Synthetic and Natural Surfactants for Potential Application in Mobilization of Organic Contaminants: Characterization and Batch Study

Round 1

Reviewer 1 Report

The authors investigated the potential of synthetic and natural surfactants for mobilization of toluene and perchloroethylene (PCE). It is a clear and well-written paper.

It provides an informative introduction. However, bio-based surfactant such as alkyl polyglycosides (APG) used in this study is nothing new. The authors should highlight what is their niche/new contribution of knowledge. What is the gap of knowledge specifically related to these bio-surfactants which motivates them to conduct this study?  

Detailed explanations are provided for the methodology which would be helpful for readers. The experimental design was clearly justified. Just some minor formatting issues such as ‘table 1’ in the text, when it should be ‘Table 1’.

The results are discussed in detailed and supported by appropriate references. However, the data plotting in Figure 2 is a bit confusing. Why there is a need for a straight line extrapolated to -3 log C when there are no data points there?

There is also a need to readjust the positioning of the sub-figures in Figure 3 and Figure 4 as the labelling of a) to e) doesn’t match the position of the sub-figures.

The conclusion clearly summarizes the work, but it needs to be checked for minor language correction.

Author Response

-The authors investigated the potential of synthetic and natural surfactants for mobilization of toluene and perchloroethylene (PCE). It is a clear and well-written paper.

Authors: We thank the Reviewer for this positive comment on the manuscript as well as for all the criticisms raised, which have been properly addressed, as described below.

- It provides an informative introduction. However, bio-based surfactant such as alkyl polyglycosides (APG) used in this study is nothing new. The authors should highlight what is their niche/new contribution of knowledge. What is the gap of knowledge specifically related to these bio-surfactants which motivates them to conduct this study? 

Authors: We agree with the Reviewer that APG surfactants have been widely investigated. However, based on the authors’ best knowledge, all the investigated research focused on the context of mobilization of hydrophobic organic compounds (i.e., NAPLs) from liquid separated phase via emulsification mechanism and have not used for mobilization of dissolved or adsorbed phase. Instead, in this work we have investigated the effect of the presence of APGs and other nonionic surfactants on the NAPLs adsorbed on a porous media. Moreover, the investigated APGs are 100% bio-based, obtained from waste row material. Lastly, we also used APG surfactants to make a comparison between the family of synthetic surfactants (i.e., APG1 and APG2) and the family of natural surfactants (the bio-surfactants SL, RL1 and RL2).

- Detailed explanations are provided for the methodology which would be helpful for readers. The experimental design was clearly justified. Just some minor formatting issues such as ‘table 1’ in the text, when it should be ‘Table 1’.

Authors: We thank the Reviewer for this comment. With respect to the reviewer’s suggestion, we modified the “table 1” in the text to “Table 1”.

- The results are discussed in detailed and supported by appropriate references. However, the data plotting in Figure 2 is a bit confusing. Why there is a need for a straight line extrapolated to -3 log C when there are no data points there?

Authors: To calculate the critical micelle concentration (CMC) values of the five surfactants, the surface tension vs the logarithm of each surfactant concentration was reported in figure 2. For each surfactant the data were fitted with two linear regressions, one below the CMC where surface tension decreases with increasing concentration and the other above the CMC where surface tension remains constant. The CMC values were calculated from the breakpoints of these two regions. As it can be seen in the graph, there are many points below the -3 log C that belong to the regions where concentrations are below the CMC. In this regard, we were forced to start the X curve from -3.5 log C.

- There is also a need to readjust the positioning of the sub-figures in Figure 3 and Figure 4 as the labelling of a) to e) doesn’t match the position of the sub-figures.

Authors: We thank the reviewer for this statement. We controlled the subfigures, and we modified the labels and fit them with the position of subfigures in Fig. 3 and Fig. 4.

- The conclusion clearly summarizes the work, but it needs to be checked for minor language correction.

