Review of Treatment Techniques for Dredged Sediments in the Context of Valorization as Secondary Raw Materials

Abstract

The valorization of dredged sediments (DS) presents a sustainable solution for managing waste while addressing resource scarcity and environmental concerns. This review explores treatment techniques and reuse options for DS, focusing on applications in the construction industry. However, disposal poses challenges due to potential contamination with heavy metals and organic pollutants. The study categorizes treatment approaches into physical, chemical, biological, and thermal processes. Physical methods, such as separation and dewatering, offer volume reduction but have limited capacities against chemically bound contaminants. Chemical treatments, including oxidation and immobilization, target specific pollutants but often entail high costs and environmental risks. Biological approaches, such as bioremediation and phytoremediation, provide sustainable, low-cost alternatives but require longer timescales. Thermal processes like pyrolysis and vitrification efficiently destroy or stabilize contaminants but involve high energy demands. Pyrolysis emerges as a particularly promising technology, combining effective decontamination with energy recovery and biochar production. Despite the advances in the area, the review identifies key barriers to large-scale DS reuse: contamination variability, lack of standardized guidelines, and limited long-term performance data. Future research should focus on integrated treatment strategies, such as combining DS with other industrial by-products, and optimization of processing, aiming to attain cost-effective, sustainable reuse. Overall, the valorization of treated DS supports circular-economy principles and offers significant environmental and economic benefits.

1. Introduction

The sustainability of the building industry is vital to the well-being of our planet and the advancement of human society. However, the production of building materials has significant environmental consequences, including high greenhouse gas emissions, depletion of natural resources, pollution, and excessive energy consumption [1,2]. In particular, recent estimates indicate that Portland cement production is responsible for approximately 8% of total carbon dioxide emissions, despite considerable efforts to improve the environmental footprint of cement manufacturing [3]. Furthermore, the extensive consumption of natural resources for cement production has led to their overexploitation, resulting in environmental degradation and ecosystem disruption [4].

In light of growing urbanization and increasingly advanced housing-quality requirements, it is crucial to seek alternative materials that can mitigate waste production and replace natural resources [5,6]. Therefore, attention has turned to the potential reuse of various types of waste, including waste glass [7], polymeric waste [8], brick waste [9], recycled concrete [10], landfill waste [11], seashell waste [12], recycled plastics [13], different types of slag [14,15], biomass ash [16], and many others. Nevertheless, identifying waste materials that are both suitable for reuse and abundantly available remains a major challenge in addressing sustainability issues in the production of building materials [17,18].

Over the past decade, the sustainable management of dredged sediments (DS) has also emerged as a critical environmental challenge, driven by the need to maintain the functionality of water bodies. In accordance with sustainable development principles, DS has attracted growing interest as a highly abundant waste material, and researchers are exploring a range of potential applications to harness their value across diverse sectors [19,20].

Sediments can generally be classified, based on their origin, into reservoir sediments, river sediments, and coastal or marine sediments [21]. Reservoir sediments, typically deposited through erosion and runoff, are mainly composed of fine particles such as silt and clay and enriched with nutrients and organic matter, but are often contaminated with pollutants. In contrast, river sediments exhibit grain sizes which are more varied, with finer particles deposited downstream and coarser sand accumulating upstream. These sediments are commonly contaminated by municipal waste, industrial discharges, and agricultural runoff [22,23]. Coastal and marine sediments, formed through tidal and riverine activities, usually contain a complex mixture of sand, silt, clay, and organic matter, and are often saline in nature. However, the high salinity of DS reduces their direct usability in service of agricultural or construction purposes, unless appropriate pretreatment measures are applied. Similarly, estuarine sediments, located at the confluence of freshwater and seawater, consist predominantly of fine particles and often serve as sinks for contaminants transported from upstream sources [21,24,25].

Periodic removal of accumulated sediments is essential to preserve reservoir functions. However, the safe and efficient disposal of dredged DS remains a significant concern [26]. Moreover, human activities such as deforestation, agriculture, and construction expose soil surfaces, leading to erosion and increased sediment deposition in rivers, lakes, and coastal zones [27,28,29].

Repurposing DS has increasingly been recognized as a sustainable and economically viable solution. Properly treated DS can offer considerable environmental benefits, such as land reclamation, erosion control, habitat restoration, and coastal protection, in addition to providing economic advantages by conserving natural resources and reducing the demand for conventional construction materials [27,30].

However, depending on the location and type of water body, DS may contain significant levels of contaminants such as heavy metals and organic pollutants [26]. In particular, DS near urban centers and industrial areas often exhibit contamination by hydrocarbons, pesticides, and chlorinated solvents [27]. Heavy metals, including cadmium, chromium, nickel, and arsenic, are typically bound to sediments through adsorption, ion exchange, or complexation mechanisms, with their retention and mobility depending on the geochemical composition of the DS material [31,32,33]. Improper disposal of contaminated sediments can lead to the leaching of pollutants into water bodies, land degradation, and major environmental and public health risks. Therefore, the reuse potential of DS largely depends on the specific characteristics, and the requirements of various industries [34,35]. Accordingly, sediment characterization is critical to determine the composition and toxicity levels, and the development of effective treatment solutions, in order to meet safety standards, is paramount prior to disposal or reuse [27,36,37,38].

Among the various reuse possibilities, non-toxic or appropriately treated DS have found promising applications as raw materials in the construction industry, agriculture, and soil filling. Identifying suitable reuse pathways not only helps to minimize disposal challenges but also contributes to reducing waste generation, cutting costs, and supporting the principles of a circular economy.

Nonetheless, the lack of standardized national guidelines leads to inconsistencies in DS management practices. As the global production of dredged sediments increases and environmental regulations continue to evolve, traditional disposal methods such as direct landfilling are becoming less viable [39]. Nowadays, landfilling is prohibited in most EU regions. However, the complexity surrounding sediment disposal still warrants focused attention, as it represents a multifaceted and critical issue. The Water Framework Directive (WFD 2013/39/EU of 24 August 2013) provides overarching principles for the management of water bodies in all EU member states. Additionally, the European Waste Directive (75/442/EEC) establishes the point at which dredged sediments are classified as waste, with disposal strategies depending on contamination levels, as defined by the European Waste Catalogue. If dredged material contains hazardous substances at levels above threshold limits, it is categorized as “dredging spoils containing dangerous substances” (item 170,505). The specific criteria for classifying materials as hazardous are outlined in Directive 2008/98/CE of 19 November 2008, Annex III.

Utilization of Sediments

The exploration of multiple applications aims to harness their potential contributions across diverse sectors. However, the feasibility of DS reuse depends strongly on the specific characteristics of the DS and the particular requirements of various industries [34,35].

The idea of repurposing DS for agricultural use has been suggested by several authors [40,41,42,43,44,45] due to the material’s potential as a valuable resource for enhancing soil fertility. In addition, construction projects often encounter challenges related to weak soils and high groundwater levels, both of which significantly affect soil stability and load-bearing capacity [46,47,48]. Therefore, DS, recognized as a potential material for soil amendment, hold promise in this context. However, these applications are also challenged by certain limitations, including poor mechanical properties, weak load-bearing capacity, high compressibility, and low permeability [49].

