photoactivity of tio 2 -coated pebbles

Photoactivity of TiO2-Coated Pebbles

Nageswara N. Rao* and Vibha Chaturvedi

Wastewater Technology DiVision, National EnVironmental Engineering Research Institute, Nehru Marg,Nagpur 440 020, India

Development of immobilized TiO2 photocatalysts for solar photocatalytic degradation of organic pollutantsis a technological need. In the present study, Degussa P-25 TiO2 photocatalyst was coated on pretreatedpebbles and the photoactivity of TiO2/pebbles is reported for the first time. Three types of pebbles, i.e., black(B), red (R), and white (W), and a mixture (M) having equal proportions of B, R, and W, were chosen. Thepretreatment of pebbles constituted washing with deionized water or leaching with EDTA solution. Coatingof TiO2 on the pretreated pebbles was carried out as per the previously made titania powder (PMTP) method.X-ray powder diffractograms (XRD) of powdered pebbles and analysis for various metals in washings andleachates using an inductively coupled plasma-optical emission spectrometer (ICP-OES) and an atomicabsorption spectrophotometer (AAS) were carried out. Solar photocatalytic decolorization of reactive black5 (RB5) and some other reactive dyes was tested using TiO2/pebbles in an open dish method. The percentdecolorization of RB5 using TiO2/white washed pebbles (TiO2/WW) and TiO2/white leached pebbles (TiO2/WL) was 59% and 99%, respectively, at the end of 5 h of exposure to sunlight. In contrast, less than 28%decolorization was found with all the other TiO2/pebble systems. On the basis of the apparent first-order rateconstants, the TiO2/WL was found 63-81 times more efficient when compared to the lowest efficiency TiO2/black leached (TiO2/BL) pebbles. The difference in photoactivity of various TiO2/pebble systems wasrationalized in terms of interaction between metal oxides/ions native to pebbles and illuminated TiO2.

1. Introduction

Titanium dioxide has aroused tremendous research interestdue to its photocatalytic activity for the degradation of organicand inorganic pollutants.1,2 The TiO2 photocatalyzed degradationof organic pollutants has received further impetus from thepossibility of combining photocatalysis and solar technologies.2,3

It may be developed into a useful process for the reduction ofwater pollution because of the milder reaction conditions andlower energy costs of solar photocatalytic technology.

As a step toward practical solar photocatalytic treatmentdevices, immobilization of TiO2 was considered especiallyuseful from the process-engineering point of view. The coatedsurface can be readily recovered and reused in the process unlikethe suspended systems.4 Various supports have been used forimmobilizing TiO2, such as ceramic,5 fiber glass,6 glass andsand,7 quartz and stainless steel,8 activated carbon,9 polyesterfabric using polyvinyl alcohol as binder, and Ti-TiO2 preparedby thermal as well as flame oxidation of Ti sheet.10 These studieswith immobilized titania revealed that the photodegradationefficiencies are somewhat low compared to those of slurry-basedprocesses. This effect was often attributed to the support-TiO2

interactions such as changes in the TiO2 energy band structuredue to chemical bonds with support and support-inducedmorphological changes (surface area and particle size) ac-companying the heat treatments.

In this paper, we prepared the TiO2-coated pebbles andexamined their photocatalytic activity toward decolorization ofsome reactive dyes. The selection of pebbles as supports forTiO2 was based on the following: (i) pebbles are readilyavailable at any civil construction site in the form of rejectsfrom sand-screening operation, (ii) because of their relativelylarger size, they can be arranged into a pebble bed reactor,

similar to a falling film reactor, and (iii) pebbles are exploredfor the first time as supports for TiO2.

2. Experimental Section

2.1. Materials. 1-Amino-8-hydroxy-2,7-bisazo[(p-vinylsul-phonic acid)]-naphthalene-(3,6-disulfonic acid) tetrasodium salt,C26H21O19N5S6Na4 (C.I. reactive black 5) was purchased fromM/s Atul Dyes, Ahmedabad. Some other reactive dyes used fortesting, viz., reactive orange 16 (RO16), reactive yellow 84(RO84), reactive red 141 (RR141), reactive red 2 (RR2), andreactive violet 13 (RV13) were a gift from M/s Color Chem.Ahmedabad. Titanium dioxide photocatalyst (P-25 TiO2, 80:20 anatase/rutile) was purchased from Degussa AG, Germany.Its specific BET surface area and mean particle diameter were50 m2 g-1 and 30 nm, respectively. Ethylenediaminetetraaceticacid disodium salt (EDTA) was a reagent grade chemical (EMerck India Pvt. Ltd., Mumbai). The metal ion standards forinductively coupled plasma-optical emission spectrometry (ICP-OES) and atomic absorption spectrophotometry (AAS) wereprocured from E. Merck India Pvt. Ltd., Mumbai. Deionizedwater (Millipore Elix 3 water purifier) was used for preparingall reagents.

