Showing papers on "Wet oxidation published in 2011"

Surface chemical modifications induced on high surface area graphite and carbon nanofibers using different oxidation and functionalization treatments

Abstract: Two graphitic carbon materials with different edge to basal plane ratio, high surface area graphite (HSAG) and graphitized carbon nanofibers (CNFs), were oxidized by two methods, aqueous-HNO3 wet oxidation and oxygen plasma oxidation. Characterization of the materials by temperature-programmed desorption, thermogravimetry and X-ray photoelectron and Raman spectroscopies indicated that the amount and nature of oxygen surface groups introduced depended on the oxidation method and on the structure of the original material. While surface sites within the layers were only oxidized by oxygen plasma, surface sites at the edges of graphene layers were oxidized by both treatments being the wet oxidation more effective. Modification of the oxidized materials with a diamine or a triamine molecule resulted in the formation of ammonium carboxylate salt species on the carbon surface.

Sewage sludge based catalysts for catalytic wet air oxidation of phenol: Preparation, characterisation and catalytic performance

Abstract: In this study the use of novel CWAO catalysts, namely potentially low cost, sewage sludge derived activated carbons was explored. Two types of municipal sludge were used: dewatered raw filter cake and dewatered mesophilic anaerobically digested sludge. The carbons were produced by: carbonisation; physical activation (steam or CO2) and chemical activation (K2CO3). Hydrochloric acid washing of some of the carbons was also investigated. The carbons were characterised in terms of their surface area, contact pH, propensity towards metal leaching, surface chemistry (via FTIR), ash content and inorganic elemental composition. Their CWAO performance was assessed at 160 °C and a partial oxygen pressure of 4.2 bar (25 bar of air) within a stirred batch reactor containing a 5 g/L phenol solution. All the carbons exhibited catalytic activity, with the K2CO3 activated and HCl washed carbons attaining a phenol and TOC removal that matched the performance of an activated carbon specifically manufactured for oxidative wastewater treatment applications. A strong correlation was found between surface area and phenol or TOC conversion, suggesting that surface area is a primary requisite for their performance in the first batch cycle. Thus, the oxidation of phenol is thought to proceed via a free radical driven mechanism. The active sites necessary to facilitate this mechanism, whether present as surface functional groups or active metals (e.g., Fe), were detected on all of the sludge based activated carbons. However, no clear correlation between phenol conversion and these active sites could be established.

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

Abstract: 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, pH pzc (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 pH pzc , 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 40 bar of total pressure. Oxalic acid is very stable under non-catalytic conditions but can be totally degraded in less than 30 min 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 45 min and complete mineralization in 120 min). 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.

Treatment of Tannery Wastewater by Various Oxidation and Combined Processes

Abstract: A general overview on the prospective of various oxidation and combined processes in the treatment of tannery industry effluent are reported. Various oxidation and combined processes such as UV/H2O2/Hypochlorites, Fenton and Electro-oxidation, photo-chemical, photo-catalytic, electro-catalytic oxidation, wet air oxidation, ozonation, biological followed by ozone/UV/ H2O2, coagulation or electro-coagulation and catalytic treatments have been considered. The tannery wastewater with sulphide as main sources of pollutant, electro-coagulation is the best removal efficiency process among the other oxidation processes, whereas for chromium, photo catalytic oxidation process using nano-TiO2 and wet air oxidation in the presence of manganese sulphate and activated carbon as a catalyst are more efficiency processes. The integrated combined processes described permit to meet disposal limits, health quality standards and the recovery of several chemicals utilized in the tanneries.

