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Applied Catalysis B: Environmental 103 (2011) 246–252

Contents lists available at ScienceDirect

Applied Catalysis B: Environmental

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

ptimized water treatment by combining catalytic Fenton reaction usingiamond supported gold and biological degradation

oberto Martín, Sergio Navalon, Mercedes Alvaro, Hermenegildo Garcia ∗

nstituto Universitario de Tecnología Química CSIC-UPV and Departamento de Química, Universidad Politécnica de Valencia, Av. De los Naranjos, s/n, 46022, Valencia, Spain

r t i c l e i n f o

rticle history:eceived 14 December 2010eceived in revised form 13 January 2011ccepted 21 January 2011vailable online 27 January 2011

eywords:

a b s t r a c t

Recently it has been reported that gold nanoparticles supported on Fenton treated diamond nanopar-ticles (Au/DNP) is a highly efficient catalyst to promote the generation of hydroxyl radicals from H2O2

[S. Navalon, R. Martin, M. Alvaro, H. Garcia, Angew. Chem. Int. Ed. 49 (2010) 8403–8407]. In the presentwork we have optimized a series of experimental parameters including initial pH, reaction tempera-ture, H2O2 concentration, phenol to gold mol ratio and oxygen pressure to achieve biodegradability ofaqueous phenol solutions with the minimum H2O2 concentration and attain a complete lack of toxicity

old nanoparticles supported on diamondanoparticleseterogeneous Fenton reactionaste water biodegradability

ibrio fischeri toxicityhenol degradation

(determined by the Vibrio fischeri bioluminescence assay). The results presented show how to combinemild Fenton degradation of bioreluctant phenol with consecutive biological treatment in such a way thatthe amount of H2O2 is kept to the minimum value. It was determined that the best conditions are pH4, 50 ◦C, H2O2 to phenol molar ratio 4 and 320 mg L−1 (1 wt% Au) catalyst under oxygen atmosphere.Au/DNP exhibits a remarkable stability under these conditions and can be used up to four times without

talyt.

observing any loose of caand H2O2 decomposition

. Introduction

The Fenton reaction consists in the generation of highly aggres-ive hydroxyl radicals (HO•) by chemical reduction of H2O2 withe2+ salts or other suitable transition metal ion [1,2]. The Fentoneaction finds wide use in water treatment for the degradationf organic pollutants [3–5]. However, its main limitation is theequirement of stoichiometric amounts of a reducing transitionetal, typically Fe2+ salts. At the end of the reaction the transi-

ion metal precipitates as the corresponding oxide generating aonsiderable amount of sludges that have to be decanted fromhe treated water and properly disposed. One important break-hrough in this area would be the development of a catalyst thatould effect the generation of HO• radicals without the need of con-umption of transition metal [6–9]. In this regard, we have recentlyeported that gold nanoparticles supported on Fenton-treated dia-ond nanoparticles (Au/DNP) is an extremely efficient catalyst for

he Fenton degradation of phenol at pH 4 [10]. Based on the analysisf the primary products it has been estimated that over 79% of the

onsumed H2O2 is converted into HO• radicals. The catalyst alsonjoys a high stability with a minimum turn over number (TON) of21,000 molecules of phenol degraded per gold atom; this corre-ponds to the degradation of 75 g L−1 of phenol by only 0.5 mg L−1

∗ Corresponding author. Tel.: +34 96 387 7807; fax: +34 96 387 78 09.E-mail address: hgarcia@qim.upv.es (H. Garcia).

926-3373/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.apcatb.2011.01.035

ic activity as determined by the temporal profiles of phenol degradation

© 2011 Elsevier B.V. All rights reserved.

of gold. It has been proposed that the high activity of Au/DNPderives from the selectivity of gold towards Fenton decompositionof H2O2 and from the inertness of the diamond support, particularlyafter being submitted to the Fenton process under harsh condi-tions [10,11]. Continuing with the study of the activity of Au/DNPas Fenton catalyst, in the present work we have determined the con-ditions for the combination of mild Fenton peroxidation of phenoland subsequent biodegradation in order to optimize the amount ofH2O2 needed in the process. H2O2 is a relatively costly commod-ity that is typically required in large excess to effect degradation oforganic pollutants in water [9]. In order to make feasible the appli-cation of Fenton treatments for water purification one necessarystep is to optimize the amount of H2O2. This can be done firstlyusing a highly selective catalyst, such a Au/DNP that minimizesthe spurious decomposition of H2O2, but also by effecting a shal-low Fenton treatment that converts bioreluctant organic pollutantsinto biodegradable and non toxic derivatives [3,9,10]. In this wayinstead of achieving complete mineralization of organic pollutantsdirectly in the Fenton treatment, a strategy that requires a largeexcess of H2O2, the target is to increase sufficiently the biodegrad-ability of the dissolved organic matter while decreasing the toxicityof the products. This methodology leads to a minimization of the

consumed H2O2, but requires the biodegradability of the resultingprimary products as well as the absence of toxicity of the Fentontreated waste waters.

