1 Introduction

Antibiotic-resistance genes (ARGs) and antibiotic-resistant bacteria (ARB) are considered as emerging pollutants and serious environmental problems (Maurya et al. 2021). According to the ‘Frontiers 2017’ report published by the United Nations Environment Program, antibiotic-resistant (AR) impact on the environment emerged as one of the six significant environmental issues worldwide. Accumulation of ARGs in environmental bacteria leads to resistance transfer to prevalent pathogens or the direct generation of new pathogens (Lin et al. 2021).

The global abuse of antibiotics is a direct cause of the continued emergence and rapid spread of ARGs and ARB (Fang et al. 2022). Antibiotics that cannot be fully metabolized by the actives are excreted into the environmental media (Li et al. 2023), and soil is considered to be the largest reservoir for ARB and ARGs (Ondon et al. 2021). Antibiotic residues in the soil environment exert selective pressure on microbial communities, leading to the development of resistance. This, in turn , promotes the diffusion of ARB and ARGs through horizontal gene transfer (HGT), thereby impacting the environmental health of soil ecosystems (Hashmi et al. 2017; Xiao et al. 2023), which poses a threat to both agricultural product safety and human health (Li et al. 2023; Zhi et al. 2019). The type and abundance of ARGs in agricultural soils vary according to soil types, and soil contaminants such as organic and chemical fertilizers (Wei et al. 2022), heavy metals (HMs) (Wang et al. 2021b), hydrocarbon (Sazykin et al. 2021) and untreated sewage sludge (Buta et al. 2021). These factors can significantly increase the abundance of ARGs and ARB in agricultural soils, thereby impacting AR (Kaviani Rad et al. 2022). Moreover, various soil-related properties including pH, texture, total nitrogen content, climate factors, and organic carbon play crucial roles in shaping the sources and fate of antibiotics and ARGs within soil ecosystems (Wang et al. 2020). The bacterial bioavailability of antibiotics is directly influenced by soil properties, thereby impacting the formation of ARGs (Wu et al. 2022a, b). The challenging removal, facile generation, and transmission of ARGs have garnered significant attention in the context of soil ARGs treatment; however, no efficient control technology has been explored to date. Revealing the mechanisms underlying the detrimental effects of ARGs on plants and humans, as well as their transmission and removal in soil, while exploring targeted and cost-effective treatment strategies, are pivotal steps towards effective pollution management in the future.

During the past years, biochar has been proposed as a tool for improving soil properties (Islam et al. 2021; Yang et al. 2021b), developed as a functional material due to its cheap and sustainable properties (Liu et al. 2015), and its application in agriculture (fertilizer), climate change (carbon sequestration), environmental remediation (adsorbent) and materials science (functionalization) are receiving increasing attention. During recent years, biochar has emerged as a promising tool for enhancing soil properties (Islam et al. 2021; Yang et al. 2021b), garnering escalating attention from the scientific community (Bolan et al. 2021; Xiao et al. 2018b). The utilization of biochar in environmental applications involves the mitigation of heavy metals (HMs) and organic contaminants in soil, along with the enhancement of soil fertility (Guo et al. 2020). The integration of biochar into soil improves its structural integrity by incorporating organic, inorganic, and mineral constituents (Lehmann et al. 2011). Furthermore, the porous structure and high specific surface area of biochar create conducive environments for microorganisms by offering nutrient-rich and water-retentive conditions (Lehmann et al. 2011; Shao et al. 2022). Furthermore, the presence of numerous functional groups in biochar enhances its ability to remediate pollutants through mechanisms such as hydrogen bonding, π–π bonding, and electrostatic interactions (Tang et al. 2021). These inherent characteristics collectively contribute to biochar’s strong adsorption capacity for pollutants.

Due to the unique characteristics of ARGs and their environmental contamination, biochar has been identified as a reliable adsorbent (Chen et al. 2023; Tang et al. 2021) that, when added to soil, enhances soil fertility and exerts a beneficial influence on ARGs, which are challenging to manage. Therefore, further research into the precise mechanisms through which biochar influences resistant bacteria in soil is a worthwhile endeavor. Due to its cost-effectiveness and sustainability, biochar has been widely studied as a versatile material with various applications in agriculture, climate change mitigation, environmental remediation, and materials science, attracting increasing interest from the scientific community. Biochar’s utilization in environmental contexts includes the stabilization of heavy metals and organic pollutants in soil, as well as enhancement of soil fertility. The addition of biochar to soil improves its structural stability by incorporating organic, inorganic, and mineral components (Lehmann et al. 2011). Furthermore, the porous structure and high specific surface area of biochar create conducive environments for microorganisms by offering nutrient-rich and water-retentive conditions (Lehmann et al. 2011; Shao et al. 2022). Additionally, the presence of various functional groups in biochar enhances its ability to remediate pollutants through mechanisms such as hydrogen bonding, π–π bonding, and electrostatic interactions (Tang et al. 2021). Together, these intrinsic qualities imbue biochar with a strong ability to adsorb pollutants.

The terms “biochar” and “antibiotic resistance gene” have been found to co-occur in 257 academic papers published between 2016 and 2023, as visualized in Fig. 1a using CiteSpace (Hu et al. 2021). The keyword clusters identified in CiteSpace provide insights into the prominent areas of research and emerging trends in the application of biochar for processing antibiotic resistance genes, with larger clusters indicating a greater volume of literature on the subject. The predominant cluster of antibiotic resistance genes is situated within the primary category of composition, encompassing bacterial community, biochar, and soil, suggesting a strong correlation between the impact of biochar on soil ARGs and the composition of bacterial community and soil structure. Additional significant keywords such as adsorption, abundance, heavy metal, and aqueous solution were identified as primary factors influencing the removal process. The quantity of literature pertaining to biochar and antibiotic resistance genes over the previous eight years is illustrated in Fig. 1b. It is evident that research in this domain has been receiving increasing attention.

