Essay: Cyanobacteria

A synthesized BiVO4/TiO2 nanocomposite by hydrothermal method was employed for the removal of MC-LR. Response surface methodology (RSM) was applied to assess the effects of operating variables (pH, contact time, and catalyst dose) on the MC-LR removal. The coefficient of determination (R2) was calculated 98.7% for the response. The obtained maximum MC-LR removal efficiency was 98% under the optimum conditions. The prepared nanocomposite was approved as a promising nano-photocatalyst for MC-LR removal under visible-light irradiation. The BiVO4/TiO2 as an important nano-catalyst with technological potential can be used directly in environmental preservation, specifically in the decontamination of MC-LR from aqueous solutions.
Keywords: Microcystin-LR (MC-LR), BiVO4/TiO2, Nanocomposite, Hydrothermal, Photocatalysis, Visible light
Introduction
Cyanobacteria are a group of microalgae that are known as the oldest oxygen-producing organisms (3.5 billion years) and called blue-green algae, Myxophyceae, Cyanophyceae and Cyanophyta [1,2]. Many cyanobacteria produce a wide variety of toxins (cyanotoxins) include Hepatotoxins, neurotoxins, cytotoxins, dermatoxins and irritant toxins [3]. Among them, Hepatotoxins are the most prevalent cyanobacterial toxins [4,5] that are included microcystins, nodularins, and cylindrospermopsins [1]. Microcystins (MCs) are potent hepatotoxins produced by the number of planktonic cyanobacteria such as Anabaena, Anabaenopsis, Oscillatoria, Nostoc, Planktothrix, and Microcystis [6]. These toxins are very soluble in water and based on their two variable amino acids have more than 80 variants [7,8]. The US Environmental Protection Agency (US-EPA) has a special concern to 4 MCs (LR, RR, YR, and LA). The main structural difference between these is the replacement of single amino acids [9]. Microcystin-leucine-arginine (MC-LR) is one of the most studied cyanotoxins due to its toxicity and abundant [10]. Moreover, the World Health Organization (WHO) is recommended a limit of 1 μg/L for total MC-LR for water for human consumptions [11]. MCs cause poisoning for livestock and wildlife and also pose a health hazard for humans through of the drinking water [12]. When MCs ingest orally absorb to hepatocytes and prohibit protein phosphatase, subsequently resulted in cell structures disruption, intrahepatic hemorrhage, and death [13]. In 1996 MC-LR was responsible for the death of least fifty Brazilian kidney dialysis patients [14]. Also, Huisman et al. reported the episodes including skin and respiratory irritations,gastrointestinal disease among swimmers on the Queensland coast from 1996-1998, and a further prevalence of illness among hemodialysis patients in 2001 [15]. Various techniques have suggested for the cyanobacteria cells and MCs control in drinking water, such as coagulation, flocculation, filtration, activated carbon, nanofiltration, oxidation processes and etc. [16,17]. Recently, in order to remove these toxins in raw waters, more studies have focused on the application of advanced oxidation processes using (AOPs) such as UV/H2O2 [18], Fenton regents [19-20], and photocatalysis [21]. In advanced oxidation processes are generated •OH, •O, and •HO2 species that considerably promised for the decomposition of MCs [22,23]. The photocatalysis has also been suggested as an effective approach for the treatment of toxic polluted waters including toxins [24,25]. Many materials such as TiO2, ZnO, ZrO2, CdS, MoS2, Fe2O3, WO3, and their various combinations have been examined as photocatalysts for the organic and inorganic pollutants degradation [26]. Among them, semiconductor nanostructures with superior optical and physicochemical characteristics are used for environmental applications. These materials due to exclusive electronic structure act as photocatalysts for photochemical reactions in presence of light [27]. The photon energy required for the photo-excitation of semiconductors depends on their band gap [28]. The region of between the valence band (VB) and the conduction band (CB) is the band gap [29]. TiO2 is one of the semiconductor photocatalysts that because of its chemical stability, chemical inertness, non-toxicity, low cost, and strong oxidizing ability is most interested [30-33], but despite of its desirable properties, because to having of large band gap (~3.2 eV) activates only in the UV light region [27,34]. Therefore, the formation of heterojunction structures between TiO2 and a