Trends and research gaps in nano-biochar soil applications: a scientometric analysis

Soil contamination caused by heavy metals, organic pollutants, and emerging contaminants (ECs) represents a critical environmental challenge that threatens soil quality, agricultural productivity, and human health. In recent years, biochar and its engineered nanoscale derivative—nano-biochar (NBC)—have emerged as promising, cost-effective amendments for soil remediation. This study conducted a comprehensive scientometric analysis of NBC research applied to soils from 2012 to 2025 using CiteSpace, VOSviewer, and Bibliometrix. The methodology combined co-occurrence mapping, thematic evolution, citation burst detection, and an author-level productivity assessment through h-index, g-index, and m-index evaluation. Additionally, the scientometric analysis was complemented by a focused mini-review addressing three conceptually relevant domains identified in the keyword clusters: adsorption mechanisms, biochar–microbial community interactions, and ecotoxicological risk assessment. Results reveal three distinct developmental phases: (i) an exploratory period (2012–2016) dominated by adsorption and physicochemical optimization; (ii) an expansion phase (2017–2021) integrating nanoparticles, microbial communities, and phytoremediation; and (iii) a recent consolidation (2022–2025) characterized by engineered nanocomposites, multifunctional NBC systems, and the emergence of risk assessment frameworks as a structurally relevant theme. Current hotspots converge on adsorption, microbial-driven remediation, and toxicity reduction, while emerging directions highlight machine-learning-assisted modeling and NBC–microbe interactions. Importantly, findings indicate that risk assessment is transitioning toward a Motor Theme, underscoring the urgent need for deeper ecotoxicological research and the incorporation of NBC within regulatory and policy frameworks that govern soil remediation and sustainable resource management. It is hoped that this work will guide future research trajectories and inform evidence-based decision making for the safe and scalable implementation of NBC technologies.

1 Introduction

Widespread industrialization, urban expansion, and the adoption of intensive agricultural practices have led to the accumulation of numerous toxic substances—including pesticides (Rajak et al., 2023), pharmaceuticals (Nand et al., 2025) and personal care products (Liu et al., 2023), organic and nanomaterials, endocrine-disrupting compounds (Cardoso et al., 2025), steroids (Merlo et al., 2024), surfactants and their metabolites, industrial additives, heavy metals (Das et al., 2025; Nuñez-Bustamante et al., 2025; Virú-Vasquez et al., 2025), and microplastics (Zhou et al., 2019; Virú-Vásquez et al., 2024). Collectively, these substances are referred to as Emerging Contaminants (ECs) due to their increasing detection in environmental matrices (Bhatt et al., 2021; Gangola et al., 2022) and potential risks to human and ecological health, threatening aquatic and terrestrial ecosystems (Jiang et al., 2023b).

The mixed matrix of pollutants in soil poses significant risks to soil health and agricultural productivity, complicating remediation efforts (Maddela et al., 2022). Furthermore, soil pollution by the emerging contaminants (ECs) deserves attention worldwide because of their toxic health effects and the need for developing regulatory guidelines. The ECs alter not only soil functionalities, also they affect the health of plants (Correia and Rasteiro, 2025) and animals (Liu et al., 2024b). Even at concentrations ranging from nmol to μmol levels, these substances exhibit toxic effects in cell cultures (Emmanuel et al., 2025) and experimental animals. Moreover, ECs tend to accumulate primarily in root vegetables (Zaman et al., 2024), posing a significant risk to human health (Maddela et al., 2022). Understanding the environmental context of the area and the potential dynamics of these contaminants is essential to comprehend the mechanisms governing their interactions. Cross-contamination in soils may occur (Wei et al., 2025), leading to complex and difficult-to-predict effects. Therefore, a proper understanding of these processes is critical for selecting the most effective soil remediation treatments.

Biochar is the product of the thermochemical conversion of biomasses without oxygen or under limited oxygen availability (Rajput et al., 2022). In recent years, biochar has gained considerable interest as a cost-effective and efficient external amendment for different applications in soil (Elad et al., 2010; Dutta et al., 2017; Jiang et al., 2019), water (Liao et al., 2022; Syarifuddin et al., 2024a), environmental management and others (Ye et al., 2019). It has evolved to be an effective instrument to address environmental and industrial challenges, thanks to its unique characteristics (Curcio et al., 2025).

With the advancement of nanotechnology, it became possible to further reduce the size of biochar particles to the nanoscale (Ramanayaka et al., 2020), thereby enhancing their physical properties and biological efficacy, knowing this product as nano-biochar (NBC), which raw materials used for fabrication include animal waste (Sani et al., 2023), municipal waste (Bhandari et al., 2023), lignocellulosic agricultural residues (such as grass, palm, peanut shells, rice husks and straw, sugarcane bagasse, bamboo, and soybean stover), woody forest by-products, and sewage sludge (Bhandari et al., 2023).

NBC exhibits particles ranging from 1 to 100 nm in at least one dimension (Ramanayaka et al., 2020; Raczkiewicz and Oleszczuk, 2025). Various methods have been developed for its synthesis (Chaubey et al., 2023; Deepshikaa et al., 2024) (Figure 1). These methods can generally be classified into two main approaches: bottom-up and top-down. The bottom-up approach involves the self-assembly of atomic or molecular components into organized nanostructures (Curcio et al., 2025). Examples of this method include chemical precipitation and microwave pyrolysis (Curcio et al., 2025). In contrast, the top-down approach involves obtaining smaller structures from a larger piece (Wallace et al., 2019).

