Cm

paaatprdos

cagrHcbet

0d

J. of Supercritical Fluids 46 (2008) 163–172

Contents lists available at ScienceDirect

The Journal of Supercritical Fluids

journa l homepage: www.e lsev ier .com/ locate /supf lu

atalytic wet air oxidation of textile industrial wastewater using

etal supported on carbon nanofibers

. Baca, Fac

of 3 wa batrem

eratureducin intlic ac

f a texst sigr a p

A. Rodrıguez ∗, G. Ovejero, M.D. Romero, C. Dıaz, MGrupo de Catalisis y Procesos de Separacion (CyPS), Departamento de Ingenierıa QuımiAvda. Complutense s/n, 28040 Madrid, Spain

a r t i c l e i n f o

Article history:Received 20 July 2007Received in revised form 28 February 2008Accepted 9 April 2008

Keywords:Carbon nanofibersCatalytic wet air oxidationCatalystPressureWashing textile wastewater

a b s t r a c t

The catalytic performancetion has been tested usingtotal organic carbon (TOC)was carried out in a tempTOC removal and toxicity180 min reaction. The maand 1,2-benzenedicarboxyCWAO to the treatment othe use of a Cu/CNF catalyconsidered as an option fo

1. Introduction

Wastewater derived from the handling of dyes is highly variablein composition, and contains a large number of different com-

ounds such as raw materials (anilines), intermediate products,nd even the dye itself [1] (Fig. 1). The dye manufacture industrynd textile industry has prompted to investigate more appropri-te and environmentally friendly treatment technologies to meethe discharge restraints that are becoming stricter everyday. Dyesroduction, textile preparation, dyeing and finishing plants are cur-ently being forced to treat their effluents at least partially prior toischarge to publicly owned treatment works because of the highrganic load, strong and resistant color as well as high dissolvedolids content of the discharged wastewater [2].

Conventional treatments of wastewater containing organicompounds include biological oxidation, chemical coagulation,dvanced oxidation and adsorption [3–7]. Biological methods areenerally cheap and simple to apply and are currently used toemove organics and color from dyeing and textile wastewater [8].owever, this dyeing wastewater cannot be readily degraded by

onventional biological processes, e.g., the activated sludge process,ecause the structures of most commercial dye compounds are gen-rally very complex and many dyes are non-biodegradable due toheir chemical nature, molecular size; thus, this results in sludge

∗ Corresponding author. Tel.: +34 91 394 4184; fax: +34 91 394 4114.E-mail address: arodri@quim.ucm.es (A. Rodrıguez).

896-8446/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.supflu.2008.04.007

rreiro, J. Garcıaultad de Ciencias Quımicas, Universidad Complutense de Madrid,

t.% copper supported on carbon nanofibers (CNFs) in liquid phase oxida-ch stirred tank microreactor in order to determine the decolorization and

oval efficiency in washing textile wastewater (WTW). A preliminary studyre range of 120–160 ◦C and two oxygen partial pressure of 6.3 and 8.7 bar.tion were as high as 74.1% and 43%, respectively at 140 ◦C and 8.7 bar, afterermediates detected in raw wastewater were decanoic acid, methyl esterid, and have been degraded by means of Cu/CNF catalyst. Application oftile effluent at 160 ◦C and 8.7 bar of oxygen partial pressure showed thatnificantly improves the TOC and color removal efficiencies and it can be

retreatment step in the treatment of these industrial effluents.© 2008 Elsevier B.V. All rights reserved.

bulking. Although dyestuffs and color materials in wastewater canbe effectively destroyed by wet oxidation, advanced chemical oxi-dation such as H2O2/UV, O3 and Fenton reagent [9,10]. On theother hand, sub- and supercritical oxidation is a promising emerg-ing technology for both the treatment of organic compounds andwastewater. There has been extensive research on the applicationof sub- and supercritical oxidation for the treatment of wastew-

ater with high organic matter [11–14] or model compounds suchas ammonia [15], phenol [16] and acetic acid [17], and they haveproved its effectiveness.

Wet air oxidation (WAO) is very useful for treating a variety ofrefractory organic pollutants in wastewater, but the high pressureand high temperature required for its operation limit its practi-cal applications, therefore, catalytic wet air oxidation (CWAO) isproposed to relax the oxidation conditions [18].

In the last decades, various heterogeneous catalysts includingnoble metals deposited on the supports and transition metal oxideshave been developed. They showed good catalytic activity in thecatalytic wet air oxidation of organic pollutants and real wastewa-ters [19–21].

Pintar and Levec [22] noticed during CWAO of some substitutedphenols, that phenols with electron withdrawing substituents aremore resistant to oxidation than phenols with electron-donatingsubstituents. This can be explained by the fact that withdrawingelectron density from the aromatic ring causes decrease in its reac-tivity. As the mechanism of CWAO is believed to be similar tononcatalytic process also similar pathways and intermediate dis-tribution should be observed. Alvarez et al. [23] have found acetic

164 A. Rodrıguez et al. / J. of Supercritical Fluids 46 (2008) 163–172

ustria

Fig. 1. Washing textile ind

acid and p-benzoquinone to be the main intermediates in CWAO ofphenol using copper oxide supported over activated carbon.

Fortuny et al. [24] oxidised phenol with a commercial coppercatalyst. They found acids (formic and acetic) and diacids (oxalic) tobe the main intermediates, responsible for almost 90% of all inter-mediates present in liquid phase at higher residence times. Theintermediates found are coherent with the mechanism proposedby Devlin and Harris [25]. A similar intermediate distribution wasfound in the work of Duprez et al. [26] over supported Ru, Pt and

Rh catalysts and Ohta et al. [27] over supported copper oxide.

Deiber et al. [28] reported the removal of nitrogenous com-pounds from p-nitrophenol, aniline, etc. using Mn/Ce compositeoxides. In the oxidation of p-nitrophenol, nitrogen mainly in formof NO2 is obtained. The C–N bond is broken and there is noother evolution of nitrogen containing group. When oxidizing ani-line, after the removal of the initial substrate, the concentrationof ammonia is decreasing and finally is totally converted to themolecular nitrogen. No other nitrogen containing intermediates areobserved. On the other hand, Oliviero et al. [29] oxidised anilineover Ru/CeO2 catalyst and classified intermediates in three groups:condensation intermediates, nitrogenous aromatic compounds andnon-nitrogenous aromatics.

