Solar photocatalytic degradation of phenol using nanosized ZnO and α-e 2 O 3

This work focuses on the solar photocatalytic degradation of phenol using nanosized ZnO and α-Fe2O3 synthesized via precipitation route. The synthesized ZnO and α-Fe2O3 were characterized using XRD, UV/Vis and surface area analysis. α-Fe2O3 was found to exhibit better solar photocatalytic activity than ZnO under identical experimental conditions due to the larger surface area and low band gap of αFe2O3. The photocatalytic degradation of phenol follows pseudo-first order kinetics on both ZnO and αFe2O3.


INTRODUCTION
Photocatalysis involves the use of semiconductors and electromagnetic radiation especially in the visible and ultraviolet regions to drive various photochemical processes (destruction of a wide variety of organic and inorganic pollutants and microorganisms, generation of hydrogen gas from water, etc).Some of the major advantages of photocatalysis include: ambient operating temperature and pressure conditions, low cost of photocatalysts, complete destruction of water pollutants and pathogens without generation of secondary pollutants, possibility of using solar energy to drive the process, etc. (Chong et al., 2010;Fujishima et al., 2007).
The photocatalytic activities of various semiconducting materials (TiO 2 , ZnO, Fe 2 O 3 , CdS, GaP, ZnS, etc) and their composites to mineralize a wide variety of water pollutants have been investigated by many researchers.TiO 2 and ZnO are the most widely used photocatalyst due to their good photocatalytic activity, non-toxicity and low cost.ZnO was found to be more photocatalytically active than TiO 2 under solar irradiation (Hayata et al., 2011).The major disadvantage of TiO 2 and ZnO is their large band gap of about 3.2 eV (~388 nm) which makes them effective only in the presence of UV light with wavelengths shorter than 380 nm (Casbeer et al., 2012).On the other hand, α-Fe 2 O 3 as a visible light active photocatalyst with a bandgap of about 2.2 eV has been applied for photocatalytic degradation of a wide range of pollutants (Jaafar et al., 2012;Bakardjieva et al., 2007).Solar energy has the highest potential of all the renewable energies especially in tropical Africa (Blanco et al., 2009).Most developing countries have problems of wastewater management.Fortunately, there are high levels of solar radiation in most of these countries.Hence, there is the need to develop materials that can harness solar energy and use it to solve the problems of wastewater treatment.Phenol and its derivatives are found in the effluents of various industries such as petrochemical, coal, pharmaceutical, wood, paint, pulp and paper industries (Ahmaruzzaman, 2008).Phenol is easily absorbed through human skin causing skin and eye burns upon contact.Phenol affects the liver, kidneys, lungs, and vascular system.Excessive exposure to phenol may cause comas, convulsions, cyanosis and even death (Ahmaruzzaman, 2008).*Corresponding author.E-mail: ahamza@abu.edu.ng.
Photocatalytic degradation of phenolic compounds by various photocatalysts has been reported by several researchers (Hayata et al., 2011;Chiou et al., 2008;Valenzuela et al., 2002).For example, Valenzuela et al. (2002) investigated photocatalytic degradation of phenol and found ZnO to be more active than α-Fe 2 O 3 under artificial ultraviolet illumination.The aim of this work was to synthesize nanosized ZnO and α-Fe 2 O 3 using precipitation route and compare their effectiveness for solar photocatalytic degradation of phenol under identical experimental conditions.

Synthesis and characterization of ZnO and α-Fe2O3
Analytical grade reagents purchased from a local chemical store in Zaria, Kaduna State, Nigeria were used throughout the work.13.75 g of zinc nitrate was dissolved in 250 ml of deionized water.13.85 ml of ammonium hydroxide was added at a constant rate to the solution with vigorous stirring at 50°C and a pH of 8.The formed precipitate was aged for 24 h, washed with deionized water and then filtered under vacuum.The product was dried overnight in an oven at 110°C.The dried product was further calcined in a furnace at 1000°C for 4 h to yield ZnO.
38.45 g of iron nitrate was dissolved in 250 ml of deionized water.21 ml of ammonium hydroxide was then added.The mixture was stirred at 50°C and a pH of 8.0 for 3 h.The formed precipitate was aged for 24 h, washed with deionized water and then filtered under vacuum.The product was dried overnight in an oven at 110°C.The dried product was further calcined in a furnace at 1000°C for 4 h to yield α-Fe2O3.
X-ray diffraction (XRD) patterns of the synthesized ZnO and α-Fe2O3 were recorded using a powder X-ray diffractometer (Shimadzu, model 6000) employing Cu Kα radiation source (λ=0.154nm).Absorption spectra of the synthesized ZnO and α-Fe2O3 were recorded using a UV/Vis spectrophotometer (Jenway 6405).The specific surface area of the photocatalyst was obtained using a modified BET method which is based on the adsorption of water molecules on the surface of solid samples (Adefila et al., 2003).

