Catalytic Wet Air Oxidation Experiments Research Articles

Preparation of porous metakaolin-based alkali-activated materials by microwave curing as supports for non-noble metal catalysts

Generally, alkali activated materials (AAMs) are prepared by curing an alkaline paste via conventional methods such as at room temperature for weeks, even months, or in oven for days to achieve the required properties. In this paper, fast electromagnetic radiation (microwaves) was investigated in detail as an alternative to conventional methods to produce environmentally friendly porous metakaolin-based AAMs as supports for catalysts (MK-AAMs). Hydrogen peroxide was used as a foaming agent to achieve porosity of the porous MK-AAMs. The non-porous MK-AAMs without a foaming agent were used as reference for the porous MK-AAMs. Curing of the MK-AAMs were conducted at room temperature (CU1), in an oven (CU2), and by microwaves (CU3). The specific surface area (SSA) of porous and non-porous MK-AAMs cured via CU1–3 was 36, 49, 90 m2/g and 9.0, 23, 24 m2/g, respectively. The average compressive strength (MPa) was 3.8, 2.7, 2.1 and 8.9, 6.4, 5.2, respectively. The catalytical performance of porous and non- porous MK-AAMs with Cu-catalyst (i.e., Cu-composites) were tested with an aqueous solution of bisphenol A (BPA) in catalytic wet air oxidation (CWAO). After 5 h of oxidation, the highest BPA conversion of 97 % was achieved with the porous Cu-composites. A slight mass loss of 1.5 wt % was observed in the Cu composites due to erosion caused by strong fluid flow in the CWAO experiments. Based on the various analyzes performed, no significant structural or chemical changes were observed in porous and non-porous MK-AAMs regardless of the curing method of CU1–3, which also confirmed that MK-AAMs can be produced by microwaves without losing the desired properties of the materials, such as porosity and mechanical durability.

Preparation of CuO/HZSM-5 catalyst based on fly ash and its catalytic wet air oxidation of phenol, quinoline and indole

This work aims to use fly ash and the organic template of tetrapropyl ammonium bromide (TPABr) to synthesize the catalyst carrier of HZSM-5 and prepare the catalyst of CuO/HZSM-5 for catalytic wet air oxidation (CWAO) of phenol, quinoline and indole in aqueous solution. The carrier and the catalyst were characterized by x-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF) and Brunauer-Emmett-Teller (BET) tests and the results indicate HZSM-5 zeolite and CuO/HZSM-5 catalyst have been successfully synthesized. The specific surface area of catalysts with copper loading from 0 to 15% decreased from 310.1 m2 g−1 to 253.8 m2 g−1. The results of catalyst performance showed that the catalyst of CuO/HZSM-5 with copper loading of 10% has the best removal effect on the mixed aqueous solution containing phenol, quinoline and indole. When the total concentrations of phenol, quinoline and indole are 200 mg.l−1 (namely 120 mg phenol·l−1, 60 mg quinoline·l−1 and 20 mg indole·l−1), the catalyst with the copper loading of 10% can remove these organic matters with 100% efficiency after reaction for 4 h at 200 °C and the COD removal rate is more than 75%. Under the same experimental conditions, if the reaction temperature drops to 120 °C, the COD removal rate will rise to 86.2%. The CWAO experiments showed the optimum reaction temperature range for the Cu-10% catalyst is from 120 °C to 150 °C.

Synthesis and characterization of carbon xerogel based iron catalyst for use in wet air oxidation of aqueous solution containing 2, 4, 6–trichlorophenol

Carbon xerogels were prepared using tannic acid, resorcinol and formaldehyde as precursors. The water content of the wet organic xerogels was replaced with t-butanol as a solvent to prevent pore collapse during ambient drying and the carbon xerogels were modified using o-phosphoric acid to improve its surface and textural properties. The xerogels and the carbon xerogel based iron catalyst were characterized for surface, textural and physico-chemical properties using various techniques. The catalytic wet air oxidation (CWAO) of aqueous solution containing 2, 4, 6-trichlorophenol (TCP) was performed using modified carbon xerogel based iron catalyst and the results were interpreted in terms of TCP and chemical oxygen demand (COD) removal efficiencies. The effect of various operating parameters on the TCP and COD removal efficiencies was studied and the maximum TCP and COD removal efficiencies of 74.49% and 24.31%, respectively were obtained at the optimum value of operating parameters. The kinetics of the degradation of TCP was studied. The reusability of the catalyst was explored by repeating CWAO experiments three times with washing and thermal treatment of used catalyst after each cycle. The leaching of iron metal from the catalyst was studied by determining the iron metal content in the CWAO effluent.

