Wet Air Oxidation of nitrogen-containing organic compounds and ammonia in aqueous media

Applied Catalysis B: Environmental 40 (2003) 163–184

Review

Wet Air Oxidation of nitrogen-containing organic compounds and ammonia in aqueous media L. Oliviero, J. Barbier, Jr., D. Duprez∗ Laboratoire de Catalyse en Chimie Organique, LACCO UMR 6503, CNRS and Université de Poitiers 40, Avenue du Recteur Pineau, 86022 Poitiers Cedex, France Received 28 April 2002; received in revised form 28 June 2002; accepted 28 June 2002

Abstract Treatment of toxic nitrogen-containing compounds is one of the major applications of the Wet Air Oxidation (WAO) processes. The aim of this paper is to review the literature dealing with the Catalytic Wet Air Oxidation (CWAO) of these nitrogenous compounds, mainly produced in chemical and pharmaceutical industries. Many studies deal with oxidation of aniline, often chosen as a model molecule of pollutant of dye industries. First, the results obtained with CWAO are compared with those obtained with other oxidation processes. Particular attention is paid to the selectivity towards organic by-products (specially, azo, nitroso and nitro compounds, phenolic compounds and carboxylic acids) as well as towards several inorganic forms of nitrogen (NH4 + , N2 , NO2 − , NO3 − ). In a second part, the review focuses on the mechanism of chemical reactions that can explain the formation of the observed products. Usually, similar catalysts can be used for CWAO of oxygen-containing (phenol, carboxylic acids) and nitrogen-containing organic compounds. Ammonia is one of the most refractory by-product formed during catalytic WAO of the nitrogen-containing organic pollutants and is itself a pollutant. For this reason, recent reports about its oxidation by the CWAO process are finally reviewed. Very high selectivities to dinitrogen can be obtained on certain noble metal catalysts. As a rule, catalysts active and selective for ammonia oxidation are different (nature of active phase, support, etc.) from the solids proven to be the best catalysts for CWAO of organic compounds. Multifunctional catalysts are, thus, required for the treatment of nitrogenous organic compounds. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Catalytic Wet Air Oxidation; Aniline; Nitrophenol; Ammonia; Ammonium ion; Azoic dyes

1. Introduction In the last 10 years, intensive research aiming at reducing the pollution in industrial wastewaters by oxidative catalytic treatments has been performed. In spite of the operating costs (high temperature and pressure), the Catalytic Wet Air Oxidation (CWAO) appears as one of the most promising wastewater treat∗ Corresponding author. Tel.: +33-549453998; fax: +33-549453499. E-mail address: [email protected] (D. Duprez).

ment. The process consists in oxidising the pollutants under oxygen pressure (5–200 bar) at elevated temperatures (125–320 ◦ C) and in the presence of a catalyst (oxides, noble metals, etc.) [1]. It may be typically applied to effluents with chemical oxygen demand (COD) in the 5–50 g l−1 [2] range of concentration and could find its main application for treating toxic or/and non-biodegradable pollutants [2]. A wide range of products has already been treated with success because this process is able to eliminate products with low solubility such as polymers or fatty acids. Depending on the reaction conditions, two different objectives can be

0926-3373/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 0 2 ) 0 0 1 5 8 - 3

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achieved: (i) a complete mineralisation of organic pollutants into CO2 , N2 and H2 O or (ii) only an increase of the effluent biodegradability by orientating the conversion of hydrophobic or/and toxic organic matter to the formation of biodegradable by-products such as carboxylic acids [3]. CWAO of nitrogen-containing compounds is subject of this review. These compounds present a very high toxicity in wastewater and, therefore, their precise identification is required all along the oxidation treatment. Aniline and azoic derivatives being an important class of nitrogenous organic compounds, different processes developed for the oxidation of these pollutants will be presented. A large part of the review will be focused on by-product identification with a particular attention paid to aniline chemistry. Ammonia being a product frequently observed in the treatment of nitrogen-containing compounds, oxidation processes of NH4 + aqueous solutions will be developed in a final part.

2. Treatment of nitrogenous organic compounds 2.1. Origin of pollution by nitrogenous compounds—standards of throwing out Before presenting the most relevant results on the treatment of waters contaminated by nitrogen-containing compounds, it seems necessary to remember the origin of the pollution by nitrogen. Some nitrogenous compounds are produced naturally like amine acids, proteins or humic substances. Also, human metabolism produces urea and ammonia. However, most of the products come from industrial activity and in particular from chemical, petrochemical and food industries [1]. In the case of aniline and its colourful derivatives, manufacturer and user of dyes, such as textile industrial processes, are more particularly concerned. It is indeed estimated that 15% of the world production of dyes are lost during their fabrication or their use; this corresponds to a world-wide throwing out of 128 tonnes per day [4]. Sewage treatment units and agriculture are also responsible for the presence in the environment of a large range of nitrogenous compounds [5]. To limit all these kind of pollution, standards were fixed taking into account the toxicity of the com-

Table 1 Toxicity of molecules Products

Toxicity

Azobenzene Phenazine Nitrobenzene

50 mg kg−1 LDLO a [6] 180 mg kg−1 LD50 b [6] 200 mg kg−1 LDLO (oral route) [6]

Aniline

250 mg kg−1 (rat: oral route) LD50 [7] 61 mg l−1 (fish toxicity) CL50 [7]

Ammonia Phenol 4-Nitrophenol Formic acid Acetic acid Oxalic acid Malonic acid Maleic acid Fumaric acid Succinic acid Acridinamine Phenylformamide Phenylacetamide

350 mg kg−1 (rat: oral route) LD50 [8] 23.5 mg l−1 CE (I) 50–48 hc [9] 20 mg l−1 CE (I) 50–48 h [9] 151.2 mg l−1 CE (I) 50–48 h [9] 190 mg l−1 CE (I) 50–48 h [9] 136.9 mg l−1 CE (I) 50–48 h [9] 275 mg l−1 CE (I) 50–48 h [9] 316.2 mg l−1 CE (I) 50–48 h [9] 212 mg l−1 CE (I) 50–48 h [9] 374.2 mg l−1 CE (I) 50–48 h [9] 45 mg kg−1 (mouse: oral route) LD50 [10] 800 ug kg−1 LDLO (frog: oral route) [11] 800 mg kg−1 (rat: oral route) LD50 d

a American standard which defines the smallest amount that can kill by one of the administering route. b Corresponds to the limit amount by oral or skin route that can lead to death 50% of the tested population. c Corresponds to the initial concentration that kills in 48 h 50% of daphnia magna. d Environmental Health and Safety Agency, USA.

