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Catalysis Today 209 (2013) 7983

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Catalysis Today

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Photodegradation ofpetrol station wastewater after coagulation/flocculationwith tannin-based coagulant

A.M. Ferrari-Lima, R.G. Marques, N.R.C. Fernandes-Machado, M.L. GimenesUniversidade Estadual deMaring, Av.Colombo, 5790 Maring, Brazil

a r t i c l e i n f o

Article history:

Received 29 June 2012

Received in revised form 6 September 2012Accepted 22 October 2012Available online 11 December 2012

Keywords:

PhotocatalysisTiO2 catalystZnO catalystPetrol station wastewater

a b s t r a c t

This work investigates the combined treatment of petrol station wastewater by coupling coagula-tion/flocculation and heterogeneous photocatalysis processes.

Coagulation/flocculation was performed using a tannin-based coagulant, Tanfloc. Commercial TiO2 ,ZnO and Nb2O5 were applied as photocatalysts, after calcination at 500 C/5 h. In order to evaluate synergiceffects, the semiconductor oxides were mixed (50 wt%) and their activities were investigated under UVand VIS radiations. High reductions ofabsorbance, chemical oxygen demand and elimination oftoxicitytoArtemia salinawas achieved with TiO2ZnO under UV radiation. Chemical oxygen demand was reducedfrom 1363 mg L1 (untreated) to 362mg L1 after coagulation/flocculation and finally to 104 mg L1 afterphotocatalytic treatment.

2012 Elsevier B.V. All rights reserved.

1. Introduction

Generated by spills during fuel supply, forecourt washing, carwashing systems and convenience stores, petrol station wastewa-

ter(PSW) hasa huge amount of hydrocarbonsthat arenot removedby oilwater separators, which is the unique treatment systemrequired for petrol stations. These hydrocarbons are formed by thesoluble fraction of petrol and diesel in water, derived mainly fromspills during fuel supply. Moreover, the presence of ethanol cancontribute by increasing the solubility of these hydrocarbons inwater [1], raising the toxicity of PSW.

Many efforts have been applied in order to eliminate severaltypes of hydrocarbons present in wastewater such as phenolicand other aromatic compounds [24]. Owing to the high toxic-ity of these compounds the oxidation of the hydrocarbons to CO2and H2O by means of advanced oxidation processes (AOPs) areof particular interest when compared to conventional processesof wastewater treatment. Among the AOPs, heterogeneous photo-

catalysis has been the subject of several studies on the degradationof toxic effluents [58] and has shown good results in the removalof several contaminants from wastewater.

When a semiconductor is subjected to radiation whose energyis equal to or greater than its band gap energy, the excitation pro-motes a valence band electron to the conduction band, creating agap in the valence band with enough positive potential to generateOH radicals from adsorbed water molecules. These radicals are

Corresponding author. Tel.: +5544 3011 4777;fax: +55 44 3261 4792.E-mail address: ana eq@hotmail.com (A.M. Ferrari-Lima).

highly oxidising and react with organic compounds mineralisingthem.

Among thehuge variety of catalyststhat have been investigated,TiO2 is undoubtedly the most used due to its well known structure

and high activity [911]. However, TiO2 activity is still not highenough to be suitable for commercial application [12], especiallydue to the charge recombination and the need of ultraviolet radi-ation for the activation of the catalyst. In this sense, many effortshavebeendedicatedinordertoenhanceitsactivitybymixingsemi-conductor oxides to achieve better charge separation and avoidrecombination centres [1214].

With this background, the aim of this work was to investi-gate the activity of mixed semiconductor oxides (TiO2, ZnO andNb2O5) in thephotodegradation ofpetrolstationwastewater underultraviolet and visible light radiations after pre-treatment by coag-ulation/flocculation process using a tannin-based coagulant. Thereduction of chemical oxygen demand and toxicity were discussedas well as the kinetics of the photodegradation and the overall

efficiency of the combined treatment.

2. Experimental

2.1. Wastewater collection and characterisation

PSW samples were collected after oilwater separation in apetrol station located in an urban area of Maringa-Parana-Braziland characterised following the Standard Methods for Examina-tion of Water and Wastewater [15]. The petrol station offers fuelsupply (ethanol, gasoline, and diesel), oil change services, vehiclewashing and convenience store.