Photocatalytic experiments
The concentration of phenol solution used in all the experiments was 500 mg/L.The pH of the solution is 4.0.100 ml of phenol solution was mixed with 0.1 g of the photocatalyst (ZnO or α-Fe2O3) in the photoreactor and magnetically stirred for 2 h in the dark in order to establish adsorption-desorption equilibrium of phenol on the photocatalyst.Thereafter, the mixture was exposed to sunlight under continuous stirring at room temperature (28±2°C) for 6 h.After every hour, samples were taken for analysis after separating the photocatalyst particles.These samples were analyzed for residual concentration of phenol at 270 nm using a UV/Vis spectrophotometer (Jenway, 6405).The experiments were performed between 10 am to 4 pm in September, 2012.Experiments were repeated in duplicate for getting the reproducibility of results.The percentage photodegradation of phenol was calculated using Equation 1: Where Co and Ct are the initial concentration of phenol and the concentration of phenol after irradiation time (t) respectively.
The crystallite sizes of ZnO and α-Fe 2 O 3 were determined using the Debye-Scherrer equation (Equation 2).
Where τ is the particle size, λ is the wavelength of the Xray radiation (Cu Kα = 0.1542 nm), k is the shape factor (k = 0.94), β is the full line width at half maximum height and θ is the angular position of the peak maximum.The estimated crystallite sizes of ZnO and α-Fe 2 O 3 were 58.4 nm and 58.2 nm, respectively.Hence, the synthesized ZnO and α-Fe 2 O 3 are nanoparticles because their crystallite sizes are less than 100 nm.The UV-Vis absorption spectra of the catalysts are shown in Figure 3. ZnO absorption is exclusively in the ultraviolet region.Whereas, α-Fe 2 O 3 exhibit broad absorption band that extends to the visible region (up to about 550 nm).The specific surface areas of ZnO and α-Fe 2 O 3 were 39.5 m 2 /g and 162.3 m 2 /g, respectively, as determined by the modified BET method.

Photocatalytic degradation of phenol over ZnO and α-Fe 2 O 3
Control experiments were performed by mixing 0.1 g of the synthesized photocatalyst (α-Fe 2 O 3 or ZnO) with 100 ml of phenol solution in the dark.The results show that 6 h of adsorption of phenol in the dark on both ZnO and α-Fe 2 O 3 resulted in the removal of less than 12 % of phenol.In the absence of photocatalyst (photolysis), the percentage degradation of phenol was only 21% in an irradiation time period of 6 h.The effect of irradiation time on the percentage degradation of phenol over ZnO and α-Fe 2 O 3 as well as in the absence of photocatalyst is depicted in Figure 4.The percentage degradation of phenol increased with increase in the irradiation time, and   reached 52% and 86% for ZnO and α-Fe 2 O 3 , respectively, within 6 h.This result indicates that the synthesized α-Fe 2 O 3 has a better photocatalytic activity for the degradation of phenol than ZnO.
The pseudo first order kinetics equation ( 3) is often used to describe the kinetics of photocatalytic processes (Hamza et al., 2011).
Where k app and t are the apparent rate constant of photocatalytic reactions and the irradiation time, respectively.The apparent rate constant is often chosen as the basic kinetic parameter for comparing different photocatalysts because it enables the determination of photocatalytic activity independent of the previous adsorption period in the dark and the concentration of solute remaining in the solution (Hamza et al., 2011).As seen in Figure 5, a linear relationship (with R 2 > 0.90) exists between -ln(C 0 /C) and irradiation times, t, for both ZnO and α-Fe 2 O 3 .Hence, the photocatalytic degradation of phenol on both ZnO and α-Fe 2 O 3 follows pseudo-first order kinetics.The apparent rate constants app ) were found to be 0.139 hr −1 and 0.310hr −1 for ZnO and α-Fe 2 O 3 , respectively.
The higher phtocatalytic activity of α-Fe 2 O 3 can be attributed to the low band gap of α-Fe 2 O 3 .The band gap α-Fe 2 O 3 is 2.1 eV (590 nm) which falls in the visible light region of the electromagnetic spectrum (400 -800 nm).Visible light constitutes about 46% of the solar spectrum (Casbeer et al., 2012).Hence, α-Fe 2 O 3 is a good absorber of solar light and utilizes the absorbed solar light photons to drive photochemical reactions under visible light irradiation better than ZnO.The band gap ZnO is 3.2 eV which falls in the ultra violet light region of the electromagnetic spectrum (200 -400 nm).This makes ZnO less active photocatalyst under solar irradiation.It should be noted Valenzuela et al. (2002) found ZnO to be more active than α-Fe 2 O 3 under ultraviolet illumination (Valenzuela et al., 2002).Ultraviolet light constitutes about 5% of the solar spectrum (Casbeer et al., 2012).The larger specific surface area of α-Fe 2 O 3 (162.3m 2 /g) may also be responsible for the observed higher photocatalytic activity of α-Fe 2 O 3 when compared with ZnO (39.5 m 2 /g).Generally, the efficiency of heterogeneous catalytic processes increases with increase in the specific surface area of the catalyst.

Conclusions
ZnO and α-Fe 2 O 3 nanoparticles were synthesized and characterized using XRD, UV/Vis and modified BET techniques.α-Fe 2 O 3 was found to exhibit better photocatalytic activity than ZnO under solar light irradiation due to the narrower band gap and larger Irradiation time (hours)  specific surface area of α-Fe 2 O 3 when compared with ZnO.The photocatalytic degradation of phenol follows pseudo-first order kinetics on both ZnO and α-Fe 2 O 3 .

Figure 4 .
Figure 4. Effect of irradiation time on the photolytic and photocatalytic degradation of phenol using ZnO and α-Fe2O3.

Figure 5 .
Figure 5. Pseudo first order kinetic plot for the photocatalytic degradation of phenol using ZnO and α-Fe2O3.