Photoactivity of TiO2-Coated Pebbles - Industrial & Engineering

DOI: 10.1021/ie0702532. Publication Date (Web): May 27, 2007. Copyright © 2007 American Chemical Society. Cite this:Ind. Eng. Chem. Res. 46, 13, 4406...
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4406

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

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 pollutants is a technological need. In the present study, Degussa P-25 TiO2 photocatalyst was coated on pretreated pebbles 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. The pretreatment of pebbles constituted washing with deionized water or leaching with EDTA solution. Coating of 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 and leachates using an inductively coupled plasma-optical emission spectrometer (ICP-OES) and an atomic absorption spectrophotometer (AAS) were carried out. Solar photocatalytic decolorization of reactive black 5 (RB5) and some other reactive dyes was tested using TiO2/pebbles in an open dish method. The percent decolorization 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 rate constants, 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 was rationalized in terms of interaction between metal oxides/ions native to pebbles and illuminated TiO2. 1. Introduction Titanium dioxide has aroused tremendous research interest due to its photocatalytic activity for the degradation of organic and inorganic pollutants.1,2 The TiO2 photocatalyzed degradation of organic pollutants has received further impetus from the possibility of combining photocatalysis and solar technologies.2,3 It may be developed into a useful process for the reduction of water pollution because of the milder reaction conditions and lower energy costs of solar photocatalytic technology. As a step toward practical solar photocatalytic treatment devices, immobilization of TiO2 was considered especially useful from the process-engineering point of view. The coated surface can be readily recovered and reused in the process unlike the suspended systems.4 Various supports have been used for immobilizing TiO2, such as ceramic,5 fiber glass,6 glass and sand,7 quartz and stainless steel,8 activated carbon,9 polyester fabric using polyvinyl alcohol as binder, and Ti-TiO2 prepared by thermal as well as flame oxidation of Ti sheet.10 These studies with immobilized titania revealed that the photodegradation efficiencies are somewhat low compared to those of slurry-based processes. This effect was often attributed to the support-TiO2 interactions such as changes in the TiO2 energy band structure due to chemical bonds with support and support-induced morphological changes (surface area and particle size) accompanying the heat treatments. In this paper, we prepared the TiO2-coated pebbles and examined their photocatalytic activity toward decolorization of some reactive dyes. The selection of pebbles as supports for TiO2 was based on the following: (i) pebbles are readily available at any civil construction site in the form of rejects from sand-screening operation, (ii) because of their relatively larger size, they can be arranged into a pebble bed reactor, * To whom correspondence should be addressed. Fax: +91-7122249900. E-mail: [email protected].

similar to a falling film reactor, and (iii) pebbles are explored for the first time as supports for TiO2. 2. Experimental Section 2.1. Materials. 1-Amino-8-hydroxy-2,7-bisazo[(p-vinylsulphonic acid)]-naphthalene-(3,6-disulfonic acid) tetrasodium salt, C26H21O19N5S6Na4 (C.I. reactive black 5) was purchased from M/s Atul Dyes, Ahmedabad. Some other reactive dyes used for testing, viz., reactive orange 16 (RO16), reactive yellow 84 (RO84), reactive red 141 (RR141), reactive red 2 (RR2), and reactive 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 were 50 m2 g-1 and 30 nm, respectively. Ethylenediaminetetraacetic acid disodium salt (EDTA) was a reagent grade chemical (E Merck India Pvt. Ltd., Mumbai). The metal ion standards for inductively coupled plasma-optical emission spectrometry (ICPOES) and atomic absorption spectrophotometry (AAS) were procured from E. Merck India Pvt. Ltd., Mumbai. Deionized water (Millipore Elix 3 water purifier) was used for preparing all reagents. 2.2. Pebbles. Pebbles were collected from the rejects of sand at a construction site. The sand was quarried from the Kanhan riverbed that falls under the Kamthi region, Maharashtra, India. This region is overlaid by coarse-grained, ferruginous sandstone with 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 proportions were also considered. 2.3. Pretreatment of Pebbles. Initially the pebbles were washed under tap water to remove the dirt present on the surface. They were then subjected to either washing with deionized water or leaching with 0.1 M EDTA solution. Washing and leaching cycles were carried out in batches for different time intervals,

