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Combination of Plackett Burman and response surface methodology experimental design to optimize Malachite Green dye removal from aqueous environment
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Combination of Plackett Burman and response surface methodology experimental design to optimize Malachite Green dye removal from aqueous environment Anupriya Sreedharan , Siew-Teng Ong PII: DOI: Reference:
S2405-8300(19)30394-5 https://doi.org/10.1016/j.cdc.2019.100317 CDC 100317
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Please cite this article as: Anupriya Sreedharan , Siew-Teng Ong , Combination of Plackett Burman and response surface methodology experimental design to optimize Malachite Green dye removal from aqueous environment, Chemical Data Collections (2019), doi: https://doi.org/10.1016/j.cdc.2019.100317
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Combination of Plackett Burman and response surface methodology experimental design to optimize Malachite Green dye removal from aqueous environment Anupriya Sreedharan1 and Siew-Teng Ong1,2* 1
Faculty of Science, 2 Centre for Biodiversity Research, Universiti Tunku Abdul Rahman, Jalan Universiti, Bandar Barat, 31900 Kampar, Perak, Malaysia
Abstract The present study focuses on the immobilization of titanium dioxide (TiO 2) for the remediation of Malachite Green (MG) dye from aqueous solution. The immobilization approach would overcome the problem associated with separation of fine TiO 2 particles that are often encountered in photodegradation process.
Surface characterization study was performed using field emission
scanning electron microscope (FESEM) and atomic force microscope (AFM). Plackett-Burman design was applied to identify the significant factors whereas Response Surface Methodology (RSM) was used to evaluate the interaction between these variables. The MG removal process was pH and concentration dependent. Kinetic studies showed that all experiments fitted well in the first order kinetics model. The percentage removal of MG by the thin films remained almost constant after five repetitive usages. Under the optimum conditions (pH = 8; initial dye concentration: 5 mg/L; catalyst dosage: 0.15 g), the percentage removal of MG was estimated to be 98.11%.
Key Words: Malachite Green; TiO2; Immoblization; Plackett Burman; Response surface methodology *Corresponding author. Tel: 605-4688888; Fax: 605-4661676 E-mail address: [email protected]; [email protected]
Specifications Table Subject area
Chemical Engineering
Compounds
Immobilization of TiO2 photocatalyst, Malachite Green dye
Data Category
Modification and preparation procedure, spectral, SEM and AFM micrographs, 3D surface plots Malachite Green photodegradation data, UV spectroscopy, Plackett-Burman, Response Surface Methodology Analyzed
Data acquisition format Data type Procedure
Description of the procedure Effect of various parameters on the photodegadation process Identification of significant variables in affecting the photodegradation process Optimization studies
Data accessibility
All data are available with this article
1. Rationale In terms of wastewater composition and discharge, textile wastewater can be considered to be the most polluting among the industrial sectors. At present, it has been estimated that there are more than 100,000 commercial dyes available and the production is around 7 × 105 to 1 × 106 tons per year [1-4]. However, the presence of dyes, even in low concentration damages the aesthetic nature of the environment. Dyes may also cause severe damage to human beings when it reaches our drinking water as some studies have shown that carcinogenicity is linked to specific types of dye intermediates or metabolites, such as benzidines [5,6]. Therefore, the removal of dyes is of utmost important in the control of environmental pollution as dyes pose not only serious environmental problems, but also a severe public health concern. Malachite Green (MG), a cationic triphenylmethane dye was chosen as the targeted pollutant in this study because it is widely used in textile, tannery, food, paper and pulp, printing, cosmetics, plastic, pharmaceuticals and dye houses [7,8]. It is also commonly used in the aquaculture industry as an effective biocide against parasites, fungal and protozoan infections in fish and fish eggs
[9,10]. However, it is well known that MG and its metabolites, leucomalachite green (LMG) are environmentally persistent and can cause a serious public health hazards. From the scientific evidence, MG is extremely hazardous and highly carcinogenic to mammalian cells as it could enhance the generation of tumor in the liver [11-13]. Although adsorption using activated carbon has emerged as an efficient process for the removal of colour from textile effluents, the regeneration of carbon often results in steep reduction in its performance and the cost is prohibitively expensive. Therefore, many studies were carried out by various researchers in order to find a promising yet efficient method for dyes removal [14-18]. Amongst all, photocatalytic degradation is one of the noteworthy treatment processes. It is an advanced oxidation process that uses a photocatalyst (such as anatase type TiO 2 or ZnO) in the presence of UV radiation to degrade pollutants. The primary advantage of using semiconductor based photoactive catalyst materials in the detoxification of pollutants is the complete mineralization into environment friendly products, without generation of waste, which is not likely to occur in the case of any other treatment method [19,20]. Although photocatalytic mineralization of pollutants, especially organic pollutants has generated much interest, most of the works have been carried out using a suspension form [21-23]. Due to the small particle size of the photocatalyst, a post-treatment catalyst recovery step involving microfiltration becomes necessary. This would be undesirable and impractical on industrial scale as it would add to the capital and operating cost of the treatment process. Therefore, in this research, focus has been devoted on the immobilization of TiO2. The immobilized TiO2 in a thin film form not only avoids the costly filtration stage, but also ease the reusability process. In order to make the process even more economical, sun light was selected as the light source to be used in this research. In conventional methods of studying a process, it is often being carried out by maintaining other factors involved at an unspecified constant level. Apart from time consuming, this method is also inadequate in describing the combined effect of all the factors involved. However, all these
limitations can be overcome by incorporating statistical experimental design in the process. With the introduction of Plackett-Burman design, the important variables in affecting Malachite Green (MG) removal process can be identified and further optimized by using response surface methodology (RSM). The optimization of effective variables by using RSM has also been reported in some of the previous works [24,25]. 2. Procedure 2.1. Adsorbates MG oxalate salt was used as the adsorbate in this study. The dye powder was purchased from Sigma-Aldrich Sdn. Bhd. and used without further purification. The molecular formula and molecular weight of MG is C50H52N4O8.C2H2O4 and 463.50 g/mol, respectively. The color index of MG is 42000 and the maximum absorption, max = 618 nm. Standard dye solutions of 100 mg/L were prepared as stock solutions and the desired concentration of the dye solution was prepared by subsequent dilution from the stock solution. 2.2. Immobilization of TiO2 into thin films TiO2 powder, which mainly consists of anatase form, having a mean particle size of 30 nm and BET surface area of 50 m2/g was used as photocatalyst in this study. The TiO2-sodium alginate film was prepared by dispersing 2.0 g of sodium alginate in 50 mL of hot water and the solution was stirred well for 1 h. Thereafter, 0.15 g of TiO2 was added in and the solution was further stirred for another 2 h until a homogeneous viscous solution without lumps was formed. The TiO 2-sodium alginate viscous solution was poured onto the glass support plates and was spread evenly on the glass plates (10 cm × 10 cm) in order to achieve a thin film with a homogeneous size and thickness. These glass plates were dried in the oven at 30 C for 24 h. The dried thin film were detached from the glass plates and immersed in 0.2 M CaCl2 solution (2 h) for crosslinking purpose. Lastly, these films were dried again in the oven for 30C for 24 h. The thickness of the TiO2-sodium alginate (T-
