Chemosphere 63 (2006) 269–276 www.elsevier.com/locate/chemosphere
Degradation of the pharmaceutical Metronidazole via UV, Fenton and photo-Fenton processes Hilla Shemer a, Yasemin Kac¸ar Kunukcu b, Karl G. Linden a
a,*
Department of Civil and Environmental Engineering, Duke University, Box 90287, Durham, NC 27708 0287, United States b Department of Environmental Engineering, Akdeniz University, 07200 Topc¸ular, Antalya, Turkey Received 17 April 2005; received in revised form 3 July 2005; accepted 19 July 2005 Available online 9 September 2005
Abstract Degradation rates and removal efficiencies of Metronidazole using UV, UV/H2O2, H2O2/Fe2+, and UV/H2O2/Fe2+ were studied in de-ionized water. The four different oxidation processes were compared for the removal kinetics of the antimicrobial pharmaceutical Metronidazole. It was found that the degradation of Metronidazole by UV and UV/ H2O2 exhibited pseudo-first order reaction kinetics. By applying H2O2/Fe2+, and UV/H2O2/Fe2+ the degradation kinetics followed a second order behavior. The quantum yields for direct photolysis, measured at 254 nm and 200– 400 nm, were 0.0033 and 0.0080 mol E1, respectively. Increasing the concentrations of hydrogen peroxide promoted the oxidation rate by UV/ H2O2. Adding more ferrous ions enhanced the oxidation rate for the H2O2/Fe2+ and UV/H2O2/Fe2+ processes. The major advantages and disadvantages of each process and the complexity of comparing the various advanced oxidation processes on an equal basis are discussed. 2005 Published by Elsevier Ltd. Keywords: Advanced oxidation; Pharmaceuticals; Fenton; Photolysis; Ultraviolet
1. Introduction Pharmaceutical substances and personal care products are an emerging class of aquatic contaminants that have been increasingly detected in ground and surface water (Halling-Sorensen et al., 1998; Stumpf et al., 1999; Zuccato et al., 2000; Jones et al., 2001; Heberer, 2002; Kolpin et al., 2002; Calamari et al., 2003). These compounds reach waterways mainly through the discharge of wastewaters and effluents. Additional pollu* Corresponding author. Tel.: +1 919 660 5196; fax: +1 919 660 5219. E-mail addresses:
[email protected] (H. Shemer),
[email protected] (Y.K. Kunukcu),
[email protected] (K.G. Linden).
0045-6535/$ - see front matter 2005 Published by Elsevier Ltd. doi:10.1016/j.chemosphere.2005.07.029
tion sources are direct emissions from production sites, improper disposal of surplus-drugs in households, medical care, and therapeutic treatment of livestock. Pharmaceuticals are often not completely removed in sewage treatment plants (Carballa et al., 2004) and therefore, are emitted into receiving water systems. Hence, it is necessary to treat the effluents containing pharmaceuticals adequately before discharge or treat intake waters for drinking water treatment plants. Metronidazole is extensively used throughout Europe for treating infections caused by anaerobic bacteria and protozoans such as Trichomonas vaginalis and Giardia lamblia (Tally and Sullivan, 1981; Lau et al., 1992). Metronidazole along with other antibacterial and anticoccidial drugs with nitroimidazole ring structure are suspected of being carcinogens and mutagents
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Fluka; ferrous sulfate from Fisher Scientific. All chemicals were used as-received. All solutions were prepared with de-ionized (DI) water.
