Landfill leachate treatment by a photoassisted fenton reaction

~

Pergamon PII: S0273-1223(97)00031-0

War. Sci. Tech. Vol. 35, No.4, pp. 239-248, 1997. © 1997 IAWQ. Published by Elsevier Science Ltd Printed in Great Britain. 0273-1223/97 $17·00 + 0'00

LANDFILL LEACHATE TREATMENT BY A PHOTOASSISTED FENTON REACTION Soo-M. Kim, Sven-U. Geissen and AlfonsVogelpohl lnstitutfiir Thermische Verfahrenstechnik der TV Clausthal, Leibnizstraj3e 15, D-38678 Clausthal-Zellerfeld, Germany

ABSTRACT A combination of the classical Fenton reaction (Fe(II)+H 20i) with UV light, the photoassisted Fenton reaction, has been investigated for the treatment of landfill leachate. The investigation has been carried out with an experimental set-up to establish the optimal treatment conditions. The degradation rate of organic pollutants is strongly promoted by the photoassisted Fenton reaction. The degradation rate depends on the amount of H 20 2 and Fe(II) added, pH value, and radiation intensity. At a specific energy input of 80 kW m- 3 the oxidation rate was increased to six times the rate without radiation (0 kW m -3). At the higher radiation intensity of 160 kW m- 3 the degradation rate was about two times faster than at that of 80 kW m- 3. Due to the regeneration of the consumed Fe(II) ions through the irradiation, the amount of ferrous salt to be added can be remarkably reduced. The optimum conditions were obtained with 1.0 x 10-3 mol 1-1 Fe(II) added, a pH value of 3, and a molar ratio of COD : H 20 2 =1:1. At a COD volume loading of less than 0.6 kg m- 3 h- l , a COD degradation of more than 70% could be obtained with an energy input of 80 kW m- 3. © 1997 IAWQ. Published by Elsevier Science Ltd

KEYWORDS Leachate treatment; photoassisted Fenton reaction; hydrogen peroxide; iron(II); irradiation; hydroxyl radical INTRODUCTION It is well known that landfill leachate contains nonbiodegradable and toxic organic compounds and that

several treatment steps must be combined in order to achieve a satisfactory removal especially of hazardous pollutants. Mostly biological processes are used for the pretreatment of landfill leachate. Other possible methods include precipitation, adsorption on activated carbon, evaporation, and incineration. These methods require high investment- and/or running costs and produce sludge that must be disposed of. For advanced wastewater treatment, chemical oxidation treatment methods which generate powerful oxidants (hydroxyl radicals) are becoming more and more important. The organic compounds may be oxidized by oxygen or free radicals and mineralized completely to water and carbon dioxide. The OH• radicals are of particular interest in this context on account of their high oxidation potential (E0::2.80 V). A combination of the classical Fenton reaction (H 20 2 + Fe(II)) with UV light, the so called photo Fenton reaction (H20 2 + Fe(II) + UV), offers the possibility to produce hydroxyl radicals. This reaction yields an increased concentration of hydroxyl radicals produced from the photolysis of aquated Fe(III) species and ultimately the classical Fenton reaction. The ferric complex ion formed from the Fenton reaction may be 239

240

S.-M. KIM et ai.

reduced to Fe(II) by near UV or even visible light (A. > 300 nm). For example, the photolysis of Fe(OH)2+ in an acidic solution produces ferrous ion with a quantum yield of 0.14 at 313 nm and a hydroxyl radical according to the following reaction (Knight and Sylva, 1975, Faust and Hoigne, 1990). Fe(OH)2+ + hv

--7

Fe2+ + OR-.

(1)

In the presence of hydrogen peroxide, the Fe(II) reduced is subsequently reoxidized by H 20 2 and produces new OH· radicals through the Fenton reaction (Walling, 1975). (2)