SA) film was measured using vernier calipers by selecting four different points on the film and the average thickness obtained was 0.05 mm. 2.3. Surface characterization study The surface morphology of the sodium alginate (SA) and T-SA film before and after photocatalytic degradation was analyzed using FESEM which is equipped with an energy dispersive X-ray spectrometer (SEM-EDX) (Model: JSM-6400) manufactured by JEOL Ltd. The surface topography of these films were also examined by using AFM (Quensant Q-Scope 250), which is a stylus-type instrument that scanned in raster fashion across the sample to detect changes in surface structure on the atomic scale. 2.4. Experimental Methods The batch experiments were carried out by immersing four T-SA films into glass tank containing 1000 mL of 25 mg/L MG solution. The solution was maintained at its natural pH which is pH 4.22. The whole set up was placed under the exposure of sunlight for 5 h. Aeration was provided to the system by using an air pump. The purpose of this step is to provide a constant oxygen supply to the system. Study was performed in duplicates and the average results were reported. At predetermined time intervals, a known volume of dye solution was withdrawn from the set up and analyzed with UV-Visible spectrophotometer to determine the percentage removal of the dye. The same experimental conditions were employed throughout the study unless otherwise stated. The percentage removal of the dye was calculated by using the following equation:
Removal (%) =
× 100%
where, = Initial concentration of dye (mg/L)
(1)
= Concentration of dye at time t (mg/L)
2.4.1. Effect of catalyst loading The influence of catalyst loading on the photocatalytic efficiency of the system was carried out by incorporating different mass of TiO2 into the sodium alginate film. The percentage removal of MG by the TiO2 -sodium alginate films containing 0.05 g, 0.10 g, 0.15 g, 0.20 g, and 0.25 g of TiO 2 was investigated while other variables remained constant. 2.4.2. Effect of initial dye concentration and contact time The effect of initial dye concentration and contact time on the percentage removal of MG was investigated with the T-SA films containing MG solutions with the concentrations of 5 mg/L, 15 mg/L and 25 mg/L. The aliquots were withdrawn at predetermined intervals of 5, 10, 15, 30, 60, 120, 180, 240 and 300 min and were analyzed for their dye concentrations. 2.4.3. Effect of pH The influence of pH on photocatalytic of MG was investigated by manipulating the pH of the system from pH 3 to 8. The desired pH of the dye solution was achieved by adjusting its natural pH with few drops HCl or NaOH solutions at various concentrations. 2.4.3. Reusability of the film The effect of film reusability was studied by repeating the photocatalytic experiments using the same films for five times. The T-SA films were rinsed with distilled water and dried in oven at 30C for 24 h after each use. The percentage removal of MG achieved by the films for each cycle was recorded in order to evaluate the photocatalytic efficiency of the reused films. 2.5. Experimental design for MG removal process 2.5.1. Plackett-Burman The determination of optimum conditions for maximum percentage removal of MG was performed using the Design Expert Version 7.1.3 software. Plackett-Burman design was utilized to
determine the significant factor(s) that affect the percentage removal of the dye. The factors studied were pH, initial dye concentration and catalyst loading. Each of the variables was assessed at three coded levels levels, -1, 0, +1, based on the experimental condition in batch study. A total of 8 sets of experimental designs produced by this software were conducted.
2.5.2. Response surface methodology (RSM) analysis Based on the significant factors identified by the Plackett-Burman design, the validation of the factors was further analyzed using the RSM design. The range and important variables selected for further optimization was based on the results obtained from Plackett-Burman design. This RSM design generates a quadratic equation which can be used to determine the optimum condition for the dye removal. A total of 15 experimental designs generated by this software were conducted and the results obtained were used to evaluate the optimized condition for maximum removal of the dye. All the experimental design and statistical analysis of the data were done by using Design Expert Version 7.1.3.
3. Data, value and validation 3.1. Surface characterization study FESEM was used to detect the changes that occur in the surface morphology of T-SA before and after photocatalytic degradation process and these are represented in Figures 1(a) and 1(b), respectively. Based on the FESEM images obtained, the surface of the T-SA film before the photocatalytic degradation process exhibited a rough surface whereas after the experiment, the surface appeared to be smoother and more compact. This can be related with the adhesion of dye molecules onto the film during the photocatalysis. Apart from FESEM, the surface characterization study was extended to AFM as well. The contact mode analysis can provide information on the surface topography and roughness of the material. Colour mapping is the usual method used in
AFM for displaying the data where light colour indicates high features or high topography and lower topography is shown by darker color. Figures 2(a) shows that a native T-SA film exhibited a rough surface with the presence of some pores. However, with the introduction of dye molecules onto the surface of T-SA film (Figure 2(b)), the surface becomes more intense and this explains why a higher topography was observed.
3.2. Comparison study The comparative percentage removal of MG by SA and T-SA film has shown that by using SA film, only 32.99% of MG can be removed. However, under the same experimental condition and with the incorporation of TiO2, the film showed a two-fold increase in the removal process. Similar obeservation was reported in the removal of MG by using banana pith with TiO2 thin film whereby with TiO2 alone, the removal efficiency is always lower than the combination method [26]. Since the objective of this study is to find a material that can remove appreciable amount of MG, therefore, subsequent studies were carried out using T-SA film.
Besides, a comparison of
maximum adsorption capacity of MG by different adsorbents was tabulated and shown in Table 1.
3.3. Effect of catalyst loading The effect of photocatalyst dosage in the removal of MG has shown that the amount of TiO2 incorporated in the T-SA film has an important role in the removal efficiency of the dye. When the amount of TiO2 in the film was being increased from 0.05 g to 0.15 g, the removal of MG increased from 46.46% to 65.70%. The increase in the percentage removal along with the increase of photocatalyst dosage in the T-SA film is because the amount of TiO2 incorporated is proportional to the number of active sites. With higher catalyst loading, this provides more number of vacant active sites and therefore, enhancing the photocatalytic efficiency of the T-SA film. However, a decrease in the percentage removal was observed when the amount of TiO 2 incorporated into the T-SA film
exceeds 0.15 g. With the same inital dye concentration, when the catalyst loading was further increase to 0.20 g and 0.25 g, the removal of MG became 56.22% and 52.97%, respectively. This is possibly because when the loading of TiO2 in the film is higher than 0.15 g, saturation occurs leading to more overlapping of the active sites. This effectively masks the available active sites and leads to a lower colour removal efficiency. Consequently, the photocatalytic efficiency of the film decreases due to low generation of •OH and •O2-. A similar observation was reported in the photocatalytic degradation of TiO2/calcium alginate composite film [33]. It was proposed that at low concentration, TiO2 nanoparticles can be evenly dispersed in the film and a small amount of TiO2 incorporated in film will achieve significant removal efficiency. However, when the catalyst loading was increased up to certain extent, the efficiency dropped and this may be associated with the congregation of TiO2. Another possible explanation for such result is that at high concentration of TiO2, the active surface of the alginate is predominantly covered by the TiO 2 molecules and therefore reducing the adsorption capacity of alginate for dye molecules. The removal of the dye proceeds mainly due to photocatalysis rather than by simultaneous adsorption and photocatalysis. As a result of low adsorption of the dye molecules on the surface of the film, the percentage removal of the dye decreases. This is in agreement with the work carried out by Kant, et al. (2014) [34]. Overall, increasing the photocatalyst dosage will increase the percentage removal of dye but when the concentration of photocatalyst is increased above the limiting value, the degradation rate decreases due to the increase in opacity and light scattering of TiO2 particles. As the results clearly showed that film containing 0.15 g TiO2 showed highest photocatalytic degradation efficiency, therefore it was chosen as the optimum dosage to be incorporated into the film and to be investigated for other parameters.
