Pergamon

Elecrrochimica Acta, Vol. 42, No. 12. pp. 1877-1882, 1997 fc 1997 Elsevier Science Ltd.

PII: SOO13-4686(96)00400-S All rights reserved. Printed in Great Britain

0013-4686/97 $17.00 + 0.00

Photocatalyzed destruction of aniline in UV-illuminated aqueous TiOz suspensions

Laura Sgnchez, JosC Peral and Xavier Domknech*

Departament de Quimica, Universitat Autbnoma de Barcelona, 08193 Bellaterra, Spain

(Received 13 February 1996; in revised form 30 September 1996)

Abstract-The photocatalytic oxidation of aniline in aqueous suspensions of Ti02 under UV-illumination is studied. The process follows a Langmuir-Hinshelwood kinetic model with an adsorption constant of 1 .I x lo3 dmm3 mol-’ and a reaction rate constant of 8.3 x IO-’ mol dmm3 ss’. The yield of photodegradation strongly depends on pH; higher yields are reported at very acid and basic media and also at pHs near the point of zero charge of Ti02. On the other hand, the yield of the photodegradation increases with the addition of small amounts of Fe*+ in solution. Hydrohydroquinone is the main intermediate product detected, phenol, paraquinone and nitrobenzene being other minor products formed during irradiation. Under illumination, the organic nitrogen is transformed to the inorganic forms: predominantly NH: at acid pH, while NO; and NO, appear at higher pHs. Percentages of TOC reduction of about 85% are attained after 8 h of irradiation of aniline solutions at pH 3 and 6 in the presence of Ti02. 0 1997 Elsevier Science Ltd. All rights reserved

INTRODUCTION

Aniline is a compound used by the chemical industry in several processes, such as the synthesis of pesticides, chemical brighteners, dyes, etc. It is also a common by-product of the petroleum, paper and coal industries. Due to the negative environmental impact of this refractory organic compound there is a need to develop methods to carry out its degradation. Electrochemical oxidation is perhaps the most studied method for the degradation of aniline [l-3]. However, some difficulties seem to arise in the practical use of this method, such as the need for high overvoltages and the poor efficiencies attained for the treatment of dilute effluents, which leads to only partial mineralization of the contaminant.

A more recent method for the destruction of refractory organic compounds in mild conditions is photocatalysis [4, 51. In this method, semiconductor materials (TiOz, ZnO, Fe203, CdS, etc.) are used as photocatalysts. By absorbing light of adequate wavelength this photocatalysis creates e--h+ pairs, which, after migration to the semiconductor surface, generate reactive species, mainly OH radicals that are responsible for the destruction of the organic contaminant [5,6]. In this paper, the degradation of aniline using Ti02 as photocatalyst under different experimental conditions is investigated. The effect

*Author to whom correspondence should be addressed.

that pH and the presence of Fe’+ Ions in solution have on the yield of the aniline photodegradation is emphasized.

EXPERIMENTAL SECTION

All chemicals used in this work 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 - 196°C (Accusorb 2100 E Microneritics) was 59.1 m* g-l. The average particle size, determined by scanning electron microscopy, was 27 nm. Unless otherwise stated, the concentration of Ti02 in suspension in the exper- iments was 2 g 1-l. All experiments were made at 25.0 f O.l”C.

Experiments were conducted in a thermostatic cylindrical Pyrex cell of 130 cm3 capacity. The reaction mixture inside the cell was maintained in suspension 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 actinometer, was 9.2 x 10m4 ein- stein dme3 min-‘.

The aniline and hydroxylated compounds formed during irradiation were analyzed by HPLC. A Metrohm 690 Chromatograph equipped with a

1877

1878 L. SBnchez et al.

vis-UV detector (795 Applied Biosystems) working at 280 nm was used. The stationary phase was an Spherisorb ODS-B column (250 nm x 4.6 mm (i.d.)], while the mobile phase was a mixture of 60% CHjCN and 40%H20. The reaction products were identified by comparing the peak areas with standard solutions. The nitrite and nitrate concentrations were deter- mined by ionic chromatography (Metrohm 690 ion chromatograph) using a Hamilton PRP-Xl00 anion column [I25 mm x 4.6 mm (i.d.)], with a solution of phthalic acid (2 mmol I-‘) and 10% acetone at pH 5 as a mobile phase. TOC of initial and irradiated samples was determined with a Shimadzu TOC 5000 total organic carbon analyser. The amount of ammonium was measured spectrophotometrically (PU 8620 Philips spectrophotometer) using Nessler reagent as colour former.

