Photocatalytic Degradation of 2,4-Dichlorophenoxyacetic Acid Using

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Photocatalytic Degradation of 2,4-Dichlorophenoxyacetic Acid Using Concentrated Solar Radiation: Batch and Continuous Operation Sanjay P. Kamble,† Sudhir P. Deosarkar,† Sudhir B. Sawant,† Jacob A. Moulijn,‡ and Vishwas G. Pangarkar*,† Chemical Engineering Department, Institute of Chemical Technology, University of Mumbai, Matunga, Mumbai-400 019, India, and Reactor and Catalysis Engineering, Delft ChemTech, Technological University of Delft, Julianalaan 136, 2628BL, Delft, The Netherlands

Solar photocatalytic oxidation (PCO) for the degradation of liquid and gaseous pollutants is receiving increasing attention. In the present investigation, an attempt has been made to demonstrate the feasibility of a novel slurry photoreactor for the mineralization of 2,4dichlorophenoxyacetic acid (2,4-D) by using a semiconductor photocatalyst (TiO2), air, and concentrated solar radiation. The photocatalytic degradation (PCD) of 2,4-D using concentrated solar radiation in both batch and continuous bubble column reactors has been studied. The photocatalytic reaction takes place on the surface of the catalyst material. Therefore, the adsorption behavior of 2,4-D on the catalyst has also been investigated. The effects of the presence of anions and the pH of the wastewaters are reported. The photocatalytic degradation studies of 2,4-D in the novel slurry bubble column indicate that this type of reactor can be used commercially for the treatment of wastewater from a plant manufacturing 2,4-D. 1. Introduction The increasing contamination of our environment with anthropogenic chloro-organic compounds is certainly one of the greatest problems for the human future. 2,4-Dichlorophenoxyacetic acid (2,4-D) is a widely used herbicide. The structural formula for 2,4-D is as follows

There are approximately 1500 registered products that contain 2,4-D in their composition, and more than 70 million pounds of this active ingredient are distributed annually. In the U.S. alone, more than 30,000 tones of herbicides are used annually.1 Different data have been reported for its toxicity with an LD50 for rats being in the range from 100 to 500 mg/kg.2 2,4-D is one of the most commonly used herbicides in controlling broadleaf weeds and other vegetation. Worldwide, 2,4-D and different salt derivatives and ester forms are used to control broadleaf weeds in a variety of places including home lawns, cereal and grain crops, commercial areas, commercial turf, rights-of-way, and forests.1,3 This widespread use of 2,4-D leads to a certain environmental impact, because, depending on the climatic conditions, the type of formulation used, and the nature of the soil, this pesticide is easily spread within the environment.3,4 It is a pollutant of great environmental concern because of its relatively high solubility * To whom correspondence should be addressed. E-mail: [email protected], [email protected]. Fax: +91-2224145614. † Mumbai University Institute of Chemical Technology. ‡ Technological University of Delft.

in water. After its application to crops, the unused portions can leach below the root zone or be washed out during precipitation and contaminate nearby water sources. In addition, various amounts of 2,4-D have been detected in surface water and groundwater not only during application of the herbicide but also after a long period of use.3 2,4-D has a mean lifetime of about 20 days, and its degradation products accumulate.5 Because of the toxicity of 2,4-D and its products, a conventional treatment method is not suitable for wastewaters containing 2,4-D, as they can destroy the microbial population of the treatment plants. Most microbial organisms lack enzyme systems to degrade these substances. Thus, these compounds tend to accumulate in water and soil, which is a reason they are termed recalcitrant or refractory compounds. Therefore, the development of an effective degradation process for this herbicide is very important. The elimination of 2,4-D from aqueous solutions has been investigated using microbial, chemical, and photochemical processes with varying success.6-8 Photocatalysis is a subject of increasing interest as a method for wastewater treatment. This approach has been shown to be a good alternative for the degradation of recalcitrant compounds.9-12 In these processes, the oxidation occurs through an attack by OH•, which has a rate constant billions of times higher than normal oxidation rate constants, using air as the oxidant.9,11 The UV radiation required for the photocatalytic processes can be obtained from artificial sources or the sun. There is a significant economic incentive for solarpowered photocatalytic reactors. The treatment of wastewater using concentrated solar radiation holds promise for regions receiving strong sunlight throughout the year such as South America, South Africa, the Indian subcontinent, Australia, and other countries.9-12 Photocatalysis is one of the advanced oxidation technologies in which OH• radicals can be produced through electrontransfer processes at the semiconductor-electrolyte interface under illumination. In these processes, the

10.1021/ie0494263 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/18/2004

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semiconductor particles absorb light of suitable energy to produce electron-hole pairs. In solution, oxygen acts as an electron scavenger while water molecules on the semiconductor surface react with photogenerated holes to produce OH• radicals.11,12

H2O + h+ f OH• + H+

(1)

