11 Titanium dioxide photocatalysis for the treatment of contaminated waters

LEAMUSZKAT

1. Introduction

The photocatalytic process takes place by a combination of photochemistry and catalysis, which implies that light and a catalyst are necessary to bring about a chemical reaction. It is well-known that ultraviolet light below about 390 nm excites the titanium dioxide semiconductor to promote photoexcited electrons (eci,) in the conduction band and form positive holes (~b) in the valence band. The holes migrate to the oxide surface, generating highly oxidising species which carry out degradation of organic water contaminants. This process enables reduction to occur at the negative sites.

Photocatalysis is found to be effective for removing heavy metals from water by reduction of metal ions to their metallic form, which plate out onto the photocatalyst. Consequently, two actions can be accomplished simultaneously: organics can be decomposed by photocatalytic oxidation while toxic metals, and also precious metals, are removed by reduction. In addition to removal by electrochemical reduction and oxidation, many pollutants, both organic and inorganic, can be removed by adsorption to the photocatalyst surface. Considerable public attention has been focused on this possibility (Carey et at., 1976; Dieter, 1989; Hafner, 1989; Prairie et at., 1993). Several reviews have recently been published which discuss the reaction mechanism and give examples of promising laboratory and field studies (Fettwell, 1989; Hecht, 1990, Robin, 1989; Thornton, 1989). However, while many of the laboratory studies demonstrated the feasibility of the method for almost all classes of hazardous chemicals, mechanistic studies have not been extensive. Titanium dioxide in its anatase form shows the highest photocatalytic detoxification efficiency. It is a wide gap semiconductor with Eg-3.2 eV. Therefore only light below 390 nm is absorbed and capable of forming the e- /h+ pairs which allow the process to occur. This amounts to 5% of the total solar energy reaching the soil surface.

2. Materials and methods

The experimental details are described in previous studies (Muszkat et at., 1992, 1995). Titanium dioxide was used at 100 mg L -1.

D. Rosen et al. (eds); Modem Agriculture and the Environment, 127-132. © 1997 Kluwer Academic Publishers.

128 Lea Muszkat

3. Results and discussion

3.1 Solar photocatalytic decomposition of polluted well water

This occurs by treatment with Fe3+, and titanium dioxide and hydrogen peroxide (Muszkat et aI., 1992).

3.1.1 Halocarbon solvents

Results are presented in Table 1. The concentration of the very common solvents 1,2-dichloroethane, trichloroethylene and tetrachloroethylene was decreased by at least one order of magnitude. The results for the very volatile chloroform were erratic (not shown), probably due to evaporation losses. At least 15 hours of sunlight were required in order to reduce the extremely high initial concentrations of some of these solvents (about 2000 ppb) by more than one order of magnitude.

3.1.2 Halobenzenes

The initial concentrations of several halobenzenes in the well water were in the range 0.5-20 ppb. As shown in Table 1, exposure shorter than 3.5 h was sufficient for complete detoxification of most compounds of this group.

3.1.3 Aromatic hydrocarbons

The two xylenes were completely degraded within less than 3.5 h to below 0.1 ppb.

3.1.4 Triazine herbicides

The triazines detected in the well water were atrazine, trietazine, propazine, terbutryn and an isomer of atrazine (m/z = 215). These molecules were decomposed to less than 0.1 ppb. It can be seen (Table 1) that 15 h exposure was sufficient to reduce the level of atrazine, of its isomer and of propazine to less than 5 ppb, but trietazine and terbutryn required a longer time.

3.1.5 Acetamides, bromacil and urea herbicides

Three widely used herbicides, alachlor, propachlor and bromacil, were found at high levels in the contaminated well. Exposure of 3.5 h resulted in complete disappearance of the urea derivatives methobromuron and I-methoxy-l-methyl-3-phenylurea. In the case of alachlor the initial concentration was relatively high and degradation required a longer exposure time.

3.1.6 Chloroxynil, bromoxynil and some benzene derivatives

The herbicides chloroxynil and bromoxynil (dichloro and dibromo hydroxybenzo­nitrile, respectively) and also dichloro benzaldehyde and di-tert-butyl hydroxy

Titanium dioxide photocatalysis for the treatment of contaminated waters 129

toluene (BHT), which were detected at levels of 0.5-6 ppb, were all completely degraded (to less than 0.1 ppb) within 15 h.

3.1.7 Phthalate esters and fatty acid derivatives

The long-chain carboxylic acids, tetradecanoic, hexadecanoic and octadecanoic acid, which appeared in rather minute concentrations « 12 ppb) in the contaminated well,

Table J. Solar photocatalytic degradation of polluted well water; treatment with Ti02 (1 g L -\), FeH

(0.3 mM) and H202 (0.1 M).