Over the last 10–15 years, continuous SCWO technology provedto be an extremely powerful treatment for all kinds and concentra-tion range of toxic organic wastewater [30,31]. Related problemswith corrosion and plant plugging due to salt precipitation weresolved on laboratory scale leading to new reactor configurationstermed hydrothermal burner and transpiring wall reactor. Long-term tests to check for the suitability of these reactors have notyet been performed. The drastic operating conditions used make

Table 1General comparison of the typical conditions of thermal oxidation [35]

Operating conditions WAO

Temperature (◦C) 200–325Pressure (bar) 20–210Residence time (min) 10–90Conversion 80–99Products CO2, H2O, N2, salts, organic acidsEstimated operating costa (DM/m3) 30–60 (Zimpro)

a Vary with wastewater type.

l wastewater production.

this technology rather expensive. Future industrial applicationsof SCWO will be most probably restricted to effluents that con-tain highly concentrated and/or extremely refractory or hazardousorganic pollutants.

For wastewater containing low to medium organic concentra-tion as textile wastewaters, batch and continuous CWAO yieldedsatisfactory results in laboratory studies [32–34]. Relatively mildoperating conditions of temperature and pressure lead to substan-tially lower investment and operation cost.

For extremely refractory compounds more severe conditionsof temperature and pressure (SCWO) can be applied and otheradvantages of catalytic reaction in SCWO is the prevention of cokeformation on catalyst. The coke precursor, if it is formed on thecatalyst surface, can be carried out by SCWO because of the highmiscibility of organic compounds with SCWO. A general compar-ison of the typical conditions of thermal oxidation was made byKrajnc and Levec [35], and is given in Table 1.

Recently Ovejero et al. [36] proved there was a beneficial effectof noble metal catalysts on the CWAO of aniline wastewater withan aniline conversion greater than 70% on a 1% Pt/CNT catalyst. Ourstudies on the oxidation of aniline wastes and phenol in subcriticaloxidation showed that many low molecular weight carboxylic acidswere formed as major intermediate products. Similar results werealso reported in some studies involving the oxidation of phenol andsubstituted phenols in sub- or supercritical water [18,37]. Recently,Bambang et al. studied the oxidizing wastewater from LCD manu-facturing in supercritical water in the temperature 396–615 ◦C and25 MPa and found conversions greater than 99.99% within 10 s.

In this context, carbon materials as activated carbon, carbonnanotubes and carbon nanofibers (CNFs) are used as catalytic sup-

CWAO SCWO

130–250 370–57020–50 220–27010–60 1–1090–98 99–99.999CO2, H2O, N2, salts CO2, H2O, N2, salts20 (Nippon Shokubai) 60–250 (EWT)

rcritic

A. Rodrıguez et al. / J. of Supe

port [38]. They are grown by decomposition of carbon-containinggases on small metal particles. Reviews concerning CNF [39,40]describe the synthesis and properties of these materials in detail.The CNF are pure, mechanically strong, and mesoporous, whichmakes them a perfect support material in liquid phase reactions[41,42]. As the properties of activated carbons are difficult to control(poor reproducibility) and the microporous nature causes diffusionproblems, CNF could replace their use.

The surface of the as-synthesized CNF support is hydropho-bic and inert in nature, thus making the incorporation of metalchallenging. By treating the surface with an oxidising agent,oxygen-containing surface groups are introduced thus increasingthe hydrophilicity [43].

In this work, the main objective is to investigate the effectivenessof carbon nanofibers supported catalysts for liquid phase oxida-tion of washing textile wastewater. This is a previous study to

application in supercritical oxidation, establishment activity andselectivity of catalyst in CWAO of wastewater in several conditions.

2. Experimental

2.1. Materials and catalyst characterization

Carbon nanofibers have diameters in the range of 20–50 nmand lengths of 50–100 nm and were obtained from Grupo AntolinIngenierıa S.A, Spain.

XRD patterns were recorded by means of a difractometerSIEMENS D-501 with a Ni-filtered Cu K� radiation in order to deter-mine some structure data. Textural characterization of the supportsand catalysts were done by using N2 adsorption–desorption at 77 Kin a Micromeritics ASAP 2010 apparatus as described in a previ-ous work [18]. Prior to the physisorption measurements, the fiberswere evacuated at 300 ◦C. Thermogravimetric analysis (TGA) wasperformed with an EXTAR 6000 Seiko thermal analyzer at a heatingrate of 10 ◦C/min. The gas evolved during analysis was monitoredby a ThermoStar Pfeifer QMS 200 quadropole mass spectrometerby using a capillary probe situated directly above the sample cup.

Fig. 2. Schematic diagram

al Fluids 46 (2008) 163–172 165

The morphology of the materials was studied by analysis on a JEOLJEM 2010 electron microscopy at 200 kV (TEM) and metal loadingwas determined by mean of X-ray fluorescence (XRF).

2.2. Preparation of catalysts

The supported catalysts were prepared by excess wetimpregnation of the CNF support with copper or iron metalprecursors (Cu(NO3)2 × 3H2O and Fe(NO3)3 × 9H2O), supplied bySigma–Aldrich. Previously, the surface chemistry of the carbonnanofibers was modified by introducing carboxylic acid groups(–COOH) by means of an acidic treatment for 6 h under reflux. Themass of precursor was calculated in order to obtain 3 wt.% metal inthe catalysts. The wet solid was dried overnight at 110 ◦C and thenactivated under N2/H2 (3:1) flowing at 350 ◦C for 3 h in a furnace,in order to reduce the metal precursor to its corresponding ground

state.

2.3. Wastewater characterization

The washing textile industrial wastewater (WTW) was col-lected from an industrial plant in Salamanca (Spain). Thiswastewater was analyzed according to the Standard Methodsfor the Examination of Water and Wastewater [44], and afterthree analyses, the following average values were obtained forthe main parameters: pH 8.7 ± 0.2; the organic matter con-tent measured as TOC = 5.9 ± 0.1 g L−1 and COD = 17.9 ± 0.1 g L−1;BOD5 = 5.5 ± 0.1 g L−1; TSS = 23.9 ± 0.05 g L−1; TDS = 12.0 ± 0.05 gL−1; the ecotoxicity measured like EC50 after 15 min of expositionwas 2.8%. Textile wastewater can be classified as strong wastewa-ter. Strength is based on classification of domestic wastewater asoutlined by Metcalf and Eddy [45].

