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Photocatalytic oxidation of acetonitrile in aqueous suspension of titanium dioxide irradiated by sun

Advances in Environmental Research 8(2004)329–335

Photocatalytic oxidation of acetonitrile in aqueous suspension of

titanium dioxide irradiated by sunlight

V .Augugliaro ,A.Bianco Prevot ,J.Caceres Vazquez ,E.Garc?a-Lopez ,A.Irico ,a b c

a b ′′′′V .Loddo ,S.Malato Rodr?guez ,G.Marc?

,L.Palmisano *,E.Pramauro a c

a a ,

b ′`Dipartimento di Ingegneria Chimica dei Processi e dei Materiali,Universita

di Palermo,viale delle Scienze,90128Palermo,a

`Italy

Dipartimento di Chimica Analitica,Universita

di Torino,via P .Giuria,5,10125Torino,Italy b

`Plataforma Solar de Almer?a,

Crta.Senes s y n,04200Tabernas,Almeria,Spain c

′′Received 8August 2002;received in revised form 17August 2002;accepted 20August 2002

Abstract

The photocatalytic oxidation of acetonitrile (CH CN )was carried out in aqueous suspensions of polycrystalline

3TiO P25Degussa irradiated by sunlight.A plug flow photoreactor in a total recycle loop was used for carrying out 2reactivity experiments in which the concentrations of acetonitrile,of its intermediate oxidation products and of not-purgeable organic carbon (NPOC )were monitored.The influence of the presence of strong oxidant species (H O ,

22S O ,ClO )on the process rate was studied.The dependence of acetonitrile photo-oxidation rate on the substrate 2y y

28concentration and on the catalyst amount was also investigated.The photodegradation rate of substrate and NPOC followed first order kinetics with respect to acetonitrile and NPOC concentrations,respectively.The presence of

S O and solar irradiation determined the occurrence of homogeneous degradation of acetonitrile and NPOC.In the 2y

presence of irradiated catalyst,a significant synergetic effect was observed for NPOC degradation while for the acetonitrile oxidation this effect was not evident.H O did not affect the process while ClO affected negatively the y 22acetonitrile oxidation rate and it inhibited the NPOC degradation.?2002Elsevier Science Ltd.All rights reserved.

Keywords:Photocatalytic oxidation;Acetonitrile;Sunlight;Titanium dioxide

1.Introduction

Heterogeneous photocatalysis is a growing field of basic and applied research especially for the case of oxidation processes of organic pollutants carried out by using aqueous oxygenated suspensions of polycrystal-line semiconductors (Schiavello,1988;Pelizzetti and Serpone,1989;Ollis and Al-Ekabi,1993).In order to improve the performance of a photocatalytic process,two main routes have been tested.The first one consists in modifying the photocatalyst by doping it with sub-*Corresponding author.Fax:q 39-91-656-7280.

E-mail address:palmisan@dicpm.unipa.it (L.Palmisano ).stances that (i )enhance the efficiency of the charge separation step or (ii )induce a shift of the absorption edge towards the visible region.The second route consists in modifying the reaction ambient by adding substances such as hydrogen peroxide (Augugliaro et

al.,1990),ozone (Sanchez et al.,1998;Piera et al.,

′2000),peroxydisulphate ions (Augugliaro et al.,1999b ),

or Fenton reagents (Piera et al.,2000;Sanchez et al.,

′1997)that eventually increase the overall oxidation rate.Acetonitrile is an extremely stable and toxic molecule.It is used as a solvent in photocatalytic oxidation reactions (Abdel Wahab and Gaber,1998;Somasundar-am and Srinivasan,1998;Fujiwara et al.,1999;O’Shea

330V .Augugliaro et al./Advances in Environmental Research 8(2004)

329–335

Fig. 1.Scheme of the photoreacting system:(a )sampling valve;(b )thermocouple;(c )not-reacting tank;(d )pump;(PFP )plug flow photoreactor.

et al.,1999)and it is often found in various civil and industrial wastewaters due to its use as eluent for HPLC analyses.

