Removal of Arsenic from Wastewater Using Hydrochar Prepared from Red Macroalgae: Investigating Its Adsorption Efficiency and Mechanism

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Comments and suggestions are shown in attached as below. 

Comments for author File: Comments.pdf

Author Response

Reviewer #1

Response: Thank you for your helpful feedback on our study.

1. After chemical hydrochar activation, an adsorbent reached a smoother surface area as well as a larger surface area. The authors should evaluate the adsorbent how much the pore improved and what exactly the pore size compared to before activation.

Response 1: We appreciate the suggestion to assess the changes in pore structure and size following chemical hydrochar activation. However, there are several valid reasons for not conducting this specific test at the moment. One primary limitation is the lack of access to the necessary equipment and resources required to perform detailed pore size analysis. This testing often demands specialized instrumentation, which is currently unavailable to us. Additionally, our research project faces budgetary constraints, and the cost associated with acquiring or outsourcing such testing may exceed our available funds.

 

2. Why did the authors use a chemical activated by FeCl3·6H2O? Previous studies used this or others else? Please make more discussion about these.

Response 2: Agreed, we added in the manuscript in:

Lines: 144-159

“FeCl3·6H2O is a well-known chemical activator for hydrochar due to its iron content, which can introduce functional groups and surface modifications to the hydrochar, making it more effective in adsorbing contaminants like arsenic (As). Zhang et al. (2023) study focuses on creating an arsenic (As) adsorption material called iron-modified hydrochar using hydrothermal carbonization (HTC) with different iron species: FeCl3·6H2O (FC), FeSO4·7H2O (FS), and Fe(NO3)3·9H2O (FN). It investigates their physicochemical properties, iron retention stability, and As adsorption capabilities. [2] used iron chloride (FeCl3) as the chemical activator to impregnate the activated carbon with iron because iron impregnation is a well-known and effective method for enhancing the adsorption properties of activated carbon. Iron impregnation can significantly improve the activated carbon's ability to remove various contaminants, including arsenic, from water. The presence of iron on the activated carbon's surface introduces active sites that can chemically bind with the target contaminants. These active sites enhance the adsorption capacity and efficiency of the activated carbon [3].”

 

3. Normally, how much the AS concentration containing in wastewater discharged to environments. Since, As concentrations prepared ranging from 0.5 mg/L to 0.25 mg. They were quite low. The authors should give the reasons and discuss.

 

Response 3: The overall concentration of arsenic in wastewater is high >1000 mg L⁻¹. The decision to use low concentrations of arsenic (As) in our batch study involving macroalgae-based hydrochar was driven by both the novelty of this approach and practical constraints. Given the relatively unexplored nature of using macroalgae-based hydrochar for As removal, lower concentrations provided a starting point to assess the material's performance and feasibility. Additionally, limited material availability necessitated the use of lower concentrations, allowing us to make the most of the resources at hand. While our chosen concentrations may not directly replicate real-world scenarios, they serve as a foundational step to understand the method's basic efficiency and potential, paving the way for future research and applications. [4] use 1mg/L for arsenic adsorption from wastewater to analyses the performance of Impregnated Activated Carbon by Fe-Oxide.

 

4. In the same way, the removal efficiency of As increased quite low (from 67% to 85.05%) as 1000 mg adsorbent were used. The authors should give reasons and discuss compared among previous studies.

Response 4: The relatively modest increase in arsenic removal efficiency when using 1000 mg of adsorbent may be attributed to a phenomenon known as adsorption saturation. When a sufficient amount of adsorbent is present, it's possible that the active surface sites of the adsorbent became nearly fully occupied, limiting further adsorption. This can result in diminishing returns in terms of adsorption efficiency with increasing adsorbent dosage. Additionally, the equilibrium concentration of adsorbate in the solution might reach a point where further adsorption is less favorable thermodynamically. It's important to note that adsorption processes often exhibit non-linear behavior and the relationship between adsorbent dosage and removal efficiency may not be strictly proportional. To better understand and interpret this phenomenon, a comprehensive analysis should be conducted, including referencing and comparing these findings with previous studies. These observations align with the tendencies reported in similar research where adsorption efficiency reaches a plateau beyond a certain adsorbent dosage, highlighting the importance of optimizing dosage for practical applications

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The authors examined the possibility of using hydrochars to remove arsenic from aqueous solution. Although the paper is not novel in terms of experimental design and scientific contribution, with certain corrections it could be considered for publication. 

