A detailed investigation of photocatalytic degradation of m-cresol purple (mCP) dye has been carried out in aqueous heterogeneous medium containing zinc oxide (ZnO) as the photocatalyst in a batch reactor. The effects of some parameters such as amount of photocatalyst, dye concentration, initial pH of solution, ethanol concentration and temperature were examined. The most efficient pH in removal of the dye with photocatalytic degradation and dark surface adsorption processes was observed to be 8. The adsorption constant calculated from the linear transform of the Langmuir isotherm model was similar to that obtained in photocatalytic degradation at pH = 8; hence, the Langmuir–Hinshelwood model was found to be accurate for photocatalytic degradation at this pH. Dark surface adsorption and degradation efficiency were increased by enhancement in the temperature at the optimum pH of 8 and the apparent activation energy (Ea) for the photocatalytic degradation of mCP was determined as 14.09 kJ/mol at this pH. The electrical energy consumption per order of magnitude (EEO) for photocatalytic degradation of mCP was also determined.

INTRODUCTION

An important source of environmental concern is dye pollutants. The release of wastewater containing dye in the ecosystem is a dramatic source of esthetic pollution and perturbations in aquatic life (Herrmann 1999; Galindo et al. 2001). Most of the organic dyes are not easily biodegradable. Nevertheless methods such as reverse osmosis, adsorption on activated carbon, ion exchange and ozonation do not destruct the pollutant molecule efficiently (Galindo et al. 2001). Heterogeneous photocatalysis appears as an emerging solution to destruction of dyes and many other organic compounds from wastewater in the presence of semiconductors (TiO2, ZnO, CdS, etc.) and ultraviolet (UV) light (Khezrianjoo & Revanasiddappa 2013). Among the semiconductors, titanium dioxide (TiO2) is found to be efficient for photocatalytic degradation of pollutants due to superior photocatalytic oxidation ability, non-photo-corrosive and non-toxic characteristics (Saien & Khezrianjoo 2008). However, widespread use of TiO2 is uneconomical for large scale water treatment; thereby interest has been drawn towards the search for suitable alternatives to TiO2. Zinc oxide (ZnO) appears to be a suitable alternative to TiO2 since its photodegradation mechanism has been proven to be similar to that of TiO2 (Dindar & Icli 2001; Pirkanniemi & Sillanpaa 2002). The biggest advantage of ZnO in comparison with TiO2 is it adsorbs over a larger fraction of the UV spectrum and the corresponding threshold of ZnO is 425 nm (Behnajady et al. 2006). Meanwhile, ZnO has been reported, sometimes, to be more efficient than TiO2 (Khodja et al. 2001; Kansal et al. 2009; Krishnakumar & Swaminathan 2011). When ZnO is irradiated with light having energy equal to or more than the band gap energy (3.37 eV), a heterogeneous photocatalytic reaction occurs at the solid solution contact surface (Sobana & Swaminathan 2007).

For engineering purposes, it is useful to find out a simple and easy-to-use rate equation that fits the photocatalytic degradation rate data. Many authors have used the modified Langmuir–Hinshelwood (L-H) kinetic expression to analyze the heterogeneous photocatalytic reaction with ZnO successfully (Chakrabarti & Dutta 2004; Vasanth Kumar et al. 2007; Habib et al. 2013). This model is subject to the assumptions that adsorption of both the oxidant and the reductant are rapid equilibrium processes and that the rate-determining step of the reaction involves both species present in a monolayer at the solid–liquid interface (Chakrabarti & Dutta 2004). The L-H kinetic model could describe the solid–liquid reaction as: 
formula
1
where and C are the initial pollution and concentration at a given time; KLH is the L-H adsorption equilibrium constant and kc is the surface kinetic rate constant of reaction. However, the fitting of experimental data with the L-H type kinetic model using zinc oxide photocatalyst has so far only been demonstrated for the photocatalytic degradation of organic substrates in single-component systems. Another point to check is dark surface adsorption; if the kinetically obtained KLH is different from that obtained by dark adsorption measurement, the L-H mechanism cannot be adopted (Ohtani 2008; Khezrianjoo & Revanasiddappa 2012). Therefore, for L-H kinetic analysis, the dark adsorption measurement is always required (Ohtani 2008; Khezrianjoo & Revanasiddappa 2012).