2.4. Experimental set-up and catalytic experiments

All experiments were conducted in a Hastelloy high-pressuremicroreactor C-276 autoclave Engineers (Fig. 2) with a volume of

of CWAO reactor.

rcritic

166 A. Rodrıguez et al. / J. of Supe

100 mL. The reactor (i.d. 50 mm) was equipped with an electricallyheated jacket, a turbine agitator and a variable speed magneticdrive. The temperature and the stirring speed were controlled bymeans of a PID controller. The gas inlet, gas release valve, cool-ing water feed line, pressure gauge and rupture disk were situated

on the top of the reaction vessel. The liquid sample line and thethermocouple well were immersed in the reaction mixture.

The reactor was first charged with 75 mL of the textile wastewa-ter and the catalyst and initially pressurize with nitrogen to ensureinert atmosphere. Afterwards, the system was heated to the desiredtemperature and a sample was withdrawn. This was consideredzero time for the reaction, and total organic carbon (TOC) conver-sion during this time can be neglected while calculating initial rates.Oxygen from the cylinder was then sparged into the liquid phaseand samples were withdrawn periodically after sufficient flushingof the sample line. Pressure drop was monitored and additionaloxygen was charged in order to maintain a constant total pressure.

Samples of the remaining reaction mixture were separated andanalyzed in terms of color, total organic carbon concentrationsand pH value. The color was detected by a UV-2401 PC, Shimadzuspectrophotometer at the wavelength range of 190–700 nm anda Rosemount-Dohrmann DC-190 TOC analyzer was used to mea-sure the TOC concentration. Additionally, toxicity and the presenceof some organic compounds in raw and treated wastewater weredetermined by means of a Microtox analyzer (model 500) and aGC–MS chromatograph (Agilent 5973 N MSD), respectively.

Fig. 3. TEM micrographs of the 3 wt.% Cu/CNF fre

al Fluids 46 (2008) 163–172

3. Results and discussion

3.1. Characterization of catalyst

3.1.1. TEM and XRD techniques

The morphology of prepared Cu/CNF catalyst was investigated

using TEM and the corresponding micrographs were shown in Fig. 3.TEM image of Cu/CNF catalyst in Fig. 3a and b shows that the diam-eter of CNFs is between 20 and 50 nm. A great number of copperparticles, obtained with mean size around 8 nm, are dispersed uni-formly on the surface of carbon nanofibers (but larger particles upto 30 nm have been found as well). TEM images revealed that theCu particles are spherical, without aggregation, highly dispersedand in intimate contact with the CNF surface (Fig. 3c). The sur-face groups on the CNF surface such as –COOH, –OH, C O whichare introduced by oxidation are important to obtain the interactionrequired for good distribution of the metal-precursor [43] on theCNF surface. Oxidation of the carbon nanofibers in nitric acid gavea larger deposition of Cu.

The interesting three-dimensional structure of Cu/CNFs andhighly dispersed copper particles may result in a large effectiveCu surface area and good catalytic properties.

Fig. 4 is the XRD of Cu/CNF. Characteristic peaks (0 0 2) and(1 0 0), of the graphite structure are visible. Cu/CNF shows charac-teristic peaks at around 43.5◦, 50.5◦ and 74.5◦, which are attributedto (1 1 1), (2 0 0) and (2 2 0) crystalline plane of Cu with face-

sh for the same area (a–c) and reused (d).

A. Rodrıguez et al. / J. of Supercritical Fluids 46 (2008) 163–172 167

Fig. 4. XRD diffractogram of Cu/CNF. (�) Cu; (�) CNF.

centered cubic (fcc) structure. The average size of the Cu particles is11.7 nm, calculated from (1 1 1) peak by the Scherrer formula [36],which is in good agreement with the result from the TEM image.

3.1.2. BET and TG techniquesIt has been well established that the porous texture and the

specific surface area of support have important effects on thefinal properties of the supported catalysts [46,47]. With this inmind, we first examined the porous texture and the specific sur-face area of the CNF support and Cu/CNF catalyst by carrying outphysical adsorption of N2 at 77 K. Fig. 5 shows the N2 adsorptionisotherms for the support and catalyst. It can be seen that the twoisotherms have the same shape in the whole range of relative pres-sure. It indicated that a sharp increase in N2 adsorption appearedbelow P/Po = 0.05 and suggested that micropores existed in CNF andCu/CNF catalyst [48]. The adsorption curve of the sample underrelative pressures of 0.05–0.8, slowly increased during adsorptionprocess due to be mesopores structure. The adsorption and desorp-tion curves completely overlap in the P/Po range of 0.05–0.4, andan adsorption hysteresis loop is observed that indicates a strongcapillary condensation in the P/Po range of 0.4–0.99.

Carbon nanofibers generally exhibit a high specific surface areadue to their high external surface area (high aspect ratio material).

Fig. 5. Nitrogen adsorption isotherms for CNF and Cu/CNF and differential pore sizedistribution for the Cu/CNF catalyst (inset).

Fig. 6. Weight loss of Cu/CNF catalyst with temperature.

The high external surface area leads to a significant increase in thesurface contact between the gaseous or liquid reactants and theactive phase supported on this nanostructured host which is a pre-requisite for its use as catalyst support, especially in liquid phasemedium where diffusion rate is predominant [42]. The specific sur-face area of the CNF and Cu/CNF are 244 and 188 m2/g, respectively.The BET surface area values of Cu/CNF are found to be less comparedto that of CNF support. This may be due to the blocking of CNF isby the Cu added. Fig. 5 (inset) shows the differential pore size dis-tribution (PSD) for Cu/CNF catalyst. There are two peaks at around3–4 nm and 26–27 nm. A t-plot analysis of Cu/CNF revealed a lowmicropore volume (0.003 cm3 g−1), which can be attributed to sur-face roughness. According to the nitrogen adsorption data, Cu/CNFis a mesoporous material with polydisperse pore distribution.

