ZnO and VUV processes for oxidation of organophosphate pesticides in water

Journal of Photochemistry and Photobiology A: Chemistry 290 (2014) 86–93

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Comparing the efficacy of UVC, UVC/ZnO and VUV processes for oxidation of organophosphate pesticides in water Gholamreza Moussavi a,∗ , Hiwa Hossaini a , Seyed Javad Jafari a , Mehrdad Farokhi b a b

Department of Environmental Health Engineering, Faculty of Medical Sciences, Tarbiat Modares University, Tehran, Iran Department of Environmental Health Engineering, Faculty of Health, Alborz University of Medical Sciences, Karaj, Iran

a r t i c l e

i n f o

Article history: Received 15 March 2014 Received in revised form 3 June 2014 Accepted 18 June 2014 Available online 25 June 2014 Keywords: Water pollution Emerging contaminants Pesticides Advanced oxidation process Vacuum UV

a b s t r a c t This study investigated the performance of UVC, UVC/ZnO and vacuum UV (VUV) processes for the degradation of diazinon as a model organophosphate pesticide. The highest diazinon degradation was obtained at a solution pH of 5 for UVC, 7.5 for UVC/ZnO, and 9 for VUV. At optimum pH and a reaction time of 30 min, the UVC process degraded 57.8% of the diazinon (10 mg/L) and the UVC/ZnO process degraded 93.3%. By comparison, the VUV process completely degraded 10 mg/L diazinon in a very short reaction time of 90 s. VUV produced a significantly greater degradation rate for diazinon than UVC and UVC/ZnO. Under similar operating conditions, the first-order degradation rate of 5 mg/L diazinon for VUV was 119 times greater than for UVC and 18 times greater than for UVC/ZnO. In continuous mode, the VUV process completely degraded 1 mg/L diazinon in natural water at a hydraulic retention time (HRT) of 2.2 min; complete mineralization was obtained at a HRT of 4.7 min. It was found that the VUV process is a very efficient and viable process for complete mineralization of organophosphate pesticides in water sources and is an appropriate technology for real scale applications. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Organophosphates are a class of pesticides that are mostly insecticides. Since organochlorine insecticides are banned, organophosphate insecticides have become the most widely-used insecticide globally [1]. Organophosphate insecticides can affect the nervous system of humans by disrupting the enzyme that regulates acetylcholine [2]. These insecticides are released into the environment and water sources in effluents from industrial producers and from agricultural drainage from application sites. Organophosphate pesticides are most frequently detected as a major emerging synthetic water contaminant in drinking water sources close to production and application sites, particularly in developing countries [3]. To protect human health, pesticides should be eliminated from contaminated water and industrial effluent before consumption. There is a lack of efficacy in conventional water treatment processes for the degradation of emerging water micropollutants including diazinon. Advanced oxidation processes (AOPs) are now considered the method of choice for treatment of such contaminated water [4,5]. Different AOPs have been developed

∗ Corresponding author. Tel.: +98 21 82883827; fax: +98 21 82883825. E-mail address: [email protected] (G. Moussavi). http://dx.doi.org/10.1016/j.jphotochem.2014.06.010 1010-6030/© 2014 Elsevier B.V. All rights reserved.

and studied over the past few decades for removal of different classes of toxic organic compound contaminants in water. These include UV/catalyst, UV/H2 O2 , UV/O3 , H2 O2 /O3 , UV/H2 O2 /O3 , catalytic ozonation, Fenton reagents and its derivatives, sonolysis, and wet air oxidation. Heterogeneous photocatalysis is one of the most successful of the emerging AOPs [6] and is used to decompose refractory organic compounds found in the air, water, and wastewater streams. Photocatalysis has unique features that make it one of the most successful AOPs [6]. It uses a semiconductor material (TiO2 or ZnO) irradiated with a UV light source to generate very reactive oxidant species (particularly • OH at E = 2.8 eV). • OH non-selectively attacks the contaminants and degrades them [7]. The main challenges to the use of photocatalysis are that it requires efficient separation of the nanocatalyst from the treated water and concern about the effect of residual nanoparticles on human and environmental health. Alternatives to photocatalysis for the generation of • OH are homogeneous AOPs, including UV/H2 O2 , UV/O3 , and H2 O2 /O3 . UV/O3 produces the highest • OH yield per mass of oxidants [8]; however, the UV/O3 process requires the external addition of ozone produced by an ozone generator to the water. Moreover, residual ozone must be removed from the treated water and the off-gas stream. These features make the process complex and costintensive.

