Heavy Metal Adsorption on Microplastics: Adsorption Mechanisms, Influencing Factors, Analytical Techniques, Toxicological Implications, and Remediation Strategies

ABSTRACT

Microplastics (MPs) act as reactive surfaces and carriers in aquatic systems, influencing the mobility, bioavailability, and toxicity of heavy metals. Recent research has clarified how heavy metals adsorb onto MPs, focusing on adsorption pathways, physicochemical factors, analytical methods, toxicological impacts, and remediation strategies. Metal binding to MPs depends on factors such as polymer type, particle size, aging, surface properties, zeta potential, pH, salinity, dissolved organic matter, temperature, and the presence of biofilms. Adsorption occurs through mechanisms including electrostatic interactions, surface complexation, ion exchange, pore filling, hydrogen bonding, van der Waals forces, cation–π interactions, surface precipitation, and biofilm‐mediated binding. MP–metal interactions vary with environmental conditions; for example, salinity and dissolved organic matter can enhance or reduce adsorption, depending on metal speciation, polymer characteristics, and experimental conditions. Toxicological studies show that MPs carrying heavy metals can increase bioaccumulation and combined toxicity by promoting transport and cellular uptake. However, strong adsorption and limited desorption may sometimes reduce the bioavailability of dissolved metals. The Mediterranean Sea is a key case study due to its semi‐enclosed nature, high urbanization, maritime activity, wastewater discharge, and significant plastic pollution, all of which heighten the importance of MP–metal interactions. There is an urgent need for standardized adsorption protocols, thorough in situ and ex situ characterization, ecologically relevant toxicological studies, and integrated remediation strategies addressing both MPs and associated heavy metals.

Summary

• Metal adsorption to microplastics was studied as a potential mechanism for pollutant removal.

• This review provides a synopsis of current research on metal adsorption to microplastics.

• The potential effects of metal adsorption on microplastics for environmental remediation.

• The potential effects of heavy metal adsorption on microplastics on human health.

• A case study on lead adsorption onto polyethylene microplastics illustrates key adsorption mechanisms and impacts.

The review examines how lead binds to polyethylene microplastics, showing how new and weathered plastics can bind metals differently through both physical and chemical mechanisms. MPs can play a dual role in the environment: they might temporarily lower dissolved metal levels by adsorbing them, but they can also help spread pollutants, increase their accumulation in organisms, and raise toxicity through the so‐called “Trojan horse” effect.

1. Introduction

Plastic pollution has increased rapidly during recent decades, becoming a major environmental concern across terrestrial, freshwater, and marine ecosystems. The large‐scale production, use, and disposal of plastics have resulted in the accumulation of persistent plastic debris in soils, rivers, coastal zones, and oceans. Under environmental conditions, larger plastic items progressively fragment into smaller particles through mechanical abrasion, photodegradation, oxidation, and weathering, forming microplastics (MPs) and nanoplastics. These particles are environmentally important not only because of their persistence and wide distribution but also because their small size, high surface‐area‐to‐volume ratio, variable polymer chemistry, and aging‐induced surface functional groups allow them to interact with co‐occurring pollutants (Galgani et al. 2013; Boucher and Friot 2017; Borrelle et al. 2020; Su et al. 2022).

When plastic is present in regular settings, it frequently contaminates the atmosphere (Xu et al. 2020), water, and soil (Su et al. 2022). It is thought that 23 × 10⁶ tons of plastic debris have entered the oceans. Between 93 and 268 tons of MPs in the present day are thought to be scattered in the aquatic ecosystem (Boucher and Friot 2017; Borrelle et al. 2020), and as noted by several writers, the current age is known as the “plasticine” because of the extent and consequences of this worldwide ocean contamination (Reed 2015). In marine ecosystems, plastic debris is commonly classified according to particle size into macroplastics (> 25 mm), mesoplastics (5–25 mm), MPs (1 μm–5 mm), and nanoplastics (< 1 μm). Under environmental conditions, larger plastic debris can progressively fragment into smaller MP and nanoplastic particles through mechanical abrasion, photodegradation, and chemical weathering (Galgani et al. 2013).

MPs are a highly diverse collection of particles with a range of sizes, shapes, colors, chemical compositions, densities, and other properties (Boucher and Friot 2017). In the maritime environment, mechanical, photolytic, and/or chemical breakdown mechanisms can cause the bigger plastic material to slowly weather or abrade into smaller fragments. Or, as main MPs (secondary MPs), they might be released instantly as microscopic particles (Mathalon and Hill 2014). In the maritime ecosystem, subsidiary MPs predominate (do Sul and Costa 2014; Duis and Coors 2016).

Interactions between MPs and other contaminants (waterborne or anthropogenic) in water are complex and influenced by many factors. It is important to (1) distinguish the MPs' size, shape, and type; (2) identify the type of the combined pollutant; (3) study different combination mechanisms; (4) explore the chance of this combination in marine settings by understanding the drivers behind it; and, finally, (5) measure its impact on aquatic ecosystems. All these steps are crucial for a better understanding of the environmental implications of the combination of MPs and for developing effective mitigation strategies. The current review focuses on the combination of MPs and metals via adsorption mechanisms and provides an overview of MPs in the Mediterranean Sea (MS).

Because of their small size, high surface‐area‐to‐volume ratio, variable polymer chemistry, and progressive environmental aging, MPs are not only physical contaminants but also reactive surfaces that interact with co‐occurring pollutants. This property is particularly important for heavy metals, which are persistent, nondegradable contaminants that can bind to MP surfaces through electrostatic attraction, surface complexation, ion exchange, precipitation, and biofilm‐mediated mechanisms. Therefore, understanding MP pollution requires moving beyond occurrence data alone toward a mechanistic assessment of how MPs modify the mobility, bioavailability, and toxicity of associated metals. Recent studies from the MS collectively indicate that MP contamination is not restricted to a single environmental compartment or organism group but occurs across sediments, reef‐associated habitats, filter‐feeding organisms, sponges, mussels, coral beds, and pelagic fish. Rather than representing isolated observations, these findings reveal a broader regional pattern in which MPs are distributed across benthic, coastal, and pelagic systems and can be ingested or retained by organisms with different feeding strategies. The frequent detection of polyethylene (PE), polystyrene (PS), and PE terephthalate (PET), particularly in small‐sized fractions, suggests that Mediterranean biota are exposed not only to visible plastic fragments but also to particles with high surface‐area‐to‐volume ratios and therefore higher potential for contaminant adsorption (Vega‐Herrera et al. 2024; C. Chen et al. 2025; Gutierrez‐Rodriguez et al. 2025). Filter‐feeding organisms such as mussels and sponges may act as useful bioindicators of local MP exposure because of their filtration capacity and direct interaction with suspended particles, whereas pelagic fish, reef‐associated sediments, and coral‐bed environments provide evidence that MPs can move across habitats and trophic levels (Hamed et al. 2023; Aguilo‐Arce et al. 2025; Torresi et al. 2024; Koutsikos et al. 2023; Bue et al. 2025; Morici et al. 2025; Bolea et al. 2025; Reuning et al. 2025). Therefore, the Mediterranean literature should be interpreted as showing an interconnected pollution problem rather than a simple list of occurrence records. This regional pattern is particularly relevant to heavy metals because the persistence, weathering, and biological uptake of MPs increase the likelihood that plastic particles will interact with dissolved or particle‐bound metals, potentially influencing metal transport, bioavailability, and ecological risk. This concern is reinforced by the recognition of the MS as a global hotspot for MPs, with reported concentrations approximately four times higher than those in the North Pacific Ocean (Sharma et al. 2021).

Accordingly, the MS provides a scientifically relevant context for examining MP–metal interactions because high plastic contamination overlaps with intense coastal urbanization, maritime transport, wastewater discharge, industrial activities, riverine inputs, and restricted water exchange. These conditions increase the likelihood that MPs will undergo aging, form biofilms, and experience repeated contact with dissolved and particulate metals. As a result, MPs may act either as temporary sinks that reduce dissolved metal concentrations or as vectors that enhance metal transport, trophic transfer, and combined toxicity. This dual role forms the basis for the present review, which critically evaluates the mechanisms, controlling factors, analytical evidence, toxicological implications, and remediation strategies associated with heavy metal adsorption onto MPs. Heavy metal adsorption on MPs presents both advantages and challenges in environmental contexts (Devi et al. 2025). On the positive side, MPs offer large surface areas and variable surface chemistries, enabling them to adsorb heavy metals and potentially temporarily reduce the concentration of free metal ions in water. This adsorption capability varies with polymer type, particle size, aging, and environmental factors, enabling targeted studies of pollutant dynamics. Additionally, MPs serve as substrates for microbial biofilms, which can further modify their adsorption properties and impact pollutant behavior (S. Liu, Shi, et al. 2021; Das et al. 2025; T. Chen et al. 2015; Alomar et al. 2025). These characteristics help researchers track pollution sources and understand contaminant distribution, especially in sensitive areas like the MS. However, there are significant downsides. MPs act as vectors that enhance the bioavailability and transport of heavy metals through aquatic food webs, a phenomenon termed the “Trojan horse” effect (Carriera et al. 2023), thereby increasing ecological and human health risks. The combined presence of MPs and heavy metals can cause synergistic toxic effects, affecting organism growth, reproduction, and survival. Adsorption behavior is influenced by environmental conditions such as pH, temperature, and exposure duration, making predictions complex. Moreover, the persistence of MPs and adsorbed metals in ecosystems raises concerns about long‐term chronic exposure and bioaccumulation. Methodological challenges remain in accurately quantifying and characterizing adsorbed metals on varied MP types (W. Liu, Zhang, et al. 2021; Das et al. 2025; T. Chen et al. 2015; M. Zhou et al. 2024). Together, these pros and cons highlight the dual role of MPs in environmental pollution studies, serving both as a tool and a risk factor, underscoring the critical need to understand their interactions with heavy metals for environmental management and policy (Table 1).

TABLE 1.

Advantages and disadvantages of heavy metal adsorption on microplastics.

Pros	Cons
MPs can adsorb heavy metals, reducing free metal ions in water temporarily	MPs act as carriers, increasing heavy metal bioavailability (Trojan horse effect) (Carriera et al. 2023)
Adsorption influenced by MP type, surface area, and environmental factors allowing targeted studies	Adsorbed metals can transfer through food chains, causing ecological risks
MPs offer surfaces for microbial biofilms which can modify adsorption positively	MPs with metals pose combined toxicological risks to aquatic life and humans
Provides a model system to study contaminant interactions and environmental behavior	Adsorption may vary with pH, temperature, and aging, complicating predictions
Helps identify pollution sources and monitor contaminant distribution	MPs and metals cause long‐term persistence and chronic exposure concerns
Advances knowledge of pollutant dynamics in regions like the Mediterranean Sea	Methodological challenges exist in accurately quantifying and characterizing adsorbed metals

Although several high‐quality reviews have addressed MP pollution, heavy metal contamination, adsorption mechanisms, toxicological effects, or treatment technologies, the current knowledge remains fragmented. What remains insufficiently understood is how polymer properties, particle size, aging/weathering, biofilm formation, metal speciation, dissolved organic matter (DOM), salinity, pH, and other environmental variables interact to control metal adsorption, desorption, bioavailability, toxicity, and removal in aquatic systems. In particular, contradictory findings in the literature, such as the promoting or inhibiting effects of salinity and DOM, have not been sufficiently reconciled within a unified mechanistic and environmental‐risk framework. This gap is especially important for the MS, which is a semi‐enclosed, highly pressured marine basin characterized by restricted water exchange, dense coastal urbanization, intensive tourism, maritime transport, wastewater discharge, riverine inputs, industrial activities, and high levels of plastic accumulation. Therefore, MP–metal interactions in the Mediterranean may have greater ecological and human‐health relevance than in more open marine systems because pollutants can persist, accumulate, and interact within coastal and semi‐enclosed environments.

The novelty of this review lies in its integrated and regionally focused synthesis of heavy metal adsorption onto MPs. Unlike previous reviews that have often treated adsorption mechanisms, environmental occurrence, toxicity, analytical techniques, or remediation strategies as separate topics, this manuscript integrates these components into a single coherent framework. Specifically, it connects adsorption mechanisms to the control of physicochemical and biological factors, evaluates the analytical techniques needed to verify these mechanisms, discusses how metal‐loaded MPs may alter toxicity through synergistic and antagonistic pathways, and assesses remediation strategies that address both MPs and associated metals. By emphasizing the MS as a priority regional context, this review provides a differentiated contribution that supports future monitoring, risk assessment, experimental harmonization, and the development of integrated management and remediation strategies for heavy metal‐laden MPs in aquatic environments.