RESULTS

In Fig. 1, the time course of the concentration of aniline in aerated aqueous solution at pH = 6 and under different experimental conditions is depicted. As can be seen, aniline is not stable under irradiation; in fact, aniline absorbs UV-light of wavelength inferior to 320 nm [7]. This phenomenon does not occur for the protonated form of the aniline (pK, = 4.6) due to the absence of nonbounded electrons that may delocalize into the aromatic system [7].

The decrease of the aniline concentration un UV-illumination is enhanced in the presence of TiOl (Fig. 1). In the dark, a slight adsorption of aniline onto the TiOz surface is observed, attaining a limiting value of about 10% of the initial concentration after 10 min of contact. The yield of aniline photodegrada- tion in presence of TiOz depends on light intensity as shown in Fig. 2; the observed decrease of the slope

I I 75

70

1 65( , , , I

0.0 0.2 0.4 0.6 0.6

time/h

Fig. 1. Time-course of aniline concentration in aerated aqueous solution under different experimental conditions:

(0) in the dark; (w) under UV illumination; (A) in the dark with TiOz (2 g dm-‘); (r) under UV illumination with Ti02 (2 g dm-j). [aniline], = 9.86 x 10e4 mol drn-‘; pH, = 6.0.

50 ,

00 0 20 40 60

Intensity / %

60 100

Fig. 2. Percentage of aniline photodegraded for different

light intensities expressed as percentages of full lamp

intensity. Experimental conditions: pH, = 6.0; [ani- line], = 9.97 x 1O-4 mol dm-); mass of TiOz = 2 g dm-“;

irradiation time = I5 min.

at high light intensities can be ascribed to a more efficient e--h+ recombination.

Figure 3 shows the time course of the aniline in solution for different initial concentrations ranging from 9.86 x lo-’ to 9.86 x 10m4 mol I-‘. From these plots the initial rate (R,) of aniline photodegradation have been determined (see Table 1). The reaction follows a Langmuir-Hinshelwood kinetic model in which R, is related to the initial concentration (C,) of aniline through the following equation:

KkC, R’ = 1 + KC,

where k and K correspond to the reaction rate and the equilibrium adsorption constants, respectively. From the data reported in Table 1, k = 5.0 x 10~smol 1-l min-’ and K = 1.1 x 10m3 1 mol-’ (cor- relation coefficient = 0.996) have been deduced.

1 .OP3

0.0 0.1 0.2 0.3

Time/h

0.4 0.5

Fig. 3. Time-course of aniline for different initial

concentrations. Experimental conditions: pHi = 6.0; mass of TiOz = 2 g dm-‘.

Photocatalyzed destruction of aniline 1879

Table 1.

Initial rate (R,) of aniline photodegradation for different

initial concentrations. Experimental conditions: pH, = 6.0;

massofTiO:=2adm-’

C, (mol drnm3) R, (mol dm-‘S-I)

9.86 x 1O-4 4.2 x lo-’ 7.50 x IO-4 3.4 X 10-7

5.13 x IO-4 3.0 X IO-’

1.97 x 10-d 1.6 x lo-’ 9.86 x 1O-5 8.0 x 1O-8

In general, a parameter that strongly affects the

yield of a photocatalytic process is the pH of the solution. Figure 4 shows the dependence of the yield of aniline removal on the initial pH, both in the dark and under irradiation. As can be seen, a maximum adsorption is reached at pH around 6. At that pH, the unprotonated form of the aniline is the predominant species in solution and the Ti02 particles are poorly charged because their point of zero charge (P.z.c.) is about 5.5 [8]. Under UV-irradiation an increment of aniline eliminated from solution respect to obscurity is observed for all the pHs studied (Fig. 4).