The thermal stability of the Cu/CNF was studied by TGA in airand the typical TGA curve is shown in Fig. 6. From this figure, it isfound that the initial burning temperature was at around 200 ◦Cand this occurred mainly due to the presence of amorphous carbonin the CNF mixture [48]. It is interesting to note that the CNF wasstable up to 450 ◦C and then the decomposition starts and reachesmaximum at 650 ◦C. At 680 ◦C, practically almost complete weightloss is noticed.

3.2. Catalytic experiments

3.2.1. Influence of stirring speedExperiments performed without catalyst at 140 ◦C and oxygen

partial pressure of 8.3 bar under nitrogen gas showed no measur-able TOC conversion (∼0.5% of initial TOC) indicating absence ornegligible thermal degradation of TOC in washing textile wastewa-ter. Low TOC conversion was obtained (7.5% and 21.5%, respectively,see Table 2), when oxygen gas was fed to the microreactor with andwithout CNF support.

The diffusional process in mass transfer depends on turbulenceintensity in liquid phase. For a given reactor geometry and typeimpeller, the rotational speed (rpm) decides the intensity of turbu-lence. In order to study the effect of agitation, experiments wereconducted at 140 ◦C and 8.7 bar (E-Cu, E-5 and E-6, see Table 2)at different impeller speed in the range of 500–1300 rpm. It wasobserved that TOC removal increased with an increasing stirringspeed from 500 to 1000 rpm. Above this value the TOC removalwas constant and independent of stirring speed. All experimentswere, therefore, conducted at 1000 rpm, in order to eliminate inter-particle mass transfer resistance.

rcritic

S (rp

1000 6.6 7.5 36.7 8.41000 6.6 21.5 45.6 7.61000 6.6 74.1 97.0 7.11000 6.6 44.5 65.4 7.41000 6.6 71.5 90.4 7.21000 6.6 72.2 94.2 7.11000 6.6 78.6 99.8 6.81000 13.3 76.4 96.4 7.21000 20.0 81.2 96.7 7.4

500 6.6 71.5 97.5 7.61300 6.6 74.8 95.8 7.31000 6.6 72.3 91.4 7.71000 6.6 37.7 84.1 7.4

ciency are very close to 97.0% and 74.1% in 180 min, respectively,which shows that Cu/CNF catalyst exhibits an excellent catalyticactivity.

It is often assumed that wet air oxidation of organic com-pounds, such as polyethylene glycol, oxalic acid, alcohol and acetic

168 A. Rodrıguez et al. / J. of Supe

Table 2Catalytic wet air oxidation of WTW

Experiment Catalyst T (◦C) PO2 (bar)

E-WAO No catalyst 140 8.7E-support Support 140 8.7E-Cu Cu/CNF 140 8.7E-Fe Fe/CNF 140 8.7E-Cu/AC Cu/AC 140 8.7E-1 Cu/CNF 120 8.7E-2 Cu/CNF 160 8.7E-3 Cu/CNF 140 8.7E-4 Cu/CNF 140 8.7E-5 Cu/CNF 140 8.7E-6 Cu/CNF 140 8.7E-7 Cu/CNF 140 6.3E-8 Cu/CNFc 140 8.7

a Accuracy of TOC is ±3.6.b Accuracy of color is ±1.8.c Reused catalyst.

3.2.2. Influence of temperature and partial pressureThe application of high pressures to industrial processes has

led to engineering operations that frequently require knowledgeof some thermodynamic properties, like solubilities of gases in liq-uids at pressures higher than those for which such data is ordinarilyavailable. Enhanced solubility of oxygen in aqueous solutions at ele-vated temperatures and pressures provides a strong driving forcefor oxidation. The elevated pressures are required to keep water inthe liquid state.

Organic compounds and gases become soluble in the reactionmedium and inorganic compounds, like salts, become insoluble[49–51]. Water behaves like a dense gas in the supercritical statebecause of increased diffusivity, decreased viscosity and dielectricconstant; at the same time, it is like a liquid in its density andsolvent properties [52–55]. Furthermore, these properties changewith temperature and pressure. When oxidation takes place belowthe critical temperature and pressure, the technique is sometimesreferred to as wet air oxidation. Again, the basic idea is enhancedcontact between molecular oxygen and the organic matter to beoxidized. Thus, the solubility of oxygen characterises the oxy-gen contribution to the kinetic expression rather than the oxygenpartial pressure. Furthermore, the oxygen solubility is not only afunction of pressure but also of temperature. Therefore, the oxygenmole fraction in the liquid phase was considered to be more rep-resentative. This mole fraction was calculated using the Henry law

[56]. In this sense, the concentration of oxygen in the liquid phaseat 120 and 160 ◦C (PO2 = 8.7 bar) is 6.08 and 1.69 × 10−5 mol frac-tion, respectively. In order to study the effect of temperature someexperiments were conducted in the temperature range from 120to 160 ◦C at 1000 rpm and 8.7 bar (E-Cu, E-1 and E-2). The catalystloading was 3 wt.%.

It was observed that temperature has a pronounced effect onTOC and color removal. Fig. 7a shows the evolution of TOC removalversus reaction time at three temperatures. The initial rate obtainedfrom these curves for TOC removal at 120, 140 and 160 ◦C was 2.8, 7.8and 12.5 gTOC h−1 gcat

−1, respectively. As a conclusion, higher tem-peratures are favorable for the CWAO of WTW (Fig. 7b). In the nextexperiments, we considered 140 ◦C as for the study of the CWAO ofWTW over Cu/CNF catalysts.

The pH of the solution decreases upon the CWAO reaction. As aresult, smaller (and stronger) acids are generated and lower pH ismeasured at the end of the reaction.

