Download - Aniline degradation by combined photocatalysis and ozonation

Aniline degradation by combinedphotocatalysis and ozonation

Laura SaÂnchez, JoseÂ Peral, Xavier DomeÁnech*

Departament de QuõÂmica, Edi®ci Cn. Universitat AutoÁnoma de Barcelona, 08193 Bellaterra, Spain

Received 2 November 1997; received in revised form 28 February 1998; accepted 22 May 1998

Abstract

The combination of TiO2-assisted photocatalysis and ozonation in the degradation of aniline in aqueous solution is

investigated. From the experimental results obtained it is observed that the ozonation pretreatment followed by photocatalysis

strongly increases the yield of TOC removal in comparison to either ozonation or photocatalysis carried out separately. The

opposite sequence (photocatalysis pretreatment followed by ozonation) does not enhance the ef®ciency of aniline degradation.

Nevertheless, the highest TOC removal was achieved by simultaneous ozonation and photocatalysis. A mechanism involving

the formation of an ozonide anion radical previous to the generation of OH radicals is suggested to explain the synergic effect

between ozone and TiO2 under illumination. # 1998 Elsevier Science B.V. All rights reserved.

Keywords: Aniline; Degradation; Photocatalysis; Ozonation

1. Introduction

Photocatalysis is a promising new methodology for

carrying out the elimination of relatively recalcitrant

organic compounds [1±3]. This methodology is based

on the production of electron±hole pairs by illumina-

tion with light of suitable energy, of a semiconductor

powder dispersed in an aqueous medium. These

charge carriers migrate to the particles surface and

react with adsorbed species of suitable redox potential.

In aerated media, adsorbed molecular oxygen accepts

photogenerated electrons, while water molecules can

react with photogenerated holes to produce hydroxyl

radicals [4,5]:

O2 � eÿ ! O�ÿ2

H2O� h� ! HO� � H�

Due to its capability to generate OH radicals, which

are powerful oxidant species [6], photocatalysis can be

considered an advanced oxidation process (AOP).

However, despite that photocatalysis has shown to

be adequate for the destruction of a wide variety of

compounds, in some cases the complete mineraliza-

tion is slowly attained and the ef®ciency of the pro-

cesses, in terms of energy consumption, is only

advantageous for very dilute ef¯uents [2,7,8]. To

overcome this dif®culty some additives with different

chemical roles such as H2O2, Fe2�, Fe3�, S2O2ÿ8 , Ag�,

etc., have been added to the photocatalytic systems

[9±14]. On the other hand, the use of ozone for the

Applied Catalysis B: Environmental 19 (1998) 59±65

*Corresponding author. Tel.: +34-3-5811702; fax: +34-3-

5812920.

0926-3373/98/$ ± see front matter # 1998 Elsevier Science B.V. All rights reserved.

P I I : S 0 9 2 6 - 3 3 7 3 ( 9 8 ) 0 0 0 5 8 - 7

destruction of organics in water is also a well known

water treatment technique and research ®eld [15,16].

Unlike photocatalysis, ozonation, due to its capability

for selectively destroying recalcitrant organics, is used

as a pretreatment step before ordinary biological

techniques, thus being more ef®cient for highly con-

centrated samples treatment. The simultaneous appli-

cation of ozonation and photocatalysis has potential

use for the ef®cient treatment of organic contaminated

waters in a wide range of concentrations. Is it for that

reason that in the present paper, the combined effect of

photocatalysis and ozonation upon the degradation of

a relatively stable organic compound, such as aniline,

is studied. The in¯uence of different experimental

parameters, i.e. ozone ¯ow, pH, aniline concentration

and mass of the semiconductor on the ef®ciency of

TOC removal is investigated.

2. Experimental

All chemicals were at least of reagent grade and

were used as received. The titanium dioxide (Degussa

P-25) was predominantly anatase (80% anatase and

20% rutile), as shown by X-ray diffraction. The BET

surface area, determined from nitrogen adsorption

at ÿ1968C (accusorb 2100 Microneritics) was

59.1 m2 gÿ1. The average particle size, determined

by scanning electron microscopy, was 27 nm. Unless

otherwise stated, the concentration of TiO2 in suspen-

sion in the experiments was 2 g lÿ1. All experiments

were made at 25.0�0.18C.

Experiments were conducted in a thermostatic

cylindrical Pyrex cell of 130 cm3 capacity. The reac-

tion mixture inside the cell was maintained in suspen-

sion by magnetic stirring. As a light source, a 125 W

Philips HPK medium pressure mercury vapour lamp

was used. The intensity of the incident light inside the

photoreactor, measured employing a uranyl actino-

meter, was 9.2�10ÿ4 einstein dmÿ3 minÿ1.