Figure 1

Figure 1. Production techniques and key properties of NBC.

Unlike bulk biochar, NBC exhibits enhanced physicochemical features, including elevated catalytic performance (Adam et al., 2021), distinctive nanostructures, much-elevated surface area-to-mass ratio (5.6–364 m²g^–1) (Jiang et al., 2023b), significantly superior adsorption capacity (Zhang, 2025), and improved mobility within soil systems (Sani et al., 2023), and others as seen in Figure 1. These properties position NBC as a highly promising material for environmental pollution remediation in the soil and water environments (Jiang et al., 2023b). NBC offers multiple potential applications in soil systems, yet remediation remains one of its most compelling and impactful uses due to its ability to immobilize, retain, or transform contaminants directly within the soil matrix (Jiang et al., 2023b; Jadhav et al., 2025; Raczkiewicz et al., 2025). This relevance becomes particularly evident when compared with conventional soil treatment technologies—such as excavation, thermal processing, and chemical treatments—which, although effective, are frequently costly, energy-intensive, and highly disruptive to soil structure and ecosystem function (Nwaichi et al., 2022; Xu et al., 2023; Mystrioti and Papassiopi, 2024). These limitations have driven increasing interest in lower-cost, in-situ alternatives, including phytoremediation and amendment-assisted stabilization (Kafle et al., 2022; Oubohssaine and Dahmani, 2024; Nuñez-Bustamante et al., 2025; Virú-Vasquez et al., 2025). Furthermore, techno-economic assessments show that phytoremediation can cost approximately US$2,500–15,000 per hectare—substantially less than excavation-based strategies, which range from US$40,500 to 48,600 per hectare for contaminants such as Pb (Islam et al., 2024). When combined with soil amendments like biochar, overall project budgets remain manageable. Typical biochar application rates of 10–20 t/ha add roughly US$4,000–8,000 per hectare at ∼ US$400/t while simultaneously enhancing metal immobilization capacity and soil quality (Tamma et al., 2025).

A key reason behind the enhanced performance of NBC in soil remediation is the wide range of physicochemical mechanisms through which it interacts with contaminants as shown in Figure 2. Depending on its composition and surface chemistry, NBC can function as an adsorbent, or biochar composite, each role associated with distinct mechanistic pathways. As an adsorbent, NBC immobilizes pollutants through physical or chemical sorption mechanisms, including physical adsorption driven by its high specific surface area and porosity (Pathak et al., 2024); ion exchange with surface functional groups and soil cations such as Pb²⁺, Cd²⁺, or NH₄⁺ (Abhishek et al., 2022; Xing et al., 2025); π–π interactions important for aromatic contaminants and pharmaceuticals (Xing et al., 2025); hydrogen bonding with polar organic molecules (Jiang et al., 2023b; Emamverdian et al., 2025); surface complexation forming inner- and outer-sphere complexes with metals and oxyanions; electrostatic attraction modulated by surface charge and soil pH; and pore filling of small or hydrophobic emerging contaminants (Wei et al., 2025).

Figure 2

Figure 2. Schematic illustration of nano-biochar adsorption and remediation mechanisms in soils.

Adsorption is widely recognized as one of the primary mechanisms through which NBC achieve soil and water remediation (Xia et al., 2023; Syarifuddin et al., 2024b). Additional evidence from recent studies further reinforces this: NBC strongly influences contaminant transport and environmental risk in soils (Zhang, 2025), modulates pollutant interactions within the rhizosphere (Sarma et al., 2024), drives the removal of heavy metals and emerging contaminants through adsorption-based mechanisms (Pathak et al., 2024), and enhances nutrient adsorption such as nitrate in soil systems (Xing et al., 2025). Together, these findings confirm that adsorption is the dominant pathway underpinning nano-biochar’s remediation performance.

Although the environmental benefits of NBC in soils are increasingly documented, important uncertainties remain regarding its risk assessment across the full application cycle (Dong et al., 2025; Omokaro et al., 2025; Zhang, 2025). Recent evidence indicates that biochar and NBC can carry both endogenous contaminants (derived from feedstock and pyrolysis conditions) and exogenous pollutants adsorbed during soil interactions, which may introduce unintended ecological and human-health risks if not properly evaluated (Dong et al., 2025). These include the potential release of heavy metals, polycyclic aromatic hydrocarbons (PAHs), and environmentally persistent free radicals (EPFRs), as well as the formation of nanoscale particles through weathering and aging processes that enhance mobility and transport to groundwater or plant tissues. Studies also show that NBC may induce oxidative stress in plants, alter microbial community structure, and facilitate co-transport of contaminants in soil–water systems (Ni et al., 2021). Despite these concerns, comprehensive risk-assessment frameworks for NBC in soils remain scarce, and no standardized protocols currently exist for evaluating its long-term ecotoxicological behavior, bioavailability dynamics, or fate and transformation in real soil environments. This gap highlights the urgent need for robust, multi-tiered ecological risk assessment approaches—including cellular bioreporters, soil ecotoxicity assays, bioavailability indicators, and long-term mesocosm evaluations—to ensure the safe and sustainable use of NBC in soil remediation.

