Reactive and Hydraulic Behavior of Granular Mixtures Composed of Zero Valent Iron

Round 1

Reviewer 1 Report

This manuscript presents a discussion of reactive and hydraulic behavior of granular mixtures composed of zero valent iron (ZVI) based on data of 24 column tests.  The authors suggested to use two performance factors: retardation factor (R) and permeability ratio (K), to assess the contaminant removal efficiency in those column tests.  They derived several relationships of R, K and three experimental parameters: groundwater flow rate, ZVI content % in two granular materials (pumice and lapillus), contaminant concentrations (water solution of Ni, Zn and/or Cu).  I saw the number of fitting data for a relation curve is scare (three points for a curve).  I doubt the usability of these curve relations.  However, I appreciated the work that the authors did and suggest the acceptance of this manuscript.

Author Response

The authors would like to express their gratitude to the Reviewer for reviewing the manuscript and for his suggestion to accept it.

We improved our manuscript by adding more information on experimental activity, and we checked other laws as well to find better correlations. In particular, we have replaced Figures 6b and 10b because we found a law that fits better the data.

Reviewer 2 Report

The topic of the paper is interesting and suitable for the journal. The authors analyse and discuss the results of 24 long term column tests of 14 granular mixtures composed of Zero Valent Iron (ZVI) and a granular material (pumice or lapillus) in terms of hydraulic permeability and reactive behaviour. Most of the results of the columns are already published in other papers, but not a discussion with all of them simultaneously.

After reading the submitted paper, my recommendation is that it can be published after moderate revision. Prior to be published, the authors are encouraged to attend these issues:

As the authors indicate in line 79 (page 2), the aim of the study is to propose a method for the choice of the optimal composition of a granular mixture containing ZVI.  However, this is not clearly explained and is left to the reader understanding how to proceed. Thus, it is suggested that the authors include towards the end of the results’ section, a clear explanation of the proposed method, step by step so that the interested reader could easily follow each step to obtain the optimal composition for a particular case.

A Materials and Methods section is missing. Please include in it:

·      the columns tests

·       Table 1

·        The parameters calculated and used for their interpretation

·       How the polluted solutions were prepared

·       Justify the pollutant concentrations used (e.g., for Ni 8-10-40-50-95-100 mg/L) and discuss in the mixture Cu-Ni-Zn:

o   which pollutant is more easily removed by ZVI,

o   which pollutant would permeate more through the Permeable Reactive Barrier,

o   and if there are any interference among the pollutants o co-precipitation before entering in contact with the ZVI.

·       Why the Ni pollutant is much more frequently used as pollutant…. in 16 out of the 24 column tests, Ni was the only pollutant. This obviously biases the results towards this pollutant, the others are barely testimonial in comparison.

·       The methods for determination the concentration of the pollutants.

·       Explain how the breakthrough time is determined

Indicate what is L (cm) in Table 1. Why some columns measure 50 cm and other only 3 cm? What affects the different length of the columns?

Provide in Table 1 (or in new table) the numerical results of the parameters that are studied: Rf and Kr.

In figures 5b, 6b, 9b and 10b a simple linear fit would be equal to or better than the exponential fitted equations shown in these figures. Notice that the 3 points shown in Figure 10b practically define a perfect straight line. Please, change to the simplest linear model, or compare both and justify why to use the exponential equation instead of the linear.

Find a better equation to fit in Figure 9a. It should be a steep descent at first followed by a smoother descent later…. like a second order kinetics.

At the end of the paper (lines 314 and 315 in page 11) there is a mention to some supplementary material that can not be seen at any part. Supplementary Materials: The following supporting information can be downloaded at: 314 www.mdpi.com/xxx/s1, Figure S1: title; Table S1: title; Video S1: title.

Author Response

Please see the attachment

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The authors have addressed all the issues raised in the initial submitted version. Therefore, the document can be published in its current form.