Toward Scaling-Up Photocatalytic Process for Multiphase Environmental Applications
, Daniela Meroni 3, Carlo Pirola 3, Claudia Letizia Bianchi 3 and Daria Camilla Boffito 1,*Round 1
Reviewer 1 Report
The authors in this review discussed the environmental issues of new materials used in photocatalysis for water and air treatment. It is a review that adequately describes the future bases that new research should take into account in order to make scaling up to a practical application feasible. However, the manuscript should be completed according to the following recommendations:
In lines 73-79, It should be indicated that catalyst blockage due to the presence of organic matter can be avoided by pretreatment in a biological system. Please include a reference with an example.
In the Introduction section as well as it has been pointed out the challenges of photocatalysis for water treatment you should mention additional information regarding air treatment.
Concerning Section 2.1, as a general comment, due to the high number of citations that you have included, it would be a helpful if you could try to summarise in a table the cases as you have classified them in the text for a easy reading. In the main text it should be discussed those aspects of the design that affect performance.
In lines 142-143, says: "The choice of the membrane materials is limited by the oxidative stress caused by the photocatalyst material" In what meaning does oxidative stress occur? Please explain.
In lines 184-199, it should be made clear the importance of the reactor design, type of inlet, whether it has a deflector plate, etc.
In the statement of lines 306-307 it should be considered the hydrophobicity.
In line 378, when it says the term "detoxification", does it mean toxicity?
In Section 2.3, it should be indicated, the cost of the reagents, investment in the capital cost of the equipment, maintenance, etc. It should be summarised in a table with a more detailed cost analysis with different photoreactor configurations and future trends.
In Section 3.3, regarding catalyst deactivation, it should be provided in more detail about each regeneration method, indicating its advantages and disadvantages.
Author Response
The authors in this review discussed the environmental issues of new materials used in photocatalysis for water and air treatment. It is a review that adequately describes the future bases that new research should take into account in order to make scaling up to a practical application feasible. However, the manuscript should be completed according to the following recommendations
Response:
Thank you for reviewing our manuscript. We have responded to your valuable comments as follows.
Comment 1
In lines 73-79, It should be indicated that catalyst blockage due to the presence of organic matter can be avoided by pretreatment in a biological system. Please include a reference with an example.
Response:
Thank you for your comment. We have indicated this point as per your suggestion as follows.
“The active sites could be maintained by prior biological treatment, which also provides a clearer medium and reduces the competition on the ROSs [23].”
Comment 2
In the Introduction section as well as it has been pointed out the challenges of photocatalysis for water treatment you should mention additional information regarding air treatment.
Response:
Thank you for your comment. We responded to this comment by adding the following paragraph.
“ Photocatalysis towards air purification faces some technological issues as well depends on the specific application (purification of industrial polluted air, photoconversion of toxic gas into valuable products, self-cleaning systems, photocatalytic filters …etc) [32]. For example, since the photocatalysis suffers from the poor mass transfer, engineering of photoactive materials with unique properties and building special photo-reactors may resolve/reduce this problem. “
Comment 3
Concerning Section 2.1, as a general comment, due to the high number of citations that you have included, it would be a helpful if you could try to summarise in a table the cases as you have classified them in the text for a easy reading. In the main text it should be discussed those aspects of the design that affect performance.
Response:
Thank you for your comment. We already provided Table 1 to summarize the studies reported in section 2.1.
Comment 4
In lines 142-143, says: "The choice of the membrane materials is limited by the oxidative stress caused by the photocatalyst material" In what meaning does oxidative stress occur? Please explain.
Response:
Thank you for your comment. The produced ROS during the photocatalytic reaction may damage the body of the membranes, so the choice of membrane material should consider the durability of the membrane under the oxidative stress caused by the photocatalytic reaction.
Comment 5
In lines 184-199, it should be made clear the importance of the reactor design, type of inlet, whether it has a deflector plate, etc.
Response:
Thank you for your comment. Actually, in these types of reactors, the catalyst fully covered the carrier surfaces and directly illuminated by near lamps with no deflectors. Also, none of these studies reported the effects of inlets or outlets, perhaps because the solution was directly contacted the coated catalyst and there is no expected influence for the inlet or the outlet.
Comment 6
In the statement of lines 306-307 it should be considered the hydrophobicity.
Response:
Thank you for your comment. We totally agree with you. Hydrophobicity may play an important role especially for the adsorption of NOM on the catalyst surface. However, it was not reported in the cited study. In our opinion, it might be unknown for the authors of this study because of the complex composition and unknown structures of the NOM in both commercial and natural matrices.
Comment 7
In line 378, when it says the term "detoxification", does it mean toxicity?
