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Jorge M. Meichtry

Hurng J. Lin

Luciana de la Fuente

Ivana K. Levy

Eduardo A. Gautier

Unidad de Actividad Química,Centro Atómico Constituyentes,

Comisión Nacional de Energía Atómica,Av. Gral. Paz 1499,

1650 San Martín, Prov. de Buenos Aires,Argentina

Miguel A. Blesa

Marta I. Littere-mail: litter@cnea.gov.ar

Unidad de Actividad Química,Centro Atómico Constituyentes,

Comisión Nacional de Energía Atómica,Av. Gral. Paz 1499,

1650 San Martín, Prov. de Buenos Aires,Argentina and

Escuela de Posgrado,Universidad Nacional de General San Martín,

Av. 25 de Mayo e Irigoyen,San Martín, Prov. de Buenos Aires, Argentina

Low-Cost TiO2 PhotocatalyticTechnology for WaterPotabilization in Plastic BottlesFor Isolated Regions.Photocatalyst FixationExperiments to evaluate the photocatalytic activity of supported TiO2 to potabilize waterin common plastic PET bottles under solar irradiation were performed. Commercialtitanium dioxide (Degussa P-25) was applied to different cheap materials—glass rings,glass rods and porcelain beads—by dip coating, or directly to the plastic wall of thebottles. The adherence and stability of TiO2 on the supports and the photocatalyticactivity in bottles under solar irradiation was evaluated using model compounds as4-chlorophenol and 2,4-dichlorophenoxyacetic acid. Rings were found to be the bestglass supports, but PET bottles were superior for this specific application, as no fragilefillings are used, and the materials can be easily fabricated on site.�DOI: 10.1115/1.2391317�

Keywords: solar photocatalysis, SODIS, titanium dioxide fixation

ntroductionLow-cost technologies are needed to bring solutions to the se-

ious problem of a safe water provision to extended groups of ansolated population in developing countries without access to a

unicipal water network. This situation is especially critical inropical regions of Africa, Asia, and Latin America. These sunnyegions are characterized by high solar radiance, in some casesore than an average of 3000 sun hours per year. As traditionalethodologies for water treatment are often too expensive to be

pplied in these places, it is imperative to develop simple, efficientnd low-cost technologies for the in situ elimination of the chemi-al and biological pollution. The technologies should be friendly,ccepted by the population and easy to handle or to be applied.

Solar irradiation in plastic PET bottles �SODIS� is a costlessechnology based on the solar exposure for some hours of com-

ercial beverage bottles containing nondrinkable water �1�. Theechnology was initiated by Acra et al. �2� and developed by

egelin et al., who confirmed its efficiency for the removal ofndicators of bacteriological pollution �1�. The combination ofV-A �315–400 nm� and infrared radiation, which raises the tem-erature of water to 50°C–55°C, destroys bacteria and virusesincluding Vibrio cholerae�. The disposable plastic bottles areeadily available, and can be used also for storage and as a drink-ng vessel, thus avoiding the risk of recontamination. Some ar-icles on the scientific grounds of SODIS appeared recently

Contributed by the Solar Energy Division of ASME for publication in the JOUR-

AL OF SOLAR ENERGY ENGINEERING. Manuscript received July 5, 2005; final manu-

cript received September 28, 2005. Review conducted by Sixto Malato.

ournal of Solar Energy Engineering Copyright © 20

�3–11�. Diverse factors as weather, type and aging of bottles, ini-tial water quality, etc. have been thoroughly analyzed �12�. Al-though SODIS is effective to abate microbial pollution of verydifferent origin and type, it is not able to provide residual protec-tion against bacterial growth after irradiation has ceased. It is alsounable to eliminate most chemical contamination.

TiO2 heterogeneous photocatalysis �HP� belongs to the groupof Advanced Oxidation Processes or Technologies �AOPs, AOTs�for water and air remediation, methods that seek the quantitativetransformation of organic matter to carbon dioxide, water, andother simple compounds, affording complete mineralization �13�.The technology uses TiO2 particles—an UV absorbingsemiconductor—and generates highly oxidizing species like hy-droxyl radicals �HO•�, which destroy chemical pollutants and pro-mote the removal of pathogens. The technology presents variousadvantages: solar light may be used, it can destroy totally theorganic pollutants, and the photocatalyst TiO2 is a cheap, reus-able, and nontoxic material. It is possible to abate organic andtoxic inorganic pollution �Cr�VI�, Hg�II�, As, etc.� �14,15� and itis effective for disinfection, destroying bacteria and viruses withpromising results �16,17�.

Therefore, it seems advantageous to make efforts to combineSODIS with HP as a means of developing a simple and inexpen-sive procedure to simultaneously reduce microbial, organic mat-ter, and metal contamination in poor regions. Recently, TiO2 hasbeen successfully tested within SODIS reactors to enhance andaccelerate the inactivation rate of bacterial pathogens �18–22� in aprocess named solar photocatalytic disinfection �SPC-DIS�. Inmost of these works, TiO2 was used as a powdered suspension,

and an additional operation is needed to eliminate the semicon-

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uctor from the treated water: as TiO2 particles are extremelymall �micrometric aggregates of nanoparticles�, filtration throughvery thin pore diameter membrane filter would be needed. Theselters are very expensive or not available in the poor localities ofpplication. Therefore, the catalyst should be used convenientlyupported: the effective fixation of TiO2 is a crucial aspect in theevelopment of the technology. Nevertheless, real and sustainablexation is very dependent on the support and on the final appli-ation of the fixed catalyst. For example, we reported TiO2 fixa-ion on very cheap porcelain beads, obtaining a stable materialith good photocatalytic activity; it presented rather superior ad-erence and very low detachment when submitted to a turbulentegime, useful then for flow or recirculation photoreactors �23�.owever, the use in bottles requires the fixation of TiO2 in smallieces, with stability against abrasion and shocks, and easy han-ling by the people. Fixation on the same plastic wall of the bottleight also be a solution.In Refs. �21,22�, TiO2 was fixed to supports by different tech-

