Ind. Eng. Chem. Res. 2003, 42, 6705-6713
6705
Batch and Continuous Photocatalytic Degradation of Benzenesulfonic Acid Using Concentrated Solar Radiation Sanjay P. Kamble, Sudhir B. Sawant, and Vishwas G. Pangarkar* University of Mumbai Institute of Chemical Technology, Matunga, Mumbai-400 019, India
The photocatalytic degradation (PCD) of benzenesulfonic acid (BSA) using concentrated solar radiation in a batch as well as a continuous bubble column reactor has been studied. The photocatalytic reaction takes place on the surface of the catalyst material. Therefore, the adsorption behavior of benzenesulfonic acid on the catalyst has also been studied. The effects of presence of anions, which are commonly present in industrial wastewaters, are reported. The effect of pH on the photocatalytic degradation of benzenesulfonic acid was also studied. The photocatalytic degradation studies of benzenesulfonic acid in the novel slurry bubble column indicate that this type of reactor can be used commercially. 1. Introduction Many toxic harmful pollutants enter the water bodies, soil, and atmosphere directly or indirectly. Aromatic sulfonated compounds are one of them. These compounds have low volatility and have high solubility in water. They are also one of several major pollutants in aquatic environment, responsible for the ever increasing deterioration of environmental quality. Therefore, elimination of these aromatic sulfonated compounds from water assumes great priority. The lethal oral dose of benzenesulfonic acid (BSA) for rats, LD50, is 890 mg kg-1 vs 3800 mg kg-1 for benzene. The biodegradation of aromatic sulfonated compounds poses several difficulties.1-3 Photocatalytic/photochemical methods offer some distinct advantages over conventional technologies, such as air stripping, vapor extraction, and carbon adsorption.4-9 In these processes, the oxidation occurs through an attack by OH•, which has a rate constant that is a billion times higher than normal oxidation rate constants.10,11 The UV radiation required for the photocatalytic processes can be obtained from artificial sources or the sun. There is a significant economic incentive for solar powered 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, Indian subcontinent, Australia, and other countries.11,12 Considerable information on the chemical aspects of photocatalytic degradation is available in the literature which has been recently reviewed by Bhatkhande et al.13 Notwithstanding the substantial advantages of photocatalytic degradation, this technique is yet to be implemented on a large scale for treatment of industrial wastes. The main hurdle in the commercialization is the lack of suitable hardware, namely a reactor, which gives high space time yields. The photocatalytic degradation process involves generation of positive holes in the valence band due to the ejection of electrons into the conductance band upon irradiation with light of wavelength (λ) greater than or equal to the band gap * To whom correspondence may be addressed. Fax: +9122-4145614. E-mail:
[email protected] [email protected].
energy, Ebg, in accordance with the following relationship.14
λ)
1240 Ebg
(1)
The electron in the conductance band needs to be removed as quickly as possible in order to avoid recombination with positive holes. This requires an equally rapid supply of oxygen (from air) as an electron acceptor. Further, it is now established that the photocatalytic degradation occurs by attack of photogenerated OH• on the adsorbed substrate.9,15 The latter needs to diffuse from the bulk liquid to the catalyst surface before it is adsorbed. The adsorption process can be considered as relatively rapid and hence at equilibrium. The mechanism outlined above indicates that a commercial reactor should have the following attributes: (1) high catalyst surface area per unit volume, (2) maximum penetration of the incident radiation to all parts of the reaction volume, and (3) high rates of oxygen and substrate mass transfer from the respective phases to the catalyst surface. Degussa P-25, a mixture of anatase (70%) and rutile (30%), is the most widely used photocatalyst. This catalyst has a very small particle size (30 nm). Therefore, its use in free form creates serious problems of filtration. In view of this, most investigators have used the catalyst in immobilized forms. Two types of immobilized photocatalysts have been used: (1) catalyst immobilized on fixed surfaces (Pyrex glass tubes, optical fibers, etc.)16-19 (2) catalyst immobilized on silica particles.20,21 In the first type, a laminar film of the liquid to be treated flows over a thin layer of the immobilized photocatalyst which is illuminated by artificial UV radiation. The poor diffusive transport and low catalyst surface area is a major drawback although immobilization avoids filtration problems. In the second case, the catalyst coated particles are fluidized allowing turbulent mass transfer. However, limited surface area and potential losses of catalyst by attrition are main drawbacks. Higher catalyst loading leads to solution opacity and poor photon penetration.