As can be seen from the data depicted in Fig. 4, there are three pH-zones where aniline removal is enhanced: below 3, above I2 and near the p.z.c. For this reason, we have studied the time course of aniline and some photodegradation products formed during irradiation of TiOl suspensions at pH 3, 6 and 12.5. Figure 5 shows the time course of aniline and hydrohydroquinone (HHQ), which is the most abundant intermediate, at the three pHs mentioned above. As can be seen, the rate of aniline removal is higher as pH increases; correspondingly, the maxi- mum observed in the concentration-time profile of HHQ concentration increases with increasing pH. In all three cases, the maximum of HHQ concentration

“1 40 -

a?

.g 30

e ‘e I

c20 : _

z .i

10 RJ

+fy<, o-

2 4 6 6 10 12 14

PH

Fig. 4. Percentage of elimination of aniline in solution at

different initial pH in the dark (0) and under illumination

(rn). Experimental conditions: [aniline], = 9.86 x 10e4 mol dme3; mass of TiOz = 2 g dm-‘; irradiation time = 30 min.

0 2 4 6 6

Time/h

Fig. 5. Time-course of aniline (0) and hydrohydroquinone (m) during illumination for three different initial pHs: (-)

12.5; (-.-,) 6.0; (- - -) 3.0. Experimental conditions:

[aniline], = 9.97 x 10m4 mol drn-‘; mass of

Ti02 = 2 g dmm3.

occurs after 3.5 h of irradiation, approximately. At pH 12.5, nitrobenzene and phenol were detected, attaining maximum concentrations of 1.20 x 10m4 mol I-’ (nitrobenzene) and 8.10 x 10m5 mol I-’ (phenol), after 2 and 3 h of illumination, approxi- mately. At pH 3, trace quantities of phenol and paraquinone were detected. On the other hand, at pH 12.5 the appearance of an initial blue coloration in the suspension is observed during the first minutes of irradiation. This colour turns to brown and fades away after 3 h of illumination. This could be an indication of the probable formation of polyanilines [3]. These changes in the colour of the suspension are not observed at the other pHs studied.

In Fig. 6, the variation of the total organic carbon as a function of the irradiation time at the three pHs is reported. Despite the relatively high efficiency of aniline removal at pH = 12.5 (Fig. 5), the TOC

,,I 70

60

50

k . 40

s 30

20

10

01 , , , I 0 2 4 6 6

Time/h

Fig. 6. Time-course of TOC during illumination of a 9.97 x 1O-4 mol dm-’ aniline solution containing Ti02

(2 g dm-‘) at three initial pHs: (A) 12.5; (W) 6.0; (a) 3.0.

L. Sanchez et al. 1880

1 .oa-3

2 9 1 s.oe-4

:

f f 4.08-4

6

2.0e4

o.oe+o 4 0 2 4 6 8

Time/h

-t

Fig. 7. Time-course of NH: formed during irradiation of an aniline solution at two initial pHs: (0) 3.0; (W) 6.0.

reduction at the pH is significantly lower than those corresponding to the other pHs; this is probably a consequence of the formation, at that pH, of the more photodegradation resistant polyanilines. The best results of TOC reduction are obtained at pH = 6 (Fig. 6). At that pH, TOC levels below 1 ppm are attained after 10 h of irradiation.

During the mineralization of aniline, organic nitrogen is transformed into the corresponding inorganic form (NH:, NO;, NO;). Figure 7 shows the time course of NH: formation during irradiation of aqueous suspensions of aniline at pHs 3 and 6. At pH = 3, practically all the organic nitrogen is transformed to NH: and the oxidized forms (NO? and NO;) have not been detected. At pHs 6 and 12.5, NO, ions have been detected in solution at several concentrations. In Table 2 the concentration of these species at different irradiation times for the two pHs is reported.

