Pharmaceutical Micropollutant Treatment with UV–LED/TiO₂ Photocatalysis under Various Lighting and Matrix Conditions

Round 1

Reviewer 1 Report

Monika Snowdon et al. provided an article focused on the influence factor on porous Ti film-TiO₂ for degradation of 18 PPCPs. They discussed the influence of pH, the amount of methanol carrier solvent and UV light irradiation parameter. This is an important theme. I suggested it to be accepted after minor revision. I consider that the authors should improve the manuscript in the following points.

1.       In line 41, “by promoting an electron from the conductance band to the valence band.” is totally wrong, the authors should correct it.

2.       In line 54, “However methanol is known to scavenge hydroxyl radicals and electron holes ones produced in photocatalysis by TiO2” is totally wrong, the authors should correct it.

3.       Some important review and research articles in this field should be cited such as Materials 2021, 14, 560; Materials 2020, 13, 1734; Org. Bio. Chem., 2021, 19, 2192-2197, Catal. Sci. Technol., 2018, 8, 2030.

4.       More carrier solvent for PPCPs other than methanol should be tested.

Author Response

1. In line 41, “by promoting an electron from the conductance band to the valence band.” is totally wrong, the authors should correct it.

Corrected - “by promoting an electron from the valence band to the conduction band.”

2. In line 54, “Methanol has been demonstrated to scavenge hydroxyl radicals and electron holes ones produced in photocatalysis by TiO2” is totally wrong, the authors should correct it.

            Corrected – “However methanol is known to scavenge hydroxyl radicals and holes produced in the photocatalysis cycle by TiO2” with added references to work by Nosaka, Yoshio, and Atsuko Y. Nosaka. "Identification and roles of the active species generated on various photocatalysts."

References: Photocatalysis and Water Purification (2013): 3-24. And Environ. Sci. Technol. 2007, 41, 13, 4720–4727. It is also discussed Arlos, M. J., Liang, R., Fong, L. C. L. C., Zhou, N. Y., Ptacek, C. J., Andrews, S. A., & Servos, M. R. (2017). Influence of methanol when used as a water-miscible carrier of pharmaceuticals in TiO2 photocatalytic degradation experiments. Journal of environmental chemical engineering, 5(5), 4497-4504.

3. Some important review and research articles in this field should be cited such as Materials 2021, 14, 560; Materials 2020, 13, 1734; Org. Bio. Chem., 2021, 19, 2192-2197, Catal. Sci. Technol., 2018, 8, 2030.

            The additional citations have been added in the introduction.

4. More carrier solvent for PPCPs other than methanol should be tested.

The main focus in our work is the UV-light rather than the methanol solvent. The use of methanol with the membranes in our work has a similar trend to those found in the powder work (Arlos, Maricor J., et al. "Influence of methanol when used as a water-miscible carrier of pharmaceuticals in TiO2 photocatalytic degradation experiments." Journal of environmental chemical engineering 5.5 (2017): 4497-4504.) wherein from Table 1, MeOH is the preferred solvent. Other works have attempted ethanol and acetonitrile, and are now also referenced to in the text.

 And Liang, Robert, et al. "Utilizing UV-LED pulse width modulation on TiO2 advanced oxidation processes to enhance the decomposition efficiency of pharmaceutical micropollutants." Chemical Engineering Journal 361 (2019): 439-449.To further justify the sole use of MeOH in the work. 

Reviewer 2 Report

The authors present a manuscript focusing on the preparation and characterization of a TiO₂-coated membrane and its photocatalytic activity in pharmaceutical micropollutants. Here are some comments that need to be addressed:

1.      The sentence in line 41-43 is not true

“Irradiation of TiO2 by UV light forms electron-hole pairs by promoting an electron from the conductance band to the valence band. The electron-hole pairs can then undergo redox reactions with water and oxygen to form reactive oxygen species (ROS).”

2.      There is no single evidence that the membrane is coated with TiO₂.

3.      The composition of the TiO₂ phase significantly impacts optical and surface properties. Please justify.

4.      Since the photocatalytic results are highly dependent on the morphology of the material, SEM, UV-Vis, Raman data should be provided.

5.      I don’t understand the expressions: “natural experimental pH”, “cumulative rates”. Please explain.

6.      Why does the negative surface charge of PTT reduce the adsorption and the degradation efficiency of positively charged compounds (line 68-71)?