Scientometrics, defined as the quantitative study of scientific communication and the dynamics of science and technology (Leydesdorff, 2001), provides valuable tools to map research trends, assess productivity, and identify gaps within a scientific domain. These techniques enable the systematic evaluation of large volumes of scholarly data, offering insights into how research outputs are distributed across disciplines, geographic regions, and publication venues (Rodríguez-Aburto et al., 2025). Although several scientometric or review studies have begun to map the global evolution of NBC research, these analyses remain predominantly focused on synthesis routes, methods (Zeng et al., 2024), and applications in water treatment or adsorption systems, (Liu et al., 2024c; Syarifuddin et al., 2024a). A few studies have touched on soil-related applications of NBC (Brar et al., 2024; Sultan et al., 2024). Given the growing scientific interest in NBC for contaminant immobilization, soil quality enhancement, and plant–soil dynamics, the absence of a focused scientometric perspective limits our understanding of how this research domain is emerging. Therefore, a domain-specific scientometric mapping is urgently needed to identify current trends, structural patterns, research gaps, and future directions for NBC applications in soil environments.

2 Materials and methods

The scientometric analysis was conducted in four main stages, as shown in Figure 3. It shows the overall workflow used for the scientometric and mini-review analysis of NBC research in soil systems.

Figure 3

Figure 3. Methods to carry out the scientometric research.

First, the process begins with the Database Selection and Search Strategy, where a structured query was applied in Scopus using TITLE-ABS-KEY terms related to nano-biochar, biochar, nanoparticles, and soil, yielding 427 records. Scopus was selected as the primary data source since it is one of the largest and most comprehensive bibliographic databases, offering broad coverage of peer-reviewed journals and extensive citation metadata (Visser et al., 2021). This database was chosen because it provides wide access to scholarly documents with a high level of reference and citation connectivity, ensuring reliable representation of scientific output in the environmental sciences.

In the Screening and Inclusion Criteria stage, all retrieved documents were limited to those published up to 6 September 2025 and written in English (n = 427). To ensure the robustness and reliability of the results, a multi-database validation was conducted by comparing the Scopus dataset with records retrieved from the Web of Science (n = 780). Although Web of Science presented a higher number of records, Scopus was preferred because the scientometric analysis aimed to exclusively consider author keywords rather than Keyword Plus, thereby maintaining greater control over the conceptual accuracy of the retrieved terms. This cross-validation revealed consistent keyword co-occurrence structures and thematic clusters across both databases, confirming the stability of the observed research trends (see Supplementary, Supplementary Figure S1; Supplementary Table S1). Such validation also demonstrates that similar analyses can be effectively replicated in alternative databases, reinforcing the methodological soundness of the study. In this stage the information was prepared for conducting the scientometric analysis.

The workflow then branches into two complementary pathways, scientometric analysis, performed on the full set of 427 records, and Mini-review, synthesizing qualitative evidence from the 22 screened papers based on different criteria like keywords “adsorption”, “microbial community” and “risk assessment”. For the scientometric analysis, in the Data Extraction and Tools Used section the data were exported in multiple formats—CSV files for VOSviewer 1.6.20 (Van Eck and Waltman, 2010) and Bibliometrix in R Studio 2024.12.1 + 563 (Aria and Cuccurullo, 2017), and BibTeX files for CiteSpace 6.3. R1 (Chen, 2016).

The analytical framework applied in this research integrates three complementary dimensions to characterize the intellectual and thematic structure of the field. First, general aspects and keyword analysis—including co-occurrence networks, density and cluster maps, and timeline-based cluster evolution—were used to identify core concepts and their temporal dynamics. Second, citation-based analysis, through strongest citation burst detection and the identification of most-cited research articles, allowed us to determine influential works and emerging high-impact topics. Finally, thematic analysis, based on thematic maps and thematic evolution mapping, provided insights into the maturity, relevance, and evolution of research themes. Together, these analytical components offer a comprehensive scientometric overview of how the field has developed, interconnected, and shifted over time.

3 Results

3.1 General aspects and keyword analysis

Figure 4 illustrates the annual evolution of scientific publications on NBC applied to soil systems from 2012 to 2025. The green bars represent the number of research articles published each year (N), while the red line shows the mean total citations per year (Mean TC per Year). The dotted trend line demonstrates a strong positive growth trajectory in publication output over time, supported by a high goodness-of-fit value (R² = 0.8499), indicating a consistent and accelerating increase in research interest. During the early period (2012–2016), publication activity remained low, with fewer than 10 articles per year. However, beginning in 2017, output increased steadily, reaching a notable rise between 2020 and 2024, when annual publications surpassed 40 articles and peaked at more than 100 in 2024. This surge aligns with the expanding recognition of NBC as a promising tool for soil remediation, pollutant immobilization, and soil quality enhancement. The red citation curve shows fluctuations in research impact, with several peaks—particularly in 2013, 2016, 2019, and 2022—suggesting the emergence of influential studies that shaped the field. A gradual decline in mean citations is observed after 2022, likely reflecting the high volume of recent publications that have not yet accumulated citations.

Figure 4

Figure 4. Temporal evolution of nano-biochar research output and citation impact (2012–2025).

Figure 5 shows a timeline-based cluster visualization generated using CiteSpace, which maps the evolution of research themes in the field of NBC and its environmental applications, particularly in soil remediation. Each colored line represents a thematic cluster, identified through co-citation or keyword analysis, with the cluster labels on the right. The horizontal timeline spans from 2012 to 2025, and the arcs connecting terms reflect the citation or keyword relationships across time, helping track how knowledge domains have evolved and interconnected.

Figure 5

Figure 5. Timeline-based cluster visualization for keywords using CiteSpace.

