The Removal of As(III) Using a Natural Laterite Fixed-Bed Column Intercalated with Activated Carbon: Solving the Clogging Problem to Achieve Better Performance

Round 1

Reviewer 1 Report (New Reviewer)

Comments and Suggestions for Authors

1.      The abstract contains claims irrelevant to that section, which are then mentioned again in the introduction. Revise the abstract and write it more informatively.

2.      The introduction is too detailed. Shorten it and include essential information related to the topic and the idea behind the experiment.

3.      Data on the instruments and their performance are missing throughout the experimental section. Regardless of previous publications (it is stated in one experimental section that it has been previously published), it is necessary to write the recording conditions, list the instruments, and explain the recording process.

4.      What is the highest concentration of As (III) detected, so the concentration for removal determines it?

5.      Adsorption isotherms for each listed column packing system should be presented and compared.

6.      Improving English throughout the work and writing style is necessary.

Comments on the Quality of English Language

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Author Response

Answers to Reviewer 1 Report

Dear Editor and Reviewer,

On behalf of my co-authors, we thank you very much for giving us an opportunity to revise our manuscript, and we appreciate Editor and Reviewer very much for their positive and constructive comments and suggestions on our manuscript entitled “Removal of As(III) using a natural laterite fixed-bed column intercalated with activated carbon: solving the clogging problem to achieve better performance”. We have studied the Reviewer’s comments carefully and have revised the manuscript thoroughly. The changes in the manuscript have been highlighted in red background for your evaluation.

The main corrections in the manuscript and the answers to the Reviewer’s comments are shown in point-to-point as follows.

 

Comments and Suggestions for Authors

1. The abstract contains claims irrelevant to that section, which are then mentioned again in the introduction. Revise the abstract and write it more informatively.

Authors' Answer: We agree with the Reviewer’s comment. After careful review, we noted that the abstract contains claims related to the clogging process, which are mentioned again in the introduction. Following the reviewer’s recommendation, these statements have been removed from the abstract section. Subsequently, slight modifications have been added in the introduction to make it more informatively.

2. The introduction is too detailed. Shorten it and include essential information related to the topic and the idea behind the experiment.

Authors' Answer: We followed the reviewer's recommendations and have therefore reduced the introduction section. As a result, the technical details that were in the introduction have been relocated in the supplementary information (SI1).

3. Data on the instruments and their performance are missing throughout the experimental section. Regardless of previous publications (it is stated in one experimental section that it has been previously published), it is necessary to write the recording conditions, list the instruments, and explain the recording process.

Authors' Answer: The following additional comments have been added in the manuscript as follows:

 

“Physical and chemical properties of the laterite sample

Elementary chemical analysis was performed by ICP (ICP- AES-IRIS Intrepid II XSP model). 0.25 g of laterite samples were digested in a microwave oven in 4 ml of HF (30 % w/w), 3 ml of H₂SO₄ (96 % w/w) and 3 ml of HNO₃ (65 % w/w).

The specific surface area and pore volume of the laterite samples were determined by the BET analysis using BelSorp-max instrument running with the Bel Japan Inc. software. The samples were degassed and dehydrated for 24 h at 200°C under vacuum. The method used is based on the nitrogen adsorption/desorption isotherms at 77K.

X-ray powder diffraction (XRPD) was carried out using a D8 Advance Davinci Bruker X-ray generator diffractometer (working at a 40 mA generator current and a 40 kV generator voltage with Cu-Kα radiation (λ = 1.54060 Å)). The XRPD data were recorded at a scan speed of 0.02° s⁻¹ and at 2θ angles with values ranging from 5° to 70°.

Using KBr pellets containing 2% in weight of natural laterite powders (or laterites residues), the FTIR spectrum were collected, between 400-4000 cm^-1 in the transmission mode, at a 4 cm^-1 resolution, on Shimadzu FTIR-8400S equipment.”

4. What is the highest concentration of As (III) detected, so the concentration for removal determines it?

Authors' Answer:

Previous studies on the physicochemical characterization of drilling waters in the Northern Region of Burkina Faso (Village of Tanlili where the filtration device is currently implemented) revealed an average As(III) concentration of 1.99 ± 0.06 mg/L [22]. This concentration, which was determined on the field of measurements, constituted our reference point. Therefore, the maximum concentration of As(III), for the fixed-bed adsorption experiments, was set at 2 mg/L during our laboratory experiments. This precision has been added in the manuscript in the experimental section.

5. Adsorption isotherms for each listed column packing system should be presented and compared.

 

Authors' Answer:

Adsorption isotherms are used, in batch experiments, to determine the maximum pollutant fixation capacities and to identify the type of adsorption. The results processed according to some mathematical models (for example Langmuir and Freundlich) make it possible to calculate the maximum adsorption capacity as well as the adsorption parameters. When it comes to fixed-bed adsorption column studies, the experiments are conducted in dynamic mode, and in this case, the maximum adsorption capacity as well as the adsorption parameters are determined, according to mathematical models, from the breakthrough curves. The maximum adsorption capacities values for the four fixed-bed column systems (packed with only laterite (DA), laterite intercalated with activated carbon (DA/BA-AC), laterite intercalated with sand (DA/DB), and packed with only activated carbon (BA-AC)) have been extracted from Tables 4 and 5 and compared. New comments have been added in the manuscript as follows:

 

 

“We also made a comparison, in terms of maximum adsorption capacity, between the following four systems: (BA-AC), (DA/BA-AC), (DA/BD), and (DA). It is noted from Tables 4 and 5 that the values are in the following order: (BA-AC) > (DA/BA-AC) > (DA) > (DA/DB). As expected, the (BA-AC) system has a value of adsorption capacity (0.127 mg/g) greater than that of the (DA/BA-AC) system (0.0530 mg/g). Although the laterite bed intercalated with sand (DA/DB) presents a higher hydraulic conductivity than the nonintercalated laterite (DA), it is characterized by the lowest adsorption capacity value (0.0130 mg/g). Consequently, this system may not be suitable, compared to the (BA-AC) and (DA/BA-AC) systems, for optimal adsorption of As(III).”

6. Improving English throughout the work and writing style is necessary.

Authors' Answer:

 

We thank the reviewer for this recommendation. Since English is not our common native language, a first improvement of the writing style has already been undertaken by using the MDPI Extensive English Editing Service, which is a paid language-editing service, before submitting the present manuscript. The result was the english-edited-78364 manuscript submitted to peer-review process. However, according to your recommendations, we agree to undertake a second moderate editing of English writing language. This can be done in agreement with the academic editor and the MDPI Extensive English Editing Service, as soon as the present manuscript meets the requirements for publication.

 

Author Response File: Author Response.pdf

Reviewer 2 Report (New Reviewer)

Comments and Suggestions for Authors

The manuscript by Ouedraogo et al. is well written, well designed  and the topic is suitable for the Journal. I have only few comments/suggestions which are appended below.

 

-Please briefly describe the analytical method used for the determination of the As concentration

 

-How were the outgoing solution collected in the tubes? Manually?  

Author Response

Answers to Reviewer 2 Report

Dear Editor and Reviewer,

On behalf of my co-authors, we thank you very much for giving us an opportunity to revise our manuscript, and we appreciate Editor and Reviewer very much for their positive and constructive comments and suggestions on our manuscript entitled “Removal of As(III) using a natural laterite fixed-bed column intercalated with activated carbon: solving the clogging problem to achieve better performance”. We have studied the Reviewer’s comments carefully and have revised the manuscript thoroughly. The changes in the manuscript have been highlighted in red background for your evaluation.

