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ISSN 00405795, Theoretical Foundations of Chemical Engineering, 2014, Vol. 48, No. 5, pp. 656666. Pleiades Publishing, Ltd., 2014.

656

1INTRODUCTION

Polycyclic aromatic hydrocarbons (PAHs) are hazardous bypass products in petrochemical industries andcoalmines. Due to PAHs presence in coal, these materials exist in coalmine dumps. Also the effluent water ofpetrochemical units, specially cracking units, containslarge amounts of PAHs [1, 2]. Among PAHs, naphthalene is the main compound that has the most solubility inaqueous media [2]. In 2007, analysis of the Jiulongcoalmine effluent showed that naphthalene is the mainorganic material existed in the coalmine effluent [3].

PAHs are very chemically stable and this trait leadsto treatment of these compounds was not accomplished completely by biological methods. Also, due todifficult conditions for surviving microorganisms, biological processes encountered various limitations [1].The mentioned problems caused to find some methods according to advanced oxidation processes(AOPs) as one of the modern technologies in wastewater treatment. AOPs act based on the production ofunstable species with high reactivity tendency (e.g.

H2O2, OH) to convert the resistant organic

compounds to the inert components [4, 5].

1 The article is published in the original.

O2

,

In previous studies, photocatalytic reaction underUV irradiation and in presence of TiO2 nanoparticleshas proved many capabilities to remove organic compounds from wastewater compared with other methods such as biological techniques [6]. Also, manykinds of nanoparticles have been investigated as photocatalyst including TiO2, ZnO, CdS, and ZnS [7].Because of high chemical stability, high efficiency andlow production cost, TiO2 can be an appropriate catalyst for the photocatalytic process [8]. Among threecrystalline structures of TiO2, anatase has the mostphoto reactivity because of the slightly higher Fermilevel, lower capacity to adsorb oxygen and higherdegree of hydroxylation [9].

Depending on the method of catalyst applicationand the source of UV irradiation, photocatalytic process can be utilized in numerous ways. JianLing et al.applied modified TiO2 nanotubes that arrays by CdS[10]. Shahmoradi et al. used photocatalytic process fortreatment the municipal wastewater based on modified neodymium doped TiO2 hybrid nanoparticles[11]. Yan et al. applied visible light for photocatalyticdecoloration of methylene blue on novel NdopedTiO2 [12]. HuiMin et al. prepared Zndoped TiO2

Photocatalytic Abatement of Naphthalene Catalyzed by Nanosized TiO2 Particles:

Assessment of Operational Parameters1

V. Mahmoodi and J. SargolzaeiDepartment of Chemical Engineering, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad, Iran

email: [email protected]; [email protected] April 24, 2012

AbstractNaphthalene removal from wastewater sources was investigated in a batch slurry system usingphotocatalytic process as a subset of advanced oxidation processes. At low concentrations of naphthalene, thepseudofirst order rate equation on the base of LangmuirHinshelwood model described the degradationkinetics very well. Also, the photocatalytic process was employed to evaluate the effect of various operationalparameters such as agitation speed (0200 rpm), injected air flow (06 L/h), TiO2 concentration (03 g/L)and UV power intensity (024 W). Experimental results demonstrated that by increment in the mass transfercoefficient, the agitation speed positively affected naphthalene degradation. Due to the screening effect,concentration of TiO2 showed an optimum amount equal to 2 g/L. Aeration to the solution affected theamount of oxygen as an electron scavenger and so enhanced naphthalene degradation rate. Meanwhile in theabsence of UV radiation to the solution, the rate of naphthalene removal decreased significantly. In otherhand, augmentation in UV radiation more than 8 W had a low influence on the amount of removed naphthalene and reaction rate.

Keywords: naphthalene removal, photocatalytic process, batch slurry system, UV irradiation, TiO2 nanoparticles, advanced oxidation processes, wastewater treatment

DOI: 10.1134/S0040579514050194

THEORETICAL FOUNDATIONS OF CHEMICAL ENGINEERING Vol. 48 No. 5 2014

PHOTOCATALYTIC ABATEMENT OF NAPHTHALENE CATALYZED 657

nanotubes electrode and assessed its application inpentachlorophenol photo electrocatalytic degradation[13]. In order to refine organic wastewater includedvarious perilous compounds, Meng et al. [14], Shon etal. [15] and Damszel et al. [16] utilized photocatalyticprocess coupled with a membrane. In addition, Parkpresented the higher ability of sonophotocatalysis inagricultural wastewater treatment rather than photocatalysis [17].

