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Solid State Sciences 12 (2010) 578–586

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Solid State Sciences

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Pristine and supported ZnO-based catalysts for phenazopyridinedegradation with direct solar light

Hikmat S. Hilal a,*, Ghazi Y.M. Al-Nour a, Ahed Zyoud a, Muath H. Helal b, Iyad Saadeddin a

a College of Sciences, An-Najah N. University, PO Box 7, Nablus, West Bank, Palestineb College of Pharmaceutical Sciences, Universiti Sains Malaysia, 11800 Pulau Penang, Penang, Malaysia

a r t i c l e i n f o

Article history:Received 25 November 2009Received in revised form8 December 2009Accepted 5 January 2010Available online 18 January 2010

Keywords:PhotodegradationDirect solar lightPhenazopyridineZnOActivated carbon

* Corresponding author. Tel.: þ970 599 273460; faE-mail address: hikmathilal@yahoo.com (H.S. Hila

1293-2558/$ – see front matter � 2010 Elsevier Massdoi:10.1016/j.solidstatesciences.2010.01.008

a b s t r a c t

In search for safe techniques to manage waste pharmaceutical compounds drained in water, solar-drivendegradation of phenazopyridine (a model drug) was investigated in aqueous media using different ZnO-based catalyst systems. Naked ZnO, CdS-sensitized ZnO (ZnO/CdS) and activated carbon-supported ZnO(AC/ZnO) have been studied. Both naked ZnO and AC/ZnO were highly efficient in mineralizing phena-zopyridine, reaching complete removal in w50 min, with AC/ZnO having the higher edge. The ZnO/CdSsystem showed lower efficiency, due to screening of light by CdS. Moreover, the tendency of CdS to leachout Cd2þ ions discouraged the use of CdS as sensitizer in this work. In both ZnO and AC/ZnO systems, thephoto-degradation reaction was induced by the UV tail of the solar light. The visible region, withwavelength longer than 400 nm, failed to induce photo-degradation. The reaction was faster with highercatalyst loading, until a maximum efficiency was reached at a certain concentration. The rate of reactionincreased with higher drug concentrations up to a certain limit. The effect of pH value was studied, andthe catalysts showed highest efficiencies at pH close to 7. Stability of ZnO to degradation was studied.Both catalyst systems showed lowered efficiencies on recovery and reuse. The results suggest thatcomplete mineralization of waste drugs, commonly dumped in sewage water, with direct solar light isa potentially feasible strategy using the AC/ZnO catalyst.

� 2010 Elsevier Masson SAS. All rights reserved.

1. Introduction

Waste pharmaceutical disposal practices involve a number ofconventional techniques, such as sewer and incineration [1].Despite the widespread utilization of these techniques, they do nothelp remove drugs from contaminated waters. Waste drugs aredisposed into water through sewer or direct disposal. Such prac-tices could be hazardous and cause serious water contamination[2]. Therefore, two strategies need to be adopted: prevention ofdisposing waste drugs into sewage system before suitable treat-ment, and purification of contaminated waters from contaminantdrugs. New techniques are thus needed to completely mineralizedrugs disposed in water at large scale. Photo-degradation by solarlight could be the ideal technique to use [3,4], like other organiccontaminants.

Large band gap semiconducting particles are being investigatedas catalysts for photo-degradation of drugs and organic contami-nants in water. TiO2 is the most widely studied system for such

x: þ970 9 2387982.l).

on SAS. All rights reserved.

purposes due to its high stability, low cost, high oxidizing powerand non-hazardous nature [5–15]. However, TiO2, with its wideband gap (3.0–3.2 eV) cannot effectively function under direct solarlight and demands UV radiation [12,16–18]. Different pharmaceu-ticals have been photodegraded using TiO2 in the presence of UVlight, H2O2 and/or O3 [19–22]. As the TiO2 process depends on UVradiations, H2O2 or O3, it would be too costly to function under largescale purification of water from pharmaceutical contaminants.

Efforts to sensitize TiO2 to visible solar light have been made,using different types of dyes [23]. Such a strategy may not be thebest choice, due to a number of reasons: Firstly, the added costs ofprocessing the sensitizers. Secondly, many sensitizers are envi-ronmentally hazardous, such as CdS which leaches Cd2þ ions.Thirdly, the sensitizers may screen oncoming UV radiation.

Using small band gap semiconductors for photo-degradationprocesses is not an alternative. Small band gap semiconductors areunstable [24] and do not survive photo-degradation conditions.Moreover, their oxidizing power is far less than that of TiO2. There isthus a need for a wide band gap semiconductor which effectivelyabsorbs in the UV region that arrives from the sun. Such a materialcombines the advantages of stability, safety, high oxidizing powerand efficient solar light utilization. ZnO is a wide band gap (3.2 eV)

Fig. 1. TGA thermographs for AC/ZnO a) pre-annealed sample, and b) non-annealedsample.