Recently, several studies with photoassisted Fenton reaction were carried out for the degradation of model substances such as 4-chlorphenol (Ruppert et al., 1993), nitrophenol (Lipczynsk-Kochany,1991), nitrobenzene and anisol (Zepp et al., 1992) as well as rnetolachlor and methyl parathion (Pignatello and Sun, 1995). However, few studies have been carried out on the treatment of real wastewater by this method. The purpose of this study was to evaluate the photoassisted Fenton reaction for the treatment of landfill leachate. The effect of the addition of hydrogen peroxide and ferrous salt, the influence of the pH value, the irradiation intensity, and the volumetric organic loading on the degradation of organic pollutants were investigated. EXPERIMENTAL METHODS Reagents. Ferrous sulphate heptahydrate (99.5%) was purchased from Riedel de Haen. Hydrogen peroxide (35 w/w) was obtained from Peroxid Chemie GmbH, Germany. The feed solution of H 20 2 and Fe(lI) were prepared with distilled water. Analyses. All samples have been taken after steady state had been reached and were immediately analysed to avoid further reaction. Oxidation was monitored by the reduction of the total organic carbon (TOC), the chemical oxygen demand (COD), and the biological oxygen demand (BODs). TOC and TIC were determined in a TOC analyser (Shimadzu, TOC-5000). For the COD and BODs test, the titrimetric method was used. Any residual H 20 2 was destroyed by enzyme catalase from beef liver (Merck) to prevent interference with COD and BODs. Hydrogen peroxide was measured by the colorimetric method at 420 nm with Ti+ IV . Analysis of Fe(II) was carried out using the modified 1.1O-phenanthroline colorimetric method at 510 nrn. All analyses were conducted according to German standard methods. Equipment. For the investigation of the oxidation by the photoassisted Fenton reaction, optimal irradiation of the reaction volume with UV light and a high oxygen input in the liquid phase is required. The experiments have been carried out using an experimental unit with a jet loop reactor for the optimal oxygen input and a UV reactor for the photolysis of Fe(lII) complex ions as shown in Fig. 1. An immersion tube is integrated in the UV reactor made from stainless steel. The immersion tube is made from synthetically prepared quartz glass which has a high transmission for UV light. The UV reactor is connected to the jet loop reactor by a PVC tube. The total volume of the experimental unit is 6.3 1, the volume of the jet loop reactor 3.9 1, and the volume of the UV reactor with the immersion tube 1 1, respectively. A mercury medium-pressure lamp of the type EQ (EQ 1023-4Z. H. Peschl) with an energy input of either 500 or 1000 W was used as the UV source. This lamp has a high emission capability in the range of near UV and visible light by iron-iodide dotation in order to provide the high energy required for the excitation of the ferric complex ions. The main range of emission is at wave lengths of 300,350, and 380 nm.

Landfill leachate treatment

241

Air

/UV reactor

r----c:::>

Waste gall

Effluent

\

Jet loop reactor

Fe(II)

Wastewater influent Figure 1. Schematic diagram of the experimental set-up for the photoassisted Fenton reaction.

The lamp is located in the immersion tube in order to avoid a direct contact of the light source with the water during operation. Experimental procedure. The investigation has been carried out with leachate from a municipal landfill. The leachate has been treated biologically and its pH has been set at the desired value by the addition of a H 2S0 4 solution before the oxidation. The composition of the landfill leachate after the biological treatment is given in Table 1. The reactor was always filled up with tap water before starting an experiment. The leachate was circulated between the jet loop reactor and the UV reactor at a flow rate of 600 I h- 1. The temperature of the water was maintained at 201°C during the reaction by cooling water from a thermostat. Oxygen was supplied at 40 I h- 1 of air resulting in its saturated concentration in water. The H20 2 dosage was based on the stoichiometric ratio with respect to COD. This was calculated assuming complete oxidation of COD as follows:

Ferrous salt was also added at a molar ratio of H 20 2 to Fe(II) using the other inlet port to avoid a direct reaction with H 20 2 .

S-M. KIM

at

l"

Table I. Composition of the landfill leachate. Concentration

Concentration COD

(mg/l)

1150

TKN

(mg/l)

5-7

BODs

tmg/I)

3-5

CI

(mg/l)

1400

TOC

(mg/l)

350

Fe(III)

(mg/l)

4

TIC

(mg/I)

740

pH

-

8.4 ± 0,2

RESULTS AND DISCUSSION The effect of UV irradiatio/l. Free radicals from hydrogen peroxide can be obtained eit.her by the catalytic reaction of iron salt or by UV-light. In the photoassisted Fenton reaction, hydroxyl radIcals can be formed following three paths. Figure 2 shows schematically the postulated chemical reactions for formation of OHo radicals and for the oxidation of the organic compound A in the photoassisted Fenton reaction.

