Statistical modeling of photocatalytic degradation of synthetic amoxicillin wastewater (SAW) in an immobilized TiO2 photocatalytic reactor using response surface methodology (RSM)

Journal of the Taiwan Institute of Chemical Engineers xxx (2014) xxx–xxx

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JTICE-822; No. of Pages 10

Statistical modeling of photocatalytic degradation of syntheticamoxicillin wastewater (SAW) in an immobilized TiO2 photocatalyticreactor using response surface methodology (RSM)

Z.M. Shaykhi, A.A.L. Zinatizadeh *

Water and Wastewater Research Center (WWRC), Department of Applied Chemistry, Faculty of Chemistry, Razi University, Kermanshah, Iran

A R T I C L E I N F O

Article history:

Received 28 July 2013

Received in revised form 20 December 2013

Accepted 29 December 2013

Available online xxx

Keywords:

Amoxicillin degradation

Immobilized photocatalyst reactor

Nano titanium dioxide

A B S T R A C T

The present research deals with degradation of amoxicillin (AMX) in an immobilized TiO2 photocatalytic

reactor under UVA irradiation (365 nm). Enhancement of photocatalysis by sequence aeration and

addition of ozone and H2O2 was also evaluated. Relationship between three numerical independent

variables (chemical oxygen demand (CODin), reaction time and initial pH) and four process responses

(COD removal efficiency, specific COD removal rate (SRR), BOD5/COD ratio, and final pH) for the synthetic

amoxicillin wastewater (SAW) photocatalyst oxidation process were analyzed and modeled using

response surface methodology (RSM). The region of exploration for the process was taken as the area

enclosed by CODin concentration (400–2000 mg/L), initial pH (3–11) and reaction time (20–240 min)

boundaries. As a result, initial COD showed different impact at different pH on the COD removal

efficiency. The maximum BOD5/COD ratio found 0.43 at initial COD 2000 mg/L and pH of 3. The

maximum COD removal efficiency for the photocatalytic reaction alone was 20%, while the value could

be improved up to 38% by sequence aeration. The photocatalytic process induced by O3 and O3/H2O2

showed COD removal efficiencies of 53 and 58%, respectively. As a conclusion, the photocatalyst process

induced by O3 and O3/H2O2 could be an appropriate pretreatment method prior to a biological treatment

process.

� 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

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Journal of the Taiwan Institute of Chemical Engineers

jou r nal h o mep age: w ww.els evier . co m/lo c ate / j t i c e

1. Introduction

Pharmaceutical chemicals are widely used for therapeutic andagricultural purposes today. A significant body of work hasidentified trace amounts of antibiotics in natural aquatic systemsaround the world, increasingly relating their occurrence towastewaters and livestock operations. Issues such as acute andchronic effects of antibiotics on ecosystems, potential rise ofantibiotic-resistant bacteria, and increasing tolerance of antibio-tics by human and livestock are not well understood, and they areat the root of increasing public concern. Amoxicillin (AMX) andother b-lactams antibiotics are one of the most importantantibiotics used in the Iran and other countries such as US andEurope, for both humans and animals [1]. Antibiotics enter theenvironment from a variety of sources including discharges fromantibiotic industrial producer, domestic wastewater and etc. Thereare only few works dealing with the advanced oxidation processes

* Corresponding author. Tel.: +98 831 4274559; fax: +98 831 4274559.

E-mail addresses: [email protected], [email protected]

(A.A.L. Zinatizadeh).

Please cite this article in press as: Shaykhi ZM, Zinatizadeh AAL.amoxicillin wastewater (SAW) in an immobilized TiO2 photocatalyticChem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2013.12.024

1876-1070/$ – see front matter � 2014 Taiwan Institute of Chemical Engineers. Publis

http://dx.doi.org/10.1016/j.jtice.2013.12.024

(AOPs) application to the AMX degradation: Fenton oxidation [2–6], ozonation [7], activated carbon adsorption [8–10] andphotocatalytic oxidation (PCO) [11,12]. Use of other AOPs suchas sonolysis [13] and combination of photocatalysis and sonolysis(sonophotocatalysis) [14] have been also extensively studied forrecalcitrant compounds. The AOPs appear as interesting tools incomparison with other techniques such as activated carbonadsorption, reverse osmosis and etc. [15]. Indeed, many of thesetechniques only transfer the pollutants from one phase to anotherwithout destroying them. Biological treatment is also limited towastewaters which contain such compounds [16].