The aim of the present work is to study the removal of an indicator dye, m-cresol purple (mCP) in the presence of ZnO photocatalyst as a suitable alternative to TiO2 irradiated by UV. Effects of operational parameters as well as degradation kinetics are investigated and ‘Electrical Energy per Order’ is also determined.

MATERIALS AND METHODS

Reagents and chemicals

All reagents were used as received. The mCP indicator dye C21H17NaO5S (CAS No. 2303-01-7, MW = 382.43) was provided from S.D. Fine Chemical Company, India, with purity of more than 97%. The molecular structure of this dye is shown in Figure 1. The ZnO catalyst was a Lobal Chemie, India product; with particle size of <5 μm and purity of more than 99%. Hydrochloric acid, sodium hydroxide and ethanol were of laboratory reagent grade (Merck products). Distilled water was used to prepare the solutions.
Figure 1

Chemical structure of mCP.

Figure 1

Chemical structure of mCP.

Photo-reactor

For the degradation process a circulating photo-reactor made of glass with a cube body shape (27 cm length, 20 cm width and 4 cm height) was used. The reactor was equipped with a water-flow jacket for regulating the temperature, by means of the external circulating flow of a thermostat bath, having an accuracy of ±0.1 °C. The 18 watt low pressure mercury UV lamp (Philips TUV PL-L) was positioned on top of the reactor with a 15 cm distance from the surface of the solution. The liquid film in the reactor was 19 mm thick. The photocatalytic reactor system was operated in a batch mode and after each experiment the reactor solution was disposed of. A pump circulated the solution; in this way, both fluidizing and good mixing of the catalyst particles were provided. The whole reactor body was covered with thin polished aluminum reflectors. Since the photocatalysis is sustained by a ready supply of dissolved oxygen, air was supplied to the center and four corners of the reactor at constant flow-rate using a micro air compressor.

Photocatalytic experimental procedure

To perform the experiments, one liter of a solution containing 20 mg/L of mCP (5.23 × 10−5 M), and a known amount of ZnO mass were prepared. The pH was adjusted to the desired value by using a pH meter (Elico LI 127). To start the degradation, the solution was transferred to the reactor, and the lamp was switched on while adjusting the temperature. During each experiment, circulation of the suspension was maintained to keep the suspension homogeneous. Samples (4 mL) were taken at regular time intervals and then centrifuged, to separate the ZnO particles from the solution. Analyses were performed with a UV-Vis spectrophotometer (Systronics 168) provided with 1 cm matched quartz cells. The degradation efficiency or conversion (X) of mCP with respect to its initial concentration at any time was obtained by: 
formula
2
where and C are the initial mCP and concentration at a given time.

Adsorption isotherm

All batch equilibrium experiments were conducted in the dark. At the time of 15 minutes, the study of mCP adsorption was performed on ZnO at different initial concentrations and pH. Data obtained from the adsorption experiments was fitted to the modified empirical Langmuir equation: 
formula
3
where Qmax is the maximum absorbable dye quantity and is the equilibrium constant for adsorption. This relationship (Equation (3)) can be written in linear form as: 
formula
4
and the adsorbed quantity (Q in mg/g) was calculated as: 
formula
5
where is the difference between the initial concentration (Co) and the equilibrium concentration (C), V is the volume (L), and m is the mass of ZnO (g).
The dimensionless separation factor (RL) (Equation (6)) has been used to describe the shape of the Langmuir isotherm (L-shape) to be either favorable , unfavorable (RL > 1), linear (RL = 1) or irreversible (RL = 0). 
formula
6
where Kads is the Langmuir adsorption constant and Co is the highest initial dye concentration (20 mg/L).