The photo-oxidation of acetonitrile adsorbed on TiO ,both in liquid and in gas phase,by using O as 22oxidant,was recently investigated (Lichtin and Avudai-thai,1996).The production of cyanogen (CN )as 2intermediate species has been reported thus indicating the formation of CN radicals which can dimerize.?Moreover,it was demonstrated that acetonitrile is much more reactive in gas phase than in liquid phase.Aceton-itrile adsorption and photo-oxidation in gas phase has been also studied by means of infrared spectroscopy (Zhuang et al.,1999).The final oxidation products in

the presence of O were CO ,H O and CO (onto 2y

2223surface ),but also isocyanate was found as a partially oxidised species.The presence of isocyanate and y or cyanate is a clear clue of the formation of free cyanide.Indeed,previous studies showed that the main oxidation products of free and complex cyanide were cyanate,nitrite,nitrate and carbonate species (Augugliaro et al.,1997,1999a,b ).In the present investigation the photo-catalytic oxidation of acetonitrile was carried out in aqueous suspensions of titanium dioxide irradiated by sunlight.The concentrations of substrate and not-pur-geable organic carbon (NPOC )were monitored.The influence of the substrate concentration and catalyst amount on the photoreaction rate was investigated.Moreover,the determination of organic and inorganic species,formed as intermediate products in the course of acetonitrile photo-oxidation,was also carried out.On the basis of the finding that sometimes the presence of strong oxidant species enhances the photo-oxidation rate of organic compounds (Augugliaro et al.,1990;Bianco Prevot et al.,2001),some photoreactivity tests were carried out by adding hydrogen peroxide or peroxydisulphate or hypochlorite ions to the reacting system.2.Experimental

The photoreactivity experiments were carried out by using not concentrating compound parabolic collectors

(CPC ),installed at the ‘Plataforma Solar of Almer?a’

′(PSA,Spain ).Two identical reacting systems were contemporarily used in order to perform photoreactivity runs at the same irradiation conditions and therefore to check the reproducibility of the results.The photoreactor configuration,reported in Fig.1,is a common one in heterogeneous photocatalysis field:a plug flow reactor in a total recycle loop with a not-reacting stirred tank whose function is that of providing aeration and samples for analyses.Each photoreactor consisted of three CPC modules in series (total irradiated surface:3.08m )2placed on fixed supports inclined 378(latitude of the PSA )with respect to the horizontal plane and facing

South,in order to maximise the daily absorption of solar radiation.The collectors are not concentrating ones,i.e.the ratio between the surface where the solar radiation impinges and that of the reactor is approxi-mately equal to 1.1.

The plug flow photoreactor (PFP )consisted of UV–transparent glass tubes (i.d.29.2mm ).All the tubes were connected in series and the aqueous suspension was continuously fed to the PFP from the not-reacting tank by means of a centrifugal pump.The suspension flow rate,maintained constant for all the runs,was 0.334dm s .The Reynolds number value was equal 3y 1to approximately 1.7=10indicating a turbulent regime 4of the flow inside the tubes.The total volume,V ,of t suspension charged in the whole system was 39dm ,3whereas the irradiated volume,V ,i.e.the volume of i suspension contained in the glass tubes,was 22dm .3The catalyst used for the photoreactivity experiments was polycrystalline TiO (P25,Degussa,approximately 280%anatase and 20%rutile,BET surface area approx-imately 50m g ).

2y 1The catalyst amounts were in the 0.2–0.4g dm y 3range.The acetonitrile initial concentrations were in the 0.122–1.220mM range and the initial pH was adjusted to approximately 11with NaOH.The experimental runs were carried out by using the following procedure:firstly,the aqueous suspension containing the organic compound and the TiO powder was circulated in the 2reacting system by maintaining the collectors covered.After 30min of this operation the cover was removed and the reactivity run started.Samples of the suspension were withdrawn at the starting of irradiation and at fixed intervals of time.It must be reminded that the samples were withdrawn from the not-reacting tank so that they are representative of the conditions at the inlet

331

V .Augugliaro et al./Advances in Environmental Research 8(2004)