Nevertheless, DS have already been successfully reused for land reclamation from water bodies, road filling, infrastructural developments, and the leveling of areas with topographical variations. Several studies, both at the laboratory and the field scale, have demonstrated the viability of DS for soil fill applications. For instance, Wang et al. [35] developed soil-filling material using contaminated DS from coastal areas in Hong Kong. Similarly, Loudini et al. [50] produced road base-layer material from DS sourced from the Moroccan harbor. Furthermore, Palaparthi et al. [36] utilized DS for dune and back barrier restoration projects in Palm Beach County, Florida. Typically, raw DS are treated to remove debris and contaminants to meet required quality standards. They are then transported, placed in layers, evenly distributed to achieve the desired flatness or topography, and compacted to attain the necessary density and mechanical stability at the target site [50].

DS can also contribute significantly to erosion control measures by stabilizing slopes, preventing soil erosion, and maintaining the integrity of surrounding infrastructure. For example, dams can be constructed from various materials, including earth, concrete, masonry, and rockfill, with the choice depending on the site’s geology and the availability of construction materials [33,49]. In addition, DS can serve as a valuable resource in building construction. The material can be incorporated into different building materials, such as bricks [51,52,53,54,55,56], blocks [35,57,58,59,60,61,62], aggregates in concrete [63,64,65,66,67,68,69,70], blended cement [71,72], and alkali-activated materials [58,73,74,75,76], among other applications.

Coarse DS fractions, such as gravel and larger particles, are particularly suitable for applications like dam cores, in which they help strengthen the structure’s integrity [77,78]. Fine-grained DS can be used as backfill material behind retaining walls, while other DS materials are suitable for grouting applications, contributing to seepage control and improving the overall performance of water-retention structures [28,79].

If properly treated and applied, DS can support environmentally friendly construction practices involving highways, roads, airports, and similar infrastructure projects, showcasing the material’s versatility and practical advantages [80,81]. Moreover, as the construction industry faces increasing pressure to secure raw materials and reduce its environmental footprint, the reuse of processed DS offers a promising shift toward more sustainable and circular construction practices [46,82]. Therefore, incorporating DS into building materials not only supports sustainable construction initiatives but also promotes the development of energy-efficient buildings and the adoption of circular-economy principles [52,55,56,83].

Nevertheless, challenges remain in meeting the stringent environmental standards required for building-related applications [34,84,85]. Considering the large volumes of DS generated worldwide and the potential hazards associated with their disposal, repurposing DS plays a critical role in future management strategies by offering both environmental benefits and economic savings through reduced dependence on traditional raw materials [38,86].

Despite considerable advances in DS treatment technologies, a clear scientific gap remains in the systematic linking of treatment methods to specific sediment properties and intended reuse applications. Most existing studies focus on isolated remediation techniques without fully considering the variability in the characteristics of the DS, site-specific conditions, or the long-term environmental and economic implications of reuse. In addition, comprehensive evaluations comparing the effectiveness, sustainability, and practical feasibility of different treatment strategies are still limited. These gaps highlight the need for integrated research that not only assesses treatment efficiency but also supports the safe, cost-effective, and sustainable valorization of DS. Therefore, this study aims to critically synthesize the existing knowledge, evaluate the suitability of various treatment approaches for building material applications, and identify priorities for future research and development.

2. Research Methodology

A systematic methodology was applied to secure transparency, validity, and clarity of communication while aiming to provide a coherent overview of treatment methods related to sediment utilization for construction purposes. In this sense, the following methodology steps are involved: 1. definition of the review hypothesis, 2. identification and selection of relevant literature sources, and 3. analysis of the selected literature. s defined in Figure 1.

The review hypothesis was consequently defined: “What techniques of water sediment treatment are available for dredged sediments to reduce their content of pollutants and meet requirements on building materials?”

This review aimed to identify and analyze existing treatment methods for dredged sediments intended for subsequent reuse, particularly in construction-related and environmental applications. A comprehensive literature search was conducted using the Web of Science database. The search terms included combinations of keywords such as the following:

“dredged sediment”, “sediment treatment”, “sediment reuse”, “sediment remediation”, “dredged material management”, and “construction applications of sediments”.

Boolean operators (AND, OR) were used to refine and expand the search results where appropriate. The search was limited to peer-reviewed journal articles, conference proceedings, and official reports published in English between 2010 and 2025 to capture recent developments and technological advances. Relevant research papers were selected based on the following criteria:

• Studies focused on treatment technologies (physical, chemical, biological, or combined) for dredged or water sediments.
• Research addressing the reuse potential of treated sediments (e.g., construction material, land reclamation, and soil amendment).
• Papers providing experimental data, field application results, or systematic evaluations of treatment methods.

Initially, the relevance of each article was assessed based on its title and abstract. Full-text copies of potentially eligible studies were then retrieved for detailed evaluation against the inclusion criteria. Studies focused solely on sediment characterization without proposing treatment techniques, and studies that dealt with sediments unrelated to reuse were excluded from the analysis. Research papers without complete datasets and those not available as full-text copies were excluded from the analysis as well. Initially, about 616 papers were identified. After removing duplicate (−45), not relevant (−328), and not-available records (−115), about 128 papers on this topic were selected.

Limitations of the Research Methodology

Despite its systematic approach, this review has several potential limitations.

First, the reliance on specific databases and English-language publications may have introduced language bias and missed relevant research published in other languages.

Second, although careful screening was employed, there remains a risk of selection bias due to subjective judgments during the inclusion/exclusion process, particularly for studies with limited information in their abstracts. Finally, the rapid nature of advancements in sediment treatment technologies means that relevant very recent studies (published after the search cut-off) may not have been captured.

An analysis of keywords, using VOSviewer 1.6.20 is provided in Figure 2, while the regional coverage is given in Figure 3.

3. Treatment Techniques

Different treatment techniques for DS have been developed over time to address the challenges associated with their safe disposal and reuse. The continued advancement of these technologies is crucial, as it will enable substantial quantities of DS to become available for safe and beneficial applications. Effective treatment solutions not only significantly reduce disposal challenges but also help to uphold environmental quality and support economic activities [26,87,88].

As discussed above, meeting the environmental protection standards and regulatory limits remains a major barrier to the wider reuse of DS. Contaminated DS are typically disposed of in specially designed landfills with appropriate soil characteristics and stringent management practices. Although this approach ensures safe disposal, the limited availability of such facilities near dredging sites complicates DS management and results in increased transportation costs. Moreover, the potential leaching of contaminants during transport or storage can pose serious environmental risks. Therefore, to overcome these challenges and promote the safe reuse of treated DS, a variety of treatment techniques have been developed [89,90,91].

DS treatment methods can generally be categorized into four main groups: physical, chemical, biological, and thermal processes [31,89,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128]; these are depicted in Figure 4.

Before selecting a specific treatment technique, it is essential to consider factors such as treatment duration, economic feasibility, and safety.