2. Adsorption of Metals Onto MPs

Multiple contaminants are prevalent in natural water and wastewater bionetworks (L. Gao, Fu, et al. 2021). Heavy metals are naturally occurring elements that become environmental pollutants at elevated concentrations in aquatic ecosystems. Reports from several studies show that metals such as Co, Zn, Mn, Fe, Ca, Ag, Pb, Cu, Cr, Cd, Ni, Al, and Hg have been observed on MPs' surfaces globally (Q. Liu et al. 2022), especially in coastal waters, harbors, and industrial areas.

Heavy metals associated with MPs in aquatic environments originate from several major source categories rather than isolated inputs (Khalid et al. 2021; Khalid et al. 2018). These include industrial discharges and untreated effluents, which contribute metals such as Al, Cr, Cu, As and Fe; urban runoff and roadside drainage, which transport traffic‐related metals such as Fe, Pb, Ni, Zn, and Cd into nearby water bodies during rainfall events; and maritime activities, including fuel combustion, shipping operations, and the weathering of antifouling paints, which release metals into harbors, estuaries, and coastal zones (Almeida et al. 2007; Canning‐Clode et al. 2011; Khalid et al. 2018, 2021; Khalid, Masood, et al. 2019; Khalid, Noman, et al. 2019; Aqeel et al. 2021; Brennecke et al. 2016).

According to current studies on MPs, metals exhibit varying sorption affinities toward MPs depending on polymer characteristics and environmental conditions. Certain metals can be found as useful additives, recyclables, or reaction leftovers inside the MP polymeric matrix (Turner et al. 2020). The extent of metals naturally present in the respective polymer form and the amount it adsorbs from the ecosystem vary (W. Li et al. 2020). Similarly, aging and weathering processes enhance the metal adsorption capacity of MPs owing to their smaller particle size, which results in a greater surface‐area‐to‐volume ratio (Wagner et al. 2014). The concentration of metals in MPs is 10‐ to 100‐fold higher than in the surrounding materials (Acosta‐Coley et al. 2019).

When metals adsorb onto MPs, ecological risk may increase because these particles can act as carriers, facilitating the entry of metals into aquatic organisms. Metal‐loaded MPs may be ingested by plankton, benthic invertebrates, mollusks, crustaceans and fish, allowing adsorbed metals to move from lower to higher trophic levels (W. S. Lee et al. 2019; F. Wang, Zhang, Zhang, Zhang, Adams, and Sun 2020; L. Gao, Fu, et al. 2021). If particles are retained in the digestive tract or if metals desorb under gut conditions, internal exposure may increase and induce oxidative stress, inflammation, enzyme inhibition, growth suppression, and reproductive impairment (Banaee et al. 2019; Lu et al. 2018; Sun et al. 2019; Wen et al. 2018; Yan et al. 2020; Y. Zhou, Liu, and Wang 2020). Repeated ingestion and predator–prey transfer may also contribute to trophic transfer and, for some persistent metals, possible biomagnification, although this depends on metal speciation, desorption potential, organism physiology, and food‐web structure (Barboza et al. 2018; Lian et al. 2020; L. Gao, Fu, et al. 2021; Q. Chen et al. 2023). From a human health perspective, MP‐associated metals may become relevant through seafood consumption, contaminated water use, and exposure to polluted coastal environments. Therefore, MPs provide a pathway by which environmental metals can enter biota and potentially reach humans, highlighting the need to evaluate MP–metal interactions in freshwater, marine, and terrestrial ecosystems (L. Gao, Fu, et al. 2021; Q. Chen et al. 2023).

Overall, these contrasting results indicate that polymer type alone cannot explain metal adsorption behavior. Differences among studies are more likely attributable to the combined influence of polymer surface chemistry, degree of aging/weathering, particle size, crystallinity, surface charge, biofilm formation, metal speciation, and the chemistry of the surrounding medium. For example, aged or polar polymers containing oxygen‐ or chlorine‐bearing functional groups may provide stronger binding sites for cationic metals, whereas nonpolar polymers may rely more strongly on physical adsorption and electrostatic interactions after surface oxidation. Therefore, apparent inconsistencies among published studies should be interpreted as evidence that metal adsorption onto MPs is system‐specific rather than universal.

2.1. Adsorption of Metal Mechanism

The dominant adsorption mechanism varies markedly with polymer type, metal speciation, and environmental conditions. For nonpolar polymers such as PE and PP, metal adsorption is primarily governed by physical adsorption, van der Waals forces, and electrostatic interactions following surface aging. In contrast, polymers containing polar or reactive groups, such as PVC, PET, PA, PMMA, and aged PS, can adsorb metals more strongly through surface complexation, polar interactions, chloride‐related interactions, and cation–π bonding. For cationic metals such as Pb²⁺, Cd²⁺, Cu²⁺, Ni²⁺, and Zn²⁺, adsorption is generally promoted by negatively charged or oxygen‐containing surface groups, particularly –COOH, –OH, and C=O groups formed during aging or biofilm development. However, anionic metal species, such as arsenic oxyanions, may follow different pathways and preferentially interact with positively polarized hydrogen sites associated with hydroxyl or carboxyl groups. Therefore, adsorption mechanisms should not be interpreted as universal but rather as polymer–metal–environment‐specific.

As shown in Figure 1, several methods have been proposed to explain how cationic and anionic contaminants adsorb onto MPs (Mo et al. 2025; Z. Li et al. 2026; Y. Gao et al. 2026; R. Wang et al. 2026; T. Yang et al. 2026). The processes documented include ion exchange, surface complexation, pore filling, hydrophobic interactions, hydrogen bonding, electrostatic attraction, π–π interactions, surface precipitation, van der Waals forces, and interactions with biofilm (Equations 1–5).

$${{\mathrm{MP}}^{-}}_{\left(\text{surface}\right)}+{{\mathrm{M}}^{n+}}_{\left(\mathrm{aq}\right)}⟶\mathrm{MP}-{{\mathrm{M}}^{n+}}_{\left(\text{adsorbed}\right)}$$ (1)

$$\mathrm{MP}-{\mathrm{FG}}_{\left(\text{surface}\right)}+{{\mathrm{M}}^{n+}}_{\left(\mathrm{aq}\right)}\rightleftharpoons {\left[\mathrm{MP}-\mathrm{FG}-{\mathrm{M}}^{n+}\right]}_{\left(\text{adsorbed}\right)}$$ (2)

$$\mathrm{MP}-\mathrm{FG}-{\mathrm{Na}}^{+}+{{\mathrm{M}}^{n+}}_{\left(\mathrm{aq}\right)}\rightleftharpoons \mathrm{MP}-\mathrm{FG}-{\mathrm{M}}^{n+}+{{\mathrm{Na}}^{+}}_{\left(\mathrm{aq}\right)}$$ (3)

$${\mathrm{MP}}^{\pi }+{\mathrm{M}}^{\pi }⟶{\mathrm{MP}}^{\pi }-{\mathrm{M}}^{\pi }$$ (4)

$${\mathrm{MP}}_{\text{bare}}+\text{microbes}\to {\mathrm{MP}}_{\text{biofilm}}$$ (5)

where FG stands for functional groups introduced on MPs by aging/oxidation (e.g., –OH, –COOH, –C=O). Metal ions form a chemical complex with these groups on the MP surface.

FIGURE 1.

Potential methods via which the MPs adsorb metals.

Mechanistically, metal adsorption onto MPs depends on interactions among polymer surface chemistry, metal speciation, and environmental conditions. Electrostatic attraction occurs when negatively charged MP surfaces attract dissolved metal cations. This mechanism is usually favored at neutral to alkaline pH because deprotonation of oxygen‐containing groups such as –COOH and –OH increases the negative surface charge, while competition from H⁺ decreases. In contrast, acidic conditions may weaken cation adsorption and promote desorption (D. Gao et al. 2020; Holmes et al. 2014; Turner et al. 2020; Y. Zhou, Yang, et al. 2020; Yona et al. 2025). Surface complexation occurs when metal ions coordinate directly with functional groups such as –COOH, –OH, C=O, C–O, or C–Cl on aged or polar polymers. Therefore, polymers such as PET, PVC, PA, PMMA, and aged PS may show stronger complexation than virgin PE or PP, although oxidation and biofilm formation can introduce new binding sites even on originally nonpolar polymers (Brennecke et al. 2016; F. Wang, Yang, et al. 2019; Tang et al. 2020; Zou et al. 2020). Ion exchange may occur when metal ions replace exchangeable ions associated with functional groups or mineral/biofilm coatings on MP surfaces, whereas cation–π and π‐related interactions are more likely for aromatic polymers such as PS because the benzene ring can interact with metal ions or metal complexes (Mo et al. 2025; Z. Li et al. 2026; Y. Gao et al. 2026; R. Wang et al. 2026; T. Yang et al. 2026). Surface precipitation may dominate when pH and metal concentration favor the formation of insoluble hydroxides, carbonates, or phosphates on MP surfaces, whereas pore filling and van der Waals interactions are more important for rough, weathered, or physically irregular particles (F. Wang, Yang, et al. 2019). Metal speciation strongly controls these mechanisms: Free cationic species such as Pb²⁺, Cd²⁺, Cu²⁺, Ni²⁺, and Zn²⁺ generally interact with negatively charged or oxygenated surfaces, whereas anionic species such as arsenic species may require positively polarized surface sites or specific hydrogen‐bearing functional groups (Dong et al. 2019; Dong et al. 2020). Chloride, carbonate, phosphate, and DOM complexes may reduce adsorption by lowering the concentration of free metal ions, but they may also modify adsorption pathways by forming complexes with different affinities for specific polymer surfaces. Thus, no single mechanism dominates under all conditions; adsorption reflects the combined effects of polymer type, degree of aging, pH, salinity, DOM, biofilm formation, and metal speciation.

Because MPs promote metal adsorption, they are believed to function as environmental metal vectors (Torres et al. 2021). MP surface control triggers this spontaneous adsorption process (Naqash et al. 2020). Though MPs do not have highly porous surfaces, they do develop negatively charged surfaces due to abrasion, long‐term weathering, and photo‐oxidation in everyday ecosystems. These surfaces may then absorb metal cations from the surrounding ecosystem (D. Gao et al. 2020; Holmes et al. 2014; Turner et al. 2020; Y. Zhou, Yang, et al. 2020; Yona et al. 2025).

During environmental exposure, MPs may develop negatively charged surfaces as a result of abrasion, weathering, and photo‐oxidation, which can enhance the adsorption of metal cations from the surrounding medium (D. Gao et al. 2020; Holmes et al. 2014; Turner et al. 2020; Y. Zhou, Yang, et al. 2020; Yona et al. 2025). For virgin or weakly weathered nonpolar polymers such as PE and PP, adsorption may be dominated mainly by physical interactions, including van der Waals forces and electrostatic attraction. However, after aging or oxidation, chemical modification of the MP surface can introduce oxygen‐containing functional groups such as –OH, –COOH, and C=O, thereby enabling stronger metal binding through surface complexation or ion exchange–like interactions. Therefore, metal adsorption onto MPs should be interpreted as a continuum from predominantly physical adsorption on unaged surfaces to chemically mediated adsorption on aged, oxidized, or biofilm‐coated particles (F. Wang, Yang, et al. 2019; Brennecke et al. 2016).

In another investigation, absorbance was strongly correlated with Pb and Mn concentrations in saltwater. These metals exhibit stronger adsorption on PVC and PP than on PA, PE, and POM MPs (F. Gao et al. 2019). Foshtomi et al. (2019) also reported a favorable association between the concentrations of 10 distinct metals along the Bandar Abbas, Iran, seashore, and the number of MPs. However, contrary to the previous findings, Rochman et al. (2014) found that after 12 months, there were no appreciable differences in the amounts of Ni, Zn, Cd, and Pb adsorbed by MPs made of PET, PVC, LDPE, PP, and HDPE in seawater.

With age, PS MPs' ability to transport metals increases dramatically, and their aging process varies depending on whether they are in freshwater or saltwater and on the atmosphere (Holmes et al. 2012; Mao et al. 2020). MPs' surface changes were due to excessive weathering/aging caused by UV light or abrasions from other environmental factors. Anionic active sites form as a result of these changes, thereby increasing the MP's surface area. According to Vedolin et al. (2018), these are the primary sites where cationic metal contaminants are extracted from the surrounding air. Furthermore, a decrease in MPs' crystallinity due to weathering might intensify adsorption by unswervingly increasing cross‐linking (Hüffer et al. 2018). F. Wang, Zhang, Zhang, Zhang, and Sun (2020) investigated the effects of UV light exposure on the sorption of Zn and Cu to PET MPs. They found that increased UV‐induced aging and weathering were associated with higher sorption. In a similar vein, age and biofouling enhanced Ag adsorption on MPs (Kalčíková et al. 2020). Another factor that affects metal adsorption by MPs is their particle size. The metal has a greater surface area for adsorption because reduced MPs have a higher surface‐area‐to‐volume ratio (F. Wang, Yang, et al. 2019; H.‐T. Wang, Ding, et al. 2019; J. Wang, Liu, et al. 2019). As a result, they may be more ecotoxic and have an unexpected capacity to adsorb metals than larger MP particles. Pb showed the greatest sorption affinity on PS MPs in a batch experiment, followed by Cu, Cd, and Ni. This suggests that the metals' affinities for the same type of polymer differ (Yuan et al. 2020).