The main corrections in the manuscript and the answers to the Reviewer’s comments are shown in point-to-point as follows.

 

Comments and Suggestions for Authors

-Please briefly describe the analytical method used for the determination of the As concentration

Authors' Answer: The following description has been added in the manuscript:

 

“The determination of As(III) was carried out using a mini potentiostat (910 PSTAT mini Metrohm) driven by the PSTAT software. The device included a 10 mL plastic cell held in place by a ring in which are placed a carbon paste electrode modified by gold nanoparticles (Au-NP/CPE), a silver chloride reference electrode, Ag/AgCl, saturated KCl (208 mV potential at 25°C compared to the normal ENH hydrogen electrode) and a platinum auxiliary electrode). The carbon paste electrode modified by gold nanoparticles (Au-NP/CPE) was prepared by introducing a paste composed of 60% graphite and 40% solid paraffin followed by a physicochemical and electrochemical modification [6]. As(III) concentrations were determined by the method by Differential Pulse Anodic Stripping Voltammetry (DPASV) in a potential range of -0.3 to 0.3 V/ECS, after a pre-concentration time of 180 s by introducing 10 ml of filtrate into the cell containing the three electrodes in a hydrochloric acid medium ( HCl, 1M). The detection limit was estimated to be 0.932 µg/L.”

-How were the outgoing solution collected in the tubes? Manually?

Authors' Answer:

The outgoing solution from the column was collected using an automatic collector (See attached image).

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report (New Reviewer)

Comments and Suggestions for Authors

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This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Editor-in-Chief.

Separations.

 

In the manuscript under review, the authors are claiming the wastewater treatment process and removal of As(III) from effluents using the laterite fixed-bed column in tercalated with activated carbon. The work is full of a lot of unresolved issues. One of the top issue is the selectivity of the column and process. The wastewater contains Arsenic in two oxidation states, As(III) and As(VI) and the two states of As are interchangeable, there is no such information which could lead to clear conclusion, does the column selectively remove the As(III) or both As(III) and As(VI)? Beside there are many grammatical mistakes and the overall English language is poor. The work is not sound scientifically. The entire work needs many experimentation to arrive a clear conclusion with proper characterizations. Similarly, the art of work presentation needs to be revised and many more.

My opinion on this manuscript is a reject.

Comments on the Quality of English Language

Dear Editor-in-Chief.

Separations.

 

In the manuscript under review, the authors are claiming the wastewater treatment process and removal of As(III) from effluents using the laterite fixed-bed column in tercalated with activated carbon. The work is full of a lot of unresolved issues. One of the top issue is the selectivity of the column and process. The wastewater contains Arsenic in two oxidation states, As(III) and As(VI) and the two states of As are interchangeable, there is no such information which could lead to clear conclusion, does the column selectively remove the As(III) or both As(III) and As(VI)? Beside there are many grammatical mistakes and the overall English language is poor. The work is not sound scientifically. The entire work needs many experimentation to arrive a clear conclusion with proper characterizations. Similarly, the art of work presentation needs to be revised and many more.

My opinion on this manuscript is a reject.

Author Response

Response to Reviewer 1 Comments

1) In the manuscript under review, the authors are claiming the wastewater treatment process and removal of As(III) from effluents using the laterite fixed-bed column intercalated with activated carbon. The work is full of a lot of unresolved issues. One of the top issue is the selectivity of the column and process. The wastewater contains Arsenic in two oxidation states, As(III) and As(VI) and the two states of As are interchangeable, there is no such information which could lead to clear conclusion, does the column selectively remove the As(III) or both As(III) and As(VI)?

Answer:

General answer to the comments

We greatly appreciate the reviewer comments and we are grateful for all the time which has been taken to review the present manuscript. The question gives us the opportunity to explain our view and the significance of the present investigations.

Before submitting a manuscript for publication, we need, as a scientific researcher, to put our data into perspective by answering a series of questions:

• What is (are) the problem(s)? What is the research question?
• Are there any existing solutions?
• Which one is the best?
• What is (are) the main limitation(s)?
• How do you hope to improve or contribute to this?

 

Background and Significance of the research

In our country, the presence of arsenic in groundwater is of major concern. Arsenic exposure due to tube-wells contamination by arsenic has been reported. Toxicological effects such as gastrointestinal problems, skin diseases and fatigue of unknown cause have been reported. Since conventional methods (chemical precipitation, physical treatment, etc.) for water purification are unsustainable in a developing country, the quest for simple, low-cost and high-performance water purification processes necessitates the use of local materials, which are natural materials, good sorbents and inexpensive and represent a viable replacement to these chemicals for the removal of dissolved trace contaminants from groundwater and waste water. According to literature survey, the most current approaches for arsenic removal include precipitation, coagulation by adding lime or coagulants to water, separation by using membranes, the use of an ion exchange process and adsorption. Among all of these approaches, adsorption has been proved to be the most promising method because of its high efficiency, ease of handling and the availability of several types of adsorbent materials. Natural laterites have been the subject of a lot of investigations since they were shown to have potential applications in arsenic removal from groundwater. In our context, natural laterites are widely available in the country, and could be used at low-cost for arsenic removal. Indeed, many papers have documented arsenic removal by adsorption on natural laterite using batch mode as well as fixed-bed column systems. However, the fixed-bed system provides flexibility compared to the batch mode. Several works focused on column percolation adsorption techniques for the removal of organic and inorganic pollutants due to their simplicity and efficiency.

 

The adsorption removal of arsenic by using natural laterites can be performed in batch mode as well as in fixed-bed column. Our previous investigations on arsenic removal using the batch mode showed that local natural laterite had high efficiency for As(III) removal. However, the fixed-bed system provides several advantages compared to the batch mode. One of the advantages of the fixed-bed system is the prediction of the breakthrough curve, which determines the functional longevity of the adsorbent beds and regeneration time. For this reason, parallel to the batch mode investigations, investigations have been carried out using a fixed-bed column filled with natural local laterites. We showed that a better adsorption capacity, during column percolation operation, could only be achieved with larger diameter and higher heights of the bed and smaller adsorbent pore sizes. However, contrary to previous investigations in the literature, our investigations showed that the fixed-bed column system for arsenic removal encountered some shortcomings, in particular the clogging of the column by small lateritic particles which can slow down the water flow through the column in the long run.

This issue constituted one of the major concerns during column percolation operations with large diameter and height of the bed. Clogging is generally defined as a process causing a drop in the performance of an adsorbent due to the deposition of suspended or dissolved materials on its outer surface or within the pores. In the case of a percolation through a laterite-lined column, small size laterite particles present low permeability to water, which leads to the clogging of the porous filter media.

In the literature, the percolation treatment process has been improved on other systems by several authors who proposed the alternated layers method to improve the system. The alternated layers could be either an inert material or an adsorbent witch also adsorb very well, without any influence on the targeted effect. We were confronted with the choice of using the following materials as alternated layers: activated carbon, manganese dioxide, fine sand or coarse sand, gravels. What is expected by using alternated layers is to improve the water permeability through the whole column by using alternated layers of either an inert material or an adsorbent material.