Fig. 1
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a A timeline map depicting the keyword network within the fields of biochar and antibiotic resistance genes was analyzed, with a Modularity Q value greater than 0.3 indicating a significant cluster structure and a Weighted Mean Silhouette S value exceeding 0.7 suggesting a strong cluster. The literature data utilized in this study was sourced from the Web of Science Core Collection, using the search terms: topic: (“biochar and antibiotic resistance gene”); document: (Article or Review); language: (English); b illustrates the annual frequency of publications pertaining to keywords “biochar” and “antibiotic resistance gene” over the past eight years

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This review initiates by analyzing the origins, dissemination, and contamination pathways of ARGs in soil. It explores the mechanisms of action, outlines the characteristics of biochar, and examines various viewpoints to clarify the numerous ways in which biochar impacts ARGs. Several mechanisms have been proposed for the biochar-mediated modulation of soil ARGs, including (1) inhibition of HGT through the influence of biochar on microbial community structure, such as providing habitat and carbon sources for microorganisms, (2) reduction of antibiotic concentrations by means of adsorption and decomposition mechanisms of biochar to attenuate the generation and dissemination of ARGs, (3) mitigation of ARGs diffusion and abundance due to the influence of biochar's presence on the selective pressure caused by HMs, (4) the potential of biochar to decrease the prevalence of ARGs by mitigating co-selective pressures through alterations in soil characteristics and the presence of pollutants such as HMs and organic substances,  and (5) the influence of biochar on microbial function mediated by its modulation of enzyme activity, resulting in a noticeable effect on ARGs. Finally, the potential and challenges of biochar in regulating soil ARGs are discussed, highlighting its significant implications for future efforts to mitigate ARGs pollution in soil environments.

2 Occurrence, transfer and risk of ARGs

2.1 The occurrence of ARGs

Antibiotic misuse is a primary contributor to the contamination of ARGs. There are five main mechanisms of action through which antibiotics operate: peptidoglycan biosynthesis, nucleic acid synthesis, protein synthesis, metabolism, and disruption of the cytoplasmic membrane (Kohanski et al. 2010). Antibiotics have the capability to exert their effects through various mechanisms concurrently. Common mechanisms contributing to bacterial antibiotic resistance include reduced antibiotic uptake, enzymatic inactivation through mechanistic changes, evasion of antibiotic targets through the synthesis of alternative proteins, and activation of efflux pumps.

Table 1 provides a comprehensive overview of the primary mechanisms of resistance exhibited towards various classes of antibiotics. The predominant forms of resistance identified include intrinsic and acquired resistance. Bacteria possess inherent capabilities to resist certain antibiotics as a result of their distinctive structural and functional attributes, which may include the lack of specific antibiotic targets, thereby conferring resistance to particular antimicrobial agents. Moreover, antibiotic resistance can be acquired through two main genetic mechanisms: chromosomal gene mutations linked to the antibiotic’s mode of action and the uptake of resistance determinants encoded by exogenous DNA through HGT. The former is predominantly passed on to progeny through vertical gene transfer (VGT), while the latter enables rapid dissemination of ARGs. HGT-mediated uptake of exogenous DNA plays a crucial role in bacterial evolution, frequently driving the emergence of antibiotic resistance (Abushaheen et al. 2020; Blair et al. 2015; Johnston et al. 2014; Munita and Arias 2016).

Table 1 The main resistance mechanisms of different types of antibiotics
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2.2 The transfer of ARGs

HGT facilitates the transfer of genetic material between bacteria of different species through mechanisms including DNA transformation, conjugation, and bacteriophage-mediated transduction, largely mediated by mobile genetic elements (MGEs) (Yin et al. 2020). Figure 2 illustrates three mechanisms by which ARGs are transferred via HGT. These mechanisms include conjugation, transformation, and transduction, which involve the transfer of genetic elements between donor and recipient cells, the uptake of naked DNA, and the utilization of phages as DNA carriers, respectively (Soucy et al. 2015). Among these mechanisms, conjugation is identified as the primary pathway facilitating the horizontal dissemination of ARGs (Virolle et al. 2020). Various research studies have explored the correlation between soil pollutants and the conjugation of ARGs in bacteria, examining regulatory mechanisms such as intracellular reactive oxygen species (ROS) production, membrane permeability, and cell–cell contact (Yu et al. 2020a, b). Nevertheless, the influence of soil composition on these three transport pathways remains inadequately understood. Soil composition can influence soil microbial communities, bacterial growth, and other factors related to coupling (Cai et al. 2018; Shi et al. 2023). The cell initiates a protective response to reduce membrane permeability and mitigate the absorption of harmful substances. Conversely, pollutants can enhance HGT of ARGs by causing an increase in membrane permeability and damage. Numerous studies have illustrated the ability of biochar to adsorb plasmids or directly deactivate bacteria, thereby effectively impeding the transmission of ARGsin soil (Fang et al. 2022). This underscores the potential of biochar as a viable approach for addressing soil ARGs contamination.

Fig. 2
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Three mechanisms (a conjugation, b transduction, c transformation) of bacteria obtaining ARGs by HGT

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HGT facilitated by MGEs plays a crucial role in the widespread dissemination of ARGs in the environment. Plasmids, extrachromosomal DNA fragments capable of autonomous replication, exert a significant influence on the distribution and propagation of ARGs through conjugation within host cells, thereby facilitating the transfer of antibiotic resistance genes (Orlek et al. 2017; Svara and Rankin 2011). Plasmids demonstrate the capacity to cross barriers between different species and genera, displaying an increased rate of transfer and aiding in the dissemination and persistence of antibiotic resistance in environmental settings that may not be directly affected by antibiotic use (Hughes and Datta 1983). In a study conducted by Mohsin et al. (2021), plasmids IncFII and IncQ harboring the tet (X4) gene were identified in E. coli strains originating from various infection sources, including poultry, chickens, game birds, and slaughterhouse effluent. Additionally, Oladeinde et al. (2019) demonstrated that the IncK2 plasmid, isolated from Salmonella enterica Serovar Heidelberg strains, facilitated the transfer of antibiotic resistance genes, enabling the acquisition of antibiotic resistance. While the current literature concerning phage and transposon-related transduction and transformation studies is, it is essential to acknowledge their crucial roles. The notable variations in the abundance of ARGs within various components of the soil-lettuce system exhibited a strong positive correlation with intI1 and intI2, as demonstrated by Wen et al. (2021). The occurrence of ARGs in agricultural soil and fresh vegetables has been documented in specific regions of phage-encapsulated DNA. These phages contain small segments that serve as vectors for the dissemination of ARGs to bacteria. Although soil-derived phages may endure in vegetables, their abundance is expected to decrease gradually over time (Larranaga et al. 2018).

