Microalgae-Mediated Biosorption for Effective Heavy Metals Removal from Wastewater: A Review

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Minor revision suggested, see attached reviewer comment.

Comments for author File: Comments.pdf

Author Response

While I acknowledge the importance of such approaches, it's essential to clarify that the focus of this review paper is primarily on microalgae-based biosorption techniques for heavy metals wastewater treatment.

The inclusion of advanced cocultures, as suggested, involving synergistic interactions between microalgae and other organisms like fungi, is indeed a valuable avenue for future research in environmental biotechnology. However, the scope of this review paper is intentionally tailored to provide a comprehensive examination of microalgae-based biosorption techniques specifically for heavy metals removal. The decision to concentrate on this specific aspect stems from the need to delve deeply into the mechanisms, applications, and potential of microalgae in addressing heavy metal pollution.

While the synergistic co-cultures you mentioned are undoubtedly a promising direction for wastewater treatment, it falls slightly outside the intended focus of this review. I appreciate your understanding and would be happy to explore this aspect in more detail in future work or research projects.

Thank you for your understanding and constructive feedback.

Reviewer 2 Report

Comments and Suggestions for Authors

Review Paper

 

Manuscript Title: Microalgae-based Biosorption Techniques for Heavy Metals Wastewater Treatment

Authors: Dumisane Mahlangu, Keletso Mphahlele, Francesco De Paola and Nomcebo Happiness Mthombeni

Manuscript ID: 283465

1.     General Remarks:

Classical methods available for heavy metal removal from wastewater have several limitations such as poor efficiency, poor selectivity and high operating costs. In this review paper, the use of microalgae, that have emerged as a promising bioremediation approach for biosorption processes, have been reviewed in detail. Thereby, the role of microalgae in the removal of heavy metals from centralized, conventional and decentralized water treatment systems (DWTS) has also been addressed. DWTS becomes particularly important in regions where centralised systems face obstacles due to geographical constraints, inadequate infrastructure or financial limitations. The authors also claim that DWTS proves to be a promising solution in disaster relief scenarios and rapidly growing urban areas.

Sytarting from the Abstract, the authors seem to lose now and then the focus of the review paper, since not only biosorpton, but also microalgae and DWTS are presented as potential solutions to the wastewater treatment problem, which is a very general conclusion to be drawn from this study. The review paper is generally well-written and comprehensive, however, the focus is –as mentioned and demonstrated above- is not so clear, may be because the “Introduction” Section is too long and addresses multiple tasks, so that concentrating the readers to the point of the topic becomes difficult. The main focus of “Removal of Heavy Metals from wastewater using microalgae” comes too late in the review paper. Some more specific comments are listed below.

 

2.     Specific Comments:

·       Manuscript title: Heavy metals or wastewater treatment? Or heavy metals in wastewater? The title is not clear in this respect. The authors may clarify and consider revision of the paper title.

·       Title, abstract and elsewhere: It should be emphasized from the very beginning on the focus of the review paper. Heavy metals removal, wastewater remediation or the beneficial use as biomass for wastewwater treatment. Apparently, it is heavy metals and their effective removal by alternative, decentralized treatment systems, with biosorption using algae as a promising and sustainable remediation method…?  It is not very clear from the above-mentioned sections until the end of the next, Introduction Section; namely as follows: “The emphasis on commercially viable and environmentally sound microalgae-based treatment strategies highlights their potential to tackle water pollution challenges (heavy metals not specifically mentioned?)”. The major focus of the review is also mentioned in the Conclusions Section; however, it comes too late.

·       Introduction: Adsorption is mentioned as an efficient, alternative method. However, it should also be noted here that adsorption is not such a robust and cost-effective method considering that it can only be applied to treated water/wastewater as a final polishing step, and adsorbent regeneration, replacement, solid waste management (treatment and disposal) costs should also be mentioned as associated operation costs of the adsorption operation.