Treatment of toxic industrial effluent containing nitrogenous organic compounds by integration of catalytic wet air oxidation at atmospheric pressure and biological processes

The treatment of non-biodegradable industrial effluent containing toxic pyridine compounds by integration of catalytic wet air oxidation (CWAO) and biological processes has been studied. The CWAO experiments were performed using ceria promoted Pt/Al2O3 catalyst. The catalyst was characterized by various techniques and the stability of the catalyst was studied. The research intended to achieve two main objectives: a) biodegradability enhancement of toxic industrial effluent by CWAO which is operated at mild conditions and b) complete mineralization of effluent obtained after CWAO treatment by biological processes. Therefore, the CWAO experiments were performed at atmospheric pressure and the effect of various operating parameters such as ceria loading, catalyst dosage and temperature on Chemical oxygen Demand (COD) removal of industrial effluent was optimized. The CWAO reaction followed a first order reaction kinetic model. The catalyst stability test was performed to determine its reusability and the catalyst was found active after 3 cycles run. The biodegradability of the industrial effluent was found to be improved from 0.07 to 0.56 and toxicity was completely removed after CWAO treatment. The effluent obtained after CWAO was further treated with biological aerobic treatment and a total COD removal of 98.4% was obtained after 10 days of aerobic treatment.

Treatment of industrial organic raffinate containing pyridine and its derivatives by coupling of catalytic wet air oxidation and biological processes

The treatment of non-biodegradable industrial organic raffinate containing refractory and toxic organic compounds pyridine, β-picoline and 3-cyanopyridine has been studied by coupling of catalytic wet air oxidation (CWAO) over alumina based platinum catalyst and biological processes. The alumina based platinum catalyst was characterized and the stability of the catalyst was studied. The CWAO experiments were performed in a glass reactor at atmospheric pressure and the effluent treatment efficiency was evaluated in terms of chemical oxygen demand (COD) removal. The effect of various parameters such as air flow rate, reaction temperature, platinum loading and catalyst dosage on the COD removal was studied. The optimum values of air flow rate, platinum loading and catalyst dosage were found to be 1 L/min, 1 wt. % and 3 g/L respectively and a COD removal of 45% was obtained at the optimum conditions at reaction temperature of 70 °C. The CWAO experimental results were found to be in agreement with the lumped kinetic model. The toxicity test using E. coli bacteria and the biodegradability study were performed. The toxicity of the effluent decreased considerably while the BOD/COD ratio was found to increase significantly after the CWAO. The aerobic biological treatment confirmed the biodegradability enhancement and a total COD removal of 98.4% was obtained after 10 days of aerobic treatment of the CWAO effluent.

Catalytic wet air oxidation of 2-chlorophenol over sewage sludge-derived carbon-based catalysts

A sewage sludge derived carbon-supported iron oxide catalyst (FeSC) was prepared and used in the Catalytic Wet Air Oxidation (CWAO) of 2-chlorophenol (2-CP). The catalysts were characterized in terms of elemental composition, surface area, pHPZC, XRD and SEM. The performances of the FeSC catalyst in the CWAO of 2-CP was assessed in a batch reactor operated at 120°C under 0.9MPa oxygen partial pressure. Complete decomposition of 2-CP was achieved within 5h and 90% Total Organic Carbon (TOC) was removed after 24h of reaction. Quite a straight correlation was observed between the 2-CP conversion, the amount of iron leached in solution and the pH of the reaction mixture at a given reaction time, indicating a strong predominance of the homogeneous catalysis contribution. The iron leaching could be efficiently prevented when the pH of the solution was maintained at values higher than 4.5, while the catalytic activity was only slightly reduced. Upon four successive batch CWAO experiments, using the same FeSC catalyst recovered by filtration after pH adjustment, only a very minor catalyst deactivation was observed. Finally, based on all the identified intermediates, a simplified reaction pathway was proposed for the CWAO of 2-CP over the FeSC catalyst.

