Research goals

The primary goal of this study is to assess the efficacy of Fenton process for the treatment of carbofuran contaminated water. In addition, the study was extended to investigate the influences of Fe²⁺ and H2O2 in the removal of carbofuran by Fenton process. The key research objectives of the present study are listed below:

ƒ Determination of the optimal Fe²⁺ and H2O2 dosages for maximum carbofuran

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removal in Fenton process at different initial carbofuran concentrations.

ƒ Enumeration of carbofuran degradation pathway in the Fenton process at the optimal conditions.

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Chapter 2 Literature review

2.1 Pesticides

Pesticide is a common term used for the chemicals employed for the pest control.

Pesticides can be classified into several groups i.e. insecticides, herbicides, fungicides, wood preservatives and disinfectants. The contamination of surface water and wastewater with pesticides is increasing day by day, and presently, it constitutes as a major problem owing to its extensive use in agriculture (Varshney et al., 1995; Hsieh and Kao, 1998; Huston and Pignatello, 1999; Benitez et al., 2002). In addition to the toxic character of the pesticides, the hazardous potential is increased by the possibility of generating organohalogen compounds through their reactions with chloro derivatives, (Benitez et al., 2002). Hence, to meet the worldwide problem of environmental protection and pollution control, it is necessary to detect, separate, identify and determine pesticide residues in the ecosystem. The disposal of pesticide wastes including equipment rinsates is a major concern and the improper disposal of such wastes can lead to the contaminations of soil, groundwater and surface water.

Pesticide contamination at farm mixing and loading sites (Habecker, 1989), agrichemical dealer sites (Norwood, 1990) and landfills were reported in the past.

Pesticide waste treatment technologies are desired to prevent water pollution and to comply with increasing regulatory pressure (Houston and Pignatello, 1999).

2.1.1 Insecticides

Insecticides are used to kill insect pests by disruption of their vital processes through chemical action. Insecticides may be inorganic or organic molecules and can be

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classified according to their mode of entry into the insect: stomach poison, contact poison and fumigation. In another way, insecticides can be classified by their mode of action. Pyrethroid, organophosphorus and carbamates insecticides all adversely affect the nervous system. However, based on their chemical composition, synthetic organic insecticides are classified in various ways, namely chlorinated hydrocarbons, cyclodine compounds, carbamates, organophosphates, etc. The target compound selected for the present study i.e. carbofuran, falls under the category of the carbamates group of insecticides.

2.2 Carbofuran

Carbofuran is one of the most toxic carbamates pesticides. It is used to control insects in a wide variety of field crops including potatoes, corn and soybeans. It is a systemic insecticide, which means that initially the plant absorbs it through the roots and later distributes it throughout its organs (mainly vessels, stems and leaves; not the fruit), where insecticidal concentrations are attained. Carbofuran also has contact activity against pests (Wang et al., 2003).

The persistence of carbofuran in water is directly related to the pH of water.

Carbofuran is stable in acidic water, but subject to increasing chemical hydrolysis as water becomes more alkaline. Microbial populations in water, sediments and flooded soils also reduce the persistence of carbofuran. Carbofuran is very toxic to fish and its reported LD50 to fish is below 1 mg/L. While accidental spraying of carbofuran poses a threat to the aquatic environment, carbofuran poses a more serious threat in runoff water from fields or orchards immediately after treatment. Killing of localized fish has been reported in such instances (Wang et al., 2003).

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The molecular structure of carbofuran is shown in Figure 2-1. Carbofuran is highly soluble in water (700 mg/L at 25 ^oC) (Worthing, 1991) as well as in other solvents like acetone (150 g/kg) and acetonitrile (140 g/kg). The detailed physical and chemical characteristics of carbofuran are listed in Table 2-1. Risks from exposure to carbofuran are especially high for persons with asthma, diabetes, cardiovascular disease, mechanical obstruction of the gastrointestinal or urogenital tracts, or those with vagotonic conditions (U.S. Department of Agriculture, 1995). As with other carbamates pesticides, carbofuran's cholinesterase-inhibiting effect is short-term and reversible. Several reports showed the chronic toxicity of carbofuran on various test organisms (Kearney and Kaufman, 1975; Hayes and Wayland Jr., 1982). Prolonged or repeated exposure to carbofuran may cause the same symptoms as an acute exposure (U.S. Department of Agriculture, 1995).