·       Table 1: Only a few chemical treatment processes and unit operations are listed here…there are more conventional/advanced options to treat wastewater. The authors may specify why they have selected specifically these ones. In addition, it is diffcult to say that all the drawbacks and advantages mentioned in this table are valid for the removal of specific pollutants from water or wastewater. Selectivity, costs and efficiency can change appreciably according to the target pollutant type and concentration. This table appears to be too general (not always applicable) and incomplete.

·                 1.2. Cultivation Techniques and Growth Conditions: In particular photobioreactors are very specialized systems and require very controlled equipment, wastewater charactieristics and operation conditions. How feasible is the use of photobioreactors in real-scale for heavy metal removal from complex water and wastewater matrixes considering economic, technical, and ecotoxicological issues? Besides, instead of indicating light intensities in “lux” units, other, more frequently used untis (SI units?) could be preferred because it will simplify comparison with alternative treatment conditions.

·                 2.2. Biosorption: This section describes basic principles of biosorption and its  mode of actions, factors affecting adsorption capacity, interaction with pollutants and the effluent medium, application ranges and biosorption capacity of various microalgae. It is too general; the removal mechanism and factors affecting adsorption capacity could be given more briefly.

·       Please delete the following sentence related to the paper template: “Proof of Theorem 1.Text of the proof. Note that the phrase “of Theorem 1” is optional if 577 it is clear which theorem is being referred to. Always finish a proof with the following 578 symbol. □”

·       Table 3 and Table 4: It is not clear why these tables are given separately and what their difference is. Heavy metals removal from water/wastewater by microalgae - ? Table headings should be more specified.

·       Future Directives and Research Perspectives: This section could be summarized with the following phrase “Investigating the scalability, cost-effectiveness, and overall environmental impact of integrating microalgae-based technologies within decentralised water treatment systems is critical. This assessment will determine these systems' practical viability and sustainability on a larger scale.” Apparently, the authors want to emphasize that there is a great potential since biosorption with microalage seems to be a promsing removal method for heavy metals. However, the next step of a real world application is yet not reported and/or very limited data is available. In  conclusion, whether algae could be used for metal removal from wastewater remains a big questionmark, since information about the performance, costs, reusability, process integration, ecotoxicological impacts is very limited and needs extensive investigation, also in this review paper...

·       Conclusions:

This paper reviews the potantial of algae-based technologies in the removal of heavy metals. The focus of this work appears to be biosorption in various water/wastewater matrices. Reaction mechanisms mainly include biosorption, bioprecipitation and bioaccumulation. It is also emhpasized that microalgae cultivation in the wastewater presents not only a cost-effective procedure but also a promising, advanced treatment alternative for sustainable water and wastewater management. In the last part of this section, recommendations for future work such as i) refining cultivation conditions, ii) exploring the potential of microalgae biochar, iii) conducting real-scale and real-world studies to explore the adoption potential. Apparently, the offered treatment approach is an interesting alternative remediation method and deserves further research work. However, it seems confusing and unclear what is innovative in the proposed remediation method and whether it really has the potential for real-scale applications and process integration with conventional treatment systems. These issues need further assessment.              

 

 

Comments for author File: Comments.pdf

Comments on the Quality of English Language

As indicated above, there are only minor English/typgraphical errors.

Author Response

Regarding concern about the clarity of the title, I understand the importance of a precise and informative title. The title has been amended and now reads as “Microalgae-Mediated Biosorption for Effective Heavy Metals Removal from Wastewater: A Review" (In a draft track changes line 3-4)

We are pleased to inform you that the title has been amended to read as follows: "Microalgae-Mediated Biosorption for Effective Heavy Metals Removal from Wastewater: A Review."   We have also made substantial improvements to the abstract and introduction to ensure a clear and early emphasis on the central focus of heavy metals removal from wastewater using microalgae-mediated biosorption. The revised abstract now explicitly communicates the key theme of the review, emphasizing the efficacy of alternative, decentralized treatment systems in the context of heavy metals remediation. (In a draft track changes line 18-35) In a draft track changes line 72-73)   In the Conclusions Section, the reason why  have reiterated and emphasized the main focus is to ensure that the reader leaves with a clear understanding of the significance of microalgae-mediated biosorption for effective heavy metals removal from wastewater.

