Photocatalytic Degradation of Phenol-4-sulfonic Acid Using an

Apr 7, 2007 - ... Institute of Chemical Technology, Matunga, Mumbai-400 019, India ... and sulfate have detrimental effects on total organic carbon re...
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Ind. Eng. Chem. Res. 2007, 46, 4257-4264

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Photocatalytic Degradation of Phenol-4-sulfonic Acid Using an Artificial UV/TiO2 System in a Slurry Bubble Column Reactor Kaushal Pujara,† Sanjay P. Kamble,‡ and Vishwas G. Pangarkar*,§ Dr. Reddy’s Laboratories, Hyderabad 500 016, India, EnVironmental Material Unit, National EnVironmental Engineering Research Institute (NEERI), Nehru Marg, Nagpur-440 020, India, and Chemical Engineering Department, UniVersity Institute of Chemical Technology, Matunga, Mumbai-400 019, India

Phenol-4-sulfonic acid (4-PSA) was photocatalytically degraded in the presence of Degussa P-25 TiO2 catalyst suspensions in a slurry annular bubble column reactor. Artificial UV radiation was employed as the source of photons. The effect of catalyst loading, pH, presence of anions, initial concentration, and photon flux on the rate of photocatalytic degradation was investigated. At the self-pH of 4-PSA (pH = 4.5) the degradation was higher. It was observed that carbonate, chloride, and sulfate have detrimental effects on total organic carbon removal. The effect of the radiation distribution of the UV lamps on quantum yield was also studied. The usage of multiple radiation sources results in higher quantum yield probably as a result of relatively better and uniform illumination inside the reactor. The kinetics of the photocatalytic degradation of 4-PSA has been reported. 1. Introduction The widespread presence of persistent organic chemicals as pollutants in wastewater effluents from industries or even households is a serious environmental problem. Advanced oxidation processes (AOPs) are considered as the best method to completely degrade low concentrations of highly refractory substances. Among these, the method using semiconductor photocatalysis (photocatalytic degradation, PCD) has been found to be very effective in mineralizing most recalcitrant pollutants.1,2 Despite the advantages claimed, PCD has not seen significant commercial utilization. The main hurdle in the commercial use of PCD is the lack of suitable equipment in the form of a photocatalytic reactor giving high space time yields.3 Photocatalysts, such as titanium dioxide (TiO2), may either be dispersed in the irradiated aqueous solution as a colloidal suspension or immobilized on a suitable support. As the primary semiconductor particle is very small, normally of the order of nanometers, semiconductors are often coated on glass beads, optical fibers, or reactor walls because of the difficulties in separating the semiconductor particles from water and because of frequent back-flushing. However, most of the published studies in water indicate that the slurry system made of small semiconductor particles exhibits a much better catalytic activity as compared to an equivalent loading of an immobilized catalyst.4-6 Hence, an annular slurry photoreactor was designed by Kamble et al.7 for PCD of m-DNB using artificial radiation. A single UV lamp was employed during PCD experiments. This study reported preliminary results on reactor performance. The main advantages of such a reactor are (a) high surface area (=200 m2/m3of reactor volume) even at very low (0.2-0.5 wt %) catalyst loading, (b) no mass transfer limitation, and (c) catalyst particles capable of being filtered with the help of a candle filter and being reused efficiently.7 A similar reactor was * Author to whom correspondence is addressed. E-mail: [email protected]. Tel.: +91 253 231 6093,+91 9423963567. Present address: Indira-Govind Smruti, Bungalow No. 16, Sahadeo Nagar, Nasik 422-013, India. † Dr. Reddy’s Laboratories. ‡ National Environmental Engineering Research Institute. § University Institute of Chemical Technology.