Extrinsic factors, such as the pH and salinity of the surrounding aqueous solution, affect the sorption of metals onto MPs (Holmes et al. 2014; F. Wang, Zhang, Zhang, Zhang, and Sun 2020). Holmes et al. (2014) evaluated the PE MPs pellets' capacity to sorb Pb, Ni, Cd, Cr, Cu, and Co, which included mixing river and seawater to create a salinity gradient. With increasing salinity, the sorption of Cr increased, whereas that of Cd, Co, and Ni decreased. Phosphorus generally increased their adsorption. pH and salinity had no discernible effects on Cu. In a similar vein, raising the pH from 3 to 7 further showed enhanced metal adsorption (F. Wang, Yang, et al. 2019; H.‐T. Wang, Ding, et al. 2019). However, in contrast, they also observed this pH effect on Cu (Holmes et al. 2014). The successful adsorption of metal cations on MPs' surfaces at high pH is due to an increase in the number of charged sites. At low pH (< 7), the solution would have a high concentration of H⁺ ions, which could displace and compete with positively charged metal cations, thereby decreasing the number of metal ions adsorbed onto MPs (T. Chen et al. 2015; F. Wang, Zhang, Zhang, Zhang, Adams, and Sun 2020; F. Wang, Zhang, Zhang, Zhang, and Sun 2020). Cu and Pb adsorption in the waters of the Musi River in Indonesia and the adjoining estuary was also strongly affected by a change in pH (Purwiyanto et al. 2020). However, because of the low salinity in these waters, salinity did not significantly affect adsorption in our investigation.

The effects of pH and salinity should therefore be interpreted mechanistically rather than descriptively. Increasing pH usually enhances the adsorption of cationic metals by reducing competition from H⁺ ions and increasing the number of negatively charged surface sites, whereas lower pH may promote metal desorption or reduce adsorption through proton competition. In contrast, anionic metal species may exhibit the opposite behavior because their interactions with MP surfaces depend on positively polarized sites and on metal speciation. Salinity also produces bidirectional effects because Na⁺, Ca²⁺, and Mg²⁺ can compete with target metals for adsorption sites, chloride ions can form soluble metal–chloride complexes, and high ionic strength can compress the electrical double layer and promote MP aggregation. However, in some systems, salinity may enhance adsorption by modifying MP surface charge or promoting cation bridging. These mechanisms explain why pH and salinity effects vary across studies and highlight the need to interpret adsorption results in light of the specific polymer, metal species, and environmental matrix (Holmes et al. 2014; T. Chen et al. 2015; F. Wang, Zhang, Zhang, Zhang, and Sun 2020; Q. Wang, Zhang, Wangjin, et al. 2020).

The presence of DOM and microbial biofilms can strongly modify metal adsorption onto MPs. After entering aquatic environments, MP surfaces may be rapidly colonized by bacteria, algae, fungi, and other microorganisms, forming a biofilm community known as the “plastisphere” (Rochman et al. 2014). During colonization, microorganisms produce extracellular polymeric substances (EPSs), including polysaccharides, proteins, lipids, and nucleic acids, which can coat MP surfaces and introduce additional binding sites. Functional groups within EPS, such as carboxyl, hydroxyl, carbonyl, phosphate, and amino groups, can interact with metal ions through electrostatic attraction, surface complexation, ion exchange, and cation bridging. As a result, biofilm‐coated MPs may exhibit greater metal affinity than clean particles, particularly for cationic metals, because the biofilm increases surface heterogeneity, hydrophilicity, and the abundance of oxygen‐containing functional groups (W. Liu, Zhang, et al. 2021). However, biofilms may also alter adsorption kinetics by creating diffusion barriers, slowing metal transport to the underlying polymer surface, or producing multiple fast and slow binding phases. Therefore, the plastisphere should be considered an active biogeochemical interface rather than a passive coating, because microbial colonization and EPS production can change both the capacity and rate of metal adsorption onto MPs (Godoy et al. 2019).

Overall, metal adsorption onto MPs should be understood as a dynamic, system‐specific process rather than a single universal mechanism. Physical adsorption, electrostatic attraction, surface complexation, ion exchange, cation–π interactions, precipitation, and biofilm‐mediated binding may occur simultaneously or competitively depending on polymer chemistry, degree of aging, particle size, surface charge, environmental pH, salinity, DOM, and metal speciation (Mo et al. 2025; Z. Li et al. 2026; Y. Gao et al. 2026; R. Wang et al. 2026; T. Yang et al. 2026). Nonpolar polymers such as PE and PP generally rely more on physical adsorption and electrostatic interactions after aging, whereas polar, chlorinated, or aromatic polymers such as PET, PVC, PA, and PS may provide stronger sites for complexation, polarity‐driven interactions, or π‐related interactions (Brennecke et al. 2016; F. Gao et al. 2019; Foshtomi et al. 2019; Rochman et al. 2014; F. Wang, Yang, et al. 2019). Aging and weathering increase surface roughness, reduce crystallinity, and introduce oxygen‐containing functional groups, thereby enhancing the affinity of MPs for cationic metals such as Pb, Cu, Cd, Zn, and Ag (Holmes et al. 2012; Mao et al. 2020; Vedolin et al. 2018; Hüffer et al. 2018; F. Wang, Zhang, Zhang, Zhang, Adams, and Sun 2020; Kalčíková et al. 2020; Yuan et al. 2020). Environmental conditions further regulate adsorption: Higher pH often promotes cationic metal adsorption by increasing negatively charged sites and reducing H⁺ competition, whereas salinity may either suppress adsorption through ionic competition and metal–chloride complexation or enhance it in specific polymer–metal systems (Holmes et al. 2014; T. Chen et al. 2015; F. Wang, Yang, et al. 2019; Q. Wang, Zhang, Wangjin, et al. 2020; Purwiyanto et al. 2020; Godoy et al. 2019). DOM and plastisphere biofilms add further complexity because they may either compete with MPs for metal binding or coat MP surfaces with additional functional groups that modify adsorption capacity and kinetics (Rochman et al. 2014; Godoy et al. 2019). Therefore, the apparent adsorption capacity of MPs reflects the combined effects of intrinsic polymer properties and external environmental conditions, which explains why different studies often report contrasting results. Figure 1 displays the mechanisms that control this process.

Physical adsorption, which involves electrostatic interactions, has always been crucial to MPs' capacity to adsorb metals, especially for MPs with basic structures and polymer types. In general, physical adsorption describes the adsorption process in which water molecules merely permeate into or onto the absorbent surface. Neither new material forms nor changes to chemical bonds were observed.

Because the negatively charged surface of the weakly acidic water solution predominated in most interactions between cationic contaminants and MPs, electrostatic attraction was the dominant force. Free ions encircled by MPs would adhere to their surfaces or become lodged in their internal or external pores due to electrostatic attraction. The largest quantity of adsorbed Cu²⁺ (0.911 mg g⁻¹) was found at pH ~ 5 when the Cu²⁺ adsorption by virgin PE in aqueous solution was investigated. This suggests a stronger electrostatic interaction between Cu²⁺ and PE in an acidic environment (F. Wang, Zhang, Zhang, Zhang, and Sun 2020; Y. Wang, Wang, et al. 2020). Shen et al. (2021) examined how the adsorption of Pb²⁺ by three different MPs was affected by surfactants. It was believed that the improved sorption capacity of the MPs was due to their increased hydrophilicity and negative (−ve) charge, as demonstrated by the addition of surfactants. Furthermore, research has demonstrated the significance of electrostatic attraction for the sorption of anionic contaminants such as As³⁺ (Dong et al. 2020; Dong et al. 2019). Dong et al. (2019) calculated the PTFE electrostatic potential both before and after adsorption using computational methods (Gaussian 16 coupled with Multiwfn and Visual Molecular Dynamics). The findings demonstrated that most PTFE surfaces had a negative electrostatic potential (−30 to 0 kcal mol⁻¹), preventing oxygen anions from being attracted to them. Moreover, the most significant element in the interaction and adsorption of As³⁺ was the hydrogen surface atom in the hydroxyl group (+82.37 kcal mol⁻¹), where the largest positive (+ve) potential was concentrated. Afterward, the PS structure was made simpler using Gaussian 16, which also showed that the electrostatic potentials of the H atoms on the carboxyl and hydroxyl groups were +46.96 and +56.60 kcal mol⁻¹, respectively, and that the electrostatic potential of the remaining PS surface was +ve (0–14 kcal mol⁻¹) (Dong et al. 2020). This facilitated sorption, allowing the arsenic anion to reach the PS surface.

2.2. Factors Influencing Adsorption

The contradictory effects of DOM and salinity reported in previous studies should not be viewed as conflicting observations but rather as the result of variation in experimental and environmental conditions. For DOM, the final effect depends strongly on its type, concentration, molecular structure, and interactions with both metal ions and the surfaces of MPs. Humic and fulvic substances may inhibit adsorption through the formation of soluble metal–DOM complexes that reduce the concentration of free metal ions available for MP binding. On the other hand, DOM may favor adsorption by coating the MP surface and adding negatively charged, oxygen‐containing functional groups such as carboxyl and phenolic groups, which increase the affinity of MPs for cations. Therefore, the different effects of DOM on the Cd²⁺ adsorption by different polymers may be due to the differences in polymer surface chemistry, DOM coating behavior, and metal–DOM complexation capacity.

Likewise, the bidirectional effect of salinity on metal adsorption by MPs can be attributed to the balance between several competing mechanisms. Na⁺, Ca²⁺, and Mg²⁺ compete with metal ions for available adsorption sites, and suppression of adsorption may occur with increasing salinity. Cl⁻ can also create soluble metal‐chloride complexes, thus reducing the concentration of free metal ions that are available for adsorption. High ionic strength may also compress the electrical double layer and promote MP aggregation, thus reducing accessible surface area. However, in some polymer–metal systems, salinity can also enhance adsorption by altering the MP surface charge, promoting cation bridging, or enabling the formation of new complexes with higher affinity for certain polymer surfaces. The results show that the adsorption behavior is not only salinity controlled but also the result of a combined effect of metal speciation, polymer type, MP aging degree, ionic composition, pH, DOM characteristics, and experimental design.

Therefore, future research should not directly compare the adsorption capacities unless the key experimental parameters are harmonized, that is, MP polymer type, particle size, degree of aging/weathering, DOM source and concentration, salinity composition, pH, contact time, metal concentration, solid‐to‐liquid ratio, and analytical extraction method. This more mechanistic interpretation helps reconcile divergent findings in the literature and strengthens the present review's role in clarifying academic disputes regarding MP–metal adsorption behavior.

MPs serve as carriers for metals in various aquatic ecosystems. The adsorption process is influenced by several factors (Figure 2), which can be categorized into three critical groups: (a) the nature of the MPs particles, including polymer type, particle density, size, shape, surface‐area, exposure time, and zeta potential; (b) bionetwork conditions like water temperature, salt content, pH, DOM, suspended materials (TSM), and microbial activities; and (c) adsorption performance and mechanisms, like adsorption kinetics and isotherms (Q. Liu et al. 2022; X. Gao, Hassan, et al. 2021).

FIGURE 2.

Summary of factors influencing the adsorption process of metals (M ⁿ⁺) on MPs particles in the aquatic environment.

Recently, in vitro studies were conducted to simulate the possible environmental variables and other factors regulating the adsorption mechanism. The following section will highlight the key related findings. It was widely held that metal sorption is pH dependent, as solution pH affects the adsorbent's accessible sorption sites and the speciation and ionization of metals. It was common for metal sorption onto the MPs to show a notable pH dependence. The quantity of adsorbed cationic contaminants increased as the pH of the solution rose owing to precipitation, higher electrostatic force, and a decrease in competitive H⁺ in the solution. On the other hand, the adsorption capacity of anionic contaminants declined with increasing solution pH (Demiraj et al. 2018; Kumar et al. 2020). It was also often observed that the solution pH had a substantial effect on metal sorption by the MPs. For example, Dong et al. (2019) reported the opposite trend for As³⁺ adsorption, whereas Zou et al. (2020) demonstrated that MPs enhanced the adsorption of Cu²⁺, Pb²⁺, and Cd²⁺ at higher solution pH.

