Copper modified iron oxide as heterogeneous photo-Fenton reagent for the degradation of
coomasie brilliant blue R-250
Noopur Ametaa, Jyoti Sharmaa, Sanyogita Sharmaa, Sudhish Kumarb & Pinki B Punjabia
aPhotochemistry Laboratory, Department of Chemistry, University College of Science, M L Sukhadia University,
Udaipur 313 002, Rajasthan, India Email: pb_punjabi@yahoo.com
bDepartment of Physics, University College of Science, M L Sukhadia University, Udaipur 313 002, Rajasthan, India Received 9 August 2011; revised and accepted 25 June 2012
The heterogeneous photo-Fenton degradation of coomasie brilliant blue R–250 under visible light has been investigated using copper modified iron oxide, which has been prepared by coprecipitation method and characterized by IR spectroscopy, scanning electron microscopy and X-ray diffraction. The rate of photocatalytic degradation of dye follows pseudo-first order kinetics. The effects of various parameters like pH, concentration of dye, amount of photocatalyst, amount of H2O2 and light intensity on the rate of the photo-Fenton degradation has also been studied. Chemical oxygen demand of the reaction mixture before and after exposure has been determined. A tentative mechanism for the photocatalytic degradation has been proposed, wherein involvement of •OHradicals has been confirmed by the drastic reduction in the reaction rate in the presence of •OH radical scavengers such as isopropanol and butylated hydroxy toluene.
The retardation with butylated hydroxy toluene is much higher than with isopropanol. Under similar conditions, efficiencies of Fe2O3 and copper modified Fe2O3 have been compared for the photocatalytic degradation of coomasie brilliant blue R-250.
Keywords: Photocatalysis, Degradation, Dye degradation, Photo-Fenton degradation, Iron oxide, Copper modified iron oxide
Dyes bring colour to our lives, but are also responsible for causing environmental pollution.
Wastewater originating from dyes production and application industries pose a major threat to surrounding ecosystems, because of their toxicity and potentially carcinogenic nature1,2. Dyes are a severe threat to the environment because of their low biodegradability3. Conventional treatment methods in the textile dyeing industry for color removal include coagulation/ flocculation and via activated carbon adsorption4, 5. Both coagulation and adsorption generate
large amounts of sludge and waste which need further treatment for disposal.
The limitations of conventional wastewater treatment methods can be overcome by the application of advanced oxidation processes (AOP’s) developed by Glaze et al.6, which are a potential alternative to decolorize and reduce recalcitrant colored wastewater loads. AOPs are based on the generation of very reactive species such as hydroxyl radicals (•OH) which have a very strong oxidation potential, second only to fluorine. Hydroxyl radicals rapidly and non-selectively oxidize a broad range of organic pollutants. Common AOPs involve photocatalytic, Fenton, photo-Fenton, ozonation, photochemical and electrochemical oxidation methods.
Photocatalytic process is well known method for wastewater treatment. Photodegradation of orange-I in the presence of heterogeneous iron-oxide oxalate complex under UV-A irradiation has been investigated by Lei et al.7 Fenton reagent is an effective and simple oxidant for various types of organic contaminants. In the Fenton reaction, ferrous salts such as FeSO4.7H2O are treated with hydrogen peroxide to generate •OH radicals8 (Fe2+ + H2O2 → Fe3+ = HO• + HO−).
Decolorisation of peach red azo dye, HF-6 by Fenton reaction has been reported by Cheng and Chern9. Degradation of phenol by heterogeneous Fenton reaction using multiwalled carbon nanotube supported Fe2O3 catalyst has been investigated by Liao et al.10 Degradation of phenol with Fenton-like treatment by using heterogeneous catalyst based on iron oxide and hydrogen peroxide was studied by Lee et al.11
The degradation of different organic pollutants by photo-Fenton reagents has been known over a century. Photo-Fenton reactions are cyclic in nature and on addition of H2O2, the process continue to generate more and more •OH radicals, while in the Fenton process, the reaction stops when all the Fe2+
ions have been consumed. In photo-Fenton reactions, two •OH radical are generated per iron ion utilized and hence, the rate of reaction is higher than in the case of Fenton reaction.
Carneiro et al.12 investigated the photodegradation of reactive blue-4 using homogeneous photo-Fenton
process under artificial and solar irradiation. In homogeneous photo-Fenton reactions, iron ions remain soluble in water after reaction is completed.
This disadvantage of the homogeneous catalytic system can be removed with the use of appropriate heterogeneous catalyst.