used in the present investigation. However, in contrast to a single lamp used by Kamble et al.,7 in this work single as well as multiple lamps were used. The present study deals with PCD of phenol-4-sulfonic acid (4-PSA) using Degussa P-25 TiO2 and artificial UV radiation. The effect of catalyst loading, pH, presence of anions, initial concentration, and photon flux on the rate of PCD has been investigated. The effect of the radiation distribution on quantum yield has been studied by varying the photon flux by using multiple sources of radiations. The kinetics of the PCD of 4-PSA has also been investigated. Sulfonated aromatics are constituent/intermediates of many industrial and pharmaceutical products, such as detergents, medicines, agrochemicals, and coloring agents. Alkylbenzenesulfonates are important components of surfactants and frequently cause water pollution; 4-PSA is one of them. The PCD of many types of anionic, cationic, and nonionic surfactants using artificial UV light and solar radiation has been reported by the group of Hidaka.8-10 Recently some studies have been reported on degradation of aromatic sulfonated compounds using AOPs.11-14 The PCD of 4-PSA in aqueous solution is industrially important because the presence of a sulfonic group confers both high water solubility and biodegradation resistance on aromatic compounds. In addition to having high water solubility, 4-PSA is a highly toxic compound with an LD 50 oral rat value of 317 mg/kg. 4-PSA is widely used in electrolytic galvanizing baths for tin plate production and in electrolytic refining for the purification of crude tin. No literature is available on PCD of 4-PSA. Therefore, PCD of 4-PSA was studied in a continuous annular slurry bubble column reactor suitable for industrial application. 2. Experimental Section 2.1. Materials. Degussa P-25 TiO2 (70:30% w/w anatase to rutile) with an average particle size of 30 nm and BET surface area of about 55 m2 g-1 was used as the photocatalyst. The Degussa P-25 TiO2 was procured as a free sample from Degussa Co., Du¨sseldorf, Germany. 4-PSA was obtained from Merck, Mumbai, India. Sodium chloride, sodium carbonate, and sodium sulfate of analytical reagent grade were obtained from S.D. Fine Chemicals, Ltd., Mumbai, India. All the reaction mixtures were

10.1021/ie061484w CCC: $37.00 © 2007 American Chemical Society Published on Web 04/07/2007

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Figure 1. Novel bubble column reactor for continuous PCD of 4-PSA using artificial UV radiation. All dimensions in m.

prepared in tap water while HPLC mobile phase solutions were prepared in deionized water. All other reagents were used as received. 2.2. Apparatus. All the experiments were carried out in a stainless steel cylindrical, annular, sectionalized slurry bubble column reactor of 0.21 m inside diameter (i.d.) and 1.6 m height. The liquid volume was 41.5 L with a gas holdup of approximately 15%. A schematic of the slurry bubble column reactor is shown in Figure 1. Low pressure, 39.5 W UV lamps (λ ) 254 nm) having a length of 0.8 m were used as source of

Figure 2. Proposed pathway for PCD of 4-PSA.

photons. The UV lamps used had identical input-output ratings (input rating ) 39.5 W and output rating ) 13.7 W). The UV lamps, enclosed within quartz sleeves, were located as shown in Figure 1 depending upon the number of lamps used. One to three lamps were used for continuous experiments. When more than two lamps were used these were arranged on a triangular pitch so that a uniform distribution of light is achieved in all parts of the reactor. To reduce back mixing, sieve plates were employed.3 A diaphragm type air compressor (model HS-2, C. P. Enterprises, Mumbai) was used to sparge air at the bottom of the column through a sintered stainless steel disk. Air was bubbled at a sufficiently high velocity (>3 cm s-1) to keep all the TiO2 in suspension. As a result of the large liquid volume surrounding the UV lamps, the temperature of the reaction mixture remained essentially constant throughout the experiment. The temperature of the reaction mixtures was in the range of 30-33 °C during the winter season of Mumbai, India. 2.3. Batch Adsorption Studies. The equilibrium adsorption experiments were performed with a high precision. In the case of dark adsorption studies, 100 cm3 aliquots were mixed with TiO2 and shaken for 24 h to allow equilibration. The glass conical flasks were covered by aluminum foil during shaking so as to prevent any radiation from entering the flasks. The solutions were then centrifuged, decanted, and filtered for analysis by HPLC. Most (approximately 70%) of the experiments were repeated for checking the reproducibility/reliability. The experimental error was observed to be within (2%. A similar procedure was used for experiments on the effect of pH on adsorption of 4-PSA. 2.4. PCD in the Continuous Flow Reactor. The reactor (mentioned in section 2.2) and the quartz sheaths were thoroughly cleaned before each experiment. The solution to be degraded was mixed with a measured amount of TiO2 powder to yield a uniform slurry. The air supply was then switched on. The solid-liquid slurry was added to the reactor using a peristaltic pump. The feed and outlet flow rates were adjusted and maintained to achieve steady state. After steady state was reached, an aliquot was taken for analysis by HPLC. This concentration was considered to be the zero time concentration [Ct)0]. After the above-mentioned procedure, the electric supply to the UV lamp was switched on. Samples were withdrawn at different intervals for analysis. 2.5. PCD in Batch Mode. The same reactor, used for the continuous flow mode (section 2.4), was also used in the batch mode to elucidate kinetics of PCD of 4-PSA. The experimental procedure was the same as explained for the continuous flow reactor. Three UV lamps having identical input-output ratings (each having an input rating ) 39.5 W and output rating ) 13.7 W) were used during the batch mode PCD experiments. The only difference was the absence of liquid inflow/outflow in the batch mode. 2.6. Analysis. Samples withdrawn during the experiments were analyzed for concentrations of 4-PSA and its intermediates