pounds. Thus, in the US, toxic products are listed by the Environmental Protection Agency. The same kind of list is nowadays prepared by EC. Some data on the toxicity of nitrogenous compounds and their oxidation by-products are given in Table 1. When no other indication is reported, the toxicity is given for daphnia magna. Environmental regulation imposed by governmental administrations being stricter and stricter, it is mandatory to develop advanced water treatments and the number of study on this topic is getting greater and greater. 2.2. Oxidation processes for aniline and azoic compounds In Table 2, the different Laboratory studies on aniline oxidation in water are reported. The efficiency is expressed in terms of TOC or COD abatement, or conversion. Attempts at obtaining good TOC abatements were made by varying the nature of the oxidant (O2 ,

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Table 2 Oxidation processes of aniline in aqueous media Process

Efficiency

Ozonation


(COD) = 64% in 40 min

CWAO

Conversion 100% (20 min)

Electrochemical


(TOC) = 97% in 11 h

UV-TiO2

Electrochemical UV-TiO2 UV-TiO2 /H2 O2 UV-TiO2 /Fe2+ /H2 O2 Electro-Fenton UV/Electro-Fenton UV-TiO2 + ozonation

[COD] = room temperature, [consumed O3 ] = 302 mg l−1 [Aniline] = 10 mmol l−1 , [H2 O2 ] = 0.15 g l−1 in the stoechiometry, [Fe2+ ] = 15 ppm, T = 200 ◦ C [Aniline] ∼ 1 mmol l−1 , pH = 10.1–12.7, anode: Pb/PbO2 , cathode: carbon polytetrafluoroethylene + O2 , I = 300 mA, T = 25 ◦ C

Reference [12] [13] [14]

[15]


(TOC) = 80% in 6 h with 1 mmol l−1 of Fe2+ 23a 68a 69a 71a 68a 92a

[Aniline] = 100 mg l−1 [H2 O2 or Fe2+ ] = 1 mmol l−1 pH = 3 TiO2 = 2 g l−1 I = 100 mA T = 25 ◦ C

[16]

[Aniline] = 9.97 × 10−1 mmol l−1 pH = 3 or 6 TiO2 = 2 g l−1 I = 9.2 × 10−4 Einstein l−1 min−1 T = 25 ◦ C

[17]


(TOC) = 27% in 2 h without ozonation
(TOC) = 96% in 2 h with ozonation (5 × 10−1 mmol min−1 )

[Aniline] = 83 mmol l−1 , T = 130 ◦ C, P(O2 ): 7 bar, PCO or H2 : 7 bar, P(N2 ) = 70 bar, [5% Pd/C] = 10 g l−1 [Aniline] = 1 mmol l−1 , work electrode: RuO2 , mediator:[Ce4+ ] = 1 mmol l−1 , E = 1.5 V (Ag/AgCl), T = 25 ◦ C [Aniline] = 1 mmol l−1 , O2 = 110 ml min−1 , pH = 6, I = 9.2 × 10−4 Einstein l−1 min−1 , T = 25 ◦ C

[18]

Conversion 62% with H2 ; conversion 87% with CO, in 16 h

Electrochemical


(TOC) = 18% in 81 h

Photo-Fenton


(TOC) = 80% in 90 min

O3 O3 /Fe2+ O3 /UVA O3 /UVA/Fe2+

78b 80b 91b 85b

b

225 mg l−1 ,

[Aniline] = 9.97 × 10−1 mmol l−1 pH = 3–6 TiO2 = 2 g l−1 I = 9.2 × 10−4 Einstein l−1 min−1 T = 25 ◦ C


(TOC) = 85% in 8 h without Fe2+

CWAO with O2 /H2 or O2 /CO

a

Conditions

[Aniline] = 1.07 mmol l−1 [Fe2+ ] = 1 mmol l−1 O3 = 0.5 g h−1 I = 8.3 × 10−7 Einstein l−1 min pH = 3, T = 25 ◦ C

[19]

[20] [21]


(TOC) in 6 h (%).
(TOC) in 1 h (%).

H2 O2 , O3 ) and the type of process (chemical, electrochemical, photochemical oxidations, catalytic or not). From these examples, aniline treatments, even at concentrations as low as 1 mmol l−1 , needs the use of strong oxidising agents (ozone, hydrogen peroxide) and long times of reaction (usually more than 2 h). This underlines the fact that aniline is a refractory compound towards oxidation. Then, its elimination can be achieved by coupling various oxidising techniques.

Some oxidative methods typically employed in the treatment of various dyes are listed in Table 3. As can be seen, usually severe conditions are needed in treating dyes. As far as Wet Air Oxidation (WAO) is concerned, it seems that the use of catalysts (either homogeneous or heterogeneous) is capable of enhancing the degradation of dyes into carbon dioxide. The choice of homogenous or heterogeneous catalysts cannot be explained in term of efficiency since

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Table 3 Oxidation processes for waste water of dye industry Process

Molecule (0.5 g l−1 )

Efficacy (%)

Conditions

Reference


(TOC) = 85 (120 min)

P(O2 = 10 bar, CuO–ZnO–Al2 O3 (42, 47, 11%), [catalyst] = 5 g l−1 , T = 190 ◦ C

[22]

[23]

)a

CWAO

Orange II

WPO

Basilien Brillant Blue (50 g l−1 )


(COD) = 81b
(TOC) = 77b

200 mg l−1 Cu2+ H2 O2 in the stoechiometry

Procion Red, Cibacron Yellow, Cibacron Brown, Procrion Black


(COD) = 90c

[Dye] = 50 g l−1


(TOC) = 80c

T = 150 ◦ C

WAO

Basilien Brillant Blue (50 g l−1 )


(COD) = 37d
(TOC) = 30d

P(O2 ) = 33 bar T = 200 ◦ C

CWAO

Basilien Brillant Blue (50 g l−1 )


(COD) = 58e
(TOC) = 58e

P(O2 ) = 33 bar 200 mg l−1 Cu2+


(COD) = 35f
(TOC) = 35f

T = 200 ◦ C T = 150 ◦ C P(O2 ) = 29.5 bar, H2 O2 = 10% of the stoechiometry 0.4 mmol l−1 Fe3+ , 0.44 mol l−1 H2 O2 ,  = 270 nm P(O2 ) = 10 bar, pH = 2, [FeSO4 ] = 0.11 g l−1 , [gallic acid] = 90 mg l−1 , T = 160 ◦ C

Photo-Fenton

Malachite Green (0.2 mmol l−1 )


(TOC) = 100 (2 h)

CWAO

Orange II (1 g l−1 )


(TOC) = 70% in 90 min

[23]