In the recent past, substantial research has been carried out on photocatalysis for the degradation of chloroorganic compounds. Table 1 gives details of the results of various investigators on the destruction of 2,4-D using advanced oxidation processes (AOPs).1-4,13-18 The development of large-scale plants for the decontamination of wastewater through the use of AOPs presents a new challenge to engineers and scientists. A considerable volume of literature has been published in the field of heterogeneous photocatalysis, with most of the experimental studies conducted in bench-scale reactors. Many laboratory reactors used for photocatalytic studies are typically small (100-500 mL in capacity), and as a result, the effect of the radiation field within the reactor was inadvertently disguised.19-20 The industrial application of photocatalysis technology is stymied by the lack of suitable photoreactor models and design procedures, size limitations, and the use of glass as a material for construction of the reactor vessel. Inadequate knowledge of secondary light-scattering effects in the reaction medium further complicates scale-up strategies.21 The geometry of the photoreactor has a major effect on the photocatalysis process. Kamble et al. used a novel slurry bubble column reactor to degrade different pollutants, using concentrated solar radiation. Their investigations clearly show that this type of reactor can be conveniently used in those parts of the world (for example, a major portion of the Indian subcontinent) where sunlight is available for a large part of the year; Figure 1 shows the configuration of the slurry bubble column used.10 In the present work, a systematic study of the photocatalytic degradation (PCD) of 2,4-D using concentrated solar radiation in batch as well continuous mode is reported. The effects of various parameters on photocatalytic degradation of 2,4-D were studied in batch reactor for its optimization. The small-scale batch experiments provide a clear idea for pilot-scale experiments in terms of the effects of catalyst loading, substrate concentration, effluent pH, and types of ions on PCD of 2,4-D, and hence, it avoided the study of these parameter at the pilot scale. In addition to pollutants, industrial effluents contain different salts at different levels of concentration. The presence of anions such as chloride, sulfate, carbonate, and bicarbonates is common in industrial effluents. These ions affect the adsorption of the degrading species, act as hydroxyl ion scavengers, and can absorb UV light as well. Therefore, the effects of these salts on the photocatalytic degradation of 2,4-D are important. Generally, industrial effluents are neutral, but sometimes, they can be acidic or basic. In the case of 2,4-D, the wastewater coming from nitration is likely to be acidic in nature. No literature studies are available on these effects except for the effect of pH. Hence, it was necessary to study these effects on the PCD of 2,4-D. All of these basic parametric effects were conveniently studied in the small-scale batch reactor. Degradation of 2,4-D was studied in two reactors of

Figure 1. Novel slurry bubble column reactor for the continuous photocatalytic degradation of 2,4-D using concentrated solar radiation.

different sizes to derive the scale-up factor. The studies with the continuous bubble column reactor work were aimed at demonstrating the viability of the novel reactor (Figure 1) proposed for the treatment of pesticide wastewaters. 2. Experimental Section 2.1. Materials. All of the reagents used for experimental studies were of analytical reagent grade. Degussa P-25 TiO2 (70:30 w/w anatase to rutile) with an average particle size of 30 nm and a BET surface area of about 55 m2 g-1 was used as the photocatalyst. 2,4Dichlorophenoxyacetic acid (2,4-D) was supplied by Atul Chemicals Ltd, Gujarat, India. Sodium chloride, sodium carbonate, sodium bicarbonate, sodium sulfate, ammonium sulfate, ferrous sulfate, and copper sulfate of analytical reagent grade were obtained from s.d. fine chemicals Ltd., Mumbai, India. All reaction mixtures and HPLC mobile phase solutions were prepared in deionized water. Plain solar radiation intensity was measured in watts per square meter using a daystar meter (Daystar Inc., Las Cruces, NM) working on the photocell principle. Solar radiation intensity at the ground level is referred to as plain intensity henceforth. A Krebsoge Excel sintered stainless steel filter (candle filter) of 1-µm rating having the dimensions 64-mm o.d. (60-mm i.d.) × 150-mm length was used as the surface filter, and a sintered stainless steel disk of 20-µm rating (100-mm o.d. × 2-mm thickness) supplied by Krebsoge Excel Pvt Ltd., Mumbai, India, was used for sparging the air at the bottom of the column. 2.2. Batch Experiments. All experiments were carried out in a quartz cylindrical reactor (8-cm i.d.) of 465-cm3 capacity fitted with a centrally mounted sparg-

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Table 1. Details of the Results of Various Investigations into the Destruction of Chloro-Organic Compounds Using AOPs compound(s)

reactor type

catalyst and/or H2O2 used

source of radiation

2,4-D

40-mL thermostatic cylindrical Pyrex cell

titanium dioxide (Degussa P-25) with surface area ) 59 m2/g and Pt-TiO2 with surface area ) 45 m2/g

125-W Philips HPK mercury vapor lamp

2,4-D

test tube quartz glass reactor (length ) 32 cm, diameter ) 6 cm, thickness of quartz wall ) 2.5 mm)

titanium dioxide

20-W black light

2,4-D

400-mL quartz reactor

iron(II) sulfate-7hydrate, ferric sulfate hydrate

low-pressure mercury lamp, λ ) 254 nm, light intensity ) 1.5 × 10-6 einstein/(L s)

2,4-D, phenoxyacetic acid, 2,4-DCP, and phenol

spiral reactor (1.6 m long, 0.86 cm i.d.)

