Bifunctional Titania Float for Metal Ion Reduction and Organics

Oct 29, 2013 - Solvent extraction,(6) ion exchange,(7) membrane separation,(8) electroplating ... were necessary to optimize for an enhanced reduction...
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Bifunctional Titania Float for Metal Ion Reduction and Organics Degradation, via Sunlight Laveena P. D’Souza,† Sindu Shree,‡ and Geetha R. Balakrishna*,† †

Center for Nano and Material Sciences, Jain University, Kanakapura, Bangalore Rural, 562112, India Sri BhagwanMahaveer Jain College, JC Road Campus, Bangalore 560027, India

‡

S Supporting Information *

ABSTRACT: The present study demonstrates the use of a bifunctional titania float in sunlight to cause the simultaneous oxidation of formic acid and reduction of Cr(VI) to Cr(III). Nanosized anatase TiO2 was synthesized by wet chemical methods and characterized by various microscopy and spectroscopy techniques. A float has been fabricated with the above nanopowder for water purification via sunlight-mediated photocatalysis. The impractical reduction of chromium in sunlight was made feasible by the use of photoactive nanotitania, unlike the UV light, which requires only the acid for complete reduction of chromium. The organic’s (formic acid) degradation and Cr(VI) reduction were observed to support each other till the completion of both oxidation and reduction, respectively. Formic acid with the catalyst (in the form of a float) in sunlight causes indirect scavenging of holes to enhance the catalytic efficiency and bring up the reduction rate on par with UV light, making the water purification an environmentally friendly and recyclable process.

1. INTRODUCTION Hexavalent chromium (Cr(VI)) is reported to be the third most common pollutant at hazardous waste sites as well as the second most common inorganic contaminant after lead (Pb).1−4 The discharge of toxic Cr(VI) as industrial effluent has ravaged our aquatic environment.5 The solubility of Cr in water (i.e., mobility in the environment) and its human toxicity are governed by its oxidation state. Although the range of Cr oxidation states ranges from +6 to −2, +6 and +3 are the most stable states found in the environment. Contrary to the highly water-soluble, toxic Cr(VI), Cr(III) is much less water-soluble where it typically forms (hydr)oxides in the absence of complexing ligand and is harmless. Therefore, reductive transformation of Cr(VI) to Cr(III) is a promising approach to remediate Cr(VI) contamination. Solvent extraction,6 ion exchange,7 membrane separation,8 electroplating,9 and sorption10 are the most common methods adapted for the disposal or recovery of metal ions in wastewater. All these methods have their own advantages and disadvantages. Photocatalysis is relatively a clean technology for removal of dissolved metal ions in the wastewater.11−17 One of the best known photocatalysts is TiO2, which is nontoxic, highly photoactive, and inexpensive.18−22 The catalyst with an appropriate band gap absorbs photons from the irradiated UV/ visible light to cause the excitation of electrons from the valence band to the conduction band and generate electron−hole pairs. These electrons and holes either migrate to the particle surface and become involved in the redox reactions or simply liberate heat. Conduction band electrons are consumed in the reaction to reduce oxidants (Ox → Oxred) whereas holes are filled via oxidation. In aqueous conditions any dissolved species with a reduction potential more than the conduction band of the photocatalyst can in principle consume electrons, and the Cr(VI) species with a redox potential of 1.3 eV is one such example of oxidant that can be reduced by TiO2 photocatalyst. Semi© 2013 American Chemical Society

conductor-mediated photoreactions induce a number of reactive oxygen species. Hydroxyl radicals are generated by the oxidation of water at the valence band of TiO2. This occurs at a standard potential of 2.8 eV, and it decreases with an increase in pH. To keep the photoreduction process going, it is necessary to avoid accumulation of holes or hydroxyl radicals. Addition of hole scavengers/proton donors, namely formic acid (FA), has the potential of simultaneous oxidation of organic compounds and reduction of metal ions. Formic acid (HCOOH), which has been extensively investigated for organic transformations,23 electrochemical oxidations,24 and hydrogen storage25 has a strong reducing property and is a promising agent for such reductions of Cr(VI) to Cr(III). Several parameters such as reactive surface area, particle size, and adsorption characteristics were necessary to optimize for an enhanced reduction rate. The key drawbacks of suspended processes are catalyst recovery and regeneration of spent catalyst. The present study reports a modified process for the reduction, particularly in sunlight via the immobilization of titania photocatalyst. A float was developed by uniform coating of catalyst particles on a conducting substrate.

