Sustain. Environ. Res., (5), 9-97 (1) 9 (Formerly, J. Environ. Eng. Manage.) INFLUENCE OF ph AND H O DOSAGE ON THE DECOMPOSITION OF CARBOFURAN DURING THE PHOTO-FENTON PROCESS Li-An Lu, 1 Ying-Shih Ma, Mathava Kumar 1 and Jih-Gaw Lin 1, * 1 Institute of Environmental Engineering National Chiao Tung University Hsinchu, Taiwan Department of Environmental Engineering and Health Yuanpei University Hsinchu, Taiwan Key Words: Carbofuran, decomposition, mineralization, photo-fenton process, UV irradiation ABSTRACT The effects of hydrogen peroxide dosage and ph on the decompostion of aqueous carbofuran (5 mg L -1 ) during the photo-fenton process, i.e. a combnination of UV irradiation with H O /Fe +, were evaluated. The efficiency of carbon removal by the photo-fenton process was estimated based on the reductions in dissolved organic carbon (DOC) and carbofuran concentrations. More than 9% carbofuran removal and % DOC reduction were observed within 1 min of the photo-fenton reaction at ph, H O dosage of.5 mg L -1 min -1 and Fe + addition of 5 mg L -1. The photo-fenton reaction was speeded-up at mg L -1 min -1 of H O dosage, beyond which a decrease in the carbofuran domposition efficiency was observed. At mg L -1 min -1 of H O dosage, 1 and 9% of carbofuran removal and DOC reduction, respectively, were observed within 1 min of the photon- Fenton reaction. Overall, a rapid decomposition as well as mineralization of carbofuran could be achieved at mg L -1 min -1 of H O dosage and ph. INTRODUCTION Carbofuran (,-dihydro-,-dimethylbenzofuran- 7-yl methylcarbamate) is a broad spectrum carbamates pesticide and nematicide, which has been used against various foliar pests observed in fruit, vegetable and forest crops. In 1995, around t of carbofuran was applied in the United States for controlling pests [1]. In the same way, carbofuran accounts about 1.% of the total insecticides produced (19 t) every year in Taiwan []. Carbofuran is highly soluble in fresh water (7 mg L -1 ); therefore, it is susceptible to leaching and percolation through agricultural fields []. Several researchers reported the presence of carbofuran in surface water and groundwater. Carbofuran is highly toxic to animals and humans both by oral and inhalation routes. For example, the oral LD 5 of carbofuran to rats is 11 mg kg -1 []. Carbofuran is highly resistant to biochemical degradation. Therefore, its rapid degradation/decontamination from an aqueous source, i.e. groundwater/surface water, could be possible only by the application of strong oxidizing agents or advanced oxida- tion processes (AOPs). In the past two decades, AOPs are widely applied to various stages of water and wastewater treatment processes. During AOPs, highly oxidative free radicals (particularly the hydroxyl radicals, OH) are generated, which could easily degrade any toxic or bio-refractory compound [5]. So far, a variety of AOPs including ultrasonic irradiation [1], direct photolysis, heterogeneous photocatalysis [6], UV irradiation in the presence of ozone or Fenton reagent [7], membrane anodic-fenton treatment [8], Fentoncoagulation [9] and TiO as a photocatalyst [1] have been developed for the treatment of pesticide contaminated water. Among these processes, the Fenton treatment has been received major attention due to its broad-spectrum of target compounds, strong oxidation ability and faster reaction rate. However, the major drawbacks of the Fenton treatment are the requirement of H O, Fe + salts and ph adjustment (mostly acidic). With the additions of H O and Fe + salts, highly reactive and unselective oxidants are produced as shown in Eq. 1 that leads to the formation of less powerful hydroperoxyl radical (HO ) as per Eq. [11]. Fe + + H O Fe + + OH + OH - (1) *Corresponding author Email: jglin@mail.nctu.edu.tw
9 Sustain. Environ. Res., (5), 9-97 (1) Fe + + H O Fe + + HO + H + () In addition, the Fenton treatment produces large volume of unwanted iron hydroxide sludge [11]. In order to overcome the sludge problem and enhance the Fenton treatment, the photo-fenton process was developed by introducing a UV light to the Fenton process. In the presence of UV-irradiation, the Fe + complex formed in Eq. 1 can be photo reduced to Fe +. This could facilitate the reaction of photo reduced Fe + with more H O molecules, produce new OH (Eq. ) and form a Fenton reaction cycle [1]. This cycle is useful in the progress of the Fenton treatment with no continuous addition of Fe + and devoid of ferric hydroxide sludge. Moreover, two OH can be produced from hydrogen peroxide under the UV-irradiation as shown in Eq. [1]. FeOH + + hν Fe + + OH (λ < nm) () H O + hν OH () So far, the photo-fenton treatment has shown greater efficiencies in the mineralization of pesticides [1]. In the present study, the photo-fenton treatment is applied for the removal of carbofuran from an aqueous system. Subsequently, the effects of H O dosage and ph on the photo-fenton treatment were evaluated. 