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Review

Soil Mercury Pollution in Nature-Based Solutions Across Various Land Uses: A Review of Trends, Treatment Outcomes, and Future Directions

Department of Civil and Environmental Engineering, Kongju National University, Cheonan 31080, Republic of Korea
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Author to whom correspondence should be addressed.
Appl. Sci. 2025, 15(12), 6502; https://doi.org/10.3390/app15126502
Submission received: 6 May 2025 / Revised: 6 June 2025 / Accepted: 7 June 2025 / Published: 9 June 2025

Abstract

Mercury (Hg) contamination in soils poses significant environmental risks. In response, various nature-based solutions (NbSs) have been developed and studied in the past to treat mercury along with other heavy metals from both point and nonpoint sources. However, various land uses present uncertainties in mercury mobility and treatment efficiency, affecting the scalability of NbS systems. In this study, a systematic review of peer-reviewed articles addressing mercury pollution in NbS soils was conducted. Results revealed that lakeside environments and mining areas are key Hg accumulation zones due to hydrological connectivity and anthropogenic pressures. Constructed wetlands were the most studied NbSs, where those with Acorus calamus and Aquarius palifolius as the main vegetation achieved >90% Hg removal efficiencies. Although NbSs achieved high Hg removal, anaerobic conditions were found to promote MeHg formation, a critical drawback. Moreover, biochar demonstrated potential for immobilizing Hg and reducing bioavailability, though certain types increased MeHg formation under specific redox conditions. Overall, the study highlighted the need for site-specific design, long-term field evaluation, and multidisciplinary strategies to optimize NbS performance for mercury removal. Furthermore, future research on the scalability of mercury-treating NbSs across diverse land uses is recommended to address mercury risks and improve effectiveness.

1. Introduction

Heavy metals as both point and nonpoint source pollutants have been the focus of numerous research endeavors in the past decades. Previous studies identified the source of heavy metals in runoff from urban environments as atmospheric deposition, road dust, tire wear, and surface wash-off from impervious areas such as roads, rooftops, and parking lots, mainly during rainfall events [1,2]. Mercury (Hg), a type of heavy metal, is a highly toxic element with no recognized safe exposure threshold, posing serious risks to human health even at low concentrations. Methylmercury (MeHg), in particular, is known to bioaccumulate and can cause neurological, reproductive, and developmental disorders, especially in children and pregnant women [3]. Chronic exposure through ingestion or inhalation, as observed in artisanal mining regions, has been linked to elevated mercury levels in blood and urine, indicating systemic toxicity and potential long-term health hazards [4,5]. In line with the confirmed presence of such heavy metals in stormwater runoff, innovative and cost-effective treatment mechanisms have also been the focus of study in recent years, particularly the removal of heavy metals in stormwater runoff using nature-based solutions (NbSs). NbSs have been defined as innovative stormwater management strategies that leverage natural processes and landscape features to enhance hydrological function and environmental resilience. Compared to conventional grey infrastructure, NbSs provide multifunctional benefits such as pollutant removal, flood mitigation, and ecological restoration [6,7]. More specifically, heavy metals have been effectively removed or stabilized in NbSs through mechanisms such as ion exchange, co-precipitation, and surface complexation, often enhanced by plant–microbe interactions and biochar amendments [8].
Despite the confirmed presence of such heavy metals in stormwater runoff, particular types such as lead, cadmium, and copper, have been numerously identified to be treated effectively in NbS systems [9]. Such technologies leveraged ecohydrological biotechnologies, including constructed wetlands, which utilize plant uptake and microbial interactions to immobilize or remove toxic metals from water matrices [10]. However, Hg has been determined to be a challenging heavy metal to treat in NbSs [11]. Additionally, Hg accumulation has been observed in both natural soils and those within NbS systems, such as constructed wetlands and urban mangroves. In natural settings like boreal peatlands and mangrove sediments, high organic matter content and anoxic conditions promote long-term retention and transformation of Hg species, including methylmercury [12]. Similarly, NbS soils accumulate mercury due to atmospheric deposition, industrial runoff, and wastewater inputs. The extent of Hg sequestration in such environments was found to be attributed to soil organic carbon, and microbial activity, leading to spatial variability across land uses and NbS designs [13,14]. Moreover, in urban wetlands, mercury was recently discovered to build up in sediments and vegetation due to industrial and residential runoff, with downstream reductions linked to retention by wetland plants [15].
Despite growing interest in plant-soil systems within NbSs, the integration of such approaches across diverse land uses remains underexplored. The unique biogeochemical challenges in urban, agricultural, industrial, and undeveloped land uses present uncertainties in mercury mobility and treatment efficiency, affecting the scalability of NbS systems. Therefore, this review aimed to synthesize existing peer-reviewed literature to characterize the occurrence and sources of mercury in NbS soils, evaluate the removal efficiencies of various NbS types and plant species, and assess emerging strategies for improving mercury treatment. A combination of bibliometric analysis and comprehensive review of peer-reviewed articles was performed to identify the trends of research on Hg in NbS soils, Hg sources, removal efficiency of NbS types and plant species, and the emergence of biochar as a treatment component. A potential value is seen in the results of the study in terms of optimizing the design of NbSs to be implemented in mercury-dominant areas amidst uncertainties brought by urbanization and land use change.

2. Materials and Methods

2.1. 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.

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.

2.2. Bibliometric Analysis

Two software tools for bibliometric review were used for the analysis of keyword co-occurrence, temporal and spatial trends of research on mercury contamination in NbS soils. First, VOS viewer was utilized to create network maps showing the co-occurrence between keywords over time. The resulting documents from the core review dataset were downloaded as a CSV file and imported into VOSviewer, where co-occurrence network maps and overlay visualizations were generated to illustrate the temporal development of research keywords [16]. The evolution of thematic focus, such as the increasing attention to phytoremediation, biochar, and constructed wetlands in recent years, was visualized using these outputs. For the analysis of spatial trends in mercury-related NbS research, a contingency matrix was developed in CorText Manager to examine the relationship between countries of publication and frequently occurring keywords. CorText Manager is a web-based platform designed for the analysis and visualization of large bibliographic and textual datasets. The web-based software is primarily used in scientometric and bibliometric research to explore co-occurrence networks, contingency tables, and topic evolution across time and space [17]. An RIS file derived from the core dataset was uploaded into CorText Manager, allowing geographic patterns and regional research emphases to be identified. Through this process, a comprehensive overview of both the temporal progression and spatial distribution of research in the field was conducted.

