Arsenate and Arsenite Sorption Using Biogenic Iron Compounds: Treatment of Real Polluted Waters in Batch and Continuous Systems

Round 1

Reviewer 1 Report

Manuscript titled: Arsenate and arsenite sorption using biogenic iron compounds: treatment of real polluted waters in batch and continuous systems written by Castro et al. presents some basic results in area of As removal from aqueous solutions. The main idea of paper and research aim of work is interesting however experiments are too simple. Therefore the conclusions are not fully adequate and reasonable. I have some recommendations:

• Abstract shoul be re-styled
• Part introduction brings just few information on topic of As removal - should be extended
• Use just one form-  As or arsenic
• Part Materials and Method should be modified
• specify chemical forms of As used in experiments (origin of chemicals)
• conditions of batch experiments should be completed (rotation speed)
• complete conditions of As determination in samples
• Table 1 - are the values average values ? Or did you analyse sample just in one replicate ? Standard deviations should be included
• Figure 1 - error bars are missing
• Units in whole manuscript need to be uniform (mg/l or mg l^-1)
• in text you are writting about metals adsorption mechanism - however As is metalloid
• sorption isotherms need to be shown in figures
• remove white line in the middle of SEM images
• the discussion is very poor prepared
• conclusions are very general
• english language has to be improved in whole manuscript

Author Response

• Abstract should be re-styled

Author’s response:

The authors changed the style of the abstract.

 

• Part introduction brings just few information on topic of As removal - should be extended

Author’s response:

The manuscript only included aspects related to As adsorption using biogenic iron compounds. Now the authors have introduced more information following the reviewer’s suggestion.

 

• Use just one form-  As or arsenic.

Author’s response:

“As” has been changed by “arsenic” throughout the text.

 

Part Materials and Method should be modified

• specify chemical forms of As used in experiments (origin of chemicals)

Author’s response:

The chemical reagents were sodium arsenite (NaAsO₂) and sodium arsenate hydrated (Na₂HAsO₄·7H₂O), both provided by Fisher Scientific, UK. These data have been included in the Materials and Methods section.

 

• conditions of batch experiments should be completed (rotation speed)

Author’s response:

The rotation speed (150 rpm) has been now included in the Materials and Methods section following the reviewer’s suggestion.

 

• complete conditions of As determination in samples

Author’s response:

The plasma gas (Ar) flow rate and the pumping rate were included in the manuscript.

 

• Table 1 - are the values average values? Or did you analyse sample just in one replicate? Standard deviations should be included

Author’s response:

All the measurements are performed in triplicate. Furthermore, the authors worked with several bottles containing water from the river. Consequently, there are some variations in the values. Now the values have been changed and standard deviations are included.

 

• Figure 1 - error bars are missing

Author’s response:

Figure 1 is a diagram obtained with a software. Error bars have been added in Figure 2.

 

• Units in whole manuscript need to be uniform (mg/l or mg l^-1)

Author’s response:

Now all the units are mg/l.

 

• in text you are writting about metals adsorption mechanism - however As is metalloid

Author’s response:

The world “metalloid” has been introduced when As is referred.

 

• sorption isotherms need to be shown in figures

Author’s response:

Sorption isotherms are now shown in figure 3.

 

• remove white line in the middle of SEM images

Author’s response:

The white line has been removed following the reviewer’s recommendation.

 

• the discussion is very poor prepared

Author’s response:

The authors improved the discussion following the reviewer’s suggestion.

 

• conclusions are very general

Author’s response:

The authors introduced more details in the conclusions.

 

• english language has to be improved in whole manuscript

Author’s response:

The manuscript has been now reviewed by a native English speaker.