In the other hand, the essays, using 3 wt.% Cu/CNF at 140 ◦C,shows that the effect of the partial pressure is negligible in thusranges (E-6 and E-7, see Table 2). In this way, with 8.7 bar oxygenpartial pressure and 140 ◦C, decolorization and TOC removal effi-

al Fluids 46 (2008) 163–172

m) CW (g L−1) XaTOC (%) Xb

color (%) pHf

Fig. 7. (a) TOC removal of WTW at various temperatures and (b) color removalefficiency. PO2 = 8.7 bar; S = 1000 rpm; Cu/CNF.

rcritic

[65]. This functionalization changes the reactivity of nanotubes and

A. Rodrıguez et al. / J. of Supe

acid, cyclohexane and cyclohexanone, and phenol [57–59], occursby means of a free-radical chain reaction. In the above CWAOprocess, •OH can be generated by a free-radical chain oxidationprocess [60] and can act to attack organic molecules present in tex-tile wastewater. In this aspect, as reported by Sadana and Katzer[61], likewise during noncatalytic WAO, the initial rate of oxida-tion exhibits an induction period, which is followed by steadystate regime. The free-radical initiation preceded by the propa-gation step was also found by Akyurtlu et al. [62,63]. A markeddependence of the induction period length on the initial additionof a free-radical inhibitor concentration essentially indicates theinvolvement of free radicals in the CWAO reaction. Another evi-dence of free-radicals participation is the dependence of rate on pH.Typically free-radical reactions in aqueous media are pH depen-dent and they typically show a maximum with pH [61]. Also, itwas observed that the induction period depends on the tempera-ture and is inversely proportional to the partial pressure of oxygen[61].

3.2.3. Influence of catalyst loadingThe influence of the catalyst loading on the TOC and color

removal was studied at standard conditions (E-Cu, E-3 and E-4)

with 6.6, 13.3 and 20.0 g L−1 of catalyst. The lower TOC removalrates may be explained by the production of small organic molecu-lar fragments along with the destruction of the organic fragmentsthat are not completely mineralized under the prevailing oxida-tion conditions. By further increasing the amount of catalyst to20 g L−1, an increase in the color and TOC removal was observed(Fig. 8). We select the concentration of 6.6 g L−1 catalyst for ourfurther experiments.

3.2.4. Reuse of catalystThe used catalysts were recovered and washed by stirring in

water for 24 h. A new reaction in a fresh volume of wastewa-ter was then carried out with the used catalyst at 140 ◦C and8.7 bar. The loss of activity observed (Table 1) was attributedto the loss of the active metal (Fig. 3d). Also, XRF measure-ment demonstrates that copper catalyst load supported on carbonnanofibers decreases after first reaction (43%). The loss of Cufrom Cu/CNF is significant when compared with platinum sup-ported on carbon nanotubes [36] and is suggestive of weakermetal/support interactions in the case of Cu supported on carbonnanofibers.

Fig. 8. TOC removal of WTW at various catalyst loads. PO2 = 8.7 bar; S = 1000 rpm;T = 140 ◦C; Cu/CNF.

al Fluids 46 (2008) 163–172 169

3.2.5. Influence of metal and supportFor comparison purposes, Cu/CNF, Fe/CNF and Cu/AC (E-Cu, E-

Fe and E-Cu/AC) were prepared under similar conditions. In thisway, Cu supported in carbon nanofibers was more active in TOCand color removal than Fe/CNF and copper supported on activecarbon. The efficiency of TOC removal decreased in the orderCu/CNF > Cu/AC > Fe/CNF.

Highly functionalized nanotubes containing acid groups of den-sity greater than 1021 sites per gram of nanotubes have beenprepared by chemical oxidation techniques [43] and it has beendemonstrated that carbon nanofibers and nanotubes decoratedwith certain metals can show higher selectivity in heterogeneouscatalysis compared with the same metals attached to other carbonsubstrates [64].

Generally, it is believed that the chemical modificationof nanotubes or nanofibers starts from the defect sites, i.e.,heptagon–pentagon pairs that are under heavy strain, while othernon-sp2 defect sites on the nanotubes, such as –CH and –CH2are attacked next. In the presence of strong oxidizing agents, theintegrated graphene structure is also attacked to create additionaldefect sites that can subsequently react with the oxidizing agent

modifies their wetting characteristics. For instance, nucleation ofmetals and attachment of metal compounds using these surfaceoxides can lead to the decoration of nanotube surfaces.

The oxidation of carbon materials (carbon nanotubes andnanofibers) has been reported by a number of researchers. Theresults indicate that oxidizing agents creates an oxidized tube thatis free from amorphous carbon. As example, the effect of chemicaloxidation on the SWNT structure by various oxidizing agents hasalso been reported by Zheng et al. [65]. Also, dilute nitric acid hasbeen shown to generate carboxylic acid groups on the defect sitesalready present on the carbon nanofibers and carbon nanotubes,while a mixture of concentrated sulphuric and nitric acid generatescarboxylic acid groups on initial and newly created defect sites.

3.2.6. Evolution of degradation process3.2.6.1. UV–vis spectral changes. The initial UV–vis spectra of tex-tile wastewater shows one major absorbance peak at 290 nm as itcan be seen in Fig. 9. This absorbance peak becomes weaker andweaker in intensity as the treatment time increases. When thecleavage of the –C C– and –N N– bonds and the aromatic rings

Fig. 9. Spectral evolution of WTW, with CNF, CWAO and WAO process. T = 140 ◦C;PO2 = 8.7 bar; S = 1000 rpm.

170 A. Rodrıguez et al. / J. of Supercritical Fluids 46 (2008) 163–172

Fig. 10. GC–MS of washing textile wastewater (a) without treatment and (b) CWAOat T = 140 ◦C; PO2 = 8.7 bar; S = 1000 rpm, Cu/CNF.

occurs, the intensity of the absorbance peak of the wastewaterdecreases [59]. Decolorization efficiency is close to 99.8% at 160 ◦Cand 8.7 bar.

3.2.6.2. Intermediates formed during CWAO of WTW. The interme-diate compounds formed during the washing textile wastewater

degradation were identified by GC/MS (Fig. 10). The main inter-mediates detected in raw wastewater were decanoic acid, methylester and 1,2-benzenedicarboxylic acid, and have been eliminatedin presence of Cu/CNF catalyst. Samples at different time werecollected during the CWAO. At a reaction time of 180 min aceticacid hydroxy-methyl ester, pentadecanoic acid methyl ester, hex-anedioic acid dimethyl ester and 2-piperidinone can be observedamong others. In the CWAO of wastewaters, polymeric compoundscan be formed and promoted by acidic sites on the catalyst [21].These polymeric compounds can be adsorbed on the surface ofthe catalyst, decreasing its activity. Several studies concerning theoxidation of lower carboxylic acids such as acetic acid, propi-onic acid and valeric (pentanoic) acid have also shown that suchcompounds are very resistant to total oxidation and that their oxi-dation to carbon dioxide is usually the rate limiting step for TOCremoval.