3. Results

3.1. Keyword Co-Occurrence and Spatial Trends

The keyword co-occurrence network map (Figure 2a) generated using VOSviewer revealed four main thematic clusters in mercury-contaminated soil and NbS research. The red cluster centered on “bioremediation” reflects microbial and plant-assisted detoxification. The green cluster, organized around “methylmercury” and “wetlands”, represented studies on biogeochemical processes in aquatic systems. The blue cluster, highlighting “soil” and “adsorption”, focused on remediation techniques in terrestrial environments. Moreover, the yellow cluster links “biochar” to soil stabilization approaches. The central positioning of “mercury” and its high connectivity across clusters underscores its interdisciplinary relevance, linking environmental, biological, and materials-focused research themes.
In addition to the cluster analysis, an overlay visualization of the keyword co-occurrence illustrating temporal evolution from 2017 to 2021 was developed, as seen in Figure 2b. The overlay network map revealed that earlier studies, represented in darker blue tones, were primarily focused on foundational topics such as “bioremediation”, “soil pollution”, and “methylmercury”, indicating a strong initial emphasis on microbial processes, contamination pathways, and wetland-based systems. In contrast, more recent keywords shown in yellow and green hues, such as “biochar”, “soil amendment”, “paddy soils”, and “charcoal”, indicated a growing research interest in the application of engineered natural materials for mercury stabilization and remediation. The term “biochar” appears as a prominent and relatively recent node, signifying its rapid emergence as a favored treatment component within NbS strategies. Overall, the timeline visualization highlighted a clear shift from classical bioremediation frameworks toward integrated, material-based approaches that support multifunctional ecosystem restoration. As visualized by the two network maps, the transition from microbially driven remediation to material-enhanced strategies like biochar reflects the field’s shift toward multifunctional and system-integrated approaches on Hg remediation in NbSs.
The spatial trend of research on mercury in NbS soils is summarized in Figure 3. A contingency matrix in Figure 3a was developed to illustrate the co-occurrence between countries and frequently occurring keywords in the research field of mercury in NbS soils. The color gradients on the contingency matrix represent standardized residuals: red cells indicate a higher-than-expected frequency of keyword use in a given country, while blue cells indicate lower co-occurrence relative to the global average. For instance, the United States showed a strong focus on “methylmercury” and “mercury (element)”, suggesting active engagement with microbial and chemical transformation pathways in mercury-contaminated systems. In contrast, Brazil, Colombia, and Indonesia were notably associated with “phytoremediation”, reflecting an emphasis on plant-based remediation strategies. Moreover, research initiatives in China more evenly distributed, with slight emphasis on “charcoal” and “soil”, aligning with the country’s growing interest in material-enhanced soil treatments. These patterns reveal regional research priorities and potential areas of expertise or underexplored niches within the global research landscape.
The stacked column chart in Figure 3b illustrates the temporal distribution of peer-reviewed articles on mercury in NbS soils across different countries, revealing a significant global increase in research activity after 2014. China, in particular, stood out as the leading contributor, particularly from 2018 onward, followed by the United States, Canada, and several emerging contributors from Asia, Europe, and South America. This pattern aligns with growing policy interest and environmental concerns related to heavy metal contamination in rapidly urbanizing and industrializing regions [18]. The accompanying heat map in Figure 3c reinforced this trend, with larger red circles concentrated in China, the United States, and Southeast Asia, indicating both high research volume and geographic clustering. These visualizations highlight the expanding international attention toward mercury remediation through NbS approaches, while also suggesting disparities in research intensity that may reflect differences in environmental urgency, funding availability, or technological readiness.