1. Materials MATERIALS AND METHODS Carbofuran was obtained from Shida chemical industries (Taoyuan, Taiwan) and used as received (HPLC grade > 98%). Hydrogen peroxide (%, w/w) was supplied by Panreac chemicals (Spain). Fe + stock solution (1 mg L -1 ) was prepared by dissolving Fe (SO ) (Yakuri pure chemicals, Japan), and the HPLC grade methanol (Panreac, Spain) was used in the HPLC analysis. All other chemicals used in this study were reagent grade and the solutions were prepared using double distilled water.. Experimental Apparatus Figure 1 shows the schematic diagram of the experimental setup. A double-walled cylindrical glass contactor with 1.6 L was used for conducting the experiments. The ph and thermo meters were equipped in the reactor to measure the variation in the reaction ph and temperature. In addition, two ports were provided in the reactor for feeding and sampling the reactants. The reactants were mixed continuously using a Teflon-coated stirrer at 175 rpm and the reaction temperature was kept constant at 5 C using a water bath. The UV-irradiation was supplied through two 8 W UV lamps with a total intensity of 6 μw cm - at 1 nm (Fig. 1). During the photo-fenton process, H O Thermal probe ph probe UV lamp ` Thermostate Stirrer ` Sampling port Syringe pump HO Fig. 1. Schematic diagram of experimental apparatus. was supplied continuously at a fixed flow rate of 1 ml min -1 by a syringe pump.. Experimental Conditions and Procedure In order to find out the effects of H O dosage and initial reaction ph on the decomposition of carbofuran, the photo-fenton experiments were conducted under different H O dosages (up to 6 mg L -1 min -1 ) and different initial ph values (-). In all the experiments, the initial carbofuran concentration and Fe + dosage were fixed at 5 and 5 mg L -1, respectively. The working volume of the carbofuran solution was maintained uniformly (1 L) in all the experiments and the initial ph of the solution was adjusted as per the requirement using.1 N H SO. The photo-fenton reaction time was restricted to 1 min. At specific time intervals, 18 ml of sample was withdrawn from the reactor, filtered through.5 μm membrane filter and subsequently, the photo-fenton reaction was quenched by the addition of sodium hydrogen sulphite.. Sample Analysis The residual carbofuran concentration in the sample was analyzed using the HPLC (Hitachi Co., Japan) equipped with Hitachi L- UV detector and a RP-18 GP 5 separation column (5 mm.6 mm i.d., Kanto Chemicals, Japan). The elution was monitored at the wavelength of 8 nm. The mobile phase was composed of methanol and water (5:5, v/v), and was pumped at a flow rate of 1 ml min -1. The injection volume of each sample was exactly μl. Under these conditions, the retention time of carbofuran was around 1 min. The removal percentage of carbofuran was expressed as shown in Eq. 5; where, C and C t are the initial and the residual concentration of carbofuran (mg L -1 ) at reaction time t (min). On the other hand, mineralization of carbofuran was esti-
Lu et al.: Carbofuran Decompostion by Photo-Fenton 95 mated from the observations of DOC concentrations in a TOC analyzer (O.I. analytical Model 1). The DOC removal percentage of carbofuran was calculated as per Eq. 6; where DOC and DOC t are the initial and residual DOC values (mg L -1 ), respectively, at reaction time t (min). The reaction ph was continuously monitored by a ph meter (Suntex TS-, Taiwan). C Ct Carbofuran removal (%) = ( ) 1 (5) C DOC DOCt DOC removal (%) = ( ) 1 DOC RESULTS AND DISUSSION (6) 1. Effect of Initial ph Level on the Degradation of Carbofuran In the photo-fenton reaction, the ph value influences the generation of OH and oxidation performance of the process. In this study, the experiments were conducted under various ph values, i.e.,.5,,.5 and, at fixed Fe + and H O dosages of 5 and mg L -1 min -1, respectively. The effects of initial ph in the photo-fenton decomposition of carbofuran and DOC removal are shown in Figs. a and b, respectively. The results demonstrate that the carbofuran decomposition and DOC removal are greatly influenced by the initial ph. Almost 1% decomposition of carbofuran was achieved in 1-1 min of the photo-fenton process under all initial ph values (Fig. a). At the end of min, 7% decomposition of carbofuran was observed at ph, whereas it was 61% at ph.5 and almost 1% at ph. This indicates that the lower ph values are better for the photo-fenton decomposition of carbofuran. However, the decomposition of carbofuran was reduced to 95 and 7% at ph.5 and, respectively at the end of min of the photo-fenton reaction. The decrease in the photo-fenton performance at higher ph values (ph > ) is attributed to the precipitation of Fe + as Fe(OH), which has the potential to hinder the reaction between Fe + and H