Photocatalytic Degradation of Diazo Dye over Suspended and Immobilized TiO₂ Catalyst in Swirl Flow Reactor: Kinetic Modeling

Round 1

Reviewer 1 Report

The manuscript presents a study of the degradation kinetics of Blue 15 (DB15) by using TiO₂ catalyst under the influence of UV light. The authors of this work investigated the effects of some factors such as  dye concentration, catalyst  loading, and light intensity on the degradation kinetics of DB15. However, this article can thus be accepted in this journal for publication, although after a minor revision. The followings are my suggestions, which the authors may choose to address:

1. The authors should modified figure 11. A, B and C and I recommend if it is possible to change the color.
2. I strongly recommended that the authors add UV-Vis absorption spectra of Blue 15 (DB15) treated by the TiO₂.
3. In page 26, the two lines above the equation 25 should be be amended.

Comments for author File: Comments.docx

Author Response

The authors are very grateful to the reviewers for their valuable time reviewing the manuscript and for the comments that have helped us improve the manuscript significantly.

1. The authors should modify figure 11. A, B and C and I recommend if it is possible to change the color.  

Response: The above comment has been taken into consideration. The authors have modified figure, and their colour has been changed to make them clearer.

2. I strongly recommended that the authors add UV-Vis absorption spectra of Blue 15 (DB15) treated by the TiO₂.

Response: The above comment has been taken into consideration. The authors have added the figure. It is referred as Figure 19 in the revised manuscript. In addition, the authors have added Table 9 and the following paragraph to support the required figure (UV-Vis absorption spectra):

“3.5 DB15 Mineralization

The DB15 mineralization at 40 mg/L was discussed from the perspective of TOC removal, and their outcomes are shown in Fig. 19 and Table 9. Fig. 19 reveals the UV-Vis spectra of DB15 decolourization at natural pH and a dye concentration of 40 mg/L. It is observed that 82.5% dye decolourization was achieved within a reaction time of 4 hrs, while a complete colour removal was obtained within 7 hrs. Although decolourization was achieved within a few hours, it did not mean the dye was completely mineralized. Decolourization refers to the colour removal of a dye solution due to a bond cleavage responsible for the colour of a dye; mineralization typically indicates conversion of carbon molecule within a dye structure to CO₂. For dye decolourization experiments, the volume of the treated dye solution was 250 ml using a catalyst concentration of 1 g/L. To reduce the time of DB15 mineralization, some parameters were changed while the dye concentration was kept constant at 40 mg/L.  To achieve this, the catalyst concentration was increased to 2 g/L while the treated volume was lowered to 200 ml. Table 9 shows the obtained results of DB15 mineralization. As can be seen, the amount of remaining carbon was 4.02 mg, registering 75.55% carbon removal after a reaction time of 14 hrs. Meanwhile, the percentage of carbon removal increases to reach 83.64% when the reaction time is extended to 16 hrs. It is noteworthy that the time of complete mineralization would be longer than 16 hrs, although DB15 decolourization was achieved within few hours. It should be noted that the model dye used in this study has a molecular weight of 992.8 g/mol, and hence would generate many reaction intermediates. The generated by-products properties could be different from the parental compound. Thus, the mineralization time would be dependant on the number and nature of the intermediates. To confirm the outcomes, the experiment of reaction time (16 hrs) was repeated. As shown in Table 9, the percentage of carbon removal was almost the same, indicating to results’ reliability and reproducibility”.

3. In page 26, the two lines above the equation 25 should be amended.

Response: The above comment has been taken into consideration. The authors have amended the mentioned lines as in the following. The authors would like to mention that the equation number has been changed to 26 instead of 25 due to the manuscript’s revision.   

“As part of the direct dye oxidation mechanism, the generated positive holes directly react with the DB15 to produce intermediate compounds that mineralize to CO₂ and H₂O eventually as final products of the oxidation process, as presented in equation 26”.   

Reviewer 2 Report

This manuscript was written by Waleed Jadaa et al. reports on suspended TiO₂ for photocatalytic degradation of diazo dye. From my perspective, this manuscript contains information that can be of interest to the scientific community and recommend its publication. However, amendments must be made before the final publication. Below are listed my observations.

1. The mechanism of the photocatalytic reaction should be further explored by the necessary experiments, such as the trapping experiments for hydroxide radical and the ESR experiments for the O₂. Some papers could be referenced to further improve the manuscript (e. g., Applied Catalysis B: Environmental 263 (2020)11730. Applied Catalysis B: Environmental 260 (2020) 118181. Applied Surface Science 498 (2019) 143850.).
2. Some characterization data of the TiO₂ should be provided, such as SEM, TEM and XRD.
3. As a comparison, the degradation performance of colorless Bisphenol A (BPA) or phenol could be provided.