[24] [25]

a

Oxygen pressure. In 30 min. c In 10 min. d In 120 min. e In 90 min. f In 90 min. b

both kinds of catalysts lead to significant mineralisation of the initial dye. However, at the end of the homogeneous process a separation step is required in order to recover the catalyst whereas a simple filtration is sufficient in the heterogeneous process. This difference is relevant from a practical point of view (need for a separated unit in the plant) as well as from an economical point of view. We can point out that though WPO and Photo-Fenton treatments show a good efficiency they suffer from the usual drawbacks homogeneous processes. 2.3. Treatment of other nitrogen-containing compounds In Tables 4 and 5, the main catalytic or non-catalytic processes used for the treatment of organic nitroge-

nous compounds other than aniline and its azoic derivatives are listed. Optimal conditions of each process as well as the nature of the most active catalyst eventually used are given. To obtain significant conversions of nitrogenous organic products, most of the non-catalytic techniques need a large excess of oxidant. As WAO is concerned, relatively high temperatures (260–300 ◦ C) should be applied. In the case of CWAO processes with iron or copper salts, Ru/C, Pd/C or Ru/C–Al2 O3 , lowest reaction temperatures (160–275 ◦ C) are required. Depending on the nature of compounds, differences in oxidability can be underlined. For instance, Chakchouk et al. [13] showed that acrylonitrile was more difficult to oxidise than nitrobenzene or pyridine. As a rule, functionalised molecules are generally more easily oxidised [32].

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Table 4 Non-catalytic treatments of nitrogenous organic compounds (other than aniline and its azoic derivatives) Process

Molecules

Optimal conditions (2 g l−1 )

300 ◦ C,

Activity

Reference

WAO O3

Nitrotoluenesulfonic acid Nitrobenzene (NB; 0.08 mmol l−1 ), 2,6-dinitrotoluene (DNT; 0.045 mmol l−1 )

T= P(O2 ) = 9.3 bar T = 10–30 ◦ C, pH > 7, [O3 ] = 0.05 mmol l−1


(TOC) = 95% (240 min) Conversion 99% (10 min)

[26] [27]

O3 /H2 O2

NB (0.132 mmol l−1 ), DNT (0.045 mmol l−1 )

Conversion 99% (60 min (DNT); 30 min (NB))

[28]

O3 /UV

DNT (0.067 mmol l−1 )

[H2 O2 ] = 1.1 mmol l−1 , T = 20 ◦ C, pH =7, O3 = 0.08 mmol min−1 T = 20 ◦ C, pH = 7, I = 3.3 × 10−8 Einstein cm−2 s−1 , O3 = 0.7 mmol min−1

WAO SWO

Quinoline (250 mg l−1 ) Nitrophenol (82 ␮mol l−1 )


(TOC) = 95% (30 min)
(TOC) = 46% (2.6 s)

[29] [30]

SWO

Fenuron (C6 H5 –NH–CO–NH(CH3 )2 ; 3 g l−1 )

T = 260 ◦ C, P(O2 ) = 20 bar T = 460 ◦ C, P(O2 ) = 253 bar, [O2 ] = 9.32 mmol l−1 T = 540 ◦ C, P(O2 ) = 250 bar


(TOC) = 100% (39 s)

[31]

2.4. Treatment of industrial waters contaminated by nitrogenous organic compounds To test inhibition phenomena linked to the simultaneous presence of various compounds, laboratory tests have also been realised on industrial waste water issued from chemical plants. The most relevant results obtained on waste water polluted by nitrogenous organic compounds are given in Table 6. The treatment of complex industrial waste waters requires an association of various processes. In order to eliminate solid particles, potential poisons of the catalyst, physico-chemical techniques (filtration, coagulation) are particularly useful. 2.5. Identification of oxidation by-products formed from WAO of nitrogenous compounds Most of the works on the oxidation of nitrogenous organic compounds mainly focuses on the initial product conversion and on COD and TOC removals. However, it is essential to identify by-products or intermediates formed during oxidation in order to evaluate their toxicity and to point out possible refractory compounds. Obviously, this is a hard task since it concerns minute amounts of compounds whose structure can be very complex. This is the reason why relatively scarce results are available about organics

Conversion 99% (5 min)

that are formed during the oxidation of nitrogenous organic compounds (Table 7). Moreover, oxidation mechanisms are seldom described and are specific to the oxidant used [14–16,42,43]. In any case, by-product identification is carried out by means of high performance liquid chromatography (HPLC) or gas chromatography–mass spectrometry (GC–MS) coupling. For the latter technique, extractions by organic solvents (dichloromethane for instance) are needed, and sometimes they have to be followed by derivation. During aniline oxidation, three main classes of organic by-products have been successfully identified. The first one consists of nitro and nitroso compounds, nitrobenzene being in particular detected in most aniline studies. The second one is composed of condensation products such as azobenzene or azoxybenzene. The rupture of the N–C bond leads to the formation of hydroquinone, benzoquinone and carboxylic acids such as maleic acid. These nitrogen-free compounds form the third class of by-products. The complexity of the reaction mechanism well appears in the chromatogram of Fig. 1 obtained by GC–MS which shows some heavy by-products identified during CWAO of aniline [42]. The analysis of the inorganic forms of nitrogen requires techniques (GC, ionic HPLC) that are less complex than those needed for the analysis of organic

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Table 5 Catalytic treatments of nitrogenous organic compounds (other than aniline and its azoic derivatives) Process CWAO CWAO CWAO

Molecules

275 ◦ C,

Activity

Reference

Acenaphtene, acroleine, acrylonitrile, nitrophenol, etc. Aliphatic amines

T= P(O2 ) = 10 bar, [CuSO4 ·5H2 O] = 0.5 g l−1 T = 200 ◦ C, [O2 ]: 550 mmol l−1 , [Co2 O3 ] = 5 g l−1

Conversion 100% (1 h)

[9]

Conversion 45–100% (1–4 h)

[32]

Nitrobenzene (1.35 g l−1 ) Pyridine 2 g l−1

T = 200 ◦ C [H2 O2 ] = 0.15–0.2 g l−1 in the stoechiometry [Fe2+ ] = 10–15 ppm

100%a 60.7%a

[13]

49.6%a

T = 30 ◦ C [Fe3+ ] = 1.4.10−4 M

5%b 27%b

[5]

T = 35 ◦ C, [H2 O2 ] = 1.2 × 10−2 M, [ZMS-5] = 1.375 g l−1 , Φ = 80 mW cm−2 T = 27 ◦ C, [WO3 /TiO2 ] = 2 g l−1 [Pollutant] = 20 mmol l−1 , T = 275 ◦ C, P(O2 ) = excess, [TiO2 ] = 2 g l−1 , Φ = 1.5–1.7 mW cm−2 T = 200–240 ◦ C, P(O2 ) = 10 bar, [Ru/C] = 1 g l−1 T = 130 ◦ C, P(O2 ) = 7 bar, PCO or H2 = 7 bar, P(N2 ) = 70 bar, [5%Pd/C] = 10 g l−1

Conversion 100% (8 h),
(TOC) = 100% (>12 h)

[33]