iques, but no comments on the stability of these fixations appear.eferences �24,25� describe TiO2 coatings on projector acetate

ransparencies, which were introduced in PET bottles for testingacteria inactivation and isopropanol oxidation under simulatedolar light. The use of the supported photocatalyst was found to beore effective than SODIS. Similar tests, indicating the validity

f the SODIS technology to abate microbial pollution in waters ofifferent origin of Argentina, arrived at the conclusion that theresence of the photocatalyst was essential for a complete disin-ection, with no regrowth, independently of the bacterial concen-ration. One of the important improvements of combining SODISnd HP is associated to better residual effects after irradiation haseased �26,27�.

In this paper, we explore the possible ways to support the cata-yst, while maintaining the basic tenet of extreme simplicity andery low cost. Results of the fixation of powdered TiO2 to differ-nt cheap materials and the evaluation of the solar photocatalyticctivity in bottles for degradation of model chemical pollutants,-chlorophenol �4-CP� and 2,4-dichlorophenoxyacetic acid �2,4-�, are presented. 4-CP is a chemical model compound represen-

ative of industrial contamination, while 2,4-D is a commonlysed herbicide in the Northwestern region of Argentina �NOAegion�, especially in the Province of Tucumán, where disinfectionests in bottles are in progress. The focus of this work is to dem-nstrate the availability and efficiency of the materials as inexpen-ive photocatalysts in very low-cost “photoreactors” �bottles� tootabilize water in isolated populations.

xperimental

Materials and Equipment. The following supports for TiO2ere used: commercial porcelain beads �Del Morro S.A., Argen-

ina, 6 mm diameter, 1.13 cm2 surface�, Pyrex glass rings �Ra-chig type, 5 mm outside diameter, 1 mm thick, 10 mm long,.51 cm2 total surface, internal and external�, Pyrex glass rods5 mm diameter, 176 mm long, 27.60 cm2 lateral surface�, andyrex glass microslides �Superior, Germany, 76�26 mm, 1 mm

hick, 19.8 cm2 per face� were used.PET soft drink and mineral water bottles of different local

rands with the highest UV transmittance were selected, on theasis of spectrophotometric measurements. Two of them resultedhe most convenient as they transmitted more than 90% of theight at ��200 nm. Finally, soft drink �Coca-Cola®� bottles werehosen because of the potential higher availability in the placeshere the technology might be used. Small �“sm,” 650 mL total

apacity� and large �“lg,” 1560 mL total capacity� PET bottlesere tested.Commercial TiO2 Degussa P-25 �Germany� was used as pro-

ided. 2,4-D �“Herbifen,” Atanor S.A., dimethylammonium salt ofhe 2,4-dichlorophenoxyacetic acid, 48.5% w/v as acid� was used.

itanium tetraisopropoxide �98%, Merck�, 4-CP �Fluka�, and all

20 / Vol. 129, FEBRUARY 2007

other chemicals were of analytical reagent grade. Deionized water�resistivity=18.0 M� cm�, obtained from a Millipore Milli Q ap-paratus, was used for the preparation of solutions and washings.

For spectrophotometric measurements, a Hewlett-Packard di-ode array UV-visible spectrophotometer, model HP 8453 A, wasused. A Konik-500-A HPLC chromatograph, with a LINEAR UV-VIS-204 detector was used for liquid chromatographic measure-ments. NPOC �nonpurgeable organic carbon� measurements werecarried out using a Shimadzu 5000-A TOC analyzer. XRD pat-terns were obtained at room temperature with a Philips PW-3710diffractometer using CuK� radiation. Ultrasonications were per-formed with a Cleanson �25 KHz� ultrasonicator, model CS-1109.It was determined that the ultrasound frequency did not deterio-rate porcelain, glass, or PET, as detected by the constancy ofweight before and after the most prolonged sonication of eachmaterial in this work.

Artificial and solar UV radiation was measured at 365 nm witha Spectroline DM-365 XA radiometer. For solar UV radiation,average values were taken from periodical measurements �eachhour�.

Impregnation Procedure

Porcelain Beads. Beads were impregnated with TiO2 accord-ing to the previously published procedure �23�. Briefly, sphereswere introduced in a 0.1 M HNO3 solution for 2 h, washed 4times with water and dried at 100°C. Then, they were immersedin a 1 M KOH solution for 48 h, washed 4 times with water, anddried again at 100°C. A 10% w/v TiO2 suspension, previouslyadjusted at pH 2.5 with HClO4, was ultrasonicated for 30 min,and the spheres immersed in the suspension for 15 min. The ex-cess of suspension was drained and the material dried at 100°Cand calcined at 450°C for 2 h at a 3.3°C min−1 heating rate.Finally, the spheres were rinsed for 15–30 min with a vigorouswater jet to eliminate loose particles, and dried at 100°C. Theimpregnation procedure was repeated 2, 4, and 6 times. Before thephotocatalytic experiments, beads were washed according to thefollowing procedure. The number of fillings to be used in eachexperiment was introduced into a “sm” bottle together with250 mL of water. Each set of beads was arranged in a longitudinaltulle bag, tied at the top with a nylon thread, to reduce shocksbetween them and with the walls. The bottle was manually shakenfor 30 s �moderate shaking�, and the 365 nm transmittance of thewashings was measured. This procedure was repeated until con-stant transmittance in the washings. In the case of the beads im-pregnated with 6 TiO2 layers, 7 washings were needed to arrive toa limit value of 80% transmittance. These materials will be called“beads.”