Author Response

The authors are very grateful to the reviewers for their valuable time reviewing the manuscript and for the comments that have helped us improve the manuscript significantly.

Reviewer #2:

1. Does the introduction provide sufficient background and include all relevant references? Can be improved.

Response:

The above comment has been taken into consideration, and the authors have added the following paragraphs into the “Introduction” section:

“From the catalyst immobilization perspective, Ray & Beenackers [32] deposited TiO₂ (P25) over Pyrex glass to examine the photocatalytic degradation of Special Brilliant Blue (SBB) dye. Their study included two different deposition configurations over & under Pyrex glass using a swirl-flow reactor. True kinetics rates and their constants were evaluated in terms of different variables such as light intensity, angle of light incidence, and amount of catalyst. The influence of mass transfer on the kinetics parameters was evaluated as well. The study found that the photocatalytic rate has the following form: . Using the same reactor type, Zhou and Ray [27] discussed the degradation kinetics of Eosin B dye. After correcting for mass transfer effect, kinetic rate constants were obtained based on various operational factors: catalyst film thickness and light intensity. The influence of different parameters on photocatalytic degradation was evaluated, such as pH, temperature, and dissolved O₂. It was concluded that the degradation kinetic rate constant could be described by the following equation  . Hashim et al. [29] used the same  swirl-flow reactor to explore the photocatalytic degradation of diclofenac (DCF) over a thin film of P25 and modified (doped and sensitized TiO₂. Two different catalysts support: Pyrex glass and fibre glass were used to achieve better immobilization. Under the influence of UV and visible light, the kinetic rate parameters were evaluated, such as pH, flowrate, DCF concentration. Their study indicated that the dye sensitized TiO₂ has higher degradation rates under visible light than doped TiO2 and P25 catalysts. In addition, the reaction rate constant had the form:   Chen et al. [33]studied the influence of external and internal mass transfer on the photocatalytic degradation of benzoic acid. Using an immobilized P25 catalyst, two different illumination configurations were applied: substrate-catalyst (SC) and liquid-catalyst (LC) illumination. Different factors such as adsorption constant, dynamic adsorption constant, effective diffusivity, and internal and external mass transfer coefficients were evaluated theoretically and experimentally. The impact of catalyst layer thickness on the degradation performance was investigated as well. It was found that the internal mass transfer resistance governs the overall degradation rates. The overall rate reached a saturation value for LC configuration while an optimum thickness was achieved for SC configuration. Zangeneh et al. [34] coated nano TiO₂ particles on quartz tubes and placed them in a photo reactor. Under UV light, the immobilized catalyst was evaluated for linear alkyl benzene (LAB) degradation. To examine the reactor performance, the influence of three independent factors: initial pH, initial chemical oxygen demand (COD), and reaction time, were studied using response surface methodology. The influence of O₃, H₂O₂, and O₃/H₂O₂ on the degradation behaviour was also discussed. For the applied range of variables, the optimum conditions were obtained. It was observed that the reactor has an efficiency of 37% for degradation without chemicals addition. The photocatalytic reaction was enhanced by adding H₂O₂, O₃, and O₃/H₂O₂, and such addition led to obtaining efficiencies of 50, 60, and 69%, respectively. Using titanium butoxide and P25, Arabatzis et al. [35] used two different methods: doctor-blade and dip-coating for depositing TiO2 on glass substrates. The thin film’s photoactivity was checked for 3,5-dichlorophenol (3,5-DCP) removal. It was revealed that the degradation registers a pseudo first order reaction kinetics. In addition, the thin films of P25 had higher removal rates than the prepared TiO₂ films, while the doctor-blade technique was more successful due to achieving a stable film photoactivity over many operation hours”.

In addition, the authors have added the following sentences to improve the last paragraph of the “Introduction” section.

“The DB15 mineralization was also discussed. For immobilized catalyst mode, P25 was deposited using a dip-coating device. The film’s surface characterization was evaluated. The P25 thin films were then studied for DB15 degradation kinetics by evaluating the effect of different operating variables such as light intensity and initial dye concentration. The degradation rate constants were determined based on the  approach. The P25 thin film’s durability was also checked to evaluate its recyclability”.    

2. Is the research design appropriate? Can be improved.

Response: The above comment has been taken into consideration, and the authors have added the following paragraphs to improve the research design.  