 

Reviewer 2 Report

This study deals with an assesment of arsenic ions adsorption performance for water remediation using the biogenic iron precipitates and with iron compounds particles encapsulated in alginate beads. Overall, the paper is well written and covers all requested aspects for the interest subject. I believe this study could be suitable for publication but will require minor revision before its consideration. The following minor aspects should be considered:

1. Why the initial concentrations of As and Zn marked in Figure 6 at pH =2.24 are not the same as the concentration of As (3.3 mg/l) and Zn (6.9 mg/l) in Caracarani river (as is seen in Table 1)?
2. What is initial porosity of columns filled with iron compounds particles encapsulated in alginate beads?
3. Please correct the number of Figure 7 (should be Figure 8) in page 9 line 312. Which mass of adsorbent in 6 cm bed height column is correct (13.35 g in Table 3 or 13.85 g in line 311 page 9)?
4. How the q_c (adsorption capacity) in column studies was calculated (give the formula)? Moreover, it is not clear why authors state „An increase of the bed height led to longer saturation and service time of the reactor due to the higher quantity of beads. However, the column effectiveness and the metal adsorption decreased slightly (Table 3)” (page 9 lines 314-316). The data presented in Table 3 indicated that parameter (q_c) was higher for As and Zn for bed height of 6 cm as compared to 3 cm. The authors should discuss obtained values of adsorption capacity for arsenic compared with other iron based sorbents.
5. Authors did not mention about the reuse of tested biogenic iron material used in the treatment of real polluted water. It is possible to regenerate?

Author Response

1. Why the initial concentrations of As and Zn marked in Figure 6 at pH =2.24 are not the same as the concentration of As (3.3 mg/l) and Zn (6.9 mg/l) in Caracarani river (as is seen in Table 1)?

Author’s response:

The authors worked with several bottles containing water from the river. Consequently, there are some variations in the values. Now the values have been changed and standard deviations are included in Table 1. In addition, a coprecipitation between Fe-Zn-As could take place.

 

 

1. What is initial porosity of columns filled with iron compounds particles encapsulated in alginate beads?

Author’s response:

The term “porosity of columns” is unclear. The authors have understood that the reviewer refers to the volume of hollows in the column. The authors have stimated that the hollow volume is 14%. According to the dimensions of the column, the hollow volume is 2.1 cm³ for 3 cm of bed height.

 

1. Please correct the number of Figure 7 (should be Figure 8) in page 9 line 312. Which mass of adsorbent in 6 cm bed height column is correct (13.35 g in Table 3 or 13.85 g in line 311 page 9)?

Author’s response:

Thank you for your comment. All the mistakes have been corrected.

 

1. How the q_c (adsorption capacity) in column studies was calculated (give the formula)? Moreover, it is not clear why authors state „An increase of the bed height led to longer saturation and service time of the reactor due to the higher quantity of beads. However, the column effectiveness and the metal adsorption decreased slightly (Table 3)” (page 9 lines 314-316). The data presented in Table 3 indicated that parameter (q_c) was higher for As and Zn for bed height of 6 cm as compared to 3 cm. The authors should discuss obtained values of adsorption capacity for arsenic compared with other iron based sorbents.

Author’s response:

The equations for calculating the adsorption capacity have been included in the text.

The authors have reviewed the discussion. The adsorption capacity increases with the bed height because the ions can diffuse into the beads during longer time. In addition, a table comparing obtained values of adsorption capacity for arsenic with other iron based adsorbents has been included.

 

 

1. Authors did not mention about the reuse of tested biogenic iron material used in the treatment of real polluted water. It is possible to regenerate?

Author’s response:

In this study, the possibility of the reuse of the adsorbent was nor tested. Nevetheless, the authors have included a discussion about this issue.

 

 

Round 2

Reviewer 1 Report

Dear authors,

 

• please modify the part- introduction. It does not meet the requirements from the previous review.
• Fig. 3 needs correction. Error bars are still missing.
• Fig. 4 - move letters a,b to the bottom of picture

 

Author Response

• please modify the part- introduction. It does not meet the requirements from the previous review.

Author’s response:

Previously the authors introduced the arsenic removal by membrane filtration and the chemical treatment (chemical coagulation and precipitation). Now some aspects about adsorption have been included such as the use of activated carbon-based adsorbents and metal-based adsorbents.

 

• 3 needs correction. Error bars are still missing.

Author’s response:

Error bars are now included in figure 3.

 

• 4 - move letters a,b to the bottom of picture

Author’s response:

Figure 4 has been modified following the reviewer’s suggestion.