VUV-photolysis of aqueous systems: Spatial differentiation between volumes of primary and secondary reactions

e:>

Pergamon

Waf. Sci. Tech. Vol. 35. No.4, pp. 25-30.1997. © 1997 IA wQ. Published by Elsevier Science Ltd

PH: S0273-1223(97)OOOO5-X

Printed in Great Britain. 0273-1223/97 $17'00 + 0·00

VUV-PHOTOLYSIS OF AQUEOUS SYSTEMS: SPATIAL DIFFERENTIATION BETWEEN VOLUMES OF PRIMARY AND SECONDARY REACTIONS G. Heit and A. M. Braun Lehrstuhl fiir Umweltmesstechnik. Engler-Bunte lnstitut, Universitiit Karlsruhe, 76128 Karlsruhe, Germany

ABSTRACT Rate and reaction pathway of the degradation of organic compounds in aqueous solution by VUV-irradiation (Xe-excimer: 172 nm) are strongly influenced by the concentration of dissolved molecular oxygen in the volume of primary reactions. Due to the very short lifetimes of the intermediates generated by the homolysis of water, this volume is almost identical with the irradiated fraction of the total reactor volume. Given the fact, that VUV-radiation is totally absorbed within less than 0.1 mm due to the high absorption cross-section of water, investigations on the concentration of dissolved molecular oxygen in the volume of primary reactions require a very high spatial resolution. This spatial resolution is reached by using a very small oxygen optode connected to the photoreactor. The measurements show that VUV-irradiation of a homogenous aqueous system is leading to pronounced heterogeneity: a very thin volume, close to the surface of the light source, which is characterised by the diffusion controlled reactions of primary radicals of very short lifetimes with the organic substrate and the trapping of the C-centered radicals by molecular oxygen, may be differentiated from the remaining major part of the reactor volume in which slower thermal reactions initiated by peroxyl radicals dominate. © 1997 IAWQ. Published by Elsevier Science Ltd KEYWORDS process analysis, oxygen optode, water photolysis, vaccum ultraviolet irradiation, hydroxyl radical INTRODUCTION Water homolyses upon vaccum-ultraviolet (VUV) excitation into hydroxyl radicals and hydrogen atoms (1). These primary species induce a series of reactions which may belong to either a reductive or oxidative process manifold (Gonzalez and Braun, 1995). Applying the VUV-photolysis of water to oxidatively degrade organic compounds dissolved in water (AOP), hydroxyl radicals must react with the organic substrate RH (Legrini et aI., 1993, Jakob et aI., 1993, Gonzalez et al.; 1995). Resulting C-centered radicals of e.g. reaction (2) react efficiently with dissolved molecular oxygen generating peroxyl radicals (3) which exhibit relatively long lifetimes. hv

)

H· + OH·

(1) 25

26

G. HEIT and A. M. BRAUN

(2)

R· + O 2 ---+ ROO·

(3)

' . f d'ISSO I d In absence or in case of . msuffi clent concentratIOn 0 ve mo Iecul ar oxygen, these C-cen tered radicals (2) may react by recombination (4) or by dismutation. (4) Such secondary reactions will finally lead to oligom ers and polym ers. pr~cipitating on and coveri ng the surface of the light source, hence, reducing the incide nt photon flux. FIlmm g (Braun et a/:, I99I! m~y be avoided by maintaining a sufficiently high concen tration of dissolv ed molec ular oxyge n m the IrradIated reactor volume. Peroxyl radicals formed after reaction (3) initiate therma l chain reactio ns e~ancing the overal l rate of mineralisation (5). For these degrad ation processes, dissolv ed molec ular oxyge n IS neede d as well. (5) (6)

In addition, as oxyge n scaven ges very efficiently hydrog en atoms (6), absenc e or insuffi cient concentration of dissolved molecular oxygen, is not only a drawb ack for an efficie nt oxidat ive degrad ation of organic pollutants; such conditions are favourable for the manifo ld of reduct ion reactio ns (Gonz alez and Braun, 1995). Consequently the local concen tration of dissolv ed oxyge n within the spatial range of prima ry reactio ns has a key-function as far as the rate of the oxidative degrad ation of organi c compo unds in aqueo us reaction systems and the pathway of reactions of these compo unds are concer ned. Within the spectral domai n of the Xe-ex cimer emissi on ( A : 172 ± 14 nm), the absorp tion cross-section of max water is very high leading to total absorption of such radiati on within less than 0,1 nun. Taking into account the relative high quantu m yield of reactio n (1) (<1> = 0.42 at 172 nm) (Heit, to be publis hed), the short lifetime of hydroxyl radicals « 1 I.l.s), limiting their wide-s pread diffusi on, their relativ ely high local permanent concentration with respect to the local polluta nt concen tration , initiat ion of oxidat ive degradation (2) yields a highly localized concen tration of C-cent ered radica ls which effects the local concentration of dissolved molecular oxygen, as replac ement of the consum ed oxyge n (3) is diffus ion contro lled and slow by comparIson. Under such conditions and following our workin g hypoth esis and corres pondin g model calcul ations , a strong heterogeneity between the volum e of primary reactions and the non-ir radiate d reacto r volum e must be observed by a spatially differentiated analysis of dissolv ed molec ular oxyge n. METH ODS

