THE OPERATIONAL PARAMETERS EFFECT ON PHOTOCATALYTIC DEGRADATION OF DIAZINON USING CARBON AND NITROGEN MODIFIED TiO2

Diazinon is one of the most active compounds containing in commercial pesticides used for agriculture application. The photocatalytic degradation of commercial emulsion diazinon using nanoparticles photocatalyst C,N-codoped TiO2 under UV irradiation was investigated. The operational parameters, i.e., the modified titania, catalyst dose, initial pH, the concentration of diazinon, H2O2, and humic acid as an additive were examined. Under optimal parameters, the diazinon with initial concentration 18 mg L-1 at pH 6 in the presence of 12 mg C,N-codoped TiO2 catalyst can be degraded 76% after 180 minutes irradiation. Whereas the mineralization of diazinon reached 37% as analyzed by total organic carbon analyzer (TOC). The additive compounds containing diazinon commercial give no effect under the photocatalysis process.


INTRODUCTION
Diazinon is an organophosphate pesticide extensively used as pest control in plantation country such as Indonesia. 1 It is effective in an application for various kinds of cultivations such as horticultural plants, tobacco, rice, corn, fruit trees, sugarcane, and palm. 2 These activities cause the diazinon residues concentration to increase in the environment. On the other hand, this organic compound is a highly toxic pesticide, nonspecific target, low persistence in the environment 3 , immunotoxic 4 , cytotoxic and genotoxic [5][6][7] and categorized as moderately hazardous class II by the World Health Organization (WHO). Thus, the removal and reduction of diazinon concentration from the water represent an emerging environmental concern. The pure diazinon is mostly used in the investigation rather than commercial ones. However, the application is more in the form of commercial insecticide emulsion which contains the organic solvent, additive, and impurities for its solubility and stability in water. The presence of those organic compounds mentioned above in emulsion can cause additional complications such as possible reactions among them and their degradation byproducts in the photocatalytic process. 1 Titania (TiO2) is one of the selected photocatalyst used for organic pollutant degradation such as diazinon into H2O and CO2 because it is effective, stable against photo-corrosion, photoactive, and environmentfriendly. 8-10 However, the wide bandgap (3.2 eV) of titania causes limitations in an extensive application under photocatalysis. Recently, modified TiO2 by codoping with non-metals (C, N, and S) is one of the most efficient techniques. Its product will have higher photocatalytic activity and special characteristics compared with single doped TiO2. Synergistic effect of dopants results in narrow bandgaps (redshift) leading to higher photoresponse and photocatalytic activity under high energy source (UV-light) 11 and low energy (visible-light). 12,13 Nanoparticles of C,N-codoped TiO2 have been synthesized by using water solvent and reported single doped and un-doped ones. 14 The pure diazinon was degraded by the photocatalysis process using modified titania such as FeFNS-TiO2 16 ,  N-doped TiO2 17  , Fe-doped TiO2 18 . While the commercial form of diazinon was studied using TiO2 PC-102 19 . This present work investigates the effect of the operational parameters on photocatalytic degradation of formulation diazinon using C,N-codoped TiO2 as a photocatalyst. The parameters are; effects of modified titania, catalyst dose, initial pH, the concentration of diazinon, H2O2, and humic acid as an additive.

EXPERIMENTAL Material and Methods
A commercial emulsion containing 600 g L -1 diazinon, (Code Name Diazinon 600 EC) was used as organic pollutant insecticide in water. A diazinon standard (CAS No. 333-41-5) was purchased from Sigma-Aldrich. Ethanol 95%, a solvent for diazinon stock solution was purchased from Sigma-Aldrich. Preparation steps and full characterization of the C,N-codoped TiO2 nanoparticles were conducted according to the previous publication 14 . The change of diazinon concentration was analyzed by and DR600 TM UV-vis spectrophotometer (Hach, USA). The total organic carbon of degraded diazinon was analyzed by the TOC analyzer (Shimadzu, Serial No. H571049). The pH of diazinon was measured by using Prolab 2000 pH meter (SCHOTT Instruments GmbH, Germany). The UV light intensity was measured by IL1400A Radiometer/Photometer (International Light Technologies, Inc).