Conversion 100% (2 h 30 min) Conversion 65–80% (8 h)

[34] [35]

Conversion 91–95% (60 min)

[36]

Conversion 80–99% (16 h)

[18]

P(O2 ) = 10 bar, [catalyst] = 5 g l−1 Ru/C 12% Ru/C–Al2 O3

1 h/220 ◦ Cc

[37]

Acrylonitrile 2 g l−1 UV Photo-Fenton

Optimal conditions

2-Nitrophenol (3.6 × 10−3 mol l−1 )

Φ = 80 mW cm−2 UV-ZMS-5

4-Nitrophenol (3.6 × 10−3 mol l−1 )

UV UV

4-Nitrophenol (50 mg l−1 ) Amino-acids, amides, succinimide, imidazole, hydroxylamine, urea etc. acetonitrile, carbamide (CO(NH2 )2 ), dimethyl formamide Nitromethane, nitrophenol, nitrobenzene, nitrosobenzene, azobenzene, dinitrobenzene, etc. (83 mmol l−1 )

CWAO CWAO

CWAO

Acetronitrile Carbamide N,N-Dimethylformamide

1 h/220 ◦ Cc , 2 h/200 ◦ Cc 1 h/220 ◦ Cc

a

Conversion in 1 h. Conversion in 2 h. c Conversion 100%. b

compounds. Moreover, it is essential to know the selectivity into ammonia, dinitrogen, nitrates and nitrites to evaluate the efficiency of a process. The desired product is of course molecular nitrogen but in most reports on WAO or CWAO of nitro compounds, N2 is not directly analysed, its final concentration being deduced from the nitrogen balance. In Table 8, the inorganic forms and the corresponding selectivity when quoted are given from the literature data. Molecular nitrogen is rarely the main product. Oxidation processes lead to nitrates or ammonia. The structure of the initial molecule, the process itself and

the catalyst have a determining role on the selectivity towards inorganic forms. As oxidation by ozone is concerned, two mechanisms, by direct or indirect routes are possible depending on the nature of the substrate [46]. In the case of aniline, aromatic compound with an unshared electron pair, the direct electrophilic attack by ozone is favoured. Then the by-products obtained are aldhehydes, ketones, acids and nitro compounds. The second mechanism, the indirect attack by OH• radicals, is not selective and lead to the formation of azo-derivatives and hydroxylated compounds. Both mechanisms are favoured at basic pH where the electrophilic character

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Table 6 Treatments of industrial waters polluted by nitrogenous organic compounds Origin of effluent

Efficacy (%)

Process/conditions

Reference

Production of rubber additives (aniline, derivatives) TNT production, COD = 665 mg l−1 , TOC = 413 mg l−1 Synthesis of dyes


(COD) = 21 (60 min)

[38]


(COD) = 97 (60 min),
(TOC) = 84
(COD) = 78,
(TOC) = 33

Ozonation: pH = 11.3, O3 = 0.845 g COD−1 g WAO: T = 260 ◦ C, P(O2 ) = 6.9 bar

Acetonitrile production, COD = 120 g l−1


(COD) = 65 (8 h; by WAO),
(COD) = 90 (at the end of process)

Production of Blue CI 79, COD = 3500 mg l−1


(COD) = 98 (2 h)

Ozonation (65 min): pH = 8.8, O3 = 659 mg l−1 ; coagulation (15 min): pH = 9.5, Ca(OH)2 = 297 mg l−1 WAO: T = 225 ◦ C, P(O2 ) = 6.9 bar; activated sludges (15 days); filtration on C Membranes + WAO on COD = 900 mg l−1 : pH = 3.5, T = 225 ◦ C, P(O2 ) = 6.9 bar, 0.37 mmol l−1 CuSO4

[39] [4]

[40]

[41]

Table 7 Identified by-products in the oxidation products of various nitrogenous pollutants Process

Initial substrate

Organic by-products

Reference

Ozonation and coagulation

Aniline + derivatives (dye synthesis)

[4]

Ozonation

Water from fabrication of rubber additives (aniline, derivatives) Aniline Nitroaniline Aniline

N-Phenyl-1,4-diimine-2,5-cyclohexadiene; phenazine; nitrophenol; nitroaniline; nitrosobenzene; azobenzene; azoxybenzene; phenoxazine; benzotriazol, etc. Nitrobenzene; 4-hydroxybenzodioxin-2phenyl; N-propylbenzenamine; N-methylazobenzenamine; azobenzene; benzothiododiazol Nitrobenzene; maleic acid Nitrobenzene; hydroquinone; benzoquinone; resorcinol; oxalic acid Hydroquinone; phenol; paraquinone; nitrobenzene

O3

Nitrobenzene 2,6-Dinitrotoluene

Benzaldehyde; nitrophenol; 1,2-dibenzoic acid 2,6-Dinitrobenzaldehyde; 2-nitrobenzaldehyde; 1,2-dibenzoic acid

[27]

O3 /UV, O3 /H2 O2

Nitrobenzene 2,6-Dinitrotoluene

Benzaldehyde; nitrophenols; 3-nitro-1,2-dibenzoic acid 1,3-Dinitrobenzene; nitrophenols; 2,6-dinitrobenzaldehyde; 2-nitrobenzaldehyde; 3-methyl-2-nitrobenzoic acid; 3-nitro-1,2-dibenzoic acid

[28]

Electrochemical, O3 , UVA CWAO

Aniline

[16,21]

Orange II

UV-TiO2 /O3

Aniline

WAO

Quinoline

CWAO

Aniline

Benzoquinone; hydroquinone; nitrobenzene; phenol; 1,2,4-benzenetriol; maleic acids; fumaric acids Benzenesulfonic acid; naphtol; 1,2-dibenzoic acid; 4-hydroxybenzenesulfonic acid; 2-hydroxymethylbenzo¨ıc acid; 1,3-isobenzofurandione; carboxylic acids Hydroquinone; benzoquinone; nitrobenzene, indoaniline; maleic acid; oxo-propanedioic acid 2-Formyl pyridine; 3-formyl pyridine; 2-acetyl pyridine; 3-acetyl pyridine; furo[3,4-b]pyridine-5(2hydroxy)one; 7-methyl-furo[3,4-b]-pyridine-5(2hydroxy)one; nicotinic acid; 2-hydroxy-nicotinic acid; 2-hydroxyquinoline; carboxylic acids Phenazine; acridinamine; formamidoacridine; azobenzene; azoxybenzene; oxanilide; phenylformamide; phenylacetamide; nitrobenzene; nitrosobenzene; nitrophenol; nitrosophenol; phenol; hydroquinone; catechol; benzoquinone; carboxylic acids

Electrochemical UV UV-TiO2

[38]

[14] [44] [15]

[22]

[17] [29]

[42,43]

