ENHANCING PHOTOCATALYTIC ACTIVITY OF BISMUTH FERRITE BY DOPING WITH COBALT AND ITS USE FOR DEGRADATION OF EVANS BLUE

An attempt has been made to enhance the photocatalytic activity of bismuth ferrite by doping it with cobalt, to prepare a catalyst to use in wide pH range for the photodegradation of Evans blue dye. The progress of the reaction has been monitored spectrophotometrically by measuring the absorbance of the reaction mixture at definite time intervals. Different parameters such as pH, the concentration of dye, amount of semiconductor and light intensity were varied to achieve the optimum rate of photodegradation. The results show that doping of bismuth ferrite by cobalt increases the rate of photocatalytic degradation due to narrowing of the band gap. Undoped bismuth ferrite has the highest catalytic activity in basic while the Co-doped catalyst in acidic medium. A tentative mechanism for the reaction has been proposed.


Introduction
Natural water resources are becoming polluted as a result of human activities, including industries causing water pollution.Polluted water may have adverse effects on animals, plant life and humans.One of the sources of polluted water is textile industries, where waste water is colored due to dye components.Azo dyes are the largest and most important class of synthetic organic dyes.It has been observed that azo dyes are used more than 50 % of all dyes because of their chemical stability and versatility. 1Azo dyes are not biodegradable by aerobic treatment processes, 2 and under anaerobic condition, they give potentially carcinogenic aromatic amines, which cause long-term health concerns. 3Evans Blue or T-1824 is an azo dye, which has a very high affinity for serum albumin.

Figure 1. Structure of Evans blue
The synthetic dyes used in a wide-range of technologies, 4 but their toxic nature generate demand for removal of dyes from wastewater.The most common methods used for the treatment of effluents from dyeing industries are membrane filtration, coagulation-flocculation, biological treatment, catalytic oxidation, sorption process, ion exchange, etc.These treatment methods for the removal of dyes from the waste water suffer from some or other drawbacks.
To develop an efficient method for converting such dyestuffs into harmless products, advanced oxidation processes (AOPs) have been widely studied in recent years.AOPs are promising methods for the treatment of wastewaters containing organic pollutants and involve two stages-first is the formation of strong oxidants and second is the reaction of these oxidants with organic contaminants in water.][7] Photocatalytic degradation has been found to be a very efficient process for mineralization of organic pollutants.The photocatalyst is a substance, which is activated by absorbing a photon and is capable of accelerating a reaction without being consumed. 8smuth ferrite (BiFeO3), is the most attractive and promising photocatalyst given photo-oxidation potential and chemical stability.Degradation of a non-biodegradable Evans blue has been carried out by the heterogeneous photo-Fenton-like processes using copper pyrovanadate (Cu3V2(OH)2O7•2H2O) as a photocatalyst. 9They also observed photocatalytic degradation of Evans blue by heterogeneous photo-Fenton-like catalysts Cu2V2O7 and Cr2V4O13. 10Electrochemical processes for the degradation of Evans Blue based on Fenton's reaction chemistry. 11The effect of Ag deposition on TiO2 for the degradation of Evans blue. 12smuth ferrite (BFO) with different particle sizes and morphologies has been synthesized by various preparation methods such as sol-gel, hydrothermal and microwave hydrothermal and studied the effects of particle size and presence of dopants on the photocatalytic activity, 13 e.g. in photocatalytic degradation of tetracycline 14 and methylene blue, 15,16 malachite green, 17 or methyl orange. 18,191][22][23][24] Now, we have studied the effect of Co-doping on the photocatalytic activity of BiFeO3 in the degradation of Evans Blue dye.

Experimental Synthesis of undoped and Co-doped bismuth ferrite
Undoped and Co-doped bismuth ferrite were synthesized by hydrothermal, and polyol methods, respectively and these were characterized by SEM-EDS techniques. 25To a solution of Bi(NO3)3.5H2O in ethylene glycol, a stoichiometric amount of Fe(NO3)3.9H2O in distilled water was slowly added under vigorous stirring for 30 min.Aqueous ammonia (approx.60 mL) was slowly dropped into the homogeneous solution to adjust pH > 10 by constant stirring to give brown colored precipitates.Then it was filtered, washed with water and dried at 80°C.The resulting solid was calcined for 3 hours at 200 °C.

Photocatalytic procedure
The photocatalytic activity of the catalyst was evaluated by measuring the rate of degradation of Evans blue.A stock solution of dye (1.0 x 10 −3 M) was prepared by dissolving (0.0960 g) of dye in 100 mL of doubly distilled water.pH of the dye solution was measured by a digital pH meter (Systronics Model 335), and the desired pH of the solution was adjusted by the addition of standard 0.1 N sulphuric acid and 0.1 N sodium hydroxide solutions.The reaction mixture containing 0.10 g photocatalyst was exposed to a 200 W tungsten lamp, and about 3 mL aliquot was taken out every 10 min.Absorbance (A) was measured at λmax=620 nm.A water filter was used to cut off thermal radiations.The intensity of light was varied by changing the distance between the light source and reaction mixture, and it was measured by Suryamapi (CEL Model SM 201).The absorbance of the solution at various time intervals was measured with the help of spectrophotometer (Systronics Model 106).
It was observed that the absorbance of the solution decreases with increasing the time of exposure, which indicates that the concentration of Evans blue dye decreases with increasing time.The calculation of degradation efficiency () was made by the relation: (1)   Here A0 is initial absorbance, and A is absorbance after degradation of dye at time t.A plot of 1 + log A versus time was linear following pseudo-first order kinetics.Typical runs are given in Table 1 and graphically presented in Figure 2.
The rate constant was calculated by using the expression:

Effect of parameters
The rate of degradation has been investigated in pH range 3.0-7.5 and 1.5-3.5 for undoped and Co-doped BiFeO3, respectively.All other parameters were kept to be identical.The results are summarized in Table 2 and Figure 3.It was observed that with an increase in pH, the rate of reaction increases.After attaining the maximum value at pH 7.0 and pH 2.5 for undoped and Co-doped BiFeO3, respectively, the rate decreases with a further increase in pH.In this case, the presence of scavenger i.e. 2-propanol does not affect the rate of reaction adversely and hence, it may be concluded that • OH radical does not participate in the degradation.It was interesting to observe that undoped BiFeO3 was active in basic range (3.0-7.5) while Co-doped BiFeO3 was active in acidic range (1.5-3.5).

Figure 3. Effect of pH on photocatalytic degradation of Evans Blue
The effect of variation of concentration of Evans blue on its degradation rate has been observed in the range from 0.4 × 10 −5 to 1.6 × 10 −5 M for both undoped and Co-doped BiFeO3 keeping all other parameters to be the same.The results are given in Table 3 and Figure 4.It has been observed that the rate of degradation increases with increasing concentration of dye up to 0.6 × 10 −5 M for both, undoped and Co-doped BiFeO3.Further increase in concentration beyond this limit results in a decrease in degradation rate.This may be explained on the basis that on increasing the concentration of dye, the reaction rate increases as more molecules of dyes were available but a further increase in concentration results appearing an internal filter effect which does not permit sufficient amount of light to reach the surface of the photocatalyst thus, decreasing the rate of photocatalytic degradation of Evans blue occurs.The effect of variation of the amount of catalyst on the rate of dye degradation has been studied in the range from 0.02 to 0.14 g in 50 mL and the results are reported in Table 4 and Figure 5.It has been observed that with an increase in the amount of catalyst, the rate of degradation increases to a certain amount of catalyst i.e. 0.10 g, for both; undoped and Co-doped BiFeO3.Beyond this point, the rate of reaction becomes virtually constant.This behavior may be explained by the fact that with an increase in the amount of catalyst, the exposed surface area of catalyst will increase.Hence, the rise in the rate of reaction has been observed, but with further increase in the amount of catalyst beyond a limit, the only thickness of the layer (and not the exposed surface area) will increase at the bottom of the reaction vessel, which was completely covered by the catalyst.The effect of light intensity on the rate of dye degradation was also studied by varying the intensity of light from 20.0 to 70.0 mWcm −2 .The observations are presented in Table 5 and Figure 6.The data indicate that with increasing light intensity, the rate of reaction increases and maximum rates were found at 70.0 and 60.0 mW cm −2 for undoped and Codoped BiFeO3, respectively.It may be explained on the basis that as the light intensity was increased, the number of photons striking per unit area also increases, resulting in higher rate of degradation for both.Further increase in the light intensity may start some thermal side reactions.

Mechanism
On the basis of the experimental observations, a tentative mechanism has been proposed for the degradation of Evans blue in the presence of bismuth ferrite (undoped and Codoped).Evans blue absorbs radiations of suitable wavelength and transforms to singlet then triplet excited state (intersystem crossing, ISC).The semiconductor also absorbs light to excite an electron from its valence band (VB) to its conduction band (CB), which will be abstracted by dissolved oxygen to generate O2 •− (in basic media, undoped BiFeO3) or HO2 • radicals (in acidic medium, Co-doped BiFeO3).These radicals can oxidize the dye to its leuco form ultimately degrading to products In acidic medium-O2 Carrying out the reaction in the presence of • OH radical scavenger, 2-propanol, the reaction rates were unaffected.This unambiguously shows that there was no involvement of • OH radicals in the reactions as an active oxidizing species.

Conclusion
At optimal conditions, the rate of degradation of Evans blue for undoped and the Co-doped BiFeO3 system was obtained as 8.95 × 10 −5 and 16.12 × 10 −5 sec −1 , respectively.Thus, the doping of BiFeO3 by cobalt ions enhances the rate of photodegradation of Evans blue almost 1.8 times (80% increase).

Figure 5 .
Figure 5.Effect of amount of catalyst on photocatalytic degradation of Evans Blue

Undoped BiFeO3 :Figure 6 .
Figure 6.Effect of light intensity on photocatalytic degradation of Evans Blue

Table 1 .
Typical runs for photocatalytic degradation of Evans blue.

Table 2 .
Effect of pH on photocatalytic degradation of Evans Blue

Table 3 .
Effect of dye concentration on photocatalytic degradation of Evans Blue

Table 4 .
Effect of amount of catalyst on photocatalytic degradation of Evans Blue

Table 5 .
Effect of light intensity on photocatalytic degradation of Evans Blue