10.1021/ie030493r CCC: $25.00 © 2003 American Chemical Society Published on Web 11/27/2003
6706 Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003
Figure 1. Batch experimental setup for photocatalytic degradation of benzenesulfonic acid using concentrated solar radiation.
located in the vicinity of the air sparger. In this region, there is a relatively high (several m s-1) liquid circulation velocity which prevents deposition of the solid on the filter surface. The liquid hydrostatic head in the column serves as the driving force for filtration allowing retention of the catalyst in the reactor. The low particle size and catalyst loading allow ease of solid suspension at relatively low (0.03-0.05 m s-1) air sparging rates. Thus, the energy requirement for air sparging can be kept low. In the present work, a systematic study of the photocatalytic degradation (PCD) of benzenesulfonic acid using concentrated solar radiation in batch as well continuous mode is reported. The photocatalytic reaction takes place on the surface of the catalyst material. Therefore, the adsorption behavior of benzenesulfonic acid on the catalyst has also been studied. The effect of various parameters on photocatalytic degradation of benzenesulfonic acid was studied to optimize the parameters. The studies with the continuous bubble column reactor work are aimed at demonstration of the viability of the novel reactor (Figure 2) proposed. Rachel et al. studied photocatalytic degradation of benzenesulfonic acids.22 They studied photocatalytic efficiency of various immobilized catalysts such as Millennium PC50, PC100, PC105, PC500 and compared them with Degussa P-25 TiO2 as the photocatalyst for the photocatalytic degradation of 3-nitrobenzenesulfonic acid (3-NBSA) and 2,5-anilinedisulfonic acid (2,5ADSA). They found that Degussa P-25 is the most efficient catalyst for the photocatalytic degradation of 3-NBSA, while in case of 2,5-ADSA it was observed that PC500 is more efficient as compared to Degussa P-25. The photocatalytic degradation of p-toluenesulfonic acid, in aqueous titanium dioxide suspensions and in systems containing TiO2 particles immobilized using a polymer on the wall of the reaction vessel, was investigated by Brezova et al.23 They observed that p-cresol was the main intermediate at the initial stage of the photocatalytic degradation of p-toluenesulfonic acid. Photocatalytic degradation of sulfonated aromatic compounds and its substituted compounds in aqueous TiO2 suspension was studied by Sangchakr et al.24 The effect of various parameters influencing the degradation rate, such as the initial concentration, temperature, and substituent group, was studied. 2. Experimental Section
Figure 2. Novel slurry bubble column reactor for continuous photocatalytic degradation of benzenesulfonic acid using concentrated solar radiation.
One alternative which can overcome this problem lies in the use of candle filters relying on surface filtration under conditions such that there is a zone of high shear near the filter surface which prevents solid deposition and plugging of the candle filter. The high surface area per unit mass of this catalyst allows low catalyst loading (2 cm s-1) to keep all 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 benzenesulfonic acid through outgoing air. Since the reactor was provided with a condenser, this loss was less than 1%. 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 by aluminum foil during shaking. The solutions were then filtered and analyzed using HPLC. 2.3. Continuous Experiments. Continuous experiments were carried out in a borosilicate glass slurry bubble column reactor of 0.1 m id × 3.0 m long and capacity of 19.5 L. A schematic of the slurry bubble column reactor is shown in Figure 2. The experimental setup was located on the terrace of a 3 story building to avoid interference of trees and other buildings. A solution of benzenesulfonic acid was prepared in tap water in a 175 L capacity agitated tank. A metering pump was used for delivering benzenesulfonic acid solutions to the top of the borosilicate glass column, while a diaphragm type air compressor (model HS-2, C. P. Enterprises, Mumbai) was used to sparge the air at the bottom of the column through a sintered stainless steel disk. A parabolic reflector (3.0 m. height and 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 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 amber colored bottle. In all the 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 benzenesulfonic acid and its intermediates were measured by HPLC (Knauer) equipped with a C-18 column (5 µm, particle diameter, Merck) and UV-vis detector. The mobile phase was determined from several combinations giving the best separation of the possible intermediates. Acetonitrile/water (70:30% v/v) was used as the mobile phase with a flow rate of 1.0 mL min-1 at λ ) 215 nm for the UV detector. 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 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 February-April, 2003, in Mumbai, India, 18.58° N, and 72.50° E. During this period, the sky was brilliant blue (no clouds), and the average solar intensity was approximately ((10%) constant at 361 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.5,7,11,15,26 In the case of aromatic compounds, the position on which OH• attacks depends on the directing effect of the original functional group in the benzene ring. For instance, the nitro group in nitrobenzene has a meta directing effect. Thus, as an intermediate nitrobenzene yields predominantly mnitrophenol.7 A similar observation was made in the case of benzenesulfonic acid. In benzenesulfonic acid, the SO3H group is meta directing, which gives meta phenolsulfonic acid as an intermediate. During the photocatalytic degradation of benzenesulfonic acid, a