1. If the applied activation is used for the first time, emphasize that, otherwise refer to the relevant literature.

2. In section 1.4, add information about the devices used

3. In part 1.5, describe in more detail the adsorption experiments, what time was used, what concentrations, etc.

4. Supplemented the tests determining the point of zero charge or Zeta potential.

5. Suggestion to the authors to include non-linear ispotherm models as well as three-parameter isotherms, besides consider the kinetics through the model of interparticle diffusion.

6. When examining the influence of concentration and time, additional points should be taken into consideration, it is necessary for the system to be in a state of equilibrium. Besides isothermal curves are not well performed, suggestion to the authors to look at similar literature

https://doi.org/10.3390/pr11051327

https://doi.org/10.1016/j.molliq.2023.121424

https://doi.org/10.1177/0734242x221093951

7. Provide FTIR spectra after adsorption, and and draw conclusions about the mechanism.

Comments on the Quality of English Language

Minor corrections are required

Author Response

 

Reviewer#2

The authors examined the possibility of using hydrochars to remove arsenic from aqueous solution. Although the paper is not novel in terms of experimental design and scientific contribution, with certain corrections it could be considered for publication.

Response: We sincerely appreciate the time and effort you dedicated to evaluating our manuscript. Your feedback is invaluable and has provided us with insightful perspectives to enhance the quality of our work.

1. If the applied activation is used for the first time, emphasize that, otherwise refer to the relevant literature.

Please check Response 2 to Reviewer#2,

2. In section 1.4, add information about the devices used

Response 2: Agreed we added this information to the manuscript in

Lines: 207-210

“Scanning Electron Microscope (SEM) imaging and Energy-Dispersive X-ray Spectroscopy (EDS) analysis were performed by Hitachi S-4800 Field Emission SEM on the hydrochar sample to observe its surface and identify the elemental composition, respectively.”

Lines: 213- 2017

“FTIR spectroscopy was also performed by Nicolet™ iS20 FTIR Spectrometer to identify the functional groups present in the sample. Infrared radiation in the range of approximately 10,000 to 100 cm^-1”

3. In part 1.5, describe in more detail the adsorption experiments, what time was used, what concentrations, etc.

Response 3: Agreed, we described the adsorption experiment in the manuscript in section 1.5. Batch adsorption

We added more details to the manuscript in

Lines: 226-232

“The study employed a range of time intervals (30, 60, 90, 120, and 150 minutes) to observe the kinetics of the adsorption process. Varied arsenic concentrations (0.05, 0.1, 0.15, 0.2, and 0.25 mg/L) were introduced to assess the impact of initial concentration on removal efficiency. Additionally, different dosages of the adsorbent (200 mg, 400 mg, 600 mg, 800 mg, and 1000 mg) were employed in separate experiments to examine their influence.”

 

4. Supplemented the tests determining the point of zero charge or Zeta potential.

Response 4: In this study, we acknowledge the significance of determining the point of zero charge (pH_pzc) or Zeta potential, as they are valuable parameters for understanding the surface charge characteristics of the adsorbent, magnetized hydrochar, and how it interacts with the adsorbate, arsenic. However, due to certain constraints, such as limitations in available resources, time, or specific instrumentation, we were unable to conduct these tests in our experimental setup. While this data would have provided additional insights into the electrostatic properties and surface charge behavior of the hydrochar, we sought to compensate for this limitation by thoroughly investigating the adsorption performance under a range of pH conditions. Our study primarily focused on examining the impact of key variables, including time, initial arsenic concentration, and adsorbent dosage, on arsenic removal efficiency. Although the absence of pH pzc or Zeta potential data restricts a comprehensive understanding of the electrochemical interactions, our findings still offer valuable insights into the practical performance of magnetized hydrochar for arsenic removal under real-world, variable pH conditions

 

5. Suggestion to the authors to include non-linear isotherm models as well as three-parameter isotherms, besides considers the kinetics through the model of intarparticle diffusion.

