Migration and Transformation of Heavy Metal and Its Fate in Intertidal Sediments: A Review

: Intertidal sediments are rich in biological resources, which are important for material circulation and energy exchange. Meanwhile, these areas can be treated as sinks as well as sources of coastal heavy metal pollutants. Due to the influence of the tide, the intertidal sediments are in a state of periodic flooding and exposure, and environmental factors such as dissolved oxygen, salinity and overlying water pressure are changeable. Heavy metals in sediments are prone to migration and transformation with the dynamic effects of tidal water and the changes in the environment factors, which increase the bioavailability of heavy metals. In this review, the characteristics of distribution and the bioavailability of heavy metals in intertidal sediments are described; the migration and transformation behavior of heavy metals and its influencing factors under tidal conditions are analyzed; and the mechanisms of heavy metal’s migration and transformation in the intertidal zone are summarized. Moreover, the bioaccumulation of heavy metal by organisms and the remediation techniques are discussed. Therefore, this review systematically summarizes the states of existence, the transport mechanisms, and the fate of heavy metals in the intertidal sediment, fills in the research gap of the cycling of heavy metal in the intertidal zone, and provides a theoretical basis for the control of heavy metal pollution.


Introduction
Coastal tidal flats, situated at the interface between the land and sea, are ecologically rich areas with significant impacts on both the marine and terrestrial environments.However, intensified human activities have led to the gradual transformation of intertidal sediments into primary receptacles for terrestrial-sourced pollutants and nearshore discharges.This, in turn, has caused disruptions in the ecosystem's structure and functionality [1].Heavy metals are widely distributed in intertidal sediments, primarily originating from the weathering of rocks and minerals; sewage discharging from industrial, agricultural, and human activities; and dry and wet deposition of dust.Among these, industrial wastewater serves as a significant source of heavy metal pollution [2,3].Heavy metals, due to their non-degradability, high accumulative nature, and potential biotoxicity, are widely distributed in various environmental media [4,5].As a result, their ecological risks and potential health hazards have garnered global attention [6,7].
Owing to the unique geographical position of intertidal zones, the processes of materials' migration and energy flow are rapid and intricate compared with other environments [8].Sediment particles are resuspended by the water flow's shear forces during tidal movements.This results in intertidal sediments being alternately submerged by seawater (resembling marine sediment) and exposed to air (resembling terrestrial sediment).The sediment's surface is subjected to continuous transitions between the mud-air interface and the mud-water interface, accompanied by changes in physicochemical factors such as dissolved oxygen (DO) and interfacial pressure [9].The oxic-anoxic interface plays an important role in the migration and transformation of heavy metals from the sediment to the surface water, where redox-sensitive processes occur, resulting in the dissolution or precipitation of heavy metal complexes [10].However, the location of the oxic-anoxic interface in the intertidal sediments varies greatly with the fluctuation of the tidal level and the local variability of the water table's levels, which complicates the migration processes.Together, all these variations affect the form and behavior of heavy metals in the sediments, trigger their migration and transformation, alter their bioavailability, and increase the ecological risks.
Despite the increasing number of studies on the migration and transformation of heavy metals, relatively little is known about the processes, mechanisms, and influencing factors of the migration of heavy metals in the intertidal sediment, which are important components of the cycling of heavy metals and the marine ecology's health.Therefore, this article reviews the distribution, forms, and migration-transformation patterns of heavy metals in intertidal sediment regions; discusses the mechanisms driving the bioaccumulation of heavy metals in these intertidal zones; and provides a theoretical foundation for the management of heavy metal pollution in intertidal areas.