Some experiments in which Fe’+ ions were added to aniline solutions at pH = 3 have been carried out. Figure 8 shows the results obtained using different experimental conditions. As can be seen, the best results are obtained with TiOz suspensions in presence of Fe*+ ions under illumination. In fdCt, the

8.044 jjI

0.0 0.2 0.4 0.6 0.6 1.0

Time I h

Fig. 8. Time-course of aniline concentration in aerated aqueous solution in presence of Fe’+ (lo-‘moldm-‘) under different experimental conditions: (a) in the dark;

(m) in the dark with Ti02 (2 g dm-)) and (A) under

illumination with TiO, (2 g dm-l). [ani- line], = 9.97 x 10m4 mol dm-j; pH, = 3.0.

presence of these ions significantly increases the yield of aniline removal; for example, after 6 h of irradiation the aniline practically disappears from the solution and the TOC suffers a 80% reduction, whereas in absence of Fe2+ the aniline remaining in solution was about 10% of the initial concentration and the TOC reduction was about 67%. The yield of aniline removal depends on the concentration of Fe2+ in solution. Table 3 summarizes the initial rate of aniline removal for different Fe*+ concentrations. From these results it appears that the optimum concentration of Fe 2+ for carrying out the aniline photodegradation is 5 x 10-4mol ll’.

DISCUSSION

As can be seen from the results in Fig. 4, the pH is a key parameter for controlling the yield of the aniline photodegradation. The relatively high yields of this process at pH near p.z.c. has been also observed for similar photodegradation processes using TiOz or ZnO as photocatalysts [9, IO].

Table 2. Nitrite and nitrate concentrations in solution at several irradiation times. Experimental conditions: [ani-

line]t = 9.97 x 10e4 mol dmm’; mass of Ti02 = 2 drn-~j

pH, = 6 pH, = 12.5

Time (h) NO; (mol dm-‘) NO; (mol dm-‘) NO; (mol dmm2) NO? (mol dm-‘)

0.0 1.0 2.0 3.0 7 X 10-e 4.5 2 X 10-e 2.0 X 10-5 6.0 5 X 10-e 3.6 x 10m5 5.1 X 10-S 2 X IO-6 7.5 9 x 10-b 7.6 x 10ms 7.3 X 10-s Y X 10-b 8.0 1.0 x 10-s 8.6 x 10ms 8.3 X 10-s I.1 X 10-s

Photocatalyzed destruction of aniline 1881

Table 3. Initial rate (R,) of aniline photodegradation in presence of different initial Fe’+ concentrations. Experimental con- ditions: [aniline], = 9.97 x lo-“ mol drn--‘; mass of TiOz = 2 g dm-‘; pH, = 3.0

C, (Fe’+) (mol dm-‘) R, (mol dm-2 s-l)

0.00 4.2 x IO-’ 5.07 x IO-5 3.6 x IO-’ 1.00 x 10-d 6.6 x IO-’ 5.00 x 10-h 1.1 x IO-6 1.00 x IO-’ 8.6 x IO-’ 1.48 x 1O-3 6.1 x lo-’ 2.00 x 10-j 3.6 x IO-’

However, the best results of the disappearance of the aniline are obtained in alkaline media. In these conditions the generation of OH’ radicals by oxidation of the OH- ions adsorbed onto the semiconductor surface becomes favoured [ 1 I]:

OH- + h+ -+ OH’ (2)

The photogenerated OH’ radicals react with aniline, leading to its transformation. However, it is clear from the results in Fig. 6 that the alkaline medium seems not to be the best environment to carry out the complete mineralization of aniline. These results show that TOC is reduced more slowly at pH = 12.5 than at neutral or acid pH.

The yield of aniline photodegradation at acid pH increases notably with the addition of Fe2+ ions to the aqueous suspensions. Several processes can account for such a behaviour. Indeed, Fe*+ ions can react with the Hz02 produced from oxygen photoreduction at the TiO2 surface to give OH radicals through the Fenton reaction [ 12, 131:

Fe2+ + HzOz -+ Fe3+ + OH- + OH (3)

In turn, Fe?+ can be reduced to Fe2+ at the semiconductor-electrolyte interface:

Fe3+ + e- + Fe2+. (4)

Fe3+ can also suffer hydrolysis giving the photolabile species FeOH2+, capable of absorbing a photon of i < 380 nm and generate Fe2+ and OH radical through the so called photo-Fenton reaction [pro- cesses (5) and (6)] [14]:

Fe’+ + H20 + FeOH2+ + H+ (5)

FeOH2+ + hv + Fe*+ + OH’ (6)

Processes (3) and (6) contribute to the build up of OH’ radicals in solution and, hence, to the enhancement of the photodegradation yield. How- ever, for high Fe2+ concentrations, the effect of the presence of this cation upon aniline photodegrada- tion seems to be detrimental (see Table 3). In fact, assuming that aniline can be photodegraded by direct reaction with photogenerated holes at the semicon- ductor electrolyte interface, a competition for the photogenerated holes between Fe2+ and aniline can be expected.