3.2. Sources of Mercury in Soils

Various land uses were identified as the subject of research on mercury contamination in the reviewed studies. The frequency distribution chart shown in Figure 4b visualized how land use types have been represented in studies on mercury in NbS contexts. Undeveloped lakeside areas such as forests and natural land were most frequently studied, comprising 32.0% (N = 24) of the total number of studied sites, followed closely by peatlands at 30.7% (N = 23), where natural biogeochemical processes are often evaluated. Mining and lakeside (commercial) areas were each included in 9.3% (N = 7) of the studies, while undeveloped land (10.7%; N = 8) and industrial zones (8.0%; N = 6) were addressed less often. This pattern suggests that research has been concentrated in relatively natural or transitional landscapes, where mercury behavior and NbS performance can be observed under less urbanized conditions.
The box plot in Figure 4a displays the distribution of total mercury (THg) concentrations in soils across various land uses, particularly industrial (IN), mining (MI), peatland (PL), undeveloped land (UD), lakeside commercial (LC), and lakeside undeveloped (LU), while Table 1 presents a summary of the reviewed studies on mercury contamination and pollution in soils from various land uses. Among these categories, lakeside areas (both LC and LU) exhibited the highest THg levels, particularly in LC sites where the range extends to nearly 1.2 mg/kg. This aligns with findings from subarctic and boreal studies, which indicated that land uses with hydrological connectivity to water bodies, such as riparian zones, peatlands, and undisturbed catchments, tend to promote mercury retention and transformation [19,20,21]. LU sites likely accumulate mercury through natural deposition and limited disturbance, while LC sites may reflect historical or diffuse anthropogenic input.
Contrastingly, mining and industrial soils presented lower median and mean THg values despite their known associations with heavy metal pollution. The relatively narrow interquartile ranges and modest outlier values in MI sites may reflect remediation efforts, natural attenuation, or localized rather than widespread contamination, consistent with studies from Zhang et al. [22] and Gosar et al. [23], which observed significant spatial variability and historical mining footprints confined to specific key accumulation zones. Peatlands and undeveloped areas showed moderate mercury concentrations, likely influenced by organic content and historical deposition without the amplifying effects of intensive land use or clear-cutting. In Yang et al. [24], mercury in lake catchment soils across England was primarily sourced from modern anthropogenic inputs, including urban runoff, waste disposal, and soil erosion exacerbated by human disturbance. While atmospheric emissions have declined, current Hg loads in sediments reflect active transfer from polluted catchment soils. For example, elevated Hg were linked to direct contamination from domestic waste, industrial runoff, and land management practices, with erosion and sedimentation facilitating Hg transport into the lakes. The study by Yang et al. [25] focused on agricultural zones and identified mercury inputs mainly from atmospheric deposition and irrigation using contaminated water. The research highlighted that modern agrochemical practices, including fertilizer and pesticide use, may indirectly contribute to soil Hg accumulation and that plowing and irrigation accelerate mercury mobilization into deeper soil layers. In Chen et al. [26], the spatial distribution of mercury in Lake Taihu sediments was strongly linked to urban and industrial pollution. The northern part of the lake showed the highest Hg concentrations due to inflows carrying untreated wastewater from nearby cities such as Wuxi and Changzhou. Non-ferrous metal processing, electroplating, and textile manufacturing were identified as key industrial contributors, while domestic sewage and agricultural runoff added to the diffuse load. Hg pollution was more pronounced near bays and tributary mouths, indicating that both point and nonpoint sources play a critical role in local Hg accumulation.
While most reviewed studies focused on natural or transitional landscapes, industrial and undeveloped areas remained underrepresented, comprising only 8.0% and 10.7% of the total, respectively. However, these settings may demonstrate distinct mercury dynamics due to site-specific anthropogenic or geochemical processes. In industrial zones, mercury can be released through historical emissions or metal processing activities, with elevated soil concentrations often persisting near facility boundaries [27,28]. In contrast, undeveloped lands, despite less disruption, still accumulate mercury through long-range atmospheric deposition and hydrological retention, especially in areas with high organic content or proximity to water bodies [29]. The limited representation of these land uses in existing studies highlights a need for broader site inclusion to improve the generalizability of NbS applications for mercury mitigation.
Table 1. Summary of the reviewed studies on mercury contamination and pollution in soils from various land uses.
Table 1. Summary of the reviewed studies on mercury contamination and pollution in soils from various land uses.
CountryStudied Land UsesResultsReference
CanadaRemote subarctic lake catchments with forested, wetland, and mixed terrainMercury concentrations in sediment were positively correlated with catchment characteristics such as dissolved organic carbon (DOC) input and forest cover. Forested and lower-elevation catchments had higher sediment THg.[30]
ChinaMining area in the Wanshan region, Guizhou ProvinceHg detected in soils at all sites; no significant bioaccumulation from soil. Hg concentrations were highest in upper watershed areas near former mercury mines. Pollution source analysis identified mining as the main source of Hg in soils.[22]
ChinaPeri-industrial and urbanized area in Gongzhuling, Northeast China24% of samples exceeded the provincial background value (0.04 mg/kg).[31]
Finland and SwedenSubarctic lake catchments along a climate–productivity gradient from pristine to intensive forestryTHg leaching from soils increases with forestry practices like peatland ditching. Higher THg baseline values found in eutrophic lakes with intensive land-use. Soil-related leaching of historical mercury due to land modification was a key factor. THg baseline positively correlated with catchment land use and the climate–productivity gradient.[21]
NorwayBoreal forest and peatland soils, with undisturbed and disturbed catchmentsForest soils had higher THg due to canopy throughfall, while MeHg levels were similar across soil types. Soil disturbance from machinery increased MeHg and MeHg-to-THg ratios in topsoil. Peatlands had higher %MeHg in deeper layers, indicating more local production. Disturbed areas showed significantly elevated MeHg production compared to non-disturbed areas.[20]
SloveniaNationwide study covering urban, rural, agricultural, and mining areas including Idrija and LitijaHg concentrations were highest in western Slovenia due to mining. Urban and industrial areas showed elevated Hg. Hg values exceeded European averages; atmospheric and river sediment transport contributed to regional dispersion.[23]
SwedenBoreal coniferous forests in watersheds with clear-cut and mature Norway spruceMeHg in organic topsoil increased after clear-cutting. Forest harvest adds ~6.6% of Sweden’s MeHg runoff. DOC-normalized MeHg levels also rose significantly after clear-cutting.[19]
United States Urban stormwater catchment treated by bioretention rain garden near San Francisco BayHg and MeHg were concentrated in the top 100 mm of soil. Subdrainage helped reduce MeHg formation. Soil near inlets exceeded EPA residential screening levels, but not for Hg. Findings highlight surface soil as the main zone of Hg retention.[32]
United StatesGreen roofs and conventional gravel roofs on campus and commercial buildingsHg concentrations in runoff were significantly higher from green roofs than gravel roofs, despite runoff reduction benefits from green roofs. Long-term accumulation or remobilization of retained Hg remains uncertain.[33]
The presence and behavior of mercury in urban green infrastructure systems have remained critically underexplored, despite the widespread adoption of these technologies for sustainable stormwater management. Systems such as bioretention ponds and permeable gardens have been widely implemented in urban areas, yet their effectiveness in retaining or transforming Hg has been evaluated in only a limited number of studies. In a multiyear investigation conducted by Gilbreath et al. [32], mercury was consistently retained in the ponding areas and engineered media of bioretention cells in the San Francisco Bay area. However, MeHg was occasionally exported during baseflow and dry-weather conditions, suggesting that microbial methylation may have occurred under favorable redox conditions. In another study by Chen et al. [34], Hg was effectively retained in permeable gardens within low impact development networks, although saturated zones with high organic matter content were found to facilitate methylmercury formation under anoxic conditions. These findings indicate that while green infrastructure systems can be effective for mercury capture and storage, the potential for MeHg generation under specific environmental conditions must be carefully considered.
Despite the limited presence of studies, irrigation with treated sewage water has also been associated with elevated mercury accumulation in soils. In the study by Pillay et al. [35], long-term irrigation with treated sewage water containing mercury concentrations between 0.1 and 1.0 ng/mL resulted in soil mercury levels reaching up to 500 ng/g, compared to only 12 ng/g in control soils irrigated with potable water. Zheng et al. [36] also reported that irrigation practices contributed to both surface deposition and vertical migration of mercury in paddy fields, especially under waterlogged and reducing conditions that favored methylation. In a more recent assessment, Salvato et al. [37] demonstrated that mercury retention and mobility in irrigated soil systems were influenced by the presence of dissolved organic carbon, which increased mercury solubility and potential bioavailability. Despite these observations, the mechanisms driving mercury transformation in irrigated agricultural landscapes remain poorly characterized. Many existing studies are limited by short-term durations, site-specific variability, or a lack of detailed chemical speciation. As a result, future research should prioritize long-term monitoring and integrated soil–water–plant assessments to better understand the risks associated with irrigation-induced mercury mobility and transformation, particularly in regions practicing water reuse.
Overall, the synthesis of studies revealed that both land use intensity and proximity to water bodies are major drivers of mercury accumulation in soils across diverse landscapes. The studies collectively show that mercury accumulation in soils is predominantly influenced by land use type, industrial legacy, and human disturbance. Apart from lakeside and downstream areas, mining activities consistently yielded the highest soil Hg concentrations, while coal-related and urban-industrial zones exhibited moderate levels, especially near traffic and industrial hubs. Forestry practices such as clear-cutting and peatland drainage significantly enhance mercury leaching and MeHg, particularly in organic-rich topsoils.