Table 1 Physical and chemical properties Metronidazole MW (g mol1) Water solubility (g l1) pKa Vp (Pa) KH (mol dm3 atm1) Molecular structure
171.2 9.5 2.55 4.07 · 107 5.92 · 107
2.2. Experimental setup
OH O2 N
N
CH3 N
2.3. UV irradiation system Irradiation experiments were carried out with low pressure mercury (LP) vapor germicidal lamps (ozonefree, General Electric # G15T8) and medium-pressure mercury (MP) lamp (Hanovia Co., Union, NJ) in a bench scale UV collimated beam apparatus. The LP UV lamp monochromatic light emission was at 253.7 nm, while the relevant MP UV output ranged from 200 to 400 nm. The MP fluence was based on 200–400 nm range due to the overlap of the Metronidazole absorbance spectrum with these wavelengths (see Fig. 1). The emission spectra of these lamps are shown in Fig. 1. UV irradiance (mW cm2) was measured with a radiometer and a UV detector (Model IL1700, SED240 detector, International Light, Co., Newburyport, MA) that had been factory-calibrated, traceable to National Institute of Standards and Technology standards with accuracy of ±7%. The measured incident irradiance at the surface of the sample was corrected for non-homogeneity of irradiation across the surface area of the Petri dish to provide the average incident
1.2
10000 9000
1.0
8000 7000
0.8
6000 0.6
5000 4000
0.4
3000
2.1. Materials Metronidazole (99% purity) was purchased from Acros Organics; hydrogen peroxide (30% w/w) from
0.0 200
250
300
350
-1
1000
2. Materials and methods
-1
2000
0.2
Molar absorption coefficient (M cm )
(Daeseleire et al., 2000). The chemical and physical characteristics of Metronidazole are summarized in Table 1. Being non-biodegradable (Richardson and Bowron, 1985) and soluble in water, Metronidazole is not removed during conventional sewage treatment hence it can accumulate in the aquatic environment (Kummerer et al., 2000). One of the novel technologies for treating polluted sources of drinking water and industrial wastewater is the advanced oxidation processes (AOPs) by which hydroxyl radicals are generated in order to degrade organic pollutants (Ku et al., 1997). AOPs can be applied to fully or partially oxidize pollutants, usually using a combination of oxidants. For example, photochemical advanced oxidation processes include UV/H2O2, UV/O3, UV/ H2O2/O3, UV/H2O2/Fe2+(Fe3+), and UV/TiO2. The efficiency of the various AOPs depend both on the rate of generation of the free radicals and the extent of contact between the radicals and the organic compound (Gogate and Pandit, 2004). It is assumed that a combination of single oxidation processes should result in better degradation rates and efficiencies as compared to individual processes. This assumption relies on (1) the similarity between the mechanisms of destruction (radical oxidation) of the different processes; (2) enhancements in the rate of generation of free radicals using combined methods; (3) synergy among the methods to minimize the drawbacks of individual methods (Gogate and Pandit, 2004). In this study the degradation of Metronidazole using UV, UV/H2O2, H2O2/Fe2+, and UV/H2O2/Fe2+ was investigated. Degradation rates and efficiencies were compared, on the basis of time, between the various oxidation processes applied. The difficulty in comparing these processes on an equal basis is discussed.
All experiments were carried out in a 70 · 50 mm crystallization dish with surface area of 34.2 cm2 open to the atmosphere. A one hundred milliliter solution of Metronidazole in DI water, at an initial concentration of 6 lM (1 mg l1), was gently stirred to maintain homogeneity. Each experiment was conducted in duplicate.
UV lamps relative Intensity
270
0 400
Wavelength (nm)
Fig. 1. Molar absorption spectra of Metronidazole (–) and emission spectra of the MP (–) and LP ( ) lamps.
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irradiance. The average irradiance in the aqueous solution was determined mathematically calculated with a spreadsheet program (Bolton and Linden, 2003). Before each experiment, lamps were warmed up for 15 min to ensure stable lamp output. Aqueous solution pH was adjusted with 0.1 M H2SO4 and 0.1 M NaOH. The pH was measured with an Orion 920 pH-meter equipped with a glass electrode. A slight reduction of the solution pH (maximum of 0.2 units) was observed during the various reactions, hence no buffer solution was required. Hydrogen peroxide assisted photodegradation was studied by adding 25, and 50 mg l1 H2O2 to DI water solution of Metronidazole at pH 6. 2.4. Fenton and photo-Fenton processes Fenton and photo-Fenton oxidation were carried out at initial pH of 3.5. Hydrogen peroxide, at initial concentration of 1 mg l1 (29.4 lM), and ferrous ions (as ferrous sulfate salt) at initial concentrations of 2.94, 5.88 and 11.76 lM were added into the solution of Metronidazole to initiate the oxidation reactions. These concentrations were chosen at levels that are both realistic and that would allow measurable degradation over a period of 5 min, which corresponded to the UV exposure time. When applying 5 mg l1 hydrogen peroxide with 11.76 lM ferrous ions, complete degradation of Metronidazole occurred within 60 s. The experiments
271
were conducted in room light and under UV MP irradiation for Fenton and photo-Fenton reactions, respectively. 2.5. Analytical methods Metronidazole concentration was determined by C18 (7.5 · 150 mm) reversed phase Varian Prostar HPLC (Varian, Inc., Palo Alto, CA) equipped with a photodiode array detector. The mobile phase was 30:70 acetonitrile:25 mM phosphate buffer pH 6.5, at flow rate of 0.8 ml min1, 100 ll injection, and absorbance detection of 220–230 nm. Sample aliquots (0.8 ml) were withdrawn from the aqueous solution at t = 0, 30, 60, 150 and 300 s for analysis. UV absorbance of the aqueous solution was measured by a Cary Bio100 (Varian, Inc., Palo Alto, CA) UV spectrophotometer using a 1 nm slit width, 1 nm step size, 0.3 nm s1 average scan rate, deuterium lamp, and quartz cell.