329–335Fig.2.CH CN concentrations versus the cumulative photon energy for runs carried out at different initial CH CN concentrations.33In the inset the corresponding values of NPOC concentrations are reported.Catalyst amount 0.2g l ;initial pH 11.

y 1of the PFP .Some runs were carried out by adding hydrogen peroxide or sodium peroxydisulphate or sodi-um hypochlorite to the reacting suspension.H O was 22added each hour in order to maintain its concentration in aqueous solution equal to the stoichiometric amount needed for the complete oxidation of the initial substrate

(CH CN:H O molar ratio:1:8).The CH CN:S O 2y

322328

molar ratio was 1:8,corresponding to the S O stoi-2y

chiometric amount needed for obtaining the complete oxidation of initial content of carbon and nitrogen atoms.

When S O was added,pH was monitored during the 2y

run and it was re-adjusted to approximately 11several times with NaOH.A few runs were carried out by adding ClO ions with a CH CN:ClO molar ratio y y 3equal to 1:8.

The quantitative determination of CH CN was rou-3tinely performed by using a gas chromatograph (Hew-lett-Packard,GC 6890system )equipped with a 5%phenyl methyl siloxane 30m =320m m =0.25m m column (Hewlett-Packard HP-5)and a FID detector.Chromatographic grade Helium was used as carrier.The sample was placed in a vial and the compounds present in the vapour phase were extracted (extraction time,5min )by using a 75m m Carboxen-PDMS SPME (Solid Phase Micro Extraction )fiber assembly (Supel-co )with a fiber holder for manual sampling.Then the holder was placed in the split y splitless injector,main-tained at a temperature of 2508C for 2min before starting the analysis.The temperature of the oven was held at 408C for the first 3min,then it was increased up to 2508C at a rate of 608C min .

y 1All the reagents were analytical grade (Fluka ).The quantitative determination of ionic species was carried out by using an ionic chromatograph system (Dionex DX 120)equipped with an Ion Pac AS144mm column (250mm long,Dionex ).Aqueous solutions of

NaHCO (1mM )and Na CO (3.5mM )were used as 323eluants at a flow rate of 1.67=10cm s .The y 23y 1samples were filtered before analyses by using Millipore MVLP filters,0.45m m.A 5050A Shimadzu total organic carbon analyser was used in order to determine the NPOC content of the samples.In order to compare experimental runs carried out on different days with different solar irradiances,the values of irradiance were monitored and recorded during the runs by using a sensor for global UV-light measurements (Eppley-TUVR )installed in the same position of the CPCs.3.Results and discussion

Preliminary photoreactivity experiments were carried out with catalyst amount in the 0.2–0.4g dm range.y 3The results did not differ appreciably so that all the runs were performed by using a constant catalyst amount of 0.2g dm .In order to compare photoreactivity y 3results obtained under different conditions of irradiation,the values of acetonitrile concentration were reported vs.the cumulative photon energy,E ,incident on the reactor.This quantity is given by:t

E s

I (t )d t (1)

in which I (t )is the instantaneous photon flow w einstein s x and t the irradiation time.The values of y 1I (t )were calculated from the recorded values of irradi-ance,UVG (t ),by using the following relationship:I (t )s UVG (t )S

(2)

332V .Augugliaro et al./Advances in Environmental Research 8(2004)

329–335

Fig.3.Concentration of acetonitrile (?),cyanate (j ),nitrate (m ),and nitrogen balance (d )vs.the cumulative photonic energy.Catalyst amount 0.2g l ,initial pH 11.

1Fig.4.Concentration of acetonitrile vs.the cumulative pho-tonic energy,E ,in the absence of chemical oxidant species

(?)and in the presence of H O (j )or S O (m )or

2228ClO (d ).In the inset the corresponding values of NPOC y

concentrations are reported.Catalyst amount 0.2g l ,initial y 1pH

11.