Figure 2-1 Molecular structure of carbofuran

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Table 2-1 Physical and chemical properties of carbofuran Parameters Description

Common name IUPAC name

Molecular weight Molecular formulae Form

Melting point Vapor pressure

Octanol /Water partition coefficient

Aqueous photolysis half-life

Carbofuran

2,3-dihydro-2,2-dimethyl-benzofuran-7-yl methylcarbamate

221.3 g/mol C12H15NO3

Colorless (white) crystalline solid 153-154^oC

3.4 x10^-6 mm Hg (25^oC) 2.32

In water 700 mg/L and in acetone 150, acetonitrile 140, benzene 40, cyclohexone 90, dimethylformade 270 and dimethyl sulfoxide 250 (all in g/kg at 25^oC).

It is stable under neutral or acidic conditions but unstable in alkaline media

3.9x10^-9 atm m³/mol 27.7 d (pH 8, 25°C) 0.54 d (pH 9, 25°C) 7.95 x 10³ d (pH 7, 28°C) (Source: HSDB, 1998; U.S. EPA, 1995)

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2.3 Transport and fate of carbofuran in water and soil

Carbofuran is moderately soluble in water. The migration of carbofuran with runoff and its presence in surface water and groundwater have been discussed by many researchers (Nicosia et al., 1991; McCall et al., 1980; Lee et al., 1990). Direct photolysis and photooxidation (via hydroxyl radicals) are thought to be the major pathways of carbofuran degradation in water. The hydrolysis half-lives of carbofuran in water were found to be 5.1 weeks at pH 7.0 and 1.2 h at pH 10. Carbofuran can be degraded to 2-hydroxyfuradan and furadan phenol when exposed to sunlight (HSDB, 1998). When compared with other insecticides, carbofuran is less persistent than organochlorine and most organophosphorus pesticides.

Lee et al. (1990) reported that carbofuran is the most mobile of eight major pesticides applied to the soils in Taiwan. Nicosia et al. (1991) estimated that more than 11% of soil-applied (around 1 to 5 cm depth) carbofuran was transported with runoff.

Carbofuran has a varied persistence rate in different soils with observed half-lives of several days to over three months. The environmental fate of carbofuran depends on the organic content, moisture content and pH of the soil. Rapid carbofuran biodegradation rate was observed in the soil with high organic content. A residue of carbofuran and its breakdown product, 3-hydroxyfuran, have been detected in raw and finished agricultural products. Tolerances have been established for carbofuran and its metabolites in various agricultural commodities and meat products (HSDB, 1998).

2.4 Carbofuran removal methods

A variety of effective treatment techniques such as irradiation, direct photolysis, UV irradiation in the presence of ozone or Fenton reagent, anodic Fenton treatment (AFT)

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and TiO2 as a photocatalyst have been proposed for carbofuran removal from drinking water. Table 2-2 summarizes the experimental conditions and the outcomes of above stated methods in aqueous carbofuran removal.

The photodegradation of carbofuran by excitation of Fe³⁺ aquacomplexes under UV irradiation has received considerable attention in the past. Katsumata et al. (2004) reported that an initial carbofuran concentration of 10 mg/L was completely degraded within 50 min at pH 2.8 with original Fe³⁺ concentration of 8 × 10⁻⁴ mol/L. This reaction was found to follow the first-order kinetics and the rate constant of 1.60 × 10⁻³ s⁻¹ was observed. The degradation rate was strongly influenced by the pH and the initial concentration of Fe³⁺. Moreover, the variation of carbofuran removal efficiency at different pH was in good proportion with the initial Fe³⁺ concentration.

The decrease of TOC content was observed during the photocatalytic process and the removal percentage obtained was about 70% after 25 h (Katsumata et al., 2004).

Purification of water using TiO2 as a photocatalyst has attracted a great deal of attention. It has been found that TiO2 microcystallites can become firmly affixed to the glass plates without any deactivation. As an example, mineralization of 222 mg/L carbofuran was tested by Tennakone et al. (1997) and they reported that complete mineralization of carbofuran can be achieved after 15 h of 400 W irradiation at pH 2.8.

This method is a low-cost process and plates can be reused without any deactivation.

However, the technical limitation of using the powder form of TiO2 is its separation from the system. AFT is a new Fenton technology for the treatment of pesticide wastewater. The substitution of an ion exchange membrane for the salt-bridge, an improvement to the practicality of the AFT without sacrificing treatment efficiency,

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has also been reported (Wang et al., 2003). The results showed that the degradation kinetics of carbofuran with different initial concentrations from 6 to 43 mg/L followed the first-order kinetics, and the treatment efficiency increased with increasing initial concentration. The increase in treatment temperature enhanced the degradation of carbofuran in solution. The pseudo-activation energy of carbofuran by membrane AFT was estimated to be 7.66 kJ mol⁻¹. The results also showed that AFT could effectively remove COD and dramatically improve the degradability of carbofuran in solution (Wang et al., 2003).