2.2. Pebbles.Pebbles were collected from the rejects of sandat a construction site. The sand was quarried from the Kanhanriverbed that falls under the Kamthi region, Maharashtra, India.This region is overlaid by coarse-grained, ferruginous sandstonewith quartz pebbles.11 Three types of pebbles, i.e., black (B),red (R), and white (W), were selected based on their appearance(color) and size (cross-sectional length at least 0.5-1.0 cm).The mixed pebbles (M) having B, R, and W in equal proportionswere also considered.

2.3. Pretreatment of Pebbles.Initially the pebbles werewashed under tap water to remove the dirt present on the surface.They were then subjected to either washing with deionized wateror leaching with 0.1 M EDTA solution. Washing and leachingcycles were carried out in batches for different time intervals,

* To whom correspondence should be addressed. Fax:+91-712-2249900. E-mail: [email protected].

4406 Ind. Eng. Chem. Res.2007,46, 4406-4414

10.1021/ie0702532 CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 05/27/2007

viz., 6, 12, and 18 h. A 150 g sample of pebbles immersed in300 mL of deionized water or 0.1 M EDTA solution wasagitated over a mechanical shaker. The washings and leachateswere collected at the end of each batch and analyzed for heavymetals.

2.4. Preparation of TiO2-Coated Pebbles (TiO2/Pebbles).The photocatalyst-coated pebbles were prepared according tothe previously made titania powder (PMTP) method.4,12 Themethod consisted of spraying a 2% TiO2 suspension in 80:20ethanol/water mixture on the surface of pretreated pebbles anddrying at 60-70 °C by blowing hot air from an electric dryer.The spray-coating-drying method was repeated five times untilsatisfactory coating on the entire surface of the pebbles wasachieved. The TiO2/pebbles were then kept in an oven at 150°Cfor 8 h. The three types of pebbles before and after coating areshown in Figure 1.

As there are no reported procedures for estimation of thequantity of coated TiO2 on irregular surfaces such as pebbles,the following method was evolved. This method was based onsticking adhesive tape and peeling off the same from the surfaceand then estimating the weight gained by the adhesive tapepiece. For this purpose, one square centimeter pieces of cellotape were cut, weighed, and adhered to the pebble’s surfacehaving TiO2 supported on it. Subsequently, it was peeled andweighed again. The procedure was repeated five times by using1 cm2 cello tape pieces on the same pebble. The weight gain ineach case was calculated and added to arrive at the amount ofcoated TiO2. Recovery of catalyst onto the adhesive tape wasconsidered to be 80%. In the present case, the amount of coatedTiO2 was estimated as 0.00013 ((0.00002) g TiO2/cm2.

2.5. Solar Photocatalytic Experiments.A typical photo-catalytic experiment involved illuminating 50 g of TiO2/pebblesimmersed in 200 mL of RB5 dye solution (25 mg L-1, 22.5µM) taken into 250 mL Pyrex glass beakers (open dish) undersunlight. The duration of exposure to sun was approximately 5h, between 10:00 a.m. and 3:00 p.m. everyday. Mixing of thedye solution during exposure to sunlight was achieved bybubbling air into beakers using an aquarium pump and airdistribution manifold. A spherical sintered glass frit fixed atthe bottom of the beakers ensured proper mixing of dyesolutions. Test samples were withdrawn hourly, and absorptionof RB5 dye was monitored at 597 nm. Similar experiments weredone using the other reactive dyes also, viz., RO16, RO84,RR141, RR2, and RV13. All photocatalytic experiments wereperformed in duplicate batches for each type of dye. The batch-to-batch error was found to be in the range of 4-7%.

Appropriate control experiments were also set up to estimatethe background contribution of pebbles and TiO2/pebbles foradsorptive and photolytic removal of color from RB5 dyesolutions. As an open dish method was used for carrying outsolar photocatalytic testing of catalysts, a certain reduction inthe volume of test solution was noticed due to evaporation.Therefore, the reduction in volume was determined at the endof each batch, made up to initial volume by addition of deionizedwater before determining the actual color reduction due tophotocatalysis.

