Nanoparticles, owing to their small size and large surface area, exhibit enhanced reactivity and adsorption capacities, making them excellent candidates for heavy metal removal (Sarma et al., 2019; Yu et al., 2021). Several studies have demonstrated that nanomaterials also possess significant redox and catalytic properties. The high surface area provides numerous active sites for interaction with heavy metal ions, facilitating efficient adsorption and transformation processes. Additionally, the tunable surface chemistry of nanomaterials allows for the modification of their properties to target specific contaminants (Garcia-Segura et al., 2020). This adaptability enhances their versatility in various environmental conditions. The potential for functionalization with specific ligands or coatings further improves their selectivity and effectiveness in complex environmental matrices. Nanoparticles are increasingly explored for environmental cleanup and sustainable water treatment technologies. Based on their magnetic properties, nanoparticles can be categorized into magnetic and non-magnetic nanoparticles.

Nanocomposites, composed of two or more distinct components at the nanoscale, offer synergistic properties for enhanced heavy metal removal. Compared to single-component systems, two distinct components at the nanoscale level offer synergistic properties that significantly enhance heavy metal removal (Singh and Bhateria, 2021). By incorporating the unique properties of each constituent nanomaterials, superior stability, adsorption capacities (Baig et al., 2021), and improved selectivity towards specific contaminants can be reached. This combination of properties allows for more efficient and effective remediation processes compared to single-component systems. Moreover, the possibility of tunability in the design of nanocomposites can be optimally repurposed for various environmental conditions. (Darwish et al., 2022; Hassan et al., 2021). This tunability is a versatile tool in the fight against heavy metal pollution. As research in this field progresses, the development of novel nanocomposite materials will provide highly extensive sustainable, and high-performance water treatment technologies. In this study, we will delve a little into the recent study of nanocomposites and their applications to remediate heavy metals from the environment. Some such nanocomposites include graphene nanocomposites, metal-organic frameworks (MOFs), and carbon nanotube composites.

Nanostructured materials, including nanofibers, nanosponges, and nanocages, offer unique architectures for efficient heavy metal removal. These structures facilitate greater interaction with molecules of the pollutants. Their porous and adaptable designs allow for the incorporation of functional groups that improve selectivity and adsorption capacity. Additionally, the adaptability of these nanostructures enables their application in various environmental conditions, making them highly effective for diverse heavy metal remediation efforts. Some of these nanostructured materials are classified as nanofibers, nano-sponges, and nanocages.

Adsorption involves the attachment of heavy metal ions onto the surface of solid adsorbents, forming a monolayer or multilayer adsorption layer (Khulbe and Matsuura, 2018). The process relies on the affinity between the surface functional groups of the adsorbent and the heavy metal ions in the solution. Recent studies have focused on enhancing the adsorption capacity and selectivity of adsorbents through the use of novel materials and surface modifications. Nanomaterials, such as graphene oxide, carbon nanotubes, and metal-organic frameworks (MOFs), exhibit high surface area and tunable surface chemistry, making them effective adsorbents for heavy metal removal (Ahsan et al., 2020; Das et al., 2021). Surface functionalization with specific functional groups, such as carboxyl, amino, or thiol groups, enhances the adsorption affinity towards target heavy metal ions (Ahmad et al., 2020; Zeng et al., 2022). Additionally, hybrid materials combining different types of nanomaterials or incorporating metal nanoparticles demonstrate synergistic effects, leading to improved adsorption capacities and kinetics. Advancements in adsorption mechanisms also include the development of continuous-flow adsorption systems such as fixed-bed columns and membrane adsorbers, for practical applications (Aydına et al., 2021; Taka et al., 2021). These systems enable efficient removal of heavy metals from large volumes of water and offer advantages in terms of scalability and cost-effectiveness.

Ion exchange involves the replacement of ions in solution with ions attached to a solid phase, typically an ion exchange resin or zeolite (Rathi et al., 2021; Ray, 2023). The process relies on the affinity between the ions in the solution and the functional groups on the surface of the ion exchange material. Recent trends in ion exchange mechanisms focus on improving the selectivity and regeneration capabilities of ion exchange materials. Functionalization of ion exchange resins with specific ligands or chelating agents enables selective removal of target heavy metal ions based on their charge and coordination chemistry (Quintas et al., 2021). Additionally, the regeneration of spent ion exchange materials through elution or desorption processes enables their reuse and reduces operational costs. Integration of ion exchange processes with other treatment methods, such as membrane filtration and adsorption (Peng and Guo, 2020; Vieira et al., 2020), enhances treatment efficiency, and offers synergistic effects. Moreover, advancements in process engineering, including continuous-flow ion exchange reactors and in-line monitoring techniques, enable precise control over ion exchange reactions and facilitate scale-up for industrial applications.