Ind. Eng. Chem. Res., Vol. 46, No. 12, 2007 4259 Table 1. Effect of pH on Adsorption of 4-PSAa pH

mmol of 4-PSA adsorbed/g catalyst

4.5 8.5 11.50

0.732 0.050 0.199

a Reaction conditions: initial concentration of 4-PSA ) 80 mg L-1; catalyst loading ) 0.02% (w/v of solution).

by HPLC (Knauer) using a C-18 column (5 µm, particle diameter, Merck) and UV-vis detector. Acetonitrile-water (30: 70% v/v) was used as the mobile phase with a flow rate of 0.6 mL min-1 at λ ) 220 nm. Total organic carbon (TOC) was calculated from HPLC analysis and verified using a total organic carbon analyzer (ANATOC-II, Australia). These values agreed within (3% indicating that the HPLC analysis could detect all the intermediates. 3. Mechanism for PCD of 4-PSA Photocatalysis can completely break down (mineralize) most organic compounds to carbon dioxide and water. Identification of the intermediates is a necessity in PCD processes. This helps in understanding the mechanism of PCD. In aqueous TiO2 suspensions, aromatic compounds are oxidized through two different mechanisms, either by hydroxylation of the aromatic ring or by direct electron transfer to TiO2 followed by the addition of a water molecule and loss of a proton.2,7 The reaction mixture was analyzed by HPLC using reference samples of various intermediates to identify individual peaks in the HPLC chart. On the basis of this identification of these intermediates the reaction mechanism/pathways proposed are shown in Figure 2. The intermediates were identified as benzoquinone and hydroquinone. The concentrations of these intermediates, although detectable, were negligible for all practical purposes. 4. Results and Discussion 4.1. Effects of the Various Factors on Adsorption of 4-PSA on TiO2. Adsorption of the substrate on the photocatalyst plays a major role in PCD. It is now well-established that the mechanisms of PCD is through the attack of OH• radicals on the adsorbed substrate.2,7 Hence, the higher the adsorption, the higher the rate of degradation.1,6 Adsorption is affected by several factors, which may include effluent composition and pH. The pH of the reaction medium has a significant effect on the surface properties of the TiO2 catalyst, which include the surface charge of the particles, the extent of aggregation of the catalyst particles, and the band edge position of TiO2. The zero charge point (pHZPC) for Degussa P-25 TiO2 is at pH ) 6.7. The extent of positive charges on the TiO2 surface decreases with increasing pH and reaches zero at pHZPC. Therefore, pH significantly affects the adsorption-desorption properties of the model compounds on the surface of the catalysts.16 At pH > pHZPC, TiO- is the predominant species, whereas at pH < pHZPC, TiOH2+ is the predominant species according to the following equilibria:

Ti-OH + OH- f TiO- + H2O

(1)

Ti-OH + H+ f TiOH2+

(2)

To determine the influence of this parameter on adsorption of 4-PSA, the pH was adjusted at the beginning of the experiments by addition of an aqueous solution of 0.1 M NaOH or 0.1 M HClO4. It was found that adsorption was highest at

Figure 3. Influence of catalyst loading on PCD of 4-PSA (Ct/Ct)0 vs time) in continuous mode. Initial concentration of 4-PSA ) 80 mg L-1. ], 0.02% (w/v of solution) of catalyst loading; 0, 0.05% (w/v of solution) of catalyst loading; and 4, 0.10% (w/v of solution) of catalyst loading.