Physical treatment techniques involve the separation of DS into different fractions according to particle size, dewatering of the fine sediments, and washing processes aimed at removing contaminants. Chemical treatments, on the other hand, include oxidation and immobilization processes that neutralize organic pollutants and reduce the mobility of heavy metals. In addition, high-temperature treatments can be applied to destroy contaminants completely, thereby enhancing the reusability of the DS. Finally, biological treatments, such as bioremediation, use living organisms to degrade or neutralize contaminants and represent a promising, environmentally friendly remediation approach [101].

3.1. Physical Treatment Process

Physical treatment of DS typically involves separating the material into different fractions, such as sand, silt, and clay. The separated fine fractions, particularly silt and clay, are then dewatered to reduce volume, as these materials have high water-retention capacities and tend to serve as major repositories for contaminants. After dewatering, DS can be washed with chemical agents such as acids, bases, or surfactants to remove surface-bound contaminants.

The applicability of physical treatment processes in large-scale DS operations largely depends on the total volume of sediment to be processed. Separation equipment such as vibrating screens, rotary screens, or hydrocyclones can be employed; however, such systems require substantial capital investment in machinery and maintenance. Therefore, operational costs, particularly those related to equipment purchase, energy consumption, and ongoing maintenance, must be carefully considered for field-scale applications. Physical separation can be cost-effective when dealing with coarser particles. However, the material’s efficiency significantly diminishes when processing fine-grained DS, such as clays and silts. Moreover, physical methods alone may not be sufficient for DS contaminated with chemicals or heavy metals, which often necessitate complementary treatment approaches [89,92].

Large-scale dewatering systems are capable of processing considerable volumes of DS, but their efficiency strongly depends on the sediment’s characteristics. Operational costs are influenced by both energy consumption and maintenance requirements. While investments in dewatering equipment and energy infrastructure can be substantial, dewatering remains essential for large DS management projects because it achieves significant volume reduction, thereby lowering the costs of subsequent treatments. Nevertheless, dewatering fine-grained DS poses particular challenges, as clays and silts are notably difficult to dewater. In addition, the residual water generated during the dewatering process often contains contaminants and must therefore undergo additional treatment before safe discharge or reuse, adding further operational expenses [31,129].

Washing treatments can be cost-effective when contaminants are primarily surface-bound and relatively easy to remove. However, this approach becomes less effective—and considerably more expensive—when dealing with contaminants that are embedded deeper within the DS matrix. Furthermore, the costs associated with the use of washing agents (e.g., water, acids, and surfactants) and the treatment of wastewater produced during the process can significantly increase the overall expenses [130,131].

The main types of physical treatment methods are further described in the following sections.

3.1.1. Separation

Physical separation techniques operate under the general assumption that the fine fractions of DS, particularly silt and clay, serve as repositories for adsorbed contaminants, whereas the sand fraction tends to have lower contaminant concentrations. Therefore, the goal of the separation process is to divide the sand from the finer silt and clay fractions. The less-contaminated sand fraction may be suitable for beneficial reuse without the need for further treatment, while the more-contaminated fine fractions require additional remediation.

Separation can be achieved either by particle size or by density differences. Size classification is commonly performed using mechanical devices such as screens and hydrocyclones, although other equipment, such as sand screws, fluidized beds, membranes, and microfiltration units, can also be employed [89,92,94,96].

Screens are particularly effective for separating DS with particle sizes greater than 1 mm, while hydrocyclones are more suitable for sediment slurries with solid-content levels of less than 20%. Hydrocyclones can remove organic contaminants and particles larger than approximately 10–20 μm through centrifugal force [89,92]. Fluidized beds are preferred for the removal of particles smaller than 50 μm, and microfiltration is often considered the most cost-effective separation technique for fine particles [92,95].

Separation based on density differences becomes applicable when there is a substantial variation in the specific gravity of the sediment components. In this method, the uncontaminated, heavier fraction settles at the bottom, effectively separating it from the lighter, more-contaminated portion [95]. For example, density-based classification can be used to separate low-specific-gravity organic contaminants from the denser mineral fraction. This technique is also suitable for isolating metallic fragments from the sediment matrix. Furthermore, froth flotation can be applied to separate coarse, low-density organic material by suspending it in a water current. The equipment commonly used in density separation processes includes mineral jigs and spiral concentrators [95,96,132].

3.1.2. Dewatering

Free water in separated DS must be removed to reduce the volume of the material, thereby facilitating easier handling. In addition, a significant proportion of water-soluble contaminants can be removed during the dewatering process, providing a preliminary level of treatment for the contaminated fraction. The performance of the dewatering process is influenced by several factors, including the material composition, environmental conditions, and the timing of dredging activities. DS rich in silt, clay, and organic matter typically exhibit high water-retention capacity, which makes dewatering more challenging. Furthermore, environmental factors such as air humidity and temperature can affect the evaporation rate; specifically, lower humidity increases water evaporation. In addition, airflow rate, pressure, fluid temperature, and the exposed surface area also significantly influence dewatering efficiency [97,98].

Existing dewatering technologies can be categorized into mechanical, thermal, chemical conditioning, and natural dehydration methods. Mechanical dewatering involves applying compression or shear forces to separate solids from free water. The equipment traditionally used for mechanical dewatering includes filter presses, belt presses, screw presses, centrifuges, and geotextile tubes. These systems are particularly suitable for confined spaces and projects with strict timelines [133].

Thermal processes, such as electro-dewatering, use heat to evaporate water from DS. However, due to the high energy costs associated with this method, its application remains limited. Chemical conditioning, on the other hand, involves the use of flocculants or coagulants to agglomerate fine particles, thus facilitating easier water removal. Often, chemical conditioning is combined with mechanical or other dewatering techniques to improve overall efficiency. Finally, natural dewatering represents a cost-effective and simple solution, wherein free water is drained from the DS in open spaces, ponds, or containers, and evaporation occurs naturally under ambient conditions. Although natural dewatering consumes little energy, it is highly climate-dependent, requires large areas of land, and is generally suitable only for projects without urgent timelines.

The effectiveness of natural dewatering in DS has been studied extensively. Zentar et al. [97] analyzed the relationship between DS evaporation rate and time. Three distinct stages were identified: an initial stage characterized by rapid water-content reduction controlled by drainage capacity; an intermediate stage governed by evaporation potential; and a final stage with a significantly reduced water removal rate. Azaiez et al. [99] reported that the texture and composition of DS significantly affect drainage capacity. Materials with a coarse texture and high organic matter content showed extended drainage capacity, with substantial free-water removal occurring within 24 h. In contrast, fine-textured DS retained a larger amount of water, with the drainage process effectively ending after approximately 8 h. Similarly, Hussan et al. [100] achieved 34% free-water removal after 12 h of natural dehydration, with negligible water removal observed thereafter.

Following dewatering, the retained fine particles still need to undergo further treatment to detach both attached and absorbed contaminants. In this context, DS can be treated with aqueous solutions containing acids, bases, chelating agents, or surfactants to solubilize heavy metals and remove organic contaminants. After chemical treatment, the DS are washed with pressurized water, and the resulting contaminated washing liquid is separated and treated before reuse [31].