Among the so-called advanced oxidation processes, homoge-neous and heterogeneous photocatalytic oxidation have shown,recently, great promise in treatment of industrial wastewaters[17]. Among the different photocatalytic processes, TiO2 photo-catalysis has emerged as a promising wastewater treatmenttechnology. The main advantages of the process are lack of masstransfer limitations, operation at ambient condition, inexpensivecatalyst, commercially available, non-toxic and photochemicallystable [18,19].

Because of the practical drawbacks in the use of suspendedprocesses in industrial scales including limited penetration of the

Statistical modeling of photocatalytic degradation of synthetic reactor using response surface methodology (RSM). J Taiwan Inst

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Table 1Experimental conditions for photocatalytic process.

Run. No Factor 1

A: CODin concentration

(mg/L)

Factor 2

B: Reaction time

(min)

Factor 3

C: Initial pH

1 400 30 3

2 1200 135 11

3 2000 240 11

4 2000 135 7

5 2000 30 11

6 400 135 7

7 2000 240 3

8 400 240 11

9 1200 135 7

10 1200 135 7

11 1200 135 7

12 1200 30 7

13 1200 135 7

14 1200 240 7

15 1200 135 7

16 1200 135 3

17 400 240 3

18 1200 135 7

19 2000 30 3

20 400 30 11

Z.M. Shaykhi, A.A.L. Zinatizadeh / Journal of the Taiwan Institute of Chemical Engineers xxx (2014) xxx–xxx2

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radiation in the suspension, fouling of UV source due to thedeposition of catalyst particles and separation of the fine solidparticles from liquid, immobilization of the catalyst on an inertsupport would be of vital importance as a practical solution.However, the over-dosage required to achieve removal efficiencycomparable to that of powder TiO2 is the main disadvantage.Therefore, development of an immobilized photocatalytic reactorfor removing AMX with a different physical design to achieve themaximum productivity in the photocatalytic activity for reducingthe over-dosage required in the attached systems was the mainmotivation and novelty of the present research. There are fewreported studies on degradation of antibiotics by TiO2 photo-catalyst [12,20]. However, no study on degradation of amoxicillinin aqueous solution by immobilized photocatalytic reactor hasbeen reported up to this date. Moreover, the published works onthe AOPs of antibiotics mainly focus on the compound removal atvery low concentrations as post treatment step [21,22]

Recently, response surface methodology (RSM) has beenemployed to optimize and evaluate interactive effects of indepen-dent factors in numerous chemical and biochemical processes[23,24]. The RSM is a statistical technique for designing experi-ment, building models, evaluating the effects of several factors, andreducing number of experiments. The present study was under-taken to examine the degradation of amoxicillin in aqueoussolution by UV/TiO2 photocatalysis with three independentvariables (initial chemical oxygen demand (CODin), reaction time,initial pH) using RSM. Enhancement of the photocatalysis by O3

and O3/H2O2 addition and reducing the catalyst poisoning bysequence aeration was also investigated.

2. Materials and methods

2.1. Wastewater preparation

The synthetic antibiotic wastewater (SAW) was prepared bydissolving two capsules of amoxicillin (AMX 500 mg) in one liter oftap water. The stock SAW was prepared with CODin of about2000 mg/L. Other solutions were prepared by dilution the stocksolution. Furthermore, the actual COD values have been verifiedeach time before initiation of experimental work. pH and BOD5/COD ratio were about 7.5 � 0.1 and 0.18–0.2, respectively.

2.2. Experimental set-up

A photocatalytic process using immobilized TiO2 was examinedin the treatment of the SAW wastewater. Fig. 1a represents animage of the experimental setup used in this study. 21 tubes with20 cm in height were used in this study. In the experiments withphotocatalyst, nano titanium dioxide with anatase structure wascoated on the body and inner wall of the quartz tubes with 3 and5 mm inner and outer diameter, respectively. The configuration ofthe tubes made was in the form of cylindrical that placed at thesurrounding of the UV lamp and was positioned in center of thevessel (Fig. 1a and b). An air pump (Q = 0.075 m3/min) was used forair supply in the cylindrical vessel. The source of UV irradiation wasa UV lamp (HITACHI, emission: 365 nm, constant intensity 60 mW/cm2) that protected by a quartz jacket, and positioned in the centerof the reactor. The lighted length of the lamp was 452 mm with aquartz sleeve diameter of 3 cm. Nano TiO2 loaded in the systemwas measured 0.279 g catalyst for the 21 quartz tubes (0.558 g/L).