H.S. Hilal et al. / Solid State Sciences 12 (2010) 578–586 579

semiconductor. The high radiation absorptivity of ZnO [18] makes itan efficient degradation catalyst under direct solar light. ZnO maycatalyze photo-degradation reactions using the UV tail that reachesthe earth, which accounts for 5–8% of solar radiation on earthsurface. Moreover, ZnO is abundant, not hazardous and not costly.For these reasons, ZnO particles have been used to degradedifferent organic contaminants in water [16,18,25–38]. As ZnOparticles are difficult to recover from aqueous suspensions, theywere supported onto different solid supports, such as activatedcarbon (AC) [39–41].

Despite the advantages of ZnO system, it has not been widelyreported for drug contaminant removal. Naked ZnO and AC/ZnOcatalyst systems will be investigated in phenazopyridine degrada-tion in aqueous solutions with direct and simulated solar lightsunder different conditions. The ZnO system could be a suitablealternative to the widely used hazardous TiO2/CdS system [42].Special attention will be paid to stability, recovery and reuse ofnaked and AC-supported ZnO catalyst systems.

2. Experimental

2.1. Materials

Organic solvents were purchased from either Aldrich–SigmaCo. or Frutarom. Phenazopyridine C11H11N5.HCl (Molar mass249.7 g/mol), with structural formula shown below, isa commonly used drug, and is chosen here as a model drugcontaminant to degrade under direct solar light here.

The presence of the azo group makes phenazopyridinea soundly stable compound [16,36]. Phenazopyridine was kindlydonated by Birzeit-Palestine Pharmaceutical Company, Birzeit,Palestine. ZnO was purchased from Merck (Catalog no. 8849, withaverage particle size 230 nm). The surface area of ZnO wasmeasured using the method of acetic acid adsorption [10,43,44]and was 47.2 m2/g.

Activated carbon powder (Aldrich) was kindly donated byCollege of Engineering, ANU. The surface area was measured byacetic acid adsorption isotherms, according to literature [10,43–46]and was 850 m2/g.

The ZnO/CdS system was prepared by deposition of nano-particles as described earlier for TiO2/CdS system [42]. The AC/ZnOcatalyst system was prepared as follows: ZnO powder (10.0 g) wasmechanically stirred with AC (2.0 g) in aqueous solution for 40 minin the dark. The suspension phase was then syringed off, and theresulting ZnO/AC solid was filtered and dried for further use.Portions of the dried ZnO/AC were calcinated at 450 �C for furtheruse. TGA analysis, Fig. 1 showed that 10% of the annealed AC/ZnOwas carbon, whereas the non-annealed system showed slightlyhigher carbon content (w13%). The 3% difference is due to incom-plete loss of carbon by annealing the AC/ZnO at 450 �C. Completeloss of carbon occurred at higher temperatures in the range525–600 �C.

2.2. Equipment

Phenazopyridine concentration was spectrophotometricallymeasured on a Shimadzu UV-1601 spectrophotometer at wave-length 480 nm. An absorbance vs. concentration calibration curvewas used to analyze remaining phenazopyridine throughout thedegradation reaction.

To analyze dissolved Zn2þ ions, anodic stripping differentialpulse polarography (ADPP) was conducted using a droppingmercury electrode (MDE 150) on a PC controlled Polarograph (POL150). The analysis parameters were: initial potential�700 mV, finalpotential �400 mV, purging time 30 s, deposition time 20 s, scanrate 20 mV/s and pulse height 25 mV. TGA analysis for AC/ZnO waskindly conducted on a TA 2950HR V5-3 instrument in the labora-tories of ICMCB, University of Bordeaux.

2.3. Photo-catalytic experiments

Aqueous solutions of known drug concentrations were placed ina 300 ml thermostated beaker. The pH was controlled by addingdilute aqueous solutions of HCl or NaOH. Known amounts ofcatalyst were then suspended in the reaction mixture in the dark.The suspension was left in the dark for 30 min to reach adsorptionequilibrium. This was to calculate the amount of adsorbed drugonto the solid surface, prior to photo-degradation. The reactionmixture was then exposed to radiation (either direct solar light orsolar light simulator). Light intensity was measured with a Lutron-LX 102 light meter and calibrated with a Kipp & Zonen CM11 pyr-anometer (Wcm�2) [47,48].

The reaction progress was followed, by spectrophotometricallymeasuring the remaining phenazopyridine with time, starting themoment the mixture was exposed to light.

