Laboratory Scale Unit for Photocatalytic Removal of Organic Micropollutants from Water and Wastewater. Methyl Orange Degradation

Laboratory Scale Unit for Photocatalytic Removal of OrganicMicropollutants from Water and Wastewater. Methyl OrangeDegradationAndrea Petrella, Mario Petrella, Giancarlo Boghetich, Piero Mastrorilli, Valentina Petruzzelli, Ezio Ranieri,and Domenico Petruzzelli*

Dipartimento di Ingegneria Civile, Ambientale, del Territorio, Edile e Chimica, Politecnico di Bari, 4, Via E. Orabona, 70125 Bari,Italy

ABSTRACT: An innovative laboratory scale unit was used to carry out UV photoinduced catalytic degradation of methylorange. For this purpose, the experimental system was made of a bottom and an upper reservoir (∼120 L each) which wereconnected by an inclined channel through which water was recirculated. TiO2 (Anatase) was deposited (∼10−2 mg/cm2) at thebottom of the connecting channel while the Methyl Orange solution was exposed to the UVB radiation (λ ≈ 300 nm) during itsrecirculation through the connecting channel.The unit was first characterized from both the hydrodynamic and the hydraulic points of view. Photodegradation kinetics werefollowed by UV−vis absorption measurements of the residual methyl orange solution concentration along time, and the synergiceffect of the catalyst and the intensity of the UV radiation in promoting degradation of the substrate was demonstrated. Theabatement efficiency of the UV/TiO2 system toward methyl orange was evaluated in the concentration range 0.3−8.5 mg/L.Kinetic patterns were described by first (or pseudofirst) order theoretical models up to the concentration of 0.7 mg/L, whereas athigher concentrations kinetic trends were better described by zero-order models independently from the substrate concentrationin the liquid-phase. The proposed solution, after an upscale field investigation, may represent a valuable alternative to themethods conventionally used for the abatement of textile dyes from wastewater, that is, water clarification, reverse osmosis,activated carbon sorption, and biosorption.

1. INTRODUCTION

Advanced oxidation processes (AOPs) of organic micro-pollutants were demonstrated to be a viable alternative fortheir removal from water and wastewater, with specificreference to those substrates showing environmental persis-tence, after their essential biorefractory nature in conventionalbiological treatment processes.1−3 Organic (and inorganic)micropollutants are today ubiquitous in most natural waterwhere they are present at trace levels; the origin of suchmicropollutants is associated to wastewater reuse practices,which are increasingly adopted in developed as well as indeveloping countries, to increment water supply from non-conventional sources.4 Conventional wastewater treatmentplants (WWTPs) are designed to remove biodegradableorganics, pathogens, and also biopersistent pollutants, withscarce success with these latter ones, to minimize their sanitaryand environmental impacts. Organic micropollutants areassociated with a wide variety of chemicals including, amongothers, steroid, phenol, brominated derivatives, and dyes,present in, for example, human contraceptives, detergents,cosmetics, flame retardants, and textile wastewater.4

Referring to dye removal processes from textile wastewater,biological methods (oxic and anoxic) have proven to beadequate, but these compounds may inhibit bacterial develop-ment thus reducing their efficiency.5,6 Physical methods ofdiscoloration include coagulation,7,8 adsorption on activatedcarbon (AC),9 biosorption on natural nonexpensive sorbents(sawdust, clam shells, shrimps exoskeleton),10,11 and membraneseparations such as reverse osmosis,12 and so forth.

AOP methods are characterized by the formation of OH•

radicals onto the catalyst surface, which promote quantitativemineralization of a variety of organic micropollutants to carbondioxide and water.1−3 Among others, photocatalytic degrada-tion represents a promising engineered process investigationarea.13−17 In some cases the use of UVB radiation might not beeconomically feasible in the absence of radical promoters, suchas heavy metals or semiconductor oxides, provided the catalyststhemselves do not influence the quality of the final effluent.Titanium dioxide (TiO2, Anatase) showed photocatalyticproperties in the oxidative degradation of a variety ofcontaminants in water, wastewater, and air.13−15,18−22 Inpractice, Anatase exhibits a stronger photocatalytic actionafter a higher exposed surface area thus leading to a strongerformation of adsorbed radicals.Although in most applications photocatalysts are applied in

the form of aqueous suspensions,23−25 more recently oxidepowders have been used after deposition onto supports, thusovercoming problems associated with separation (and recov-ery) of the catalyst particles from the liquid-phase.19,26,27