0920-5861/$ see front matter 2012 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.cattod.2012.10.022

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2.2. Coagulation/flocculationprocess

Coagulation/flocculation was performed in jar test Milan JT101.Wastewater was stirred (100rpm) for 1 min, after addingTanflocat500mg L1 concentration and followed by 30 min of slow agitation(50 rpm). After 2 h flocs settling down, the supernatant aqueousphase was separated for characterisation.

2.3. Catalysts preparation and characterisation

The photocatalysts TiO2 (Kronos), ZnO (Dynamics) and Nb2O5(Companhia Brasileira de Mineraco e Metalurgia) were used plainand in simple mixture (50wt%) after calcination at 500C/5h. Priorto calcination, the catalysts were submitted to agglomeration pro-cess (particle size 0.1500.300mm).

Textural analysis was carried out by nitrogen adsorptionisotherm at 77K using QuantaChrome Nova 1200 equipment. X-ray diffraction (XRD) was performed in a Bruker D8 Advance X-raydiffractometer with Cu 40 K radiation, 30 mA and Ni filter. Theobserved peaks were compared with the data published by JCPDS(10). Scanning electron microscopy (SEM) analysis was carried outon Shimadzu SS-550 Superscan Microscope.

Photoacoustic spectroscopy (PAS) was performed in equipmentconsisting of a monochromatic light provided by a xenon lamp of1000W (Oriel Corporation 68820), a monochromator (Oriel Instru-ments 77,250), a high sensitivity capacitive microphone (Brueland Kjaer) and a lock-in amplifier (EG & G 5110). The light beamwas modulated with a mechanical modulator (Stanford ResearchSystems SR540). The photoacoustic spectrum was obtained onmodulation frequencyof 21Hz at wavelength from 200 to 800 nm.Emission spectra of the lamps were acquired in a VS140 LinearArray UV-VIS & VIS Spectrometer (HORIBA).

2.4. Photocatalytic reactions

The reaction system was formed by a 500mL glass beakerequipped with magnetic agitation, temperature control and radia-

tionisolation.Thephotographicillustrationofthereactorisshowedin Fig. 1. Radiation was provided by a 250W low pressure mer-cury lamp (EMPALUX) whose bulb has been removed to obtainUV radiation or maintained to obtain VIS radiation. The distancebetween the lamp and the surface of the suspension was 15cm.The UV light intensity was measured by an Instrutherm UV lightmeter MRU-201 and the VIS light intensity was measured byan Instrutherm luximeter LD300. Photocatalytic assays were per-formed with 1g L1 of catalyst concentration, in accordance withprevious work [16]. Catalyst activity was evaluated by chemicaloxygen demand(COD), biotoxicitytoArtemiasalinaand absorbancereductions after 300min of irradiation. Absorbance was measuredat 254, 280, 310 and 500 nm (HACH DR5000 spectrophotometer),corresponding to aliphatic groups, aromatic groups, conjugated

aromatic rings and light-absorbing molecules, respectively[16,17].

2.5. Biotoxicity tests

Toxicity tests were performed following the methodologydescribed in Santana and Fernandes-Machado [14]. Artemia saline(brine shrimp) was used as bioindicator of the toxicity. Cysts ofArtemia saline were incubated in a 35g L1 marine salt solution for24h at 28 C with continuous illumination and aeration. After 24 hthe most active organisms were transferred to a new marine saltsolution at the same concentration of the previous one and main-tained for more 24h at the same conditions. For toxicity testing,the followingsolutionswere prepared: forthe control, a solution ofpotassium dichromate (1 g L1) was prepared and diluted in saline

solution at the concentrations of 0, 10, 20, 40 and 60mLL1

. 2 m L

Fig. 1. Photograph of the photocatalytic reactor.

aliquots of these solutions were added to the wells. For the waste-water, aliquots of 2 mL(before and after treatment) were added tothe wells at the following dilutions in saline solution: 0, 1/2, 1/4,1/8, 1/16 and 1/32 and 1, corresponding to percent concentrationsof 100, 50, 25, 12.5, 6.25, 3.12 and 0% of wastewater, respectively.About 10 nauplii were added to each well and maintained in thesesolutions for 24h under illumination (triplicate). The mortality ofthe nauplii for each dilution was evaluated and the lethal concen-tration for 50% of the nauplii (LC50) after 24h of exposition was

determined by probit analysis.