Fig.5.Acetonitrile concentration vs.the cumulative photonic energy,E ,for runs carried out in the absence of chemical oxi-dant species (m ),in the presence of S O (j )and in the 2y

presence of S O in homogeneous phase (?).In the inset 2y

the corresponding values of NPOC concentrations are reported.Catalyst amount 0.2g l ,initial pH 11.

y 1in which S is the total irradiated surface and UVG is the irradiance expressed as W m .The UVG dimen-y 2sions were transformed in w einstein s m x by using y 1y 2the Planck’s equation (E s hc y l ).By considering that the TiO (anatase )band gap energy corresponds to a 2wavelength of 387nm,the conversion factor from w W m x to w einstein s m x has the value of y 2y 1y 23.23=10.

y 6It is worth noting that 6einstein correspond approx-imately to 5h of sunlight irradiation (10.00–15.30h ).Fig.2shows the substrate and NPOC (in the inset )concentration values (expressed as mmoles l of sub-y 1strate or organic carbon )respectively,measured at the inlet of the photoreactor,vs.the cumulative photon energy,E ,for selected experiments.The runs were carried out at different initial CH CN concentrations 3and an initial pH of 11.Degraded acetonitrile was oxidised to carbonate and nitrate and the main inorganic intermediate detected was cyanate,while only traces of cyanide and nitrite were found.The main organic intermediate detected was HCOO ,probably derived y from the presence of methanol (formed by oxidation of the methyl group of acetonitrile )which can be oxidised to methanol and then to methanoic acid that in alkaline medium is found as methanoate.

Fig.3reports,for a typical run carried out with a catalyst amount of 0.2g l and an initial pH of 11,y 1the experimental values of acetonitrile,cyanate and nitrate concentrations versus E .In Fig.3the nitrogen balance is also reported.It can be observed that the nitrogen mass balance is quite well satisfied during the course of the run thus indicating that no significant formation of additional nitrogen-containing species occurred.Acetonitrile degradation was quite low,prob-ably due to the competition of OCN with acetonitrile y on the same active sites.Moreover,it is reported in the pertinent literature (Augugliaro et al.,1997,1999a,b )that OCN is slowly photo-oxidised.

y Fig.4shows the results obtained in the presence of oxidant species such as hydrogen peroxide or peroxy-disulphate or hypochlorite;the results obtained without oxidant species are also reported.For the run carried

out in the presence of S O the pH value was main-2y

tained equal to 11during the course of the experiment.From the observation of this figure it can be noticed

that the reaction rate increases by adding S O ,where-2y

28as the presence of H O does not affect the photoreac-22tivity and that of ClO decreases the activity.

y Fig.5reports the reactivity data obtained:(i )in the presence of catalyst;(ii )with homogeneous solution

containing S O ;and (iii )in the presence of catalyst

28and of dissolved S O .Preliminary runs,carried out 2y

28in the dark with homogeneous solution containing

S O or H O or ClO ,showed a not measurable 2y

y 28

22reactivity.

333

V .Augugliaro et al./Advances in Environmental Research 8(2004)329–335Table 1

Values of true kinetic constants obtained for runs with different initial acetonitrile concentrations and with different oxidant species

k 9(einstein )10y 12obs TiO 2

TiO q H O 222TiO q S O 2y

228

TiO q ClO y 2S O 2y

homogeneous

C 0,ACN CH CN 3NPOC CH CN 3NPOC CH CN 3NPOC CH CN

3NPOC

CH CN 3NPOC (mM )0.12 4.98.2––––––0.24 4.7 6.6––16249.5 2.50.49 4.77.3––––––1.2

4.8

6.7

4.8

8.3

3.80.0

–

It may be observed that the homogeneous photode-gradation of acetonitrile with S O is comparable to 2y

the heterogeneous photocatalytic one.In the presence

of S O and catalyst the decrease of acetonitrile 2y

concentration seems due to the cumulative effects of the heterogeneous photo-oxidation reaction and the homogeneous photoreaction.