3.4. Effect of initial dye concentration and contact time Figure 3 shows that prolonging the exposure time leads to a better removal of MG from the aqueous solution. Increasing the exposure time permits more number of hydroxyl and superoxide radicals to be generated and this in turn enhanced the removal efficiency. It is evident that the removal of MG was rapid at the beginning, followed by a more gradual process. The fast removal at the beginning may be attributed to the rapid attachment of the dye molecules to the surface of the adsorbent and the following slower portion is most probably due to intraparticle diffusion. With an initial dye concentration of 5 mg/L, 98.55% of MG removal was achieved within 300 min. However, under the same experimental condition, only 65.28% of MG was removed with the initial dye concentration of 25 mg/L. The decrease of removal percentage with increasing initial dye concentration may be explained by the large amount of adsorbed dye on the surface of TiO 2. This phenomenon is thought to have an inhibitive effect in photodegradation because it absorbs light and photons, thereby reducing the generation of •OH and •O2 - to remove the dye. As the formation of radicals on the surface of TiO2 decreases, there are insufficient •OH and •O2- to degrade the dye, thus resulting in lower percentage of dye removal. Another possible reason for such trend is that it may be due to the UV-screening effect of the dye molecules itself. This causes the light photons being absorbed by the dye molecules and prevented it from reaching the catalyst surface. At high concentrations, more dye molecules absorb photons resulting in lower absorption of photons by catalyst molecule. This decreases the concentration hydroxyl radical in the aqueous solution and consequently the percentage removal of MG decreases. This finding is in agreement with other reported works [35-37].
3.5. Kinetics study Basically, the degradation of dyes with respect to first order can be expressed as below: Rate =
= k[C]
(2)
To perform kinetic analysis, Equation 2 was integrated to give: ln ( ) = kt
(3)
where, C0 = initial concentration of the dye (mg/L) C = concentration of the dye at time t (mg/L) t = time of exposure (min) k = rate constant (min-1) By plotting the data using the equation ln (C0/C) against time, a linear relationship was obtained for all the studied concentrations. The rate constant was derived from the gradient of the straight line whereas the half-life of MG, which is the time taken for MG to degrade to half of its initial concentration, was calculated according to Equation 4. Half-life, t1/2 = ln 2 / k
(4)
The rate constants, correlation coefficients and half-life obtained are shown in Table 2. By increasing the initial dye concentration from 5 to 25 mg/L, the rate constant decreases nearly 75 % from 0.0138 to 0.0035 min-1. Again, this is related to the formation of radicals which are crucial in the photocatalytic degradation process. It is generally acceptable that as the initial dye concentration increases, the colour of the irradiating solution becomes more intense and the penetration of light to the surface of photocatalyst decreases. As a consequence, this retarded the generation of hydroxyl radicals (OH) and superoxide radicals (O2 -) and a drop in photocatalytic performance is anticipated.
3.6. Effect of pH The effect of pH was investigated within the pH range of 38. This range of pH was chosen because a colour change was observed when pH was below 3 (gradually turns yellow) and above 8
(gradually turns colourless) which indicates that the MG dye solution is unstable outside this range. Besides, it is also impractical if the pH of the actual wastewater has to be adjusted to extreme values for industrial applications [38]. Figure 4 shows that the maximum degradation of MG occurs at pH 8 which is 89.27%. Below this pH, the percentage removal exhibits a decreasing trend till it reaches the lowest point at pH 3. This trend showed that the photocatalytic degradation of MG was more favorable in alkaline conditions. Similar pH removal efficiency profile for MG at high pH of the system was also reported in some of the previous publications [22,26]. This finding can be explained in terms of electrostatic interaction between the photocatalyst and targeted pollutant. In general, pH influences the surface state of a catalyst and also the ionization state of ionizable organic compounds. As a result, this affects the interfacial electron transfer and overall photoredox process. In other words, photocatalysis rate of a system can either be enhanced or retarded depending on the dominance of either the attractive or repulsive intermolecular forces [39]. Besides, the pH at zero-point of charge also plays an important role. When the pH of the system was greater than the zero-point charge or isoelectric point for TiO2, the surface of the photocatalyst becomes predominantly negatively charged. Since MG is cationic dye, electrostatic attraction causes more MG adsorption or migration of it towards the catalyst surface. Such conditions would therefore cause the pollutant to be more susceptible to photodegradation [40]. Not only that, an alkaline medium would also contain more OH which are readily available to be oxidized into OH on the photocatalyst surface that will later diffuse through the bulk solution leading to an expected increase in the photocatalytic process. On the contrary, under acidic conditions (pH < pHzpc), the surface of TiO2 is positively charged and MG will be repelled from the surface of the catalyst due to electrostatic repulsion. This causes the degradation process to be reduced greatly. It is postulated that the photodegradation process is still occuring but at a lower rate. It is suggested that the positive holes are the major oxidation species in low pH systems [41].
3.7. Reusability of the film The evaluation on the reusability of the T-SA film is necessary because the application of photocatalysis to treat dye contaminated wastewater will be more practical and desirable at industrial scale if the photocatalyst can be reused. In this study, the results showed that the photocatalytic activity of the T-SA films remains high after five repetitive usages (Figure 5). A slight increase in the percentage removal of MG was observed at third cycle and this may probably due to the variation of the sunlight intensity. Although the removal of MG is still above 60%, T-SA film was not subjected for any further usage after the 5th cycles because thereafter the film became brittle. A similar observation was reported in TiO2/calcium alginate composite film whereby the changes in the mechanical properties of the film were quite minimal after five consecutive usages [33]. This is because alginate exhibits a good compatibility with TiO 2 as the latter can interact with the former through hydrogen bonding of the hydroxyl groups present in both molecules.
3.8. Plackett-Burman (PB) design Plackett Burman design was utilized to evaluate the factors which have a significant effect on the percentage removal of MG. This experimental design is a handy screening tool as it reduces the number of experiments and determines influential variables for further optimization. The function of desirability was applied using Design Expert Version 7.1.3.1 in order to validate the model. The experimental conditions to be verified were selected based on the highest desirability. This technique is useful for constructing designs for various experimental situations and enables the determination of important parameters for further optimization study [42]. Different variables, namely effect of pH, initial dye concentration and catalyst dosage were used in the screening process and a total of 8 sets of experimental conditions were generated by the Plackett- Burman program. These experiments were carried out according to the conditions generated and the results obtained from these experiments are presented in Table 3. Based on the percentage error, the
experimental percentage removal apparently did not deviate much from the predicted values and the differences were within 0.28-3.65%. The differences observed between the experimental and predicted percentage removal is closely related to the involvement of insignificant variables in the analysis. This kind of deviation has also been reported in some of the previous works [6,32]. The analysis of variance (ANOVA) was performed to test the validation of the three parameters and the result obtained was shown in Table 4. The photocatalytic degradation model was significant with the Prob>F value of <0.0001. The variables were identified as significant when the Prob>F value is below 0.05. Based on the results obtained, all the three parameters studied in this experiment are significant with Prob>F values of <0.0001, indicating all three parameters investigated in this batch experiments influences the percentage removal of MG. This finding agrees well with most of the reported works whereby pH, catalyst loading and the initial dye concentrations affect the percentage removal of dye due to the charge, amount of active sites and path length of the photons entering the dye solution.