2.6. Analyses.Solar illumination was measured using a CarlZeiss luxmeter from 10:00 a.m. to 4:00 p.m. in the November-December period during which the photocatalytic experimentswere carried out. Generally, illuminance during November-December in Nagpur corresponds to a typical hazy day. Thesolar radiation intensity varied from 190 to 240× 104 lux (lx)from 10:00 to 11:30 a.m., 245-265× 104 lx from 11:30 a.m.to 2:00 p.m., 210-243 × 104 lx from 2:00 to 3:00 p.m., and

226-153 × 104 lx from 3:00 to 4:00 p.m. The average lightintensity during the study period was estimated as 36 W m-2

based on the average illuminance of 237× 104 lx.13

Test samples were analyzed for color after filtration througha 0.2µm syringe microfilter. All spectrophotometric measure-ments were carried out in the range of 200-700 nm using adouble-beam UV-vis spectrophotometer (Perkin-Elmerλ 900).The reduction in the color band intensity was determined fromthe time-overlaid UV-vis spectra. The specific color removalrates were obtained by photometric determination at the colorband of RB5, i.e.,λmax ) 597 nm,ε ) 0.0321 mg-1 L cm-1).Each data point was an average of data from duplicate runs.The corresponding wavelengths of maximum absorption for theother reactive dyes were as follows: RO16,λmax ) 493 nm,ε) 0.0214 mg-1 L cm-1; RR141,λmax ) 547 nm,ε ) 0.022mg-1 L cm-1; RO84,λmax ) 409 nm,ε ) 0.012 mg-1 L cm-1;

Figure 1.

Ind. Eng. Chem. Res., Vol. 46, No. 13, 20074407

RR2,λmax ) 538 nm,ε ) 0.019 mg-1 L cm-1; RV13, λmax )538 nm,ε ) 0.0237 mg-1 L cm-1. The washings and leachateswere analyzed for heavy metals using ICP-OES (Optima 4100DV, Perkin-Elmer, U.S.A.) and AAS (Perkin-Elmer, Analyst800). The test samples were appropriately digested as perStandard Methods.14 The average concentration of each metalion from two batches was reported. The batch-to-batch errorwas less than 5%. Simultaneously, the black, red, and whitepebbles were crushed into fine powders and X-ray powderdiffraction (XRD) analysis was performed to identify majorphases. X-ray diffraction for powdered pebbles in the 2θ rangeof 10-60° was performed using a Phillips Analytical Xpert PROdiffractometer (Cu KR).

3. Results and Discussion

3.1. Metals in Washings and Leachates of Pebbles.TheICP-OES data indicated the presence of several metal ions, viz.,Mn, Fe, Zn, Ni, Cd, Co, Cr, Mo, V, and Cu in both washingsand leachates, but the concentrations of Fe, Mn, and Co weresignificant. The concentrations of Fe, Mn, and Co in thewashings and leachates of different pebbles at different timeintervals are given in Table 1a-c, and the relative concentrationof the Fe, Mn, and Co in washings and leachates of pebblesafter 12 h of pretreatment are presented in Table 1d. In eachcase, an increase in metal concentration with increase in washingor leaching time was observed. In addition, the metal ionconcentrations in washings was less than that in leachates. Theconcentrations of metals present in washings/leachates were inthe order of Fe> Mn > Co (Table 1). In the case of mixedpebbles, the concentrations of Fe, Mn, and Co were higher than

the average of their concentrations in washings/leachates of red,black, and white pebbles.

The data in Table 1d indicates which of the metals was morereadily leached or washed from the surface of pebbles. Due tothe lowest concentrations of all metals found in red pebbles,the relative concentration in this case was found to be 1. Amongthe three metals, the relative concentration of Mn was thehighest, i.e., 20.89 in washings and 53.18 in leachates. Thisimplies that the Mn in pebbles was more readily washed orleached. Thus, the order of washability or leachability of Fe,Mn, and Co phases in pebbles may be written as Mn> Co >Fe. It may be noted that this order is opposite to the order ofabsolute concentrations of the metals in washings and leachates,i.e., Fe> Mn > Co. Apparently this must have originated fromtheir relative concentrations of phases in the pebbles, i.e., Fe>Mn > Co.

3.2. XRD Analysis.Figure 2a-c shows the XRD patternsof powders of white, red, and black pebbles, respectively. The2θ values for most intense peaks of expected phases arecollected from the literature and given in Table 2. The mostintense peak in the XRD pattern of white pebbles was at 2θ )26.5° corresponding to interplanar spacing (d) equal to 3.35.This was attributed to low quartz. Adjacent to this, a small peakat 2θ ) 26.1° (d ) 3.39) was attributed to high quartz. Theratio of these two peaks was 9:1, suggesting predominance oflow quartz in white pebbles. The peaks at 2θ equal to 28.6°,36.5°, 42.5°, 44.5°, 50.1°, 55.0°, and 60° corresponding tointerplanar spacing,d ) 3.13, 2.47, 2.13, 2.04, 1.82, 1.68, 1.66,and 1.54, respectively, were ascribed toâ-MnO2. The peak at2θ ) 20.8° (d ) 4.28) was assigned to goethite (R-FeOOH).Several other low-intensity peaks could not be clearly assigneddue to the fact they belong to more than one phase. However,excepting the low-quartz peak, the XRD pattern closelyresembles that of ferruginous manganese ore.15

The XRD of red pebbles represented mainly high quartz (2θ) 26.1°, d ) 3.39). The peaks at 2θ ) 17.8° and 20° wereascribed toγ-FeOOH. The peak at 2θ ) 20.5° (d ) 4.33) maybe assigned to goethite. The peak at 2θ ) 33.5° was assignedto Fe2O3, by analogy with the reported peak (cf., Table 2). Asmall amount ofâ-MnO2 was also present in the red pebbles,as some of the 2θ values matched with those assigned beforefor MnO2 in white pebbles.