Furthermore, the solution temperature significantly influences adsorption behavior. The adsorption of MIs was shown to be endothermic, indicating that sorption capacity increased with temperature (Ahmad et al. 2014). Also, Tang et al. (2020) investigated the thermodynamics of Pb²⁺ adsorption on aged nylon MPs at various temperatures. According to the data, the enthalpy change (ΔH, kJ mol⁻¹) was positive. The calculated values of the Gibbs free energy (ΔG, kJ mol⁻¹ K⁻¹) were −1.491, −2.528, and −5.205 kJ mol⁻¹ at 15°C, 27°C, and 40°C, respectively. These findings suggested that the sorption process was endothermic and spontaneous. Additionally, Tang et al. (2021) conducted further research on the thermodynamic characteristics of sorption of Ni²⁺, Zn²⁺, and Cu²⁺ onto nylon MPs, yielding similar results. One explanation is the frequent chemisorption between cationic ions and nylon MPs. One explanation is the frequent chemisorption between cationic ions and nylon MPs. At elevated temperatures, more energy was required to transfer contaminants to the MPs' surface (Inyang et al. 2016). Conversely, elevated temperature may adversely affect As³⁺ sorption. Dong et al. (2020, 2019) investigations revealed that all particle sizes showed negative estimated ΔH values for As³⁺ sorption onto PS and PTFE, suggesting that the processes were exothermic and favored at lower temperatures. According to the previously mentioned results, there may or may not be a relationship between solution temperature and MPs' capacity to adsorb metals, depending on the polymer used in the MPs and the pollutants in question. Therefore, it is crucial to examine how solution temperature affects the sorption of different metals by MPs.

The WWTP and surface water exhibit high and widespread salt levels, which should affect MPs' metal adsorption behavior (Guo et al. 2020; Zou et al. 2020). F. Wang, Yang, et al. (2019) examined the effects of varying NaCl concentrations on Cd adsorption by HDPE (0, 1, 10, and 100 mg L⁻¹). The results showed that Cd adsorption was significantly reduced by the addition of NaCl. Adsorbed Cd exchanged with the salt, and the growing competitive impact of coexisting ions on the few adsorption sites may be the main cause of the significantly reduced sorption capacity of Cd. Increasing the NaCl dose did not affect the experiment. The moderate salt content (1.0 mg L⁻¹ of NaCl) was thought to successfully compete for sorption sites, given Cd has a relatively poor sorption capacity on HDPE. An increase in Cl concentration may alter the distribution of metal species in solution, according to the authors' hypothesis in the Tang et al. (2021) work. As the quantity of NaCl increased, fewer bivalent ions were adsorbed onto the MPs, and more Cl‐containing complexes were generated. Additionally, when salty ions coexisted, the MPs' elemental properties and polymer types (PA and PMMA) had distinct effects on Cu²⁺ adsorption (J. Yang et al. 2019). Specifically, PMMA was more amenable to manipulation than PA with respect to Cu²⁺ ion sorption. Mg²⁺ ions exerted a significant inhibitory effect on Cu²⁺ adsorption for both MPs, compared with Ca²⁺. In summary, several studies have shown that MPs' ability to take up certain cationic contaminants (Cd²⁺, Cu²⁺, and Pb²⁺) depends on the NaCl concentration (Tang et al. 2021; Tang et al. 2020). During the sorption process, calcium, magnesium, and sodium ions competed with the metals, particularly the bivalent ions, thereby reducing the activity of the charged pollutants. According to B. Wang et al. (2017), a high concentration of the anion (e.g., Cl⁻) promoted the formation of complexes such as CuCl₃ ⁻ and CdCl₃ ⁻, thereby reducing the sorption capacity of the MPs. Moreover, increased ionic concentration due to salinity was reported to affect how MPs accumulate, as it reduced repulsive forces and compressed the electrical double layer, causing MPs to aggregate, exhibit “stacking effects,” and lose surface area and sorption capacity (Tang et al. 2021).

The effects of metal adsorption onto MPs in water‐soluble solutions and in saltwater were investigated in a recent study (Godoy et al. 2019). Salinity affects MPs' ability to adsorb certain metals, both positively and negatively. The adsorption capacities of Cr₂O₇ ²⁻ and Co²⁺ on PE and PS decreased upon saltwater adsorption; however, the addition of saltwater increased the adsorption capacities of Cr₂O₇ ²⁻ and Cu²⁺ on PP and PVC. The enhanced sorption capabilities might originate from the coupling of cations onto new molecules that promote adsorption onto polymers. Holmes (2013) showed the same results. These results demonstrate that, due to significant differences between ideal water‐soluble solutions and actual seawater, the impact of coexisting salinity on the adsorption behavior of MPs in natural settings remains highly complex and unresolved.

The literature suggests that DOM may affect the adsorption behavior of metals (P. Li et al. 2019; Peng, Sun, Fan, et al. 2021). For instance, Y. Zhou, Yang, et al. (2020) discovered that the adsorbed Cd²⁺ level on the PA, ABS, and PET decreased as the humic acid (HA) concentration climbed, in contrast to Guo et al. (2020), who established that the adsorption of Cd²⁺ by the PP, PVC, PE, and PS amplified with the increase in HA level. Fulvic acids (FAs) and HAs, which are negatively charged, exhibited a robust attraction for cationic contaminants because of their π electrons and FGs (mitogen, phenolic hydroxyl, and carboxyl). As a result, the solution's sorption capacity decreased as cationic pollutants (PA, ABS, and PET) sank into it (Y. Zhou, Yang, et al. 2020). As an alternative, excess FAs and HAs could bind to certain MPs (PVC, PE, PP, and PP) and facilitate electrostatic interactions between cationic pollutants and these MPs (Guo et al. 2020). Moreover, Tang et al. (2021) conducted a systematic study to explore the effects of DOM on the adsorption of bivalent MIs, such as Cu²⁺, Ni²⁺, and Zn²⁺, and found a range of effects on sorption capacities. This paper suggests that FA may bind bivalent metals via several mechanisms and simulates the DOM–metal complex using the Stockholm humic mimic. To form a compound with MPs, FANi⁺ needed to occupy only one carboxyl site, whereas free Ni²⁺ required two carboxyl sites. This indicated that the FA solution contained most of the Ni, which was favorable for sorption onto the MPs. Based on published studies, we have determined that the impact of DOM on the adsorption of metals by metal–MPs is multifaceted and may vary with pollutants, MP types, and DOM characteristics. Yet, few studies have shown that particulate matter can affect MPs' capacity to adsorb metals, both positively and negatively. Shen et al. (2021) investigated the impact of surfactants on Pb²⁺ sorption by PE, PP, and PMMA, noting the inconsistent presence of surfactants in the effluent. Triton X‐100, sodium dodecyl benzenesulfonate, and 1‐hexadecyl pyridinium bromide were among these surfactants. The experiment found that the increased hydrophilicity of the MPs might improve Pb²⁺ sorption capacity when three surfactants coexisted.

Additionally, free chlorine and corrosion inhibitors, which are commonly used compounds in WWTPs and water distribution systems, could influence the interactions between MPs and metals. According to Huang et al. (2020), the condensed concentrations of metals such as Zn²⁺, Cu²⁺, Pb²⁺, and Mn²⁺ on the PE pellets in the presence of free chlorine (sodium hypochlorite) and corrosion inhibitors (sodium dihydrogen phosphate) were explained by ClO⁻‐induced metal ion oxidation and PO₄ ³⁻ metal ion competition, respectively. However, it remains unclear how the co‐occurrence of particulate matter in the natural environment affects MPs' sorption patterns (Horton et al. 2017). For this reason, it is critical to advance this field's understanding in the future.

Furthermore, Zhu et al. (2023) noted that the polymer type in MPs affects their capacity to adsorb pollutants. For example, they found that polyvinyl alcohol (PVA) has a higher affinity for adsorbing Pb and Cd relative to other polymers. They also highlighted that metal concentrations, pH, and the adsorption environment (natural water, wastewater, or sediments) influence this process.

On the other hand, biotic factors, such as microbial biofilms, play a critical role in altering the morphology of MPs' surfaces and positively impacting adsorption by increasing the abundance of carboxyl and ketone groups (W. Liu, Zhang, et al. 2021).

The adsorption mechanisms may act synergistically or competitively depending on environmental conditions. Increasing pH usually enhances the adsorption of cationic metals by reducing H⁺ competition and increasing the number of negatively charged binding sites, whereas the adsorption of anionic metal species may decrease at higher pH. Salinity can inhibit adsorption by promoting competition from Na⁺, Ca²⁺, and Mg²⁺ ions or by forming soluble chloride–metal complexes; however, it may also enhance adsorption in specific polymer–metal systems by modifying surface charge or promoting bridging interactions. DOM has a dual role: It can compete with MPs for metal binding by forming soluble metal–DOM complexes, but it can also coat MP surfaces and introduce additional functional groups that enhance metal adsorption. Biofilms further complicate this behavior by increasing the number of oxygen‐containing functional groups and creating new binding sites. Thus, the apparent adsorption mechanism reflects the balance between electrostatic attraction, surface complexation, ion exchange, cation–π bonding, DOM complexation, salinity‐driven competition, and biofilm‐mediated surface modification.

3. Analytical Techniques

To directly and thoroughly understand the surface and structural changes that occurred both before and after the adsorption process, methods such as X‐ray photoelectron spectroscopy (XPS), X‐ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy‐dispersive X‐ray spectroscopy (EDX), Raman spectra, and Fourier transform infrared spectroscopy (FTIR) are frequently employed (Figure 3). Using these methods, sorption processes could be accurately determined from different perspectives (Peng, Sun, Hanif, et al. 2021; Y. Zhou et al. 2021). Numerous studies have confirmed that selective sorption (specifically surface complexation and cation–π bonding) dominates metal adsorption on MPs, alongside physical adsorption. The C=O band proportions on MPs (excluding PVC) and the C=C/C–C band proportions on PA and ABS increased after Cd²⁺ sorption. This indicates that oxygen FGs (C=O and C–O groups) and the cation–π interaction were involved in the sorption of Cd²⁺. An examination of the FTIR and XPS C 1s spectra of PA, PVC, PS, ABS, and PET provided evidence for this finding (Y. Zhou, Yang, et al. 2020). In the FTIR examination of nylon MPs, the IR adsorption peak of the C–O bond was observed to shift from 1369 to 1339 cm⁻¹, suggesting that Pb²⁺ interacted with the carboxylate anion (–COO–) and that the primary mechanism for Pb²⁺ adsorption was surface complexation (Tang et al. 2020). In related research, the chemical characteristics of PVC, PE, and chlorinated PE (CPE) were examined using FTIR and XPS methods (Zou et al. 2020). The findings demonstrated that once Pb²⁺, Cu²⁺, and Cd²⁺ were adsorbed, there was a discernible change in the C=O/C–O bonds (for all MPs) and C–Cl bonds (particularly for PVC and CPE). This suggests that the sorption process may be influenced by the electronegative chloride groups on CPE and PVC, as well as by interactions between metals and oxygen‐containing FGs on MPs (W. Liu et al. 2020).

FIGURE 3.

Analytical techniques for the detection of heavy metal adsorption on microplastics.

Polar contacts and van der Waals forces, among other nonspecific forces, sometimes drive the adsorption of MPs onto metals. Guo et al. (2020) showed that the main mechanism of Cd²⁺ adsorption on PP and PE is the van der Waals force because of nonspecific FGs, whereas polar interactions produced by the benzene structure were exploited in the adsorption process on PS.

FTIR is particularly useful for identifying the involvement of functional groups such as –OH, –COOH, C=O, C–O, and C–Cl and therefore provides evidence for surface complexation, hydrogen bonding, and functional group–mediated adsorption. XPS provides complementary information on surface elemental composition, chemical states, and binding‐energy shifts, making it suitable for confirming metal–oxygen interactions, changes in C=O/C–O bonds, and the participation of chloride‐containing groups in PVC and chlorinated polymers. SEM–EDX can reveal surface roughness, aging‐related morphological changes, and the spatial distribution of adsorbed metals, while XRD can help identify crystalline precipitates formed through surface precipitation. Raman spectroscopy can provide information on changes in the polymer backbone and π‐related interactions, particularly for aromatic polymers such as PS. Zeta potential analysis is essential for evaluating electrostatic attraction or repulsion under different pH, salinity, DOM, and surfactant conditions.

Therefore, each adsorption mechanism requires a specific combination of analytical evidence. Electrostatic attraction should be supported by changes in zeta potential, together with pH‐dependent adsorption behavior. Surface complexation should be verified by FTIR peak shifts and XPS binding‐energy changes. Cation–π or π–π interactions require evidence from aromatic polymer structures, Raman spectra, and changes in XPS and FTIR after adsorption. Ion exchange can be inferred from changes in exchangeable ions in solution and surface elemental composition. Surface precipitation should be confirmed using SEM–EDX mapping and XRD identification of newly formed phases. Biofilm‐mediated adsorption should be supported by microscopic observation, FTIR and XPS evidence of biofilm‐related functional groups, and changes in oxygen‐containing groups such as carboxyl and ketone groups.