3. Results

3.1. Catalysts characterisation

Textural analysis (Table 1) showed that surface area and poresize distributions of mixed catalysts were approximately the aver-age of those for plain oxides. A slight reduction on the surface areawith the thermal treatment means an increase in the crystal sizeby the sintering of small particles.

XRDanalysis(Fig.2) identifiedanatasephaseforTiO 2,hexagonalstructure for Nb2O5 and wurzita phase for ZnO after calcination at500 C/5 h. New phases were not identified by mixing the oxides.

PAS analysis showed that all the catalysts have similar bandgapenergies, around 3.2 eV (Fig. 3), it means that the catalysts need

Table 1

Surface area and band gapof thecatalysts.

Catalyst Surface area BET (m2 g1)

As p repared Afterannealing at500 C/5h

Band gap(eV)

Absorptionthreshold(nm)

TiO2 10.6 8.8 3.20 388ZnO 6.8 6.5 3.12 397Nb2O5 69.6 47.8 3.17 391TiO2ZnO 8.7 8.5 3.17 391Nb2O5ZnO 49.0 39.6 3.15 394TiO2Nb2O5 47.5 33.0 3.21 386

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Fig. 2. XRD ofthe catalysts (x= TiO2 anatase,o =ZnO, *= Nb2O5).

around the same amount of energy to promote the electron. Thisenergy is located in the ultraviolet region of the electromagneticspectra,asitiswellknownforTiO2,ZnOandNb2O5 oxides(Table1).

SEM analysis showed that all the catalysts have similar mor-phology, consistingof well distributed sphericalgrains. Some grainagglomerates were found and can be associated with the sinteringcaused by the heat treatment. The particle size inside the reactorwas in the interval of 0.1500.300mm.

By measuring the emission spectra of the lamps (Fig. 4) it ispossible to note that when the bulb is removed both UV and VISradiations are emitted with main peaks located at the ultravioletregion and also strong peaks located at 440, 550 and 580nm (visi-ble region). When the bulb is maintained the majority of the peaksarefound at the visible region with two peaks corresponding to theultraviolet region, 365 and 410 nm. In this way, the emission pro-vided by the lamp with and without the bulb was called VIS and UVradiations, respectively. Comparingthe peaks of the lamps withtheabsorption threshold of the catalysts (Table 1) it can be proposedthat the 365 nm peakis the responsible for theactivation ofthe cat-

alysts whereas at this wavelength the minimum energy requiredby the semiconductors to promote the electron is provided.

3.2. Wastewater characterisation and coagulation

In general, as depicted in Table 2, the effluent has neutral pH,high turbidity and COD. Reductions in all the parameters aftercoagulation/flocculation certify the capability ofTanflocto remove

Fig. 3. UV/VIS photoacoustic spectra of the catalysts.

Fig. 4. UV/VIS and VISlampsemission spectra.

organic and colloidal matter from the effluent. Beltrn-Herediaand Snchez-Martn [18] determined the adsorption capacity (q)

as being an efficiency parameter for the coagulation process. It wasdefined as q= (C0 C1)V/W, where C0 and C1 are, respectively, theinitial and final COD concentrations (mg L1), V is the volume ofsolution (L) and W is Tanfloc mass (g). In the present work, theequation gave a q value of 2002mg of COD per g ofTanflocfor PSWcoagulation/flocculaltion while other authors achieved q values of490mg and 359mg for dye and surfactant removal, respectively,applying the same coagulant [19,20].

Pre-treatment can represent a fundamental step in heteroge-neousphotocatalysis systems, giventhat high turbidity and organicmatter concentration canaffectthe scattering of light into the solu-tion. Some authors report that for successful photocatalysis, CODvalues must be lower than 800 mgL1 [16,17,21]. Tanfloc can beconsidered a good alternative to inorganic coagulants since thecoagulant did not drastically reduce the pH of the effluent andshowed satisfactory COD, TOC and turbidity removal capability.Additionally, the advantages associated with the use of organiccoagulants such as no pH adjustment requirements, no addition ofmetal ions to the system,and biodegradable sludge generation [22]can represent a promising use of organic coagulants in wastewatertreatment.