2.3. Photocatalyst reactor (PCR) operation

In a typical photocatalytic run, 500 mL of the aqueous solutioncontaining the desired concentration of AMX (400, 1200 and2000 mg/L as COD) was loaded in the photocatalyst reactor (PCR).


After that, the UV lamp was turned on, while air was continuouslysparged in the reaction mixture. Most experiments wereperformed at inherent solution pH which was left uncontrolledduring the reaction; the inherent solution pH was around neutralpH (7.5 � 0.1). For those runs where the initial pH had to be adjusted,this was done by adding the appropriate amount of 1 M NaOH or 1 MHCl solutions, as necessary.

In order to evaluate the synergistic effects of O3 and H2O2 withthe photocalatytic process, the photocatalytic-ozonation (O3/UV/TiO2) and photocatalytic-perozonation (O3/H2O2/UV/TiO2) atalkaline pH (11) were also examined for treatment of the SAW.The flow diagram of photocatalyst ozonation set up is shown inFig. 1b. The air flow rate was adjusted at 5 L/min. The ozonecontent of the input air stream was measured as 0.27 g O3/h. Theozone content of offgas was also measured and the consumedozone was obtained (2.3 g ozoneconsumed/gCODremoved).

The photocatalytic reaction at the optimum condition wasrepeated with regular sequence regeneration using aeration (every30-min reaction). This experiment was carried out using rawsamples of SAW with COD content of 400 mg/L and pH 11. Afterevery 30 min, the photocatalyst was regenerated with aeration anddistilled water.

2.4. Experimental design and mathematical modeling

The statistical method of factorial design of experiments (DOE)eliminates systematic errors with an estimate of the experimentalerror and minimizes the number of experiments [25–27]. Effects ofthree independent numerical factors, initial COD concentration,reaction time and initial pH on the PCR performance wereinvestigated. The response surface methodology (RSM) used wasa central composite face-centered design (CCFD) involving thethree different factors. Initial COD concentration varied from 400to 2000 mg/L at 3 levels (400, 1200 and 2000 mg/L), reaction timefrom 30 to 240 min at 3 levels (30, 135 and 240 min), and initial pHat 3 levels (3, 7, and 11). The need for study of the effects of acidicand basic conditions on the process performance was the mainreason for the relatively wide range of initial pH selected. Similarrange has been studied in other studies [11].

The photocatalytic process was assessed based on the full CCFDexperimental plan. The design consisted of 2k factorial pointsaugmented by 2k axial points and a center point where k is thenumber of variables. Accordingly, 20 experiments were conducted



Fig. 1. Laboratory-scale experimental set-up (a) photocatalytic reactor (PCR) and (b) schematic diagram of the experimental set-up; photocatalytic ozonation system.

Z.M. Shaykhi, A.A.L. Zinatizadeh / Journal of the Taiwan Institute of Chemical Engineers xxx (2014) xxx–xxx 3

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with 16 experiments organized in a factorial design (including 9factorial points, 6 axial points and 1 center point) and theremaining 4 involving the replication of the central point to getgood estimate of the experimental error. Repetition experimentswere carried out after other experiments followed by order of runsdesigned by DOE as shown in Table 1. Table 1 shows theexperimental conditions for the photocatalytic process. CODremoval, specific COD removal rate (SRR), BOD5/COD ratio andfinal pH were dependent responses. Data analysis was carried outusing response surface methodology (RSM). The results werecompletely analyzed using analysis of variance (ANOVA) whichwas performed by Design Expert Software (version 6.0, State-Ease,Inc., Minneapolis, MN).

After conducting the experiments, the coefficients of thepolynomial model were calculated using the following equation,Khuri and Cornell [25]:

Y ¼ b0 þ biXi þ b jX j þ biiX2i þ b j jX

2j þ bi jXiX j þ . . . (1)


where i and j are the linear and quadratic coefficients, respectively,and b is the regression coefficient. Model terms were selected orrejected based on the P value with 95% confidence level. The resultswere completely analyzed using analysis of variance (ANOVA) byDesign Expert software. Three-dimensional plots were obtainedbased on the effect of the levels of the two factors. From thesethree-dimensional plots, the simultaneous interaction of the twofactors on the responses was studied.