Photolysis of Fe(III) complex

Photolysis of H 20 2

Direct photolysis A+hv

Radical reaction OR-+A

A> 300 nm A < 300 nm

A oxiJ Fenton reaction Fe(m + H 2 0 2

A oxiJ

OR-•

radical

Figure 2. Scheme of chemical reactions in the photoassisted Fenton reaction.

The oxidation of the organic compounds by OHo radicals follows two reaction steps. OH' radicals can react with organic species through many steps in a chain reaction. Peyton has reported that OHo radicals react unselectively within a millisecond with organic substances, disignated as HRH (Peyton, 1988). After the oxidation of HRH with an OHo radical, the organic radical (RHo) is generated as follows: HRH + OR• RR• + O 2 RH· + RHo

~

H2 0 + RHo, 02 RH ·,

~

HRRH.

~

(3)

(4) (5)

The organic radical reacts subsequently with dissolved oxygen in the water and is converted to a peroxyl radical (02 RH o) (von Sonntag and Schuchmann, 1991). Therefore, the presence of oxygen is very important for the oxidation. If oxygen is not present in the reaction, the undesirable recombination of organic radicals follows as reaction (5), and the degradation of organic compounds can not be promoted. Weichgrebe and Vogelpohl have reported that the specific consumption of hydrogen peroxide may be drastically reduced by the addition of pure oxygen or air (Weichgrebe and Vogelpohl, 1994). In this work, all investigations have been also carried out at a saturated concentration of oxygen.

Landfill leachate treatment

243

In order to determine the effects of UV irradiation on the oxidation, experiments were carried out at three differe~t specific volumetric energy inputs of 0, 80 and 160 kW m- 3 with a molar ratio of COD: H 20 2 = 1: 1 and an lron(lI) concentration of 1.2 x 10- 3 moll-I. Figure 3 demonstrates the effect of UV irradiation on the concentration of TOC in the effluent. The measurements were carried out continuously and the steady state was reached after three times the retention time. In the dark Fenton reaction (0 kW m- 3), the TOC degradation obtained was 17%. In this case, due to the limited ferrous ion concentration (1.2 xlO- 3 moll-I), the concentration of OH- radicals produced may be limited, i.e., one molecule of Fe 2 + produces only one OH- radical as shown by reaction (2).

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400

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Time in min

• • NN kWm-3 k

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•

Conditions

0

80

160 COD:HzO z = 1:1, 't = 2 h, pH =3

1.7

9.6

18.7 HzOz:Fe(ll) = 50: I, T=20 ± I DC

Figure 3. The effect of UV irradiation on the TOe degradation.

In contrast to the dark Fenton reaction, the TOC degradation was increased about three times in the photoassisted Fenton reaction with a specific energy input of 80 kW m- 3. In the irradiated reaction, the UV irradiation contributes by photolysis of Fe(III) complex ions and H 20 2 to the formation of OH- radicals. In the presence of H 20 2 , the regenerated Fe(lI) from the photolysis of Fe(III) species is subsequently reoxidized by H20 2 and produces new OH- radicals, so that the oxidation of organic compounds is accelerated. The TOC degradation is also promoted by a higher energy input of 160 kW m- 3. i.e., the irradiation improves the efficiency of the dark Fenton reaction significantly. The effect of irradiation on the reaction can be also explained by the degradation kinetics. The organic degradation kinetics in the photochemical oxidation may be expressed by a pseudo-first order kinetics according to the following relationship

(6)

244

S.-M. KIM et al.

in which C A is the concentration of the organic substances, C OH the concentration of OH- radicals and k exp the expected pseudo-first order rate constant. The rate constant is a function of radiation intensity, temperature, and pH. In this experiment, the results were obtained from a continuous operation at 2 hours retention time(t). The degradation rate may be calculated from the mass balance as follows (7)

in which CA.i and C A are the influent and effluent concentration, respectively, Vi the wastewater flowrate, and V R the reactor volume. Under the hypothesis of a steady state, the degradation rate r A of the organic substance follows from the relationship

(8) Under the above mentioned conditions, the rate constants obtained were k ::: 3.4 x 1O- 3/min for the specific energy input of 0 kW m- 3 , k::: 19.1 x 10- 3 min- I for 80 kW m- 3 , and k::: 37.3 X 10- 3 min,l for 160 kW m- 3 as indicated by the results in Fig. 3. The degradation rate at 80 kW m- 3 was about six times larger than that with no energy input (0 kW m- 3). At the higher radiation intensity of 160 kW m- 3 the degradation is about two times faster than at 80 kW m- 3 . The effect ofpH. The pH value has a decisive effect on the oxidation potential of OH· radicals because of the reciprocal relation of the oxidation potential to the pH value (EO::: 2.8 V and E14::: 1.95 V). Furthermore, the concentration of inorganic carbon and the hydrolytic speciation of Fe(III) species are strongly affected by the pH value. Therefore, it is required to determine the role of pH in the photoassisted Fenton reaction.