In our first set of investigations, we focused on adsorbent alternated layers, bearing in mind that inert alternated layers (coarse and fine sand, gravels) will also be investigated. The possibility to use alternated layers of an adsorbent with a high specific area and a mechanical stability could improve the adsorptive performance of the column. The system can be adapted at a large scale in order to treat a large quantity of solution by percolating the pollutant solution through the column. This finding constituted the starting point of our own solutions. However, the question that rises is which adsorbent is going to be more appropriate to be intercalated between the laterite layers in view of the increase of the permeability of the adsorptive porous media. Among several adsorbents which are currently being used, activated carbon seems more suitable because of its high adsorption capacity, which is due to its large specific surface and the presence of surface charges induced by its chemical composition. In addition, the use of available wastes as raw materials to prepare activated carbon appears as a good alternative to achieve a low-cost pollutant removal process. To the best of our knowledge, this is the first time that investigations are directed toward the understanding of those factors controlling the clogging problem during As(III) removal in a laterite fixed-bed column, with a view to getting an insight into a rational design of a new low-cost adsorptive porous system based on laterite layers alternated with activated carbon layers. So, our expectations were to solve this clogging problem by setting up this new low-cost filter for arsenic (III) removal.

The novelty of the present work: It can be seen from the use of a natural laterite fixed-bed column intercalated with activated carbon, which solves for the first time the clogging problem occurring during the percolation process through a fixed-bed column filled with laterite particles. Moreover, the characterization of the hydraulic property of the natural laterite fixed-bed column intercalated with activated carbon may lead to an appropriate justification when the screening of such an adsorptive porous column for use in pollutant removal is made. The present paper is the first of a set of innovative results which will be published as soon as possible. The following research plan was then envisaged:

• Arsenic (III) removal from aqueous solution (synthetic solution of As(III)) using batch mode: our previous work in reference [28],
• Arsenic (V) removal from aqueous solution (synthetic solution of As(V)) using batch mode: our previous work in reference [28],
• Arsenic (III) removal from aqueous solution (synthetic solution of As(III)) using fixed-bed column (this the present work),
• Simultaneous removal of As(III) and As(V) from aqueous solution and influence of co-existing cations and anions (forthcoming paper),
• Applications to real contaminated ground water (forthcoming paper).

 

Specific answers to the questions

We are now going to give answers to specific questions.

First of all, we would like to state (according to the Pourbaix Diagram, See the Figure under) that the oxidation states that are likely to occur for arsenic in aqueous solution are the following: +III, +V. Arsenic doesn’t exist in the oxidation state of +VI. The oxidation state of +VII may exist in the form of , but it is not taken into account when considering the Pourbaix Diaagram.

 

Pourbaix Diagram of arsenic in aqueous solution

Secondly, we would like to specify again our experimental method which is the following (See in the manuscript paragraph 2.2):

2.2. Chemicals

“The 1000 mg/L arsenic (III) stock solution was prepared by dissolving an appropriate mass of  (99%, Merck Ltd) in 1 L volume of milli-Q water. All working solutions, whose concentrations are 0.5, 1, and 2 mg/L, were obtained by dilution of the stock solution.”

From this experimental method, it can easily be seen that in the present investigation, we are not concerned with a wastewater which contains both oxidation states of Arsenic. Our experience consists of a synthetic solution containing only arsenic (III), that is to say only one oxidation state of arsenic, and not both oxidation states of Arsenic (As(III) and As(V)). So, the fixed-bed column is only removing one oxidation state of As(III), and there is no concern about any type of selectivity. Dear reviewer, the selectivity issue will be the subject of our next investigations, as we planned above. In these forthcoming investigations, the matter will be to see how the adsorptive porous system works when both arsenic (III) and arsenic (V) are present in the same solution. These are ongoing investigations, and we would like to say that the partial obtained results are very satisfactory, since with this low-cost adsorptive porous system, we were already able to achieve, over a period of six months, arsenic removal from real contaminated groundwater sites. In the following figure, you can see the upscale of what has been achieved at the laboratory scale.

 

 

Pilot station of fixed-bed column installed at a real contaminated site in a village

Indeed, all the residual concentrations of arsenic solution after the percolation through the column are under the limit of 10 µg/L set by WHO, which shows that the systems performs well as for arsenic removal from a real contaminated groundwater containing As(III) and As(V); the method used for arsenic detection is based on AuNPs modified Carbon Paste Electrode. However, we need to evaluate the selectivity of the system. Dear reviewer, this is an ongoing activity which needs to be validated. 

 

As a matter of fact, the present work is part of a global research on the adsorption of arsenic by natural laterite soils of Burkina Faso. Significant results have already been obtained in our previous paper [28] (R. D. OUEDRAOGO et al. / Int. J. Biol. Chem. Sci. 13(6): 2959-2977, 2019) as for the adsorption of arsenic (III) solution and the adsorption of arsenic (V) from aqueous solution onto natural laterites by using the batch mode. The kinetics studies have shown that equilibrium is achieved within 16 hours of contact time between the laterite and the solution containing arsenic (III or V) and their adsorption follows a pseudo-second order kinetic model. This suggested the existence of chemisorption of both arsenic species. The results of the dose effect showed an elimination reaching 99.69% and 97.30%, for As (V) and As (III) respectively, for a dose of 0.75 g of laterite.

This paper constitutes indeed the follow-up of our previous paper [28]. Since we were successful in the batch mode regarding the removal of arsenic (As(III) solution or As(V) solution), the next step was to envisage the fixed-bed system which provides flexibility compared to the batch mode. In the present investigations, we are only concerned with arsenic (III) solution. However, the results for arsenic (V) removal using the fixed-bed system are also available and will be the subject of our next paper. Another step will be to carry out investigations on simultaneous removal of As(III) and As(V) using the fixed-bed column. As already said, this is the topic of our forthcoming paper which will also investigate the influence of co-existing cations and anions. The applications will then be set-up for real contaminated ground water in villages.

Dear reviewer, we had to plan all of these results which cannot be published in a single paper. It is then appropriate to investigate all of them step by step in order to achieve a general conclusion.

2) Beside there are many grammatical mistakes and the overall English language is poor.

Answer:

Once again, we greatly appreciate the reviewer comments and we are grateful for all the time which has been taken to review the present manuscript. As a matter of fact, the paper was proofread by an English Professional. However, we will correct the grammatical mistakes and will use another service of English language. 

3) The work is not sound scientifically.

Answer: Please refer to the above answer in the general comment.

 

4) The entire work needs many experimentation to arrive a clear conclusion with proper characterizations. Similarly, the art of work presentation needs to be revised and many more.

 

Answer:

Once again, we appreciate the reviewer’s comment and would like to highlight the fact that this work is part of a global research on the adsorption of arsenic by natural laterite soils of Burkina Faso. This paper constitutes the second step of our investigations, after the batch adsorption which showed an elimination rate of 99.69% and 97.30%, for As(V) and As(III) respectively, for a dose of 0.75 g laterite [28]. Several investigations are planned and are currently being carried out. Some additional results are already available, in particular those related to As(V) removal by using the fixed-system. The next step will be to simultaneously investigate both oxidation stated in the same solution and see if the system is more selective to one oxidation state or both oxidation states. It is evident that all of these results could not appear in a single paper.

The main objective of the present paper was the understanding of the clogging process which occurs during synthetic arsenic (III) solution percolation and this constitutes the key element in our present study. Then, to achieve our goal, the following investigations have been undertaken:

 

• The synthesis and the characterizations of the activated carbon which was used as alternated layers between the laterite layers. Moreover proper characterizations were carried out regarding the activated carbon. We refer the reviewer to the paragraph 3.1.2. Preparation and characterization of the activated carbon and the paragraph 3.2. Characterization of activated carbon
• Regarding the natural laterites, the characterization of the laterite sample, named DA laterite, was already described in our previous paper [28]. We showed that it contained major phases such as quartz (SiO₂), kaolinite (Al₂Si₂O₅(OH)₄), hematite (Fe₂O₃), and goethite (FeO(OH)) [28]. Moreover, the physical properties of the DA laterite are shown in Table 1 and its chemical composition is shown in Table 2 of the present paper.

