jees11-0096_offprint 96..107

Journal of Environmental Engineering and Science Volume 11 Issue JS4 Photocatalytic degradation of red dye: optimisation using RSM Ossman, Farouq and Abdelfattah

Journal of Environmental Engineering and Science, 2016, 11(4), 96–107 http://dx.doi.org/10.1680/jenes.16.00016 Paper 16.00016 Received 28/06/2016; accepted 18/10/2016 Published online 15/11/2016

ICE Publishing: All rights reserved

Photocatalytic degradation of red dye: optimisation using RSM Mona E. Ossman PhD

Marwa Abdelfattah PhD

Professor, Petrochemical Engineering Department, Pharos University, Alexandria, Egypt; City for Scientific Research and Technological Application, Alexandria, Egypt

Assistant Professor, Petrochemical Engineering Department, Pharos University, Alexandria, Egypt

Rania Farouq PhD Assistant Professor, Petrochemical Engineering Department, Pharos University, Alexandria, Egypt (corresponding author: [email protected])

The aim of this research was to apply experimental design methodology to the optimisation of the photocatalytic degradation of red dye present in waste water. This paper reports the broad range of several photocatalyst composite efficiencies for photocatalytic degradation of red dye in waste water samples from textile industries. Three composites, which were graphitic carbon nitride (g-carbon nitride (C3N4))/zinc oxide (ZnO), g-carbon nitride/titanium dioxide (TiO2) and zinc oxide nanoparticles, were prepared using different precursors (zinc chloride, zinc nitrate etc.). The catalysts were characterised by Fourier transform infrared spectroscopy, scanning electron microscopy and transmission electron microscopy. The catalytic performance of different photocatalysts was tested using different variables such as dosage, stirring speed, composite structure and dye concentration. Two methods were used to optimise the operating conditions. The results indicate that the photocatalytic degradation of the red dye solution at the resulting optimum condition, which was found to be after 90 min of ultraviolet irradiation, can reach 68, 57 and 53% when using zinc oxide nanoparticles, titanium dioxide/carbon nitride and zinc oxide/carbon nitride, respectively, as catalysts.

Notation A B C

stirring speed weight of the catalyst (g/ml) concentration of the dye after a certain time period t (mg/ml) Co initial concentration of the dye (mg/ml) Ka adsorption constant kr rate constant Xi uncoded value of the ith test variable X0 uncoded value of the ith test variable at the centre point xi coded value of the ith variable Y predicted response βi, βj, βij coefficients estimated from the regression η percentage of photocatalytic removal

Introduction Organic dyes contain large groups of organic compound pollutants. Owing to their high chemical stability, dyes can have a toxic or an inhibitory effect on living organisms when combined with various heavy metal oxides. Conventional waste water treatment plants cannot degrade most of these pollutants, as it is difficult to treat dye waste water by chemical and physical processes because of its complex molecular structures. Alternative methods for the decontamination of dye waste waters are the biological methods (Rai et al., 2014; Raj et al., 2012). However, photocatalysis has been considered as a cost-effective alternative to biological treatment processes for the treatment of dye-containing 96

waste water (Fu et al., 2012; Li et al., 2011; Liu et al., 2013; Priac et al., 2014). Heterogeneous photocatalysis involves photoreactions that occur at the surface of the catalyst (Priac et al., 2014). Depending on where the initial excitation occurs, photocatalysis can be generally divided into two classes of processes: a catalysed photoreaction and a sensitised photoreaction (Linsebigler et al., 1995; Litter, 1999). Semiconductors (titanium dioxide (TiO2), zinc oxide (ZnO), iron (III) oxide (Fe2O3), cadmium sulfide (CdS), zinc sulfide (ZnS) etc.) may act as photocatalysts for the redox process due to their electronic structures, characterised by a filled valence band and an empty conduction one (Chen et al., 2013; Fang et al., 2014; Guo et al., 2011). The semiconductor titanium dioxide is a non-toxic, lowcost photocatalyst and is used as an additive in various products, such as cosmetics, food and pharmaceuticals. Interest in photocatalysis has focused on the use of titanium dioxide as a photocatalyst for the removal of organic and inorganic pollutants from water because of the ability of titanium dioxide to oxidise organic and inorganic substrates in air and water (Bonetta et al., 2013; Ibhadon and Fitzpatrick, 2013). However, this compound is widely used as a self-cleaning material for surface coating in many applications (Bonetta et al., 2013; Yu et al., 2010). Zinc oxide has been often considered a valid alternative to titanium dioxide because of its good optoelectronic, catalytic and photochemical properties along with its low cost. The photocatalytic activity of zinc oxide is almost similar to that of titanium dioxide, as the band gap energy of zinc oxide is the same as that of titanium dioxide. In aqueous solution, zinc oxide shows a photocorrosion tendency with illumination by ultraviolet (UV) light. Zinc oxide has been used as a photocatalyst in waste