3.3. Photocatalytic degradation

Aftercoagulation/flocculationprocess,the supernatant was sep-arated from sludge and submitted to photodegradation under UVand VIS radiations with an irradiation time of 300 min. Radiationintensity on the suspension surface was 3570W cm2 at 390 nmfor the UVlight and for the VIS light itwas 700W cm2 at390nmand 15,780lx.

Despite the similarity of the catalysts characteristics, mixingTiO2 andZnO enhanced bothTiO2 andZnO activities under UV radi-ation (Fig. 5). Although the reduction of COD was 68%for both TiO2

Table 2

PSW characterisation before and after coagulation/flocculation with Tanfloc.

Parameter In natura After coagulation Reduction (%)

pH 6.9 6.8 COD (mgL1) 1363 362 73TOC(mg L1) 187 90 52Turbidity (NTU) 326 34 90Total solids (mgL1) 1287 632 51Grease (mgL1) 56 45 18

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Fig. 5. Residual absorbance of pre-clarified PSW after 300min of UV irradiation.

and TiO2ZnO catalysts, the reduction of absorbance was higherfor TiO2ZnO, reaching 76, 78, 81 and 88% for the wavelengths of254, 280, 310 and 500nm, respectively.These values show that thehydrocarbons are being degraded and the organic matter is beingremoved from the wastewater by means of photocatalysis. Otherauthors have also reported synergic effect between semiconductoroxides at the same proportion of 50% [14]. This synergic behaviourcanbe attributed to the electron transfer from the conduction bandof ZnO to the conduction band of TiO2 under illumination and thehole transfer from the valence band of TiO2 to the valence band ofZnO, leading to an decrease in the recombination rate and increasein the lifetime of the electron-hole pair, improving the occurrenceof redox processes [23,24].

A slight reduction in COD and absorbance was observed whenreaction was carried out without the catalyst, indicating that UV-absorbing compounds are probably present in the PSW and can

compete with catalyst for absorption of radiation [25].When the pre-clarified wastewater was submitted to VIS radi-

ation the reductions of COD and absorbance were lower than thatobtained when UV radiation was applied (Fig. 6). This fact is clearlyrelated to the band gaps of the catalystswhich according to Table 1are located in the UV region. Nevertheless, 53% of COD reductionwas reached with TiO2 and VIS radiation. Absorbance reductionsgreater than 40% were achieved for all the wavelengths monitored

Fig. 6. Residual absorbance of pre-clarified PSW after 300min of VISradiation.

Table 3

Rate constantsof apparent first order (k1), half-lifetime(t1/2) and R2 forCOD reduc-tion of PSW.

Selected system k1 (103 min1) t1/2 (min) R2

TiO2/UV 2.38 291 0.9974TiO2ZnO/UV 5.73 121 0.9986TiO2/VIS 2.34 296 0.9823

for both TiO2 and TiO2ZnO catalysts. Some authors associate thelower activity of Nb2O5 to the low stability of its hydrocolloid,keeping the catalyst usually precipitated in water [26,27], thus theloweractivityoftheNb2O5 containingcatalystsevenwithitshighersurface area.

In general, absorbance reduction occurs at the following order:500nm>310nm>280nm>254nm.This fact occursbecause as thearomatic hydrocarbons are degraded, aromatic rings are brokendown into smaller compounds (single rings or aliphatic groups),increasing theabsorbance in the wavelengths related to these com-pounds. Herrmann [28] points out that the aromatics rings areeasilydearomatised andif there arealiphatic groupsattachedto therings, this bond is quickly broken. Furthermore, the oxidation pro-cesspreferentiallyoccursatthehighestelectronicdensitypositions

[16].