In the present work, an innovative laboratory scale unit wasused for the photocatalytic degradation of Methyl Orange(MO), a typical micropollutant resulting from the textileindustry. The sodium salt of p-[[p-(dimethylamino)phenyl]-

Received: October 4, 2012Revised: January 11, 2013Accepted: January 19, 2013Published: January 19, 2013

Article

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azo]benzenesulfonic acid is used as a dye in the textile industryunder the commercial names of Acid Orange III or AcidOrange 52. The compound is toxic to humans if swallowed orinhaled beyond its general impacts in different environmentalcompartments.28,29

A tap water solution of the dye was recirculated from anupper to a bottom reservoir which were connected by aninclined channel. The flowing MO containing solution wasexposed to UVB lamps for known exposure times dependingon the hydrodynamics of the system. The catalyst (TiO2,Anatase) was embedded into a cement mortar and uniformlyspread in a thin layer (0.5 mm) at the bottom of the connectingchannel to activate the photodegradation reactions of thechemical substrate. Preliminary hydraulic and hydrodynamiccharacterization of the experimental unit was carried out toevaluate speed and flow-rate of the recirculating solution underdifferent hydraulic loads and gradients. Exposure of the liquidphase to the UV radiation changed sensibly with thehydrodynamic conditions of the unit which were accuratelyevaluated. Photodegradation kinetics were monitored by UV−vis absorption measurements of the MO residual concen-trations in the liquid-phase along time; data were correlated tocredited theoretical models.

2. EXPERIMENTAL SECTION2.1. The Experimental Unit. Figure.1A shows a schematic

view of the laboratory scale unit used to carry out kineticexperiments. Design of the unit was based on literaturedata,30,31 and hydraulic and hydrodynamic parameters wereaccurately modulated by controlled iteration of the operativeconditions to get the general control of the system.Specifically, the unit includes the following components:• upper reservoir (∼120 L), containing 60 L of the tap water

MO solution at variable concentrations ready for kineticexperiments (Figure 1B). The reservoir is provided ofmanifolds to control the hydraulic loads in the unit(Figure.1D);• bottom reservoir for the collection of the solution (Figure

1C), provided with an overflow and a piezometric tube for theperiodic backup (with demineralized water) of the recirculatingliquid in case of unsustainable evaporation (Figure.1H);• U-shaped inclined channel between reservoirs layered with

the TiO2 catalyst (10−2 mg/cm2) (Figure.1E);

• recirculation pump through reservoirs (Figure.1F);• three UV lamps (40 W each) covered by a case

overlooking the length of the channel (Figure.1G);• connecting pipes and regulators to control the hydraulic

gradient of the channel (Figure.1I).2.2. The Bottom Reservoir. Figures.1C and H show

different views of the bottom reservoir which is provided with alower outlet connected to the recirculation pump, an overflow,and a stopcock allowing for discharge of the exhausted solutionor, after connection to a piezometric tube, to get a precisecontrol of the level of the liquids in case of unsustainableevaporation of the solution during prolonged kinetic experi-ments. The inside of the reservoir is layered with an epoxyenamel to minimize surface adsorption of the MO substrate.2.3. The Upper Reservoir. Figure 1D shows the lateral

view of the upper reservoir including the manifolds into whichto spill the liquid at specified height to control the hydraulicload of the liquid overflowing to the underlying channel. Finecontrol of the liquid flow-rate, at constant hydraulic load, isobtained through a slot (11 cm wide, 0.2 cm thick), placed at

the bottom of the upper reservoir, whose opening is controlledby the insertion, and proper positioning, of 11 × 70 cm plate ofvariable thickness (1.5; 1.0; 0.5 mm).Needless to say, the accurate setting of the liquid flow-rate in

the channel is functional to the strict control of the exposuretime of the liquid-phase to the UV radiation. As before, thereservoir is internally lined with an epoxy enamel to minimizesurface adsorption of the substrate.