Authors: As suggested the conclusion section was modified as follows:

“In order to preliminary evaluate the capacity of different surfactants for remediation purposes and mobilization of sorbed NAPLs onto soil and aquifer we have characterized the interfacial properties of five nonionic biodegradable surfactants. Then, by an adsorption study, we have deeply investigated the effect of the surfactant on the adsorption behavior of PCE and toluene onto a reference adsorbent material (PWB). Overall, the investigated surfactants, i.e., two synthetic surfactants (APG1 and APG2) and three biosurfactants (SL, RL1, and RL2), significantly improved the potential mobilization of both organic target compounds. The results from CMC characterization indicated the lowest CMC among the synthetic and bio-surfactants were belonged to APG2 and RL1, 0.0071 wt% and 0.0013 wt%, respectively. Nevertheless, the surfactants APG2 and RL2 offered the highest values of efficiency, respectively. Moreover, results from the batch study confirmed the results of the characterization study, as APG2 and RL2 have shown the highest abilities to reduce the adsorption of contaminants on the PWB. In general, the results demonstrated the significant capacities of synthetic surfactant APG2 and biosurfactant RL2 in the mobilization of hydrophobic contaminants. Interestingly, these two surfactants at just 5 times of their CMC reached to reduce maximum adsorption capacities of toluene on PWB to 74% and 65%, and PCE on PWB up to 65% and 86%, respectively for APG2 and RL2. In conclusion, the biosurfactant RL2 could be the best candidate for surfactant enhanced aquifer remediation technology (SEAR) due to the higher effectiveness of this material and the lowest environmental impact respect to its natural base. Future research will be focused on column study to simulate the applicability of different surfactants in the in-situ flushing remediation technology.”

Author Response File: Author Response.pdf

Reviewer 2 Report

In my opinion, it is a practical article but it needs to be considered the following comments before publishing.

The title needed significant modification. Characterization is not justified throughout the manuscript.

The result in abstract part must be extended.

All abbreviations were defined when used for the first time in the manuscript.

Introduction: This work lack novelty to be considered as an original in this paper. At the end of the introduction, the necessity and novelty of the work must be expressed well. The manuscript does not illustrate great attention and activity in the field. The purpose of the study is not well expressed.

Add more instrumental characterization in the manuscript.

Relook your batch configuration: adsorption isotherms, adsorption kinetics and thermodynamics are different things.

Analytical methods: Why and how the said parameters were selected for this work. More specific details are needed to be added with use of latest reference.

QA/QC information has to be included in manuscript.

Provide error analysis and error bars in the experimental figures.

What do you mean by discussions?

Good to have a separate section for the mechanism of the process

Future scope of this study can be added as well as social impact can also be discussed in this paper.

Conclusions: State main findings only

References are not unified and systematic.

Author Response

- In my opinion, it is a practical article but it needs to be considered the following comments before publishing.

Authors: We thank the Reviewer for this positive comment on the manuscript. We have carefully taken into consideration the Reviewer’s comments as reported below.

- The title needed significant modification. Characterization is not justified throughout the manuscript.

Authors: We thank the reviewer for this statement. Although, we believe that the title well represents the aim of the article. As this study has been started from physical-chemical characterization of five surfactants from different families by measuring their critical micelle concentration (CMC) values and interfacial properties. The second part of the study was focusing on evaluating the mobilization capacities of the surfactants in a batch mode.

-The result in abstract part must be extended.

Authors: We thank the Reviewer for this advice. We revised the abstract to be sharper by adding the important results and numbers as follow:

“In this paper, the abilities of five sugar-based synthetic and bio-surfactants from three different families (i.e., alkylpolyglycoside (APG), sophorolipid (SL), and rhamnolipid (RL)) to dissolve and mobilize non-aqueous phase liquids (NAPL) components, i.e., toluene and perchloroethylene (PCE), adsorbed on porous matrices, were investigated. The objective of this study was to establish a benchmark for the selection of suitable surfactant for the flushing aquifer remediation technique. The study involved a physico-chemical characterization of the surfactants to determine the critical micelle concentration (CMC) and interfacial properties. Subsequently, a batch study, through the construction of adsorption isotherms, made it possible to evaluate the surfactants' capacities in contaminant mobilization by the reduction in their adsorption onto a reference ad-sorbent material, a pine wood biochar (PWB). The results indicate synthetic surfactant from the APG family with a long fatty acid chain and di-rhamnolipid biosurfactant with a shorter hydrophobic group offered the highest values of efficiencies, they reduced water surface tension up to 54.7% and 52%, respectively. These two surfactants had a very low critical micelle concentration (CMC), 0.0071 wt% and 0.0173 wt%, respectively, which is critical from the economical point of view. The batch experiments showed that these two surfactants, at concentration just five times their CMC, were able to reduce adsorption on PWB of toluene up to 74% and 65% and of PCE up to 65% and 86%, with APG and RL, respectively. In general, these results clearly suggest the possibility of using these two surfactants in the surfactant enhanced aquifer remediation technology.”

- All abbreviations were defined when used for the first time in the manuscript.

Authors: We thank the reviewer for the comment. We have taken this into consideration.

- Introduction: This work lack novelty to be considered as an original in this paper. At the end of the introduction, the necessity and novelty of the work must be expressed well. The manuscript does not illustrate great attention and activity in the field. The purpose of the study is not well expressed.

Authors: We thank the Reviewer for this comment. Changes have been provided in the revised version of the manuscript that, in our opinion, make the introduction more comprehensible for the reader. Also, we completely agree with the Reviewer that the novelty of the manuscript is a particularly relevant aspect, that has been clearly highlighted in the last paragraph of the introduction section, as follows:

“Overall, this research aimed to provide a benchmark for the identification and characterization of potential surfactants for in-situ flushing of contaminated sites. In this regard, the ability of five nonionic and biodegradable surfactants to solubilize and mobilize NAPLs components adsorbed to a porous medium was investigated. The study initially involved the chemical-physical characterization of surfactants to determine their CMCs and surface behavior parameters by measuring the liquid-air surface tension at different surfactant concentrations. In the second phase, a study on the mobilization of the adsorbed pollutants was carried out. This involved adsorption isotherm batch studies to determine the effectiveness of the surfactants in enhancing the mobilization of NAPLs by reducing their adsorption on a porous reference material.”

 

- Add more instrumental characterization in the manuscript.

Authors: This work focuses on an approach for selection of suitable surfactant for remediation purposes which is divided into two phases. The first phase concentrated on the characterization of the surfactants through the CMC determination and in the second part the capacity of surfactant for mobilization of contaminant investigated in a series of batch tests. The characterization part was done by using a “MWG LAUDA tensiometer” for measuring surface tensions in different concentrations of surfactants. In the second part, the results were analyzed using a “gas-chromatograph (GC)” with a flame ionizing detector (FID). Both of these two instruments are wildly discussed in the manuscript in section 2.2 and 2.4.

- Relook your batch configuration: adsorption isotherms, adsorption kinetics and thermodynamics are different things.

Authors: We apologize for the confusion. Our purpose was to assess the capacities of different surfactants in mobilization of the contaminants. In this regard, this study suggests a new approach for evaluating the mobilization capacity of the five surfactants through the adsorption isotherm study, so we replaced the thermodynamic study with adsorption study throughout the whole manuscript. 

- Analytical methods: Why and how the said parameters were selected for this work. More specific details are needed to be added with use of latest reference.

Authors: This method and parameters were selected based on the work by Rossi et al. 2021. We added the reference in the text (line 236).

- QA/QC information has to be included in manuscript.

Authors: In this article, we investigated five different surfactants with diverse formulations that have been kindly formulated and prepared by CHIMEC SpA, where the only target was the evaluation of the environmental applicability of these surfactants and introduce a benchmark for the selection of suitable surfactant for remediation purposes. Moreover, these surfactants are completely biodegradable and 100% bio-based, which warranty environmental safety. In this regard, we did not do QA/QC analysis, although further studies are still needed to evaluate whether these materials comply with all regulations.

