application range, high organic matter removal rate, low secondary pollution and high efficiency, etc., and it has significant effects on the treatment of difficult biodegradable organic waste-water. According to the homogeneity of the phases of the catalytic reaction system, the catalytic reactions can be divided into two categories: homogeneous catalysis and heterogeneous catalysis. The reported catalysts or their active components mainly include four categories: metal salts, metals/supports, oxides/supports, and composite oxides/supports.

Application and Improvement of Wet Oxidation Technology

The common treatment methods for industrial biodegradable industrial waste-water are physical, chemical and biological methods, but the conventional methods have complicated and inefficient treatment processes. Wet air Oxidation (WAO) technology can effectively treat industrial biodegradable industrial waste-water. WAO has achieved good results in the treatment of pesticide waste-water, dye waste-water, activated sludge and phenol-containing waste-water. The CODCr removal rate of waste-water reaches 72.3 %∼95.3 %. In order to further improve the application effect of WAO technology, three improvement approaches have been developed: Catalytic Wet Air Oxidation, Catalytic Wet Supercritical Oxidation, and Catalytic Wet Peroxide Oxidation.

Study on the treatment of actual waste-water by CWAO method

The use of catalytic wet air oxidation (CWAO) technology to treat organic waste-water that is difficult to biodegrade had a significant effect. The following 12 types of waste-water have achieved good results with the CWAO method. The 12 types of waste-water include: papermaking waste-water, dye waste-water, pesticide waste-water, perfume waste-water, phenol waste-water, carbon-containing waste-water, ethanol waste-water, petrochemical company alkali residue waste-water, and oil extraction waste-water, ammonia-containing coking waste-water and pesticide waste-water, cleaning workshop emulsified waste-water and desizing waste-water. A typical homogeneous catalytic wet oxidation system includes 5 processes: Ciba Geigy, Bayer Loprox, WPO, Orcan, and IT Enviroscience. Heterogeneous catalysts lose their activity due to the low pH of the reaction system and the accumulation of activated carbon.

A generalized kinetic model for electro-assisted catalytic wet air oxidation of triclosan on Ni@NiO/graphite electrode

Electro-assisted catalytic wet air oxidation (ECWAO) is an efficient technique for wastewater treatment under ambient condition. In this work, the mineralization kinetics of triclosan (TCS) in an ECWAO process with Ni@NiO/graphite-felt (GF) electrode is studied. The TCS mineralization follows a pseudo-first order kinetics in the ECWAO process. A general kinetic model, based on the depletion of total organic carbon (TOC) as a lumped parameter, is developed. Such a kinetic model suggests the optimized ECWAO operating conditions of initial concentration of TOC at 40 mg L−1, applied current at 25 mA, and temperature at 308 K. In addition, the ECWAO is also effective for the treatment of biosphenol A (BPA) and sulfamethoxazole (SMX). Under the optimal conditions, TCS, BPA and SMX are mineralized by 92.9 ± 1.5, 90.8 ± 1.7 and 90.6 ± 1.0% within 120 min, with specific energy consumptions (SECs) of 2.2 ± 0.3, 1.4 ± 0.5, and 2.8 ± 0.6 kW h kg-TOC−1, respectively.

Effect of calcination temperature on catalyst performance in landfill leachate treatment

In this study, the landfill leachate was treated by Catalytic wet air oxidation, and the heterogeneous catalyst Fe-Co-Ru-Ce/γ-Al2O3 was prepared. The calcination temperatures were respectively set at 300, 450, 600, 750 and 900 °C, and the Fe-Co-Ru-Ce/γ-Al2O3 impregnated with the four metal salt solutions was calcined at the set temperature for 3 h to study the effect of calcination temperature on the activity of the catalyst.The results showed that when the calcination temperature was 450 °C, the activity of the catalyst reached the maximum. At this time, the COD removal rate, decolorization rate and turbidity removal rate of the treated water samples were the highest, which were 78.1%, 60.4% and 92.7%, respectively.

