Scoping Review on Soil Contamination from Pb–Zn Slag and Environmental Assessment Methods

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Overall, the manuscript addresses a relevant topic and presents a structured overview of soil contamination from lead-zinc slag. However, several aspects require minor revision to improve clarity and presentation. In particular, the quality and consistency of the figures should be enhanced to meet publication standards. The methodological description, although following PRISMA-ScR guidelines, would benefit from clearer and more detailed explanations to improve transparency and reproducibility. Additionally, some sections could be streamlined to reduce redundancy and improve logical flow. Minor issues related to terminology consistency and English expression are also present and should be carefully revised. Finally, the discussion could be strengthened by incorporating more critical insights into the limitations of current assessment methods and providing clearer perspectives for future research.

Author Response

Comment 1: Quality and Consistency of Figures

"The quality and consistency of the figures should be enhanced to meet publication standards."

Response 1: We appreciate this comment and agree with your assessment. We have carefully reviewed all figures in the manuscript and made the following improvements:

• All figures have been re-examined for consistency in color schemes, font sizes, and labeling conventions.
• Figure resolution has been enhanced to ensure clarity and readability in both print and digital formats.
• Captions have been revised to be more descriptive and informative.
• Axis labels and legends have been standardized across all figures for improved consistency.

These revisions are highlighted in red throughout the manuscript and are visible in the revised figure files submitted with this response.

Comment 2: Methodological Description and PRISMA-ScR Transparency

"The methodological description, although following PRISMA-ScR guidelines, would benefit from clearer and more detailed explanations to improve transparency and reproducibility."

Response 2: We agree with this observation and have substantially improved the methodological section. Specifically, we have:

• Expanded the Methods section to provide more detailed descriptions of the search strategy, including specific databases consulted, search terms used, and date ranges of the literature search (Pages 4-5, paragraphs 2-3).
• Added a detailed inclusion/exclusion criteria table to clarify which studies were selected and why (Page 5, Table 1).
• Enhanced the data extraction process description to specify how information was systematically collected from each included study (Page 5, paragraph 4).
• Included a PRISMA-ScR checklist as supplementary material to ensure full transparency and compliance with reporting guidelines.

These enhancements improve the reproducibility and transparency of our methodology in line with PRISMA-ScR standards.

Comment 3: Redundancy and Logical Flow

"Some sections could be streamlined to reduce redundancy and improve logical flow."

Response 3: Thank you for pointing out areas of redundancy. We have carefully reviewed the manuscript and made the following revisions:

• Condensed the Introduction section by removing redundant statements about lead-zinc contamination while maintaining essential context (Pages 2-3).
• Reorganized the Results section to improve the logical progression from contamination sources to environmental impacts to human health risks (Pages 6-8).
• Removed overlapping content between the Results and Discussion sections to avoid repetition (Pages 8-10, marked in red).

The revised manuscript now has improved clarity and a more streamlined narrative structure.

Comment 4: Terminology Consistency and Language Quality

"Minor issues related to terminology consistency and English expression are also present and should be carefully revised."

Response 4: We have meticulously reviewed the entire manuscript for terminology consistency and English language quality. The following improvements were made:

• Standardized terminology: 'bioavailability' replaced with 'bioaccessibility' where appropriate, and 'contamination' is used consistently instead of the occasional use of 'pollution' (marked in red throughout).
• Corrected grammatical errors and awkward phrasing for improved readability.
• Enhanced verb-tense consistency, particularly in the Methods and Results sections.
• Improved sentence structure to ensure clarity and precision throughout the manuscript.

All corrections are highlighted in the revised manuscript for your review.

Comment 5: Strengthening the Discussion Section

"The discussion could be strengthened by incorporating more critical insights into the limitations of current assessment methods and providing clearer perspectives for future research."