3.9. Response Surface Methodology From the screening results obtained through Plackett-Burman design, the interaction between the important variables and their optimum levels for maximum percentage removal of MG were further studied using response surface methodology (RSM). Similar to the previously reported works [43,44], a factorial central composite design (CCD) model for significant variables with replicates was used in this study. All experiments were conducted in duplicate and the mean value of percentage removal was used as response. Cubic equation used for the optimization of percentage removal of MG is shown as follows:
2
2
2
i 1
i 1
i 1
1
Y i X i ii X i2 iii X i3 1
2
i 1 j i 1
1
ijj
Xi X 2 j
iij
i 1 j i 1
2
i 1 j i 1
2
ij
Xi X j
(5)
2 i
X Xj
where, βo, βi, βii, βiii, βij, βijj and βijj are the constant coefficients, and Xi, and Xj are the independent variables. A total of 15 sets of experimental conditions generated by the program were conducted and the results obtained from these experiments were presented in Table 5. The experimental values agreed well with the predicted values and the differences were within 0.05-4.82%. The results of ANOVA analysis was presented in Table 5. The model F value of 27.89 and Prob>F value < 0.0001 indicates that the model for the removal of MG by photocatalytic degradation was significant (Table 6). All the three parameters studied were found to be significant. The Prob>F value was less than 0.0001 for both pH and initial dye concentration, whereas for catalyst dosage, it is 0.0001. The R2 for this model was 0.9617 which indicates that there was a good agreement between the observed and predicted values. The coefficient of variance (CV) for this model is 4.99. This shows a good reliability and precision of the model as when the CV value is lower, the precision and reliability of the experiments are higher. Adequate precision measure the signal to noise ratio and value greater than 4 is preferred. Since the adequate precision for the system is 20.521 therefore, an adequate signal was obtained. Figures 6-8 show the 3D surface plot on the interactions between pH and catalyst loading, pH and initial dye concentration and catalyst loading and initial dye concentration in the removal MG, respectively. The region highlighted in red shows the highest percentage removal of MG whereas; the blue region shows the lowest percentage removal of the dye. The maximum removal of the dye was achieved in the condition of high pH, moderate catalyst loading and low initial dye concentration. The optimum conditions for maximum removal MG was at pH 8, catalyst dosage of
0.15 g and initial dye concentration of 5 mg/L. Under the optimum conditions, the predicted and experimental percentage removal of MG was 98.11% and 98.55%, respectively.
4. Conclusion This study proved the potential of T-SA film as a promising catalyst for the remediation of MG from aqueous solution. Based on the findings, the percentage removal of the MG was influenced by pH, catalyst loading and initial dye concentrations. The removal of MG increased with contact time but decreased with initial dye concentrations. Higher catalyst loading is beneficial up to certain extent only. The photocatalytic degradation of MG followed first order kinetics model. The rate constant was found to be 0.0138, 0.0052 and 0.0350 for 5, 15 and 25 mg/L of MG, respectively. It is interesting to note that the phototocatalytic degradation ability of T-SA film remains high after 5 repetitive usages. Plackett-Burman experimental design successfully identify that all the three parameters to be significant factors in affecting the percentage removal of MG. Based on the RSM analysis, under the optimized conditions which were pH 8, catalyst loading of 0.15 g and initial dye concentration of 5 mg/L, the removal of MG is higher than 98%. A good agreement was found between the experimental and predicted values by the generated model.
Acknowledgements The authors are thankful for the financial support and research facilities provided Universiti Tunku Abdul Rahman (UTAR).
Declaration of competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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(a)
(b)
Fig. 1. SEM micrograph of T-SA film (a) before photocatalytic degradation; (b) after photocatalytic degradation
Fig. 2. Surface topography of T-SA film (a) before photocatalytic degradation; (b) after photocatalytic degradation
Removal efficiency (%)
100
80 60 40 20
5 mg/L
15 mg/L
25 mg/L
0 0
50
100
150 200 Time (min)
250
300
Fig. 3. Effect of initial dye concentrations on percentage removal of MG
Removal efficiency (%)
100
80
60
40
20
0 3
4
5
pH
Fig. 4. Effect of pH on percentage removal of MG
6
7
8
Removal efficiency (%)
100
80
60
40
20
0 0
1
2
3
4
5
6
Number of reuse cycle Fig. 5. Effect of repetitive usage of the T-SA film on percentage removal of MG
Fig. 6. 3D surface plot for removal of MG from aqueous solution using T-SA film by photocatalytic degradation as a function of catalyst dosage and pH of solution.
Fig. 7. 3D surface plot for removal of MG from aqueous solution using T-SA film by photocatalytic degradation as a function of initial dye concentration and pH of solution.
Fig. 8. 3D surface plot for removal of MG from aqueous solution using T-SA film by photocatalytic degradation as a function of initial dye concentration and catalyst dosage.
Table 1. Comparison of maximum adsorption capacity for MG by different adsorbents Adsorbent
Maximum adsorption capacity (mg/g)
Reference
MOF nanocomposite
329.61
[27]
Sodium dodecylbenzene sulfonate (SDBS)-modified sepiolite
270.3
Alginate coated perlite
74.63
Carboxylate functionalized multiwalled carbon nanotubes
49.45
Superparamagnetic sodium alginatecoated Fe3O4 nanoparticles
47.84
Corn cob thin film
35.34
[28] [29] [30]
[31] [32]
Table 2. Pseudo first order rate constants, correlation coefficients and half-life values for the photocatalytic degradation MG
R2
5
Rate, k (min-1) 0.0138
0.9770
50.23
15
0.0052
0.9649
133.30
25
0.0035
0.9905
198.04
Initial dye concentration (mg/L)
Half-life, t1/2 (min)
Table 3. Plackett-Burman design and results for remediation of MG from aqueous solution. pH Differences (%) Catalyst Initial Dye Experimental Predicted Dosage Concentration Removal (%) Removal (%) 8.00
0.25
5.00
83.98
83.70
0.28
8.00
0.05
5.00
76.37
78.76
2.39
8.00
0.05
25.00
59.41
60.43
1.02
3.00
0.25
25.00
49.81
50.28
0.47
3.00
0.05
25.00
44.46
43.34
1.12
3.00
0.05
5.00
60.32
56.67
3.65
3.00
0.25
5.00
65.14
66.61
1.47
8.00
0.25
25.00
69.22
67.37
1.85
Table 4. Regression analysis (ANOVA) of Plackett-Burman for the remediation of MG from aqueous solution Source
Sum of Squares
Degree of freedom
Mean Square
F-Value
p-value Prob > F
Model
1820.42
3
606.81
461.70
< 0.0001
A-pH
875.69
1
875.69
660.29
< 0.0001
144.56
109.99
< 0.0001
B- Catalyst Dosage
1 144.56
C- Initial Dye
1
Concentration
800.17
Residual
10.51
8
1830.94
11
Total
800.17
< 0.0001 608.83
1.31
Table 5. RSM design and results for remediation of MG from aqueous solution pH
Predicted Removal (%)
Differences (%)
8.00
Catalyst Initial Dye Experimental Dosage Concentration Removal (%) 0.05 25.00 59.41
62.94
3.53
8.00
0.25
25.00
69.22
66.56
2.66
3.00
0.25
5.00
83.98
80.96
3.02
5.50
0.15
5.00
98.11
94.00
4.11
3.00
0.05
25.00
44.46
40.96
3.50
5.50
0.15
15.00
79.67
80.35
0.68
3.00
0.15
15.00
71.29
74.08
2.79
5.50
0.05
15.00
60.43
57.28
3.15
8.00
0.05
5.00
76.37
76.32
0.05
5.50
0.25
15.00
66.71
67.83
1.12
8.00
0.15
15.00
93.41
88.59
4.82
5.50
0.15
25.00
69.93
72.01
2.08
8.00
0.25
5.00
83.98
87.99
4.01
3.00
0.25
25.00
49.81
50.37
0.56
3.00
0.05
5.00
60.32
63.49
3.17
Table 6. Regression analysis (ANOVA) of RSM for the remediation of MG from aqueous solution Source
Sum of Degree of Mean Squares freedom Square
F-Value
p-value Prob > F
Model
3353.38
9
372.60
27.89
< 0.0001
A-pH
526.06
1
526.06
39.37
< 0.0001
277.83
20.79
0.0010
B-
Catalyst
Dosage
1 277.83
C- Initial Dye
1
Concentration
1208.46
Residual
133.61
10
Total
3486.99
19
1208.46
< 0.0001 90.45
13.36
Combination of Plackett Burman and response surface methodology experimental design to optimize Malachite Green dye removal from aqueous environment Anupriya Sreedharan , Siew-Teng Ong PII: DOI: Reference:
S2405-8300(19)30394-5 https://doi.org/10.1016/j.cdc.2019.100317 CDC 100317
To appear in:
Chemical Data Collections
Received date: Revised date: Accepted date:
3 October 2019 21 November 2019 10 December 2019
Please cite this article as: Anupriya Sreedharan , Siew-Teng Ong , Combination of Plackett Burman and response surface methodology experimental design to optimize Malachite Green dye removal from aqueous environment, Chemical Data Collections (2019), doi: https://doi.org/10.1016/j.cdc.2019.100317
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Combination of Plackett Burman and response surface methodology experimental design to optimize Malachite Green dye removal from aqueous environment Anupriya Sreedharan1 and Siew-Teng Ong1,2* 1
Faculty of Science, 2 Centre for Biodiversity Research, Universiti Tunku Abdul Rahman, Jalan Universiti, Bandar Barat, 31900 Kampar, Perak, Malaysia
Abstract The present study focuses on the immobilization of titanium dioxide (TiO 2) for the remediation of Malachite Green (MG) dye from aqueous solution. The immobilization approach would overcome the problem associated with separation of fine TiO 2 particles that are often encountered in photodegradation process.