In this work, we have first determined the influence of thetemperature on the phenol degradation, H2O2 decomposition and

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R. Martín et al. / Applied Catalysis

iodegradability promoted by Au/DNP as catalyst. Then, at theelected temperature we have determined the influence of the2O2 concentration, atmosphere and catalyst amount on the effi-iency of the process and biodegradability. In addition to theiodegradability for the optimal H2O2 amount under suitablexperimental conditions we have determined the toxicity of theater resulting from the combined Fenton and subsequent biolog-

cal treatment.

. Experimental

.1. Materials

Hydrogen peroxide solution in water (30%, v/v), phenol (≥99%urity), hydroquinone (≥99% purity), quinone (≥99% purity),atechol (≥99% purity), hydrochloric acid (37%, ACS reagent),imethyl sulfoxide (DMSO, ≥99.9% purity), sodium hydroxideACS reagent), potassium phosphate dibasic, potassium phos-hate monobasic and N,O-bis(trimethylsilyl)trifluoroacetamideBSTFA) with trimethylchlorosilane (TMCS) (10%) were supplied byigma–Aldrich. Milli-Q water was used in all the experiments. Thether reagents used were of analytical or HPLC grade.

.2. Catalyst preparation

The catalyst used in the present work, Au/DNP (1 wt% Au), comesrom the same batch used in previous work. Catalyst preparationrocedure as well as characterization can be found in the literature10].

.3. Catalytic tests

100 mL of Milli-Q water containing 1 g L−1 (10.6 mM) of phenolas placed in a round bottom flask. The required amount of catalystas introduced and magnetically stirred. Then, the initial pH was

djusted to the required value using HCl (0.1 M) or NaOH (0.1 M).nce the reaction temperature was reached the required amountf H2O2 (30%, v/v) was added to the flask.

Pure oxygen or nitrogen atmosphere, when necessary, was bub-led through the phenol solution at the corresponding initial pHnd in the presence of the catalyst during 45 min. At this time aalloon containing of O2 or N2 was placed on the top of the con-enser in order to maintain the required atmosphere. At this timehe desired amount of H2O2 was added.

.4. Activated sludges biological treatment

Biological treatments were performed in the absence of H2O2hat can act as bactericide. This requires prolonging the Fentoneaction for the necessary time to ensure H2O2 absence. This timeepends on reaction temperature, initial pH, presence of oxygennd other experimental conditions and can be deduced from theemporal profiles of H2O2 concentration. When H2O2 concentra-ion of the catalytic tests was zero, the sample was filtrated through

0.2 �m nylon filter. Then, the sample was aerated for 1 h byagnetically stirring at 20 ◦C. Then, the sample was introduced

n a commercially available 300 mL BOD-bottle (Lovibond®) anduffered at pH 7 using K2HPO4/KH2PO4 (0.01 M). Subsequently,ctivated sludges were added to the solution, to achieve a concen-

ration between 1 and 1.5 g L−1 [12]. At this point, the BOD-bottleas closed by a BOD-Sensor (Lovibond® OxiDirect®) and the solu-

ion was incubated in the dark, under continuous stirring for 5 dayst 20 ◦C. Finally, the sample was filtrated through 0.2 �m nylonlters and analyzed by HPLC–RF–UV (see Section 2.5).

ironmental 103 (2011) 246–252 247

2.5. Product identification

Phenol and aromatic intermediates (hydroquinone, quinoneand catechol) were analyzed by reverse-phase chromatog-raphy using Kromasil-C18 column as stationary phase andH2O:MeOH:CH3COOH /69:30:1 as eluent under isocratic condi-tions and UV detector (monitoring wavelength 254 nm). Prior toanalysis, aliquots (2 mL, filtered through 0.2 �m Nylon filter) of thereaction were 10-fold diluted.

