Wet air and catalytic wet air oxidation of several azodyes

from wastewaters: the beneficial role of catalysis

A. Rodrıguez, J. Garcıa, G. Ovejero and M. Mestanza

ABSTRACT

A. Rodrıguez (corresponding author)

J. Garcıa

G. Ovejero

M. Mestanza

Grupo de Catalisis y Procesos de Separacion

(CyPS),

Departamento de Ingenierıa Quımica,

Facultad de Ciencias Quımicas,

Universidad Complutense de Madrid,

Avda. Complutense s/n, 28040 Madrid,

Spain

E-mail: arodri@quim.ucm.es;

juangcia@quim.ucm.es

Degradation of several azo dyes, Acid Orange 7 (AO7), Acid Orange 74 (AO74), Direct

Blue 71 (DB71), Reactive Black 5 (RB5) and Eriochrome Blue Black B (EBBB), well-known

non-biodegradable mono, di and tri azo dyes has been studied using, wet-air oxidation (WAO)

and catalytic wet air oxidation (CWAO). The efficiency of substrate decolorization and

mineralization in each process has been comparatively discussed by evolution concentration,

chemical oxygen demand, total organic carbon content and toxicity of dyes solutions.

The most efficient method on decolorization and mineralization (TOC) was observed to be

CWAO process. Mineralization efficiency with wet air and catalytic wet air oxidation essays

was observed in the order of mono-azo . di-azo . tri-azo dye. Final solutions of CWAO

applications after 180min treatment can be disposed safely to environment.

Key words | catalytic wet air oxidation, dyes, heterogeneous catalysts, wet air oxidation

INTRODUCTION

Printing and textile wastewater includes a large variety of

inks, dyes and chemicals additions that make the environ-

mental challenge for printing and textile industry not

only as liquid waste but also in its chemical composition.

Organic dyes are one of the largest groups of pollutants in

wastewaters produced from textile and other industrial

processes. The textile industry consumes considerable

amount of water during the dyeing and finishing operations

and the fixation efficiency of dyes typically ranges between

60% and 90% (Camp & Sturrock 1990); therefore the main

pollution in these wastewaters came from these processes.

It has been documented that residual colour is usually due

to insoluble inks and dyes which have low biodegradability

(Scheeren et al. 2002; Arslan-Alaton 2004). These highly

colored compounds stop reoxygenation capacity of the

receiving water and cut–off sunlight, thereby, upsetting

biological activity in aquatic life. The potential toxicity of

some organic dyes such as azo dyes has long been known.

Furthermore, dye effluent may contain chemicals, which are

toxic, carcinogenic, mutagenic or tetratogenic in various

microbiologic, fish species (Altibas et al. 1995; Daneshvar

et al. 2003). Therefore, it is necessary to find an effective

method of wastewater treatment in order to remove color

from textile effluents.

The conventional methods used for textile wastewater

purification are biological oxidation and physical–chemical

treatment (e.g. coagulation–flocculation, and activated

carbon adsorption). These processes are not enough

efficient since dyes are hardly removable due to their low

molecular weight and high water solubility. To reduce the

stated problems, advanced oxidation processes (AOPs)

have been developed to generate hydroxyl free radicals

by different techniques as WAO and CWAO. WAO can be

defined as the oxidation or organic and inorganic

substances in an aqueous solution, or suspension by

means of oxygen or air at elevated temperatures and

pressures. However, CWAO reduce the stringent operating

conditions of WAO. Another major benefit of using

catalysts in WAO is the oxidation of the refractory

compounds, namely acetic acid and ammonia, at much

doi: 10.2166/wst.2009.526

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lower temperatures than in the absence of catalysts. Hu et al.

1999, supported Cu as active metal on a commercial AC

from Norit for oxidation treatment of a dyeing and printing

wastewater from a textile company. The COD and TOC

reduction of the wastewater was higher with the Cu/AC

catalyst than with the catalyst made with the same metal

supported on alumina or with the homogenous catalyst,

copper nitrate in solution, i.e. reduction of 91% of COD and

64% of TOC. Gomes et al. (2000), used a carbon supported

Pt catalyst to study the CWAO of low molecular weight

carboxylic acids as model compounds. Their results showed

that, at around 2008C and at 7 bar of oxygen partial

pressure, a very high catalytic activity is obtained, with

selectivity to gaseous products and water near 100%. Also,

Alvarez et al. (2002a,b) and Wu et al. (2005), used copper

supported activated carbons as catalysts in the catalytic

oxidation of phenol, and despite good phenol conversions

were achieved in their studies; the copper leaching was

responsible for deactivation in both cases.