Precipitation involves the chemical reaction between heavy metal ions and precipitating agents to form insoluble precipitates, which can then be separated from the solution (Pillai and Thombre, 2024; Zhang et al., 2023). The process relies on the solubility product of the metal precipitate and the pH and temperature conditions of the solution. Recent trends in precipitation mechanisms focus on enhancing the efficiency and selectivity of precipitation reactions while minimizing the generation of secondary wastes. Advanced precipitation methods, such as co-precipitation and hydrothermal synthesis (Gholamrezaei et al., 2019), enable the synthesis of highly crystalline and uniform precipitates with controlled morphologies and compositions. The use of novel precipitating agents, including natural polymers, biodegradable chelating agents, and green chemistry reagents, reduces the environmental impact of precipitation processes and enhances the sustainability of heavy metal removal systems. Moreover, recent advancements in process engineering, such as continuous-flow precipitation reactors and in-line monitoring techniques, enable precise control over precipitation reactions and facilitate scale-up for industrial applications.

Redox reactions involve the transfer of electrons between chemical species, leading to the transformation of heavy metal ions into less toxic or insoluble forms (Xu et al., 2022). The process relies on the redox potential of the metal ions and the reducing or oxidizing agents present in the solution (Gulcin and Alwasel, 2022). Recent trends in redox reactions for heavy metal removal focus on developing sustainable and energy-efficient processes with minimal environmental impact. Electrochemical methods, such as electrocoagulation, electrooxidation, and electrochemical reduction, offer selective and precise control over redox reactions, enabling the removal of specific heavy metal contaminants from complex water matrices (Feng et al., 2023; Martínez-Huitle et al., 2018; Yasri and Gunasekaran, 2017). Advanced electrode materials, including carbon-based electrodes, metal oxide nanoparticles, and conductive polymers, exhibit enhanced electrochemical activity and stability, leading to improved treatment efficiency and durability of electrochemical reactors. Integration of renewable energy sources, such as solar and wind power, into electrochemical systems reduces the carbon footprint and energy consumption of heavy metal removal processes (Ganiyu and Martinez-Huitle, 2020; Klemeš et al., 2019; Zahmatkesh et al., 2022).

Unlike traditional adsorption techniques, which are limited by saturation and slow kinetic rates, nanomaterial-driven cascade systems propose a new mechanism where adsorption is not an endpoint but a trigger for secondary electrochemical reactions (Y. Zhang et al., 2023). For instance, zero-valent iron nanoparticles (ZVINPs) serve as a starting point by reducing heavy metal ions, such as Pb²⁺ and As³⁺, and simultaneously generating electrons that catalyze subsequent redox reactions in an electrochemical process (Silva-Calpa et al., 2020; Tarekegn et al., 2021; Yang et al., 2021). This cascading system ensures that once the adsorptive sites of the nanomaterials are saturated, electrochemical reduction and oxidation cycles are initiated, allowing continuous heavy metal transformation and immobilization, thus enhancing overall efficiency.

The concept of coupling photocatalytic nanomaterials, such as TiO₂ nanoparticles, with electromagnetic fields opens up new avenues for intensifying contaminant degradation. In this hybrid approach, UV-activated TiO₂ nanoparticles generate reactive oxygen species (ROS) capable of breaking down organic pollutants (Park et al., 2021; Sun et al., 2020). However, instead of limiting the process to the photocatalytic activity alone, the addition of a low-frequency electromagnetic field enhances nanoparticle dispersion and accelerates the electron transfer processes involved in pollutant degradation (Cai et al., 2019; Rawat et al., 2021). This strategy increases the reaction rate and extends the lifetime of ROS, effectively expanding the scope of contaminants that can be addressed, including both heavy metals and organic pollutants.

Another concept emerging in environmental remediation is nanomaterial-enhanced phytomining, where engineered nanomaterials are combined with hyperaccumulating plants to extract valuable metals from contaminated environments (Li et al., 2020; Tognacchini et al., 2019). Traditional phytoremediation is often hindered by the slow uptake of metals and limited bioavailability (Khalid et al., 2021). By introducing metal-seeking nanoparticles, such as functionalized carbon nanotubes or metal-organic frameworks (MOFs), to the root zone of these plants, the uptake of metals such as nickel, cadmium, and zinc is greatly amplified (Doria-Manzur et al., 2022; Rossi et al., 2019). These nanoparticles enhance the bioavailability of metals by chelating them into soluble forms that are more easily transported through the plant’s vascular system, ultimately resulting in higher metal yields. This synergistic approach not only cleanses contaminated soils but also offers a novel method for recovering valuable metals, providing a sustainable alternative to traditional mining processes.

A pioneering concept is the design of dynamic self-regenerating systems, in which nanomaterials and microbial consortia work together to sustain long-term contaminant degradation without requiring external inputs (Fu et al., 2021; Xia et al., 2023). For example, magnetic nanoparticles (MNPs) can be combined with microbial biofilms capable of biodegrading organic pollutants (Liu et al., 2022; Zhu et al., 2020). The MNPs serve two primary functions: first, they adsorb heavy metals, preventing them from inhibiting microbial activity; second, they act as catalysts that regenerate the microbial degradation capacity by facilitating electron transfer between the bacteria and the contaminants. This interaction sets up a dynamic feedback loop where contaminants are continuously broken down and the system regenerates itself, maintaining efficiency over extended operational periods.