For carboxylic acid identification [13] the reaction mixtureswere acidified to pH 2 with HCl, concentrated at 40 ◦C andlyophilized. The residue was suspended in BSTFA+ 10% TMCS andthe solution stirred at 80 ◦C for 8 h. The resulting silylated mixturewas dissolved in anhydrous acetonitrile, filtered through a 0.45 �mmembrane and injected in a gas chromatography–mass spectrom-etry (GC–MS) system (Hewlett Packard HP6890 Chromatographand mass detector Agilent 5973). The capillary column (30 m)contains crosslinked (5%) phenylmethylsilicone (HP-5MS) as sta-tionary phase. Helium was used as a carrier gas (1.2 mL min−1).The injection volume was 1 �L. The injection and detector temper-atures were 250 and 280 ◦C, respectively. The oven temperatureprogramme starts at 50 ◦C for 3 min, then it increases at a rate of8 ◦C min−1 up to 90 ◦C, maintains this temperature for 2 min andsubsequently rises again at a rate of 15 ◦C min−1 up to 280 ◦C for10 min. Product identification was done comparing the retentiontime and the mass spectra of the silylated commercial samples(malonic, fumaric, malic and tartaric acids) with that correspondingto the sample treated after the Fenton reaction.

The residual H2O2 was determined by 20-fold dilution of thereaction mixture aliquots and using K2(TiO)(C2O4)2 (Aldrich) inH2SO4/HNO3 as colorimetric titrator. The solution was allowed toreact for 10 min before monitoring at 420 nm.

2.6. Biodegradability and toxicity measurements

The biological oxygen demand (BOD) to the chemical oxygendemand (COD) ratio was used as biodegradability index. COD wasdetermined by closed reflux colorimetric method (Standard Meth-ods, 5220D) [14]. BOD was determined by manometric method(Standard Methods, 5210) [14] using commercial Lovibond OxiDi-rect system. Activated sludges for BOD analysis were taken from thesettle of a waste water treatment plant (activated sludges biolog-ical treatment) and a concentration around 1–1.5 g L−1 [12] wereused for BOD bottle inoculation. BOD blank controls were carriedout according Standard Methods by using a solution containingglucose (150 mg L−1) plus glutamic acid (150 mg L−1) whereby thecorresponding BOD value of 198 mg L−1 ± 20 was measured.

Toxicity tests against Vibrio fischeri were carried out using aMicrotox® M500 Toxicity analyzer. The results of toxicity wereexpressed in Toxicity Units (T.U.) defined as T.U. = 100/EC50, whereEC50 is the percentage of solution under analysis that causes 50%inactivation, measured as the reduction of the 50% light emissionby bacteria, in 50 min of contact.

2.7. Quenching experiments

HO• radical scavenging experiments were performed by usingDMSO as quencher. Experiments were carried out under deter-

mined optimal conditions (see Section 3) but adding DMSO. Theexact conditions were the following: 1 g L−1 phenol (10.6 mM),1.44 g L−1 H2O2 (42.3 mM), oxygen atmosphere, pH 4, 50 ◦C,320 mg L−1 Au/DNP (1%), DMSO to H2O2 molar ratio 10 and 6 hreaction time.

248 R. Martín et al. / Applied Catalysis B: Env

25201510500.0

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0.4

0.6

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100ºC 80ºC 60ºC 50ºC 40ºC

Ea = 46 Kcal·mol-1

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ig. 1. Influence of the temperature on the phenol disappearance (top) and H2O2

ecomposition (bottom) catalyzed by Au/DNP. The insets present the relationshipetween ln(r0) and 1/T. Reaction conditions: 320 mg L−1 catalyst (0.016 mM as gold),g L−1 phenol (10.64 mM), 2 g L−1 H2O2 (58.82 mM), pH 4.

.8. Leaching measurements

At the end of the reactions the suspensions were filtered through0.2 �m Nylon filter and analyzed by inductively coupled plasma

tomic emission spectroscopy to determine the presence of Au. Theetection limit of the technique was 0.1 �g L−1.

. Results and discussion

.1. Temperature effect

One important point to be addressed is the time required toffect the Fenton reaction under a given phenol to H2O2 molaratio, pH value and catalyst amount. The time required dependsainly on the reaction temperature and should be long enough to

ncrease the biodegradability of the resulting mixture above the tar-et threshold. Biodegradability is usually determined by measuringhe biological (BOD) and chemical (COD) oxygen demand and con-idering that biodegradability is achieved when the BOD/COD ratios higher than 0.4 [12,15]. To achieve this objective we have initiallyetermined the time conversion plot for phenol degradation and2O2 decomposition in the temperature range from 50 to 100 ◦C.ig. 1 shows the corresponding temporal profiles.