On the other hand, the oxidation of aromatic com-

pounds and their transformation to lower molecular

weight fractions and organic acids in wet air oxidation has

been explained as a free radical chain auto-oxidation

process. It has been suggested that wet air oxidation

might occur through the formation of alkyl, alkyl peroxyl

radicals and hydroperoxides with the latter being respon-

sible for the autocatalytic decomposition of the original

compound. Transition metal ion catalysts such as Cu have

been used to promote wet air oxidation. It is thought that at

high temperatures, Cu ion may facilitate the process,

possibly by direct electron transfer between the compound

and metal in a new initiating step (Wu et al. 2000, 2005;

Quintanilla et al. 2006).

The extensive literature in this field has been reviewed

(Venkatadri & Peters 1993), and during the last decade,

some investigators have reported the beneficial applications

of the AOPs processes of dye wastewater treatment.

In recent years, we have contributed to develop a new

generation of carbon materials for catalytic oxidation

processes of organic compounds present in wastewaters

(Sotelo et al. 2002; Garcia et al. 2005; Ovejero et al. 2007).

In this way, the use of carbon nanotubes (CNTs), carbon

nanofibers (CNFs) and activated carbon (AC), as supports

in heterogeneous catalysts is still increasing because of the

wide versatility of these materials (Serp et al. 2003; Ovejero

et al. 2006). Granular activated carbons (GAC) are used as

support for a great number of active phases, including noble

metals, with the advantages of the easy recovery of the

metal by means of simply burning and the chemical stability

due to their higher resistance to leaching in acidic solutions

under wet air oxidation conditions (Fuente et al. 2001).

The manuscript is the continuation of previously

published work (Rodrıguez et al. 2008), but in this case

we study the effect of several azo dyes with different

structure. In addition, to our knowledge the effect of the

oxidation with copper supported carbon nanofibers

(nanosupport) catalysts on dye molecular structure has

not been investigated. In this study, five commercial dyes,

Acid Orange 7 (AO7), Acid Orange 74 (AO74), Direct Blue

71 (DB71), Reactive Black 5 (RB5) and Eriochrome

Blue Black B (EBBB), were degraded by wet air oxidation

and by Cu/CNF using a batch reactor.

MATERIALS AND METHODS

Materials

Carbon nanofibers (CNFs) have diameters in the range

20–50nm and lengths of 50–100nm and were obtained

from Grupo Antolın Ingenierıa S.A., Spain.

Acid Orange 7 (AO7), Acid Orange 74 (AO74),

Eriochrome Blue Black B (EBBB); Direct Blue 71 (DB71)

and Reactive Black 5 (RB5) were selected as the model

pollutants because they are hardly biodegradable by

the conventional biological process, but widely used in the

textile, color solvent, ink, paint, paper, and plastic indus-

tries. The azo dyes were purchased from Sigma–Aldrich

(Steinheim, Germany) and used without further purifi-

cation. The main characteristics of dyes used in this work

and the structure are shown in Table 1.

Catalyst characterization

Textural characterization of the supports and catalysts

were done by using N2 adsorption–desorption at 77K in

a Micromeritics ASAP 2010 (Monchengladbach, Germany).