General Procedure
Photocatalytic reactions were performed in a batch system under 13 Watt UV lamps irradiation (Germicidal CEG13 Base BFC 11004, λ=254 nm). The solution of diazinon with concentration 18 mg L -1 was added by photocatalyst (10 at.% C,N-codoped TiO2 after calcination at 500 ᵒC) and irradiated under 1.7 mW cm -2 UV-lamp. During the photocatalytic reaction, the optimal C,N-codoped TiO2 dose was set at experiments with different diazinon concentrations, initial diazinon pH, H2O2, and humic acid addition were conducted to investigate their effects on the degradation efficiencies of diazinon. Aliquots were sampled every interval times (30, 60, 120, and 180 minutes) and diazinon residue was detected by a DR600 TM UV-vis spectrophotometer at 247 nm. The degraded diazinon solution under optimal condition was also measured by SCL-10A VP HPLC (Shimadzu) equipped with Poroshell 120 SB-Aq 2.7 (4.6 x 150 mm) column, using 80/20 (methanol/water, v/v), volume injection and flow rates were 20 µL and 0.5 mL min -1 , respectively. The mineralization percentage of degraded diazinon was analyzed by using a TOC analyzer (Shimadzu, Serial No. H571049).

RESULTS AND DISCUSSION
The Effect of Modified Titania on Diazinon Degradation Figure-1 represents the effect of carbon and nitrogen as dopant on the degradation of diazinon. Photocatalysis also conducted using single doped TiO2 carbon, nitrogen, and un-doped as a control. As seen in Fig.-3 diazinon efficiency is greater under photocatalysis using carbon and nitrogen modified titania than single doped. This result can be elucidated by the small band gap and high crystalline of C,N-codoped TiO2 than C-doped TiO2, N-doped TiO2 and un-doped TiO2 with the bandgap is 2.87 eV, 2.97 eV, 3.02 eV, and 3.09 eV respectively as reported in our previous research. 14 Carbon and nitrogen give synergy effect on modified titania with having high crystallinity and low bandgap. Three opinions regard to the mechanism of modified TiO2 by doping using non-metals: 1. The dopant incorporates into the TiO2 crystalline lattice and alters the structure of TiO2 electronic band which leads the level of the dopant above the O 2p valence band and the bandgap of TiO2 becomes narrower. Some Ti 4+ are converted to be Ti 3+ by non-metal dopant through charge compensation and generated Ti 3+ forms donor energy below the conduction band of TiO2. 2. The co-doped carbon produces a photosensitizer by forming carbonaceous species on the surface of TiO2. 3. Nitrogen dopant substitutes the oxygen site of TiO2 and forms the impurity energy level above the valence band 12 .

The Effect of Catalyst Dose on Diazinon Degradation
Catalyst dose is one of the most factor influences photocatalysis degradations. To optimize C,N-codoped TiO2 dose, the catalyst with an amount (0, 6, 12, 18, and 24 mg) was added into the diazinon solution while another parameter is keeping constant. The experiment was also carried out under UV photolysis without catalyst addition as a control. Diazinon was removed 11% for 180 min irradiation times indicating the direct photolysis is not effective. As presented in Fig.-2 the efficiency increases significantly in the presence of 6-18 mg catalyst and decreases at amount 24 mg. The improvement in degradation at higher photocatalyst concentration can be explained by increasing the catalyst providing a higher active site and more photons adsorbed on catalyst surface for the photocatalytic reaction which increased in the oxidative species formation. Further increase in TiO2 concentration at 24 mg addition, however, gives a negative effect. The exceeded optimal catalyst dose causes increasing turbidity, lower light penetration, and resulting agglomeration causes difficulty in the suspension homogenous form and lowering the number of active sites. 20 Thus, the chosen catalyst as an optimal condition for the degradation process was 12 mg. The Effect of Initial pH on Diazinon Degradation The effect of pH on the photocatalysis degradation of diazinon using 12 mg C,N-codoped TiO2 was investigated at pH 3, 5, 6, 7, and 9 for 180 min irradiation. The results are shown in Fig. 3. As observed the optimal diazinon degradation was achieved at pH 6. It is reported that the pHzpc of nano-titania is at range 6.3-6.9. 21 The titania surface will be positive at lower the pHzpc and negative at higher the pHzpc. On the other hand, the pKa of diazinon is 2.6 which is negatively charged above pH 2.6. Low diazinon degradation at higher pH is caused by the negatively charged of nano-titania catalyst preventing the negatively charged diazinon and hydroxide anion to adsorp in catalyst surface. Conversely, there is an electrostatic attraction between positive diazinon and catalyst in acidic conditions leading to a low degradation of diazinon. Therefore, the optimal condition is found at pKa diazinon < pH < pH zpc catalyst.