Response 5: We greatly appreciate this suggestion and have substantially revised the Discussion section. The improvements include:

• Added a new subsection (Page 9, 'Limitations of Current Assessment Methods') that critically evaluates existing methodologies for assessing lead-zinc contamination, including their strengths and limitations.
• Included critical insights regarding the gaps between laboratory-based risk assessments and real-world environmental conditions.
• Expanded the 'Future Research Directions' subsection (Page 10) with specific, actionable recommendations for advancing the field, including the need for standardized protocols and long-term monitoring studies.
• Integrated discussion of emerging technologies and novel approaches for contamination assessment that warrant future investigation.

These revisions strengthen the manuscript by providing a more critical and forward-looking perspective on the challenges and opportunities in this research area.

Response to Comments on the Quality of English Language

The manuscript has been thoroughly reviewed and corrected for English language quality. The original English was generally acceptable, and we've refined several phrases for clarity and precision. All language revisions are marked in red in the resubmitted manuscript, making them easy to identify.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The study presents a synthesis of assessment methods for Pb–Zn slag-contaminated soils. It has strong potential but suffers from methodological weaknesses and insufficient analytical depth, and requires revision before it can be considered for publication

1. Ensure consistent use of key terms (eg, “slag-contaminated soils”, “Pb–Zn slag”) throughout the manuscript
2. The claim that “ No systematic mapping 10 of environmental assessment methods for slag-contaminated soils exists” requires stronger justification. The authors should clarify how this work differs from existing reviews and provide a comparison with prior literature to substantiate its novelty
3. Clarify the truly new points: (i) slag-specific, (ii) method mapping, (iii) Central Asia gap.
4. The novelty of the review is not sufficiently emphasize, it is advised to clearly differentiate this work from existing reviews by explicitly outlining its unique contribution (eg, slag-specific focus, methodological mapping, regional gap...)
5. The literature search is limited to three databases. The inclusion of additional databases (eg., Scopus, WoS) or a justification for their exclusion is advised
6. Lack of in-depth analysis; the results section mainly lists percentages (73.2%, 67.9%, etc.), it lacks a truly scientific synthesis, please compare the advantages/disadvantages of the methods and analyze when to use each method.
7. Only 7.1% of studies used the model, but the discussion did not analyze the reasons in depth. Please explain why it is less frequently applied and suggest specific research directions
8. The review lacks assessment of study quality or risk of bias. Even for a scoping review, a basic evaluation of methodological robustness would strengthen the reliability of theirconclusions
9. The conclusion that cadmium is the primary ecological risk driver should be more cautiously stated or supported with stronger comparative evidence

Author Response

Comment 1. Ensure consistent use of key terms (eg, “slag-contaminated soils”, “Pb–Zn slag”) throughout the manuscript

Response: We thank the reviewer for this observation. A terminological audit has been conducted throughout the manuscript. The term "Pb–Zn slag" is now used consistently to refer to the material, and "Pb–Zn slag-contaminated soils" to refer to the affected environment. Variant forms (e.g., "lead-zinc smelters") have been standardized. A typographical error ("CCritically") has also been corrected. Changes are highlighted in the revised manuscript.

Comment 2.The claim that “ No systematic mapping 10 of environmental assessment methods for slag-contaminated soils exists” requires stronger justification. The authors should clarify how this work differs from existing reviews and provide a comparison with prior literature to substantiate its novelty

Response: We appreciate this comment. The revised manuscript now explicitly acknowledges three relevant prior reviews: Zhou et al. (2022) on heavy metal assessment near Pb–Zn smelters [10], Han et al. (2025) on human health risks at industrial sites [110], and Kowalska et al. (2018) on pollution indices [121]. We clarify that none of these reviews focused specifically on Pb–Zn slag as a distinct contamination source, nor did they provide a systematic mapping of the full range of assessment methods applied in this context. This comparison has been added to both the Abstract and the Introduction (Section 1, final paragraph).

Comment 3. Clarify the truly new points: (i) slag-specific, (ii) method mapping, (iii) Central Asia gap.

Response: We have added an explicit statement of the three novel contributions at the end of the Introduction, immediately before the objectives: (i) slag-specific focus, (ii) comprehensive method mapping, and (iii) systematic identification of the Central Asian gap. This clarifies the unique contribution of our work.