Surface characterization study was performed using field emission
scanning electron microscope (FESEM) and atomic force microscope (AFM). Plackett-Burman design was applied to identify the significant factors whereas Response Surface Methodology (RSM) was used to evaluate the interaction between these variables. The MG removal process was pH and concentration dependent. Kinetic studies showed that all experiments fitted well in the first order kinetics model. The percentage removal of MG by the thin films remained almost constant after five repetitive usages. Under the optimum conditions (pH = 8; initial dye concentration: 5 mg/L; catalyst dosage: 0.15 g), the percentage removal of MG was estimated to be 98.11%.
Key Words: Malachite Green; TiO2; Immoblization; Plackett Burman; Response surface methodology *Corresponding author. Tel: 605-4688888; Fax: 605-4661676 E-mail address: [email protected]; [email protected]
Specifications Table Subject area
Chemical Engineering
Compounds
Immobilization of TiO2 photocatalyst, Malachite Green dye
Data Category
Modification and preparation procedure, spectral, SEM and AFM micrographs, 3D surface plots Malachite Green photodegradation data, UV spectroscopy, Plackett-Burman, Response Surface Methodology Analyzed
Data acquisition format Data type Procedure
Description of the procedure Effect of various parameters on the photodegadation process Identification of significant variables in affecting the photodegradation process Optimization studies
Data accessibility
All data are available with this article
1. Rationale In terms of wastewater composition and discharge, textile wastewater can be considered to be the most polluting among the industrial sectors. At present, it has been estimated that there are more than 100,000 commercial dyes available and the production is around 7 × 105 to 1 × 106 tons per year [1-4]. However, the presence of dyes, even in low concentration damages the aesthetic nature of the environment. Dyes may also cause severe damage to human beings when it reaches our drinking water as some studies have shown that carcinogenicity is linked to specific types of dye intermediates or metabolites, such as benzidines [5,6]. Therefore, the removal of dyes is of utmost important in the control of environmental pollution as dyes pose not only serious environmental problems, but also a severe public health concern. Malachite Green (MG), a cationic triphenylmethane dye was chosen as the targeted pollutant in this study because it is widely used in textile, tannery, food, paper and pulp, printing, cosmetics, plastic, pharmaceuticals and dye houses [7,8]. It is also commonly used in the aquaculture industry as an effective biocide against parasites, fungal and protozoan infections in fish and fish eggs
[9,10]. However, it is well known that MG and its metabolites, leucomalachite green (LMG) are environmentally persistent and can cause a serious public health hazards. From the scientific evidence, MG is extremely hazardous and highly carcinogenic to mammalian cells as it could enhance the generation of tumor in the liver [11-13]. Although adsorption using activated carbon has emerged as an efficient process for the removal of colour from textile effluents, the regeneration of carbon often results in steep reduction in its performance and the cost is prohibitively expensive. Therefore, many studies were carried out by various researchers in order to find a promising yet efficient method for dyes removal [14-18]. Amongst all, photocatalytic degradation is one of the noteworthy treatment processes. It is an advanced oxidation process that uses a photocatalyst (such as anatase type TiO 2 or ZnO) in the presence of UV radiation to degrade pollutants. The primary advantage of using semiconductor based photoactive catalyst materials in the detoxification of pollutants is the complete mineralization into environment friendly products, without generation of waste, which is not likely to occur in the case of any other treatment method [19,20]. Although photocatalytic mineralization of pollutants, especially organic pollutants has generated much interest, most of the works have been carried out using a suspension form [21-23]. Due to the small particle size of the photocatalyst, a post-treatment catalyst recovery step involving microfiltration becomes necessary. This would be undesirable and impractical on industrial scale as it would add to the capital and operating cost of the treatment process. Therefore, in this research, focus has been devoted on the immobilization of TiO2. The immobilized TiO2 in a thin film form not only avoids the costly filtration stage, but also ease the reusability process. In order to make the process even more economical, sun light was selected as the light source to be used in this research. In conventional methods of studying a process, it is often being carried out by maintaining other factors involved at an unspecified constant level. Apart from time consuming, this method is also inadequate in describing the combined effect of all the factors involved. However, all these
limitations can be overcome by incorporating statistical experimental design in the process. With the introduction of Plackett-Burman design, the important variables in affecting Malachite Green (MG) removal process can be identified and further optimized by using response surface methodology (RSM). The optimization of effective variables by using RSM has also been reported in some of the previous works [24,25]. 2. Procedure 2.1. Adsorbates MG oxalate salt was used as the adsorbate in this study. The dye powder was purchased from Sigma-Aldrich Sdn. Bhd. and used without further purification. The molecular formula and molecular weight of MG is C50H52N4O8.C2H2O4 and 463.50 g/mol, respectively. The color index of MG is 42000 and the maximum absorption, max = 618 nm. Standard dye solutions of 100 mg/L were prepared as stock solutions and the desired concentration of the dye solution was prepared by subsequent dilution from the stock solution. 2.2. Immobilization of TiO2 into thin films TiO2 powder, which mainly consists of anatase form, having a mean particle size of 30 nm and BET surface area of 50 m2/g was used as photocatalyst in this study. The TiO2-sodium alginate film was prepared by dispersing 2.0 g of sodium alginate in 50 mL of hot water and the solution was stirred well for 1 h. Thereafter, 0.15 g of TiO2 was added in and the solution was further stirred for another 2 h until a homogeneous viscous solution without lumps was formed. The TiO 2-sodium alginate viscous solution was poured onto the glass support plates and was spread evenly on the glass plates (10 cm × 10 cm) in order to achieve a thin film with a homogeneous size and thickness. These glass plates were dried in the oven at 30 C for 24 h. The dried thin film were detached from the glass plates and immersed in 0.2 M CaCl2 solution (2 h) for crosslinking purpose. Lastly, these films were dried again in the oven for 30C for 24 h. The thickness of the TiO2-sodium alginate (T-
SA) film was measured using vernier calipers by selecting four different points on the film and the average thickness obtained was 0.05 mm. 2.3. Surface characterization study The surface morphology of the sodium alginate (SA) and T-SA film before and after photocatalytic degradation was analyzed using FESEM which is equipped with an energy dispersive X-ray spectrometer (SEM-EDX) (Model: JSM-6400) manufactured by JEOL Ltd. The surface topography of these films were also examined by using AFM (Quensant Q-Scope 250), which is a stylus-type instrument that scanned in raster fashion across the sample to detect changes in surface structure on the atomic scale. 2.4. Experimental Methods The batch experiments were carried out by immersing four T-SA films into glass tank containing 1000 mL of 25 mg/L MG solution. The solution was maintained at its natural pH which is pH 4.22. The whole set up was placed under the exposure of sunlight for 5 h. Aeration was provided to the system by using an air pump. The purpose of this step is to provide a constant oxygen supply to the system. Study was performed in duplicates and the average results were reported. At predetermined time intervals, a known volume of dye solution was withdrawn from the set up and analyzed with UV-Visible spectrophotometer to determine the percentage removal of the dye. The same experimental conditions were employed throughout the study unless otherwise stated. The percentage removal of the dye was calculated by using the following equation:
Removal (%) =
× 100%
where, = Initial concentration of dye (mg/L)
(1)
= Concentration of dye at time t (mg/L)
2.4.1. Effect of catalyst loading The influence of catalyst loading on the photocatalytic efficiency of the system was carried out by incorporating different mass of TiO2 into the sodium alginate film. The percentage removal of MG by the TiO2 -sodium alginate films containing 0.05 g, 0.10 g, 0.15 g, 0.20 g, and 0.25 g of TiO 2 was investigated while other variables remained constant. 2.4.2. Effect of initial dye concentration and contact time The effect of initial dye concentration and contact time on the percentage removal of MG was investigated with the T-SA films containing MG solutions with the concentrations of 5 mg/L, 15 mg/L and 25 mg/L. The aliquots were withdrawn at predetermined intervals of 5, 10, 15, 30, 60, 120, 180, 240 and 300 min and were analyzed for their dye concentrations. 2.4.3. Effect of pH The influence of pH on photocatalytic of MG was investigated by manipulating the pH of the system from pH 3 to 8. The desired pH of the dye solution was achieved by adjusting its natural pH with few drops HCl or NaOH solutions at various concentrations. 2.4.3. Reusability of the film The effect of film reusability was studied by repeating the photocatalytic experiments using the same films for five times. The T-SA films were rinsed with distilled water and dried in oven at 30C for 24 h after each use. The percentage removal of MG achieved by the films for each cycle was recorded in order to evaluate the photocatalytic efficiency of the reused films. 2.5. Experimental design for MG removal process 2.5.1. Plackett-Burman The determination of optimum conditions for maximum percentage removal of MG was performed using the Design Expert Version 7.1.3 software. Plackett-Burman design was utilized to
determine the significant factor(s) that affect the percentage removal of the dye. The factors studied were pH, initial dye concentration and catalyst loading. Each of the variables was assessed at three coded levels levels, -1, 0, +1, based on the experimental condition in batch study. A total of 8 sets of experimental designs produced by this software were conducted.
2.5.2. Response surface methodology (RSM) analysis Based on the significant factors identified by the Plackett-Burman design, the validation of the factors was further analyzed using the RSM design. The range and important variables selected for further optimization was based on the results obtained from Plackett-Burman design. This RSM design generates a quadratic equation which can be used to determine the optimum condition for the dye removal. A total of 15 experimental designs generated by this software were conducted and the results obtained were used to evaluate the optimized condition for maximum removal of the dye. All the experimental design and statistical analysis of the data were done by using Design Expert Version 7.1.3.
3. Data, value and validation 3.1. Surface characterization study FESEM was used to detect the changes that occur in the surface morphology of T-SA before and after photocatalytic degradation process and these are represented in Figures 1(a) and 1(b), respectively. Based on the FESEM images obtained, the surface of the T-SA film before the photocatalytic degradation process exhibited a rough surface whereas after the experiment, the surface appeared to be smoother and more compact. This can be related with the adhesion of dye molecules onto the film during the photocatalysis. Apart from FESEM, the surface characterization study was extended to AFM as well. The contact mode analysis can provide information on the surface topography and roughness of the material. Colour mapping is the usual method used in
AFM for displaying the data where light colour indicates high features or high topography and lower topography is shown by darker color. Figures 2(a) shows that a native T-SA film exhibited a rough surface with the presence of some pores. However, with the introduction of dye molecules onto the surface of T-SA film (Figure 2(b)), the surface becomes more intense and this explains why a higher topography was observed.
3.2. Comparison study The comparative percentage removal of MG by SA and T-SA film has shown that by using SA film, only 32.99% of MG can be removed. However, under the same experimental condition and with the incorporation of TiO2, the film showed a two-fold increase in the removal process. Similar obeservation was reported in the removal of MG by using banana pith with TiO2 thin film whereby with TiO2 alone, the removal efficiency is always lower than the combination method [26]. Since the objective of this study is to find a material that can remove appreciable amount of MG, therefore, subsequent studies were carried out using T-SA film.
Besides, a comparison of
maximum adsorption capacity of MG by different adsorbents was tabulated and shown in Table 1.
3.3. Effect of catalyst loading The effect of photocatalyst dosage in the removal of MG has shown that the amount of TiO2 incorporated in the T-SA film has an important role in the removal efficiency of the dye. When the amount of TiO2 in the film was being increased from 0.05 g to 0.15 g, the removal of MG increased from 46.46% to 65.70%. The increase in the percentage removal along with the increase of photocatalyst dosage in the T-SA film is because the amount of TiO2 incorporated is proportional to the number of active sites. With higher catalyst loading, this provides more number of vacant active sites and therefore, enhancing the photocatalytic efficiency of the T-SA film. However, a decrease in the percentage removal was observed when the amount of TiO 2 incorporated into the T-SA film
exceeds 0.15 g. With the same inital dye concentration, when the catalyst loading was further increase to 0.20 g and 0.25 g, the removal of MG became 56.22% and 52.97%, respectively. This is possibly because when the loading of TiO2 in the film is higher than 0.15 g, saturation occurs leading to more overlapping of the active sites. This effectively masks the available active sites and leads to a lower colour removal efficiency. Consequently, the photocatalytic efficiency of the film decreases due to low generation of •OH and •O2-. A similar observation was reported in the photocatalytic degradation of TiO2/calcium alginate composite film [33]. It was proposed that at low concentration, TiO2 nanoparticles can be evenly dispersed in the film and a small amount of TiO2 incorporated in film will achieve significant removal efficiency. However, when the catalyst loading was increased up to certain extent, the efficiency dropped and this may be associated with the congregation of TiO2. Another possible explanation for such result is that at high concentration of TiO2, the active surface of the alginate is predominantly covered by the TiO 2 molecules and therefore reducing the adsorption capacity of alginate for dye molecules. The removal of the dye proceeds mainly due to photocatalysis rather than by simultaneous adsorption and photocatalysis. As a result of low adsorption of the dye molecules on the surface of the film, the percentage removal of the dye decreases. This is in agreement with the work carried out by Kant, et al. (2014) [34]. Overall, increasing the photocatalyst dosage will increase the percentage removal of dye but when the concentration of photocatalyst is increased above the limiting value, the degradation rate decreases due to the increase in opacity and light scattering of TiO2 particles. As the results clearly showed that film containing 0.15 g TiO2 showed highest photocatalytic degradation efficiency, therefore it was chosen as the optimum dosage to be incorporated into the film and to be investigated for other parameters.