2.3 The risk of ARGs

The dissemination of ARGs can transpire between plant-associated microorganisms and their external milieu, predominantly originating from soil (Chen et al. 2017). The act of cultivating plants in soil is linked to the existence of ARGs, whereas the utilization of biochar has the potential to influence antibiotic-resistant soil populations by modifying microbial community structures (Shao et al. 2022; Wang et al. 2022a, b). The rhizosphere soils are identified as key sites for the presence of ARGs, MGEs, bacterial communities, and interactions with antibiotic residues (Shen et al. 2021). Additionally, the soil-root interface constitutes a complex and interconnected ecosystem. Research on the MGE profile in the soil–plant continuum has revealed that increased levels of MGEs may promote the spread of the resistome through HGT (Wang et al. 2021a). Plant roots are acknowledged as important reservoirs of ARGs. Crop plants grown in polluted agricultural settings can uptake and store pollutants in their tissues, potentially acting as vectors for the transfer of contaminants into the food chain (Christou et al. 2019). Research has shown that the spread of ARGs and pathogenic bacteria through the consumption of raw vegetables is a notable pathway, and the presence and variety of ARGs in edible plant components are linked to soil-based ARGs (Cerqueira et al. 2019; Mao et al. 2015). The application of poultry manure has been demonstrated to significantly enhance the abundance of ARGs in samples of deciduous trees, soil, and root endophytes. The ARGs originating from both soil and manure play a vital role as a reservoir of resistance determinants in vegetables. The composition of the microbial community has a significant impact on the plant microbiome, with specific bacteria originating in the soil and later transferring to vegetable tissues, where they establish colonization as root and leaf endophytes. ARGs can be transferred to plant tissues via internal pathways, with shared operational taxonomic units (OTUs) potentially serving as bacterial hosts for ARGs, thereby influencing their presence in lettuce root endophytes, leaf endophytes, and leaf bulbs (Zhang et al. 2019). The movement of ARGs from soil to plant roots and surfaces plays a significant role in the spread of resistance among organic fertilizers, plants, and bacteria. Additionally, bacterial populations, including ARB, have the ability to migrate between the rhizosphere and leaf levels. ARGs are first introduced into the rhizosphere soil through a bottom-up process before being assimilated by endophytic and rhizospheric bacteria that are linked with plants (Wang et al. 2022a, b). Previous research has demonstrated the presence of sul1, sul2, tetC, and tetG in endophytes located in roots and leaves, as well as in leaf-like structures of lettuce cultivated in soils rich in fats (Chen et al. 2017; Fang et al. 2015); the study conducted by Jauregi et al. (2021) revealed a substantial increase in the absolute abundance of MGEs and ARGs in plant samples following the application of manure in soil. Specifically, the absolute abundance of MGEs in lettuce plants ranged from 1.08 × 108 to 2.56 × 109 copies g–1, while the absolute abundance of ARGs in wheat grains ranged from 4.04 × 109 to 1.47 × 1010 copies g–1. Moreover, the absolute abundance of MGEs in both plant types exhibited a significant increase, suggesting a heightened risk of antibiotic resistance transmission in the crops under investigation.

3 Characteristics of biochar and its mechanism of remediation of soil ARGs

3.1 Basic properties of biochar

Biochar is a complex carbonaceous material composed predominantly of coalescing aromatic rings and some polymeric aliphatic hydrocarbons, along with various other functional groups such as oxygen (Yang et al. 2019). Figure 3 illustrates that the fundamental properties of biochar are influenced by factors such as pyrolysis temperature, basic characteristics, and primary mechanisms of action. The characteristics of biochar are primarily influenced by the source material used for its production, the conditions of pyrolysis such as temperature and duration, retention time, heating rate, and other factors as noted by Tu et al. (2022) and Liao et al. (2022). The application of biochar has been shown to provide significant ecological and economic advantages by improving soil fertility, increasing crop productivity, and effectively mitigating heavy metals and organic contaminants in the soil, ultimately enhancing overall soil quality as demonstrated by Singh et al. (2022) and Zhang et al. (2021). Biochar has the capacity to modify the abundance, composition, and activity of soil microbial communities through soil aeration and the provision of essential nutrients and water (Razzaghi et al. 2020; Singh et al. 2022). Biochar is produced through the thermal decomposition and stabilization of organic materials such as agricultural solid waste, residual sludge, straw, woody materials, and livestock manure under high temperature and oxygen-limited conditions. In addition to carbon, biochar consists of a variety of elements that form specific structures and play a role in determining its physical characteristics. The chemical composition of biochar is essential, as it influences the functions and potential applications of this material.

Fig. 3
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Basic properties of biochar and its impact on various pollutants

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3.1.1 Physical properties

The adsorption capacity of water and nutrients in soil is enhanced by the high specific surface area of biochar’s pore structure, as noted by Uzoma et al. (2011). This characteristic is influenced by various factors including the composition of the raw material, heating temperature, and heating rate, as highlighted by Pandey et al. (2020). Higher heating rates have been found to promote increased porosity, while increasing pyrolysis temperatures results in the gradual removal of simple molecules such as ethylene and esters from the outer surface of the feedstock, facilitating the development of biochar's structure and pore area, as discussed by Lian and Xing (2017) and Xiao et al. (2018a). When the pyrolysis temperature reaches a certain threshold, aliphatic alkyl and ether groups undergo detachment, leading to the release of volatile compounds such as cellulose and hemicellulose (Kim et al. 2013). Consequently, this process facilitates the creation of micropores, thermal condensation within the material, and a rapid increase in the specific surface area of biochar (Li et al. 2019a, b). The biochar is comprised of a mixture of open and contracted gas-phase nanopores, along with residual macropores (Gray et al. 2014; Lawal et al. 2021), which limit the total specific surface area and impede the attainment of maximum adsorption capacity (Lawal et al. 2021). As a result, biochar produced through low-temperature pyrolysis of biomass exhibits improved adsorption properties. Changes in the surface properties (Jiang et al. 2021), electrochemical characteristics, and π–π electron donor–acceptor interactions of biochar further impact its adsorption efficiency (Klupfel et al. 2014; Sorrenti et al. 2016; Zhao and Zhou 2019).