3.2.6.3. Toxicity evaluation. Frequently, the products obtained dur-ing treatment show higher toxicity than the precursor compounds[66]. As the total organic content was not completely removed,the capacity of the process was finally evaluated by determining

Fig. 11. Inhibition curves after 15 min incubation by the bacterium Vibrio fisheri ofraw wastewater (inset), and treated at 140 ◦C. PO2 = 8.7 bar and S = 1000 rpm.

the acute toxicity. Toxicity assays based on bioluminescence inVibrio fisheri can provide a rapid assessment of chemical toxicity.Thousands of publications have reported favorably results obtainedfrom bioluminescence assays [67]. In this study, the acute toxicityendpoint was determined for 5, 15 and 30 min as the effective con-centration (EC50) of a chemical that causes a 50% of reduction inthe bioluminescence of the bacteria. In this sense, Fig. 11 shows anexample for the raw wastewater and the treated at 140 ◦C. The inhi-bition values for WTW were 2.7 for raw wastewater, 1.6% and 1.2%for treated at 140 and 160 ◦C, respectively. EC50 values decreasein the order: raw wastewater > Cu/CNF (140 ◦C) > Cu/CNF (160 ◦C).Based on the 15 min percentage inhibition values, the sample after180 min of treatment with the 1.2% of inhibition indicates the dis-appearance of the toxic compounds formed in the early stagesof reaction and can be assigned as sample with the lowest toxiccapacity. This toxic assay can be used in the regulations as a toolexpressed as toxicity units to indicate if wastewater treatment wasefficient or not, or if toxic discharges from industry have increased[68].

4. Conclusions

Carbon nanofibers can be efficiently used as metal support forcatalytic wet air oxidation of washing textile wastewater. Cu par-ticles were distributed on tube walls uniformly. Increasing thereaction temperature can enhance the activity of the copper cat-alyst. More than 97% color reduction and 81.2% TOC removal canbe obtained by properly choosing the reaction conditions after180 min of reaction. Catalyst load, oxygen partial pressure andagitation speed produced a positive effect in the TOC and colorremoval efficiency. Copper catalyst was more active than iron sup-ported on carbon nanofibers. The toxicity has been evaluated andthe inhibition values decrease with the temperature. Catalytic wetair oxidation would thus provide a cost effective environmen-tally attractive option to manage the growing organic sludge andtoxic wastewater treatment problems. Applications of catalyticabatement to real wastewater require highly active, nonselectivecatalysts, capable of long-life operation in hot water, without struc-tural and/or performance degradation. CWAO becomes especiallyattractive when coupled with a biological or physical-chemicaltreatment avoiding the need for a complete mineralization of theorganic pollutants often difficult to achieve under CWAO reactionconditions.

rcritic

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

A. Rodrıguez et al. / J. of Supe

Acknowledgements

The authors gratefully acknowledge the financial support fromMinisterio de Educacion y Ciencia (Grant: CTQ2004/05141), byCONSOLIDER Program through TRAGUA Network CSD2006-44,and by Comunidad de Madrid through REMTAVARES Network S-505/AMB/0395.

Appendix A

BOD5 biochemical oxygen demand (g L−1)CNF raw carbon nanofibersCOD chemical oxygen demand (g L−1)CWAO catalytic wet oxidationCW catalyst load (g L−1)EC50 molar concentration of a compound where 50% of its max-

imal effect is observed (%)PSD pore size distribution (nm)S stirrer rate (rpm)TDS total dissolved solids (g L−1)TOC total organic carbon (g L−1)TSS total suspended solids (g L−1)X conversion (%)WAO wet air oxidationWTW washing textile wastewater

References

[1] T. Robinson, G. McMullan, R. Marchant, P. Nigam, Remediation of dyes in textileeffluents: a critical review on current treatment technologies with a proposedalternative, Bioresour. Technol. 77 (2001) 247–255.

[2] I. Arslan, Treatability of a simulated disperses dye-bath by ferrous iron coagu-lation, ozonation, and ferrous iron-catalyzed ozonation, J. Hazard. Mater. B85(2001) 229–241.

[3] F.J. Beltran, J.F. Garcia-Araya, J. Frades, P. Alvarez, O. Gimeno, Effects of sin-gle and combined ozonation with hydrogen peroxide or UV radiation on thechemical degradation and biodegradability of debittering table olive industrialwastewater, Water Res. 33 (3) (1999) 723–732.

[4] F.J. Benitez, J.L. Acero, F.J. Real, J. Garcia, Kinetics of photodegradation and ozona-tion of pentachlorophenol, Chemosphere 51 (2003) 651–662.

[5] E. Chamarro, A. Marco, S. Esplugas, Use of fenton reagent to improve organicchemical biodegradability, Water Res. 35 (4) (2001) 1047–1051.

[6] C. Dominguez, J. Garcia, M.A. Pedraz, A. Torres, M.A. Galan, Photocat-alytic oxidation of organic pollutants in water, Catal. Today 40 (1998) 85–101.

[7] J.P. Chen, M. Lin, Equilibrium and kinetics of metal ion adsorption onto a com-

mercial H-type granular activated carbon: experimental and modeling studies,Water Res. 35 (2001) 2385–2394.

[8] N.A. Lina, I.J. Ahmad, Biological treatment of textile wastewater using sequenc-ing batch reactor technology, Environ. Model Assess. 11 (2006) 333–343.

[9] J. Sarasa, M.P. Roche, M.P. Ormad, E. Gimeno, A. Puig, J.L. Ovelleiro, Treatmentof a wastewater resulting from dyes manufacturing with ozone and chemicalcoagulation, Water Res. 32 (9) (1998) 2721–2727.

[10] L. Szyrkowicz, C. Juzzolino, S.N.A. Kaul, Comparative study on oxidation of dis-perse dyes by electrochemical process, ozone, hypochlorite and fenton reagent,Water Res. 35 (2001) 2129–2136.