the self-pH of 4-PSA (Table 1). This may be because at alkaline pH, 4-PSA exists in dissociated form which decreases its adsorption. The effect of pH on adsorption of 4-PSA is twofold: (i) change in the properties of the species and (ii) change in the surface charge of TiO2. These two effects are difficult to separate, and, hence, only the overall effect is reported. 4.2. PCD in the Continuous Flow Reactor. 4.2.1. Effect of Catalyst Concentration. The effect of the catalyst loading on the rate of reaction was investigated for the PCD of 4-PSA in the continuous annular slurry bubble column reactor. The experiments were conducted with different values of catalyst loading (0.02-0.1% w/v) while keeping other parameters constant (initial concentration of 4-PSA ) 80 mg L-1, lamp power, pH). As shown in Figure 3, the extent of PCD decreased with increasing catalyst loading. The degradation was highest at the lowest catalyst loading studied. This is in stark contrast to an optimum value observed in most studies with concentrated solar radiation.17 This may be due to the fact that the opacity of the solution decreases with decreasing catalyst loading. For the mild radiation dose employed in the present study (as against the strong concentrated solar radiation) the UV transmittance decreases sharply with increasing catalyst loading.18 Although no study was made at still lower loadings, it is possible that the degradation may increase further with decrease in catalyst loading reaching a maximum and then decreasing again because of lower catalyst surface available at the relatively lower catalyst loadings. 4.2.2. Effect of pH. The effect of initial pH on the PCD of 4-PSA was studied by varying the pH using aqueous NaOH or HClO4. An initial 4-PSA concentration of 80 mg L-1 and a catalyst loading of 0.02% (w/v) were used in these experiments. Figure 4 shows that, as the pH increases from acidic to basic, the degradation rate increases with increase in pH. Because the PCD of 4-PSA occurs through the formation of the previously referred intermediates the rates of disappearance of 4-PSA do not reflect the effectiveness of the process. The latter can be more clearly judged from the rate of disappearance of the TOC. Figure 5 shows the effect of pH on TOC removal rate. Although the rate of PCD of 4-PSA was highest at pH of 11.5, TOC

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Figure 6. Effect of variation in radiation photon flux on TOC reduction of 4-PSA in continuous mode.

not reported TOC reduced during PCD of 2-chlorophenol. Figure 4. Effect of pH on the PCD of 4-PSA on TiO2 in continuous mode (Ct/Ct)0 vs time). Catalyst loading ) 0.02% (w/v of solution), initial concentration of 4-PSA ) 80 mg L-1. ], pH ) 4.5; 0, pH ) 8.5; and 4, pH ) 11.5.

Figure 5. Effect of pH on TOC reduction of 4-PSA on TiO2 (TOC/ (TOC)t)0 vs time) in continuous mode. Catalyst loading ) 0.02% (w/v of solution), initial concentration of 4-PSA ) 80 mg L-1. ], pH ) 4.5; 0, pH ) 8.5; and 4, pH ) 11.5.

removal was highest at the self-pH (pH = 4.5) of 4-PSA. A decrease in TOC reflects a decrease in dissolved organic carbon (4-PSA/its intermediates), which can only be attributed to formation of CO2 through cleavage of the aromatic ring prior to mineralization of the organic carbon. Thus, at self-pH, the PCD favors direct ring cleavage. Higher PCD of the parent molecule at alkaline pH has also been reported, for instance, by Doong et al.19 for PCD of 2-chlorophenol. A higher pH value can provide higher concentration of hydroxyl ions to react with holes to form hydroxyl radicals, subsequently enhancing the photodegradation rate of 2-chlorophenol. But these authors have

OH- + h+ f OH•

(3)

O- + hν f OH• + eaq-

(4)

eaq- + O2 f O2•

(5)

High PCD rate but low TOC removal rate at alkaline pH shows that, in alkaline media, degradation occurs by only functional group substitution and not by direct ring cleavage. This is also supported by the fact that, in alkaline pH, the concentration of intermediates formed was higher. These observations show that it will be better to treat any acidic compound in an effluent before neutralizing the same.13,14 On the other hand, effluents containing basic refractory compounds are easier to treat under basic pH.17 4.2.3. Effect of Photon Flux. The photon flux is inevitably connected with the photocatalytic activity. Photon flux can be increased either by using a single lamp having higher wattage or by using multiple lamps of same wattage. Pareek et al.18 have studied the intensity distribution in an annular reactor. For a 300 W medium-pressure mercury lamp, the findings of Pareek et al.18 indicate that even for very low catalyst loadings (∼0.05 wt %) the effective UV radiation penetration is only up to a few centimeters into the solution from the lamp surface. Pareek et al.18 also conclude that for the annular reactor used by them, the annular depth (ro - ri) should not be more than 2 cm. In the present case the geometry used mandates distribution of the lamps so that the entire reactor cross section is illuminated as uniformly as possible. Established engineering practices clearly advise that in such a case properly distributed multiple sources can result in better coverage (and utilization) of the reactor cross section by the incident radiation. This requirement is similar to that for a liquid distributor for counter-currently operated packed towers.20,21 To ascertain this argument, the number of UV lamps was varied. Experiments were carried out for an initial concentration of 4-PSA ) 80 mg L-1 with continuous reactor flow rate ) 55 mL min-1. Figure 6 shows a plot of log [(TOC)i/ (TOC)f] at different radiation fluxes. With increase in radiation flux as well as distribution of the lamps, percentage removal of