This controversial result in adsorption mechanism studies may arise from differences between ex situ and in situ characterization. Ex situ methods such as dried FTIR, XPS, SEM–EDX, and XRD provide strong evidence of stable surface interactions after adsorption, but washing, drying, and vacuum conditions may remove weakly bound ions or alter hydrated complexes. In situ or near‐in situ approaches, including ATR‐FTIR in aqueous media, zeta potential measurements at environmentally relevant pH and salinity, time‐resolved adsorption experiments, and metal speciation analysis, are better suited to resolving dynamic mechanisms such as reversible electrostatic attraction, DOM‐mediated complexation, salinity‐driven competition, and pH‐dependent adsorption. Combining in situ and ex situ characterization is therefore necessary to distinguish true surface complexation from temporary physical adsorption or solution‐phase complexation.

4. Toxicological Implications

The way MPs and metals interact can alter MPs' hazard and bioaccumulation (Figure 4). For instance, when MPs and/or metals combine, the negative effects on organisms may be amplified, depending on dose or size (W. S. Lee et al. 2019; F. Wang, Zhang, Zhang, Zhang, Adams, and Sun 2020). Although exposure to 1‐mg L⁻¹ PS nanoplastics or 1‐μg L⁻¹ Ag⁺ individually did not induce toxicity in Daphnia magna , combined exposure produced significant toxic effects (Monikh et al. 2020). Animal growth suppression is another example; yellow seahorses and earthworms with larger Cd accumulations have been shown to exhibit this (Banaee et al. 2019; Lu et al. 2018; Sun et al. 2019; F. Wang, Zhang, Zhang, Zhang, Adams, and Sun 2020; Yan et al. 2020; Y. Zhou, Liu, and Wang 2020). This could be explained by the inflammatory responses and oxidative damage induced by the coexistence of these two pollutants (W. S. Lee et al. 2019; Lu et al. 2018; Wen et al. 2018). It is worth noting that several studies have also shown the opposite effect: Interactions between chelating MPs and Cd may protect organisms. For instance, it was found that PVC reduced Cd toxicity to worms; this effect may have been due to the strong chelating activity of PVC and its polymers (Wakkaf et al. 2020). Additionally, the presence of MPs decreased the amounts of Hg and Cd that Dicentrarchus labrax and Symphysodon aequifasciatus accumulated, as well as the Cd contents in Triticum aestivum L. leaves (Barboza et al. 2018; Lian et al. 2020; Wen et al. 2018). Additionally, the presence of MPs facilitated the removal of Hg from the mussel Mytilus galloprovincialis (Fernández et al. 2020). Furthermore, MPs did not suppress algal growth following simultaneous exposure to Cu, nor did they increase Cd bioconcentrations in plant tissues (Bellingeri et al. 2019; F. Wang, Zhang, Zhang, Zhang, Adams, and Sun 2020; F. Wang, Zhang, Zhang, Zhang, and Sun 2020). Furthermore, findings demonstrated that the presence of species such as fish also affected the quantity of Hg in solution and the mechanisms by which MPs and metals interacted (Barboza et al. 2018). These changes would impact the noxiousness and bioaccumulation of metals in organisms. On the other hand, MPs can be fragmented and transferred from one trophic level to a higher one in the food chain, ultimately affecting human health significantly (X. Gao, Hassan, et al. 2021). Q. Chen et al. (2023) reviewed the toxic effects of metals adsorbed on MPs in molecular, cellular, and individual levels of fish.

FIGURE 4.

Toxicological implications of heavy metal–laden microplastics.

The toxicological outcome of combined exposure to MPs and heavy metals is not universal but is governed by interactions among physicochemical, environmental, and biological factors. One of the most important drivers is the exposure concentration and the ratio of MPs to dissolved or adsorbed metals. At low metal concentrations, MPs may reduce the fraction of metal dissolved in solution through adsorption, thereby decreasing immediate metal bioavailability. However, at higher concentrations, or when the MP surface becomes saturated with metals, metal‐loaded MPs may increase the total contaminant dose available for ingestion and internal exposure. Therefore, the same MP–metal system may show either antagonistic or synergistic effects depending on the concentration range and the balance between adsorption in the external medium and desorption inside organisms (Monikh et al. 2020; W. Liu, Zhang, et al. 2021; X. Gao, Hassan, et al. 2021).

MP particle size is another critical factor. Smaller MPs and nanoplastics usually have a larger surface‐area‐to‐volume ratio, higher adsorption capacity, and greater probability of ingestion by aquatic organisms. They may also cross biological barriers more easily than larger particles and can be retained for longer periods in tissues or digestive compartments. Consequently, fine MPs and nanoplastics are more likely to serve as efficient carriers of heavy metals than larger particles, especially when aged, oxidized, or biofilm coated. Aging and weathering increase surface roughness and introduce oxygen‐containing functional groups such as –OH, –COOH, and C=O, which can enhance metal adsorption and modify the toxicity of the combined pollutants (W. S. Lee et al. 2019; F. Wang, Zhang, Zhang, Zhang, and Sun 2020; W. Liu, Zhang et al. 2021; Monikh et al. 2020).

The type and chemical species of the heavy metal also strongly determine the toxicological response. Metals such as Cd, Pb, Hg, Cu, and Ag differ in adsorption affinity, redox behavior, biological essentiality, and intrinsic toxicity. For example, nonessential metals such as Cd, Pb, and Hg may induce toxic effects even at relatively low internal doses, whereas essential metals such as Cu, and Zn may become toxic mainly when their concentrations exceed the capacity of physiological regulation. Metal speciation is also important because free ions, chloride complexes, organic complexes, and particle‐bound forms differ in mobility, desorption potential, and biological availability. Therefore, the toxicity of metal‐loaded MPs should be interpreted according to both the metal type and its environmental speciation (Barboza et al. 2018; Wen et al. 2018; Monikh et al. 2020; Yan et al. 2020).

The biological characteristics of the exposed organism represent another key determinant. Filter feeders, deposit feeders, and planktonic organisms are generally more exposed to suspended or sediment‐associated MPs than organisms with selective feeding behavior. Species‐specific differences in ingestion rate, gut residence time, digestive pH, enzyme activity, detoxification capacity, and life stage can explain why similar MP–metal combinations produce different toxicological responses among algae, crustaceans, mollusks, fish, plants, and terrestrial organisms. Early life stages are usually more sensitive due to their rapid development, limited detoxification capacity, and greater vulnerability to oxidative stress and growth inhibition (W. S. Lee et al. 2019; Wen et al. 2018; Lian et al. 2020; Yan et al. 2020; Q. Chen et al. 2023).

Exposure duration should also be considered when interpreting synergistic or antagonistic effects. Short‐term exposure may primarily reflect acute oxidative stress, membrane damage, or feeding inhibition, whereas long‐term exposure may lead to chronic bioaccumulation, growth suppression, reproductive impairment, immune disturbance, and trophic transfer. In chronic exposure scenarios, even moderate metal adsorption onto MPs may become environmentally relevant if particles are continuously ingested and retained by organisms (Wen et al. 2018; F. Wang, Zhang, Zhang, Zhang, Adams, and Sun 2020; Yan et al. 2020; X. Gao, Hassan, et al. 2021).

Environmental conditions further regulate the toxicological behavior of MP–metal mixtures. pH, salinity, DOM, temperature, and microbial biofilms can alter metal adsorption, desorption, and speciation. Low‐pH conditions, such as those in some digestive systems, may promote metal desorption from ingested MPs and increase internal metal exposure. Salinity may either reduce metal adsorption by competing with Na⁺, Ca²⁺, and Mg²⁺ ions or alter metal speciation through chloride complex formation. DOM may reduce toxicity by complexing free metals in solution, but it may also coat MP surfaces, increasing their affinity for cationic metals. Biofilms can increase the number of binding sites on MP surfaces and may also enhance ingestion by making particles more similar to natural food items (Monikh et al. 2020; F. Wang, Zhang, Zhang, Zhang, and Sun 2020; W. Liu, Zhang, et al. 2021).

Accordingly, the “Trojan horse effect” should not be considered an automatic consequence of metal adsorption onto MPs. It is more likely to occur when several conditions are fulfilled simultaneously: (i) The MP surface has a sufficient metal load under environmental conditions; (ii) the particle size is small enough to be ingested and retained by the organism; (iii) the organism is exposed for a sufficient duration to allow internal transfer; (iv) digestive conditions promote desorption of the adsorbed metal; and (v) the released metal exceeds the organism‐specific biological tolerance threshold. In contrast, if the MP strongly retains the metal and desorption under gut conditions is limited, or if adsorption reduces the dissolved metal fraction in the surrounding medium, the combined exposure may show an antagonistic or protective effect, as reported for some MP–Cd, MP–Hg and MP–Cu systems (Wakkaf et al. 2020; Barboza et al. 2018; Fernández et al. 2020; Bellingeri et al. 2019; F. Wang, Zhang, Zhang, Zhang, Adams, and Sun 2020).

Therefore, synergistic and antagonistic toxic effects of MP–metal mixtures should be interpreted as the result of a dynamic balance among adsorption capacity, metal speciation, particle ingestion, internal desorption, exposure duration, and organism sensitivity. This framework helps explain the apparently contradictory results reported in the literature and provides a more useful basis for environmental risk assessment of heavy metal–laden MPs (W. Liu, Zhang, et al. 2021; X. Gao, Hassan, et al. 2021; Q. Chen et al. 2023).

5. Remediation Strategies

MP control policies should prioritize prevention, followed by the 7Rs approach (reduce, reuse, recycle, refuse, rethink, regift, and recover), and end with MPS dumping. Two main strategies can control aquatic pollution with MPs and their adsorbed pollutants: the preventive and curative strategies (Figure 5). The infrastructure, economic climate, treatment costs, kinds of MPs released into the environment, scope of application, and public willingness to transition from a plastic‐dependent to a nonplastic economy are all important considerations for choosing an appropriate solution (Osman et al. 2023).

FIGURE 5.

Summary of preventive and curative measures for controlling and remediation of MPs and associated metals.

The preventive approach can be divided into short‐term practical actions and long‐term policy strategies. Short‐term approaches aim to rapidly reduce MP inputs into aquatic environments through measures such as reducing single‐use plastics, improving waste collection and source separation, strengthening public awareness, controlling plastic leakage from wastewater and stormwater systems, and applying the 7Rs approach: reduce, reuse, recycle, refuse, rethink, regift, and recover. Long‐term policy strategies target structural reduction of plastic pollution through extended producer responsibility, eco‐design of plastic products, restrictions on unnecessary plastic use, development of biodegradable or safer alternatives, circular‐economy systems, improved recycling infrastructure, and continuous regulatory monitoring. These preventive measures complement curative strategies, which are based on physical, chemical, or biological treatment technologies to remove MPs and associated pollutants (Osman et al. 2023). The efficiency of these treatment technologies depends on MP type, size, shape, concentration, wastewater flow rate, and environmental conditions such as pH and salinity (Figure 5).

Table 2 shows a clearer comparison among physical separation, coagulation/flocculation, electrocoagulation/electroflotation, membrane filtration, oxidation‐based treatments, and biological/nature‐based systems.

TABLE 2.

Comparative summary of remediation technologies for microplastics and associated heavy metals (Ayoub et al. 2001; Garcia‐Segura et al. 2017; Perren et al. 2018; Ma et al. 2019; Poerio et al. 2019; S. Liu, Shi, et al. 2021; W. Liu, Zhang, et al. 2021).