 

• It is worth highlighting the fact that breakthrough curves analysis are among the most important techniques used to investigate the effects of operational parameters (natural laterite particles grain sizes, bed height, and initial arsenic concentration) on As(III) removal and to correlate the experimental data with dynamic models to predict overall adsorption behaviors (See the following references in the manuscript: [24, 41, 42, 50, 51]).

 

• Another method which is currently being used is the mathematical modeling. Indeed, in order to upscale this technology and realize its full potential, a comprehensive understanding of the dependence of filter life on operating regime (flow rate, arsenic concentration and filter size) is essential. We will investigate a mathematical model that characterizes arsenic removal, circumventing the need for time-consuming experiments. The model will incorporate inter- and intra-particle mass transport within the filter medium. The model will enable prediction of the filter lifetime in a specified role, such as on a domestic or community scale, and should assist in future filter deployment and maintenance. This constitutes our next challenges and will be the subject of our forthcoming paper.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

In this paper, authors investigated natural laterite fixed-bed column intercalated with activated carbon prepared from Balanites aegyptiaca (BA-AC) for As(III) removal from an aqueous solution. The comprehensive study was performed to solve the problem of column clogging which appears during water percolation through the column. The new low-cost filtration system, based on laterite layers alternated with BA-AC layers, showed as useful for the treatment of arsenic-contaminated water, to provide cleaner water in rural areas.

The paper might be interesting and useful for researchers in that field and I recommend accepting the paper for publication after minor revision.

- Nitrogen adsorption and desorption isotherms and Barrett-Joyner-Halenda (BJH) pore-size distribution for sample BA-A should be presented. The values of the average pore diameter and specific surface area of BA-AC have too much significant numbers.  Instead of 2.7834 nm should be 2.78 nm, as it was given in Table 4.  The value of BA-AC specific surface area value determined by BET method should be 666 m2/g instead of 666.464 m2/g.

Author Response

Review2 Report Form

 

In this paper, authors investigated natural laterite fixed-bed column intercalated with activated carbon prepared from Balanites aegyptiaca (BA-AC) for As(III) removal from an aqueous solution. The comprehensive study was performed to solve the problem of column clogging which appears during water percolation through the column. The new low-cost filtration system, based on laterite layers alternated with BA-AC layers, showed as useful for the treatment of arsenic-contaminated water, to provide cleaner water in rural areas.

The paper might be interesting and useful for researchers in that field and I recommend accepting the paper for publication after minor revision.

 

1) Nitrogen adsorption and desorption isotherms and Barrett-Joyner-Halenda (BJH) pore-size distribution for sample BA-A should be presented. The values of the average pore diameter and specific surface area of BA-AC have too much significant numbers. 

 

Authors' Answer:

 

We greatly appreciate the reviewer comments and we are grateful for all the time which has been taken to review the present manuscript. New figures have been added to the manuscript:

 

- Figure 4 shows the adsorption-desorption isotherms for nitrogen at 77 K;

- Figure 5 shows the pore size distribution for the BA-AC sample.

 

Figure 4: Adsorption-desorption isotherm curves for nitrogen at 77 K on BA-AC activated carbon.

 

 

Figure 5: Pore size distribution curves of activated carbon BA-AC

 

 

2) Instead of 2.7834 nm should be 2.78 nm, as it was given in Table 4.  The value of BA-AC specific surface area value determined by BET method should be 666 m2/g instead of 666.464 m²/g.

 

Authors' Answer:

The appropriate modifications have been undertaken as recommended by the reviewer:

• 7834 nm was modified to give to 2.78 nm,
• 666 464 m²/g was modified to give 666.46 m²/g.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Journal: Separations

Title: Removal of As(III) using a natural laterite fixed-bed column in tercalated with activated carbon: solving the clogging problem to achieve better performance

 

The main goal of the research is the removal of As(III) using the obtained biochar and natural laterite in a fixed bed column. By applying alternating layers of biochar and laterite, the authors prevent the clogging of the column that occurs when only laterite is applied for heights of 30 and 40 cm. This research has a lot of ambiguities in terms of meaningfulness. Therefore, I do not want to reject the paper, but give the authors a chance to upgrade.

 

First, I have a question about the title of the paper. I think that the contribution of biochar, i.e. activated carbon as the authors call it, is greater compared to laterite for As(III) removal.

Therefore, the maximum sorption capacity of natural laterite, biochar and natural laterite-biochar mixture (mass ratios as used in the column) should be determined by batch experiment.

In this way, the answer will be obtained as to the contribution of individual sorbents as well as the mixture of sorbents. In fact, the answer will be whether it makes sense to use a mixture of sorbents at all. If we look at and compare the BET of natural laterite (35.08 m2/g) and biochar (666.46 m2/g), it is clear that biochar should be better for As(III) sorption.

 

Furthermore, the goal is certainly to process the largest possible volume of wastewater to the breakthrough point as specified by the authors of 10ug/L. However, at the same time, the goal is to use the sorbent as much as possible, to saturate it as much as possible.

For this purpose, the results shown in Figure 8 should be completed with an additional experiment, a column with a bed depth of 20 cm, but filled exclusively with biochar.

 

For the results shown in Figure 9 and 11, it is necessary to provide a comparison of the sorption capacities (mg/g) at the breakthrough point and the exhaustion point of the column. Then compare these results with the capacities obtained by the batch experiment.

The question arises whether the flow rate of 50 mL/min is too high. Would a 50% lower flow rate process a larger volume of waste water and achieve greater utilization (saturation) of the sorbent.

These are the basic questions that arise for me. 1. Does it make sense to use natural laterite alternated with biochar or is it better to use only biochar. 2. It is necessary to find an optimization solution, achieve the highest possible saturation of the sorbent and process the highest possible amount of wastewater.

 

 

Specific comments:

 

Line 28: „solve the problem of column clogging“ - clearly state why the clogging problem occurs.

Line 38: „filtration system“ - I think that this term is not appropriate because it is about adsorption.

Line 97: „low-cost filter system“- I think that this term is not appropriate because it is about adsorption.

Line 140: Define masses.

Table 1: pHpcn – meaning of pcn. Is it point zero charge (pzc)?

 

Define and explain in the discussion what is the purpose of impregnation with phosphoric acid. To define the sorption mechanism of As(IIII) binding to biochar and laterite.

 

In the experiment shown in Figure 3, why is Z=5, 10 and 15 cm, and not 10, 20 and 30, as well as the bed depth in the columns.

Line 350: affluent change to effluent

In the figure 11 ppm to mg/L

 

Conclusions need to be improved.

Lines 590-592: „The parameters of the fixed-bed column were studied to find the optimal experimental operating conditions.“ - you will get an answer and a solution when you consider my suggestions.

Lines 602-604: „hese results are a step towards the large-scale use of  this type of laterite-filled column intercalated  with activated carbon to provide

cleaner water in rural areas.“ - I'm not really sure, you MAY get an answer and a solution when you consider my suggestions.

The literature should be cited carefully according to the propositions of this journal.

Author Response

Reviewer's n°3 comment

Title: Removal of As(III) using a natural laterite fixed-bed column in tercalated with activated carbon: solving the clogging problem to achieve better performance

The main goal of the research is the removal of As(III) using the obtained biochar and natural laterite in a fixed bed column. By applying alternating layers of biochar and laterite, the authors prevent the clogging of the column that occurs when only laterite is applied for heights of 30 and 40 cm. This research has a lot of ambiguities in terms of meaningfulness. Therefore, I do not want to reject the paper, but give the authors a chance to upgrade.