The Heavy Metal Contents of Intertidal Sediments
Heavy metals are widespread in intertidal zones (Table 1).The contents of copper (Cu), lead (Pb), zinc (Zn), cadmium (Cd), chromium (Cr), nickel (Ni), arsenic (As), and mercury (Hg) in intertidal sediment differ greatly in different estuaries or bays.The content of Cu in the East China Sea, the content of Pb in the East and South China Sea, and the content of Hg in the East China Sea and Suez Bay in Egypt were higher than the level of probable adverse biological effects.The sediment's properties, such as the grain size, total organic carbon (TOC), and Fe/Mn oxides, have certain regulatory effects on the content of heavy metal in intertidal sediment, with grain size being the dominant factor, while pH and temperature showed little correlation with the distribution of heavy metal [12,18].A study showed that the sediment in the intertidal zone is composed primarily of fine sand.For example, the surface sediment consists mainly of sand (38.0%) and fine sand (55.4%) in the Hangzhou Bay and its adjacent areas, and the content of heavy metals is significantly positively correlated with the content of fine sand and clay [19].The contents of TOC in sediments were also related to the particle size.Research has shown that the contents of TOC increased with a decrease in the particle size, which produces a large specific surface area, and the contents of heavy metal (Pb, Cd, Cu) had positive relationships with the contents of TOC and fine particles [20].It has been proven that Hg and As are positively related to TOC, while concentrations of Zn, Cr, Pb, Cd, and Cu are positive related to clay contents in the sediment of Beibu Bay (South China Sea) [21].In the dry sediment environment, Fe/Mn oxides are easily formed, which could adsorb heavy metals (especially Cd, Cu, Pb, and Zn) and increase the precipitation of heavy metals in the sediments while the rewetting of the sediment will reduce the Fe/Mn oxides and release the adsorbed heavy metals [18].The excellent adsorption capacity of Fe-Mn dioxides for As (III) is due to the synergistic effect of manganese oxide as an oxidant to oxidize As (III) to As (V), and iron oxide as an adsorbent for As (V) [22].Hydrodynamic conditions also have important effects on the sediment's characteristics.The heavy metal content is usually high in silted-up coasts with weak hydrodynamic conditions, while it is usually low in coasts with intense and complex hydrodynamic forces [23].

The Fractions of Heavy Metal in Intertidal Sediments
Heavy metal in the sediment can be present in different forms, such as dissolved and particulate form.A study showed that the content of metal in the solution phase (Cr, Ni, Cu, Zn, Pb, and Cd) was 55, 391, 1673, 740, 24, and 15 µg/L, while the content of solid phase metals was 695, 244, 1482, 980, 66, and 5 µg/g, respectively, in intertidal sediment in southern China [24].The dissolved forms of heavy metals in intertidal sediments are the most readily taken up by organisms, exhibiting the highest bioavailability [7].Heavy metals generally bind to sediments in four ways: 1  ⃝ adsorption, 2 ⃝ coprecipitation with hydrous iron and manganese oxides, 3  ⃝ complexation with organic molecules, and 4  ⃝ incorporation into crystalline minerals [25].The adsorption process primarily involves chemical affinities, residual valences, and van der Waals forces that lead to adsorption onto clay minerals (such as kaolinite, illite, and montmorillonite), hydrous iron, and manganese oxides, as well as other substances (such as humic acids) [26].The heavy metals bound through adsorption and coprecipitation with hydrous iron and manganese oxides are also easily decomposable and have high bioavailability.A study about the intertidal sediment in Zhoushan, China, found significant variations in the fraction and composition of different heavy metals, with the dominant proportion of Cd being the acid-soluble form, that of Pb being primarily the reducible form, Cu remaining evenly in an extractable and residual state, and Zn, Ni, Cr, and As being mainly present as residual forms [27].A study in Zhelin Bay (South China) showed that many metals in the intertidal surface sediment were dominated by residual fractions, while Cd and Mn had highest bioaccessibility [28].
Diethylene triamine pentaacetic acid (DTPA) was used for extracting the bioavailable states of heavy metal [29].Diffusive gradients in thin films (DGT) have been developed for measuring the bioavailable heavy metal [30].Some researchers believed that the DGT-labile metals had positive relationships with the acid-soluble metals in the coastal sediment [31].A study showed that the average values of DGT-labile Cd, Pb, Ni, Cu, Zn, and Mn were 0.78, 0.29, 0.23, 0.13, 34.2, and 12.4 µg/L, respectively, in coastal sediment in Hainan Island, South China Sea [32].The mean DGT-labile concentrations of Cd, Pb, Ni, Cu, Zn, and Mn were 0.79, 1.46, 10.2, 2.90, 52.1, and 2328 µg/L, respectively, in the intertidal zone of the Pearl River Estuary, China [33].The bioaccessible heavy metals can also be tested by in vitro digestion models using acid to simulate the digestive condition [34].A study showed that the concentrations of bioaccessible heavy metals, namely Cd, Cr, Cu, Mn, Ni, Pb, and Zn were up to 0.11, 0.53, 0.45, 52.5, 0.27, 1.05, and 2.4 µg/g, respectively, and the bioaccessibility of these metals was up to 52.4, 11.8, 8.57, 55.0, 21.8, 15.2, and 29.4%, respectively, in the intertidal sediments of the Persian Gulf in Iran [35].