Degradation of phenol via wet-air oxidation over CuO/CeO2–ZrO2 nanocatalyst synthesized employing ultrasound energy: physicochemical characterization and catalytic performance

Catalytic wet air oxidation (CWAO) of phenol was carried out under atmospheric pressure of oxygen at 160°C in a stirred batch reactor over copper catalysts supported by CeO2–ZrO2. The copper with different loadings were impregnated over the composite support by a sonication process. The catalysts were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray analysis (EDX), Brunauer-Emmett-Teller (BET) specific surface area and Fourier-transformed infrared analyses. Characteristic peaks attributed to copper were not found in XRD patterns even at high loadings, but based on EDX results, the existence of copper particles was confirmed. It means that sonochemical synthesis method even at high loadings produced small copper particles with low crystallinity and excellent dispersion over the CeO2–ZrO2 composite. FESEM micrographs indicated just slight enhancement in particle size at high loadings of Cu. Blank CWAO experiments illustrated low conversion of phenol using bare CeO2–ZrO2 support. Although some agglomeration of particles was found at high loadings of copper but owning to the fact that almost all ZrO2 particles incorporated into the CeO2 lattice at high contents of Cu, catalyst activity not only did not decrease but also the phenol conversion reached to the higher values. The optimal catalyst loading for phenol degradation was found to be 9 g/l. Complete conversion of phenol was achieved using CuO/CeO2–ZrO2 in 9 g/l catalyst loading with initial phenol concentration of 1000 ppm after 3 h of reaction.

Catalytic wet air oxidation of bisphenol A model solution in a trickle-bed reactor over titanate nanotube-based catalysts

Titanate nanotube-based catalysts were prepared via alkaline hydrothermal synthesis route followed by heat-treatment at different temperatures, ranging from 300 to 700°C. The resulting metal-free solids were then applied as a catalyst in a three-phase trickle-bed reactor, where catalytic wet air oxidation (CWAO) reactions of model aqueous bisphenol A (BPA) solution were performed. Mainly, the CWAO experiments were conducted at 200°C with oxygen partial pressure of 10bar over 300mg of a catalyst. It was observed in the given range of operating conditions that BPA undergoes both non-catalytic as well as catalytic oxidation routes, while the latter is far more pronounced. At 210°C and in the presence of 0.5g of titanate nanotube-based catalyst, which was annealed at 600°C, complete BPA removal was obtained. From TOC point of view, approximately 70% conversion was achieved indicating the persistence of refractory intermediates of lower carboxylic acids. The cross-section of results derived from various analytical techniques, which were used to identify surface, textural and morphological properties, revealed that balanced physicochemical properties are required to achieve meaningful extent of BPA removal. During 2–4 day time on stream, no catalyst deactivation occurred that could be attributed to the dissolution of active powders, or to the carbonaceous deposits accumulated on the catalyst surface. Therefore, these nanotubular materials can be regarded as innocuous and efficient long-term catalysts for oxidation of hazardous organic compounds (such as BPA) in the CWAO process.

Chemical and toxicological evaluation of an emerging pollutant (enrofloxacin) by catalytic wet air oxidation and ozonation in aqueous solution