However, it is important to note that DS washing is generally more effective for the removal of weakly bound contaminants. Heavy metals absorbed within DS fine fractions may be difficult to extract using washing alone. Therefore, alternative methods such as thermal treatment, electromagnetic field treatment, or froth flotation may be necessary [95]. According to Dermont et al. [134], froth flotation has been successfully applied to remove heavy metals such as cadmium (Cd), copper (Cu), lead (Pb), and zinc (Zn) from DS.

3.1.3. Washing

The properties of DS, such as texture, organic matter content, and cation exchange capacity, play critical roles in the efficiency of contaminant removal. For example, the valency of heavy-metal ions influences their adsorption and desorption behavior. Metals with cationic valency, such as Pb, Cu, Zn, and Cd, are more weakly adsorbed onto DS at low pH levels. As pH decreases, the competition for ligands between H⁺ ions and dissolved metals increases, thereby reducing heavy-metal adsorption and enhancing heavy-metal mobility, which facilitates easier removal [135].

In addition to sediment properties, several process conditions must be considered to achieve effective contaminant removal. These include contact time, temperature, reagent composition, and the dosage applied [136]. Strong acids are widely recognized for their effectiveness in heavy-metal removal. For instance, hydrochloric acid (HCl) forms metallic chloride complexes with heavy metals, while nitric acid (HNO₃) leverages its oxidizing properties to break down metallic–organic complexes [31,136,137]. Beolchini et al. [138] reported a chromium (Cr) and zinc (Zn) removal efficiency of approximately 30% using sulfuric acid (H₂SO₄) at a pH of 2, along with 40% removal for nickel (Ni) and 58% for arsenic (As).

Environmentally friendly organic acids, such as citric acid, ascorbic acid, and ethylenediaminetetraacetic acid (EDTA), have also been used for heavy-metal removal. Their ability to chelate metals and form soluble metallic–ligand complexes makes them attractive alternatives [31,139]. According to Beolchini et al. [138], citric acid exhibited removal efficiencies comparable to H₂SO₄ for many metals, although it achieved only 35% efficiency for arsenic. However, it should be noted that acid washing can adversely affect the mineral composition of treated DS, potentially limiting the material’s applicability for reuse. In this context, washing with milder alternatives such as surfactants can maintain DS properties while facilitating the transfer of contaminants into the aqueous phase for easier removal [140].

Surfactants are biodegradable compounds capable of solubilizing both organic and inorganic contaminants by reducing surface tension. Their hydrophobic tail binds to contaminants, while the hydrophilic head remains in the aqueous phase, forming micelles that detach and encapsulate contaminants for removal [141]. Chemically synthesized surfactants, including both ionic and non-ionic types, have been extensively used in soil washing applications [142]. However, biosurfactants derived from microbial sources (e.g., glycolipids) and plant sources (e.g., saponins) offer several advantages, including low toxicity and biodegradability, and high efficiency in reducing surface tension and critical micelle concentration (CMC) values. Biosurfactants can lower the surface tension of water from 72 to 27 mN/m at a concentration as low as 0.005% [135,142].

The application of biosurfactants in DS decontamination is conceptually similar to bioremediation techniques used for oil-contaminated soils [142]. Biosurfactant molecules facilitate heavy-metal removal by weakening interfacial energies, enabling adsorption onto micelles, and promoting detachment into the solution phase [31]. For example, Dahrazma and Mulligan [143] successfully solubilized copper, nickel, and zinc at rates of 37%, 27%, and 13%, respectively, using rhamnolipid, a glycolipid-based biosurfactant. Mulligan et al. [142] achieved 25% removal of Cu and 60% removal of Zn using 4% sophorolipid. Moreover, combining biosurfactants with other reagents has been shown to significantly enhance solubilization efficiency. For instance, a mixture of 4% sophorolipid and 0.7% hydrochloric acid achieved 100% removal of Cu and Zn, while combining 0.1% surfactant with 1% sodium hydroxide removed 70% of Cu and 40% of Zn, demonstrating the potential effectiveness of biosurfactant-assisted DS decontamination strategies.

Following washing, the treatment liquid must be further processed to separate and remove the dissolved contaminants, thus allowing the extractant to be reused. However, synthetic surfactants present drawbacks, including low biodegradability, potential toxicity, and poor environmental compatibility. Therefore, treated wash water must be carefully managed before its discharge to the environment. Alternatively, the use of organic extractants offers several advantages, including lower chemical consumption, reduced energy requirements, cost-effectiveness, and minimized effluent discharge [93].

3.2. Chemical Treatment Process

Chemical treatment is a versatile approach to mitigating the environmental impacts of contaminants by using various chemical agents to eliminate or stabilize pollutants within the DS matrix, enabling the material’s safe reuse. The selection of a specific treatment method depends largely on the type of contaminants present and the properties of the DS. Chemical treatment methods can be categorized into the techniques of chemical oxidation and immobilization.

In large-scale applications, achieving uniform distribution of the oxidant within the DS can be challenging during chemical oxidation. Moreover, determining the exact dosage to apply is often difficult, which may result in inconsistent treatment outcomes across the site. In addition, handling and transporting bulk quantities of chemicals can be logistically demanding, and managing the by-products generated during the treatment process presents an additional burden.

However, immobilization methods are generally more suitable for large-scale applications due to their straightforward operation and implementation. Immobilization typically relies on inexpensive industrial wastes or binders, which are physically mixed with DS to bind contaminants and reduce their mobility. In comparison, chemical oxidation tends to be more expensive, primarily due to the costs of chemical reagents and the need for careful application. Nevertheless, both the chemical oxidation and the immobilization methods often require additional post-treatment processes, including disposal or secondary waste management [31,102].

Despite their effectiveness, chemical treatment methods face certain challenges. In particular, the potential environmental impacts of chemical additives and the need for long-term monitoring to ensure treatment stability remain significant concerns.

3.2.1. Chemical Oxidation

Chemical oxidation is a widely applied treatment technique applied to contaminants of concern, particularly organic pollutants such as hydrocarbons, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and certain heavy metals. This method utilizes strong oxidizing agents to generate highly reactive radicals capable of mineralizing organic compounds. Among the commonly used oxidants, hydrogen peroxide, permanganates, ozone, persulfate, and Fenton’s reagent are the most prominent [101,102,103]. According to the findings reported by Ferrarese et al. [102], chemical oxidation is an effective technique for DS treatment; however, the material’s performance varies depending on the specific oxidant employed. The best results were achieved using, severally, modified Fenton’s reagent, hydrogen peroxide, and potassium permanganate, although determining the optimum dosage for each particular set of conditions remains necessary.

Hydrogen peroxide (H₂O₂) is a widely used oxidizing agent applied at concentrations ranging from 3% to 35% to neutralize organic and inorganic contaminants. Upon decomposition, H₂O₂ rapidly generates hydroxyl radicals which can react with contaminant molecules. To enhance radical formation and improve the oxidative strength, H₂O₂ is often combined with iron salts in a Fenton catalytic reaction [102,144].