G. Moussavi et al. / Journal of Photochemistry and Photobiology A: Chemistry 290 (2014) 86–93

Vacuum UV (VUV) lamps have recently been considered to generate • OH and overcome the defects of classic AOPs. In the VUV process, radical species, including • H and • OH, are generated mainly by homolysis and photo-ionization of water molecules using a UV lamp emitting radiation at a wavelength of 185 nm [4,9,10]. Ozone is also generated in situ during the process from the interaction of dissolved oxygen and VUV radiation [11]. In situ generation of radical-type oxidant species [7] make VUV a very efficient AOP; overcoming the need for external addition of a catalyst or chemical to the reactor [5,8] and making the process efficient, simple to operate, energy-intensive, and cost-effective. VUV can be applied for oxidizing toxic organic contaminants in streams; however, a review of the literature found no reports comparing the efficacy of photocatalysis and VUV for oxidation of pesticides as toxic water contaminants. This study compared the UVC process (conventional oxidation process), UVC/ZnO process (classic AOP photocatalytic process), and VUV process (novel AOP) for the oxidation and mineralization of diazinon organophosphate pesticide under different experimental conditions. The most efficient process (VUV) was also tested in the continuous mode for oxidizing and mineralizing diazinon in distilled water and tap water. The findings of this study can be useful in establishing the appropriate technology for treating water contaminated with pesticides. Diazinon is a common organophosphate insecticide used in residential, agricultural, and ranching applications [12]. It is released into the environment via human activity and is most frequently detected in water as an emerging synthetic water contaminant [3]. Because diazinon is toxic to humans, the WHO has recommended a maximum acceptable concentration of 20 ␮g/L in drinking water. A drinking water equivalent level of 7 for diazinon has been established by the USEPA [13]. Developing an appropriate and efficient technology to decrease the amount of diazinon in contaminated water to recommended levels is critical for public health protection. 2. Materials and methods 2.1. Diazinon-contaminated water samples Two types of diazinon-laden water samples were used in the present study: distilled water and tap water. The concentration of diazinon in the test samples was regulated by diluting aliquots of diazinon stock solution (25 mg/L) with distilled and tap water. 2.2. Experimental set up and procedure The experimental setup was a tubular glass reactor with an internal diameter of 25 mm and a height of 400 mm into which a quartz sleeve 15 mm in diameter was longitudinally inserted at the axial center. A UV lamp was placed in the sleeve. The reactor was also equipped with an air-supplying system for mixing the contents of the reactor. For VUV oxidation experiments, a 5.7 W low-pressure mercury UV lamp (Heraeus Co.) was used that emitted radiation at 254 nm (flux of 56 ␮W/cm2 at 1 cm from lamp surface) and 185 nm (<10%; flux of 5 ␮W/cm2 at 1 cm from lamp surface). UVC and UVC/ZnO tests used a 9 W low-pressure mercury lamp (Philips Co.) emitting radiation mainly at 254 nm. The ZnO nanoparticles used as a photocatalyst in the UVC/ZnO process was purchased from Nanoamerican Co. Manufacturer information stated that ZnO particles had a mean particle size of 20 nm and BET of 90 m2 /g. The photo-reactor was wrapped in an aluminum sheet to prevent the emission of UV radiation into the lab. The total volume of the photo-reactor was 120 mL; the volume of solution used in the oxidation tests was 100 mL.

87

The study was divided into two phases. In the first phase, the photo-reactor operated in batch mode and the influence of pH, diazinon concentration, ZnO concentration (in UVC/ZnO process only), and reaction time on diazinon degradation in the VUV, UVC, and UVC/ZnO processes using contaminated distilled water was recorded. The effect of water components and tert-butanol radical scavenger on the degradation of diazinon in the VUV process (the most efficient process) was also evaluated. In the VUV and UVC batch tests, 100 mL of contaminated water at a given pH and concentration of diazinon was transferred to the reactor, the lamp was switched on and the reactor was run for a specified time. The same operational procedure used for UVC was followed for the UVC/ZnO batch tests except that a predetermined amount of ZnO was added to the diazinon solution. The content of the reactor at all the selected processes was completely mixed using compressedair aeration. At the end of each test, the lamp was switched off and the content of the reactors was analyzed for residual diazinon and total organic carbon (TOC). In the UVC/ZnO tests, the content of the reactor was first centrifuged at 15,000 × g for 10 min, then filtered using cellulose acetate filter with a pore size of 0.2 ␮m, and the filtrate was analyzed. In the second phase, the most efficient process for degradation and mineralization of diazinon (VUV) was further examined in continuous mode and the influence of hydraulic retention time (HRT = volume/flow) was tested for degradation of diazinon in contaminated tap water. At each HRT, the VUV process was fed at a known concentration of diazinon. The effluent was sampled after 3 HRT periods and the sample was analyzed for residual diazinon. 2.3. Analysis The diazinon concentration was measured using a Knauer HPLC (C18 ODS reverse phase column; 250 × 4.6 × 5) with a UV-PDA detector at a wavelength of 247.5 nm. The mobile phase was a mixture of acetonitrile and water with a volumetric ratio of 65/35 and an injection flow rate of 1 mL/min. The performance of UVC, UVC/ZnO and VUV processes for the degradation of diazinon in distilled water at different experimental conditions was evaluated based on the degradation percentage and the degradation rate. The percentages of diazinon degradation in the selected processes were calculated from the following equation: diazinon degradation (%) =