narrow band gap semiconductor can efficiently extend the photosensitivity of TiO2 into the visible region [35]. Using photocatalysts with a narrow band gap that active in visible-light irradiation is interested recently. Many visible light active photocatalysts that have good photocatalytic performance are CdS, CdSe, WO3, AgVO4, Bi2WO6, BiVO4, and etc [36]. Bismuth vanadate (BiVO4) is a low-cost semiconductor that has attracted special great attention for the organic pollutants degradation [37,38]. BiVO4 compounds have been prepared in three crystalline phases including monoclinic and tetragonal scheelite, and tetragonal zircon. Monoclinic form with the band gap~2.4 eV has the best photocatalytic performance in visible-light region [39-43]. Bismuth-based semiconductors have considered as the kinds of emerging and promising photocatalysts due to their unique crystal configurations and properties, and photo-induced charge carriers formation [44]. TiO2 coupled by BiVO4 is alternated as a way for enhancement of photocatalytic performance. BiVO4 due to its high visible-light absorption ability has been chosen as a sensitizer [45]. Absorption of a photon by semiconductors excites an electron [20] from the VB to the CB, if the photon energy (hv) equals or exceeds from the semiconductor/photocatalyst band gap. Simultaneously, a positive charge called a hole (h+) is also generated in the VB and resulted inthe formation of electron-hole pairs (Eq. (1)) [26]. Then these pairs (e-–h+ pair) move to the photocatalyst surface and in redox reactions with the adsorbed pollutants on the photocatalyst recombines, producing thermal energy or participate. The lifetime of an e-–h+ pair is very little [46] but is still enough for the promotion of redox reactions in the solution or gas phase in the semiconductor surface [26]. Then the photo-excited electrons react with molecular oxygen (O2) to the production of superoxide radical anions (•O2-) [47] (Eq. (2)), and the photo generated holes react with water to produce hydroxyl radicals (•OH) (Eq. (3)). Therefore, nanocomposites are enabled to have a powerful and durable photo-oxidation capability through generation of these strong radicals [48]. As these radicals are the main active species in the photo-oxidation process under visible light [49], they play crucial roles in the decomposition of the toxic and persistent organic pollutants (R) present at the surface of the photocatalyst and convert them into harmless species. The products due to the photodegradation are reduced to mineral compounds and finally CO2 and H2O are released (Eq. (4)) [50].
〖Photocatalyst〗_ □(→┴hν e^-+h^+ ) (1)
e^-+ O_2→ •O_2^- (2)
h^++ 〖OH〗^-→ •OH (3)
•O_2^-+ • OH + (R)→ 〖CO〗_(2 )+ H_(2 ) O (4)
Hu et al. reported that the addition of BiVO4 to TiO2 is accelerated the degradation of gaseous benzene even higher than the pure BiVO4 and TiO2 only [51]. The successful removal of rhodamine B (RhB) was performed by using BiVO4/TiO2 heterojunction structure under UV light and simulated sunlight irradiation [52]. The photocatalytic activity of the BiVO4/TiO2 nanocomposite was investigated for the degradation of gaseous isopropanol under indoor illumination [53]. In Wetchakun et al. study, the BiVO4/TiO2nanocomposites showed good photocatalytic degradation for methylene blue under simulated solar light irradiation [45]. Bao et al. showed that the BiVO4/TiO2 ceramic fibers were novel photocatalysts with high activity for the degradation of the azo dye of X-3B [54]. In Guo et al. study BiVO4/TiO2 nanocomposite exhibited the high photocatalytic activity for the decomposition of RhB under visible light illumination [55]. Li et al. study showed that the degradation processes of MB by BiVO4/TiO2 composites were synergistic reactions by combination photosensitization and photocatalysis under visible light irradiation and were driven by the excitation of MB and BiVO4. The TiO2 as a transporter was played important role in the transfer processes of carriers (electron-hole pairs) on the composite [56]. Sun et al. by using V2O5/BiVO4/TiO2 nanocomposites exhibited a higher photocatalytic activity for the decomposition of gaseous toluene compared to pure TiO2 and V2O5/BiVO4 under visible light irradiation [35]. In another study in 2017, degradation of phenol under visible light by BiVO4/TiO2carried out successfully. This good photocatalytic performance in BiVO4/TiO2 nanocomposite enhances the separation of photogeneration of electron and hole pairs [57].