Control experiments were conducted using aqueous phenazo-pyridine solutions in the dark, or in the absence of catalyst.Adsorption of phenazopyridine on solid surface was also studied. Incontrol experiments, phenazopyridine concentrations did notchange after prolonged times in the dark, except when AC waspresent. ZnO/AC system exhibited relatively high drug adsorptiononto AC surface which accounted to about 10 ppm of drugconcentration per 0.02 g AC.

In case of naked ZnO and AC/ZnO, exposure of phenazopyridineto direct solar light caused solution decolorization. When a cut-offfilter (removing 400 nm or shorter wavelengths) was placedbetween solution mixture and light, no solution decolorizationoccurred. This indicated that the degradation reaction was inducedby the UV tail, not the visible range, available in the solar light.

H.S. Hilal et al. / Solid State Sciences 12 (2010) 578–586580

3. Results

The ZnO/CdS system showed lower efficiency than either nakedor AC-supported ZnO systems. This indicated little sensitizingeffect, if any, by CdS particles. Moreover, the CdS particles exhibiteda screening effect on the oncoming UV light. CdS is known todegrade under PEC conditions into the hazardous Cd2þ ions [42].Therefore, the use of CdS as sensitizer was avoided. ZnO particleswere efficient under direct solar light and did not need sensitizers.Thus, unless otherwise stated, only naked ZnO and AC/ZnO catalystsystems were used.

Fig. 3. Polarographic peaks for NO3- ion resulting at different time intervals (10 min

each) from photo-degradation of phenazopyridine. Reaction was conducted underdirect solar light with ZnO (0.2 g) using 300 mL of phenazopyridine (25 ppm) underneutral conditions.

3.1. Naked ZnO catalyst system

Complete mineralization of phenazopyridine under photo-catalytic experimental conditions, using naked ZnO, was confirmedby absence of any phenyl groups in the reaction mixture afterreaction completion. The absence of bands longer than 230 nmindicated complete removal of phenyl rings, as indicated by elec-tronic absorption spectral analysis, Fig. 2.

Moreover, polarographic analysis of the reaction mixture showedcontinued production of nitrate ion with time, Fig. 3, in parallel withcontinued disappearance of phenazopyridine. The appearance ofnitrate ion and disappearance of phenyl rings are evidence forcomplete mineralization of phenazopyridine. Complete mineraliza-tion of other contaminants, such as methyl orange, by similar pho-tocatalytic systems, is well documented [16,36,42,49].

The degradation reaction was conducted under differentconditions. Effects of pH, catalyst amount, drug concentration andtemperature on reaction rates were all studied.

3.1.1. Effect of pHThe effect of pH on reaction rate of phenazopyridine degrada-

tion under solar simulator radiation was investigated. The rate ofreaction was not much affected with the value of pH. At highbasicity, a slightly different behavior occurred as shown in Fig. 4.Turnover number values (T.N., moles drug reacted per mole cata-lyst) calculated after 30 min at different pH values 3.3, 6.4, 8 and11.4, were: 0.00597, 0.00797, 0.00978 and 0.00752, respectively.The T.N. values indicated the activity was higher when the condi-tions were close to neutral conditions.

Fig. 2. Disappearance of phenazopyridine (neutral 300 mL suspension, 25 ppm) withtime under photo-degradation experimental conditions, as observed from measuringelectronic absorption spectra. Reaction was conducted under direct solar light withZnO (0.2 g) using 300 mL of phenazopyridine (25 ppm) under neutral conditions.

3.1.2. Temperature effectThe temperature effect on rate of phenazopyridine degradation

was studied under solar simulator radiation over the temperaturerange 10–40 �C. There was no significant temperature effect oncatalyst efficiency with a slight increase in the rate at the highestused temperature, Fig. 5. The reaction exhibited only small activa-tion energy (w6.1 kJ/mol).

3.1.3. Effect of catalyst concentrationThe effect of ZnO catalyst concentration on phenazopyridine

degradation rate was investigated. The reaction rate increased withincreasing catalyst amount, Fig. 6. At higher concentrations the ratebecame independent of catalyst concentration. Turnover numbervalues calculated after 60 min using different nominal catalystamounts 0.09 g, 0.15 g, 0.2 g and 0.25 g were: 0.00125, 0.00135,0.000758 and 0.000613, respectively. The T.N. values indicate theuselessness of increasing the amount of ZnO over 0.15 g/100 mLsolution of phenazopyridine.

3.1.4. Effect of drug concentrationEffect of initial phenazopyridine concentration on initial reac-

tion rate was studied under direct solar light. The degradation ratewas higher with higher drug concentration as shown in Fig. 7. T.N.values calculated after 20 min using different phenazopyridine

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Fig. 4. Effect of pH value on rate of photo-degradation of phenazopyridine (100 mLsolution 12 ppm) under Xe solar simulator radiation (overall radiation intensity 70 000Lux, 0.01 W/cm2) using 0.15 g ZnO at room temperature. Solution pH values were:a) 4.9 b) 6 c) 8 d) 11.6.