2.5. Catalyst coating procedure

Quartz tubes washed by water and detergent and rinsed withde-ionized (DI) water. They were further cleaned with extra pureacetone (ScharlauChemie S.A., 99.5% purity), and subsequentlywere rinsed with DI water and dried. They were then placedin electric furnace at 400 8C for 30 min in order to removeresidual organic contaminates and enhance wettability. A perox-otitanium complex solution was prepared by mixing titanium tetra



Fig. 2. (a) AFM and (b) SEM image of nano TiO2 coated respectively.


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isopropoxide (TTIP) (Merck, purity g 99.5%), H2O2 (Merck, 30%) andH2O, with volume proportions of 12:90:200, respectively. SolutionpH was then raised to 7.0. The resulting solution was refluxed at90 8C for 10 h to obtain crystalline anatase sol. Gel films were


formed on the substrates from the 1 wt% TiO2 sol by dip coatingwith withdrawal speed of 9.2 mm/s. For all the samples, a pre-coatof the peroxotitanium complex solution (the sol before reflux) wasapplied to enhance the adhesion. Subsequent layers of crystalline




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TiO2 were deposited by dip coating 4 times. The samples weredried after each dip at 100 8C for 15 min. Finally, the samples wereannealed in the range between 100 and 500 8C for 1 h in air usingan electric furnace (Azar Furnaces M2L 1200) [28]. The thickness ofTiO2 films was estimated to be 330 � 5 nm.

2.6. Characterization of TiO2 coated on quartz tubes

The thin films of TiO2 coated on quartz tubes were character-ized by scanning electron microscope (SEM) and atomic forcemicroscope (AFM) to evaluate the surface morphology and theeffectiveness of the coating technique. Fig. 2a–b represents theAFM and SEM images of the nano TiO2 coated on the quartz tubes,respectively.

2.6.1. Atomic force microscope (AFM)

A surface morphology of coated photocatalyst thin films wasvisualized using an atomic force microscope (Mobile S, Nanosurf,Switzerland). Explorer atomic force microscopy was in thenoncontact mode, using high resonant frequency (F0 170 kHz)15 pyramidal cantilevers with silicon probes having dynamic force.As can be seen in Fig. 2a, AFM image indicated that the film surfaceis smooth and uniform, and thickness of the film was estimated tobe around 330 nm. With an increase in the number of multilayer,the roughness of some parts of the surface increased because offarther deposition of TiO2 in these sites, which is due to theincrease in the amount of TiO2 deposited per each layer.

2.6.2. Scanning electron microscopy (SEM)

A surface morphology of titania thin films was studied byscanning electron microscopy using a Philips XL30 microscope atan accelerating voltage of 10 kV. After oven-drying of the thin filmfor 12 h, the sample was coated with a platinum layer using anSCDOOS sputter coater (BAL-TEC, Sweden) in an argon atmosphere.Subsequently, the sample was scanned and 12 photomicrographswere obtained.

Fig. 2b shows the top view surface morphology of coatedphotocatalyst thin films that were examined by scanning electronmicroscope. A close view surface of the film shows a flat and densesurface morphology of distributed TiO2 nanoparticles embedded inthe film (Fig. 2b). The film shows porous structure between thenanoparticles to permit free diffusion of pollutant in and out of thefilm, and also these porous structures can effectively trap light intothe inner layers.

2.7. Analytical methods

Antibiotic concentration was determined by a High Perfor-mance Liquid Chromatograph (HPLC) equipped with a micro-vacuum degasser, quaternary pump, diode array and multiplewavelength detector at wavelength 254 nm. The column wasECLIPSE XDD-C18 (4.6 mm � 150 mm, 5 mm) and its temperaturewas 60 8C. The mobile phase was 98% of KH2PO4 (0.05 M) as buffersolution in ultrapure water and 2% acetonitrile at a flow rate of1 mL/min. All the chemicals used in the analysis were analyticalgrade (Merck, Darmstadt, Germany). COD was measured accordingto the Standard Methods [29]. A colorimetric method with closedreflux method was developed. Spectrophotometer (DR 5000, Hach,Jenway, USA) at 600 nm was used to measure the absorbance ofCOD samples. A pH meter (JENWAY 3510) was used for pHmeasurement. Biodegradability was measured by 5-day biochem-ical oxygen demand (BOD5) test in a BOD meter (OxiTop IS 6)according to the Standard Methods [29].

In the runs with H2O2, MnO2 powder was used for eliminationof the interference of residual H2O2 in COD test. Then, the samplewas centrifuged to remove MnO2 powders; the supernatant was


used for COD test [30].

