Effect of chemical additives on electrokinetic remediation of Cr-contaminated soil coupled with a permeable reactive barrier

Chromium (Cr) contamination in soil, especially Cr(VI), is a serious threat to the environment and human health. The electrokinetic remediation (EKR) is a promising technology to remediate the Cr(VI). Therefore, in this study, EKR coupled with a permeable reactive barrier (PRB) was used to treat the Cr(VI)-contaminated soil. The CTMAB-Z, a modified zeolite (prepared with cetyltrimethyl ammonium bromide) alone and a mixture of CTMAB-Z and Fe(0) were used as PRB-1 and PRB-2 reactive media, respectively. The effect of chemical enhancers/additives, i.e. DL-tartaric acid and Tween 80 on EKR of Cr(VI) was also analysed in the contrasting experiments. While the effects of repair time, voltage gradient and DL-tartaric acid concentration on Cr(VI) remediation were investigated by using the multifactor orthogonal experiment which was based on contrasting experiments. The contrasting experiment results showed that the highest Cr(VI) removal rate (66.27%) and leaching efficiency (71.29%) were observed in the experimental group which had DL-tartaric acid and PRB-2. Furthermore, the multifactor orthogonal experiment results had depicted that the highest Cr(VI) removal rate (80.92%) and leaching efficiency (85.25%) were achieved after treating the samples at a voltage gradient of 2.5 V cm−1 for 8 days in the presence of 0.15 M concentration of DL-tartaric acid. This study demonstrated that Cr(VI) remediation through EKR process could be significantly enhanced by the use of PRB and additives.

reasoning? P7L31, why do you choose samples in the A4-S1 region to analyze Cr morphology? Please add your reason in manuscript. P7L41, please simplify and improve the conclusion, and delete experiment processes and methods which is written in the conclusion.

19-Feb-2019
Dear Professor Li: Title: Effect of chemical additives on electrokinetic remediation of Cr-contaminated soil coupled with a permeable reactive barrier Manuscript ID: RSOS-182138 Thank you for your submission to Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
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Reviewer: 2
Comments to the Author(s) 1. The manuscript is not well written, many poor sentence construction and grammatical errors. It is strongly recommended that the manuscript is further revised or reviewed by colleagues with English as first language as it is sometimes hard to understand. 2. What type of soil was used in this study? Please classify. 3. Lot of studies have been carryout on Cr contaminated soil and the some more recent literature survey in this area is needed.It is always better to correlate the findings of the study with the reported Literature. 4. The error analysis of test data is needed 5. The removal mechanisms of Cr(VI) is deficiency 6. The Cr morphology/forms is deficiency, other group soil is need Reviewer: 3 Comments to the Author(s) This research by Xu Yu et al. provides an approach in the effect of chemical additives on EKR of Cr-contaminated soil coupled with a PRB. Remediation technologies of contaminated soil are an important topic and a cutting-edge research direction, especially EKR technology. Although there are several studies in literature that have reported EKR of heavy metal in soil, there is still work to be done regarding the choice of electrolyte solutions, the application of permeable reactive barrier/PRB, the energy-saving of EKR and the mechanism of action of agent, etc. In this study, CTMAB-Z and mixture of CTMAB-Z and Fe (0) were used as PRB reactive media, the effect of chemical additives, such as di-tartaric and tween 80, on EKR of Cr-contaminated soil coupled with PRB, and relatedly reactive mechanisms were discussed. In addition, the three factors including repair time, voltage gradient and dl-tartaric acid concentration were investigated by utilizing multifactor orthogonal experiment, and an optimal reactive condition was disclosed. The manuscript is written clearly and concisely in good English, and it has certain academic value. I would recommend publication of the article if the authors are willing to proceed to a few changes/additions/corrections/ answers indicated below. P3L43, why do you only add chemical additives into cathode electrolyte? Please add your explanation in manuscript. P3L56, what does the leaching efficiency refer to? How do you test and calculate? It's not clear, please add relatedly information in your manuscript. P5L17, Explain why the electrolyte solution in anode compartment became darker with passage of time, and please add your reasons in your manuscript. P5L21, besides precipitation of heavy metal ions, I believe the tested soil became hardened is mainly because of volatilization of soil moisture caused by thermal effect. P5L41, the absolutely decrease of ion concentration and quantity in the system may be the main reason, please you rethink the reason why the lowest current was observed in A5 group. P5L58, please delete or correct the sentence "which indicated that soil was alkaline in nature and had strong pH buffering capacity"; the logic is incorrect, and tested soil properties is not derived from EKR experiments. P6L6, have you given any thought to the reduction of Cr (VI) by dl-tartaric acid? P7L20, did you characterize CTMAB-Z supported by Fe (0) after the reaction to verify your reasoning? P7L31, why do you choose samples in the A4-S1 region to analyze Cr morphology? Please add your reason in manuscript. P7L41, please simplify and improve the conclusion, and delete experiment processes and methods which is written in the conclusion.