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Fig. 5. Effect of temperature on rate of photo-degradation of phenazopyridine (neutral100 mL solution, 10 ppm) using 0.05 g ZnO at: a) 10 �C b) 20 �C c) 30 �C, and d) 40 �C.All reactions were conducted in a carefully thermostated beaker under solar simulatorradiation (overall radiation intensity 0.01 W/cm2).

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Fig. 7. Effect of drug concentration on photo-degradation reaction using 0.1 g ZnOsuspended in 250 mL neutral solutions at room temperature under direct solar light(0.0168 W/cm2). Phenazopyridine concentrations were: a) 5 ppm b) 10 ppm c) 15 ppmand d) 20 ppm.

H.S. Hilal et al. / Solid State Sciences 12 (2010) 578–586 581

concentrations 10, 15 and 20 ppm were: 0.00755, 0.00941 and 0.01,respectively. Plots of (ln initial rate) vs. (ln initial concentration)showed linear relation with order 0.4 with respect to phenazo-pyridine concentration.

3.1.5. Catalyst recovery and reuseAfter reaction cessation, naked ZnO catalyst powder was recov-

ered and reused in fresh phenazopyridine photo-degradationreactions. First, second and third recovery samples showed 13%, 21%and 63% catalyst efficiency losses, respectively, compared to freshcatalyst samples. Fig. 8 summarizes the results. Quantum yieldvalues (reacted drug molecules per incident photon) for freshsample, first, second and third recovered samples were: 0.00582,0.00501, 0.00455 and 0.00215, respectively.

3.2. AC/ZnO catalyst

Unlike naked ZnO catalyst, the AC/ZnO system exhibited rela-tively high drug adsorption onto the AC surface. This accounted for

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Fig. 6. Effect of nominal catalyst ZnO amount on rate photo-degradation of phena-zopyridine (neutral 100 mL solution, 12 ppm), at room temperature using Xe lamp(overall radiation intensity 0.01 W/cm2). Catalyst ZnO nominal amounts were: a) 0.09 gb) 0.15 g c) 0.20 g, and d) 0.25 g

about 10 ppm of drug adsorption on the AC surface (0.02 g). Forthis reason, relatively high drug concentrations were used in theAC/ZnO catalyst system experiments. To reach equilibrium, thecatalyst and drug mixture was allowed to stand in the dark forprolonged time intervals before irradiation started. Any contami-nant loss after that was due to photo-degradation.

3.2.1. Effect of pHPhenazopyridine degradation reaction using AC/ZnO system was

investigated in solutions with different pH values. The catalyticefficiency did not vary with value of pH in the range 4–10. At higherpH value the reaction rate was slightly lower as shown in Fig. 9. T.N.values calculated after 20 min at different pH values (3.3, 6.4, 8, 11.4)were: 0.00424, 0.00797, 0.0096 and 0.00752 respectively. The resultsshow that the photo-degradation process is more favored underconditions close to neutral media. The AC/ZnO system resembled thenaked ZnO system in terms of pH effect on reaction rates.

3.2.2. Temperature effectPhenazopyridine degradation reaction using AC/ZnO system

was investigated at different temperatures. As temperature was

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Fig. 8. Photo-degradation reaction profiles for fresh and recovered ZnO catalysts.Reactions were conducted at room temperature under Xe lamp solar simulator(intensity 0.01 W/cm2) using a fresh neutral 100 mL solution (10 ppm) in each run. Theoriginal catalyst loading was (a) Fresh ZnO (0.09 g) (b) First recovery (c) Secondrecovery and (d) Third recovery.

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Fig. 9. Effect of pH on photo-degradation of 300 mL solution of phenazopyridine(25 ppm) using 0.12 g ZnO/AC (with 0.1 g net ZnO) at room temperature under directsolar light (0.0134 W/cm2). Solution pH values were: a) 3.3 b) 6.4 c) 8 d) 11.4.

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Fig. 11. Effect of AC/ZnO amount on rate of photo-degradation of phenazopyridine(400 mL neutral solution) at room temperature under direct solar light (0.0141 W/cm2).Net ZnO catalyst amounts were: a) 0.7 g b) 0.8 g c) 0.9 g and d) 1.0 g, with different drugconcentrations 100, 115, 130 and 140 ppm, respectively.

H.S. Hilal et al. / Solid State Sciences 12 (2010) 578–586582

increased the reaction rate was lowered. This behavior occurred atup to 30 �C, Fig. 10. At 40 �C, different behavior occurred. T.N. valuescalculated after 60 min at temperatures 10, 20, 30 and 40 �C were:0.00264, 0.00233, 0.00182 and 0.00212, respectively.