From these plots, data of the initial reaction rate for phenol dis-ppearance and H2O2 decomposition can be obtained, allowing toetermine the activation energy from the Arrhenius plot of ln(r0)ersus 1/T of these two processes. Interestingly, the estimated acti-ation energy value for these two processes is coincident under

hese conditions. The coincidence of the energy barrier for phenolegradation and H2O2 decomposition suggests that the generationf HO• radicals, a process controlled by the H2O2 decomposition, ishe rate determining step and once these HO• radicals are formedhey will attack, essentially without activation barrier, to phenol.

ironmental 103 (2011) 246–252

According to this rationalization the activation energy determinedexperimentally for phenol and H2O2 disappearance should corre-spond exclusively to the activation energy required to form HO•

radicals.Biodegradability of the resulting solutions after the Fenton

treatment at different temperatures in the range 40–100 ◦C wasdetermined by measuring the BOD and the COD at the time in whichthe concentration of H2O2 was zero. Biological treatment requiresthe total absence of H2O2. Although generally the Fenton reac-tion can be carried out under H2O2 excess that is quenched beforebiological treatment, since our study is aimed at optimization ofH2O2 concentration we have avoided destruction of the excess ofH2O2 and preferred allowing the catalytic reaction to run until thecomplete disappearance of H2O2. This time varies depending onthe reaction temperature, initial pH, presence of oxygen and otherexperimental conditions and can be deduced from the temporalprofiles of H2O2 concentration.

The criterion for biodegradability is to have a BOD/COD ratiohigher than 0.4 while simultaneously the treated water should befree of any residual phenol. This general criterion derives fromthe lack of phenol biodegradation and for the fact that in sampleswhere phenol is not present the rest of organic compounds canundergo biological degradation, this being reflected by a relativelyhigh BOD value with respect to the COD. In the present study, itwas calculated that in all cases the BOD/COD ratio was around 0.4.Additionally, analysis of the product distribution at the final timealso indicates that for all the temperatures all aromatic products,namely phenol, hydroquinone, catechol and quinone, are absent inthe reaction mixture. From the above data it was concluded thatoperation at 50 ◦C is convenient from the experimental point ofview since it increases the reaction rate and the biodegradabilitybut still allows to determine the influence of other parameters onthe biodegradability of phenol.

3.2. Optimization of H2O2 concentration

As commented in the introduction H2O2 is a relatively expen-sive commodity and optimization of cost of the Fenton treatmentnecessarily requires a minimization of the amount of H2O2 used.Moreover, precedents in the literature [9] have clearly establishedthat excess of H2O2 can even be detrimental for the efficiency ofthe Fenton treatment since H2O2 can quench HO• radicals con-verting them into less reactive hydroperoxyl (HOO•) radicals [9].Additionally, the target in the present work is not the completemineralization of phenol but rather to increase biodegradability sothat Fenton and biological treatments can be combined.

In order to assess the minimal H2O2 concentration for theoptimal effect we performed the Fenton treatment under the pre-vious conditions (see Fig. 1 caption) at 50 ◦C and increase theH2O2/phenol molar ratio from 1 to 5.5. The results are shown inFig. 2A where the product distribution of the Fenton treated phenolsolution before and after the biological treatment is presented. Asit can be seen in this plot increasing the amount of H2O2 decreasessignificantly the amount at the final reaction time (12 h) of phenolas well as other aromatic by-products degradation. Also this plotshows that phenol does not undergo biological degradation whilethe other three aromatic by-products (hydroquinone, quinone andcatechol) can be biodegraded under these conditions. This point isalso clearly manifested when the BOD/COD ratio versus the amountof H2O2 is plotted (see Fig. 2B). These results led us to select asoptimal H2O2/phenol molar ratio a value of 4. This value is sig-

nificantly lower than those that have been used in literature inanalogous studies that can be as high as around 400 [16] and againreveal the high efficiency of Au/DNP for the generation of HO• radi-cals minimizing the spurious decomposition. The high efficiency ofAu/DNP for the generation of free HO• radicals has been attributed

R. Martín et al. / Applied Catalysis B: Environmental 103 (2011) 246–252 249

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compromise between reasonable amount of catalyst and adequatereaction rate has to be reached. In our case, we have selected as anoptimum catalyst concentration a value of 320 mg L−1 that in thepresence of oxygen can produce complete phenol degradation inless than 5 h.