Glass Supports

Microslides Preliminary tests to evaluate the best procedure forTiO2 fixation on glass fillings were carried out on microscopeslides �plain and frosted�. For frosting, carborundum powder wasrubbed directly on the glass surface on only one face �the otherremaining unfrosted�. Several impregnation methods were testedin different conditions. Only 75% of the microslide surface �i.e.,14.85 cm2 per face� was impregnated with TiO2 in all cases. Cal-culations with noncoated slides have been made to discount theamount of TiO2 fixed to the nonfrosted surface. The following fiveprocedures were found to be the best.

�1� The plate was immersed in KOH �pH 13.5� for 24 h,washed with water and dried at 130°C for 24 h. Then, a 2% �w/v�TiO2 aqueous suspension at pH 2.5 �HClO4� was applied by dipcoating �at 3 mm/s�. The material was dried at room temperaturefor 10 min and then at 130°C for 24 h. The procedure was re-peated four times. Finally, the microslide was calcined at 550°C

−1

for 2 h at a 3.3°C min heating rate.

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�2� The same procedure as in �1�, but HNO3 was used instead ofClO4 to adjust the pH to 2.5.�3� The microslide was washed for 30 min with water under

ltrasound, and dried at 130°C for 24 h. Then, a 2% �w/v� TiO2uspension in isopropanol was applied by dip coating �atmm/s�. The drying procedure was the same as in �1�. The coat-

ng was repeated four times. The slide was finally calcined at50°C for 2 h at a 3.3°C min−1 heating rate.�4� The microslide was washed for 30 min with water under

ltrasound, and dried at 130°C for 24 h. Then, the support wasip coated �at 1 mm/s� with a solution of titanium tetraisopro-oxide in isopropanol at a molar 1:3 ratio. The equivalent TiO2oncentration provided by the Ti precursor was 15% �w/v�. Theaterial was dried at 130°C for 24 h, followed by calcination at

50°C for 6 h at a 3.3°C min−1 heating rate. A transparent, com-act film was obtained over the glass surface.

�5� The same procedure as in �4�, followed, after calcination, byip coating �3 mm/s� of a 2% �w/v� TiO2 aqueous suspension atH 2.5 �HClO4�. Drying was at room temperature for 10 min andt 130°C for 24 h. The impregnation and drying procedure wereepeated four times. Finally, the microslide was calcined at 550°Cor 2 h at a 3.3°C min−1 heating rate.

As the material prepared according to procedure �1� showed theest adherence and photocatalytic activity �see later�, this proce-ure was applied for the impregnation of glass rods and ringsboth frosted�, with some slight modifications as follows:

Glass Rods. Rods were introduced in a sanding machine torost the surface, washed with water under ultrasound for 1 h, left8 h in KOH �pH 13.5�, rinsed abundantly with water and driedh at 130°C. Then, each rod was dip coated �at 3 mm s−1, 85%

f the surface, i.e., 23.56 cm2� with a 2% w/v aqueous TiO2 sus-ension, pH 2.5 �HClO4�, previously ultrasonicated for 30 min.fter coating, rods were dried 24 h at 130°C. The impregnationrocedure was repeated four times. Finally, supports were cal-ined at 450°C for 2 h at a 3.3°C min−1heating rate. Before thehotocatalytic experiments, fillings were washed as in the case ofbeads;” only 2 washings were needed to attain a constant 365 nmransmittance �99%� in the washings. These materials will bealled “rods.”

Glass Rings. The TiO2 impregnation procedure was the sames in the case of “rods.” Internal surface frosting was not neces-ary, as abrasion by contact, causing TiO2 detachment, would notake place. For dip coating, 11 rings were threaded in a cottontring. Before the photocatalytic experiments, the fillings wereashed as the previous materials; in this case, 8 washings wereeeded to attain a constant 365 nm transmittance �100%�. Theseaterials will be called “ringsk.” One lot �“rings”� was prepared

kipping the KOH immersion step.

PET Bottles. “Sm” and “lg” bottles were washed with waternder ultrasound for 1 h, and dried 48 h at room temperature. TenL of a 2% w/v TiO2 suspension, pH 2.5 �HClO4�, previously

ltrasonicated for 30 min, were introduced in the bottle, whichas then shaken and rolled long enough to obtain a homogeneouslm over the entire wall. The remaining suspension was drainednd the bottle left inverted, taking care that no drops or irregulari-ies were formed on the surface. The bottle was dried at roomemperature for 24 h. The impregnation was repeated twice, re-ulting finally a thin, semitransparent, and homogeneous TiO2lm. To eliminate loose particles, the bottle was half filled withater and vigorously �manually� shaken for 0.5 min. This proce-ure was repeated twice; no TiO2 detachment was noticed. Beforehe photocatalytic experiments, the bottle was washed as in pre-ious cases; only 2 washings were needed to attain a constant65 nm transmittance �100%�. These materials will be called “2%ot.”

As an alternative, a 10% w/v TiO2 suspension was used, but

ournal of Solar Energy Engineering

only 1 layer was applied; the resulting TiO2 film was not as regu-lar as the one obtained by the previous procedure, but the detach-ment behavior was the same. These materials will be called “10%bot.” A 100% of transmittance in the washings was obtained afterwashing as in the case of “2% bot.”