The OH·radicals are scavenged in the secondary reaction at a high reaction rate by scavengers such as carbonate or bicarbonate (Buxton et al., 1988): ~"

OR- + C0 3-

-7

OR" + CO]"",

OH· + HCO J

-7

H20 + CO}·".

(9)

(10)

To insure a high availability of OH· radicals for the destruction of the organic pollutants, a removal of carbonate from the wastewater is, therefore, very advantageous. Inorganic carbon can be easily removed by controlling the pH. In the leachate used, the concentrations of total inorganic carbon (TIC) were 0 mg I-I at a pH::: 3, 170 mg 1-1 at a pH::: 5.5, and 740 mg I-I at a pH=8.2, respectively. In our experiments, the effect of inorganic carbon was not investigated. For the photolysis of Fe(IlI), the hydrolytic speciation of Fe(I11) is very important as Fe 3+, Fe(OH)2+, Fe(OH)2+ and Fe2(OH)24+ have different absorption spectra and photochemical behavior. Faust and Hoigne have reported that the hydrolytic speciation of Fe(III) is dependent on the pH value and that Fe(OH)2+ is predominant at somewhat acidic pH values (pH::: 2.5 - 5) (Faust and Hoigne, 1990). Figure 4 illustrates the effect of pH on the COD and H 20 2 degradation. The experiments have been carried out at pH range from 3 to 8.2 (pH value of the untreated leachate). 70% of the COD was removed at a pH ::: 3 whereas only 20% of the COD was eliminated at a pH ::: 8.2. At acidic pH values, it has been shown that the H 20 2 decomposition to produce OH· radicals has a similar tendency as the COD degradation. However, H 2 0 2 decomposition was drastically increased at a pH:::8.2. In this case, the H 20 2 may not have been decomposed by the COD degradation, but consumed by an autodecomposition. At a pH:::5.5, the COD degradation is drastically decreased and the formation of scale on the immersion tube was observed. The formation of scale may be the result of a precipitation of ferric complexes. Sylva has reported that the precipitation of Fe 3 + to amorphous oxyhydroxide (FeZ03' nHzO) occurs at pH > 3 (Sylva, 1972). The formation of scale on the immersion tube prevents the transmission of UV light into the water, so that the photoassisted Fenton reaction can not take place.

Landfill leachate treatment

245

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~ 4)

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//1

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o

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a>

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:I:

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o L-...---ll....-----L_--.L_---L._.....I-_...L-_..L--_l----J_--.L_-L._-L------J 3

4

5

6

7

8

0

9

pH Figure 4. The effect of the pH value on the COD and H20 2 degradation.(COD o = 1150 mg I-I, NN=80 kW m- 3, 't =2 h, T=20°C, pH=3, Fe(II)=1.2xlO- 3 mol 1-1).

The effect ofH 20 2. Figure 5 illustrates the COD degradation as a function of the stoichiometric H 20 2 input. The investigation has been carried out at a stoichiometric H 20 2 input of 0, 25, 50, 100 and 150%. The diagonal line in Fig. 5 indicates the stoichiometric ratio between COD degradation and the H 20 2 input. 100

r----~----r---...----

-----..._--_--_

.,':::-- stoichiometric ratio ~ o

c

00

c o

Cl

o

()

20

50

100

150

S to i chi 0 met ric H 2 O 2 in put in % Figure 5. The COD dergradation as a function of the H 20 2 input (CODo= 1150 mg 1-1, NN=80 kW m- 3, 't =2 h, T=20°C, pH=3, Fe(II)=1.2xlO- 3 moll-I).