RESULTS AND DISCUSSION

Effect of catalyst concentration

In the presence of both ZnO and UV light, 97.2% of dye was degraded at the irradiation time of 240 min. This was contrasted with only 3.6% decolorization for the same experiment performed without ZnO as photocatalyst. Figure 2 shows a plot of the reaction rate constant kobs as a function of the catalyst mass for the mCP degradation under UV illumination. The rate constant increases with an increase in the amount of catalyst up to a level corresponding to the complete absorption of incident light by the catalyst. With an increase in the catalyst mass, the density of particles in the area of illumination increases; meanwhile, the greater number of the dye molecules that will be adsorbed on catalyst surface. Above 1.5 g, the degradation rate constant of mCP decreases mildly. The reason for this can be due to the screening effect of excess catalyst particles, which masks reduction in the light intensity throughout the solution (Daneshvar et al. 2004).
Figure 2

Effect of the amount of ZnO on degradation rate constant of mCP. Inside: relationship between ln(kobs) and ln(ZnO mass) [mCP]o = 20 mg/L, [ZnO] = 1.5 g/L, T = 25 °C and pH = 6.7.

Figure 2

Effect of the amount of ZnO on degradation rate constant of mCP. Inside: relationship between ln(kobs) and ln(ZnO mass) [mCP]o = 20 mg/L, [ZnO] = 1.5 g/L, T = 25 °C and pH = 6.7.

Hypothesizing that the rate of photocatalytic degradation depends on catalyst concentration, the initial rate of photocatalytic degradation of dye (ro) with the homogeneous photochemical reaction can be expressed in the following form (Galindo et al. 2001): 
formula
7
Since the photocatalytic degradation of the dyes in aqueous ZnO can be described by the pseudo-first-order kinetic, the amount of m exponent is around one. The quantity of the n exponent can be evaluated from the dependence of values and the ZnO mass (Figure 2 inside). Consequently, in our case Equation (7) can be expressed as where n is an exponent observed to be less than one for all the dyes studied, relative to low concentration of photocatalyst (Galindo et al. 2001; Sauer et al. 2002; Khezrianjoo & Revanasiddappa 2012).

UV-Vis spectra

In the UV-Vis spectra of mCP dye, two maximum absorption peaks have been observed (Figure 3). Since mCP is an indicator dye, the appropriate maximum wavelength (λmax) and absorbance at different pH values was measured; the results are indicated in Table 1.
Table 1

Observed maximum wavelength (λmax) at different pHs for mCP; [mCP]o = 20 mg/L and T = 25 °C

pH46.7 (natural)810
λmax (nm) 465 440 455 560 
Absorbance 0.26 0.25 0.28 0.36 
pH46.7 (natural)810
λmax (nm) 465 440 455 560 
Absorbance 0.26 0.25 0.28 0.36 
Figure 3

Changes in UV-Vis spectra of mCP at different irradiation times; [mCP]o = 20 mg/L, [ZnO] = 1.5 g/L, T = 25 °C and initial pH of 6.7.

Figure 3

Changes in UV-Vis spectra of mCP at different irradiation times; [mCP]o = 20 mg/L, [ZnO] = 1.5 g/L, T = 25 °C and initial pH of 6.7.

Figure 3 shows the changes in the optical densities at 440 nm and at 300 nm of mCP at different irradiation times under natural pH of 6.7, T = 25 °C and ZnO concentration of 1.5 g/L. The decrease of the absorption peak at λ = 440 nm of the dye reveals decolorization in the presence of ZnO suspension after about 240 min. However, the degradation of the aromatic part of the dye molecule at λ = 300 takes more time.