Thermal remediation methods, such as soil heating, have traditionally been used for organic pollutant removal but are less effective for heavy metal immobilization. A novel approach involves using thermally activated nanoparticles, such as aluminum oxide (Al₂O₃) or zirconium oxide (ZrO₂), which, when heated, change their surface chemistry to selectively bind and sequester heavy metals like lead and mercury (Allwin Mabes Raj et al., 2022; Fan et al., 2020; Guo et al., 2021; Huang et al., 2021). This nanoparticle-enhanced thermal remediation technique leverages the temperature-induced phase transformation in the nanoparticles, enhancing their adsorptive capacities and providing an innovative method for metal recovery during remediation processes (Al-Najar et al., 2021; Williams and Peterson, 2021).

Another concept is the nanomaterial-enabled electrohydrodynamic remediation, which combines electric fields with fluid dynamics to direct the flow of contaminants toward reactive nanomaterials embedded in porous media (Ji et al., 2023; Sprocati and Rolle, 2019). In this system, nanomaterials like functionalized graphene oxide are integrated within a hydrodynamic framework that is manipulated through external electric fields. This enables selective capture of contaminants by steering them towards nanomaterials with high affinity for specific heavy metals, ensuring high capture rates and fast remediation times (Ahmad et al., 2020; Nisola et al., 2020). Additionally, the fluid flow generated by electrohydrodynamics can be fine-tuned to optimize the contact time between the contaminants and the nanomaterials, leading to a highly efficient and targeted removal process.

Surface functionalization plays a pivotal role in tailoring the selectivity of nanomaterials by modifying their surface chemistry to preferentially adsorb specific heavy metal ions (Sarma et al., 2019). Recent research has demonstrated the efficacy of surface functionalization strategies for enhancing the selectivity of nanomaterials in heavy metal removal applications (Jawed et al., 2020). Functional groups such as carboxyl (-COOH), amino (-NH2), thiol (-SH), and hydroxyl (-OH) are commonly introduced onto the surface of nanomaterials to impart specific binding affinity towards target heavy metal ions (Rai et al., 2022). For example, graphene oxide (GO) and carbon nanotubes (CNTs) functionalized with carboxyl groups exhibit enhanced selectivity for heavy metal ions such as lead (Pb), cadmium (Cd), and mercury (Hg) due to the formation of strong metal-carboxylate complexes (Mohan et al., 2023). Similarly, metal-organic frameworks (MOFs) can be functionalized with tailored ligands to selectively adsorb specific heavy metal ions based on their size, charge, and coordination chemistry (Wen et al., 2018).

Furthermore, the development of multifunctional nanomaterials through the simultaneous incorporation of multiple functional groups enables synergistic effects and enhanced selectivity toward target heavy metal contaminants. For instance, hybrid nanocomposites composed of graphene oxide and magnetic nanoparticles functionalized with both carboxyl and amino groups demonstrate superior selectivity and adsorption capacity for heavy metal ions compared to individual components (Natarajan et al., 2023). Advancements in surface functionalization techniques, including covalent bonding, electrostatic interactions, and chemical modification, enable precise control over the surface chemistry of nanomaterials, thereby facilitating the design of highly selective adsorbents for heavy metal removal (Liu et al., 2019).

Ligand design represents a versatile approach for enhancing the selectivity of nanomaterials by incorporating specific chelating agents or receptors that exhibit high affinity towards target heavy metal ions (Dzhardimalieva and Uflyand, 2018). Recent studies have focused on the rational design and synthesis of ligands with tailored properties to selectively form complexes with specific heavy metal contaminants. Ligands such as crown ethers, cyclodextrins, and calixarenes offer unique binding cavities and coordination sites for selective recognition and capture of heavy metal ions (Liang et al., 2022). Functionalization of nanomaterials with ligands featuring complementary functional groups enables the formation of stable metal-ligand complexes, thereby enhancing the selectivity and affinity of nanomaterials for target heavy metal ions (de Oliveira et al., 2021).

Moreover, the integration of molecular modeling techniques, such as density functional theory (DFT) calculations and molecular dynamics simulations, aids in the rational design and optimization of ligands for selective heavy metal removal (Ebenezer and Solomon, 2024). By elucidating the underlying interactions between ligands and heavy metal ions at the molecular level, researchers can tailor the structure and properties of ligands to maximize selectivity and binding affinity (Li et al., 2023a). Furthermore, the development of stimuli-responsive ligands that undergo conformational changes or structural transformations in response to specific environmental cues offers dynamic control over the selectivity of nanomaterials (Grzelczak et al., 2019). Stimuli-responsive ligands can selectively bind heavy metal ions under certain conditions, such as pH, temperature, or redox potential, enabling on-demand capture and release of target contaminants (Mamidi et al., 2021; Sharma et al., 2024).