Evaluation of Adherence and Amount of TiO2 DepositedThe amount of titania attached to the different supports and to

PET bottles was determined gravimetrically. In the case of“beads,” the procedure has been described previously �23�, mainlyconsisting in the ultrasonication of some beads in water for30 min, rinsing with water, and drying for 24 h at 100°C; theTiO2 mass was the difference in weight before and after the de-tachment. The adherence of this material was evaluated in Ref.�23�.

In microslides, the amount of fixed TiO2 was calculated as thedifference in weight after and before impregnation. To evaluatethe fixation stability and adherence, the plates were immersed in50 mL water and ultrasonicated for 30 min. The amount of TiO2remaining at the end of the ultrasound treatment was calculated asthe difference in weight of the support after sonication and beforeimpregnation.

The amount of TiO2 fixed in rings, rods, and bottles was esti-mated as in the case of “beads.” “Rods” �4�, “ringsk” �40�, and“rings” �40� were dried at 130°C for 15 h, while “sm” bottles ofboth classes were dried 2 h at 35°C. The mass of each set ofcoated supports and of each coated bottle was registered. Then,the glass fillings were introduced in a “sm” clean bottle togetherwith 200 mL of water and ultrasonicated for 4 h; the same wasmade with the 2% “sm” bottle, while it was necessary to sonicate8 h the 10% “sm” bottle to eliminate all TiO2. In each case, thewater was filtered through a previously weighed 0.45 �m mem-brane filter, and the supports or bottles were dried as before andweighed again. In addition, each filter was dried in an oven at60°C for 12 h, and then weighed to have independent measure-ments. In all cases, both sets of data differed in no more than 15%and average values were taken.

Aging of TiO2-Coated PET Bottles. TiO2-coated “sm” PETbottles were totally filled �650 mL� with water and left in therooftop for a prolonged period �30 days�. Five mL samples of thewater were withdrawn after 15 and 30 days for NPOC measure-ments and 2 mL for UV-vis spectrophotometric analysis, previ-ously filtered by 0.45 �m filters.

Photocatalytic TestsPreliminary tests to evaluate the activity of TiO2-coated mi-

croslides were done under artificial UV irradiation using a PhilipsHPA 400S 9G lamp, with a cutoff filter ���300 nm�; the 365 nmlight intensity was 45 W/m2. Here 25 mL of a 0.2 mM 4-CPsolution, adjusted at pH 3 with diluted HClO4, were poured in aPyrex Petri dish and irradiated from above under magnetic stir-ring, taking periodically 1 mL samples to follow 4-CP concentra-tion. The same photocatalytic system was used to determine thebest number of TiO2 layers on porcelain beads.

Solar tests were performed on sunny days of June 2005, on therooftop of the laboratory in Buenos Aires �34° 38� S, 58° 28� W�,between 10 a.m. and 4 p.m. The average insolation in the cityduring June 2005 can be estimated in 2.1 KWh/m2 �monthlymean global energy, in the range 0.3–3 �m�, from which 4.5%,i.e. around 0.1 KWh/m2, is UV light �28�. The ambient daytimetemperature during the test period was 11–14°C.

PET bottles were used as solar photoreactors. Tests were per-formed with 4-CP �the same conditions as before� and 2,4-D�0.5 mM, pH 6.8�. “Sm” and “lg” bottles containing coated fill-ings or TiO2 on their wall, were filled with 250 and 600 mL,respectively, of the pollutant solution. The bottles were placed

directly on the rooftop floor over aluminum wrapping foil, and

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xposed to sunlight for several hours. During irradiation, the tem-erature in the bottles never exceeded 40°C. In “sm” bottles, 165beads” were arranged in a longitudinal tulle bag �200 mm length,0 mm width�; in “lg” bottles, 396 “beads” were arranged in anrregular shape �480 mm total length, 20 mm width�. “Rings” andringsk” were threaded in a cotton string, 36 in the “sm” bottlend 87 in the “lg” one. “Rods” were put free inside the bottle, 4 inhe “sm” and 8 in the “lg.” In all cases, the total amount of TiO2n the supports in “sm” and “lg” bottles was proportional, withhe exception of the “rods,” whose number was merely duplicatedn “lg” bottles.

4-CP concentration was followed spectrophotometrically at25 nm, taking 1 mL samples diluted to 10 mL. The validity ofhe measurement methodology has been explained in Ref. �23�.

2,4-D concentration was followed by HPLC taking 3 mLamples that were injected to the chromatograph �injection loop:00 �L�, previous filtration through 0.45 �m membrane filters.he measurement was made, modifying slightly the method re-orted in �29�. A THERMO AQUASTAR RP-18 column �length:50 mm, diameter: 4.6 mm� was employed. The eluent wasH3CN-H2O �1+4�, 0.3 M NaOH, pH 2.95 �H3PO4�, at a.2 mL/min flow; the measurement was taken at 280 nm.