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Table 2-2 Carbofuran degradation treatment techniques

Reference Method

Carbofuran

conc. (mg/L) Condition

Reaction

(2001) Ultrasonic 30

1800 W, 20 kHz

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2.5 Carbofuran degradation by physico-chemical processes 2.5.1 Carbofuran degradation pathway under AFT

Wang et al. (2003) analyzed the degradation products generated after 2 min of membrane AFT in a GC-MS. The results indicated the formation of 2,3-dihydro-2,2-dimethylbenzofuran-7-ol, which is the product formed by the cleavage of the carbamates group from the parent compound. Further, it was oxidized to 2,3-dihydro-2,2-dimethylbenzofuran-7-yl formate and then to 2,3-dihydro-3-oxo-2,2-dimethylbenzofuran-7-ol and 2,3-dihydro-3-hydroxyl-2,2- dimethylbenzofuran-7-ol. In addition to these four compounds, other degradation products still possibly exist in the oxidation system but were not detected because of their low concentration, low extraction efficiency and/or limited sensitivity in GC-MS.

With these identified products, a suggested oxidation pathway of carbofuran by membrane AFT is shown in Figure 2-2.

The degradation products of carbofuran i.e. intermediates, stated above were also detected during the hydrolysis (Wei et al., 2001), photolysis (Bachman and Patterson, 1999) and TiO2 catalyzed photolysis (Kuo and Lin, 2000) of carbofuran. It can be observed from the literatures that carbamates group appears to be the primary attack site by the OH radical and it is also the first group removed during the AFT. After the removal of carbamates group, the OH radical continues to attack by substituting an OH group for one of the H atoms at 3-C of the furan ring. Further oxidation eliminates another H atom at 3-C and a carbonyl group is formed. Based on the decrease of COD during the AFT, it can be anticipated that the furan ring and/or benzene ring is opened and further oxidative products are formed in AFT.

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Figure 2-2 Carbofuran degradation pathway under AFT

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2.5.2 Carbofuran degradation pathway under photocatalytic process

Katsumata et al. (2004) used GC-MS to investigate the photoproducts formed in the photocatalytic degradation of carbofuran in the aqueous solution after 20 min. By interpreting the mass spectrum, the photo-products of carbofuran were assigned as 2,2-dimethyl-2,3-dihydro-benzofuran-7-ol, 7-hydroxy-2,2-dimethyl-benzofuran-3-one, 2,2-dimethyl-2,3-dihydro-benzofuran-3,7-diol and 3-hydroxy-2-methoxy- benzaldehyde. The possible degradation pathway of carbofuran under photocatalytic process is shown in Figure 2-3 (Bachman and Patterson, 1999).

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Figure 2-3 Carbofuran degradation pathway under photocatalytic process

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2.6 Fenton process

Fenton’s reagent was discovered about 100 years ago, but its application as an oxidizing process for destroying toxic organics was not applied until the late 1960s (Huang et al., 1993). The wastewater treatment processes using the Fenton reaction are known to be very effective in the removal of many hazardous organic pollutants from water. The main advantage is the complete destruction of contaminants to harmless compounds, e.g. CO2, water and inorganic salts. The Fenton reaction causes the dissociation of the oxidant and the formation of highly reactive hydroxyl radicals that attack and destroy the organic pollutants (Neyens and Baeyens, 2003).

Fenton’s reagent is a mixture of H2O2 and ferrous iron (Fe²⁺), which generates hydroxyl radicals according to the reaction in Eq. 2-1 (Kitis et al., 1999). The ferrous iron initiates and catalyses the decomposition of H2O2, resulting in the generation of hydroxyl radicals. The generation of the radicals involves a complex reaction sequence in an aqueous solution.

2 3

-2 2

Fe⁺+H O →Fe⁺+iOH OH+ ………...……….(2-1) One of the advantages of Fenton’s reagent is that no energy input is necessary to activate hydrogen peroxide. Therefore, this method offers a cost-effective source of OH radicals. Furthermore, it commonly requires a relatively short reaction time compared with other advanced oxidation processes (AOPs). The Fenton process has been employed successfully to treat different industrial wastewaters, including textile (Fang et al., 2002), paper pulp (P´erez et al., 2002), pharmaceutical (H¨ofl et al., 1997), dyes (Szpyrkowicz et al., 2001), olive oil (Rivas et al., 2001) and petroleum industrial wastewaters (Gao et al., 2004).

