New Reactor Design for Photocatalytic Wastewater Treatment with


Andreas Haarstrick, Oemer M. Kut, and Elmar Heinzle. Environmental Science & Technology 1996 30 (3), 817-824. Abstract | Full Text HTML | PDF | PDF w/...
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Environ. Sci. Technol. 1994, 28, 670-674

New Reactor Design for Photocatalytic Wastewater Treatment with Ti02 Immobilized on Fused-Silica Glass Fibers: Photomineralization of 4-Chlorophenol K. Hofstadlert and Rupert Bauer’ Institute of Physical Chemistry, Vienna University of Technology, Getreidemarkt 9, A-1 060 Vienna, Austria S. Novallc and 0. Heisler Austrian Energy and Environment, SGP/Waagner-Bir6 GmbH, Siemensstrasse 89, A-12 10 Vienna, Austria

Anew reactor design for the use of titanium dioxide-coated fused-silica glass fibers for wastewater treatment is described. The manufacture of the coating of the fibers is explained in detail. 4-Chlorophenol was used as the test compound. The influences of temperature and irradiation wavelength and the effect of hydrogen peroxide addition were investigated. Activation energy for the initial attack of an OH radical on 4-chlorophenol was calculated to be 20.6 kJ/mol. Degradation rates and quantum yields obtained with this reactor were compared with results measured with Degussa P25 TiO2-slurry treatment. With the present design, the degradation rate of 4-chlorophenol is 1.6 times higher and the destruction of the total organic carbon (TOO is 2.8 times faster. Introduction The most common water treatment technologies for hazardous organic pollutants (I) use adsorption on granulated activated carbon and air stripping, neither of them destroys the contaminants. Negative public perception and the potential hazard of incineration of organic toxic compounds prevented its implementation on a large scale. Hence, the most effective approach to resolve this problem is the mineralization of organic contaminants by using strong oxidizing agents. Chlorine gas, hydrogen peroxide ( 2 ) , the photo-Fenton reaction (3, 4 ) , ozone (51, and photocatalytic semiconductors (6, 7) (SrTiOa, TiO2, ZnS, CdS) are all viable alternative processes. The bandgap of titanium dioxide (TiO2) is 3.2 eV. Excitation with UV light (A C 380 nm) in aqueous solution generates electronhole pairs (e- and h+; eq 1)with an oxidation potential of TiO,

+ hv

-

Ti02(e- + h+)

(1)

2.9 V versus NHE (8,9). The potential is high enough to destroy most organic compounds to carbon dioxide and mineral acids (IO, 11). The mechanism of photocatalytic destruction of hazardous organic compounds on UVirradiated Ti02 in aerated water is widely known and wellexamined in Ti02 suspensions (12-17). The photogenerated hole-electron pairs are able to react with adsorbed compounds (eqs 2 and 3) (18).The reaction Ti02(hf)+ OH,, Ti02(e-) + X

-

X’-

+ TiO,

-

+ TiO,

(2)

X...02, H,O,, 0,

(3)

OH,,’

* Author to whom correspondence should be addressed.

+ Present address: Dipartimento di Chimico “G. Ciamician”, Universita di Bologna, Via Selmi 2, 1-40126 Bologna, Italy. 670

Envlron. Sci. Technoi., Vol. 28, No. 4, 1994

of an hole with OH- leads to the reactive hydroxyl radical (eq 21, which is able to attack most organic compounds. The kinetics of the destruction of organic compoundi on the Ti02 surface is analyzed in terms of the LangmuirHinshelwood mechanism (19). The use of slurries in wastewater treatment systems has some disadvantages: the separation of fine particles is a slow and expensive process and the depth of penetration of UV light is limited because of strong absorptions by Ti02 and dissolved organic species. Different researchers have tried to minimize these problems by immobilizing Ti02 on various materials; for example, on glass plates (201,porous glass (21),Teflon tubes ( I I ) ,silica beads (221, and fiberglass mesh (23, 24). The immobilization of a semiconductor on a support generates unique problems. During the heating process, which is used for fixing the coating, a part of the porous structure gets lost through a sintering process. Furthermore, only a part of the semiconductor is in contact with the solution. Caused by the stirring of the suspensions, the illuminated particles change quickly, in contrast to immobilized Ti02 in the reactor where the whole surface is always “irradiated”. Therefore, there is a need for a reactor whose design provides a high ratio of illuminated immobilized Ti02 to illuminated surface and provides the possibility of total reactor illumination. The present work describes a candidate solution to this problem. Experimental Section Materials. Reagent-grade potassium iodide, phthalic acid, tetrapropyl orthotitanate (97% ), and analytical-grade methanol, tris(hydroxymethyl)aminomethane,and H202 (30% ) were obtained from Riedel de-Haen. Reagent-grade 4-chlorophenol (4-CP), sulfuric acid (9597%1, and analytical-grade ammonium heptamolybdate were obtained from Merck AG. TiOz-P25 was from Degussa. The reagents were used without further purification. Twicedistilled water was used. Pure oxygen (99.995%)was from AGA. All other chemicals were of reagent-grade quality and were used as received. Equipment and Analyses. Irradiations were performed with a universal lamp housing LH151 (Spectral Energy) equipped with a 3-in. fused-silica condenser, a 100-mm fused-silica water filter, a universal filter holder, and a 400-W high-pressure Hg lamp (Osram Ultramed). The low wavelength cutoff filters (WG 280, WG 305 and WG 320) that were used were from Schott. For analyses of 4-CP, an LDC (Model 111)high-pressure liquid chromatograph (HPLC) was used coupled to a Spectra Monitor I11 (Model 1204A) with UV absorbance detection at 225 nm and a Spherisorb S5 ODS column. Elution was done with methanol-water 40-60 vol % at a 0013-936X/94/0928-0670$04.50/0