(C0 − Ct ) × 100 C0

(1)

The observed rate of diazinon degradation (robs ) in the UVC, VUV and UVC/ZnO processes was also determined from the following first-order linearized reaction equation: robs = −kobs · C

(2)

where C0 and Ct are the diazinon concentrations at the beginning and at time t after the reaction has started, respectively, and kobs is the observed first-order rate constant of diazinon degradation in the oxidation processes. The degree of diazinon mineralization was determined by measuring the TOC of the solution using a TOC analyzer (Shimadzu Co.) before and after oxidation. The degree of mineralization of diazinon was calculated as: diazinon mineralization (%) =

(TOC0 − TOCt ) × 100 TOC0

(3)

where TOC0 and TOCt denote the TOC concentrations before and after oxidation, respectively. The pH levels of the samples were measured using a pH meter (SenseIon 37.58, Hack). The temperature of the solutions was determined using a thermometer.

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100 100

Diazinon degradaon (%)

Diazinon degradaon (%)

pH=5

80

pH=7.5 pH=9

60

40

80 60 40

pH=5 pH=7.5

20

20

pH=9 0

0 0

5

10

15

20

25

30

0

35

5

10

Reacon me (min) Fig. 1. Effect of solution pH on diazinon degradation in UVC process (diazinon: 10 mg/L; solution pH: 5, 7.5 and 9; reaction time: 5–30 min).

15 20 Reacon me (min)

25

30

35

Fig. 2. Effect of solution pH on diazinon degradation in UVC/ZnO process (diazinon: 10 mg/L; solution pH: 5, 7.5 and 9; catalyst concentration: 100 mg/L; reaction time: 5–30 min).

3.2. UVC/ZnO process 3. Results and discussion The influence of water pH (5, 7.5, 9) on the performance of the VUV-, UVC-, and UVC/ZnO processes for the degradation of diazinon (10 mg/L) in distilled water was examined.

The performance of the UVC/ZnO process in the degradation of diazinon (10 mg/L) in distilled water was investigated at pHs of 5, 7.5 and 9 at reaction times from 5 min to 30 min. The irradiation of ZnO with UV resulted in the generation of • OH as the main oxidation agent as:

3.1. UVC process

ZnO + UV photons + H2 O → • OH

The performance of the UVC process for degradation of diazinon (10 mg/L) in distilled water was investigated in acidic (pH = 5), neutral (pH = 7.5), and alkaline (pH = 9) solutions as a function of reaction time (Fig. 1). As seen in Fig. 1, the efficiency of UVC process in diazinon degradation at solution pHs of 5, 7.5 and 9 was 57.8 ± 1.5%, 51.4 ± 1.1% and 44.8 ± 2.4%, respectively, at a reaction time of 30 min. The rate of diazinon degradation in the UVC process calculated from Eq. (2) is given in Table 1. As shown in Table 1, the rate of degradation decreased with the increase of solution pH confirming that the degradation of diazinon decreased as the pH increased in this process. It is known that • OH is not generated in the UVC process [9] and thus the diazinon might degraded in this process mainly UV photolysis, as shown in Eq. (4): diazinon + UV photons → products

(4)

The pKa of diazinon is 2.4 for carboxyl meaning that the carboxylic anionic form was the predominant form of diazinon in water at all pH values. The nearer the solution pH was to pKa , the greater the molecular fraction of undissosiated diazinon in the water sample. The acidic form of diazinon was more reactive and its degradation potential was higher; thus, diazinon degradation increased as pH decreased [14,15] due to an increase in the quantum yield [14].