This study evaluates the degradation efficiency of enrofloxacin (ENR) by catalytic wet air oxidation (CWAO) and ozonation. Results obtained by CWAO experiments show that 99.5% degradation, 37.0% chemical oxidation demand (COD) removal and 51.0% total organic carbon (TOC) conversion were obtained when 100mol% FeCl3 and 25mol% NaNO2 at 150°C under 0.5MPa oxygen pressure after 120min are used. The degradation products are identified by liquid chromatography–tandem mass spectrometry (LC–MS/MS), gas chromatography–mass spectrometry (GC–MS) and ion chromatography (IC). The oxidation end products, F−, NO3- and NH4+ were determined by IC. The BOD5/COD ratio as a measure of the biodegradability of the parent compound increased from 0.01 to 0.12 after 120min of reaction time, indicating an improved biodegradability of the parent compound. The inhibition of bioluminescence of the marine bacteria V. fischeri decreased from 43% to 12% demonstrating a loss in toxicity of ENR during CWAO. Ozonation of 0.2mM ENR was carried out with an ozone concentration of 7.3gm−3 at pH 7. ENR decomposition with a degradation rate of 87% was obtained corresponding to the reaction time. Moderate changes in COD (18%) and TOC (17%) removal has been observed. The bioluminescence inhibition increased from 8% to 50%, due to the generation of toxic degradation products during ozonation. In comparison to the widely use of well developed method of ozonation CWAO exhibits better performance in terms of COD, TOC removals and generates less toxic products.

Ru/TiO2 catalyst for efficient removal of estrogens from aqueous samples by means of wet-air oxidation

The ubiquitous presence of endocrine-disrupting compounds (EDCs) in the environment is mainly the consequence of their incomplete removal from biological wastewater treatment plants. One of the promising options for EDC removal is catalytic wet-air oxidation (CWAO), where pollutants are oxidized by activated O2 species in the presence of a solid catalyst. 17β-estradiol (E2) was used in CWAO experiments conducted in a trickle-bed reactor up to 230 °C over TiO2 and Ru/TiO2 solids. In the given range of operating conditions, E2 undergoes noncatalytic and catalytic oxidation routes. The employed Ru(3.0 wt.%)/TiO2 catalyst enabled complete and long lasting efficiency of E2 degradation as well as removal of estrogenicity from the feed solution. No deactivation occurred and no carbonaceous deposits were accumulated on the catalyst surface.

Degradation of trinitrophenol by sequential catalytic wet air oxidation and solar TiO2 photocatalysis

Catalytic wet air oxidation (CWAO) and solar TiO2 photocatalysis were investigated as advanced oxidation processes to degrade trinitrophenol (TNP) in model aqueous solutions. An activated carbon (AC) treated with sulphuric acid of different concentrations (5, 10 and 18M) at two different temperatures (353 and 423K) was investigated as a metal-free CWAO catalyst, while a commercially available P25 TiO2 powder was used as a photocatalyst. CWAO experiments were conducted at 448K, 0.7MPa oxygen pressure (4.7MPa of total pressure), 1.3gL−1 AC loading and 270mgL−1 TNP concentration, while photocatalytic experiments were conducted at ambient temperature, 1gL−1 photocatalyst loading, 500–1000Wm−2 irradiance provided by a solar simulator and 32–270mgL−1 TNP concentration. Treatment efficiency was assessed by measuring the concentrations of TNP and nitrates, total organic carbon (TOC) and biochemical oxygen demand (BOD5). Up to 90% TNP degradation was attained during CWAO over 120min, from an initial concentration of 270mgL−1. For the same TNP concentration, TiO2 photocatalysis gives only 13% conversion over the same 120min. However, for TNP concentrations below 144mgL−1, photocatalysis can be effectively used: 100 and 80% TNP degradation obtained in 120min of irradiation for initial TNP concentrations of 64 and 144mgL−1, respectively. In this respect, CWAO and photocatalysis were employed sequentially to treat TNP; complete TNP conversion being achieved after 120min of CWAO followed by 60min of photocatalysis at 1000Wm−2 irradiance, and this was accompanied by 82% TOC reduction, as well as an increase of BOD5/TOC ratio from 0 to 2.28.