3. Results and discussion 3.1. UV and UV/H2O2 processes Photodegradation of Metronidazole exhibited pseudo-first order reaction kinetics. Time based rate constants (s1) are summarized in Table 2. A plot of
Table 2 Rate constants of degradation of Metronidazole by UV, UV/H2O2, Fenton and photo-Fenton processes Process
Oxidant/ catalyst
Rate constant
R2
t1/2 (min)
Removal (%)
1
k1 (s )
UV
LP lamp MP lamp
9.36 · 105 3.84 · 104
0.826 0.927
123.4 30.1
6 12
UV/H2O2
H2O2 (mg l1) LP 25 50 MP 25 50
5.28 · 103 6.10 · 103 6.18 · 103 7.77 · 103
0.994 0.998 0.987 0.999
2.2 1.9 1.9 1.5
59 67 58 64
k2 (lM s1) H2O2/Fe2+
UV/H2O2/Fe2+
Fe2+ (lM) 0.00 2.94 5.88 11.76
No degradation 4.56 · 104 9.48 · 104 1.10 · 103
– 0.920 0.942 0.883
– 4.9 2.1 1.0
0 53 73 76
Fe2+ (lM) 0.00 2.94 5.88 11.76
1.56 · 104 6.71 · 104 2.48 · 103 3.83 · 103
0.996 0.997 0.958 0.960
9.1 1.8 0.8 0.5
36 74 91 94
Average irradiance 1.5 mW cm2 for the LP lamp and 1.9 mW cm2 for MP lamp.
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LP lamp
0.0
ln [metronidazole/metronidazole0]
-0.2
d½M=dt U½M ¼ P ð k k s ðkÞÞ k s ðkÞ ¼
-0.4
0 mg/l H2O2
-0.6
25 mg/l H2O2 50 mg/l H2O2
-0.8 -1.0 -1.2 0
100
200
300
400
500
600
MP lamp
0.0 -0.2 -0.4
0 mg/l H2O2
-0.6
25 mg/l H2O2 50 mg/l H2O2
-0.8 -1.0 -1.2 0
100
200
300
400
500
600
UV dose (mJ cm-2) Fig. 2. Metronidazole UV photolysis with and without H2O2 addition using LP (1.5 mW cm2) and MP (1.9 mW cm2) lamps. Metronidazole 6.0 lM; pH 6.0.
ln([M/M0]) versus UV fluence gives a straight line passing through the origin and whose slope gives the fluence (UV dose) based rate constant (mJ1 cm2), as presented in Fig. 2. M and M0 represents the concentration of Metronidazole at UV fluences from 0 to 600 mJ cm2 and its initial concentration, respectively. The error bars indicate the standard deviation of Metronidazole concentration determined experimentally in duplicates. The polychromatic medium pressure lamp appears to be more effective than the UV 254 nm LP lamp for the degradation of Metronidazole, as indicated by the degradation rates and quantum yields. The pseudo-first order rate constants of the degradation of Metronidazole increased by five fold using the MP lamp as compared to the LP lamp, 2.04 · 104 and 3.74 · 105 mJ1 cm2, respectively. Correspondingly, the quantum yields, calculated using Eqs. (1) and (2) (Sharpless and Linden, 2003), were determined to be 0.0033 and 0.0080 mol E1 for the LP and MP lamps, respectively. As seen in Fig. 1 there is substantial overlap of the emission from the medium pressure UV lamp with the absorption band of Metronidazole centered at 310 nm. Whereas, the low pressure lamp emits only at 254 nm.