Modified Fenton processes have been developed to address the limitations of the traditional Fenton reaction, such as narrow operational pH ranges and rapid iron precipitation. Modifications allow for a broader range of working pH, extending into neutral and alkaline conditions. In addition, the addition of chelating agents, such as ethylenediaminetetraacetic acid (EDTA), reduces iron precipitation. Although the use of highly concentrated H₂O₂ can enhance hydroxyl radical generation, it also introduces the risk of complex side reactions. In particular, excess hydrogen peroxide can generate additional reactive species, such as hydroperoxide radicals, and cause oxidant self-consumption, ultimately reducing treatment efficiency if not properly managed [102].

It is important to note that the addition of iron salts is not always necessary to promote the catalytic reaction. Naturally occurring iron minerals, such as the hematite and magnetite present in DS, can catalyze the formation of hydroxyl radicals [102,103]. Previous studies have demonstrated the effectiveness of modified Fenton processes in removing organic contaminants from DS. For example, Usman et al. [104] achieved substantial hydrocarbon degradation rates ranging from 40% to 70% by applying magnetite-catalyzed Fenton reagents, while the use of oxidants alone without catalysts resulted in a degradation of less than 5%. Similarly, in another study, 84.7% to 99.9% of PAHs were successfully removed from contaminated soils [145]. Ferrarese et al. [102] also reported excellent PAH removal efficiencies of approximately 95% when applying modified Fenton’s reagent with a catalyst-to-oxidant molar ratio of 1:50.

3.2.2. Immobilization

Unlike organic contaminants, heavy metals are not degradable, but they can be transformed into less harmful forms. Their bioavailability depends on the properties of the DS, such as pH, cation exchange capacity, organic matter content, clay mineral presence, metal oxide phases, carbonate content, buffering capacity, redox potential, water content, and temperature, as well as the chemical properties of the metals themselves and the microbial activities within the DS [105]. Ambient temperature generally promotes metal transformation and microbial activity; however, excessively high temperatures can disrupt stable bonds and induce metal remobilization. Consistent moisture levels are essential for heavy-metal immobilization and stability. Dry conditions introduce oxygen and promote metal release, while high moisture under anaerobic conditions can foster the formation of stable metal sulfides. Furthermore, alkaline environments promote metal precipitation and stability, whereas acidic conditions enhance metal remobilization.

Competition for binding sites in the DS is another critical factor, as increased salinity can displace previously bound metals, leading to their release. Metals with higher charges tend to bind more strongly to DS particles, thus promoting immobilization. Stable, insoluble metals are generally easier to immobilize than more soluble forms. Organic matter can form complexes with metals, aiding in immobilization; however, decomposition of organic matter over time may release the bound metals back into the environment [105,146].

Mobilization of heavy metals can occur when environmental changes or disturbances, such as sediment dredging, disrupt the DS matrix. Mobilization may happen through physical desorption or chemical transformation of contaminants into more mobile forms, consequently increasing the toxicity of the DS. Therefore, several remediation techniques have been developed to reduce heavy-metal mobility by stabilizing the contaminants within the DS matrix [147].

The immobilization techniques involve binding the heavy metals and locking them within the DS structure using mineral binders. The binder is uniformly mixed with DS and allowed to react over a period during which toxic metals are chemically stabilized, thereby reducing their leachability and bioavailability [31,106,107].

Various physical and chemical processes have successfully demonstrated heavy-metal stabilization in DS. Cementitious binders such as lime, fly ash, and Portland cement; mineral-based agents like phosphates; iron-bearing materials (e.g., zero-valent iron, goethite, hematite, and ferrihydrite); and adsorbents such as aluminosilicates (e.g., clays and zeolites) have all been utilized. Different stabilization mechanisms have been reported, including precipitation, surface complexation, ion exchange, and formation of amorphous solids [107].

Heavy metals bond with different fractions within DS, and their toxicity and mobility depend on various environmental factors, such as pH and binding method [106,108]. As sediment pH decreases, competition between hydrogen ions (H⁺) and dissolved metal ions for available ligands (e.g., OH⁻, SO₄²⁻, CO₃²⁻, S²⁻, Cl⁻, and phosphates) intensifies. This leads to reduced heavy-metal adsorption and increased mobility. For example, Covelo et al. [148] demonstrated that in acidic soils with pH values between 4.6 and 6.6, Zn and Cd exhibited higher mobility compared to Pb. Similarly, the increased solubility of organic matter enhances heavy-metal mobility by forming soluble metal–organic complexes.

DS sites are often contaminated by multiple heavy metals with varying chemical properties, making the selection of an appropriate stabilizer and dosage challenging. Therefore, mixtures of stabilizing agents are frequently used to address multi-element contamination. For instance, Qian et al. [106] reported that while phosphate minerals such as apatite effectively immobilized Pb²⁺, they were less effective against other heavy metals. Moreover, the presence of soluble organics could induce contaminant desorption by forming soluble metal complexes. Consequently, combining zero-valent iron (Fe⁰) with apatite was found to stabilize multiple metals, including Pb²⁺, Zn²⁺, Cu²⁺, Cd²⁺, and Ni²⁺, in multi-contaminated DS. Additionally, Fe²⁺/Fe³⁺ salts combined with lime proved to be more effective stabilizing agents for arsenic-contaminated DS than were Fe⁰ or iron compounds alone [110].

Using lime alone is an effective technique in stabilizing cationic pollutants such as Cd, Cu, Ni, and Zn. However, it can also promote the mobility of anionic species such as As, Mo, and Cr⁶⁺ [107]. Similarly, cementitious materials, including Portland cement, alkali-activated materials, and clays, have been shown to effectively stabilize a broad range of inorganic contaminants. These materials physically encapsulate and chemically bind with metals to form solid aggregates, thus preventing leaching. Ordinary Portland cement (OPC) is widely used due to its low cost, availability, reliable hardening properties, and consistent composition [147,149].

Nevertheless, the hydration process of cement can be adversely affected by the presence of certain contaminants. Metals may interfere with the cement matrix, altering hydration rates, while high sulfate concentrations can negatively impact solidification. Therefore, careful cement mix design is critical to ensuring successful stabilization. Guo et al. [126] reported that Pb compounds were effectively immobilized in fly ash through both physical encapsulation and chemical bonding; additionally, precipitates were formed that inhibited the leaching of Pb. Furthermore, Zhang et al. [150] confirmed that kaolinite and limestone effectively reduced the bioavailability of Zn, Pb, and Cu by 11.61%, 2.73%, and 3.85%, respectively. However, limestone generally exhibited a stronger immobilization effect, compared to kaolinite [151].

3.3. Biological Treatment Process

Organisms such as microbes and plants are employed to reduce contaminants, with the processes producing substances that are less harmful. The efficiency of biological treatment processes depends on the properties of the DS, including moisture content, pH, nutrient levels, and the types of contaminants present. In addition, environmental factors such as temperature, light availability, and salinity can significantly influence treatment performance. Compared to physical and chemical treatments, biological methods are relatively cost-effective, as they typically require fewer chemical inputs and consume less energy. However, one notable limitation is that biological treatments generally require longer periods to achieve desired outcomes.