The structural metrics analysis demonstrate that the knowledge domain of NBC research exhibits a robust and interpretable intellectual organization since the modularity score (Q = 0.49) indicates a moderately strong community structure (Newman and Girvan, 2004; Blondel et al., 2008) suggesting that the field is divided into several coherent thematic clusters that nonetheless maintain meaningful interconnections—an expected pattern for an emerging and interdisciplinary research area. This is reinforced by the very high weighted mean silhouette value (S = 0.88), which reflects excellent internal consistency and clear separation among clusters (Rousseeuw, 1987), ensuring that each thematic group is topically homogeneous and analytically reliable. Additionally, the network’s harmonic mean (Q, S) of 0.63 further confirms the overall robustness and stability of the cluster configuration (Chen, 2006). The largest connected component, encompassing 72% of all nodes, highlights the high degree of conceptual integration within the field, indicating that most research efforts converge around shared core themes such as soil remediation, heavy metals, adsorption processes, and engineered biochar materials. Although the overall network density is low (0.032), this is typical of scientometric maps and reflects a diversified and expanding research landscape rather than structural fragmentation. Collectively, these metrics validate the reliability of the scientometric mapping and support the use of cluster-based analyses to interpret how NBC research is evolving within soil science and environmental remediation. Respect to clusters, cluster #0“soil health” and cluster #1“heavy metal” show continuous activity and connection over the years, indicating their foundational importance and sustained interest in biochar studies. Moreover, cluster #2“particle size” and #4“bacteria” have emerged more recently, suggesting a shift toward understanding the mechanistic and microbiological effects of biochar at finer scales—possibly influenced by advances in nanotechnology and microbial ecology. The prominence of keywords such as “adsorption,” “pyrolysis,” and “bioremediation” reflects core processes and applications associated with biochar. The emergence of “acidic soil” (cluster #5) also suggests a recent niche focusing on agronomic implications. This time map provides a retrospective view of the thematic landscape, and also forecasts future directions, as seen in the extended trajectories of certain clusters reaching into 2025.

Figure 6 represents a keyword cluster network generated using CiteSpace. In this visualization, clusters of keywords are grouped and color-coded using the Louvain or LLR (log-likelihood ratio) algorithm, which allows for the identification of thematic groupings (Zang et al., 2022). Each cluster—numbered and labeled (e.g., #0 soil amendment, #1 soil remediation, #2 cation exchange capacity)—is composed of closely related terms that frequently co-occur in the same articles, indicating a shared research focus. The proximity of the nodes and the thickness of the connecting lines (edges) represent the strength of the co-occurrence relationships among the keywords. Cluster #0, labeled “soil amendment”, emerges as a central and foundational topic in biochar-related research, reflecting its broad application in improving soil quality (Zhang, 2025). Similarly, cluster #1 “soil remediation” and cluster #3 “plant growth” are also prominently placed, indicating their significance in both environmental and agricultural contexts (Rajput et al., 2022; Raczkiewicz et al., 2025). On the other hand, cluster #4 “hexavalent chromium” and cluster #2 “cation exchange capacity” reflect more specific mechanisms and contaminants that have garnered attention in recent years, suggesting a growing specialization in the field. The presence of cluster #6 “wheat”, which lies somewhat isolated in the upper-right section, signifies the increasing interest in crop-specific responses to biochar and nanoparticles under stress conditions like salinity or oxidative damage (Muhammad Mehmood et al., 2024; Shani et al., 2024). Such niche themes show the expanding interdisciplinary nature of biochar research—bridging soil science, plant physiology, and nanotechnology.

Figure 6

Figure 6. Keyword Co-occurrence cluster map.

Figure 7 shows the Top 15 keywords with the strongest citation bursts generated in CiteSpace. A citation burst indicates a sudden increase in the frequency of a keyword within a specific time window, suggesting heightened scholarly attention. The red bars in the timeline (2011–2025) mark the active burst period, while the light blue segments indicate the rest of the time range (Baca-Neglia et al., 2025). These keywords are thus considered influential within their respective burst windows. The strongest burst was observed for the keyword “heavy metals” with a strength of 5.32, beginning in 2021 and continuing beyond 2025 (word: “heavy metal”, highlighting the recent and ongoing interest in this area—likely in connection with environmental remediation using nanomaterial. Similarly, emerging topics such as “soil health” (2023–2025) and “sustainable agriculture” (2023–2025) show high burst strengths (2.86 and 2.48, respectively), indicating a shift toward holistic and sustainability-oriented research themes in recent years. These newer bursts suggest that the current frontier in biochar or environmental studies is increasingly focused on ecosystem services and soil function enhancement, in line with global sustainability goals (e.g., SDG 15: Life on Land). Earlier bursts such as “adsorption mechanisms” (2011–2018), “Cr (VI)-contaminated soil” (2016–2018), and “in situ remediation” (2016–2019) reflect foundational research that laid the groundwork for pollutant removal techniques. The mid-phase keywords like “electron transfer” (2019–2020) and “magnetic biochar” (2021–2023) denote a transition to more advanced materials and mechanisms. The continuity of certain themes (e.g., “biochar”, “bacterial community”, “hexavalent chromium”) reflects a sustained research interest, while the rise of new terms highlights the dynamic and interdisciplinary nature of the field. These insights allow researchers and decision-makers to identify past research hotspots and align future investigations with growing thematic areas.

Figure 7

Figure 7. Top 15 keywords with the Strongest Citation Bursts.

