## 2 Materials and Methods  
### Systematic Review Process  
The peer-reviewed articles synthesized in this review were retrieved from the Scopus database. As illustrated in the PRISMA flow diagram (Figure 1), the identification phase began with a search for articles with titles including the keywords mercury and soils, yielding 23,679 records. During screening, the search was refined to identify articles whose titles also included references to NbSs, narrowing the dataset to 108 records. No duplicate entries were found, and only open-access articles were retained for eligibility assessment.  
Bibliometric analysis was conducted on the 108 open-access articles to examine research trends and keyword co-occurrence. To assess relevance to the three primary review objectives, 'AND ABS' Boolean filters were applied to the abstracts of these 108 records. This abstract-level filtering was used to classify articles into three subsets aligned with each objective: 69 articles addressed mercury sources and land uses, 38 focused on NbS removal efficiencies, and 18 examined biochar as a treatment component. Within each subset, non-relevant articles were subsequently excluded based on full-text evaluation. Exclusion criteria included lack of soil context, absence of mercury-related data, unclear NbS system descriptions, or use of biochar unrelated to mercury treatment.  
In addition to the database-derived articles, nine relevant studies were identified using other sources such as manual reference checks and expert consultation and were included after full-text screening. In total, 93 articles met the eligibility criteria and were included in the final synthesis. A full summary of search terms, filtering logic, inclusion/exclusion criteria, and the Boolean syntax used is provided in Table S1 of the Supplementary Material. The selected articles were synthesized to evaluate current knowledge, identify key research gaps, and provide future directions for mercury remediation in soils using NbSs.  
Figure 1: PRISMA-based flow diagram illustrating the identification, screening, eligibility, and inclusion process for selecting peer-reviewed articles on mercury contamination in soils treated with NbSs. “ indicates truncation to include word variants (e.g., “remov*” captures “removal”, “removing”, etc.).  
Review Scope and Limitations  
The resulting documents for the comprehensive review were filtered to articles written in English and open access articles. Only peer-reviewed articles written in English and accessible through open access platforms were included to ensure reproducibility, transparency, and equitable access to the reviewed content. It is acknowledged that some relevant findings from non-English or subscription-based sources may not have been captured. However, such criterion allowed consistent application of search algorithms and full-text screening tools without institutional access limitations. Furthermore, while this review focused on peer-reviewed, open-access English-language articles, no formal risk of bias or quality appraisal tool was applied. However, studies were screened based on methodological clarity, presence of quantitative data, and relevance to mercury removal in NbSs. Despite these inclusion criteria, heterogeneity in experimental design, site conditions, and reporting depth may affect the generalizability of findings. Notably, variability in measurement methods for total and methylmercury, differences in scale, and lack of long-term monitoring were recurrent limitations among the reviewed studies.