Comment 4. The novelty of the review is not sufficiently emphasize, it is advised to clearly differentiate this work from existing reviews by explicitly outlining its unique contribution (eg, slag-specific focus, methodological mapping, regional gap...)

Response: We have strengthened the novelty framing in the Discussion. Section 4 now opens with an explicit statement that this is the first scoping review with a Pb–Zn slag-specific focus. Section 4.1 further explains why this focus is warranted, citing the distinct geochemical properties of Pb–Zn slag (high residual metal content, alkaline buffering, long-term weathering) that differentiate it from other metallurgical wastes [14], [17]. Together with the changes made in response to Comments 2 and 3 in the Introduction, the novelty of this review is now clearly articulated throughout the manuscript.

Comment 5. The literature search is limited to three databases. The inclusion of additional databases (eg., Scopus, WoS) or a justification for their exclusion is advised

Response: We acknowledge this limitation. The revised manuscript now provides an explicit justification for the database selection in Section 2.3: Dimensions indexes over 130 million publications with demonstrated comparable recall to Scopus and Web of Science (Hook et al., 2018), and OpenAlex extends coverage through aggregation of Crossref, PubMed, and institutional repositories (Visser et al., 2021). The exclusion of Scopus and Web of Science due to institutional access restrictions is acknowledged as a limitation in Section 4.5, with a recommendation for their inclusion in future reviews. Two supporting references have been added [161], [162].

Comment 6. Lack of in-depth analysis; the results section mainly lists percentages (73.2%, 67.9%, etc.), it lacks a truly scientific synthesis, please compare the advantages/disadvantages of the methods and analyze when to use each method.

Response: We thank the reviewer for this important suggestion. We have added Table 3, which provides a systematic comparative analysis of each assessment method category, including strengths, limitations, and recommended applications, with supporting references. A synthesizing paragraph has been added after Table 3 that critically analyzes when each method is most appropriate, why no single method is sufficient, and what an optimal integrated assessment strategy would look like. This transforms the Results section from a descriptive inventory into an analytical synthesis.

Comment  7. Only 7.1% of studies used the model, but the discussion did not analyze the reasons in depth. Please explain why it is less frequently applied and suggest specific research directions

Response: We have substantially expanded the discussion of predictive modeling in Section 4.3. The revised text now identifies three specific reasons for the low adoption rate: (i) high parameterization requirements of process-based models, (ii) the geochemical complexity of multi-phase slag dissolution, and (iii) disciplinary traditions favouring empirical field approaches over modeling. We have also added a dedicated paragraph outlining three specific future research directions: coupled reactive transport modeling at slag sites, machine learning as a computationally efficient alternative, and climate-scenario modeling for Central Asian slag deposits. All claims are supported by existing references.

Comment  8. The review lacks assessment of study quality or risk of bias. Even for a scoping review, a basic evaluation of methodological robustness would strengthen the reliability of their conclusions

Response: We appreciate this suggestion. While PRISMA-ScR 2018 guidelines explicitly state that a formal risk-of-bias assessment is not required for scoping reviews, as their primary objective is to map the extent and nature of available evidence rather than to synthesize effect estimates (Tricco et al., 2018), we agree that a basic quality appraisal strengthens our conclusions. Accordingly, we have added Section 2.6 (Methodological Quality Appraisal), which describes four quality criteria adapted from established environmental research frameworks: (1) adequacy of sampling design (n ≥ 20, spatial coverage documented); (2) use of validated analytical methods with QA/QC procedures; (3) inclusion of reference or background samples; and (4) application of at least one standardized assessment framework. Results are reported in the new Section 3.1.1: 34 studies (60.7%) were classified as high quality, 18 (32.1%) as moderate, and 4 (7.1%) as low. All low-quality studies were published before 2018, indicating improvement in methodological rigour over time. Importantly, the key findings — cadmium as the primary ecological risk driver and lead as the dominant health risk — were consistent across quality categories, confirming the robustness of our conclusions. The limitations of this appraisal are acknowledged in Section 4.5.

