Bacteria might adapt or be resistant to metal toxicity using various approaches, i.e., synthesis of metallothioneins, active transport or efflux mechanisms, morphological changes, synthesis of siderophores, biotransformation of toxic metals, and synthesis of exopolymeric compounds (Funtikova et al., 2023; Liaqat et al., 2023; Sevak et al., 2023; Wang et al., 2023; Zhou et al., 2023). Figure 1 presents the few microbial approaches used for HM remediation from the soil.

Bacteria are well-known for their ability to degrade or mineralize pollutants through enzyme-catalyzed catabolic action (Tables 6, 7). Numerous catabolic genes that rely on PAHs and are remarkably conserved have been identified during bioremediation studies (Barker and Bryson, 2002; Ali et al., 2022). These genes are present in gram-positive bacterial species and include phd, pdo, nid, and nar, and in gram-negative bacterial species and include phn, pah, ndo, nah, and nag (Sakshi and Haritash, 2020). Analysis showed that crude oil might be biodegraded by the bacteria Pseudoalteromonas agarivorans, Isoptericola chiayiensis, Rhodococcus soli, and Bacillus algicola in less than 2 weeks (Lee et al., 2018). Most small molecules with carbon atoms between C9 and C14 were entirely decomposed, while those with C15 and C20 were almost completely broken down. The larger-sized molecules, on the other hand, were substantially broken down. Another study reported that after 30 days of remediation, bacterial consortiums obtained from China’s Yangtze River Delta broke down 44.5% of total petroleum hydrocarbon (TPH) (Jia et al., 2018). Chlorpyrifos (CPF) breakdown was studied in two different bacterial species Bacteroides megaterium CM-Z19 and Pseudomonas syringae CM-Z6. Five days of incubation at 37°C resulted in a 92.6% and 99.1% degradation of chlorpyrifos-methyl-Z19 and chlorpyrifos-methyl-Z6, respectively, at an initial 100 mg/L concentration (Zhu et al., 2019). Nevertheless, 200 μg per liter of CPF can be degraded by Cupriavidus nantongensis X1T in about 48 h. C. nantongensis X1T species may survive in temperatures between 30°C and 42°C, and pH ranges between 5 and 9. It has a CPF tolerance of 500 mg/L (Shi et al., 2019). Cupriavidus sp. DT-1 has been shown to successfully degrade CPF in liquid media, mineralizing both CPF entirely after 14 h at pH 7°C and 30°C. In the same settings, 90% of chlorpyrifos in soil media degrades after 30 days (Lu et al., 2013). The most efficient microorganisms, P. putida MAS, can break down 90% of CPF in 24 h (Kamika and Momba, 2013). Recent studies indicate that aerobic bacteria, such as Sphingobium sp. strain BHC-A and Schistosoma japonicum UT26, can fully degrade lindane in an aerobic environment (Perera and Hemamali, 2022). Atrazine-contaminated soil from the Vetiver rhizosphere reported two bacterial species: Paenarthrobacter aurescens TC1 and Arthrobacter MCM B-436. Atrazine degradation in P. aurescens TC1 is controlled by trzN, atzC, and atzB genes. Arthrobacter sp CW-1 breaks down dimethyl phthalate (DMP) in anaerobic environments (Jia et al., 2021). Nicotine degradation in Arthrobacter nicotinovorans is mediated by the plasmid pAO1 (Guo et al., 2019). Rhodococcus pyridinivorans SS2 and Rhodococcus ruber SS1 can effectively remediate triCB and dichlorobiphenyl (diCB) (Xiang et al., 2020). Pseudomonas sp. breaks down - Hexabromocyclododecanes (HBCD) at concentrations as low as 50 mg/L in about 5 days, but at 640 mg/L, it takes 8 days to break down (Huang L. et al., 2021; Huang et al., 2022). Bacillus sp. can break down HBCDs at 320 mg/L in about 4 days (Huang et al., 2022). Pseudomonas aeruginosa HS9 can degrade 69% of 1.7 mg/L HBCDs in 14 days. Table 6 provides a detailed overview of diverse bioremediation strategies utilized by microorganisms to effectively eliminate Heavy Metals (HMs) and Persistent Organic Pollutants (POPs) from contaminated environments.