Figure 1. (A) Schematic view of the laboratory scale unit. (B) Upperreservoir. (C) Bottom reservoir. (D) Manifolds for hydraulic loads ofthe unit. (E) U-shaped channel. (F) Recirculation pump. (G) UVlamps. (H) Piezometric tube. (I) Connecting pipes and regulators forhydraulic gradients of the channel.

Industrial & Engineering Chemistry Research Article

dx.doi.org/10.1021/ie3027026 | Ind. Eng. Chem. Res. 2013, 52, 2201−22082202

2.4. The Channel. The inclined channel, connecting thereservoirs, allows for the accurate control of the hydraulicgradient to the recirculating solution (seven settings) which, inturn, control the speed of the liquid and its exposure time tothe UV radiation (Figure.1E). The channel is made of a U-shaped vibrated cement structure with the bottom uniformlylayered with a consolidated (0.5 mm thick, determined by amicrometer) cement mortar containing 50% TiO2 (Anatase)for an approximate surface density of 10−2 mg/cm2 and anoverall amount exceeding 23 mg.Dimensions of the channel are 15 cm (width) and 185 cm

(length); the average surface of the liquid exposed to the UVradiation is in the range 140 × 14 cm, depending on the flowrate of the water in the channel.2.5. The Recirculation Pump. A centrifugal recirculation

pump (Mod. CPm 130, Pedrollo, Milan, Italy; 0.37 kW, 230 V,50 Hz) operates between the upper and bottom reservoirs.Operating flow rates vary in the range 10−80 L/min,corresponding to a maximum and minimal pump headlossexceeding 23 and 14 m, respectively (Figure 1F).2.6. UV Lamps. To activate photocatalytic reactions, three

1.2 m cylindrical UV−B lamps, 40 W each, from ViberLourmat, France, were adopted (Figure 1G). Lamps showed apeak wavelenght at 312 nm.

3. MATERIALS AND METHODS

Commercial titanium dioxide (99% Anatase), from AdriaticaS.r.l., Fasano, Italy, was used throughout the experiments. Theaverage particle size was in the range 0.15 μm, specific gravity of3.85 g/cm3, corresponding to an apparent density exceeding 0.7g/cm3. A cement mortar including titanium dioxide (50:50weight) was spread onto the surface of the channel for anaverage thickness of 0.5 mm, determined by the use of amicrometer. The catalyst concentration in the consolidatedmortar exceeded 0.78 g/cm3 of which only the surface layerresulted to be active to photocatalytic purposes. MO fromSigma Aldrich, Europe, was used to prepare tap water solutionsin the range 0.3 to 8.5 mg/L (pH = 7.2) to simulate typicalindustrial wastewater.X-ray determinations were carried out by the use of a

Mod.PW 1050/04 goniometer equipped with a PW 3710 mpddata controller and a PW1830 voltage generator, all fromPhilips, Eindoven, The Netherlands.UV−vis adsorption for MO determinations in the liquid-

phase were based on an automatic scanning spectrophotometerMod.UVIKON 942 from Kontron Instruments, Germany.

4. RESULTS AND DISCUSSION

4.1. Hydraulic Characterization of the Unit. Hydraulicand hydrodynamic characterization of the unit was carried outto evaluate operative parameters such as the average liquid flowrates, Q, (l/s), the average water velocity in the channel, v, (m/s), the exposed surface of the liquid flowing through thechannel, lc, (cm), the dynamic hydraulic loads, hw, (cm) which,as mentioned, are functional in determining the kineticexposure times of the liquid-phase to the UV radiation. Table1 summarizes the average operative parameters of the unitunder which kinetic experiments were carried out.The average speed of single liquid streamlines in the channel,

under the given hydraulic load, assumes particular impor-tance.30,31 The speed of the water, v, through the channel wasexperimentally determined by the evaluation of the average

running times through it, as well as by the application of theknown relationship between the flow rate, Q, and the section ofwater flowing through the channel, A:

=v Q A/ (1)

where the section, A, was experimentally evaluated by (seeTable 1):

= ×A l hc w (2)

where lc is the channel length and hw is the hydraulic load.4.2. Photocatalytic Experiments. Figure.2A shows the X-

ray diffraction (XRD) pattern of commercial TiO2, whoseappearance is shown in the inset. XRD patterns exhibitedstrong diffraction peaks at 25° and 48° indicating TiO2 in theAnatase phase. All peaks are included in the standard spectrum.Figure 2B shows MO UV−vis absorption spectrum. This lattershows a maximum corresponding to the π→π* transitions ofthe dimethylamino electron donors (470 nm) in the visiblerange, and a secondary peak at 270 nm determined by π → π*transitions of the aromatic rings.Photocatalytic degradation kinetics were determined by the

evaluation of MO residual concentration in the liquid-phasealong time, with the liquid continuously recirculating throughthe channel under the UV lamps. The irradiated solution wassampled at prefixed times, and the residual MO concentrationdetermined by UV−vis spectrometry. First, the initial MOconcentration in the solution was set at 0.7 mg/L. The halftimes of photodegradation reactions were determined from theexperimental kinetic curves.Table 2 summarizes kinetic tests carried out and the

experimental conditions adopted in each case with and withoutthe presence of the catalyst at constant UV irradiation.Figure.3A shows a comparison of the kinetic trends for

experiments carried out by UV irradiation with and without thepresence of the catalyst. Apparently, photocatalytic degradationin the presence of the catalyst is faster than the correspondingone in absence of it, as in this latter case degradation of thesubstrate occurs only after the slow photolytic phenomenaoperated by UV radiation alone. The synergic effect of thecatalyst and UV radiation in promoting photodegradationphenomena of the dye is evident.No leaching of TiO2 moieties was detected along the

experiments carried out as titania is truly embedded into thecement mortar.

Table 1. Average Operative Parameters of the Unit

Hydrodynamic Parameters of the Experimental Unit

volume of the recirculating solution (L) V = 60active wet and irradiated length of the channel (cm) lc = 140width of the liquid in channel (cm) Ic = 14depth of the liquid in the channel (cm) D = 0.7headloss in the channel (cm) ΔH = 0.25hydraulic gradient (%) i = 0.0015hydraulic load at first setting in the channel (cm) hw1 = 13.5flow rate (L/s) Q = 0.066speed of the liquid in the channel (m/s) V = 0.07running time of the liquid in the channel (s) T = 17.9power of the UV lamps (W) P = 120irradiated UV specific (surface) power applied to the liquid(kW/m2)

Pa = 0.53

irradiated UV specific (volume) power applied to the liquid(kW/m3)

Pv = 76.53



From mechanistic point of view, the catalyst photogeneratedelectrons react with electron acceptors such as O2, incidentallyadsorbed onto the TiO2 hydrated surface or dissolved in water,thus reducing it to the superoxide anion radical O2

•− (seeScheme 1). The photogenerated holes onto the semiconductorsurface oxidize the organic substrate to the correspondingmolecular carbocation (R+), or incidentally react with hydroxylanions or water, thus leading to the oxidation product, OH•

radicals. Both O2•− and OH• species result to be highly reactive

toward organic molecules which are broken down (oxidized) tocarbon dioxide and water (Scheme 1).13,16,17

Experimental data confirm that UV-induced photocatalyticdegradation rates, via oxidation of various dye substrates, fitproperly the Langmuir−Hinshelwood (L−H) kineticsmodel:13,21,32,33

= =+

rCt

kKCKC

dd 1 (3)

with r (mg/L min), the oxidation rate of the substrate; C (mg/L), the time concentration of the substrate; t (s), the UVirradiation time; k (mg/L min), the reaction rate constant; andK (l/mg), the sorption coefficient of the substrate onto theTiO2 catalyst. For small figures of the initial substrateconcentration, C0, eq 3 may be approximated by a pseudofirst-order rate equation:32−34

= = = −⎜ ⎟⎛⎝

⎞⎠

CC

kKt k t C C tln or etk0

app 0 app(4)

For the case at hand, Figure.3B shows the good correlation ofthe experimental data corresponding to test no.2 (Table 2),where a linear correlation of ln C0/C vs t is obtained. From theslope an apparent first-order rate constant kapp is obtained (see

Figure 2. (A) XRD pattern of TiO2, in the inset: TiO2 powder. (B)Methyl Orange UV−vis absorption spectrum.