3.4. Effect of initial dye concentration and contact time Figure 3 shows that prolonging the exposure time leads to a better removal of MG from the aqueous solution. Increasing the exposure time permits more number of hydroxyl and superoxide radicals to be generated and this in turn enhanced the removal efficiency. It is evident that the removal of MG was rapid at the beginning, followed by a more gradual process. The fast removal at the beginning may be attributed to the rapid attachment of the dye molecules to the surface of the adsorbent and the following slower portion is most probably due to intraparticle diffusion. With an initial dye concentration of 5 mg/L, 98.55% of MG removal was achieved within 300 min. However, under the same experimental condition, only 65.28% of MG was removed with the initial dye concentration of 25 mg/L. The decrease of removal percentage with increasing initial dye concentration may be explained by the large amount of adsorbed dye on the surface of TiO 2. This phenomenon is thought to have an inhibitive effect in photodegradation because it absorbs light and photons, thereby reducing the generation of •OH and •O2 - to remove the dye. As the formation of radicals on the surface of TiO2 decreases, there are insufficient •OH and •O2- to degrade the dye, thus resulting in lower percentage of dye removal. Another possible reason for such trend is that it may be due to the UV-screening effect of the dye molecules itself. This causes the light photons being absorbed by the dye molecules and prevented it from reaching the catalyst surface. At high concentrations, more dye molecules absorb photons resulting in lower absorption of photons by catalyst molecule. This decreases the concentration hydroxyl radical in the aqueous solution and consequently the percentage removal of MG decreases. This finding is in agreement with other reported works [35-37].
3.5. Kinetics study Basically, the degradation of dyes with respect to first order can be expressed as below: Rate =
= k[C]
(2)
To perform kinetic analysis, Equation 2 was integrated to give: ln ( ) = kt
(3)
where, C0 = initial concentration of the dye (mg/L) C = concentration of the dye at time t (mg/L) t = time of exposure (min) k = rate constant (min-1) By plotting the data using the equation ln (C0/C) against time, a linear relationship was obtained for all the studied concentrations. The rate constant was derived from the gradient of the straight line whereas the half-life of MG, which is the time taken for MG to degrade to half of its initial concentration, was calculated according to Equation 4. Half-life, t1/2 = ln 2 / k
(4)
The rate constants, correlation coefficients and half-life obtained are shown in Table 2. By increasing the initial dye concentration from 5 to 25 mg/L, the rate constant decreases nearly 75 % from 0.0138 to 0.0035 min-1. Again, this is related to the formation of radicals which are crucial in the photocatalytic degradation process. It is generally acceptable that as the initial dye concentration increases, the colour of the irradiating solution becomes more intense and the penetration of light to the surface of photocatalyst decreases. As a consequence, this retarded the generation of hydroxyl radicals (OH) and superoxide radicals (O2 -) and a drop in photocatalytic performance is anticipated.
3.6. Effect of pH The effect of pH was investigated within the pH range of 38. This range of pH was chosen because a colour change was observed when pH was below 3 (gradually turns yellow) and above 8
(gradually turns colourless) which indicates that the MG dye solution is unstable outside this range. Besides, it is also impractical if the pH of the actual wastewater has to be adjusted to extreme values for industrial applications [38]. Figure 4 shows that the maximum degradation of MG occurs at pH 8 which is 89.27%. Below this pH, the percentage removal exhibits a decreasing trend till it reaches the lowest point at pH 3. This trend showed that the photocatalytic degradation of MG was more favorable in alkaline conditions. Similar pH removal efficiency profile for MG at high pH of the system was also reported in some of the previous publications [22,26]. This finding can be explained in terms of electrostatic interaction between the photocatalyst and targeted pollutant. In general, pH influences the surface state of a catalyst and also the ionization state of ionizable organic compounds. As a result, this affects the interfacial electron transfer and overall photoredox process. In other words, photocatalysis rate of a system can either be enhanced or retarded depending on the dominance of either the attractive or repulsive intermolecular forces [39]. Besides, the pH at zero-point of charge also plays an important role. When the pH of the system was greater than the zero-point charge or isoelectric point for TiO2, the surface of the photocatalyst becomes predominantly negatively charged. Since MG is cationic dye, electrostatic attraction causes more MG adsorption or migration of it towards the catalyst surface. Such conditions would therefore cause the pollutant to be more susceptible to photodegradation [40]. Not only that, an alkaline medium would also contain more OH which are readily available to be oxidized into OH on the photocatalyst surface that will later diffuse through the bulk solution leading to an expected increase in the photocatalytic process. On the contrary, under acidic conditions (pH < pHzpc), the surface of TiO2 is positively charged and MG will be repelled from the surface of the catalyst due to electrostatic repulsion. This causes the degradation process to be reduced greatly. It is postulated that the photodegradation process is still occuring but at a lower rate. It is suggested that the positive holes are the major oxidation species in low pH systems [41].
3.7. Reusability of the film The evaluation on the reusability of the T-SA film is necessary because the application of photocatalysis to treat dye contaminated wastewater will be more practical and desirable at industrial scale if the photocatalyst can be reused. In this study, the results showed that the photocatalytic activity of the T-SA films remains high after five repetitive usages (Figure 5). A slight increase in the percentage removal of MG was observed at third cycle and this may probably due to the variation of the sunlight intensity. Although the removal of MG is still above 60%, T-SA film was not subjected for any further usage after the 5th cycles because thereafter the film became brittle. A similar observation was reported in TiO2/calcium alginate composite film whereby the changes in the mechanical properties of the film were quite minimal after five consecutive usages [33]. This is because alginate exhibits a good compatibility with TiO 2 as the latter can interact with the former through hydrogen bonding of the hydroxyl groups present in both molecules.
3.8. Plackett-Burman (PB) design Plackett Burman design was utilized to evaluate the factors which have a significant effect on the percentage removal of MG. This experimental design is a handy screening tool as it reduces the number of experiments and determines influential variables for further optimization. The function of desirability was applied using Design Expert Version 7.1.3.1 in order to validate the model. The experimental conditions to be verified were selected based on the highest desirability. This technique is useful for constructing designs for various experimental situations and enables the determination of important parameters for further optimization study [42]. Different variables, namely effect of pH, initial dye concentration and catalyst dosage were used in the screening process and a total of 8 sets of experimental conditions were generated by the Plackett- Burman program. These experiments were carried out according to the conditions generated and the results obtained from these experiments are presented in Table 3. Based on the percentage error, the
experimental percentage removal apparently did not deviate much from the predicted values and the differences were within 0.28-3.65%. The differences observed between the experimental and predicted percentage removal is closely related to the involvement of insignificant variables in the analysis. This kind of deviation has also been reported in some of the previous works [6,32]. The analysis of variance (ANOVA) was performed to test the validation of the three parameters and the result obtained was shown in Table 4. The photocatalytic degradation model was significant with the Prob>F value of <0.0001. The variables were identified as significant when the Prob>F value is below 0.05. Based on the results obtained, all the three parameters studied in this experiment are significant with Prob>F values of <0.0001, indicating all three parameters investigated in this batch experiments influences the percentage removal of MG. This finding agrees well with most of the reported works whereby pH, catalyst loading and the initial dye concentrations affect the percentage removal of dye due to the charge, amount of active sites and path length of the photons entering the dye solution.