2. EXPERIMENTAL SECTION 2.1. Materials. TiCl4 (99.5% loba Chemie) was used as a titanium source for the preparation of TiO2. The source for Cr(VI), potassium dichromate (99.5%), and other chemicals namely formic acid (100%, 26 N), sulfuric acid (98%, 36 N), perchloric acid (70%, 11.63 N), sodium hydroxide (97%), and ammonium hydroxide (30%) were from Merck. Received: Revised: Accepted: Published: 16162

August 8, 2013 October 15, 2013 October 29, 2013 October 29, 2013 dx.doi.org/10.1021/ie402592k | Ind. Eng. Chem. Res. 2013, 52, 16162−16168

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2.2. Synthesis of Photocatalyst. A 100 mL aliquot of TiCl4 was added dropwise to 1 L of double distilled water. A reaction temperature of 7 gives different λmax of 250 and 370 nm, respectively.31,32 It was observed that potassium dichromate at pH 9 does not respond significantly to photocatalytic reductions and hence was not considered for further studies. The surface charge properties of TiO2 change with acidic pH. Adsorption is the first thing that happens when TiO2 is contacted with dissolved metal ions. In the low pH range, Cr(VI) exists in aqueous solution predominantly as an anion. Anions adsorb through a ligand exchange reaction, favored at a low pH, where the surface is positively charged and site hydration is favorable. Considering TiO2 has a positive charge, at a pH lower than the isoelectric point of charge, it would be expected that dichromate would be electrostatically bonded to the surface of TiO2 and the possible reaction between the active

Figure 4. Absorption spectra of TiO2. Inset: reflectance spectra of TiO2.

the photocatalyst. The sharp decrease in the spectra at the absorption edge of ≅390 nm (indicated by an arrow) can be assigned to the band gap of pure anatase nano TiO2. The band gap energy was calculated (inset of Figure 4) to be 3.1 eV. The AFM image in Figure 5 represents the surface roughness and thickness of the coating to be around 0.23 μm on the float. The mesoporous nature of this sample was confirmed by nitrogen

Figure 5. AFM image indicating (a) surface roughness and (b) 3D view of nano TiO2. 16164

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Figure 6. BET surface analysis of nano TiO2: (a) nitrogen adsorption−desorption isotherm; (b) pore size (BJH) distribution.

Figure 7. EDS pattern of nano TiO2.

Figure 8. Raman spectrum of nano and anatase TiO2. Figure 9. Reduction of Cr(VI) in acidic and basic medium.

groups of the TiO2 surface and dichromate would be as below, favoring photocatalytic reactions.

Also the H+ ions adsorbed on the surface of TiO2 have been reported to have large surface proton exchange capacity and the

TiOH 2+ + Cr2O7 2 − → TiOH ·Cr2O7 2 − + H+ 16165

dx.doi.org/10.1021/ie402592k | Ind. Eng. Chem. Res. 2013, 52, 16162−16168

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Figure 10. Exponential reduction of Cr(VI) in UV light under different experimental conditions.

Figure 11. Exponential reduction of Cr(VI) in sunlight light under different experimental conditions.

process allows the adsorbed H+ to trap electrons to form H°ads and facilitate reduction of Cr(VI).22 Figure 10 represents the UV-mediated reductions followed by UV−vis spectroscopy at an absorbance of 349 nm. Lone FA was sufficient to cause 97% reduction within 60 min of UV irradiation, as observed in line 5 of plot b. The low pH caused by the addition of formic acid facilitates the formation of HCrO4−.33 The HCrO4− species has a chromophore CrO3O34 and hence is responsible for the peak at 350 nm, as evident in Figure 9 (due to transitions occurring from πp of the ligand to the 3d of the metal). When high-energy UV radiation of 350 nm is directed on HCrO4−, it allows the chromophore to respond to UV-irradiated light and undergo rapid reduction with the nascent hydrogen produced by formic acid. In aqueous solution, FA can be decomposed in the presence of light as per the equation

presence of catalyst alone enhances the rate of the reduction by 50% (line 7). The photocatalyst with a band gap of 3.1 eV responds well to the light of the solar spectrum to produce the electron−hole pair and cause redox reactions. Considering a nanoparticle, the concept of deep traps and defects may not be applicable. The defects or the irregularities are only with surface states, and the conducting substrate for TiO2 allows easy induction of electrons, thus maximizing the charge separation time and acting as a carrier trap, avoiding recombination that later releases the same electrons from the nanoparticle’s surface, leading to a good interfacial charge transfer.35,36 Thus, the importance of a conducting substrate on which the nano TiO2 has been coated contributes to charge carrier separation and effectively causes redox reactions. TiO2-catalyzed photoreactions in aqueous media lead to the formation of a number of hydroxyl radicals and many reactive oxidizing species, and most of the reports indicate oxidation processes to be feasible for pollutant degradation.35 But the present study involves reduction, and the reduction chances are very low unless the holes and hydroxyl radicals that are potent oxidizing species are scavenged/converted to reducing species. Acids were used as hole scavengers through indirect reduction of hydroxyl radicals. Also, addition of FA facilitates the formation of CO2• by scavenging OH•.34