3.2.3. Effect of catalyst amountThe phenazopyridine degradation reaction was studied using

different AC/ZnO nominal amounts, Fig. 11. Using higher catalystamounts did not increase the reaction rate. Quantum yieldscalculated after 10 min, using different catalyst amounts (totalAC/ZnO 0.84 g, 0.96 g, 1.08 g, 1.2 g) were 0.05, 0.0575, 0.0575 and0.0478, respectively. Thus highest AC/ZnO amounts showed lowestQuantum yields. The same tendency also occurred in T.N. valueswhich were 0.00050, 0.00054, 0.00048, and 0.00034, respectively.

3.2.4. Effect of drug concentrationThe effect of phenazopyridine concentration on the AC/ZnO

catalyzed reaction was studied. The rate of reaction increased as thedrug initial concentration increased up to 30 ppm, Fig. 12. T.N.values measured after 20 min for different phenazopyridine drugconcentrations (20, 25, 30 and 35 ppm) were 0.00495, 0.00862,0.008944 and 0.00828, respectively. The T.N. values indicated thatthe reaction was faster with higher drug concentration up to30 ppm. Using higher drug amounts lowered the reaction rate.

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Fig. 10. Effect of temperature on photo-degradation of phenazopyridine (nominal40 ppm, neutral 100 mL) ZnO/AC (using 0.12 g, containing 0.1 g pure ZnO) under solarsimulator (0.01 W/cm2). The reaction temperature was thermostated at: a) 10 �Cb) 20 �C c) 30 �C, and d) 40 �C.

3.2.5. Catalyst recovery and reuseThe catalyst AC/ZnO was recovered from the phenazopyridine

degradation reaction and reused for three successive times. Contraryto naked ZnO, the AC/ZnO system was much easier to recover bysimple methods like filtration. The recovered catalyst systemshowed lowered efficiency on recovery, Fig. 13. T.N. values calculatedafter 40 min for the fresh, first, second and third recovered sampleswere 0.003, 0.00183, 0.00154 and 0.00127, respectively. About 40%,50% and 58% efficiency loss occurred in the first, second and the thirdrecovery, respectively, compared to the fresh catalyst.

3.2.6. Calcination of AC/ZnOSamples of AC/ZnO catalyst system were annealed at 450 �C prior

to use. Effect of calcination on catalyst efficiency was investigated.Phenazopyridine degradation reaction using calcinated AC/ZnO andnon-calcinated AC/ZnO fresh catalyst systems was investigated. Thedegradation rate using pre-calcinated AC/ZnO system was higherthan that for non-calcinated, Fig. 14. T.N. values calculated after40 min for pre-calcinated and non-calcinated ZnO/AC systems,under otherwise identical conditions, were 0.00433 and 0.00324,respectively. AC/ZnO catalyst efficiency was increased by 25% due tocalcination.

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Fig. 12. Effect of drug concentration on photo-degradation reaction of phenazopyr-idine in neutral solutions (300 mL) at room temperature under direct solar light(intensity: 0.0165 W/cm2) using AC/ZnO 0.15 g (containing 0. 0.125 g ZnO). Drugconcentrations were: A) 20 ppm b) 25 ppm c) 30 ppm d) 35 ppm.

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Fig. 13. Reaction profiles showing photo-degradation of phenazopyridine (30 ppm,100 mL neutral solutions) at room temperature under Xe lamp (0.01 W/cm2) using a)0.1 g fresh AC/ZnO catatyst, containing 0.083 g ZnO, b) First recovery c) Secondrecovery and d) Third recovery.

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Fig. 15. Reaction profiles showing photo-degradation of phenazopyridine (100 mlneutral solutions, nominal 45 ppm) at room temperature under Xe solar simulatorradiation (intensity 0.01 W/cm2) using pre-calcinated ZnO/AC catalyst. Reactions wereconducted using AC/ZnO samples: a) Fresh with net ZnO 0.166 g, b) First recovery with0.1 g ZnO, c) Second recovery with 0.096 g ZnO and d) Third recovery with 0.085 g ZnO.

H.S. Hilal et al. / Solid State Sciences 12 (2010) 578–586 583

Effect of calcination on retention of recovered AC/ZnO catalystefficiency was investigated. The calcinated catalyst AC/ZnO wasrecovered from the phenzopyridine degradation reaction andreused for three successive times. Calcination did not enhanceefficiency of recovered catalyst, Fig. 15. Efficiency loss of about 30%,on first recovery of pre-calcinated system, was observed. T.N. valuescalculated after 60 min for the fresh, first, second and third recoverysamples were: 0.002378, 0.001668, 0.001635 and 0.00134, respec-tively. The pre-calcinated system showed higher efficiency loss onrecovery than the non-calcinated counterpart.