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

Ind. Eng. Chem. Res., Vol. 46, No. 13, 2007 4407

viz., 6, 12, and 18 h. A 150 g sample of pebbles immersed in 300 mL of deionized water or 0.1 M EDTA solution was agitated over a mechanical shaker. The washings and leachates were collected at the end of each batch and analyzed for heavy metals. 2.4. Preparation of TiO2-Coated Pebbles (TiO2/Pebbles). The photocatalyst-coated pebbles were prepared according to the previously made titania powder (PMTP) method.4,12 The method consisted of spraying a 2% TiO2 suspension in 80:20 ethanol/water mixture on the surface of pretreated pebbles and drying at 60-70 °C by blowing hot air from an electric dryer. The spray-coating-drying method was repeated five times until satisfactory coating on the entire surface of the pebbles was achieved. The TiO2/pebbles were then kept in an oven at 150 °C for 8 h. The three types of pebbles before and after coating are shown in Figure 1. As there are no reported procedures for estimation of the quantity of coated TiO2 on irregular surfaces such as pebbles, the following method was evolved. This method was based on sticking adhesive tape and peeling off the same from the surface and then estimating the weight gained by the adhesive tape piece. For this purpose, one square centimeter pieces of cello tape were cut, weighed, and adhered to the pebble’s surface having TiO2 supported on it. Subsequently, it was peeled and weighed again. The procedure was repeated five times by using 1 cm2 cello tape pieces on the same pebble. The weight gain in each case was calculated and added to arrive at the amount of coated TiO2. Recovery of catalyst onto the adhesive tape was considered to be 80%. In the present case, the amount of coated TiO2 was estimated as 0.00013 ((0.00002) g TiO2/cm2. 2.5. Solar Photocatalytic Experiments. A typical photocatalytic experiment involved illuminating 50 g of TiO2/pebbles immersed in 200 mL of RB5 dye solution (25 mg L-1, 22.5 µM) taken into 250 mL Pyrex glass beakers (open dish) under sunlight. The duration of exposure to sun was approximately 5 h, between 10:00 a.m. and 3:00 p.m. everyday. Mixing of the dye solution during exposure to sunlight was achieved by bubbling air into beakers using an aquarium pump and air distribution manifold. A spherical sintered glass frit fixed at the bottom of the beakers ensured proper mixing of dye solutions. Test samples were withdrawn hourly, and absorption of RB5 dye was monitored at 597 nm. Similar experiments were done using the other reactive dyes also, viz., RO16, RO84, RR141, RR2, and RV13. All photocatalytic experiments were performed in duplicate batches for each type of dye. The batchto-batch error was found to be in the range of 4-7%. Appropriate control experiments were also set up to estimate the background contribution of pebbles and TiO2/pebbles for adsorptive and photolytic removal of color from RB5 dye solutions. As an open dish method was used for carrying out solar photocatalytic testing of catalysts, a certain reduction in the volume of test solution was noticed due to evaporation. Therefore, the reduction in volume was determined at the end of each batch, made up to initial volume by addition of deionized water before determining the actual color reduction due to photocatalysis. 2.6. Analyses. Solar illumination was measured using a Carl Zeiss luxmeter from 10:00 a.m. to 4:00 p.m. in the NovemberDecember period during which the photocatalytic experiments were carried out. Generally, illuminance during NovemberDecember in Nagpur corresponds to a typical hazy day. The solar 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

Figure 1.

226-153 × 104 lx from 3:00 to 4:00 p.m. The average light intensity 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 through a 0.2 µm syringe microfilter. All spectrophotometric measurements were carried out in the range of 200-700 nm using a double-beam UV-vis spectrophotometer (Perkin-Elmer λ 900). The reduction in the color band intensity was determined from the time-overlaid UV-vis spectra. The specific color removal rates were obtained by photometric determination at the color band of RB5, i.e., λmax ) 597 nm,  ) 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 the other reactive dyes were as follows: RO16, λmax ) 493 nm,  ) 0.0214 mg-1 L cm-1; RR141, λmax ) 547 nm,  ) 0.022 mg-1 L cm-1; RO84, λmax ) 409 nm,  ) 0.012 mg-1 L cm-1;

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Table 1. Concentration of Different Metal Ions in Washings/ Leachates of Pebblesa (a) Fe (mg L-1) time (W/L) (h) 6 12 18

WW

RW

BW

MW

WL

RL

BL

ML

5.482 5.356 26.093 13.223 10.582 6.573 21.484 16.684 8.004 5.483 27.883 17.953 11.787 7.346 29.954 27.304 11.83 7.706 30.493 28.863 11.845 10.282 35.034 30.334 (b) Mn (mg L-1)

time (W/L) (h)

WW

RW

BW

MW

WL

RL

BL

ML

6 12 18

0.461 0.920 1.247

0.316 0.381 0.419

7.405 7.960 8.583

0.914 2.744 4.019

3.520 4.246 4.591

1.58 3.90 4.05

20.120 20.262 23.154

13.912 19.690 20.042

(c) Co (mg L-1) time (W/L) (h)