Fe2O3-pillared rectorite as an efficient and stable Fenton-like heterogeneous catalyst for photodegradation of organic contaminants has been investigated by Zhang et al.13. Plata et al.14 studied the heterogeneous photo-Fenton reaction using goethite as catalyst.
Ramirez et al.15 investigated heterogeneous photo- Fenton oxidation with pillared clay-based catalyst for wastewater treatment. Wastewater treatment by catalytic wet oxidation using H2O2 and pillared clays containing iron as heterogeneous catalyst has been widely investigated16,17. Fenton and photo-Fenton-like degradation of a textile dye by heterogeneous process with Fe/ZSM-5 zeolite has been studied by Duarte and Madeira18. Phenol degradation in water through a heterogeneous photo-Fenton process catalyzed by Fe-treated laponite was studied by Iurascu et al.19
The present investigation focuses on the preparation of a heterogeneous photo-Fenton like reagent by co-precipitation method and its catalytic properties in the photo-Fenton degradation of the commercial dye, coomasie brilliant blue R-250. The effects of different factors, such as pH, amount of catalyst, concentration of dye, light intensity and amount of H2O2 have been studied on the rate of the reaction. The catalyst has been characterized using scanning electron microscopy (SEM), FT-IR and X-Ray diffraction (XRD).
Experimental
The modified iron oxide catalyst was prepared as follows: FeSO4.7H2O (50 g) and CuSO4.5H2O (25 g) were dissolved in 250 mL of water separately and then mixed together. Then, aqueous NaOH was added dropwise and the pH was adjusted to 9.0. The contents were agitated for 40 mins. The precipitate was filtered, washed with water to ensure complete removal of the NaOH and sulphate ions (SO42-) and air dried. The precipitate was kept in a muffle furnace at 400 °C for 2 hours. The copper and iron ratio in the catalyst was estimated to be 1:2.068.
Scanning electron microscopy (model Leo 430 Cambridge) was used to observe the morphological changes caused by loading copper ions in Fe2O3. Stability of the catalyst was checked by atomic absorption spectroscopy using ECTL 4129A atomic
absorption spectrophotometer. IR spectra of Cu modified and pure Fe2O3 were recorded on 8400 S FTIR spectrophotometer.
XRD diffraction patterns of the samples were recorded on 18 KW X-Ray diffractometer using Cu-Kα radiation. Diffraction patterns were recorded over the 2θ range from 10º to 90º with a step size of 0.05º.
Stock solution of coomasie brilliant blue R-250 (Himedia, 10-3 M) was prepared. Degradation of the dye was observed by taking 40.0 mL of a mixture of dye solution (1.25 × 10-5 M), H2O2 (0.25 mL, 30 % vol., CBH) and Cu modified Fe2O3 (0.03 g). The reaction mixture was irradiated with a 200 W tungsten lamp (Philips). The intensity of light at various distances from the lamp was measured using a solarimeter (SM CEL 201). A water filter was used to cut off thermal radiations. A digital pH meter (model 232) was used to measure the pH of the reaction mixture. The pH of the solution was adjusted by the addition of previously standardized 0.1 N sulphuric acid and 0.1 N sodium hydroxide solution. The progress of the photo-Fenton degradation was monitored by measuring the absorbance of the reaction mixture at regular time intervals using UV-visible spectrophotometer (Systronics, model 106).
An aliquot of 3.0 mL was taken out from the reaction mixture at definite time intervals and the absorbance was measured at 620 nm. It was observed that the absorbance of the solution decreases with increasing time intervals, which indicates that the concentration of coomasie brilliant blue-R 250 decreases with increasing time of exposure. A plot of 2 + log A against time was linear and follows pseudo-first order kinetics (Fig. 1). The rate constant calculated from the expression, k = 2.303 × slope.
Fig. 1 – A typical run for photodegradation of coomasie brilliant blue R-250 in presence of Cu modified Fe2O3. {pH = 9.0;
[Brilliant blue R-250] = 1.25 × 10-5M; Photocatalyst = 0.03 g;
H2O2 = 0.25 mL; Light intensity = 60mW cm-2}.
The photodegradation efficiency of the catalyst was calculated from the following expression: η = [(CODbefore – CODafter)/CODbefore] × 100, where η = photodegradation efficiency (%), CODbefore = COD of dye solution before illumination and CODafter = COD of dye solution after illumination.