TiO2 (Degussa P-25) with surface area ) 57.9 m2/g

125-W Philips HPK mercury vapor lamp

2,4-D

250-mL cylindrical Pyrex reactor and double quartz cylindrical plasma photoreactor (diameter ) 10 cm, i.d. ) 1 cm)

Degussa P-25 with surface area ) 53 m2/g

Ushio 250-W mercury lamp

2,4-D

130-cm3 thermostatic cylindrical Pyrex cell

ZnO photocatalyst with surface area ) 34.6 m2/g, FeIISO4

125-W Philips HPK medium vapor pressure lamp

2,4-D

2000-cm3stirredtank photoreactor

H2O2 with 30.4% (p/v)

15-W G15T8 low-pressure mercury lamp

2,4-D

600-mL Pyrex Erlenmeyer flask

H2O2 (35%) ferric sulfate hydrate (Fe3+)

no light

2,4-D

1.5-dm3 annular photoreactor (quartz inner wall)

no catalyst

Uviarc UA-3 360-W mercury lamp

2,4-D

400-cm3 quartz cylinder, kept in a prewarmed RPR-20 Rayonet photochemical reactor

ferrous oxalate/H2O2

two 253.7-nm phosphor-coated low-pressure mercury lamps (35 W each)

comments 2,4-D photodegradation depends on the mass of the semiconductor, temperature, and light intensity. The highest conversion rates were obtained at pH ) 3.5, which is close to the pKa of the acid and lower than the point of zero charge of TiO2. Photocatalytic degradation of 2,4-D using a combination of ozonolysis and photocatalysis gave better degradation results with lower intermediate concentrations, whereas photocatalytic treatment of 2,4-D-polluted waters led to the buildup of high concentrations of the long-lived intermediate DCP. Oxidation of 2,4-D by different iron-mediated processes, with or without ultraviolet radiation (at 253.7 nm) and oxalate, was investigated. The rate of degradation can be accelerated by exposure to UV irradiation, which can increase the initial 2,4-D decay rate from 10 to more than 100 times PCD of 2,4-D, phenoxyacetic acid, 2,4-DCP, and phenol was conducted in continuous mode by means of a flow system, in which TiO2 remains fixed on glass pearls. The rate of degradation was high except for 2,4-DCP. It was observed that the rate of degradation depends on the solution pH. The intermediate formed during PCD was also analyzed. Photocatalytic degradation of 2,4-D using integrated microwave/UV radiation was studied. The effect of oxygen on PCD was also studied. The greater efficiency of the microwaveassisted process was ascribed to a nonthermal effect of microwave radiation on the breakup of the aromatic ring of 2,4-D (oxidation) but evidently not in the dechlorination process (reduction) for which microwave radiation seemed to have a negligible influence, if any. The oxidation of 2,4-D using ZnO as photocatalyst was investigated. This rate of photodegradation depends on the catalytic loading, the light intensity and the pH of the solution. The rate of photodegradation increases with addition of a small amount of Fe(II). The photooxidation of 2,4-D in aqueous solution using H2O2 and UV radiation was studied. 2,4-DCP and chlorohydroquinone (CHQ) major intermediates were formed during oxidation. The Langmuir-Hinshelwood model with consideration of a major intermediate was fitted. Photochemical degradation of 2,4-D using Fe3+/H2O2 was investigated. The effect of high temperature on degradation of 2,4-D was studied, they found that degradation rate increases with increasing temperature. The photodegradation of 2,4-D in aqueous solutions was studied by using UV radiation in an annular batch reactor. The effect of pH on photodegradation was investigated. It was observed that the rate of degradation was higher at acidic than alkaline pH. The effects of pH and the oxalate, and hydrogen peroxide concentrations on the degradation performance of the 2,4-D was studied in a novel ferrous oxalate/H2O2/UV system. The effects of oxalate addition and pH on PCD of 2,4-D were investigated. The 2,4-D transformation was faster at a lower initial pH than at a higher pH.

ref Trillas et al.1

Muller et al.2

Kwan et al.3

Trillas et al.4

Horikoshi et al.13

Sanchez et al.14

Alfano et al.15

Lee et al.16

Chamarro et al.17

Kwan et al.18


Figure 2. Batch experimental setup for the photocatalytic degradation of 2,4-D using concentrated solar radiation.

er surrounded by a cooling coil (4-cm i.d.), both made of borosilicate glass, and a water-cooled glass condenser for the outgoing gases/vapors. The initial temperatures of the reaction mixtures for the PCD of 2,4-D were in the range of 28-32 °C, and the final temperature was in the range of 32-35 °C. The temperature of the reaction mixture was maintained near ambient by passing water through the cooling coil. The experimental setup is shown in Figure 2. Before the start of a batch experiment, the 2,4-D solution was equilibrated by being stirred for 15 min in the dark before the reactor assembly was exposed to the concentrated sunlight from a compound parabolic reflector (projected area . 1400 cm2). The concentration of 2,4-D in this filtered sample was treated as the zerotime concentration (Ct)0) in each experiment before exposure to radiation. The reflector was used to concentrate the solar radiation, forming a continuous glowing band of concentric light surrounding the wall of the reactor. After every 15 min, the position of the reflector was tracked with respect to the sun so as to maintain the band of light surrounding the reactor wall. Air presaturated with water was bubbled at a sufficiently high velocity (>2 cm s-1) to keep the TiO2 in suspension. The batch experiments were started at 10: 30 a.m. Indian Standard Time (IST). Samples were taken periodically, centrifuged, filtered, and stored in amber-colored bottles. Blank experiments (without TiO2 and solar radiation) were also carried out to assess the loss of 2,4-dichlorophenoxyacetic acid through outgoing air. Because the reactor was provided with a condenser, this loss was less than 1%. The equilibrium adsorption experiment was performed with high precision. In the case of dark adsorption studies, 50-mL aliquots were mixed with TiO2 and shaken for 24 h to allow equilibration. The glass conical flasks were covered with aluminum foil during shaking so as to prevent light from entering the flasks. The solutions were then centrifuged, decanted, and filtered for analysis by HPLC. Most of the experiments were repeated for better accuracy. The experimental error was observed to be within (2%. A similar procedure was used for experiments on the effect of pH on the adsorption of 2,4-D. 2.3. Continuous Experiments. Continuous experiments were carried out in borosilicate glass slurry bubble column reactors of 0.1-m i.d. × 3.0-m length and 0.15-m i.d. × 3.0-m length with capacities of 20 and 54 L, respectively. A schematic of the slurry bubble column reactor is shown in Figure 1. The experimental setup was located on the terrace of a three-story building to