3.3. Mercury Removal Performance of NbSs

3.3.1. Hg Removal and MeHg Changes in NbSs

In line with confirmations of mercury pollution in soils, there has also been a research focus on cost-effective and sustainable treatment technologies like NbSs in recent years. Among the studied articles for the review on NbS treatment efficiency of mercury, 64% focused on constructed wetlands. Upon synthesis, constructed wetland performance was found to vary by configuration, with engineered systems supplemented with wetlands having unconventional soils and coagulants achieving the highest removal, while simpler plant-based systems still offered substantial uptake and ecological benefits. As summarized in Table 2, an inverse relationship between total mercury (THg) removal and MeHg production in NbSs was revealed. Wang et al. [38], using a surface flow constructed wetland amended with Fe, reported 98.9% THg removal and a 33.7% MeHg reduction (p < 0.01). Ma et al. [39] observed similar trends in a vertical flow constructed wetland with biochar mesocosms, with 92% THg removal and 43% MeHg reduction (p < 0.05). In contrast, Bachand et al. [40], working with constructed wetland mesocosms, achieved only 50% THg removal and noted up to an eightfold MeHg increase in summer (p = 0.0001). King et al. [41], using biochar-amended pots, reported 63% THg removal with a similar increase in MeHg (p < 0.05). These findings suggest that lower THg retention may leave more bioavailable Hg for methylation, highlighting the need to assess both THg and MeHg in NbS evaluations. These findings align with established principles on methylation. According to the key literature, the process of mercury methylation in NbSs is largely driven by microbial and geochemical conditions. Sulfate-reducing bacteria (SRB) and iron-reducing bacteria (IRB) are known to be the primary methylators of inorganic mercury under anoxic conditions, converting Hg2+ into the more toxic and bioaccumulative methylmercury [42,43].

3.3.2. Hg Removal Through Plant Uptake

In addition to NbS types, the influence of specific vegetation on THg removal and MeHg dynamics in NbSs has been extensively examined in recent studies, which are summarized in Table 3. In the study by Siswanti et al. [45], Aquarius palifolius was used in a free water surface constructed wetland system to remediate mercury-contaminated water, particularly from artisanal gold mining sources. The plant was found to function effectively as a hyperaccumulator, absorbing Hg from the water primarily through its submerged roots. The process followed a Langmuir isotherm model, indicating monolayer adsorption on a homogenous surface, and achieved up to 91.8% mercury removal. This model assumes that mercury binds to the plant surface in a uniform way, forming only one layer of adsorption. Moreover, in the study by Feng et al. [47], a constructed wetland in South Florida was investigated for its role in mercury methylation, particularly under varying inflow sulfate concentrations and dry–rewet cycles. The study demonstrated how environmental factors such as sulfate influx and drought-induced dryouts enhance in situ microbial methylation of Hg. Cells experiencing dryouts showed significantly higher MeHg levels, with inflow sulfate identified as a major stimulant of sulfate-reducing bacteria that facilitate Hg methylation. Meanwhile, Wang et al. [48] implemented a novel engineered system using zero-valent iron and pyrrhotite in a vertical flow constructed wetland planted with Acorus calamus. This configuration achieved 98.9% total Hg removal by enhancing redox reactions, which allowed efficient Hg uptake by the substrate and plants while suppressing MeHg formation, making it one of the most effective configurations reported. The study by King et al. [41] evaluated a pilot-scale constructed wetland treatment cell amended with gypsum (CaSO4) to reduce mercury in contaminated water. While the system achieved ~50% removal of THg, it also stimulated microbial activity that increased MeHg concentrations eightfold, especially in sulfate-rich conditions. However, planting Potamogeton pusillus was shown to mitigate MeHg buildup through demethylation, highlighting the importance of vegetation in balancing treatment outcomes. This pattern in MeHg buildup was similarly observed in a study by Bachand et al. [40], where the untreated control wetland increased MeHg concentrations by 11%, indicating enhanced methylation activity in the absence of chemical coagulants. Both findings emphasized that while wetlands can effectively lower total Hg, they also have the potential to promote MeHg production if not properly managed.
Though less frequent, non-wetland treatment systems such as phytoaccumulation reactors were also studied in the obtained articles. Gomez et al. [46] studied the effect of four native aquatic plant species (Eichhornia crassipes, Ludwigia helminthorrhiza, Marsilea quadrifolia, and Lemna minor) in a pilot-scale phytoaccumulation systems for mercury removal from mining wastewater. The study found that L. helminthorrhiza and E. crassipes showed the highest THg accumulation, achieving up to 74.4% and 50.9% removal, respectively. Root systems played a crucial role in uptake, and evapotranspiration and sediment deposition were also quantified to track Hg distribution across the system. The meta-analysis by Ma et al. [39] synthesized data from over 30 studies and confirmed that root uptake is the dominant pathway for mercury absorption in aquatic phytoremediation systems. Root retention and cell wall precipitation were highlighted as major mechanisms limiting Hg translocation to aerial tissues. The effectiveness varied by plant family, with species from Araceae, Hydrocharitaceae, and Salviniaceae showing strong uptake potential. Environmental factors like water pH and plant lifeform also significantly influenced Hg absorption outcomes.
The alluvial diagram in Figure 5 visualizes the relationship among NbS types, plant species, and mercury uptake levels categorized by concentration. The data used to construct this figure were extracted and synthesized from peer-reviewed studies included in this review, which reported plant-based Hg uptake values across different NbS systems. Uptake values were classified into four categories: very high (>100 mg/kg), high (20–100 mg/kg), medium (2–20 mg/kg), and low (<2 mg/kg). Non-constructed wetland systems, primarily comprising pilot-scale and experimental setups, showed strong associations with plants exhibiting very high Hg uptake. For instance, Salvinia natans and Lemna minor achieved elevated bioaccumulation under controlled mesocosm conditions with continuous water column Hg exposure and nutrient availability [39,49]. These systems typically used spiked water inflows and maintained ideal redox conditions for metal bioavailability, which likely enhanced uptake. Eichhornia crassipes and Ludwigia helminthorrhiza also showed high uptake values (58.8 and 51.89 mg/kg, respectively) in pilot-scale systems with shallow water columns and high organic content, potentially facilitating Hg mobility [50].
In contrast, most species in full-scale constructed wetlands were associated with low Hg accumulation. Studies by Gomez et al. [46] and Molisani et al. [51] documented that plants such as Phragmites australis, Elodea densa, and Sagittaria montevidensis were grown in wetland substrates characterized by high clay content or residual organic soils with slightly acidic to neutral pH levels, which are known to bind mercury and reduce its bioavailability. However, Typha domingensis exhibited very high Hg uptake, possibly due to its presence in a tropical wetland with fluctuating water levels and organic-rich peaty sediments that favor Hg methylation and plant assimilation.
The synthesis of the articles on the mercury removal performance of NbSs revealed that constructed wetlands and non-constructed systems both rely on plant uptake and sediment interaction for mercury removal, with root absorption and retention being key mechanisms in both. However, constructed wetlands showed greater variability in performance due to factors like design complexity, substrate type, and potential for MeHg production, while non-constructed systems such as phytoaccumulation reactors were simpler and more controlled but generally limited to short-term or small-scale applications. Overall, constructed wetlands could offer potentially higher scalability and customization but would require careful management to avoid promoting mercury methylation in soils.