Reviewer Recommendation and Comments
The manuscript "Effect of chemical additives on electrokinetic remediation of Cr-contaminated soil coupled with a permeable reactive barrier" is clear and follow a logic path. However, before it was accepted for publication in this journal, there are some questions. We have made correction according to comments, which are described below.
Thanks again for your time and letter. We would like to express our sincere appreciation for your careful reading and helpful comments.
Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have addressed the points. Reply: Thank you for your valuable comments. Although zeolite has been widely used as PRB materials, its adsorption capacity of Cr(VI) anions is limited by its native surface characteristics. These negatively charged zeolite can be further modified with cationic surfactants to form micelle-like bilayer structures, changing the zeta potentials, therefore enhancing the adsorbing capacity of anions. To further improve the adsorption capacity, the zeolite in this study was modified with cetyl trimethyl ammonium bromide (CTMAB), resulting in a better performance. Now, we have added some information to show the innovation of this research in the introduction section according to your guidelines. Now you can see on page (2) at the line (26-35).

Comment 2:
The specific analytical methods for Cr(VI) concentration in the soil have been missed.
And Cr(III) concentration need be obtained during the experiment.
(2)The concentration of Cr(III) in soil was indirectly determined by measuring the concentration difference between the total Cr and the Cr(VI). The concentration of total Cr in soil was determined by inductively coupled plasma emission spectrometer(ICP-OES). The initial concentration of total Cr and Cr(III) in soil were 36475.32 mg/kg and 35971.32 mg/kg respectively. Now, we presented the date of total Cr and Cr(III) in the two replicates of experiments under the optimum combination, which was shown in the following table. This paper focuses more on the remediation efficiency of Cr(VI) instead of total Cr and Cr(III).
From the viewpoint of removal rate, the removal rates of total Cr and Cr(III) are low due to their chemical stability and resistivity of migration. According to the above table, even under the optimal conditions, the removal rates of total Cr and Cr(III) are relatively low, the average removal rate of total Cr and Cr(III) were 6.53% and 5.49% respectively. Luckily, the Cr(III) is stable in the soil and its low toxicity brings less impact on the safety of environment. On the other hand, the concentration of Cr(VI) is critical due to its high toxic, although it only represents a small part of total Cr. The remediation efficiency of Cr(VI) is high as it is easy to migrate with EKR. Overall, this paper discussed the Cr(VI) removal rate and leaching toxicity as the main indicators, but did not consider the two indicators of total Cr and Cr(III). Reply：(1) The LOD in this experiment was 0.007ug/ml, and the LOQ was 0.023 ug/ml.
(2) We used K 2 Cr 2 O 7 and kaolin to prepare the simulated soil with the Cr(VI) concentration of 500 mg/kg. The experiment was carried out under the condition of a voltage gradient of 2.0V/cm, repair time of 5d. The average removal rate of Cr(VI) reached 90%. Therefore, it can be concluded that the EKR has a good effect on the remediation of the simulated soil. Based on this, the actual soil was studied in this paper.. Reply: In this paper, we used actual Cr-contaminated soil which had a high total Cr content and our research focused on the Cr(VI). After the experiment, the removal rate of Cr(VI) was very high, but more than 90% of total Cr remained in the soil after the experiment. On the other hand, it is difficult to measure the heavy metal precipitation in the PRB and cathode regions. Therefore, we directly disposed of the waste without analyzing the total Cr in each part. Thanks for your advice, we have found our shortcomings in my current work. We will improve my scientific research level in accordance with your suggestions in future work and make more achievements! Responses to Reviewer #2: Comment 1: The manuscript is not well written, many poor sentence construction and grammatical errors. It is strongly recommended that the manuscript is further revised or reviewed by colleagues with English as first language as it is sometimes hard to understand.
Reply: Thank you for your advice. We have made a detailed revision to the manuscript and we hope these will be satisfactory.
Comment 2: What type of soil was used in this study? Please classify.

Reply:
The soil was derived from a chemical plant which produced chromium salts. According to the date of XRF (Table 4) , the major elements in soil are Ca、 O、 Si、 Cr etc. The Cr(VI) content(504 mg/kg) of the soil is high and the soil is classified as solid waste according to the environmental quality standards for soils (GB15618-2008); According to the amount of sand, the soil is classified as loam; Furthermore, according to the property of pollutants, the soil can be divided into inorganic pollutantsheavy metal contaminated soil.

Comment 3:
Lot of studies have been carryout on Cr contaminated soil and the some more recent literature survey in this area is needed.It is always better to correlate the findings of the study with the reported Literature.