Effect of pH

The pH is a complex parameter. It is related to the ionization state of the catalyst surface; the zero point charge (pHzpc) of ZnO particles is 9.0 ± 0.3 (Anandan et al. 2006) so the catalyst surface is positively charged in acidic media (pH < 9) whereas it is negatively charged under alkaline conditions (pH > 9). Meanwhile, pH can change the substrate properties. For the diprotic sulfonephthalein dye mCP, the reaction of interest at the pH range of 4 to 12 is the second dissociation: 
formula
8
where HI and I2− are the protonated (negative one ion) and unprotonated (negative two ion) forms of the indicator species and is the second apparent dissociation constant of the dye. The is a function of temperature T and salinity S. 
formula
9
and valid for temperatures in the range from 293 °K to 303 °K and salinities from 30 to 37 psu (Abmann et al. 2011). According to Equation (9), the value of the is around 8 ± 1%; hence, the ratio of the negative two ion form of mCP concentration to its negative one ion form can be estimated by Equation (10). 
formula
10

Effect of pH on surface adsorption

To study the influence of substrate adsorption on the catalyst particle surface, a series of experiments was carried out at room temperature (25 °C) and various pHs, while the UV light was absent. All isotherms showed (Figure 4) a type of Langmuir-shape.
Figure 4

Adsorption isotherm of mCP on catalyst surface at different pHs; [ZnO] = 1.5 g/L and T = 25 °C.

Figure 4

Adsorption isotherm of mCP on catalyst surface at different pHs; [ZnO] = 1.5 g/L and T = 25 °C.

The L-shape isotherms mean that there is no strong competition between solvent and the dye to occupy the ZnO surface sites. The data obtained from the adsorption experiments were fitted to the linear form of the Langmuir equation (Equation (4)); from which the Kads, Qmax and RL factors have been calculated; results are presented in Table 2.

Table 2

Effect of pH on the Langmuir equilibrium constant (Kads), maximum absorbable dye quantity (Qmax) and RL factor of mCP adsorption on ZnO surface; [ZnO] = 1.5 g/L and T = 25 °C

pHKadsQmaxRL
0.462 1.547 0.91 
6.7 0.377 1.959 0.92 
0.292 2.541 0.95 
10 4.383 0.405 0.53 
pHKadsQmaxRL
0.462 1.547 0.91 
6.7 0.377 1.959 0.92 
0.292 2.541 0.95 
10 4.383 0.405 0.53 

According to the Langmuir model, adsorption of mCP on ZnO at pH of 4, natural (6.7) and 8 was fairly good; however best fitting with the L-shape model was observed at pH of 8 (RL = 0.95); meanwhile maximum amount of dye adsorption on the catalyst surface has been observed at this pH (Qmax = 2.541 mg/g). Since at pH = 8 the dye molecules are negatively charged , strong adsorption of dye molecules on the positively charged catalyst surface was observed. As the pH of the solution decreases from 7 to 4, the will decrease from 10−1 to 10−4, so the dye adsorption amount on the positively charged catalyst surface decreases. At pH = 10, the catalyst surface is negatively charged. On the other hand, the mCP molecules lose the proton under this condition , hence the low tendency (Qmax = 0.405 mg/g) to be adsorbed on the catalyst surface due to electrostatic repulsion forces.

Effect of pH on photocatalytic degradation

As illustrated in Figure 5, the maximum degradation (97%) was observed at pH 8 after 150 min of irradiation. Generally, in alkaline solutions photodegradation efficiency was greater than that in acidic solutions. This is because photodecomposition of ZnO particles takes place in acidic and neutral solutions (Khodja et al. 2001). Meanwhile, increasing the pH up to 8 provides a higher level of adsorption on the catalyst surface. It should be mentioned that more efficient formation of hydroxyl radicals also occurs in alkaline solutions (Khodja et al. 2001). The photodegradation efficiency decreases at pH = 10; the reason for this can be due to the low-level of dye adsorption on the catalyst surface. The sharp decrease in the photodegradation efficiency at the pH of 4 can be related to the complete photodecomposition of ZnO under highly acidic (pH at or lower than 4) conditions (Khodja et al. 2001).
Figure 5

Effect of pH on degradation of mCP at different irradiation times, [mCP]o = 20 mg/L, T = 25 °C and [ZnO] = 1.5 g/L.