3.3.3. Microbial Dynamics and MeHg Risk in NbS Soils

Alongside vegetation components, microbial communities were also found to influence Hg transformation processes in NbSs. Particularly, sulfate-reducing bacteria (SRB), such as Desulfovibrio spp., were noted as significant drivers of MeHg formation under anoxic and sulfate-rich conditions [29,52]. Other groups such as iron-reducing bacteria, methanogens, and fermentative anaerobes also showed contribution to Hg methylation, especially in organic-rich substrates where redox gradients are pronounced [53]. The methylation process is highly sensitive to biogeochemical factors like sulfate, organic carbon, and redox potential, making it a dynamic function of both microbial composition and environmental conditions. Conversely, seasonal and spatial heterogeneity in microbial activity can shift wetlands from MeHg sinks to sources, as demonstrated in Chavan et al. [54], where higher summer microbial activity correlated with increased MeHg production.
The structure and function of microbial communities involved in Hg cycling were observed to be strongly influenced by NbS design parameters. Substrate amendments such as biochar or iron compounds, vegetation type, and oxygen availability have been shown to alter microbial assemblages and their metabolic pathways. For instance, Chang et al. [55] reported that aerated wetlands amended with biochar promoted the growth of Hg-detoxifying bacteria such as Hydrogenophaga, Arenimonas, and Lysobacter, which corresponded with lower MeHg levels. Similarly, Du et al. [56] found that manipulating redox conditions in vertical flow wetlands suppressed MeHg formation while maintaining total Hg removal. Moreover, Li et al. [57] further emphasized that aeration disrupts anaerobic niches required for SRB and thus serves as a potential control strategy for MeHg suppression. These findings collectively highlighted the importance of integrating microbial considerations into NbS design, as microbial-mediated Hg transformations are not only biologically driven but also highly sensitive to system configuration and operational dynamics.