In the control experiment, the dye (0.0826 g) was dissolved in 100 mL of doubly distilled water, so that the concentration of dye solution was 1.0 × 10-3 M, which was used as stock solution. The dye solution was placed in equal amounts in four beakers: In the first beaker, only dye solution was taken, in the second beaker, photocatalyst was added, in the third beaker, catalyst and H2O2 were added and in the fourth beaker, catalyst and H2O2 were added and exposed to the light. The optical density of the solution of each beaker was measured with the help of a spectrophotometer. It was found that rates of degradation of dye in beaker (1-3) were comparatively slower than the rate of reaction of beaker (4).
Results and discussion
The SEM images of the modified and pure Fe2O3
are shown in Figs 2 and 3, respectively. SEM images show that loading of copper ions leads to the formation of smaller and more regular particles of the catalyst, increasing the surface area of the catalyst, and hence, increased rate of photo-Fenton degradation. The average particle sizes of Cu modified Fe2O3 and Fe2O3 was observed to be 5 µm and 40 µm, respectively.
AAS data show that even after one month, leaching of copper ions from the catalyst was negligible, indicating that the catalyst has good stability for its use as heterogeneous photo-Fenton like reagent under visible range.
A characteristic band at 459 cm-1 due to Fe-O stretching is present in the FT-IR spectrum of the pure sample of Fe2O320.A band at 592 cm-1 due to Cu-O stretching is obtained in the FT-IR of Cu modified Fe2O3 in addition to the band due to Fe-O stretching, which clearly indicates that the loading of copper in Fe2O3 has taken place successfully.
Figures 4 and 5 illustrate the indexed XRD patterns of the Cu modified Fe2O3 and pure Fe2O3; all the Bragg reflections have been indexed in rhombohedral structure in the hexagonal setting. (space group: R-3C No. 167). The obtained values of the cell parameters for the pure Fe2O3 (a = 5.035 (5) Å, c = 13.799
(17) Å) are in very good agreement with reported standard values (a = 5.035 Å, c = 13.748 Å). Relative changes in the peak position and peak intensity in the Cu modified Fe2O3 clearly indicate that Cu atoms are well incorporated in the Fe2O3 matrix. The obtained values of the cell parameters in Cu modified Fe2O3
are a = 5.059 (5) Å and c = 13.8222 (15) Å.
The unit cell slightly expands relative to pure Fe2O3, which is consistent with the substitution of larger cation Cu2+ (0.72 Å) on the Fe3+ (0.64 Å) sites.
The partial occupancy of the interstitial octahedral site would be expected to further increase in the expansion.
The effects of varying parameters such as pH, concentration of dye, amount of catalyst, amount of H2O2 and light intensity, on the photocatalytic activity have been investigated.
The effect of pH on the rate of photo-Fenton degradation has been investigated in pH range 6.5 – 10.0. It has been observed that with an increase in pH, rate of reaction increases and after attaining the
Fig. 2 – SEM image of Cu modified Fe2O3.
Fig. 3 – SEM image of Fe2O3.
maximum value at pH 9.0, decreases with further increase in pH. The rate of reaction increases on increasing the pH of the medium as the number of
‾OH ions increases with increase in pH. As a consequence, the number of •OH radicals also increases, resulting in higher rate of degradation of dye. However, on increasing the pH above 9.0, the number ‾OH ions increase to a greater extent and will repel the anionic dye (overall anionic in nature, as it has one positively and two negatively charged centers). Hence, on increasing the pH of the medium above 9.0, the rate of degradation of dye decreases.
The effect of varying concentration of dye on the rate of photo-Fenton degradation has been observed in the range of 0.25 × 10-5 M to 2.0 × 10-5 M. It has been observed that the rate of degradation increases with increasing concentration of coomasie brilliant blue R-250 up to 1.25 × 10-5 M. Further increase in concentration beyond 1.25 × 10-5 M, decreases the rate of degradation. This may be explained as follows: on increasing the concentration of dye, the reaction rate increases as more molecules of dyes are available for degradation. However, further increase in concentration beyond 1.25 × 10-5 M causes retardation of the reaction due to increase in the number of collisions among dye molecules, whereas collisions amongst the dye and •OH radicals decrease. As a consequence, rate of reaction is retarded.
The effect of varying amount of photocatalyst has also been observed on the rate of photo-Fenton degradation. With increase in the amount of catalyst, the rate of photo-Fenton reaction increases up to a certain amount of catalyst (0.03 g), which may be regarded as the saturation point. Beyond this, the rate of reaction decreases with increase in amount of
catalyst. This may be explained by the fact that with the increase in the amount of catalyst, there is an increase in the surface area of catalyst, leading to an increase in the rate of reaction. However, after a certain limiting amount of catalyst (0.03 g), further increase in the amount of photocatalyst would also increase the number of iron and copper ions in which case there would be a possibility of short circuiting of ferrous and ferric and cuprous and cupric ions21-23. As a result, lesser number of hydroxyl radicals would be formed and retarding the reaction rate.