Molecular imprinting is an emerging technique for imprinting specific binding sites or cavities within the structure of nanomaterials to selectively recognize and capture target heavy metal ions (Tchekwagep et al., 2022). Recent advancements in molecular imprinting technology have led to the development of highly selective nanomaterials for heavy metal removal (Ali et al., 2024). Molecularly imprinted polymers (MIPs) represent a promising class of materials that mimic the molecular recognition properties of natural receptors or antibodies (Parisi et al., 2022). By imprinting the desired heavy metal ions within a polymer matrix through template-assisted polymerization or sol-gel methods, MIPs can selectively bind and remove target contaminants from aqueous solutions (ul Gani Mir et al., 2022). The precise control over the size, shape, and functionality of imprinted cavities enables high selectivity and affinity towards specific heavy metal ions (El Ouardi et al., 2021).

Moreover, the integration of molecular imprinting technology with nanomaterials, such as graphene-based nanocomposites, metal oxide nanoparticles, and carbon nanotubes, enhances the stability, reusability, and performance of imprinted materials for heavy metal removal applications (Yahaya et al., 2021). Functionalization of nanomaterials with template molecules and cross-linking agents facilitates the formation of well-defined imprinted cavities with tailored recognition properties (Parisi et al., 2020). Furthermore, the development of smart and stimuli-responsive molecularly imprinted nanomaterials offers dynamic control over the selectivity and release of captured heavy metal ions (Li et al., 2023b). Stimuli-responsive MIPs can undergo reversible changes in their structure or properties in response to external stimuli, enabling the on-demand release of adsorbed contaminants under specific conditions (Mintz Hemed et al., 2023).

Desorption techniques involve the removal of adsorbed heavy metal ions from the surface of adsorbent materials, allowing for the regeneration and reuse of the adsorbents. Various desorption methods have been developed to efficiently recover heavy metal ions while minimizing the generation of secondary wastes (Lata et al., 2015). Recent trends in desorption techniques focus on enhancing the efficiency and selectivity of desorption processes while reducing energy consumption and environmental impact (Kopac, 2021). Thermal desorption, which involves heating the adsorbent material to release adsorbed contaminants, remains a commonly used method due to its simplicity and effectiveness. However, advancements in alternative desorption methods, such as chemical desorption, solvent extraction, and microwave-assisted desorption, offer advantages in terms of selectivity, speed, and energy efficiency (Jalili et al., 2020).

Chemical desorption techniques involve the use of desorbing agents, such as acids, bases, or complexing agents, to break the bonds between the adsorbent and the heavy metal ions (Chatterjee and Abraham, 2019). Recent studies have investigated the use of environmentally friendly desorbing agents, such as citric acid, EDTA, and hydrochloric acid, to minimize the generation of hazardous wastes and reduce environmental impact (Golmaei et al., 2018; Yaashikaa et al., 2021). Furthermore, the development of innovative desorption techniques, such as ultrasonic-assisted desorption and supercritical fluid extraction, enables rapid and efficient recovery of heavy metal ions from adsorbent materials (Wang J. et al., 2024). These techniques offer advantages in terms of selectivity, speed, and energy efficiency, making them promising candidates for the regeneration of adsorbents in heavy metal removal systems (Shrestha et al., 2021).

Electrochemical regeneration involves the application of an electric current to regenerate adsorbent materials or electrodes used in heavy metal removal processes (Ganzoury et al., 2020). The process relies on electrochemical reactions to desorb and recover heavy metal ions from the surface of the electrodes or adsorbents. Recent advancements in electrochemical regeneration techniques focus on optimizing electrode materials, operating conditions, and regeneration protocols to enhance efficiency and reduce energy consumption (Romano et al., 2020). Electrochemical regeneration methods, such as electrochemical desorption and electrodialysis, offer advantages in terms of selectivity, scalability, and environmental sustainability (Sedighi et al., 2023).

Advanced electrode materials, including carbon-based electrodes, metal oxide nanoparticles, and conductive polymers, exhibit enhanced electrochemical activity and stability, leading to improved regeneration efficiency and durability of electrochemical regeneration systems (Pan et al., 2019). Moreover, the integration of renewable energy sources, such as solar and wind power, into electrochemical regeneration systems reduces the carbon footprint and energy consumption of heavy metal removal processes (Zahmatkesh et al., 2022). Additionally, the development of smart and adaptive electrochemical regeneration systems enables real-time monitoring and control of regeneration processes, leading to enhanced efficiency and reliability (Lv et al., 2023). These systems offer advantages in terms of process optimization, resource utilization, and environmental sustainability, making them promising candidates for the regeneration of adsorbents and electrodes in heavy metal removal systems (Shrestha et al., 2021; Younas et al., 2021).

Photocatalytic regeneration involves the use of photocatalysts to facilitate the degradation or transformation of adsorbed contaminants under irradiation with light, typically ultraviolet (UV) or visible light (Liao et al., 2022). The process relies on the generation of reactive oxygen species (ROS) or photoinduced electrons and holes to oxidize or reduce the adsorbed contaminants (Afreen et al., 2020). Recent trends in photocatalytic regeneration focus on developing efficient photocatalysts with enhanced activity and stability for the degradation of heavy metal ions (Pang et al., 2024). Advanced photocatalytic materials, such as metal oxides, semiconductor nanoparticles, and carbon-based nanomaterials, exhibit high photocatalytic activity and selectivity towards specific heavy metal contaminants (Farhan et al., 2023).