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2.6.1 Factors affecting Fenton process

The key features of the Fenton process are believed to be its reagent condition, i.e.

[Fe²⁺], [Fe³⁺], [H2O2] and the reaction characteristics (pH, temperature and the quantity of organic and inorganic constituents). Because these parameters determine the overall reaction efficiency, it is important to understand the mutual relationships between these parameters in terms of hydroxyl radical production and consumption (Neyens and Baeyens, 2003).

H

2

O

2

concentration

As the concentration of H2O2 increases, the degradation of organic compounds also increases because the amount of oxidant present in the reaction system is higher. This finding is in agreement with the results found in other studies (Catalkaya and Kargi, 2007; Bautista et al., 2007; Oliveria et al., 2006). However, for higher H2O2 loads the degradation efficiency decays. This detrimental effect at high H2O2 concentrations can be explained by the fact that hydrogen peroxide competes with the organic matter for the OH radicals, and in this way it reduces the amount of these highly oxidative radicals present in the system (Oliveria et al., 2006). Thus, an increase of H2O2

concentration will result in an increase in the rate of radical scavenging, leading to the reduction in treatment efficiency. Glaze et al. (1995) and Beltran et al. (1996) reported a reduction in pesticide removal at high H2O2 concentrations indicating the adverse effects of excess H2O2.

Fe

²⁺

concentration

The increase in the initial concentration of Fe²⁺ considerably improved the degradation efficiency of herbicide tebuthiuron (Silva et al., 2007). These results can

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be explained by an increase in the reaction kinetics due to the higher iron concentration.

Characteristics of the target compound

Ruppert and Bauer (1993) studied the influence of the structure of several organic pollutants as they were mineralized by OH radicals. All of the aromatic substances studied were strongly degraded after several hours, while the organic carbon of cyclohexanol and cyclohexanone was hardly attacked. In alicyclic compounds the attack of the electrophilic OH radicals cannot occur at conjugated C = C double bonds in contrast to aromatic compounds where ring opening and further degradation takes place. The ordered H2O2 decrease during reaction was in good correlation with the total organic carbon (TOC) degradation. For all aromatic substances studied, degradation curves became linear after the first 30 min, until H2O2 was completely exhausted. During degradation of cyclohexanol and cyclohexanone only a slight decrease of the oxidant could be observed. The continued destruction of nitroaniline after exhaustion of H2O2 was attributed to photo-Fenton reactions (Ruppert and Bauer, 1993).

Initial concentration of the target compound

Initial pesticide concentration was the most important parameter affecting the efficiency of pesticide removal in the Fenton reaction. Mineralization efficiencies obtained at low pesticide concentrations were higher than those obtained at high pesticide concentrations. Increases in initial pesticide concentrations yielded decreasing pesticide removal at all Fe²⁺ concentrations indicating limitations of H2O2

concentration at high initial pesticide concentrations. The percentage pesticide

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removals were 35%, 48% and 72% with an initial pesticide doses of 15, 10 and 1 mg/L, respectively, at a constant Fe²⁺/H2O2 dose of 15/170 mg/L (Catalkaya and Kargi, 2007).

Initial pH

Walling et al. (1975) simplified the overall chemistry of Fenton process by accounting the dissociation of water, as seen in Eq. 2-2.

2 3

2 2 2 2 2 2

Fe⁺ +H O + H⁺ → Fe ⁺+ H O………...……(2-2) This equation suggests that the presence of H⁺ is required in the decomposition of H2O2, indicating the need for an acidic environment to produce the maximum amount of hydroxyl radicals. Previous Fenton studies have shown that acidic pH levels near 3 are usually optimum for the Fenton reactions (Hickey et al., 1995). In the presence of organic substrates (RH), excess Fe²⁺ and at low pH, hydroxyl radicals can add to the aromatic or heterocyclic rings.

At low pH, H2O2 is stabilized as H3O²⁺ (Kwon et al., 1999) and moreover, the regeneration of Fe²⁺ by reaction of Fe³⁺ with H2O2 is inhibited and the reaction between ^•OH and H⁺ becomes important. On the other hand, the decrease of oxidation yield of the process at higher pH values (pH>3) is due to the precipitation of Fe³⁺ as Fe(OH)3, hindering the reaction between Fe³⁺ and H2O2. Besides, Fe(OH)3 catalyzes the decomposition of H2O2 to O2 and H2O, thus decreasing the production of ^•OH (H¨ofl et al., 1997). Moreover, at higher pH values it is possible that highly stable Fe²⁺ complexes are formed (Tang and Huang, 1996). Therefore, in the present investigation all the experiments were carried out at an initial pH value of 3.