0 5 10 15 200.0

0.2

0.4

0.6

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1.0 O2 atmosphere

Air atmosphere N

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ig. 2. Effect of H2O2 on product distribution and biodegradability (BOD/COD ratio)2O2 as indicated in the axis, air atmosphere, pH 4, 50 ◦C. The labels “a” and “b” ref

o the inertness of the diamond surface that does not interact withO• radicals and also the efficiency of gold injecting electrons into2O2. This implies that the catalytic cycle swings between Au0 (the

pecies injecting electrons to H2O2) to positive gold ions (Au+ oru3+ that will be the species that will be reduced by H2O2) [10].

.3. Influence of the presence of oxygen

The Fenton reaction generates HO• radicals that, in turn,an form carbon centred radicals either by hydrogen abstrac-ion from C–H bonds or by electrophilic addition to unsaturatedarbon–carbon multiple bonds [9]. Therefore it is clear that theenton reaction triggers an array of carbon centred radicals thatre able to react with oxygen forming peroxyl radicals (see Eqs.1)–(5)) [9].

H + HO• → H2O + R• → oxygenated compounds

→ CO2 + H2O (1)

• + O2 → ROO• (2)

OO• + RH → ROOH + R• (3)

ROOH + 3 H+ + 3 e− → ROH + R O + 2 H2O (4)

OOH + RO• → ROH + R O + H2O (5)

It can be assumed that considering the similarity of H2O2nd organic hydroperoxides formed through oxygen quenchingf carbon radicals, also Au/DNP can decompose these organicydroperoxides leading to additional decomposition promoted byhe presence of oxygen. A general observation is that the presencef oxygen increases the decomposition of organic substrates in theenton reaction [17,18]. For this reason and since our study is per-ormed under conditions in which minimal concentrations of H2O2re used, it is of interest to determine what is the influence ofhe oxygen pressure on the phenol disappearance, H2O2 decom-osition and biodegradability. The results obtained are plotted inig. 3.

As it can be seen there, increasing the partial pressure of oxy-en from 0 to 1 atm produces a remarkable increase in the phenolegradation and H2O2 decomposition. Moreover, when the initialeaction rate, determined from the slope of the time conversionurve at zero time is plotted versus oxygen partial pressure atraight line is obtained indicating that the reaction is first orderith respect to oxygen as reagent. These results indicate that the

resence of oxygen enhances significantly the degradation andecomposition of phenol and H2O2, respectively.

It was also observed that while lower H2O2 to substrate molaratio can lead to BOD/COD ratios higher than the 0.4 threshold, theresence of residual phenol determines that the treated water does

H2O2 / phenol molar ratio

tion conditions: 320 mg L−1 catalyst (0.016 mM as gold), 1 g L−1 phenol (10.64 mM),roduct distribution before and after the biological treatment, respectively.

not meet all the criteria to be consider biodegradable, particularlythe point related to absence of non biodegradable phenol.

3.4. Influence of the phenol to catalyst ratio

Besides the temperature, the concentration of H2O2 and oxygenpressure another parameter that can play a role in the rate of phe-nol degradation is the amount of catalyst. As expected increasingthe amount of catalyst in the range of 50–320 mg L−1 increases pro-portionally the initial reaction rates of both phenol degradation andH2O2 decomposition (see Fig. 4). We notice, however, that the slopeof the initial reaction rate versus the Au/DNP amount is about fourtimes larger for H2O2 decomposition than for phenol degradation.Since the initial reaction rate and the final phenol degradation at agiven reaction time increases as the amount of catalyst increases, a

N2 atmosphere Air atmosphere O

2 atmosphere

Fig. 3. Influence of the oxygen presence in the phenol degradation and H2O2 decom-position. The insets show the influence of the oxygen parcial pressure on the initialreaction rate. Reaction conditions: 320 mg L−1 catalyst (0.016 mM as gold), 1 g L−1

phenol (10.64 mM), 1.44 g L−1 H2O2 (42.35 mM), pH 4, 50 ◦C.

250 R. Martín et al. / Applied Catalysis B: Env

0 100 200 300 400

0

5

10

15

20

25

R2 = 0.9281r 0

, mM

·h-1

Catalyst amount, mg L-1

R2 = 0.9890

Fig. 4. Influence of the catalyst amount on the initial reaction rate for phenoldna

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egradation (©) and H2O2 decomposition (�). Reaction conditions: 1 g L−1 phe-ol (10.64 mM), 1.44 g L−1 H2O2, catalyst amount (as indicated), pH 4, pure oxygentmosphere, 50 ◦C.