Katsoni et al. utilized solar UV for photocatalyticdegradation (PCD) of the trinitrophenol [18]. Song etal. studied photocatalytic removal of phenol using thephotocatalyst poly(fluorinecobithiophene)/TiO2under visible light [19]. Gang et al. modeled PCDkinetics of Rhodamine B on nanosized TiO2 film andobtained an exponential kinetic model [20].

Mechanism of the PCD of naphthalene isdescribed in previous studies [6, 21]. Theurich et al.demonstrated that during the photocatalytic process, naphthalene changed to different intermediates e.g. couramine, cinnamic acid, phthalic acidand 1,2benzenedicarboxaldehyde [1]. However,Ohno et al. confirmed that in PCD of naphthalene,the main product is 2formylcinnamaldehyde [22].Nevertheless, different parameters such as solutiontemperature, catalyst loading, initial concentration ofthe contaminant, initial pH, intensity of the mixing

solution, amount of dissolved oxygen in the solution,UV light intensity, and wavelength of UV light canaffect performance of PCD [6]. In similar researches,the effects of solution temperature, pristine pH andnaphthalene concentration were investigated [23, 24].

In current paper, the kinetic study of photocatalyticreaction and the effect of key parameters such as agitation speed (), TiO2 concentration, aeration flow(Q) and light intensity (I) were investigated on naphthalene removal in the batch slurry system.

MATERIALS AND METHODS

Materials and characterization instrument. In allexperiment test runs, TiO2 in anatase crystalline phasewith greater than 99.5% purity was purchased from USResearch Nanomaterials Inc. Naphthalene with analytical grade was purchased from Merck. In order toprepare the naphthalene solution, raw naphthalenewas mixed with double distillated water (Abtab Co.,Iran) at 45C. TiO2 nanoparticles were analyzed usingXray powder diffraction with model of XRD, PhilipsPW 1800. The morphology/structure of TiO2 nanoparticles was obtained by scanning electron microscopy (SEM, Cambridge S360) and Transmissionelectron microscopy (TEM, Philips CM120).

Flow meter Temp. controller

Air pump

Voltageregulator

To 220 V

Sampling

(1)

(2)

(3)

(4)

T

(5)

(1) (1)

Fig. 1. Schematic diagram of the photocatalytic reactor system: 1UV lamps; 2thermocouple; 3magnetic stirrer; 4waterbath; 5suspension.

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JOURNAL OF CTHEORETICAL FOUNDATIONS OF CHEMICAL ENGINEERING Vol. 48 No. 5 2014

MAHMOODI, SARGOLZAEI

Apparatus. The photocatalytic reactor system isschematically exhibited in Fig. 1.

A magnetic stirrer (Taksan Co., Iran) was used tomix the solution at different intensities. The reactor was equipped with three 8 W UV lamps at365 nm wavelength. The temperature was measured using a type K thermocouple and was monitored by a temperature controller (Abtin Co.,United Kingdom).

An aquarium air pump (Khanemahi Co., Iran) wasused for air injection into the solution weakly. Air flowwas controlled using an accurate flow meter (VgtlinCo., Switzerland). Water bath between the beaker andstainless steel container was utilized to keep the solution temperature constant during the process. A digitalpH meter (Hanna Co., USA) was used to measurethe pH of the solution during the photocatalyticreaction.

Procedure. First, 100 mL of naphthalene solutionwas poured inside a 150 mL beaker that was placedinto a cylindrical stainless steel container. In order toreach thermal equilibrium, the solution was stirred fortwo minutes extremely. Then the required amount ofTiO2 was added to the solution and the suspension wasstirred in the dark for five minutes. After that, the UVirradiation was started and the solution was sampled indesired time intervals. For separation of TiO2 nanoparticles, the resulted samples were filtered by PTFEsyringe membrane (0.45 m pore size).