“2.3 Catalyst Immobilization

2.3.1 Dip-coating apparatus

The dip-coating apparatus consists of a vertical structural frame connected to two horizontal frames and a horizontal plate. An electrical motor (Leeson Electric) with 1 H.P. is placed on the horizontal plate and connected to a lead screw shaft (threaded screw rod) mounted on a ball-bearing at both ends. The screw shaft is vertically positioned between the vertical frame and horizontal plate and fixed using small pins. A sample holder is mounted on the screw shaft using a ball bearing, which conveys through the shaft’s level limits. The upper and lower limits of the sample holder, cycle limits, are determined using two position sensors placed on a vertical rod located beside the screw shaft. The position of the mentioned sensors can be adjusted based on the cycle level requirements. Under the horizontal plate, a movable horizontal frame includes four heat lamps (Philips, 175W) distributed in different directions; thus, the horizontal plate’s height can be adjusted. Besides, the position of the heat lamps can be relocated according to the coating requirements. The lower horizontal frame includes four wheels so that the device can be moved easily between different places. The purpose of this frame is to provide support for the device. The dip-coating device was built by University Machine Shop (UMS) previously used by our group (e.g. [29]) and was initially designed for tube coating. Therefore, the device is modified before it’s used in the circular glass coating. The modification includes a cylindrical shaft connected to a circular frame. The rod fits well the sample holder of the device while the circular glass is placed on the circular frame and fixing it by four small nuts and bolts. The details of the dip-coating device and its modification are presented in Fig. 3”.

2.3.2 Catalyst immobilization procedure

The Pyrex glass was first sandblasted to increase the surface roughness and enhance the strength of catalyst fixation. The Pyrex glass was then soaked in a 5% Nitric acid solution overnight to remove fragile surface parts due to the sandblasting process. To minimize the acidic solution’s influence, the Pyrex glass was rinsed many times with MQ water then was dried in an oven at 105°C. An appropriate amount of TiO2 was added to MQ water to achieve a 1.5% catalyst concentration. The solution was then sonicated using an ultrasonic cleaner (Fisher Scientific; model F560) for 1 hr, resulting in a milky solution [29]. The milky solution was stirred at 200 rpm using a mixer (Eurostar) for about 30 min to obtain better homogeneity. After the milky solution was prepared, the Pyrex glass was inserted into the milky solution and pulled out at a dipping speed of about eight mm/s using the dip-coating device. The coated thin film was dried in an oven at 100°C for 1 hr to remove the water content. To keep the coated film from cracking, the coating catalyst was then calcined in a vacuum oven for 0.5 hr at 250°C with a low-temperature ramp of about 0.03°C/s under a vacuum pressure up to about -0.15 bar. Under the same vacuum pressure range, the thin film was cooled down to room temperature, about 21°C. The cooling down of the thin film temperature under a controlled vacuum pressure ensures a fixed temperature reduction. Thus, the presence of film cracking was avoided. The entire procedure mentioned above was repeated depending on the required film thickness. To determine the amount of coated catalyst, the glass substrate is weighed before and after the coating process. 

It is important to mention that the swirl flow reactor includes two rubber O’rings on both sides of the reactor housing to seal the reactor from any possible leakage. The presence of those O’rings leads to cover circular parts of the glass and create a shaded area on both sides of the reactor frame because of cutting off the emitted light from the lamp. Thus, it is assumed that there are no sufficient photoreactions over the created shaded area compared to uncovered areas. Based on this, reducing the coating area to the actual area of those O’rings is quite reasonable to maximize the light distribution per the coated area and lower the related cost of TiO₂ amount reduction. To achieve this objective, a similar rubber O’ring is placed on the modified circular frame of the dip-coating device during the coating process to simulate the reactor during its normal operation. A schematic diagram of the approach used for the Pyrex glass coating is presented in Fig. 4. Additionally, one face of the circular frame was covered using parafilm and heat tape as the coating for one glass face is required. For the same reason, two heat lamps instead of four lamps were used for quick drying during the coating process. 

2.3.3 Surface characterization of thin film

The thin film’s surface characterization at the maximum amount of coated TiO₂ was investigated at the Western Nanofabrication Facility at Western University. Morphological properties: surface morphology and texture were studied by scanning electron microscopy (SEM) using a microscope LEO (Zeiss) 1540 XB FIB/SEM. The thin film of TiO₂ was analyzed at an accelerating voltage of 6 kV, with different working distances and magnification factors. Prior to analysis, the film was dried and coated with a thin layer of conductive material (Osmium) using an Osmium Plasma Coater (Filgen OPC80T).

In addition, the authors have added the following paragraphs to improve the sections of “Photodegradation Experiments” and “Analytical methods”, respectively.