Halophilic bacteria possess extremozymes that can operate effectively in severe conditions, which makes them a promising choice for bioremediation applications. These halophilic bacteria produce an extremozyme with unusual properties, including resistance to heat, acidity, organic solvents, and strong ions. Microprecipitation or proton exchange aids in attaching these bacteria to HMs via an extracellular polymeric material (Kaushik et al., 2021). The negative charge on cell surfaces can be attributed to various functional groups, including sulfate, carboxyl, phosphoryl, and amino functional groups. These functional groups possess negatively charged atoms or groups of atoms, such as oxygen or sulfur, which contribute to the overall negative charge of the cell surface (Dawwam et al., 2023; Syed et al., 2022). The negative charge of these functional groups plays an essential role in the interaction of biomass with metal ions. These groups serve as ion exchange sites and can bind metal ions through a process known as cation exchange. During cation exchange, metal ions, such as hydrogen ions, are exchanged for positively charged ions on the biomass surface (Kaushik et al., 2021). Microorganisms employ various mechanisms to eliminate heavy metals from contaminated soils. These include precipitation, biosorption by sequestering them in intracellular metal-binding proteins (metallothioneins), and converting them into harmless forms through an enzymatic transformation, as presented in Figure 2.

In addition, the anionic functional groups in the cell walls of gram-positive and gram-negative bacteria have an essential function in binding metals. The negatively charged groups allow the bacteria to bind metal ions on the surface or within the cell wall. Binding metals is vital for bacteria’s survival as metal ions are necessary for many cellular processes, such as enzyme activity and energy metabolism (Pachaiappan et al., 2021). It has been reported that Methanothermobacter thermautotrophicus can convert chromium (VI) to chromium (III) and then immobilize chromium (III) as hydroxide or oxide. Bacillus cereus and Shewanella have been shown to decrease Cr (VI) and its immobilization (Chen et al., 2012) as shown in Figure 2. Table 7 enumerates diverse bacterial species employed in soil for the remediation of heavy metals (HMs), showcasing their effectiveness in mitigating environmental contamination.

Fungi are a diverse group of eukaryotic organisms that obtain their nutrients from organic matter in their environment, and they are often referred to as saprophytic organisms. Fungi have been present on Earth for millions of years, and they play essential roles in various ecological processes such as decomposition, nutrient cycling, and symbiosis. They are exceptionally efficient in decomposing PAHs and HMs due to their specificity for excessive refractory chemicals and survival potential in harsh natural habitats, i.e., elevated temperatures and reduced pH (Arwidsson et al., 2010; Bano et al., 2018; Chen et al., 2022; de Moura Dickel et al., 2022). Moreover, the fungus may digest PAHs and HMs “in situ” by producing extracellular enzymes (Liu et al., 2017), which can be done due to the extensively branched mycelia. Fungi have two main approaches for metal detoxification: biosorption (Bano et al., 2018), which includes adhering metals to the membrane, and bioaccumulation, which includes absorbing metals into the cell and metabolizing them (Soleimani et al., 2010). It has been shown that certain fungi, including Gloeophyllum sepiarium, Penicillium chrysogenum, Aspergillus versicolor, Aspergillus terreus, Aspergillus niger, Aspergillus fumigatus, and Rhizopus oryzae can breakdown PAHs and HMs (Hota et al., 2021). Recently, Chen et al. (2022) evaluated the potential of white rot fungus for the remediation of heavy metal contamination. However, heavy metal concentrations, organic pollutants, and unfavorable environmental conditions can slow this remediation process. Table 8 presents a comprehensive overview of various fungi, algae, and plants utilized for remediating heavy metals (HMs) in soil, emphasizing their mechanisms and target HMs. These organisms play crucial roles in bioremediation by absorbing, accumulating, or transforming HMs through mechanisms such as biosorption, bioaccumulation, and biotransformation. Understanding these biological agents and their specific interactions with HMs is essential for developing effective strategies for soil remediation and environmental protection.