very negligible amount of m-phenolsulfonic acid was formed. It is known that the aromatic ring yields at the most trihydroxy derivatives.13 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 Adsorption of Benzenesulfonic Acid on TiO2. Adsorption of the substrate on the photocatalyst has a major role in its photocatalytic degradation. According to the mechanism suggested, the attack of OH• radicals takes place on the adsorbed substrate.8,26-27 Hence the species which is adsorbed in a greater amount will degrade more rapidly. Adsorption is affected by several factors, which may include effluent composition and pH. Industrial effluents commonly contain one or several salts. The anion part of these salts is known to have a deleterious effect on adsorption of organics on TiO2 and hence on photocatalytic degradation.28,29 At pH < pHZPC, the TiO2 particle has a positive charge. This positive charge attracts the negatively charged anions through an electrostatic force [∝ (q1 × q2)/l2]. The charge on the ions with a given valency being same the electrostatic force would also be the same under identical conditions. Smaller anions will be preferably accommodated than larger ions. For pH > pHZPC, the situation is reversed, and a repulsive force particularly for the anions devel-
6708 Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003
Figure 3. Effect of presence of various anions on the adsorption of benzenesulfonic acid on TiO2: (b) adsorption isotherm of benzenesulfonic acid in the absence of anions; (9) adsorption isotherm of benzenesulfonic acid in the presence of 0.1 M Na2SO4; (2) adsorption isotherm of benzenesulfonic acid in the presence of 0.1 M NaHCO3; (O) adsorption isotherm of benzenesulfonic acid in the presence of 0.1 M (NH4)2SO4; (]) adsorption isotherm of benzenesulfonic acid in the presence of 0.1 M Na2CO3; (4) adsorption isotherm of benzenesulfonic acid in the presence of 0.1 M NaCl.
ops. However, this situation mostly results in alkaline pH, and BSA forms a salt which is not adsorbed as well as free BSA. Further, anions can act as OH• scavengers. The chloride effect is pronounced in the case of substances which are weakly adsorbed whereas in the case of strongly adsorbing species the effect of carbonate and bicarbonate is predominant.13 Carbonate, bicarbonate, and chloride ions act as hydroxyl ion scavengers and absorb UV light through the following reactions:
OH• + Cl- 98 Cl• + OH-
(2)
• •CO23 + HO 98 CO3 + H2O
(3)
• • HCO23 + HO 98 CO3 + H2O
(4)
These ions might also block the active sites of the catalyst surface thus deactivating the catalyst toward the organic molecules.28 It can be seen from the adsorption equilibria that the presence of anions has some effect on the adsorption of benzenesulfonic acid on TiO2 (Figure 3), and this is reflected in the degradation as well. Na2SO4, (NH4)2SO4, NaCl, and NaHCO3 have lesser effect on adsorption, and hence also on the photocatalytic degradation. The presence of Na2CO3 has comparatively larger effect on the adsorption, and photocatalytic degradation as discussed later. From our previous studies of adsorption and photocatalytic degradation of aniline,11 PHBA,30 benzoic acid,31 and benzenesulfonic acid adsorption in the present studies, the adsorption of benzenesulfonic acid on the TiO2 surface is greater than aniline, PHBA, and benzoic acid, and hence, the photocatalytic degradation rates of benzenesulfonic acid are effectively also high. Figure 4 shows the effect of pH on the adsorption. The pH of the benzenesulfonic acid solution was varied in the alkaline range by adding aqueous NaOH. It is evident that the solution pH has a significant effect on
Figure 4. Effect of pH on adsorption of benzenesulfonic acid (g/g TiO2 v/s pH).
adsorption of benzenesulfonic acid. Maximum adsorption occurs at the self pH of benzenesulfonic acid. Similar results were obtained in the photocatalytic degradation of 2,4,6-trichlorophenol (TCP).32 It was found that although a significant amount of adsorption was observed at lower pH values (3.1-4.2), at higher pH values very small adsorption was noted. 4. Batch Photocatalytic Degradation with Concentrated Solar Radiation 4.1. Effect of Reflector Shape. In all the experiments, a parabolic reflector was employed for photocatalytic degradation of benzenesulfonic acid. In one of the experiments this was replaced by an anodized aluminum reflector of the spherical shape. Initial benzenesulfonic acid concentration of 100 mg L-1 and catalyst loading of 0.20% w/v of solution were used in both the experiments. Significant change in the photocatalytic degradation of benzenesulfonic acid was observed as seen from Figure 5. 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 reactor. On the other hand the spherical reflector yields a vertical beam parallel to the vessel wall. The surface area of the spherical reflector is 3620 cm2 whereas that of the parabolic reflector is 1400 cm2. The surface area of spherical reflector is 2.6 times greater than that of parabolic reflector. Thus, although the spherical reflector has a much higher surface area, its effectiveness in terms of photon penetration in the reactor is very poor. In view of the above, it can be concluded that the spherical reflector is far less efficient than the parabolic reflector. 4.2. Influence of Catalyst Concentration. In static, slurry, or dynamic flow photoreactors, the initial reaction rates were found to be directly proportional to catalyst loading. This indicates a truly heterogeneous catalytic regime. However, above a certain value, the reaction rate levels off and becomes independent of catalyst loading. This limit depends on the geometry and reaction conditions of the photoreactor and implies a definite amount of TiO2 in which all the particles, i.e., the entire exposed surface, is totally illuminated.
Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 6709
Figure 5. Comparison of photocatalytic degradation of benzenesulfonic acid with different reflector shape: (b) photocatalytic degradation of benzenesulfonic acid with parabolic reflector; (9) photocatalytic degradation of benzenesulfonic acid with spherical reflector.
Figure 6. Influence of catalyst loading on photocatalytic degradation of benzenesulfonic acid (C/Ct)0 vs time): (b) 0.1% (w/vol of solution) of catalyst loading; (9) 0.2% (w/vol of solution) of catalyst loading; (2) 0.3% (w/vol of solution) of catalyst loading.
When catalyst concentration is very high, after traveling a certain distance on an optical path, turbidity impedes further penetration of light in the reactor. Therefore the optimum catalyst loading has to be found in order to avoid excess catalyst and ensure total absorption of photons.7 With initial concentration of 100 mg L-1 of benzenesulfonic acid, optimum loading of the catalyst was found by varying the catalyst loading. As can be seen from Figure 6 the optimum catalyst loading is 0.20% (w/vol of the solution). As the catalyst loading is increased, there is an increase in the surface area of the catalyst available for adsorption and degradation. But an increase in the catalyst loading increases the solution opacity leading to decrease in the penetration of the photon flux in the reactor and thereby decreasing the photocatalytic degradation rate. In all subsequent experiments, 0.20% (w/vol of the solution) TiO2 loading was used.
Figure 7. Effect of initial concentration on the photocatalytic degradation of benzenesulfonic acid (C/Ct)0 vs time), catalyst loading 0.20% (w/vol of solution): (b) initial concentration 100 mg L-1; (9) initial concentration 200 mg L-1; (2) initial concentration 300 mg L-1.
4.3. Effect of Initial Concentration of Benzenesulfonic Acid. Photocatalytic degradation studies were carried out using 100, 200, and 300 mg L-1 initial concentration of benzenesulfonic acid and 0.20% (w/vol of solution) catalyst loading. Figure 7 shows a plot of normalized benzenesulfonic acid concentration (C/Ct)0) with time. In the case of initial concentrations of 100, 200, and 300 mg L-1, the benzenesulfonic acid concentration was reduced by 99%, 81%, and 65%, respectively, within 4 h. In case of initial concentrations of 100, 200, and 300 mg L-1 benzenesulfonic acid, the TOC was reduced by 95%, 77%, and 63%, respectively, within 4 h. Figure 7 shows that an increase in the feed concentration of benzenesulfonic acid causes a decrease in the rate of photocatalytic degradation. As mentioned earlier. the reaction occurs between the adsorbed benzenesulfonic acid and OH• generated on a TiO2 surface. The concentration of adsorbed benzenesulfonic acid increases with an increase in feed benzenesulfonic acid concentration. However, for constant light intensity, TiO2 loading and dissolved oxygen concentration, the concentration of OH• remains practically the same. Thus, although the adsorbed benzenesulfonic acid concentration increases, the rate of photocatalytic degradation decreases due to a lower OH•/BSA ratio. 4.4. Kinetic Modeling of Photocatalytic Reaction. The Langmuir-Hinshelwood kinetic treatment was successfully used as a qualitative model to describe solid-liquid reactions.7,33 According to the L-H model, the reaction rate (ra) for a surface reaction, where the reactant is significantly more strongly adsorbed than the product, follows eq 533
ra )
krKC 1 + KC + KsCs
(5)
where kr is the reaction rate constant, K is the substrate/ reactant adsorption constant, C is the concentration of reactant at any time t, Ks is the solvent adsorption constant, and Cs is the concentration of the solvent (in water, Cs ≈ 55.50 M). As Cs . C and Cs remains practically constant, the part of catalyst covered by
6710 Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003
water is unaltered over the whole range of concentration C, and the above eq 5 can be integrated as follows:
ln
C0 krK K + (C - C) ) t C 1 + KsCs 0 1 + KsCs
(6)
C0 + Kapp(C0 - C) ) krKappt C
(7)
ln
This expression represents the sum of zero order and first order rate equations, and their contribution to the overall reaction depends on the initial concentration C0 where Kapp ) K/1 + KsCs. Equation 5 reduces to the zero order asymptote for the case when KC . 1 + KsCs.