Prior to the physisorption measurements, the fibers were

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Table 1 | Main characteristics and structure of dyes

Dye C.I. name Structure Chem. class

Commercial

name C.I. number

MW

(gmol21)

Dye

content

(%) Molecular formulae

lmax

(nm)

Acid Orange 7(AO7)

Acid dye Orange II 15510 350.3 85 C16H11N2NaO4S 309

Acid Orange 74(AO74)

Acid dye 201812_SIAL

18745 508.3 60 C16H11CrN5NaO8S 324

Eriochrome BlueBlack B(EBBB)

Acid dye MordantBlack 3

14640 416.4 – C20H13N2NaO5S 306

ReactiveBlack 5(RB5)

Reactivedye

RemazolBlack B

20505 991.8 55 C26H21N5Na4O19S6 596

Direct Blue 71(DB71)

Direct dye 212407_ALDRICH

34140 965.9 50 C40H28N7NaO13S4 587

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evacuated at 3008C. Thermogravimetric analysis (TGA) was

performed with an EXTAR 6000 Seiko Instruments Inc.

thermal analyzer at a heating rate of 108C/min. The gas

evolved during analysis was monitored by a ThermoStar

Pfeifer QMS 200 (Asslar, Germany) quadropole mass

spectrometer by using a capillary probe situated directly

above the sample cup. The morphology of the materials was

studied by analysis on a JEOL JEM 2010 electron

microscopy (Hertfordshire, United Kingdom) at 200kV

(TEM) and metal loading was determined by mean of X-Ray

Fluorescence (XRF) (Broker S4 Explorer). It consists of the

Philips high-voltage generator and another module with

contains the X-ray source and the detector.

Catalysts preparation

The supported catalysts were prepared by wet impregnation

of the CNF support with copper metal precursor

(Cu(NO3)2·3H2O), supplied by Sigma–Aldrich (Steinheim,

Germany). Previously, the surface chemistry of the carbon

nanofibers was modified with carboxylic acid groups

(ZCOOH) by means of an acidic treatment for 6h under

reflux. The mass of precursor was calculated in order to

obtain 3wt.% metal in the catalysts. The wet solid was

dried overnight at 1108C and then activated under N2/H2

(3:1) flowing at 3508C for 3h in a furnace, in order to

reduce the metal precursor to its corresponding ground

state (Garcia et al. 2005).

Reaction procedures and analytical methods

All experiments were conducted in a Hastelloy high-

pressure microreactor C-276 autoclave Engineers with a

volume of 100mL. The reactor (i.d. 50mm) was equipped

with an electrically heated jacket, a turbine agitator and a

variable speed magnetic drive. The temperature and the

stirring speed were controlled by means of a PID controller.

The gas inlet, gas release valve, pressure gauge, rupture

disk and cooling water feed line were situated on the top

of the reaction vessel. The liquid sample line and the

thermocouple well were immersed in the reaction mixture.

The reactor was first loaded with 75mL of solutions of

commercial dyes mono, di and tri azo at a concentration

of 1.0 g/L with or without catalyst, and initially pressurize

with nitrogen to ensure inert atmosphere. Afterwards, the

system was heated to the desired temperature and a sample

was withdrawn. This was considered zero time for the

reaction, and TOC conversion during this time can be

neglected while calculating initial rates. Oxygen from

the cylinder was then sparged into the liquid phase and

samples were withdrawn periodically after sufficient

flushing of the sample line. Pressure drop was monitored

and additional oxygen was charged in order to maintain a

constant total pressure.

Each dye concentration was determined by the wave-

length corresponding to its maximum UV–vis absorption

(lAO7 ¼ 483nm, lAO74 ¼ 474nm, lEBBB ¼ 539nm,

lRB5 ¼ 596nm, lDB71 ¼ 587nm). The reactions were mon-

itored by UV–vis spectrophotometer using a Shimadzu

spectrophotometer. Total organic carbon measurements

were carried out on a Shimadzu TOC analyzer, after

filtration (pore diameter 10mm). Chemical oxygen demand

was measured on a PF11 photometer Macherey–Nagel

according to Standard Methods (APHA 2005). Toxicity was

determined by means of a Microtox analyzer model 500.

The EC50 value was employed to compare the toxicities of

the solutions before and after the reaction. Once the target

temperature was reached (1408C), oxygen was introduced

and maintained at a constant partial pressure of 0.8MPa

(PO2) for the duration of the test (3 h).

RESULTS AND DISCUSSION

Supports and catalysts characterization

The treatment of carbon nanofibers with acids oxidant,

such as HNO3 and HCl, apart from mineral matter

removal, introduces oxygen surface complexes that

modify the surface chemistry and can alter the surface

area and porosity of the original sample. The influence of

support features in catalyst preparation is strong (Rodrıguez

et al. 2008).