Journal of Environmental Engineering and Science Volume 11 Issue JS4

Photocatalytic degradation of red dye: optimisation using RSM Ossman, Farouq and Abdelfattah

Offprint provided courtesy of www.icevirtuallibrary.com Author copy for personal use, not for distribution water for decomposing halogen compounds and azo dye compounds (Benhebal et al., 2013; Muthirulan et al., 2013). Zinc oxide nanoparticles can be synthesised using various techniques such as hydrothermal synthesis, sol–gel synthesis, wet chemical synthesis, precipitation, microemulsion, chemical vapour deposition, solid-state reaction and laser ablation (Benhebal et al., 2013; Yu et al., 2010). Carbon nitride (C3N4) has been identified as a new form of organic polymer-like material with diverse properties and is considered to be the most stable allotrope of various cyanide (CN) structures under ambient conditions (Sridharan et al., 2013). It has been reported that graphitic carbon nitride (g-carbon nitride) is the most stable allotrope of carbon nitride at ambient atmosphere, and it has rich surface properties which make it an attractive alternative for use as a catalyst, due to the presence of basic surface sites. The ideal g-carbon nitride consists solely of an assembly of C–N bonds without electron localisation in the π state (Zhu et al., 2014). Several composite materials in combination with g-carbon nitride have been also reported, such as g-carbon nitride–bismuth tungstate (Bi2WO6) (Liu et al., 2012; Miranda et al., 2013; Nishikawa et al., 2013; Zhao et al., 2012). The aim of this work was to study the degradation of red dye using g-carbon nitride/zinc oxide, g-carbon nitride/titanium dioxide and zinc oxide nanoparticles and characterise these composites by using techniques such as Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The effect of various parameters such as dye concentration, photocatalyst dosage and stirring speed were investigated.

Experimental work The photocatalytic activities of different photocatalysts were evaluated by degradation of red dye under UV light (λ = 365 nm). The UV light was provided by a 12-W UV light lamp. Chemical reagents Melamine, titanium dioxide, zinc chloride (ZnCl2), sodium carbonate (Na2CO3), zinc nitrate, sodium hydroxide and soluble starch were used as precursors for the synthesis of photocatalyst composites. All chemicals were of analytical grade and were used as purchased without further purification. The dyestuff used to model waste water pollution is red dye with λmax = 487 nm and a molecular weight of 373·9 g/mol. Its scientific name is carminic acid; its structure is shown in Figure 1.

CH2OH HO

O H

OH

O

OH

O

HO OH HO

CH3 O C OH OH

Figure 1. Structure of red dye

Graphitic carbon nitride/titanium dioxide composite Graphitic carbon nitride/titanium dioxide composite was obtained by adding an 0·2 g of g-carbon nitride and 9·8 g of titanium dioxide to methanol and stirring at room temperature for 24 h. Afterwards, the composite photocatalyst was obtained by evaporating the methanol (Miranda et al., 2013). Graphitic carbon nitride/zinc oxide composite Melamine, 0·074 g, was dispersed in 25 ml of zinc chloride solution (0·5 mol/ml) in a 250-ml glass beaker, and the suspension was vigorously stirred for 20 min. Then, 25 ml of sodium carbonate (0·5 mol/ml) solution was added dropwise into the suspension and stirred magnetically for 30 min. Subsequently, the mixture was filtered, washed with deionised water for several times and dried at 60°C for 24 h. The precursor of g-carbon nitride/zinc oxide photocatalyst in a g-carbon nitride ratio of 5·0 weight % (wt.%) was obtained (Liu et al., 2012). Zinc oxide nanoparticles The zinc oxide nanoparticles were prepared by the precipitation method using zinc nitrate and sodium hydroxide as precursors and soluble starch as a stabilising agent. Starch, 0·3 g, was dissolved in 100 ml of distilled water at 27°C. Zinc nitrate, 10 ml of 0·1 mol/l solution, was added to the starch solution. The obtained solution was stirred constantly using a magnetic stirrer for 2 h until complete dissolution occurred. Subsequently, 10 ml of 0·2 mol/l sodium hydroxide solution was added drop by drop to the solution from the side-walls. The reaction was allowed to proceed for 2 h after the complete addition of sodium hydroxide solution. After the completion of the reaction, the solution was allowed to settle down for 24 h. The supernatant solution was discarded carefully, and the remaining solution was centrifuged. In order to remove the by-products and excessive starch bound to the nanoparticles, the precipitate was washed with ethanol repeatedly. Zinc oxide nanoparticle powder was obtained after drying at 100°C (Lanje et al., 2013).