The particle size of biochar is influenced by the characteristics of the original biomass, the type of pyrolysis system utilized, and the specific process conditions employed. These factors not only dictate the potential applications of the biochar but also impact its transport and interactions within the environment. Consequently, the handling, storage, and transportation of biochar dust are critical considerations with significant implications for human health and safety, as highlighted by previous research (Aller 2016; Amonette and Joseph 2009a; Brewer et al. 2014). Biochar typically exhibits higher solid density compared to virgin material, with solid density increasing as pyrolysis temperatures rise which increases as the pyrolysis temperature rises. Additionally, the presence of micropores in biochar contributes positively to its overall density. The bulk density of biochar, however, is not influenced by pyrolysis temperature but rather by the raw material utilized, with a direct correlation between the bulk weight of prepared biochar and the raw material used (Byrne and Nagle 1997; Kazemi Shariat Panahi et al. 2020).

3.1.2 Chemical properties

During the pyrolysis of biomass to produce biochar, various physicochemical processes occur, leading to the formation of a diverse array of biochar compositions. The predominant component of biochar is carbon, with the proportion of stable and unstable carbon forms contingent upon the specific pyrolysis temperature and mineral composition of the initial biomass (Mašek et al. 2013). Carbon, hydrogen, oxygen, and nitrogen are among the most prevalent elements in biochar, typically comprising the primary structural components (Aller 2016).Pyrolysis facilitates the elimination of H and O, resulting in a change in the proportion of elements in the biochar (Crombie et al. 2013), and the O/C and H/C ratios are directly related to polarity, aromaticity, and biodegradability, with lower ratios at higher pyrolysis temperatures leading to increased aromaticity and carbon stability and reduced polarity, which is the key to removing organic pollutants. The process of pyrolysis facilitates the removal of hydrogen and oxygen, resulting in a shift in the elemental composition of biochar (Crombie et al. 2013). The relationship between the ratios of oxygen to carbon (O/C) and hydrogen to carbon (H/C) and the characteristics of polarity, aromaticity, and biodegradability is well-established in the literature. Elevated pyrolysis temperatures have been shown to decrease O/C and H/C ratios, resulting in heightened aromaticity, improved carbon stability, decreased polarity, and enhanced efficacy in the removal of organic pollutants (Oliveira et al. 2017; Suliman et al. 2016). The cation exchange capacity (CEC) of biochar is determined by its surface negative charge, which is influenced by the pyrolysis temperature and the presence of functional groups carrying a net negative charge (Ahmad Bhat et al. 2022). Biochar produced at lower temperatures tends to have a higher CEC (Al-Wabel et al. 2018), with variations also observed based on the original biomass source. Furthermore, biochar derived from biomass with higher ash content, such as manure biochar, may exhibit a greater CEC (Ro et al. 2010). The electrical conductivity (EC) of a substance is defined as its ability to conduct an electric current, a property that is influenced by the concentration of dissolved salts in water (Laghari et al. 2016). It has been hypothesized that the increased EC of biochar can be attributed to the presence of Na+, K+, and Mg2+ ions (Mukherjee et al. 2011). Research indicates that biochar produced from livestock manure as its primary source material displays elevated EC values due to its significant salt content, potentially posing toxic effects on young plants with excessive application (Song and Guo 2012). The amphiphilic characteristics of biochar’s surface, attributed to the presence of oxide minerals, lead to a pH-dependent fluctuation in electric charge. As demonstrated by Kazemi Shariat Panahi et al. (2020), the surface of biochar is positively charged in acidic environments and negatively charged in alkaline environments. Furthermore, the pyrolysis temperature employed in biochar production is positively associated with the pH of the resulting biochar, suggesting its alkaline nature can be leveraged to ameliorate soil acidity and impact the movement of metal ions within the soil matrix. The pH of biochar is significantly correlated with oxygen content, as indicated by previous research linking biochar alkalinity to the presence of oxygen-rich functional groups such as diketones, quinones, gamma-pyrrolidone, and chromium (Montes-Morán et al. 2004; Mukome et al. 2013). Biochar derived from locust wood or pine typically exhibits slight acidity or neutrality, with acidic biochar serving as a potential soil amendment for alkaline soils (Laghari et al. 2016).

The chemical reactions taking place on the surface of biochar are largely influenced by its surface functional groups, as noted by Xiao et al. (2018a). The feedstock utilized in the production of biochar contains a high abundance of oxygen-containing functional groups, including phenols, carbonyl groups, carboxylic acids, alcohols, and heterocycles. These functional groups play a significant role in determining the distribution of functional groups found in cellulosic and lignin biomass, which are predominantly composed of carboxyl groups; some of these functional groups persist in the biochar even after undergoing pyrolysis (Lian and Xing 2017; Zhao and Zhou 2019). The electrostatic interaction is dependent on the specific functional group that is present. At higher pyrolysis temperatures, the increase in oxygen-containing functional groups in biochar can be linked to a rise in ion exchange capacity due to a greater concentration of positively charged functional groups on the surface (Zhang et al. 2017). Conversely, this correlation remains consistent at lower pyrolysis temperatures (Banik et al. 2018). The quantity of functional groups diminishes as the pyrolysis temperature rises (Yang et al. 2018), and the functional groups present on the surface of biochar can generally be categorized into two groups: (1) electron donors, such as NH2, OH, O (C=O) R, or OR, which contain α or π electrons; and (2) electron acceptors, such as (C=O) OH, NO2, or (C=O) H groups, which contain empty orbitals (Amonette and Joseph 2009b; Kazemi Shariat Panahi et al. 2020). Cryogenic biochar typically exhibits a diverse array of functional groups, including a significant presence of alkyl C–H and C=C bonds, as well as hydroxyl, carboxyl, carbonyl, and methyl groups (Yang et al. 2018; Trigo et al. 2016).