Ozone was produced by a Sander Labor-Ozonisator

301.7 and was immediately bubbled through the sus-

pensions. The amount of ozone generated was deter-

mined by iodometry. In order to know the amount of

ozone consumed, the reactor outlet gas was bubbled

through a KI (0.125 mol dmÿ3) tamponed solution

and ozone was measured by iodometry. Total organic

carbon (TOC) of initial and irradiated samples was

determined with a Shimadzu TOC-5000 analyzer. The

concentration of aniline was measured by HPLC. A

Metrohm 690 Chromatograph equipped with a vis±

UV detector (795 Applied Biosystems) working at

280 nm was used. The stationary phase employed was

an Spherisorb ODS-B column [250 mm�4.6 mm

(i.d.)], while the mobile phase was a mixture of

60% CH3CN and 40% H2O.

3. Results and discussion

Different experiments using ozone, TiO2, and com-

binations of them have been performed to carry out the

degradation of aniline in aqueous solution. Fig. 1

shows the TOC values found in solution at different

times after treatment of an aniline aqueous solution of

approximately 80 ppm (1.0�10ÿ3 mol dmÿ3) with

O3, O3�TiO2, TiO2, TiO2�light and TiO2�O3�light.

For these experiments the mean ¯ow of O3 through the

suspension was (5.0�0.5)�10ÿ4 mol minÿ1, the

initial pH 3.0, and the mass of TiO2 2 g dmÿ3. From

Fig. 1 it is worth to note that a synergic effect exists

Fig. 1. Time-course of TOC in aqueous solution under different

experimental conditions: (a) in the dark with TiO2, (b) under

illumination with TiO2, (c) in the dark with O3, (d) in the dark with

TiO2 and O3, and (e) under illumination with TiO2 and O3.

Experimental conditions: [aniline]i�10ÿ3 mol lÿ1; pHi�3.0; mass

of TiO2�2 g lÿ1; mean flow of O3�5.0�10ÿ4 mol minÿ1.

60 L. SaÂnchez et al. / Applied Catalysis B: Environmental 19 (1998) 59±65

between TiO2 and O3, specially remarkable when

comparing curve e (simultaneous effect of TiO2, O3

and light) with curves b and c. Indeed, after 2 h of

irradiation, 96% TOC reduction is detected, while in

the absence of O3 the TOC reduction is only 27%.

Comparing curve d (O3�TiO2 in the dark) with the

cumulative effect of O3 alone and TiO2 alone (curves

a�c), an increment of approximately 10% of TOC

reduction is observed with respect to the addition of

the separated O3 and TiO2 systems. This aniline

degradation enhancement can be attributed to 10%

increment of O3 ¯ow (5.3�10ÿ4 mol minÿ1) in the

experiments represented in the curve d with respect to

that of experiments of curve c (4.8�10ÿ4 mol minÿ1).

On the other hand, the ozone consumption during

irradiation (initial aniline concentration: 350 ppm)

has been determined, giving the following ratios in

terms of O3 consumed per ppm of TOC degraded:

2.3�10ÿ2 g ppmÿ1 for aniline ozonation in the dark

(the same value is obtained under illumination) and

1.1�10ÿ2 g ppmÿ1 for aniline degradation in the pre-

sence of TiO2 and O3 under illumination. The experi-

ments with combinations of O3 and light did not give

further improvement in TOC removal when compared

to the ozonation in the dark. This could be explained

by the Pyrex glass light absorption which ®lters

wavelengths below 300 nm, those required to directly

photoactivate O3.