0 1994 American Chemical Society

GL 45 cap

uv

imaottm

GL 45 cap Shitina nlates

\

\ \ opt n2 and sin 01 (measured from the vertical line to the interface) is larger than (n2/n1).In our case nl = nb and n2 = w o n . The refractive index of Ti02 is higher than that of fused-silica glass in the nonahsorhing (long-wavelength) region (X > 400 nm, na < nnoJ. The refractive index of fused-silica glass is in a wavelength range 400 to 200 nm, practically constant and approximately 1.5. In the range from 400 to 380 nm, the change in the refractive index of Ti02 is 2.4-2.6 (27). Going to shorter wavelengths, the refractive index of Ti02 shows a stronger increase (28,ZS). For this reason, it is impossible that total reflection occurs at the interface (fused-silica glass/TiOp coating), and the light flux is divided one part of it is reflected and the other part leaves the fiber. The reflected part (approximately 20-30%, ref 30) transports light energy through the fiber withalossof intensity after eachcontact with the interface. Theother part reaches theTi0zcoating and generates e--h+ pairs for wastewater treatment. Irradiation was performed with a collimated beam to obtainaconstant illuminationofthe fibers. Measurements at the end of a TiOz-coated fiber (2 mm diameter, 480 mm length) displayed a decrease of the light intensity down to 10% of the initial value. The photoreactor consisted of a glass tube and was filled with apacketofthesecoated fused-silica fibers (140pieces)

UV b ~ m m plexglass t Coaled bsed slica glass fibws wndow Figure 2. Photocatalytic rendor wkh cooling jacket. light window, and iiquld flow

similar to a heat exchanger with tube bundles (see Figure 2). Thefibers werefixedwithfiveperforatedTeflonplates (each with 140 holes) used as baffles. Four of them (with a cut segment) had to divert the stream of the 4-CP solution. A plane Plexiglass window (irradiated area 908 mm2) was attached to the illuminated front side of the reactor in front of the Tiorfree ends of the fibers. A t 5 mm behind this window, a Teflon head plate was fixed to the fibers to avoid direct photochemistry of the 4-CP solution. The free reactor volume was 0.22 L, and the geometrical surface of Ti02 was 4.22 X lo5 mm2. Theoretical calculations concerning coated fibers were made by Marinangeli and Ollis (31). Immobilization of TiOt. After the fused-silica fibers were rinsed with a detergent solution, 10% HCI, HzO, and dried at 100 "C, they were coated hy dipping them into a solution of 10% tetrapropyl orthotitanate in absolute methanol. After removal, the tetrapropyl orthotitanate hydrolyzed in an atmosphere of approximately 50% humidity, producing a white coating on the surface of the fibers. This procedure was repeated five times for each hacth. The coated fibers were calcinated in a furnace (in vertical position). The temperature was raised to 100 'C (200 "Cih) and held for 60 min. Subsequently, the temperature was raised to 600 OC at a rate of 500 "C/h and held for 180 min. All non-fixed particles were removed from the surface, and the fibers were coated a second time. Both first and second dipcoating and calcination procedures were the same, except the final calcination for the second coating was performed at 600 OC for only 70 min. The fronts of the fibers were cleaned and polished with 1rm of diamond paste to free the surface of TiOz. The resulting coating had a thickness between 0.4 and 3.2 pm along the length of the fibers measured with a scanning electron microscope. Using X-ray diffraction, itwas found that the coating consisted of anatase with less than 5 % rutile. Flow Diagram of Photocatalytic Reactor System a n d Experimental Conditions. The 4-CP solution, 2 L for each experiment, was circulated using a membrane pump (rate, 41 L/h). In a storage flask, the solution was mixed and saturated with oxygen. The samples for analysis were taken out of this flask (see Figure 3). All experiments were performed at an initial pH of 5.8 and 25 OC, except for experiments where temperature and pH were varied. The initial concentration of 4-CP was Envkcm scl Techmi, Val 28. No. 4,1994