Moreover, the integration of advanced reactor designs, such as photocatalytic membranes, immobilized photocatalysts, and flow-through photocatalytic reactors, enables efficient utilization of light energy and enhancement of mass transfer kinetics (Molinari et al., 2021; Wang Z. et al., 2021). These reactor designs offer advantages in terms of scalability, continuous operation, and process integration, making them suitable for practical applications in heavy metal removal systems (Qasem et al., 2021). Hence, the development of hybrid photocatalytic systems, such as photocatalytic-adsorption and photocatalytic-electrochemical systems, enables synergistic effects and enhanced regeneration efficiency. These hybrid systems offer advantages in terms of selectivity, efficiency, and versatility, making them promising candidates for the regeneration of adsorbents and electrodes in heavy metal removal systems.

Synthesis methods are crucial in determining the scalability, reproducibility, and properties of nanomaterials used in heavy metal removal technologies (Wawata and Fabiyi, 2024). Recent advancements in synthesis techniques focus on enhancing the scalability and cost-effectiveness of nanomaterial production while maintaining control over the properties and performance of the materials (Gohar et al., 2024; Saleh, 2024). Traditional synthesis methods, such as chemical precipitation, sol-gel, and hydrothermal synthesis, offer advantages in terms of simplicity, versatility, and scalability (Zohrabi, 2024). However, these methods may suffer from limitations in terms of particle size control, uniformity, and reproducibility (Li et al., 2023a; Lin et al., 2023). Recent trends in synthesis methods include the development of scalable and environmentally friendly approaches, such as green synthesis, microwave-assisted synthesis, and continuous-flow synthesis, which offer advantages in terms of energy efficiency, reaction control, and waste reduction (Adeola et al., 2023; Ahmed S. F. et al., 2022; Kaur et al., 2023). Moreover, advancements in bottom-up and top-down fabrication techniques enable precise control over the size, shape, and surface properties of nanomaterials, leading to improved performance and selectivity in heavy metal removal applications. Integration of advanced characterization techniques, such as electron microscopy, X-ray diffraction, and spectroscopy, facilitates real-time monitoring and optimization of synthesis processes, enabling rapid scale-up and commercialization of nanomaterial-based technologies (Abid et al., 2022; Baig et al., 2021; Chen et al., 2022; El-Khawaga et al., 2023).

Stability and longevity are critical factors in the performance and sustainability of nanomaterials used for heavy metal removal, particularly in long-term and continuous water treatment applications (Gul Zaman et al., 2021; Yu et al., 2022). While significant advancements have been made in improving the stability and durability of nanomaterials, challenges remain. Nanomaterials are prone to aggregation due to weaker intermolecular interactions, which can reduce their surface area and subsequently diminish their removal efficiency (Ahmed S. F. et al., 2022). In addition, nanomaterials, particularly nanomembranes, often suffer from mechanical instability, which results in performance degradation over time, limiting their effectiveness in long-term applications. One major issue is the environmental fate of nanomaterials, especially when they interact with microorganisms or are released into aquatic environments. Nanoparticles, such as carbon nanotubes and graphene-based materials, are prone to settling in sediments where they can harm benthic organisms. For instance, graphene oxide can produce reactive oxygen species that damage cell membranes in marine life (Saleem and Zaidi, 2020). These interactions with environmental microorganisms could lead to unintended ecological consequences, complicating the large-scale deployment of nanomaterials in water treatment systems.

The aggregation of nanomaterials in wastewater treatment plants is another significant concern. Studies have shown that a substantial proportion of nanoparticles are removed during the activated sludge process via bioadsorption onto the sludge surface. For example, nanoparticles like WO₃ and TiO₂ were found to accumulate in sludge (Simelane and Dlamini, 2019). The aggregation of nanoparticles in sludge complicates the management of waste sludge, as it can lead to potential environmental risks if not properly managed. To address these challenges, surface modification techniques have been developed to enhance the stability and dispersibility of nanomaterials. Functionalization with stabilizing agents, encapsulation within protective coatings, and immobilization on support matrices can extend the operational lifespan of nanomaterials by preventing aggregation and degradation in harsh water treatment environments (Alipour Atmianlu et al., 2021). For instance, by reducing aggregation and enhancing dispersibility, these modifications can significantly extend the operational lifespan of nanomaterials. However, long-term environmental assessments are still necessary, as exposure to various environmental factors can alter the size, composition, and stability of these materials, potentially changing their behaviour and environmental impact (Simeonidis et al., 2019). Nanoparticles such as Ag and ZnO have been reported to negatively affect phytoplankton and diatoms, demonstrating the need for more research on the environmental impact of spent nanomaterials.