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Fig. 1. GC–MS analysis of the organic intermediates of aniline CWAO after 3 h of reaction. Reaction conditions: T = 160 ◦ C; P (O2 ) = 20 bar; initial aniline concentration = 20 mmol l−1 ; catalyst: 5% Ru/CeO2 (160 m2 g−1 , 4 g l−1 ).

of the substrate is exalted and radical reactions are enhanced. Sauleda and Brillas have proposed an oxidation scheme for aniline oxidation by ozone different either if there is or not an UVA or Fe2+ activation [21]. This scheme is given in Fig. 2. A route through benzoquinoneimine can be envisaged. This molecule is unstable and gives very easily benzoquinone and ammonia. The oxidant used in WAO is oxygen. Delanoë [47] has tested several catalysts for this reaction: a 1% Pt/CeO2 catalyst active for phenolic compound oxidation, a 5% Ru/CeO2 catalyst active for the oxidation of acetic acid and a 5% Ru/Mn/Ce catalyst whose Mn/Ce composite support is active for the oxidation of ammonia according to Imamura and Dol [48]. The Ru/CeO2 catalyst appeared as the most active: the rate of aniline transformation, at 170 ◦ C, is then initially of 7.5 mmol h−1 g−1 ; total conversion is reached

in 150 min and COD abatement is of 79% after 3 h. However, a strong coloration of the final solution was noticed. This colour is more intense and accompanied of a precipitate when other catalysts are used. Higher temperatures should be used to obtain clearer solutions, total oxidation of refractory by-products being then possible. COD abatement is quasi total at 230 ◦ C. To propose a global mechanism of aniline oxidation on Ru/CeO2 , the end products have been analysed after 3 h of reaction [42]. The distribution of C and N reaction on the Ru/CeO2 catalyst versus temperature is given Figs. 3 and 4, respectively. These distributions include the carbon and the nitrogen analysed in the coke deposit on the catalyst. The amount of carbon deposited on the catalyst passes through a maximum at 160 ◦ C. A temperature of 200 ◦ C is needed to reach a total conversion of or-

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Table 8 Inorganic forms of nitrogen in the oxidation products of various nitrogenous pollutants Process

Initial substrate

Mineral by-products +

Reference

WAO WAO WAO Electrochemical

Lineary amines Nitrotoluenesulfonic acid Water from TNT fabrication Aniline

NH4 NH4 + , N2 57% NO3 − , 31% NH4 + , N2 conversion >85% 75–80% NH4 +

[32] [26] [39] [16]

UV-TiO2

Nitrobenzene Nitrosobenzene Phenylhydroxylamine Aniline

4/1a 1/2a 1/4a 1/6a

[45]

UV-TiO2

Aniline

Ammonium (acidic pH), NO3 − and NO2 − (basic pH)

[15]

CWAO

Acetonitrile-N,N-dimethyl formamide Carbamide (CO(NH2 )2 )

Ru/graphite = 55–60%, conversion 90%b Ru/graphite = 70%, RuCeOx /graphite = 50%b

[36]

CWAO

Aniline Nitrobenzene Nitrosobenzene

32/26/62c 28/16/99c 8/32/99c

[18]

SWO

Fenuron (C6 H5 –NH–CO–NH(CH3 )2 )

100% N2

[31]

O3 O3 /Fe2+ O3 /UVA O3 /UVA/Fe2+

Aniline

40/13d 69/7d 45/35d 64/33d

[32]

Ratio: NH4 + /NO3 − . Selectivity into N2 at 200 ◦ C. c NH + (%)/NO − (%)/conversion (%) at 130 ◦ C in 16 h. 4 3 d NH + (%)/NO − (%) compared to initial N. 4 3 a

b

ganic carbon into carbon dioxide after 3 h of reaction. As far as nitrogen is concerned, there is an optimum temperature of 200 ◦ C for the formation of molecular nitrogen. Above this temperature, a significant amount of nitrates and nitrites is produced. At lower temperatures, the inorganic form of nitrogen is ammonia. From this figure, it seems that ammonia is one of the intermediate of formation of molecular nitrogen and of nitrites and nitrates. The oxidation scheme of ammonia is discussed in [49]. As for the carbon, the amount of N deposited on the catalyst is the highest at 160 ◦ C. The ratio C/N of the compounds on the surface is close to 6. These compounds can be either polymers of aniline or oxidised by-product such as aminophenol or nitrobenzene [42]. At 160 ◦ C, experiments have been carried out to follow the deposition on the catalyst with the reaction time [49]. The amount of carbon and nitrogen on the surface increases along the 3 h of reaction. It is speculated that the layer of com-

pounds deposited on the catalyst surface hinders the diffusion of organics and oxygen from the solution to the active sites. At 200 ◦ C, the deposit has a maximum amount after 20 min of reaction; then the catalyst is able to oxidise the compounds on its own surface [42]. As already reported in Table 7, major by-products of CWAO identified after 3 h of reaction are in any case: aminophenol, nitrophenol, nitrobenzene, nitrosobenzene, phenol, hydroquinone, carboxylic acids and nitrates. Compared to the blank experiment, the effect of the catalyst on the product formation appears for instance in the better selectivity in nitrobenzene in detriment of nitrosobenzene. The catalyst improves the oxidation state of the intermediates. These by-products present different reactivity towards oxidation. At a temperature of 200 ◦ C, the decreasing order of reactivity is aminophenol > nitrophenol > nitrobenzene [42]. Phenol is easily oxidised at 200 ◦ C

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be linked to the metal/support interaction and the ability of ceria to feed the metal in active O species [52]. From these results, a reaction scheme for aniline oxidation on the Ru/CeO2 catalyst has been proposed [43]. This scheme is given in Fig. 5. Route 1 explained the formation of colourful compounds of condensation observed in particular at 160 ◦ C. Routes 2 and 3 should be favoured at 200 and at 230 ◦ C, respectively. At this high temperature, a significant production of nitrates occurs compared to the temperature of 200 ◦ C. 3. Chemistry of aniline oxidation To explain the formation of the intermediary organic compounds, it is useful to recall information available about aniline oxidation. The mechanism of non-catalysed oxidation of aniline in presence of oxygen donor species is known since 1956 [53]. The corresponding scheme is given in Fig. 6. Fig. 2. Aniline oxidation by ozone. Adapted from [21].

3.1. Reactions between aniline and its partial oxidation products and above, in accordance with further work on phenol oxidation [50]. Acetic acid, especially refractory to oxidation [51], is detected at this temperature and above. The oxidation of carboxylic acids is particularly favoured on the Ru/CeO2 catalyst and this can

The partial oxidation products of aniline depollution (phenylhydroxylamine, nitrosobenzene, etc.) can react with aniline itself or one with each other and the possible reactions are given in Table 9.