Figure 5

Effect of pH on degradation of mCP at different irradiation times, [mCP]o = 20 mg/L, T = 25 °C and [ZnO] = 1.5 g/L.

Kinetics of photocatalytic degradation of mCP

Photodegradation experiments of mCP by UV/ZnO process at pH = 8 exhibited pseudo-first-order kinetics with respect to the initial concentration of the dye (Equation (11)). 
formula
11
where kobs is the pseudo-first-order rate constant, and [mCP] and [mCP]o are the concentration at time (t) and (t = 0), respectively. Figure 6 shows a plot of ln([mCP]o/[mCP]) versus time for the experiments with different initial concentrations of mCP.
Figure 6

Linear variation of versus time for the photocatalytic degradation of mCP at different initial concentrations, [ZnO] = 1.5 g/L, T = 25 °C and pH = 8.

Figure 6

Linear variation of versus time for the photocatalytic degradation of mCP at different initial concentrations, [ZnO] = 1.5 g/L, T = 25 °C and pH = 8.

The values corresponding to different initial concentration rate constants, along with the regression coefficients are listed in Table 3.

Table 3

Pseudo-first-order kinetic rate constants in photocatalytic degradation of mCP with different initial concentrations at pH = 8, [ZnO] = 1.5 g/L and T = 25 °C, along with the regression coefficients

Co (mg/L)kobs × 102 (1/min)R2
6.03 0.9992 
10 3.98 0.9975 
15 2.74 0.9928 
20 2.20 0.9903 
Co (mg/L)kobs × 102 (1/min)R2
6.03 0.9992 
10 3.98 0.9975 
15 2.74 0.9928 
20 2.20 0.9903 

As can be seen in Table 3, the lower mCP concentrations provide the better agreement with the first-order reaction.

The linearity plotted of the L-H kinetic model (Equation (1)) can be used as: 
formula
12
The data reported in Table 3 were plotted in Figure 7 as 1/kobs versus [mCP]o. Using a least square best fitting procedure, at pH of 8, the values of the adsorption equilibrium constant, KLH, and the kinetic rate constant of surface reaction, kc, were calculated as KLH = 0.302 L/mg and kc = 0.508 mg/L min.
Figure 7

Variation of reciprocal of constant rate versus different initial concentrations of mCP; [ZnO] = 1.5 g/L, T = 25 °C and pH = 8.

Figure 7

Variation of reciprocal of constant rate versus different initial concentrations of mCP; [ZnO] = 1.5 g/L, T = 25 °C and pH = 8.

According to the L-H model, if KHL truly reflected the adsorption affinity of the dye on the catalyst surface, Kads and KLH would be identical (Ohtani 2008; Khezrianjoo & Revanasiddappa 2012). Since the adsorption constant in the kinetic model was determined in our case to be close to that obtained in the dark (KLH = 1.03Kads), the photocatalytic degradation of 20 mg/L of mCP at pH = 8, T = 25 °C and 1.5 g/L of ZnO follows the L-H model satisfactorily. It should be mentioned that, in other studies, it was reported that KLH measured under irradiation could be substantially different from Kads measured in the dark (Sauer et al. 2002; Kusvuran et al. 2005; Gora et al. 2006).

Effect of ethanol as radical scavenger

The activity of two main species (electron and hole, and hydroxyl radical) can be distinguished using ethyl alcohol as a radical scavenger (Saien & Soleymani 2007). The importance of these species depends on the substrate structure and operational parameters. It was observed in Figure 8 that small amounts of ethanol inhibited the photocatalytic degradation of 20 mg/L of mCP in the presence of 1.5 g/L of catalyst, T = 25 °C and pH of 8. The photodegradation efficiency decreases with an increase of ethanol until 0.6% (v/v). On the other hand, adding an extra amount of ethanol leads to a mild increase in the process efficiency due to the formation of ethoxy radicals (C2H5O) from direct photocatalytic oxidation of ethanol; meanwhile, the ethanol molecules can produce hydroxyl radicals in direct photolysis with respect to the level of C–O energy bond (Saien & Soleymani 2007).
Figure 8

Inhibitory effect of ethanol on photocatalytic degradation efficiency of mCP, [mCP]o = 20 mg/L, [ZnO] = 1.5 g/L, T = 25 °C and pH = 8.