Sewage sludge based carbons for catalytic wet air oxidation of phenolic compounds in batch and trickle bed reactors

Abstract: The potential of using sludge based activated carbons (SBACs) for catalysing the wet air oxidation (WAO) of phenol, o-cresol, o-chlorophenol and p-nitrophenol was assessed in both a batch slurry reactor and a continuous trickle-bed reactor. In the batch reactor, the activity of two powdered carbons prepared from, respectively, dewatered raw (DRAW) sludge and dewatered, mesophilic anaerobically digested (DMAD) sludge was tested at 160 °C and 4.2 bar of oxygen partial pressure. Continuous, trickle-bed reactor experiments of up to 72 h were conducted at similar operating conditions to study the durability and catalytic wet air oxidation (CWAO) performance of three economically promising steam activated SBACs. Due to their low mechanical strength, the two DRAW derived SBACs tested were produced using two different attrition resistance enhancement techniques. A commercial activated carbon (Chemviron, AP4-X) was employed as the reference catalyst for all of the tests. In the batch runs, the SBACs and AP4-X achieved high levels of pollutant conversion in the case of phenol, o-cresol and o-chlorophenol. However, irrespective of the carbon tested, p-nitrophenol was resistant to oxidation. When employed in the trickle-bed reactor, the DRAW derived SBAC pelletised using a lignosulphonate binder was found to be the most stable carbon. With this carbon the order of compound reactivity was as observed in the batch experiments.

Pt-catalysts supported on activated carbons for catalytic wet air oxidation of aniline: Activity and stability

Abstract: Three activated carbons (ACs) were prepared from olive stones using KOH as activation agent at different temperatures. These materials present similar surface chemistry, but a larger porosity (mainly microporosity) is developed by increasing the activation temperature. The ACs were used as supports for Pt-catalysts and those with a larger porosity led to the highest Pt dispersion. The ACs and the supported Pt-catalysts were tested in the degradation of aniline by catalytic wet air oxidation. Aniline conversion and mineralization was found to depend on the porosity and Pt dispersion of the catalysts. The materials were characterized before and after reaction by different techniques (N 2 and CO 2 adsorption, elemental analysis, TPD and XPS). Analysis of the catalysts after reaction indicated some loss of microporosity, accompanied by an increase of the oxygenated surface groups and changes in the Pt oxidation state. In addition, no Pt leaching was detected. The performance of the catalysts was evaluated during three consecutive cycles without any reactivation treatment. The activity decreased after the first cycle, which is due to the saturation of the adsorption capacity; however, the catalytic performance was constant during the second and third cycles, showing the high stability of the tested catalysts.

Catalytic Wet Air Oxidation of p-Coumaric Acid over Carbon Nanotubes and Activated Carbon

Abstract: The catalytic wet air oxidation of p-coumaric acid, a representative substrate of biorecalcitrant phenolic compounds typically found in olive oil processing wastewater, was carried out under mild c...

Multiphase wet oxidation catalyst and preparation method thereof

Abstract: The invention provides a multiphase wet oxidation catalyst, in which noble metal elements, which are selected from one or more than one of the group consisting of platinum, ruthenium, rhodium, iridium and aurum, are taken as active components; and rare earth elements, which are selected from one or more than one of the group consisting of lanthanum, cerium, praseodymium and neodymium, are taken as auxiliary components; the active components and the auxiliary components are carried on a ceramic-activated carbon carrier, wherein the content of activated carbon accounts for 1 to 15% of the totalweight of the ceramic-activated carbon carrier, and the weight of elements in the active components and the auxiliary components accounts for 0.25 to 5% of the weight of the activated carbon respectively. The invention further provides a preparation method and uses thereof. The catalyst of the invention is particularly suitable for the treatment of high-concentration organic waste water which is hardly degraded.

TiO2-supported gold catalysts in the catalytic wet air oxidation of succinic acid: influence of the preparation, the storage and the pre-treatment conditions

Abstract: Titania-supported gold catalysts were prepared via the so-called deposition–precipitation method, using either urea or ammonia as the precipitation agent and evaluated in the catalytic wet air oxidation of succinic acid (a model pollutant often found in the degradation pathway of phenolic compounds) at 463 K under 0.75 MPa oxygen partial pressure. The preparation parameter with the strongest impact on the gold particle size, the gold deposition yield, the chlorine removal efficiency and the gold catalyst performances was shown to be the final pH of the preparation, which strongly influenced the gold precursor speciation in solution and the interaction of the gold precursor with the titania support. Gold catalysts were shown to be very active, with about 80% of the succinic acid which is converted being directly mineralized. Furthermore, gold catalysts appeared to be somehow very selective in acetic acid with only traces of acrylic acid being intermediately produced upon succinic acid degradation. The most active catalyst was prepared via deposition–precipitation using ammonia and was shown to be only 3 times less active than the best performing ruthenium catalysts. Unfortunately, the gold catalyst rapidly deactivated upon time on stream, probably due to the intrinsic gold instability under the applied reaction conditions.