Ionised water was used throughout this study.

Results and discussion

Synthesis Carbone nitride preparation Graphitic carbon nitride was obtained by simple calcination of melamine at 580°C for 4 h in a covered alumina crucible in order to prevent sublimation of melamine (Miranda et al., 2013).

Characterisation of the photocatalyst FTIR FTIR spectra were collected by means of an infrared (IR) spectrometer. The morphology of the prepared samples was analysed using high-resolution SEM, and high-resolution TEM was performed in a transmission electron microscope operating at 97



Offprint provided courtesy of www.icevirtuallibrary.com Author copy for personal use, not for distribution 20 kV. The samples for TEM were prepared by dissolving them in ethanol. The crystal structure of g-carbon nitride can be confirmed by FTIR, as shown in Figure 2. Thus, pristine carbon nitride exhibits the typical band at c. 717·1 cm−1. A wide band could also be observed between 2918·92 and 3360 cm−1, which corresponds to the N–H stretching vibration of the residual NH2 group attached to the sp2hybridised carbon (C). The occurrence of such a band clearly indicates that the presence of this NH2 group may increase the nitrogen (N)/carbon ratio. For composite materials, this band would also correspond to the O–H stretching of the free OH groups on the surface of the sample. The crystal structure of g-carbon nitride (5·0 wt.% zinc oxide) can be confirmed by FTIR. As shown in Figure 3, the absorption band near 1508·08 cm−1 is attributed to C–N stretching. The band near 834·8 cm−1 is attributed to out-of-plane bending modes of C–N heterocycles. A broad band near 3401·51 cm−1 corresponds to the stretching modes of NH2 or NH groups. The crystal structure of zinc oxide nanoparticles can be confirmed by FTIR. As shown in Figure 4, prominent peaks at 523·51, 1082·1, 1647·29 and 3409 cm−1 are clearly observed. The peak at 523 cm−1 corresponds to Zn–O, and the remaining peaks at 1082·1, 1647·29 and 3409 cm−1 are due to the absorption of water during the preparation of IR pellets or O–H present in the starch molecules. The peaks at 931·25, 1021·9 and 2929·6 cm−1 are due to the presence of C–O and C–H vibration modes of starch, which act as a capping agent.

SEM Figure 5 shows the SEM photographs of zinc oxide nanoparticles; it is clearly seen that the zinc oxide nanoparticles display mainly sphere-like agglomerates with a diameter in the range between 34 and 61 nm, which is smaller than 100 nm, the average particle size of nanocrystalline powder. SEM was used to investigate the morphologies of the composite samples. Figure 6 shows the SEM images of the titanium dioxide/gcarbon nitride samples. It is shown that the composite is assembled in the form of microspheres. From SEM images, it can be inferred that the titanium dioxide aggregates cover the large g-carbon nitride particles. Figure 7 shows the high-magnification SEM photographs of g-carbon nitride/zinc oxide (5·0 wt.%). From Figure 7, it is clearly seen that the g-carbon nitride/zinc oxide composite displays mainly sphere-like agglomerates with a diameter of 0·5 µm, which are composed of several small spherical nanoparticles, probably because melamine plays a conglomeration role in the chemical precipitation process. In addition, the small spherical nanoparticles have a mean size of about 57·47 nm. It can be concluded that zinc oxide is evenly distributed on the surface of the sphere-like composite, which favours the formation of a heterojunction and results in an enhanced photocatalytic activity. TEM As shown in Figure 8, the diameter of the zinc oxide particles was found to be smaller than 90 nm. Moderately aggregated particles seem to be present.

51∙8 50 48 46 44 Transmittance: %

42 38 36 34 32 30 28 26 24 22 20 19 4400 4000

3266∙96 2818∙92 3360∙11 3200∙83 2860∙42 2360∙89 2337∙70 3117∙58 2260∙11

1676∙75

665∙90 717∙00

2918∙92

547∙99 717∙1

3000

2000

1500 cm–1

Figure 2. FTIR analysis of titanium dioxide and carbon nitride

98

261∙93

1337∙83 1289∙18 1272∙00 1150∙00

40

1000

500 350



Offprint provided courtesy of www.icevirtuallibrary.com Author copy for personal use, not for distribution 46·5 44 42 40 38 36

4376·25 4336·02 3904·41 3949·93 4303·71 3857·93 3803·05 4166·89 4026·48 3749·49

1947·96

2419·46 2309·48 2340·68

2105·00

354·86 372·52

34 32 Transmittance: %

30 28 739·83

26

707·45

24

893·40

22

472·21

1047·78

20

953·89

18 16

834·80

14 12 10 8

1584·66

6

1508·68

3401·51

4 2·5 4400

4000

3000

2000

1500

1000

500 350

cm–1

Figure 3. FTIR analysis of g-carbon nitride (5·0 wt.% zinc oxide)