avoid interference of trees and other buildings. A solution of 2,4-D was prepared in tap water in a 250L-capacity agitated tank. A metering pump was used for delivering 2,4-D solutions to the top of the borosilicate glass column, and a diaphragm-type air compressor (model HS-2, C. P. Enterprises, Mumbai, India) was used to sparge the air at the bottom of the column through a sintered stainless steel disk (Krebsoge Excel Pvt Ltd., Mumbai, India). A parabolic reflector (3.0 m in height with a total surface area of 6.0 m2) was used to concentrate the solar radiation, forming a continuous glowing band of concentric light surrounding the wall of the bubble column reactor. After every 15 min, the position of the reflector was changed with respect to the sun so as to maintain the band of light surrounding the column reactor wall. Air was bubbled at sufficiently high velocity (>2 cm s-1) to keep all of the TiO2 in suspension. The air passed into the reactor was presaturated with water. Each experiment was started at 9:30 a.m. Indian Standard Time (IST). After steady state was reached, a sample was taken for analysis and stored in an ambercolored bottle. In all of these experiments, pH was not controlled. 2.4. Analysis. In the case of photocatalytic experiments, samples were centrifuged and filtered through a membrane filter to separate out TiO2 particles. Concentrations of 2,4-D and its intermediates were measured by an HPLC instrument (Knauer) equipped with a C-18 column (5-µm particle diameter, Merck) and a UV-vis detector. The mobile phase was determined from several combinations giving the best separation of the possible intermediates. Acetonitrile/water/acetic acid (50:50:0.2% v/v) was used as the mobile phase. The flow rate of mobile phase for analysis was kept at 0.5 mL min-1 using λ ) 280 nm for UV detection. Total organic carbon (TOC) was calculated from HPLC analysis and verified using a total organic carbon analyzer (ANATOC-II, Australia). These values matched within (2%, indicating that the HPLC analysis could detect all of the intermediates. Pure nitrogen was bubbled through the samples before TOC analysis to remove the dissolved CO2. 3. Results and Discussion The experiments were carried out in the months of Nov 2003 to Feb 2004 in Mumbai, India (18.58° N, 72.50° E). During this period, the sky was brilliant blue (no clouds), and the average solar intensity was approximately ((10%) constant at 415 W m-2 as measured at the ground level. This intensity is referred to as the plain solar intensity. 3.1. Mechanism of Photocatalytic Degradation. From previous studies, it is known that the mechanism of photocatalytic degradation is through the attack of OH• on the substrate.12,20 In the case of aromatic compounds, the position at which OH• attacks depends on the directing effect of the original functional group in the benzene ring. For instance, the amino group in aniline has an ortho/para directing effect. Thus, as intermediate aniline yields predominantly o-and pamiophenol.9 In the case of 2,4-D, the -OCH2COOH group is replaced by an OH• radical. During the photocatalytic degradation of 2,4-D, 2,4-dichlorophenol (concentration ranges from 44.5 to 3.50 mg/L for 100 mg/L of 2,4-D solution) was found to be a major intermediate, whereas chlorohydroquinone (5.50-2 mg/L) was found in very

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small quantities. Similar intermediates were observed during the photochemical degradation of 2,4-D by Alfano et al.15 Thus, during the PCD of 2,4-D in solution, the attack of OH• radical on the aromatic ring leads to the dechlorination of the chloroaromatic compounds, producing hydroxylated aromatic compounds. It is known that the aromatic ring yields at most trihydroxy derivatives.12 Generally, after the attachment of two OH groups, the aromatic ring breaks. The subsequent products are rapidly oxidized to CO2 and H2O, as evidenced by the absence of aliphatic moieties. 3.2. Effects of Various Factors on the Adsorption of 2,4-D on TiO2. The pH of the aqueous solution significantly affects titanium oxide (TiO2) in terms of the surface charge on the photocatalyst, the size of the aggregates it forms, and also the state of ionization of the substrate and hence its adsorption. The pH at which the surface of an oxide is uncharged is defined as the zero-point pH (pHzpc), which is around 7 for TiO2. Above and below this value, the catalyst is negatively and positively charged, respectively, according to the following equilibria10,22

-TiO2- T TiOH + H+

(2)

-TiOH T TiO- + H+

(3)

Figure 3. Effect of the presence of various anions on the adsorption 2,4-D on TiO2. Adsorption isotherms of 2,4-D (9) in the absence of anions and in the presence of (2) 0.1 M Na2SO4, (0) 0.1 M NaHCO3, (4) 0.1 M Na2CO3, and (() 0.1 M NaCl.