Materials. Phenol (Fluka) 99.5% was used withou t furthe r purification. Soluti ons prepar ed with tridistilled water were saturat ed with synthetic air (Mess er Griesh eim).

Experiments. An immer sion type photoc hemic al reactor of 65 ml equipp ed with a (max.) 200 W Xe-excimer

lamp and linked to a reserv oir of IL was used for the photol ysis of contin uously air satura ted aqueo us phenol solutions of 5.10-5-8 ·10-4 M in a semi batch mode. The Xe-ex cimer lamp Was build of two concentric Suprasil® quartz tubes (30.4/2 6.2 mm outer diame ters and 18.0/15.1 nun inner diame ters) and driven by a high voltage power supply (ENI Model HPG-2 ). The effecti ve length of the lamp is 250 rom. The inner electrode (phase) consis ts of an alumin ium foil, cooled with distille d water. The outer electro de was a metal

VUV-photolysis of aqueous systems

27

net fixed around the outer wall of the reactor and connected to the ground. The reaction solutions were pumped from the reservoir into the reactor, passing a static mixer (Figure 1). Analysis. The local concentration of dissolved molecular oxygen has been measured on-line in deviating a

very small part of the solution from the photochemical reactor through a needle of 0,1 mm inner diameter to an oxygen sensitive optode (Heit, to be published). The measuring principle of the optode is based on the quenching of the luminescence of Ru-complexes by oxygen (Camara et al. 1991). The oxygen optode was placed into the needle, and the whole device mounted in an angle of 90° to the surface of the light source and coupled with a micrometric screw. Moving the device towards or away from the surface of the light source allows high resolution of this radial displacement and, hence, precise definition of the location where the reaction mixture is taken for analysis. Measured luminescence lifetimes were converted and stored for later data processing. The degradation of phenol was followed by HPLC-analysis, the diminution of dissolved organic matter was measured by a DOC-analyzer.

-tJ

inner electrode (water cooled) outlet

I~-_

light source

sonde with oxygen optode static mixer inlet

Figure 1. Photochemical reactor RESULTS Figure 2 shows the oxygen concentration, expressed in partial pressure units and measured at intervals of 0.1 mm beginning at distance 0 (at the lamp's surface, bold line), as a function of irradiation time !or different initial concentrations of phenol. As expected, most important oxygen depletions occur at short dIstance from the light source and with increasing concentrations of the organic substrate. Our spatially reso~ved oxygen measurements show that oxygen depletion extends into the non-irradiated volume, where reactIOns (1) and (2) do not take place. Higher local concentrations of C-cent~red radicals l~ad t? a wide-spread ~epleti?n of molecular oxygen which is diffusing towards the productIOn and trappmg SItes o.f the organ~c radIcals. Consecutive thermal oxidation reactions become more important when the concentratIOn of organIC substrate

28

G. HEIT and A. M. BRAUN

" 0 f d"ISS0 1ved molecular oxyger is increased. The results show that under these conditions, the concentratIOn cannot be maintained at optimal value with the reactor in use. 200 180 180 140 120 100 80 80 40 20 0

a) 0

i:' C'I

~

E

= ....

10

20

30

40

50

80

70

80

90

100

110

120

130

200 180 160 140 120 100 80 60 40 20 0

140

150

b) 0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

~

~

lo'l

Q

't:l ~

~ Q

'"

.~

....

't:l

--=...

200 180 160 140 120 100 80 60 40 20 0

Q

= .~ C'I

~

CJ

=

Q

U

c) 0

200 180 160 140 120 100 80 60 40 20 0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

~

d) 0

10

20

30

40

50

80

70

80

90

100

110

120

130

140

150

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

200 180 160 140 120 100 80 80 40 20 0

Irradiation time [min]

Figure 2. Dissolved molecular oxygen concentration in function of irradiation time. Bold line: Measurement at distance 0 at the surface of the light source, consecutive curves at distance intervals of 0.1 mm. Xe• excimer lamp: 100 W, flow rate: 37 ml·s- l , phenol concentrations: 5 (a) 5xlO- M, (b) lxlO-4 M, (c) 2xl0-4 M, (d) 4xlO-4 M, (e) 8xl0-4 M. Higher photon rate densities will also lead to higher local concentrations of primary radicals. Consequently, oxygen depletion should become more important. However, figure 3 shows practically the same profiles of the concentration of dissolved molecular oxygen at distance 0 for three different regimes of the light source. We ded'lce from these curves that already at 55W of electrical power, the rate of water homolysis is leading to a permanent hydroxyl radical concentration which exceeds largely the concentrations of phenol and its oxidation products. In moving the location of analysis outward, it is interesting to note that the slope of oxygen depletion is more important at higher regimes of the light source indicating that fast oxygen consumption takes place in a more important fraction of the reactor volume.