The XRD of black pebbles exhibited the most intense peakat 2θ )28.5° (d ) 3.15). This was attributed toâ-MnO2. Thepeak at 22° is probably associated with feldspar. The low-intensity peak at 2θ ) 26.5° (3.38) can be ascribed to quartz.The peak at 2θ ) 29.8° (3.01) belongs to calcite. The peak at2θ ) 33.6° (2.66) once again was ascribed to Fe2O3. Severalother peaks may be assigned to a small amount of clay, quartz,and calcite, etc. The black pebbles did not contain FeOOH insufficient concentration to give rise to characteristic diffractionpeaks either at 2θ ) 20.5° (d ) 4.33) that may be assigned togoethite or at 2θ ) 17.8° and 20° that may be ascribed toγ-FeOOH. Although the heavy metal analysis (cf., section 3.1.)indicated the presence of Co on the surface of pebbles, the XRDdata did not reveal characteristic peaks of cobalt oxides. Thismay be due to its very low concentration in the pebbles.

3.3. Solar Photocatalytic Activity of TiO2/Pebbles. 3.3.1.Control Experiments. The dark adsorption experiments in-volved 50 g of pebbles or TiO2/pebbles immersed in 200 mLof RB5 dye solution (25 mg L-1, 22.5µM) taken into 250 mLPyrex glass beakers (open dish). A 5 h contact time foradsorption was maintained. There was 3.4-3.6% adsorption ofdye on washed pebbles, whereas it ranged between 2.5% and

Table 1. Concentration of Different Metal Ions in Washings/Leachates of Pebblesa

(a) Fe (mg L-1)

time(W/L)

(h) WW RW BW MW WL RL BL ML

6 5.482 5.356 26.093 13.223 10.582 6.573 21.484 16.68412 8.004 5.483 27.883 17.953 11.787 7.346 29.954 27.30418 11.83 7.706 30.493 28.863 11.845 10.282 35.034 30.334

(b) Mn (mg L-1)

time(W/L)


6 0.461 0.316 7.405 0.914 3.520 1.58 20.120 13.91212 0.920 0.381 7.960 2.744 4.246 3.90 20.262 19.69018 1.247 0.419 8.583 4.019 4.591 4.05 23.154 20.042

(c) Co (mg L-1)

time(W/L)


6 0.032 0.005 0.062 0.025 0.052 0.088 0.235 0.15412 0.033 0.009 0.066 0.039 0.058 0.094 0.286 0.27618 0.034 0.01 0.080 0.056 0.060 0.122 0.322 0.311

(d) Relative Concentration of Fe, Mn, and Co in Washings andLeachates (12 h) of Pebbles

metal WW RW BW MW WL RL BL ML

Fe 1.46 1.00 5.09 3.27 2.15 1.34 5.46 4.98Mn 2.41 1.00 20.89 7.20 11.14 10.23 53.18 51.68Co 3.67 1.00 7.33 4.33 6.44 10.44 31.78 30.67

a WW ) washings of white pebbles, RW) washings of red pebbles,BW ) washings of black pebbles, MW) washings of mixed pebbles, WL) leachates of white pebbles, RL) leachates of red pebbles, BL) leachatesof black pebbles, ML) leachates of mixed pebbles.

4408 Ind. Eng. Chem. Res., Vol. 46, No. 13, 2007

3.5% on leached pebbles. On the other hand, the TiO2/washedpebbles adsorbed 4-5% dye, as against 3.8-4.5% on TiO2/leached pebbles. Similarly, for assessing the contribution ofdirect photolysis to decolorization, control experiments wereperformed under the same conditions but with or without 50 gof pebbles immersed in RB5 dye solution and exposed tosunlight for 5 h. The aqueous dye solutions did not show loss

of color under direct photolysis. But, the color reduced by 3-5%under direct photolysis in the presence of pebbles, and most ofit could be attributed to adsorption over pebbles as found earlier.We may infer that the pebbles do not exert any photocatalyticeffect.