The Migration and Transformation Behavior of Heavy Metals in Intertidal Sediments and Its Influencing Factors
Intertidal sediments are reservoirs for heavy metals; however, when the environmental conditions change, heavy metals within the sediments can be released through metal ion exchange reactions, migration with sediment groups and colloids, and other mechanisms [36].Heavy metals in the overlying water may stay in liquid media (interstitial water/overlying water) or adsorb on solid media (sediment/suspended particles) [37].In the process of resuspension, suspended solids are important carriers of heavy metals.The migration and transformation of heavy metals in intertidal sediments are affected by many factors, which can be divided into three categories [38], as follows.

Properties of the Sediment
These include the composition and content of organic matter, as well as the content of Fe/Mn oxides and sulfides, and particle size.These factors are related to the transformation of the form and the bioavailability of heavy metals in sediments [14] For instance, exchangeable and carbonate-bound forms and Fe/Mn oxide-bound forms are easily decomposed into ionic forms and accumulated by organisms [39].Heavy metals bound to organic matter are more stable and can only be decomposed under strong oxidative conditions.Meanwhile, dissolved organic matter (DOM) is a complex heterogeneous substance that can be degraded by microorganisms into humic and fulvic acids, which can adsorb metal ions and thus alter the migration of heavy metals [40].In the intertidal zone, DOMs are more complex, as they have diverse sources.The adsorption ability, humic contents, and aromaticity of DOMs in coastal marshes increase at low tide [41].
The particle size and composition of the sediment, including the content of clay and silt, are also important factors.Research has shown that the sand content gradually decreased from the sea toward the land, while silt and clay had an inverse pattern compared with sand under tidal conditions [19].Sediments containing clay minerals such as montmorillonite have multiple active binding and ion exchange points on their surfaces and are recognized as high-performance adsorbents [42].

Physicochemical Conditions in the Aquatic Environment
These include the pH, DO, and redox potential (Eh) in both the water and sediment.The influencing mechanisms involved include adsorption, desorption, coagulation, precipitation, and coprecipitation, as well as ion exchange.For instance, higher pH levels can enhance the adsorption and precipitation of free heavy metals in water, while lower pH levels can weaken the binding capacity of heavy metals [43].This relates to the competition for adsorption by H + , as well as the dissolution of some sparingly soluble metal salts and its complexes when the pH decreases [44].High salinity favors the solubility of weakly-bound metals because of the strong potential of major cations, such as Ca 2+ and Mg 2+ , can easily substitute for heavy metals on negatively charged sediment surfaces, although the effect of this factor on the adsorption-desorption behavior of heavy metals is less pronounced compared with pH [26].Studies have shown that the salinity of the water at the surface of the sediment may regulate the metals' diffusion flux at the water-sediment interface and control the metals' bioaccessibility [31].In the intertidal area, where marine and fresh water are mixed, salinity could increase the particulate phase of heavy metals by promoting the complexation of metal and organic matter [45].An increase in DO in the aquatic environment can accelerate the oxidation rate of sulfides, decompose organic compounds, and facilitate the desorption of heavy metals and the breakdown of metal complexes, leading to the release of heavy metal ions [46].The study of Calmano et al. showed that changes in processes such as the oxidation of sulfides in sediments can lead to the release of heavy metals from the sediment into overlying water, while some of the heavy metals may be readsorbed or precipitated on the sediment [47].De Jonge et al. found that an increase in the dissolved oxygen (DO) content in the water can promote the degradation of acid-volatile sulfide (AVS), thereby facilitating the release of heavy metals from the sediment [48].The impact of the redox conditions on the release of heavy metals varies with different types of sediment.The sediment's properties, including the grain size, the content of sulfides and organic matter, and the presence of hydrous metal oxides of iron and manganese, significantly influence the release of heavy metals during these processes [49].Furthermore, an increase in Eh can inhibit the activity of anaerobic bacteria, promote the growth of denitrifying and desulfurizing bacteria, causing sulfides to oxidize, thereby increasing the bioavailability of heavy metals [50].