In the case of 2,4-D, NPOC measurements were made on 5 mLamples, only before and at the end of irradiation.

esults and Discussion

Deposition Technique and Amount of TiO2 Deposited. Topply TiO2 suspensions to glass surfaces, the dip-coating methodas used because the most uniform layers were obtained in com-arison with simple immersion, spraying, or brushing. Thisethod has the advantage that the rate of film growth and there-

ore its thickness can be controlled �14�. It was found that speedsower than 3 mm/s gave nonuniform TiO2 films, while higherpeeds gave thicker films with bad adherence. The application ofore than four layers did not increase the amount of fixed TiO2.

n the case of titanium tetraisopropoxide solutions, dip-coatingpeeds lower than 1 mm/s gave also nonuniform films, and higherpeeds formed unstable coatings after calcination, made of veryiny particles that were easily detached from the surface; the sameffect was observed with the application of more than one layer ofhe solution. Based on all these preliminary results, the final cho-en conditions for fixation on microslides were procedures 1–5escribed in the experimental section.

TiO2 mass on the supports is shown in Table 1. The amount ofatalyst remaining on the solid after 30 min ultrasonication is alsoresented and allows evaluating the adherence and stability of thexation. The results show that most materials prepared withrosted surfaces fix more TiO2 than nonfrosted surfaces, with thexception of microslides 4 and 5. Slides 1 and 2 �frosted� pre-ented similar amounts of fixed TiO2. The use of isopropanol

able 1 Amount of TiO2 fixed to the microslides and remain-ng after ultrasonication

lideFixed TiO2

�g�g

TiO2/m2

TiO2remainingmass �g�

% remainingTiO2

�frosted� 0.0011 0.74 0.0011 100�nonfrosted� 0.0002 0.13 0.0000 0�frosted� 0.0011 0.74 0.0001 7.1�nonfrosted� 0.0004 0.27 0.0001 14.3�frosted� 0.0020 1.35 0.0000 0�nonfrosted� 0.0011 0.74 0.0000 0�frosted� 0.0010 0.67 0.0010 100�nonfrosted� 0.0012 0.81 0.0010 82.6�frosted� 0.0024 1.62 0.0008 33.3�nonfrosted� 0.0020 1.35 0.0010 47.5

nstead of water �slide 3� increases the quantity fixed in frosted as

22 / Vol. 129, FEBRUARY 2007

well as in unfrosted surfaces, indicating that isopropanol wouldprovide a better medium to link glass with TiO2. Procedures 4 and5, which form an initial TiO2 layer by titanium isopropoxide hy-drolysis, yielded, as said, similar results independently of theroughness of the surface. The highest amount of TiO2 was fixedby method 5, which combines both techniques, but, as we willsee, the final adherence by this procedure is not good. After30 min of ultrasonication, the whole mass or an important quan-tity of TiO2 was lost in various cases, mainly from unfrostedsurfaces �1, 2, 3�. This did not occur with materials prepared byprocedures 1 and 4 on frosted surfaces. Procedure 4 was the onlyone that did not lead to a considerable loss of TiO2 when preparedover an unfrosted surface. Although procedure 2, which usedHNO3 to acidify the TiO2 suspension, fixed an equal amount ofthe catalyst than procedure 1, which used HClO4, this fixation wasnot stable. In case 3, the frosted and nonfrosted surfaces retainedsimilar amounts of catalyst, which were completely lost after ul-trasonication, indicating an unstable fixation. In the case of mate-rials prepared by procedure 5, the amount of retained TiO2 wassimilar to that retained by coatings prepared by procedure 4,meaning that the TiO2 fixed in the second step is very unstableand detaches easily; the first TiO2 layer, prepared by titaniumisopropoxide hydrolysis, seems to be strongly attached to the glasssurface �frosted or not�. Of course, titanium isopropoxide is amore expensive precursor than Degussa P-25 and, as we will seelater, the photocatalytic efficiency of this material is not good.

After these preliminary results, procedure 1 was chosen forcoating TiO2 on rods and rings. It was also used for coating PET,eliminating all heating treatments. In Table 2, the amount of TiO2fixed to the different supports is shown. Surface areas covered byTiO2 were calculated according to the geometrical shape of eachsupport; in the case of PET Coca-Cola bottles, a cylindrical plus atruncated conical surface was taken as an approximation.

It can be seen that immersion in KOH favors impregnation, andthis effect will be also observed in photocatalytic experiments.Rods and bottles present a lower amount of TiO2.

Evaluation of the Performance of Different Fixation Meth-odologies. Photocatalytic Tests. Photocatalytic tests to evaluatethe activity of the different TiO2-coated microslides �all frosted�were made following 4-CP degradation, before and after treatingcoated slides by sonication for 30 min �these last will be named“treated”� �Fig. 1�. “Treated” 2 and 3 were not evaluated becauseit did not contain TiO2. For comparison, results without TiO2 andwith TiO2 suspensions at two concentrations �0.044 g/L, the sameTiO2 amount of slides 1 and 2, and 0.44 g/L� are shown. Initialrates �R0� have been calculated from the plots and are presented inTable 3. Of course, suspended TiO2 reacted much faster because,as known, the activity is very much reduced when the catalyst isimmobilized, due mainly to mass transfer limitations and less lightavailability �23,22�. The results show the highest �similar� R0 formicroslides 1, “treated” 1 and 5, which present also the highestdegradation extent after 120 min of irradiation. “Treated” 5 shows

Table 2 Amount of TiO2 fixed to the different supports used inthe solar photocatalytic experiments

SupportImpregnatedarea �cm2�

mgTiO2/g

of supportg

TiO2/m2

“Beads” �six layers� 1.13 2.10 8.00“Ringsk”a 2.51 1.24 2.57“Rings”a 2.51 1.01 2.12“Rods” 23.56 0.08 0.27“Sm 2% bot” 525.00 0.18 0.09“Sm 10% bot” 525.00 0.21 0.10

aIncludes the whole surface, internal and external.