WW

RW

BW

MW

WL

RL

BL

ML

6 12 18

0.032 0.033 0.034

0.005 0.009 0.01

0.062 0.066 0.080

0.025 0.039 0.056

0.052 0.058 0.060

0.088 0.094 0.122

0.235 0.286 0.322

0.154 0.276 0.311

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

WW

RW

BW

MW

WL

RL

BL

ML

Fe Mn Co

1.46 2.41 3.67

1.00 1.00 1.00

5.09 20.89 7.33

3.27 7.20 4.33

2.15 11.14 6.44

1.34 10.23 10.44

5.46 53.18 31.78

4.98 51.68 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 ) leachates of black pebbles, ML ) leachates of mixed pebbles.

RR2, λmax ) 538 nm,  ) 0.019 mg-1 L cm-1; RV13, λmax ) 538 nm,  ) 0.0237 mg-1 L cm-1. The washings and leachates were analyzed for heavy metals using ICP-OES (Optima 4100 DV, Perkin-Elmer, U.S.A.) and AAS (Perkin-Elmer, Analyst 800). The test samples were appropriately digested as per Standard Methods.14 The average concentration of each metal ion from two batches was reported. The batch-to-batch error was less than 5%. Simultaneously, the black, red, and white pebbles were crushed into fine powders and X-ray powder diffraction (XRD) analysis was performed to identify major phases. X-ray diffraction for powdered pebbles in the 2θ range of 10-60° was performed using a Phillips Analytical Xpert PRO diffractometer (Cu KR). 3. Results and Discussion 3.1. Metals in Washings and Leachates of Pebbles. The ICP-OES data indicated the presence of several metal ions, viz., Mn, Fe, Zn, Ni, Cd, Co, Cr, Mo, V, and Cu in both washings and leachates, but the concentrations of Fe, Mn, and Co were significant. The concentrations of Fe, Mn, and Co in the washings and leachates of different pebbles at different time intervals are given in Table 1a-c, and the relative concentration of the Fe, Mn, and Co in washings and leachates of pebbles after 12 h of pretreatment are presented in Table 1d. In each case, an increase in metal concentration with increase in washing or leaching time was observed. In addition, the metal ion concentrations in washings was less than that in leachates. The concentrations of metals present in washings/leachates were in the order of Fe > Mn > Co (Table 1). In the case of mixed pebbles, 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 more readily leached or washed from the surface of pebbles. Due to the lowest concentrations of all metals found in red pebbles, the relative concentration in this case was found to be 1. Among the three metals, the relative concentration of Mn was the highest, i.e., 20.89 in washings and 53.18 in leachates. This implies that the Mn in pebbles was more readily washed or leached. 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 of absolute concentrations of the metals in washings and leachates, i.e., Fe > Mn > Co. Apparently this must have originated from their relative concentrations of phases in the pebbles, i.e., Fe > Mn > Co. 3.2. XRD Analysis. Figure 2a-c shows the XRD patterns of powders of white, red, and black pebbles, respectively. The 2θ values for most intense peaks of expected phases are collected from the literature and given in Table 2. The most intense 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 peak at 2θ ) 26.1° (d ) 3.39) was attributed to high quartz. The ratio of these two peaks was 9:1, suggesting predominance of low 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 to interplanar spacing, d ) 3.13, 2.47, 2.13, 2.04, 1.82, 1.68, 1.66, and 1.54, respectively, were ascribed to β-MnO2. The peak at 2θ ) 20.8° (d ) 4.28) was assigned to goethite (R-FeOOH). Several other low-intensity peaks could not be clearly assigned due to the fact they belong to more than one phase. However, excepting the low-quartz peak, the XRD pattern closely resembles 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° were ascribed to γ-FeOOH. The peak at 2θ ) 20.5° (d ) 4.33) may be assigned to goethite. The peak at 2θ ) 33.5° was assigned to Fe2O3, by analogy with the reported peak (cf., Table 2). A small amount of β-MnO2 was also present in the red pebbles, as some of the 2θ values matched with those assigned before for MnO2 in white pebbles. The XRD of black pebbles exhibited the most intense peak at 2θ )28.5° (d ) 3.15). This was attributed to β-MnO2. The peak at 22° is probably associated with feldspar. The lowintensity 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 at 2θ ) 33.6° (2.66) once again was ascribed to Fe2O3. Several other peaks may be assigned to a small amount of clay, quartz, and calcite, etc. The black pebbles did not contain FeOOH in sufficient concentration to give rise to characteristic diffraction peaks either at 2θ ) 20.5° (d ) 4.33) that may be assigned to goethite or at 2θ ) 17.8° and 20° that may be ascribed to γ-FeOOH. Although the heavy metal analysis (cf., section 3.1.) indicated the presence of Co on the surface of pebbles, the XRD data did not reveal characteristic peaks of cobalt oxides. This may 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 involved 50 g of pebbles or TiO2/pebbles immersed in 200 mL of RB5 dye solution (25 mg L-1, 22.5 µM) taken into 250 mL Pyrex glass beakers (open dish). A 5 h contact time for adsorption was maintained. There was 3.4-3.6% adsorption of dye on washed pebbles, whereas it ranged between 2.5% and

Ind. Eng. Chem. Res., Vol. 46, No. 13, 2007 4409

Figure 2.