28·5 26 2168·44 2148·79

24

Transmittance: %

22 20 18 16

860·52

14 1647·29

12 8

4 2·4 4400·0 4000

763·36 931·25 706·54

10 6

1240·30

1417·65 1377·77

2929·60

1021·90 1155·64 1082·10

3409·88

3000

2000

1500

1000

568·77 523·51 476·82 432·81

500 350

cm–1

Figure 4. FTIR analysis of zinc oxide nanoparticles

99



Offprint provided courtesy of www.icevirtuallibrary.com Author copy for personal use, not for distribution

86·21 nm

50·56 nm 51·72 nm

40·23 nm

61·80 nm

20 kV

×35 000

57·47 nm

33·71nm

0∙5 μm

023313

20 kV

×35 000

0·5 µm

023317

Figure 5. SEM photographs of zinc oxide nanoparticles

Figure 6. SEM photographs of titanium dioxide/carbon nitride

Figure 9 shows a TEM image of the composite photocatalyst g-carbon nitride/zinc oxide. It clearly shows that the mean size of the composite particles is approximately 67 nm.

Photocatalytic activity of the different types of photocatalysts To study dye degradation, some parameters which affect removal efficiency, such as the amount of catalyst and the initial concentration, were studied.

The morphology of g-carbon nitride/titanium dioxide was observed by TEM. As shown in Figure 10 from the TEM image of the g-carbon nitride/titanium dioxide hybrid, it can be seen that the titanium dioxide particles disperse on the surface of g-carbon nitride. Graphitic carbon nitride/titanium dioxide displayed a firm connection between titanium dioxide and g-carbon nitride, indicating that the formation of a heterojunction was possible. Figure 10 shows that the mean size of the composite particles is in the range of 50–72 nm.

20 kV

×35 000

0·5 µm

023315

Figure 7. SEM photographs of g-carbon nitride/zinc oxide

100

The degradation of red dye was estimated by applying the following equation

Zn-1.tif Print Mag: 138 000× @7∙0 in 9:22:02 a 02/07/15

100 nm HV = 80∙0kV Direct Mag: 10 000× AMT camera system

Figure 8. TEM analysis of zinc oxide nanoparticles



Offprint provided courtesy of www.icevirtuallibrary.com Author copy for personal use, not for distribution probability of the red dye molecules reacting with hydroxyl radicals decreases.

74·3 nm

57·9 nm

66·2 nm 70·4 nm

Zn C3N4-2.jpg Print Mag: 138 000× @7·0 in 9:30:24 a 02/07/15

100 nm HV = 80·0 kV Direct Mag: 10 000× AMT camera system

Figure 9. TEM analysis of g-carbon nitride/zinc oxide

1:

ηð%Þ ¼

Effect of dye initial concentration The effect of increasing the initial concentration of the dye from 5 to 25 parts per million (ppm) at a catalyst dosage of 0·1 g at 200 revolutions per minute (rpm) was investigated. The results are illustrated in Figure 12. As can be seen, when the dye concentration increases from 5 to 10 ppm, the percentage of removal increases; however, it decreases with increasing initial concentrations. This induces an inner filter effect, and the path length of the photons entering the solution decreases and the quantity of intermediates increases as well, competing through side reactions with the parent dye decomposition. Molecules will be adsorbed on the surface of the photocatalyst, and the active sites of the catalyst will be reduced; thereby, the generation of hydroxyl radicals decreases. Another reason may be that at higher dye concentration, UV light might be absorbed by the dye rather than by the g-carbon nitride/zinc oxide composite particles, and this reduces the photodegradation efficiency. Effect of stirring speed The stirring speed in the experiment ranged from 200 to 800 rpm. The results illustrated in Figure 13 show that by increasing stirring speed, photocatalyst degradation is increased slightly from around 54% at 200 rpm to 63% at 800 rpm, which means that the effect of the stirring speed is insignificant.

ð C C C Þ100 o

o

50·4 nm

where η is the percentage of photocatalytic removal, Co is the initial concentration of the dye (mg/ml) and C is the concentration of the dye after a certain time period t (mg/ml). Effect of photocatalyst dosage In photocatalysis, one of the main parameters of the discoloration of the dye solution is photocatalyst dosage. Different dosages of g-carbon nitride/zinc oxide composite were used to degrade red dye. The results obtained for a radiation time of 120 min are shown in Figure 11. As the loading of the photocatalyst increases from 0·2 to 0·3 g/ml, the photodegradation rate increases from 44 to 60% after 90 min. In this case, the number of photons and the adsorbed molecules increase and the density of the molecule in the area of illumination also increases, which causes an increase in the degradation rate (Senthilkumaar et al., 2005). Whereas in the case of catalyst loading above 0·3 g/ml, the degradation rate falls, which may be due to the fact that the added catalyst blocks light transmission through the solution and increases light scattering. Further increase in the catalyst amount may result in the deactivation of the activated molecules due to collision with the ground-state molecules (Fotiadis et al., 2007). Therefore, the generation of hydroxyl radicals will be decreased and the