A great number of copper particles, obtained with mean

size around 8.0 nm, are dispersed uniformly on the surface

of carbon nanofibers (but larger particles up to 30nm have

been found as well). The surface groups on the CNF surface

such as ZCOOH, ZOH, CvO which are introduced by

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oxidation are important to obtain the interaction required

for good distribution of the metal precursor (Garcia et al.

2006) on the CNF surface. Oxidation of the carbon

nanofibers in nitric acid gave a large deposition of Cu.

The thermal stability of the Cu/CNF was studied by

TGA in air. It is interesting to note that the CNF was stable

up to 4508C and then the decomposition starts and reaches

maximum at 6508C. At 6808C, practically almost complete

weight loss is noticed.

On the other hand, carbon nanofibers generally exhibit

a high specific surface area due to their high external

surface area (high aspect ratio material). The high external

surface area leads to a significant increase in the surface

contact between the gaseous or liquid reactants and the

active phase supported on this nanostructured host which is

a prerequisite for its use as catalyst support, especially in

liquid phase medium where diffusion rate is predominant

(Ovejero et al. 2007). The specific surface area of the CNF

and Cu/CNF are 244m2 g21 and 188m2 g21, respectively.

The BET surface area values of Cu/CNF are found to be less

than the corresponding to CNF support. This may be due to

the blocking of CNF by the Cu particles added.

Wet air and catalytic wet air oxidation of dyes

compounds

Generally, organics degradation by WAO were recognized

as a free-radical mechanism (Levec & Pintar 2007), and

hydroxyl radical is an extremely potent oxidizing agent

with a short life which is able to oxidize organic compounds

and generate other free radicals in the presence of organic

compounds (RH). The process can convert organic con-

taminants to CO2, water, and biodegradable short-chain

organic acids. WAO can involve any or of the following

reactions:

RHþO2 ! RzþHOz2 ð1Þ

RzþO2 ! ROz2 ð2Þ

ROz2 þ RH! ROOHþ Rz ð3Þ

ROOH! ROzþOHz ð4Þ

ROzþ RH! ROHþ Rz ð5Þ

OHzþ RH! RzþH2O ð6Þ

2ROOz! ROORþO2 ð7Þ

These reactions generate organic radicals and other free

radicals which in turn initiate chain reactions of dye

oxidative degradation with the help of dissolved oxygen.

Thus in the WAO system, the reaction intermediates,

especially the organic radicals increase, accelerate the

reaction rate.

Dyes removal depends on the operation parameters as

temperature, pH, oxygen pressure, stirring speed and dye

initial concentration. The investigation about the degra-

dation of 1 gL21 of several dyes (AO7, AO74, EBBB, RB5

and DB71 by wet air and catalytic wet air oxidation has

been carried out at 1408C and 8.7 bar partial pressure. For

this study, we select the parameters values obtained in a

previous work (Rodrıguez et al. 2008).

In the traditional organic compounds, the five or six

element ring is the most stable in chemical reaction, while

the linear structure, especially the azo bond is relatively

easier to be attacked. As for the color formation theory, the

color of dye formed by the cooperation of the chromophore

and the auxochrome group mainly containing phenyl

annulus (Zollinger 2003). In the AO7, AO74, EBBB, RB5

and DB71, the chromophore is the azo bond and when the

cooperation of the azo bond and the auxochrome is broken,

the dye absorbance detected at 483, 474, 539, 596 and

587nm, respectively, would disappear, which means the

removal of color. In WAO system, at first, the azo bond

(ZNvNZ) would first be attacked by heat energy and free

radicals. After the azo bond was attacked, most of the

nitrogen element went away from the liquid phase as

nitrogen and little portion amount came into the solution,

which led to the destruction of the dye molecule. CWAO

improves conversion of compounds that are refractory to

noncatalytic oxidation and accelerates rates of oxidation.

One more advantage of catalyst is that nitrogen-containing

compounds are almost fully converted into dinitrogen gas.