3.5% on leached pebbles. On the other hand, the TiO2/washed pebbles adsorbed 4-5% dye, as against 3.8-4.5% on TiO2/ leached pebbles. Similarly, for assessing the contribution of direct photolysis to decolorization, control experiments were performed under the same conditions but with or without 50 g of pebbles immersed in RB5 dye solution and exposed to sunlight 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 of it could be attributed to adsorption over pebbles as found earlier. We may infer that the pebbles do not exert any photocatalytic effect. 3.3.2. Photocatalytic Decolorization. The decolorization of RB5 was observed with illuminated TiO2/pebbles. Figure 3

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

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

rate ) kKC ) kappC

(2)

where k is the reaction constant (min-1), K the Langmuir adsorption constant (mg-1 L), C the initial concentration, and kapp is the apparent rate constant. Equation 2 may be integrated, and a linear form can be obtained as shown in eq 3

-ln(Ct/C0) ) kappt

Figure 3. Table 2. 2θ Values for the Most Intense Peaks of Expected Phases in Different Pebble Powder Samples phase

2θ (deg)

ref

low quartz high quartz γ-FeO(OH) R-FeO(OH) Fe2O3 MnO2 (pyrolusite)

26.5 26.1 18.0 20.0 33.5 28.6

calcite feldspar clay

29.7 22.0 13.5

16 16 17 http://www.mindat.org/ http://www.mindat.org/ http://www.mindat.org/ and JCPDS 24-0735 18 and http://www.mindat.org/ http://www.mindat.org/ http://www.mindat.org/

illustrates the decolorization of RB5 with washed (Figure 3a) and leached (Figure 3b) pebbles. The rate of decolorization for all the types of TiO2-coated pebbles was similar until 2 h (Figure 3a). After 3 h of exposure to sunlight, the difference in the decolorization rate became visible. At the end of 5 h, the color removal was 59% with TiO2/WW, while only 28% decolorization was observed with the TiO2/BW pebbles. In the case of TiO2-coated leached pebbles (Figure 3b), remarkable increase in the photocatalytic efficiency was observed with TiO2/WL. Complete decolorization was found after 5 h of exposure to sunlight using TiO2/WL. In contrast, for other TiO2-coated pebbles (TiO2/BL, TiO2/RL, and TiO2/ML) the decolorization efficiency was 24-36%, which was similar to that of their counterparts from the washing sequence. The kinetics of photocatalytic degradation reactions1,19 have been often found to obey the Langmuir-Hinshelwood kinetic model (eq 1).

rate ) kKC/(1 + KC)

(1)

(3)

In the present case, the trend of decrease in concentration with time of illumination (Figure 3a) is not distinctly exponential like as expected for a typical first-order reaction. Nevertheless, the corresponding -log(Ct/C0) versus time plots were linear with R2 values better than 0.90, except two cases. We may consider that the data matches approximately with pseudo-first-order kinetics. The kapp values with respect to each type of TiO2/pebble systems and the corresponding regression coefficients are presented in Table 3a; the rate constants for TiO2/WL are higher than that of TiO2/WW, but on the contrary rate constants for TiO2/RL, TiO2/BL, and TiO2/ML are lower than those of TiO2/ RW, TiO2/BW, and TiO2/MW. The relative photocatalytic efficiencies of the TiO2/pebble systems are compared in Table 3b. The TiO2/WL system is 63-81 times more efficient compared to TiO2/BL. All the other TiO2/pebble systems showed comparatively lower activity, and the following order of photoactivity can be generalized based on the data in Table 3b: TiO2/WL . TiO2/WW > TiO2/RW > TiO2/MW > TiO2/ RL > TiO2/ML > TiO2/BW > TiO2/BL. Leaching of red and black pebbles appears to lower the photoactivity. The solar photocatalytic experiments were also extended to five other reactive dyes, viz., RO16, RR2, RY84, RV13, and RR141 using TiO2/WL. The corresponding first-order rate constants were compared with that of RB5 (Table 4). The order of photoactivity may be written as RB5 > RY84 > RV13 > RR141 > RO16 > RR2. Both RO16 and RR2 photodegrade slowly at a kapp that is approximately an order of magnitude lower. The data confirms that the TiO2/WL pebble system decolorizes all the chosen dyes. Generally, many supported TiO2 photocatalysts showed lower photoactivity compared to the suspended forms of photocatalysts. In a related study, Matthews and McEvoy7,20 used titania immobilized on beach sand for photodegradation of phenol and color removal. They reported 3 times lower efficiency with the immobilized 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 of a suspended system (0.2 g) making it difficult to compare the rate constant data. Nevertheless, qualitatively the data in Table 5 indicates that a 30 min exposure of the TiO2/pebble system was not as efficient as the suspended TiO2 system. One of the reasons could be the lower amount of TiO2 on the pebbles. On the other hand, the TiO2/pebble system also showed considerable efficiency 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 performed repetitively over several batches to ascertain the reusability of TiO2-coated pebbles. After each batch, the TiO2/pebbles were washed in deionized water, dried in an oven at 100 °C, and reused repeatedly. There was an approximately 8-10% decrease in color removal efficiency over 20 batches. Moreover, the