°

From to 50% of the H 20 2 required for a stoichiometric conversion, the COD degradation is higher than the theoretical value. At 25% of the stoichiometric H 20 2 input, the COD is degraded about two times more than to be expected theoretically. This can be explained by the effect of the additionally produced OH- radicals JWST 35:4-1

246

S.-M. KIM et al.

due to the photolysis of the Fe(n!) hydroxy complex and the direct effect of the UV radiation on the molecules of the pollutants. Even at no addition of H 2 0 2 , the COD degradation obtained is about 25%. Here we can see the effect of the Fe(n). The Fe(n) added is oxidized by the dissolved oxygen and produces Fe(nI) and the superoxide (° 2--) which generates hydrogen peroxide in the reaction with H+ as follows: Fez+ + Oz ~ Fe 3+ + OZ'-, 20 2 ,- + 2H+ ~ H 20 Z + Oz.

(I 1) (12)

Fe(lH) formed in reaction (11) is reformed to the Fe(III) hydroxy complex ion by hydrolysis. This Fe(lII) hydroxy complex generates the OH- radicals through the photolysis reaction (1). The degradation at no addition of H20 2 can be attributed to photolysis of the ferric hydroxy complex and to direct photolysis of the organic compounds. Addition of H2 0 2 exceeding the stoichiometric ratio of 100% did not improve the respective maximum degradation. This may be due to autodecomposition of H 20 2 to oxygen and water and the recombination of OH- radicals as follows: 2H 2 0 Z OR- + H 20 2

~

~

2H 2 0 + O2 , H 20 + H02·.

(13) (14)

Since the OH- radical reacts with H 20 2, the H 20 2 itself contributes to the OH- scavenging capacity (Buxton et ai., 1988). Therefore, H 2 0 2 should be added at the optimal concentration to achieve the best degradation. The effect of added ferrous ion. To obtain the optimal Fe(n) amounts, investigations were carried out with molar ratios of H20 2 : Fe(II) = 1:0, 100: 1, 50: 1, and 25: 1. Figure 6 shows the COD degradation as a function of the added Fe(II). Between the ratio of H 20 2 : Fe(n) = 00 and 50: I (1.2 x 10- 3 mol Fe(II) 1- I) the COD degradation distinctly increases with increasing amounts of Fe(n). Addition of Fe(n) above the ratio H 20 2 : Fe(II) = 25: 1 did not affect the degradation, even when the concentration of the Fe(II) was doubled. A higher addition of Fe(1I) resulted in a brown turbidity that hinders the absorption of the UV light required for photolysis and causes the recombination of OH- radicals. In this case, Fe 2 + reacts with OH- radicals as a scavenger (Walling, 1975). (15)

It is desirable that the ratio of H 2 0 2 to Fe(II) should be as small as possible, so that the recombination can be avoided and the sludge production from the iron complex is also reduced.

Even at no addition of Fe(n), the COD degradation is 35%. This reduction may be due to a photolysis of H2 0 2 and the direct photolysis of the organic pollutants. When hydrogen peroxide absorbs the UV light with a wavelength < 300 nm, H 20 2 is converted to OH- radicals as follows (Jacob et al., 1977). (16)

Because the lamp used for this investigation emits in the UV-C range at a small flux, the photolysis of H 20 2 takes place simultaneously during the reaction. By the direct photolysis of the organic pollutants the molecules may reach an excited state and can be oxidized partly by the oxygen. The molecules in the excited state are more reactive than before the absorption of the UV light. However, the effectiveness of direct UV quanta is small.

247

Landfill leachate treatment

'#.

100

c:: c::

80

tI3 "0 tI3

60

0

...

0) Q)

"0

40

0

0

()

20

0'-----'---"-----'---"-----'---.L..----'---.L..--~---1

0,0

0,5

1,0

1,5

I n flu e n t [F e ( II )] / 1 0

2,0

-3

2,5

mol 1-1

Figure 6. The COD degradation as a function of Fe(ll) input. (COD O= 11S0 mg 1-1, NN=80 kW m- 3, 't = 2 h, T=20°C, pH=3, COD: H 20 2=1:1).

The effect of organic volume loading. Figure 7 shows the COD degradation and the concentration of BODs in the effluent as a function of the organic volume loading. The experiments have been carried out at 1, 2, 3 and 4 hours retention time (-r) and a variation of the COD volume loading. A COD volume loading of 0.6 kg m- 3 h- 1 was required to reduce the COD by about 70%. The maximum degradation was about 80% at 0.29 kg COD m- 3 h- 1 and the minimum degradation was 43% at 1.15 kg COD m- 3 h- 1.