a much lower activity, in accordance with the loss of an important

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mount of TiO2. Interestingly, slide 2, containing the same TiO2mount than slide 1, is less active, taking into account that thenly experimental difference between them is the use of HNO3nstead of HClO4 to adjust pH in the TiO2 suspension used formpregnation. This might be related with the observation byRMC of a decreased recombination of electron and holes wheniO2 supported on trans-decalin was irradiated in the presence ofaClO4 �30�. Slide 3 is less efficient than slide 1, although con-

aining more TiO2; as it was prepared with isopropanol instead ofater, some rests of organic matter might have remained even

fter calcination, competing with 4-CP. The activity of plate 4,ontaining TiO2 prepared from titanium isopropoxide hydrolysis,as lower than those containing P-25, a result frequently observed

n photocatalytic works. In agreement, DRX patterns showed thathe anatase/rutile ratio of P-25 �ca. 80:20� was maintained in theoating of slide 1, while TiO2 on slide 4 was pure anatase. Nor-alized initial rates, taking into account the mass of TiO2 on each

lide, are presented also in Table 3. Of course, this is a very roughomparison because not all TiO2 participates in the photocatalyticeaction, as the deep-lying particles are badly exposed to the lightnd far away from the pollutant solution; however, this calculationllows making a different comparison of the results. The bestample is microslide 1, because it holds the lowest amount ofiO2 and produces the highest rate. In addition, it shows a similarfficiency before and after the ultrasound treatment, which meanshat the fixation procedure would be suitable for supports sub-ected to extreme mechanical stress and shocking. Note that theormalized rate for suspended TiO2 at the highest concentration isower than the rate using a TiO2 suspension ten times more di-uted.

Fig. 1 4-CP photocatalytic degradatTiO2-coated microslides. †4-CP‡=0.2 m

able 3 Initial rate, normalized initial rate and extent of degra-ation at 120 min for 4-CP degradation over TiO2-coatedicroslides

aterialg

TiO2/L%

degradationR0�107

�M min−1�R0Norm�104

�M min−1 g−1�

iO2 suspension 0.044 100 22.6 20.5iO2 suspension 0.440 100 29.4 2.7lide 1 0.044 66.1 12.8 11.6lide 1 “treated” 0.044 63.0 10.6 9.6lide 2 0.044 31.4 6.8 6.2lide 3 0.080 39.5 7.2 3.6lide 4 0.040 17.4 3.8 3.8lide 4 “treated” 0.040 17.4 2.8 2.8lide 5 0.096 69.0 13.0 5.4lide 5 “treated” 0.032 14.0 2.4 3.0

ournal of Solar Energy Engineering

Porcelain Beads. Evaluation of the Activity According to theNumber of Applied Layers. The evaluation was made also with4-CP in the same conditions and setup as in the case of the mi-croslides. Beads coated two, four, and six times were tested. Re-sults are shown in Fig. 2.

As shown, the best results were those obtained with beads im-pregnated six times. Consequently, this material was the one usedin the final solar tests in bottles.

Aging of TiO2-Coated PET Bottles. NPOC and 254 nm ab-sorbances were measured in samples taken from TiO2-coated PETbottles �“2% bot” and “10% bot”� completely filled with waterand left for 30 days on the rooftop. In all cases, the measuredamount of organic carbon was under the limit of determination ofthe equipment or very close to it �0.5–1.5 mg/L�, while absor-bances ranged 0.006–0.010. Therefore, it is concluded that TiO2does not deteriorate the plastic and that no strange substances areintroduced to the water, a condition already established for SODIS�8�. Besides, it is not expected that water be stored in the bottlesfor a long time before consumption.

Solar Photocatalytic Tests in BottlesThe bottles were used as the photoreactors, using “ringsk,”

“rings,” “beads,” “rods,” and TiO2-coated bottles.The rational design of the modified bottle as a solar photoreac-

tor is not easy. The disinfection or decontamination process musttake place in the interface catalyst/substrate, practically in thesame moment the photon is absorbed by the catalyst. Any delay in

under artificial light irradiation onpH 3, I0„365…=45 W/m2.

Fig. 2 Influence of the number of layers on the 4-CP photo-catalytic degradation on “beads.” †4-CP‡=0.2 mM, pH 3, I0„365…

2

ionM,

=45 W/m .

FEBRUARY 2007, Vol. 129 / 123

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he time required for the electron/hole pair to reach the surfaceesults in a substantial decrease of the efficiency. Similarly, anyltering effect by intervening material results also in dark, uselessegions in the unshaken bottle. An important variable is thus theirection of impingement of solar radiation on the catalyst cover-ng the inner surface of the bottle. In the case of bottles withompletely impregnated walls, the film thickness must be lownough to permit the radiation to reach the catalyst from above.ith fillings as impregnated rings, rods, or beads, the way to pack

hem in the bottle affects the efficiency in unpredictable ways. Forhese reasons, in this paper the efficiency was tested experimen-ally, without any attempt to model the photoreactor. The bottlesere put on the floor and the collection of the most possible

mount of solar irradiation was facilitated by the use of aluminumoil. Considerations about the oxygen availability into the bottlend about the treated volume and bottle size have been also made.