ð1Þ
E0p ðkÞeðkÞ½1 10aðkÞz aðkÞz
ð2Þ
where ks(k) is the specific rate of light absorption by Metronidazole (E mol1 s1), U is the quantum yield for removal (mol E1), E0p(k) is the incident photon irradiance (103 E cm2 s1), e(k) is the molar absorption coefficient of Metronidazole (M1 cm1), a(k) is the solution absorbance (cm1), and z is the depth of solution. The summation in Eq. (2) was taken over the wavelength range 200–400 nm for the MP lamp, and at k = 254 nm for the LP lamp. Removal efficiency (%) due to direct photolysis was calculated at the end of 5 min at UV dose of 600 mJ cm2. Only 6% and 12% of the initial concentration of Metronidazole were removed using the LP and MP lamps, respectively (Table 2). Addition of hydrogen peroxide to the aqueous solution resulted in faster degradation of Metronidazole, as compared to its direct photolysis. The enhancement was by a factor of 56 and 16 using LP and MP lamps, respectively in the presence of 25 mg l1 hydrogen peroxide. Degradation mechanism of Metronidazole in the presence of H2O2 is oxidation by the very strong oxidizing agent hydroxyl radicals (E0 = 2.8 V). The quantum yield for generating hydroxyl radicals using UV/ H2O2 process is 1.0 (Yu and Barker, 2003), hence, the rapid degradation of the Metronidazole in the presence of H2O2. Only a minor increase of the degradation rate was observed between 25 and 50 mg l1 H2O2 in both lamps used. These results suggest that by adding greater than 25 mg l1 hydrogen peroxide, the excess HO may act to scavenge the H2O2 making the UV/H2O2 process less effective at high hydrogen peroxide concentrations, as shown in Eq. (3). OH þ H O ) HO þ H O ð3Þ 2
2
2
2
The difference in efficiency for degradation of Metronidazole between the LP and MP lamps became less distinct in the presence hydrogen peroxide, as shown in Fig. 2, probably due to the excess in the concentration of H2O2 added to the aqueous solution. For both lamps, approximately 60% and 65% of the initial concentration of Metronidazole was removed with the addition of 25 and 50 mg l1 H2O2, respectively, at the end of 2.5 min irradiation, or a UV dose of 250 mJ cm2 (Table 2). 3.2. Fenton and photo-Fenton process The pH has a significant role determining the efficiency of Fenton and photo-Fenton oxidation. Maximum degradation carried out by H2O2/Fe or UV/ H2O2/Fe is expected at pH between 2 and 4. At pH < 2.5, (FeOH)2+ is formed, which reacts more slowly
with hydrogen peroxide and therefore reduces the degradation efficiency. In addition, the scavenging effect of hydroxyl radicals by hydrogen ions becomes significant at very low pH. At pH > 4, degradation rates decrease due to the formation of Fe(II) complexes and precipitation of ferric oxyhydroxides. Furthermore, the oxidation potential of hydroxyl radicals is known to decrease with an increase in the pH (Gogate and Pandit, 2004). Fenton and photo-Fenton reactions were carried out at pH 3.5, at initial hydrogen peroxide concentration of 29.4 lM (1 mg l1), and ferrous ions at initial concentrations of 2.94, 5.88 and 11.76 lM which resulted in molar ratio of 1:10, 1:5 and 1:2.5 Fe2+:H2O2, respectively. When ferrous ions were added to the Metronidazole aqueous solution containing hydrogen peroxide, the color of the solution changed immediately from colorless to yellowish brown. As the reaction progressed the color became darker. Degradation of Metronidazole by Fenton and photo-Fenton oxidation exhibited second order reaction kinetics. A plot of 1/[M] versus the reaction time (s) gives a straight line whose slope presents the second order rate constant k2 (Table 2). Degradation rate and efficiency of Metronidazole was promoted with increasing Fe2+ concentration from 2.9 to 5.8 lM, by both Fenton and photo-Fenton reactions. For example the removal of Metronidazole, by Fenton reaction, increased from 53% at molar ratio of 1:10 Fe2+:H2O2 to 76% at molar ratio of 1:2.5 Fe2+:H2O2, at a 5 min reaction time. Only a minor increase of the degradation rate was observed between 5.8 and 11.8 lM Fe2+, as shown in Fig. 3a and b. The feature of an optimal dose of ferrous ions, where further addition becomes inefficient, is characteristic of FentonÕs reactions (Pera-Titus et al., 2004). It was found that varying the aqueous solution pH to 3, 4, 6, 7, 8, or 9.5 had no significant effect on the direct photolysis (using LP UV lamp) of Metronidazole. At these pH values (above its pKa of 2.4) there is no change in the ionic charge of the organic molecule and therefore the pH of the solution did not alter the structure or subsequent degradation of Metronidazole. Based on these results it can be concluded that the variations in the initial aqueous solution pH between the oxidation processes studied (6.0 for UV and UV/H2O2 and 3.5 for H2O2/Fe2+ and UV/H2O2/Fe2+) were not factors in the different rates and efficacies obtained. 3.3. Processes comparison The same operating parameters (temperature, volume, reaction time, etc.) were applied in UV, UV/ H2O2, H2O2/Fe2+ and UV/H2O2/Fe2+ processes in order to compare their efficiency for the degradation of Metronidazole using time-based kinetics. The comparison between the UV and UV/H2O2 processes are summarized in Fig. 4a. Fig. 4b presents the removal effi-
(Metronidazole)/(Metronidazole)0
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1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
273
0 μM
2.94 μM
5.88 μM
11.76 μM
(a)
0 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
50
100
150
200
250
0 μM 5.88 μM
300
350
2.94 μM 11.76 μM
(b)
0
50
100
150 200 Time (s)
250
300
350
Fig. 3. Degradation of Metronidazole by Fenton (a) and photo-Fenton (b) oxidation using various concentrations of ferrous ions. Metronidazole 6.0 lM; H2O2 29.4 lM; pH 3.5.