After filtration, the naphthalene solution sampleswere analyzed immediately by an UV/vis Spectropho

tometer (CECIL Co. 9000 series, United Kingdom) atmax = 275.4 nm with a calibration curve based on theBeerLamberts law. At 275.4 nm wavelength, naphthalene can only absorb UV rays significantly [2].

Figure 2 presents the UV absorbance spectrum ofthe standard naphthalene solution at C0 = 20 mg/L.The temperature of the solution was maintained at 32 2C during all experimental test runs. The operation conditions of all experiments are summarized in Table 1.

It is noteworthy that before performing the maintests, in a series of experiments in the dark, werealized that the adsorption equilibrium is attainedafter five minutes in the naphthalene/TiO2/watersystem (Fig. 3).

In the first moments, the TiO2 showed considerableadsorption of the naphthalene (area 1), as shown byFig. 3. Then the naphthalene concentration increasedslightly that may occur due to the partial desorption ofnaphthalene from the surface of TiO2 (area 2) [25]. Atarea 3, the concentration stayed approximately constant and so it can be concluded that the adsorptionhas reached to the dynamic equilibrium after 5 min.Also, the results showed that less than 10% of thenaphthalene was adsorbed on the surface of TiO2 in thedark under specified conditions and no considerabledegradation of naphthalene was observed without UVirradiation.

RESULTS AND DISCUSSION

TiO2 Morphology and Characteristics. As shown inFig. 4, TiO2 was analyzed by XRD which showed thepresence of anatase as the nanoparticle crystalline phase.

TiO2 nanoparticles were also analyzed by scanningelectron microphotography (SEM). Figure 5 showsthe morphology of the TiO2 nanoparticles.

1.5

1.0

0.5

0

(275.4, 0.604)

400350300250200, nm

A2.0

Fig. 2. UV absorbance spectrum of the naphthalenesolution.

20

15

10

550100 20 30

t, min

a

C, mg/L

(1)

(2)(3) b

40

Fig. 3. Evolution of naphthalene adsorption on the TiO2surface in the dark ( = 100 rpm; = 2 g/L; Q = 0):

aC0 = 10 mg/L; bC0 = 20 mg/L.

CTiO2



As seen in Fig. 5, the purity and porosity of TiO2nanoparticles are very appropriate. Figure 6 shows theTransmission Electron Microscope (TEM) image ofTiO2. Transmission electron microscopy could be thefirst method used to establish the size distribution ofnanoparticles. Accordingly, this figure shows that theaverage particle size of pristine TiO2 is less than 30 nm.

This range of size can cause significant surface areaand high degree of conversion. Table 2 presents general characteristics of TiO2 nanoparticles.

Kinetics modeling of photocatalytic reaction. Inorder to conduct kinetic investigation, a series ofexperiments was performed with different initialnaphthalene concentrations (C0) while keeping allother factors constant. Results are shown in Fig. 7.

LangmuirHinshelwood (LH) model is the mostwidely used expression to explain the kinetics of heterogeneous catalytic systems [2629]. LH mechanismis related to the reaction that different agents areadsorbed on a porous surface and then react togetheron the surface [30].

According to the LH model, the reaction rate(r) for a surface reaction where the pollutant is substantially more adsorbed than the product follows theequation below [31, 32]:

(1)

where kr, KI, CI, K, C, KW, and CW are the reaction rateconstant, intermediate species adsorption constant,concentration of the intermediates at any time, naphthalene adsorption constant, the concentration ofnaphthalene at any time t, water adsorption constantand concentration of the water, respectively. Since theconcentration of intermediates is negligible comparedto that of the reactants, the intermediate term can beneglected in Eq. (1), which results as follows:

(2)

As known, CW is much greater than C (in water CW 55.5 M) and it is evident that CW stays actually constant during the treatment process. For low liquidphase concentrations of the organic pollutant, Eq. (2)can be simplified to a pseudofirstorder reaction(Eq. (3)), where Kapp represents the pseudofirst orderreaction rate constant and C0 is the initial concentration of the pollutant:

(3)

where t is the reaction time and Kapp =

r krKC

1 KWCW KICI KC+ + +,=

r krKC

1 KWCW KC+ + .=

CC0 ln Kappt,=

krK

1 KWCW+.

It is noteworthy that Eq. (3) applies at low concentrations of substrate and here, the initial naphthaleneconcentration is negligible compared with the waterconcentration. So we used the simplified LH modelto describe the degradation of naphthalene.