The correction has been noted and amended the section by adding “However, it is crucial to acknowledge that adsorption has certain limitations. While it is advantageous for final polishing steps in treated water/wastewater, the associated operation costs, including adsorbent regeneration, replacement, and solid waste management, should be considered. These factors can affect the overall cost-effectiveness and robustness of adsorption as a heavy metal remediation method.  Despite these considerations, adsorption remains an economical and uncomplicated approach with benefits such as straightforward operation, cost-effectiveness, robust pH adaptability, and suitability for large-scale industrial applications.” In a draft track changes line 117-127)

We appreciate your insightful comments and recognize the need for additional specificity in our selection of chemical treatment processes and unit operations. The table aims to provide a broad overview of commonly used waste treatment techniques, highlighting their general advantages and disadvantages. We understand your concern about the need for specificity in the selection process. In the revised version, we included a brief explanation in the table caption/ introduction to clarify that these techniques have been chosen for their widespread use and representation of different categories of waste treatment methods rather than an exhaustive list of all available options. The terms used in the report serve as general indicators to facilitate a comparative understanding of waste treatment techniques, considering selectivity and cost factors, while acknowledging the nuanced and context-dependent nature of these considerations. In a draft track changes line 121-127)

The feasibility of utilizing photobioreactors at a real-scale for heavy metal removal from complex water and wastewater matrices is a multifaceted consideration, taking into account economic, technical, and ecotoxicological factors discussed in the provided section. The economic feasibility is addressed through the acknowledgment of the challenge in attaining cost-competitive algal products, emphasizing the need for economically viable growth media and the optimization of conditions to minimize operational costs, especially in cultivation and harvesting phases. The section also highlights the cost-effectiveness of utilizing wastewater as a growth medium, presenting it as an optimal resource for algal biomass production. From a technical standpoint, the section delves into the detailed characteristics and applications of photobioreactors, both aerobic and anaerobic, demonstrating the complexity of these specialized systems. It discusses the controlled environmental conditions maintained by photobioreactors, including light intensity, temperature, pH, and nutrient availability. The versatility of photobioreactors in providing a regulated environment enhances their potential for optimizing growth parameters and improving production efficiency.     The lux (symbol: lx) was used because itis the standard unit of illuminance in the International System of Units (SI).

Thank you for your insightful feedback on the bioaccumulation section (Section 2.2). We have incorporated your suggestions to provide a more concise overview of the removal mechanism and factors affecting adsorption capacity, while maintaining clarity and relevance to the overall context. We believe these modifications enhance the clarity and conciseness of the section, addressing your valuable comments. In a draft track changes line 509-521)

Noted and delated. (In a draft track changes line 605-608)

We appreciate your feedback regarding the clarity and specificity of Table 3 and Table 4. To enhance clarity and provide a more concise presentation, we have revised the tables and their headings accordingly. The modifications aim to clearly distinguish the content and purpose of each table. Below are the updated tables with more specified headings: Table 3: Microalgae-Driven Heavy Metal Removal Efficiency (In a draft track changes line 425-426) and Table 4: Biosorption Efficiency of Microalgae for Heavy Metal Removal . (In a draft track changes line 604-605)

We have carefully considered your suggestion and made the necessary modifications to improve clarity and alignment with your proposed phrase. (In a draft track changes line 689-710)

I would like to draw your attention to the modification made in the conclusion, specifically the inclusion of the sentence, "However, it is crucial to note that these issues need further assessment to fully comprehend their practical implications and refine the proposed strategies." This addition aims to acknowledge the ongoing nature of research in this field and the need for continuous assessment and refinement of proposed strategies. By highlighting this, we emphasize the dynamic nature of the subject matter and the importance of future studies to delve deeper into the practical implications of the discussed methodologies. (In a draft track changes line 785-787)

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have answered most of the reviewer comments and considered suggested notes for revision. The manuscript has been sufficiently improved. However, there are still several typographical (spelling) errors and I recommend to carefully revise for language once more. Thank you.

Comments on the Quality of English Language

The quality is fine, but there are some typographical errors that need another careful check by the authors.