A photocatalytic run was carried out with peroxydi-sulphate but without re-adjusting the pH to 11during the occurrence of the reaction.It was observed that the pH decreased to approximately 3,due to the reduction

of S O to sulphuric acid,and the disappearance rates 2y

of CH CN and NPOC increased considerably.The 3positive effect of acidic pH on the acetonitrile degra-dation rate is difficult to clarify.A tentative explanation could be the formation and the release in the gas phase of HCN from CN at acidic pH before the oxidation y of CN to OCN .Then,the competition between y y OCN and acetonitrile on the same active sites would y be less important,while likely explanations for the improvement of NPOC disappearance rate could be the transformation of CN ,formed during the acetonitrile y photo-oxidation,in volatile hydrogen cyanide and y or the transformation of CNO produced by the cyanide y photocatalytic oxidation,in cyanic acid,HCNO,which in solution decomposes to form ammonia,carbon diox-ide and water (Cotton and Wilkinson,1972).

After testing different kinetic models to the data of CH CN (ACN )and NPOC concentrations vs.E ,it was 3found that the first order kinetics,expressed by the following equations dC ACN

s k C obs ,ACN ACN

dE dC NPOC y s k C (3)

obs ,NPOC NPOC

dE best fitted the experimental data.The integration of Eq.(3)with the limiting conditions that at the start of the reaction (E s 0)acetonitrile and NPOC concentrations are equal to the initial ones (C s C ;C s ACN 0,ACN NPOC C )gives:

0,NPOC y k E (obs,ACN )

C s C e ACN 0,ACN y k E (obs,NPOC )

C s C e (4)

NPOC 0,NPOC By applying a least-square best fitting procedure to the experimental data,the values of k and obs,ACN k were determined for all the runs.The photo-obs,NPOC reacting system used in this work is a total recycling one composed by a PFP and a mixing tank.The reaction of substrate oxidation only occurs inside the irradiated PFP while the not-reacting tank determines that the substrate concentration at the PFP inlet is different from the outlet one.The system is not at steady state so that the inlet and outlet PFP concentrations are changing with irradiation time.The dynamics of this system have been deeply discussed (Wolfrum and Turchi,1992;Augugliaro et al.,1999b ).In order to obtain the true kinetic constants,the observed reactivity data of CH CN 3and NPOC concentrations must be normalised by mul-tiplying them by the ratio of the total volume to the irradiated volume,V y V .In the present case,being the t i kinetics of first order,the true kinetic constants are obtained from the observed ones by means of the following simple relationships:V V t t k 9s k k 9s k (5)

obs ,ACN obs ,ACN

obs ,NPOC obs ,NPOC

V V i

The calculated values of true kinetic constants,k 9,both for CH CN and NPOC degradation are reported in 3Table 1.

For the photocatalytic runs carried out without the addition of oxidant compounds Table 1shows that the k 9values are independent of the initial concentra-obs,ACN tion of CH CN.Indeed by using the Langmuir–Hin-3shelwood model,the kinetic equation for a first order reaction occurring on the catalyst surface is:dC k K C ACN ACN ACN ACN

s (6)

dE 1q K C q K C

ACN

334V .Augugliaro et al./Advances in Environmental Research 8(2004)329–335

in which k indicates the first order rate constant,K the equilibrium adsorption constant and C the concentration in the aqueous phase.The ‘ACN’and ‘i ’subscripts refer to CH CN and its intermediate degradation prod-3ucts,respectively.If the organic species are scarcely adsorbed on the catalytic surface,the K C and ACN ACN K C terms are negligible so that equations can be i i rewritten as:dC ACN

s k K C (7)

ACN ACN ACN

By comparing Eq.(7)with Eq.(3)(for ACN )it may be noted that the k K term corresponds to ACN ACN k .The addition of peroxydisulphate ion increases obs,ACN the values of the k and k but it does not obs,ACN obs,NPOC modify the independence of the kinetic constant of the substrate concentrations.In the contemporary presence

of catalyst and S O the value of k is quite 2y

obs,ACN similar to the sum of k values obtained by only obs,ACN heterogeneous photocatalysis and homogeneous photo-reaction.This feature suggests that these two processes are independent,parallel ways for acetonitrile degrada-tion.On the contrary the k value obtained with

obs,NPOC catalyst and S O is greater than the sum of those 2y

obtained with photocatalysis and homogeneous photo-reaction.It may be therefore concluded that S O ions 2y

do not participate in the photocatalytic degradation of acetonitrile:however,they positively affect the degra-dation rate of the intermediate degradation products of acetonitrile.