Ind. Eng. Chem. Res., Vol. 46, No. 13, 2007 4411 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 time (W/L) (h)

TiO2/WW (× 10-3 min-1)

TiO2/RW (× 10-3 min-1)

TiO2/BW (× 10-3 min-1)

a TiO2/MW -3 (× 10 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 12 18

1.45 (0.97)a 1.60 (0.96) 1.80 (0.93)

0.63 (0.98) 0.98 (0.90) 1.00 (0.94)

0.40 (0.98) 0.45 (0.99) 0.72 (0.96)

0.79 (0.98) 0.78 (0.97) 0.64 (0.99)

3.80 (0.90) 4.50 (0.94) 3.50 (0.88)

0.67 (0.97) 0.65 (0.98) 0.39 (0.77)

0.055 (0.93) 0.40 (0.96) 0.16 (0.93)

1.00 (0.98) 0.63 (0.98) 0.40 (0.96)

b time (W/L) (h)

TiO2/WW

TiO2/RW

TiO2/BW

TiO2/MW

TiO2/WL

TiO2/RL

TiO2/BL

TiO2/ML

6 12 18

26.36 29.09 32.70

11.45 17.80 18.18

7.27 8.18 13.09

14.47 14.18 11.63

69.09 81.81 63.63

12.18 11.81 7.09

1.00 7.27 2.90

18.18 11.45 7.27

a

Values in parenthesis are regression coefficients.

Table 4. Rate Constants (min-1) for Solar Photocatalytic Degradation of Various Dyes in Contact with Titania-Deposited White Leached Pebbles dye

k (× 10-3 min-1)

R2

reactive black 5 reactive orange 16 reactive red 2 reactive yellow 84 reactive violet 13 reactive red 141

4.50 0.46 0.18 2.10 2.00 1.40

0.99 0.86 0.86 0.93 0.86 0.90

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

Figure 4.

percent color reduction suspended TiO2

TiO2/WL

dyes

30 min

30 min

4h

reactive black 5 reactive orange 16 reactive red 2 reactive yellow 84 reactive violet 13 reactive red 141