Dye solutions treated catalytically may be more amenable to

aerobic biodegradation as smaller quantities of oxidation

intermediates formed in the catalytic process.

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After the CZN (ZCZNvNZ) was broken, the air at

high temperature continued to react with the intermediates

with the help of free radicals, which led to the formation of

several intermediates and kinds of organic acids. With the

continuous air and the reaction time at high temperature,

these intermediates including different types of organic

acids could be oxidized to small molecule organic acids,

part of these intermediate would convert to the final

oxidation products, such as water and carbon dioxide

during to the total organic carbon removal.

On the other hand, it is possible to consider several

alternative steps involved in the CWAO of dyes:

(a) cleavage of cleavage of NZN bond of heteropolyaro-

matic; (b) cleavage of CZS bond of heteropolyaromatic;

(c) cleavage of CZN bond of the substituting group of

the heteropolyaromatic; (d) small molecules formed.

In this sense, dyes breaks thermally into fragments that

are most probably nitrogen (gas), naphtol and benzene

sulphonic acid via electrophilic cleavage of the labile azo

bond (ZNvNZ), provided that a pollutant-specific

threshold temperature and/or a steady state free radical

concentration has been achieved (Arslan-Alaton & Ferry

2002). The exact mechanism of the reaction is, however, not

known. A key question should be to determine whether it

is the C atoms bearing the azo group, or theZNvNZ atoms

themselves, that are attacked.

Decoloration, COD and TOC removal

The pH final, total organic carbon, chemical oxygen

demand, dye conversion and toxicity as EC50 after 3 h

reactions are summarized in Table 2. After 3 h reaction, the

dyes were highly decomposed in the presence of the Cu/

CNF catalysts but the TOC and COD removal were lowest

in the absence of the catalyst. In absence of catalyst (WAO

experiments) at 1408C TOC and COD removal are between

4.1–30.0% and 6.1–30.0%, respectively, for all dyes.

Figure 1 shows the concentration decay of AO7, AO74,

EBBB (mono azo examples), RB5 (di-azo example) and

DB71 (three-azo example) in WAO and CWAO process.

At 1408C no oxidation or dye conversion was observed

in the case of AO74 (monoazo structure), while for the

diazo dye RB5 and triazo dye DB71 a 34.0% and 58.2%

conversion was registered, respectively. At 45min of

reaction dyes removal were very low (except for EBBB

with 94.2%). In all case the toxicity was high in the final

effluent of WAO process.

In the presence of the Cu/CNF catalyst (CWAO

process), complete dye conversion was observed, practically

after 3 h, and 45min in any case as EBBB, RB5 and DB71.

The removal rate of dye, TOC and COD was significantly

enhanced with respect to the WAO process. After 3 h of

reaction, TOC and COD removal was between 6.8–71%

and 43.0–75.8%, respectively. Thus, the dyes can effectively

Table 2 | Wet air and catalytic wet air oxidation of azo dyes in aqueous solutions. [Dye]0 ¼ 1 g L21; T ¼ 1408C; Partial pressure ¼ 8.7 bar; stirring speed (S) ¼ 1,000 rpm;

Reaction time ¼ 3 h

WAO CWAO

Experiment pHi pHf XTOC (%) XCOD (%) XDye (%) EC50 (%) pHf XTOC (%) XCOD (%) XDye (%) EC50 (%)

AO7 5.3 7.3 12.0 18.0 24.1 104.7 6.0 24.1 43.0 99.6 3.2

3.1p 98.1p

AO74 Monoazo 10.3 6.8 30.0 25.0 0.0 19.3 6.6 71.1 75.8 99.1 8.9

0.0p 97.4p

EBBB 4.0 5.8 4.1 20.5 97.3 – 5.8 50.0 51.2 100.0 –

94.2p 100.0p

RB5 Diazo 4.8 3.1 18.6 30.0 34.0 46.4 7.2 25.0 54.9 100.0 3.4

12.2p 100.0p

DB71 Triazo 6.3 3.9 1.0 6.1 58.2 31.7 5.9 6.8 57.3 100.0 5.0

15.1p 100.0p

pConversion after 45min.