(C0 - C) ) krt
(8)
The general form of eq 5 to be used for estimating the various parameters is33
1 1 1 + KsCs 1 ) + r0 krK C0 kr
Figure 8. Effect of pH on the photocatalytic degradation of benzenesulfonic acid on TiO2. [TOC/(TOC)t)0 vs time], catalyst loading 0.20% (w/vol of solution): (O) pH ) 3.38; (0) pH ) 7.0; (4) pH ) 11.0.
(9)
where r0 is the initial rate at C ) C0, kr is reaction rate constant, and Kapp ) K/1 + KsCs, suggesting that competitive adsorption by a solvent present at constant concentration leads to a diminution of the true binding constant K to an apparent value Kapp. The values of kr and Kapp were obtained by regressing the data according to eq 9 and were found to be 0.753 mg L-1 min and 0.0254 L mg-1, respectively, with a regression coefficient of R2 ) 0.96. 4.5. Effect of pH. The effect of initial pH on the photocatalytic degradation of benzenesulfonic acid was studied. In the alkaline range, the pH was varied using aqueous NaOH, whereas in the acidic range, pH was varied using HClO4. Initial benzenesulfonic acid concentration of 100 mg L-1 and catalyst loading of 0.20% (w/vol of solution) were used in all 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 may be basic or acidic, and therefore, this pH effect needs to be considered. Figure 8 shows a plot of 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 benzenesulfonic acid increases and is at its maximum at pH 3.38. This observation tallies with greater adsorption of benzenesulfonic acid under nonionized, low pH conditions (Figure 4). Similar results were obtained in the photocatalytic degradation of 4-chlorophenol and 4-nitrophenol.18,34 On the other hand, aniline which is a basic compound and is ionized under the acidic pH conditions shows the opposite effect of pH. Thus, the photocatalytic degradation of aniline increases with increase in pH and is at its maximum at pH 12.11 4.6. Effect of Presence of Anions. Industrial effluents contain, apart from pollutants, different salts at different levels of concentration. The salts are generally ionized under the condition of photocatalytic degradation. The anion and cation parts of the salt have different effects on the photocatalytic degradation.35 The effect of the presence of various anions such as chloride, sulfate, bicarbonate, and carbonate was studied using
Figure 9. Effect of presence of various anions on photocatalytic degradation of benzenesulfonic acid on TiO2 [TOC/(TOC)t)0 vs time] catalyst loading 0.20% (w/vol of solution): (]) photocatalytic degradation of benzenesulfonic acid in the absence of anions; (0) photocatalytic degradation of benzenesulfonic acid in the presence of 0.1 M Na2SO4; (4) photocatalytic degradation of benzenesulfonic acid in the presence of 0.1 M (NH4)2SO4; (O) photocatalytic degradation of benzenesulfonic acid in the presence of 0.1 M NaHCO3; (/) photocatalytic degradation of benzenesulfonic acid in the presence of 0.1 M NaCl; (×) photocatalytic degradation of benzenesulfonic acid in the presence of 0.1 M Na2CO3.
0.1 M solution of their sodium salts and initial concentration of 100 mg L-1 of benzenesulfonic acid with 0.20% (w/vol of solution) TiO2 loading. In the case of sodium sulfate, ammonium sulfate, sodium bicarbonate, sodium chloride, and sodium carbonate, the reduction in the benzenesulfonic acid concentrations was 85%, 63%, 35%, 26%, and 13%, respectively, whereas in the absence of anions 99% reduction in the concentration of benzenesulfonic acid was observed. The corresponding TOC reductions were 80%, 58%, 30%, 21%, and 9%, respectively, in the presence of sodium sulfate, ammonium sulfate, sodium bicarbonate, sodium chloride, and sodium carbonate as shown in Figure 9. Previous studies with phenol,5 nitrobenzene,7 p-hydroxy benzoic acid,30 and benzoic acid31 have concluded that all the above anions have a negative effect on photocatalytic degradation. The very strong negative effect of Na2CO3 is attributed to the fact that
Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 6711
Figure 10. Variation of extent of photocatalytic degradation of benzenesulfonic acid with sieve plate and without sieve plate with respect to residence time in the slurry bubble column (initial concentration 100 mg L-1, catalyst loading 0.2 w/v of the solution): (b) benzenesulfonic acid with sieve plate; (0) total organic carbon with sieve plate; (2) benzenesulfonic acid without sieve plate; (4) total organic carbon without sieve plate.