A novel molybdenum-based nanocrystal decorated ceramic membrane for organics degradation via catalytic wet air oxidation (CWAO) at ambient conditions

Catalytic wet air oxidation (CWAO), one of the best-known methods for water treatment, has been intensely investigated for dyes degradation. However, the extreme operation conditions as well as the recovery of suspended catalyst are economically unattractive. In the current work, a novel Mo-based nanocrystal decorated ceramic membrane (Mo/Al2O3) has been prepared and applied for organics degradation via CWAO at ambient conditions for the first time. The catalytic Mo/Al2O3 membranes were prepared by an in-situ hydrothermal followed by calcination method. Their physical and chemical properties were characterized by field emission scanning electron microscope (FESEM), X-rays diffractometer (XRD) and X-ray photoelectron spectroscopy (XPS). The catalytic performance of Mo/Al2O3 membranes were evaluated via organics (safranine O and humic acid) degradation under a home-made membrane filtration system. The effects of calcination temperature, catalyst loading amount, and trans-membrane pressure (TMP) were systematically investigated. The stability and durability of the catalytic Mo/Al2O3 membrane were examined in a long-term filtration system. Results showed that the performance of Mo/Al2O3 membrane would decrease with increasing of calcination temperature as well as TMP. However, the catalyst loading amount was not a major effect on the removal of organics in the system. For one-time loading membrane calcined at 300 °C (1x-Mo/Al2O3 membrane@300), the removal efficiency could achieve higher than 90 % in 40 min in a recycled filtration system with an initial safranine O concentration of 10 mg L−1. The chemical quenching experiment as well as radical quantification verified the main reactive oxygen species were 1O2 and O2− in the system. The ROS generation mechanism was proposed via the characterization of the catalyst after reaction. The deliberate combination of CWAO and membrane separation represents a new strategy that offers exciting possibilities for water treatment under ambient conditions.

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.

Preparation of Cu-Loaded Biomass-Derived Activated Carbon Catalysts for Catalytic Wet Air Oxidation of Phenol

A series of Cu-loaded biomass-derived activated carbon (Cu/AC) catalysts with various impregnation ratios were prepared by precursor impregnation (PI) and activated carbon impregnation (ACI) method...

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.

The catalytic degradation of nitrobenzene by the Cu–Co–Fe-LDH through activated oxygen under ambient conditions

Efficient and low-cost catalysts for catalytic wet air oxidation (CWAO) under ambient conditions are of great significance for the degradation of hydrophobic organic contaminants. In this study, four LDH catalysts were prepared and their catalytic performance was studied by the degradation of nitrobenzene. The CuCoFe-LDH shows the best catalytic activity with an NB removal efficiency of 41.2%. The CuCoFe-LDH exhibited a typical layer structure, with a specific surface area of 167.32 m2 g-1, and Cu2+, Co2+ and Fe3+ were evenly dispersed on the crystal. The NB removal efficiency was increased by 12.5% through adding formic acid. After five recycling processes, the NB removal efficiency was 18.9% because 3.8 mg g-1 of Co was leached out of the LDH. In the CWAO process, H2O2, ˙OH, ˙O2- and 1O2 were successfully formed through activated oxygen by the CuCoFe-LDH catalyst under ambient conditions. This work further broadens the application scope of layered double hydroxides (LDHs) in the degradation of organic pollutants by CWAO under ambient conditions.

Preparation and performance analysis of γ-Al2O3 supported Cu-Ru bimetallic catalysts for the selective Wet Air Oxidation of Aqueous Ammonia to Nitrogen.

Series of Copper Ruthenium (Cu-Ru) bimetallic catalysts supported on γ-Al2O3 with different metal loading are prepared and investigated for catalytic wet air oxidation of ammonia to nitrogen. The ammonia decomposition activity was studied at three different temperatures i.e. 150oC, 200oC, and 230 oC and it is found that catalytic activity increases with the increase in temperature along with the high selectivity towards nitrogen production. The results also revealed that the bimetallic Cu-Ru/ γ-Al2O3 are much more efficient especially stable than the corresponding monometallic Cu and Ru catalysts. Up to 99 % ammonia decomposed to N2 without any undesirable nitrites and nitrates at 230 oC by optimizing catalysts to ammonia ratio. So, it can be considered as a potential method to remove a large amount of ammonia from wastewater. Furthermore, the catalysts characterization results strongly suggested that there is a strong relationship between catalytic activity and Ru and Cu contents in bimetallic catalysts. The presences of both metals (Cu and Ru) affect the reactivity and coverage of oxygen species, as well as protecting each other from leaching. The combination of all effects including the strong interaction between metals, synergistic effect, proper oxygen coverage and resistance against leaching could be attributed to the enhancement of the catalytic activity of Cu-Ru/ γ-Al2O3 catalyst.