This study was performed with the aim to degradation of microcystin-LR by BiVO4/TiO2nanocomposite by Response Surface Methodology (RSM) based on Central Composite Design (CCD) to optimize the performance of nanocomposites with consideration of effective variables on process efficiency. This prepared nanocomposites already was used for some organic pollutants removal but up to date there was not seen a study that uses this nanocomposite for MC-LR removal.
2.Experimental
2.1. Materials
MC-LR standard solution (10 µg/ml) was purchased from Sigma-Aldrich Co. (USA). In addition, MC-LR standard powder (100 µg) also was purchased from Enzo Life Sciences, Inc., USA. Then the solutions were prepared by dissolving the standard powder in 100% methanol (1 mL) before diluting to working solutions using high-purity water (Merck, Germany). All standards and the working solutions were stored at −20°C until use. The MC-LR physicochemical properties such as molecular structure and weight (g/mol), chemical formula and solubility in water and density are demonstrated in Table 1. Bi (NO3)3.5H2O (bismuth nitrate), NH4VO3 (ammonium metavanadate) and TiO2 (anatase, purity: 99.9 wt%, particle size: 20 nm, and BET>200 m2/g) were purchased from Sigma-Aldrich Co. (USA). High-performance liquid chromatography (HPLC)-grade methanol, acetonitrile, and trifluoroacetic acid (TFA) were provided by Merck (Germany).
Table 1. Physicochemical properties of MC-LR.
Physicochemical properties MC-LR
Molecular structure [58]
Molecular weight (g/mol) 995
Chemical formula C49H74N10O12
Solubility in water (g/l) >1
Density (g/cm3) 1.299
2.2. Preparation of BiVO4/TiO2 nanocomposite
BiVO4/TiO2 nanocomposite was synthesized by hydrothermal method. Briefly, 0.02 mol of Bi(NO3)3.5H2O and 0.02 mol of NH4VO3 were dissolved in 20 ml of 4 M HNO3 and 2 M NaOH aqueous solutions, respectively, to form two transparent solutions. These solutions were mixed together with Bi: V molar ratio of 1:1. The orange slurry was obtained by adjusting pH to 7 with 4 M NaOH. Then an appropriate amount of TiO2 powder was subsequently added to the pre-prepared BiVO4 solution (50:50 W/Wratio) and was stirred for 2 h under ambient air. After this time, the suspension was transferred into a Teflon-lined stainless steel autoclave with a capacity of 100 ml and the hydrothermal reaction was carried out at 180°C for 24 h, and then cooled down to room temperature. The final product was filtered and washed with deionized water several times and then was dried at 80°C for several hours. Finally, the resultant sample was calcined at 500°C for 6 h. Furthermore, the pure BiVO4was prepared without the addition of TiO2 powder and was used to the next experiments.
2.3. Characterization of nanocomposite
Field emission scanning electron microscope (FESEM) (FEI Quanta 200, USA) was used to investigate the BiVO4and TiO2nanoparticles and BiVO4/TiO2nanocomposite structure and surface morphology. The crystalline nature was also revealed by X-ray diffractometer (XRD, Bruker D8 Advance, Germany).FTIR spectrum of BiVO4/TiO2 was recorded using an IR spectrometer (Jasco 6300, Japan).