100 ~ 0

c::

-

250

• •

80

COD BODs

200

C>

c::

E

0

tI3 "0 tI3

j

60

...

0) Q)

"0

150

c::

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0

50

0,2

c::

::J

20

o0,0

al

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0

()

,

~

0,4

0,6

0,8

1,0

1,2

-w

0

1,4

Organic volume loading in kg CODm-3 h- 1 Figure 7. The effect of the organic volume loading on the degradation (CODo=lISOmg 1-1, NN=80 kW m- 3, T=20°C, pH=3, COD: H 20 2=1:1, H 20 2: Fe(II)=SO:l).

S.-M. KIM et al.

248

The concentration of BOD 5 in the effluent was obviously increased after the oxidation. ~he nonbiodegradable organic compounds were converted to biodegradable substances by the che~I1lCal oxidation. Therefore, it is possible to obtain a satisfactory water quality by a biological treatment. In spIte of the increasing degradation, the BOD 5 concentrations were reduced as shown in Fig. 6. This may be due to a higher mineralisation. CONCLUSION The results in this study indicate that the degradation of organic pollutants in leachate is strongly accelerated by the photoassisted Fenton reaction. The oxidation rate is influenced by many factors, like the radiation intensity, the pH value, the amount of hydrogen peroxide and ferrous salt, and the organic volume loading resulting in the formation of OH· radicals. A higher radiation intensity does accelerate the degradation of the organic pollutants significantly. The optimum conditions obtained for the best degradation are a Fe(lI) concentration of about 1.0 x 10- 3 mol I-I , a pH=3, and a molar ratio COD: H 20 2 = 1: 1, respectively. A COD degradation of above 70% could be obtained at less than 0.6 kg COD m- 3 h- I of COD volume loading. The amount of ferrous salt to be added could be minimized by the regeneration of the consumed Fe(II). With the photo Fenton reaction, it is also possible to use UV light including near UV and visible light effectively, which leads to a more efficient oxidation process. REFERENCES Buxton, G. V., Greenstock, C. L., Helman, W. P. and Ross, A. B. (1988). Critical review of rate constants for reaction of hydrated electrons, hydrogen atoms and hydroxyl radicals (oOHloO-) in aqueous solution. J. Phys. Chem. Ref Dat., 17,513-886. Faust, B. C. and Hoigne, J. (1990). Photolysis of Fe(III)-hydroxy complexes as sources of OHo radicals in clouds, fog and rain. Atoms. Environ., 24A(1), 79-89. Jacob, N., Balakrishnan, I. and Reddy, M. P. (1977). Characterization of the hydroxyl radical in some photochemical reactions. 1. Phys. Chem., 81(1), 17-22. Lipczynska-Kochany, E. (1991). Novel method for a photocatalytic degradation of 4-nitrophenol in homogeneous aqueous solution. Envir. Techno/., 12, 87-92. Peyton, G. R. (1988). Understanding and optimizing ozonelUV treatment for the destruction of hazardous organic compounds in water: mechanism efficiency and by-product. Detoxif. Hazard. Wastes., 1, 353-368. Pignatello, J. J. and Sun, Y. (1995). Complete oxidation of metolachlor and methyl parathion in water by the photoassisted Fenton reaction. Wat. Res., 29(8), 1837-1844. Ruppert, G., Bauer, R. and Heisler, G. (1993). The photo-Fenton reaction - an effective photochemical wastewater treatment process. J. Photochem. Photobio/. A: Chem., 73, 75-78. Slyva, R. N. (1972). The Hydrolysis of Iron(III). Rev. Pure Appl. Chem., 22, 115-132. von Sonntag, C. and Schuchmann, H. -Po (1991). AufkHirung von Peroxyl-Radikalreaktionen In waBriger L6sung mit strahlenchemischen Techniken. Angew. Chemie.. 103, 1255-1279. Walling, C. (1975). Fenton's reagent revisited. Ace. Chem. Res., 8,125-131. Weichgrebe, D. and Vogelpohl, A. (1994). A comparative study of wastewater treatment by chemical wet oxidation. Chem. Eng. Process., 33, 199-203. Zepp, R. G., Faust, B. ~. and. Hoigne, J. (1992). Hydroxyl radical formation in aqueous reactions (pH 3-8) of iron(II) with hydrogen peroxIde: The photo-Fenton reaction. Environ. Sci. Techno/. 26, 313-319.