For solutions with initial pH lower than the pHzpc of TiO2, the positively charged surface favors the adsorption of the negatively charged species in solution, the concentration of which depends on the pKa of the substrate being considered. On the contrary, for solutions of initial pH higher than the pHzpc of TiO2, the negatively charged surface favors the adsorption of the positive species. At this pH, a lower percentage of the substrate is adsorbed on the catalyst surface, because of the increasing percentage of anionic species in solution with increasing pH. Hydroxyl groups on the TiO2 surface are known to undergo the following acid-base equilibria4

>TiOH2+ S TiOH + H+

(4)

>TiOH S TiO- + H+

(5)

Figure 4. Effect of pH on adsorption of 2,4-D (g/g of TiO2 vs pH)

As a consequence, the photocatalytic degradation of the ionizable organic compounds is affected by the pH. In many cases, a very important feature of photocatalysis that is generally not taken into account when it is to be used for the decontamination of water is that, during the reaction, a multitude of intermediate products are produced that can behave differently depending on the pH of the solution.22 The intermediates detected in the present work were already reported in section 3.1. It can be seen from the adsorption equilibria that the presence of anions has some effect on the adsorption of 2,4-D on TiO2 (Figure 3) and this is reflected in the degradation as well. The solid line represents the best fit for each adsorption isotherm. The presence of Na2SO4, NaCl, Na2CO3, and NaHCO3 has a large effect on the adsorption and hence also on the photocatalytic degradation as discussed later. A comparison of our previous studies on the adsorption and photocatalytic degradation of aniline,9 PHBA,23 benzoic acid,24 and benzenesulfonic acid10 and that on 2,4-D adsorption in the present study lead to an interesting conclusion: the adsorption of 2,4-D on the

TiO2 surface is greater than that of aniline, PHBA, benzoic acid, and benzenesulfonic acid, and this explains why the photocatalytic degradation rate of 2,4-D also is the highest. Figure 4 shows the effect of pH on the adsorption. The pH of the 2,4-D solution was varied in the alkaline range by adding aqueous NaOH. It is evident that the solution pH has a significant effect on the adsorption of 2,4-D. The maximum adsorption occurs at the self-pH of 2,4D. This pH equals 2.68. Similar results were obtained in the photocatalytic degradations of BSA10 and 2,4,6trichlorophenol (TCP).25 It was found that, although a significant amount of adsorption was observed at lower pH values (2.64-4.2), at higher pH values, very little adsorption was noted. This is probably due to the fact that, at higher pH, 2,4-D exists in the ionized state, which is poorly adsorbed. 4. Comparison of the Photocatalytic, Photochemical, and Photolytic Degradation of 2,4-D Photocatalytic, photochemical, and photolytic degradation studies were carried out using a 100 mg L-1


Figure 5. Comparison of photocatalytic degradation of 2,4-D with photochemical and photolysis method [TOC/(TOC)t)0 vs time]. Initial concentration ) 100 mg L-1, catalyst loading ) 0.20% (w/v of solution). (9) TiO2/air/solar radiation. (O) H2O2 /solar radiation. (4) Solar radiation.

initial concentration of 2,4-D. In the photocatalytic degradation experiments, Degussa P-25 TiO2, concentrated solar radiation, and air were used. For the photochemical degradation experiments, a stoichiometric amount of hydrogen peroxide with concentrated solar radiation was used, and for the photolytic degradation experiments, only concentrated solar radiation was used. Figure 5 shows the course of the photochemical, photocatalytic, and photolytic degradations of 2,4-D using concentrated solar radiation. The percentage degradations of 2,4-D in the photocatalytic, photochemical, and photolytic processes were 98.5, 78.7, and 47.4, respectively, after 4 h. The corresponding percentage TOC reductions were 88.6, 24.4, and 11.8, respectively, after 4 h. The rates of 2,4-D degradation in the case of the photocatalytic method in terms of TOC were found to be about 64.1% and 76.8% higher than the rates by the photochemical and photolysis method, respectively. This is because (i) the rate of generation of OH• radical in the photocatalytic method is higher than that in the photochemical and photolysis methods and (ii) higher quantities of intermediates are present in the photochemical and photolysis methods than in the photocatalytic method. Therefore, further studies were restricted to the photocatalytic oxidation method. 5. Batch Photocatalytic Degradation with Concentrated Solar Radiation 5.1. Effect of Reflector Shape. In all of the experiments, a parabolic reflector was employed for the photocatalytic degradation of 2,4-D. In one of the experiments, this reflector was replaced by an anodized aluminum reflector of spherical shape. An initial 2,4-D concentration of 100 mg L-1 and a catalyst loading of 0.20% w/v of solution was used in both experiments. A significant change in the photocatalytic degradation of 2,4-D was observed, as seen in Figure 6. The parabolic reflector is a unique reflector shape that focuses a collimated beam of radiation. Thus, a circular concentrated band of light is formed on the surface of the