The remediation outcomes appear to be highly sensitive to the types of strains, the types of pollutants, and the reaction conditions. Concentrations of heavy metals or organic pollutants that are too high or too low, or reaction conditions that are too slow or too fast, might impede the cleaning process (Kumar et al., 2023; Palanivel et al., 2023; Tessaro et al., 2023). Zhuo and Fan (2021) have reported an in-depth analysis to evaluate the most current developments in using white rot fungus to degrade organic pollutants. In addition, they deduced that most current bioremediation investigations of white rot fungus are undertaken in controlled laboratory settings. Further research must consider the challenges associated with treating pollution in practice. The white-rot fungus most likely uses laccases, lipases, lignin peroxidase (LiP), manganese peroxidase (MnP), and cytochrome P450 versatile peroxidase to breakdown and decrease PAHs and HMs (El-Khoury et al., 2022; Sanchez-Hernandez et al., 2023; Syed et al., 2014). Lasiodiplodia theobromae, isolated from PAH-polluted soil in Beijing, China, removed 53% of benzo[a]pyrene (BaP) within 10 days of incubation (Punetha et al., 2022). Increasing the incubation period to 2 weeks enhanced the biodegradation potential of Peniophora incarnate strain KUC8836, resulting in the removal of 97.9% of pyrene, 95.3% of phenanthrene, and 95% of fluoranthene, attributed to elevated laccase, LiP, and MnP production (Lee et al., 2014). Within 30 days, Rhizoctonia zeae SOL3, Scopulariopsis brevicaulis, and Pleurotus pulmonarius FO43 achieved near-complete decomposition of pyrene at concentrations of 42%, 64%, and 99%, respectively (Bhattacharya et al., 2014; Mao and Guan, 2016). Aspergillus terreus, Trichoderma viride, Trichoderma longibrachiatum, and Aspergillus niger were observed to absorb Pb, Cd, Cr, and Ni at rates of 59.67 mg/g, 16.25 mg/g, 0.55 mg/g, and 0.55 mg/g, respectively (Dell’Anno et al., 2022; Kumar and Dwivedi, 2021). In another study, the highest remediation potentials for Cr (III), Pb (II), Cr (VI), and Cu (II) were found to be 226.6 mg/g, 208.5 mg/g, 207.3 mg/g, and 205.1 mg/g, respectively, when the fungus was immobilized in living form (Hanif et al., 2015). The fungus Aspergillus fumigatus (FS6) and Aspergillus flavus (FS4) eliminated almost 70% of the Cr (VI) from the liquid PDB medium. Cd (II) removal by Aspergillus fumigatus (FS9) was as high as 74% (Talukdar et al., 2020). Sterigmatomyces halophilus, A. restrictus, A. penicillioides, A. gracilis, and A. flavus and all of which are obligate halophilic fungi, showed efficient biosorption for cadmium, copper, ferrous, manganese, zinc, and lead (de Moura Dickel et al., 2022; Hota et al., 2021; Kumar and Singh, 2023; Tessaro et al., 2023). S. halophilus and A. flavus demonstrated the highest adsorption levels, averaging 83%–86%. The fungi Mucor alternans, Phanerochaete chrysosporium, Trichoderma viride, Rhizopus arrhizus FBL 578, Fusarium oxysporum, and Trichoderma hamatum FBL 587 are long-established as DDT degraders (Russo et al., 2019). When endosulfan is exposed to Trichoderma harzianum, it is oxidized to endosulfan sulfate and then degraded naturally (Landa-faz et al., 2021). In PAH-contaminated industrial soil, the fungus Irpex lacteus and Pleurotus ostreatus can break down the PAHs. In 5 days at 26.8°C and pH 6.5, Cladosporium cladosporioides degrades 50 mg/L CPF (Bhattacharya et al., 2014).