Compound Concentration (Ppb)

Untreated Exposure time (h)

3.5 15 75

CHCh-CH2CI 3 1 0 1 CH3-CCh 5.5 0 0 0 CH2CI-CH2CI 1950 750 70 105 CC4 75 55 0 0 CHCI =CCh 1900 160 4 1 CCh =CCh 2400 160 4 1 Chlorobenzene 21 0 0 0 Dichlorobenzene I 13 0.6 0 0 Dichlorobenzene II 3 0 0 0 Bromoxylene I 0.5 0 0 0 Bromoxylene II 0.5 0 0 0 Trichlorobenzene I 12 1 0 0 Trichlorobenzene II 8.5 0.5 0 0 Bromo-t-butylbenzene 5.5 0.6 0 0 Toluene 7 0 0 0 Xylene-I 3.5 0 0 0 Xylene-II 5.5 0 0 0 Atrazine 43 8.5 4 0 Atrazine isomer 2.5 0 0 0 Trietazine 23 9.5 5 0.3 Propazine 1 0 0 0 Terbutryn 4 3 3 0 Alachlor 640 3.5 0 0 Propachlor 1 0 0 0 Bromacil 115 40 6 0 Metobromuron 22 0 0 0 l-Methoxy-l-methyl-3-phenylurea 11 0 0 0 Chloroxynil 6 4.5 0 0 Bromoxynil 3 0 0 0 Oichlorobenzaldehyde 0.5 0 0 0 Dibutyl hydroxy toluene 0.5 0 0 0 Tetradecanoic acid 4 4.5 6 2 Hexadecanoic acid 12 0 0 0 Octadecanoic acid 2 1.5 3.5 4 9-Octadecenarnide 3.5 0 0 0 Diethyl phthalate 0.5 0 0 0.2 Dioctyl phthalate 2.5 2 2 2 Hydroxymethyl coumarin I 36 0 0 0 Hydroxymethyl coumarin II 1 0 0 0 Dibutoxymethane 1.5 0.3 0 0

130 Lea Muszkat

were essentially unaffected by photocatalysis. Octadeceneamide was degraded within 3.5 h to below 0.1 ppb, possibly by photolysis to octadecenoic acid. Diethyl and diisooctyl esters of phthalic acid were not appreciably degraded.

3.1.8 Hydroxymethyl coumarins and dibutoxy methane

These compounds were completely removed during illumination. The two isomers of hydroxymethyl coumarin and butylmethyl pyrimidinedione (butyl methyl uracil) were degraded within 3.5 h but dibutoxymethane (formaldehyde dibutyl acetal) disappeared only after 15 h.

3.2 Titanium dioxide - solar photocatalysis in the absence of hydrogen peroxide (Muszkat et al., 1995)

Field studies of polluted well water point to the possibility of inhibition of the photodegradation by some unknown factors. This inhibitory effect is prevented by hydrogen peroxide (Muszkat et al., 1995). The data in Table 2 present the photodegradation of dilute malathion and atrazine solutions. It can be noted that photodegradation of these solutions is also effective in the absence of hydrogen peroxide. Comparison with the direct excitation, non-catalytic photodecomposition, emphasises the decisive contribution of titanium dioxide. Photocatalysis of malathion formulations (diluted to 70 ppm) was also examined (Table 3) in open and in polyethylene-covered systems. The half-life value for malathion is about 1 h and the reaction follows first order kinetics over several half-life times. The reaction efficiency seems to be satisfactory in closed systems. While the role of hydrogen peroxide in the photodegradation of trace organics at the sub-ppm levels is not clear, we consider that it is indispensable for the decomposition of inhibitory constituents of polluted water. We have encountered one such case when examining the photodegradation of groundwater from the polluted Nir-Galim well (Table 4).

3.3 Photocatalytic degradation of surjactants

The removal of surfactants which reach waste water in increasingly growing amounts can be achieved by biodegradation. However, although this is the most important process in the waste water treatment plants, biodegradation may be problematic when dealing with some cationic and non-ionic surfactants which are not easily biodegraded. Thus alkylbenzenesulphonates and alkylphenol-polyethox­ylates are resistant to degradation by bacteria. In the case of nonylphenolethoxylate, biodegradation results in formation of nonyl phenol, a toxic product (Giger et at., 1984; Swisher, 1986). Recently it has been shown that titanium dioxide photocatalysed degradation is one of the more promising methods to mineralise these substances. Hidaka et al. (1993), who studied the kinetics of surfactant photodegradation, classified it into fast and slow events. The fast events include the formation of the electron hole pair and of hydroxyl radicals, whereas the slow

Titanium dioxide photocatalysis for the treatment of contaminated waters 131

Table 2. TiOz Photocatalysed decomposition.

Residual concentration (Ppb)

Exposure (h)

o 4

12

Atrazine

90 17 0.1

Malathion

104 0.34 0.2

Blank (no TiOz)

Atrazine

90 81 73

Malathion

104 83 84

Table 3. Effect of solar irradiation through polyethylene film on TiOz-photocatalysed decomposition of malathion (70 ppm)abc.