Figure 6. Comparison of photocatalytic degradation of 2,4-D with differently shaped reflector [TOC/(TOC)t)0 vs time]. Initial concentration ) 100 mg L-1, catalyst loading ) 0.20% (w/v of solution). (0) Photocatalytic degradation of 2,4-D with a parabolic reflector. (9) Photocatalytic degradation of 2,4-D with a spherical reflector.

reactor. On the other hand, the spherical reflector yields a vertical beam parallel to the vessel wall. Therefore, the parabolic reflector gives a higher number of photons in the reactor as compared to the spherical reflector. The surface area of the spherical reflector is 3620 cm2, whereas that of the parabolic reflector is 1400 cm2. Thus, although the surface area of the spherical reflector is 2.6 times greater than that of the parabolic reflector, its effectiveness in terms of photon penetration in the reactor is relatively poor. 5.2. Influence of Catalyst Concentration. In slurry photocatalytic processes, the TiO2 dosage is an important control parameter that can affect the degradation rate. When the catalyst concentration is very high, turbidity impedes further penetration of light into the reactor. Therefore, the optimum catalyst loading has to be found to avoid excess catalyst and ensure effective utilization of the incident photons.20 With an initial concentration of 100 mg L-1 of 2,4-D, the optimum loading of the catalyst was found by varying the catalyst loading. As can be seen in Figure 7, the optimum catalyst loading is 0.20% (w/v of solution). As the catalyst loading is increased, there is an increase in the surface area of the catalyst available for adsorption and degradation. However, increasing the catalyst loading increases the solution opacity, leading to a decrease in the penetration of the photon flux in the reactor and thereby decreasing the photocatalytic degradation rate. In all subsequent experiments, a 0.20% (w/v of solution) TiO2 loading was used. 5.3. Effect of the Initial Concentration of 2,4-D. Photocatalytic degradation studies were carried out using 100, 200, 300, and 400 mg L-1 initial concentrations of 2,4-D and 0.20% (w/v of solution) catalyst loading. Figure 8 shows a plot of the normalized 2,4-D concentration (C/Ct)0) with time. For the initial concentrations of 100, 200, 300, and 400 mg L-1, the 2,4-D concentration was reduced by 99.5%, 99.1%, 99%, and 98%, respectively, within 4 h. For the initial concentra-

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Figure 7. Influence of catalyst loading on photocatalytic degradation of 2,4-D [TOC/(TOC)t)0 vs time]. Initial concentration ) 100 mg L-1. ()) 0.1% (w/v of solution) catalyst loading. (9) 0.2% (w/v of solution) catalyst loading. (4) 0.3% (w/v of solution) catalyst loading.

Figure 8. Effect of initial concentration on the photocatalytic degradation of 2,4-D (C/Ct)0 vs time). Catalyst loading ) 0.20% (w/v of solution). Initial concentration ) (9) 100, (() 200, (2) 300, and (0) 400 mg L-1.

tions of 100, 200, 300, and 400 mg L-1 2,4-D, the TOC was reduced by 88.6%, 77.9% 74.0%, and 69.9%, respectively, within 4 h. Figure 8 shows that an increase in the feed concentration of 2,4-D causes a decrease in the rate of photocatalytic degradation. As mentioned earlier, the reaction occurs between the adsorbed 2,4-D and OH• generated on the TiO2 surface. The concentration of adsorbed 2,4-D increases with increasing feed 2,4-D concentration, but at high concentration of 2,4-D, the concentration of intermediates formed is higher. These intermediates adsorb on the surface of the catalyst and compete with the parent compound. However, for constant light intensity, TiO2 loading, and dissolved oxygen concentration, the concentration of OH• remains practically same. Thus, even though the adsorbed 2,4-D concentration increases, the rate of photocatalytic degradation decreases because of a lower OH•/2,4-D ratio. 5.4. Effect of pH. The effect of the initial pH on the photocatalytic degradation of 2,4-D was studied. In the

Figure 9. Effect of pH on the photocatalytic degradation of 2,4-D on TiO2 (C/Ct)0 vs time). Initial concentration ) 100 mg L-1, catalyst loading ) 0.20% (w/v of solution). pH ) (9) 2.68, (b) 7.0, and (2) 11.0.

Figure 10. Effect of pH on the photocatalytic degradation of 2,4-D on TiO2 [TOC/(TOC)t)0 vs time]. Initial concentration ) 100 mg L-1, catalyst loading ) 0.20% (w/v of solution). pH ) (0) 2.68, (b) 7.0, and (2) 11.0.

alkaline range, the pH was varied using aqueous NaOH, whereas in the acidic range, the pH was varied using HClO4. An initial 2,4-D concentration of 100 mg L-1 and a catalyst loading of 0.20% (w/v of solution) were used in all of these experiments. The pH affects the surface charge on the photocatalyst and also the state of ionization of the substrate and hence its adsorption. Also, industrial effluents can be basic or acidic, and therefore, this pH effect needs to be investigated. In Figure 9, it is observed that, as the pH increases from acidic to alkaline, the rate of PCD is maximum at the self-pH of 2,4-D. Figure 10 shows a plot of the normalized total organic carbon concentration [TOC/ (TOC)t)0] with time. It is observed that, as the pH decreases from alkaline to acidic, the rate of photocatalytic degradation of 2,4-D increases and is a maximum at pH 2.68. This observation agrees with the greater adsorption of 2,4-D under nonionized, low-pH conditions


Figure 11. Effect of the presence of various anions on the photocatalytic degradation (PCD) of 2,4-D on TiO2 [TOC/(TOC)t)0 vs time]. Initial concentration ) 100 mg L-1, catalyst loading ) 0.20% (w/v of solution). PCD of 2,4-D ()) in the absence of anions and in the presence of (0) 0.1 M NaCl, (×) 0.1 M (NH4)2SO4, (O) 0.1 M NaHCO3, (2) 0.1 M Na2SO4, and (() 0.1 M NaHCO3.