2.4. Photocatalytic setup
All of the experiments were performed in 25 ml Pyrex beakers involving 10 ml of aquatic solutions with 500 μg/l MC-LR concentration and catalyst mixture. The pH of the samples was adjusted by 0.01 M NaOH and HCl. Then, the samples were put in an ultrasonic bath for 10 min to eliminate aggregates. Photo-catalyst suspensions were illuminated with 15 W LED lamps (white light). These lamps were located in 10 cm above beakers. Firstly, and the samples were stirred for 30 min to reach equilibrium in darkness. Then, lamps were turned on and samples were placed under magnetic stirring to maintaining agitation during the reaction to keep suspension uniformity. At the end of contact time, lamps were turned off and samples were taken and filtrated through 0.22 μm syringe filters to separate the catalyst particles before measuring the residual MC-LR content by HPLC.
2.5. Design of Experiment: RSM approach
Respond surface methodology (RSM) was used for the experimental design by Design Expert software 10 by using central composite design (CCD) as a most widely used method to evaluate the interactive effects of the significant operating parameters and to optimize the MC-LR degradation by BiVO4/TiO2photocatalyticnanocomposite. The three main parameters pH, contact time (min), and catalyst dose (g/l) were chosen as model variables to study their interaction effects on the ability of BiVO4/TiO2nanocomposite for MC-LR removal. Also, the each of BiVO4 andTiO2nanoparticles only were tested under specified terms by Design Expert software 10.
Each independent actual parameter was varied over three coded levels (−1, 0, +1), which composed the batch experimental design matrix derived from the CCD summarized in Table 2.
Table 2. Independent variables and their levels for the CCD used in this study.
Overall 20 different experimental runs (including six replicates of the center point for error evaluation) were designed, along with the predicted and actual MC-LR removal values are shown in Table 3. These numbers of runs were employed to investigate the interaction between the independent and the dependent variables through following regression model (Eq. (5)):
Y = β0 + ∑ βiXi+ ∑ βiiXi2+ ∑ βijXiXj (5)
WhereY is the predicted response of MC-LR removal (response), β0 is the model constant, βiis the linear coefficient, βii is the quadratic coefficient and the βijis the cross product coefficient.
Table 3. Experimental conditions & results of central composite design (CCD).
2.6. MC-LR quantification
The concentration of MC-LR in the samples was determined by an HPLC system (Jasco PU-2080, Tokyo, Japan) which was equipped with a quaternary mixing pump, an inline vacuum degasser, automatic injector (AS-2055 Plus), a C18 column (150 × 4.6 mm, Germany), packed with 5 µm particles for the separation, and an UV-Vis detector (UV-2075 plus). The used mobile phase was a gradient of Mili-Q water and acetonitrile (50:50, v/v), both with 0.1% of TFA. Wavelength was set at 238 nm to the typical absorption spectra of MC-LR. The injection volume was 100 μl with a flow rate of 1 ml/min. Total run time was 15 min. Data acquisition and processing were done using BORWIN Chromatography software (Version 1.50). The degradation rate of MC-LR was calculated by the (Eq. (6)):
Degradation (%)=(C_0-C)/C_0 × 100 (6)
where C0 and C are the initial and the residual concentrations of MC-LR.
2.7. Validation of the method
The validation of the analytical method was done according to International Conference on Harmonization (ICH) recommendations for relative standard deviation (RSD), relative recovery, limit of detection (LOD), and limit of quantification (LOQ), (Table 4).
Table 4. Method validation parameters for determination of MC-LR by HPLC.
3. Results and discussions
3.1. Catalysts characterization
Field emission scanning electron microscope (FESEM) graphs of the studied photocatalysts are presented in Fig. 1. It can be shown that symmetric spherical nanoparticles of BiVO4 (Fig. 1a), and angled asymmetric TiO2nanoparticles (Fig. 1b) were synthesized to form BiVO4/TiO2nanocomposite (Fig. 1c). Therefore, the obtained nanocomposite has very different structure and more porous surface than its ingredients.
For each of pristineBiVO4, and TiO2nanoparticles; and BiVO4/TiO2nanocomposite, separately, X-ray diffraction (XRD) patterns were measured using an X-ray diffractometer at 40 kV and 40 mA with Ni-filtered Cu Kα radiation (λ= 0.15406 nm) at a scan rate of 1°/min in 2θ ranging from 5° to 80° (Fig. 2). The crystallite size was estimated using the Debye-Scherrer formula (Eq. (7)):
D = Kλ/(βcos θ) (7)
where D is the crystallite size, λ is the wavelength of the X-ray radiation, K is a constant taken as 0.89, β is the full width at half maximum corresponding to the Bragg’s angle θ. Therefore, based on XRD, the average crystal sizes of BiVO4 and TiO2 in synthesized nanocomposite were 35 and 20 nm, respectively.