3.2 Mechanisms of removal of soil ARGs by biochar

The incorporation of biochar into agricultural soils has been shown to improve soil properties, as it is frequently utilized for the mitigation of organic contaminants in soil through mechanisms such as direct immobilization and modulation of soil metabolic processes. Biochar can effectively immobilize organic pollutants through various interactions, including electrostatic interactions, pore filling, π–π electron donor–acceptor interactions, and hydrophobic interactions (Xiao et al. 2018a). The adsorption efficiency of biochar in soil primarily relies on its inherent properties, the characteristics of organic pollutants, as well as the structure and properties of the soil. Additionally, other environmental factors that govern pollutant transport also play a crucial role. Currently, extensive research is being conducted on the modification of biochar, encompassing metal salt/metal oxide modification, organic modification, acid/base modification, loading oxidizing agents and other approaches (Chen et al. 2018). Table 2 provides a comprehensive summary of relevant studies involving modified biochar for controlling ARGs.

Table 2 Removal mechanisms of soil ARGs by modified biochar
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In the realm of soil remediation, it is essential to thoroughly assess potential adverse effects and conduct toxicity testing before proceeding with the additional utilization of biochar. Specifically, the incorporation of heavy metals (HMs) into biochar via feedstock, organic pollutants, and the pyrolysis process has the potential to create environmentally persistent free radicals that may negatively impact soil health (Odinga et al. 2020; Xiao et al. 2018a). The extended presence of these contaminants poses a significant toxic hazard to soil microorganisms, consequently affecting soil organisms and plant life.

The incorporation of biochar into soil has a diverse range of impacts on the abundance of resistance genes, as illustrated in Fig. 4. It can directly influence bacterial populations and their genetic characteristics, such as gene duplication and transfer (Ejileugha 2022), as well as alter microbial composition (Cheng et al. 2021), heavy metal concentrations, soil physical properties (Hu et al. 2022), chemical properties, and co-selection pressures (An et al. 2018). The synergistic actions of these mechanisms lead to a decrease in both commensal and pathogenic bacteria harboring antimicrobial resistance genes, thus efficiently managing the proliferation of resistance genes and antibiotics in agricultural crops and soil (Jiao et al. 2018).

Fig. 4
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The main ways of biochar affecting soil ARGs by different factors (antibiotics; non-antibiotics; microbial community structure and soil physico-chemical properties)

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The specific ways in which biochar affects ARGs abundance in soil are as follows.

3.2.1 Biochar alters ARGs abundance by affecting microbial diversity and activity

The bacterial community composition plays a significant role in determining the content of resistance genes in the environment, with human activities contributing to the evolution and proliferation of ARGs in microbial communities. This, in turn, facilitates the exchange of ARGs between bacteria. The addition of biochar to soil has been shown to impact the microbial community and its function in agricultural soil (Cole et al. 2019). The application of biochar has been found to have an impact on the relative abundance of soil bacteria. In particular, higher concentrations of cornstalk biochar have been shown to be associated with a reduction in the relative abundance of ignavibacterium and acidobacteria (Fan et al. 2020). The addition of biochar to agricultural soil has been shown to stimulate the growth of functional bacteria, leading to their dominance within the microbial community. This enhancement of bacterial stability serves to counteract perturbation effects (Liu et al. 2020b).

Possible mechanisms for adding biochar to improve microbial diversity and activity in soil are as follows.

  • 1. Biochar can serve as a porous habitat that may protect microbial growth and predators. Its porous structure contributes to a high surface area and enhanced adsorption capacity, enabling it to offer a protective environment for less competitive microbes against protists. Additionally, microorganisms can utilize biochar particles as favorable habitats for colonization, growth, and reproduction (Warnock et al. 2007).

  • 2. Offering a carbon source and mineral nutrients to microorganisms is essential for their growth and metabolism. Unstable carbon can act as a substrate for microbial processes, impacting microbial populations and community dynamics by providing energy in the form of carbohydrates. The ash found in biochar plays a dual role in stabilizing microbial metabolites through the formation of organic-mineral bonds and supplying necessary inorganic nutrients for microbial growth (Dai et al. 2021; Xu et al. 2020).

The study conducted by Wang et al. (2019) revealed that soil subjected to long-term fertilization exhibited a greater level of bacterial resistance compared to unfertilized soil, attributed to the diverse HGT mechanisms employed by bacteria. Elevated concentrations of macrolide-resistant bacteria in such environments can promote HGT among microorganisms in agricultural fields, leading to an escalation in the prevalence of macrolide ARGs. For example, a study by Gao et al. (2020) demonstrated notable changes in the microbial community structure in soil contaminated with pig manure, showing positive relationships among proteobacteria, firmicutes, and bacteroidetes, along with an increase in the overall abundance of ARGs and antibiotics. Additionally, enterococcus and sphingobacter were found to be positively associated with these changes, while other bacterial taxa that exhibit positive correlations with different environmental factors may serve as potential reservoirs for the migration of ARGs, as suggested by Berendsen et al. (2021). Crops also influence the microbial community, thereby affecting the abundance of ARGs and MGEs in soil. (Jauregi et al. 2021) revealed that wheat soil exhibited a higher absolute abundance of ARGs and MGE genes compared to lettuce soil. Moreover, the researchers pinpointed various factors that influence the microbial community's composition, as well as the dispersion and destiny of MGEs and ARGs in soil. These factors include the types of crops, the time elapsed from plant growth to harvest, the types of roots, and the amount and makeup of rootstock. The results indicated that bacteria were responsible for approximately 79.76% of the variability in ARGs. Other factors, such as antibiotics, HMs, and soil nutrients, may exert influence on the abundance and composition of ARGs by altering the community structure and activity of agricultural soil bacteria (Yang et al. 2021a). Moreover, soil animals function as hidden reservoirs of antimicrobial resistance genes (ARGs), as demonstrated by the discovery of various antimicrobial substances and antibiotic gene clusters in the gastrointestinal tract of soil animals following the use of organic fertilizers (Zhu et al. 2022). Furthermore, both plant-derived and fecal-derived biochar have notable impacts on the abundance of ARGs by influencing soil bacterial communities and gut properties (Ding et al. 2019).