Table 1. Operating conditions of all experimental test runs

Run no. , rpm g/L C, mg/L I, W Q, L/h

1 0 2 20 16 4

2 50 2 20 16 4

3 100 2 20 16 4

4 150 2 20 16 4

5 200 2 20 16 4

6 100 0 20 16 4

7 100 0.5 20 16 4

8 100 1 20 16 4

9 100 1.5 20 16 4

10 100 2.5 20 16 4

11 100 3 20 16 4

12 100 2 5 16 4

13 100 2 10 16 4

14 100 2 15 16 4

15 100 2 20 0 4

16 100 2 20 8 4

17 100 2 20 24 4

18 100 2 20 16 0

19 100 2 20 16 1

20 100 2 20 16 2

21 100 2 20 16 3

22 100 2 20 16 5

23 100 2 20 16 6

CTiO2,

1600

900

400

100

0

Titanium oxide (anatase), TiO2

706030 40 50Position 2, deg

Co

un

ts

Crystallographic system: tetragonalAna

lyse,

syn/

TiO

2

Fig. 4. XRay diffraction pattern of TiO2 nanoparticles.

660



The pseudofirst order reaction rate constants wereequal to 0.0042, 0.0046, 0.0051 and 0.0065 min1 forinitial naphthalene concentrations of 20, 15, 10, and 5mg/L, respectively (Fig. 8). The decrease of Kapp asincreasing the initial naphthalene concentration indi

cates that there is a competition between the producedintermediates and naphthalene for the active sites onthe surface of TiO2 [33]. So, the adsorption processcan be excluded from the ratedetermining step.Hence the surface reaction is probably the ratedeter

5 m

Fig. 5. Scanning electron microphotography of TiO2 nanoparticles.

100 nm

Fig. 6. Transmission electron microscope image of TiO2 nanoparticles.



mining step and it is clearly consistent with the LHmechanism assuming a surfacereaction limited process [34]. Actually, modified LH kinetic model hassuccessfully described many similar photocatalyticsystems [46, 26, 28, 31] and generally is recognizedas a basic kinetic equation of heterogeneous photocatalysis today [30].

pH history. As shown by Fig. 9, at C0 = 20 mg/L,pH variation was evaluated during the process. pHwas reduced about 2 units in one hour passed formthe process that it can be attributed to the hydration of the TiO2 surface as Lair et al. reported intheir study [23]. Produced hydroxide ions (fromwater molecules dissociation) adsorb at the TiO2surface strongly and protons are released in the liquid phase, lead to reduce the solution pH from 7.5to 4.7.

In the following, pH decreased slightly due to theproduction of unstable acidic species e.g. carboxylicacid and phthalic acid as other researchers mentionedin their observations [1, 2, 23].

Effect of agitation speed. For chemical reactionsthat affected by mass transfer, the overall rate resistance is the sum of the mass transfer resistance and thechemical reaction resistance [35]:

(4)

where kC is the mass transfer coefficient and kO is theoverall rate constant. Because of fine particle size ofthe TiO2 nanoparticles, the chemical reaction rate onthe surface is very fast, so the mass transfer resistancemay determine the overall rate process. Assuming thatthe reaction rate constant is much larger than the masstransfer coefficient:

(5)

1kO 1

kC 1

kr,+=

kr kC1kr 1

kC.

Thus the overall reaction rate is controlled by the masstransfer resistance:

(6)

Considering the spherical TiO2 nanoparticles suspended in the solution, equation related to kC is likeequation as follows [36]:

(7)

where DAB is the diffusion coefficient of the naphthalene in water and dp is the nanoparticle diameter. WithRe and Sc substitution in Eq. (7):

(8)

where is the kinematic viscosity of water and U is thevelocity of the fluid passing from the spherical particle.Considering Eq. (8), there are two ways to increase kCor kO. First, reducing dp and, second, increasing in Uvalue. By enhancing the agitation speed, U willincrease, so kC enhances.