it is basic and reacts with benzenesulfonic acid to yield NaBSA and is also a scavenger of OH•. This NaBSA being ionic is poorly adsorbed. 5. Continuous Photocatalytic Degradation in Slurry Bubble Column Reactor with Concentrated Solar Radiation 5.1. Variation of Extent of Photocatalytic Degradation with Residence Time in the Column. Figure 10 shows the variation of extent of photocatalytic degradation with residence time for benzenesulfonic acid in the column. As expected, an increase in the residence time yields higher photocatalytic degradation (PCD). Figure 10 also shows the reduction in TOC with residence time. Figure 11 also shows the effect of two salts on the photocatalytic degradation of benzenesulfonic acid. In general, an effluent is brought to neutral pH before it is introduced in the effluent treatment plant (ETP). In the case of benzenesulfonic acid, with the original solution being acidic, an alkali is added to raise the pH to neural pH. Ammonia and sodium carbonate/hydroxide were considered to be used for this purpose. The neutralization process will generate ammonium sulfate and sodium sulfate, respectively, in these two cases. The effect of the salts on the photocatalytic degradation of benzenesulfonic acid shown in Figure 11 clearly indicates that the salts have a substantially detrimental effect on the photocatalytic degradation. Thus, it can be concluded that if photocatalytic degradation is to be adopted it will be desirable to carry out photocatalytic degradation before neutralization. The output of the photocatalytic degradation process can subsequently be introduced into the ETP. 5.2. Effect of Reduced Backmixing 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. Sectionalization of the bubble column can reduce the backmixing. To study this effect, acrylic sieve plates were introduced in the column.
Figure 11. Effect of the presence of 1% (wt %) Na2SO4 and (NH4)2SO4 on photocatalytic degradation of benzenesulfonic acid in slurry bubble column (initial concentration 100 mg L-1, catalyst loading 0.20 w/v of the solution): (0) benzenesulfonic acid; (9) total organic carbon (TOC).
Mixing between the two sections can be completely prevented if the sieve plates operate above the weep point, which has been found to be given by36
Fh ) VhxFG ) 10 (SI units)
(10)
In the present case, no downcomer for the liquid was provided, and therefore, some weeping had to be allowed. Further, this study was exploratory in nature. The sieve plates were thus designed for Fh ) 5 to allow passage of liquid (weeping) between the sections. Figure 10 shows the effect of sectionalization on the photocatalytic degradation of benzenesulfonic acid. The presence of the sieve plates increase the percentage degradation by about 13% over that in their absence. Thus, it is obvious that reduction in axial mixing has a beneficial effect on the extent of photocatalytic degradation. 6. Conclusion The photocatalytic degradation of benzenesulfonic acid using concentrated solar radiation and titanium oxide is a viable technique. The adsorption of benzenesulfonic acid 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 deleterious effects on the photocatalytic degradation of benzenesulfonic acid. The photocatalytic degradation rate of benzenesulfonic acid at its self pH is higher than alkaline pH. 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. The efficiency of photocatalytic degradation can be increased by curtailing the axial mixing in the column. It will be desirable to carry out the photocatalytic degradation of benzenesulfonic acid before neutralization. The output of the photocatalytic degradation process can subsequently be introduced into the effluent treatment plant.