7.      To what objective were the deuterated standards applied?

8.      Has the isoelectric point value and band gap value been measured for the synthesized membrane? Explain on what basis there were given.

9.      How the apparent reaction rate constants were determined. Please present the kinetic plots.

10.  Since the reaction rate constants strongly depend on the charge of both the catalyst surface and the test compound, the ionic composition (mole fractions) of the reaction mixture should be considered.

11.  The correlation between structural and morphological findings and photocatalytic performance should be discussed.

12.  The tested mixture contains compounds that may undergo reductive degradation. The addition of methanol can promote this process by scavenging hydroxyl radicals. The influence of methanol addition on the mechanism of micropollutants degradation should be thoroughly discussed.

13.  Line 297-299:

“Based on these results decreasing UV light exposure in UV/TiO2 reactors for PPCPs has little to no effect on degradation rates, and could greatly increase photonic efficiency.”

Explain it.

14.  Why are the apparent rate constant in Tables S6, S9, and S10 negative? I also do not understand where the negative R² values in these tables come from. Furthermore, low R² values indicate that the chosen kinetic model is probably not appropriate. Perhaps the rate of reaction is significantly influenced by other factors, not included in this model.

Based on the comments given above, additional experiments and a careful analysis of the results obtained are needed.

Author Response

1. The sentence in line 41-43 is not true

“Irradiation of TiO2 by UV light forms electron-hole pairs by promoting an electron from the conductance band to the valence band. The electron-hole pairs can then undergo redox reactions with water and oxygen to form reactive oxygen species (ROS).”

Corrected - “by promoting an electron from the valence band to the conduction band.”

2. There is no single evidence that the membrane is coated with TiO2.

The Raman spectra show the PTT surface has anatase phase TiO2, and uncoated PTi has no peaks indicative of crystalline TiO2. XRD shows the PTT also has rutile and titanium TiO2 crystal structures. The bandgap energy, derived from a Tauc plot, is 3.0eV. This bandgap corresponds to crystalline TiO2 for the nanomaterials.

The process is similar to Liang, Robert, et al. "Utilizing UV-LED pulse width modulation on TiO2 advanced oxidation processes to enhance the decomposition efficiency of pharmaceutical micropollutants." Chemical Engineering Journal 361 (2019): 439-449.

The composition of the TiO2 phase significantly impacts optical and surface properties. Please justify.

TiO2 is a multifunctional material. We identified that the porous titanium-titanium dioxide has an anatase surface based on Raman spectroscopy, which is a better photocatalyst than rutile TiO2 as addressed in Scientific Reports 2014,4, 4043. Anatase has an indirect bandgap (~3.2 eV) whereas rutile has a direct bandgap (~ 3.0 eV).

4. Since the photocatalytic results are highly dependent on the morphology of the material, SEM, UV-Vis, Raman data should be provided.

We have added reference to previous work on our TiO₂ membranes Liang, Robert, et al. "Utilizing UV-LED pulse width modulation on TiO2 advanced oxidation processes to enhance the decomposition efficiency of pharmaceutical micropollutants." Chemical Engineering Journal 361 (2019): 439-449.

5. I don’t understand the expressions: “natural experimental pH”, “cumulative rates”. Please explain.

“natural experimental rates:, we agree that this is an unclear term, we edited now as “experimental pH”

“Cumulative rate constant” is the same terminology we used in a previous paper (Liang, 2018) and it is the kinetic rate constant that was calculated from the average of all compound concentrations to get the total parent compound concentration at each time point. The normalized total parent compound concentration is then used to obtain the cumulative kinetic rate constant from the slope of the In(C/Co) vs. t plot.   We have change cumulative degradation rate to cumulative kinetic rate constant based on the comment.

6. Why does the negative surface charge of PTT reduce the adsorption and the degradation efficiency of positively charged compounds (line 68-71)?

We have corrected the sentence, the PTT is positive at pH 5 as per the zeta potential in the supplemental information that we have now added.

7. To what objective were the deuterated standards applied?

Samples were spiked with deuterated standards to a concentration of 20 µg/L and then underwent solid phase extraction following the method from Arlos et al. (2017)

8. Has the isoelectric point value and band gap value been measured for the synthesized membrane? Explain on what basis there were given.

Yes, the isoelectric point values have been measured as per our answer to question 6 and the band gaps were measure in Liang, 2019.