Answer:

General answers to the comments

We greatly appreciate the reviewer comments and we are grateful for all the time which has been taken to review the present manuscript. This question gives us the opportunity to explain our view and the significance of the present investigations.

Before submitting a manuscript for publication, we need to put our data into perspective by answering a series of questions:

• What is (are) the problem(s)? What is the research question?
• Are there any existing solutions?
• Which one is the best?
• What is (are) the main limitation(s)?
• How do you hope to improve or contribute to this?

 

Background and Significance of the research

In our country, the presence of arsenic in groundwater is of major concern. Arsenic exposure due to tube-wells contamination by arsenic has been reported. Toxicological effects such as gastrointestinal problems, skin diseases and fatigue of unknown cause have been reported. Since conventional methods (chemical precipitation, physical treatment, etc.) for water purification are unsustainable in a developing country, the quest for simple, low-cost and high-performance water purification processes necessitates the use of local materials, which are natural materials, good sorbents and inexpensive and represent a viable replacement to these chemicals for the removal of dissolved trace contaminants from groundwater and waste water. According to literature survey, the most current approaches for arsenic removal include precipitation, coagulation by adding lime or coagulants to water, separation by using membranes, the use of an ion exchange process and adsorption. Among all of these approaches, adsorption has been proved to be the most promising method because of its high efficiency, ease of handling and the availability of several types of adsorbent materials. Natural laterites have been the subject of a lot of investigations since they were shown to have potential applications in arsenic removal from groundwater. In our context, natural laterites are widely available in the country, and could be used at low-cost for arsenic removal. Indeed, many papers have documented arsenic removal by adsorption on natural laterite using batch mode as well as fixed-bed column systems. However, the fixed-bed system provides flexibility compared to the batch mode. Several works focused on column percolation adsorption techniques for the removal of organic and inorganic pollutants due to their simplicity and efficiency.

 

Our previous investigations on arsenic removal using the batch mode showed that local natural laterite had high efficiency for As(III) removal [28]. We achieved in batch mode an elimination rate of 99.69% and 97.30%, for As(V) and As(III) respectively, for a dose of 0.75 g of laterite [28]. The adsorption capacities were 0.33 mg/g and 0.30 mg/g, for As(V) and As(III) respectively. Since we were successful with the batch experiment regarding arsenic removal from aqueous solutions, we envisaged the experiments with the fixed-bed systems. Indeed, the fixed-bed system provides several advantages compared to the batch mode. One of the advantages of the fixed-bed system is the prediction of the breakthrough curve, which determines the functional longevity of the adsorbent beds and regeneration time. For this reason, parallel to the batch mode investigations, investigations have been carried out using a fixed-bed column filled with natural local laterites. We showed that, in the case of column percolation operation, a better adsorption capacity could only be achieved with larger diameter and higher heights of the bed and smaller adsorbent pore sizes. However, contrary to previous investigations in the literature, our investigations showed that the fixed-bed column system for arsenic removal encountered some shortcomings, in particular the clogging of the column by small lateritic particles which can slow down the water flow through the column in the long run.

This issue constituted one of the major concerns during column percolation operations with large diameter and height of the bed. Clogging is generally defined as a process causing a drop in the performance of an adsorbent due to the deposition of suspended or dissolved materials on its outer surface or within the pores. In the case of a percolation through a laterite-lined column, small size laterite particles present low permeability to water, which leads to the clogging of the porous filter media.

In the literature, the percolation treatment process has been improved on other systems by several authors who proposed the alternated layers method to improve their system. Let’s precise that the alternated layers method is not based on a mixture of two compounds, but it is based on superimposed layers of two compounds as given by the following Figure:

 

Figure 2. Experimental device for solution percolation through a natural laterite fixed-bed column alternated with activated carbon layers

The concept of mixture of compounds has nothing to do with the alternated layers method. In the alternated layers method, the goal is to improve the water permeability through the whole column, since we are confronted with a clogging of the column with smaller adsorbent pore sizes. It is not a matter of mixing the compounds. We experimented the mixture and found that the mixture is compacted in the form of sludge which cannot favor the solution percolation through the fixed-bed column. To verify and confirm that the layers superimpose well or are really and easily stacked, we performed hydraulic conductivity of the porous media, the hydraulic conductivity being linked to the water permeability of the adsorptive porous system.

The alternated layers could be either an inert material or an adsorbent material which also adsorbs very well, without any negative influence on the targeted pollutant. We had the choice of using the following materials as alternated layers: activated carbon, manganese dioxide, fine sand or coarse sand, gravels. Activated carbon and manganese dioxide are adsorbent materials, whereas fine sand, coarse sand, and gravels are inert material. What is expected by using the alternated layers method is to improve the water permeability through the column.

 Laterite is well known as a swelling material. Basing on literature, it is also known that an intercalation of a swelling adsorbent by a non-swelling adsorbent material, such as activated carbon, manganese dioxide, fine or coarse sand, gravels, etc., could improve the permeability to water of the porous filter media. A higher hydraulic conductivity is linked to a higher flow rate, and a lower residence time of the water inside the porous material, diminishing the risk of clogging.

In our first set of investigations, we focused on a fixed-bed column filled with laterites layers alternated with an adsorbent material, bearing in mind that inert alternated layers (coarse and fine sand, gravels) will also be investigated. These alternated layers that are intercalated between the laterite layers are only secondary materials, the primary material being the laterite which is available at a low-cost and has been proved to remove arsenic in batch experiment. The possibility to use alternated layers of an adsorbent with a high specific area and a mechanical stability could improve the adsorptive performance of the column. The system can be adapted at a large scale in order to treat a large quantity of solution by percolating the pollutant solution through the column. This finding constituted the starting point of our own solutions. However, the question that rises is which adsorbent is going to be more appropriate to be intercalated between the laterite layers in view of the increase of the permeability of the adsorptive porous media. Among several adsorbents which are currently used, activated carbon seems more suitable because of its high adsorption capacity, which is due to its large specific surface and the presence of surface charges induced by its chemical composition. We had the choice of using commercial activated carbon from the commerce or preparing it by our own means. However, the commercial activated carbon is very expensive. Indeed, in all experiences where such activated carbon has been used, the limit of the investigations was the cost of the starting activated carbon material. The use of available wastes available in our country as raw materials to prepare activated carbon appears as a good alternative to achieve a low-cost pollutant removal process. Although synthesizing the activated carbon by our own means can be energy consumer, its cost appears relatively lower than the commercial activated carbon. As a result, alternating the laterites layers, which are widely available in the country, with the synthesized activated carbon layers allows us to achieve a low-cost system. We must bear in mind that this system is to be implemented in rural environment, i.e. it must be prepared at low-cost. To the best of our knowledge, this is the first time that investigations are directed toward the understanding of those factors controlling the clogging problem during As(III) removal in a laterite fixed-bed column, with a view to getting an insight into a rational design of a new low-cost adsorptive porous system based on laterite layers alternated with activated carbon layers. So, our expectations were to solve this clogging problem by setting up this new low-cost adsorptive porous system for arsenic (III) removal. We would like to point out that we are currently investigating other types of alternated layers (coarse or fine sand, gravels, manganese dioxide, etc.) to see whether their implementation could be achieved at a low-cost and improve the fixed-bed column performance by preventing the clogging of the column that occurs when only laterite is applied.