Table 1 summarizes the journals that have contributed most actively to NBC research, highlighting their bibliometric performance indicators such as h-index, g-index and m-index (Hirsch, 2005; Egghe, 2013), total citations and publication volume. High-impact multidisciplinary environmental journals—including Science of the Total Environment, Chemosphere, and the Journal of Hazardous Materials—lead the field, which aligns with their long-standing reputation for publishing research on environmental remediation, nanomaterials, and soil pollution. These journals also show the highest citation counts, reflecting their central role in disseminating cutting-edge studies on NBC synthesis, characterization, and performance. Also, in Table 2 presents the top contributing authors in the field, revealing a research landscape led by influential scientists with consistent publication trajectories and strong citation performance. Authors such as Li Y, Wang Y, and Zhang Y show high h-, g-, and m-index values, suggesting both productivity and sustained scientific influence within NBC research.

Table 1

Table 1. Top contributing journals in nano-biochar research and their bibliometric indicators.

Table 2

Table 2. Top contributing authors in nano-biochar research and their bibliometric indicators.

3.2 Period analysis

Figure 8 illustrates the evolution of NBC research from 2012 to 2025 through a thematic map, revealing a clear trajectory from foundational studies to increasingly specialized and technologically advanced themes. In the first period (2012–2016), the field was in its formative stage, focusing on core concepts such as adsorption, sorption, and remediation—laying the groundwork for environmental applications, particularly soil detoxification. General topics are common to find in the initial periods (Rodríguez-Aburto et al., 2024; 2025; Baca-Neglia et al., 2025). Themes were broad and general, with limited development and centrality, reflecting an exploratory phase with modest innovation. By the second period (2017–2021), the research landscape diversified significantly. New keywords like magnetic biochar, microwave, graphene oxide, and plastic particles appeared, showing the field’s expansion into advanced material engineering and broader environmental concerns such as organic pollutant removal and electron transfer mechanisms. In the most recent period (2022–2025), the field has reached a stage of high thematic maturity and specialization. Dominant themes now include biochar nanoparticles, aging, and biochar composites, reflecting a shift toward engineered NBC with controlled properties. There is a clear trend toward nano-enabled agriculture, nutrient management, and interactions with the microbial community, pointing to precision applications in agri-environmental systems. Furthermore, the appearance of terms like oxidant enzymes, stabilization, and environmental risk illustrates not only advanced functionalization techniques but also a growing interest in assessing the environmental safety and long-term behavior of these materials. This third period marks a technological consolidation, with NBC research now embracing molecular-level functionality and risk evaluation, signifying a matured and multidisciplinary research frontier.

Figure 8

Figure 8. Clusters of keywords for different research periods.

In Table 3, the titles were grouped by periods (I, II, and III), and trends were inferred from their thematic focus and citation behavior. In period I, the most cited papers are characterized by relatively lower average citation years (between 26.46 and 29.80), indicating older publications that have accumulated considerable attention over time. The normalized citation values range from 1.93 to 3.60, suggesting foundational contributions with growing relevance. These studies predominantly explore the development and physicochemical optimization of biochar-based materials, with a strong emphasis on enhancing adsorption capabilities and integrating nanostructures. The topics revolve around the synthesis of composite materials and their potential use in contaminant removal, reflecting a research phase focused on material innovation and laboratory-scale testing. The second period includes works with higher normalized citation values (from 3.41 to 4.14) and total citations surpassing 330 in all cases. With average citation years between 41.75 and 61.00, these publications are more recent and have shown rapid citation growth, indicating significant impact in a shorter time. Research during this phase concentrates on combining biochar with other reactive agents such as zero-valent iron and applying these combinations to complex contamination scenarios, including multi-metal polluted soils and aquatic systems. There is a clear shift toward application-oriented studies, often involving synergistic remediation mechanisms and biological interactions. Additionally, the inclusion of systematic reviews reflects the need to consolidate findings and propose standardized approaches. The most recent period is marked by the presence of highly normalized citation scores (ranging from 4.25 to 4.46) despite slightly lower total citation counts, which is expected due to their recency (average citation years between 45.00 and 47.25). These works focus primarily on reviews and analyses of functionalized or magnetic biochars, emphasizing not only removal efficiency but also reusability and environmental safety. The high normalized impact suggests increasing scholarly attention to the refinement and practical deployment of biochar technologies. There is a notable emphasis on systematic evaluation, practical challenges, and the need for scaling up successful remediation strategies.

Table 3

Table 3. Most cited articles.

3.3 Research trends

Figure 9 shows a timeline-based cluster visualization for keywords conducted with VOSviewer, revealing the evolution of scientific research on NBC applications in soil remediation across three distinct periods. Each visual cluster represents co-occurring keywords derived from published literature, allowing the identification of major research trends and thematic shifts over time.

Figure 9

Figure 9. Timeline-based cluster visualization for keywords.

In Period I (2012–2016), the research landscape was dominated by foundational studies focusing on the physicochemical properties of biochar. Keywords such as sorption, adsorption, dye, and remediation reflect the early interest in using biochar as a simple adsorbent for removing contaminants like Cd and Cr (VI) from polluted soils. The network is relatively simple and compact, indicating that research during this time was still emerging and largely focus on material behavior and pollutant immobilization in controlled settings.

As research advanced into Period II (2017–2021), the network became denser and more diverse. New keywords appeared—nanoparticles, zero-valent iron (nZVI), cadmium, phytoremediation, bacterial community—reflecting the growing complexity of remediation approaches. During this phase, researchers began combining biochar with nanomaterials and biological strategies, exploring their synergistic effects on contaminant stabilization and soil health. This period marked a shift toward more interdisciplinary approaches, integrating knowledge from environmental chemistry, microbiology, and plant sciences.