33·9 nm 59·3 nm

72·1 nm

TiO2 C3N4-2.jpg Print Mag: 138 000× @7·0 in 9:36:31 a 02/07/15

68·8 nm

100 nm HV = 80·0 kV Direct Mag: 10 000× AMT camera system

Figure 10. TEM analysis of g-carbon nitride/titanium dioxide

101



70

70

60

60

50

0·1 g

40

0·2 g

30

0·3 g

20

0·4 g 0·5 g

10

Dye degradation: %

Dye degradation: %


0 0

20

40 60 Time: min

80

20 ppm

30

15 ppm

20

10 ppm

10

5 ppm 0

Comparison of photocatalytic activities of different composites To investigate the efficiency of the different composites, experiments were carried out at constant conditions of initial concentration, catalyst dosage and stirring speed. The values of efficiency of titanium dioxide/carbon nitride, zinc oxide/carbon nitride and zinc oxide nanoparticles in the photocatalytic degradation of red dye were examined; the results are presented in Figure 14. The results clearly indicate that zinc oxide is found to be the most active in the degradation of red dye. The order of photocatalytic activity of various photocatalysts for red dye is zinc oxide nanoparticles > titanium dioxide/carbon nitride > zinc oxide/carbon nitride. As shown in Figure 14, the results indicate that the photocatalytic degradation of the red dye solution, after 90 min of UV irradiation, can reach percentages of 68, 57 and 53% when using the catalysts zinc oxide nanoparticles, titanium dioxide/carbon nitride and zinc oxide/carbon nitride, respectively, which means that the zinc oxide nanoparticles have a higher degradation efficiency.

100 90 80 70 60 50 40 30 20 10 0

20

40 60 Time: min

80

100

Figure 12. Comparison diagram of photocatalytic degradation of dye with different dye concentrations

Reaction kinetics The photocatalytic reaction follows the Langmuir–Hinshelwood model

2:

r¼

KakrC 1 þ KaC

where kr and Ka represent the rate constant and adsorption constant, respectively. The equation can be simplified to an apparent firstorder equation

3:

ð CC Þ ¼ K k

ln

o

a r

t ¼ kot

A plot of ln(Co/C) against time is represented in Figure 15. The values of ko and the linear regression coefficients of the photodegradation of red dye which correspond to the different composites are listed in Table 1.

Photocatalytic degradation: %

Dye degradation: %

25 ppm

40

0

100

Figure 11. Comparison diagram of photocatalytic degradation of red dye with different dosages of g-carbon nitride/zinc oxide composite

80 70 60 50

TiO2/C3N4

40 30

ZnO/C3N4

20 ZnO nanoparticles

10 0

0

200

400

600 800 Speed: rpm

1000

1200

Figure 13. Effect of stirring speed on photocatalyst degradation of red dye

102

50

0

15

30

45 60 Time: min

75

90

Figure 14. Comparison of efficiencies of different composites on the degradation of red dye



Offprint provided courtesy of www.icevirtuallibrary.com Author copy for personal use, not for distribution 1·4 y = 0·0093x + 0·2494 R2 = 0·9517

1·2

ln(C0/C)

1·0

TiO2/C3N4 ZnO/C3N4

0·8

ZnO nanoparticles 0·6

Linear TiO2/C3N4 Linear ZnO/C3N4

0·4 y = 0·007x + 0·2597 0·2

Linear ZnO nanoparticles

y = 0·0076x – 0·0153

R2 = 0·9495

R² = 0·8867

0 0

20

40

60

80

100

Time: min

Figure 15. ln(Co/C) against time for different composites

According to Table 1, zinc oxide nanoparticles provide better agreement with the first-order reaction. Statistical methods Multiple linear regression modelling Multiple linear regression (MLR) analysis is commonly used in environmental studies in order to correlate the independent variables on the examined dependent response. The concentration, catalyst dosage and stirring speed of the solution were considered in the development of mathematical models for the red dye removal rate. The correlation between the factors (concentration, catalyst dosage and stirring speed) and the red dye removal rate was obtained by MLRs. In the study, the Minitab program was used to derive the models of the form

4:

% degradation ¼ 692 161A 231B þ 000111C

variables, while the removal percentage of red dye was the dependent response variable in the central composite design. In order to describe the effects of stirring speed, catalyst dosage and initial red dye ccentration on percentage removal of red dye, batch experiments were conducted. The coded values of the process parameters were determined by the following equation