- Provide error analysis and error bars in the experimental figures.

Authors: In agreement with the Reviewer’s suggestion, we modified the figure 2 a & b and we added error bars.

- What do you mean by discussions?

Authors: We apologize to the reviewer for this error, in this case, we modified discussions by discussion.

- Good to have a separate section for the mechanism of the process

Authors: In agreement with the Reviewer’s suggestion, the main attention has been paid to explaining the underlying mechanisms of the process. In general, surfactants can reduce the interfacial tension between NAPL and water and the attractive force between NAPL and soil, thus improving the displacement and mobility of entrapped NAPL by emulsification. When the surfactant concentration is equal to or greater than the critical micelle concentration (CMC), the monomers spontaneously aggregate into micelles, providing a favorable environment for the dispersion of organic compounds. Therefore, micellation can enhance the solubilization of NAPLs from residual trapped ganglia and/or adsorbed phases. This is widely discussed in the introduction section, and we do not think a separate section would be necessary.

- Future scope of this study can be added as well as social impact can also be discussed in this paper.

Authors: Thank you for this statement. Since in the market, there are numerous mobilizer products and selecting the right and effective material for the specific application is a fundamental step, this work suggested a benchmark for selection of suitable surfactants that could be applicable for surfactant enhanced remediation technology, which starts from the characterization of the chemical-physical properties of the surfactants then passes through batch experimentation and the future research will be focused on column study to simulate applicability of different surfactants in the in-situ flushing. In this regard, five different nonionic bio-based surfactants from diverse families have been selected for this purpose.  The future study has already been added in the conclusion section.

- Conclusions: State main findings only

Authors: We thank the Reviewer for this comment. Consequently, we modified the conclusion section summarizing and better specifying the main objectives of the study. Modifications are as follow:

“In order to preliminary evaluate the capacity of different surfactants for remediation purposes and mobilization of sorbed NAPLs onto soil and aquifer we have characterized the interfacial properties of five nonionic biodegradable surfactants. Then, by an adsorption study, we have deeply investigated the effect of the surfactant on the adsorption behavior of PCE and toluene onto a reference adsorbent material (PWB). Overall, the investigated surfactants, i.e., two synthetic surfactants (APG1 and APG2) and three biosurfactants (SL, RL1, and RL2), significantly improved the potential mobilization of both organic target compounds. The results from CMC characterization indicated the lowest CMC among the synthetic and bio-surfactants belonged to APG2 and RL1, 0.0071 wt% and 0.0013 wt%, respectively. Nevertheless, the surfactants APG2 and RL2 offered the highest values of efficiency, respectively. Moreover, results from the batch study confirmed the results of the characterization study, as APG2 and RL2 have shown the highest abilities to reduce the adsorption of contaminants on the PWB. In general, the results demonstrated the significant capacities of synthetic surfactant APG2 and biosurfactant RL2 in the mobilization of hydrophobic contaminants. Interestingly, these two surfactants at just 5 times their CMC reached to reduce maximum adsorption capacities of toluene on PWB to 74% and 65%, and PCE on PWB up to 65% and 86%, respectively for APG2 and RL2. In conclusion, the biosurfactant RL2 could be the best candidate for surfactant enhanced aquifer remediation technology (SEAR) due to the higher effectiveness of this material and the lowest environmental impact with respect to its natural base. Future research will be focused on column study to simulate the applicability of different surfactants in the in-situ flushing remediation technology.”

- References are not unified and systematic

Authors: Thanks to the Reviewer’s suggestion, we have correctly formatted all references in the revised version of the manuscript.

Author Response File: Author Response.pdf

Reviewer 3 Report

This paper presents a series of investigations on establishing a benchmark for the selection of surfactants that can be used in the flushing aquifer remediation technique.

There are several points which needs to be seriously taken care off and then the manuscript could be considered for its publication. 