H2O2 �!MnO2

H2O þ O2 (2)

The ozone dosage was determined by an iodometery methodusing a washing bottle containing 2 wt% KI solution [31].

All the experiments were carried out with the same TiO2

immobilized tubes set and each experimental condition wasrepeated three times and the data reported are average of threemeasurements. The differences between the measurements foreach were less than 1%.

3. Results and discussion

3.1. Process performance

3.1.1. Statistical analysis

As various responses were investigated in this study, differentdegree polynomial models were used for data fitting (Table 2). Theregression equations obtained are presented in Table 2. In order toquantify the curvature effects, the data from the experimentalresults were fitted to higher degree polynomial equations, i.e.

quadratic and reduced cubic model. The ANOVA results for allresponses have been summarized in Table 2. The model terms inthe equations are after elimination of insignificant variables andtheir interactions. Based on the statistical analysis, the modelswere highly significant with very low probability values (<0.0001).It was shown that the model terms of independent variables weresignificant at the 99% confidence level. The square of correlationcoefficient for each response was computed as the coefficient ofdetermination (R2). It showed high significant regression at 95%confidence level. The R2 values obtained indicate an adequateagreement between real data and the ones obtained from themodels. The models adequacy was tested through lack-of-fit F-tests [32]. The lack of fit F-statistic was not statistically significantas the P-values were greater than 0.05.

3.1.2. COD removal

In this process, COD was measured as a response representingthe organic content of the SAW. The effect of the variables on CODremoval efficiency is shown as three dimensional plots in Fig. 3a–cat three levels of initial pH. Relationship between the response andthe variables is described by the following equation. The equationis based on the coded values.

COD removal ¼ 10:07 � 1:2A þ 1:94C � 7:25AC � 5:06A2

� 3:24C2 (3)

where A is CODin and C is initial pH. From Eq. (3), B (reaction time)did not show any effect on the response in the design spacestudied. The results obtained from HPLC analysis from two reactiontimes (30 and 240 min) at the same condition (CODin 2000 mg/Land initial pH 11) also showed no significant change after 240 minrelative to the results at 30 min, implying that the AMXphotocatalytic degradation is an instantaneous reaction. It canbe seen from the Fig. 3a, at initial pH 3, the response increasedupon increasing the CODin up to about 1200 mg/L and it remainedalmost constant by further increment of the CODin. In thiscondition, the maximum efficiency was obtained about 14% inthe highest value of CODin (2000 mg/L). As mentioned earlier, thereaction time was not effective. It is attributed to poisoning of thecatalyst surface caused by AMX adsorption on the photocatalyst[14]. Elmolla and Chaudhuri studied the removal of amoxicillin,ampicillin and cloxacillin (CODin = 520 mg/L) using TiO2 photo-catalyst (suspended system) and no significant degradationoccurred as the time progressed to 300 min. COD removal



Table 2ANOVA for response surface models applied.

Response Model type ANOVA

Source Sum of squares Degrees of freedom (DF) Mean square F-value Prob > F

COD removal (%) Reduced two factor

interaction model

Model 554.75 5 110.95 46.67 <0.0001

A 14.4 1 14.4 6.06 0.0274

C 37.64 1 37.64 15.83 0.0014

AC 420.5 1 420.5 176.9 <0.0001

A2 82.01 1 82.01 34.5 <0.0001

C2 33.54 1 33.54 14.11 0.0021

Residual 33.28 14 2.38 – –

(R2 = 0.9434, Adj. R2 = 0.9232, Adeq. Precision = 21.765, Std. Dev. = 1.54, C.V.% = 16.83, PRESS = 52.28)

SRR (mg COD

removal/g cat.h)

Reduced cubic model Model 644996.7 8 80624.6 36.00 <0.0001

A 62206.3 1 62206.3 27.78 0.0003

B 202769.4 1 202769.4 90.54 <0.0001

C 15488.9 1 15488.9 6.91 0.0234

AB 69226.6 1 69226.6 30.91 0.0002

AC 127246.1 1 12724.1 56.81 <0.0001

BC 30472.2 1 30472.2 13.61 0.0036

B2 27749.8 1 27749.8 12.40 0.0048

ABC 109837.1 1 109837.1 49.04 <0.0001

Residual 246635.4 11 2239.6 – –

(R2 = 0.9632, Adj. R2 = 0.9364, Adeq. Precision = 26.157, Std. Dev. = 47.32, C.V.% = 35.76, PRESS = 375084)