During Period III (2022–2025), the research field had matured significantly. The visual network displays high complexity, with numerous interconnected clusters and topics such as soil remediation, microbial community, antioxidant enzymes, abiotic stress, and nano-biochar. The literature expanded to include ecotoxicological assessment, crop performance, soil-plant-microbe interactions, and human health risks. This evolution indicates a transition from purely technical remediation methods to more holistic frameworks, emphasizing environmental sustainability, food security, and systemic resilience.

Across the three periods, clear structural changes can be observed in the keyword co-occurrence networks. In Period I, the network displays only six clusters—groups of keywords that represent distinct thematic areas—and a limited number of links (112), which indicate how often keywords co-occur and therefore how strongly topics are conceptually connected. The low Total Link Strength (TLS = 116), a metric that captures the overall intensity of these connections, reflects a field that was still emerging and thematically fragmented. In Period II, the number of clusters increases to 11 and link density nearly doubles (235 links; TLS = 333), showing greater diversification of topics and stronger interconnections among them. By Period III, the network reaches 16 clusters with very high connectivity (656 links; TLS = 875), revealing a mature and well-integrated research landscape in which NBC and soil-related themes are strongly interlinked across publications.

4 Discussion

Time-line based cluster visualization for keywords are key to better understand the main topics in cluster categories. As Wei et al. (2021) points out, a combination of keywords and time can help to accurately reveal the trend of research. Similar research had divided timeline for analyzing each one (Rodríguez-Aburto et al., 2024; 2025; Baca-Neglia et al., 2025). On the other hand, scientometric studies on NBC are scarce (Zeng et al., 2024), most have focused on understanding the general aspects of biochar (Fikri et al., 2025; Iwuozor et al., 2025) and remediation (Jiang et al., 2023a; Liu et al., 2024c; Phiri et al., 2024). Therefore, future scientometric analyses could adopt a more specific focus on NBC, enabling the identification of key research gaps and the projection of scientific directions with potential global relevance.

The analysis of keyword trends reveals that adsorption has consistently served as the foundational concept driving early research on NBC applications. Its sustained prominence in the literature across different time periods reflects a strong research focus on emerging and conventional soil contaminants (Syarifuddin et al., 2024a; Wang et al., 2024), including heavy metals such as cadmium (Cd) (Yue et al., 2019; Liu et al., 2020), arsenic (As) (Ghassemi-Golezani and Rahimzadeh, 2024), and organic pollutants like PFAS (Hassan et al., 2025) and tylosin (Guo et al., 2016). Table 4 shows different studies for adsorption processes and key parameters and description in research periods. In Period I, research concentrated on organic pollutants (antibiotics, phenols, dyes) and already achieved strong removals using surface-engineered biochars (Ghaffar and Younis, 2014; Ying et al., 2015a; Guo et al., 2016). In this research period, materials operated primarily through adsorption, a process dominated by physical or chemical interactions—such as π–π interactions, hydrogen bonding, and electrostatic attraction—without altering the chemical structure of the pollutant.

Table 4

Table 4. Representative studies on biochar adsorption: Key parameters and models across research periods.

In contrast, Period II marks a transition toward the use of NBC engineered with nanomaterials such as zero-valent iron (nZVI) (Fan et al., 2020), calcium polysulfide (CPS) (Hou et al., 2021), or modified through high-temperature treatments (e.g., CB900) (Guaya et al., 2025). These materials were increasingly applied to more complex or emerging soil contaminants, including heavy metals (As, Cr(VI)) and notably microplastics, which did not appear in the previous period. While NBC showed promising removal efficiencies—often exceeding 90%—several studies in this phase did not report adsorption kinetics or isotherm models and instead focused on overall reduction percentages. This may indicate a shift in research priorities, emphasizing practical effectiveness over mechanistic modelling.

Thus, Period II illustrates both the technological advancement in material design and the methodological divergence from earlier model-driven approaches. In Period III, research on biochar adsorption advances toward multifunctional and nanostructured materials, such as trimetallic and magnetic biochars. Studies target emerging contaminants like phosphate, ammonium, and complex mixtures of PFAS, using advanced materials like coffee-ground-derived biochar and magnetic sugarcane bagasse biochar. While traditional kinetic and isotherm models are still applied in some cases, others adopt machine learning regressions to model adsorption behavior—marking a shift toward data-driven approaches. This period reflects a growing emphasis on tailored NBC design, complex pollutant removal, and integration of novel analytical tools in adsorption studies.

Although the keyword “nanoparticle catalysts”, does not explicitly appear in the scientometric analysis results (Figure 9), it is important to highlight that such terms were indeed found in the database. This reflects emerging research directions, such as the use of cobalt-based nanoparticles embedded in doped biochar to activate oxidants like peroxymonosulfate (PMS) (Liu et al., 2022). These types of catalysts have demonstrated high efficiency in the degradation of persistent organic pollutants, including PCB28 and bisphenol A (BPA) (Yang et al., 2024). Recent studies are exploring the interaction between NBC and enzymes for potential applications in PET degradation (Han et al., 2024). Although no current studies directly combine NBC and enzymes for PET degradation, this gap highlights a promising research frontier. Considering NBC´s high surface area, functional groups, and biocompatibility, it stands as a strong candidate for future enzyme immobilization strategies aimed at plastic biodegradation, particularly in the context of circular bio-based remediation technologies (Jia et al., 2021; Sui et al., 2023).