[11] J. Fraga-Dubreuil, J. Garcia-Serna, E. Garcia-Verdugo, L.M. Dudd, R. Graham, W.Aird, B. Thomas, M. Poliakoff, The catalytic oxidation of benzoic acid to phenolin high temperature water, J. Supercrit. Fluids 39 (2006) 220–227.

12] T.-J. Park, J.S. Lim, Y.-W. Lee, S.-H. Kim, Catalytic supercritical water oxidation ofwastewater from terephthalic acid manufacturing process, J. Supercrit. Fluids26 (2003) 201–213.

13] O.O. Sogut, M. Akgun, Treatment of textile wastewater by SCWO in a tubereactor, J. Supercrit. Fluids 43 (2007) 106–111.

[14] V. Bambang, T.-J. Park, J.-S. Lim, Y.-W. Lee, Supercritical water oxidationof wastewater from LCD manufacturing process: kinetic and formation ofchromium oxide nanoparticles, J. Supercrit. Fluids 34 (2005) 51–61.

[15] P.A. Webley, H.R. Holgate, D.M. Stevenson, J.W. Tester, Oxidation kinetics ofmodel compounds of metabolic waste in supercritical water, in: SAE TechnicalPaper Series #901333, 1990, p. 928.

[16] T.D. Thornton, P.E. Savage, Phenol oxidation in supercritical water, J. Supercrit.Fluids 3 (1990) 240–248.

[17] P.E. Savage, M.A. Smith, Kinetics of acetic acid oxidation in supercritical water,Environ. Sci. Technol. 9 (1995) 216–221.

[

[

[

[

[

[

[

[

[

al Fluids 46 (2008) 163–172 171

[18] G. Ovejero, J.L. Sotelo, J. Garcia, A. Rodrıguez, Catalytic removal of phenol fromaqueous solutions in a trickle bed reactor, J. Chem. Technol. Biotechnol. 80(2005) 406–412.

[19] A. Pintar, J. Levec, Catalytic oxidation of organics in aqueous solutions. I. Kineticsof phenol oxidation, J. Catal. 135 (1992) 345–357.

20] K. Belkacemi, F. Larachi, S. Hamoudi, A. Safari, Catalytic wet oxidation of high-strength alcohol-distillery liquors, Appl. Catal. A 199 (2000) 199–209.

21] H.T. Gomes, J.L. Figueiredo, J.L. Faria, P. Serp, P. Kalck, Carbon-supported iridiumcatalysts in the catalytic wet air oxidation of carboxylic acids: kinetics andmechanistic interpretation, J. Mol. Catal. A 182–183 (2002) 47–60.

22] A. Pintar, J. Levec, Catalytic liquid-phase oxidation of phenol aqueous solutions.A kinetic investigation, Ind. Eng. Chem. Res. 33 (1994) 3070–3077.

23] P.M. Alvarez, D. McLurgh, P. Plucinski, Copper oxide mounted on activatedcarbon as catalyst for wet air oxidation of aqueous phenol. 1. Kinetic and mech-anistic approaches, Ind. Eng. Chem. Res. 41 (9) (2002) 2153–2158.

24] A. Fortuny, C. Bengoa, J. Font, F. Castells, A. Fabregat, Water pollution abatementby catalytic wet air oxidation in a trickle bed reactor, Catal. Today 53 (1999)107–114.

25] H.R. Devlin, I.J. Harris, Mechanism of the oxidation of aqueous phenol withdissolved oxygen, Ind. Eng. Chem. Fundam. 23 (4) (1984) 387–392.

26] D. Duprez, F. Delanoe, J. Barbier jr., P. Isnard, G. Blanchard, Catalytic oxidationof organic compounds in aqueous media, Catal. Today 29 (1996) 221–317.

27] H. Ohta, S. Goto, H. Teshima, Liquid-phase oxidation of phenol in a rotatingcatalytic basket reactor, Ind. Eng. Chem. Fundam. 19 (1980) 180–185.

28] G. Deiber, J.N. Foussard, H. Debellefontaine, Removal of nitrogeneous com-pounds by catalytic wet air oxidation, Environ. Pollut. 96 (3) (1997) 311–319.

29] L. Oliviero, J. Barbier Jr., D. Duprez, Wet air oxidation of nitrogen-containingorganic compounds and ammonia in aqueous media, Appl. Catal. B 40 (2003)163–184.

30] E.E. Gloyna, L. Li, Supercritical water oxidation research and developmentupdate, Environ. Prog. 14 (1995) 182–192.

31] P. Kritzer, E. Dinjus, An assessment of supercritical water oxidation (SCWO):existing problems, possible solutions and new reactor concepts, Chem. Eng. J.83 (2001) 207.

32] V.S. Mishra, V.V. Mahajani, J.B. Joshi, Wet air oxidation, Ind. Eng. Chem. Res. 34(1995) 2.

33] S. Imamura, Catalytic and noncatalytic wet oxidation, Ind. Eng. Chem. Res. 38(1999) 1743.

34] A. Pintar, Catalytic processes for the purification of drinking water and indus-trial effluents, Catal. Today 77 (2003) 451.

35] M. Krajnc, J. Levec, The role of catalyst in supercritical water oxidation of aceticacid, Appl. Catal. B: Environ. 13 (2) (1997) 93–103.

36] G. Ovejero, J.L. Sotelo, M.D. Romero, A. Rodrıguez, M.A. Ocana, G. Rodrıguez, J.Garcia, Multiwalled carbon nanotubes for liquid-phase oxidation. Functional-ization, characterization, and catalytic activity, Ind. Eng. Chem. Res. 45 (2006)2206–2212.

37] J.L. DiNaro, J.W. Tester, J.B. Howard, K.C. Swallow, Experimental measurementsof benzene oxidation in supercritical water, AICHE J. 46 (2000) 2274–2284.

38] F. Stuber, J. Font, A. Fortuny, C. Bengoa, A. Eftaxias, A. Fabregat, Carbon materialsand catalytic wet air oxidation of organic pollutants in wastewater, Top. Catal.33 (1–4) (2005) 3–50.

39] P. Serp, M. Corrias, P. Kalck, Carbon nanotubes and nanofibers in catalysis, Appl.Catal. A: Gen. 253 (2003) 337–358.