3.3.2. Photocatalytic Decolorization.The decolorization ofRB5 was observed with illuminated TiO2/pebbles. Figure 3

Figure 2.


illustrates the decolorization of RB5 with washed (Figure 3a)and leached (Figure 3b) pebbles. The rate of decolorization forall the types of TiO2-coated pebbles was similar until 2 h (Figure3a). After 3 h of exposure to sunlight, the difference in thedecolorization rate became visible. At the end of 5 h, the colorremoval was 59% with TiO2/WW, while only 28% decolori-zation was observed with the TiO2/BW pebbles. In the case ofTiO2-coated leached pebbles (Figure 3b), remarkable increasein the photocatalytic efficiency was observed with TiO2/WL.Complete decolorization was found after 5 h of exposure tosunlight using TiO2/WL. In contrast, for other TiO2-coatedpebbles (TiO2/BL, TiO2/RL, and TiO2/ML) the decolorizationefficiency was 24-36%, which was similar to that of theircounterparts from the washing sequence.

The kinetics of photocatalytic degradation reactions1,19 havebeen often found to obey the Langmuir-Hinshelwood kineticmodel (eq 1).

In the present case, whereCo ) 25 mg L-1 (∼22.5 µM) thedenominator term in eq 1 can be assumed to be 1; then eq 1transforms into eq 2

where k is the reaction constant (min-1), K the Langmuiradsorption constant (mg-1 L), C the initial concentration, andkapp is the apparent rate constant.

Equation 2 may be integrated, and a linear form can beobtained as shown in eq 3

In the present case, the trend of decrease in concentrationwith time of illumination (Figure 3a) is not distinctly exponentiallike as expected for a typical first-order reaction. Nevertheless,the corresponding-log(Ct/C0) versus time plots were linear withR2 values better than 0.90, except two cases. We may considerthat the data matches approximately with pseudo-first-orderkinetics. Thekappvalues with respect to each type of TiO2/pebblesystems and the corresponding regression coefficients arepresented in Table 3a; the rate constants for TiO2/WL are higherthan that of TiO2/WW, but on the contrary rate constants forTiO2/RL, TiO2/BL, and TiO2/ML are lower than those of TiO2/RW, TiO2/BW, and TiO2/MW. The relative photocatalyticefficiencies of the TiO2/pebble systems are compared in Table3b. The TiO2/WL system is 63-81 times more efficientcompared to TiO2/BL. All the other TiO2/pebble systemsshowed comparatively lower activity, and the following orderof photoactivity can be generalized based on the data in Table3b: TiO2/WL . TiO2/WW > TiO2/RW > TiO2/MW > TiO2/RL > TiO2/ML > TiO2/BW > TiO2/BL. Leaching of red andblack pebbles appears to lower the photoactivity.

The solar photocatalytic experiments were also extended tofive other reactive dyes, viz., RO16, RR2, RY84, RV13, andRR141 using TiO2/WL. The corresponding first-order rateconstants were compared with that of RB5 (Table 4). The orderof photoactivity may be written as RB5> RY84 > RV13 >RR141> RO16 > RR2. Both RO16 and RR2 photodegradeslowly at akapp that is approximately an order of magnitudelower. The data confirms that the TiO2/WL pebble systemdecolorizes all the chosen dyes.

Generally, many supported TiO2 photocatalysts showed lowerphotoactivity compared to the suspended forms of photocata-lysts. In a related study, Matthews and McEvoy7,20 used titaniaimmobilized on beach sand for photodegradation of phenol andcolor removal. They reported 3 times lower efficiency with theimmobilized catalyst when compared to using a free suspension.In the present case, it was not possible to weigh out a TiO2/pebble sample to provide the same quantity of TiO2 as that ofa suspended system (0.2 g) making it difficult to compare therate constant data. Nevertheless, qualitatively the data in Table5 indicates that a 30 min exposure of the TiO2/pebble systemwas not as efficient as the suspended TiO2 system. One of thereasons could be the lower amount of TiO2 on the pebbles. Onthe other hand, the TiO2/pebble system also showed considerableefficiency at the end of 4 h.

3.3.3. Reusability of TiO2/WL. Some studies with TiO2/WL and using 25 mg L-1 RB5 solution were performedrepetitively over several batches to ascertain the reusability ofTiO2-coated pebbles. After each batch, the TiO2/pebbles werewashed in deionized water, dried in an oven at 100°C, andreused repeatedly. There was an approximately 8-10% decreasein color removal efficiency over 20 batches. Moreover, the

Figure 3.