3.4. Biochar as an NbS Component for Treating Mercury

The application of biochar in mercury-contaminated soils has shown highly variable outcomes in terms of THg removal and MeHg production, with performance strongly influenced by biochar properties, system configuration, and prevailing redox conditions. As summarized in Table 4, a cross-study comparison of seven experimental systems showed THg removal efficiencies ranging from approximately 30% to over 95%, demonstrating biochar’s potential as a stabilizing agent for mercury. In particular, Fe-modified rice straw biochar in mesocosms achieved up to 98% THg removal, while also reducing MeHg by around 60% under moderately oxic conditions [58]. Similarly, column systems using combined oak woodchip and Fe-based biochar also achieved high THg removal (>90%) and suppressed MeHg formation by over 70% [59]. However, one consistent trend across the literature is that MeHg concentrations increased in a majority of the systems, particularly those operating under anoxic or fluctuating redox conditions. For instance, in field-based mesocosms mimicking constructed wetland redox dynamics, aged rice husk biochar was associated with up to a twofold increase in MeHg levels [60]. This increase was attributed to changes in the microbial community, with enhanced activity of methylating organisms such as sulfate-reducing and iron-reducing bacteria. Likewise, Xing et al. [61] and Man et al. [60] observed MeHg elevation in rice paddy or flooded soil systems amended with biochar. These findings indicate that the introduction of biochar into redox-unstable or anaerobic environments may unintentionally enhance MeHg production, especially in systems with high organic matter and microbial activity.
The type of system used (microcosm, mesocosm, column, or pot experiments) was also observed to be critical in interpreting treatment outcomes. Microcosms and columns allowed tighter control of redox boundaries, enabling better THg immobilization under designed conditions. For example, pine-derived biochar in anaerobic sediment microcosms reduced aqueous THg concentrations significantly due to its high surface area and redox buffering capacity [62]. On the other hand, field-like systems and pot trials, though more realistic, exhibited greater microbial MeHg generation, suggesting that operational complexity and real-world variability increase risk when applying biochar in NbSs. Furthermore, the functional design of the NbSs, such as whether the wetland system is surface flow or vertical flow, influences Hg fate by dictating oxygen gradients, substrate contact time, and microbial niche development. In surface flow wetlands or flooded soils, biochar often encounters strongly reduced zones where methylating bacteria thrive, while aerated or vertically flowing systems may offer better control over microbial activity and redox interfaces. These distinctions could be critical, as the dual role of biochar, as both a sorbent for inorganic Hg and a potential stimulant of MeHg production, depends on how the system manages sulfate levels and organic matter decomposition.
Altogether, the review findings emphasize that while biochar has strong potential to enhance Hg immobilization in NbSs, its deployment must consider not only the type of biochar and Hg species targeted but also the hydrologic regime, redox status, and microbial ecology of the system. Without such context-specific tuning, biochar amendments may exacerbate, rather than mitigate, mercury risks through unintentional stimulation of MeHg production pathways.
The conceptual framework presented in Figure 6 synthesizes the major factors influencing Hg dynamics in NbSs, including sources, plant uptake mechanisms, and treatment interactions. Mercury inputs into NbS systems were found to commonly originate from mining activities, atmospheric deposition, and industrial emissions, particularly coal-related sources. Lakeside environments, especially undeveloped zones, often function as passive THg sinks. Once introduced into wetland systems, mercury can be taken up and retained by aquatic vegetation. Species such as Acorus calamus, Eichhornia crassipes, Aquarius palifolius, and Ludwigia helminthorrhiza have been shown to exhibit strong Hg accumulation potential through root-zone uptake. However, conditions that favor accumulation, such as high organic content and low redox potential, may also facilitate MeHg production via microbial processes in sulfate- or iron-rich substrates.
It was also noted that biochar may have a dual role as both a stabilizing and potentially risk-enhancing treatment component. Biochar, upon synthesis, was revealed to improve soil physicochemical properties, such as pH and cation exchange capacity, thus enhancing THg immobilization. However, under reducing or flooded conditions, it may inadvertently stimulate microbial MeHg production, particularly if SRB are active. System design considerations, such as aeration, plant species selection, and organic matter management, are seen as strongly influential in these outcomes. Furthermore, the presence of DOC in NbS systems can enhance mercury mobility, posing challenges for long-term retention. Advanced monitoring of both THg and MeHg, alongside microbial and redox dynamics, is essential for understanding treatment performance and minimizing unintended risks.
Table 4. Summary of the reviewed studies on biochar as a treatment component for mercury contamination and pollution.
Table 4. Summary of the reviewed studies on biochar as a treatment component for mercury contamination and pollution.
Biochar TypeSystem TypeExperimental DescriptionTHg Removal Statistical SignificanceMeHg ChangeRedox ConditionsKey FindingsReference
Oak woodchip + Fe biocharConstructed wetland columnBiochar and iron amendments tested in column-type constructed wetland~90%p < 0.05IncreasedControlled aerobicCombined amendment reduced THg and MeHg synergistically[59]
Fe-modified rice strawSurface flow constructed wetland mesocosmSurface flow mesocosms tested with Fe-doped biochar~98%p < 0.05IncreasedOxic–anoxic gradientIron doping enhanced Hg immobilization and suppressed MeHg[63]
Maize stalkSimulated constructed wetland substrateStatic pots with maize biochar and Hg-spiked solutions57.6%p < 0.01Not studiedAnoxicBiochar adsorbed Hg, effectiveness varied with pH and DOC[62]
Rice strawPaddy soil potsRice biochar added to flooded soil pots with rice plants~30%Not reportedIncreasedAnaerobic, floodedBiochar increased MeHg due to microbial methylation activation[60]
Rice husk biocharSoils from Hg mining areaSoil microcosmsAdsorbed, not quantifiedNot reportedIncreasedFluctuating redoxBiochar increased MeHg under reducing conditions; THg adsorbed[61]
Pine woodAnaerobic sediment microcosmsLab-scale microcosms with biochar–sediment mixtureSignificantly reduced; but no specific valuep < 0.05IncreasedAnaerobicBiochar reduced THg and MeHg in porewater through sorption and redox buffering[64]
Aged rice huskField wetland mesocosmsField mesocosms with continuous flow and aged biochar amendmentNot specified-IncreasedFluctuating/anoxicBiochar increased MeHg production; shifted microbial composition[58]