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Figure 7. Effect of variation in photon flux on quantum yield.

TOC increased. When the photon flux ) 0.594 mol of photons m-2 h-1 (i.e., when three lamps, each lamp contributing radiation flux ) 0.198 mol of photons m-2 h-1) was used, the percentage TOC removal was 57%. 4.2.4. Quantum Yield. The quantum yields (the ratio of grams carbon oxidized per hour to moles photon input per hour) for the PCD of 4-PSA were observed to be 0.33, 0.19, and 0.1 for the photon fluxes of 0.594, 0.396, and 0.198 mol of photons m-2 h-1, respectively. The increase in photon flux was accompanied by distribution of the lamps. Thus, instead of a single high wattage lamp, two or three lamps of the same low wattage but distributed over the cross section were used. The results (Figure 7) show that as the number of distributed lamps increases, the quantum yield also increases. The relationship can be described by

Qy ) 0.5303 × photon flux (R2 ) 0.99)

(6)

The increase in Qy with more and distributed lamps is due to an effective coverage of the reactor cross section by the radiation from the lamps. Although not experimentally demonstrated in this work, it is most unlikely that a centrally placed single high wattage lamp equivalent to two or three lamps used in this work would yield equivalent quantum efficiency as in Figure 7. This argument is supported by the model calculations of Pareek et al.,18 which show rapid decrease of radiation intensity with increasing distance from the lamp. Thus, the high-intensity radiation from this single lamp would be effective in a very small zone in contrast to the distributed lamp system used in the present work. The increased quantum efficiency, however, is at the expense of higher capital cost due to more lamps/quartz sheaths, electrical connectors, and so forth and a more elaborate cleaning system for the quartz sheaths. The last mentioned, the cleaning system, is required because of the strong tendency of TiO2 to adhere to quartz surfaces. 4.3. PCD in Batch Mode. 4.3.1. Effect of Initial 4-PSA Concentration. The initial concentration of the pollutant is always an important parameter in any process water treatment. Therefore, it is essential to examine the effect of the initial concentration. Degradation studies were carried out using 50, 80, and 150 mg L-1 initial concentration of 4-PSA and 0.02% (w/v) catalyst loading in batch mode. Figure 8 shows that an increase in the feed concentration of 4-PSA causes a decrease

Figure 8. Effect of initial concentration on PCD of 4-PSA in batch mode. Catalyst loading ) 0.02% (w/v of solution). ], Initial concentration of 4-PSA ) 50 ppm; 0, initial concentration of 4-PSA ) 80 ppm; and 4, initial concentration of 4-PSA ) 150 ppm.

in the rate of PCD. Turchi and Ollis22 have postulated four different mechanisms of the reaction between the substrate and the hydroxyl radical depending upon whether the substrate and/ or hydroxyl radical is adsorbed on the catalyst surface or in the bulk. As the hydroxyl radicals are extremely reactive and shortlived, their diffusion into bulk is negligible. The evidence available in the literature points to a reaction between the substrate molecules and OH• generated on the TiO2 surface.2 With increase in initial feed concentration of 4-PSA, concentration of intermediates also increases with time. These intermediates compete for the sites on the catalyst surface. For constant light intensity, TiO2 loading, and dissolved oxygen concentration, the concentration of OH• remains practically the same. Thus, as the initial concentration increases, OH• becomes the limiting reactant. Further, parts of the active sites are occupied by the intermediates formed during PCD of the substrate. Thus, the availability of adsorbed substrate also decreases. The overall effect is a reduced rate of PCD of the substrate with its increased concentration in the feed. 4.3.2. Kinetics of PCD of 4-PSA. The initial rate of degradation increases with increase in initial concentration of 4-PSA, which indicates concentration dependent degradation kinetics. This dependence on concentration is confirmed by LHHW kinetics. The following assumptions are made for the LHHW approach. 1. At equilibrium, the number of surface adsorption sites is fixed. 2. Only one substrate may bind at each surface site. 3. The heat of adsorption of the substrate is identical for each site and is independent of the surface coverage. 4. There is no interaction between adjacent adsorbed molecules. 5. The rate of surface adsorption of the substrate is greater than the rate of any subsequent chemical reactions; that is, there is no diffusional limitation. 6. No irreversible blocking of active sites occurs because of binding by product(s).