As can be seen, the crystal phase of anatase TiO2 nanoparticles is included in the diffraction peaks at about 2θ = 25.33˚, 37.8˚, 48.1˚, 53.9˚ and 55.1˚ (PDF number 21-1272, JCPDS). Also, all of the peaks match well to the monoclinic scheelite BiVO4 with the diffraction peaks at about 2θ = 18.99˚, 28.95˚, 30.57˚, 39.87˚, and 53.22˚ (PDF number 14-0688, JCPDS). Moreover, there is no any other phase or impurity peak in the resulted XRD pattern (Fig. 2), which confirms that the synthesized BiVO4/TiO2nanocomposite sample by the hydrothermal method has high purity and is only composed of monoclinic BiVO4 and TiO2 nanoparticles.
Fig. 3 shows the FTIR spectra of BiVO4and TiO2 nanoparticles; and BiVO4/TiO2 nanocomposite. According to this Figure, in the FTIR spectra of BiVO4, the region of 730-410 cm-1 was assigned to oxygen bonds with bismuth (Bi-O) and vanadium metals (V-O). Also, the band of 3436 cm-1 was related to O-H bonds due topresenceof water molecules in synthesized BiVO4 nanoparticles. In the FTIR spectra of TiO2, the band of 455 cm-1 was representative of metal oxide (Ti-O). In the region of 1632-1062, the peaks are negligible and 3408 cm-1 band was assigned to O-H bonds. Finally, in the FTIR spectra of BiVO4/TiO2 nanocomposite, The band at 3432 cm-1was attributed to O-H stretching vibration of adsorbed water molecules on composites. The adsorption band at 1624 cm-1also was related to hydroxyl groups (Ti-O-H bending mode) on the surface of the synthesized sample. The region of 730-470 cm-1was due to the chemical or structural effects of the Bi3+ or VOx ion in BiVO4/TiO2nanocomposite lattice, demonstrating an interconnection between BiVO4 and TiO2.
3.2. Statistical analysis
The analysis of variance (ANOVA) was carried out based on the suggested model to show the interaction among three variables (pH, contact time, and catalyst dose) and MC-LR degradation as the response parameters. In addition, the coefficient of determination (R2), adjusted R2 (R2adj), p-value, and F-value which show the goodness of fit of the obtained regression model are summarized in Table 5.
Std. Dev.: 1.49; Mean: 79.82; C.V. %: 1.87; PRESS: 113.97; R2: 0.9872; Adj R2: 0.9756; Pred R2: 0.9341;
Adeq. Precision: 30.161.
*not significant
According to Table 5, the model F-value of 85.58 implies that the model is significant. In addition, the values of “Prob>F” less than 0.05 indicate model terms are significant. On the other hand, A, B, C, AB, AC, A2, B2, and C2 symbols are significant model terms. However, the values greater than 0.05 indicate the model terms are not significant. For example, the BC symbol is not significant. The Lack of Fit F-value=1.57 implies the lack of fit is not significant relative to the pure error. The non-significant lack of fit value for the fitting model (p-value>0.05) suggests that the quadratic model is valid for this process and errors of experiments are low. This relation shows that the model is significant.
3.3. Model Fitting
To study the combined effect of all the variables, experiments were performed with different combinations of the variables, which were statistically designed by using CCD. The following equation represents the first model that was developed with all linear, two-way, and quadratic interaction of predictors:
Y= 72.35 – 5.56 A + 5.07 B + 4.41 C + 1.91 AB – 1.23 AC – 0.14 BC + 1.18 A2 + 3.42 B2 + 7.22 C2 (8)
The predicted R2 of this preliminary model was 0.9341. To achieve a final model with significant predictors, the only not significant interaction, BC: (p-value=0.8), was removed from this model. Thus, all of the other variables were significant and retained in the final regression model. Table 6 shows the final regression model variables for MC-LR removal.