Table 2. 2θ Values for the Most Intense Peaks of Expected Phasesin Different Pebble Powder Samples

phase 2θ (deg) ref

low quartz 26.5 16high quartz 26.1 16γ-FeO(OH) 18.0 17R-FeO(OH) 20.0 http://www.mindat.org/Fe2O3 33.5 http://www.mindat.org/MnO2 (pyrolusite) 28.6 http://www.mindat.org/

and JCPDS 24-0735calcite 29.7 18 and http://www.mindat.org/feldspar 22.0 http://www.mindat.org/clay 13.5 http://www.mindat.org/

rate) kKC ) kappC (2)

-ln(Ct/C0) ) kappt (3)

rate) kKC/(1 + KC) (1)


treated dye solutions did not contain detached TiO2 as deter-mined by the absence of absorption at 380 nm in the UV-visspectra. This suggests that the TiO2/pebbles can be reusedsatisfactorily. Metal analysis of treated dye solution indicatedmuch lower concentration of Fe (2.5 mg L-1), Mn (0.45 mgL-1), and Co (0.012 mg L-1) when compared to the corre-sponding concentrations in washings (after a 6 hperiod, Table1). The comparison with 6 h washings is appropriate becausethe duration of exposure to sunlight was also of the same order(5 h).

3.3.4. Possible Origin of the Influence of the Support uponPhotoactivity of TiO2. The extensive photoactivity testingstudies on TiO2-coated pebbles revealed that TiO2/pebbles (whiteleached) are more efficient in removing color from dye solutions.It is also found that leaching pretreatment given to white pebblesfollowed by TiO2 coating ensures higher photoactivity, whilethe same pretreatment imparted to red and black pebbles failedto enhance the photoactivity. The result can be attributed to theinteraction of TiO2 with the native metal ions that were exposedon the surface of pebbles after longer leaching time. The metalion and XRD analysis indicated the presence of substantiallyhigher concentrations of Fe, Mn, and Co apart from the otherphases such as quartz, clay, and calcite on the surfaces ofpebbles.

On the basis of the intensity of XRD peaks corresponding toquartz (SiO2), iron oxides, and manganese oxide present in W,R, and B pebbles, the ratios of Si/Fe/Mn were deduced in each

case, and we attempted to classify them as quartzitic andmanganese-rich materials (Table 6). The sum of intensities ofXRD peaks due to different iron oxides was considered for ratioestimation in the case of red pebbles. The white pebblescomprise mainly quartz. It also contains goethite in about one-sixth concentration of quartz, while MnO2 was present as a smallimpurity. On the other hand, the red pebbles contain quartz andiron oxides approximately in equal proportion. A small quantityof MnO2 is also associated with these pebbles. On the contrary,the black pebbles are Mn-rich with appreciable amounts ofhematite. A small fraction of clay, calcite, and quartz are alsopresent in black pebbles.

In the case of white pebbles that are actually constituted ofquartz, the support appears to be quite stable and much moreinert than the other two types (black and red). The goethite (R-FeOOH) phase present on the surface of white pebbles isprobably leached out, or it may be present in very lowconcentration on the surface of white pebbles so that the largeamount of coated TiO2 did not make contact with it. This mayhave resulted in significantly enhanced photoactivity of TiO2/WL, whereas these oxides are available in substantially higherconcentration on the surface of red and black pebbles so thateven after the leaching pretreatment, their concentration on thesurface was adequate to inhibit the photoactivity. Both blackand red pebbles have higher concentrations of the iron oxideson the surface as can be understood from their higher concentra-tions in washings and leachates as well as in the light of thedata in Table 6.

In the TiO2/red pebble and TiO2/black pebble systems, twodifferent pathways of mechanism may be considered to accountfor the recombination of photogenerated electrons and holes. Itmay be recalled that the red pebbles contain a certain amountof Fe2O3 also apart from FeOOH. On the other hand, the blackpebbles contained mainly MnO2. First, there is a possibility ofshunting of electrons and holes from the illuminated TiO2 tothe metal oxide components (â-MnO2, Fe2O3, etc.) present at

Table 3. (a) Rate Constants (min-1) for Solar Photocatalytic Degradation of RB5 in Contact with TiO2/Pebbles; (b) Relative Photoactivity(kTiO2/pebbles/kTiO2/BL) for Solar Photocatalytic Degradation of RB5

atime

(W/L) (h)TiO2/WW

(× 10-3 min-1)TiO2/RW

(× 10-3 min-1)TiO2/BW

(× 10-3 min-1)TiO2/MW

(× 10-3 min-1)TiO2/WL

(× 10-3 min-1)TiO2/RL

(× 10-3 min-1)TiO2/BL

(× 10-3 min-1)TiO2/ML

(× 10-3 min-1)

6 1.45 (0.97)a 0.63 (0.98) 0.40 (0.98) 0.79 (0.98) 3.80 (0.90) 0.67 (0.97) 0.055(0.93) 1.00 (0.98)12 1.60 (0.96) 0.98 (0.90) 0.45 (0.99) 0.78 (0.97) 4.50(0.94) 0.65 (0.98) 0.40 (0.96) 0.63 (0.98)18 1.80 (0.93) 1.00 (0.94) 0.72 (0.96) 0.64 (0.99) 3.50 (0.88) 0.39 (0.77) 0.16 (0.93) 0.40 (0.96)

btime

(W/L) (h) TiO2/WW TiO2/RW TiO2/BW TiO2/MW TiO2/WL TiO2/RL TiO2/BL TiO2/ML

6 26.36 11.45 7.27 14.47 69.09 12.18 1.00 18.1812 29.09 17.80 8.18 14.18 81.81 11.81 7.27 11.4518 32.70 18.18 13.09 11.63 63.63 7.09 2.90 7.27

a Values in parenthesis are regression coefficients.