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With these assumptions, the surface coverage Θ of substrate S, on the catalyst particle, is related to its concentration [C] in bulk and to the apparent adsorption equilibrium constant KS:23

ΘS )

KS[C] 1 + KS[C]

(7)

The second-order rate expression can be written as

r ) k′ΘOHΘS

(8)

where ΘOH and ΘS are the fractional surface coverages by hydroxyl radicals and substrate, respectively. These variables can be expressed by eqs 9 and 10, respectively.

ΘOH )

ΘS )

KO2PO2 1 + KO2PO2

(9)

KS[C] 1 + KS[C] +

∑i Ki[Ii]

(10) Figure 9. LHHW kinetics plot (reciprocal of initial rate vs reciprocal of initial concentration of 4-PSA).

in which KO2, KS, and Ki are equilibrium adsorption rate constants and Ii refers to the various intermediate products formed during the PCD. During the photocatalytic reaction, intermediates can also bind competitively to the catalyst active sites and their concentrations also change with time, inasmuch as they are eventually and completely mineralized. Owing to the fact that the oxygen partial pressure remained constant during the reaction, the fractional site coverage by hydroxyl radicals can be assumed to be constant.24 Therefore, we can write

KO2PO2 )k k′ 1 + KO2PO2

(11)

At time t ) 0, there are no intermediates present. Therefore, the initial rate of PCD can be expressed as follows:

KS[C0] r0 ) k 1 + KS[C0]

(12)

Equation 12 can also be expressed in linear form as shown below.

1 1 1 1 ) + r0 kKS [C0] k

(13)

A plot of reciprocal of initial rate versus reciprocal of initial concentration (Figure 9) shows that the degradation of 4-PSA follows pseudo-first-order kinetics with a rate constant of 0.3 s-1. 4.3.3. Effect of Anions. In addition to pollutants, industrial effluents contain different salts at different levels of concentrations. The salts are generally ionized under the conditions of PCD. The anionic and cationic parts of the salt have different effects on the PCD process. Studies were carried out to determine the effect of the presence of anions commonly found in industrial wastewaters on the rate of degradation of 4-PSA in batch mode. The effects of presence of various anions such as chloride, sulfate, and carbonate were studied using 0.1 M solutions of their sodium salts and an initial concentration of 80 mg L-1 of 4-PSA and 0.02% (w/v) catalyst loading in batch mode.

Figure 10. Effect of presence of anions on TOC reduction [TOC/(TOC)t)0 vs time] in batch mode. Catalyst loading ) 0.02% (w/v of solution), initial concentration of 4-PSA) 80 mg L-1. ], PCD of 4-PSA in the presence of 0.1 M NaCl; 0, PCD of 4-PSA in the presence of 0.1 M Na2SO4; 4, PCD of 4-PSA in the presence of 0.1 M Na2CO3; and ×, PCD of 4-PSA in the absence of anions.

Figure 10 shows the effect of chloride, sulfate, and carbonate on TOC reduction. It was observed that TOC reduction was affected significantly in the presence of carbonate, sulfate, and chloride. TOC reduction was 55% in the absence of anions while it was 37%, 30%, and 25% in the presence of chloride, sulfate, and carbonate, respectively. Carbonate, bicarbonate, and chloride ions act as hydroxyl ion scavengers and absorb UV light through the following reactions:2

OH• + Cl- f Cl• + OH-

(14)

CO32- + HO• f CO3•- + H2O

(15)

In addition to scavenging of hydroxyl radicals by anions, they may hinder the adsorption of substrate on catalyst particle.