4. Discussion and Future Perspective

Despite the limitation of information retrieval from a single database and open-access articles, research on mercury contamination in NbS soils has gradually increased over the past decade, with a notable rise in publications addressing both treatment mechanisms and environmental implications. Spatially, the research has been dominated by countries in Asia, North America, and Europe, with strong thematic linkages observed between keywords such as “biochar”, “constructed wetlands”, and “remediation”. These trends indicated a growing global interest in sustainable remediation approaches, though regional disparities in research focus suggest the need for expanded studies in underrepresented areas. Furthermore, significant knowledge gaps were observed, particularly regarding the long-term performance and reliability of NbSs in arid or semi-arid climate areas, where water availability and evapotranspiration rates may compromise treatment efficacy. Additionally, limited research exists on how NbS function under extreme weather events or shifting seasonal hydrology, which are increasingly relevant under climate change. Particularly, differences in NbS application between climate zones have also not been systematically examined. For instance, tropical regions may exhibit higher biological productivity and faster pollutant cycling, while temperate zones may offer more stable hydrologic regimes that support consistent treatment performance. The absence of comparative studies across climatic contexts limits the transferability of design principles and may lead to site-specific inefficiencies. These regional and climatic deviations highlight a critical research gap and should be more explicitly acknowledged in future investigations and policy guidance.
The synthesis revealed that mercury contamination in soils originates from diverse sources, including industrial emissions, mining, agricultural runoff, and atmospheric deposition, with lakeside environments acting as significant sinks due to sedimentation and hydrological connectivity. In such aquatic-adjacent areas, mercury can accumulate in surrounding soils through periodic flooding and surface runoff, increasing the risk of methylation. However, key challenges remain in accurately quantifying diffuse mercury inputs and implementing nature-based remediation approaches that are efficient across heterogeneous land uses. Amidst uncertainties, NbSs have shown strong potential for mercury treatment through mechanisms like plant uptake, root retention, redox transformation, and sediment binding. Aquatic plants such as Acorus calamus, Eichhornia crassipes, and Aquarius palifolius demonstrate high mercury removal efficiencies, often exceeding 70–90%, with certain species also capable of minimizing MeHg formation. However, the synthesis conducted in this study suggest that challenges still remain, particularly the risk of enhanced MeHg production under anaerobic conditions, which can offset the benefits of THg removal. Optimal design, plant selection, and substrate control were found critical for the long-term effectivity of NbSs.
A more focused review revealed that biochar has strong potential as a treatment component for mercury-contaminated environments by reducing total mercury mobility, lowering plant uptake, and converting Hg into more stable, less bioavailable forms. However, under certain conditions, particularly in fluctuating redox environments, biochar can unintentionally enhance mercury methylation, increasing the production of toxic MeHg. These risks, along with variability in biochar properties and site-specific soil chemistry, highlight the need for careful selection, modification, and monitoring of biochar applications in real-world treatment systems.
Overall, a potential is seen in NbSs in terms of offering a more cost-effective alternative to conventional Hg removal technologies. The high cost of such conventional technologies, such as activated carbon filtration and chemical stabilization, often limits their application at scale, with some estimates exceeding USD 1000 per gram of Hg removed depending on site conditions and treatment complexity [65]. In contrast, NbSs, including constructed wetlands and biochar-amended systems, show promise as cost-effective alternatives due to lower operational demands and co-benefits like ecosystem enhancement [66,67]. However, their large-scale adoption requires supportive regulatory frameworks that incentivize low-cost, decentralized technologies.
A comparative analysis of mercury removal efficiency, cost, and operational advantages is summarized in Table 5. While information on installation and production costs was obtained from external literature sources beyond the primary studies reviewed, the analysis of benefits and drawbacks was synthesized from the findings of this review. Constructed wetlands were found to exhibit high removal efficiencies (30–98%), with installation costs ranging from 50 to 120 USD/m2 [68]. These systems have been recognized for their applicability in large-scale, passive treatment scenarios. However, risks of MeHg formation under anaerobic conditions were also identified, particularly in sulfate- or iron-rich environments. Bioretention systems demonstrated removal efficiencies between 24% and 78%, with installation costs between 100 and 200 USD/m2 [69]. Their compact footprint and design flexibility have supported widespread use in urban contexts, though baseflow conditions have been associated with MeHg export due to the formation of localized anoxic microsites. Moreover, biochar amendments were observed to offer high removal potential (57–95%) and improvements in soil chemical properties, including pH and cation exchange capacity. Production costs were reported in the range of 300–500 USD per ton, depending on feedstock and processing method [70,71]. While Hg immobilization has generally been enhanced through sorption processes, increased MeHg formation has also been reported under reducing conditions, likely a result of elevated dissolved organic carbon and altered microbial dynamics. These findings underscore the need for careful evaluation of each NbS option in terms of cost-effectiveness, site conditions, and environmental risk. Integrating economic data with system-specific benefits and limitations, as summarized in this study, is essential for guiding sustainable and context-sensitive NbS implementation.
In terms of policy, initiatives such as performance-based subsidies, flexible permitting, and integration into national Hg reduction plans could accelerate NbS deployment while maintaining environmental safeguards. As such, aligning economic assessments with policy development is critical to mainstreaming NbSs for mercury-contaminated sites. Furthermore, the implementation of NbSs aligns with the goals of the Minamata Convention on Mercury, which promotes the reduction of mercury emissions and releases to the environment [72]. Incorporating NbS strategies into national action plans under this treaty may support compliance while offering nature-based co-benefits and localized resilience. To strengthen this alignment, financial mechanisms have been recommended for adaptation in order to reflect the multi-functional value of NbSs. Although support has been provided under the Minamata Convention through mechanisms such as the Global Environment Facility and the Specific International Programme, NbSs have not been consistently recognized as eligible for funding. It has been suggested that expanding eligibility to include NbS interventions, particularly those that demonstrate both Hg removal and ecosystem restoration, may allow access to blended financing from climate, biodiversity, and pollution-related funds. Long-term adoption may be facilitated if NbSs are formally integrated into national mercury management strategies, supported by dedicated funding channels and outcome-based monitoring frameworks
Reflecting the bibliometric analysis conducted in this study, the application of NbSs for mercury mitigation in developing countries has remained limited due to financial, technical, and institutional challenges. Conventional engineering solutions have typically been prioritized in infrastructure investment, while understanding of NbS implementation methods and their associated benefits has often been lacking. Barriers such as insecure land tenure, limited availability of baseline mercury data, and weak monitoring capacity have further constrained progress. It is proposed that these technologies could be advanced using capacity-building programs, policy incentives, and the demonstration of successful pilot projects. Greater integration of NbSs into national mercury control and climate adaptation plans has been viewed as an important step toward enabling equitable and sustainable remediation efforts in resource-constrained regions.
Additionally, the potential application of NbSs in artisanal and small-scale gold mining (ASGM) regions, particularly in Africa, has garnered increasing attention due to the growing environmental burden posed by mercury contamination. In Mauritania, elevated levels of Hg were detected in soils near ASGM sites in Chami town, with dispersion patterns driven by wind and tailings transport [73]. Similar contamination was documented in Ghana and Zambia [74,75], where limited regulatory oversight and hyper-arid or savannah environments complicate remediation. In such settings, constructed wetlands and biochar-based soil remediation systems offer promising low-cost alternatives that can be locally managed with minimal infrastructure. However, these must be tailored to hydrologic and climatic contexts, especially where water is scarce. Adapting NbSs to such mining zones not only addresses mercury pollution but also provides ancillary benefits such as soil rehabilitation, water reuse, and livelihood support, particularly where ecological engineering and local stewardship are integrated.
The findings of this study may also inform NbS-based treatment strategies for these related contaminants, particularly in contexts where Hg-specific data remain limited. In future research endeavors on NbS soils, it is recommended that the long-term field assessments across diverse land uses be prioritized to better understand pollutant dynamics, enhance treatment reliability, and guide adaptive design strategies. Advancing standardized monitoring frameworks and integrating multidisciplinary approaches is seen as beneficial in improving the sustainability and scalability of NbS applications in mercury-impacted environments.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/app15126502/s1, Table S1: Summary of search strings used for article selection in the systematic review of mercury-contaminated soils in NbS.

Author Contributions

Conceptualization, M.E.R. and L.-H.K.; methodology, M.E.R.; software, Y.O.; validation, L.-H.K. formal analysis, M.E.R.; investigation, M.E.R., Y.O. and M.T.H.; resources, L.-H.K.; data curation, M.E.R., Y.O. and M.T.H.; writing—original draft preparation, M.E.R.; writing—review and editing, M.E.R. and M.J.; visualization, M.E.R.; supervision, L.-H.K.; project administration, Y.O. and L.-H.K.; funding acquisition, L.-H.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Not applicable.