Table 4. Rate Constants (min-1) for Solar PhotocatalyticDegradation of Various Dyes in Contact with Titania-DepositedWhite Leached Pebbles

dyek

(× 10-3 min-1) R2

reactive black 5 4.50 0.99reactive orange 16 0.46 0.86reactive red 2 0.18 0.86reactive yellow 84 2.10 0.93reactive violet 13 2.00 0.86reactive red 141 1.40 0.90

Table 5. Comparison of Photoactivity of P-25 TiO2 in Suspensionand Supported on White Leached Pebbles (12 h)

percent color reduction

suspended TiO2 TiO2/WL

dyes 30 min 30 min 4 h

reactive black 5 65 39 92reactive orange 16 45 08 15reactive red 2 94 12 57reactive yellow 84 88 25 73reactive violet 13 86 20 55reactive red 141 98 30 80

Figure 4.


the TiO2/pebble interface. Figure 4 illustrates qualitatively theshunting of photogenerated carriers in illuminated TiO2 toadventitious metal oxide particles on the surface of pebbles.The system resembles that of reported heterojunction compositesemiconductors,21,22wherein the charge carriers generated in awide band gap material (e.g., TiO2) may be driven to lowerband gap metal oxide impurities, viz., Fe2O3 or MnO2. BothFe2O3 and â-MnO2 are n-type semiconductors with band gapenergies of 2.2 and 2.1 eV, respectively.22,23 The band gap ofâ-MnO2 was estimated using the onset wavelength in its DRIFTspectrum reported by Lamaita et al.23 In comparison, the bandgap of TiO2 is much higher (3.20 eV). This manifests into settingnew valence and conduction band levels with reduced band gapas well as lowered ability to mediate redox reactions on thesurface. This also can lead to significant recombination ofphotocharges.

On the other hand, the metal oxide components present onthe surface of pebbles may undergo a certain dissolution in situand set up redox couples (e.g., Fe3+/Fe2+ and Mn3+/Mn2+, etc.).Alternatively, these metal ions may adsorb on the surface ofTiO2

22 and act as surface states or otherwise influence adsorptionof dyes. The redox couples at the TiO2/aqueous solutioninterface may abstract photoelectrons and holes from theilluminated TiO2 depending upon their redox potentials relativeto band edge positions of TiO2.24 This may be illustratedqualitatively through the energy level schemes shown in Figure5, parts a and b. Figure 5a relates to energy levels in TiO2 versusexpected redox levels in solution; theEredoxrepresents a solution

redox potential under equilibrium. The different redox systemshave energy levels22,24-27 located in the bang gap region of TiO2,and electron exchange may be fast enough to establish equi-librium such that a depletion layer (band bending) is formed inthe dark. It may be noted that the redox couples with morenegative potential thanEf (Fermi level in TiO2) equilibrate withEf and lead to band bending in the dark (Figure 5a). However,upon illumination the band bending is removed as theEf levelis now shifted to a more negative potential relative toEredox insolution. Thus, the quasi-Fermi levels of both charge carriers(nEf and pEf) would snap the different redox couples as shownin Figure 5b. As a result, many of the redox couples will undergoredox reactions utilizing photogenerated holes and electrons.These redox reactions at the interface compete with usefulreactions, viz., O2 f O2

-‚ reaction and the hole-mediatedreaction of OH radical generation. This may account for theobserved lower photoactivity of TiO2/red and black pebbles.Brezova et al.24 also discussed a similar mechanism to explainthe influence of metals, viz., Ca2+, Mg2+, Zn2+, Ni2+, Mn2+,and Co2+ on the rate of photocatalytic degradation of phenol.

Many reports reveal that photoactivity of TiO2/support isextremely dependent on the structural, electronic, and chemicalproperties of supports.4,28-30 A complete retention of slurry-type photoactivity may not be possible because the constituentsof supports may interact with illuminated TiO2 and drain awaya large part of the charge carriers, which otherwise would havereacted to generate‚OH or O2

-‚ radicals. Thus, chemically inertsupports should be preferred for obtaining high efficiency of

Figure 5. (a) TiO2/solution interface in dark. Several metal ion redox couples are also shown for comparison (vs SHE). (b) TiO2/solution interface underillumination. The recombination pathways through metal ion redox couples are indicated (vs SHE).