(Figure 4). Similar results were obtained in the photocatalytic degradations of benzenesulfonic acid,10 2,4dichlorophenoxyethanoic acid,18 and 4-chlorophenol.26 On the other hand, aniline, which is a basic compound and is ionized under acidic pH conditions, shows the opposite effect of pH. Thus, the photocatalytic degradation of aniline increases with increasing pH and is a maximum at pH 12.9 These observations point to a better strategy for more efficient PCD. Thus, generally, it will be better to treat any acidic compound in an effluent before neutralizing the same. On the other hand, effluents containing basic refractory compounds are easier to treat under basic pH. 5.5. Effect of the Presence of Anions. In addition to pollutants, industrial effluents contain different salts at different levels of concentration. The salts are generally ionized under the conditions of photocatalytic degradation. The anionic and cationic parts of the salt have different effects on the photocatalytic degradation process.27-29 The effects of the presence of various anions such as chloride, sulfate, bicarbonate, and carbonate were studied using 0.1 M solutions of their sodium salts and an initial concentration of 100 mg L-1 of 2,4-D with a 0.20% (w/v of solution) TiO2 loading. In general, an effluent is brought to neutral pH before it is introduced into the effluent treatment plant (ETP). In the case of 2,4-D, because the original solution is acidic, an alkali is added to raise the pH to the neutral pH. Ammonia and sodium carbonate/hydroxide were considered for this purpose. The neutralization process will generate ammonium sulfate and sodium sulfate, respectively, in these two cases. The results of studies of the photocatalytic degradation of 2,4-D in the presence of these salts, shown in Figure 11, clearly indicate that the salts have a substantially detrimental effect on the photocatalytic degradation. For sodium sulfate, ammonium sulfate, sodium chloride, sodium bicarbonate, and sodium carbonate, the reductions in the TOC were 75.7%, 54.1%, 59.6%, 25.5%, and 24.4%, respectively, whereas in the absence of anions, the reduction

in TOC was 88.6%. Previous studies with benzenesulfonic acid,10 p-hydroxybenzoic acid,23 and benzoic acid24 have also concluded that all of the above anions have a negative effect on photocatalytic degradation. Thus, it can be concluded that, if photocatalytic degradation is to be adopted for the treatment of wastewaters, it will be desirable to carry out the photocatalytic degradation prior to neutralization. The output of the photocatalytic degradation process can subsequently be introduced into the effluent treatment plant (ETP). 5.6. Effect of Cations. The presence of various cations is common in industrial effluents. These cations can have a positive or negative effect on the PCD rate. Bhatkhande et al.20 studied the effect of cations on the photocatalytic degradation of nitrobenzene (NB) in the presence of 0.1 and 0.01 M FeSO4 solutions. A reduction in the PCD rate was observed in the presence of 0.1 M FeSO4, which can be attributed to the negative effect on the adsorption of NB due to SO42-. However, in the presence of the 0.01 M FeSO4 solution, negligible improvement in the degradation was observed. Sclafani et al.30 studied the effect of Fe3+, Fe2+, and Ag+ on the PCD of phenol in aqueous polycrystalline TiO2 dispersions. They observed a maximum photoreactivity for TiO2 (anatase) in the presence oxygen and an Fe3+ concentration of 5 × 10-4 M. The effect of Fe2+ was similar. The anatase photoactivity was beneficially influenced in the presence of oxygen and a Ag+ concentration of 1 × 10-4 M. The presence of cations in general has been shown to have a detrimental effect on PCD rates. Previous studies on the PCD of aniline,9 phydroxybenzoic acid,23 and benzoic acid24 indicate that Fe2+ and Cu2+ have detrimental effects on the PCD rate at 0.1 M concentrations. However, as mentioned earlier, some investigators have reported a positive effect of Fe, Ag, and Cu if present in very small concentrations in the range of (1-5) × 10-6 mM.30 Therefore, the degradation of 2,4-D was studied in the presence of 0.01 M solutions of FeSO4 and CuSO4. It was found that the degradation of 2,4-D in the presence of these concentrations of FeSO4 and CuSO4 decreased. That is, the degradations of 2,4-D in the presence of FeSO4 and CuSO4 were 22.4% and 17.1%, respectively, as against 88.6% in their absence, as shown in Figure 12. It is likely that the positive effect of the cation is more than offset by the negative effect of the SO42- anion. 6. Continuous Photocatalytic Degradation in a Slurry Bubble Column Reactor with Concentrated Solar Radiation 6.1. Variation of the Extent of Photocatalytic Degradation with Residence Time in the Column. PCD experiments were performed in reactors of two different sizes (0.1- and 0.15-m i.d.) using the same reflector. As the size of the reactor increased, the PCD was found to decrease. The relative increase in the volume was greater than the increase in the area receiving the concentrated light from the parabolic reflector surrounding the reactor. Further, for the larger-diameter column, because the path length for the photons into the core of the reactor was larger, the penetration of photons decreased, causing a reduction in the PCD. Figures 13 and 14 show the variation of extent of the photocatalytic degradation with the 2,4-D residence time in the two columns. As expected, an increase in the residence time yields higher photocata-

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Figure 12. Effect of the presence of various cations on the PCD of 2,4-D on TiO2 (C/Ct)0 vs time). Initial concentration ) 100 mg L-1, catalyst loading ) 0.20% (w/v of solution). PCD of 2,4-D (0) in the absence of cations and in the presence of (4) 0.01 M FeSO4 and (×) 0.01 M CuSO4.