Algae offers various advantages as a decontaminating agent, including low cost, easy handling, non-pollution, quick metal contamination removal for recovery, and no additional wastage. Microalgae exhibit the capability of bio-remediating environmental-contaminants (ECs) through three distinct approaches: bio-uptake, bio-adsorption, and bio-degradation (Table 8) (Goswami et al., 2022; Kashem et al., 2023; Tambat et al., 2023). Bio-adsorption is the process by which contaminants are adsorbed onto the surface of the microalgae cells without any cellular uptake or degradation. This process depends on the physicochemical properties of both the contaminant and the microalgae cells, and it can be affected by factors such as pH, temperature, salinity, and ionic strength (Goswami et al., 2022). Bio-adsorption can be an effective method for removing low concentrations of contaminants from the environment, but it is not a long-term solution, as the contaminants can be released back into the environment over time (Dubey et al., 2023). Bio-uptake is the process by which contaminants are taken into the microalgae cells and accumulated within the cells (Dubey et al., 2023). This process can occur through passive diffusion or active transport, depending on the contaminant’s physicochemical properties and the cellular membrane. Bio-uptake can be an effective method for removing moderate to high concentrations of contaminants from the environment, as the contaminants are sequestered within the cells and are not released back into the environment. Bio-degradation is the process of metabolizing contaminants and breaking them down by the microalgae cells into less toxic or non-toxic compounds (Cameron et al., 2018; Goswami et al., 2022). This process depends on the microalgae cells’ metabolic pathways and the contaminants’ nature. Bio-degradation can effectively remove complex or persistent contaminants from the environment, but it requires specific conditions and nutrients to support the growth and metabolism of the microalgae cells. Bio-adsorption occurs when environmental contaminants (ECs) link to organic substances released by cells or components of the cell wall (Das et al., 2022; Satya et al., 2023). Alternatively, bio-uptake occurs when pollutants bind to intracellular proteins and other substances and involve the subsequent intracellular transit through active transport, assisted diffusion, or simple diffusion. Microalgae use a catalytic metabolic process to break down the chemicals into their parts to biodegrade ECs. Bio-degradation is an essential approach for cleaning up hazardous toxins, which works more like a bioreactor than a biofilter by breaking down the contamination into less dangerous chemicals (Sher and Rehman, 2019; Leon-Vaz et al., 2021; Chebotaryova et al., 2023). It could take place inside cells, outside cells, or in a hybrid form. Spirulina, Scenedesmus, Phormidium, Oscillatoria, Nodularia, Desmodesmus, Cyanothece, Chlorella, Botryococcus, and Arthrospira are a few of the microalgal genera used in bioremediation (Dwivedi, 2012; Dubey et al., 2023). Chlorella vulgaris is effective at degrading acenaphthene and fluoranthene, according to research by Touliabah et al. (2022). The microbes Lyngbya digueti, Phormidium mucicola, Oscillatoria princeps, Anabaena variabilis, and Westiellopsis prolific were helpful in the reduction of quantity of various petroleum hydrocarbons in oil refinery effluent, which ranged from 24% to 92% reduction (Takáčová et al., 2014). It was reported that Chlorella kessleri could degrade 3,4-benzpyrene (29%) when exposed to light at a strength of 13.5 W per square meter. Similarly, Chlamydomonas reinhardtii was reported to decompose benz(a)anthracene at a rate of 10 mg/L in 11 days (Luo et al., 2020; Luo et al., 2020). The breakdown of homogentisate resulted in a rise of gene expression that encodes for ubiquinol oxidase, carboxy-methylene-butenolide, carboxylase/oxygenase (Rubisco), ribulose 1,5-bisphosphate, and homogentisate 1,2-dioxygenase (HGD) enzymes. Chlorella vulgaris biomass is an effective biosorbent for the removal of copper (Cu²⁺), Cadmium (Cd²⁺), and lead (Pb²⁺) from a mixed solution containing 50 mg dm³ of each metal ion (Goher et al., 2016). After being treated with Spirulina sp, Ca²⁺ was reduced by 98% and Cu²⁺ by 91% in municipal wastewater. Another study (Yang et al., 2015) found that Chlorella minutissima could remove 84% of Cu²⁺, 84% of Mn²⁺, 74% of Cd²⁺, and 62% of Zn²⁺ from municipal garbage. Microalgal biochar has been shown to remove Cr (VI) from water with 100 percent efficiency by Daneshvar et al. (2019), while Cheng et al. (2017) have studied the biosorption and kinetics of Cd (II) removal using both live and dead C. vulgaris. The research findings indicate that both viable and decaying cells of C. vulgaris exhibit a notable ability for adsorbing Cd, demonstrating efficiencies of 95.2% and 96.8% respectively.

Plants are used in phytoremediation, a form of bioremediation, to clean up polluted environments such as soil, water, and air. Methods include phytotransformation, phytostabilization, rhizofiltration, phytostimulation, rhizodegradation, phytodegradation, phytovolatilization, and phytoextraction are all part of the broader field of phytoremediation (Nedjimi, 2021; Shabaan et al., 2021; Oladoye et al., 2022) as shown in Figure 3 and Table 8.

Many plant species can absorb, bioaccumulate, immobilize, and degrade environmental pollutants. Some plants that can be utilized for HMs and POPs phytoremediation of soil include Cucurbita pepo, Zea mays, Nicotiana tabacum, Medicago sativa, Alyssum murale, Achillea millefolium, Aeolanthus biformifolius, Arabis gemmifera, Phytolacca americana, and Pteris vittata (Kurniawan et al., 2022; Li et al., 2023; Rahman and Singh, 2020; Tauqeer et al., 2016). In order to effectively remove pollutants from water, certain plant species are utilized, i.e., Eichhornia crassipes, Ipomoea aquatica, Phragmites australis, Potamogeton natans, Ruppia maritima, Vallisneria americana, Hygrophila corymbosa, Nuphar lutea, Salvinia minima, Pistia stratiotes, and Lemna minor (Wei et al., 2021). The impact of certain bacterial species, which are associated with the growth of plants underground, can enhance plant development, promote metal translocation within the plant, alter the bioavailability of metals in the soil, and reduce metal phytotoxicity. This leads to an increase in the effectiveness of phytoremediation. Evidence from a few research shows that bacterial inoculations considerably alter the expression pattern of various metal transporters, including the ZIP, NRAMP, HMA, F-box, and AtALS3 gene families, employing these shared and unique growth-promoting functions (Dash and Osborne, 2023; Dash et al., 2018; Manobala et al., 2021; Dash and Osborne, 2023).