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2.7 Application of Fenton process for water and wastewater treatment

Over the last years, a great number of studies on the Fenton degradation of organic compounds have been reported (Table 2-3). The advanced oxidation of diuron in aqueous solution by Fenton’s reagent using FeSO4 as the source of Fe²⁺ was investigated in the absence of light. The effect of operating parameters such as concentrations of pesticide (diuron), H2O2 and Fe²⁺ on oxidation of diuron was investigated using the Box-Behnken statistical experiment design and the surface response analysis. Diuron removal increased with increasing H2O2 and Fe²⁺

concentrations up to a certain level. Around 98.5% diuron removal was achieved after 15 min reaction period. However, only 58% of diuron was mineralized after 240 min under optimal operating conditions indicating the formation of some intermediate products. The optimal H2O2/Fe²⁺/diuron concentration resulting in the maximum diuron removal (98.5%) was found to be 302/38/20 mg/L (Catalkaya and Kargi, 2007)

Bautista et al. (2007) evaluated the removal of organic matter (TOC and COD) from a cosmetic wastewater by the Fenton process. With the operating conditions of initial pH equal to 3.0, Fe²⁺ concentration of 200 mg/L, and H2O2 concentration to COD initial weight ratio corresponding to the theoretical stoichiometric value (2.12), TOC conversions higher than 45% at 25°C and 60% at 50°C were achieved. A simple kinetics analysis based on TOC was also studied. The second-order kinetics well described the overall process within a wide TOC conversion range covering up to 80 to 90% of the maximum achievable conversion.

The degradation of wheat straw black liquor (WSBL) by Fenton’s reagent was

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investigated by Torrades et al. (2007). The use of irradiation providing the conditions needed for the occurrence of photo-Fenton reaction along with the main parameters that govern the complex reactive system have been studied. Concentrations of Fe²⁺

(20-1000 mg/L) and H2O2 (500-2000 mg/L) were chosen for the experiments. The best results were obtained when using 1500 mg/L H2O2 and 200 mg/L Fe²⁺ (12:1 molar ratio of H2O2/Fe²⁺). The use of Fenton and photo-Fenton reactions have been proven to be highly effective in the treatment of WSBL. Also, the high levels of COD, aromatic and lignin removals were observed in the system.

Oliveria et al. (2006) used an experimental design methodology for designing 2,4-dichlorophenol (2,4-DCP) oxidation experiments using Fenton’s reagent. The multivariable and multilevel approaches of the experimental design methodology were useful to quantify the effects between the experimental variables (temperature, Fe²⁺, and H2O2 concentrations) in the process performance, with the minimum number of experiments. Response factors considered were 2,4-DCP degradation after 5, 10, and 20 min of reaction time, for an initial 2,4-DCP concentration of 100 mg/L. It was found that the Fe²⁺ concentration had a positive effect on the oxidation performance.

Besides, the optimal conditions depend on the response considered, with it being advisable to use less-aggressive conditions if responses are taken at longer reaction times. Finally, the kinetic model proposed was useful for predicting the evolution of 2,4-DCP concentration within the batch reactor over time.

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Table 2-3 Literature review of Fenton process

Reference Method Compound Condition

Reaction

Fenton and wet oxidation

Bautista et al.

(2007) Fenton

Torrades et al.

(2007) Photo-Fenton

Oliveria et al.

(2006) Fenton

*Wheat straw black liquor

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Chapter 3

Materials and Methods

3.1 Chemicals

Carbofuran was obtained from Shida Chemical Industries (Taoyuan, Taiwan) and was used as received (HPLC grade＞98%). Fe²⁺ solution was prepared by dissolving FeSO4．7H2O (Panreac, E.U.) in ultra pure water. Hydrogen peroxide (30%, w/w in water) was supplied by Panreac, E.U. All other chemicals and solvents were of the purest grade commercially available and were used without further purification.

3.2 Experimental procedure

The schematic diagram of the Fenton process experimental set-up is shown in Figure 3-1. The stock carbofuran solution (100 mg/L) was prepared in ultrapure water just prior to the experiments. The Fenton degradation experiments were conducted at two carbofuran concentrations i.e. 10 and 50 mg/L with a working volume of one litre.

The schematic diagram of the Fenton process experimental set-up is shown in Figure 3-1. The stock carbofuran solution (100 mg/L) was prepared in ultrapure water just prior to the experiments. The Fenton degradation experiments were conducted at two carbofuran concentrations i.e. 10 and 50 mg/L with a working volume of one litre.

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