Figure 8

Inhibitory effect of ethanol on photocatalytic degradation efficiency of mCP, [mCP]o = 20 mg/L, [ZnO] = 1.5 g/L, T = 25 °C and pH = 8.

From data given in Figure 8, the contributions of hydroxyl radicals and electron-holes in the photocatalytic degradation process at different times are determined and compared in Figure 9. As can be seen in Figure 9, hydroxyl radicals play a major role in photocatalytic degradation of mCP for all times. Another species involved which does not react with alcohols most probably provides the positive holes formed on the irradiated photocatalyst, which react with the adsorbed dye molecules. Since, the direct oxidation part with increases with respect to the time, it can be assumed that the dye is more strongly adsorbed on the catalyst surface than the intermediate products.
Figure 9

The contribution of active species in degradation efficiency of mCP, [mCP]o = 20 mg/L, [ZnO] = 1.5 g/L, T = 25 °C and pH = 8.

Figure 9

The contribution of active species in degradation efficiency of mCP, [mCP]o = 20 mg/L, [ZnO] = 1.5 g/L, T = 25 °C and pH = 8.

Effect of temperature

It is generally accepted that temperature is a critical parameter determining the photocatalytic reaction rate (Nakashima et al. 2003). The effect of temperature on the photocatalytic degradation rate constant and dark surface adsorption of mCP at the initial pH of 8 has been studied and results are presented in Table 4. At 60 min, about 33% enhancement in the dye degradation efficiency was observed; and 45% increase for surface adsorption was achieved at 15 min, between 5 and 45 °C. Temperatures higher than 45 °C caused significant evaporation of the solution during the experiments; so a higher operating temperature was not applied.

Table 4

Effect of temperature on the rate constant and dye removal percentage with photocatalytic degradation and surface adsorption processes; [mCP]n = 20 mg/L, [ZnO] = 1.5 g/L, pH = 8

Temperature (°C)kobs × 102 (1/min)Degradation% (t = 60 min)Adsorption% (t = 15 min)
1.42 62.3 11 
15 1.87 66.1 14 
25 2.20 71.4 16 
35 2.36 76.2 18 
45 3.09 82.9 20 
Temperature (°C)kobs × 102 (1/min)Degradation% (t = 60 min)Adsorption% (t = 15 min)
1.42 62.3 11 
15 1.87 66.1 14 
25 2.20 71.4 16 
35 2.36 76.2 18 
45 3.09 82.9 20 

Increase in temperature increases the amount of mCP adsorbed on the surface of the catalyst. On the other hand, an increase in temperature decreases the solubility of oxygen in water and helps the reaction to compete more efficiently with electron and hole pairs recombination, which is not desirable (Saien & Khezrianjoo 2008).

The apparent activation energy (Ea) for the photocatalytic degradation of mCP has been calculated from the Arrhenius equation: 
formula
13
where kobs (1/min) is the rate constant, A is the temperature independent frequency factor, Ea is the apparent activation energy of the photocatalytic degradation (J/mol), R is the gas constant (8.31 J/K mol) and T is the solution temperature in Kelvin. The linear transform ln(kobs) versus (1/T), which is shown in Figure 10, gives a straight line whose slope is equal to –Ea/R. From the data obtained, the apparent activation energy was determined (Ea = 14.09 kJ/mol). This apparent energy represents the total activation energy of adsorption and photocatalytic degradation of mCP.
Figure 10

Linear variation of ln(kobs) versus 1/T; [ZnO] = 1.5 g/L, pH = 8.

Figure 10

Linear variation of ln(kobs) versus 1/T; [ZnO] = 1.5 g/L, pH = 8.