O. Under excess H +, i.e. at low ph condition (ph < ), H + will react with OH via Eq. 7 and produce water [11]. OH + H + + e - H O (7) Similar trends can be seen in Fig. b, where the DOC removal at the end of 1 min increased from 9% at ph to 9% at ph, and then decreased to 8% at ph. The consumption of OH can decrease the oxidation performance of target compounds. Owing to these observations, the further photo-fenton experiments were conducted at ph. These observations are in good agreement with the previous photo-fenton studies conducted for the destruction of different organic compounds [15,16]. Carbofuran removal (%) DOC removal (%) 1 8 6 1 8 6 (a) (b) 6 8 1 1.5.5 Fig.. Variations of (a) carbofuran and (b) DOC removals in photo-fenton reaction at different initial ph values (carbofuran 5 mg L -1, Fe + 5 mg L -1 and H O mg L -1 min -1 ).. Effect of HO Addition on the Degradation of Carbofuran Fe + and H O dosages are the two most important parameters controlling the efficiency of the photo- Fenton treatment for organic pollutants. Therefore, it is necessary to estimate the appropriate Fe + and H O dosages for achieving better removal efficiency. To understand the effect of H O dosages on the decomposition of carbofuran, the photo-fenton process was carried out under different H O dosages at a fixed Fe + dosage, i.e. 5 mg L -1. Based on the earlier observation, the initial ph of the reaction was adjusted to. Figure shows the ph profiles during the photo- Fenton reaction at different H O dosages. It can be noticed in Fig. that insignificant variations in ph was observed throughout the experiments, i.e..-.. The profiles of carbofuran and DOC removals under various H O dosages are shown in Figs. a and b, respectively. In the absence of H O, i.e. UV/Fe + process, the decomposition and mineralization of carbofuran were reached around 77 and 1%, respectively, within 1 min of reaction. At a H O dosage of.5 mg L -1 min -1, more than 9% carbofuran removal was observed corresponding to a DOC removal of % at the end of 1 min reaction. Both carbofuran and DOC removals were distinctlyamplified with the increased addition of H O. When the H O dosage was increased to mg L -1 min -1,
96 Sustain. Environ. Res., (5), 9-97 (1) ph.5....1..9.5.5.8 1.5.8 6 8 1 1 Fig.. Variation of ph in photo-fenton reaction at different H O dosages in mg L -1 min -1 (carbofuran 5 mg L -1, Fe + 5 mg L -1 and ph ). Carbofuran removal (%) DOC removal (%) 1 8 6 1 8 6 (a) (b) 6 8 1 1.5.5.8 1.5 5 6 Fig.. Variations of (a) carbofuran and (b) DOC removals in photo-fenton reaction at different H O dosages in mg L -1 min -1 (carbofuran 5 mg L -1, Fe + 5 mg L -1 and ph ). almost 1% of carbofuran was decomposed within min reaction and a maximum carbofuran mineralization of 9% was observed after 1 min of reaction (Fig. b). At higher H O dosage conditions, the DOC removal percentage has shown a sharp increase within -6 min (1-6%), and then followed a gradual increase (6-9%). In contrast, the highest H O dosages such as 5 or 6 mg L -1 min -1 were ineffective in the decomposition and mineralization of carbofuran, which could be attributed to the competition between overdosed H O (Eq. 8) and the organic 5 6 matter for OH. At the same time, the generation of HO reduced the efficiency of the photo-fenton process by decreasing the amount of OH present in the system [15]. H O + OH H O + HO (8) The experimental outcomes, i.e. carbofuran degradation, mineralization and ratio of DOC removal/carbofuran degradation, at the end of 1 min of the photo-fenton process are shown in Table 1. The ratio of DOC removal/carbofuran degradation increases with increasing H O additions (Table 1), which demonstrates that the addition of H O is useful for enhancing the mineralization of carbofuran in the photo-fenton process. Table 1. Effect of H O dosages on carbofuran degradation and DOC removal in the photo- Fenton process at ph H O Carbofuran degradation DOC removal (mg L -1 min -1 ) (%) [A] (%) [B] B/A 77 1.17.5 97 7.8.5 1 9.9.8 1 6.6 1.5 1 76.76 1 89.89 1 9.9 1 9.9 5 1 9.9 6 1 79.79 CONCLUSIONS The efficiency of carbofuran decomposition by the photo-fenton process at different initial ph values and H O dosages was investigated. The experimental outcomes indicate that the photo-fenton efficiency was maximized at ph and H O dosage of mg L -1 min -1. Under these conditions, almost 1% of carbofuran decomposition and 9% DOC removal were achieved within 1 min of reaction. The H O dosage beyond mg L -1 min -1 was ineffective in the enhancement of carbofuran decomposition during the photo-fenton process. REFERENCES 1. Hua, I. and U.P. Thompson, Ultrasonic irradiation of carbofuran: Decomposition kinetics and reactor characterization. Water Res. 5(6), 15-15 (1).. Council of Agriculture, Annual Report of Taiwan s Agriculture, Council of Agriculture.
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