6712 Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003
Acknowledgment The authors are grateful to Department of Science and Technology (DST), Government of India, for providing financial support for this work. We wish to thank Degussa Co., Germany. for free sample of Degussa P-25 titanium dioxide catalyst. Nomenclature l ) distance between q1 and q2 (m) (TOC)t)0 ) total organic carbon at time t, before the benzenesulfonic acid solution was exposed to sunlight (mg L-1) C ) concentration of benzenesulfonic acid after time t (mg L-1) C0 ) initial concentration of the benzenesulfonic acid (mg L-1) Cs ) concentration of the solvent (for water ≈ 55.5 M) Ct)0 ) concentration of benzenesulfonic acid at time t)0, before the benzenesulfonic acid solution was exposed to sunlight (mg L-1) Ebg ) band gap energy of the photocatalyst (eV) Fh ) F factor through holes defined by eq 10 K ) benzenesulfonic acid adsorption constant Kapp ) K/1+KsCs (L mg-1) kr ) reaction rate constant (mg L-1 min-1) Ks ) adsorption constant of the solvent q1 ) charge on ion (Coulomb) q2 ) charge on TiO2 surface (Coulomb) TOC ) total organic carbon after time t (mg L-1) Vh ) velocity of gas through the hole in the sieve plate (m s-1) Greek Letters FG ) density of gas (kg m-3) λ ) wavelength of light (nm)
Literature Cited (1) Gerhartz, W.; Yamamoto, Y. S.; Campbell, F. T.; Pfefterkorn, R.; Rounsaville J. F. Benzenesulfonic Acids and Their Derivatives. Ullmann’s Encyclopedia of Industrial Chemistry; VCH Publishers: Germany, 1985. (2) Tully, P. S. Benzenesulfonic Acids. In Encyclopedia of Chemical Technology (Kirk Othmer); Kroschwitz, JI., Howe-Grant, M., Humphreys, L., Eds.; John Wiley and Sons: New York, 1992. (3) Fruscella, W. Benzene. In Encyclopedia of Chemical Technology (Kirk Othmer); Kroschwitz, JI., Howe-Grant, M., Humphreys, L., Eds.; John Wiley and Sons: New York, 1992. (4) Legrini, O.; Oliveros, E.; Braun, A. M. Photochemical Processes for Water Treatment. Chem. Rev. 1993, 93, 671. (5) Yawalkar, A. A.; Bhatkhande, D. S.; Pangarkar, V. G.; Beenackers, A. A. C. M. Solar Assisted Photocatalytic and Photochemical Degradation of Phenol. J. Chem. Technol. Biotechnol. 2001, 76, 363. (6) Bhatkhande, D. S.; Sawant, S. B.; Schouten, J. C.; Pangarkar, V. G. Photocatalytic Degradation of Chlorobenzene Using Solar and Artificial UV-Radiation. J. Chem. Technol. Biotechnol., under review. (7) Bhatkhande, D. S.; Pangarkar, V. G.; Beenackers, A. A. C. M. Photocatalytic Degradation of Nitrobenzene Using TiO2: Chemical Effects. Water Res. 2003, 37 (6), 1223. (8) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Environmental Applications of Semiconductor Photocatalysis. Chem. Rev. 1995, 95, 69. (9) Mills, A.; Davies, R. H.; Worsley, D. Water Purification by Semiconductor Photocatalysis. Chem. Soc. Rev. 1993, 22, 417. (10) Malato, S.; Blanco, J.; Campos, A.; Caceres, J.; Guillard, C.; Herrmann, J. M.; Fernandez-Alba, A. R. Effect of Operating Parameters on the Testing of New Industrial Titania Catalysts at Solar Pilot Plant Scale. Appl. Catal., B 2003, 42, 349.
(11) Kamble, S. P.; Sawant, S. B.; Schouten, J. C.; Pangarkar, V. G. Photocatalytic and Photochemical Degradation of Aniline Using Titanium Dioxide and Concentrated Solar Radiations. J. Chem. Technol. Biotechnol. 2003, 78 (8), 865. (12) Chan, A. H. C.; Chan, C. K.; Barford, J. P.; Porter, J. F. Solar Photocatalytic Thin Film Cascade Reactor for Treatment of Benzoic Acid Containing Wastewater. Water Res. 2003, 37, 1125. (13) Bhatkhande, D. S.; Pangarkar, V. G.; Beenackers, A. A. C. M. Photocatalytic Degradation for Environmental Applicationss A Review. J. Chem. Technol. Biotechnol. 2001, 77, 102. (14) Lea, J.; Adesina, A. A. Oxidative Degradation of 4-Nitrophenol in UV-Illuminated Titania Suspension. J. Chem. Technol. Biotechnol. 2001, 76, 803. (15) Turchi, C. S.; Ollis, D. F. Photocatalytic Degradation of Organic Contaminants: Mechanisms Involving Hydroxyl Radical Attack. J. Catal. 