An interesting aspect observed during periods II and III is the emergence of the keywords “bacterial community” and “microbial community”, respectively (see Figure 9), which reflects growing interest in the role of microorganisms in environmental remediation strategies. This conceptual trend is supported by recent experimental findings on the use of biochar—particularly in its modified forms such as magnetic biochar and biochar enriched with microbial consortia—as an effective tool for the degradation of microplastics and other soil contaminants. For instance, in waterlogged soils contaminated with polyethylene and PVC microplastics, magnetic biochar has been shown to promote advanced oxidation processes, which not only increased organic matter and the accumulation of humified compounds, but also facilitated the cooperation of microbial species such as Nocardioides and Rhizobium in plastic degradation (Ji et al., 2023). This biochemical and microbial interaction was also observed with biochars derived from poultry manure and wood waste, which significantly accelerated the degradation of polylactic acid (PLA) microplastics through combined mechanisms of oxidation, aminolysis, and microbial activity, all of which were influenced by the specific type of biochar used (Zou et al., 2024). Additionally, the integration of biochar with a bacterial consortium (QY2Y) showed high efficiency in degrading persistent organic pollutants such as BDE-47, while enhancing soil enzymatic activity, improving physicochemical properties, and fostering key symbiotic interactions within the microbiota—demonstrating how microbial immobilization on biochar can optimize remediation efforts in contaminated soils (Guo et al., 2024). Additionally, research on the application of NBC in soil and its effects on microbial communities are shown in Table 5.

Table 5

Table 5. Research on effects on microbial communities applying NBC.

The thematic evolution across the three periods shows a remarkable shift in the role of risk assessment within NBC research. In Period II (2017–2021), risk assessment appears as an Emerging or Declining Theme, indicating that although studies had begun to address ecological and toxicological implications of NBC, the topic had not yet achieved strong conceptual or structural centrality in the field. This status reflects early concerns about the release of nanoparticles, potential soil–plant transfer pathways, and uncertainties regarding environmental fate. However, by Period III (2022–2025), environmental risk transitions into a Niche Theme, characterized by both high development and high relevance. This shift suggests that the field now recognizes NBC safety evaluation as fundamental for scaling environmental applications, regulatory acceptance, and environmental engineering implementation. Risk assessment is not limited to a single type of pollutant; rather, it can be applied across various contamination scenarios—including microplastics, organic pollutants, and heavy metals. Its limited presence in keyword frequency may reflect a lag between conceptual relevance and bibliometric visibility, suggesting that this topic could gain greater prominence in future research trends. In particular, soil contamination by heavy metals and metalloids such as arsenic (As), cadmium (Cd), chromium (Cr), and lead (Pb) pose a major environmental and public health concern due to their persistence, non-degradability, and potential bioaccumulation in the food chain (Mitra et al., 2022; Yin et al., 2024; Abdelmonem et al., 2025). In this context, ecotoxicological risk assessment (ERA) has become essential to evaluate not only the concentrations of pollutants in soils, but also their bioavailability, plant uptake, and subsequent impacts on food safety and human exposure (Schaap et al., 2025). Indicators such as Residual Soil Availability (RSA), Relative Plant Bioaccumulation (RPB), and Risk Reduction (RR) have been increasingly adopted to quantify the effectiveness of remediation strategies—particularly those using biochar, magnetic biochar, and engineered nanocomposites, which can immobilize contaminants and mitigate their transfer to crops.

These indicators offer an integrated framework to assess how soil amendments contribute to reducing both carcinogenic and non-carcinogenic risks, effectively linking soil geochemistry, plant physiology, and human health outcomes (Ngole-Jeme and Fantke, 2017). As shown in Table 6, recent studies have applied these ecotoxicological tools to assess the impact of NBC-based amendments in soils contaminated with heavy metals such as Cd, As, Cr, Ni, Cu, and Pb. The results demonstrate that engineered biochars—including magnetic NBC (nZVI-BC), silicon-doped biochar (nano-Si–BC), and ZnO-based nanocomposites—can significantly reduce metal bioavailability in both soils and crops. Removal efficiencies (RSA, RPB) are often accompanied by reductions in toxicological risk indicators such as Hazard Quotient (HQ), Health Risk Index (HRI), or cancer risk thresholds.

Table 6

Table 6. Risk assessment research with the application of NBC in soils.

NBC contributes to the development of Circular Bioeconomy and despite these scientific advances, a critical gap becomes evident when examining the thematic evolution, there is almost no discussion of policy frameworks, governance instruments, or regulatory inclusion of NBC technologies (Sani et al., 2023; Velasquez-Pinas et al., 2025). This omission is significant because the transition of “environmental risk” from an emerging theme in Period II to a niche and highly developed theme in Period III suggests that risk assessment maturity is increasing scientifically, but its translation into environmental policy, national soil quality standards, or technology certification frameworks remains underdeveloped.