The activity for degradation was measured in partially filledottles of two different sizes �“lg,” 1560 mL and “sm,” 650 mL�,o evaluate the effect of the optical pathlength into the bottle. Allxperiments were performed simultaneously to assure the samehoton flux and ambient conditions, together with control experi-ents �illuminated, no TiO2�. Bottles were periodically opened

nd shaken to renew the amount of oxygen inside the solution,nd samples were taken at regular times for analysis. A first set ofhotocatalytic tests was performed with 4-CP. Further experi-ents with 2,4-D were made only with the two most efficientaterials �“ringsk” and “10% bot”�.The use of similar amounts of TiO2 on each support, although

ttempted, was not possible because of the many variables ruling

ig. 3 4-CP photocatalytic degradation under solar irradiationn different supports in “sm” bottles. †4-CP‡=0.2 mM, pH 3.verage UV light intensity at 365 nm: 5.9 W/m2.

ig. 4 4-CP photocatalytic degradation under solar irradiationn different supports in “lg” bottles. †4-CP‡=0.2 mM, pH 3. Av-

2

rage UV light intensity at 365 nm: 5.9 W/m .

24 / Vol. 129, FEBRUARY 2007

this parameter, mainly the impregnation procedure. To comparethe efficiency of the different materials, the amount of TiO2reached by solar light and in contact with the pollutant solution ineach bottle �named “active” surface� was calculated. The follow-ing considerations and assumptions were made, taking into ac-count the disposition of glass pieces into the bottles and the vol-ume of water into the TiO2 coated bottles:

− For “rings” and “ringsk,” 10% of the total surface was incontact with the bottom of the bottle, totally in the darkness. Thus,the “active” surface is 90% of the total.

− Ten percent of “beads” were covered by the rest and notirradiated. Only 50% of the remaining surface was illuminated,meaning that only 45% of the total surface is “active.”

− If 4 “rods” were used, the internal two were 50% irradiatedand the external two, 75%. In the case of 8 “rods,” 6 were innerand 2 external. The “active” surface is around 56% of the total.

− In both “2% bot” and “10% bot,” it was calculated that 47%of the surface was covered by the solution �the same for “sm” and“lg”� and 10% was in contact with the bottom; this leads to only37% of “active” surface.

In Figs. 3 and 4, the results of 4-CP solar degradation tests in“sm” and “lg” bottles, respectively, are presented. In all cases,plots could be approximated to a very good first order decay, fromwhich rate constants were calculated and presented in Table 4,together with the extent of 4-CP degradation after 18 h undersunlight. Small induction periods could be seen in some degrada-tion profiles, but they were not taken into account in the analysis.

Rate constants and degradation extent in large and small bottleswere comparable, only somewhat lower in large bottles. As watervolumes were proportional, the results indicate that the amount ofoxygen and light inside the bottles was enough for the oxidationand that the size is not a crucial parameter, at least working inbottles of the tested sizes. The best rates and conversions wereobtained using “beads.” However, after around 6 h of solar irra-diation and handling of the bottles, a large amount ofTiO2—visually seen—was detached from these fillings, owing tothe abrasion and shocks between the small pieces. It was notpossible, in this case, to accurately measure 4-CP concentrationbecause of the turbidity caused by TiO2 coming in suspension,corrupting the UV measurements. Moreover, if the evaluation hadbeen done after filtration, the results would have been affected byan error in excess because the more efficient TiO2 in suspensionwould have contributed to the activity. This effect was not ob-served with the other fillings, very much stable. It seems that thecylindrical shape of rods and rings, having less exposed area for

Table 4 “Active” TiO2 surface per bottle, concentration of irra-diated TiO2, first order rate constant and percentage of 4-CPdegradation after 18 h of solar irradiation in bottles

Material

“Active”surface�cm2�

gTiO2

irradiated/Lk�104

�min−1�%

degradation

“Sm” bottles

“Beads” 83.90 0.268 40 88 �at 360 min�“Ringsk” 81.32 0.084 19 88“Rings” 81.32 0.069 17 82“Rods” 58.90 0.006 9 60“2% bot” 194.25 0.007 10 66“10% bot” 194.25 0.008 13 77

“Lg” bottles

“Beads” 201.37 0.268 21 55 �at 360 min�“Rings” 196.53 0.070 15 83“Rods” 106.02 0.005 7 48“2% bot” 357.79 0.005 9 63“10% bot” 357.79 0.006 8 59

shocks and scratches between pieces, is better than a spherical

Transactions of the ASME

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hape. “Ringsk” yielded very good results; the activity of “rings”not treated with KOH� was somewhat lower, indicating the im-ortance of this step for the immobilization, as the deposit of theatalyst is facilitated by electrostatic attraction �23�. The activityf “rods” was low, and this material appears to be not very goods a photocatalyst, probably due to the low amount of coatediO2. Both “2% bot” and “10% bot” bottles are in between. Com-aring the different fillings �rods, beads, and rings�, rings have theargest “active”/total area ratio �90%� and also the largest “active”rea/volume of fillings ratio. However, if “active” surfaces andiO2 concentration are considered, bottles and “rods” are muchore active than rings, as they hold less TiO2. In particular,

ottles have a larger “active” surface than that of “rings” orringsk,” suggesting that in these supports some TiO2 is not prof-ted and useless. From both coated bottles, “10% bot” is slightlyetter, with the additional advantage of only one impregnationayer.

In previous experiments in small “2% bot” and “10% bot” per-ormed in March 2005, the oxygen availability had been testedTable 5�. Completely filled bottles �650 mL� were left on theooftop for 7 days, exposed in total to 56 h of solar radiation.lthough care was taken to open filled bottles each day for oxy-enation, it took a very long time to reach a high degradationxtent, while these values were rapidly attained in partially filledottles �250 mL�, indicating that the availability of dissolved oxy-en is crucial for the photocatalytic degradation. Note that theigher sunlight intensity available in March leads to a faster 4-CPegradation �cf. Tables 4 and 5�.