1994 A. Rodrıguez et al. | Wet air and catalytic wet air oxidation of several azodyes from wastewaters Water Science & Technology—WST | 60.8 | 2009

Figure 1 | Plot of dye concentration vs reaction time of dyes AO7, AO74, EBBB, RB5 and DB71 with WAO and CWAO processes.

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be degraded within a reasonable time at this moderate

temperature with the Cu/CNF catalyst. The removal of TOC

concentration decreased in the following order:

monoazo . dioazo . triazo, with toxicity lowest that

WAO process. TOC and COD were not completely

destroyed indicating the presence of refractory organic

compounds in the reaction mixture.

Nevertheless, total color removal was observed in the

degradation of all azo dyes, practically, indicating that their

destructionshouldoccurbyattackof reactiveoxidationspecies

on the NvN bonds, generating small intermediate molecules.

In this sense, the lowest selectivity towards non-organic

compounds formation was observed in the degradation of

the triazo dye (DB71), with low because a great complexity

of oxidation intermediates should be expected to occur.

Changes in TOC and COD are not overlapped (the

changes in the COD are faster than those of the TOC), and

there is an important change in the shape of the UV–vis

spectra. Both observations indicate the formation of large

amounts of intermediates, and so the existence of mild

oxidation conditions. In addition, there is a certain

accumulation of WAO-refractory compounds at the end

of the treatment. This accumulation has been observed in

the treatment of many types of pollutants, and it is

associated to the formation of some types of carboxylic

acids whose oxidation by hydroxyl radicals is very slow.

As example, the RB5 is a di-azo dye in which the

chromophore part of molecular structure contains azo

linkage and shows a strong absorbance in the visible region,

while the absorbance peaks of the benzene and naphtha-

lene rings appeared in the UV region. The absorbance peaks

at UV region and at 596nm are respectively attributed to

these aromatic rings and azo linkage (Feng et al. 2000).

The colour changes from light blue to light pink at

180min. These changes are shown in Figure 2. The change

in colour that occurred in the first 60 minutes was most

likely due to the formation of compounds such as coloured

condensation products from the oxidation of dye. In the

Table 2, we can observe that with Cu/CNF as catalyst,

decolorization efficiency increase with respect to WAO

process in all cases. In this sense, we can observe in Figure 3,

the comparison of UV–vis spectra in WAO and CWAO

processes for AO7 (mono-azodye). The decrease of absorp-

tion peaks at lmax was highest in CWAO process.

COD values have been related to the total concen-

tration of organics in the solutions (Daneshvar et al. 2005).

Using this criterion, the mineralization of dyes was

investigated under WAO and CWAO processes. For the

initial concentration of 1 gL21 of dye and under the

optimum conditions, the COD values were measured and

the appropriate efficiencies were calculated with respect to

its initial value. The results are presented in Figure 4. Using

CWAO process, after 180min, for instance, 43.0–75.8% of

COD ranges removal efficiency was achieved; however,

these values are reduced to 6.1–30.0% ranges, respectively,

when using WAO; significantly less than the case when Cu

supported on carbon nanofibers as catalyst is used.

It is seen that the COD degradation yields of DB71

(tri-azo dye) in the absence of catalyst was less than 6.1% at

Figure 2 | Color removal efficiency with WAO process on RB5.

Figure 3 | Comparison of WAO and CWAO processes on absorbance spectra for AO7.

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180min by WAO, but the COD elimination of AO7, AO74,

EBBB and RB5 is significant with a yield of 18.0, 25.0, 20.5

and 30.0%., respectively. Higher COD removal can be

expected for the CWAO process. In this case, we have

obtained yield between 43.0–75.8% ranges. The reason

could be due to the stable intermediates which are not more

degradable by WAO. The presence of Cu catalyst is vital for

an effective mineralization of dyes, especially for short times

of oxidation. The order of decreasing COD for mono azo

dyes compounds in WAO is as follows: AO74 . EBBB .

AO7, and in general mono-azo . di-azo . tri-azo dye.