(5)

The diazinon molecules degraded mainly via oxidation with • OH (Eq. (6)), although some diazinon might have been degraded by direct oxidation with UV photons (Eq. (4)). diazinon + • OH → products

(6)

In the UVC process diazinon absorb photons and the energy released drives degradation process induced by light. Diazinon absorbs light at the wavelength greater than 290 nm [16,17] whereas the UVC lamp used in the study emitted UV at the maximum wavelength of 254 nm which is not coincided with the maximum absorption wavelength of diazinon. Therefore, the direct photolysis using UVC radiation is not an efficient process for oxidation of diazinon. In comparison, the UV/ZnO process generates • OH with a reaction rate constant of 109–1011 M−1 s−1 for degradation of organic molecules [17] which noneselectively attack the organic molecules resulted in greater diazinon degradation percentages in the UVC/ZnO process than in the UVC process. An increase in reaction time increased the total amount of holes (h+) and • OH which in turn increased the diazinon degradation efficiency [18]. As observed in Fig. 2, the best performance for UVC/ZnO was obtained at pH = 7.5. For a reaction time of 30 min, diazinon degradation was 88.4% at pH = 5, 93.3% at pH = 7.5, and 83.7% at pH = 9. The greatest diazinon degradation was obtained for the neutral pH of 7.5. As shown in Table 1, the value of diazinon degradation rate in UVC/ZnO process was the highest at solution pH of 7.5 verifying that the process attained the highest rate in the diazinon

Table 1 The kinetic information for degradation of diazinon in UVC, UVC/ZnO, and VUV processes under different solution pHs. Solution pH

Degradation process VUV

5 7.5 9

UV/ZnO

UVC

R2

kobs (min−1 )

rVUV (mg/L min)

R2

kobs (min−1 )

rUVC/ZnO (mg/L min)

R2

kobs (min−1 )

rUVC (mg/L min)

0.992 0.997 0.993

1.02 1.141 1.380

10.20 11.41 13.80

0.993 0.996 0.998

0.064 0.084 0.052

0.64 0.84 0.52

0.993 0.995 0.998

0.031 0.027 0.022

0.31 0.27 0.22

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89

100 100 Diazinon degradaon (%)

Diazinon removal (%)

80 UVC/ZnO 60

Adsorpon Synergy

40

80 60

pH=5 pH=7.5

40

pH=9

20

20

0

0 0

25

50

75

100

125

150

175

200

0

225

30

60

90 120 Reacon me (s)

150

180

210

ZnO concentraon (mg/L) Fig. 3. Effect of ZnO concentration on diazinon degradation in UVC/ZnO process (diazinon: 10 mg/L; solution pH: 7.5; catalyst concentration: 10–200 mg/L; reaction time: 30 min).

degradation at this pH. This finding is self-evident, considering that the pKa of diazinon is 2.6 for carboxyl and the pHpzc of ZnO is about 9 [19]. Diazinon molecules are negatively charged at pH > 2.6 and the ZnO surface is positively charged at pH < 9, so a greater amount of diazinon was adsorbed onto the ZnO as the pH of the solution increased from 5 to 7.5 (below pH = 9, at which the charge on the ZnO surface is zero), improving the photo catalytic efficacy. Zn(OH)2 may form on the surface of the photocatalyst at an alkaline pH, which makes it nonreactive and inhibits the formation of OH radicals [20]. As the ZnO dissolves at an alkaline pH, it limits the holes (h+ ) transfer and thus its availability for • OH generation [20]. This can be another reason for the decrease in diazinon degradation at the alkaline pH value. Daneshvar et al. [21] investigated the degradation of diazinon using the UVC/ZnO process and found that optimum degradation occurred at a solution pH of 5. Since the pH of natural water is approximately neutral, the remaining experiments were carried out at neutral pH (pH = 7.5) to resemble real field conditions. The effect of ZnO concentration was investigated on the performance of the UVC/ZnO process for degradation of diazinon (10 mg/L; pH = 7.5). Fig. 3 shows the diazinon degradation in UVC/ZnO compared to diazinon adsorption onto the ZnO catalyst (dark condition) as a function of ZnO concentration. As shown, the adsorption of diazinon increased from 15% at a ZnO concentration of 10 mg/L to 35% at a ZnO concentration of 200 mg/L. Diazinon degradation in the absence of ZnO (UVC photolysis) was 51.4% (Fig. 1). The removal of diazinon increased from 70.7% in the presence of 10 mg/L ZnO to 93% in the presence of 100 mg/L ZnO. The increase in diazinon removal with the increase in ZnO concentration (up to 100 mg/L) can be attributed to the increase of active adsorption sites and increased interaction between the ZnO particles and diazinon molecules [19,22,23]. Moreover, an increase in ZnO catalyst concentration increased the absorbance of photons and increased the generation of • OH [24,25]. The increase in ZnO concentration to 200 mg/L decreased the diazinon removal to below 88% for the UVC/ZnO process. The decrease in diazinon removal with the increase in the ZnO concentration to above 100 mg/L may be related to the increase in turbidity in the suspension and the resulting decrease in UV light penetration in the solution, which decreases • OH generation [25,26]. The synergistic effect of combining UVC and ZnO nanoparticles was calculated as: synergy =