40] K.P. De Jong, J.W. Geus, Carbon nanofibers: catalytic synthesis and applications,Catal. Rev. Sci. Eng. 42 (2000) 481–510.

41] T.G. Ros, D.E. Keller, A.J. Van Dillen, J.W. Geus, D.C. Koningsberger, Preparationactivity of small rhodium metal particles on fishbone carbon nanofibres, J. Catal.

211 (2002) 85–102.

42] M.L. Toebes, J.M.P. Van Heeswijk, J.H. Bitter, A.J. Van Dillen, K.P. De Jong, Theinfluence of oxidation on the texture and the number of oxygen-containingsurface groups of carbon nanofibers, Carbon 42 (2004) 307–315.

43] J. Garcia, H.T. Gomes, P. Serp, P. Kalck, J.L. Figueiredo, J.L. Faria, Carbon nanotubessupported ruthenium catalysts for the treatment of high strength wastewaterwith aniline using wet air oxidation, Carbon 44 (2006) 2384–2391.

44] American Public Health Association/American Water Works Association/WaterEnvironment Federation, Standard Methods for the Examination of Water andWastewater, 21st ed., American Public Health Association/American WaterWorks Association/Water Environment Federation, Washington, DC, USA,2005.

45] Metcalf Eddy Inc., Wastewater Engineering: Treatment, Disposal and Reuse, 3rded., McGraw-Hill, New York, USA, 1985.

46] Z. Li, Z. Pan, S. Dai, Nitrogen adsorption characterization of aligned multi-walled carbon nanotubes and their acid modification, J. Colloid Interface Sci.277 (2004) 35–42.

47] M. Jaroniec, K. Kaneko, Physicochemical foundations for characterization ofadsorbents by using high-resolution comparative plots, Langmuir 13 (1997)6589–6596.

48] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press,London, 1982.

49] J.F. Connolly, Solubility of hydrocarbons in water near the critical solution tem-peratures, J. Chem. Eng. Data 11 (1) (1966) 13–16.

50] M.L. Japas, E.U. Franck, High pressure phase equilibria and PVT-data of thewater–oxygen system including water–air to 673 K and 250 MPa, Ber. Bunsen-Ges. Phys. Chem. 89 (1985) 1268–1275.

[

[

[

[

[

[

[

[

172 A. Rodrıguez et al. / J. of Supercritic

[51] P.A. Webley, J.W. Tester, H.R. Holgate, Oxidation kinetics of ammonia andammonia–methanol mixtures in supercritical water in the temperature range530–700 ◦C at 246 bar, Ind. Eng. Chem. Res. 30 (8) (1991) 1745–1754.

52] E.U. Franck, Water and aqueous solutions at high pressures and temperatures,Pure Appl. Chem. 24 (1970) 13–30.

53] K.H. Dudziak, E.U. Franck, Messungen der viskositat des wassers bis 560 ◦C und3500 bar, Ber. Bunsen-Ges. Phys. Chem. 70 (1966) 1120–1128.

54] A.G. Kalinichev, J.D. Bass, Hydrogen bonding in supercritical water. 2. Computersimulations, J. Phys. Chem. A 101 (1997) 9720–9727.

55] M. Uematsu, E.U. Franck, Static dielectric constant of water and steam, J. Phys.Chem. Ref. Data 9 (1960) 1291–1306.

56] D.R. Lide, Handbook of Chemistry and Physics, 73rd ed., CRC, Boca Raton Florida,1992.

[57] Y.I. Matatov-Meytal, M. Sheintuch, Catalytic abatement of water pollutants, Ind.Eng. Chem. Res. 37 (1998) 309–326.

58] J. Levec, A. Pintar, Catalytic oxidation of aqueous solutions of organics. An effec-tive method for removal of toxic pollutants from wastewaters, Catal. Today 24(1995) 51–58.

59] J. Garcia, H.T. Gomes, P. Serp, P. Kalck, J.L. Figueiredo, J.L. Faria, Platinum cata-lysts supported on MWNT for catalytic wet air oxidation of nitrogen containingcompounds, Catal. Today 102–103 (2005) 101–109.

60] F. Luck, Wet air oxidation: past, present and future, Catal. Today 53 (1999) 81–91.

[

[

[

[

[

[

[

al Fluids 46 (2008) 163–172

[61] A. Sadana, J.R. Katzer, Involvement of free radicals in the aqueous phase catalyticoxidation of phenol over copper oxide, J. Catal. 35 (1974) 140–152.

62] J.F. Akyurtlu, A. Akyurtlu, S. Kovenklioglu, Catalytic oxidation of phenol in aque-ous solutions, Catal. Today 40 (4) (1998) 343–352.

63] J. Levec, A. Pintar, Catalytic wet-air oxidation processes: a review, Catal. Today124 (3/4) (2007) 172–184.

64] B.C. Satishkumar, A. Govindaraj, J. Mofokeng, G.N. Subbanna, C.N.R. Rao, Novelexperiments with carbon nanotubes: opening, filling, closing and functional-izing nanotubes, J. Phys. B 29 (21) (1996) 4925–4934.

65] B. Zheng, Y. Li, J. Liu, CVD synthesis and purification of single-walled carbonnanotubes on aerogel supported catalyst, Appl. Phys. Lett. 74 (2002) 345–348.

66] M.D. Hernando, O. Malato, M. Farre, A.R. Fernandez-Alba, D. Barcelo, Appli-cation of ring study: water toxicity determinations by bioluminescence assaywith Vibrio fischeri, Talanta 69 (2006) 370–376.

67] A.R. Fernandez-Alba, M.D. Hernando, L. Piedra, Y. Chisti, Toxicity evaluation ofsingle and mixed antifouling biocides measured with acute toxicity bioassays,Anal. Chim. Acta 456 (2002) 303–312.

68] M. Farre, E. Martinez, M.D. Hernando, A.R. Fernandez-Alba, J. Fritz, E. Unruh, O.Mihail, V. Sakkas, A. Morbey, T. Albanis, D. Barcelo, European ring exercise onwater toxicity using different bioluminescence inhibition tests based on Vibriofischeri in support to the implementation of the Water Framework Directive,Talanta 69 (2006) 323–333.