Table 2. Summary of the Kinetic Test

testno.

influent concentration(mg/L)

temperature(°C)

TiO2 surface amount(mg)

1 0.3 20 232 0.7 20 233 0.7 20 04 1.0 20 235 3.0 20 236 5.0 20 237 8.5 20 23

Figure 3. (A) Kinetic trends for experiments carried out by UV withand without catalyst. [MO] = 0.7 mg/L; Q = 0.066 L/s. (B) Kinetictrend of test no. 2, in the inset: ln C0/C vs t linear correlation. (C)UV−vis quenching of the Methyl Orange spectrum during irradiation.



inset to Figure 3B). Based on our experience, a pseudo first-order kinetic model resulted more appropriate for datacorrelation up to 1 mg/L initial concentration of the substrate,a finding confirmed by several authors.15,20,35 Finally, Figure 3Cshows the corresponding UV−vis quenching of the MOabsorption peak during kinetic experiments, together with acharacteristic blue-shift thus confirming photocatalytic degra-dation of the dye substrate. Accordingly, quantitative removalwas obtained, as clearly evidenced from Figures 3B and Cwhere the relative residual MO concentration (C0/C) alongtime approaches zero asymptotically together with thementioned complete quenching of the UV−vis peak.The half reaction and total decay times of MO in our

experiments are comparable to literature data for tests carriedout with the catalyst suspended in the liquid-phase33,36−45

under comparable operating conditions. In our case, in fact, themolar ratio TiO2/substrate was pretty low (≈2) if compared tothe mentioned literature data where the above ratio was 2 or 3orders of magnitude higher, thus leading to a higherinvolvement of the catalyst. In all cases literature data consistedof nanostructured semiconductor suspensions with averageparticle size 1 order of magnitude lower than the particles usedin present work (higher specific surface).33,36−45

The key points of the paper are as follows: (a) use of aninnovative laboratory scale unit; (b) the catalyst wasimmobilized onto a solid support thus avoiding its finalseparation (and recovery) from the liquid-phase;23−25 (c)operations in tap water, although anions may induce inhibitioneffects on photocatalytic reactions;27,32 (d) operations atneutral pH although, based on literature data,19 pH lowerthan TiO2 zero point charge (pzc) may favor sorption ofnegatively charged intermediates such as the radical promoters,that is, O2

•− and OH•; (e) the operations were carried out withlow pressure UVB lamps;19,36 (f) no particle annealing which isreported to affect photobleaching efficiency;36,44 (g) no need ofthermal activation of the film which is reported to improvesorption of the substrate at the catalyst surface;19,46 (h) no useof surfactants which is reported to favor (orient) the substrateinteraction at the catalyst surface;19,44 (i) no additions of extraoxidants (O2, H2O2, S2O8

−2) which is reported to favor theoxidation of azoic dyes.32,35,41,47−49 The above points aresummarized in Table 3.All the above points will be better substantiated as part of an

upscale investigation which is underway.

MO photodegradation kinetics were carried out in the range0.3−8.5 mg/L with faster rates in the range 0.7 mg/L. Asignificant decrease in the kinetic performance is observed atboth lower concentrations (0.3 mg/L) and higher concen-trations (1−8.5 mg/L), with this latter finding contrary toexpectations based on the application of the L-H model(Figures.4A and B).It is known that the overall rate of photodegradation is

associated with the probability of formation of the OH• radicalsonto the catalyst surface, followed by the probability of reactionof these latter with the substrate. On the basis of thesepremises, kinetic trends are properly correlated by the first (orpseudofirst) order L-H theoretical models and, accordingly, aspeed-up of the kinetic trends should be expected with theincreasing initial substrate concentration. In this context, it isapparent from Figure.4A and B showing kinetic trends forexperiments carried out at variable initial concentration of thesubstrate, that a sensible peak performance occurred atconcentrations ranging 0.7 mg/L.At higher substrate concentrations this unexpected result

may be associated with the inhibition operated by the excessamount of MO molecules to the formation of OH• radicalsafter occupation of the surface active sites of the catalyst.Moreover, a kind of screening effect, operated by the excesspresence of MO molecules, to the UV radiation onto thecatalyst surface may not be excluded, thus hindering themassive formation of reactive radicals and, as a consequence,