5:

xi ¼

Xi Xo ΔX

where xi is the coded value of the ith variable, Xi is the uncoded value of the ith test variable and X0 is the uncoded value of the ith test variable at the centre point. The experimental range and levels of independent variables are given in Table 2. The regression analysis was performed to estimate the response function as a second-order polynomial k

k1

k1 k

i¼1

i¼1

i¼1 j¼1

% removal ¼ β0 þ ∑ X i þ

∑ βii X i 2 þ ∑ ∑ βi j X i X j

where A is the stirring speed, B is the weight of the catalyst (g/ml) and C is the red dye concentration (mg/ml).

6:

Experimental design by response surface methodology The response surface methodology (RSM) was used to optimise the three parameters (stirring speed, catalyst dosage and initial dye concentration). The three parameters were selected as independent

where Y is the predicted response. βi, βj and βij are coefficients estimated from the regression; they represent the linear, quadratic and cross-products of X1, X2 and X3 on response. Minitab 14 was

Composite

ko: min−1

R2

0·007 0·0076 0·0093

0·949 0·886 0·951

Factor Titanium dioxide/carbon nitride Zinc oxide/carbon nitride Zinc oxide nanoparticles

Table 1. Pseudo-first-order apparent constant values for the different composites

Initial concentration: ppm Dosage: gm Stirring speed: rpm

Low (−1)

Central (0)

High (+1)

5 0·1 200

15 0·3 600

25 0·5 1000

Table 2. Design matrix for the central composite designs

103




Run

Initial concentration: ppm

Dosage: g

Stirring speed: rpm

Removal (observed): %

Removal (predicted): %

SE

Residual

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

5 10 15 20 25 5 5 5 5 5 5 5 5 25 25 25 5

0·1 0·1 0·1 0·1 0·1 0·2 0·3 0·4 0·5 0·1 0·1 0·1 0·1 0·1 0·5 0·5 0·5

200 200 200 200 200 200 200 200 200 400 600 800 1000 1000 1000 200 1000

60·44 66·12 33·493333 28·41 20·78 44·26 60·02 46·54 42·3 57·32 61·54 64·4 68·62 23·5923 82 16·5115 45 29·6913 28 48·0249 17

59·758 51·497 42·098 31·561 19·885 55·015 51·023 47·782 45·293 58·837 60·39 64·417 70·918 21·478 18·626 27·577 45·91

7·214 5·112 6.071 5·157 7·737 5·112 6·071 5·157 7·737 5·112 6·071 5·157 7·737 8·078 8·078 8·078 8·078

0·682 14·623 −8·604 −3·151 0·895 −10·755 8·997 −1·242 −2·993 −1·517 1·15 −0·017 −2·298 2·115 −2·115 2·115 2·115

SE, standard error

Table 3. The experimental design with variable responses

used for regression analysis of the data obtained and for estimating the coefficient of the regression equation. Statistical combinations of the independent variables with the measured and predicted yield are shown in Table 3. It was observed that, for most experiments, the predicted responses express well the variation in the observed responses.

By analysing the experimental results through RSM, an empirical correlation for the best photodegradation efficiency was found to be

dye degradation ¼ 345049 167892A 43291B þ 05523C 22759A2 þ 15033B2 þ 49484C 2 þ 55391AB 23918AC 26357BC 7:

The quadratic response surface model was validated by the analysis of variance (Anova). Anova is required to test the significance and adequacy of the model, which can be used to estimate the goodness of fit in each case by using the coefficient of determination (R2). The coefficient of determination was calculated to be 0·883.

For estimating the effects of the factors, a t-test was performed. The significance of each factor was determined and is listed in Table 5.

Term

Coefficient

SE coefficient

t

P

Constant A B C A2 B2 C2 AB AC BC

34·5049 −16·7892 −4·3291 0·5523 −2·2759 1·5033 4·9484 5·5391 −2·3918 −2·6357

18·017 2·984 2·984 2·984 9·023 9·023 9·023 2·918 2·918 2·918

1·915 −5·626 −1·451 0·185 −0·252 0·167 0·548 1·898 −0·820 −0·903

0·097 0·001 0·190 0·858 0·808 0·872 0·600 0·099 0·439 0·396

Table 4 shows the results of the Anova analysis of the model.