removal in UVC − (adsorption onto ZnO ZnO + removal in UVC process)

(7)

Fig. 4. Effect of solution pH on diazinon degradation in VUV process (diazinon: 10 mg/L; solution pH: 5, 7.5 and 9; reaction time: 10–200 s).

As seen in Fig. 3, the greatest synergistic effect was obtained at a ZnO concentration of 100 mg/L. Accordingly, the optimum concentration of ZnO in the UVC/ZnO process for the degradation of diazinon was 100 mg/L. 3.3. VUV process The performance of the VUV process for the degradation of diazinon (10 mg/L) in distilled water was investigated using acidic (pH = 5), neutral (pH = 7.5), and alkaline (pH = 9) solutions as a function of reaction times of 10–200 s. As seen in Fig. 4, diazinon degradation in the VUV process was not considerably affected by pH values of 5 and 7.5. At the solution pHs of 5 and 7.5, the degradation of diazinon increased from about 15.5% and 16.6% for a reaction time of 10 s to about 96.1% and 97.7%, respectively, for a reaction time of 200 s. The degradation of diazinon was higher at pH of 9 than that at the solution pHs of 5 and 7.5. Diazinon degradation increased at the solution pH of 9 as reaction time increased and complete degradation was attained at a reaction time of 90 s. Also, the rate of diazinon degradation at solution pHs of 5, 7.5 and 9 was 10.20, 11.40 and 13.80 mg/L min, respectively (Table 1), which confirms the higher degradation rate of diazinon in UVC/ZnO process at pH of 9 than those of 7.5 and 5. Imoberdorf and Mohseni [9] reported that the variation of solution pH (5, 7.5, 9) had little effect on the degradation of natural organic matter using VUV. The degradation of 4-tert-octylphenol in the VUV process has been found to be higher at an acidic pH than that at an alkaline pH [27]. This discrepancy can be related to the difference in the experimental conditions, particularly the target reactant properties and the degradation mechanism. The effect of solution pH on the degradation level of diazinon in the VUV process can be explained by homolysis and photochemical ionization of water molecules in the presence of VUV and the disintegration of oxygen molecules in the presence of VUV [9,11,28–32] as: hv 185 nm ˙ + H˙ H2 O −→ OH

(8)

hv 185 nm ˙ + H+ + eeq − H2 O −→ OH

(9)

hv 185 nm

−→ 2O˙

(10)

O˙ + O2 → O3

(11)

O2

For neutral pH (7.5) and acidic pH (5) solutions, diazinon molecules were mainly oxidized by • OH generated by homolysis

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and photochemical ionization of water, as shown in Eqs. (8) and (9) and partly by direct ozone oxidation (Eqs. (10)–(12)). diazinon + O3 → products

2OH˙ + O2

(13)

This phenomenon increased the concentration of • OH at an alkaline pH (9) over values of 5 and 7.5, resulting in an increase in degradation efficiency. Note that the pH of a solution did not noticeably change before and after the VUV reactions, indicating there was no need for pH modification of treated water, which decreases treatment cost. Huang et al. [27] reported that the degradation of 4-tert-octylphenol in a VUV reactor was lower at pH = 10 than that at pH values of 3 and 6. Imoberdorf and Mohseni [10] found that pH values of 5, 7.5 and 9 had little effect on NOM degradation efficiency in the VUV process. Comparing the results of this study with the cited literature indicates that the effect of pH on the performance of the VUV process strongly depends on the type and properties of the contaminant and on the composition of the solution under reaction. 3.4. Comparison of UVC, UVC/ZnO and VUV processes