The novelty of the present work: It can be seen from the use of a natural laterite fixed-bed column intercalated with activated carbon, which solves for the first time the clogging problem occurring during the percolation process through a fixed-bed column filled with laterite particles. Moreover, the characterization of the hydraulic property of the natural laterite fixed-bed column intercalated with activated carbon may lead to an appropriate justification when the screening of such an adsorptive porous column for use in pollutant removal is made. The present paper is the first of a set of innovative results which will be published as soon as possible. The following research plan was then envisaged for the global investigations:

• Arsenic (III) removal from aqueous solution using batch mode: this has been already completed in reference [28],
• Arsenic (V) removal from aqueous solution using batch mode this has been already completed in reference [28],
• Arsenic (III) removal from aqueous solution using fixed-bed column (this is the present work),
• Simultaneous removal of As(III) and As(V) from aqueous solution and influence of co-existing cations and anions using fixed-bed column (forthcoming paper),
• Applications of the findings to real contaminated ground water sites in villages.

Specific answers to the questions

We are now going to give answers to specific questions.

1) First, I have a question about the title of the paper. I think that the contribution of biochar, i.e. activated carbon as the authors call it, is greater compared to laterite for As(III) removal.

Therefore, the maximum sorption capacity of natural laterite, biochar and natural laterite-biochar mixture (mass ratios as used in the column) should be determined by batch experiment.

In this way, the answer will be obtained as to the contribution of individual sorbents as well as the mixture of sorbents. In fact, the answer will be whether it makes sense to use a mixture of sorbents at all. If we look at and compare the BET of natural laterite (35.08 m2/g) and biochar (666.46 m2/g), it is clear that biochar should be better for As(III) sorption.

Answer:

Dear reviewer, as we said in our general answers, the initial goal was to use low-cost laterite soils which are widely available in the country and have been proved to have a good efficiency for arsenic removal from aqueous solutions. We began our investigations by using the batch mode [28]. Significant results have been obtained in our previous paper [28] (R. D. OUEDRAOGO et al. / Int. J. Biol. Chem. Sci. 13(6): 2959-2977, 2019) as for the adsorption of arsenic (III) onto natural laterites by using the batch mode. As we already said, we achieved in batch mode an elimination rate of 97.30% for As(III), for a dose of 0.75 g of laterite [28]. The adsorption capacity was 0.30 mg/g for As(III).The kinetics studies have shown that equilibrium is achieved within 16 hours of contact time between the laterite and the solution containing arsenic (III or V) and their adsorption follows a pseudo-second order kinetic model. This suggested the existence of chemisorption of both arsenic species.

Following the reviewer’s suggestion, we carried out the batch experiment of the mixture as follows (See the photos under):

1) 0.75 g of BA-AC (activated carbon), with a removal rate of 100% and an adsorption capacity of 0.333 mg/g

2) 0.5 g of BA-AC + 0.5 g of laterite, with a removal rate of 100% and an adsorption capacity of 0.333 mg/g

3) 0.375 g of BA-AC + 0.375 g of laterite, with a removal rate of 100% and an adsorption capacity of 0.333 mg/g

4) 0.25 g of BA-AC + 0.5 g of laterite, with a removal rate of 100% and an adsorption capacity of 0.333 mg/g

 

 

0,25g BA-AC +0,5 DA

0,375g BA-AC +0,375 DA

0,375g BA-AC +0 ,375DA

0,5g BA-AC +0,25 DA

0,75g BA-AC

 

 

 

The above results have been compared with the results obtained for the batch experiment of laterite which is the following: an elimination rate of 97.30% and an adsorption capacity of 0.30 mg/g, for a laterite dose of 0.75 g. As we can notice, the presence of the activated carbon positively contributes since we achieved an elimination rate of 100% compared to the elimination of 97.30% when we have the laterite only. In term of the adsorption capacity, the results are similar: 0.333 mg/g (different mixtures) compared to 0.30 mg/g (laterite only). Although the activated carbon presents a great value of a specific surface as determined by the BET method (666.46 m2/g), the obtained result of 100% as elimination rate, for all the different mixtures, is not far from the one of the laterite which is 97.30% (specific surface of 35.08 m2/g). Indeed, these results are not surprising, since it has been shown that the main factor, when we are dealing with arsenic elimination, is not only the specific surface, but the mineral phases contained in the compound. Table 2 gives the chemical composition of DA laterite:

 

Table 2: Chemical composition of DA laterite [28]

Chemical composition (%)
Fe2O3	20.40
Al2O3	19.10
SiO2	45.00
K2O	0.36
Na2O	0.20
TiO2	1.40
MgO; MnO2; BaO; CaO	Traces

 

As we can see, the laterite contains some mineral phases, such as Fe₂O₃ and Al₂O₃ which are the main phases responsible for arsenic elimination through chemical binding at the surface of the laterite between As(III) and Fe in Fe₂O₃ and Al in Al₂O₃. In the case of activated carbon, although the specific surface is higher, we have the presence of surface charges induced by its chemical composition [34-36]. Compared to the laterite mineral phases, these surface charges are less likely to establish chemical binding. The chemical binding of the mineral phases in the laterite favors the arsenic elimination, although the specific surface is lower than the one of the activated carbon.

In conclusion, the results obtained indicates that the use of the laterite would be beneficial for the elimination of arsenic in the sense that its utilization could be made at a low-cost compared to an exclusive utilization of an activated carbon which shows limitations when it comes to the upscale of the technology at real contaminated groundwater sites. The combination of the laterite with the activated carbon also shows a high elimination rate compared to the laterite only. This combination could also be prepared at a low-cost compared to the exclusive utilization of the activated carbon. It allows a minimization of the consummation of the activated carbon.

The next step was to envisage the fixed-bed system which provides flexibility compared to the batch mode. We showed that, for a fixed-bed column filled only with laterite, a better adsorption capacity could only be achieved with larger diameter and higher heights of the bed and smaller adsorbent pore sizes. However, contrary to previous investigations in the literature, our investigations showed that the fixed-bed column system for arsenic removal encountered some shortcomings, in particular the clogging of the column by lateritic particles which can slow down the water flow through the column in the long run.

To solve the clogging issue which was encountered with a fixed-bed system filled only laterite layers, we used the alternated layers, which means layers superimposed in the system. We cannot fill the fixed-bed column with the mixture since it represents a physical combination of two or more substances in which the identities are retained and are mixed in the form of solutions, suspensions and colloids. Such a configuration doesn’t favor the solution percolation through the fixed-bed column. That is why we use a superimposition of layers in the fixed-bed column (not a mixture). In this configuration, the percolation occurs through the whole column.

The fundamental question that rises here was to see whether the layers are easy to stack so that the permeability to water of the porous system is improved and the percolation of the solution occurs through the column without any risk of clogging. Basing on literature, it is known that an intercalation of a swelling adsorbent (laterite) by a non-swelling adsorbent material (activated carbon) could improve the permeability to water of the porous filter media.As expected, the intercalated laterite bed exhibits a higher hydraulic conductivity compared to the non-intercalated laterite bed, and consequently performs well. A higher hydraulic conductivity is linked to a higher flow rate, and a lower residence time of the water inside the porous material, diminishing the risk of clogging. On the other side, an exclusive utilization of activated carbon in a fixed-bed column is not appropriate since it will turn out very expensive when it comes to the upscale of the technology. The combination of the activated carbon and the laterite in the form of alternating layers not only minimizes the cost of the adsorptive porous system, but prevents the clogging of the system.

2) Furthermore, the goal is certainly to process the largest possible volume of wastewater to the breakthrough point as specified by the authors of 10ug/L. However, at the same time, the goal is to use the sorbent as much as possible, to saturate it as much as possible.