Source

DF Seq. SS

Adj. SS

Adj. MS

F

P

Regression 9 4251·59 4251·59 472·399 6·20 0·013 Linear 3 3856·28 2576·46 858·820 11·28 0·005 Square 3 25·75 37·23 12·411 0·16 0·918 Interaction 3 369·56 369·56 123·186 1·62 0·270 Residual error 7 532·97 532·97 76·139 Total 16 4784·56 DF, degree of freedom

SE, standard error S = 8·726; R2 = 88·9%; R2(adjusted) = 74·5%

Table 4. Anova for percentage removal

Table 5. Factor effect and associated P values

104



Offprint provided courtesy of www.icevirtuallibrary.com Author copy for personal use, not for distribution Surface plot

Contour plot 0·5

% Removal 60

Dosage: g

0·4

60

Hold values Stirring speed: 600 rpm

0·3

Removal: % 40 20

0·2

0·1

6

12 18 24 Initial concentration: ppm 5

10 20 15 Initial concentration: ppm

25 (a)

1000

% Removal 50

Stirring speed: rpm

900 800 700

Hold values Dosage: 0·3 g

600

Removal: %

500 400

50 40 30 20 6

12 18 Initial concentration: ppm

300 200

5

15 20 10 Initial concentration: ppm

24

900 600 300 Stirring speed: rpm

25 (b)

1000

% Removal 48

900 800 Stirring speed: rpm

0·55 0·40 Dosage: g 0·25 0·10

700

45

Hold values Initial concentration: 15 ppm

600

40 Removal: % 35

500

30 400

900 600 0·10 0·25 0·40 0·55 300 Dosage: g Stirring speed: rpm

300 200 0·1

0·2

0·3 Dosage: g

0·4

0·5 (c)

Figure 16. Graphs of surfaces and contours for the percentage dye degradation: (a) initial concentration and dosage, (b) initial concentration and stirring speed and (c) dosage and stirring speed

105



Offprint provided courtesy of www.icevirtuallibrary.com Author copy for personal use, not for distribution As shown in Table 5, the response was significantly affected by the initial concentration and the interaction term between the initial concentration and dosage. Surfaces and contours for percentage degradation For optimisation of the removal efficiency, the response surfaces and contour diagrams can be used. Each graph represents an infinite number of combinations of two variables, with one maintained at its average values corresponding to the central point of each variable. The graphs of surfaces and contours are given in Figure 16, which represent the effect of initial concentration and dosage (Figure 16(a)), initial concentration and stirring speed (Figure 16(b)) and finally dosage and stirring speed of the solution (Figure 16(c)). The efficiency of the photocatalytic degradation decreases when the initial concentration increases. However, a yield close to 58·75% can be obtained for a concentration of 10 ppm. These figures show that 100% of degradation can be reached for the values of the following parameters: initial concentration of 10 ppm, stirring speed of 200 rpm and dosage of 0·1 g.

Conclusion Different photocatalysts (zinc oxide/g-carbon nitride, titanium dioxide/g-carbon nitride and zinc oxide nanoparticles) were prepared using different precursors. The composites were characterised by FTIR, SEM and TEM. Their photocatalytic efficiencies were investigated through degradation of a red dye solution. The results indicate that the photocatalytic degradation of the red dye solution, after 90 min of UV irradiation, can reach 68, 57 and 53% when using the catalysts zinc oxide nanoparticles, titanium dioxide/carbon nitride and zinc oxide/carbon nitride, respectively, which suggest that zinc oxide nanoparticles have a higher degradation efficiency. The reactions were mathematically described as a function of variables such as stirring speed, catalyst dosage and initial concentration and were modelled by the use of MLR and RSM. The optimised conditions correspond to an initial concentration of 10 ppm, a stirring speed of 200 rpm and a dosage of 0·1 g. The experimental values agreed with the values predicted by RSM. REFERENCES

Benhebal H, Chaib M, Salmon T, Geens J et al. (2013)

Photocatalytic degradation of phenol and benzoic acid using zinc oxide powders prepared by the sol–gel process. Alexandria Engineering Journal 52: 517–523. Bonetta S, Bonetta S, Motta F, Strini A and Carraro E (2013) Photocatalytic bacterial inactivation by TiO2-coated surfaces. AMB Express 3: 1–8. Chen G, Li F, Fan Y et al. (2013) A novel noble metal-free ZnS– WS2/CdS composite photocatalyst for H2 evolution under visible light irradiation. Catalysis Communications 40: 51–54. Fang Z, Li S, Gong Y et al. (2014) Comparison of catalytic activity of carbon-based AgBr nanocomposites for conversion of CO2 under visible light. Journal of Saudi Chemical Society 18: 299–307. 106

Fotiadis C, Xekoukoulotakis NP and Mantzavinos D (2007)