Diazinon degradaon (%)

−→

60 50 40 30

Diazinon = 5 mg/L

20

Diazinon = 10 mg/L

10

Diazinon = 20 mg/L

0 0

10

20

30

40

50

60

70

Reacon me (min)

(B)

100 Diazinon degradaon (%)

alkaline pH

(A)

70

(12)

Under alkaline conditions (pH = 9), more hydroxide was available, which accelerated the decomposition of ozone generated from the reaction of VUV with the oxygen present in the solution (Eq. (9)) to • OH (Eq. (13)) [8]: O3 + H2 O

80

80 Diazinon = 5 mg/L

60

Diazinon = 10 mg/L Diazinon = 20 mg/L

40 20 0 10

0

20 30 Reacon me (min)

40

50

(C)

100 Diazinon degradaon (%)

The performance of UVC, UVC/ZnO and VUV processes for the degradation of diazinon was compared at different diazinon concentrations in distilled water based on the degradation percentage and rate indexes calculated from Eqs. (1) and (2) [33]. The percentage of degradation for different concentrations of diazinon as a function of reaction time for the UVC, UVC/ZnO and VUV processes are shown in Fig. 5(A)–(C). The values of diazinon degradation rate are shown in Table 2 and indicate that the first-order reaction model precisely matched the experimental data of diazinon degradation in the experiments with a determination coefficient (R2 ) greater than 0.98 for all oxidation processes. The degradation percentage (Fig. 5) and rate (Table 2) were in order of VUV>UVC/ZnO>UVC for all the three tested concentration. This means, for example, that the rate of diazinon degradation at 5 mg/L and optimum pH of 7.5 in the VUV process is 118 times greater than that in the UVC and 17 times greater than that in the UVC/ZnO. It is seen therefore that the VUV process performed considerably better than the UVC and UVC/ZnO processes at the degradation of diazinon under similar conditions. Another point observed in Table 2 is that the rate of diazinon degradation in all the three selected processes increased with the increase of initial diazinon concentration. This finding can be such explained that as the first order oxidation reaction model was best fitted with the experimental data (Table 2), the rate of degradation increased directly with the increase of initial diazinon concentration. The higher rate of diazinon degradation attained for VUV over UVC can be attributed to the difference in oxidizing agents and thus the degradation mechanism which was described in Section 3.2. In

80 60 Diazinon = 5 mg/L

40

Diazinon = 10 mg/L 20

Diazinon = 20 mg/L

0 0

50

100

150

200

250

Reacon me (s) Fig. 5. Performance of (A) UVC, (B) UVC/ZnO, and (C) VUV processes for different concentrations of diazinon as a function of reaction time (diazinon: 5, 10, 20 mg/L; solution pH: 7.5; ZnO concentration in UVC/ZnO process: 100 mg/L).

Table 2 The kinetic information for degradation of various concentrations of diazinon in UVC, UVC/ZnO, and VUV processes. Concentration (mg/L)

Degradation process VUV

5 10 20

UV/ZnO

UVC

R2

kobs (min−1 )

robs (mg/L min)

R2

kobs (min−1 )

robs (mg/L min)

R2

kobs (min−1 )

robs (mg/L min)

0.992 0.995 0.992

1.896 1.128 0.682

9.48 11.28 13.64

0.993 0.989 0.989

0.103 0.094 0.053

0.52 0.94 1.06

0.986 0.983 0.990

0.016 0.018 0.019

0.08 0.18 0.38

G. Moussavi et al. / Journal of Photochemistry and Photobiology A: Chemistry 290 (2014) 86–93

100

Diazinon degradaon (%)

80

60

no ion (dislled water)

40

sulfate chloride bicrbonate

20

nitrate carbonate phosphate

0 0

30

60

90

120

150

150

210

Reacon me (s) Fig. 6. Effect of anions on diazinon degradation in VUV process (diazinon: 10 mg/L; solution pH: 7.5; reaction time: 10–200 s; anion concentration: 0.1 M each).