Scheme 1. Scheme of Photocatalytic Reactions

Table 3. Main Features of the Present Investigation AsCompared to Related Literature Data

laboratory scale unit operations 38, 39, 45immobilized catalyst onto a rigid support thus minimizingpotential release of TiO2 moieties from the solid support

23−25

commercial TiO2 (incidentally wastes from the paint industry) isused

33, 36−45

tap water operation thus allowing for evaluation potentialinhibition of ionic species

27, 32

operation at pH > pHTiO2pzc when radicals inhibition effects areobserved

19

use of low pressure UVB lamp 19, 36no particles annealing 36, 44no thermal activation of the catalyst 19, 46no use of surfactants during catalyst preparation 19, 44no addition of extra oxidants beyond dissolved oxygen 32, 35, 41,

47−49



leading to a sensible slowing down of the overall kineticperformance.34,49,50

When the liquid-phase concentration of the substrate issufficiently high to saturate all the catalyst surface active sites,stationary kinetic performances are observed which areindependent from the substrate liquid-phase concentration intrue agreement with a zero-order rate model, a pure catalyticcontrol of the overall kinetic process.It goes without saying that catalyst efficiency evaluations will

be carried out in the undergoing upscale experiments asreliability of the reference data are strictly associated to the longrun experiments.

5. CONCLUSIONSAn innovative laboratory scale unit was used to carry out UV-induced photocatalytic degradation kinetics of methyl orangeincidentally present in textile wastewater. The catalyst (TiO2,Anatase crystals) was mixed in a cement mortar and uniformlydeposited onto an inclined channel through which a tap watersolution of the chemical substrate was recirculated undercontrolled UVB irradiation. Preliminary hydraulic and hydro-dynamic tests of the unit allowed for the evaluation of mainoperative conditions, that is, flow rates, water velocity, hydraulicloads and gradients, allowing for evaluation of exposure timesfor the UV irradiation of the solution.The synergic effect of the catalyst and UV radiation in

promoting degradation of the substrate was demonstrated.Kinetic trends were correlated by the use of a first (or

pseudofirst) order theoretical model up to concentrationsranging 0.7 mg/L. At higher concentrations kinetic trends werebetter described on the assumption of a zero-order kineticmodel, which was independent from the initial liquid-phaseconcentration of the dye substrate, in terms of a purely catalytickinetic system. No leaching of TiO2 moieties was detectedalong the experiments carried out as titania is truly embeddedinto the cement mortar. Catalyst efficiency in the long run willbe evaluated in the undergoing upscale experiments.Based on the experience gained and data collected from the

laboratory scale unit operation we optimized the main operativeconditions to be adopted by designers during the developmentof a full scale project which is planned to be carried out bydeposition of the TiO2 containing mortar at the bottom of aside-stream of a river bed irradiated by the proper UVB light.This was done without ignoring details on the pertinentreaction mechanisms for the benefit of basic scientists andexperts of the area.The fate of the target biopersistent pollutant will be

monitored upstream and downstream of the full scale systemto verify the overall efficiency of the method for the abatementof persistent pollutants.

■ AUTHOR INFORMATION

Corresponding Author*Tel: +39(0)805963777. Fax: +39(0)805963635. E-mail: [email protected].

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe financial support of the EU project “Research anddevelopment” under grant no. 01-01480 is kindly acknowl-edged.

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Figure 4. (A) Photodegradation kinetics of Methyl Orange in theconcentration range 0.3−8.5 mg/L. (Q = 0.066 L/s). (B)Corresponding t0.5 vs C0.



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