The most common biological treatment approaches include bioremediation, phytoremediation, and natural attenuation. Each strategy offers specific advantages and limitations and is suitable for different environmental conditions. Bioremediation is highly versatile and effective against a wide range of organic and inorganic contaminants. However, its success depends on maintaining favorable conditions to ensure optimum microbial functionality. Key factors affecting bioremediation efficiency include temperature, pH, moisture content, nutrient availability, and the types and concentrations of contaminants. Microbial activity tends to be most efficient under ambient temperatures and neutral-to-alkaline pH conditions. Conversely, high contaminant concentrations or high toxicity levels can overwhelm microbial populations, thus reducing treatment efficiency.

Bioremediation can be applied to large volumes of DS at a large scale through techniques such as landfarming or the use of bioreactors. However, achieving consistent results remains challenging, especially when dealing with heterogeneous contamination and fluctuating environmental conditions. For instance, highly acidic or alkaline environments can inhibit microbial activity; drought conditions can reduce microbial populations due to desiccation; cold weather slows down microbial processes; and extreme temperatures may even eliminate microbial communities altogether [89,152].

Phytoremediation, by contrast, relies on plant growth and thus represents a slower treatment process compared to bioremediation, and one requiring a long-term commitment. The success of phytoremediation depends on selecting the appropriate plant species, as well as on the characteristics of the DS, ambient temperature, and moisture conditions. Moreover, phytoremediation is not suitable for all types of contaminants but has shown effectiveness in removing heavy metals. However, its success is limited by the depths at which contaminants reside within the DS, as root systems may not always reach deeper layers [153,154].

Natural attenuation is another sustainable biological treatment option, relying on natural processes such as microbial degradation and chemical transformation to reduce contaminant concentrations without human intervention. It is a cost-effective method suitable for managing low levels of contamination. However, natural attenuation is generally slow and may be insufficient for treatment of highly contaminated DS [111,155].

Details on the individual biological treatment processes are further elucidated in the following sections.

3.3.1. Biodegradation

Biodegradation is a cost-effective and environmentally friendly in situ remediation approach that employs living microorganisms, such as bacteria, fungi, algae, and cyanobacteria, for contaminant removal [101]. It uses metal-reducing microbes under aerobic conditions to break down organic contaminants into less-toxic substances, such as H₂O and CO₂, and under anaerobic conditions, in the absence of oxygen, to produce methane and hydrosulfide. Microorganisms typically utilize these contaminants as sources of carbon and energy, and this approach has proven effective against a wide range of heavy metals and organic pollutants.

According to Ayilara et al. [111], bacterial species such as Brevibacterium iodinum, Pseudomonas aeruginosa, Pseudomonas fluorescens, and Alcaligenes faecalis, along with fungal species such as Saccharomyces cerevisiae, are highly active soil bioremediators. The effectiveness of other species, including Anaeromyxobacter sp., Comamonas sp., Saccharibacteria sp., Desulfomicrobium sp., Acinetobacter sp., Zoogloea sp., and Thiobacillus sp., in the bioremediation of organic compounds has also been reported. Furthermore, El-Sheekh et al. [156] demonstrated the ability of algal species such as Scenedesmus obliquus and Chlorella vulgaris to degrade petroleum hydrocarbons.

Microorganisms have evolved several mechanisms to reduce heavy-metal toxicity. They achieve this through various biological processes, including bio-transformation, in which enzymes break down complex metal structures into simpler and less harmful forms [89]. Metals can either be oxidized, such as with mercury and cadmium, or reduced, such as with arsenic and iron, depending on the microbial enzymatic pathways [142,157]. Additional microbial mechanisms for decontamination include transportation across cell membranes, biosorption or bioaccumulation of metals onto cell walls, entrapment within extracellular capsules, precipitation, complexation, and oxidation–reduction reactions [105]. Through these processes, microbes limit the cellular uptake of metals, form physical barriers to metallic ion penetration, and perform enzymatic detoxification and degradation of contaminants [89,158].

Although many microbes are capable of surviving in extreme conditions, their bioremediation effectiveness depends heavily on environmental parameters such as temperature, nutrient levels (particularly phosphorus and nitrogen), DS pH, oxygen availability, contaminant toxicity, and moisture content. For instance, highly acidic or alkaline conditions can inhibit microbial activity, drought-induced dryness can reduce microbial populations, and cold temperatures can significantly impair microbial performance. Moreover, microbial decontamination efficiency can vary across contaminant types, and treatment times may be extended and less predictable. Consequently, it is crucial to assess the biodegradability potential of selected microbes for specific contaminants before application [112].

In this regard, Ahemand [105] highlighted that the use of specific plant growth-promoting bacteria with heavy-metal detoxification abilities as biofertilizers can significantly enhance plant growth in contaminated soils. For example, the introduction of Sphingomonas macrogoltabidus and Microbacterium liquefaciens into Alyssum murale led to increased nickel (Ni) uptake and improved plant growth. Similarly, the mung bean plant (Vigna radiata) was protected from cadmium (Cd) and lead (Pb) toxicity by inoculation with the siderophore-producing bacterium Pseudomonas putida KNP9 [159].

Therefore, the use of microbes for DS decontamination represents an ecologically and economically viable remediation strategy [112]. However, practical field applications face challenges. Processing costs may rise due to site preparation, biostimulation activities, and the need for ongoing monitoring and maintenance over extended periods, thereby increasing overall operational expenses.

3.3.2. Phytoremediation

Research has shown that plants possess the genetic potential to reduce contaminant toxicity in the environment. Phytoremediation techniques include phytoextraction, phytostabilization, phytovolatilization, phytodegradation, and phytofiltration. Among these, phytoextraction remains the most promising method for the removal of heavy metals and certain organic pollutants [113]. In phytoextraction, contaminants are taken up by plant roots, translocated to the shoots via biochemical processes, and accumulated in vacuoles, from where they can be removed through biomass harvesting [160]. For example, mycorrhizal fungi hosted near plant roots can enhance water and nutrient uptake and simultaneously facilitate the removal of heavy metals and organic contaminants [112,113].

The phytoextraction process can be divided into three main stages: planting suitable species in contaminated soil, harvesting the metal-accumulated biomass, and managing the harvested plant material along with the soil [114]. The effectiveness of phytoextraction depends on several factors, including soil properties, heavy-metal speciation, and plant species selection. Therefore, site-specific studies are essential in order to identify the appropriate plants for particular contaminants. Selected plants must be easy to cultivate, tolerant to toxicity and climatic stress, and characterized by a high growth rate.

Recent research has highlighted the effectiveness of certain hydrophyte species, such as Eichhornia crassipes and Hydrilla verticillata, in the biosorption of heavy metals from DS. Their remediation mechanisms rely on the precipitation of insoluble metal compounds and redox reactions, which limit the movement of heavy metals through plant tissues. Eichhornia crassipes has shown strong cadmium-absorption capabilities, while Hydrilla verticillata has been reported to effectively remove copper (Cu) and lead (Pb) from DS [105,115].