ciency of Metronidazole using Fenton and photo-Fenton oxidation. It is evident from Fig. 4a and Table 2 that the UV photolysis is not as effective as the UV/ H2O2 oxidation for the degradation of Metronidazole. Approximately 88% of the Metronidazole remained in the solution at the end of 5 min irradiation (UV dose of 600 mJ cm2) using the MP lamp, as compared to only 40% residue after 2.5 min of MP-UV/H2O2 oxidation. Addition of only 1 mg l1 hydrogen peroxide enhanced the removal of Metronidazole by a factor of 3 from 12% to 36%. When UV light is absorbed directly by hydrogen peroxide OH radicals are generated by photolysis of the peroxidic bond (Eq. (4)). The highest hydroxyl radical yields are obtained when short-wave ultraviolet wavelengths (200–280 nm) are used due to the stronger absorption by the peroxide at lower wavelengths. H2 O2 + hm ) 2 OH
ð4Þ
Based on the observations described above it can be assumed that Metronidazole degrades more efficiently by hydroxyl radicals rather than by direct photolysis. In FentonÕs process ferrous ions are used as catalyst for hydrogen peroxide degradation, at acidic pH, for production of hydroxyl radicals according to Eq. (5):
274
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compare the iron and UV processes using these rate constants given the experimental set-ups used. An increase of 20% in the removal efficiency of Metronidazole occurred with the photo-Fenton reaction as compared to the Fenton oxidation, in all three initial ferrous ions concentrations (Fig. 4b). By applying UV light to the Fenton process (photo-Fenton) additional hydroxyl radicals are generated and ferric ions are oxidized into ferrous as follows:
(a)
90
MP
80
MP/H2O2
70
LP
60 50
LP/H2O2
Metronidazole removal (%)
40 30 20
Fe3þ + H2 O + hm ) Fe2þ + OH + Hþ
10 0 0 100 90 80 70 60 50 40 30 20 10 0
50
100
150
200
250
300
350
(b)
Fenton Photo-Fenton
0
50
100
150
200
250
300
350
Time (s) Fig. 4. Degradation of 6.0 lM Metronidazole by (a) UV, UV/ 25 mg l1 H2O2 using LP and MP UV lamps, and (b) Fenton and photo-Fenton processes H2O2 29.4 lM (1 mg l1); Fe2+ 5.9 lM.