HCOOH → H 2 + CO2

Chances of adverse effect by the addition of catalyst, due the formation of H• and OH• (instead of nascent hydrogen) and Cr(VI) recovery thereof, do exist to some extent in high-energy UV-mediated catalytic reductions. Figure 11 depicts plots of exponential reduction from Cr(VI) to Cr(III) in presence of sunlight. Addition of lone FA to target species (in noncatalytic reduction) had a negligible effect on reduction, as observed in plot b. Sunlight fails to cause reduction in the absence of catalyst. Photoreduction appears impossible without catalyst, as seen in lines 6, 9, and 10. Line 8 in plot a shows a complete reduction of Cr(VI) within 240 min of sunlight irradiation in the presence of the catalyst and formic acid. The

OH• + HCO2− → H 2O + CO2• Cr(VI) + 3CO2• → Cr(III) + 3CO2 16166

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of FA along with metal ion reduction. The results clearly indicate the organic’s degradation within 120 min of sunlight irradiation with a constant increase in pH. It can, however, be stated that FA plays a different role in UV and in sunlight to cause reduction of Cr(VI). In the former, FA directly participates in the reduction of Cr, unlike the latter, wherein such reduction does not occur in sunlight. FA, however, with a catalyst causes indirect scavenging of holes to enhance catalytic efficiency and bring up the reduction rate on par with UV light, making it an environmentally feasible process.

The carboxyl ion formed in the process may also act as one of the main reducing species for Cr(VI). The above photocatalytic reduction proceeds by three subsequent one-electron-transfer processes as shown below e−

e−

e−

Cr(VI) → Cr(V) → Cr(IV) → Cr(III)

which results in the formation of stable Cr(III).37,38 The possibility of the intermediates, Cr(IV) and Cr(V) formed during the process have been reported.37 They are, however, unstable and the corresponding UV-absorption peaks at 510 and 520 nm could not be traced during the course of the reduction. The catalytic efficiency is given by k/Ccatalyst, where k (slope obtained by the plot of C/C0 vs time) represents the rate of reduction and C is the concentration of catalyst. The catalytic efficiency of TiO2 is enhanced by 3 times from 3.39 × 10−3 to 9.96 × 10−3 min−1 mg−1 in the presence of formic acid. The nascent hydrogen generated by the decomposition of HCOOH in the presence of light is responsible for such a significant increase in the rate of reduction of Cr(VI).

5. CONCLUSION The light active float of nano TiO2 effectively reduces Cr(VI) of 10 ppm concentration completely in contaminated waters, in the presence of formic acid within 4 h of sunlight exposure. The surface states in the nanostructure and the presence of conducting substrate act as charge carrier traps suppressing recombination, thus effectively contributing to redox reactions. Formic acid facilitates both the formation of reducing species and charge separation due to indirect hole scavenging. The study gains significance in demonstration of the concept of immobilization of titania on an appropriate conducting substrate for multiple applications as treatment applications (reduction of metal ions, degradation of organics) and economic recyclability.

Cr2O7 2 − + 8H+ + 3H 2 → 2Cr 3 + + 7H 2O

The addition of FA was observed to cause a decrease of absorbance in the initial concentration of the Cr(VI) target solution even without the treatment, as observed in lines 3, 5, 8, and 10 of Figures 10 and 11. At low pH, the formed HCrO4− species has a standard electrode potential of 1.33 V, and this is indicative of the chances of direct reduction with a strong reducing agent such as formic acid. However, this was only at initial stages and further reduction was possible only in the presence of a catalyst; Figure 11b is clearly indicative of this situation. All the catalytic plots had a common observation that the rate of reduction was very high at initial stages of reduction. At high substrate concentration, i.e., during the initial stage of photoreduction all the catalytic sites of semiconductor surface area are free and photoreduction takes place at a very fast rate. As time progresses, the catalytic surface sites get covered with substrate and the reduction pace decreases. Further at later stages of reaction, when most of the substrate is reacted, the reduction pace again enhances due to free catalytic surface sites being available for reduction. Figure 12 represents the total organic carbon and pH measured at different intervals of time for catalytic degradation

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This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*G. R. Balakrishna: e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge DST and Jain university, for support and funding to carry out the above research work. REFERENCES

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Figure 12. Changes in the pH and the exponential degradation of formic acid during chromium reduction in presence of titania float and sunlight. 16167

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