.5. pH effect

The Fenton reaction is highly sensitive to the solution pH [9].articularly, when using Fe2+ or other transition metal ions itas been said that acid pH values (below pH 5) are necessary inrder to make compatible the solvated metal ion and avoiding for-ation/precipitation of the corresponding hydroxides. However,

tudies using solid catalyst have shown that even in those cases inhich no metal ion in solution is present, generation of HO• rad-

cals from H2O2 occurs at acidic pH [10]. One of the reasons forhis is that under basic conditions H2O2 tend to disproportionatepontaneously into O2 and H2O at high reaction rates. This dispro-ortionation is responsible for the disappearance of H2O2 withoutarticipating in the Fenton reaction. In a previous report [10], work-

ng at room temperature we have shown that Au/DNPs is an activeatalyst in a pH window that is limited by gold leaching (pH lowerhan 3) on one side and the failure of Fenton reaction (pH > 5) onhe other side. Since we have seen that working at 50 ◦C the Fentoneaction proceeds at high rate and also considering that our goalere is not to achieve the maximum mineralization using Fentonut to increase biodegradability of the phenol aqueous solution,e have performed a study of the influence of the initial pH value

n the phenol degradation, H2O2 decomposition, biodegradabilitynd product distribution after the Fenton reaction combined or notith biological treatment. In these experiments it should be noted

hat the pH values appearing in the Figs. 5 and 6 correspond to thenitial pH and because there is not buffer in the solution and due tohe natural pH evolution during the Fenton, the pH will decreasepontaneously during the course of the reaction. In this context,t is worth to mention that HO• radicals readily react with mostf the inorganic and organic anions that are used for buffering, andherefore some buffers are incompatible with the generation of HO•

adicals since they act as quenchers of these species. The results arehown in Figs. 5 and 6.

As it can be seen in Fig. 5 the initial pH value plays an importantole controlling the reaction rate that for phenol disappearance isigher at pH 4 and gradually decreases as the pH increases stoppinghe disappearance at initial pH values higher than 7. In the rangerom 5 to 7, an induction period whose length increases along theH is observed. This induction period is most probably due to theime required to lower the pH value near 4 and once the appropriate

H value is achieved the phenol disappearance undergo remarkableate acceleration.

Concerning H2O2 decomposition similar trends to phenol disap-earance, i.e. higher initial reaction rates at pH 4 and presence of an

ironmental 103 (2011) 246–252

induction period at near neutral pH values, are observed from pH4 to 8. However, at the highest pH value studied (pH 9) we noticea remarkable increase in H2O2 decomposition that reaches an ini-tial reaction rate similar to that of pH 4. However, since no phenoldisappearance is observed at this pH it is clear that H2O2 decompo-sition beyond pH 7 is due to spurious disproportionation and notto the generation of HO• radicals.

The considerations commented for Fig. 5 lead us to limit thebiodegradability study and product analysis to pH below 7 (seeFig. 6A). With respect to biodegradability two different phenol toH2O2 molar ratios were assayed. We observed that pH 4 gives thehigher BOD/COD ratio that decreases by one half or more when thepH increases up to 5. However, when higher concentration of H2O2is used suitable BOD/COD near to 0.4 or higher is achieved even atpH 7. The importance of using phenol to H2O2 molar ratio of 1 to5.5 is evidenced in Fig. 6B where it can be seen that except at pH4 at the other pH values the presence of a residual concentrationof phenol that impedes the complete mineralization by biologicaltreatment is still observed in the reaction mixture after the Fen-ton treatment. When the phenol to H2O2 molar ratio was 1 to 5.5phenol was absent in all the range of pH values (see Fig. 6B, bar b,for the product distribution) but, even more important in all thecases the biological treatment carried out subsequently to the Fen-ton reaction leads to the complete disappearance of any aromaticproduct in the aqueous solution.

3.6. Biodegradability and toxicity under optimal reactionconditions

The previous studies have allowed us determining the opti-mal experimental conditions to perform phenol degradation usingAu/DNP as catalyst. Once the set of experimental parameters wereselected we proceed to determine the biodegradability of theresulting solution after the mild Fenton treatment and to evalu-ate the toxicity after the Fenton and biological treatment. It wasdetermined that while under the optimum conditions on air theBOD/COD ratio was 0.4, when the atmosphere was pure oxygena ratio of 0.73 was obtained indicating that a qualitative jump inbiodegradability occurs under these conditions.