As intermediate products resulting from the photodegradation of aniline, some hydroxylated aromatic compounds such as phenol and hydrohy- droquinone have been detected together with paraquinone and nitrobenzene. The formation of these intermediates is consistent with the mechanism proposed recently by Brillas et al. [3] for OH radical mediated degradation of aniline. For this process two different pathways can be formulated: (a) through hydrogen abstraction and formation of an imino radical, which is transformed to benzoquinonimine and eventually to paraquinone:

and (b) through the oxidation to nitrobenzene, which is further hydroxylated to phenol and hydrohydroquinone:

1882 L. Sanchez et al.

During illumination, the organic nitrogen is transformed to NH:. This species remain stable in solution at acid pH, as observed in the results depicted in Fig. 7. However, at neutral to alkaline media, NH; can be photocatalytically oxidized to NO; according to processes (7) and (8) [I 51:

2HzO + NH; + 6h+ + NO; + 8H+ (7)

Hz0 + NO; + 2h+ + NO; + 2H+. (8)

These processes lead to the generation of H+ ions. In fact, at pHs 6 and 12.5, where the photooxidation of NH: takes place, a certain acidification of the solution has been detected during illumination (eg 0.24.3 pH units).

It must be noted that the sum of NO,, NO; and NH; concentrations obtained after 8 h of irradiation of aniline suspensions at pH = 6 does not correspond to the stoichiometric amount of organic nitrogen destroyed, as calculated from TOC reduction (about 82%) observed at these experimental conditions (see Fig. 6). This fact can be explained by considering that

NO; and NO; strongly adsorbed onto TiO? surface [l5]. The adsorption of the oxidized species of nitrogen occurs even in basic medium [ 151.

ACKNOWLEDGEMENTS

This work was financially supported by a research

grant from CICYT (AMB95-0885X02-01), for which we are very grateful

5.

6.

7.

8.

9.

IO.

Il. 12.

13. 14.

IS.

REFERENCES

D. W. Kirk. H. Sharifian and F. R. Foulkes, J. Aup/. . . EIectrochem. 15, 285 (1985). C. Comninellis and E. Plattner. Chimiu 42, 250 (1988). E. Brillas, R. M. Bastida, E. Llosa and J: Casado, j. Elrctrochem. Sot. 142, 1733 (1995). X. Domcnech, Photocatalytic Degradation of Contami- nnnts, in Trends in Photochemistry and Photobiology, Vol. I, p. 105. Council of Scientific Research Integration (1991). M. Hoffmann, S. Martin, W. Choi and D. Bahnemann, Chcm. Rev. 95, 69 (1995). J. Herrmann, C. Guillard and P. Pichat, Catalysis Toduy 17, 7 (1993). R. P. Schwarzenbach, P. M. Gschwend and D. M. Imboden, Environmental Organic Chemistry, p. 446. John Wilev, New York (1993). N. Jaffrezic-Renault, P. Pichat, A. Foissy and R. Mercer. J. Phvs. Chem. 90. 2733 (1986). M. Tillas, J. P&al and X. Domenech, A&. Catal. 3,45 (1993). L. Sanchez, J. Peral and X. Domenech, Electrochim. Ac,tu (in press). C. S. Turchi and D. F. Ollis, J. Coral. 122. 178 (1990). J. R. Harbour and M. L. Hair, J. Phys. &em. 83,652 (1979). J. Pignatello, J. Environ. Sci. Technol. 26, 944 (1992). P. Warneck, Fresenius, J. Anal. Chem. 340, 585 (1991). A. Bravo, J. Garcia, X. Domenech and J. Peral, J. Chem. Res. 1993, 376 (1993).