Acknowledgments

This research was supported by the National University Development Project of the Ministry of Education, Korea in 2024.

Conflicts of Interest

The authors declare no conflicts of interest.

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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.).
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.).
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Figure 2. Keyword co-occurrence network related to mercury pollution in NbS soils: (a) cluster view showing thematic groupings, and (b) overlay visualization indicating the temporal evolution of keyword usage.
Figure 2. Keyword co-occurrence network related to mercury pollution in NbS soils: (a) cluster view showing thematic groupings, and (b) overlay visualization indicating the temporal evolution of keyword usage.
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Figure 3. Spatial trends of research on mercury pollution in NbS soils using a (a) contingency matrix of keywords and countries, (b) number of articles per country from 2005 to 2024, and (c) heat map of the cumulative number of articles.
Figure 3. Spatial trends of research on mercury pollution in NbS soils using a (a) contingency matrix of keywords and countries, (b) number of articles per country from 2005 to 2024, and (c) heat map of the cumulative number of articles.
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Figure 4. Summary of the sources of mercury in soils visualized with a (a) box plot of soil THg concentration across different land use types and (b) frequency distribution of land use types reported in the reviewed studies (data from 13 studies, 75 sites).
Figure 4. Summary of the sources of mercury in soils visualized with a (a) box plot of soil THg concentration across different land use types and (b) frequency distribution of land use types reported in the reviewed studies (data from 13 studies, 75 sites).
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Figure 5. Alluvial diagram showing the relationship among NbS type, plant species, and mercury uptake level, categorized by concentration ranges: very high (>100 mg/kg), high (20–100 mg/kg), medium (2–20 mg/kg), and low (<2 mg/kg).
Figure 5. Alluvial diagram showing the relationship among NbS type, plant species, and mercury uptake level, categorized by concentration ranges: very high (>100 mg/kg), high (20–100 mg/kg), medium (2–20 mg/kg), and low (<2 mg/kg).
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Figure 6. Summary of research findings and potential challenges on the different components of mercury-treating NbSs.
Figure 6. Summary of research findings and potential challenges on the different components of mercury-treating NbSs.
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Table 2. Summary of Hg removal efficiencies and MeHg responses across various NbS configurations.
Table 2. Summary of Hg removal efficiencies and MeHg responses across various NbS configurations.
NbS TypeTHg Removal EfficiencyMeHg ChangeStatistical SignificanceReference
Free water surface flow constructed wetland86.3Not reportedNot reported[44]
Free water surface flow constructed wetland with amendments98.9Decreasedp < 0.01[38]
Constructed wetland mesocosm50Increasedp = 0.0001[40]
Free water surface flow constructed wetland91.8Not reportedNot reported[45]
Biochar cells63Increasedp < 0.05[41]
Biochar mesocosms92Decreasedp < 0.05[39]
Floating wetland78Not reportedNot reported[46]
Table 3. Summary of the plant species and mercury treatment performance by constructed wetlands in the reviewed studies.
Table 3. Summary of the plant species and mercury treatment performance by constructed wetlands in the reviewed studies.
NbS TypePlant SpeciesRemoval EfficiencyReference
Constructed wetlandT. latifolia, T. angustifolia, T. domingensis14% removal of THg; 11% increase in TMeHg[40]
Pilot-scale constructed wetlandLimnocharis flava90%[44]
Constructed wetland with pyrrhotite and zero-valent ironAcorus calamus98.9%[38]
Free water surface constructed wetlandAquarius palifolius91.8%[45]
Pilot-scale constructed wetland treatment cellScirpus californicus; Potamogeton pusillus~50% THg removal; 8% increase in TMeHg[41]
Table 5. Comparison of cost, mercury removal efficiency, scale, advantages, and risks of selected NbS technologies.
Table 5. Comparison of cost, mercury removal efficiency, scale, advantages, and risks of selected NbS technologies.
NbS TypeRelative Capital CostsHg Removal EfficiencyAdvantagesDrawbacks/Risks
Constructed wetland50–120 (installation) [68]30–98%
  • Passive long-term treatment
  • High retention capacity
  • Effective in saturated, low-energy environments
  • Potential MeHg formation under low redox
  • Performance sensitive to vegetation and microbial activity
Bioretention cells100–200 (installation) [69]24–78%
  • Proven THg retention
  • Widely used in urban areas
  • Customizable soil and plant media
  • Risk of MeHg export under baseflow
  • Sensitive to hydrologic changes and media composition
Biochar-amended soils300–500 (per ton, production cost) [70,71]57–95%
  • Enhances soil pH
  • Strong sorption for Hg
  • Flexible application options
  • Variable performance based on feedstock
  • Risk of MeHg generation under reducing conditions
  • May require periodic replacement
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Robles, M.E.; Oh, Y.; Haque, M.T.; Jeon, M.; Kim, L.-H. Soil Mercury Pollution in Nature-Based Solutions Across Various Land Uses: A Review of Trends, Treatment Outcomes, and Future Directions. Appl. Sci. 2025, 15, 6502. https://doi.org/10.3390/app15126502

AMA Style

Robles ME, Oh Y, Haque MT, Jeon M, Kim L-H. Soil Mercury Pollution in Nature-Based Solutions Across Various Land Uses: A Review of Trends, Treatment Outcomes, and Future Directions. Applied Sciences. 2025; 15(12):6502. https://doi.org/10.3390/app15126502

Chicago/Turabian Style

Robles, Miguel Enrico, Yugyeong Oh, Md Tashdedul Haque, Minsu Jeon, and Lee-Hyung Kim. 2025. "Soil Mercury Pollution in Nature-Based Solutions Across Various Land Uses: A Review of Trends, Treatment Outcomes, and Future Directions" Applied Sciences 15, no. 12: 6502. https://doi.org/10.3390/app15126502

APA Style

Robles, M. E., Oh, Y., Haque, M. T., Jeon, M., & Kim, L.-H. (2025). Soil Mercury Pollution in Nature-Based Solutions Across Various Land Uses: A Review of Trends, Treatment Outcomes, and Future Directions. Applied Sciences, 15(12), 6502. https://doi.org/10.3390/app15126502

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