Disturbance Effects
These include physical processes in the water environment (such as tidal currents and human activities) as well as the activities of saltmarsh organisms (such as burrowing and excretion).Biological activities can disrupt the physical structure of the sediment-water microinterface, alter the oxygen content at the interface, and induce the movement of the sediment's particles.They can also carry interstitial water out of the sediment interface, all of which promote the release of heavy metals from sediments [51,52].Physical disturbances are stronger, as the energy they generate propagates downward through the water until it reaches the sediment-water interface.This causes changes in the physicochemical properties of both the sediment and overlying water.When the shear stress on the surface of the sediment reaches the critical shear stress, resuspension of the sediment occurs, leading to alterations in the distribution of heavy metals in the sediment and overlying water, making the particles and colloids carrying heavy metals more likely to attach to surfaces or enter aquatic organisms' tissues [53,54].The contents of heavy metals in the suspended particle matter were twice as large than those in the sediment of the coast under hydrodynamic conditions [55].Additionally, during the resuspension process, the forms of heavy metals are less stable, and particulate-bound heavy metals are prone to transform into dissolved forms, significantly increasing their bioavailability.A study showed that dissolved heavy metal fractions increased significantly when the sediment was resuspended [56].While some studies have shown that the suspended solids, especially aggregated colloids, could remove dissolved metals and DOMs [53].Field studies have shown that in estuarine intertidal areas under long-term inundation conditions, the migration rate of heavy metals tends to be low [57].Furthermore, tidal water currents can affect the particle size composition, subsequently influencing the distribution and forms of heavy metals [58].

Mechanisms of the Migration and Transformation of Heavy Metals in the Intertidal Zone
The impact of tidal action on the intertidal sediment is a complex process, with the resuspension and settling of sediment particles being a crucial factor contributing to the migration of heavy metals within the sediment.The shear forces reaching the sedimentwater interface initially disrupt the oxidative layer on the sediment's surface, where soluble iron/manganese oxidize into insoluble iron/manganese oxides under oxidizing conditions, with a larger surface area and a stronger ability to adsorb heavy metals [59].When disturbance activities generate a sufficiently large shear stress on the sediment's surface, surpassing the adhesive forces between the sediment's particles, resuspension of the sediment occurs [60].The resuspension process exposes both the surface sediments and the suspended particles to oxidizing conditions, altering the physicochemical properties of the sediment environment, including changes in pH and Eh, which lead to the release of heavy metals from the surfaces of the sediments and suspended particles into the overlying water [61].Simultaneously, hydrodynamic forces disrupt the equilibrium conditions of heavy metals at the sediment-water interface, causing changes in the water pressure and facilitating the diffusion of interstitial water within the sediment, which also results in the dispersion of heavy metals from the sediment into the overlying water [62].
When intertidal sediments are subjected to periodic inundation, there might be specific patterns of the migration and transformation of heavy metal under tidal conditions (as shown in Figure 1).During low tide, when the sediments are exposed to air, the Eh of the Processes 2024, 12, 311 6 of 13 surface sediments increases, which leads to the oxidation of organic matter and sulfides in the sediment, releasing the associated heavy metals.However, during this time, Fe/Mn ions in the sediment tend to form oxides, which can bind with heavy metals, reducing their mobility [63].The reverse is true as well.Moreover, the reduced pressure from overlying water during low tide can lead to the migration of interstitial water from the sediments.Research has indicated that DOMs and heavy metals in the intertidal zone can vary with tidal fluctuations, with DOMs and heavy metals being more likely to be released from sediments during low tide [64,65].When the sediment is submerged by water during the high tide, the Eh decreases.Heavy metals in the interstitial water of the surface sediments move into the overlying water due to differences in the concentration.Additionally, heavy metals in ionic, organic-bound, and particulate forms may undergo resuspension, mutual transformation, and precipitation processes during this process.Salinity in the intertidal zone varies significantly with the flowing of the fresh water and tidal processes, and thus influences the solubility of the heavy metals [66].
(as shown in Figure 1).During low tide, when the sediments are exposed to air, the Eh the surface sediments increases, which leads to the oxidation of organic matter and fides in the sediment, releasing the associated heavy metals.However, during this ti Fe/Mn ions in the sediment tend to form oxides, which can bind with heavy metals, ducing their mobility [63].The reverse is true as well.Moreover, the reduced press from overlying water during low tide can lead to the migration of interstitial water fr the sediments.Research has indicated that DOMs and heavy metals in the intertidal z can vary with tidal fluctuations, with DOMs and heavy metals being more likely to released from sediments during low tide [64,65].When the sediment is submerged by ter during the high tide, the Eh decreases.Heavy metals in the interstitial water of surface sediments move into the overlying water due to differences in the concentrat Additionally, heavy metals in ionic, organic-bound, and particulate forms may unde resuspension, mutual transformation, and precipitation processes during this process.linity in the intertidal zone varies significantly with the flowing of the fresh water tidal processes, and thus influences the solubility of the heavy metals [66].