Comment  9.The conclusion that cadmium is the primary ecological risk driver should be more cautiously stated or supported with stronger comparative evidence

Response: We thank the reviewer for this important observation. The conclusion regarding cadmium has been qualified throughout the manuscript. We now explicitly acknowledge that the identification of Cd as the primary ecological risk driver is largely attributable to its high toxicity response coefficient (Tr = 30) in the Hakanson (1980) framework, which assigns Cd a weight substantially higher than Pb (Tr = 5), Zn (Tr = 1), and As (Tr = 10). We note that this ranking is partly an artefact of the coefficient system and that site-specific risk rankings may vary when alternative weighting schemes are applied (Cao et al., 2023 [125]). This qualification has been added in Sections 3.4.2, 4.2, and 5 (Conclusions).

 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

After reviewing the article, I make the following comments: 

Under the standards of a systematic review following the PRISMA-ScR guidelines. Ensure that the "background" values cited for Kazakhstan (Ref. [33], [75]) come from sources with methodologies comparable to international standards to avoid bias in the discussion of 3,000 mg/kg of Pb. In the article, some minor inconsistencies are observed in the formatting of names (some with initials, others full). They should be standardized according to the journal's style. 

The bibliography is sufficient and very appropriate. It is well balanced between the classics of toxicology and the current technological frontier. It fully justifies why it is urgent to carry out this study in the context of Kazakhstan.

Regarding the tables, a Figure 5 (Gap Pyramid) is mentioned in the text (line 366), but it appears labeled again as Figure 4. The numbering should be corrected to avoid confusion.

Figure 4 (the pyramid) is an excellent idea, but it should be supported by a proactive discussion.

What is suggested is a 'Public Policy Recommendations' section (Policy Implications). Not just mentioning what is missing, but mentioning how the authorities of Kazakhstan should apply the detected methods (e.g., prioritize the PMF model to separate plant responsibility from natural pollution).

Regarding tables. In the methodological details: it is suggested that Table 2 include a brief column with examples of software used (e.g., ArcGIS for GIS, EPA ExpoBox for health, Hydrus for modeling) to guide future researchers in Kazakhstan. 

What is missing is a table (or supplementary material) that summarizes the concentration levels of Pb, Zn, and Cd reported in the most critical studies in order to compare them with the 3,000 mg/kg in Shymkent. 

Regarding the statistical method, it is suggested to consider: a Co-occurrence Analysis (Network Analysis) since tools like VOSviewer could be used; you could include a keyword map. This would statistically show how the term "Lead-zinc slag" is connected with "Human Health Risk" or "Kazakhstan".

It is suggested to create a comparative table showing the concentration ranges (minimum, maximum, mean) of Pb, Zn, and Cd found in China, Poland, and Brazil compared to the data reported in Shymkent.

Regarding the metadata of the findings, although it is a Scoping Review and not a Meta-analysis, a small descriptive statistic (mean, median, range) of the lead concentrations found in the 56 studies would be very useful. This would allow one to state statistically: "The global average value of Pb in slags is X, while in Shymkent it is 3,000 mg/kg."

The article briefly mentions the climate in the introduction (line 73), but the results do not reflect it. What is suggested is to discuss how Predictive Modeling methods (such as Hydrus-1D) are critical for Central Asia due to its aridity and dust storms. The article is methodologically solid and comprehensive, but it is suggested to expand the discussion in the discussion section. It is suggested to carry out or integrate a direct comparison of the Shymkent concentrations against the global average from the review.

Author Response

Comment 1: The background values and analytical methods for the 3,000 mg/kg Pb concentration at the Shymkent site should be justified with methodological details.

Response: We agree. We have added a detailed methodological note in the Introduction (lines 76–84, pp. 2–3) explaining that the Shymkent Pb concentration was determined using AAS following GOST-standardized protocols, which are methodologically comparable to ICP-based international methods. We also clarified that the South Korean background values [75] were obtained using validated ICP-MS procedures consistent with ISO 17294 standards.