Photocatalytic treatment of wastewater from cottonseed processing: effect of operating conditions, aerobic biodegradability and ecotoxicity. Catalysis Today 124: 247–253. Fu Y, Chen H, Sun X and Wang X (2012) Combination of cobalt ferrite and graphene: high-performance and recyclable visible-light photocatalysis. Applied Catalysis B: Environmental 111–112: 280–287. Guo J, Ma B, Yin A, Fan K and Dai W (2011) Photodegradation of rhodamine B and 4-chlorophenol using plasmonic photocatalyst of Ag–AgI/Fe3O4@SiO2 magnetic nanoparticle under visible light irradiation. Applied Catalysis B: Environmental 101: 580–586. Ibhadon A and Fitzpatrick P (2013) Heterogeneous photocatalysis: recent advances and applications. Catalysts 3: 189–218. Lanje A, Sharma S, Ningthoujam R, Ahn J and Pode R (2013) Low temperature dielectric studies of zinc oxide (ZnO) nanoparticles prepared by precipitation method. Advanced Powder Technology 24: 331–335. Li Y, Wang J, Yao H, Dang L and Li Z (2011) Efficient decomposition of organic compounds and reaction mechanism with biophotocatalyst under visible light irradiation. Journal of Molecular Catalysis A: Chemical 334: 116–122. Linsebigler A, Lu G and Yates J (1995) Photocatalysis on TiO2 on surfaces: principles, mechanisms, and selected Results. Chemical Reviews 95: 735–758. Litter M (1999) Heterogeneous photocatalysis: transition metal ions in photocatalytic systems. Applied Catalysis B: Environmental 23: 89–114. Liu W, Wang M Xu, C and Chen S (2012) Facile synthesis of gC3N4/ZnO composite with enhanced visible light photooxidation and photoreduction properties. Chemical Engineering Journal 209: 386–393. Liu W, Wang M, Xu C, Chen S and Fu X (2013) Significantly enhanced visible-light photocatalytic activity of g-C3N4 via ZnO modification and the mechanism study. Journal of Molecular Catalysis A: Chemical 368–369. Miranda C, Mansilla H, Yánˇ ez J, Obregón S and Colón G (2013) Improved photocatalytic activity of g-C3N4/TiO2 composites prepared by a simple impregnation method. Journal of Photochemistry and Photobiology A: Chemistry 253: 16–21. Muthirulan P, Meenakshisundararam M and Kannan N (2013) Beneficial role of ZnO photocatalyst supported with porous activated carbon for the mineralization of alizarin cyanin green dye in aqueous solution. Journal of Advanced Research 4: 479–484. Nishikawa M, Hiura S, Mitani Y and Nosaka Y (2013) Enhanced photocatalytic activity of BiVO4 by co-grafting of metal ions and combining with CuBi2O4. Journal of Photochemistry and Photobiology A: Chemistry 262: 52–56. Priac A, Morin-Crini N, Druart C et al. (2014) Alkylphenol and alkylphenol polyethoxylates in water and wastewater: a review of options for their elimination. Arabian Journal of Chemistry.



Offprint provided courtesy of www.icevirtuallibrary.com Author copy for personal use, not for distribution Rai SM, Bhat RP, Prajna PS, Jayadev K and Rao PSV (2014)

Degradation of malachite green and congo red using Aloe barabadensis Mill. extract. International Journal of Current Microbiology and Applied Sciences 3–4: 330–340. Raj S D, Prabha JV and Leena V (2012) Analysis of bacterial degradation of Azo dye Congo Red using HPLC. Journal of Industrial Pollution Control 28: 57–62. Senthilkumaar S, Porkodi K and Vidyalakshmi R (2005) Photodegradation of a textile dye catalyzed by sol–gel derived nanocrystalline TiO2 via ultrasonic irradiation. Journal of Photochemistry and Photobiology A: Chemistry 170: 225–232. Sridharan K, Jang E and Park T (2013) Novel visible light active graphitic C3N4–TiO2 composite photocatalyst: synergistic

synthesis, growth and photocatalytic treatment of hazardous pollutants. Applied Catalysis B: Environmental 142–143: 718–728. Yu T, Tan X and Zhao L (2010) Characterization and mechanistic analysis of the visible light response of cerium and nitrogen co-doped TiO2 nano-photocatalyst synthesized using a one-step technique. Journal of Hazardous Materials 176: 829–835. Zhao S, Chen S, Hongtao Y, Quan and X (2012) g-C3N4/TiO2 hybrid photocatalyst with wide absorption wavelength range and effective photogenerated charge separation. Separation and Purification Technology 99: 50–54. Zhu J, Xiao P, Li H and Carabineiro S (2014) Graphitic carbon nitride: synthesis, properties, and applications in catalysis. ACS Applied Materials & Interfaces.

HOW CAN YOU CONTRIBUTE?

To discuss this paper, please submit up to 500 words to the editor at [email protected]. Your contribution will be forwarded to the author(s) for a reply and, if considered appropriate by the editorial board, it will be published as a discussion in a future issue of the journal. 107