9. How the apparent reaction rate constants were determined. Please present the kinetic plots.

Please refer to the supplemental information which is based on a previous publication (Liang 2018, and Arlos et al. 2017) that addresses this issue. Degradation rates are presented in Table S6 with the R2 values so the kinetic plots can be inferred. 

10. Since the reaction rate constants strongly depend on the charge of both the catalyst surface and the test compound, the ionic composition (mole fractions) of the reaction mixture should be considered.

We conducted the testing under MilliQ water and adjusted the pH with HCl or NaOH accordingly.

11. The correlation between structural and morphological findings and photocatalytic performance should be discussed.

We chose not to engage in this part because it had been detailed in previous studies (Liang 2018, Arlos et al. 2017)

12. The tested mixture contains compounds that may undergo reductive degradation. The addition of methanol can promote this process by scavenging hydroxyl radicals. The influence of methanol addition on the mechanism of micropollutants degradation should be thoroughly discussed.

The Arlos paper has previously discussed this event. We have added some relevant references to the introduction: Nosaka, Yoshio, and Atsuko Y. Nosaka. "Identification and roles of the active species generated on various photocatalysts." Photocatalysis and Water Purification (2013): 3-24. And Environ. Sci. Technol. 2007, 41, 13, 4720–4727

13. Line 297-299:

“Based on these results decreasing UV light exposure in UV/TiO2 reactors for PPCPs has little to no effect on degradation rates, and could greatly increase photonic efficiency.”

Explain it.

Based on work in Liang, 2019, there are mass transfer limitations. These observations indicate that powdered samples have more of a surface area for pharmaceuticals reactivity whereas the membranes, such as those presented in this work, have limited surface sites for reaction. The light irradiation has to pass through the water to hit the membrane instead of stirring with powder that has a tendency to remain on the water surface level where the light reacts the most. Therefore, the reaction rate is faster.

14. Why are the apparent rate constant in Tables S6, S9, and S10 negative? I also do not understand where the negative R2 values in these tables come from. Furthermore, low R2 values indicate that the chosen kinetic model is probably not appropriate. Perhaps the rate of reaction is significantly influenced by other factors, not included in this model.

The data is based on the electrostatic interactions of the material. We have changed Kapp to |Kapp| to show magnitude and removed the negative sign. All the supplementary tables have been updated to reflect this change. The negative sign was due to how the first-order equation was applied in – In(C/Co)=kt. The negative sign is no longer missing. The results will not show a perfect R² for the PPCPs due to interactions at the water interface. For instance, a solution with a pH of 5 has the negatively charged compounds degrading first compared to neutral and positive compounds due to electrostatics, as shown in our work.

Based on the comments given above, additional experiments and a careful analysis of the results obtained are needed.

Reviewer 3 Report

This paper deals with a porous titanium-titanium dioxide membrane for removing a cocktail of 18 pharmaceuticals and personal care products (PPCPs) by UV-LEDs. The natural organic matter removal from waste water processes were investigated by several characterization techniques.

The studies were quite systematic and the resulted were well organized by the authors. I’d like to recommend the publication of this paper in photochem after revision.

1. Author should provide the crystalline structure, morphology, and surface area of the porous titanium-titanium dioxide membrane for understanding the catalytic characterizations.
2. The constants of rate and adsorption should be calculated from Langmuir-Hinshelwood kinetics by different concentrations.
3. The authors should explain if the intermediate products were found in the processes.

Author Response

1. Author should provide the crystalline structure, morphology, and surface area of the porous titanium-titanium dioxide membrane for understanding the catalytic characterizations.

We have added reference to previous work on our TiO2 membranes Liang, Robert, et al. "Utilizing UV-LED pulse width modulation on TiO2 advanced oxidation processes to enhance the decomposition efficiency of pharmaceutical micropollutants." Chemical Engineering Journal 361 (2019): 439-449.

1. The constants of rate and adsorption should be calculated from Langmuir-Hinshelwood kinetics by different concentrations.

We understand that certain molecules behave in the first order manner, however, we have used the abbreviated Langmuir-Hinshelwood kinetics model due to the low concentration and shown in Table S7 different concentrations. The neutral positive compounds do not fit the first order rate.