Bioaccumulation of Heavy Metals by the Organisms in Intertidal Sediments
The bioavailability of heavy metals in intertidal sediments may increase under t conditions.The organisms that live in the intertidal area are important carriers of he metal.Studies showed that large amounts of heavy metals, such as Cu, Pb, Zn, Cd, and Hg were detected in different species in the intertidal zone (Table 2), including valves (Crassostrea ariakensis, Chlamys farreri, Sinonovacula constricta, and Meretrix meret crabs (Calappa lophos, Charybdis anisodon, Portunus trituberculatus), and fishes (Lates ca rifer, Chanos chanos, Mugil cephalus).Biological enrichment serves as a significant mean reducing the heavy metal content in sediments.However, the transfer of heavy me through the food chain can introduce ecological crises and health hazards for huma demanding more attention [67].

Bioaccumulation of Heavy Metals by the Organisms in Intertidal Sediments
The bioavailability of heavy metals in intertidal sediments may increase under tidal conditions.The organisms that live in the intertidal area are important carriers of heavy metal.Studies showed that large amounts of heavy metals, such as Cu, Pb, Zn, Cd, Cr, and Hg were detected in different species in the intertidal zone (Table 2), including bivalves (Crassostrea ariakensis, Chlamys farreri, Sinonovacula constricta, and Meretrix meretrix) crabs (Calappa lophos, Charybdis anisodon, Portunus trituberculatus), and fishes (Lates calcarifer, Chanos chanos, Mugil cephalus).Biological enrichment serves as a significant means of reducing the heavy metal content in sediments.However, the transfer of heavy metals through the food chain can introduce ecological crises and health hazards for humans, demanding more attention [67].The accumulation of heavy metals within organisms primarily depends on the bioavailability of heavy metals in the environment and the metabolic efficiency of the organisms toward the metals, including the assimilation rate, absorption rate (uptake rate), and excretion rate.Research on the enrichment of heavy metals in bivalves in the Yangtze River Estuary, China, indicated that the heavy metal content in bivalve tissues was positively correlated with the content of heavy metals in the sediment and water, especially the sediment [74].Various models were developed to describe and predict the process of accumulation of heavy metals in aquatic organisms, and the biokinetic model indicated that the accumulated heavy metals are from water and food [75,76].The contents of heavy metal in crustaceans and shellfish were significantly higher than those in fish in the intertidal zone [77].There was considerable variation in the distribution of heavy metals among different tissues of the organisms.Typically, the highest concentration of metal (dry weight) was found in the gills and mantle tissue, followed by the visceral mass, and finally the muscle tissue, which related to the moisture content of different tissues [78].A study has shown that the muscle tissue of bivalves exposed to the air during low tide can still absorb the heavy metal Cd, as the dissolved Cd that remains in the mantle tissue could be continually absorbed by the muscle tissue [79].
Heavy metals in the environment can enter organisms through the following three ways: (1) direct absorption through the gills' epithelial cells (aqueous absorption); (2) ingestion of food or particles containing heavy metals, followed by digestive absorption (particulate absorption); and (3) adsorption of dissolved or small particulate forms of heavy metals onto the surface of the organism's body (multiphase adsorption) [80].Heavy metal ions bind with sulfur-or nitrogen-containing amino acids on cell membranes and diffuse into the cell's body, aided by concentration gradients, a process that continues when the concentration of free metal ions