65 45 94 88 86 98

39 08 12 25 20 30

92 15 57 73 55 80

treated dye solutions did not contain detached TiO2 as determined by the absence of absorption at 380 nm in the UV-vis spectra. This suggests that the TiO2/pebbles can be reused satisfactorily. Metal analysis of treated dye solution indicated much lower concentration of Fe (2.5 mg L-1), Mn (0.45 mg L-1), and Co (0.012 mg L-1) when compared to the corresponding concentrations in washings (after a 6 h period, Table 1). The comparison with 6 h washings is appropriate because the duration of exposure to sunlight was also of the same order (5 h). 3.3.4. Possible Origin of the Influence of the Support upon Photoactivity of TiO2. The extensive photoactivity testing studies on TiO2-coated pebbles revealed that TiO2/pebbles (white leached) are more efficient in removing color from dye solutions. It is also found that leaching pretreatment given to white pebbles followed by TiO2 coating ensures higher photoactivity, while the same pretreatment imparted to red and black pebbles failed to enhance the photoactivity. The result can be attributed to the interaction of TiO2 with the native metal ions that were exposed on the surface of pebbles after longer leaching time. The metal ion and XRD analysis indicated the presence of substantially higher concentrations of Fe, Mn, and Co apart from the other phases such as quartz, clay, and calcite on the surfaces of pebbles. On the basis of the intensity of XRD peaks corresponding to quartz (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 and manganese-rich materials (Table 6). The sum of intensities of XRD peaks due to different iron oxides was considered for ratio estimation in the case of red pebbles. The white pebbles comprise mainly quartz. It also contains goethite in about onesixth concentration of quartz, while MnO2 was present as a small impurity. On the other hand, the red pebbles contain quartz and iron oxides approximately in equal proportion. A small quantity of MnO2 is also associated with these pebbles. On the contrary, the black pebbles are Mn-rich with appreciable amounts of hematite. A small fraction of clay, calcite, and quartz are also present in black pebbles. In the case of white pebbles that are actually constituted of quartz, the support appears to be quite stable and much more inert than the other two types (black and red). The goethite (RFeOOH) phase present on the surface of white pebbles is probably leached out, or it may be present in very low concentration on the surface of white pebbles so that the large amount of coated TiO2 did not make contact with it. This may have resulted in significantly enhanced photoactivity of TiO2/ WL, whereas these oxides are available in substantially higher concentration on the surface of red and black pebbles so that even after the leaching pretreatment, their concentration on the surface was adequate to inhibit the photoactivity. Both black and red pebbles have higher concentrations of the iron oxides on the surface as can be understood from their higher concentrations in washings and leachates as well as in the light of the data in Table 6. In the TiO2/red pebble and TiO2/black pebble systems, two different pathways of mechanism may be considered to account for the recombination of photogenerated electrons and holes. It may be recalled that the red pebbles contain a certain amount of Fe2O3 also apart from FeOOH. On the other hand, the black pebbles contained mainly MnO2. First, there is a possibility of shunting of electrons and holes from the illuminated TiO2 to the metal oxide components (β-MnO2, Fe2O3, etc.) present at

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Figure 5. (a) TiO2/solution interface in dark. Several metal ion redox couples are also shown for comparison (vs SHE). (b) TiO2/solution interface under illumination. 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

R

I 2θ

1500 26.1

10 28.3

12.5/12/1

mainly contains quartz and Fe-oxides approximately in equalratio and MnO2 impurities

B

I 2θ

250 26.1

250 17.8 (γ-FeOOH) + 20 (R-FeOOH) + 33.5 (Fe2O3) 240 33.6 (Fe2O3)

20 28.3

1/5/40

MnO2-rich material with Fe2O3, clay, and very small amount of quartz

I

10

50

400

the TiO2/pebble interface. Figure 4 illustrates qualitatively the shunting of photogenerated carriers in illuminated TiO2 to adventitious metal oxide particles on the surface of pebbles. The system resembles that of reported heterojunction composite semiconductors,21,22 wherein the charge carriers generated in a wide band gap material (e.g., TiO2) may be driven to lower band gap metal oxide impurities, viz., Fe2O3 or MnO2. Both Fe2O3 and β-MnO2 are n-type semiconductors with band gap energies of 2.2 and 2.1 eV, respectively.22,23 The band gap of β-MnO2 was estimated using the onset wavelength in its DRIFT spectrum reported by Lamaita et al.23 In comparison, the band gap of TiO2 is much higher (3.20 eV). This manifests into setting new valence and conduction band levels with reduced band gap as well as lowered ability to mediate redox reactions on the surface. This also can lead to significant recombination of photocharges. On the other hand, the metal oxide components present on the surface of pebbles may undergo a certain dissolution in situ and set up redox couples (e.g., Fe3+/Fe2+ and Mn3+/Mn2+, etc.). Alternatively, these metal ions may adsorb on the surface of TiO222 and act as surface states or otherwise influence adsorption of dyes. The redox couples at the TiO2/aqueous solution interface may abstract photoelectrons and holes from the illuminated TiO2 depending upon their redox potentials relative to band edge positions of TiO2.24 This may be illustrated qualitatively through the energy level schemes shown in Figure 5, parts a and b. Figure 5a relates to energy levels in TiO2 versus expected redox levels in solution; the Eredox represents a solution

redox potential under equilibrium. The different redox systems have energy levels22,24-27 located in the bang gap region of TiO2, and electron exchange may be fast enough to establish equilibrium such that a depletion layer (band bending) is formed in the dark. It may be noted that the redox couples with more negative potential than Ef (Fermi level in TiO2) equilibrate with Ef and lead to band bending in the dark (Figure 5a). However, upon illumination the band bending is removed as the Ef level is now shifted to a more negative potential relative to Eredox in solution. Thus, the quasi-Fermi levels of both charge carriers (nEf and pEf) would snap the different redox couples as shown in Figure 5b. As a result, many of the redox couples will undergo redox reactions utilizing photogenerated holes and electrons. These redox reactions at the interface compete with useful reactions, viz., O2 f O2-‚ reaction and the hole-mediated reaction of OH radical generation. This may account for the observed lower photoactivity of TiO2/red and black pebbles. Brezova et al.24 also discussed a similar mechanism to explain the 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 is extremely dependent on the structural, electronic, and chemical properties of supports.4,28-30 A complete retention of slurrytype photoactivity may not be possible because the constituents of supports may interact with illuminated TiO2 and drain away a large part of the charge carriers, which otherwise would have reacted to generate ‚OH or O2-‚ radicals. Thus, chemically inert supports should be preferred for obtaining high efficiency of