Response:
Thank you for your comment. The “detoxification” term means the minimization of toxicity. It was assessed by the survival of certain species of microorganisms. Some studies use the detoxification for evaluating the photocatalytic performance.
Comment 8
In Section 2.3, it should be indicated, the cost of the reagents, investment in the capital cost of the equipment, maintenance, etc. It should be summarised in a table with a more detailed cost analysis with different photoreactor configurations and future trends.
Response:
Thank you for your comment. The costs of water/wastewater treatment by photocatalysts were scarcely reported in the literature. Most of recent studies reported novel complicated catalysts, which are not commercially produced. Furthermore, the ignorance of reactor design in most of the studies perpetuated the difficulty of cost estimation. We have discussed this point by adding the following paragraph, but there is no substantial data in the literature to present in a table.
“The cost estimation of photocatalytic water/wastewater treatment on large scale was hindered by the lack of information about the commercial prices of novel catalysts. In fact, most of the recent composites used for enhancing the photocatalysis under visible light are not produced in commercial scales. Furthermore, the required amounts for the treatment of industrial wastewaters are elusive to identify due to the rare studies on real wastewater as discussed in the previous section. Therefore, only a few studies estimated the costs of wastewater treatment by photocatalysis [14,89,90]. However, these few studies are for specific cases regarding the pollutants and photocatalysts, and hence they cannot present substantial generic costs.”
Comment 9
In Section 3.3, regarding catalyst deactivation, it should be provided in more detail about each regeneration method, indicating its advantages and disadvantages.
Response:
Thank you for your comment. As suggested more details regarding the deactivation, regeneration and preventing the quick deactivation were added as follows.
“Throughout the literature, the deactivation process depends mainly on the characteristics of the photocatalysts, the type and concentration of pollutants. Deactivation is most severe in the presence of aromatic compounds [143,144], however it has been reported for a wide range of species [145]. For instance, van Dijk et al. reported photocatalyst deactivation within 80 min of photocatalytic operation for the oxidation of cyclohexane [128]. Despite the importance of this topic for the commercial application of photocatalytic technologies, comparatively few studies have investigated the mechanisms of deactivation, regeneration and the photocatalyst lifetime.
Several strategies to reactivate the photocatalyst have been proposed, including treatment with water vapor [128], high temperature treatment [128,145], oxidation with H2O2 [145] and UV irradiation [146]. The use of a more oxidizing atmosphere and better mass transfer have been reported to slow down deactivation [144]. Simple washing with water or reagents and organic solvents could be used also to regenerate the deactivated photocatalysts. Djellabi et al. reported the deactivation of TiO2 P25 during the photocatalytic reduction of Cr(VI) under natural solar light [147]. It was found that 39% of reduced Cr(VI) was deposited as Cr(III) on the surface of P25 which turns the surface of P25 into green. The regeneration of P25 surface was carried out by the sequential extraction, and the leaching of Cr(III) was found to be 90 and 42% after three washing by citric acid and EDTA, respectively. However, the regeneration of photocatalysts at large scale is not recommended due to the time consuming and cost effective. The use of vacuum ultraviolet or nonthermal plasma have been suggested to prevent the deactivation of photocatalysts or to enhance their lifetime. In fact, the application of the last two routes is quite hard at large scale for the treatment of large volumes. Engineering of photocatalytic materials may reduce the deactivation process. Chen et al. reported that the deactivation of TiO2 P25 during the decomposition of VOCs in air phase is due to the adsorption of benzaldehyde intermediate on P25’ surface which characterized by their highly reaction energy of aromatic ring opening [148]. The deactivation causes a dramatic decrease in VOCs reduction from 72.4% to 12.3% with prolonged time. However, under the same conditions, β-Ga2O3 remains photocatalytically active along with the experiment without decrease in the efficiency. The authors reported that β-Ga2O3 is more effective for opening the aromatic ring of intermediates which prevents the deactivation of photoactive surface.”
Reviewer 2 Report
This is a nice, clearly structures review article. I recommend publication.
Author Response
This is a nice, clearly structures review article. I recommend publication
Response:
Thank you for reviewing our manuscript. We appreciate your recommendation.
Reviewer 3 Report
The authors raised the important issue of the practical application of photocatalysis in water and air purification. Important information on photocatalytic systems, especially reactors, was presented. The manuscript is well prepared.
Author Response
The authors raised the important issue of the practical application of photocatalysis in water and air purification. Important information on photocatalytic systems, especially reactors, was presented. The manuscript is well prepared.
Response:
Thank you for praising our manuscript. We are pleased for your interest in the manuscript.
Round 2
Reviewer 1 Report
The authors adequately responded to all recommendations and improved the quality of the manuscript. Consequently, I recommend that the manuscript be accepted for publication.