Although NBC exhibits promising performance at laboratory scale, its practical deployment requires a broader evaluation of scalability, cost, and environmental risks. The production of modified or doped NBC often involves high-temperature pyrolysis, chemical activation, or incorporation of metals (e.g., Fe, Zn, Co), all of which increase energy demand, reagent use, and operational complexity (Gallego-Ramírez et al., 2023; Saraugi et al., 2025; Zhang et al., 2025). These steps raise the overall production cost compared with bulk biochar, limiting feasibility for large-scale soil remediation unless optimized through waste-derived feedstocks or low-energy synthesis routes. Life cycle assessments (LCA) performed for biochar systems indicate that pyrolysis and post-modification stages are the largest contributors to environmental footprint, largely due to electricity use, transport of biomass, and emissions associated with activation processes (Goswami et al., 2022; Zhang et al., 2025). Recent studies demonstrate that although NBC offers significant potential for environmental remediation, its true sustainability must be assessed through Life Cycle Assessment (LCA), covering production, modification, use, and end-of-life phases. For example, a full cradle-to-grave LCA of potassium-ferrate-modified biochar shows that chemical activation, energy consumption, and modifying agents (e.g., K₂FeO₄) represent major contributors to carbon emissions and system costs, even though reusing the material for multiple treatment cycles substantially reduces both environmental impacts and operational expenses (Zhang et al., 2025). Additional LCA comparisons highlight that metal-modified biochars introduce extra burdens linked to iron extraction, reagent consumption, and wastewater generation, emphasizing the need for cleaner synthesis routes and greater resource efficiency (Gallego-Ramírez et al., 2023). Despite its strong performance, the industrial scalability of NBC remains constrained by economic and technological barriers or emerging applications such as NBC-assisted anaerobic digestion (Gamaralalage et al., 2025), authors highlight that robust techno-economic analysis (TEA) and integrated LCA (Gamaralalage et al., 2025; Koné et al., 2025; Trollip and Merckel, 2025) studies are still lacking to determine whether large-scale production can maintain cost-effectiveness and sustainability.

Despite the burgeoning interest in NBC for soil amendment, very few empirical studies have tracked its functional performance across multiple soil treatment cycles or seasons. Recent works such as (Saini et al., 2025) demonstrate that nitrogen-fortified NBC improves soil moisture retention, nutrient availability and crop yield in a single season, yet they stop short of evaluating whether such benefits persist beyond one crop cycle or through soil ageing processes. Similarly, (Faisal et al., 2025), show NBC ameliorating arsenic toxicity in soybean, but the study does not explore how the NBC behaves in situ over successive plantings or under repeated stress exposure. Furthermore, the review by Kumar et al. (2025) highlights that although NBC offers enhanced surface area, nutrient-retention capacity and remediation potential, there is a critical lack of long-term soil field trials assessing how N ages, degrades, or loses effectiveness. Moreover (Zhang, 2025), raises concerns about the mobility and negative environmental risks of BC in soil meshes, signalling that long-term stability and fate remain under-explored.

5 Conclusion

The temporal evolution of NBC research shows a clear shift from purely adsorption-driven mechanisms toward multifunctional materials capable of simultaneously enabling adsorption, redox-based degradation, catalytic activation, and microbially mediated transformations. Early studies (Period I) were dominated by π–π interactions, hydrogen bonding, and electrostatic adsorption, but Periods II–III reveal the emergence of nanostructured NBC (e.g., nZVI-BC, CPS-BC, magnetic biochars) capable of degrading pollutants such as Cr(VI), PFAS, and microplastics—reflecting a diversification of mechanisms and contaminant types.

The thematic progression from basic adsorption concepts to complex topics such as microbial community dynamics and environmental risk assessment demonstrates a maturing research landscape that increasingly integrates ecological, biochemical, and health-oriented perspectives. The appearance of “bacterial community,” “microbial community,” and the transition of “risk assessment” from an emerging theme (Period II) to a niche/high-development theme (Period III) indicate that scientific priorities are moving beyond removal efficiency toward understanding long-term soil interactions, pollutant fate, and implications for ecosystem functioning.

Despite significant advances in NBC engineering and pollutant removal performance, substantial methodological and scalability gaps persist—particularly regarding life-cycle sustainability, long-term stability, and techno-economic feasibility. Current evidence shows that high-temperature modifications, metal doping, and chemical activation improve NBC reactivity but also increase environmental footprint and production cost. Life Cycle Assessment (LCA) and Techno-Economic Analysis (TEA) remain underdeveloped, making it difficult to determine whether NBC can be sustainably deployed at industrial or field scales.

Long-term soil behavior, ageing effects, and repeated-cycle performance of NBC represent major unresolved challenges that require urgent experimental attention before NBC can transition from laboratory innovation to real-world soil remediation technology. The absence of multi-season field trials, limited understanding of NBC mobility in soil matrices, and insufficient evaluation of post-application transformations (e.g., dissolution, aggregation, loss of active sites) highlight critical knowledge gaps. Addressing these issues will be essential for ensuring environmental safety, regulatory approval, and durable remediation outcomes.

Data availability statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

Author contributions

PV-V: Software, Methodology, Investigation, Supervision, Funding acquisition, Conceptualization, Writing – original draft, Validation, Resources, Data curation, Visualization, Project administration, Formal Analysis. MB-N: Conceptualization, Writing – review and editing. EB-R: Conceptualization, Writing – review and editing. MC-C: Writing – review and editing, Conceptualization, Validation. JC: Validation, Writing – review and editing. AS-M: Validation, Writing – review and editing. JS-V: Writing – review and editing, Validation. EN: Validation, Writing – review and editing. HA-E: Validation, Writing – review and editing. JA-C: Validation, Writing – review and editing. MA-A: Validation, Writing – review and editing. RC-M: Validation, Writing – review and editing. TQ-O: Validation, Writing – review and editing, Conceptualization.

Funding

The author(s) declared that financial support was not received for this work and/or its publication.

Acknowledgements

The authors are grateful to the reviewers for their insightful remarks for enlightening the manuscript.

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Generative AI statement

The author(s) declared that generative AI was not used in the creation of this manuscript.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fenvs.2025.1723114/full#supplementary-material

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