In Arabidopsis tissues, Bacillus amyloliquefaciens alters the transcriptional activity of the IRT1, FRO2, and FIT1 genes, increasing Fe and Cd accumulation (Sukweenadhi et al., 2015). It has been shown that the endophytic Pseudomonas fluorescens Sasm05 strain significantly increases Cd accumulation and tissue growth after inoculation, a process that mimics the overexpression of the SaHMAs, SaNRAMPs, and SaZIPs gene families (Chen et al., 2017). Much research has been conducted on the ZIP transporter gene family, which regulates zinc transportation through membranes and cytoplasmic concentrations in plant cells. Enterobacter cloacae–Zn solubilizing bacterial inoculation in rice plants had altered OsZIP1, OsZIP4, and OsZIP5 gene expression, resulting in enhanced Zn accumulation in plant tissues (Krithika and Balachandar, 2016). In Arabidopsis thaliana (Sukweenadhi et al., 2015), inoculations with Plant Growth-Promoting Rhizobacteria (PGRP) under Aluminum (Al) stress offer a promising strategy to alleviate heavy metal toxicity and enhance plant development. This is achieved by modulating the expression of key genes, including AtAIP, AtALMt1, and AtALS3. Notably, the activation of AtALS3 gene in response to Al stress results in synthesizing an ABC transporter-like protein within phloem cell membranes. This protein aids in effectively relocating Aluminum away from vulnerable areas, thus protecting the plant. While the specific role of the AtALP gene in Aluminum tolerance remains uncertain, it likely contributes to the overall adaptive response. Additionally, the collaboration between the HMA gene family and AtALS3 plays a vital role in facilitating the translocation of Heavy Metals (HMs) from the plant’s roots to the shoots, primarily accomplished through the xylem. This mechanism assists in regulating the distribution of HMs within the plant, ultimately supporting its resilience to metal-induced stress.

Unfortunately, plant cells do not include any natural transporters specific to organic environmental pollutants. Hence, they move around without actively doing anything. Root microbiome components such as rhizosphere bacteria and endophytes have long been appreciated for their role in the phytoremediation of PAHs. Diesel-contaminated soil remediation using petroleum hydrocarbons was less harmful due to the presence of endophytic microbial species such as Stenotrophomonas spp., Pseudomonas spp., Pantoea spp., and Flavobacterium spp (Agarwal et al., 2019; Pinel-Cabello et al., 2023). To remove organic pollutants from the environment, rhizobial symbiotic consortiums use organic molecules as a C and N source in phytoremediation. Rhizobium strains that nodulate the hyperaccumulator plant Leucaena have been shown to aid in rhizoremediation by using plant toxins (such as the aromatic chemical Mimosine) as C and N sources (Sytar et al., 2021). The bacterial species help plants detoxify HMs and PAHs by increasing their metabolic growth rate. Plant growth and metabolic gene regulation by PGPR inoculations facilitate the systematic development of plant physiology (specifically, “biomass, bushiness, lateral root production, lateral root number, surface area, and thickness”). Inoculating rice seedlings with Bacillus altitudinis, for instance, improves root architecture by regulating auxin metabolism and modulating the expression of OsIAA1, OsIAA4, OsIAA11, and OsIAA13 (Ambreetha et al., 2018). The antioxidant defenses of the host plant are also strengthened by PGPR inoculations, making them more effective in combating stress.

The improvement of Solanum tuberosum Zn tolerance by Bacillus isolates is achieved by adjusting the expression of SOD, GR, DHAR, CAT, and APX genes (Gururani et al., 2013). Moreover, the growth-promoting qualities of ACC-deaminase are critical in aiding hosts to resist the toxicity of petroleum hydrocarbons, which is just one of the many ways endophytic bacteria can help. The use of arbuscular mycorrhizal fungus (AMF) and endophytic fungi to bio-enhance plant growth is a current focus in phytoremediation (Kumar and Saxena, 2019; Ordookhani et al., 2010). The underground network of mycelium belonging to AMF aids in phytoremediation by expanding the rhizosphere, enabling plants to access contaminants and nutrients. This is facilitated by a symbiotic relationship between AMF species, such as Rhizophagus irregularis, Glomus versiforme, and Funneliformis mosseae, which can increase the GRSP (Glomalin related soil protein) in soil (González-Chávez et al., 2004). As a direct result of this, the levels of lead and cadmium in maize decrease while the pH of the soil rises.