Wet air oxidation and catalytic wet air oxidation for dyes degradation

Abstract: Background, aim, and scope Textile industry produces wastewater which contributes to water pollution since it utilizes a lot of chemicals. Preliminary studies show that the wastewater from textile industries contains grease, wax, surfactant, and dyes. The objective of this study was to determine the treatment efficiency of the nickel catalysts supported on hydrotalcites in three-dye model compounds and two types of wastewater.

Process optimisation using the combination of simulation and experimental design approach: Application to wet air oxidation

Abstract: This study develops a coupling of energetic and experimental design approaches on a given configuration of wet air oxidation process (WAO), applied for wastewater containing a hard chemical oxygen demand (phenol for instance). Taking into account thermodynamic principles and process simulation, the calculation of minimum heat required by the process, exergetic efficiency and work balance is presented. Five parameters are considered: pressure (20–30 MPa); temperature (200–300 °C); chemical oxygen demand (23–143 g l−1); air ratio (1.2–2) and temperature of exiting steam utilities (160–200 °C). Using the surface response method, it appears that initial chemical oxygen demand and temperature are the two parameters that mainly influence the result. With the modelling, good conditions for the functioning of the presented process are the following: pressure of 19.4 MPa, temperature of 283 °C, chemical oxygen demand of 54.9 g l−1, air ratio of 1.7 and vapour temperature of 183 °C.

Co-treatment and utilisation of liquid pharmaceutical wastes

Abstract: The pharmaceutical industry produces carbon-rich liquid wastes which have been generally qualified as hazardous. A significant proportion of these carbon-rich wastes are currently sent for incineration, although they could be utilised. It was found that the majority of the liquid wastes investigated in this study could be used in biological N-removal as carbon sources for denitrification in domestic wastewater treatment, or for anaerobic biogas production. The volatile content could be separated and the solvents re-utilised, the residual toxic organic compounds could be decomposed by wet oxidation and subsequently sent for biological treatment.

Understanding Hydrogen in Bayer Process Emissions. 1. Hydrogen Production during the Degradation of Hydroxycarboxylic Acids in Sodium Hydroxide Solutions

Abstract: The formation of potentially explosive gas mixtures during Bayer process digestion and the wet oxidation of Bayer process liquors underscores the need for an improved understanding of the degradation reactions of organic compounds that produce flammable gases. This study is the first of a series investigating the production of hydrogen from different classes of organic compounds in sodium hydroxide solutions. The alkaline degradation of a range of aliphatic and aromatic carboxylates and hydroxycarboxylates was investigated under anaerobic conditions in an autoclave. It was found that aliphatic C4 carboxylates possessing a single β-hydroxy substituent are particularly reactive under these conditions, generating hydrogen gas and a range of low molecular weight (LMW) carboxylates. The effect of temperature (175–275 °C) and NaOH concentration (0–6 M) on the degradation of 3-hydroxybutanoate and hydroxybutanedioate (malate) was investigated in detail for reaction times up to 120 min. Under conditions that prom...

Application of Ni supported over mixed Mg–Al oxides to crystal violet wet air oxidation: The role of the reaction conditions and the catalyst

Abstract: This study explores the ability of nickel catalysts supported over Mg–Al mixed oxides (Ni/HT) by incipient wetness impregnation technique for the catalytic wet air oxidation of aqueous crystal violet solutions. The effect of operational conditions such as temperature, pressure, pH, presence of the catalyst and initial dye concentration has been tested. It has been demonstrated that Ni/HT is an efficient catalyst for the CWAO of crystal violet (CV) at mild reaction conditions. The reaction which was carried out at its natural pH, showed to be the most efficient when Ni/HT was employed. The catalyst does not lose any of its activity in the recycling experiments performed, when CV degradation is considered.