Figure 10 shows the effect of agitation speed onPCD of naphthalene at 0 to 200 rpm. As seen inFig. 10, by increasing agitation speed the naphthalene

1kO 1

kC .

kC 0.6DABdp

Re0.5Sc0.33,=

kC 0.6DAB

0.67

0.17 U

dp 0.5,=

Table 2. Characteristics of TiO2 nanoparticles

Type of nanoparticle Size, nm Specific surface area, m2/g

TiO2, anatase 50

25

20

15

10

5

0 30025015010050 200t, min

a

C, mg/L

350

bcd

Fig. 7. Effect of initial naphthalene concentration ( =100 rpm; Q = 4 L/h; I = 16 W; = 2 g/L): C0 = (a) 20,

(b) 15, (c) 10, and (d) 5 mg/L.

CTiO2

0

0.5

1.0

1.5

2.0

2.53002501501000 50 200

t, min

a

ln(C/C0)

350

bcdLinear (a)Linear (b)Linear (c)Linear (d)

Fig. 8. Assessment of naphthalene removal kinetics: a20 mg/L, Kapp = 0.0042 min

1, R2 = 0.9; b15 mg/L,Kapp = 0.0046 min

1, R2 = 0.88; c10 mg/L, Kapp =0.0051 min1, R2 = 0.94; d5 mg/L, Kapp = 0.0065 min

1,R2 = 0.88.

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degradation rate enhances due to increase of the masstransfer coefficient.

Also, variation of pseudofirst order reaction rateconstant with agitation speed was investigated andthe results are depicted in Fig. 11. As shown by Fig.11, a higher reaction rate constant (0.0069 min1)was observed for higher agitation speed that can berelated to the higher pollutant adsorption rate onthe catalyst.

Effect of injected air flow. Air injection into thesolution affects the amount of oxygen as an electronscavenger and so it was anticipated that air injectioninto the naphthalene solution is a positive parameter.Figure 12 shows the effect of the injected air flow onthe naphthalene degradation.

As shown by Fig. 12, aeration to the solution is veryimpressive. The aeration flow equal to 1 L/h enhancednaphthalene conversion by about 20%. Also, at 5 and6 L/h of air flow, naphthalene conversion increased upto 70% sharply. Although this abnormal enhancementcan be related to interference the term of volatility ofnaphthalene for more than 56 L/h of injected airflow, as other researchers mentioned in their studies [2].

Also, variation in pseudofirstorder reactionrate constant with air flow was evaluated from thediagram slopes in Fig. 13. As shown by Fig. 13,reaction rate constant increases with the air flowup to 6 L/h (0.0088 min1) due to increase of theoxygen concentration in the solution as an electron scavenger.

Effect of catalyst loading. After comprehending theeffect of the agitation speed and injected air flow onthe naphthalene degradation kinetics, another seriesof experiments was carried out in which the TiO2 concentration was varied from 0 to 3 g/L. TiO2 was used inpure anatase phase and it was demonstrated that thebest TiO2 concentration in anatase phase for photocatalytic naphthalene refining is equal to 2 g/L, as shownby Fig. 14.

Also, Fig. 14 shows that without TiO2 nanoparticles, naphthalene cannot be degraded well. Whilethe amount of TiO2 increases in the solution, levelof the photocatalytic activity becomes greater andthe naphthalene adsorption enhances. So it wasexpected that by enhancement of catalyst loading,the amount of naphthalene was reduced and theconversion percent was increased. But anotherfactor that affected the degradation process wasthe level of solution turbidity. TiO2 loadingenhancement led to increment the solution turbidity, so the light penetration into the solutiondecreased due to the screening effect of the catalyst nanoparticles [35]. So interaction between twoopposing factors e.g. availability of active sites on

8

7

6

5

43002501501000 50 200

t, min

pH

350

Fig. 9. pH history: = 100 rpm; = 2 g/L; I = 16 W;

Q = 4 L/h; C0 = 20 mg/L.

CTiO2

20

15

10

5

0 1601208040t, min

a

C, mg/L

bcde

Fig. 10. Effect of the agitation speed on the amount ofdegraded naphthalene (Q = 4 L/h; I = 16 W; TiO2 loading= 2 g/L; C0 = 20 mg/L): a0; b50; c100; d150;e200 rpm.