1990, 122, 178. (16) Yue, P. L.; Puma, G. L. Enhanced Photocatalysis in a Pilot Laminar Falling Film Slurry Reactor. Ind. Eng. Chem. Res. 1999, 38 (9), 3246. (17) Yatmaz, H. C.; Howorth, C. R.; Wallis, C. Photocatalysis of Organic Effluents in Falling Film Reactor. In Photocatalytic Purification and Treatment of Water and Air; Ollis, D. F., Al-Ekabi, H., Eds.; Elsevier Science Publishers: Amsterdam, 1993. (18) Hofstadler, K.; Bauer, R.; Novalic, S.; Heisler, G. New Reactor Design for Photocatalytic Wastewater Treatment with TiO2 Immobilized on Fused Silica Glass Fibers: Photomineralization of 4-Chlorophenol. Environ. Sci. Technol. 1994, 28, 670. (19) Anderson, M. A.; Hill, C. G., Jr.; Zeltner, W. A. Supported Titania for Photodegradation. Chemtech May 1993, p 21. (20) Haarstrick, A.; Kut, O. M.; Heinzle, E. TiO2-Assisted Degradation of Environmentally Relevant Organic Compounds in Wastewater Using a Novel Fluidized Bed Photoreactor. Environ. Sci. Technol. 1996, 30, 817. (21) Ray, A. K. Design, Modeling and Experimentation of a New Large-Scale Photocatalytic Reactor for Water Treatment. Chem. Eng. Sci. 1999, 54, 3113. (22) Rachel, A.; Sarakha, M.; Subrahmanyam, M.; Booule, P. Comparison of Several Titanium Dioxides for the Photocatalytic Degradation of Benzenesulfonic Acids. Appl. Catal. B 2002, 37, 293. (23) Brezova, V.; Jankovicova, M.; Soldan, M.; Blazkova, A.; Rchakova, M.; Surina, I.; Ceppan, M.; Havlinova, B. Photocatalytic Degradation of p-toluenesulfonic Acid in Aqueous Systems Containing Powered and Immobilized Titanium Dioxide. J. Photochem. Photobiol., A 1994, 83, 69. (24) Sangchakr, B.; Hisanaga, T.; Tanaka, K. Photocatalytic Degradation of Sulfonated Aromatics in Aqueous TiO2 Suspension. J. Photochem. Photobiol., A 1995, 85, 187. (25) Mills, A.; Morris, S.; Davis, R. Photomineralization of 4-Chlorophenol Sensitised by Titanium Dioxide: A Study of the Intermediates. J. Photochem. Photobiol., A 1993, 70, 183. (26) Pelizzetti, E.; Minero, C.; Borgarello, E.; Tinucci, L. Photocatalytic Activity and Selectivity of Titania Colloids and Particles Prepared by the Sol-Gel Technique: Photooxidation of Phenol and Atrazine. Langmuir 1993, 9, 2995. (27) Turchi, C. S.; Ollis, D. F. Mixed Reactant Photocatalysis: Intermediates and Mutual Rate Inhibition. J. Catal. 1989, 119, 483. (28) Abdullah, M.; Gary, K. C. L.; Matthews, R. W. Effect of Common Inorganic Anions on Rates of Photocatalytic Oxidation of Hydrocarbons over Illuminated TiO2. J. Phys. Chem. 1990, 94, 6620. (29) Calza, P.; Pelizzetti, E. Photocatalytic Transformation of Organic Compounds in the Presence of Inorganic Ions. Pure Appl. Chem. 2001, 73, 1839. (30) Subramanian, V.; Pangarkar, V. G.; Beenackers, A. A. C. M. Photocatalytic Degradation of PHBA: Relationship between Substrate Adsorption and Photocatalytic Degradation. Clean Products and Processes 2000, 2, 149. (31) Ajmera, A. A.; Sawant, S. B.; Pangarkar, V. G.; Beenackers, A. A. C. M. Solar Assisted Photocatalytic Degradation of Benzoic Acid Using Titanium Dioxide as a Photocatalyst. Chem. Eng. Technol. 2002, 25, 173. (32) Tanaka, S.; Saha, U. K. Effects of pH on Photocatalysis of 2,4,6-Trichlorophenol in Aqueous TiO2 Suspensions. Water Sci. Technol. 1994, 30, 47. (33) Al-Ekabi, H.; Serpone, N. Kinetic Studies in Heterogeneous Photocatalysis. 2. TiO2-Mediated Degradation of 4-Chlorophenol Alone and in a Three-Component Mixture of 4-Chlorophenol, 2,4-
Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 6713 Dichlorophenol, and 2,4,5-Trichlorophenol in Air-Equilibrated Aqueous Media. Langmuir 1989, 5, 250. (34) Andreozzi, R.; Caprio, V.; Insola, A.; Longo, G.; Tufano, V. Photocatalytic Oxidation of 4-Nitrophenol in Aqueous TiO2 Slurries: an Experimental Validation of Literature Kinetic Models. J. Chem. Technol. Biotechnol. 2000, 75, 131. (35) Choi, W.; Hoffmann, M. R. Novel Photocatalytic Mechanisms for CHCl3, CHBr3, and CCl3CO2- Degradation and the Fate of Photogenerated Trihalomethyl Radicals on TiO2. Environ. Sci. Technol. 1997, 37, 889.
(36) Prince, R. G. H. Characteristics and Design of Perforated Plate Columns. International symposium on distillation; Institution of Chemical Engineers: United Kingdom, 1960.
Received for review June 11, 2003 Revised manuscript received October 1, 2003 Accepted October 7, 2003 IE030493R