29

VUV-photolysis of aqueous systems

a) o

10

20

40

30

50

60

70

80

90

100

.......---..----.

200 ~-""T""-....,.--~-..,....-...,...-......,.--.,.....180 160 140 120 100 80 60 40 20

0'--_......._.....I._ _....._ . . l . - _......_

o

10

20

30

40

50

b)

......._ _....._..I-_-l.._...J

60

70

80

90

100

200~~~~~ 180 160 140 120 100 80 • 60 40 20 0 ....._

o

c) ......._.....1._ _....._ . . l . - _......_

10

20

40

30

50

......._ _.l..-_..1-_-'-_....J

60

70

80

90

100

Irradiation time [min]

Figure 3. Dissolved molecular oxygen concentration in function of irradiation time. Bold line: measurement at distance 0, at the surface of the light source, consecutive curves at distance intervals of 0, I mm. Phenol: 4xl0-4 M, flow rate: 37·ml S·l, Xe-excimer lamp: (a) 55 W, (b) 100 W, (c) 155 W. The flow rate of the aqueous phenol solution is of secondary importance as far as the overall rate of degradation is concerned. However, our spatially resolved oxygen measurements indicate that oxygen depletion is more important and oflarger radial extension at lower flow rates (Figures 4).

200 180 160 140 120 100 80 60 40 20 %.70 6.• '\).50 4 1111 • 0 3 Ct! IJ . 0 2 0 1 IIJ III lQ . 0 .0

J)jsl

"0

.

p

s""'I'llc

t!/IIJ

a) lQJ

lJ HEIT and A. 1\1. BRAUN

30

c

'"

OJ;

..

~

o

"'0

'"

;>

o

'I> 'I>

"'0

'•

o c o

... ......c o:l

'"
o

V

200 180 160 140

120 100

80 60 40

20

'b.70 6_ DiSt '\).5 4 0• 0 3
1<1

'02 .

/J]IJ

0.1 0 0• SiI,.r,
//J]/J]/

b)

Figure 4. Dissolved molecular oxygen concentration in function of irradiation time and 4 distance from the light source. Xe-excimer lamp: 100 W, phenol: 4x 10. M, I l flow rates: (a) 18 ml·s· , (b) 38,5 ml·s· .

CONCLUSION Spatially resolved analysis of the dissolved molecular oxygen concentration during VUV-photolysis of aqueous solutions containing phenol as a model pollutant confirms a very distinct heterogeneity between reaction zones of fast. light-induced primary reactions and slower thermal oxidation reactions, respectively. Experiments and results presented are an example of successful optode application in process analysis and will be of importance in further development of photochemical reactor design (Braun et al., 1993).

REFERENCES Braun. A M. Jakob. L, Oliveros, E, Oller de Nascimento, C A (1993). Up-scaling photochemical reactions. In: Admnces in Photochemistry, D H Volman, G S Hammond, 0 C Neckers (Eds.), Vol. 18, Wiley, New York. pp. 235-313. Braun. A M. Maurette. M T, Oliveros, E (1991). Photochemical Technology, Wiley, Chichester. Camara. c. Moreno. M C. Orellana, G (1991). Chemical sensing with fiberoptic devices. In: Biosensor.\' with fiberoptics. D L Wise, L B Wingard (Eds.), Humana Press, Clifton, New Jersey. Gonzalez. M C. Braun, AM (1995), VUV photolysis of aqueous solutions of nitrate and nitrite, Res. Chern. Intermed. 21,837-859, Gonzalez. M C , Hashem. T M , Jakob, L, Braun, A M (1995). Oxidative degradation of nitrogen containing organic compounds: vacuum-ultraviolet (VUV) photolysis of aqueous solutions of 3-amino 5• methylisoxazole. Fresenius' J Anal. Chern. 351,92-97. Jakob. L, Hashem, T M. Burki, S, Guindy, N M , Braun, AM (1993). Vacuum-ultraviolet (VUV) photolysis of water: degradation of 4-chlorphenol. J Photochem. Photobiol. A: Chern. 75,97-103. Legrini. O. Oliveros. E, Braun, A M (1993). Photochemical processes for water treatment. Chern. Revs. 93, b71-698.