4. Discussion

Exposure of aqueous solutions of phenazopyridine, at differentpH values, to direct or simulated solar light, in the presence ofnaked ZnO or AC/ZnO particles, caused complete decolorization ofphenazopyridine in reasonably short times. Control experimentsindicated that the photo-degradation reaction was catalyzed byZnO itself. Cut-off filter experiments indicated that the degradationreaction was caused by the UV tail (400 nm or shorter) of solar lightrather than the visible region.

Literature showed that organic contaminants undergo completemineralization under similar PEC conditions [8,10,12–18].

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Fig. 14. Effect of pre-calcination of ZnO/AC (at 450 �C) on degradation of phenazo-pyridine (neutral 100 mL phenazopyridine solution, 35 ppm) at room temperatureusing Xe lamp (radiation intensity: 0.01 W/cm2). Used fresh AC/ZnO catalyst 0.12 g(containing 0.1 g ZnO), using: a) pre-calcinated, and b) non-calcinated samples.

Therefore, drug decolorization here is assumed to be due tocomplete mineralization. Complete mineralization was furtherevidenced by the disappearance of absorbance bands longer than230 nm, which are attributed to organic phenyl groups. Theappearance of the NO3

� polarographic peak is another evidence ofdrug mineralization. In case of phenazopyridine, the amine groupsare the source for NO3

� ions. The azo group does not produce NO3�

but escapes as N2 gas [16,36].The theoretical basis for complete mineralization is the high

oxidizing power of the ZnO particles due to their wide band gap.When a semiconductor is irradiated with photons of energy higherthan, or equal to, its band gap energy, electron–hole pairs arecreated. The photo-generated electrons could reduce a species suchas O2 (adsorbed on the catalyst surface or dissolved in water) tosuper oxide radical anion O2

��. The photo-generated holes maydirectly oxidize the organic molecules or may react with OH� (orH2O) to produce the highly oxidizing OH� radicals. Detailed mech-anisms showing mode of action of catalysts in organic contaminantphoto-degradation processes are reported [18].

Due to the high positive potential of the valence band of ZnO,(aboutþ3.0 V vs. NHE), [17], the resulting holes have highly positivepotentials. This explains the high oxidizing power of ZnO thatresults in complete drug mineralization.

The ability of ZnO to degrade phenazopyridine under directsolar light is the focal issue here. Effects of catalyst type, pH,temperature, catalyst nominal concentration and drug concentra-tion were all studied. Moreover, the effect of supporting ZnO ontoAC is discussed.

4.1. Naked ZnO catalyst system

The ZnO/CdS system showed lower efficiency, under direct solarlight, than the naked system. With higher CdS uptakes, moreinhibition occurred. The inhibition is attributed to screening the UVradiation away from the ZnO particle surfaces. This parallels otherreports, where CdS screened UV light away from SC catalystparticles and lowered the efficiency in the UV [31]. Mixing ZnOwith other systems like TiO2, SnO2 also decreased ZnO catalyticefficiency [50]. However, contradicting results about CdS ability to

H.S. Hilal et al. / Solid State Sciences 12 (2010) 578–586584

enhance ZnO catalyst efficiency are reported [32,50]. In addition toscreening, CdS may inhibit degradation reaction by producingsulfur which may in turn poison the catalyst, as suggested earlier[51]. In any case, its tendency to yield hazardous Cr2þ ions underPEC conditions was a good reason to exclude CdS from this work.Such tendency was confirmed here by polarographic analysis ofreaction mixture, which showed continued appearance of Cd2þ

ions throughout the reaction progress. The ability of ZnO to catalyzedecomposition under direct solar light, without sensitization,shows its advantage over TiO2 which demands sensitization [42].

4.1.1. Effect of reaction conditionsThe pH value did not significantly affect ZnO catalyst efficiency.

Changing solution pH is expected to cause positive or negativecharges on particle surface, and to affect organic molecule adsorp-tion rate [35]. This would then affect overall degradation rates. Theresults showed that pH did not significantly affect reaction rate here.Therefore, phenazopyridine degradation experiments were con-ducted using neutral solutions, unless otherwise stated.

Under highly acidic or highly basic conditions, the reaction wasslower. In a strongly alkaline environment ZnO undergoes disso-lution into Zn(OH)4

2�. While under acidic conditions, ZnO powderdissolves readily to yield Zn2þ ions [27,30,35,37]. This explains thedecrease in the rate of photo-degradation under extreme acidic orbasic conditions. Such behavior should not be counted against ZnOcatalyst systems, because the natural working conditions for waterpurification involve pH ranges closer to 7.