BOD5/COD Reduce quadratic model Model 0.046 3 0.015 24.68 <0.0001

A 0.022 1 0.022 35.53 <0.0001

AB 0.003 1 0.003 4.52 0.0493

A2 0.021 1 0.021 33.98 <0.0001

Residual 0.001 16 0.001 – –

(R2 = 0.8223, Adj. R2 = 0.7890, Adeq. Precision = 11.793, Std. Dev. = 0.025, C.V.% = 7.98, PRESS = 0.02)

Final pH Reduced quadratic model Model 91.54 2 19.64 1057.19 <0.0001

C 90.06 1 90.06 2080.08 <0.0001

C2 1.01.48 1 1.48 34.30 <0.0001

Residual 0.6 14 0.04 – –

(R2 = 0.9920, Adj. R2 = 0.9910, Adeq. Precision = 74.48, Std. Dev. = 0.21, C.V. % = 2.69, PRESS = 1.15)


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efficiency obtained 11.7, 9.2, 10.2 and 11.2% at initial pH of 3, 5, 8and 11, respectively [11].

At pH 7 (Fig. 3b), CODin had an inverse effect on the response.By increasing in CODin from 400 to 1200 mg/L, the response wasincreased while further increase in the variable up to 2000 mg/Lcaused a decrease in COD removal efficiency where the adsorptionwas probably predominant mechanism due to the increase inCODin and limited repulsion between AMX and photocatalyst atneutral pH [11,14]. The minimum and maximum response wasobtained to be 2 and 11%, respectively.

Fig. 3c demonstrates the response variation as a function of thevariables (CODin and reaction time) at alkaline condition (pH 11).As represented in the Fig., by increasing CODin from 400 to2000 mg/L, COD removal was decreased from 22 to 2%. It might bedue to the higher OH�/CODin ratio at low CODin (400 mg/L) relativeto the ratio at high CODin (2000 mg/L). This result is inverse to thatobtained at pH 3, indicating an interactive effect of initial pH-CODin

on the response. However, at the alkaline pH the indirect oxidationusing OH� was the dominant mechanism [33]. The effect of initialpH is justified by the mechanism presented in the followingreactions [34].

TiO2ðhnÞ ! hþ þ e� (4)

TiO2ðe�Þ þ O2 ! TiO2þ O2�� (5)

O2�� þ H2O ! HO2

� þ HO� (6)

2HO2� ! H2O2þ O2 (7)

As a result, two possible mechanisms can justify the interactiveeffects of initial pH-initial COD on the response: (1) AMXadsorption on the photocatalyst and (2) the hydroxylation reactiondue to AMX-OH� interaction in the reactor bulk [11,13,14,19].Initial pH showed significant effect only at minimum and


maximum levels of CODin. While, for CODin of 1200 mg/L, almostsimilar efficiency was obtained at different initial pHs. In otherword, maximum efficiency for high level of CODin was found atacidic condition while it was at alkaline condition for the low levelof CODin, implying different mechanism at different pH. At acidicpH, both TiO2 and amoxicillin are positively charged and hence, theadsorption on the surface of TiO2 is limited [35]. The highdegradation of antibiotics at acidic pH compared to that at neutralpH may be due to the hydrolysis of antibiotics. High degradation ofantibiotics in alkaline condition may be due to two facts. First is theenhancement of hydroxyl radical formation at high pH due to theavailability of hydroxyl ions on TiO2 surface that can easily beoxidized to form hydroxyl radical. Second is the hydrolysis of theantibiotics due to instability of the b-lactam ring at high pH [11].

3.1.3. Specific COD removal rate (SRR)

In order to assess the overall performance of the system,specific COD removal rate (SRR) was calculated as anotherresponse. A reduced cubic model described the variation of thespecific removal rate for COD as a function of the variables (CODin,reaction time and initial pH) in the system (Table 2). Multipleregression coefficients of the model are summarized in Table 2.The significance of each coefficient was determined by F-value andP-value. From Table 2, A, B, C, AB, AC, BC, B2 and ABC are selected asthe effective terms with confident level less than 0.05. Other modelterms are not significant. The following equation describes therelationship between the response and the variables as coded form.

SRR; mg CODrem=gcat:h ¼ 95:09 þ 78:87A � 142:4B

� 39:4C � 93:02AB � 126:12AC

þ 61:7BC þ 74:49B2

þ 117:17ABC (8)



Fig. 3. Three-dimensional graphs for COD removal as a function of CODin and reaction time at different initial pH; (a) 3, (b) 7 and (c) 11.