Exposure (h)

1 3 6

12 18

a Polyethylene film, 100 /Lm thick.

Residual concentration (ppm)

Polyethylene cover

34 9 2.5 0.2

> 0.2 ppb

35 13 4.5 0.3 0.8 ppb

b In the presence of 1.15 mg L -\ detergent TPBS. Detergent is decomposed completely after 1 h of irradiation.

c Irradiation in crystallizer (diameter, 22.5 em); solution volume, 2 L; in the absence of HzOz. d Concentrations corrected for evaporation.

Table 4. Solar, TiOz -photocatalysed decomposition of organic contaminants in water of well Nir Galim 1.

Compound Concentration (ppb) after exposure for

Oh 1 h 5h lOh

Dichloroaniline 45 13 12 4 Benzopyran (mlz 176) 10 4 3.5 3 Atrazine 23 9 6 5 Propazine 6 3 3 3 Alachlor 273 86 83 83 Prometryn 25 12 11 11 Bromacil 27 13 12 10 Cyanobenzoate (mlz 253) 24 12 10 8

132 Lea Muszkat

processes are the benzene ring opening, the intennediate fonnation of peroxides, aldehydes and carboxylic acids, and finally the mineralisation to carbon dioxide. A significant factor in the photodegradation kinetics is the adsorption of surfactants onto the photocatalyst surface. The following order of photodegradation rate has been noticed: anionic> nonionic > cationic surfactants. In the surfactant groups the rate of photooxidation changes according to the specific structure, in the order, aromatic> oxyethylene > alkyl group.

3.4 Intermediate products in the photocatalysed oxidation of organics

The fonnation of carboxylic acid intennediates, mainly formic acid, was observed in the photocatalysis of surfactants (Hidaka et al., 1993). Phosphorus containing surfactant was mineralised to fonn PO~- ions, and nitrogen containing surfactant yielded ammonium, NHt, which is very stable at acidic pH. These results may indicate the necessity of a post-treatment or the combination with other methods. In the photocatalytic oxidation of 1,2,4-trichlorobenzene, the fonnation of biphenyl derivatives has been observed (pelizzetti et ai., 1993). However, these potentially detrimental intennediates were completely degraded following an irradiation of two additional hours. In the photocatalysis of aromatic pollutants (Pichat et aI., 1993) it has been shown that hydroxylation of the aromatic nucleus is a general pattern which is sometimes accompanied by side chain abstraction and decarboxylation. The photocatalytic degradability of chlorophenols was shown to be correlated with descriptors such as the Hammet coefficients and with the octanol-water partition coefficient. The rate of pollutant disappearance is slowed by chloride, sulphate and phosphate, which interact with the photocatalyst at pH below the zero charge point (6.3 for titanium dioxide) and at concentrations higher than those of natural water.

References Carey J H, Lawrence J and Tosine H M 1976 Bull. Environ. ContaIn. Toxicol., 16,697. Dieter H H 1989 Wechselwirkung, 43, 4. Fettwell J 1989 New Sci. 10 June, 36. Giger W, Brunner PH and Shaffner C 1984 Science 225,623. Hafner M 1989 Nature 10, 20. Hecht J 1990 J. New Sci. 14 April, 28. Hidaka H, Zhao J, Nohara K et al. Photocatalyzed mineralization of non-ionic, cationic and anionic

surfactants at Ti<h1H20 interfaces 1993 In Photocatalytic Purification and Treatment of Water and Air. Eds D F Ollis and H Al-Ekabi. pp 251-259 Elsevier.

Muszkat L, Hallman M and Raucher D 1992 J. Photochem. Photobiol. Chern. 65, 409. Muszkat L, Bir L and Feigelson L 1995 J. Photochem. Photobiol. Chern (in press). Pelizzetti E, Minero C, Sega M and Vincenti M 1993 Formation and disappearance of biphenyl

derivatives in photocatalytic transformation of trichlorobenzene In Photocatalytic Purification and Treatment of Water and Air. Eds D F Ollis and H Al-Ekabi. pp 291-300. Elsevier.

Pichat P, Guillard C, Maillard C, Amairic L and D'Oliveira J 1993 Ti02 photocatalytic destruction of water aromatic pollutants, In Photocatalytic Purification and Treatment of Water and Air. Eds D FOllis and H Al-Ekabi. pp 207-223. Elsevier.

Prairie M R, Stange B M and Evans L R 1993 Ti02 photocatalysis for the destruction of organics and the reduction of heavy metals In Photocatalytic Purification and Treatment of Water and Air. Eds D FOllis and H Al-Ekabi. P 353. Elsevier.

Robin E 1989 San Francisco Examiner, 20 May, D-3. Swisher R D Ed. 1986 Surfactant Biodegradation. M Dekker, New York. Thornton J 1989 SERI S&T in Review, 8.