As far as the H O and ClO roles are concerned,y 22the data of Table 1indicate that hydrogen peroxide does not affect the photodegradation process while hypochlo-rite ion negatively affects the acetonitrile oxidation rate and in addition it seems to inhibit the NPOC degradation.4.Conclusions

The photocatalytic oxidation of acetonitrile was car-ried out in the presence of polycrystalline TiO irradi-2ated by sunlight.The substrate and NPOC photo-oxidation rates follow first order kinetics with a low fractional sites coverage both for the substrate and its organic intermediates under the used experimental conditions.The presence of H O in the reacting medi-22um did not influence the acetonitrile photodegradation rate,while the addition of peroxydisulphate had a beneficial effect.The presence of ClO is detrimental y for both acetonitrile and NPOC photodegradation.The

final oxidation products were CO and NO ,while 2y

y 33the main intermediate products were HCOO and y CNO .

y Acknowledgments

The authors like to thank the ‘Improving Human Potential’programme of EU-DGXII for financial sup-port and availability of the facilities located at the

Plataforma Solar of Almer?a (Spain ).′References

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Vincenzo Augugliaro is full professor of Transport Phenomena at Palermo University,Faculty of Engineer-ing.He is expert of photoreactors and kinetics of chemical processes and he is involved in several national and international projects on photocatalysis.He is author of many scientific publications in international and national journals and books.

Alessandra Bianco Prevot Ph.D.of research,is researcher at the Department of Analytical Chemistry of the University of Torino.Her main activity concerns the analytical and environmental problem of the removal of pollutants from water and solids,with particular attention to the use of surfactants as solvent system and to the photocatalytic degradation of organic pollutants. Julia Caceres Vazquez is finishing her Ph.D.of ′′

research at the Plataforma Solar de Almer?a,institution

′belonging to the Spanish Ministry of Science and Technology.She has been involved in5National and European Projects related with the development of Solar Technologies applied to wastewater treatment.She is author of some publications in international journals. Elisa Garc?a-Lopez obtained the degree in Chemistry ′′

in1993at the Universidad Autonoma of Madrid.Cur-rently she is researcher of Chemistry at Palermo Uni-versity,Faculty of Engineering.Her research work is in the field of photocatalysis both in gas-phase and liquid-phase regimens and she is author of some publications in international journals.

Alessandra Irico Ph.D.of research,is technician at the Department of Analytical Chemistry of the Univer-sity of Torino.Her main activity concerns the develop-ment of analytical procedures for the analysis of organic molecules,mainly using the mass spectrometry technique.

Vittorio Loddo is Ph.D.of research in Chemical Engineering.Currently he is researcher of Principle of Chemical Engineering at Palermo University,Faculty of Engineering.His research work is in the field of reactor modelling for photocatalytic reactions and he is author of some publications in international and national jour-nals and books.

Sixto Malato Rodr?guez is Ph.D.of research in Chem-

′

ical Engineering.At present he has a permanent position as Senior Researcher of the Spanish Ministry of Science and Technology.He has been involved in9European Union,9National R&D Projects and5R&D Contracts related with the development of Solar Technologies applied to wastewater treatment.He is author of many scientific publications in international and national jour-nals and books.

Giuseppe Marc?is Ph.D.of research in Chemical

Technology.Currently he is researcher of Chemistry at Palermo University,Faculty of Engineering.His research work is in the field of photocatalytic reactions involving organic and inorganic molecules in heteroge-neous systems.He is author of some scientific publications in international and national journals. Leonardo Palmisano is full professor of Chemistry at Palermo University,Faculty of Engineering.He is well experienced in co-ordination chemistry and expert in photocatalysis and he is involved in several national and international research projects.He is author of many scientific publications in international and national jour-nals and books.

Edmondo Pramauro is full professor of Analytical Chemistry at the University of Torino.His main research fields are:the study of surfactant applications in analyt-ical chemistry and separation science,with particular attention devoted to micelle-based extraction and ultra-filtration procedures,and the investigation of pollutants degradation in treatments based on photocatalysis over semiconductors dispersions.