Integrated Photocatalytic Oxidation and Adsorption Approach for the Robust Treatment of Refinery Wastewater Using Hybrid TiO₂/AC

Round 1

Reviewer 1 Report

This work evaluated the removal of hydrocarbon (HC) pollutants from petroleum refinery wastewater by integrated photocatalytic oxidation and adsorption using TiO2/AC hybrid material. Developing the integrated photocatalytic oxidation and adsorption system applied in refinery wastewater is interesting, and some minor issues need to be addressed before publishing.

 

(1) The curves in  Figure 1 are unclear, and Figure 1 needs to be polished.

(2) The words in  Figure  S1 could be clearer, and Figure S1 needs to be polished.

(3) The contribution rate of adsorption and photocatalytic oxidation needs to be given.

Author Response

Reviewer 1

We are thankful to the honourable reviewer for considering our work for evaluation. We believe that all the comments suggested by the reviewers are highly valuable for the improvement of the submitted manuscript. We have carefully studied all of the comments and have made respective changes in the revised manuscript which are highlighted in yellow text. The detailed explanations to each comment are sequentially provided below. We hope the explanations and answers provided below would fulfill the requirements of the worthy reviewer.

Comments

This work evaluated the removal of hydrocarbon (HC) pollutants from petroleum refinery wastewater by integrated photocatalytic oxidation and adsorption using TiO₂/AC hybrid material. Developing the integrated photocatalytic oxidation and adsorption system applied in refinery wastewater is interesting, and some minor issues need to be addressed before publishing.

• The curves in  Figure 1 are unclear, and Figure 1 needs to be polished.

Reply: The resolution of the Figure 1 has been improved, as suggested by the worthy reviewer.

• The words in  Figure  S1 could be clearer, and Figure S1 needs to be polished.

Reply: The  Figure 1 has been carefully edited and polished to make the words visible.

(3) The contribution rate of adsorption and photocatalytic oxidation needs to be given.

Reply: The contribution rate of adsorption and photocatalytic oxidation has been given in table 8, in which the data shows that only adsorption over AC attains 92 % COD removal in 120 min adsorption time, whereas only photocatalytic oxidation realizes 93 % COD removal in 90 min. However simultaneous or integrated photocatalytic oxidation and adsorption achieve 95% COD removal in merely 50 min.   

Author Response File: Author Response.pdf

Reviewer 2 Report

This work portrays the current theme of the search for the treatment of a pollutant stream derived from the refinery activity. This study tests novelty treatment to reduce the organic load of the effluent and decrease the negative impact of this industrial activity.

This work has the potential to be published, however, it could be improved in several aspects until its final publication, namely with the following suggestions (small suggestions):

1) The authors should add a table with a typical composition of the pollutant effluent in the Introduction section.

2) Do you think that this technology can be applied for other streams? If yes, mentioned that in the conclusion.

Author Response

Reviewer 2

We are thankful to the honourable reviewer for considering our work for evaluation. We believe that all the comments suggested by the reviewers are highly valuable for the improvement of the submitted manuscript. We have carefully studied all of the comments and have made respective changes in the revised manuscript which are highlighted in yellow text. The detailed explanations to each comment are sequentially provided below. We hope the explanations and answers provided below would fulfill the requirements of the worthy reviewer.

Comments

This work has the potential to be published, however, it could be improved in several aspects until its final publication, namely with the following suggestions (small suggestions):

• The authors should add a table with a typical composition of the pollutant effluent in the Introduction section.

Reply: In section introduction, table 1 has been included which shows typical composition of the refinery waste water.

• Do you think that this technology can be applied for other streams? If yes, mentioned that in the conclusion.

Reply: Yes, we strongly believe that this technology can also be applied for other streams, containing various types of organic pollutants. The statement has been included in the conclusion section.

Author Response File: Author Response.pdf

Reviewer 3 Report

This manuscript reported the removal of hydrocarbon pollutants from petrochemical industry with AC and TiO2/AC. The control experiments for adsorption and adsorption coupled with photocatalysis were investigate in details and adsorption isotherms behavior was also included. However, there are several major parts should be investigated and discussed more in details to clarify the reaction mechanism.

1.       What the AC? Is it active carbon? Please provide more in details the chemicals where they are from and their purity. Also, the author claims that the AC and TiO2/AC are porous structures, so what’s the porous data based on N2 adsorption and desorption experiments? The characterization is necessary to any porous materials.