0

0.4

0.8

1.2

160120800 40t, min

a

ln(C/C0)

bc

eLinear (a)Linear (b)Linear (c)Linear (d)Linear (e)

d

Fig. 11. Effect of agitation speed on naphthalene removalkinetics: a0, Kapp = 0.0028 min

1, R2 = 0.99; b

50 rpm, Kapp = 0.0052 min1, R2 = 0.98; c100 rpm,

Kapp = 0.0057 min1, R2 = 0.98; d150 rpm, Kapp =

0.0062 min1, R2 = 0.92; e200 rpm, Kapp = 0.0069 min1,

R2 = 0.92.



the TiO2 surface and the level of turbidity, led tothe fact that the catalyst concentration had anoptimum value.

As shown in Fig. 15, the rate of photocatalyticreaction increased up to 2 g/L and then the pseudofirst order constant decreased. At the optimum concentration of TiO2, the reaction rate constant wasequal to 0.0062 min1. This trait can be attributed tothe enhanced turbidity in the solution as was previously explained.

These results are consistent with the results of previous studies. Wu et al. [35] and Mahvi et al. [37]determined that development in the value of catalystto the level consistent with the optimized level of lightabsorption increases the amount of degradation.However, any further increase in the amount of catalyst has negative effect on the photodegradation performance.

Effect of light intensity. In general, PCD of thenaphthalene is a heterogeneous reaction that isoccurred at the active sites on the TiO2 particle surface.So a higher UV power intensity is expected to givemore active sites and a higher degradation rate.A series of experiments was carried out under the sameoperating conditions but varying UV light power, asWu et al. performed in their study [35]. As shown byFig. 16, absence of UV light acquired a very low conversion about 15 percent. When just one UV lampturned on, the conversion percent significantlyincreased up to 50 percent.

Variation in degradation rates of naphthalene wasevaluated by assessment pseudofirst order reactionrate constant and the results are summarized in Fig.17. As shown in Fig. 17, reaction rate constantincreases significantly after UV irradiation, and then

the UV power intensity has little influence on thenaphthalene degradation kinetics.

These results are quite consistent with otherresearch results. Ollis et al. [38] expressed that at lowUV light intensities, the reaction rate has a linear relation with the light intensity, at moderate light intensities beyond a certain value, the reaction rate dependson the square root of the light intensity and at the high

20

15

10

5

0 1601208040t, min

a

C, mg/L

bcdefg

Fig. 12. Effect of variation in air flow rate on the naphthalene degradation ( = 100 rpm; = 2 g/L; I = 16 W;

C0 = 20 mg/L): a0; b1; c2; d3; e4; f5; g6 L/h.

CTiO2

0

0.4

0.8

1.2

160120800 40t, min

a

ln(C/C0)

bc

e

Linear (a)Linear (b)Linear (c)Linear (d)Linear (e)

d

1.6

fg

Linear (f)Linear (g)

Fig. 13. Effect of air flow rate on naphthalene removalkinetics: a0, Kapp = 0.0027 min

1, R2 = 0.98; b1 L/h,

Kapp = 0.0046 min1, R2 = 0.98; c2 L/h, Kapp = 0.0053

min1, R2 = 0.98; d3 L/h, Kapp = 0.0055 min1, R2 =

0.99; e4 L/h, Kapp = 0.006 min1, R2 = 0.93; f5 L/h,

Kapp = 0.0071 min1, R2 = 0.95; g6 L/h, Kapp =

0.0088 min1, R2 = 0.95.

20

15

10

5

0 1601208040t, min

a

C, mg/L

bcdefg

Fig. 14. Effect of TiO2 loading ( = 100 rpm; Q = 4 L/h;I = 16 W; C0 = 20 mg/L): a0; b0.5; c1.0; d1.5;e2.0; f2.5; g3.0 g/L.

664



light intensities the reaction rate has no relation withthe light intensity.

CONCLUSIONS

In this research, photocatalytic decomposition ofnaphthalene by TiO2 nanoparticles and UV was evaluated in a batch slurry reactor. TiO2 nanoparticles wereanalyzed by XRD and SEM which results showed thepresence of anatase as catalyst crystalline phase andhigh porosity. The process kinetics was investigatedusing LangmuirHinshelwood mechanism that at lownaphthalene concentrations, the LH equation converted to a pseudofirst order kinetic equation. Thereaction rate constants were obtained equal to 0.0042,0.0046, 0.0051 and 0.0065 min1 for initial naphthalene concentrations of 20, 15, 10, and 5 mg/L, respectively. The pH of the aqueous solution decreased during the process due to the adsorption of the hydroxideions on the TiO2 surface and production of the unstable acidic species during the reaction.