addition, the photon energy of VUV (6.71 eV) is higher than that of UVC (4.89 eV) [27,28], and resulted in greater diazinon degradation through direct photolysis in the VUV process. The higher diazinon degradation in the VUV than in the UVC/ZnO resulted from the increased reactions in the formation of • OH in the VUV process (Eqs. (6) and (7)). Imoberdorf and Mohseni [10] reported obtaining zero order kinetics for degradation of 2,4-D using VUV. The discrepancy can be related to the difference in the nature of the tested contaminant as well as the experimental conditions. 3.5. Effect of water anions on diazinon degradation in VUV process Different mineral salts are present in water which may affect the performance of UV oxidation processes used for degradation of the contaminants in water. The efficiency of the VUV process was examined in the presence of 0.1 M of different anions (Cl− , CO3 2− , HCO3 − , SO4 2− , NO3 − , and PO4 3− ) individually and in combination at a solution pH of 7.5 and diazinon concentration of 10 mg/L. Fig. 6 compares the time-course results for degradation of diazinon in the VUV process in the presence of anions. As shown, the degradation level of diazinon decreased considerably in the presence of all anions in tap water. The kinetics of diazinon degradation was determined based on the data in Fig. 6 to better illustrate the effect of anions on the VUV process. The values of diazinon degradation rate are shown in Fig. 7 and indicate that all anions strongly suppressed the rate of diazinon degradation reaction. The greatest decrease was 12

rVUV (mg/L.min)

10 8 6 4 2 0 No anion

Bicarbonate

Sulfate

Chloride

Carbonate

Nitrate

Phosphate

Mixture

Type of ingredient Fig. 7. The rate of diazinon degradation in VUV process in the presence of anions (diazinon: 10 mg/L; solution pH: 7.5; reaction time: 200 s; anion concentration: 0.1 M each).

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observed at the presence of PO4 3− , NO3 − and CO3 2− . The degradation rate of diazinon in the absence of anions was 11.46 mg/L min while that in the presence of PO4 3− , NO3 − and CO3 2− was 2.85, 3.36 and 4.38 mg/L min, respectively. The anions depending their types interfered with the VUV process in two ways: (a) some of them absorbed UV radiation at wavelengths of 185 nm, inhibiting homolysis and photochemical ionization of water molecules and constraining the generation of • OH; and (b) some scavenged • OH, limiting the amount of • OH available for diazinon oxidation [9,10,34,35]. The significant decrease in diazinon degradation rate in the presence of PO4 3− and CO3 2− can be related to scavenging • OH by these anions [34,36–38], confirming • OH as the main oxidation agent in the VUV process. Although the anion radicals form from the reaction of these anions with • OH, they have less oxidation potential than • OH [34] and thus their reaction with diazinon molecules is less efficient. NO3 − absorbed VUV radiation and thus inhibited the homolysis and photochemical ionization of water molecules resulted in limiting the generation of • OH. Therefore, the reduction of diazinon degradation rate at the presence of NO3 − strongly re-confirms the generation of • OH as the main working oxidant specie. The generation of • OH in the VUV process was further verified by adding 0.1 M tert-butanol. The results indicated that the degradation of diazinon decreased by 41% at the presence of 0.1 M tert-butanol. As tert-butanol is a wellknown • OH scavenger, markedly decrease of diazinon degradation at the presence of tert-butanol clearly implies that the indirect reaction with • OH was the main mechanism contributed in oxidation [39] of diazinon in the VUV. The effect of the anion mixture was also determined for diazinon degradation in the VUV process (Fig. 7). Fig. 7 shows that the mixture of Cl− , CO3 3− , HCO3 − , SO4 2− , PO4 3− and NO3 − (the concentration of each anion was 0.1 M) strongly decreased degradation rate from 11.28 mg/L min in the absence of anions (distilled water) to 2.97 mg/L min in the presence of the anion mixture. The strong decrease in diazinon degradation in the VUV process in the presence of mixture of anions is due to both absorption of VUV radiations by some anions and thus retarding the generation of • OH and to scavenging the • OH generated in the reactor. Ocampo-Pérez et al. [40] reported that the reaction rate constant for cytarabine degradation was 50% lower for groundwater in the presence of radical scavenging species than for ultrapure water. 3.6. VUV process for treatment of contaminated natural water Anions in the water affected the performance of the VUV reactor for the degradation of diazinon. Since these ions are frequently found together in water, the degradation of diazinon in an actual tap water sample was investigated to determine their influence in water on the VUV process. The tap water had a pH of 7.4, total alkalinity of 85 mg/L CaCO3 , electrical conductivity of 269 ␮S/cm, and Cl− , SO4 2− , PO4 3− and NO3 − concentrations of 45, 29, 0.7, and 6.5 mg/L, respectively. Fig. 8 compares the degradation of diazinon in distilled water and in tap water using VUV for similar pH values (∼7.5). As seen, as the reaction time increased to 200 s, the degradation of diazinon increased to 97.7% for distilled water and 87.9% for tap water. Although the constituents in tap water decreased the degradation of diazinon by less than 10%, VUV still attained a high degree of degradation in tap water at a very short reaction time, suggesting that the VUV process is an efficient system for oxidation and removal of organophosphate pesticides in a real contaminated water source. The decreased diazinon degradation in tap water could be a result of the complex water matrix. Giri et al. [14] observed a marked decrease in photodegradation of perfluorooctanoic acid in tap water over that in ultrapure water using a VUV photoreactor.