Catalytic activity and stability of multiwalled carbon nanotubes in catalytic wet air oxidation of oxalic acid: The role of the basic nature induced by the surface chemistry

Different chemical and thermal treatments (nitric acid at boiling temperature, liquid-phase urea treatment at 200°C, and gas-phase thermal treatment with nitrogen at 600°C) were applied to multiwalled carbon nanotubes (MWCNTs) in order to produce materials with different textural and chemical properties. Nitrogen adsorption isotherms, temperature programmed desorption, pHpzc (point of zero charge), elemental analysis and X-ray photoelectron spectroscopy were used to characterize these materials. The chemical and thermal treatments have influence on the materials pHpzc, which decreases upon nitric acid treatment and increases with urea or gas-phase thermal treatments. The original and modified MWCNTs, without any impregnated metal, were investigated as catalysts in the catalytic wet air oxidation (CWAO) process, using oxalic acid as model compound at 140°C and 40bar of total pressure. Oxalic acid is very stable under non-catalytic conditions but can be totally degraded in less than 30min in the presence of MWCNTs. The rate of oxidation of oxalic acid depends on the chemical properties of MWCNTs, the apparent initial first-order rate constants being lower for the MWCNTs with a marked acid character. The textural properties of MWCNTs are stable in cyclic CWAO experiments, but a decrease of their basic character leads to the reduction of their catalytic activity, even if this activity is still high with reused catalysts (nearly total oxalic acid degradation in 45min and complete mineralization in 120min). Therefore, MWCNTs without any impregnated metal, especially those of more basic character, are very active catalysts for CWAO, their activity strongly depending on the stability of their surface chemistry.

Manganese and Copper Catalysts for the Phenolic Wastewaters Remediation by Catalytic Wet Air Oxidation

The catalytic wet air oxidation (CWAO) of a simulated phenolic wastewater was investigated at temperatures up to 200ºC and 15 bar of air partial pressure. Laboratorial (Mn-Ce-O 70/30 IMP, Mn-Ce-O 70/30 CP, MnCu 70/30, MnCu 50/50) and a commercial catalyst (N140: CuO-MnOx 22/50) were used to evaluate the catalytic activity in terms of Total Organic Carbon (TOC). Among the manganese/cerium and manganese/copper catalysts tested with different molar ratios, Mn-Ce-O was the most active leading to the complete mineralization after 120 min of an aqueous solution containing six representative phenolic acids. The co-oxidation effect was identified when comparing the oxidation of global phenolic mixture and the individual parent compounds' depletion. The catalyst preparation method had no significant differences in terms of catalytic activity. During the CWAO experiments, acetic acid and phenol were detected and measured by HPLC as the main reaction intermediate compounds. Concerning catalytic stability, manganese and copper catalyst revealed good properties in terms of active metal leaching to the bulk liquid as well as in the deposition of only 3.152% (W/W) of carbonaceous material on the catalyst surface for the co-precipitated Mn-Ce-O 70/30 catalyst.

Catalytic wet air oxidation of olive oil mill effluents: 4. Treatment and detoxification of real effluents

Olive oil mill wastewater (OMW) generated by the olive oil extraction industry constitutes a major pollutant, posing severe environmental threats. It contains a high organic load and phytotoxic and antibacterial phenolic compounds which resist biological degradation. Platinum and ruthenium supported titania or zirconia were studied in the catalytic wet air oxidation (CWAO) of OMWs in a batch reactor and in a continuous trickle-bed reactor. CWAO experiments at 190°C and 70bar total air pressure confirmed the effective elimination of the TOC (total organic carbon) and of the phenolic content of actual diluted OMW. Simultaneously, toxicity towards Vibrio fischeri was reduced and a decrease in phytotoxicity occurred. The ruthenium catalysts were found stable over a long period of operation in a trickle-bed reactor.The biodegradability of the oxidized waste has been enhanced and this study also examined the feasibility of coupling CWAO and an anaerobic digestion treatment. The pretreatment of the OMW in the presence of a ruthenium catalyst reduced considerably the total phenolic contents of the wastewater, and produced an effluent suitable to be treated by anaerobic treatment with increased biomethane production compared to the untreated effluent.