In thermally induced catalytic reactions the rates progressivelyincrease with higher temperature. On the contrary, photochemicaldegradation reaction rates, with high band gap SC catalysts, arenormally independent of temperature [16,17,36], as observed here.Thermal energy gained by heating at ambient temperatures(w60 �C) is far less than 3.0 eV, viz. heat energy lies within the IRregion. Moreover, higher temperatures encourage the dissolvedoxygen to escape. Therefore, OH� radical formation is inhibited bydissolved oxygen deficiency.

At lower catalyst concentrations, the reaction rate increasedwith catalyst amount, to a certain limit. This is due to increasedtotal surface area, available for adsorption and degradation, withincreased concentration.

At higher concentrations, the rate was independent of catalystconcentration. When higher catalyst concentrations are used, largeproportions of the catalyst sites are screened away from incidentlight, and the efficiency decreases. Similar behaviors are known inliterature [11].

At lower phenazopyridine concentrations, the rate of degrada-tion increased with increasing the concentration. The value of thereaction order (0.4) with respect to the phenazopyridine concen-tration does not show a linear relation. At higher drug concentra-tions, the catalyst efficiency decreased. The results suggestscreening of light away from ZnO particles, by drug molecules.Similar behaviors are known for other photo-degradation systems[8,11,16,37,52]. Another acceptable explanation is the fact that athigher contaminant concentration, the contaminant molecules maycompete with oxygen (or other adsorbed intermediates) [10,17,42].

4.1.2. Catalyst recovery and reuseThe lowering in recovered catalyst efficiency is possibly due to

coagulation of ZnO powder caused by interaction with drugmolecules or their intermediates. Coagulation lowers the effectivesurface area of ZnO particles. Nano-scale inorganic oxides have hightendency to aggregate [38]. In TiO2 systems, interaction withpollutant molecules or their intermediates forces the TiO2 powderto coagulate, which lowers the amount of UV radiation fromreaching the TiO2 active centers, and thus reduces its catalytic

efficiency [39]. Any possibility of sulfur poisoning is ruled out here,because no source of sulfur is present.

Loss of efficiency on recovery and reuse is known in othercatalytic systems [10,34]. The difficulty, in recovering naked ZnOpowder out of the reaction mixture is another technical challengewhich faces micro- and nano-scale catalysts. Here comes the needto use solid supports as discussed below.

4.2. AC/ZnO system

The AC/ZnO system was a dark solid, with 83% (or higher insome cases) ZnO and the remaining was AC. The AC/ZnO ishydrophilic in nature as it readily mixes with water. The system isalso easy to isolate from water by simple filtration. This alonemakes up for any other shortcoming that could possibly be asso-ciated with using AC support. The efficiency of AC/ZnO system hasbeen investigated using different parameters, keeping an eye on itsfeasibility to use at large scale in the future.

4.2.1. Effect of reaction conditionsThe measured initial drug concentration values did not match

the nominal used concentrations, due to adsorption onto ACsurface. The initial rate calculations could not thus be accuratelymeasured, and efficiency comparisons were performed based onT.N. and quantum yield values.

Based quantum yield values, higher overall reaction rateoccurred with increased catalyst concentration. After a certainconcentration the rate decreased. The results parallel thoseobtained for naked ZnO catalytic systems. As mentioned earlier,light screening is responsible for the decrease in the efficiency ofphoto-degradation with higher catalyst amounts. This is consistentwith earlier studies [40].

The reaction was faster with higher drug concentrations, up toinitial concentration of 30 ppm, above which the reaction sloweddown. This is consistent with earlier results on different catalystsystems. Different explanations are proposed, all of which rely onthe adsorption of contaminant molecules on the solid surface ina Langmuire Hinshelwood model [53]. As contaminant initialconcentration increases, more contaminant molecules are adsor-bed onto the surface of AC/ZnO. This prohibits adsorption of otherspecies such as O2, which is necessary for the reaction to occur.Moreover, with higher contaminant concentrations more lightscreening is expected to occur [10,15,17,40,41,54].

The reaction was slower with increased temperature up to 30 �C.At 40 �C the rate was slightly higher than that at 30 �C. Thetemperature effect in AC/ZnO catalyst resembles that for naked ZnOcatalysts. The rate lowering with increased temperature is thusrationalized based on adsorption. At higher temperature, lesscontaminant adsorption occurs. Oxygen removal from the reactionmixture at higher temperatures is another factor. The slight increasein rate at 40 �C is presumably due to enhanced diffusion rate athigher temperature, which balances off adsorption and oxygenremoval effects.

The degradation rate was slightly affected by pH. Based onturnover number values, the lowering in the rate at pH 3.3 isattributed to the amphoteric behavior of ZnO and its tendency todissolve at lower pH [30], as discussed earlier.

Similar to naked ZnO systems, under highly basic conditions thereaction rate was slower than under neutral conditions. Earlierreports show that the photo-degradation of Direct Blue 53 wasaffected by pH change in the range 3–11. The photo-degradationrate increased with increasing pH up to 9 and then it remainedconstant [41].