Fig. 3. Effect of the temperature on aniline CWAO—carbon distribution in the products after 3 h of reaction. Reaction conditions: P (O2 ) = 20 bar; initial aniline concentration = 20 mmol l−1 ; catalyst: 5% Ru/CeO2 (160 m2 g−1 , 4 g l−1 ).

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173

Fig. 4. Effect of the temperature on aniline CWAO—nitrogen distribution in the products after 3 h of reaction. Reaction conditions: P (O2 ) = 20 bar; initial aniline concentration = 20 mmol l−1 ; catalyst: 5% Ru/CeO2 (160 m2 g−1 , 4 g l−1 ).

3.2. Major reactions of aniline conversion in connection with its oxidation In addition to these reactions, aniline is also used as raw product for several synthesis processes. In order to explain the formation of some of the by-products during the oxidation of aniline by WAO, established mechanism for specific reactions are specified. These oxidations can be carried out either in an aqueous or non-aqueous media and in the presence of various homogeneous or heterogeneous catalysts.

3.2.1. Formation of polymers from aniline Aniline polymers are used for their electrical conduction and their anti-corrosive properties. They are prepared by chemical [56] or electrochemical [57] steps. The intermediate of polyaniline formation is a cation-radical: C6 H5 –NH2 •+ . Dimers are the primary forms obtained during the preparation. Head to tail dimerisation gives p-aminodiphenylamine, tail to tail benzidine and head to head hydrazobenzene [58]. The different structures of head to tail polyanilines are given as follows:

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Fig. 5. Aniline oxidation scheme. Adapted from [53].

The blue pernigraniline is stable at pH lower than 0.5–1 and green emeraldine at pH lower than 6–7. Values of the pH above these are in favour of basic forms. Generally speaking, aniline dimerisation leads

to the decrease of the pH. Polymerisation of substituted anilines shows that nitroaniline only forms dimers since the nitro function decreases the ring reactivity.

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175

Fig. 6. Aniline oxidation by oxygen. Adapted from [43].

3.2.2. Synthesis of dimers or trimers from aromatic amines Various reactions involving aromatic amines are described in the literature. They lead to the formation of specific compounds whose structures are given in Table 10. These structures show the reactivity of the nitrogen atom and the stability of radicals based on this element when they involve an aromatic ring. The intermediate of o-aminophenol oxidation is o-benzoquinonemonoimine which structure is given as follows [59,60]:

In their study, Simándi et al. have also shown that during the formation of the molecules described earlier production of a tar occurs [61]. The structure of this tar has been determined to be as followed:

3.2.3. Partial oxidation of aniline Aniline oxidation by hydrogen peroxide has been studied by Gontier and Tuel [62]. From a general

point of view, oxidation process by H2 O2 of aromatic amines involve transition metal salts and more specifically Ti4+ , V4+ , Mo6+ , and Fe3+ . This oxidation is realised at low temperature (<100 ◦ C) and with a low H2 O2 /amine ratio (<0.2) to obtain a high selectivity in nitrosobenzene and azoxybenzene. The catalyst used by Gontier and Tuel [62] is a titanium silicate (TS-1) known for its oxidising capability. The reaction is carried out at 70 ◦ C with 20 ml of solution containing 0.5 g of catalyst and 4.6 ml of aniline. The major products obtained are then nitrosobenzene and azoxybenzene. The presence of azobenzene has also been pointed out, but there is no nitrobenzene or phenylhydroxylamine. The changes with time of the concentration of nitrosobenzene (which totally disappears in 1 h) and of azoxybenzene (formed at the end of the reaction with a selectivity of 95%) allow to establish that azoxybenzene is formed by reaction 1, left route (Table 9). The reaction between aniline and nitrosobenzene involves neither H2 O2 nor the presence of a catalyst and leads to azobenzene, oxidised itself in azoxybenzene by H2 O2 . This reaction has been excluded because of the non-reactivity of azobenzene in catalysed reactions. The effect of temperature has also been studied: a high temperature favours the formation of azoxybenzene but does not modify the nature of products. More than kinetic consideration, extra granular diffusion can be considered as the responsible phenomena for this effect.

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177

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Table 10 Condensation products (dimerisation and trimerisation) of various amines Initial amines

Catalysts

o-Aminophenol

K3 Mn(C2 O4 )3 a

o-Phenylenediamine

K3 Co(C2 O4 )3 a

p-Phenylenediamine

K3 Cu(C2 O4 )3 a

o-Phenylenediamine

Co(II)/O2 in MeOH or THF

Formed molecule

Reference

[59]

2,3-Diaminophenazine

[60]

o-Aminophenol

a

T = 25/45 ◦ C.

Aniline oxidation in acidic medium by an excess of phenyliodosoacetate (PIA) has been investigated by Pati et al. [63]. The catalyst is the RuCl3 and the radical process is described as followed:

The products formed are hydrazobenzene and azobenzene via successive reactions. In their study, Pati et al. [63] have established a reactivity classification

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179

Table 11 Products formed in aniline partial oxidationa Oxidants

Major product

Reference

Tl(OAc)3 + Ru(III) in water + acetic acid + ClO4 + H2 SO4 NaIO3 + perchloric acid + Ru(III) in water + acetic acid Pyridiniumchlorochromate + dichloroacetic acid in PhNO2 and PhCl Ru(III) + phenyliodosoacetate Cu(III) or Au(III) in ethanol + water H2 O2 (2%), peroxotungstophosphates (10%)

Azobenzene Azobenzene Phenylhydroxylamine, aminophenol, azobenzene, benzoquinone Azobenzene Azobenzene Azoxybenzene, nitrobenzene, nitrosobenzene

[64] [65] [66]

MnO2 Anhydres peracides Aqueous peracides V, Mo complexes + t-butylhydroperoxyde WO3 , H2 O2 RuCl3 or N+ X4

Azobenzene Nitrobenzene Azo-azoxybenzene Nitrobenzene Nitrosobenzene + azoxybenzene Azoxybenzene + nitrobenzene

[69]

H2 O2 or TBHP/zeolites TS-1/Ti-ß

Nitrosobenzene + azoxybenzene, traces of azobenzene

[62,69,70]

a

[63] [67] [68]

These products are potential intermediates in CWAO of aniline.

of several substituted anilines:

This order can be linked to the electro-donor or electro-withdrawing character of substituents: an electro-donor substituent is favourable to the first step of complex formation and stabilises the involved radicals. An outline of the overall work on aniline partial oxidation is reported in Table 11. Whatever the oxidant used, the major product the most often produced from partial oxidation of aniline is azobenzene. The conditions of oxidation can also lead to the formation of azoxybenzene and nitrosobenzene. 4. Ammonia oxidation 4.1. Origin of pollution by ammonia and possible treatments In term of reactivity in CWAO, ammonia is the equivalent for nitrogenous compounds to acetic acid

for organic compounds: NH4 + appears as a particularly refractory nitro-compound by-product [18]. Indeed, this molecule is often the end product of more complicated compounds oxidation (Table 8). Moreover, ammonia is by itself a pollutant, toxic for aquatic life and present in various industrial waste waters. This is the case for waters issued from fine chemistry, fertiliser making, petrochemical and metallurgy industries. However, pollution by ammonia comes principally from agriculture. In Great Britain, it is estimated that agriculture contributes for 85% of the ammonia emissions [71]. One of the negative effect of ammonia is the eutrophication of rivers even though nitrates are mainly responsible for this problem [72]. Biological processes can be used in order to treat industrial water containing low amounts of ammonia (<80 mg l−1 ) like refinery effluents [73]. In 7–14 days, 90% of initial ammonia can be transformed in nitrites by Nitrosomona. Then these nitrites will be oxidised

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in nitrates by Nitrobacter after 1 week [73]. Nitrates will thereafter be themselves biologically treated to give molecular nitrogen. However, industrial effluents polluted by ammonia are generally too concentrated to be treated by biological processes so that physical and chemical techniques are necessary. It is possible to realise the stripping of ammonia contained in the effluent and to further oxidise it in gas phase. This method has been used by Huang [74] with an oxidation step activated by an hydrophobic catalyst (a Pt-based catalyst supported on styrene–divinyl–benzene polymers). The advantage of this process is that it requires milder conditions than WAO. The reaction is typically carried out at 120 ◦ C and under a pressure of 9 bar in a triphasic reactor. A conventional gas phase cannot work for this process. In a gas containing ammonia, water vapour and nitrogenous oxides, ammonium salts are formed: they behave as poisons at the catalyst surface and deactivate it rapidly. If the reaction is carried out in a triphasic reactor, liquid water is able to dissolve these salts and to maintain the catalyst free from inorganic deposits. The authors underline that in the case of the triphasic reactor, oxidation is in fact limited by the liquid–gas equilibrium of ammonia. Ammonia oxidation in supercritical medium has been carried out by Helling and Tester [75]. He points out the refractory character of this molecule (conversion of about 5% at 540 ◦ C in 6–13 s). Moreover, he remarks that the presence of organic substances like ethanol does not influence the oxidation of ammonia. More recently, Dell’Orco et al. have carried out the reaction between nitrate salts and ammonia at temperatures ranging from 450 to 530 ◦ C and under 300 bar [76]. The reaction leads both to the formation of molecular nitrogen and to nitrites, nitrogen monoxide, and nitrogen dioxide. The proposed mechanism involves radicals and the limiting step appears to be the hydrolysis of nitrate salts. Ammonium ions can also be removed on ion exchangers such as clays or zeolites [77]. This process can eliminate 61% of ammonium ions from a solution initially concentrated at 100 mg l−1 in 1 h. 4.2. CWAO of ammonia WAO processes have been applied to the elimination of ammonia with a good efficiency. However, most authors conclude that a catalyst is necessary to

activate the oxidation [78]. Many oxides have been tested in CWAO of ammonia. Inoue et al. [79] showed the particular efficiency of the cobalt(II) oxide (CoO), compared to other tested oxides: Cr2 O3 , MnO2 , NiO, N2 O3 , V2 O5 ZnO Co2 O3 and Fe2 O3 . Moreover, Inoue et al. [79] underline that the catalytic oxidation on CoO is very selective in molecular nitrogen. The same catalyst was used in the oxidation of ammonium acetate. It appears that the formation of carbon dioxide occurs first, far before that of molecular nitrogen. However, the rate of N2 formation in ammonium acetate oxidation is equal to the rate observed in ammonia oxidation. This fact is also observed during the oxidation of organic nitrogenous compounds such as acetonitrile and aniline. The competitive adsorption between carbonaceous molecules and ammonia would be responsible of this difference in reactivity. Imamura has compared the activity of numerous oxide catalysts for the oxidation of ammonia at 263 ◦ C and under 10 bar of oxygen. The composite oxide catalyst Mn/Ce (7/3) is more efficient than copper nitrates or than Co/Bi mixed oxides; it leads to a 70% conversion in 1 h [80]. The adsorption capacity of ammonia would be the determining factor for the catalytic activity. Thus, catalysts having in gas phase a great affinity for ammonia are the most active in liquid phase. Likewise, Chakchouk used the same composite oxide of manganese and cerium for the oxidation of ammonia. He confirmed that a temperature of 260 ◦ C is necessary to reach a significant conversion of ammonia, as illustrated in Table 12 [81]. The reaction is particularly selective in dinitrogen on this Mn/Ce catalyst. A surface mechanism has been proposed which explains that nitrites and nitrates present after 1 h disappear and are transformed in N2 after 4 h. Traces of N2 O have also been detected in the gas phase of ammonia oxidation. In 1977, a patent of Okada et al. (Osaka Gas Company, Japan) claimed the use of transition and noble metals for ammonia oxidation [82]. In this patent, it is indicated that iridium, platinum, gold, palladium, rhodium, iron, nickel, tungsten, copper and cobalt have all an activity for ammonia oxidation at 265 ◦ C and 70 bar when they are supported on alumina. In 1987, Takahashi et al. used noble metals deposited on honeycomb ceramic to oxidise the filtered solution obtained after WAO of sludge produced in sewage purification units [83]. Without any catalyst, nitrogen remains

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181

Table 12 Ammonia oxidation in the presence of a Mn/Ce catalysta [81] T (◦ C)

Reaction time (min)

Conversion of NH3 (%)

N–NO2 − (%)

N–NO3 − (%)

N–N2 (%)

200 260 260

60 60 240

22 66 91

10 1 0

6 0.5 0

7 65 91

a

Concentrations: [Mn/Ce (7/3)] = 20 mmol l−1 , [NH3 ] = 2 g l−1 ; pH = 10.5.