1. The authors should explain if the intermediate products were found in the processes.

The line 294 “Intermediate products in the processes were not detected in this iteration of the work.” Has been added

Round 2

Reviewer 2 Report

Comments for author File: Comments.pdf

Author Response

1. Accepted

2. Did the authors synthesize a titanium-coated membrane? If so, they should show images confirming obtaining this material.

If the process is similar to Liang R. (2018), how can you be sure that the material obtained is identical? I emphasize once again that the results of XRD and UV-vis measurements should be included in the paper (please include appropriate figures).

We have included the figures in the SI.

3. We come back to the points mentioned above again. Images confirming the obtained results (here Raman spectra) should be included in the work.

We have added the Raman to the SI.

4.If the authors synthesized the membrane according to Liang's procedure (see lines 101-102), they must show the physicochemical characteristics of the material obtained (appropriate figures and images must be included in the paper or SI). It is not the same material as obtained by Liang et al. 2018. The study lacks an analysis of the surface morphology of the obtained membrane. The surface morphology can significantly affect the kinetic rate constants.

The surface characterization has been added to the SI.

Please provide an explanation of the cumulative rate constant in the paper.

“Cumulative kinetic rate constant is the kinetic rate constant that was calculated from the average of all compound concentrations to get the total parent compound concentration at each time point. The normalized total concentration of the parent compound is then used to obtain the cumulative kinetic rate constant from the slope of the ln(C/Co) vs. t plot.”

We have added the explanation in the text.

6. Accepted

7. Please explain why deuterated standards were used in the analysis of the products. Both the HPLC and LCMS techniques do not require the use of deuterated compounds. The authors should explain in the text what was the purpose of using deuterated compounds in the analysis.

We have added the explanation: “Deuterated samples used to decrease chromatography time because they increase the throughput and lower the rejection rate.”

8.Please put the Tauc diagrams used to determine the material's Ebg in the paper or SI.

We have placed them in the SI.

9. The kinetic rate constants along with R2 were determined from the kinetic curves. Please, put these graphs in your paper or SI. The curves in Arlos et al. 2016 are not the results obtained by the authors.

We have added the kinetic curves with linear fitting for the different pHs and light sources used in the SI as well as for the compounds CBZ, GFZ and NFLX that we discussed in depth in the paper.

10. “These fractional charges of the different functional groups of each compound were added to give the over- all charge of the compound” (line 234-236).

Calculating the net charge of a mixture is not appropriate as each component in a mixture will interact differently with the membrane surface. Depending on the pH of the solution, the tested compounds appear in the reaction mixture in various forms (H2A +, HA, A-). Similarly, the membrane surface has a different charge depending on the pH of the reaction mixture. This results in the presence or absence of membrane surface-organic compound interactions, which can significantly affect the efficiency and rate of degradation of the organic compound. Please discuss this issue.

We have added this explanation to the discussion. The negatively charged compounds were observed to be pH dependant.

11. The authors should discuss the correlation between structural and morphological findings and photocatalytic performance and then compare their results with those described in the literature (Arlos et al. 2017; Liang et al. 2018).

We have added to the discussion the different characteristics of the anatase surface and its correlation to photocatalytic performance. We identified that the porous titanium-titanium dioxide has an anatase surface based on Raman spectroscopy, which is a better photocatalyst than rutile TiO2 as addressed in Scientific Reports 2014,4, 4043. Anatase has an indirect bandgap (~3.2 eV) whereas rutile has a direct bandgap (~ 3.0 eV).

12. And again:

The tested mixture contains compounds that may undergo reductive degradation. The addition of methanol can promote this process by scavenging hydroxyl radicals. The influence of methanol addition on the mechanism of micropollutants degradation should be thoroughly discussed.

We have added to the introduction section.

13. Please put this explanation in the paper.

It is now in the paper.

14. The R2 coefficient is a measure of the fit of the model. To talk about the correct fit of the model, R2 should have a value of at least 0.9. The kinetic curves drawn from the measuring points should indicate the correct kinetic model.

“The results will not show a perfect R2 for the PPCPs due to interactions at the water interface. For instance, a solution with a pH of 5 has the negatively charged compounds degrading first compared to neutral and positive compounds due to electrostatics, as shown in our work.” – see Comment 10.

The compounds that do not have opposite charges to the membrane will generally not follow first order models which we have observed from the data we obtained and added a sentence explaining such.

On the basis of the comments given above, major revision is needed

Author Response File: Author Response.docx