In addition to these stability issues, proper disposal and recyclability of spent nanomaterials are crucial for ensuring sustainable applications. Recycling methods, such as using spent nanoparticles in brick manufacturing or disposing them in controlled landfills, have been suggested, but long-term evaluation is required, as many approaches have only been tested at the laboratory scale (Prathna et al., 2018). Clear guidelines on nanomaterial disposal are also necessary to avoid unintended environmental contamination when scaling up their use in industrial applications. Despite these challenges, advancements in nanocomposite and hybrid materials show promise for enhancing stability and longevity. For example, combining different nanomaterials can create synergistic effects, improving their structural integrity and resistance to environmental degradation (Hassan et al., 2021). Integrating nanomaterials with biodegradable or natural matrices also enhances their environmental compatibility while maintaining high removal efficiency.

Cost-effectiveness is a critical consideration in developing and implementing heavy metal removal technologies, particularly for large-scale and decentralized water treatment systems (Ayach et al., 2024; Neisan et al., 2023). Several economic factors can mitigate against investing in nanomaterial-based remediation technologies. The running cost of the technologies can be huge. This may be due to the raw materials used, the synthesis method adopted, specialized equipment and facilities, energy consumption cost, labour, and environmental and health risk assessment costs (Asghar et al., 2024).

However, recent advancements in materials synthesis, process engineering, and system design focus on reducing costs while maintaining performance and reliability (Chai et al., 2021; Gupta et al., 2021). The cost of nanomaterials, energy consumption, and operational maintenance are primary factors influencing the overall cost-effectiveness of heavy metal removal technologies. Recent trends in materials synthesis emphasize scalable and low-cost approaches, such as template-assisted synthesis, self-assembly, and waste-derived materials, to reduce production costs and enhance affordability (Abdullahi et al., 2024; Luo et al., 2021).

Furthermore, advancements in process engineering, such as optimization of reactor design, flow rate control, and automation, enable efficient utilization of resources and energy, leading to reduced operational costs and improved cost-effectiveness (Constance Obiuto et al., 2024). Integration of renewable energy sources, such as solar and wind power, into water treatment systems further enhances sustainability and reduces operating expenses (Shokri and Sanavi Fard, 2022). Moreover, life cycle cost analysis and techno-economic assessments provide valuable insights into the cost-effectiveness and feasibility of heavy metal removal technologies, guiding decision-making and investment strategies for water treatment infrastructure (Ćetković et al., 2022; Ilyas et al., 2021; Kehrein et al., 2021).

Specifically, to address scalability, cost-effectiveness, and environmental sustainability challenges associated with the use of nanoparticles for heavy metal remediation, there is a need to improve surface functionalization. When nanoparticle surfaces are optimally functionalized, the tendency for aggregation is reduced. This will ultimately improve its stability. Also, using green chemistry and biological methods of synthesis would tremendously reduce the cost associated with the use of nano-based heavy metal remediation. Furthermore, optimizing the magnetic properties of nanoparticles to improve their separation from the environment will increase recovery and reusability (Asghar et al., 2024).

Integrating nanomaterials with existing water treatment technologies, such as membrane filtration, electrochemical treatment, and biological remediation, presents an opportunity to enhance the efficiency of heavy metal removal (Elgarahy et al., 2021; Khan et al., 2021). Recent efforts focus on hybrid systems that leverage the unique properties of nanomaterials alongside traditional remediation techniques, offering synergistic effects that can improve selectivity, efficiency, and fouling resistance (Adeola and Forbes, 2021; Pérez et al., 2023; Thakur and Kumar, 2022). For instance, membrane-based nanocomposites embedded with functionalized nanoparticles have demonstrated superior performance in selectively removing heavy metals from water while also improving membrane durability and reducing fouling.

The successful incorporation of nanomaterials into current water treatment systems is exemplified by ceramic disk filters coated with nano-ZnO. These filters, which utilize the photocatalytic antibacterial properties of ZnO to reduce Escherichia coli in drinking water, offer a cost-effective solution, particularly beneficial for remote and rural communities (Huang et al., 2018). This hybrid approach, combining traditional filtration methods with nanomaterials, provides a low-cost upgrade to existing systems, enhancing water safety without requiring substantial modifications. However, integrating nanomaterials into large-scale WWTPs presents unique challenges, as demonstrated by studies on the behaviour of AgNPs in wastewater treatment processes. Research indicates that during wastewater treatment, AgNPs undergo sulfidation, particularly in anaerobic zones, converting them to Ag₂S. This transformation reduces their effectiveness in contaminant removal (Kent et al., 2014). This transformation complicates the integration of nanomaterials in large-scale systems, due to the altered adsorption properties of the nanoparticles.