For this purpose, the results shown in Figure 8 should be completed with an additional experiment, a column with a bed depth of 20 cm, but filled exclusively with biochar. 

Answer:

As already stated, the implementation of a fixed-bed column filled exclusively with activated carbon was not the initial goal of the present work. The main objective was to implement natural laterite widely available in the country. The key issue in this work was to solve the clogging which occurs when we used a fixed-bed system only filled with natural laterite. The alternated layers allows us to solve the clogging problem and to minimize the cost of the system compared to a fixed-bed column exclusively filled with activated carbon. The additional experience with a column filled exclusively filled with activated carbon will certainly performs well, but cannot be compared with the results of the columns in figure 8 which are linked to the natural laterites we are using.

3) For the results shown in Figure 9 and 11, it is necessary to provide a comparison of the sorption capacities (mg/g) at the breakthrough point and the exhaustion point of the column. Then compare these results with the capacities obtained by the batch experiment.

Answer:

Dear reviewer, in this first set of experimental results, we are only concerned with the breakthrough point which characterizes the limit of 10 mg/g as set by the WHO. At this breakthrough point, the adsorption capacities are in the Table 6 and Table 9 (See the quantity Q_p in mg/g).

 

Table 6: Fixed-bed column parameters for adsorption of As(III) in DA laterite packed column intercalated with activated carbon at different bed depths.

 

Z (cm)	Mass (kg)	tp (min)	Vp (L)	Qtot(mg)	Qp(mg/g)	R(%)
20	0.839	900	45	44.450	0.053	99
30	1.425	3360	168	166.320	0.116	99
40	1.780	5100	255	252.450	0.141	99

 

 

Table 9: Fixed-bed column parameters for As(III) adsorption in DA laterite packed column intercalated with activated carbon at different concentrations.

C0 (mg/L)	Q (mL/min)	Z (cm)	Mass(kg)	tp (min)	Vp (L)	Qtot(mg)	Qp(mg/g)	R(%)
0.5	50	40	1.780	14820	741	295.362	0.165	99
1	50	40	1.780	5100	255	252.450	0.141	99
2	50	40	1.780	1920	96	191.011	0.107	99

 

As for the exhaustion point of the column, this means that the column is considered exhausted when the arsenic (III) concentration at the column outlet reaches the initial concentration which was used. Generally, these experiences are carried out with laboratory column with diameter £ 3 cm, which allow a saturation in an acceptable time (Roxanne Brion-Roby et al 2017):

10. Brion-roby, J. Gagnon, J. Deschênes, and B. Chabot, “Journal of Environmental Chemical Engineering Investigation of fixed bed adsorption column operation parameters using a chitosan material for treatment of arsenate contaminated water,” J. Environ. Chem. Eng., vol. 6, no. 1, pp. 505–511, 2018, doi: 10.1016/j.jece.2017.12.032.

These results have already been the subject of previous results in our laboratory (Bakouan, 2018 in [29]).

In our context, since we are using field-scale columns with diameters ³ 7 cm, the saturation is not expected to be reached. Indeed, because of the mass utilized in such large columns, it may take over months to reach the saturation. In our case, since we are not reaching the saturation point, we consider that the column is exhausted when we reach the breakthrough point which is set to a value of 10 mg/l by the WHO. This consideration takes into account real working conditions at the contaminated sites.

The adsorption capacities obtained on a fixed bed cannot be compared with those obtained by the batch experiment for several reasons: batch experiment are carried out in discontinuous mode; whereas fixed-bed system is carried out in a continuous dynamic mode; the samples grain sizes are not identical since in fixed-bed column we used grain size that are greater than the ones in batch experiment; in batch mode, due to mass transfer and driving force, the adsorption rate decreases during sequestration of the pollutant, because the concentration decreases in the medium, whereas in fixed-bed systems, the initial concentration is always the same.

4) The question arises whether the flow rate of 50 mL/min is too high. Would a 50% lower flow rate process a larger volume of waste water and achieve greater utilization (saturation) of the sorbent.

Answer:

The flow rate in this study was set to 50 mL/min. Indeed we experimented the fact that a 50% lower flow rate makes it possible to treat a larger volume of solution. But as we demonstrated in our previous investigations [BAKOUAN 2018 in [29]], an operation at a lower flow rate of 20 mL/min lasted 120 min and we treated a volume of water of 2.400 mL. However, in our context, we had to make a compromise between a short time (with a flow rate of 50 mL/min) needed to treat a daily amount of water acceptable for the rural population and a long time (with a lower flow rate smaller than 50 mL/min) needed to achieve a larger volume of solution. We found that a 50 mL/min flow rate was the optimal flow rate in our study at real sites working conditions in villages.

5) These are the basic questions that arise for me. 1. Does it make sense to use natural laterite alternated with biochar or is it better to use only biochar. 2. It is necessary to find an optimization solution, achieve the highest possible saturation of the sorbent and process the highest possible amount of wastewater.

 Answer:

This question was already addressed above. The goal of using natural laterite was based on the fact the fact this material is widely available in the country and could be used at low-cost. Moreover, this material has been proved to be an excellent adsorbent for arsenic removal (literature and our own investigations). Since conventional methods, which include adsorption on expensive activated carbon in batch experiment, for water purification are unsustainable in a developing country, the quest for simple, low-cost and high-performance water purification processes necessitates the use of local materials, which are natural materials, good sorbents and inexpensive and represent a viable replacement to expensive materials which are usually used for arsenic removal. Although activated carbon has a high BET value, its implementation has been proved to be very expensive in our context. We remind that we are looking for a low-cost system which can be implemented in rural environment. Investigations are ongoing at real contaminated sites and are very encouraging in terms of the quantity of water which is currently treated. The process has already been optimized and transferred at real contaminated sites. These are ongoing investigations, and we would like to say that the partial obtained results are very satisfactory, since with this low-cost adsorptive porous system, we were already able to achieve, over a period of six months, arsenic removal from real contaminated groundwater sites. In the following figure, you can see the upscale of what has been achieved at the laboratory scale.

Indeed, all the residual concentrations of arsenic solution after the percolation through the column are under the limit of 10 µg/L set by WHO, which shows that the systems performs well as for arsenic removal from a real contaminated groundwater containing As(III) and As(V); the method used for arsenic detection is based on AuNPs modified Carbon Paste Electrode. However, we need to evaluate the selectivity of the system in a forthcoming paper. Dear reviewer, this is an ongoing activity which needs to be validated. 

 

This is a pilot station of the adsorptive porous system which was installed at a real contaminated site in a rural village. The system consists of laterite layers alternated with activated carbon layers. Characteristics of the column: height = 1 m, diameter = 20 cm

 

Pilot station of fixed-bed column installed at a real contaminated site in a village

Specific comments: 

6) Line 28: „solve the problem of column clogging“ - clearly state why the clogging problem occurs.

Answer:

In the abstract, the clogging definition cannot be detailed since we have already reached the limit of 200 words. However, the clogging problem is stated as follows in the introduction.

 “A better adsorption capacity, during column percolation operation, could only be achieved with larger diameter and higher heights of the bed and smaller adsorbent pore sizes. However, contrary to previous investigations in the literature, our investigations showed that the fixed-bed column system for arsenic removal encountered some shortcomings, in particular the clogging of the column by lateritic particles which can slow down the water flow through the column in the long run. Clogging is generally defined as a process causing a drop in the performance of an adsorbent due to the deposition of suspended or dissolved materials on its outer surface or within the pores. In the case of a percolation through a laterite-lined column, small size laterite particles present low permeability to water, which leads to the clogging of the porous filter media.”