To further reduce heavy metal toxicity on host plants, AMF secrete extracellular polymeric substances (EPS) from their fungal surface through surface precipitation, ion exchange, and chelation (More et al., 2014; Riaz et al., 2021). EPS can absorb minerals and elements that are smaller than plant roots. Recent research suggests that phosphate groups with negative charges can cause Cr (III) to precipitate on the surface of fungi (Wu et al., 2021). The importance of glomalin and organic acid excretion by fungi and plants cannot be overstated when it comes to immobilizing 85% of heavy metals (HMs) in soil. By manipulating endophytic fungi, it is possible to minimize metal toxicity to plants, and some of these fungi can even flourish in environments rich in metals. Endophytic fungi possess a range of tolerance mechanisms contributing to their effectiveness in phytoremediation. These mechanisms include extracellular metal sequestration and precipitation, internal metal sequestration and complexation, compartmentation, volatilization, and metal binding to fungal cell walls. Such diverse strategies bolster the potential of phytoremediation efforts (Aly et al., 2011; Sharma and Kumar, 2021). The Festuca pratensis and Festuca arundinacea, infested with endophytic fungi, grew more biomass in their roots and shoots while significantly degrading the petroleum hydrocarbons in the soil despite growing in ancient petroleum-contaminated soil (Soleimani et al., 2010). The gibberellin-producing endophyte Penicillium janthinellum LK5 protects host plants from Cd-induced oxidative stress and membrane damage by decreasing lipid peroxidation and electrolytes and increasing reduced glutathione content and catalase activity (Khan et al., 2014). Canola biomass and Cd extraction efficiency were both increased when the endophytic fungus Lasiodiplodia sp. MXSF31 was introduced to Portulaca oleracea stems grown in Pb and Cd-contaminated soils (Zanganeh et al., 2022).

Heavy metals (HMs) and nanoparticles (NPs) coexist in agricultural settings can have devastating consequences for the soil, crop yields, and microbiota. However, the behavior of NPs can be influenced by several biological and environmental factors due to their distinct physicochemical properties. Therefore, various biotic variables may affect the interaction between HMs and NPs (da Silva et al., 2023; Sabourian et al., 2020). Moreover, the uptake, transport, and accumulation of NPs in different plant organs are also influenced by biotic factors. When present in polluted areas, heavy metals interact with NPs through physical adsorption, chemical interactions, and electrostatic binding (Noman et al., 2020). Such interactions can significantly impact the environment, accumulating these harmful substances in the soil and the food chain. Therefore, it is essential to understand the complex interactions between HMs and NPs in agricultural settings to minimize their adverse effects on the environment and public health. Examples include the adsorption of Cd from the soil by FeO NPs, which were able to do so because of their unique qualities, including reactivity, electrostatic attraction, a wide surface area, and the ability to cap molecules (Manzoor et al., 2021). In this situation, Ca and Fe transporters bring Cd into plant cells. This results in a lower metal concentration within the plant tissues as Cd and FeO NPs compete to enter the plant systems via the same transporter channel (Ahmed et al., 2021).

Similarly, Noman et al. (2020) found that Cu-NPs reduced Cd translocation from soil to aerial parts of wheat because of their wide surface area, reactivity, and electrostatic attraction. Hence, the biogenic CuNPs’ capping molecules boosted soil Cd immobilization. As a result, the wheat’s development was aided by the plant’s ability to absorb Cu-bound nutrients. It functions as a coenzyme in essential reactions and stimulates plant growth and development in polluted soil. As another example, graphene oxide (GO), which has a similarly huge surface area, has been utilized to clean up HMs contaminated areas (Etemadi et al., 2017). For instance, graphene oxide sheets, which, due to their functional group, may conjugate with metals like Cr (VI), speed up the adsorption kinetics of HMs ions (Wang et al., 2017). In conclusion, NPs’ metal complexing abilities are anticipated to aid in elucidating how the NPs may effectively reduce metal toxicity.