“For the immobilized catalyst experiments, the kinetics experiments were performed at natural pH by examining the influence of light intensity and dye concentration on the degradation performance. The dye concentration ranged between 9-25 mg/L, while the extent of light intensity ranged from 1 to 11.2 W/m². The thin film durability was evaluated as well. To achieve the objective, a group of experiments were conducted at a dye concentration of 13 mg/L to examine the P25 film recyclability. After each experiment, the reactor was cleaned, and the dye solution was removed using MQ water. An amount of fresh MQ water was then circulated through the reactor while the UV light was ON for half hour to remove any remaining adsorbed dye molecules from the catalyst surface. After that, the immobilized catalyst was dried in an oven at 110C° for 3 hrs”.

“For the TOC test, samples of about 5 ml were taken using a syringe, filtered (0.45 µm) and assessed using a Shimadzu TOC-V CPN analyzer (Shimadzu, Japan). The obtained findings were then used to evaluate the dye mineralization as a percentage of carbon removal for TOC measurements”.

3. Are the results clearly presented? Can be improved.

Response: The above comment has been taken into consideration, and the authors have added the following paragraphs to improve the “results” section.

“3.4 Immobilized catalyst

3.4.1 Surface characterization  

Fig. 14 shows the thin film’s surface microstructure at A) low and B) high magnifications. It is observed that the TiO₂ nanoparticles were deposited on the glass substrate with relatively non-uniform distribution. The thin film microstructure mainly consists of agglomerated spheres that accumulate on each other, resulting in spherical particles’ multilayers, as shown in Fig. 14-A. In Fig. 14-B, the magnified image indicates that the texture was a rough surface with an irregular shape, resulting in surface heterogeneity. It is also obvious that the film microstructure refers to the presence of many voids of different sizes and shapes. These characteristics lead to obtaining a highly porous structure with different surface roughness. However, the presence of such surface properties is advantageous in terms of enhancing surface adsorption properties. The organic pollutants can easily attach to sharp and irregular edges compared to a smooth surface. Besides, the presence of such sharp and irregular edges increases the surface area available for molecules adsorption. Moreover, the light would easily penetrate the structure's surface pores, resulting in absorb large amounts of light and subsequently more degradation”. 

“Fig. 15 shows the SEM micrograph of the surface morphology of TiO2 thin film. It was noticed that the surface morphology mainly consists of a single or chain of like-cluster structures without a specific size that is uniformly distributed on the surface substrate. The captured image shows that the distribution of like-cluster structures leads to empty areas among them without particle aggregation. Such variance in areas appears a visible difference between the height of the like-cluster structures and actual film thickness. The inset in Fig. 15 displays a high magnification of a selected area, which presents both single and chain of like-cluster structures are formed due to TiO₂ particles agglomeration. Based on different chosen areas, the height of like-cluster structures was also determined. The SEM images showed that the height of like-cluster structures is ranged between 132 to 140 µm, as shown in Fig. 16”.

“3.4.2 Kinetics using a fixed catalyst 

The obtained results of DB15 degradation kinetics at different conditions for catalyst immobilization are shown in Table 8 and Fig 17. Table 8 demonstrates that linear fit was acquired for different initial concentrations. This refers to the pseudo first-order model that can describe the degradation kinetics of DB15 dye. For a catalyst density of 8.72 x 10⁴ Kg/m², it is observed the  value increases as the initial concentration is decreased. The  value increases from 5.65 to 35.11 x 10^-3 when the initial concentration is decreased from 25 to 9 mg/L. It is noteworthy that a significant variance was registered for ka values between slurry and immobilized catalyst. Such variation could be attributed to many factors. The main reason is that there is a clear difference in the experiments’ pH values. While the slurry catalyst experiments were performed at the optimized pH value (pH 4), the immobilized catalyst experiments were conducted at natural pH (about 5.5). As discussed earlier in the pH optimization section, this pH difference significantly impacts dye degradation. When the pH value decreases from 5.5 to 4, the dye degradation is improved dramatically. This is mainly attributed to enhancing adsorption properties because of the attractive force between the dye (included four sulfonate groups) and the positively charged catalyst surface. Hence, the degradation would be further increased. Another reason is the internal and external mass transfer effect. There are no limitations and effects for mass transfer between the bulk solution and catalyst surface for slurry catalysts. In contrast, there are some limitations for external mass transfer between the bulk concentration and catalyst surface. In addition, the internal mass transfer has major effects on the overall reaction rates. These limitations mainly control some reactions, as widely reported in the relevant literature [33].          