Concerning the toxicity while the starting phenol solution (afterneutralization at pH 7 using phosphate buffer) has a toxicity indexof 51 T.U. determined by bioluminescence of V. fischeri, indepen-dently of the atmosphere, either air or oxygen, under which theFenton reaction and subsequent biological treatment is carried outa negligible value for the toxicity index (<3 T.U.) was determined.This lack of toxicity is a consequence of the final product distribu-tion present in the resulting solution after the Fenton treatment. Infact, HPLC analysis of the solution at final reaction time after phenoltreatment with H2O2 in the presence of Au/DNP, under the opti-mal conditions did not allow observing any aromatic compoundas assessed by monitoring 254 nm wavelength with diode arraydetector. Product analysis was carried out by removal of water ofthe final solution and derivatization using a mixture of BSTFA withCTMS followed by GC–MS analysis of the resulting silylated deriva-tives. The compounds detected after silylation of the degradationmixture are shown in Fig. 7. As it can be seen there the mixture isformed by dicarboxilic acids and hydroxylated dicarboxilic acids ofthree or four carbons. Interestingly, two of this products malic andtartaric acid have also been detected adsorbed on Au/DNP catalystafter extensive use upon extraction of this solid with basic aqueoussolutions.

3.7. Reuse of Au/DNP catalyst

Working under the optimized conditions a series of four con-secutive uses were carried out. After each reaction, the Au/DNP

R. Martín et al. / Applied Catalysis B: Environmental 103 (2011) 246–252 251

0 5 10 15 20 25 30 35

pH 4 pH 5 pH 6 pH 7 pH 8 pH 9Ph

enol

, g L

-1

t (h)0 5 10 15 20 25 30 35

pH 4 pH 5 pH 6 pH 7 pH 8 pH 9H

2O2, g

L-1

t (h)

A B

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.4

0.8

1.2

Fig. 5. Influence of initial pH on phenol disappearance (A) and H2O2 decomposition (B). Reaction conditions:, 1 g L−1 phenol (10.64 mM), 1.44 g L−1 H2O2, 320 mg L−1 catalyst(0.016 mM as gold), pH as indicated, pure oxygen atmosphere, 50 ◦C.

A B

-- 1:4 1:4 1:5.5 1:4 1.5.5 1:4 1.5.5 00

100

200

300

400

mg

L-1

Phenol Catechol Quinone Hydroquinone

4 5 6 7 a a b a b a b

pH 0

0.0

0.2

0.4

0.6

0.8

1.0

1:4 1:5.5

BO

D/C

OD

pH0

Phenol to H2O2 molar ratio

4 5 6 7

F lity (le1 , purer

waiutaspdottho

F1

ig. 6. Influence of the initial pH and H2O2 concentration on the phenol biodegradabi.44 g L−1 H2O2 (42.35 mM), 320 mg L−1 catalyst (0.016 mM as gold), pH as indicatedespectively.

as recovered by filtration and exhaustively washed with watert pH 10 and a final washing with distilled water. Precedentsn the literature have shown that supported gold catalyst canndergo deactivation by poisoning with carboxylates [19]. Thisype of poisons can be removed by washing the deactivated cat-lyst with strong basic solutions that, in addition, do not dissolveupported gold nanoparticles or effects the leaching of gold asreviously assessed [19]. In fact, GC–MS analysis of the silylatederivatives present in the washing waters reveal the presencef significant amount of malic and tartaric acid indicating that

hese acids are strongly adsorbed in the catalyst. In addition,he lack of observation of fumaric acid is probably due to theigh flux of HO• radicals by H2O2 decomposition on the surfacef the Au/DNP. It is known that HO• radicals can act as elec-

HOOC COOH HOOCCOOH

malonic acid fumaric acid

HOOCCOOH

OHHOOC

COOH

OH

OH

tartaric acidmalic acid

(B) Products desorbed from catalyst after NaOH

(A) Reaction products from Fenton reaction

ig. 7. Products identified by GC–MS after silylation of the reaction mixture resulting frg L−1 phenol (10.64 mM), 1.44 g L−1 H2O2 (42.35 mM), 320 mg L−1 catalyst (0.016 mM as

ft) and product distribution (right). Reaction conditions:, 1 g L−1 phenol (10.64 mM),oxygen atmosphere, 50 ◦C. (a) and (b) refer to a phenol/H2O2 molar ratio 4 and 5.5,

trophile and add to multiple C C double bonds [9] as the onepresent in fumaric that will be converted into malic or tartaricacids.