Response 5: we agreed and described the kinetics through the model of intarparticle diffusion in the manuscript Kinetic study section

Lines: 491-497

“Three different kinetic models, the models the pseudo-first order, pseudo-second order and Intra-particle diffusion model were examined in this study. Table 2 provides the kinetic constant values obtained from the first and second order and Intra-particle diffusion mod-els. these models are commonly used in solid-liquid systems [85]. In past numerous studies has been done that investigates the hydrochar performance based on kinetic models using first and second order and Intra-particle diffusion models for different heavy metal and other contaminant removal [86–88].” 

Lines: 500-512

“Intra-particle diffusion model is of-ten employed to gain insights into the adsorption process and to determine if it is con-trolled by intra-particle diffusion. This model is the primary rate-limiting step, the qt versus √t plot should exhibit a linear relationship, starting from the origin with a small C value. Figure 5 c) investigates the inter diffusion model within the studied medium, derived critical parameters to assess the process. The (R2) yielded a value of 0.61899, indicating the model's ability to explain approximately 61.899% of the variance in the adsorption data. The diffusion rate constant (k diff) was measured at 0.00322, representing the rate at which adsorption occurs, with an associated uncertainty of ± 0.00146. 'C', serving as the intercept in our adsorption equation, was determined to be 0.17051, with an uncertainty of ± 0.01383. These findings provide fundamental insights into the adsorption process within the medium and highlight the role of the intercept 'C' in our adsorption equation, despite this is not a modest fit of the model to the data. Further investigation and potential model refinement may be required to improve the goodness of fit.”

(As per reviewer suggestion we have added intar particle diffusion model, but due to time constrain we will look forward to add remaining models in our next manuscript)

 

Figure 5. Kinetic study a) pseudo-first-order model b) pseudo-second-order model c) intra particle diffusion model

 

6. When examining the influence of concentration and time, additional points should be taken into consideration, it is necessary for the system to be in a state of equilibrium. Besides isothermal curves are not well performed, suggestion to the authors to look at similar literature

https://doi.org/10.3390/pr11051327

https://doi.org/10.1016/j.molliq.2023.121424

https://doi.org/10.1177/0734242x221093951

Response 6: Your insights regarding the necessity of the system being in a state of equilibrium for a comprehensive analysis are duly noted. Thank you for providing these valuable resources. We added all references in the manuscript in lines:

“Three different kinetic models, the models the pseudo-first order, pseudo-second order and Intra-particle diffusion model were examined in this study. Table 2 provides the kinetic constant values obtained from the first and second order and Intra-particle diffusion models. Both models are commonly used in solid-liquid systems [11]. In past numerous studies has been done that investigates the hydrochar performance based on kinetic models using first and second order and Intra-particle diffusion models for different heavy metal and other contaminant removal [12–14].” 

 

7. Provide FTIR spectra after adsorption, and and draw conclusions about the mechanism.

Response 7: we agreed and described the mechanism in the manuscript in lines: 

 

Figure 2. FTIR spectra adsorbent:  hydrochar sample before activation, FTIR spectra of hydrochar sample after activation and Activated hydrochar spectra after As adsorption.

We added this part to the manuscript in:

Lines: 573-793

“When As comes into contact with the hydrochar surface, it can form strong chemical bonds with functional groups, such as carboxyl (HCOO-), hydroxyl (-OH), and amino (-NH2) groups [85]. These functional groups can act as electron donors and can form a complex with the As through a process known as ligand exchange Figure 6. The complex formed by hydrochar and As is believed to be stable and can remain in place for a long time. As can interact with these electrons, forming a stable complex. The understanding of these mechanisms has been applied to the development of activated biochar materials for water remediation. For instance, the incorporation of nano-zero valent iron (Fe0) into biochar can improve the immobilization of As (V) in water by creating strong chemisorption interactions through an oxidation and reduction reaction on the surface of the biochar [83]. To investigate the binding mechanism between As and AHC, we conducted a comparison of FTIR spectra before and after adsorption, as illustrated in Figure 2. In Figure 2, we also compared the FTIR spectra of, AHC, and HC prior to the adsorption of arsenic ions. All three materials displayed broad absorption bands at 1395 and 1660 cm−1, which can be attributed to the vibrations of C=O bonds [86]. Notably, this absorption bands exhibited minimal changes following arsenic adsorption, suggesting that the O-H and the C=O bond were not significant contributors to the adsorption process.