Ind. Eng. Chem. Res., Vol. 46, No. 13, 2007 4413

TiO2/support systems. Litter and Navio,22 while reviewing the photocatalytic properties of iron-doped titania powders, emphasized that formation of multiphasic samples (viz., TiO2Fe2O3) during Fe doping and heat treatment is detrimental and the observed photoactivity is closely linked to the ionic state of iron: Fe2+, Fe3+, or Fe. The effect of several types of metal ions (Cu2+, Zn2+, Fe3+, Al3+, and Cd2+) on the photodegradation of non-azo dyes in TiO2 suspensions under visible light illumination has been investigated by Chen et al.31 The study concluded that Cu2+ and Fe3+ ions suppress photodegradation of the dyes by competing for photoelectrons and thereby inhibiting the formation of reactive oxygen species (‚OH or O2-‚, ‚O2H). Other metal ions, Zn2+, Al3+, and Cd2+, affected the adsorption of the dyes and caused slight variation in photoactivity. On the other hand, the beneficial effect of Fe3+ ions was also documented. For example, an increase in the rate of photodegradation of acid red 1 was observed in TiO2 suspensions containing Fe3+ aquo ions.32 This effect was traced to increase in dye adsorption on Fe3+-modified TiO2. A similar effect was also found on the photodegradation of rhodamine B in aqueous TiO2 suspensions.33 On the other hand, the addition of Fe2+ ions in the TiO2 aqueous suspensions did not affect the photocatalytic mineralization of aniline, and higher concentrations were reported to be detrimental.34 The inhibition was attributed to the competition of Fe2+ with the organic substrate for the oxidant species. In a study that was aimed at understanding the effect of metal ions on the photocatalytic degradation of phenol, Brillas et al.35 reported that Mn2+ and Co2+ inhibited the degradation. The detrimental effect of these ions was explained by electron transfer involving metal ions and holes (decreasing ‚OH production) and by competitive adsorption with phenol 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 activity of the TiO2/pebble photocatalyst. We fabricated a falling film type pebble bed reactor using TiO2/WL pebbles, and the results of solar photocatalytic decolorization of dyes will be published elsewhere. 4. Conclusions Pebbles can be used as supports for TiO2. The TiO2-coated pebbles exhibit photoactivity, which is dependent on the composition of pebbles in respect to various metal compounds present on the surface and hence is also a function of type of pretreatment provided to the pebbles. The TiO2/WL system exhibits superior photoactivity when compared to the other TiO2/ pebble systems. Leaching of white pebbles may have reduced concentration of surface metal ions and exposed fresh quartz/ silica surface that minimized the support-TiO2 interaction. The difference in photoactivity of various TiO2/pebble systems can be attributed to the interaction between native metal ions on the surface of pebbles and illuminated TiO2. It is of particular importance to avoid using pebbles that undergo substantial chemical dissolution leading to formation of solution redox couples or surface states at the interface. Significant recombination of photogenerated charge carriers can be expected due to intervention of these surface states/redox couples. Acknowledgment The authors (V.C.) thank the Ministry of Non-conventional Energy Sources (MNES), New Delhi, India and Indian Institute of Technology (IIT), New Delhi, India for the National Renewable Energy (NRE) Fellowship. The authors are grateful