Changes made: Added methodological justification, lines 76–84, pp. 2–3.

Comment 3: Figure numbering should be checked for consistency.

Response: We have verified and corrected figure numbering throughout the manuscript. Figure 8 (Research Gaps Pyramid) is now correctly numbered on p. 21.

Changes made: Corrected Figure 8 numbering, p. 21.

Comment 4: The discussion of the research gaps pyramid (Figure 8) needs expansion.

Response: We have substantially expanded the discussion of Figure 8 in Section 4.3 (lines 548–559, p. 20). The expanded text now explains the hierarchical organization of research gaps: foundational geographic gaps (Central Asia) at the base, methodological gaps (predictive modeling, biological methods) at the intermediate level, and standardization gaps at the apex.

Changes made: Expanded discussion, lines 537–559, p. 20.

Comment 5: Policy Implications should include specific tools/models (PMF, Hydrus-1D, GIS).

Response: We have added a detailed paragraph in Section 4.6 Policy Implications (lines 635–647, p. 23) specifying that PMF should be prioritized for source apportionment at the Shymkent site, Hydrus-1D for long-term metal migration forecasting under semi-arid conditions, and GIS-based kriging for delineating exclusion zones.

Changes made: Added specific tool recommendations, lines 631–647, pp. 22–23.

Comment 6: Table 2 should include a column with example software/tools for each assessment method.

Response: We have added a new column “Example Software/Tools” to Table 2 (p. 12), listing specific software for each method category (e.g., ArcGIS Pro, QGIS for GIS; EPA PMF 5.0 for source apportionment; Hydrus-1D, PHREEQC for transport models).

Changes made: Table 2 expanded with software column, p. 12.

Comment 9: A comparative table of heavy metal concentrations across study regions should be provided.

Response: We have added Table 4 (pp. 16–17) presenting comparative Pb, Zn, and Cd concentration ranges, means, sample sizes, and regulatory limits for China, Poland, Brazil, France, and Shymkent (Kazakhstan). Data were extracted from included studies via Elicit.

Changes made: New Table 4 added, pp. 16–17, lines 394–406.

Comment 10: Descriptive statistics for Pb concentrations across reviewed studies should be reported.

Response: We have added descriptive statistics in Section 3.5 (lines 363–376, p. 15): among studies reporting absolute values (n = 32), Pb ranged from 58 to 59,000 mg/kg, with a mean of 9,878 mg/kg and a median of 2,397 mg/kg (IQR: 503–16,038 mg/kg). The Shymkent value of 3,000 mg/kg places it at the 53rd percentile globally.

Changes made: Added descriptive statistics, lines 363–376, p. 15.

Comment 11: The discussion should address climate-specific factors (arid/semi-arid) and their implications for Hydrus-1D modeling in Central Asia.

Response: We have added a detailed paragraph in Section 4.3 (lines 527–537, pp. 19–20) discussing the semi-arid climate of southern Kazakhstan (300–400 mm annual precipitation, high evapotranspiration, dust storms) and explaining how these conditions create distinct metal transport dynamics requiring adapted Hydrus-1D parameterization with wind erosion modules and episodic recharge functions.

Changes made: Added climate-adapted modeling discussion, lines 516–526, pp. 19–20.

Comment 12: The Shymkent contamination level should be compared to the global average from the reviewed studies.

Response: We have added a detailed comparison in Section 4.1 (lines 444–454, pp. 17–18) using data from Table 4: the Shymkent Pb level of 3,000 mg/kg exceeds the global median (2,397 mg/kg) by a factor of 1.3, while falling below extreme values at Santo Amaro, Brazil (37,460 mg/kg) and Upper Silesia, Poland (31,200 mg/kg).

Changes made: Added quantitative comparison, lines 433–443, pp. 17–18.

Author Response File: Author Response.pdf