It is interesting to emphasize that in all the experi-

ments carried out in the presence of O3 in solution the

chromatograms obtained after only 15 min of reaction

reveal the total absence of aromatic compounds such

as aniline and its oxidation derivatives like phenol,

hydroquinone, benzoquinone, nitrobenzene, etc.,

which otherwise are detected in experiments with

illuminated TiO2 suspensions in the absence of O3

in solution [14]. In fact, it is known that O3 reacts very

ef®ciently with aromatic compounds [17]. Concretely,

ozone readily reacts with aniline giving iodoaniline,

maleic acid and oxo-propanedioic acid as main inter-

mediates [18,19]. This fact suggests that an ozonation

pretreatment in the dark followed by photocatalysis

would be a fast way to perform the initial steps of

aniline degradation. Fig. 2 shows the TOC concentra-

tion vs. reaction time for an experiment where ozona-

tion of an aniline solution is followed by TiO2-assisted

photocatalysis. As can be seen, the TOC reduction at

the initial stages of photocatalysis (solid lines in

Fig. 2) is signi®cantly increased with respect to photo-

catalysis without previous ozonation treatment (see

curve b in Fig. 1). The kinetics of photocatalysis

seems to be independent of the extent of ozonation

pretreatment. In fact, the experimental points onto the

solid lines in Fig. 2 follow ®rst-order kinetics with rate

constants of 7.9�10ÿ3 and 7.8�10ÿ3 minÿ1 (correla-

tion coef®cients: 0.994 and 0.996, respectively) for 30

and 120 min preozonation, respectively. On the other

hand, the opposite sequence of events (photocatalysis

pretreatment followed by ozonation) does not appre-

ciably increase the yield of ozonation without photo-

catalytic pretreatment.

Although the strategy of ozonation pretreatment

followed by photocatalysis would be a satisfactory

route for aniline degradation, the simultaneous ozona-

tion and photocatalysis methodology leads to a larger

TOC removal which, despite being more energy and

material demanding, could be preferred from an

applied point of view. In fact, besides the direct

ozonation of the intermediate compounds, in the pre-

sence of TiO2 under illumination ozone can generate

Fig. 2. TOC vs. time in dark ozonized (- - -) samples without TiO2

followed by photocatalysis with TiO2 (ÐÐÐ). (&) First

experience: 30 min ozonation, (~) second experience: 120 min

ozonation. Experimental conditions: [aniline]i�10ÿ3 mol lÿ1;

pHi�3.0; mass of TiO2 (during photocatalysis)�2 g lÿ1; mean

flow of O3 (during ozonation)�5.0�10ÿ4 mol minÿ1. After

ozonation air was bubbled through the suspension for 10 min to

eliminate any ozone remaining in the solution.

L. SaÂnchez et al. / Applied Catalysis B: Environmental 19 (1998) 59±65 61

OH�

radicals through the formation of an ozonide

radical (O�ÿ3 ) in the adsorption layer [20]:

TiO2 � h� ! eÿ � h�

O3 � eÿ ! O�ÿ3

The generated O�ÿ3 species rapidly reacts with H� in

the solution to give HO�3 radicals (rate constant:

5�1010 molÿ1 dm3 sÿ1 [21]), which evolves to give

O2 and OH�

(rate constant: 1.4�105 sÿ1 [21]):

O�ÿ3 � H� ! HO

�3

HO�3 ! O2 � HO

�

In photocatalysis, the hydroxyl radical is generally

considered to be mainly responsible for the organic

attack. It must be considered that the OH�

radicals can

react with O3 (rate constant: 2�109 molÿ1 dm3 sÿ1

[22]),

OH� � O3 ! O2 � HO

�2

in competition with aniline and the corresponding

organic intermediates (reaction rate for aniline reac-

tion with OH�: 8.6�109 molÿ1 dm3 sÿ1 [23]). Further,

the O�ÿ2 species participates in a closed loop reaction

scheme, which leads to a continuous consumption of

ozone [24]. In the absence of O3, dissolved O2 itself

can accept TiO2 conduction band electron and gen-

erate O�ÿ2 ,

O2 � eÿ ! O�ÿ2

which in turn can be protonated to form HO�2 (rate

constant: 2�10ÿ9 molÿ1 dm3 sÿ1 [22]):

O�ÿ2 � H� ! HO

�2

In contrast with HO�3, this species cannot give OH

radicals in a single step [22] and an alternative reaction

pathway has been proposed to account for OH radicals

generation from HO�2 [25]:

2HO�2 ! H2O2 � O2

H2O2 � O�ÿ2 ! HO

� � HOÿ � O2

This mechanism requires a total of three electrons

for the generation of a single OH�

species, which is a

less favored situation if compared with the one elec-

tron needed through the O�ÿ3 reaction pathway.

According to the above mentioned mechanism the

generation of OH radicals from O3 is pH dependent. In

Fig. 3, the time-course of TOC in solution at initial pH

of 3 and 12 is depicted. The ef®ciency of TOC removal

is higher for the lower pH, which is the expected result

considering that the increase in H� concentration

produces more OH radicals.

Fig. 4 shows the time-course of TOC in solution at

different initial concentrations of aniline after irradia-

tion of TiO2 slurries containing O3 in solution at

pH�3. As can be seen, the practical mineralization

of aniline (around 99%) is attained in all the cases; this

mineralization is achieved at longer irradiation times

with increasing initial aniline concentration. On the

other hand, a shoulder appears in each curve of Fig. 4,

which becomes more pronounced by increasing the

initial aniline concentration. This shoulder may prob-

ably correspond to the formation of increasing

amounts of relatively stable intermediates.