3.2.2 Biochar affects microbial community structure by altering soil physicochemical properties, resulting in changes in ARGs abundance

Biochar exerts a significant influence on various soil chemistry parameters, such as soil pH, cation exchange capacity (CEC), electrical conductivity (EC), organic matter (OM), total carbon (TC), carbon–nitrogen ratio (C:N ratio), and soil organic carbon (SOC) (Sun et al. 2022). These changes in soil physicochemical properties and nutrient availability have been documented in previous studies (Warnock et al. 2007). The lasting effects of biochar on soil properties are expected to persist, potentially impacting microbial communities through their influence on physicochemical processes within the soil (Fan et al. 2020). Soil pH is a significant factor in this process (Sheng and Zhu 2018), as the incorporation of biochar results in an elevation of soil pH. This, in turn, leads to an increase in the population of gram-negative bacteria (Aciego Pietri and Brookes 2009); the microorganisms in the soil aid in the oxidation of the surface of biochar particles, resulting in an augmentation of negative charge density and consequently enhancing the CEC of biochar-amended soil as time progresses (Cole et al. 2019). This process induces changes in ionic equilibrium, promotes microbial adhesion, and stimulates the growth of soil microorganisms, thereby increasing both diversity and activity levels (Huang et al. 2022); the enhancement of soil nutrition content has been shown to positively impact the diversity and growth of the soil microbial community (Song et al. 2020); the improved water holding capacity of the soil has been found to influence the biota within the soil (Cole et al. 2019); the physicochemical properties of biochar have been shown to impact soil nutrient levels through increased adsorption on the biochar surface (Herrmann et al. 2019); changes in pH resulting from this process can alter the availability of nutrients, particularly phosphorus, rendering them more accessible to microorganisms (Ding et al. 2016); alterations in soil organic matter content can directly or indirectly influence the composition and activity of the microbial community, particularly fungal populations (Herrmann et al. 2019). Elevated pH levels can also impact microbial enzyme activity and biomass, thereby influencing the dynamics of soil organic matter (Maestrini et al. 2014).

The physicochemical characteristics of environmental media play a crucial role in influencing the enrichment, migration, and transformation of ARGs (Lin et al. 2021). Soil parameters such as pH, CEC, OM, and OC have been found to have a significant impact on the abundance of ARGs, leading to variations in their distribution within the soil matrix. Metagenomic analysis has revealed a significant and positive correlation between the abundance of tet and sul genes and soil properties, particularly pH and SOM (Guo et al. 2018). The microbial community's structure and abundance of ARGs can be influenced by the physical and chemical properties of the environment, as demonstrated by Chand et al. (2023). Specifically, OM, pH, TN, and TP are identified as key factors in this interaction in various soil types, such as clay, silt, and sand, as highlighted by Wang et al. (2020). The distribution of tetracycline resistance genes in paddy fields, particularly in areas where organic manure has been applied, is predominantly shaped by the physicochemical characteristics of the soil. TN and OM are important properties that have the ability to influence bacterial community structure and modify antibiotic concentrations, thereby impacting the presence and abundance of antibiotic resistance genes (Qing et al. 2022; Tang et al. 2015).

3.2.3 Biochar influences antibiotic content at source and thus ARGs abundance

AR has been primarily attributed to the induction of antibiotics, resulting in their persistence in the environmental medium (Zhou et al. 2020). The release of antibiotics into the environment has been shown to influence the regulation of ARGs and provoke microbial responses, thereby increasing bacterial mutation rates and promoting HGT of ARGs from ARB to non-resistant strains (Zhu et al. 2013). The persistence of antibiotic-resistant bacteria in soil is influenced by the physicochemical properties of antibiotics (Berendsen et al. 2021). Antibiotics are frequently utilized in animal disease management and as growth-promoting additives in animal feed. The persistence of antibiotics in the environment is a significant contributor to the escalation of ARGs in soil. Antibiotics have the potential to selectively support the proliferation of bacteria carrying mutagenic genes, thereby facilitating the expansion of ARG populations. Additionally, antibiotics can increase the prevalence of antibiotic-resistant bacteria by facilitating the horizontal transfer of ARGs through conjugation (Johnston et al. 2014).

Elevated levels of antibiotics have a profound impact on the activity and diversity of soil microorganisms. Tetracycline, for instance, imposes a significant antimicrobial selection pressure on soil bacteria, leading to the proliferation of resistant strains when present in concentrated quantities (Wang et al. 2019). Nevertheless, the chelation, absorption, and degradation of antibiotics in soil result in a transient nature of the selective pressure on microorganisms. Research indicates that antibiotics account for a mere 4.76% of the variation in ARGs, suggesting that the presence of ARGs is not solely dependent on the presence of corresponding antibiotics (Gao et al. 2020; Guo et al. 2018; Yang et al. 2021a). The significance of low antibiotic concentrations on the dissemination and amplification of ARGs is a critical area of study. Numerous studies have consistently detected the presence of low levels of antibiotics in diverse environmental matrices. For example, Wu et al. (2019) reported that hygromycin, even at subinhibitory concentrations, can facilitate the persistence and horizontal transfer of ARGs. Furthermore, Yin et al. (2020) illustrated the phenomenon of cross-induction between antibiotic stress responses and sublethal antibiotic concentrations, which leads to the promotion of bacterial antibiotic resistance. Moreover, a limited number of studies have revealed that subminimal inhibitory concentrations of antibiotics contribute to the transfer of ARGs (Jutkina et al. 2018).

Biochar will influence the content of antibiotics in the following ways, and the ARB specific influencing mechanisms are shown in Table 3.

  1. 1.

    Inherent properties of biochar: the primary mechanism of antibiotic adsorption by biochar involves the π–π bond interaction between the antibiotic and the aromatic ring present on the biochar, as supported by previous studies (Peng et al. 2016; Rajapaksha et al. 2014). Additionally, electrostatic interactions, hydrogen bonding, pore filling (Tang et al. 2021), cation bridging, and surface complexation have been identified to play a crucial role in the adsorption process for certain antibiotics (Pan 2020). The aromaticity of biochar is shown to significantly impact the π–π electron donor–acceptor interaction between the adsorbent and biochar, thereby influencing the overall adsorption process (Pignatello et al. 2017). Moreover, electrostatic interactions are a significant factor in the adsorption process of ionic organic pollutants onto biochar. The presence of negatively charged molecules on the biochar surface facilitates the electrostatic binding of cationic dyes, such as brilliant blue and rhodamine B, as demonstrated in studies by Xu et al. (2011) and Qiu et al. (2009). Furthermore, biochar has been shown to decrease the bioavailability of antibiotics and reduce the transfer of antibiotics from contaminated soil to plant tissues, as evidenced by research conducted by Hurtado et al. (2017). The mechanism of action of biochar can vary depending on the type of antibiotics used. In instances where the pH is elevated, sulfamethoxazole (SMZ) forms hydrogen bonds with the COOH or OH groups of biochar. Conversely, at a pH below 8, the interaction between SMZ and biochar is primarily characterized by electrostatic π–π electron donor interaction and cation exchange interaction (Vithanage et al. 2014); biochar has the potential to enhance the adsorption of tetracycline through mechanisms such as π–π bonds and metal bridging, leading to the conversion of free tetracyclines to bound forms. This process ultimately results in reduced mobility, bioavailability, and selective pressure of tetracycline in soil, consequently lowering tetracycline content and the prevalence of its resistance gene (Qin et al. 2022).