With this simple treatment it was possible to use four times theAu/DNP catalyst observing exactly not only the same final conver-sion of phenol but also exactly the same temporal profile, indicatingthat Au/DNP is a remarkably stable catalyst under these conditions(see Fig. 8). It is interesting to note that chemical analysis of goldin the solutions indicate that the percentage of gold present in theAu/DNP catalyst and leached to the solution decreases with the

consecutive reuses from the first use (<3 wt%) to the fourth use(<0.1 wt%). This small percentage of gold leaching does not affectto the catalytic activity of Au/DNP in the reuse tests carried out inthe present work.

HOOCCOOH

OHHOOC

COOH

OH

OH

tartaric acidmalic acid

washings

om the Fenton reaction (A) and from catalyst washings (B). Reaction conditions:,gold), pH 4, pure oxygen atmosphere, 50 ◦C.

252 R. Martín et al. / Applied Catalysis B: Environmental 103 (2011) 246–252

A B

0 5 10 15 20 25 300.0

0.5

1.0

1.5

H2O

2 (g

L-1)

t (h)

0 5 10 15 20 25 300.0

0.5

1.0

Phe

nol (

g L

-1)

t (h)

EQUATIONS

RH + HO· H2O + R· oxygenated compounds CO2 + H2O (Eq. 1)

R· + O2 ROO· (Eq. 2)

ROO· + RH ROOH + R· (Eq. 3)2 ROOH + 3 H+ + 3 e- ROH + R=O + 2 H2O (Eq. 4)

ROOH + RO· ROH + R=O + H2O (Eq. 5)

F ve useH phered

3

toprparDtHi(npiu

4

poddoitlpsaarwi

[

[

[

[[

[

[

[

[18] B. Utset, J. Garcia, J. Casado, X. Domenech, J. Peral, Chemosphere 41 (2000)

ig. 8. Phenol degradation (A) and H2O2 decomposition (B) during four consecuti2O2 (42.35 mM), 320 mg L−1 catalyst (0.016 mM as gold), pH 4, pure oxygen atmosistilled water before reuse.

.8. Quenching of the reaction by DMSO

Concerning the reaction mechanism previous studies [10] withhe same Au/DNP catalyst have allowed to detect the generationf HO• radicals by trapping these species with N-tert-butyl-�-henylnitrone (PBN) and recording the electron paramagneticesonance (EPR) spectrum of the corresponding adduct. In theresent case, for the optimum conditions, we have performed andditional experiment using an excess of DMSO, acting as HO•

adical quencher, respect to H2O2 concentration (1:10 H2O2 toMSO molar ratio). Reports in the literature have shown that under

hese conditions DMSO is an efficient scavenger of HO• radicals. If2O2 decomposes through other alternative mechanisms like those

mplying metal hydroperoxide where no HO• radicals are generatedcatalase–peroxidase routes), then the presence of DMSO shouldot have any effect [10,20,21]. In our case it was observed that theresence of DMSO completely stops phenol degradation support-

ng the intermediacy of HO• radicals in the H2O2 decompositionsing Au/DNP as catalyst.

. Conclusions

In the present work, we have optimized several experimentalarameters influencing the decomposition of H2O2 by Au/DNP inrder to minimize the amount of H2O2 necessary to effect phenolecomposition in reasonable reaction times. In this way, we haveetermined that operating at an initial pH of 4 and 50 ◦C under air orxygen it is possible to increase the biodegradability of the result-ng solution with a H2O2 to phenol molar ratio of 4 in such a wayhat a combination of mild Fenton reaction and subsequent bio-ogical treatment can lead to complete detoxification of the initialhenol solution. Analysis of the by-products at final reaction timeshows the complete absence of aromatics and the presence of three

nd four carbon dicarboxilic acids. The catalyst exhibits a remark-ble stability if adequately washed with aqueous base and can beeused without noticeable loss of catalytic activity. The quenchingith DMSO indicates that HO• radicals are the most likely reaction

ntermediates in the H2O2 decomposition.

[[

[

s catalyzed by Au/DNP. Reaction conditions 1 g L−1 phenol (10.64 mM), 1.44 g L−1

, 50 ◦C. Au/DNP was exhaustively washed with basic aqueous solution (pH 10) and

Acknowledgements

Financial support by the Spanish DGI (CTQ-2009-11587) isgratefully acknowledged. SN thanks to the Technical Universityof Valencia for a postgraduate research contract (Cantera Pro-gramme).

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