3.9. Response Surface Methodology From the screening results obtained through Plackett-Burman design, the interaction between the important variables and their optimum levels for maximum percentage removal of MG were further studied using response surface methodology (RSM). Similar to the previously reported works [43,44], a factorial central composite design (CCD) model for significant variables with replicates was used in this study. All experiments were conducted in duplicate and the mean value of percentage removal was used as response. Cubic equation used for the optimization of percentage removal of MG is shown as follows:
2
2
2
i 1
i 1
i 1
1
Y i X i ii X i2 iii X i3 1
2
i 1 j i 1
1
ijj
Xi X 2 j
iij
i 1 j i 1
2
i 1 j i 1
2
ij
Xi X j
(5)
2 i
X Xj
where, βo, βi, βii, βiii, βij, βijj and βijj are the constant coefficients, and Xi, and Xj are the independent variables. A total of 15 sets of experimental conditions generated by the program were conducted and the results obtained from these experiments were presented in Table 5. The experimental values agreed well with the predicted values and the differences were within 0.05-4.82%. The results of ANOVA analysis was presented in Table 5. The model F value of 27.89 and Prob>F value < 0.0001 indicates that the model for the removal of MG by photocatalytic degradation was significant (Table 6). All the three parameters studied were found to be significant. The Prob>F value was less than 0.0001 for both pH and initial dye concentration, whereas for catalyst dosage, it is 0.0001. The R2 for this model was 0.9617 which indicates that there was a good agreement between the observed and predicted values. The coefficient of variance (CV) for this model is 4.99. This shows a good reliability and precision of the model as when the CV value is lower, the precision and reliability of the experiments are higher. Adequate precision measure the signal to noise ratio and value greater than 4 is preferred. Since the adequate precision for the system is 20.521 therefore, an adequate signal was obtained. Figures 6-8 show the 3D surface plot on the interactions between pH and catalyst loading, pH and initial dye concentration and catalyst loading and initial dye concentration in the removal MG, respectively. The region highlighted in red shows the highest percentage removal of MG whereas; the blue region shows the lowest percentage removal of the dye. The maximum removal of the dye was achieved in the condition of high pH, moderate catalyst loading and low initial dye concentration. The optimum conditions for maximum removal MG was at pH 8, catalyst dosage of
0.15 g and initial dye concentration of 5 mg/L. Under the optimum conditions, the predicted and experimental percentage removal of MG was 98.11% and 98.55%, respectively.
4. Conclusion This study proved the potential of T-SA film as a promising catalyst for the remediation of MG from aqueous solution. Based on the findings, the percentage removal of the MG was influenced by pH, catalyst loading and initial dye concentrations. The removal of MG increased with contact time but decreased with initial dye concentrations. Higher catalyst loading is beneficial up to certain extent only. The photocatalytic degradation of MG followed first order kinetics model. The rate constant was found to be 0.0138, 0.0052 and 0.0350 for 5, 15 and 25 mg/L of MG, respectively. It is interesting to note that the phototocatalytic degradation ability of T-SA film remains high after 5 repetitive usages. Plackett-Burman experimental design successfully identify that all the three parameters to be significant factors in affecting the percentage removal of MG. Based on the RSM analysis, under the optimized conditions which were pH 8, catalyst loading of 0.15 g and initial dye concentration of 5 mg/L, the removal of MG is higher than 98%. A good agreement was found between the experimental and predicted values by the generated model.
Acknowledgements The authors are thankful for the financial support and research facilities provided Universiti Tunku Abdul Rahman (UTAR).
Declaration of competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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(a)
(b)
Fig. 1. SEM micrograph of T-SA film (a) before photocatalytic degradation; (b) after photocatalytic degradation
Fig. 2. Surface topography of T-SA film (a) before photocatalytic degradation; (b) after photocatalytic degradation
Removal efficiency (%)
100
80 60 40 20
5 mg/L
15 mg/L
25 mg/L
0 0
50
100
150 200 Time (min)
250
300
Fig. 3. Effect of initial dye concentrations on percentage removal of MG
Removal efficiency (%)
100
80
60
40
20
0 3
4
5
pH
Fig. 4. Effect of pH on percentage removal of MG
6
7
8
Removal efficiency (%)
100
80
60
40
20
0 0
1
2
3
4
5
6
Number of reuse cycle Fig. 5. Effect of repetitive usage of the T-SA film on percentage removal of MG
Fig. 6. 3D surface plot for removal of MG from aqueous solution using T-SA film by photocatalytic degradation as a function of catalyst dosage and pH of solution.
Fig. 7. 3D surface plot for removal of MG from aqueous solution using T-SA film by photocatalytic degradation as a function of initial dye concentration and pH of solution.
Fig. 8. 3D surface plot for removal of MG from aqueous solution using T-SA film by photocatalytic degradation as a function of initial dye concentration and catalyst dosage.
Table 1. Comparison of maximum adsorption capacity for MG by different adsorbents Adsorbent
Maximum adsorption capacity (mg/g)
Reference
MOF nanocomposite
329.61
[27]
Sodium dodecylbenzene sulfonate (SDBS)-modified sepiolite
270.3
Alginate coated perlite
74.63
Carboxylate functionalized multiwalled carbon nanotubes
49.45
Superparamagnetic sodium alginatecoated Fe3O4 nanoparticles
47.84
Corn cob thin film
35.34
[28] [29] [30]
[31] [32]
Table 2. Pseudo first order rate constants, correlation coefficients and half-life values for the photocatalytic degradation MG
R2
5
Rate, k (min-1) 0.0138
0.9770
50.23
15
0.0052
0.9649
133.30
25
0.0035
0.9905
198.04
Initial dye concentration (mg/L)
Half-life, t1/2 (min)
Table 3. Plackett-Burman design and results for remediation of MG from aqueous solution. pH Differences (%) Catalyst Initial Dye Experimental Predicted Dosage Concentration Removal (%) Removal (%) 8.00
0.25
5.00
83.98
83.70
0.28
8.00
0.05
5.00
76.37
78.76
2.39
8.00
0.05
25.00
59.41
60.43
1.02
3.00
0.25
25.00
49.81
50.28
0.47
3.00
0.05
25.00
44.46
43.34
1.12
3.00
0.05
5.00
60.32
56.67
3.65
3.00
0.25
5.00
65.14
66.61
1.47
8.00
0.25
25.00
69.22
67.37
1.85
Table 4. Regression analysis (ANOVA) of Plackett-Burman for the remediation of MG from aqueous solution Source
Sum of Squares
Degree of freedom
Mean Square
F-Value
p-value Prob > F
Model
1820.42
3
606.81
461.70
< 0.0001
A-pH
875.69
1
875.69
660.29
< 0.0001
144.56
109.99
< 0.0001
B- Catalyst Dosage
1 144.56
C- Initial Dye
1
Concentration
800.17
Residual
10.51
8
1830.94
11
Total
800.17
< 0.0001 608.83
1.31
Table 5. RSM design and results for remediation of MG from aqueous solution pH
Predicted Removal (%)
Differences (%)
8.00
Catalyst Initial Dye Experimental Dosage Concentration Removal (%) 0.05 25.00 59.41
62.94
3.53
8.00
0.25
25.00
69.22
66.56
2.66
3.00
0.25
5.00
83.98
80.96
3.02
5.50
0.15
5.00
98.11
94.00
4.11
3.00
0.05
25.00
44.46
40.96
3.50
5.50
0.15
15.00
79.67
80.35
0.68
3.00
0.15
15.00
71.29
74.08
2.79
5.50
0.05
15.00
60.43
57.28
3.15
8.00
0.05
5.00
76.37
76.32
0.05
5.50
0.25
15.00
66.71
67.83
1.12
8.00
0.15
15.00
93.41
88.59
4.82
5.50
0.15
25.00
69.93
72.01
2.08
8.00
0.25
5.00
83.98
87.99
4.01
3.00
0.25
25.00
49.81
50.37
0.56
3.00
0.05
5.00
60.32
63.49
3.17
Table 6. Regression analysis (ANOVA) of RSM for the remediation of MG from aqueous solution Source
Sum of Degree of Mean Squares freedom Square
F-Value
p-value Prob > F
Model
3353.38
9
372.60
27.89
< 0.0001
A-pH
526.06
1
526.06
39.37
< 0.0001
277.83
20.79
0.0010
B-
Catalyst
Dosage
1 277.83
C- Initial Dye
1
Concentration
1208.46
Residual
133.61
10
Total
3486.99
19
1208.46
< 0.0001 90.45
13.36