The Effect of H2O2 Addition on Diazinon Degradation
The effect of H2O2 addition on diazinon degradation efficiency was examined at a series of hydrogen peroxide concentrations in range 0 mmol L -1 -0.5 mmol L -1 as exhibited in Table 1. The H2O2 addition gives a positive effect on the diazinon degradation at 30-60 minutes photocatalysis. The degradation efficiency decreases with increasing initial H2O2 concentration and the optimal concentration is found at 0.25 mmol L -1 H2O2 addition. The diazinon degradation is 37.91% without H2O2 and increases to be 51.85% at 0.25 mmol L -1 H2O2 addition. However, the addition of 0.5 mmol L -1 H2O2 causes the descent of diazinon degradation to be 35.77%. The presence of H2O2 at low concentration on the photocatalytic system could be an option electron acceptor to oxygen which inhibits the recombination process 24,25 . Also, H2O2 is can be as an additional free radical source through direct photolysis of H2O2 by UV-light as shown 26 . Those reactions would improve the efficiency of diazinon degradation. While H2O2 may act as a powerful  The Effect of Humic Acid Addition on Diazinon Degradation The effect of humic acid is important to be studied as its occurrence in natural water is in range 0.03-30 mgCL -1 . 28 The effect of humic acid effect on diazinon degradation was investigated at concentrations of 0-18 mg L 1 . The diazinon removal decreases from 76.55% to be 29.41% in the presence of humic acid at concentration 3-18 mg L -1 . This decline could be attributed to the occupied catalyst surface by humic acid blocking the active sites for the access of diazinon. 26,27 Besides, humic acid can be an optical filter as it has strong absorbance at 190 to 800 nm and competitively absorbs light and photons. 28  The first two peaks can be attributed to the organic solvent in the commercial diazinon emulsion. 1 The intensity of those peaks keeps constant as increasing irradiation times, suggesting there is no degradation of those compounds. At the same time, the peak of diazinon at tR = 5.3 min decreases in prolonging irradiation time, indicating the reduction and degradation of diazinon. As shown in Fig.-5b, the process produces a new peak at tR = 3.3 min and its intensity raises with decreasing diazinon concentration in longer photocatalysis. This result suggests that the photocatalysis process may transform diazinon and forms a new compound as degradation products. This compound peak appears at shorter retention times than that the diazinon, implying that the generated intermediate is smaller molecular weight and less polarity than that of diazinon. 1 Figure-5c shows that the degradation and mineralization degree of diazinon increase as the reaction time increased with efficiency 76% and 37 %, respectively. The amount of degraded diazinon is higher than that of the mineralized. This much difference indicated that the degradation processes produced transient organic intermediates on the photocatalysis system caused un-complete mineralization. 29 This result is confirmed by the HPLC analysis that found a diazinon intermediate during photocatalytic degradation. When the titania catalyst was irradiated by photon, its electron will excite from valence band to conduction band which generates electron-hole. The reaction of oxygen and water with the generated electron-hole will form the strong oxidative hydroxyl radical. The radical and hole oxidize the organic pollutant to be CO2, H2O, and some intermediates. 26,30,31

CONCLUSION
The degradation of commercial diazinon emulsion using C,N-codoped TiO2 catalyst under UV-light irradiation was investigated. The diazinon degradation efficiency was strongly influenced by modified titania, catalyst dose, initial pH, the concentration of diazinon, H2O2, and humic acid addition. The addition of 0.25 mmol L -1 H2O2 improves diazinon efficiency at 30-60 minutes photocatalysis. While the presence of humic acid on the system decreases diazinon degradation. Photocatalysis of 18 mg L -1 diazinon at pH 6 in the addition of 12 mg C,N-codoped TiO2 catalyst can be degraded 76% and mineralized 37% after 180 minutes irradiation. The additive compounds containing diazinon commercial give no effect under the photocatalytic process.