Furthermore, the effect of operational parameterssuch as agitation speed, injected air flow, TiO2 catalystloading, and UV light intensity on the refining processwas investigated. The experimental results showed thatthe agitation speed varied from 50 to 200 rpm has apositive influence on the naphthalene degradation dueto the enhancement of mass transfer coefficient.Naphthalene degradation rate increased with the TiO2suspension concentration up to 2 g/L and in greateramounts, the TiO2 concentration had a negative effecton the reaction rate due to poor light penetration. Thepseudofirst order constant enhanced with increasingair flow due to the augmentation of dissolved oxygenloading into the solution. Also, in absence of UV irradiation to the solution, amount of the degraded naphthalene was very low. On the other hand, increase ofthe UV intensity from 8 to 24 W has low influence onthe amount of removed naphthalene.

ACKNOWLEDGMENTS

The authors wish to acknowledge the FerdowsiUniversity of Mashhad for funding this research workand also Dr.Z. Khashayarmanesh andDr.H. Moallemzadeh Haghighi from the pharmaceutical faculty for providing the laboratory facilities. Alsowe like to appreciate Mrs. N. Binesh for her cooperation in performing the experiments.

NOTATION

Aabsorbance, nm;

Cconcentration of component (naphthalene),mg/L;

20

15

10

5

0 1601208040t, min

a

C, mg/L

bcd

Fig. 16. Effect of light intensity ( = 100 rpm; Q = 4 L/h; = 2 g/L; C0 = 20 mg/L): a0; b8; c16; d24 W.CTiO2

Fig. 17. Effect of UV power intensity on naphthalene removalkinetics: a0, Kapp = 0.0001 min

1, R2 = 0.76; b8 W,

Kapp = 0.0053 min1, R2 = 0.99; c16 W, Kapp =

0.0057 min1, R2 = 0.98; d24 W, Kapp = 0.0066 min1,

R2 = 0.95.

0

0.2

0.8

1.0

160120800 40t, min

a

ln(C/C0)

bc

e

Linear (a)Linear (b)Linear (c)Linear (d)Linear (e)

d

1.2

fg

Linear (f)Linear (g)

0.4

0.8

Fig. 15. Effect of TiO2 loading on naphthalene removal

kinetics: a0, Kapp = 0.0009 min1, R2 = 0.98; b

0.5 g/L, Kapp = 0.003 min1, R2 = 0.97; c1.0 g/L, Kapp =

0.0039 min1, R2 = 0.99; d1.5 g/L, Kapp = 0.0051 min1,

R2 = 0.99; e2.0 g/L, Kapp = 0.0062 min1, R2 = 0.9; f

2.5 g/L, Kapp = 0.0055 min1, R2 = 0.91; g3.0 g/L,

Kapp = 0.0052 min1, R2 = 0.94.

0

0.2

0.8

1.0

160120800 40t, min

a

ln(C/C0)

bc

Linear (a)Linear (b)Linear (c)Linear (d)

d

1.2

0.4

0.8



concentration of TiO2 in the suspension,

g/L;dpnanoparticle diameter, m;DABdiffusion coefficient, m2/s;IUV light intensity, W;krreaction rate constant, mg/(L min);kCmass transfer coefficient, mg/(L min);kOoverall rate constant, mg/(L min);Kapppseudofirstorder reaction rate constant,

min1;Kadsorption equilibrium constant, L/mg;Qair flow rate, L/h;rreaction rate, mg/(L min);treaction time, min;Uvelocity of the fluid passing from the spherical

particle, m/s;wavelength, nm;kinematic viscosity of water, kg/(m s);agitation speed, rpm;

ReReynolds number

ScSchmidt number

SUBSCRIPTS AND SUPERSCRIPTS

0initial value;Iintermediate species;Wwater.

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CTiO2

ReUdp

= ;

Sc = DAB

.

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