within the cell is lower than that outside the cell [81].There are primarily three methods by which heavy metals are stored in organisms.The first is binding with biomolecules such as proteins, which are not easily exported out of cells due to their size, resulting in intracellular accumulation.The second is the synthesis of metallothioneins (MTs) induced within the organism's body, followed by binding with heavy metals.The third is in the form of ions or small molecular complexes within the organism's body, typically involving less toxic heavy metals such as Zn [70].Additionally, there is an intracellular detoxification mechanism called the sequestration mechanism.When particles containing heavy metals enter the cells, the cells enclose them with membranes, isolating them from other cellular components.This phenomenon is common in liver and kidney cells, and is a manifestation of the accumulation of heavy metals within organisms [82].The research by Wang et al. [83] indicated that dissolved forms of Ag, Cd, and Zn primarily enter the cells of the blue mussel (Mytilus edulis) through carrier-mediated processes, which are associated with specific proteins or sulfur-containing compounds within the cells.Dissolved cobalt (Co) enters through passive diffusion, selenium (Se) uses anion channels, while heavy metals such as Fe and Pb may use phagocytosis or endocytosis to enter the cells.The concentrations of heavy metals in the interstitial water of sediments are typically relatively high.Meanwhile, there are folds or cilia in the surfaces of organisms, such as the gill tissues of benthic organisms, that allow dissolved or small particulate forms of heavy metals to be adsorbed by these organisms [78].

Heavy Metal Remediation of Intertidal Sediments
Two ideas are being considered for in situ remediation of heavy metals in intertidal sediments: reducing the heavy metal content and decreasing the bioavailability of heavy metals [84].Fixation/stabilization, plant extraction, and microbial remediation are environmentally friendly methods for in situ remediation of heavy metal pollution (Figure 2).The fixation/stabilization process is based on the characteristics of the large specific surface area and functional groups of passivators, which can alter the morphology of heavy metals through surface adsorption, ion exchange, and complex precipitation to reduce their biological availability [85].An in situ thin-layer coating technology is usually utilized by applying one or more layers of passivators onto the sediment as a physical barrier and to provide physical and chemical stabilization [86].Phytoremediation involves reducing the concentrations of heavy metal in the sediment by absorbing heavy metals through transporters associated with phosphorus and silicon, or transmembrane transport mechanisms [87].Additionally, root exudates such as organic acids could modify the forms of heavy metals to activate or stabilize heavy metal compounds.Microbial remediation operates through functional groups (such as hydroxyl and carbonyl groups) present on the surfaces of the microorganisms, which interact with heavy metal ions via ion exchange, complexation, chelation, or electrostatic adsorption [88].Microorganisms could also secrete organic acids that dissolve or form complexes with heavy metals, produce basic compounds leading to precipitation reactions with heavy metal, and produce various enzymes capable of redox reactions (e.g., oxidizing As 3+ into As 5+ or reducing Cr 6+ into Cr 3+ ) [89].The advantages and the disadvantages of the three remediation techniques are shown in Table 3.
of sediments are typically relatively high.Meanwhile, there are folds or cilia in the surfaces of organisms, such as the gill tissues of benthic organisms, that allow dissolved or small particulate forms of heavy metals to be adsorbed by these organisms [78].