Wet air oxidation of acetic acid over platinum catalysts supported on cerium-based materials: Influence of metal and oxide crystallite size

Platinum catalysts (2.5 wt%) prepared by impregnation on pure commercial ceria and on Zr 0.1(Ce 0.75Pr 0.25) 0.9O 2 sol–gel mixed oxide were synthesized and used for catalytic wet air oxidation (CWAO) of acetic acid. The influence of the platinum sintering (reducing treatment) was studied for both supports to modify the oxygen transfer from the gas phase onto the metallic active site of the catalyst and also to determine the effect of this treatment on the catalytic activity and stability. The catalysts were characterized by BET, TEM, XRD, FT-IR and hydrogen chemisorption before and after CWAO reaction, and oxygen storage capacity (OSC) was measured. The nature of the support [CeO 2 and Zr 0.1(Ce 0.75Pr 0.25) 0.9O 2 compounds] and the preparation conditions led to samples displaying a Pt dispersion ranging from 0.4 to 41%, and BET surface area varying from 4 to 45 m 2 g −1. The results demonstrate poisoning of the catalysts during the CWAO experiments due to the formation of carbonate species. This limits the performance of the mixed oxide-based catalysts displaying the highest OSC values. A correlation was established between the OSC of the catalysts and the formation of carbonate species. Nevertheless, there is no correlation between OSC and catalytic activity, with the ceria-based samples showing the highest reaction rate. An optimal platinum crystallite size is required to achieve the highest conversion of acetic acid in terms of turnover frequency values.

Catalytic and non-catalytic wet air oxidation of sodium dodecylbenzene sulfonate: Kinetics and biodegradability enhancement

Wet air oxidation (WAO) and catalytic wet air oxidation (CWAO) were investigated as suitable precursors for the biological treatment of industrial wastewater containing sodium dodecylbenzene sulfonate (DBS). Two hours WAO semi-batch experiments were conducted at 15 bar of oxygen partial pressure ( P O 2 ) and at 180, 200 and 220 °C. It was found that the highest temperature provides appreciable total organic carbon (TOC) and chemical oxygen demand (COD) abatement of about 42 and 47%, correspondingly. Based on the main identified intermediates (acetic acid and sulfobenzoic acid) a reaction pathway for DBS and a kinetic model in WAO were proposed. In the case of CWAO experiments, seventy-two hours tests were done in a fixed bed reactor in continuous trickle flow regime, using a commercial activated carbon (AC) as catalyst. The temperature and P O 2 were 140–160 °C and 2–9 bar, respectively. The influence of the operating conditions on the DBS oxidation, the occurrence of oxidative coupling reactions over the AC, and the catalytic activity (in terms of substrate removal) were established. The results show that the AC without any supported active metal behaves bi-functional as adsorbent and catalyst, giving TOC conversions up to 52% at 160 °C and 2 bar of P O 2 , which were comparable to those obtained in WAO experiments. Respirometric tests were completed before and after CWAO and to the main intermediates identified through the WAO and CWAO oxidation route. Then, the readily biodegradable COD (COD RB) of the CWAO and WAO effluents were found. Taking into account these results it was possible to compare whether or not the CWAO or WAO effluents were suitable for a conventional activated sludge plant inoculated with non adapted culture.

Reaction pathway of the catalytic wet air oxidation of phenol with a Fe/activated carbon catalyst

Catalytic wet air oxidation (CWAO) of phenol with molecular oxygen using a home-made Fe/activated carbon catalyst at mild operating conditions (100–127°C; 8atm) has been studied in a trickle-bed reactor. Ring compounds (hydroquinone, p-benzoquinone and p-hydroxybenzoic acid) and short-chain organic acids (maleic, malonic, oxalic, acetic and formic) have been identified as intermediate oxidation products. CWAO experiments using each one of these intermediates as starting compound have been carried out (at 127°C and 8atm) in order to elucidate the reaction pathway. It was found that phenol is oxidized through two different ways. It can be either para-hydroxylated to hydroquinone, which is instantaneously oxidized to p-benzoquinone or para-carboxylated to p-hydroxybenzoic acid. p-Benzoquinone is majorly mineralized to CO2 and H2O through oxalic acid formation whereas p-hydroxybenzoic acid gives rise to short-chain acids. Only acetic acid showed to be refractory to CWAO under the operating conditions used in this work. The catalyst avoids the presence of ring-condensation products in the reactor effluent which were formed in absence of it. This is an additional important feature because of the ecotoxicity of such compounds.