Similar to naked ZnO, the results indicate the ability of the AC/ZnO system to effectively function, under different mild pH values,

H.S. Hilal et al. / Solid State Sciences 12 (2010) 578–586 585

almost indifferently. The AC/nO would thus be suitable to purifynatural waters, which have pH values closer to 7. This shreds lighton future feasibility of AC/ZnO.

4.2.2. Catalyst stability and recoveryRecovery of AC/ZnO from phenazopyridine degradation reaction

mixture, after reaction cessation, was achieved effectivelyby simple filtration. This shows the potential applicability of usingAC/ZnO at future large scale.

The catalyst was recovered and reused three successive times.Efficiency loss occurred in both recovered ZnO systems, naked andsupported ones. The AC support failed to retain efficiency andstability of supported ZnO.

Both naked ZnO and AC/ZnO systems degraded to yield aqueousZn2þ ions. This was confirmed by polarographic analysis of thecatalytic reaction mixture with time. Such leaching tendency wasmore obvious in acidic conditions than in basic conditions. At pH 12,only 3.3% of the supported ZnO degraded into Zn2þ ions. At pH 3.2, upto 6.3% degradation occurred. At higher values (pH 12) precipitationof Zn(OH)2 is possible [11]. This accounts for low Zn2þ tendency toleach out.

Naked ZnO systems also showed similar tendency. Under neutralor slightly basic conditions, 3.3% of originally used naked ZnO leachedout under PEC conditions. However, efficiency lowering does notundermine the major advantage of the supported system. Moreover,the Zn2þ is not considered hazardous [55,56]. Thus leaching out ofZn2þ ions should not be counted against using AC/ZnO systems, ifcarefully calculated.

Calcination was attempted to improve efficiency and stabilityof the supported ZnO catalyst. The 25% higher efficiency of cal-cinated AC/ZnO over non-calcinated system was evident bycalculated T.N. values after 60 min. Such enhancement is possiblydue to ZnO particle agglomeration and flattening on the ACsurface. Calcination is expected to increase relative surface areaof supported ZnO particles. This is due to their tendency tochange from spherical shape into plate-like shape by sinteringwith the surface. Similar observations are reported for otherdifferent supported semiconducting particles [42]. Such shape-change increases surface area, and consequently the efficiency ofZnO catalyst.

AC surface adsorbed drug molecules. With improved ZnOattachment on the surface, the adsorbed drug molecules moveeasier to the ZnO active catalyst sites. Better attachment betweenAC surface and ZnO particles allows more synergistic effectbetween them. Thus high photocatalytic efficiency would beenhanced by calcination. Similar observations and discussionswere reported for AC/TiO2 catalyst systems in degradation reac-tions [29].

Calcination, however, failed to improve the activity andstability of the catalyst on recovery. A 30% efficiency loss in thefirst recovered system, compared to fresh sample, was observed.T.N. values indicated that considerable loss of activity occurred incalcinated AC/ZnO, parallel to the non-calcinated ZnO/AC system.Despite the enhanced attachment with the surface, calcination didnot stabilize the supported ZnO. This is due to the flattening of theattached spheres, with higher relative surface area. Therefore, thesupported particles become more prone to degradation aftercalcination.

While exhibiting the advantages of easy recovery and highactivity, the AC/ZnO catalyst system must also show higher stabilityand efficiency on recovery in order to compete in future purifica-tion strategies. To achieve such goals together, work is underwayhere to chemically deposit ZnO nano-scale particles onto differentsolid supports. Such technique would yield combined advantagesof easy separation, efficiency and stability.

5. Conclusion

Naked ZnO and carbon-supported ZnO systems showed soundcatalytic activity in photo-degradation of phenazopyridine, usingthe UV tail of direct solar light. The carbon support made catalystrecovery easier, without sacrificing it efficiency, but did notenhance its stability on recovery. Calcinating the carbon-supportedZnO enhanced catalytic efficiency, but not stability on recovery. Thecarbon/ZnO system is a promising candidate for future applicationin direct solar light driven pharmaceutical waste management,should its stability be enhanced on recovery.

Acknowledgements

The authors wish to thank Dr. Guy Campet and DaeHoon Park,both of ICMCB, University of Bordeaux, France, for TGA analysis.Support to this activity by French-Palestine University CooperationProgram (Al-Maqdisi) is acknowledged. Help from the technicalstaff at ANU is acknowledged. Thanks are due to Dr. Amer El-Hamouz (ANU) and Birzeit-Palestine Pharmaceutical factory for freegifts of chemicals. G. Al-Nour wishes to thank Mr. Abdul-RahmanNour and Miss. Manal Nour for technical help.

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