Heavy Metal Remediation of Intertidal Sediments
Two ideas are being considered for in situ remediation of heavy metals in intertidal sediments: reducing the heavy metal content and decreasing the bioavailability of heavy metals [84].Fixation/stabilization, plant extraction, and microbial remediation are environmentally friendly methods for in situ remediation of heavy metal pollution (Figure 2).The fixation/stabilization process is based on the characteristics of the large specific surface area and functional groups of passivators, which can alter the morphology of heavy metals through surface adsorption, ion exchange, and complex precipitation to reduce their biological availability [85].An in situ thin-layer coating technology is usually utilized by applying one or more layers of passivators onto the sediment as a physical barrier and to provide physical and chemical stabilization [86].Phytoremediation involves reducing the concentrations of heavy metal in the sediment by absorbing heavy metals through transporters associated with phosphorus and silicon, or transmembrane transport mechanisms [87].Additionally, root exudates such as organic acids could modify the forms of heavy metals to activate or stabilize heavy metal compounds.Microbial remediation operates through functional groups (such as hydroxyl and carbonyl groups) present on the surfaces of the microorganisms, which interact with heavy metal ions via ion exchange, complexation, chelation, or electrostatic adsorption [88].Microorganisms could also secrete organic acids that dissolve or form complexes with heavy metals, produce basic compounds leading to precipitation reactions with heavy metal, and produce various enzymes capable of redox reactions (e.g., oxidizing As 3+ into As 5+ or reducing Cr 6+ into Cr 3+ ) [89].The advantages and the disadvantages of the three remediation techniques are shown in Table 3.

Conclusions
Based on a review of the speciation and distribution, the mechanisms of migration and transformation, and the bioaccumulation processes of heavy metals in intertidal sediment, this study analyzed the impact mechanisms of environmental changes in the intertidal zone on the migration of heavy metals within sediment, which fills a research gap on the cycling of heavy metals in the intertidal zone.However, due to tidal influences, sediments in different elevation zones of tidal flats may exhibit varying characteristics due to differences in the duration of submergence.Therefore, changes in the speciation and distribution of heavy metals require systematic investigation.Tidal disturbances can enhance the bioavailability of heavy metals within the sediment, yet the life activities of intertidal organisms are also influenced by tidal water currents.Organisms can respond to flow effects and suspended particle concentrations by reducing their feeding or filtration activity.Therefore, further research is necessary to elucidate the mechanisms and influencing factors of heavy metal enrichment in intertidal organisms.This will provide a theoretical foundation for the management of heavy metal pollution and ecological risk assessments in intertidal zones.

Figure 1 .
Figure 1.Migration and transformation of heavy metals from sediments with/without surface wa

Figure 1 .
Figure 1.Migration and transformation of heavy metals from sediments with/without surface water.

Figure 2 .Table 3 .
Figure 2. Conceptual graphs of the three methods of in situ remediation of heavy metal pollution.

Figure 2 .
Figure 2. Conceptual graphs of the three methods of in situ remediation of heavy metal pollution.

Author
Contributions: Conceptualization, Y.X.; investigation, N.G.; resources, D.L. and F.B.; writing-original draft preparation, N.G.; writing-review and editing, Y.X.; supervision, C.X.; funding acquisition, D.L.All authors have read and agreed to the published version of the manuscript.Funding: This research was funded by National Natural Science Foundation of China (grant number 22006138), Zhejiang Public Welfare Technology Application Research Project (grant number LZJWD22E090001), the Joint Funds of the Zhejiang Provincial Natural Science Foundation of China (grant number LZJWZ22C030001), the Key Technology Research and Development Program of Zhejiang (grant number 2021C03019), the Scientific Research Foundation of Zhejiang University of Water Resources and Electric Power (grant numbers xky2022006 and xky2022008), and Nanxun Scholars' Foundation.

Table 1 .
Concentration of heavy metals in intertidal sediment samples from typical estuaries or bays (µg/g dw).
TEL, level of threshold effects, the concentration below which adverse biological effects rarely occur.PEL, level of probable effects, the concentration above which adverse biological effects frequently occur.

Table 2 .
Concentrations of heavy metal (µg/g, ww) reported in the organisms in the intertidal zone and values of international guidelines.

Table 3 .
Description of three remediation methods, with their advantages and disadvantages.