Furthermore, Dahmani-Muller et al. [117] reported on the metal uptake and accumulation capacities of metallophyte and pseudometallophyte species. High metal concentrations were observed in both the leaves and the roots of metallophytes, whereas pseudometallophytes predominantly immobilized metals within the root system. Plant families such as Brassicaceae, Euphorbiaceae, Asteraceae, Lamiaceae, and Scrophulariaceae have similarly been identified relative to their potential in lead (Pb) uptake [116].

However, several limitations hinder the performance of phytoextraction processes. Phytoextraction is inherently a slow and selective process, heavily influenced by climatic conditions, soil pH, and contaminant toxicity levels. High toxicity can restrict plant growth rates, while hyperaccumulator plants, which absorb large quantities of contaminants, often grow slowly and produce limited biomass. Moreover, the harvested plant biomass itself becomes hazardous and must be handled carefully and safely disposed of [154,161].

3.3.3. Natural Attenuation

Natural attenuation represents the simplest form of biological treatment, relying on the natural degradation or transformation of contaminants without direct human intervention. In this approach, contaminants are left in a monitored natural state over time, allowing native microbial populations to drive self-amendment processes. Natural attenuation is typically a slow remediation method that requires extensive long-term monitoring and is most applicable under conditions of low contamination.

The effectiveness of natural attenuation depends heavily on site-specific factors, including pH, temperature, nutrient levels, and the types and concentrations of contaminants present. However, one major concern with this approach is the potential migration of contaminants into surrounding environments, including groundwater systems, during the extended remediation timeframe. Contaminants can also spread via dilution, contaminating larger volumes of groundwater. Therefore, it is critical to assess and estimate the potential movement of contaminants before applying natural attenuation strategies [162,163].

Despite these challenges, natural attenuation has been successfully applied in hydrocarbon-contaminated soils. For instance, Erkelens et al. [164] reported a 70% success rate in remediating trinitrotoluene (TNT) contamination, while Guarino et al. [165] documented a 57% reduction in total petroleum hydrocarbons (TPH) via natural attenuation.

The performance of natural attenuation can be further enhanced by combining it with techniques such as biostimulation or bioaugmentation. In biostimulation, the native microbial populations are stimulated by removing growth-limiting factors. This is achieved by supplementing nutrients, such as phosphate, nitrate, and salts, often sourced from fertilizers or nitrogen-rich organic matter, to enhance microbial degradation activity. However, degradation of contaminants in DS often relies on anaerobic metabolism, necessitating the addition of electron acceptors like Fe(III), sulfate, nitrate, or humic substances. For example, Perelo [118] successfully degraded benzene in petroleum-contaminated soil under anaerobic conditions through the addition of electron acceptors.

Although biostimulation can yield high success rates, it is not without challenges. The addition of nutrients and electron acceptors can be costly and may introduce long-term environmental risks, such as eutrophication, acidification, or increased soil salinity if excess nutrients are not fully utilized by the microbial communities. Furthermore, competitive nutrient uptake may allow certain fast-growing microbes to dominate the community, potentially reducing overall degradation functionality over time. In addition, anaerobic processes can produce methane and nitrogen oxide gases, contributing to greenhouse gas emissions [93,119].

Another challenge is that indigenous microbial communities may have a limited ability to tolerate, absorb, and degrade specific contaminants, reducing the potential reliance on natural microbial degradation for complete bioremediation. Therefore, a targeted approach is necessary. Site-specific studies should be conducted to identify native microbes with the best adaptability and biodegradation potential. Where indigenous populations are insufficient, laboratory-engineered microbes with enhanced biodegradation capabilities can be introduced through bioaugmentation to strengthen the microbial community. This approach can significantly improve the breakdown of contaminants that native microbes might otherwise be unable to effectively degrade [166].

3.4. Thermal Treatment Process

Thermal treatment requires the application of heat to immobilize heavy metals or destroy organic contaminants. At elevated temperatures, inorganic contaminants such as heavy metals can melt and become encapsulated within the DS matrix, while organic contaminants, including molecular hydrocarbons and polycyclic aromatic hydrocarbons (PAHs), are oxidized. However, unstable organic compounds, such as chlorinated hydrocarbons, may produce more toxic by-products, like furans and dioxins, during the thermal process, necessitating special handling and disposal measures. Although contaminants can be totally or partially disintegrated over a wide range of temperatures within a short period, the thermal process can induce changes in the physical properties of DS, potentially affecting the material’s subsequent engineering applications [120].

Organic contaminants can also be mobilized into the gaseous phase through thermal desorption (TD) or broken down into simpler compounds via pyrolysis. Alternatively, at sufficiently high temperatures, contaminants can be immobilized through vitrification. Organic contaminants generally vaporize at temperatures below 500 °C and are completely decomposed at higher temperatures. In addition, certain heavy metals such as mercury (Hg), cadmium (Cd), and arsenic (As) can be vitrified at temperatures around 1200 °C [31,89].

However, thermal methods face considerable challenges when applied at large scales. High energy requirements, substantial operational costs, and technical complexities make these methods best suited for targeted applications where specific contaminants necessitate specialized treatment. Thermal desorption typically operates within a temperature range of 100 °C to 600 °C and consumes large amounts of energy, coupled with high equipment costs. Moreover, its operation requires air pollution control systems to manage gaseous emissions [31,167]. Similarly, vitrification processes are highly energy-intensive, operating at even higher temperatures (1000–1500 °C), and present technical challenges related to specialized equipment needs and the difficulty of maintaining uniform melting conditions [123,125].

Therefore, both thermal desorption and vitrification are considered less suitable for large-scale DS treatments. However, pyrolysis presents a more cost-effective and sustainable thermal treatment solution, particularly for large-scale remediation processes where energy recovery is desired [127,167].

Further elucidation of each thermal treatment method is presented in the following sections.

3.4.1. Thermal Desorption (TD)

Thermal desorption (TD) is a highly efficient treatment technique suitable for a wide range of contaminants, particularly volatile organic compounds (VOCs). The method offers short treatment times and produces minimal toxic secondary pollution, making it well-suited for treating highly contaminated soils and DS with relatively little deterioration of their physical properties [121].

The working mechanism involves heating DS in equipment such as rotary dryers, fluidized beds, microwaves, or vacuum-enhanced infrared-ray systems to mobilize and remove volatile contaminants. As the temperature rises between 100 °C and 600 °C, contaminants are desorbed, mobilized, and evaporated into a carrier gas stream [167]. Thermal desorption processes are typically classified into high-temperature thermal desorption (HTTD) and low-temperature thermal desorption (LTTD), based on a theoretical boundary at 300 °C, with contaminants’ volatilization being determined by their boiling points.

LTTD is suitable for the removal of low-boiling-point VOCs such as gasoline and benzene, whereas contaminants like PAHs, PCBs, and volatile metals such as lead (Pb) require higher temperatures and are therefore treated under HTTD conditions. The resulting volatile compounds, including particulates, volatile heavy metals, and acid gases, are captured and condensed into concentrated forms; this is followed by treatment of the off-gas via filtration and activated carbon systems before final disposal [31,122].