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As observed in Eq. (8), effect of A, second-order B, interactioneffects of B and C, interaction of ABC showed an increasing impacton the response while the terms B, C, AB and AC effects of thevariables causes a decrease trend in the response. Fig. 4a and brepresents the response surface plots for the SRR as a function ofCODin concentration and reaction times at different initial pHs. Inoverall, the reaction time had a decreasing impact on the SRR,indicating surface poisoning of the catalyst by AMX/intermediatesadsorbed. As can be seen in the Fig., two maximum SRR (860 and290 mg CODremoved/gcat.h) was found at the maximum andminimum CODin, respectively for pH 3 and 11 at reaction time30 min. In comparison with the literature results, about 12 mgCODremoved/gcat.h was reported as maximum SRR obtained in asuspended TiO2-photocatalytic process treating a mixture ofantibiotics (amoxicillin, ampicillin, and cloxacillin) with CODin

of 520 mg/L [11], indicating more photocatalyst productivity in thepresent work compared to that reported in the literature.

3.1.4. BOD5 to COD ratio

AMX as non biodegradable soluble COD (nbsCOD) is brokendown and may be converted to the biodegradable soluble COD


(bsCOD). So, BOD5/COD ratio was calculated to indicate biode-gradable fraction of total COD after the oxidation process asanother response. In the line with this, a reduced quadratic modelwas selected to describe the variation of the BOD5/COD ratio as afunction of the variables (CODin and pH). The relationship isdescribed by the following equation. The equation is based on thecoded values.

BOD5=COD ¼ 0:28 þ 0:047A � 0:01875AB þ 0:065A2 (9)

Initial pH had no effect on the response. Fig. 5a represents thechanges in the BOD5/COD ratio as a function of the CODin andreaction time at neutral pH. As can be seen in the Fig., maximumBOD5/COD ratio was modeled about 0.42 at the conditions withCODin 2000 mg/L and reaction time 30 min. Minimum ratio ofBOD5/COD determined 0.26 for the CODin about 800 mg/L at anyreaction time in the range studied. Fig. 5b shows the interactiveeffect of CODin-reaction time on BOD5/COD. As is seen in Fig. 5 byincreasing reaction time, the BOD5/COD was increased while CODremoval did not change as time progressed. It might be due togradual degradation of AMX during the reaction. Whereas, in a



Fig. 4. Three-dimensional graphs for SRR as a function of CODin and reaction time at (a) pH 3 and (b) 11, respectively.

Fig. 5. (a) BOD5/COD as function of CODin and reaction time and (b) interactive effect of CODin–reaction time on BOD5/COD at neutral pH.

Fig. 6. Three-dimensional graph for final pH.


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research work treating a mixture of antibiotics (amoxicillin,ampicillin and cloxacillin) with CODin of 520 mg/L at pH 5 nosignificant improvement in BOD5/COD ratio (0.05–0.1) wasoccurred [11]. In other research works, treating antibioticscontaining AMX, BOD5/COD has been obtained 0.37, �0.4 and0–0.4 by using Fenton process with CODin 520 [5], photo-Fentonprocess with CODin 790 [36] and photo-Fenton process with CODin

520 mg/L [2], respectively.

3.1.5. Final pH

In order to evaluate the variation in pH in the effect of thephotocatalytic reaction, pH after reaction was measured as aresponse. A reduced quadratic model described the variation of thefinal pH as a function of the variables (CODin, reaction time andinitial pH). The following regression equation (built with codifiedfactors) was obtained for the variation of final pH.

Effluent pH ¼ 8 þ 3:001C � 0:545C2 (10)

Please cite this article in press as: Shaykhi ZM, Zinatizadeh AAL. Statistical modeling of photocatalytic degradation of syntheticamoxicillin wastewater (SAW) in an immobilized TiO2 photocatalytic reactor using response surface methodology (RSM). J Taiwan InstChem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2013.12.024


200

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300

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360

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400

0 10 20 30 40 50 60 70 80 90 100 110 120 130

CO

D c

once

ntra

tion

(mg/

L)

Time (min)

First 30 minSecond 30 min

Third 30 min

Fourth 30 min

Fig. 8. Performance of photocatalytic process with regular sequence regeneration

by aeration.


G Model


Fig. 6 depicts the changes in the final pH as a function of CODin

and initial pH. From Fig. 6, a small increase in the response wasobserved at the low level of initial pH which is justified by themechanism presented in reactions given earlier (reactions 4–7).Conversely, the solution pH was slightly decreased at the highinitial pH (11). This result was from another mechanism which isdescribed by the following reactions (reactions 11 and 12) [34].