Table 6. Intensity (XRD)-Based Si/Fe/Mn Ratios in White, Red, and Black Pebbles

pebble type SiO2 Fe MnO2 Si/Fe/Mn remarks

W 2θ 26.5 20.8 (R-FeOOH) 28.6 150/25/1 mainly quartz with significant amount of (R-FeOOH)and MnO2 impurities

I 1500 250 10R 2θ 26.1 17.8 (γ-FeOOH)+ 20

(R-FeOOH)+ 33.5 (Fe2O3)28.3 12.5/12/1 mainly contains quartz and Fe-oxides approximately

in equalratio and MnO2 impuritiesI 250 240 20

B 2θ 26.1 33.6 (Fe2O3) 28.3 1/5/40 MnO2-rich material with Fe2O3, clay, and very smallamount of quartz

I 10 50 400


TiO2/support systems. Litter and Navio,22 while reviewing thephotocatalytic properties of iron-doped titania powders, em-phasized that formation of multiphasic samples (viz., TiO2-Fe2O3) during Fe doping and heat treatment is detrimental andthe observed photoactivity is closely linked to the ionic stateof iron: Fe2+, Fe3+, or Fe. The effect of several types of metalions (Cu2+, Zn2+, Fe3+, Al3+, and Cd2+) on the photodegradationof non-azo dyes in TiO2 suspensions under visible lightillumination has been investigated by Chen et al.31 The studyconcluded that Cu2+ and Fe3+ ions suppress photodegradationof the dyes by competing for photoelectrons and therebyinhibiting the formation of reactive oxygen species (‚OH orO2

-‚, ‚O2H). Other metal ions, Zn2+, Al3+, and Cd2+, affectedthe adsorption of the dyes and caused slight variation inphotoactivity. On the other hand, the beneficial effect of Fe3+

ions was also documented. For example, an increase in the rateof photodegradation of acid red 1 was observed in TiO2

suspensions containing Fe3+ aquo ions.32 This effect was tracedto increase in dye adsorption on Fe3+-modified TiO2. A similareffect was also found on the photodegradation of rhodamine Bin aqueous TiO2 suspensions.33 On the other hand, the additionof Fe2+ ions in the TiO2 aqueous suspensions did not affect thephotocatalytic mineralization of aniline, and higher concentra-tions were reported to be detrimental.34 The inhibition wasattributed to the competition of Fe2+ with the organic substratefor the oxidant species. In a study that was aimed at understand-ing the effect of metal ions on the photocatalytic degradationof phenol, Brillas et al.35 reported that Mn2+ and Co2+ inhibitedthe degradation. The detrimental effect of these ions wasexplained by electron transfer involving metal ions and holes(decreasing‚OH production) and by competitive adsorption withphenol for the TiO2 surface.

In the present case too, interaction of the native metal oxides/ions on the pebble support with titania explains the lower activityof the TiO2/pebble photocatalyst. We fabricated a falling filmtype pebble bed reactor using TiO2/WL pebbles, and the resultsof solar photocatalytic decolorization of dyes will be publishedelsewhere.

4. Conclusions

Pebbles can be used as supports for TiO2. The TiO2-coatedpebbles exhibit photoactivity, which is dependent on thecomposition of pebbles in respect to various metal compoundspresent on the surface and hence is also a function of type ofpretreatment provided to the pebbles. The TiO2/WL systemexhibits superior photoactivity when compared to the other TiO2/pebble systems. Leaching of white pebbles may have reducedconcentration of surface metal ions and exposed fresh quartz/silica surface that minimized the support-TiO2 interaction. Thedifference in photoactivity of various TiO2/pebble systems canbe attributed to the interaction between native metal ions onthe surface of pebbles and illuminated TiO2. It is of particularimportance to avoid using pebbles that undergo substantialchemical dissolution leading to formation of solution redoxcouples or surface states at the interface. Significant recombina-tion of photogenerated charge carriers can be expected due tointervention of these surface states/redox couples.

Acknowledgment

The authors (V.C.) thank the Ministry of Non-conventionalEnergy Sources (MNES), New Delhi, India and Indian Instituteof Technology (IIT), New Delhi, India for the NationalRenewable Energy (NRE) Fellowship. The authors are grateful

to Dr. Sukumar Devotta, Director, NEERI for his kind permis-sion to publish this work and Dr. Tapas Nandy, Scientist andHead, WWT Division, NEERI for encouragement.

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ReceiVed for reView February 17, 2007ReVised manuscript receiVed April 17, 2007

AcceptedApril 19, 2007

IE0702532


photoactivity of tio 2 -coated pebbles

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