Results of 2,4-D photocatalytic degradation with the best fill-ngs �“ringsk” and “10% bot”� in “sm” bottles are presented inig. 5. A pH decrease from 6.8 to 4.5 was measured in theringsk” system after 10 h of irradiation. The curves can be fittedo a very good first order, in agreement with the results of Her-mann et al. in a different photocatalytic setup �31�. A slight de-rease �20%� in the control �irradiated, no TiO2� was observed at

able 5 Behavior of completely and partially filled “sm”iO2-coated bottles in the solar degradation of 4-CP

aterial % degradationSolar irradiation

time

2% bot”, 650 mL 83a56 h

10% bot”, 650 mL 72a56 h

2% bot”, 250 mL 98b12 h

10% bot”, 250 mL 100b12 h

average UV light intensity at 365 nm: 1.7 W/m2.average UV light intensity at 365 nm: 2.0 W/m2.

ig. 5 2,4-D photocatalytic degradation under solar light irra-iation on different supports in small bottles. †2,4-D‡=0.5 mM,

2

H 6.8. Average UV light intensity at 365 nm: 5.3 W/m .

ournal of Solar Energy Engineering

the beginning; as 2,4-D presents a band in the UV, centeredaround 290 nm, the degradation can be attributed to direct homo-geneous photolysis under sunlight. However, the decrease ceasedat longer times. Both supports yielded similar results, althoughslightly better with “ringsk.” In Table 6, rate constants, 2,4-Ddegradation extent and NPOC decrease are presented. In the chro-matograms, an increasing unknown peak was noticed since thefirst times of irradiation, belonging probably to 2,4-dichlorophenol, a known intermediate of 2,4-D degradation �31�;however, its detection was not attempted. NPOC results validatedthis assumption, as the decrease was slower than that of 2,4-D.

It was found again that although in absolute terms constantrates and conversion are similar for both materials, coated bottlesare much more efficient because of a better profit of TiO2 in alarger area.

It is remarkable the good degradation obtained for both pollut-ants after only a few irradiation hours under the low solar irradi-ance in the winter of Buenos Aires.

ConclusionsTo obtain stable films of TiO2 on supports for use in bottles to

potabilize water, various materials and deposition techniques weretested. TiO2-coated porcelain beads, proved in a previous work tobe very stable in turbulent recirculation regimes, were not foundto be stable to abrasion and shocks, as needed for application inbottles. In contrast, TiO2-coated glass rings, rods, or TiO2 directlyfixed to the wall of PET bottles, were better materials for thisspecific use. The preparation of the best materials only needs anaqueous suspension of the low-cost Degussa P-25, without the useof other more expensive precursors. Although the suspensionswere acidified with the costly and unsafe HClO4, only a lowamount of the acid was used. All the materials were stable for thisparticular application, cheap and easy to fabricate. In addition,they presented a good ability to degrade model compounds, evenunder the low sunlight radiance of winter days in Buenos Aires.With respect to glass materials, rings were the best fillings, pre-senting resistance to shocks and scratches due to their convenientgeometrical shape, and good “active”/total area and “active” area/volume of fillings ratios.

TiO2-impregnated PET bottles were superior to glass materialsbecause they exhibited similar activity, but with the advantage ofa lower TiO2 content, a more extended TiO2 area exposed to light,and an easier preparation: only one layer of a TiO2 suspension andno calcination or other thermal treatment are needed. The TiO2film is very stable, and no fragile fillings are used. The adherenceis optimal and no deterioration of PET by TiO2 was observed aftera prolonged time under weather conditions. Although a half im-pregnated bottle would collect more solar light, the formation of ahomogeneous TiO2 layer on the entire surface of the bottle mayresult easier for people; besides, this upper TiO2 layer not in con-tact with water might eliminate very rapidly bad odors or toxiccompounds in the gas phase. The main advantage of the materialis that it can be easily handled and directly prepared by conve-niently trained people in isolated settlements. Of course, tests withreal waters must be done to evaluate probable interferences and

Table 6 “Active” surface per bottle, concentration of irradi-ated TiO2, first order rate constant and percentage of 2,4-D deg-radation after 12 h of solar irradiation in bottles

Material

“Active”surface�cm2�

gTiO2

irradiated/Lk�104

�min−1�%

degradation% NPOCdecrease

“Ringsk” 81.32 0.084 15 58 2710% bot 194.25 0.008 12 65 29

inhibitions of the photocatalytic process.

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Efforts to optimize the plastic material, i.e., to find the rightiO2 concentration in the impregnating suspension, the effect ofainting completely the surface with the catalyst or only the halfr in stripes, etc. are underway, together with disinfection experi-ents and tests with real waters.On the other hand, “ringsk” or even “rings” could be used as

iO2-coated supports in other technological applications in whichhotoreactors must be packaged with small glass fillings. Tests aren progress.

cknowledgmentWork was performed as part of Comisión Nacional de Energía

tómica P5-PID-36-4 Program, American States OrganizationE/141/2001 Project and Agencia Nacional de Promoción Cien-

ifica y Tecnología, PICT98-13-03672. We thank A.G. Leyva �Un-dad de Actividad Física, CNEA� for help with XRD spectra and. Babay for help with HPLC measurements. M.I.L. and M.A.B.re members of CONICET. J.M.M. thanks CONICET for a doc-oral fellowship. We thank Atanor S.A. for providing the 2,4-Dample.

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Benzofuran,” Appl. Catal., B, 17, pp. 15–23.

Transactions of the ASME