TiO2ðhþÞ þ H2Oads ! TiO2þ HO�adsþ Hþ (11)

TiO2ðhþÞ þ HO� ! TiO2þ HO�ads (12)

3.2. Photocatalyst process induced by O3 and O3/H2O2

Photocatalytic ozonation is a new advanced ozonation processallowing high removal of COD by combining the beneficial effectsof ozonation (direct ozone reactions) with the generation ofhydroxyl radicals via electron–hole formation (free radicaloxidation) [37]. In the line with this and in order to evaluatethe synergistic effects of O3 and H2O2 with the photocalatyticprocess, the photocatalytic ozonation (O3/UV/TiO2) and photo-catalytic perozonation (O3/H2O2/UV/TiO2) at the alkaline pH (11)were also examined treating the SAW. COD removal efficiencieswere achieved 20, 53 and 58%, respectively for the O2/UV/TiO2, O3/UV/TiO2 and O3/H2O2/UV/TiO2 processes (Fig. 7). In a researchwork carried out by Beltran et al. [38], 93% TOC removal has beenreported for sulphametoxazol (SMX) with an initial concentrationof 30 mg/L and 1.5 g/L TiO2 photocatalyst in the form ofsuspension. BOD5/COD ratio was also determined for the processesand obtained 0.44, 0.48 and 0.42, respectively for O2/UV/TiO2, O3/UV/TiO2 and O3/H2O2/UV/TiO2 processes.

3.3. Photocatalytic process with regular sequence regeneration

Less effect of reaction time on the response was convinced bypoisoning of the catalyst surface. Therefore, in order to prove theclaim, the photocatalytic reaction was repeated with regularsequence regeneration using aeration every 30-min reaction. Thisexperiment was carried out using a raw sample of SAW with CODcontent 400 mg/L and pH 11. Fig. 8 shows the performance of thephotocatalytic process with regular sequence regeneration byaeration. As can be seen in Fig. 8, COD removal efficiency wasobtained 38% in the condition when the catalyst was regeneratedby sequence aeration after reaction time 120 min against 19%

0

10

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0

CO

D r

emov

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%)

Time (min)

O3/ UV/TiO2, pH=11 , CODin = 500 mg/L .

O3/H2O2 /UV/TiO2 , pH= 11, COD in= 500 mg/L

O2/UV/TiO2 , pH=11 , CO D=400 mg/ L

30025020015010050

Fig. 7. Performance of the combined processes (O2/UV/TiO2, O3/UV/TiO2 and O3/

H2O2/UV/TiO2).


obtained for the same condition without aeration regeneration. Asa result, the procedure could improve the process performance byabout 19% increase in COD removal efficiency.

4. Conclusion

This research work showed that the advanced oxidationprocesses could be applicable for treatment of AMX wastewaters.However, it needs more studies in details to introduce the bestpractice. The main aim in use of advanced oxidation processes fortreatment of refractory wastewaters can be COD mineralizationand/or COD to BOD conversion which could be well achieved in thisstudy. It is noted that the photocatalyst process was found timeindependent due to the catalyst poisoning. Sequence regenerationusing aeration (every 30 min once) as a method for de-poisoningcould improve the process performance by about 19% increase inCOD removal efficiency. Combination of O3 and O3/H2O2 processeswith photocatalyst process was also showed as a good method toreactivate the photocatalyst. As a result, combination of variousadvanced oxidation processes with photocatalyst process (i.e.

O3 + PCR, O3/H2O2 + PCR) could be an appropriate pretreatmentmethod prior to a biological treatment process. However, it is alsosuggested that other possible solution such as (O3/H2O2 + PCR)system intermittently regenerated by O3 or O3/H2O2, ultrasound/O3/H2O2, and etc. to be examined.

Acknowledgements

The authors would like to acknowledge Kermanshah Depart-ment of Environment for providing analytical equipment tomeasure. The authors would also like to acknowledge thecooperation of Mr. S. Ghanbari for his fantastic and unique jobin the fabrication of the immobilized photocatalytic reactor set up.Special thanks also goes to Dr. R. Heydari for his assistance in HPLCanalysis.

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Statistical modeling of photocatalytic degradation of synthetic amoxicillin wastewater (SAW) in an immobilized TiO2 photocatalytic reactor using response surface methodology (RSM)