Std. Dev.: 1.42; Mean: 79.82; C.V. %: 1.79; PRESS: 87.09; R2: 0.9871; Adj. R2: 0.9777;
Pred. R2: 0.9497; Adeq. Precision: 33.374.
The final equation was derived using the coefficient of the coded variables for MC-LR degradation as follow (Eq. 9):
MC-LR degradation (%) =72.35 – 5.56 A + 5.07 B + 4.41 C + 1.91 AB – 1.23 AC + 1.18 A2 + 3.42 B2 + 7.22 C2 (9)
This equation has relatively higher fitness (the lack of fit F-value=1.32) and confirms a proper fit of this model. Also, the other coefficients and factors were improved in order to the model fitness. The correlation between the actual and predicted MC-LR removal efficiencies and the normal plot of residual related to experimental data are shown in Fig. 4a and b, respectively.
(a) (b)
Fig. 4. (a) Correlation of actual versus predicted MC-LR removal efficiencies; (b) Normal plot of residual related to experimental data.
According to these figures, the data points were well distributed close to a straight line, which indicated an excellent relationship between the experimental and predicted values of the response for BiVO4/TiO2 nanocomposite. These results also demonstrated that the chosen model was appropriate in assuming the response variables for the experimental data.
3.4. Effect of variables on photocatalytic degradation of MC-LR
3.4.1 Single variables
According to Table 5, between the single variables (pH, contact time, and catalyst dose), each factor that poses the highest mean square and F-value has the most effect in the degradation of MC-LR. Therefore, pH was the most important parameter (mean square and F-value were equal 390.96 and 176.25, respectively). Also, pH parameter has a negative effect on MC-LR removal (negative sign in (Eq. (9)). The contact time and catalyst dose parameters have lower importance than pH. Of course, both have a positive effect on MC-LR removal (positive sign in (Eq. (9)).
3.4.2. Effect of pH
As mentioned earlier, the pH variable has a negative effect in MC-LR removal. Namely, the removal efficiency of MC-LR is increased with decreasing pH in the range of 5 to 9 (Fig. 5).
The pH is an important factor in the organic pollutants photocatalytic removal from aqueous solutions [59]. This parameter may effect on the photocatalyst surface charge and also the state of ionization of the substrate [60] and hence its photo-degradation. In photocatalytic systems, the optimal pH was determined in the acidic range, which strong electrostatic adsorption could happen between the positively charged catalyst and the negatively charged toxin [61].
The main reasons for the better MC-LR removal in acidic pH are:
The tendency of MC-LR molecule to wrapping and reducing molecular dimensions and hydrogen bonds forming between these molecules and photocatalysts surface [62].
Hydrophobic characteristic of the MC-LR due to being a large molecule (MW = 995 g/mol) and having the complex of amino acids [62]. So that, Lawton et al. reported that hydrophobicity was the most important factor controlling the adsorption of MCs on the surface of TiO2 catalysts [63]. Also, Chen et al observed that the hydrophobic property of the Adda group in MCs helps its sorption on the surface of Bi2WO6 nanoparticles [64].
The increasing of MC-LR solubility in water that leads to the reaction of between the MC-LR and catalysts through the ion pairing effect [65].
The formation of •H radicals. According to Liu et al. study, solution pH has an important effect on MC-LR removal, so that when pH was in the acidic conditions, a better removal rate by••H radicals was found, due to the main path of MC-LR degradation and Adda strain removal [66].
3.4.3. Effect of contact time
The contact time has a positive effect on MC-LR removal. On the other word, the removal efficiency of MC-LR was enhanced with increasing contact time in the range of 30 to 90 min (Fig. 6).
Fig. 6. Effect of contact time on MC-LR degradation.
Because of increasing contact time enhances the reaction time between the MC-LR and photocatalysts and then its effective removal [67,68]. Zhang etal. showed that the removal efficiency of the MC-LR increased overthe time [65]. In Nasseri et al. study, Malathion adsorption and photocatalytic removal were increased with enhancing of contact time [59].