to Dr. Sukumar Devotta, Director, NEERI for his kind permission to publish this work and Dr. Tapas Nandy, Scientist and Head, WWT Division, NEERI for encouragement. Literature Cited (1) Herrmann, J. M. Heterogeneous photocatalysis: fundamentals and applications to the removal of various types of aqueous pollutants. Catal. Today 1999, 53, 115-129. (2) Blake, D. M. Bibliography of Work on the Heterogeneous Photocatalytic RemoVal of Hazardous Compounds from Water and Air; NREL/ TP- 570-26797; National Renewable Energy Laboratory: Golden, CO, 1999. (3) Adesina, A. A. Industrial exploitation of photocatalysis: Progress, perspectives and prospects. Catal. SurV. Asia 2004, 8, 265-273. (4) Pozzo, R. L.; Baltanas, M. A.; Cassano, A. E. Supported titanium dioxide as photocatalyst in water decontamination: State-of-art. Catal. Today 1997, 39, 219-231. (5) Sunada, F.; Heller, A. Effects of water, salt water and silicone over coating of the TiO2 photocatalyst on the rates and products of photocatalytic oxidation of liquid 3-octanol and 3-octanone. EnViron. Sci. Technol. 1998, 32, 282-286. (6) Blazkova, A.; Karpinsky, L.; Groskova, J.; Havlinova, B.; Jorik, V.; Ceppan, M. Phenol decomposition using Mn+/TiO2 photocatalysts supported by the sol-gel technique on glass fibres. J. Photochem. Photobiol. 1997, 109, 177-183. (7) Matthews, R. W. Photooxidative degradation of colored organics in water using supported catalysts. TiO2 on sand. Water Res. 1991, 25, 11691176. (8) Fernandez, A.; Lasaletta, G.; Jimenez, V. M.; Justo, A.; GonzallezElipe, A. R.; Herrmann, J. M.; Tahiri, H.; Ait-Ichou, Y. Preparation and characterization of TiO2 photocatalysts supported on various rigid supports (glass, quartz and stainless steel). Comparative studies of photocatalytic activity in water purification. Appl. Catal., B 1995, 7, 49-63. (9) Takeda, N.; Iwatta, N.; Torinoto, T.; Yoneyama, H. Influence of carbon black as an adsorbent used in TiO2 photocatalyst films on photodegradation behaviour of propyzamide. J. Catal. 1998, 177, 240-246. (10) Rao, N. N.; Dube, S. Application of Indian commercial TiO2 powder for destruction of organic pollutants. Photocatalytic degradation of 2,4dichlorophenoxy acetic acid (2,4-D) using suspended and supported TiO2 catalysts. Indian J. Chem. Technol. 1995, 2, 241-248. (11) Sengupta, S. Gondwana sedimentation in the Pranhita-Godavari valley: a review. J. Asian Earth Sci. 2003, 21 (6), 633-642. (12) Bideau, M.; Claudel, B.; Dubien, C.; Faure, L.; Kazouan, H. On the “immobilization” of titanium dioxide in the photocatalytic oxidation of spent waters. J. Photochem. Photobiol., A 1995, 91, 137-144. (13) Online conversions-Asknumbers.com, Luminance unit conversion at http://www.asknumbers.com (accessed April 7, 2006). (14) APHA. Standard Methods for Examination of Water and Wastewater, 17th ed.; American Public Health Association: Washington, DC, 1989. (15) Chakravarty, S.; Dureja, V.; Bhattachayya, G.; Maity, S.; Bhattacharjee, S. Removal of arsenic from ground water using low cost ferruginous manganese ore. Water Res. 2002, 36, 625-632. (16) Jerden, J. L., Jr.; Sinha, A. K. Phosphate based immobilization of uranium in an oxidizing bedrock aquifer. Appl. Geochem. 2003, 18, 823843. (17) Balasubramaniam, R.; Ramesh Kumar, A. V.; Dillmann, P. Characterization of rust on ancient Indian iron. Curr. Sci. 2003, 85, 15461555. (18) Kraimer, R. A.; Curtis Monger, H.; Steiner, R. L. Mineralogical distinctions of carbonates in desert soils. J. Soil Sci. Soc. Am. 2005, 69, 1773-1781. (19) Konstantinou, I. K.; Albanis, T. A. TiO2-assisted photocatalytic degradation of azo dyes in aqueous solution: Kinetics and mechanistic considerations. Appl. Catal., B 2004, 49, 1-14. (20) Matthews, R. W.; McEvoy, S. R. Destruction of phenol in water with sun, sand, and silica. Sol. Energy 1992, 49, 507-513. (21) Navio, J. A.; Colon, G.; Litter, M. I.; Bianco, G. N. Synthesis, characterization and photocatalytic properties of iron-doped titania semiconductors prepared from TiO2 and iron(III) acetylacetonate. J. Mol. Catal. A: Chem. 1996, 106, 267-276. (22) Litter, M. I.; Navio, J. A. Photocatalytic properties of iron-doped titania semiconductors. J. Photochem. Photobiol., A 1996, 98, 171-181. (23) Lamaita, L.; Peluso, M. A.; Sambeth, J. E.; Thomas, H. J. Synthesis and characterization of manganese dioxide employed in VOCs abatement. Appl. Catal., B 2005, 61, 114-119.

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ReceiVed for reView February 17, 2007 ReVised manuscript receiVed April 17, 2007 Accepted April 19, 2007 IE0702532