As can be seen in Figure 4, less than 30% of the initial

TOC could be removed regardless of the species of dyes in

the absence of catalyst. When Cu catalyst was fed into the

batch reactor together with air, the TOC removal efficien-

cies increased significantly. More than 70% of the initial

TOC could be removed within 180min (Table 2). From the

results in Figure 4, it is evident that small amount of Cu

catalyst must have played an important role on accelerating

the wet oxidation rate. In the absence of catalyst there

existed slight differences in the TOC removal efficiencies

obtained from the Cu/CNF catalyst. In the presence of

catalyst, however, remarkable enhancement of TOC

removal efficiencies was observed. Highest removal of

TOC from the AO74 aqueous solution could be achieved

in 180min (71.1%). Even in the case of the DB71 (tri-azo),

which was the most difficult to be oxidized, it took 180min

for the 6.1% removal of TOC.

It can be concluded from the results of CWAO that

the TOC degradation efficiency can be improved by about

50.2% in AO7, 57.8 in AO74, 91.8 in EBBB, 25.6%

in RB5 and 85.3% in DB71 by using Cu/CNF catalyst

with respect WAO process. It has been evidenced that

the presence of Cu leads to a significant increase of the

rate of oxidation and mineralization in CWAO (Levec &

Pintar 2007).

Evaluation of toxicity

Some Spanish autonomic and regional administrations or

water companies that operate municipal sewage plants use

standards to classify effluents in terms of their toxicity

values in order to limit the discharge of effluents to the

sewer system. To evaluate the toxicities of the solutions after

the reaction, Microtox tests were performed (Bennett &

Cubbage 1992). The EC50 values of the solutions after

reaction are summarized in Table 2. Toxicity results are

shown on an effective concentration scale based on a 50%

response (EC50 concentration). EC50 values decrease with

CWAO process. Based on the 15min percentage inhibition

values, the sample after 180min of treatment indicates the

disappearance of the toxic compounds formed in the early

stages of reaction (Figure 5).

Figure 4 | Influence of WAO and CWAO processes on color, COD and TOC removal.

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These results indicate that the toxicity of the solution

containing AO7, AO74, RB5 and DB71 is increased as a

result of oxidation without copper catalyst.

On the basis of the toxicity test results, it can be

concluded that the use of CWAO process as a treatment

technology has successfully detoxified the dyes effluent and

removed its undesirable compounds.

Finally, during the CWAO process, the components

may be leached out from the catalysts. To investigate the

stability of Cu/CNF with respect to Cu leaching, the

concentration of dissolved Cu in the solution after catalytic

oxidation for 180min was analyzed. Under the reaction

conditions employed for this research, the catalyst under

study show a moderate chemical stability with slow

leaching copper. Leaching is detrimental not only for the

catalyst, some toxic substances are eluted, and they can be

harmful for aquatic systems. Obviously, the leaching rate of

copper is strongly enhanced by the acidic conditions.

An attempt to limit copper leaching by the modification

of the catalyst composition was made by Hocevar et al.

(1999). They prepared the mixed Ce12xCuxO22a catalysts

support (by precipitation and sol–gel method), which are

much less soluble in hot acidic solutions. If prepared in the

form with highly dispersed copper oxide phases on CeO2,

they are catalytically very active under mild reaction

conditions and are selective toward the CO2 formation.

In a future work we will study this aspect.

CONCLUSIONS

The treatment of carbon nanofibers with acids oxidant, such

as HNO3, introduces oxygen surface complexes to act as

anchoring centres that change the surface chemistry. The

results corroborate the high efficiency of these processes,

speciality CWAO process that is the most effective method

for decolorization and mineralization of all the azo dyes

essayed. The order of decreasing color removal for mono

azo dyes compounds has been as follows: EBBB .

AO7 . AO74, in WAO and CWAO, in general mono-

azo . di-azo . tri-azo dye. For the last, the decrease in

the toxicity of the dye solution has been higher in the

catalytic wet air oxidation process than in wet air oxidation

process.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the financial support

from Ministerio de Educacion y Ciencia by CONSOLIDER

Program through TRAGUA Network CSD2006-44,

CTQ2008-02728, by Comunidad de Madrid through

REMTAVARES Network S-505/AMB/0395 and by Com-

plutense/Santander project PR34/07-15913. We also thank

M. Barreiro for your assistance in experiments preparation.

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