Electrical energy efficiency

Photodegradation with a UV lamp is an electric-energy-intensive process. Accordingly, a figure-of-merit of the process based on electric energy consumption can be very useful and informative. Recently, the Photochemistry Commission of the International Union of Pure and Applied Chemistry (IUPAC) proposed the parameter for advanced oxidation processes (AOPs) on the use of electrical energy. The energy demand, in a certain reactor, is described by the ‘Electrical Energy per Order’ (EEO). For the case of low pollutant concentration (i.e., cases that are overall pseudo-first-order in concentration of pollutant), this parameter is the electric energy in kilowatt hours (kWh) required to remove the concentration of pollutant by one order of magnitude (90%) in 1 m3 of contaminated water or air (Mahamuni & Adewuyi 2010). The simplified form of the equation that gives EEO (kWh/m3) is: 
formula
14
where P is the lamp power (kW), V is the treated volume (L), Co and C are the initial and final mCP concentrations and t is the time of irradiation (min). The time required for 90% degradation of the pollutant, in the pseudo-first-order degradation of pollutants is given by (Mahamuni & Adewuyi 2010): 
formula
15
where kobs is the pseudo-first-order rate constant (1/min). The EEO values for photocatalytic decolorization of mCP at different pH levels are listed in Table 5.
Table 5

The EEO values for photocatalytic decolorization of mCP at different pHs; [mCP]o = 20 mg/L, [ZnO] = 1.5 g/L and T = 25 °C

pH 6.7 10 
EEO (kWh/m347.4 31.2 45.0 
pH 6.7 10 
EEO (kWh/m347.4 31.2 45.0 

The higher electrical energy consumption means lower process efficiency. The low level of energy consumption at pH = 8 confirms the optimum pH as discussed above. The actual electrical energy consumption for photocatalytic degradation of organic pollutions depends on the operational parameters and initial concentration of pollutant; increase in the pollution initial concentration causes an increase in the electrical energy consumption (Mahmoud & Ismail 2012; Mohammadi et al. 2015). The EEO values for photocatalytic degradation of 20 mg/L of amoxicillin trihydrate in the presence of different catalysts have been reported in the range of 10.15 to 40.66 kWh/m3 (Mohammadi et al. 2015) and the EEO value for photocatalytic degradation of 50 mg/L of Acid Yellow 36 has been determined as 27.9 kWh/m3 (Khezrianjoo & Revanasiddappa 2013).

CONCLUSIONS

The dye mCP is easily degraded by zinc oxide (ZnO) assisted photocatalysis in aqueous dispersion under irradiation by UV light. Dye removal efficiency was negligible when the photolysis was carried out and it was small in the presence of ZnO in darkness. We learned that optimal amount of photocatalyst in the natural pH of 6.7 was 1.5 g/L, with dye concentration of 20 mg/L, and dependence of the initial decolorization rate on the ZnO concentration can be explained as . The most efficient pH for the photocatalytic decomposition and dark surface adsorption of mCP was 8, and zinc oxide cannot be used in highly acidic solution (pH ≤ 4). The L-H kinetic model showed a good agreement for the initial rates of degradation at pH of 8 with the appropriate rate constant of surface reaction and the substrate adsorption constant values of kc = 0.508 mg/L min and KLH = 0.302 L/mg, respectively. The adsorption constant in the kinetic model at pH of 8 was found to be similar to that obtained in the dark; thus, the photocatalytic degradation of mCP follows the L-H model satisfactorily at this pH. Our results showed that ethanol inhibited the photodegradation of dye, and we concluded from the inhibitory effect of ethanol that hydroxyl radicals play a major role in photocatalytic degradation of mCP with UV/ZnO processes for all times. The degradation and dark surface adsorption efficiency were affected by temperature; at the pH of 8, the activation energy (Ea) of this degradation was calculated as 14.09 kJ/mol. The complete removal of color, after selection of optimal concentration of catalyst and pH of 8 could be achieved in a relatively short time of about 150 min. The lowest energy consumption for photocatalytic decolorization of dye was also observed at this pH.

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