Figure 14. Variation of the extent of the photocatalytic degradation of 2,4-D with and without a sieve plate with respect to residence time in a 6-in. slurry bubble column (initial concentration ) 100 mg L-1, catalyst loading 0.2 w/v of solution). (2) 2,4-D degradation and (4) total organic carbon in the presence of the sieve plate with downcomer. (9) 2,4-D degradation and (0) total organic carbon in the absence of the sieve plate with downcomer.

Mixing between the two sections can be completely prevented if the sieve plates operate above the weep point, which is given by31

Fh ) VhxFG ) 10

Figure 13. Variation of the extent of the photocatalytic degradation of 2,4-D with and without a sieve plate with respect to residence time in a 4-in. slurry bubble column (initial concentration ) 100 mg L-1, catalyst loading 0.2 w/v of solution). (9) 2,4-D degradation and (0) total organic carbon in the presence of the sieve plate. (2) 2,4-D degradation and (4) total organic carbon in the absence of the sieve plate.

lytic degradation (PCD). Figures 13 and 14 also show the reduction in TOC with residence time. 6.2. Effect of Sieve Plates with Downcomers in the Slurry Bubble Column on Photocatalytic Degradation. Bubble columns exhibit significant axial mixing in the liquid phase. This mixing is detrimental to their performance as a reactor. Sectioning of the bubble column can reduce the backmixing. In our previous publication, we demonstrated that, in the presence of sieve plates in a bubble column, the percentage degradation of refractory pollutants increases.31. However, these sieve plates allowed some weeping, as no downcomer was provided and hence axial mixing could not be completely prevented. To overcome this drawback, acrylic sieve plates with downcomers were introduced into the column.

(SI units)

(6)

In the present case, the downcomer allowed the liquid to move from one plate to another. The sieve plates were thus designed for Fh ) 10, so that only air could flow through the small holes (no. of sieve plates with downcomer ) 5, size of the holes ) 3 mm). Figure 14 shows the effect of sectioning the column on the photocatalytic degradation of 2,4-D. The presence of the sieve plates with downcomers increases the percentage degradation by about 10% over that obtained in their absence. Thus, it is obvious that a reduction in axial mixing has a beneficial effect on the extent of photocatalytic degradation. 7. Conclusions The photocatalytic degradation of 2,4-D using concentrated solar radiation and titanium oxide is a viable technique. The adsorption of 2,4-D on the surface of the catalyst is critical; it depends on the pH of the solution as well as the concentration and type of anions. It was found that the presence of anions has a deleterious effect on the photocatalytic degradation of 2,4-D. The photocatalytic degradation rate of 2,4-D at its self-pH (∼2.68) is higher than that under alkaline pH. This is because of the positive charge of the surface at low pH, resulting in high adsorption. Therefore, it is desirable to carry out the photocatalytic degradation of 2,4-D before neutralization. The output of the photocatalytic degradation process can subsequently be introduced into the effluent treatment plant. Analogously, for basic impurities such as aniline, a high pH is favorable, and again, neutralization should not be done before treatment. A novel slurry bubble column reactor with in-line catalyst filtration is shown to be a viable proposition


for photocatalytic degradation (PCD) using concentrated solar radiation. Curtailing the axial mixing in the column can increase the efficiency of photocatalytic degradation. The rate of PCD of 2,4-D in a 0.15-mdiameter column is less than that in a 0.1-m-diameter glass column. The photocatalytic degradation studies of 2,4-D in the novel slurry bubble column (4- and 6-in. glass columns) indicate that this type of reactor can be used commercially for the treatment of pesticide-containing industrial wastewater. Acknowledgment Financial assistance from the Department of Science and Technology (DST), Government of India, for this work is gratefully acknowledged. We also thank Degussa, Du¨sseldorf, Germany, for a free sample of Degussa P-25 titanium dioxide catalyst. Nomenclature C ) concentration of 2,4-D at time t, mg L-1 Ct)0 ) concentration of 2,4-D at time t ) 0, before the 2,4-D solution was exposed to sunlight, mg L-1 Fh ) factor through holes defined by eq 6 TOC ) total organic carbon at time t, mg L-1 (TOC)t)0 ) total organic carbon at time t ) 0, before the 2,4-D solution was exposed to sunlight, mg L-1 Vh ) velocity of gas through the holes in the sieve plate, m s-1 Greek Letter FG ) density of gas (kg m-3)

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Received for review June 30, 2004 Revised manuscript received September 8, 2004 Accepted September 14, 2004 IE0494263

Photocatalytic Degradation of 2,4-Dichlorophenoxyacetic Acid Using

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