  2. 2.

    Effect on soil physicochemical properties: the incorporation of biochar resulted in an elevation of soil pH and a decrease in bio-rich tetracycline levels, as demonstrated by Yue et al. (2019). The modification of fungal community composition by biochar is attributed to its alkaline impact on soil and electrostatic repulsion, which in turn hinders the interaction between tetracycline and microbial organisms, as discussed by Liu et al. (2020a); TC is associated with SOM via H-bonding, cation bridging and cation exchange, with the latter probably being the dominant mechanism (Pollard and Morra 2018); nitrogen is one of the important nutrients of soil microorganisms, and biochar contains primarily heterocyclic nitrogen; increasing N in the soil may enhance the excretion of tetracycline antibiotics (Yue et al. 2019).

  3. 3.

    Effect on surface microorganisms: the concentration of antibiotics at the onset can have an impact on biodegradation and soil microorganisms through modifications in enzyme activity and metabolic capacity, albeit with variability among bacterial species and their growth stages (Cycon et al. 2019; Katiyar et al. 2022). Additionally, bacteria attached to biochar surfaces demonstrate increased effectiveness in antibiotic degradation, as biochar provides carbon and other nutrients that facilitate microbial colonization and activity, thereby accelerating the biodegradation of organic pollutants in soil (Adelaide and José Luís da Silva 2021).

Table 3 Removal mechanisms of common antibiotics and their corresponding ARGs by biochar
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3.2.4 Biochar affects antibiotic resistance through the adsorption of HMs

The distribution and propagation of AR can be significantly influenced by environmental metal pollution due to the antimicrobial properties of metals and the long-term selection pressure they impose (Cao et al. 2020). Figure 5 demonstrates that HMs can promote HGT of ARGs among environmental bacteria. Metal ions have the ability to induce oxidative stress and bacterial reactive oxygen species (ROS), leading to a bacterial SOS response that is considered a key mechanism in the evolution of AR. This response can activate integrases and transferases, facilitating the transfer of ARGs through HGT (Guerin et al. 2009; Lin et al. 2019; Zhang et al. 2018). Numerous research studies have illustrated the antimicrobial and growth-promoting attributes of specific HMs commonly incorporated into animal feed. As a result, pollution from heavy metals aids in the dissemination of ARGs through mechanisms of co-selection and co-regulation. Co-selection arises from the interplay between resistance mechanisms for antibiotics and heavy metals, leading to the development of co-resistance, wherein multiple resistance genes are clustered in close proximity on a mobile genetic element. Moreover, metal ions have the ability to simultaneously modulate genes associated with antibiotic resistance and diminish susceptibility to antibiotics (Berendsen et al. 2021; Seiler and Berendonk 2012). The occurrence of ARGs demonstrated a significant and favorable relationship with HMs such as As, Cd, Cu, and Zn. Specifically, tetracycline readily formed complexes with HMs and frequently co-localized within the same MGEs. Conversely, HMs also impacted the transferability of tetracycline, leading to a positive association between the two variables. This observation highlighted a synergistic selection pressure on ARGs due to long-term co-selection exerted by metals (Guo et al. 2018). The sul gene displayed greater sensitivity to metal contamination in comparison to the tet gene, particularly in response to heavy metals Cu and Hg which exhibited a stronger capacity for induction compared to other metals. These two metals are likely the main triggers of microbial SOS responses, leading to an upregulation of the sul gene (Deng et al. 2020). Concentrations of Zn (II), Cr (VI), Ag (I), and Cu (II) were found to be comparable to sub-inhibitory levels observed in contaminated environments and in treated animals and humans, potentially facilitating horizontal gene transfer of antibiotic resistance genes (Zhang et al. 2018).

Fig. 5
figure 5

The way HMs contribute to ARGs pollution

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The utilization of biochar for the stabilization of HMs in soils encompasses a multitude of potential mechanisms, whereby biochar can directly or indirectly influence the chemical speciation of HMs, as shown in Table 4, encompassing: (1) Direct interaction. Furthermore, aside from physical adsorption and surface precipitation as discussed by Zeng et al. (2015), the interaction of metal ions and cations with biochar is influenced by humic complexation and co-precipitated inner sphere complexation of mineral oxides with biochar. The biochar is characterized by numerous surface functional groups such as carbonyl groups, carboxyl, and phenolic hydroxyls, in addition to a well-defined porous structure. These attributes possess the capacity to improve ion exchange processes on the biochar surface and reduce the bioavailability of heavy metals (Li et al. 2021; Mohamed et al. 2017). The charge properties of these characteristics facilitate interactions between various functional groups and the surfaces of heavy metals, resulting in internal spherical complexation with the available hydroxyl groups of mineral oxides and other surface precipitates, consequently impacting the adsorption efficacy (Puga et al. 2015). (2) Indirect interaction. The elevation of soil pH through the utilization of biochar and the subsequent liberation of dissolved organic carbon have been found to enhance the adsorption and precipitation of heavy metals in forms such as phosphates, carbonates, oxides, and hydroxides, ultimately diminishing their bioavailability via adsorption and/or precipitation reactions (Beesley et al. 2011; Li et al. 2021; Zeng et al. 2015). Furthermore, the composite nature of biochar, encompassing both mineral and organic constituents, imparts a substantial binding capacity for heavy metals and furnishes supplementary sites for adsorption (Uchimiya et al. 2010).