mainly in the ammonia form. Palladium and platinum are the most promising metals for ammonia oxidation with both a good activity and a good selectivity to N2 . Gold, rhodium and ruthenium are less interesting metals for this reaction: they are either less active or less selective or both. However, whatever the metal, a complete oxidation is not obtained. These works underline the fact that during ammonia oxidation, an optimum of the oxidising capacity of the catalyst must be found. The catalyst must mildly activate the oxidation of ammonia in order to avoid the deep oxidation into nitrite and nitrate ions. The catalytic properties of the different metals for ammonia oxidation are recalled in Table 13. Noble metal and transition metal catalysts supported on alumina have also been tested by Qin and Aika [84] at a lower temperature than in the work of Okada et al. [82]. At 230 ◦ C and under 15 bar of air, after 2 h of reaction, noble metal catalysts are generally more active than transition metal catalysts, except manganese. Ruthenium and palladium are particularly active. At 180 ◦ C and 27 bar, Ru/Al2 O3 allows to obtain a 99% conversion of ammonia and still remains active after four cycles of oxidation. The very good performances of Ru can linked to the excellent activity of this metal for ammonia synthesis. Ruthenium, able to dissociate molecular nitrogen, should be active for the recombination reaction of two nitrogen atoms. In ammonia synthesis, the nature of the metallic precursor and of the support strongly influences the activity of ruthenium. In particular, chlorinated precursors

have to be avoided [85]. There is no similar report in the literature concerning ammonia oxidation. Owing to the presence of large amounts of water, we may just infer that the role of Cl precursors should be less critical than in NH3 synthesis. The ammonia oxidation reaction does not seem to be sensitive to particle size of metals [85]. More recently, the reaction was revisited by Taguchi and Okuhara with noble metal catalysts deposited on TiO2 [86]. He shows that the order of activity for the formation of molecular nitrogen is: Pt > Ru > Pd > Rh Cu Cu, Co and Ni. For Qin and Aika, the performance of the catalyst is linked to the affinity of the active phase for oxygen, quantified by the heat of formation of the oxide per mole of oxygen,
Hf0 [84].
Hf0 is the enthalpy of formation of the metal oxide but in order to facilitate the comparison in M–O bond strength, the value for the Mx Oy oxide is divided by the number of M–O bonds in the oxides. Fig. 7 shows that those metals having a moderate affinity for oxygen are the most active for oxidation of ammonia and the most selective towards molecular nitrogen formation. In the work of Qin and Aika [84], a great activity is linked to a great selectivity, which is not in agreement with previous studies [81,83]. However, the idea that the best catalyst should have a moderate oxidising capability must be kept in mind. A comparison between the data of Qin and Aika [84] and Taguchi and Okuhara [86] shows that the activity of noble metals strongly depends on the nature

Table 13 Oxidising capacity and selectivity of metals tested by Takahashi et al. [83]

Oxidation capacity Selectivity in

Rh

Ru

Pd

Pt

Ru + Rh

+++ NH4 + + N2

+ NO3 − + N2

+ N2

− N2

+++ N2 + NO3 −

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Fig. 7. Selectivity in molecular nitrogen (䊏) and conversion (䊉) of ammonia with respect to the heat of formation of metal oxides used as catalysts. Adapted from [84].

of the support. This was confirmed by Takayama et al. for Pd catalysts [87]. Various 3 wt.% palladium catalysts prepared on active carbon, TiO2 , alumina, MgO, CeO2 , Sm2 O3 and La2 O3 have been tested at 170 ◦ C under 20 bar of air for the oxidation of a solution initially loaded with 1.5 g l−1 of ammonia. The Pd/active carbon is the most efficient catalyst with the highest activity and selectivity towards molecular nitrogen. It is even more promising than the Ru/Al2 O3 catalyst. However, the presence of large amounts of chloride (the reaction was carried out with NH4 Cl solutions) might bias the comparison. All the supports lead to a selectivity greater than 90% in molecular nitrogen, excepted La2 O3 which favours the formation of nitrates. The activity of active carbon is probably linked to its hydrophobicity and to its greater specific surface compared to the other supports. Finally, it is shown that the activation of the catalyst which leads to the best activity is the reduction. This pre-treatment enables the elimination of hydrophilic functional groups of active carbon and metal in its reduced state is more active than the corresponding oxide. The effect of the pH on the oxidation of ammonia has been studied on a RuO2 /Al2 O3 catalyst by Qin and Aika [84] and on a Pd/active carbon by Takayama et a. [87]. An acidic pH inhibits the reaction which is favoured at a pH higher than the pKa of ammonia, 9.25. This phenomena is explained by the fact that the

molecular form of ammonia is more reactive than the ionic form. In the previous studies [84–87], ammonia oxidation products are exclusively molecular nitrogen, nitrite and nitrate ions. Nitrogen oxides are rarely detected. This is not in agreement with a recent report by Ukropec et al. [88] who showed that N2 O is formed in large amounts on a platinum catalyst supported on graphite, at 160 ◦ C and with an excess of oxygen.

5. Conclusion Among the treatments of nitrogen-containing organic compounds in water, WAO appears as one of the most promising. Nevertheless, in order to obtain high conversions of nitrogenous pollutants, the non-catalytic techniques need severe operating conditions in terms of temperature and pressure. CWAO processes, with homogeneous or heterogeneous catalysts, require milder reaction conditions. The complexity and the potential toxicity of numerous nitrogen-containing intermediates formed during these oxidation processes have to be taken into consideration. Global parameters like COD or TOC are necessary but not sufficient to evaluate the performances, in terms of detoxification, of a waste water treatment process. Kinetic schemes and reaction mechanisms should be investigated to

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search for very toxic by-products, even though they are formed in small amounts (azo, nitroso, amines compounds, etc.). With the help of sophisticated analytical techniques, the identification of intermediary and refractory by-products can be performed. Globally, the different steps of the WAO of these pollutants, involved in the process can be summarised as follows:

It clearly appears that acetic acid and ammonia are the most refractory compounds. Both oxides and supported noble metals can be used in CWAO of organic compounds. However, catalysts active in steps I and II could be different from those which are active and selective to N2 in step III. The higher cost of the noble metals is balanced by a very high stability in liquid phase making a recovery step useless contrary to the oxide catalyst. References [1] S.V. Mishra, V.M. Mahajani, J.B. Joshi, Ind. Eng. Chem. Res. 34 (1995) 2. [2] R. Andreozzi, V. Caprio, A. Insola, R. Marotta, Catal. Today 34 (1999) 51. [3] J. Donlagic, J. Levec, Appl. Catal. B 17 (1998) L1. [4] P. Roche, P. Ormad, J. Sarasa, A. Martin, A. Puig, J.L. Ovelleiro, Combinated preozonation and chemical coagulation of a spillage composed of aniline and its derivatives, in: Proceedings of the GRUTEE, Poitiers, France, 29–30 September 1994. [5] J. Kiwi, C. Pulgarin, P. Peringer, Appl. Catal. B 3 (1994) 335. [6] Sax, in: R.J. Lewis (Eds.), Dangerous Properties of Industrial Materials, 9th Edition, Van Nostrand Reinhold, New York, 1996. [7] Security Data Prolabo, 1995. [8] Security Data Prolabo, 1998. [9] T.L. Randall, P.V. Knopp, Detoxification of specific organic substances by wet oxidation, J. WPCF 52 (1980) 2117. [10] ROINEU Radiation Oncology Investigations, Vol. 1, 1993, p. 206. [11] XPHBAO US Public Health Service, Public Health Bulletin, Vol. 271, 1941, p. 19. [12] E. Gilbert, Water Res. 10 (1987) 1273.

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