Technology	Reported removal efficiency	Pollutant type targeted	Advantages	Limitations	Relative cost
Primary settling, skimming, and gravity separation	4%–99% decrease in MP abundance reported for settling, depending on particle size, density, and wastewater conditions.	Mainly large MPs, floating plastics, fibers, and MP aggregates.	Simple, widely available, low energy demand, and suitable as a first treatment step.	Limited removal of small MPs and nanoplastics; does not remove dissolved metals; retained sludge may contain metal‐loaded MPs.	Low
Coagulation/flocculation using Al‐, Fe‐, or Mg‐based coagulants	High for some MPs; liquid bittern removed As and Cd by almost 90% in wastewater treatment applications.	MPs, MP aggregates, suspended particles, and some dissolved/adsorbed metals.	Aggregates MPs and immobilizes metals through floc formation, adsorption, and coprecipitation; applicable to municipal and industrial wastewater.	Chemical consumption; residual coagulants; sludge enriched with MPs and metals requires safe management.	Low–medium
Electrocoagulation/electroflotation	High but system dependent; exact efficiency varies with electrode type, current density, conductivity, pH, and MP properties.	MPs, suspended solids, and some metals.	Produces coagulants in situ; reduces need for added chemical reagents; can remove MPs and metals simultaneously.	Energy demand; electrode consumption/passivation; operational optimization required; sludge still requires disposal.	Medium
Membrane filtration, ultrafiltration, nanofiltration, and reverse osmosis	Generally high for MPs; metal removal depends on membrane pore size, charge, and configuration.	Small MPs, nanoplastics, dissolved metals, and final polishing of treated wastewater.	High separation efficiency; suitable for advanced wastewater treatment, desalination, and industrial effluents.	Membrane fouling; high operational cost; concentrate/backwash streams may contain MPs and metals.	High
Ozonation, ultrasound, and advanced oxidation processes	Variable; can modify MP surfaces and reduce some associated pollutants but may require downstream separation.	MPs, oxidizable organic pollutants, and some MP‐associated contaminants.	Can degrade or alter pollutants and reduce microbial load; useful as an advanced treatment step.	May fragment MPs; may release adsorbed metals; should be combined with coagulation, adsorption, or membrane separation.	Medium–high
Biological degradation and microbial treatment	Usually slow and polymer specific; quantitative efficiencies are not consistently reported.	Biodegradable MPs, biofilm‐associated MPs, and some metals through biosorption.	Environmentally friendly; potential for complete degradation of selected polymers; low energy demand.	Slow kinetics; limited polymer range; performance depends on microbial community and environmental conditions.	Low–medium
Constructed wetlands and nature‐based systems	Moderate and site specific; quantitative efficiency depends on hydraulic retention time, substrate, vegetation, and influent quality.	MPs, suspended particles, nutrient‐rich waters, and MP‐associated metals.	Sustainable, low cost, suitable for agricultural runoff, drainage water, and coastal wetland applications; supports biosorption and sedimentation.	Requires large area; slower than advanced technologies; sediments/biomass may accumulate metals and need safe handling.	Low
Biochar, algae, and microalgal EPS‐based systems	Variable; can immobilize metals and trap MPs, but efficiency depends on biomass/biochar properties and water chemistry.	MP‐metal combined pollution, especially in runoff and drainage systems.	Low‐cost adsorptive and biological options; can promote aggregation, biosorption, and precipitation of metals.	Regeneration and disposal of spent biomass/biochar required; site‐specific performance; possible secondary waste.	Low–medium
Nanoparticle/nanomaterial‐based remediation	Potentially high at laboratory scale, but field‐scale efficiency and safety remain uncertain.	MPs, dissolved metals, and mixed contaminants.	High surface area and tunable surface chemistry; useful for targeted remediation.	Cost of nanomaterials; recovery difficulty; possible ecotoxicological concerns.	High

The bulk of lightweight fibers is removed using the typical primary settling process, which relies on gravity separation. When the fibers stick to the flocs and float together, they form scum‐like accumulations that may be flicked off the surface (K. E. Lee et al. 2012). As demonstrated by W. Liu, Zhang, et al. (2021), efficient settling removes MPs, resulting in a 4%–99% drop in MP abundance from 1.26 to 0.22 particles L⁻¹.

PE MPs were successfully removed by coagulation using aluminum‐ and iron‐based coagulants (Ma et al. 2019). Less expensive options include electrochemical techniques such as electroflotation and electrocoagulation, which do not depend on chemical reagents or microbes (Perren et al. 2018). When added to wastewater, liquid bittern (LB), a saltwater, serves as an affordable magnesium‐based coagulant that can remove As and Cd (almost 90%) (Ayoub et al. 2001). Another technique is electrocoagulation, which uses sacrificial electrodes to produce MIs that act as coagulants (Garcia‐Segura et al. 2017). On the other hand, membrane fouling can develop in successive membrane filtration, particularly for tiny PE particles, following coagulation (Ma et al. 2019). Drinking water treatment might benefit from the MPs' removal properties observed during coagulation and ultrafiltration processes. In industrial and municipal water treatment systems, reverse osmosis (pore size > 2 nm) is a technology that pushes water through a semipermeable membrane to remove pollutants, metals, and other impurities (Poerio et al. 2019). Due to the membrane's narrow pore size, PE particles were completely removed during ultrafiltration, resulting in relatively little membrane fouling (Ma et al. 2019). Flocculants can successfully cause MPs and flocs to aggregate when their charges are opposite to those of MPs (Larue et al. 2003). Synthetic silica‐alumina oxide with a mesoporous structure (97% SiO₂, 3% Al₂O₃) produced by the high‐temperature method is used to develop sol–gel‐induced agglomeration techniques. This adsorbent is particularly effective at removing metals and polymers from wastewater (Wawrzkiewicz et al. 2017).

Ozone technology was found to effectively eliminate 89.9% of MPs in the research by Hidayaturrahman and Lee (2019), compared with removal rates of quick sand filters (73.8%) and membrane disc filters (79.4%). Ozone nanobubbles are more effective in adsorbing metals (especially Cr) when paired with ultrasound than when used alone (Batagoda et al. 2019). Furthermore, Kyllönen et al. (2004) established that ozonation and ultrasound may be used to remove Pb and Zn from soil. When impurities, mainly copper oxide (CuO), in bricks were removed by ultrasonic sound (Newman et al. 1997). According to research by Tripathi and Tripathi (2011), harmful metals were effectively removed with a 55%–75% efficiency rate using ozonation (10‐mg L⁻¹ dose for 5 min) and biofiltration using Eichornia crassipes and Lemna minor with a 262‐mL min⁻¹ flow rate for 48 h at a plant density of 30 mg⁻¹.

The average MPs abundance in conventional wastewater treatment plants (WWTPs) for the influent, primary, secondary, and tertiary treatment stages was reported to be 1–31,400, 0.2–12,580, 0.002–7863, and 0.003–447 items perliter of wastewater, respectively (Azizi et al. 2022). Yet, it has been shown to increase its effectiveness when cutting‐edge methods such as backwashing (Rocher et al. 2012), membrane filtration (Baker 2024), A₂O bioreactor system Jeong et al. (2016), membrane bioreactor (Talvitie et al. 2017), and UV‐oxidation and chlorination (Kelkar et al. 2019) are combined with traditional WWTPs. Mahvi et al. (2005) extracted lead from wastewater using tea trash, a naturally occurring adsorbent. MPs are more difficult to remove from WWTPs than metals because of their diverse physical and chemical properties (e.g., form, density, size, and zeta potential).

In effluents, naturally occurring bacteria and fungi degrade synthetic polymers. Several microorganisms consume plastic waste (Ahmed et al. 2018). Feeding bivalves using a filter PE and PA polymers are degraded by Scapharca cornea (Ibrahim et al. 2016); Aneurinibacillus and Brevibacillus species show promise in the biodegradation of PE and PP (Skariyachan et al. 2018), and Ideonella sakaiensis has been demonstrated to degrade PET film MPs (Yoshida et al. 2016). According to Y. Yang et al. (2014), Pseudomonas, Bacillus, and Actinomycetes, which are members of the Proteobacteria, Firmicutes, and Actinobacteria, respectively, are the most common bacteria found in anaerobic digesters. During methane oxidation in landfills, Methanomicrobia, Methylococcus sp., Methylocystis sp., and Methylocella sp. enhanced MP polymer breakdown (Muenmee et al. 2016). Cunha et al. (2020) found that dangerous flocculants that conventional wastewater treatment methods cannot eliminate can be replaced with microalgal‐based biopolymers, such as EPSs. Sargassum horneri (X. Wang et al. 2021) is well known for removing Sr, Co, and Mn, among other metals, from seawater. M. galloprovincialis can also be treated with an environmentally friendly cellulose‐based nanosponge (CNS) technology (Liberatori et al. 2020), using zeolite carriers until the sulfate‐reducing bacteria (SRB) became immobile (Kim et al. 2015). An environmentally friendly remediation method that eliminates 95% and 93.8% of As, respectively, using biochar derived from Phoenix dactylifera and Delonix regia seeds (Pal et al. 2021). Another ecologically acceptable remediation strategy is the wetland treatment method, but it is only 50% as effective as other cutting‐edge treatment methods (Crites et al. 2010; W. Li et al. 2020; Sarkar et al. 2021).

However, because they outperform traditional approaches, nanoparticles (NPs) are currently used to eliminate MPs and associated pollutants (Ouda et al. 2023). The production of NP composites, such as magPOM‐SILP, involves the combination of imidazole‐zeolite framework material (Fe‐ZIF‐8) with polyoxometalate ionic liquid material (POM‐IL). MPs, organic and inorganic compounds, and bacteria are only a few of the pollutants that Misra et al. (2020) can remove from water. In contrast to pure forms, which demonstrated incredibly low removal efficiencies of 10–50 mol% for Pb²⁺, Ni²⁺, Co²⁺, and MnO₄, the magPOM‐SILP achieved 75–99 mol% metal removal efficiency (Misra et al. 2020).

Numerous investigations are underway on the adsorption properties of chitosan and its byproducts (Abu‐Saied et al. 2017). To effectively remove metals, chitosan‐based adsorbents were developed, with a focus on their microsphere structure. Naskar et al. (2019) comprise alginate‐immobilized chitosan (Kuczajowska‐Zadrożna et al. 2020), chitosan‐based nanofibers, such as PE oxide nanofibers treated with pentetic acid (Surgutskaia et al. 2020), chitosan matrix embedded with magnetite/maghemite (Peralta et al. 2019), chitosan‐g‐poly (acrylic acid) matrices hydrogel composites (F. H. A. Rodrigues et al. 2019), grafted carboxylated chitosan with methacrylic acid and glycidyl methacrylate (Dev et al. 2020), chitosan NPs functionalized with salicylaldehyde (Hussein et al. 2012), and N‐aminorhodanine‐modified chitosan hydrogel (Zidan et al. 2020).

Due to their increased surface area and resultant affinity for certain MIs, carbon nanotube (NT) adsorbents (CNTs) are also acknowledged to eliminate metals such as Cu²⁺ > Pb²⁺ > Co²⁺ > Mn²⁺ (Stafiej and Pyrzynska 2007; Vuković et al. 2010). Numerous studies have demonstrated that metals can be effectively removed from water by combining inorganic NPs such as metal oxides (MeO) (Mohamadiun et al. 2018), mesoporous silica (Sheet et al. 2014), and quantum dots (Jaiswal et al. 2012) with various organic nanomaterials such as nanofibers (Rodríguez et al. 2020) and NTs (Jlassi et al. 2021). In addition, bubble barriers are tools that separate floating and nonfloating particles by forming a vertical curtain of air bubbles in a collecting device. Furthermore, superhydrophobic materials have demonstrated remarkable effectiveness in removing MPs, metals, and other contaminants from water. Zirconium phosphate (ZrP) nanoplate modified with octadecylamine and acrylic resin and TiO₂ containing SiO₂, Fe₃O₄, and dodecylamine (DDA) are examples of these materials (Rius‐Ayra et al. 2023).

Electrokinetic‐assisted filtration is an advanced alternative to address fouling issues in microfiltration. It prevents MPs from passing through a filter by combining physical filtration with electrokinetic help (M. Lee et al. 2023). No matter the types, sizes, concentrations, or chemical makeup of the MPs, consistent filtering performance is achieved because it operates on the electrokinetic principle of ion mobility. A more topical technique, known as “Constructed Wetland Microbial Fuel Cells (CW‐MFCs),” employs a microbial fuel cell (MFC) within a constructed wetland (CW) to cleanly and sustainably remove metals associated with MPs from wastewater (Kesarwani et al. 2022).

It remains unclear how effective traditional procedures are; new, cutting‐edge approaches must be developed to remove metal‐enriched MPs as efficiently as possible. By implementing cutting‐edge technologies, the serious threat posed by metal‐enriched MPs can be reduced, safeguarding the marine ecosystem and mitigating secondary pollution while leveraging worldwide technological breakthroughs.

Generally, applying a single conventional method for removing MPs is inefficient, but using a combined technology with a simple, low‐cost approach and a high MP removal rate is recommended. Table 3 shows the processing techniques used, their advantages and disadvantages, and potential applications in the MS region.

TABLE 3.

Processing techniques, their advantages, disadvantages, scope of application, practical engineering applications, and potential applications in the Mediterranean region.