Fe2þ + H2 O2 ) Fe3þ + OH + OH
ð5Þ
No degradation of Metronidazole was obtained using only hydrogen peroxide without ferrous ions or UV addition, as shown in Fig. 3a. On a time basis, the removal of Metronidazole was more effective using Fenton reaction as compared to UV/H2O2 although lower concentrations of H2O2 were used during the Fenton reaction; 1 mg l1 as compared to 25 mg l1 for H2O2/Fe2+ and UV/H2O2, respectively. Despite the finding that the formation of hydroxyl radicals by decomposition of hydrogen peroxide catalyzed by ferrous ions appears more efficient as compared to UV light, this direct comparison is not appropriate. The variable of UV intensity, which would affect the time based rate constant, was not assessed. In fact, in UV reactors, the UV intensity would be approximately 100 times greater than that used in this study with a bench scale collimated beam system, resulting in a vastly different time-based rate constant. Thus, the time based rate constants can be used to compare the Fenton reactions and the UV dose based rate constants can be used to compare the UV photolysis and oxidation processes, but it is not possible to directly
ð6Þ
Recycling of Fe2+ (Eq. (6)) leads to further generation of OH according to Eq. (5) (Ghaly et al., 2001). Increase in the gross hydroxyl radical concentration results in acceleration of the degradation of Metronidazole using the photo-Fenton reaction as compared to Fenton oxidation. At UV dose of 600 mJ cm2 12% of the initial concentration of Metronidazole was removed using the MP lamp, while at the same dose, over 90% of the Metronidazole was degraded by photo-Fenton reaction conducted by 29.4 lM H2O2 and 5.88 and 11.76 lM ferrous ions. Second order kinetics, which was determined for the Fenton and photo-Fenton, as compared to pseudo-first order for UV and UV/H2O2, implies that the degradation rate of Metronidazole using the former is controlled by the organic compound initial concentration. Whereas, the UV based treatments are independent of the Metronidazole concentration in the aqueous solution. Half-life (t1/2) expresses the time (min) it takes for half of the initial concentration of Metronidazole to react. Addition of 1 mg l1 hydrogen peroxide decreased the half live of Metronidazole from 30.1 to 9.1 min using the MP lamp. Further increases in the hydrogen peroxide concentration to 25 and 50 mg l1 reduced the half live to 1.9 and 1.5 min, respectively. t1/2 was also decreased with an increase in the ferrous ion concentration in both the Fenton and photo-Fenton reactions (Table 2). 3.4. Complexity in comparison of degradation processes The parameters available to compare various degradation processes include time-based kinetics, cost estimation, and energy demand. Most often time-based kinetics is used since all the different processes can be determined based on this parameter. Nevertheless, in photolysis processes the reaction rate depends strongly on the intensity and wavelength of irradiation. These parameters are completely ignored when time-based kinetics is applied, thus it is not possible to fairly compare these different processes on a time basis from bench scale data. The overall costs estimation should represent the sum of the capital costs, the operating costs and maintenance. For a full-scale system these costs strongly
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depend on flow rate and configuration of the reactor as well as the characteristics of the treated water (Azbar et al., 2004). Usually, cost estimations do not consider the ultimate treatment level required. When based on laboratory scale tests, such costs estimations can be very inaccurate for full-scale operations. Also, energy demand does not include hydrogen peroxide and iron salt consumption, which would exclude the Fenton regent treatment from this calculation because there are no easily transferable energy costs for iron and hydrogen peroxide. Therefore, the advantages and drawbacks of each process should be considered in addition to the traditional comparison parameters described above. A major advantage of the photolytic oxidation based processes are operation at conditions of room temperature and pressure and the possibility to effectively use sunlight or near UV light for irradiation, which should result in considerable economic savings especially for large-scale operations (Gogate and Pandit, 2004). Nevertheless, photolytic methods may result in expensive high energy requirements (Ghaly et al., 2001). In Fenton and photo-Fenton processes the reagents are safe to handle and non-threatening to the environment. Highly complicated apparatus are not required which can help the transition from laboratory scale to a large scale (Kavitha and Palanivelu, 2004). A major disadvantage of these processes is strong dependence on the aqueous solution pH and the concentrations of hydrogen peroxide and ferric/ferrous ion. The requirement for acidic pH 2–4 in order to achieve high degradation efficiency and minimize sludge production is a major drawback in many natural waters. Iron salts may be considered a pollution source therefore, neutralization is essential in order to precipitate the dissolved iron as Fe(OH)3. Hence, formation of iron sludge may cause waste disposal issues. 4. Conclusions Degradation of Metronidazole using UV LP and MP lamps and UV/H2O2 exhibited pseudo-first order reaction kinetics. Quantum yields were found to be 0.0033 and 0.0080 mol E1 for LP (254 nm) and MP (200– 400 nm) lamps, respectively. Photolysis was found to be less effective as compared to UV/H2O2 oxidation for the degradation of Metronidazole. Increasing the concentration of hydrogen peroxide promoted the oxidation under UV/H2O2 treatment. Oxidation of Metronidazole carried out by H2O2/ Fe2+ and UV/H2O2/Fe2+ followed second order behavior. Enhancement of destruction was observed when the concentration of the ferrous ions was increased using these processes. An increase of 20% in the removal efficiency of Metronidazole occurred with the photo-Fenton reaction as compared to the Fenton oxidation.
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