871

Ill 0,

Table 1. Temperature Dependence: Disappeared (4-CP, TOC) and Appeared (C1-) in Percent after 25-h Irradiation (A > 320 nm)

T(W

4-CP

TOC

C1-

pH

02(mg/L)

11.7

25.7 34.9 46.0 59.0

23.0 33.8 41.9 49.5

6.9 9.9 14.9 17.8

4.4 4.2 4.0

43 32 27

4.0

22

H,O

25.0 42.0 56.7

Table 2. Influence of Wavelength Disappeared (4-CP, TOC) and Appeared (Cl-) in Percent after 25-h Irradiation (T= 25 "C)

Opbcai lalie

Figure 3. Photocatalytic reactor with flow line, thermostat,and storage bulb.

60

I

I

I

I

10

20

30

40

.c

Lo N

50

i

D

a W 0

5

40

O W Q

a o w

6 x

30

20 Temperature

/

50

5 60

"C

Figure 4. Disappearanceof 4-CPand TOC as a functionof temperature.

1.01 X 10-3 mol/L (=72.9 ppm TOC). The 4-CP solution was stirred and purged with pure oxygen during the reaction. The molar ratios of Hz02:4-CP were either 1O:l or 50:l. Experimental Conditions for Suspensions. The experiments with the Degussa P25 suspension were made under identical conditions (irradiation intensity and wavelength, temperature, initial pH) in a flask equipped with a plane Duran window with an area identical to the front window of the flow reactor. The solution (2 L) was stirred, thermostated at 25 "C, and gassed with oxygen during the experiment. The concentration of suspended Ti02 was 2 g/L. Results and Discussion

Influence of Temperature. A linear dependence on temperature of both disappearance of 4-CP and decomposition of TOC was observed in the temperature range from 10 to 60 "C (see Figure 4 and Table 1). The rate of decomposition increased with temperature from 0.28 mg L-l h-l TOC at 11.7 OC to 1.08 mg L-1 h-l TOC at 56.7 "C, calculated for the linear region of the curve (from 2 to 25 h). The amount of free chloride ions was also increased by the temperature, affected by the faster degradation rates and a shift of the C1- adsorption-desorption equilibrium. 672

Environ. Sci. Technol.. Vol. 28, No. 4, 1994

A > (nm)

4-CP

TOC

I at 365 nm (mW/cm2)

C1-

02(mg/L)

280 305 320

48.5 33.2 34.9

24.0 23.1 33.8

22.7 20.2 19.9

33 11.9 9.9

33 33 32

The Arrhenius equation allowed a calculation of the activation energy of the initial hydroxyl radical attack on the 4-CP molecules of 20.6 kJ/mol. Influence of Light Intensity and Wavelength. A comparison was made using three different ranges of wavelength (see Table 2). The experiments using shorter wavelengths seemed necessary, because the refractive index of the coating increases (see Introduction). Direct irradiation of the solution was prevented by the first Teflon plate, and irradiation passing through the fibers should be absorbed by the Ti02coating. During excitation with light (A > 280 nm), the solution became slightly pink because of generated byproducts which have higher polarities than 4-CP (indicated by HPLC and in ref 32). The use of shorter excitation wavelengths resulted in a higher oxidation rate of 4-CP but not of the TOC, suggesting higher stabilities against oxidation to COz of the generated byproducts than 4-CP. Similar byproducts were found during irradiation of a Ti02 suspension with X > 280 nm or direct phototreatment of the substrate in aqueous solution without TiOz. Influence of Added Hydrogen Peroxide. Hydrogen peroxide acts both as an additional e- scavenger (eq 5) in addition to 0 2 and as an oxidizing agent (eq 6). The effect of added H202 depends on its quantity (33).

(7)

The degradation rate of 4-CP could be increased by H202 addition (4-CP:HzOz = 150; see Figure 5 and Table 3). The degradation rates of TOC were almost equal (see Figure 6) in the presence and absence of H202, but with a dependence on the irradiation wavelength (see Table 3). A possible explanation of this complicated behavior could be the recombination reaction of two OH' to H202 in competition with the OH' attack on the oxidation products of 4-CP. According to eq 6, hydroxyl radicals can also be consumed by excess H202, or by HOz' (eq 7). Influence of Initial pH. Lowering the pH from 5.8 to 3.0 resulted in increasing quantities of degraded 4-CP and TOC (see Table 4). At an initial pH of 11, only degradation of 4-CP was affected but none of TOC. The