Remediation technology	Advantages	Disadvantages	Applicable scope	Practical engineering applications	Mediterranean region applications
Biological degradation	Environmentally friendly—Potential for complete degradation	Slow process—Limited to certain types of MPs	Natural waters—Landfill sites	Use of bacteria and fungi to degrade MPs	Research on local microbial species for biodegradation in Mediterranean
Wetland treatment	Natural and sustainable—Supports biodiversity	Lower efficiency compared with advanced methods—Space requirements	Agricultural runoff—Urban wastewater	Constructed wetlands for municipal wastewater	Used in rural Mediterranean areas for sustainable wastewater management
Physical methods	Simple and cost‐effective—Effective for larger MPs	Limited for smaller MPs—May require further treatment	Wastewater treatment plants (WWTPs)—Surface water bodies	Gravity settling in WWTPs—Skimming of floating debris	Used in coastal areas to reduce visible MP pollution
Membrane filtration	High removal efficiency—Can target smaller MPs	High operational costs—Membrane fouling issues	Advanced wastewater treatment	Reverse osmosis and ultrafiltration systems	Increasingly adopted in Mediterranean desalination plants
Chemical coagulation	High efficiency for certain MPs—Can remove associated heavy metals	Chemical use may have environmental impacts—Potential for residual chemicals	Industrial effluents—Municipal wastewater	Coagulation with aluminum or iron‐based coagulants	Applied in Mediterranean industries for treating municipal sewage
Ozonation	High removal efficiency for MPs and pollutants—Reduces microbial load	Ozone generation can be costly—Requires careful handling	Wastewater treatment—Soil remediation	Ozone treatment systems in WWTPs	Pilot studies in Mediterranean regions for treating contaminated sites
Electrochemical techniques	Cost‐effective—No need for chemical reagents	Potential for membrane fouling—Limited to certain types of MPs	Municipal and industrial wastewater	Electrocoagulation and electroflotation processes	Emerging technology in Mediterranean urban water treatment
Nanoparticle technologies	High efficiency for a wide range of pollutants—Versatile applications	Cost of nanoparticles—Potential environmental impact	Targeted pollution remediation—Laboratory settings	Use of tailored nanoparticles for water treatment	Innovative research projects in Mediterranean universities
Bubble barriers	Simple to implement—Effective for floating MPs	Limited to surface MPs—May require additional systems for complete removal	Surface waters—Rivers and lakes	Bubble curtains in coastal cleanup operations	Eco‐friendly initiatives along Mediterranean coastlines

Remediation of MPs–heavy metal combined pollution requires a different strategy from treating MPs or dissolved metals alone. In this case, MPs act not only as particulate pollutants but also as mobile carriers of adsorbed metals. Therefore, an effective technology should ideally remove both the MP particle and the associated metal load while preventing metal desorption during treatment. If only MPs are separated while adsorbed metals are released into the water phase, the treatment may transfer pollution from a particulate to a dissolved, more bioavailable form. Similarly, if dissolved metals are removed but MP particles remain, the remaining MPs may continue to adsorb and transport metals in aquatic systems.

Physical separation methods, including settling, skimming, flotation, and filtration, are useful for removing large MPs or MP aggregates, particularly in WWTPs and surface‐water pretreatment. However, these methods mainly target the particle phase and may not remove dissolved metals released from MP surfaces. Therefore, physical separation should be combined with coagulation, adsorption, membrane filtration, or chemical stabilization when metal‐loaded MPs are present. The separated sludge or retained particles should also be treated as metal‐bearing waste to prevent secondary pollution.

Coagulation, flocculation, and electrocoagulation are more suitable for MP–metal combined pollution because they can aggregate MPs and simultaneously immobilize some metals through adsorption, coprecipitation, or incorporation into flocs. Aluminum‐, iron‐, and magnesium‐based coagulants can enhance the removal of PE MPs and some metals such as As and Cd. However, the generated sludge may contain concentrated MPs and heavy metals, and improper disposal may cause secondary contamination. Therefore, sludge dewatering, stabilization, and safe disposal should be considered essential parts of the treatment process.

Membrane technologies, including ultrafiltration, nanofiltration, and reverse osmosis, can efficiently retain MPs and, depending on membrane pore size and surface chemistry, may also remove dissolved metals. These technologies are particularly appropriate for advanced wastewater treatment, desalination, and final polishing of industrial effluents. Their main limitations are membrane fouling, high operational cost, and the production of concentrate or backwash streams enriched with MPs and metals. These streams require further treatment to avoid re‐release of adsorbed metals.

Ozonation, ultrasound, and advanced oxidation processes can alter MP surfaces and reduce some associated pollutants, but they should be applied cautiously. Oxidation may increase the number of oxygen‐containing functional groups and enhance subsequent metal binding, but it may also fragment MPs or release adsorbed metals into solution. Therefore, oxidation‐based treatment should preferably be integrated with downstream coagulation, adsorption, or membrane separation to capture both oxidized MPs and desorbed metals.

Biological and nature‐based systems, including CWs, microalgal EPSs, biochar, algae, and CW‐MFCs, may be useful for low‐cost and sustainable treatment of MP–metal combined pollution in agricultural runoff, coastal wetlands, and Mediterranean drainage systems. These systems can trap MPs, promote biofilm‐mediated aggregation, and immobilize metals through biosorption or precipitation. However, their efficiency is slower and more site specific than advanced technologies, and harvested biomass or sediments may become enriched with metals and must be managed safely.

The selection of an MP treatment method in the Mediterranean region should consider economic factors, local environmental conditions, and the readiness of local communities to adopt effective technologies (Table 3). Oxidation may increase the number of oxygen‐containing functional groups and enhance subsequent metal binding, but it may also fragment MPs or release adsorbed metals into solution. Combining several techniques often yields the best results, particularly in areas with high levels of plastic pollution.

6. MP Pollution: The MS

The busiest and most significant maritime routes in the biosphere and a global hotspot for biodiversity is the MS (Boucher and Billard 2020). Its high concentrations of industry and cities along the coast and near rivers render it vulnerable to the buildup of substantial volumes of marine litter (Cózar et al. 2015; Llorca et al. 2020; Boucher and Billard 2020; Papadimitriu and Allinson 2022). The MS is the sixth‐largest region where marine debris accumulates (Cózar et al. 2014; Gesamp 2016). The most heavily affected regions include the Gulf of Lion, the NW Adriatic Sea, the Catalan Sea, and the Cilician Sea. The largest amounts of plastic were found near the most populous and urbanized areas. Additionally, local areas with high concentrations of plastics have been identified. Among these relatively small areas close to the contaminants with restricted water circulation are the Buna‐Bojana plume in the Adriatic, the Saronic Gulf in the Aegean Sea, the Gulf of Arzew and the Bay of Malaga in the Alboran Sea, the Gulf of Naples in the Tyrrhenian Sea, the Gulf of Tunis, and the Abu Qir Bay (Liubartseva et al. 2018).

This buildup between 1 × 10³ and 3 × 10³ tons is also a result of the basin's hydrodynamics (Cózar et al. 2015). To be more precise, there is limited subsurface seepage of somewhat salty, cold Mediterranean water, and a surface introduction of fresh, warm Atlantic water with substantial plastic inputs into the MS (Béranger et al. 2010). This hydrodynamic model, according to Cózar et al. (2015), identifies the MS as a sink for floating plastic debris from the Atlantic and recommends that part of the plastic effluent may originate from sources beyond the basin. Along with other global‐scale concerns like ocean acidification and global warming, the effects of excessive human activity have led to significant habitat degradation and a concerning rise in pollution. Because of their high concentration (among the highest in the ecosphere) at the surface and their well‐documented occurrence at all depths across almost the whole basin, the scientific community is now very concerned about plastics and MPs among the numerous pollutants (Suaria et al. 2016).

It is also critical to remember that a substantial body of research suggests that plastic may be found in the stomachs of a wide variety of species across various marine domains and trophic levels, demonstrating that marine life is not immune to plastic pollution. Hydrodynamic analyses of the basin indicate that some of the floating plastic in the MS may have originated in the Atlantic Ocean (Soto‐Navarro et al. 2010). However, most of the plastic contamination is directly linked to numerous rivers (such as the Po, Nile, and Rhone) that flow into the basin carrying a variety of pollutants and waste after passing through densely inhabited and industrialized areas (Lechner et al. 2014; Lebreton et al. 2012; Prevenios et al. 2018; Liubartseva et al. 2018). Furthermore, a significant amount of plastic debris has been found on the MS's seafloor at varying depths, with accumulation zones documented in many places like the Spanish continental shelf, the Tyrrhenian Sea, the French Mediterranean coast, the Eastern Mediterranean, the Cilician Coast, and the Sardinian coast (Angiolillo et al. 2015; Tubau et al. 2015; García‐Rivera et al. 2017; Cau et al. 2017; Vlachogianni et al. 2018; Galgani et al. 2000; Pierdomenico et al. 2020).

A survey of the published literature on MS was conducted in Scopus from 2011 to 2024, with a focus on MPs and their associated metals. Two sets of keywords were used to explore the published information: the first set included MPs OR MPs AND Pollution AND Mediterranean AND Sea. The second set included MPs OR MP AND Metals OR Metal AND Pollution AND Mediterranean AND Sea. The search encompassed the article title, abstract, and keywords. The findings are summarized in Table 4.

TABLE 4.

MPs' pollution in the Mediterranean Sea.

Keywords used	MPs and metals or metal and pollution and Mediterranean sea	MPs and pollution and Mediterranean sea
Search within	Article title, abstract, keywords	Article title, abstract, keywords
Number of documents results	17 documents	381 documents
Document by year
Documents by type
Document by subject area
Documents by country or territory

Overall, the first set of keywords yielded a significantly higher number of documents (381) than the second (17), indicating a substantial focus on MP pollution in the MS. There is also growing research interest in correlating MPs with different metal levels. Over 80% of the documents were research articles, followed by reviews and book chapters. Authors affiliated with Italy had the highest number of publications in the MS on MPs in general (Vianello et al. 2013) and metals associated with MPs specifically.

Deep insight into metal‐related MPs topics in the MS reveals that most studies focused on monitoring and relating the concentration of metal(loid)s with size, shape, color, and polymer type of MPs and how the degradation status of MPs affects metal(loid)s adsorption process in the Mediterranean coast of Spain (J. P. Rodrigues et al. 2023). In the same context, Fajković et al. (2022) and Maršić‐Lučić et al. (2018) indicated that PS had the highest MPs‐metal adsorption affinity relative to polypropylene and low‐density PE, and Zn showed the strongest adsorption affinity for all MP types in the Croatian Adriatic coastal and estuarine area.

Other studies demonstrated the occurrence of metals adsorbed on MP particle surfaces isolated from the gills and stomachs of spiny lobster in Greece (Kampouris et al. 2023), as well as from zooplankton collected from the Italian Seas (Squadrone et al. 2022), and from common octopus ( Octopus vulgaris ) in the MS (Pedà et al. 2022). These investigations highlight MPs' role as drivers of metal cycling in a marine food web and how to use those organisms as bioindicators of pollution.

On the other hand, the in‐vitro experiments were conducted to test the role of MPs in concentrating metals such as palladium (Pd) from water and augmenting their bioaccumulation in the Mediterranean mussel (J. P. Rodrigues et al. 2023). Other experiments correlate metal levels (as Cu) with biofilm production and microbial community composition on MP particles in the northwestern part of the MS (Onrubia et al. 2021; Djaoudi et al. 2022).

Isotope ratio mass spectrometry (IRMS) was recently used by Kuznetsova et al. (2023) on the beach at Viareggio, Italy, to detect polymer material in seawater and examine compositional alterations resulting from its chemical breakdown.

7. Case Study: Lead (Pb) Adsorption on PE MPs

Lead (Pb) is a widespread toxic heavy metal commonly found in aquatic environments due to industrial and urban pollution. PE, one of the most abundant MP polymers, is hydrophobic and chemically stable, making it a key vector for adsorbing heavy metals in marine and freshwater systems. Understanding Pb adsorption on PE MPs elucidates pollutant behavior and environmental risks.

The Pb–PE case study was expanded to provide a quantitative and comparative interpretation of adsorption behavior across different particle sizes, degrees of aging, and environmental media. Pristine PE generally exhibits limited Pb adsorption due to its hydrophobic, chemically stable, and weakly functionalized surface. In contrast, aged or weathered PE exhibits higher Pb affinity due to surface oxidation, increased roughness, and the formation of oxygen‐containing functional groups such as –OH, C=O, and –COOH. Smaller PE particles are expected to adsorb more Pb per unit mass than larger particles because of their higher surface‐area‐to‐volume ratio and greater number of accessible binding sites. Therefore, particle size and degree of aging should be considered together when assessing the environmental role of PE as a Pb carrier.

The adsorption models were also compared more critically. The Langmuir model is useful for estimating a theoretical maximum adsorption capacity under ideal monolayer adsorption conditions, but it may oversimplify Pb adsorption onto environmentally weathered PE. The Freundlich model is often more appropriate for aged or biofilm‐coated PE because it accounts for heterogeneous surface energies and multilayer adsorption. Kinetic models, including pseudo–first‐order and pseudo–second‐order models, can further help distinguish whether Pb adsorption is dominated by physical diffusion‐controlled adsorption or by chemisorption/surface complexation with oxidized functional groups.

Environmental media strongly influence Pb adsorption onto PE. In freshwater, lower ionic strength may favor Pb interaction with available PE surface sites. In seawater and Mediterranean coastal waters, high salinity may reduce adsorption by competing with Na⁺, Ca²⁺, and Mg²⁺ and by forming soluble metal–chloride complexes. However, aging, DOM coating, suspended matter, and biofilm formation may counteract this inhibition by introducing additional binding sites. In wastewater‐impacted zones, organic matter may either compete with PE for Pb complexation or enhance Pb retention by coating PE surfaces with reactive functional groups. Thus, the net Pb adsorption capacity of PE depends on the balance between surface activation, ion competition, Pb speciation, DOM complexation, and biofilm‐mediated binding.