Further challenges include the handling of nanomaterials in the sludge generated by wastewater treatment processes. A recent study (Cervantes-Avilés and Keller, 2021) demonstrated that while WWTPs can remove between 84% and 99% of metal-based nanoparticles from influent wastewater, substantial concentrations of nanoparticles, especially Mg, Ni, and Cd, still accumulate in the waste sludge. The accumulation of nanoparticles in waste sludge requires careful consideration during disposal, as the potential release of nanomaterials into the environment could pose additional risks. This was affirmed by another study that researched integrating WO₃ and TiO₂ mixtures into a wastewater treatment plant (Simelane and Dlamini, 2019). The nanoparticles were mainly adsorbed onto the sludge and removed from the wastewater, with the activated sludge process proving effective in their elimination. However, the long-term fate of these nanoparticles, including the stability of their polymorphs like monoclinic WO₃ and anatase TiO₂, remains a concern for sludge management strategies. Moreover, a study on silver nanoparticles in municipal WWTPs demonstrated that both mechanical and biological treatments are effective in reducing nanoscale silver particles from wastewater, with a 95% reduction achieved by combining these processes (Li et al., 2013). However, despite this high reduction rate, residual concentrations of n-Ag-Ps in the effluent still pose a challenge for WWTPs, particularly when scaling up the technology for larger plants.

System optimization and advancements in process integration have facilitated the incorporation of nanomaterials into conventional water treatment infrastructure. For instance, modular approaches allow for easier retrofitting of remediation units, enabling the deployment of nanomaterials without extensive changes to existing systems (Ruíz-Baltazar, 2024; Vasoya, 2023). However, the transformation, stability, and long-term behaviour of nanomaterials within these systems, particularly under varying environmental conditions, remain areas of concern. Collaborative efforts between nanotechnology experts, environmental engineers, and materials scientists are essential for overcoming these challenges and realizing the full potential of nanomaterials in integrated water treatment solutions.

Overall, integrating nanomaterial with existing water treatment infrastructure is essential for the practical implementation and adoption of heavy metal removal technologies in real-world settings (Singh et al., 2023). Recent advancements in system design, modularization, and process optimization focus on compatibility and interoperability with existing treatment processes and infrastructure (Brad et al., 2021). Heavy metal removal technologies should be designed to complement and integrate seamlessly with conventional water treatment processes, such as coagulation-flocculation, sedimentation, filtration, and disinfection (Khan Khanzada et al., 2023; Vidu et al., 2020). Modular and scalable designs enable flexible deployment and retrofitting of treatment units within existing infrastructure, facilitating gradual upgrades and expansions according to specific water quality and capacity requirements (Brears, 2021; Daigger et al., 2020; Leigh and Lee, 2019). Moreover, advancements in sensor technology, remote monitoring, and automation enable real-time data acquisition, process control, and performance optimization, enhancing the operational efficiency and reliability of integrated water treatment systems (Martínez et al., 2020; Park et al., 2020; Zainurin et al., 2022; Zhang W. et al., 2020). Integration of smart sensors and IoT-enabled devices facilitates remote monitoring and control of treatment processes, enabling predictive maintenance and early detection of system failures.

Nanoparticles could be optimized to become highly reusable. For instance, platinum magnesium and copper oxide nanoparticles have been reused in literature. However, when nanoparticles are not properly managed, they could accumulate in the environment due to their long-term stability. They can react with organic matter in the environment to cause environmental damage, and health issues such as apoptosis, oxidative stress, etc (Fu et al., 2020; Imran et al., 2021; Wahl et al., 2021; Zhang X. et al., 2020).

One of the primary challenges in heavy metal remediation is conducting comprehensive environmental impact assessments to evaluate the potential risks and benefits associated with remediation technologies (Khatun et al., 2024; Rashid et al., 2023). While nanomaterials offer promising solutions for heavy metal removal, concerns remain regarding their environmental fate, toxicity, and long-term effects on ecosystems. Recent studies have focused on assessing the environmental implications of nanomaterial-based remediation technologies through rigorous toxicity testing, fate and transport modelling, and ecological risk assessments (Isibor, 2024; Isibor et al., 2024; Prasad and Gupta, 2024; Shanker, 2024). These efforts aim to identify potential environmental hotspots, evaluate exposure pathways, and mitigate adverse impacts on aquatic organisms, soil microbiota, and human health. Moreover, advancements in life cycle assessment (LCA) and environmental footprint analysis enable holistic evaluation of the environmental impacts of heavy metal remediation technologies, considering factors such as energy consumption, resource utilization, and waste generation (Ding et al., 2024; Pandit et al., 2024; Rothee et al., 2024). Integration of environmental sustainability criteria into the design and implementation of remediation strategies ensures responsible stewardship of natural resources and minimizes unintended consequences.

Regulatory compliance is another significant challenge in the development and deployment of heavy metal remediation technologies, as stringent regulations govern the use, disposal, and discharge of contaminants in water sources. Recent trends in regulatory compliance focus on harmonizing standards and guidelines for heavy metal concentrations in drinking water, surface water, and wastewater effluents to ensure the protection of human health and the environment (Kumar and Samadder, 2023; Shaikh and Birajdar, 2024). Regulatory agencies, such as the Environmental Protection Agency (EPA) in the United States and the European Chemicals Agency (ECHA) in Europe, play a crucial role in setting and enforcing regulatory requirements for heavy metal remediation technologies (Nwokediegwu et al., 2024). Moreover, advancements in risk-based approaches and adaptive management strategies enable flexible and pragmatic regulatory frameworks that balance environmental protection with technological innovation (Gikay, 2024; Dada et al., 2024). Collaborative efforts between government agencies, industry stakeholders, and research institutions facilitate knowledge exchange, capacity building, and continuous improvement in regulatory compliance.