Table 4 Adsorption mechanisms and influence between biochar and HMs and impacts on soil ARGs
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3.2.5 Biochar affects antibiotic resistance through influencing enzyme activity

The assessment of soil enzyme activity is essential for evaluating microbial functionality, as all biochemical processes in the soil are interconnected with it (Gianfreda and Rao 2008). The potential influence of enzyme activity on mitigating ARGs contamination through biochar application has not been extensively investigated. The enzymatic activity plays a crucial role in the resistance mechanisms against aminoglycosides and chloramphenicol antibiotics (Ogawara 2019; Costello et al. 2019). A novel dihydrofolate reductase found in the soil microbiome has been identified as resistant to trimethoprim. Enzyme activity in soil is subject to alterations stemming from variations in its physical, chemical, and biological constituents (Gianfreda and Rao 2008). Nonetheless, the introduction of biochar can wield an indirect impact on soil enzyme activity by regulating these factors (Zheng et al. 2022). Additionally, biochar harbors vital nutrients conducive to microbial proliferation, including phosphorus, potassium, and magnesium. These nutrients can bolster soil enzyme activity by fostering heightened microbial activity within the soil (Lehmann et al. 2011). Current research in the field has explored the effects of biochar on soil enzyme activity and microorganisms, with a particular focus on the regulatory role of ARGs (Zheng et al. 2022). The mechanisms by which biochar impacts enzyme activity are complex and varied, with different types of biochar yielding differing effects on enzyme activity. Additionally, the supplementation of biochar with nutrients has been found to enhance microbial activity, supporting the growth and utilization of microorganisms and serving as a substrate for specific enzymes. Ultimately, these findings contribute to a deeper understanding of the role of biochar in soil health and microbial processes. For example, Imparato et al. (2016) found that elevated levels of wheatgrass gasification biochar can augment soil phenoloxidase activity while diminishing cellulase levels; the incorporation of poultry litter biochar led to heightened soil enzyme activity for cellulase, β-glucosidase, and arylsulfatase (Paz-Ferreiro et al. 2015). The impact of biochar on soil ARGs was examined, with a specific focus on the influence of enzyme activity. The results of this investigation revealed strong negative associations between enzyme activity and extractable concentrations of Cd and Pb, as well as positive correlations with SOC, DOC contents, microbial activity, and biomass (Azadi and Raiesi 2021). The impact of straw biochar on dehydrogenase, catalase, and urease activities was observed to be influenced by the availability of As, Cu, Cd, and Zn (Tang et al. 2020). It is postulated that the addition of biochar may have a beneficial effect on the elimination of soil ARGs through its effects on soil enzyme activity, reduction of co-selection pressure, and modulation of microbial activity.

3.3 Biochar affects antibiotic resistance through other factors

In addition to HMs found in soil, various pollutants, including microplastics, inorganic salts, and non-antibiotics, also affect the occurrence and transmission of ARGs within soil. The presence of microplastics in the soil environment not only alters the physical, chemical, and biological properties of the soil and its functions but also results in a notable increase in the relative abundance of ARGs due to a higher prevalence of bacteria hosting more than 5 ARGs (Lu and Chen 2022; Su et al. 2024; Wang et al. 2022a, b). The impact of microplastics and biochar on soil ARGs is predominantly shaped by indirect variables such as soil characteristics, microbial populations, and the prevalence of MGEs, with soil properties emerging as the most influential factor (Su et al. 2024). The presence of inorganic salts in the soil undoubtedly contributes to the effects of biochar on ARGs, as certain anions and antibiotics occupy adsorption sites where ARGs, being negatively charged, may experience electrostatic repulsion from the anions. Therefore, the cations present enhance the adsorption of ARGs by biochar (Du et al. 2023). Wang (2013) suggested that Ca2+ can form inner sphere complexes with phosphate groups of DNA, forming a compact molecular structure, which is more suitable for the microporous size of the biochar. It is hypothesized that pesticides, disinfectants, detergents, nanomaterials, and other non-antibiotics play a substantial role in the reduction of soil ARGs. However, there is a lack of research investigating the relationship between these complex agricultural chemicals and pharmaceutical pollutants with soil ARGs. The potential benefits and mechanisms of action of biochar as an eco-friendly amendment require thorough examination, posing new challenges for future research efforts.

4 Conclusions and future prospects

To address the challenges posed by AR, it is crucial to conduct thorough investigations into the origins, selection pressures, enrichment mechanisms, and transmission pathways of ARGs in environmental media. The soil environment is consistently exposed to antibiotics, ARGs, and ARB from sources including livestock manure, wastewater, and sewage sludge. The potential transmission of ARGs and ARB to humans via contaminated vegetables and crops poses a significant threat to human health. Despite this risk, research on the removal of ARGs from agricultural soils has been limited, with even fewer studies investigating environmentally sustainable soil treatment methods. The application of biochar as a soil conditioner has shown promise in influencing the abundance and diversity of soil ARGs due to its rich functional groups, extensive specific surface area, and porous structure. It is imperative to comprehend the mechanism through which biochar regulates antibiotics and ARGs, as it presents an innovative strategy for soil remediation. The concurrent implementation of measures to prohibit antibiotic abuse and the advancement of efficient technical treatment methods can effectively mitigate antibiotic resistance in environmental media.

Despite the known health risks posed by ARGs pollution, there is a lack of reliable methods for accurately measuring the extent and frequency of this risk. Due to the microscopic nature of ARGs pollution, visual detection is not possible. The absence of statistical data on soil ARGs, coupled with the difficulty in detection methods, hinders the assessment of associated risks and the allocation of appropriate attention to them. Current available methods primarily focus on mitigating soil ARGs through biochar application. The comprehensive elimination of antibiotic resistance genes (ARGs) from soil has not been thoroughly investigated, presenting an ongoing challenge that requires additional examination and resolution. The effectiveness of biochar in remediating soil ARGs is affected by factors such as the amount, composition, and timing of application. The scarcity of research has impeded the development of a systematic approach for categorizing and summarizing findings for future application. Some studies have indicated that prolonged use of specific biochar types in soil may have a contradictory effect on ARGs; however, this issue has yet to be resolved. The results of these foundational studies underscore the need for the development of management and regulatory measures designed to minimize potential long-term health hazards for humans.