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Diazinon degradaon (%)

100 80 60 40

Disled water Tap water

20 0 0

50

100

150

200

250

Reacon me (s) Fig. 8. The performance of VUV process in degradation of diazinon in distilled water and tap water as a function of reaction time (diazinon: 10 mg/L; solution pH: 7.5; reaction time: 10–200 s).

3.7. Effect of HRT on continuous operation of VUV process For practical water treatment, a process should usually operate in continuous mode. The VUV process was investigated for fullscale treatment of water contaminated with toxic compounds. The effect of HRT on the degradation and mineralization of diazinon at concentrations of 1 mg/L and 10 mg/L in contaminated tap water was investigated. Fig. 9 shows the degree of degradation (Fig. 9(A)) and mineralization (Fig. 9(B)) of diazinon as a function of HRT from 0.6 to 8.5 min. The degree of degradation and mineralization of

(A)

Diazinon degradaon (%)

100 80 60 40

Diazinon=1 mg/L

Diazinon=10 mg/L

20 0 0

2

4

6

8

10

HRT (min)

(B)

Diazinon mineralizaon(%)

100 80 60

diazinon for both low and high concentrations increased as HRT increased. This increase was a result of the increase in exposure time of the diazinon molecules to VUV radiation, which increased the formation of radical species and the oxidation rate. Fig. 9 also shows the complete degradation (Fig. 9(A)) and mineralization (Fig. 9(B)) of 1 mg/L diazinon obtained at HRTs of 2.2 min and 4.7 min, respectively. For a diazinon concentration of 10 mg/L, complete degradation was observed at HRT = 8.5 min and 81% mineralization. These findings clearly reveal that the VUV process efficiently eliminated the pesticides present in low-to-moderate concentrations in a short retention time. No report was found in the literature on the operation of VUV reactor in continuous-flow mode for comparison of the obtained results. In reviewing the related literature, no result was found on the operation of VUV reactor in continuous-flow mode to compare with the results of this study. It is worth noting that the pH of the water did not change significantly after passing through the VUV reactor. Unique features of the VUV process include no chemicals being required and no change in the pH of the treated water, eliminating the need to adjust the pH. The low retention time, small size, low energy consumption, and minimal required operation and maintenance make the VUV process cost-effective and efficient. The VUV process is a promising and appropriate system in practice for purifying contaminated drinking water. 4. Conclusion The degradation of diazinon as a model organophosphate pesticide was investigated and compared in UVC, UVC/ZnO, and VUV processes. Following conclusions were drawn from this study: a. UVC, UVC/ZnO, and VUV processes attained the highest rates of diazinon degradation in acidic, neutral, and alkaline solutions, respectively. b. In the UVC/ZnO process, maximum degradation of diazinon was obtained for a ZnO concentration of 100 mg/L; greater concentrations of ZnO decreased degradation. c. The performances of the UVC, UVC/ZnO and VUV processes were compared for different concentrations of diazinon. VUV had a much greater degradation rate for all diazinon concentrations than the other processes. Therefore, VUV might be more costeffective than the UVC or UVC/ZnO processes for oxidation of pesticides in water. d. The VUV process efficiently oxidized diazinon in contaminated natural water. e. The VUV process was used in a continuous flow treatment of natural water contaminated with diazinon onto evaluate the effect of HRT. Complete degradation and mineralization of diazinon (1 mg/L) in natural water was attained at short HRTs of 2.2 and 4.7 min, respectively. f. In conclusion, the VUV process was found to be very efficient for complete degradation of organophosphate pesticides in contaminated drinking water sources. References

40 Diazinon=1 mg/L

Diazinon=10 mg/L

20 0 0

2

4

6

8

10

HRT (min) Fig. 9. Effect of HRT on the performance of continuous VUV process for (A) degradation and (B) mineralization of different concentrations of diazinon in tap water (diazinon: 1 and 10 mg/L; solution pH: 7.4; HRT: 0.6–8.5 min).

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