Scaling up the production of nanomaterial-based remediation technologies from laboratory-scale prototypes to commercially viable systems poses practical challenges, including cost considerations, process optimization, and supply chain management. Recent advancements in scaling up production focus on optimizing synthesis methods, improving materials efficiency, and streamlining manufacturing processes to reduce production costs and enhance scalability (Ganguly et al., 2024; Palit and Ranjit, 2024; Tyagi et al., 2024). Automation, robotics, and process intensification techniques enable high-throughput production of nanomaterials with consistent quality and performance (Aithal and Aithal, 2024; Darwish et al., 2024). Furthermore, partnerships between academia, industry, and government agencies facilitate technology transfer, knowledge dissemination, and capacity building to accelerate the commercialization of nanomaterial-based remediation technologies (Kumar et al., 2023). Investment in research infrastructure, pilot-scale testing facilities, and demonstration projects enables validation of scalability and performance under real-world conditions.

Multifunctional nanomaterials offer exciting opportunities for addressing multiple challenges in heavy metal remediation simultaneously, including selectivity, stability, and regeneration (Feisal et al., 2024; Nikić et al., 2024). Recent trends in multifunctional nanomaterials focus on integrating multiple functionalities, such as adsorption, catalysis, and sensing, into a single platform to enhance performance and versatility in heavy metal removal applications (Asghar et al., 2024; Godja and Munteanu, 2024; Shanmugavel et al., 2024). For example, nanocomposites composed of graphene oxide, metal nanoparticles, and molecularly imprinted polymers exhibit synergistic effects and enhanced selectivity for specific heavy metal contaminants. Moreover, advancements in responsive and stimuli-triggered nanomaterials enable dynamic control over adsorption, desorption, and regeneration processes, enhancing efficiency and sustainability (Makaev et al., 2023; Shahrokhinia et al., 2024; Zhou et al., 2021).

However, designing multifunctional nanomaterials also presents several challenges. The complexity of synthesizing materials that combine multiple functionalities while maintaining structural integrity and long-term stability is a significant barrier. For instance, some multifunctional materials, such as those used for both adsorption and catalysis, must balance these functions without compromising their effectiveness in any particular role (Li and Liu, 2023). Additionally, the recovery and recyclability of these materials are critical for large-scale applications. Magnetic nanomaterials, such as ZnO-Fe₃O₄ composites, have shown promise in this regard due to their ability to be easily separated from treated water using external magnets, thus enhancing the regeneration process and overall cost-effectiveness (Goyal et al., 2018). Furthermore, multifunctional nanomaterials must maintain high selectivity in complex water matrices containing competing ions, a factor that often limits their performance (Baby et al., 2022).

Despite these challenges, advancements in stimuli-responsive and smart nanomaterials have opened new avenues for overcoming these limitations. Materials that can dynamically alter their adsorption properties based on environmental conditions, such as pH-responsive poly 4-hydroxyphenyl methacrylate single-walled carbon nanotube (PHPMA-SWCNT) nanocomposites, have demonstrated the ability to selectively target heavy metals like Pb²⁺ and Cd²⁺, even in the presence of multiple interfering ions (Mamidi et al., 2021). This adaptability enhances the material’s performance in real-world applications, where water compositions can vary significantly. A practical example of multifunctionality can be seen in the work by Goyal et al. (2018), where ZnO-Fe₃O₄ (ZF) nanocomposites were successfully used for simultaneous adsorption of heavy metals, degradation of organic dyes, and antibacterial activity. This multifunctional performance is crucial for dealing with complex contamination scenarios, such as those found in industrial effluents, where pollutants often coexist. Moreover, the ZF nanocomposite could be easily recovered using magnetic separation and reused multiple times, offering a sustainable solution for water treatment.

Further practical examples of multifunctional nanomaterials include the multifunctional nanomaterials is the biomaterial-functionalized graphene-magnetite (Bio-GM) nanocomposite, developed by Ramalingam et al., 2018, which addresses the challenges of colloidal stability and recyclability in graphene-based materials. By incorporating Shewanella oneidensis cells into the nanocomposite, Bio-GM efficiently adsorbed both dyes and Cr⁶⁺, with removal capacities of 189.63 mg/g for dyes and 222.2 mg/g for Cr⁶⁺. Additionally, the nanocomposite facilitated the biocatalytic reduction of Cr⁶⁺ to Cr³⁺, demonstrating its multifunctionality. Bio-GM could be regenerated and reused without releasing harmful components, making it a sustainable option for water treatment. In another example, El Mouden et al., 2023 synthesized NC@Co₃O₄ nanocomposites by co-precipitating natural clay with cobalt oxide nanoparticles for heavy metal removal. These nanocomposites exhibited high adsorption efficiencies for Pb²⁺ and Cd²⁺, with rates of 86.89% and 82.06%, respectively.