2.       The author claims that the strong adsorption peak at 500 cm-1 is ascribed to the vibration of Ti-O bonds in the TiO2 lattice but failed to present in the Figure 1. These data should be provided and label it clearly.

3.       What’s the difference in SEM image between AC and TiO2/AC? Do the brighter sites belong to TiO2 in Fig 2b?  The scale bar for both images in Figure2 should be constant?

4.       Is EDX done on SEM instrument? SEM-EDX is not a quantitative analysis tool to give the composition of as-achieved techniques. Other analysis techniques such as ICP should be used to quantify the composition. Besides, it seems that there are a lot of Si and Zn elements as well, these are also active sites for adsorbing HCs.

5.       The author studied the adsorption over AC by control experiments but failed to list in details the experiment conditions. For example, what’s the temperature and adsorption time in Fig 4? What’s the weight of AC and time for Fig 5 and what’s the weight of AC and temperature in Fig6? Please state them accordingly.

6.       Why did the model HCs solution have a pH value of 3? Did any acid added into the solution? Also, since the presence of heteroatoms such as S and N are abundant in the petrochemical wastewater, what about these species affect the purification reaction? Will they contaminate the TiO2/AC or be competitive for adsorption or reaction?

7.        Did the author characterize the catalysts after adsorption ? The author claimed the oxidation reaction happened with TiO2/AC but failed to provide the directly data for the oxidation products. Besides, how about the behavior and durability of catalysts after recycling?

8.       What’s the difference between Figure9B and Figure 9D? It seems there is no difference for purified water between AC and TiO2/AC.

9.       Experiments should be provided more in details such as the detailed operation condition for SEM, FT-IR, XRD and their brands. Also, what’s the wavelength for UV used? TIO2 is sensitive to the UV wavelength based on their TiO2 nanoparticle size.

10.   The Figures are poorly organized. Should be inserted into the main text accordingly rather than just being assembled them together.

Author Response

Reviewer 3

We are thankful to the honourable reviewer for considering our work for evaluation. We believe that all the comments suggested by the reviewers are highly valuable for the improvement of the submitted manuscript. We have carefully studied all of the comments and have made respective changes in the revised manuscript which are highlighted in yellow text. The detailed explanations to each comment are sequentially provided below. We hope the explanations and answers provided below would fulfill the requirements of the worthy reviewer.

Comments

This manuscript reported the removal of hydrocarbon pollutants from petrochemical industry with AC and TiO2/AC. The control experiments for adsorption and adsorption coupled with photocatalysis were investigate in details and adsorption isotherms behavior was also included. However, there are several major parts should be investigated and discussed more in details to clarify the reaction mechanism.

1. What the AC? Is it active carbon? Please provide more in details the chemicals where they are from and their purity. Also, the author claims that the AC and TiO2/AC are porous structures, so what’s the porous data based on N2 adsorption and desorption experiments? The characterization is necessary to any porous materials.

Reply: Yes, AC stands for Activated Carbon. The AC was purchased from Duksan Chemicals. The BET surface area of the AC was about 3000 m²/g. The information has been included in section experimental,   

2. The author claims that the strong adsorption peak at 500 cm-1 is ascribed to the vibration of Ti-O bonds in the TiO2 lattice but failed to present in the Figure 1. These data should be provided and label it clearly.

Reply: The peaks in FTIR spectra has been labeled accordingly, as suggested by the worthy reviewer.

3. What’s the difference in SEM image between AC and TiO2/AC? Do the brighter sites belong to TiO2 in Fig 2b?  The scale bar for both images in Figure2 should be constant?

Reply: Thank you for the correction, the bright sites show the TiO₂ crystallites dispersed on the surface of AC, which is the main difference between the SEM images of AC and TiO₂/AC. The scale bar for both the images has been kept constant.

We agree with the opinion of the worthy reviewer that scale bar of both images should be same however, due to very poor resolution of 1000X SEM images of TiO₂/AC, the 500X micrograph  was selected for display.

4. Is EDX done on SEM instrument? SEM-EDX is not a quantitative analysis tool to give the composition of as-achieved techniques. Other analysis techniques such as ICP should be used to quantify the composition. Besides, it seems that there are a lot of Si and Zn elements as well, these are also active sites for adsorbing HCs.

Reply: Since the AC used was commercial, the EDX analysis was merely aimed to confirm presence of Ti. The amount of Si and Zn is very small (Less than 0.7 %), to be considered as major active sites for adsorption.   