1.1

I

I

1

n

Suspension u Reactor without H,O, m Reactor with HO ,, in 1:50 molar excess

A

1

2 E E

1.0 0.9

n 0

4 0.8

Table 3, Influence of HzOz: Disappeared 4-CP and TOC in Percent after 25-h Irradiation at 26 OC molar excess

4-CP

TOC

A(nm)

HzOz consumed (mmol/L)

02(mg/L)

1O:l 50:l 501

47.9 55.1 50.0

24.9 25.8 34.4

305 305 320

3.9 5.4 3.1

32 32 32

LC

0 p)

0.7

Table 4. Influence of Initial pH: Disappeared (4-CP, TOC) and Appeared (Cl-) in Percent after 25-h Irradiation (X > 320 nm, T = 25 "C)

U

c

0.4 I

'

0

5

10

15

Irradiation time Flgure 5.

-

75 A

E 70

a a

%..A...

Suspension Reactor without H,O, Reactor with H,O, in 1:50 molar excess

65

%...A

0

60

C

O

b

55

a a 0

2 50 t,

45 0

5

10

15

Irradiation time Flgure 6.

3.0 5.8 11.0

52.4 34.9 14.8

TOC

C1-

35.9 33.8 not measurable

17.8 9.9 19.8

0~(mg/L) 30 32 32

processes. Based on excitation light in the range of 320380 nm, an effective quantum yield for the destruction of 4-CP in the reactor (for light entering the fibers, 440 mm2) was 2 X lo4, compared to the suspension experiment 5 X (irradiated area 908 mm2). These values are in good agreement with those in ref 35.

Conclusion

...

L

w

25

4-CP

h

Disappearance of 4-CP as a function of irradiation time.

U

P

20

/

I 30

pH

20

/

25

30

h

Disappearance of TOC as a function of irradiation time.

variation in pH values entailed an alteration in the properties of the semiconductor/liquid interface: in addition to the redox potentials, adsorption and desorption properties are also different. The observed effects are similar to results reported and explained for the photomineralization of phenol using Ti02 (34). Influence of Pump Speed. A decreased flow rate of 20 L/h (usual 41 L/h) had almost no effect on the decomposition efficiency, indicating turbulent flow conditions between the single fibers and no limits caused by mass transfer in this pump speed region. Experiments without irradiation performed in this reactor after 25 h of pumping gave, with exception of the usual adsorption of 4-CP on the semiconductor surface in the beginning, no measurable decomposition of the substrate and TOC. During an experiment under identical conditions (25 "C, 320 nm cutoff filter) but with uncoated fused-silica glass fibers, 18% of the initial 4-CP concentration and 16?6 of TOC disappeared by direct photolysis after 25 h. The effect of direct photochemistry using coated fibers is minimal because of a high UV absorption by the semiconductor. Data obtained with this reactor and with Ti02 suspensions on the destruction of 4-CP in combination with information on light flux allowed the determination of quantum yields for these photocatalyzed destruction

A simple comparison of these results with experiments obtained with Degussa P-25suspensions showed that the use of the present reactor design by using equal irradiation areas gave yields 1.6 times higher for the amount of destroyed 4-CP; the mineralized amount of TOC was 2.8 times larger (neglecting the losses of light, which did not enter the fibers). This comparison is not completely valid because using a higher local light intensity lowers the quantum yield in a suspension. Furthermore, only a small part of the catalyst is illuminated. A comparison of the disappeared amount of 4-CP and TOC indicates that a part of the free chloride is adsorbed on the surface of the Ti02 coating. The Ti02 coating on fused-silica fibers is very stable, and no losses of activity could be observed. The amount of degraded TOC after 600 h on the same catalyst (equal to a flow of 25 m3 through the reactor) changed approximately 3 % ,which is in the experimental error. The idea to illuminate the total reactor volume using optical fibers with an immobilized photocatalyst could be realized. Prospectives Currently, efforts are underway to prepare different types of Ti02 coatings with the aim of a reduction of e--h+ pair recombination, which is the reason for the small quantum efficiencies. Furthermore, ozone will be tested as an alternative electron scavenger.

Acknowledgments The authors are grateful to K. Eichinger (Institute of Organic Chemistry, TU-Vienna), A. Haunold, 0. M. E. El-Dusouqui, G. Ruppert (all of the Institute of Physical Chemistry, TU-Vienna), and W. D. Schubert (Institute of Inorganic Chemistry, TU-Vienna) for their interest and help. We would like to thank the Forschungsforderungsfonds der GewerblichenWirtschaft Osterreichs (P 3/8669), the Austrian Energy and Environment, the Theodor Envlron. Sci. Technol., Vol. 28, No. 4, 1994 673

Korner Fonds, and the Bundeskammer der gewerblichen Wirtschaft Osterreichs for their financial support.

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