Fig. 5 shows the representation of the inverse of the

initial rate of TOC removal as a function of the inverse

of initial aniline concentration. The straight line

obtained (correlation coef®cient: 0.999) suggests that

the experimental points in Fig. 5 follow a Langmuir±

Hinshelwood kinetics:

Ri � kKCi

1� KCi

Fig. 3. Time-course of TOC during irradiation in the presence of

TiO2 and O3 at two different initial pH: (*) 3.0 and (&) 12.0.

Experimental conditions: [aniline]i�10ÿ3 mol lÿ1; mass of

TiO2�2 g lÿ1; mean flow of O3�5.0�10ÿ4 mol minÿ1.


where k and K are the ®rst-order reaction rate and

adsorption equilibrium constants, respectively. The

values of these constants, deducted from the plot,

are 85.5 ppm minÿ1 and 3.47�10ÿ4 ppmÿ1 for k

and K, respectively. Because the kinetic constants

have been obtained using initial time data, it can be

assumed that these parameters are referred to aniline.

The results depicted in Fig. 4 show that mineraliza-

tion of aniline at high concentrations can be accom-

plished by simultaneous ozonation and photocatalysis,

something that either photocatalysis or ozonation

separately cannot afford. This fact is clearly shown

in Fig. 6, in which the time-course of TOC removal of

an approximately 1300 ppm (1.8�10ÿ2 mol lÿ1)

initial aniline solution is depicted. The TOC removal

was carried out by means of three different routes:

photocatalysis, ozonation, and photocataly-

sis�ozonation. As can be seen, while photocatalysis

with ozonation leads to 97% TOC removal after 7 h,

photocatalysis and ozonation separately give only

8.8% and 44% TOC reduction, respectively, for the

same time interval.

In Fig. 7, the TOC remaining in solution after

simultaneous photocatalysis and ozonation treatment

is plotted as a function of the irradiation time for

1300 ppm aniline solutions at pH 3, and in the pre-

sence of several semiconductor masses. The results

Fig. 4. Time-course of TOC for illuminated aqueous aniline

solutions of different initial concentrations. Experimental condi-

tions: pHi�3.0; mass of TiO2�2 g lÿ1; mean flow of

O3�5.0�10ÿ4 mol minÿ1.

Fig. 5. Inverse of the initial rate of TOC removal as a function of

the inverse of the initial aniline concentration. Experimental

conditions: the same as in Fig. 4.

Fig. 6. Time-course of TOC in illuminated aqueous solution under

different experimental conditions: (&) under illumination with

TiO2, (~) in the dark with O3 and (*) under illumination with

TiO2 and O3. Experimental conditions: [aniline]i�1300 ppm;

pHi�3.0; mass of TiO2�2 g lÿ1; mean flow of O3�5.0�10ÿ4 mol minÿ1.


show that the mass of TiO2 has a limited in¯uence on

the degradation yield except for long irradiation times,

for which the suspensions with lower semiconductor

content give lower degree of TOC removal; this fact is

probably due to the gradual poisoning of the catalyst

surface with time. Similarly, the amount of ozone in

solution does not in¯uence the yield of TOC removal

during the ®rst 90 min of irradiation (see Fig. 8). For

longer irradiation times, the ef®ciency of TOC

removal clearly increases with increasing ozone ¯ow

through the suspension.

4. Conclusions

The combination of ozonation and photocatalysis

with TiO2 gives high yields of aniline degradation in

aqueous solutions. Particularly, an ozonation pretreat-

ment followed by photocatalysis signi®cantly

increases the yield of TOC removal in comparison

to either ozonation or photocatalysis acting separately.

This enhanced ef®ciency is not observed for photo-

catalysis pretreatment followed by ozonation. This

behavior can be rationalized considering that ozone

is an effective agent for the degradation of aniline and

the intermediate aromatic compounds, which, other-

wise would be slowly degraded by photocatalysis. In

this way, after ozonation, photocatalysis can progress

at high rates in an aqueous medium free of aromatic

compounds. On the other hand, even higher ef®cien-

cies of TOC removal are obtained with simultaneous

ozonation and photocatalysis. It is proposed that ozone

acts by accepting a photogenerated electron of TiO2

to form an ozonide anion radical, which is an inter-

mediate species in the formation of OH radicals.

Acknowledgements

This work was ®nancially supported by CICYT

(AMB96-0742) and FundacioÂn Domingo MartõÂnez,

to whom we are very grateful.

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