Across the globe, researchers have employed multiple approaches, such as physical, chemical, and biological techniques - including phyto and microbial remediation - to decontaminate soil polluted with heavy metals (Yadav et al., 2017; Singh et al., 2020; Sunanda et al., 2022). This is done to ensure that the soil becomes suitable for farming, considering the risks heavy metals pose to diverse life forms. Nevertheless, most of these techniques have only been tested in the lab at bench scales, and those tested in real-world settings have met with scant success for various reasons. Physicochemical methods include excavation and landfill (Funtikova et al., 2023), chemical reduction, evaporation acid leaching, soil washing, soil flushing, precipitation, electrokinetic extraction, vitrification, thermal treatment, and surface capping pose significant issues as mentioned in Table 9 (Rahman and Singh, 2020). The adverse impacts on soil, microbiota, and plant ecosystems are a direct result of the production of secondary metabolites, which can be costly and difficult to eradicate (Gaur et al., 2014; Wang P. et al., 2018). Table 10 outlines the utilization of nanoparticles in the remediation of heavy metals (HMs) and persistent organic pollutants (POPs), showcasing their efficacy in tackling environmental contaminants through innovative nanotechnology-based solutions.

For instance, According to Lambert et al. (2000), FRTR (Federal Remediation-Technologies Roundtable) statistics suggest that excavation and disposal costs $270 to $460 per tonne (Feng et al., 2020; Hu et al., 2021). The United States Environmental Protection Agency (USEPA) estimates that, depending on the size of the polluted site, the cost of soil cleaning ranges from $150 per tonne up to $250 per tonne.

Selection and placement of plants, irrigation, soil amendment, field monitoring, harvesting, and residue management all add to the price tag of a treatment that relies on phytoextraction. The cost of remediation could range from $10 to $35 per ton of soil with low levels of toxins, which also depends on the contamination level and size of the site (Fulekar et al., 2012). Due to the plant-based nature of phytoremediation, the soil treatment process, which typically takes 3 months to 5 years, becomes more expensive and time-consuming (Gavrilescu, 2022; Oladoye et al., 2022). However, the clean-up has failed under natural field conditions due to reliance on specific pollutant characteristics, soil qualities, low efficiency, changing environments, and site conditions. Metal clean-up programs rely heavily on nanotechnology because of the unique physicochemical properties of nanosized particles (NPs) ranging in size from 1 to 100 nm. Furthermore, the nano remediation procedure has successfully removed heavy metals from soil ecosystems and other habitats by exploiting NPs’ potential mobility, reactivity (catalysis), and adsorption properties (Corsi et al., 2018; Baragaño et al., 2020; Del Prado-Audelo et al., 2021). Nanoremediation technology is one of the most promising remediation alternatives, and it removes toxic metals through a variety of mechanisms, including

(i) absorption,

Various materials such as polymers, carbon-based compounds, metallic oxides, metals, and nanocomposites have remarkably removed metals (Baragaño et al., 2020). However, the type of metal and pollution source (e.g., biogenic) can affect the efficacy of these materials. Such materials include carbon nanoparticles (fullerenes), semiconductors, noble metals, and magnetic nanoparticles (such as zinc oxide and titanium dioxide). One specific example is Spirulina platensis supported PdNP, which removed between 12%–90% of Pd from polluted environments (Sayadi et al., 2018). On the other hand, an iron oxide nanoparticle-based on Geobacter sulfurreducens could remove chromium from chromium-polluted soils altogether (O’Neil et al., 2008). Overall, these findings highlight the potential of using different materials and approaches for effective metal removal, which could be tailored based on the type of pollutant and the specific environmental conditions. Nanoscale metal oxide particles (MONPs) made of iron, silver, nickel, and palladium have been remarkably successful in removing toxic metals and other chemicals from polluted areas. Detoxifying Cd stress in wheat plants using FeO NPs increased plant growth, antioxidants, and chlorophyll levels (Manzoor et al., 2021). Cd-phosphate production appears to be the leading cause for decreased bioaccumulation of Cd in soil, and treatment of the soil with Fe₃ (PO₄)₂ NPs successfully immobilized the Cd by 70% (Gong et al., 2018).

Similarly, another study found that applying biochar-supported FeNPs reduced plant Cr bioavailability (Neeli et al., 2020). Wheat’s development and nutrient profile were found to be improved when Cu NPs were present, and vice versa (Noman et al., 2020). Notwithstanding the progress, new NPs that are effective in nano remediation technologies must be discovered. However, this requires researchers to collaborate with local governments, which can back innovations and fund research to identify sustainable nano-solutions for contaminated soils. Compared to traditional clean-up methods, nano remediation technology is typically swift, may be deployed over a broad contamination region, and costs less. According to the USEPA, about 70 potentially harmful trace elements have been effectively cleaned worldwide using nano-remediation techniques, considerably reducing time and operational costs (Feng et al., 2023; Tufail et al., 2022; Yang et al., 2023).