The Pb–PE case study is directly relevant to pollution control in the Mediterranean. PE is one of the dominant MP polymers detected in Mediterranean biota and sediments, while Pb is commonly associated with urban runoff, industrial discharge, port activities, antifouling sources, and atmospheric deposition. Weathered PE particles in Mediterranean coastal waters may therefore serve as mobile Pb carriers, contributing to the redistribution of metal pollution across harbors, estuaries, wastewater outfalls, sediments, and food webs. This case study supports the need to include metal‐loaded MPs, especially aged PE particles, in Mediterranean monitoring programs and to combine MP removal with heavy‐metal control in wastewater treatment, harbor management, and coastal remediation strategies.

7.1. Adsorption Mechanism and Equation

Pb adsorption onto PE MPs primarily occurs via physical adsorption (physisorption) and surface complexation facilitated by surface oxidation of PE particles. Weathering introduces oxygen‐containing functional groups (carbonyl and hydroxyl) on PE surfaces, increasing reactive sites for Pb binding (Figure 6). The adsorption can be described by the Langmuir isotherm model (Equation 6):

$$q_{\mathrm{e}}=\frac{q_{\max }{K}_L{C}_{\mathrm{e}}}{1+K_{\mathrm{L}}{C}_{\mathrm{e}}}$$ (6)

where q _e is the adsorbed Pb amount per gram of PE, q _max is the maximum adsorption capacity, K _L is the Langmuir constant indicating affinity, and C _e is the equilibrium Pb concentration (Q. Chen et al. 2023). However, the Freundlich model is generally more suitable for describing adsorption on heterogeneous surfaces, such as weathered MPs. The Langmuir model is used to approximate the “theoretical maximum absorption capacity.”

FIGURE 6.

Mechanism of lead adsorption on pristine and weathered polyethylene microplastics and environmental influences on adsorption dynamics.

7.2. Factors Influencing Adsorption

• Particle size: Smaller PE MPs provide higher surface‐to‐volume ratios, enhancing adsorption capacity.

• Surface weathering/aging: Oxidative processes generate functional groups that increase adsorption affinity for Pb.

• pH: Acidic conditions promote greater Pb solubility and adsorption, whereas alkaline pH may reduce Pb adsorption via precipitation effects. This may mean that the measured removal value represents the complete “removal.”

• Ionic strength: High ionic concentration competes with Pb ions for adsorption sites, lowering adsorption efficiency.

• Organic matter: Natural organic materials can complex with Pb, modifying adsorption on PE surfaces.

7.3. Type of Adsorption

Adsorption involves both physisorption, driven by van der Waals forces, and chemisorption via surface complexation with oxidized PE functional groups, with chemisorption more prominent in aged MPs (Q. Chen et al. 2023; Z. Li et al. 2026; Y. Gao et al. 2026). The environmental dynamics of Pb adsorption on MPs is twofold. Positively, MPs function as efficient sorbents for lead (Pb), binding the heavy metal via surface interactions strengthened by HA, pH increases, and polymer aging. By sequestering Pb from solution, MPs can momentarily lower Pb bioavailability in water and soil. Utilizing the large surface area and oxygenated sites created by UV weathering of MPs, this adsorption capacity is often comparable with that of natural colloids. Cons, however, predominate because MPs act as carriers of Pb‐contaminated particles within aquatic food webs, increasing the toxicity to organisms via ingestion and bioaccumulation while complicating remediation.

Tire wear particles (TWPs), UV‐aged polylactic acid (UV‐PLA), UV‐aged PS (UV‐PS), pristine PLA, and pristine PS are examples of aged MPs. Table 5 likely compares lead (Pb²⁺) adsorption capacities across these MPs, highlighting TWP's superior performance, followed by UV‐PLA and UV‐PS. Adsorption isotherms (e.g., Freundlich KF coefficients) and kinetics (e.g., pseudo–second‐order rates) are important parameters from model fits that show how UV aging increases oxygen‐containing functional groups, thereby increasing electrostatic interactions and complexation, leading to higher Pb uptake and often surpassing pristine forms by significant margins. These results highlight the role of MPs as vectors in aquatic systems, and the ecological hazards associated with biodegradable polymers such as UV‐PLA are further underscored by desorption behaviors in sediment versus water columns (Carriera et al. 2023).

TABLE 5.

Pros and cons of Pb adsorption on microplastics.

Pros	Cons
PE microplastics facilitate Pb transport and increase bioavailability via the Trojan horse effect	Bioaccumulated Pb on microplastics causes ecological and health hazards upon ingestion
Reduces immediate free Pb ion availability, potentially lowering acute toxicity risks	Persistent microplastics with adsorbed Pb lead to long‐term chronic exposure for organisms
Provides indicators for Pb pollution monitoring in aquatic environments
Informs the development of targeted remediation using engineered polymers or adsorbents

8. Future Directions and Research Gaps

Future research on heavy metal adsorption onto MPs should move beyond general laboratory observations and focus on specific, environmentally relevant and operable research priorities. First, Mediterranean‐specific monitoring programs are urgently needed because the MS is an MP pollution hotspot affected by dense coastal urbanization, industrial discharge, shipping activities, wastewater inputs, limited water exchange, and high plastic accumulation. Future studies should quantify metal‐loaded MPs in coastal hotspots such as harbors, estuaries, wastewater discharge areas, semi‐enclosed bays, sediments, and biota. Particular attention should be given to dominant Mediterranean polymers such as PE, PP, PS, PVC, and PET and to toxic metals commonly associated with urban, industrial and port activities, including Pb, Cd, Cu, Zn, Ni, Cr, and Hg (Suaria et al. 2016; Sharma et al. 2021; Squadrone et al. 2022; Hamed et al. 2023; Vega‐Herrera et al. 2024; Torresi et al. 2024).

Second, future experimental work should be designed to resolve the contradictory results reported for the effects of DOM, salinity, pH, and aging on metal adsorption by MPs. This requires harmonized experimental schemes in which polymer type, particle size, degree of aging, metal concentration, metal speciation, pH, salinity composition, DOM type and concentration, contact time, solid‐to‐liquid ratio, and analytical extraction methods are systematically controlled. Such standardized designs would allow direct comparison among studies and help distinguish whether observed differences are caused by polymer chemistry, metal speciation, environmental matrix effects, or methodological variation (Godoy et al. 2019; F. Wang, Yang, et al. 2019; H.‐T. Wang, Ding, et al. 2019; Y. Zhou, Yang, et al. 2020; Guo et al. 2020; Tang et al. 2020; Tang et al. 2021; X. Gao, Hassan, et al. 2021; W. Liu, Zhang, et al. 2021; Z. Li et al. 2026).

Third, more attention should be given to environmentally aged and biofilm‐coated MPs instead of relying mainly on pristine commercial particles. Weathering, UV exposure, mechanical abrasion, and biofilm formation can increase surface roughness, alter zeta potential, and introduce oxygen‐containing functional groups such as –OH, –COOH, and C=O, thereby altering adsorption mechanisms and metal bioavailability. Future studies should therefore compare pristine, UV‐aged, mechanically weathered, and biofilm‐colonized MPs under freshwater, seawater, sediment, and wastewater conditions to better simulate real environmental behavior (Holmes et al. 2012; Vedolin et al. 2018; Mao et al. 2020; Kalčíková et al. 2020; X. Gao, Hassan, et al. 2021; S. Liu, Shi, et al. 2021; W. Liu, Zhang, et al. 2021; Alomar et al. 2025).

Fourth, future research should combine in situ and ex situ analytical techniques to more rigorously verify adsorption mechanisms. FTIR, XPS, SEM–EDX, XRD, and Raman spectroscopy are useful for identifying functional groups, surface morphology, metal distribution, and surface precipitation after adsorption. However, ex situ procedures such as washing, drying, and vacuum analysis may alter weakly bound ions or hydrated complexes. Therefore, future studies should include near‐in situ methods such as ATR‐FTIR in aqueous media, zeta potential analysis at environmentally relevant pH and salinity, time‐resolved adsorption/desorption experiments, and metal speciation analysis. This combined approach would help distinguish true surface complexation from temporary electrostatic adsorption, DOM‐mediated complexation, or solution‐phase precipitation (Peng, Sun, Hanif, et al. 2021; Y. Zhou et al. 2021; S. Liu, Shi, et al. 2021; W. Liu, Zhang, et al. 2021; M. Zhou et al. 2024; Z. Li et al. 2026; Y. Gao et al. 2026).

Fifth, future ecotoxicological studies should directly link adsorption results with biological endpoints. The “Trojan horse effect” should be tested under realistic exposure scenarios by measuring not only metal adsorption onto MPs but also ingestion, gut desorption, tissue accumulation, oxidative stress, inflammation, growth inhibition, reproduction, and trophic transfer. Such studies should include diverse organisms, life stages, and feeding strategies, especially Mediterranean‐relevant species such as mussels, fish, sponges, and benthic organisms. This would clarify when MPs increase metal toxicity and when they reduce dissolved metal bioavailability through external adsorption (W. S. Lee et al. 2019; Monikh et al. 2020; F. Wang, Zhang, Zhang, Zhang, Adams, and Sun 2020; Wen et al. 2018; Barboza et al. 2018; Fernández et al. 2020; Q. Chen et al. 2023).

Sixth, forward‐looking studies should integrate advanced technologies, including high‐resolution imaging, microspectroscopy, metal speciation tools, omics‐based ecotoxicology, plastisphere microbial analysis, and machine‐learning models. Machine‐learning approaches could be used to predict metal adsorption capacity and toxicity based on polymer type, particle size, aging index, zeta potential, DOM, salinity, pH, and metal speciation. These predictive tools would be especially useful for screening high‐risk MP–metal combinations and identifying priority monitoring sites in the Mediterranean basin (S. Liu, Shi, et al. 2021; W. Liu, et al. 2021; Das et al. 2025; M. Zhou et al. 2024; Y. Gao et al. 2026; Z. Li et al. 2026).

Finally, future remediation studies should evaluate combined treatment systems capable of removing both MPs and adsorbed metals. Conventional wastewater treatment, coagulation, electrocoagulation, membrane filtration, ozonation, biofiltration, wetlands, biochar, nanomaterials, and microalgal or microbial systems should be assessed not only for MP removal efficiency but also for their ability to prevent metal desorption and secondary pollution. Pilot‐scale studies in Mediterranean WWTPs, harbor waters, and coastal discharge zones would provide more practical evidence for managing metal‐loaded MPs under real environmental conditions (Ma et al. 2019; Perren et al. 2018; Hidayaturrahman and Lee 2019; Batagoda et al. 2019; Talvitie et al. 2017; Pal et al. 2021; Osman et al. 2023).

9. Conclusion

The adsorption of heavy metals onto MPs poses a significant environmental challenge due to the potential for bioaccumulation and the toxicity of these complexes in aquatic ecosystems. MPs have been shown to act as vectors for transporting metal pollutants, increasing exposure risks to aquatic organisms, and potentially magnifying toxic effects. The adsorption process is influenced by multiple factors, including the chemical composition of the metals, the surface properties of the MPs, and environmental conditions such as pH, temperature, and ionic strength. A notable example is the adsorption of lead (Pb) onto PE MPs, in which both pristine and weathered PE surfaces adsorb Pb ions via physical and chemical interactions, influenced by factors including surface functional groups and environmental conditions. The adsorption capacity of pristine PE for heavy metals is usually minimal via physical adsorption, as adsorption depends primarily on functional groups generated on the PE by aging or the presence of biofilms. Despite advances in knowledge, critical gaps remain in fully understanding adsorption mechanisms and their long‐term ecological consequences. To effectively mitigate the environmental risks posed by MPs–metal composites, comprehensive research is required to elucidate the underlying physicochemical interactions and evaluate their impacts on biological communities over time. Moreover, the development of sustainable, eco‐friendly remediation approaches is essential to reduce the influx of metals and MPs into natural waters. Proactive strategies, including strict regulations and controls on pollution sources, are also needed to safeguard aquatic ecosystems and public health. In summary, addressing these complexities holistically—with case studies such as Pb adsorption on PE MPs that provide focused insights—will enable the design of targeted interventions to minimize the environmental footprint of combined MP and heavy‐metal pollution. This enhanced perspective underscores the urgency of multidisciplinary research and informed policy measures to tackle this dual‐contaminant threat effectively.

Author Contributions

Amr G. Dardeer: writing – original draft. Nashwa A. Shaaban: writing – original draft. Ahmed Tawfik: investigation, validation. Mohamed A. Hassaan: conceptualization, writing – original draft, validation, software, data curation. Uyiosa Osagie Aigbe: writing – original draft. Ahmed El Nemr: conceptualization, writing – review and editing, supervision, validation.

Funding

The authors have nothing to report.

Ethics Statement

The authors have nothing to report.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

The corresponding author of the study can provide access to the datasets utilized in this inquiry upon request.