3.4. Kinetics of COD reduction

Kinetics studies were performed with the following systems:TiO2/UV, TiO2ZnO/UV and TiO2/VIS. For the selected systems,the data fit the LangmuirHinshelwood kinetics model for reac-tions of apparent first order for COD oxidation (Table 3). Thetotal irradiation time considered for kinetic studies was 300min.The same behaviour has been reported for the photodegrada-tion of several compounds [2,2932]. The more active catalystwas TiO2ZnO/UV which showed the shortest half-life time forCOD reduction (121min). The rate constants showed the same

magnitude found by An et al. [33] for COD removal from reac-tive dye-containing wastewater applying TiO2. The cited authorsreported a rate constant of 0.0046min1 for the mentioned pro-cess while in the present work the rate constants were 0.0024,0.0057 and 0.0023 min1 for the systems TiO2/UV, TiO2ZnO/UVand TiO2/VIS, respectively.

3.5. Toxicity reduction and combined treatment

It is known that photocatalysis can generate side-compoundsthat are toxic to aquatic environment. Therefore, biotoxicity assayswere carried out withA. salina (brine shrimp) in order to evaluatethe final toxicity of the treated water after the combined treat-ment. The brine shrimp A. salina has been successfully employed

to detect the toxicity of several media including vinasse, cyanobac-teria, aldehydes and biocides [14,3437]. Toxicity was evaluatedin terms of LC50 percentage, which corresponds to the lethal con-centration in percentage of wastewater that causes death of 50% ofArtemianauplii after 24h of exposition. The toxicity was measuredonly after the end of the reaction (after 300 min of irradiation),so there is no toxicity data to predict the behaviour of the forma-tion of more or less toxic oxidation intermediates throughout thephotodegradation.

In the case of TiO2 and TiO2ZnO catalysts applied under UVradiation, the LC50 was greater than 100% after 300min of irradia-tion (Table 4), values with different letters are statistically differentwhile values with the same letter are statistically identical. Thisfact indicates that when the nauplii were submitted to the effluent

treated with these catalysts the percentage of deaths was lower

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Table 4

PSW LC50%a under various conditions after 300 in of irradiation.

PSW LC50(%)b

Untreated 1.31a

After coagulation 2.91b

Catalyst UV VIS

After photocatalysisTiO2 102.08c 22.69e

ZnO 11.04d 9.30h

Nb2O5 21.38e 9.20h

TiO2ZnO 115.1f 14.22i

TiO2Nb2O5 60.37g 13.23i

Without catalyst 2.97b 2.97b

a Lethal concentration of PSWthat causesdeathof 50%Artemia nauplii after 24hofexposition.

b Differentletters implyvalues statistically different(P< 0.05 byTukeytest).n= 3samples analysed.

than 50% for the effluent undiluted. It can be considered that thetoxicity of the effluent was eliminated.

For Nb2O5 catalyst no synergic effect has been observed withTiO2 . However the final result showed to be the average betweenboth catalysts concerning toxicity reduction (Table 4).

No significant pH variation occurred during the photocat-alytic processes and significant reductions of turbidity and CODwere achieved. The COD of the effluent in natura reduced from1363 to 362 mg L1 after coagulation/flocculation and to 152and 104mg L1 after photocatalysis with the systems TiO2/VISand TiO2ZnO/UV, respectively. The final turbidity of the treatedwater was 3.78 and 9.58NTU after combination of coagula-tion/flocculation and photocatalysis with the systems TiO2/VIS andTiO2ZnO/UV, respectively. After combined treatment, no colourand no odour remained in the treated water.

4. Conclusions

The combined treatment of petrol station effluent by coagula-

tion/flocculation followed by heterogeneous photocatalysis allowsthe reduction of pollutants and removal of toxic compounds, espe-cially with TiO2 and ZnOTiO2 photocatalysts. Additionally, a goodefficiency was achieved under visible radiation, which would facil-itate the practical use of the process. Synergic effect occurredbetween TiO2 and ZnO, leading to a higher activity to TiO2ZnOcatalyst when compared with the respective activities of TiO2 andZnO singly. The elimination of toxicity to A. salina was achievedafter combined treatment. Considering the overall efficiency of theinvestigated process, the combined treatment exhibited promis-ing potential for full scale application. Compared to the exitingPSW treatment systems some drawbacks like available space forinstallation and catalyst separation would have to be considered.

Acknowledgments

This work was sponsored by the Brazilian agencies CNPq andCAPES.

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