1. First of all, the confusion between thermodynamic study and equilibrium study of adsorption process must be eliminated. The adsorption isotherms (Langmuir, Freundlich, Temkin, Dubinin-Radushkevich models) describe the equilibrium relationship between the adsorbate in solution and in the adsorbent at constant temperature. Adsorption thermodynamics means to evaluate the effect of temperature and determining the thermodynamic parameters (ΔG °, ΔH °, ΔS °) for the adsorption process. The thermodynamic parameters are essential to better understand the effect of temperature on the adsorption process, indicating if the  process is spontaneous, endothermic or physical adsorbtion. The whole manuscript will have to be corrected in this regard. 
   
2. In the section 2.1.2 the following must be specified: - where does the pine wood come from; - the methods and equipment used for its characterization;
3. Why only the Langmuir model was tested?
4. How do you explain the value for qmax of 142.32 ± 30 (mg g− 1) for RL1 (PCE test)? since the reference is 114.12 ± 7.8( g g− 1)
   
5. Conclusions section is too long. The conclusions should be concise and appropriate. This section should be reviewed.

Author Response

- This paper presents a series of investigations on establishing a benchmark for the selection of surfactants that can be used in the flushing aquifer remediation technique.

There are several points which needs to be seriously taken care off and then the manuscript could be considered for its publication.

- First of all, the confusion between thermodynamic study and equilibrium study of adsorption process must be eliminated. The adsorption isotherms (Langmuir, Freundlich, Temkin, Dubinin-Radushkevich models) describe the equilibrium relationship between the adsorbate in solution and in the adsorbent at constant temperature. Adsorption thermodynamics means to evaluate the effect of temperature and determining the thermodynamic parameters (ΔG °, ΔH °, ΔS °) for the adsorption process. The thermodynamic parameters are essential to better understand the effect of temperature on the adsorption process, indicating if the process is spontaneous, endothermic or physical adsorbtion. The whole manuscript will have to be corrected in this regard.

Authors: We thank the reviewer for the clarification. This study suggests a new approach for evaluating the mobilization capacity of different surfactants through the adsorption isotherm study, so we replaced the thermodynamic study with adsorption study throughout the whole manuscript.  

- In the section 2.1.2 the following must be specified: - where does the pine wood come from; - the methods and equipment used for its characterization;

Authors: Pine wood biochar (PWB) was obtained in a gasification unit in Germany and was kindly provided by the University of Chemistry and Technology of Prague. It was obtained from a V 3.90 Burkhardt ECO 180 HG wood gas generator as a thermal and electrical plant at about 850 ^◦C. Details about biochar production, syngas composition, and electricity production of this unit have already been reported in Silvani et al. 2017. The removal capacity of PWB was investigated by Rossi and coworkers (2021), both in batch and column configurations, and compared with biochar from other feedstocks and treatments.  We cited these two papers in section 2.1.2.

- Why only the Langmuir model was tested?

Authors: As explained before, the aim of the batch experiment was to study a new approach for evaluating the mobilization capacity of different surfactants through the adsorption isotherm study so one model could be used just for fitting the data and in our opinion more models could make confusion for the readers. Moreover, we chose the best model that fitted our data which was Langmuir model.

- How do you explain the value for qmax of 142.32 ± 30 (mg g− 1) for RL1 (PCE test)? since the reference is 114.12 ± 7.8 (g g− 1).

Authors: We thank the reviewer for this comment, that gives us the chance to explain more about this issue. The higher q_max value for RL1-PCE test than the reference is due to the formation of hemimicelles onto the surface of adsorbent material (i.e., pine wood biochar). Hemimicelles are surfactant monomers aggregates with the hydrophilic and hydrophobic moieties oriented toward the aqueous phase and solid surface, respectively. In this way, due to the amphiphilic nature of surfactants, hemimicelles can solubilize and retain hydrophobic compounds.