“The effect of light intensity on  values for catalyst immobilization are presented in Fig. 17. It is well known that light intensity plays a significant influence on the degradation reaction rate. The degradation rate constant typically has a power-law relation with light intensity. It has been reported that the degradation rate constant has a square root relation with the light intensity at the high range while the relation follows first order at the lower intensities range [27]. By plotting  values versus their corresponding light intensities, the light intensity-dependent equation was obtained. With the use of non-linear regression, the best fit equation of the experimental values was then determined. Based on the MINITAB software package (Version 19.2020.1) analysis, the obtained equation was: 

 ……………. (19)

It is concluded that the obtained equation is consistent with the relevant literature. Yatmaz et al. [30] found that  constant correlates to the square root of the light intensity through the degradation of RR dye over ZnO. As discussed earlier, the values of a and b for the slurry catalyst were 0.14 and 0.91, respectively. Chen and Ray [46] found a value of 0.84 for the photocatalytic degradation of 4-nitrophenol”.  

“3.4.3 Catalyst recyclability      

The reusability test considers of great importance for immobilized photocatalysts. Thus, a group of experiments were carried out at the same operating conditions to evaluate the extent of catalyst reusability. The recyclability was examined for up to four cycles, and the findings are displayed in Fig. 18. As can be seen, no significant change, only 2.6%, in the catalyst activity after the second cycle. Compared to the original activity, the third cycle shows the catalyst activity reduced by 5.6%. After the fourth cycle, it appears that the catalyst lost 9.71% of its activity, registering a higher loss of activity. In this cycle, the DB15 degradation removal was reduced to 82.33% from 91.18% for the first use. The obtained results indicated that the immobilized catalyst possesses high reusability. However, the loss of catalyst activity could be explained by two reasons. The catalyst loses some of its particles over the cycles, and the catalyst washing after cycles. The second reason is catalyst surface contamination. Some dye molecules could be accumulated over the cycles that could compete with the following experiments’ molecules for the limited active sites of the catalyst surface, resulting in a reduction in catalyst activity [48]”.

4. Are the conclusions supported by the results? Can be improved.

Response: The above comment has been taken into consideration, and the authors have added the following sentences to improve the “conclusions” section.

“The DB15 mineralization was also investigated. In terms of immobilized catalyst, P25 was coated on a Pyrex glass using a dip-coating device. The coated thin film surface was characterized using an SEM test. Using an immobilized catalyst, the influence of initial dye concentration and light intensity on the DB15 degradation kinetics were examined. The thin film catalyst reusability was also evaluated”.

“The results showed that DB15 mineralization at 40 mg/L reaches up to 83.64% after a reaction time of 16 hrs. For immobilized catalysts, the pseudo first-order model fits well the DB15 kinetics for different dye concentrations, while a power-law model describes the influence of light intensity on the degradation kinetics. The findings also revealed that the thin film was recyclable for up to four cycles with high durability”. 

5. The mechanism of the photocatalytic reaction should be further explored by the necessary experiments, such as the trapping experiments for hydroxide radical and the ESR experiments for the O₂. Some papers could be referenced to further improve the manuscript (e. g., Applied Catalysis B: Environmental 263 (2020)11730. Applied Catalysis B: Environmental 260 (2020) 118181. Applied Surface Science 498 (2019) 143850.).

Response: The authors really appreciate the reviewer’s comment, and they want to clarify the following:

The authors have finished an extensive investigation of the photocatalytic reaction mechanism for this model dye compound using the GCMS test. We have identified many intermediates. The manuscript is under preparation, and it will be submitted soon. However, the authors believe that the addition of such a large study would change the manuscript’s main objective from kinetics to degradation pathways. This would lead to making the manuscript inconsistent with the focus of the current manuscript. Hence, the authors have added a whole kinetic section regarding the use of immobilized catalysts. In addition, the authors have added/improved the related sections corresponding to the new section’s addition from the introduction until the conclusions. The details of the added section have been explained in the previous comments.   

5. Some characterization data of the TiO₂should be provided, such as SEM, TEM and XRD.

Response: The above comment has been taken into consideration. The authors have added a group of SEM micrographs regrading the surface characterization of the thin film of coated catalyst.

6. As a comparison, the degradation performance of colorless Bisphenol A (BPA) or phenol could be provided.

Response: The above comment has been taken into consideration, and the authors have added a comparison with phenol, or its derivatives as requested by the reviewer. For example, the authors refer to Line 720 as follows: 

“As discussed earlier, the values of a and b for the slurry catalyst were 0.14 and 0.91, respectively. Chen and Ray [46] found a value of 0.84 for the photocatalytic degradation of 4-nitrophenol”.

Round 2

Reviewer 2 Report

I would like to recommend this paper for publication, as it meets the publication criterion.