5. The author studied the adsorption over AC by control experiments but failed to list in details the experiment conditions. For example, what’s the temperature and adsorption time in Fig 4? What’s the weight of AC and time for Fig 5 and what’s the weight of AC and temperature in Fig6? Please state them accordingly.

Reply: The experimental conditions has been provided in all the captions of figures.

6. Why did the model HCs solution have a pH value of 3? Did any acid added into the solution? Also, since the presence of heteroatoms such as S and N are abundant in the petrochemical wastewater, what about these species affect the purification reaction? Will they contaminate the TiO2/AC or be competitive for adsorption or reaction?

Reply: Yes, acid (Sulphuric acid) was added to achieve pH 3 of model HCs solution. pH can greatly influence the surface charge of the catalyst, the state of the organic compounds and the mechanism of the oxidation process. The surface charge of the catalyst can be characterized by zero point charge pH (pH_zpc), which has been reported to be 6.25 for TiO₂. To absorb the anions on the surface of TiO₂, and for and electron-hole pair generation, the pH must be lower than its pH_zpc. As the OH^. radicals are formed by the interaction of OH^- (hydroxyl ions) with the +ive holes, therefore adsorption of OH^- will be favored at low pH, and hence maximum degradation of model hydrocarbons was achieved at pH 3. Heteroatoms will not affect the purification reaction as they are compatible with the hybrid TiO₂/AC,

7. Did the author characterize the catalysts after adsorption? The author claimed the oxidation reaction happened with TiO2/AC but failed to provide the directly data for the oxidation products. Besides, how about the behavior and durability of catalysts after recycling?

Reply:  Although the catalyst was not characterized after adsorption however the results of % removal of hydrocarbons and % COD removal directly supports the mechanism of phenomena (adsorption or oxidation process) involved. In our previous study (Water Environment Research, 92(12), 2086-2094.) we have already described that during photocatalytic oxidation of refinery wastewater not all the parent hydrocarbons are completely mineralized, rather give various oxidation products. Here we have provided GCMS analysis of the refinery wastewater sample before and after treatment (oxidation and adsorption with TiO₂/AC) to identify the individual hydrocarbon compounds in both  samples.

Regeneration studies of hybrid adsorbent materials TiO₂/AC were carried out by chemical treatment. Various organic solvents including methanol, ethanol and methyl ethyl ketone were used for the washing of TiO₂/AC, out of which Methyl ethyl ketone (MEK) give good result, the regenerated TiO₂/AC was reused without no loss in efficiency.    

8. What’s the difference between Figure9B and Figure 9D? It seems there is no difference for purified water between AC and TiO2/AC.

Reply: Figure 9B and Figure 9D shows near complete removal of all the hydrocarbons from the sample. However, the adsorption over AC completes in 90 min (Fig. 9B), on the other hand simultaneous oxidation and adsorption over TiO₂/AC takes in case of Figure 9B, sample takes only 50 min.    

9. Experiments should be provided more in details such as the detailed operation condition for SEM, FT-IR, XRD and their brands. Also, what’s the wavelength for UV used? TIO2 is sensitive to the UV wavelength based on their TiO2 nanoparticle size.

Reply: The details of the operational conditions for SEM, FTIR, XRD has been included in the paper as given below;

FTIR analysis of the samples was carried out by FTIR spectrophotometer (Perkin Elmer, Spectrum II), integrated with ATR sample base diamond plate. The spectra of the samples were recorded at resolution 4 cm^-1 and in scan range 4000 to 450 cm^-1 by NTOS2 software. The surface morphology of the materials was analyzed by scanning electron microscope (Hitachi S-4700). The elemental analysis of the materials was carried out by energy dispersive X-ray analysis, through X-ray source equipped with SEM. XRD analysis of the materials was performed by using X- ray diffractometer (Xpert Philips). The source of radiation of X-ray diffractometer was CuKα and λ = 1.54 Å was the radiation wavelength. The diffraction patterns of the powder samples were recorded at the angle range of 10 to 80 degrees.

The UV lamp used was mercury 400W (200 nm-550 nm), having highest irradiation peak at 365 nm.

10. The Figures are poorly organized. Should be inserted into the main text accordingly rather than just being assembled them together.

Reply: All the figures have been rearranged to positions where they appear in t

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

The revised version answered most of my comments. I agree to publish it now.