References:

- Ballesteros-Gómez, A.; Rubio, S. Hemimicelles of Alkyl Carboxylates Chemisorbed onto Magnetic Nanoparticles: Study and Application to the Extraction of Carcinogenic Polycyclic Aromatic Hydrocarbons in Environmental Water Samples. Anal. Chem. 2009, 81, 9012–9020, doi:10.1021/AC9016264.

- Zeng, Z.; Liu, Y.; Zhong, H.; Xiao, R.; Zeng, G.; Liu, Z.; Cheng, M.; Lai, C.; Zhang, C.; Liu, G.; et al. Mechanisms for Rhamnoipids-Mediated Biodegradation of Hydrophobic Organic Compounds. Sci. Total Environ. 2018, 634, 1–11, doi:10.1016/J.SCITOTENV.2018.03.349.

- Guo, Y.P.; Hu, Y.Y.; Lin, H.; Ou, X.L. Sorption and Desorption of 17α-Ethinylestradiol onto Sediments Affected by Rhamnolipidic Biosurfactants. J. Hazard. Mater. 2018, 344, 707–715, doi:10.1016/J.JHAZMAT.2017.11.005.

- Rosen, M.J.; Kunjappu, J.T. Surfactants and Interfacial Phenomena: Fourth Edition. Surfactants Interfacial Phenom. Fourth Ed. 2012, doi:10.1002/9781118228920.

All this information has been included in the revised version of the manuscript (Line 385-390).

- Conclusions section is too long. The conclusions should be concise and appropriate. This section should be reviewed.

Authors: We thank the Reviewer for this comment, that gives us the possibility to simplify the conclusion section. In this regard, we summarized the conclusion and emphasized our findings.

“In order to preliminary evaluate the capacity of different surfactants for remediation purposes and mobilization of sorbed NAPLs onto soil and aquifer we have characterized the interfacial properties of five nonionic biodegradable surfactants. Then, by an adsorption study, we have deeply investigated the effect of the surfactant on the adsorption behavior of PCE and toluene onto a reference adsorbent material (PWB). Overall, the investigated surfactants, i.e., two synthetic surfactants (APG1 and APG2) and three biosurfactants (SL, RL1, and RL2), significantly improved the potential mobilization of both organic target compounds. The results from CMC characterization indicated the lowest CMC among the synthetic and bio-surfactants belonged to APG2 and RL1, 0.0071 wt% and 0.0013 wt%, respectively. Nevertheless, the surfactants APG2 and RL2 offered the highest values ​​of efficiency, respectively. Moreover, results from the batch study confirmed the results of the characterization study, as APG2 and RL2 have shown the highest abilities to reduce the adsorption of contaminants on the PWB. In general, the results demonstrated the significant capacities of synthetic surfactant APG2 and biosurfactant RL2 in the mobilization of hydrophobic contaminants. Interestingly, these two surfactants at just 5 times their CMC reached to reduce maximum adsorption capacities of toluene on PWB to 74% and 65%, and PCE on PWB up to 65% and 86%, respectively for APG2 and RL2. In conclusion, the biosurfactant RL2 could be the best candidate for surfactant enhanced aquifer remediation technology (SEAR) due to the higher effectiveness of this material and the lowest environmental impact with respect to its natural base. Future research will be focused on column study to simulate the applicability of different surfactants in the in-situ flushing remediation technology.”

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

The authors answered the points specified in the revision except for the justification given to the question "Why only the Langmuir model was tested?" which is not convincing to me. 

Author Response

- The authors answered the points specified in the revision except for the justification given to the question "Why only the Langmuir model was tested?" which is not convincing to me. 

Authors: Since the aim of the work was not to identify the model that represents the adsorption behavior but instead to evaluate the effectiveness of the different surfactants, we decided not to burden the work by inserting the comparison of the various tested models. In any case, other isotherm models have been evaluated but the Langmuir model was the one that allowed the best comparison. We also added this to the text as follows (Line 326-328):

“The Langmuir isotherm was used with the sole purpose of comparing the results obtained in the various experimental conditions and besides the adsorption data showed a plateau trend that can be well fitted by this model.”