Microbes have long been used in various settings, including the medical, agricultural, and environmental sectors. However, as explained below, the application of microbiome to further optimize nanoparticle usage in the nano-bioremediation process has also shown promising results in detoxifying various inorganic contaminants, hence reducing the limiting potential of bioremediation (Bhatt et al., 2022). Microbiome-based nano-bioremediation has demonstrated substantial progress in detoxifying carcinogenic and mutagenic chromium by employing palladium nanoparticles (Alexakis, 2016). These nanoparticles are synthesized using Pd (II) ions, with the mediation of Clostridium pasteurianum. The process involves the conversion of hexavalent chromium into an insoluble trivalent form, resulting in hydrogen gas production.

Similarly, a matrix composed of carbon nanotubes (CNTs), sodium alginate, and polyvinyl alcohol (PVA) immobilized on P. aeruginosa has been shown to detoxify Cr (VI) selectively (Pang et al., 2011). At 80 mg/L Cr (VI), the immobilized bacterial cells converted 84% of the compound to the soluble Cr (III), and this process was completed within 24 h. Biotransformation of poisonous Cr (VI) into less harmful Cr (III) has been demonstrated by immobilised cells of Shewanella oneidensis stabilised with CNTs (Yan et al., 2013). Immobilized S. oneidensis and carbon nanotubes were four times more effective at removing hexavalent chromium from a solution than the test bacteria or calcium alginate beads alone. Based on these results, it is plausible that nano-bioremediation methods targeting habitats contaminated with inorganic pollutants could benefit from incorporating CNTs with bacteria. Magnetic iron oxide nanoparticles, known as MIONPs, have become popular for removing metals due to their extensive surface area, strong reactivity, adjustable features, distinctive magnetic qualities, potent reducing ability and capacity to soak up various dangerous metals and metalloids (Kumar et al., 2019; Verma et al., 2023). For instance, the Lysinibacillus sphaericus prepared magnetic oxide nanoparticles have been shown to release exopolysaccharides (EPS) that act as a complexing, stabilizing, and capping agent and have several binding sites for different metal ions. The EPS-functionalized magnetic oxide nanoparticles (VI) improve the ability to absorb Cr (Kumar et al., 2019).

Similarly, adding iron nanoparticles, produced by Chlorococcum sp. green algae, led to a 92% reduction of Cr (VI) to Cr (III). These nanoparticles were highly reactive, stable, and had a practical ability to reduce (Subramaniyam et al., 2015). Further strengthening algae’s role in detoxifying carcinogenic chromium is the incorporation of C. vulgaris as a functionalized agent in ultrafine bi-metallic (TiO₂/Ag) chitosan nanofiber mats. According to this study, combining C. vulgaris algae with TiO₂/Ag chitosan nanofiber mats significantly boosted the photocatalytic reduction of hexavalent chromium. The researchers observed that various organic compounds secreted by the algae played a crucial role in enhancing the process. As a result, the study implies that the synergistic effect between the algae and TiO₂/Ag hybrid nanomaterial could offer a cost-effective solution for removing chromium from polluted environments (Goher et al., 2016; Awasthi et al., 2018). Rhodosporidium diobovatum was responsible for generating lead sulfide (PbS) nanoparticles, which effectively converted toxic Pb (II) ions into less harmful and advantageous compounds (Seshadri et al., 2011). Combining B. subtilis and nanohydroxyapatite and the production of CdS nanoparticles from P. aeruginosa (NHAP) successfully eliminated Cd from a Cd-contaminated environment. Implementing this remediation approach stimulated the rhizosphere community, leading to a notable rise in bacterial diversity in rapeseed (Brassica campestris L.) cultivated in previously contaminated soil (Liu et al., 2018).

Economic and technological variables must be considered when selecting a treatment technique for POP removal, as they significantly impact POP destiny, transport, and degradation (Zhou et al., 2023). With the rise of nanotechnology, a powerful tool is now available to tackle environmental problems, specifically in purifying polluted treatment solutions. Nanoremediation is a state-of-the-art method that may safely and effectively remove organic contaminants from the environment. Nanomaterials are highly beneficial in many fields due to their extraordinary electromagnetic, structural, mechanical, thermal, and optical capabilities, i.e., in wastewater treatment (Del Prado-Audelo et al., 2021). Various forms of nanomaterials can be created through different methods, including physical, chemical, or biological processes. Many researchers also utilize green chemistry principles to ensure environmentally friendly synthesis. Cutting-edge multifunctional nanomaterials such as nanowires, nanoflowers, and nanocomposites are designed to optimize performance and address existing obstacles (Corsi et al., 2018). Higher specific surface area (SVR) of nanomaterials enhances their reactivity with POPs. In the upcoming sections, we will delve into nanocatalysis, nano adsorbents, and nanomembranes in POP treatment.