The effect of varying the amount of H2O2 on the photo-Fenton degradation of coomasie brilliant blue R-250 has also been investigated in the range from 1.00 × 10-2 M to 2.65 × 10-2 M .It has been observed that initially upon increase of H2O2 up to 1.66 × 10-2 M, the rate of degradation increases.
However, beyond 1.66 × 10-2 M, the rate of photo-Fenton degradation decreases. This is because the propagation step in the oxidative cycle would be hindered by excess H2O2 scavenging the
•OH radicals in solution (H2O2 + HO• → H2O + HO•2).
As a result, a decrease in rate of reaction final step is observed beyond 1.66 × 10-2 M of H2O2.
As light intensity was increased, the rate of reaction also increased and maximum rate was observed at 60.0 mW cm-2. As light intensity was increased, the number of photons striking per unit area also increased, resulting in higher rate of degradation. Further increase in the light intensity beyond 60.0 mW cm-2 results in a decrease in the rate of reaction, which may probably be due to thermal side-reactions.
Under optimal conditions, rate of degradation of coomasie brilliant blue R-250 dye is found to be
Fig. 4 – XRD pattern of Cu modified Fe2O3. Fig. 5 – XRD pattern of Fe2O3.
3.07 × 10-4 s-1, while in presence of pure Fe2O3, the rate of photo-Fenton degradation is found to be 2.16 × 10-4 s-1.
On the basis of the experimental observations and corroboration with the existing literature, a tentative mechanism has been proposed for the degradation of coomasie brilliant blue R-250 by photo-Fenton reagent (Scheme 1).
+ +
++ H O → Fe + HO + H
Fe3 2 Visible light 2 . … (1)
− +
++ H O → Fe + HO + HO
Fe2 2 2 3 . … (2)
+ +
++ H O→ Cu + HO + H
Cu2 2 Visiblelight . … (3)
− +
++ H O → Cu +HO + HO
Cu 2 2 2 . … (4)
Scheme 1
The •OH radical is non-selective and a strong oxidizing agent with relatively high oxidation potential, as compared to common oxidizing agents like H2O2,O3, O2, etc. These •OH radicals react with dye and degrade it into smaller products like CO2, NO3-, SO32− ions, etc. The products have been identified by usual chemical tests and spot tests.
[ ] [ ]
1 Dye →hν 1 Dye * … (5)
[ ] [ ]
1 Dye* →ISC 3 Dye * … (6)
[ ]
3 *
Dye + •OH →Smaller products … (7) The scavenger studies in presence of 2-propanol and butylated hydroxy toluene (BHT) indicate the involvement of •OH radicals in the present reaction.
In presence of 2-propanol and BHT, reaction rate has been found to be drastically reduced24-26.
Chemical oxygen demand (COD) of dye solution before and after illumination has been determined by iodometric method. COD of dye solution before and after exposure was found to be 256 mg/L and 96 mg/L, respectively. The photodegradation efficiency after 2 hours of illumination has been found to be 63 %.
In the present study, Cu modified Fe2O3 catalyst has been prepared by co-precipitation method, using ferrous sulphate and copper sulphate as precursors.
The amount of photo-Fenton catalyst required (typically around 0.03 g in 40 mL) is much less than that usually used. The effects of the amount of photo-
Fenton catalyst, hydrogen peroxide, concentration of dye, pH of the reaction medium and light intensity on the observed rate of reactions have been studied.
Under optimal conditions, rate of degradation for coomasie brilliant blue R-250 dye was found to be k = 3.07 × 10-4 s-1. However, in presence of pure Fe2O3, rate of photo-Fenton degradation was to be 2.16 x 10-4 s-1.During heterogeneous photo-Fenton process, •OH radicals react with dye and degrade it into smaller products like H2O, CO2, SO32-, NO3-
ions, etc.
Acknowledgement
The authors (PBP and NA) are highly thankful Dr Mukul Gupta, UGC-DAE Consortium, Indore, India, for providing SEM and XRD facilities.
Supplementary data
Supplementary data associated with this article are available in electronic form at http://www.niscair.res.
in/jinfo/ijca/IJCA 51A(07) 943-948 Suppl Data.pdf.
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