3.4.4. Effect of catalyst dose
Fig. 7 shows the effect of catalyst dose on MC-LR degradation. The removal efficiency of MC-LR is increased with increasing catalyst dose in the range of 0.2 to 0.5 g/l.
Fig. 7. Effect of catalyst dose on MC-LR degradation.
Zhang et al. reported that the removal efficiency of MC-LR was improved with an increase in catalyst dosage because of the surface area of the catalysts increased with increasing dosage. Therefore, the selection of the optimum catalyst dose was important with consideration of its cost and effects [65].
3.4.5. Interaction between variables
Fig. 8 shows the interaction between pH and contact time as a 2-dimensional graph. According to this figure, the MC-LR removal efficiency was increased by increasing of contact time and decreasing of pH. Of course, the coefficient of this term in Eq. 9 is less than the coefficient of each variable.
Fig. 8. 2-dimensional graph of interaction between variables of pH and contact time in the MC-LR removal (%).
Fig. 9 illustrates the interaction between pH and catalyst dosage. In this Figure, it can be seen by pH reduction and increasing of catalyst dose, the MC-LR removal efficiency was increased. However, the model coefficient of this term in Eq. 9 is lower than each of the variables only.
Fig. 9. 2-dimensional graph of interaction between pH and catalyst dose in the MC-LR removal (%)
3.5. Optimization the process and model verification
The aim of this study was to determine the optimum amount of parameters with the RSM model which lead to the highest MC-LR removal efficiency. In this regard, the optimization of influencing variables was necessary. The desirable goal in MC-LR degradation was set on the maximum value. The synthesized BiVO4 nanoparticles had highest MC-LR removal efficiency about 93% at these conditions: pH= 5, contact time= 180 min and catalyst dose= 0.5 g/l. While, purchased TiO2 nanoparticles in the best conditions (pH= 5, contact time= 30 min and catalyst dose= 1 g/l) had about 95%MC-LR removal efficiency. The prepared nanocomposite BiVO4/TiO2 showed extremely enhanced photocatalysis characteristics compared to the individual BiVO4and TiO2 [69]. So that, the highest MC-LR removal efficiency (about 98%) by the BiVO4/TiO2 nanocomposite was obtained at the optimum conditions: pH= 5, contact time= 90 min and catalyst dose= 0.5 g/l. Therefore, synergetic effect of the porous nanostructures can enhance the photocatalytic activity of nanocomposite photocatalysts [44] .Ultimately, in order to estimate the validity of these optimal conditions, a series of MC-LR removal experiments were implemented which their results were in good accordance with Design Expert predictions.
4. Conclusions
The produced BiVO4/TiO2 nanocomposite through the hydrothermal method was applied for MC-LR photocatalytic degradation under visible light. Important parameters such as pH, contact time, and catalyst dose were evaluated using RSM technique based on CCD design. The results showed that the MC-LR decomposition efficiency by BiVO4/TiO2 nanocomposite was enhanced by increasing of contact time and catalyst dose, while pH reduction was led to the increase of degradation rate. The best conditions for MC-LR removal by the BiVO4/TiO2nanocomposite were pH= 5, contact time= 90 min, and catalyst dose= 0.5 g/l. Under these circumstances, MC-LR removal efficiency was about 98%. In this case, BiVO4/TiO2 nanocomposite was approved as a promising nano-photocatalyst for MC-LR removal. Therefore, this study presented a new approach for the application of more efficient and inexpensive photocatalytic nanocomposites in order to the pollutants removal for instance MC-LR. Also, in synthesis of BiVO4/TiO2 nanocomposite by integration of BiVO4 andTiO2 nanoparticles, will have a narrow band gap which is activated in visible-light range. Thus, prepared nanocomposite can active by LED lamps. Using of these lamps for activation of BiVO4/TiO2nanocomposite due to their inexpensive, low energy requirements, high photon efficiency, long-term stability and emission in broader spectral wavelengths is an innovative approach for replacing the conventional visible-light sources in many photocatalytic applications.
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