Simultaneously, we observed the appearance of prominent and well-defined peak at 820 cm−1, mainly associated with As-O. These symmetric and asymmetric As-O vibrations within the Fe structure imply the formation of iron arsenate precipitation on the material's surface [87,88].”

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Dear authors,

I reviewed your article and found it very interesting and certainly a good perspective for applications close to devices for absorbing harmful elements, such as arsenic.

However, I found serious deficiencies in the insertion part of the basic biomass, i.e. macroalgae; the chemical analysis part, however, seemed coherent and well prepared.

By virtue of this I request a minor revision and attach a pdf file with all the specific comments.

Comments for author File: Comments.pdf

Author Response

 

Reviewer #3:

Thank you for your valuable comments and suggestions regarding our manuscript. We appreciate the insightful feedback and have made significant revisions based on your points. Here is our response addressing each of the raised concerns:

Comment 1 write in more detail why macroalgae should used? Macroalgae is a often a source of bioactive molecules, which and which algae used for HTC?

Response 2: we agreed and added this part to the manuscript in:

Lines: 117-142

“Macroalgae, distinct from microalgae, possess plant-like attributes with higher protein content and a composition rich in cellulose, lignin, and hemicellulose. Several studies have explored the potential of hydrothermal carbonization (HTC) as an environmentally friendly and versatile technique for sustainable biomass processing. Biller and Ross (2012) investigated the application of HTC for producing biochar, biocrude, and syngas from algal biomass, showcasing its versatility in converting various biomass sources [5]. Patel et al. (2021) addressed ecological issues caused by waste seaweed through HTC, emphasizing the circular green solution of producing hydrochar and nutrient-rich liquid slurry from the seaweed [6]. Rasam et al. (2021) examined the impact of different parameters on the HTC process for Sargassum horneri macroalgae and used machine learning techniques to estimate key process parameters [7]. Spagnuolo et al. (2023) furthered the exploration of HTC by investigating hydrochar recovery from Sargassum muticum macroalgae and assessing its potential in adsorbing water organic pollutants for environmental remediation [8]. Spagnuolo, Bressi, et al. (2023) explored the practical applications of HTC solutions by investigating the reuse of discarded liquid phases as seed-priming treatments for Phaseolus vulgaris L., highlighting the benefits of HTC in agriculture and resource management[9]. The utilization of macroalgae offers a sustainable and renewable resource, promising applications in hydrochar, biofuels, and bioproducts, potentially transforming the sustainable landscape [10]. In this study, we explored the novel application of macroalgae-based hydrochar for arsenic removal, a previously unexplored avenue. Our findings reveal macroalgae based hydrochar promising potential as an effective method for mitigating arsenic contamination from wastewater.”

Comment 2: Which species? And how much the contamination level?

Response 2: Certainly, I understand your concern regarding the lack of specific information on the species and contamination levels in our study. In our current research, we didn't conduct a microbial analysis that would provide that level of detail. However, we acknowledge the importance of this information and plan to address it in future studies. We aim to incorporate microbial analysis to identify the specific species involved and quantify contamination levels more accurately. This will undoubtedly enhance the depth and precision of our future investigations.

Comment 3: why didn’t you use frozen or fresh biomass?

Response 3: Drying the biomass removes a significant portion of the water content, which reduces the energy required to heat and process the material. The HTC process typically involves heating the biomass to high temperatures, and wet biomass would require more energy to evaporate the water. Also, Wet biomass would require a longer processing time to remove the water content through hydrothermal carbonization, extending the overall processing time. Using dried biomass can expedite the process. In this study biomass naturally dried for 48 hours to reduce the time and energy and it’s easy to recover hydrochar. Also, it was easy to handle it as, we had limited amount of biomass available and we could not afford to lose it.

 

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Authors corrected manuscript, so now it can be considered for publication.