7) Line 38: „filtration system“ - I think that this term is not appropriate because it is about adsorption.

8) Line 97: „low-cost filter system“- I think that this term is not appropriate because it is about adsorption.

Answer:

Dear reviewer, we would like to point out the fact although it is about adsorption in a fixed-bed column, the term of filter is used by several authors [44]. However, we have modified this term to be “adsorptive porous system”.

9) Line 140: Define masses.

Answer: The masses are defined as follows:

 The following masses were defined in the manuscript.

Masse =163g, 290g, 439g.

10) Table 1: pHpcn – meaning of pcn. Is it point zero charge (pzc)?

 Answer:

This is the point of zero charge (pzc). This has been modified in the manuscript.

11) Define and explain in the discussion what is the purpose of impregnation with phosphoric acid. To define the sorption mechanism of As(IIII) binding to biochar and laterite.

Answer:

The following explanations justify the choice of phosphoric acid as the activator.

Phosphoric acid is the best impregnation reactant used in most cases, including the activation of carbon. According to the literature, the result of the impregnation is the formation of C–O-P bindings at the carbon active surfaces sites, which lead to activated carbon with a high specific surface [37].  

Since we didn’t carry out FTIR and XRD measurements on adsorbent residues after arsenic adsorption, at this step, we cannot give detailed adsorption mechanism regarding As(IIII) binding to biochar and laterite. Taken into account the structural characteristics of laterite and activated carbon, the most accepted mechanisms are as follows. The laterite contains some mineral phases, such as Fe₂O₃ and Al₂O₃, which are the main phases responsible for arsenic elimination through chemical binding at the surface of the laterite between As(III) and Fe in Fe₂O₃ and Al in Al₂O₃. In the case of activated carbon, we have the presence of surface charges induced by its chemical composition [34-36] which can be subject to electrostatic interactions.

12) In the experiment shown in Figure 3, why is Z=5, 10 and 15 cm, and not 10, 20 and 30, as well as the bed depth in the columns.

Answer:

Dear reviewer, the term Z (Figure 3) which is used in the paragraph 3.1 “Free swelling index of DA laterite”  doesn’t have the same meaning as the Z which is used in the fixed-bed experiments (paragraph 3.4.2). The Z used in the determination of the free swelling index is a standard test already described in the literature to determine a physical property. This determination technique uses a graduated test tube of total height of 20 cm, whereas the Z used in the fixed-bed column experiments describes the bed height of the column.

13) In the figure 11 ppm to mg/L

Authors' Answer:

The suggestion of the reviewer has been taken into account. Correction has been made in Figure 11 by replacing ppm with mg/L.

 

 

Figure 11. Breakthrough curves of As(III) adsorption in laterite packed column intercalated with activated carbon at different concentrations (Bed depth Z = 40cm, flow rate = 50mL/min; diameter =7cm; 0.900 ≤ G ≤ 1.250 mm).

Conclusions need to be improved.

14) Lines 590-592: „The parameters of the fixed-bed column were studied to find the optimal experimental operating conditions.“ - you will get an answer and a solution when you consider my suggestions.

15) Lines 602-604: „hese results are a step towards the large-scale use of  this type of laterite-filled column intercalated  with activated carbon to provide cleaner water in rural areas.“ - I'm not really sure, you MAY get an answer and a solution when you consider my suggestions.

Answer:

Dear reviewer, as we already said what we see in the fixed-bed column is not a mixture of two compounds, but a superimposing of layers of two compounds. The concept of mixture of compounds has nothing to do with the alternated layers method. In the alternated layers method, the goal is to improve the water permeability through the whole column, since we are confronted with a clogging with smaller adsorbent pore sizes.

We refer the reviewer to the detailed answer above in question 1). To solve the clogging issue which was encountered with a fixed-bed system filled only laterite layers, we used the alternated layers, which means layers superimposed in the system. We cannot fill the fixed-bed column with the mixture since it represents a physical combination of two or more substances in which the identities are retained and are mixed in the form of solutions, suspensions and colloids. Such a configuration (mixture compacted in the form of sludge) doesn’t favor the solution percolation through the fixed-bed column. That is why we use a superimposition of layers in the fixed-bed column (not a mixture). In this configuration, the percolation occurs through the whole column.

The intercalated layers method is currently being applied at real contaminated groundwater sites. A pilot station of the adsorptive porous system was installed at a real contaminated site in a rural village. The system consists of laterite layers alternated with activated carbon layers, and the characteristics of the column are the following: height = 1 m, diameter = 20 cm. The results obtained showed that all the residual concentrations of arsenic solution after the percolation through the column are under the limit of 10 µg/L set by WHO, which shows that the systems performs well as for arsenic removal from a real contaminated groundwater containing As(III) and As(V). Dear reviewer, this is an ongoing activity which needs to be validated in a forthcoming paper. Although we achieved satisfactory results, they cannot be put in a single paper. The present paper deals with the clogging issue when arsenic (III) solution is percolating through a fixed-bed column filled with natural laterite. The next papers will deal with other aspects, including the real mechanism of arsenic binding to laterite and activated carbon as well as the selectivity of arsenic oxidation states, the influence of co-existing cations and anions, which are present in real contaminated water.

To verify and confirm that the layers superimpose well or are really and easily stacked, we performed hydraulic conductivity of the porous media. In this context of dynamic mode, the Bohart-Adams and the BDST models allows us, according to the literature, to find the optima parameters as stated in the manuscript.

16) The literature should be cited carefully according to the propositions of this journal.

Following the reviewer’s recommendation, the literature has been cited carefully according to the guidelines of the journal.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

English quality must be improved and introduction should be presented in a way to provide sufficient and relevant information in proper sequence. 

Comments on the Quality of English Language

English quality of the manuscript must be improved.

 

Reviewer 3 Report

Comments and Suggestions for Authors

Dear Authors,

thank you for the answers.

First of all, I must say that the quantity of answers over quality of answers.

Changes in the manuscript are minimal, i.e. insignificant. The main thrust of my suggestions was not included in the manuscript.

 

Based on Author's answers like:

 

!Our previous investigations on arsenic removal using the batch mode showed that local natural laterite  had  high  efficiency  for  As(III)  removal  [28].  We  achieved  in  batch  mode  an elimination rate of 99.69% and 97.30%, for As(V) and As(III) respectively, for a dose of 0.75 g of laterite [28]. The adsorption capacities were 0.33 mg/g and 0.30 mg/g, for As(V) and As(III) respectively.!

 

„The alternated layers could  be either an inert  material or an adsorbent  material  which also adsorbs very well, without any negative influence on the targeted pollutant. We had the choice of using the following materials as alternated layers: activated carbon, manganese dioxide,

fine sand or coarse sand, gravels.“

 

„The combination of the activated carbon and the laterite in the form of alternating layers not only minimizes the cost of the adsorptive porous system, but prevents the clogging of the system.“

 

„The main objective was to implement natural laterite widely available in the country.“

 

„The goal is to improve the water permeability through the whole column“

 

 

I conclude the following:

1. Any synthesis of either activated carbon or bio-carbon is expensive. If treatment with phosphoric acid is also carried out, these are additional costs.

2. According to the authors, the purpose of biochar is to increase the permeability of the column, that is, to prevent clogging of the column.

3. Based on batch experiments, natural laterite has excellent arsenic removal efficiency, similar to biochar.

4. So the main conclusion of the manuscript is that it makes no sense to use biochar to increase permeability and to prevent column clogging. It is better to use gravel, which is much cheaper than biochar.