Ind. Eng. Chem. Res., Vol. 46, No. 12, 2007 4263 Table 2. System Sizing, Cost Estimation, and Scale-up Parameters for TOC Reduction during PCD of 4-PSAa no. of UV lamps

electricity consumption (W)

log((TOC)i/(TOC)f)

UV dose (kW h/m3)

EE/O (kW h/(m3 order))

cost (USD/(m3 order))

1 2 3

39.5 79 118.5

0.03 0.13 0.42

11.97 23.94 35.91

398.9 184.1 85.5

120 55 25.6

a Reaction conditions: flow rate ) 0.0033 m3 h-1; initial concentration of 4-PSA ) 80 mg L-1; catalyst loading ) 0.02% (w/v of solution); and electrical cost per unit ) 0.30 USD/(kW h).

Lesser adsorption of the substrate and loss of hydroxyl radicals in the presence of anions cause reduction in % TOC removal. 4.4. System Sizing, Cost Estimation, and Scale-up Parameters. (a) Electrical Energy Determination. The evaluation of the treatment costs is one of the important aspects which needs greater attention. For the selection of any waste treatment technology, various parameters such as cost of treatment, economy of scale, regulations, effluent quality goals, operational maintenance, safety, and flexibility to change/upsets need to be considered. Although all these factors are important, economics is often dominant. Because the photocatalytic process is electrical-energy intensive and electrical energy can represent a major fraction of the operating costs, simple figures-of-merit based on electric energy consumption can be very useful and informative.25 The UV dose, when applied to an AOP, is a measure of the total lamp electrical energy applied to a fixed volume of water. This parameter combines flow rate, residence time, and light intensity into a single term. Electrical energy required (UV dose) for a continuous flow system can be calculated as

UV dose )

lamp power (kW h) flow rate (m3/h)

(16)

(b) EE/O. EE/O is a powerful scale up-parameter and is defined as kilowatt hours required to remove the TOC of a compound in 1 m3 by 1 order of magnitude (or 90%). The unit for EE/O is kW h/(m3 order).26 The relationship between UV dose and EE/O is given by

UV dose )

EE log{(TOC)i/(TOC)f} O

(17)

[

]

[ ]

USD USD EE ) × power cost O kW h m × order (18) 3

Acknowledgment We wish to thank Degussa Co., Germany, for the free sample of Degussa P-25 titanium dioxide catalyst. Nomenclature λ ) wavelength of light (nm) Θ ) surface coverage (m2 g-1) ΘS ) surface coverage of substrate S (m2 g-1) ΘOH ) surface coverage of hydroxyl radicals (m2 g-1) Ct)0 ) initial concentration of substrate (mg L-1) Ct ) concentration of substrate at any time t (mg L-1) KS ) adsorption equilibrium constant of substrate (L mg-1) KO2 ) adsorption equilibrium constant of oxygen (L mg-1) Ki ) adsorption equilibrium constant of intermediate i formed during PCD (L mg-1) k′ ) second-order reaction rate constant (L mg-1 s-1) k ) pseudo-first-order reaction rate constant (s-1) r ) rate of reaction (mg L-1 s-1) r0 ) initial reaction rate (mg L-1 s-1) φl ) quantum yield at wavelength (TOC)t)0 ) TOC at time t ) 0 before the substrate solution was exposed to UV radiation (mg L-1) TOC ) TOC at any time t when the substrate solution was exposed to UV radiation (mg L-1) AbbreViations

The EE/O values calculated from the above equation are given in Table 2. (c) Operating Cost (Lamp Electricity). Basis: TOC removal by one order in 1 m3 of effluent to be treated. The cost of 1 kW h is considered to be 0.30 USD (U.S. dollars).

electrical cost

It was observed that the usage of multiple sources of radiation increases the coverage of illumination inside the reactor. This effective distribution of radiation increases the quantum yield. The PCD of 4-PSA follows pseudo-first-order kinetics.

The estimated operating cost (electrical cost) for the PCD of 4-PSA is given in Table 2 which indicates that the cost of treatment decreases with increasing number of equally spaced UV lamps as concluded earlier in section 4.2.3. 5. Conclusions The results obtained in the continuous flow studies demonstrate the potential of PCD as a promising technology for treating aqueous streams containing 4-PSA. At self-pH (pH = 4.5) the degradation of 4-PSA was higher as compared to alkaline pH. It was also observed that carbonate, chloride, and sulfate have detrimental effects on TOC removal. The effect of the radiation distribution of the UV lamps on quantum yield was also studied.

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ReceiVed for reView November 21, 2006 ReVised manuscript receiVed February 27, 2007 Accepted February 28, 2007 IE061484W