TD can be operated either as an in situ or ex situ process. The in situ method is relatively simple and cost-effective but typically requires longer remediation times compared to ex situ operations. Ex situ thermal desorption, while more complex, achieves higher treatment efficiencies and is better suited for highly contaminated DS, although it typically handles smaller batches of material at a time.

In ex situ setups, the heating process can be either direct or indirect. Direct heating, involving direct contact with a flame, increases heat transfer efficiency and lowers heating costs compared to indirect heating methods, which rely on heat conduction. Although direct heating produces a smaller volume of off-gas, it requires only a relatively simple and compact off-gas treatment system [121,122].

3.4.2. Vitrification

Vitrification transforms DS into stable glass-like or crystalline materials when subjected to high temperatures, causing contaminants such as heavy metals to melt within the DS matrix. The material is heated in an incinerator, together with glass-forming precursors as additives, at temperatures ranging from 1000 °C to 1500 °C, producing an amorphous, homogeneous, glass-like single-phase solid upon cooling [123].

For example, Li et al. [124] successfully converted residue fly ash into a useful construction material through vitrification. The residue was heated at 1450 °C for 1.5 h using bottom ash or cullet as an additive. It was observed that increasing the amount of the additives reduced the basicity of the mixture, leading to a more amorphous glassy matrix. Vitrification achieved a volume reduction of over 50%, and the resulting product exhibited small porosity, low water absorption, and high compressive strength.

Vitrification offers several advantages, including high destruction efficiency as to organic contaminants, effective immobilization of toxic elements, and significant volume reduction. However, it is associated with a major disadvantage—high processing energy costs [125].

The main factors influencing the immobilization of heavy metals through vitrification are pH, the chemical speciation of the metals, and the redox potential of the system. Regarding organic compounds, immobilization can occur through chemical reactions that destroy or alter organic structures, or through physical processes such as adsorption and encapsulation.

It is noteworthy that certain solid wastes, such as municipal solid waste or DS, can undergo vitrification without the need for additional additives. Although the resulting glass may not be perfectly homogeneous, the vitrified product can still be utilized as a construction material [126].

3.4.3. Pyrolysis

Pyrolysis is a widely used thermal process, commonly applied in charcoal or biochar production, and it can also be used to remediate contaminated soil, DS, and biomass [127]. Pyrolysis presents opportunities for the rapid remediation of contaminated DS without significantly altering the material’s physicochemical properties, thus allowing the treated material to serve as a plant-growing medium [127,168]. Compared to other thermal processes, pyrolysis is a low-energy alternative, capable of saving approximately 40% to 60% of the heating energy. It operates at relatively low temperatures, ranging from 200 °C to 500 °C, in the absence of oxygen, making it a more sustainable option than traditional thermal treatment techniques [167].

During the pyrolytic process, toxic and carcinogenic volatile organic chemicals (VOCs) with high vapor pressure are readily evaporated into the air. Concurrently, biochar, a carbon-rich material derived from the combustion of DS, is formed. This process retains organic matter and significantly reduces the bioavailability of heavy metals. For example, pyrolytic treatment of diesel-contaminated soil for 10 min at 250 °C reduced the total hydrocarbon content from 6272 mg/kg to 359 mg/kg [169]. Similarly, Kim et al. [170] reported a reduction of petroleum hydrocarbons from 4922 mg/kg to as low as 50 mg/kg. Furthermore, the process stabilized heavy metals within DS, reducing zinc and copper concentrations by up to 58.9% and 89.6%, respectively. In addition to contaminant reduction, biochar can contribute to soil nutrients and enhance nutrient retention, thereby offering agronomic benefits not typically associated with other thermal remediation methods [127].

Nevertheless, variables such as heating rate, residence time, and pyrolytic temperature must be carefully considered, as these factors can affect the physicochemical characteristics of the treated DS. For instance, Li et al. [171] reported that high pyrolysis temperatures significantly reduced soil organic matter, increased pH, and altered nutrient availability. However, ref. [167] observed an increase in the water-holding capacity of contaminated soil treated at 400 °C for 30 min, particularly when supported with the addition of 5% hematite. Therefore, optimizing the pyrolytic temperature and heat duration is essential for preserving organic properties and ensuring sufficiently high product quality for use as a plant growth medium, while simultaneously achieving the desired treatment efficiency.

On this note, Kim et al. [170] conducted plant germination and growth experiments to assess the quality of treated DS as a plant growth medium for barley cultivation. Their findings indicated that products derived from pyrolysis at 400 °C were of significantly higher quality.

In summary, treating contaminated DS and transforming the material into a valuable resource offers considerable environmental and economic benefits. Repurposing DS for a range of applications is increasingly recognized as a promising practice for resource conservation, land reclamation and development, erosion control, coastal protection, the production of construction materials, habitat restoration, innovation, and research [27]. DS have already found applications in soil filling, agriculture, coastal nourishment, and construction. Identifying appropriate reuse strategies will help reduce waste generation, save costs, and support circular-economy principles.

However, despite the advantages, several challenges remain, including issues related to public acceptance, regulatory compliance, and the need for continued technological advancement. Therefore, it is crucial to ensure the safe reuse of DS by developing cost-effective treatment solutions capable of mitigating contaminants in order to meet established safety standards [36].

A summary of the different DS treatment techniques, along with key operational parameters, is presented in

Table 1. Summary of treatment methods.

Resource Recovery Process	Treatment Techniques	Purpose	Contents Removed	Pros	Cons	Source
Physical	Separation using centrifuge, screening, hydrocyclone.	Separation of small and fine contaminated particles.	Contaminated fine particles.	Efficiency.	Energy consumption.	[89,92]
	Dewatering using filter presses, draining screens, natural evaporation and drainage.	Removal of free and interstitial water. Treatment of contaminated fraction.	Water and contaminants.	Reduces volume of DS. Natural dewatering is more cost-effective and simple. Energy efficient.	Climate-dependent; requires large area.	[129,172]
	Washing with water or chemicals—acids, bases, surfactants.	Neutralization of toxic contaminants and salts.	Heavy metals and salts.	Depends on the reagent used, dosage, pH, contact time.	High energy; water and chemical consumption.	[130,131,173]
Chemical	Oxidation, uses hydrogen peroxide, Fenton reagent, ozone, potassium permanganate.	Neutralization of organic contaminants.	Organic contaminants.	Fenton reagents produce excellent performance when catalyzed.	Consumption of chemicals; poor heavy-metal removal.	[101,174,175]
	Immobilization, uses cement, lime.	Reduction in heavy-metal mobility.	Heavy metals	Less expensive.	Possible escape of contaminants.	[31,120,176]
Thermal	Thermal desorption, operates at 100 to 600 °C with a boundary temperature of 300 °C.	Removal of volatile organic compounds (VOCs) and metals.	Mostly organic contaminants.	Short treatment time with low secondary pollution. Suitable for highly contaminated DS, with low effect on the material’s properties.	Air pollution with environmental impact.	[121,177]
	Pyrolysis, operates at low temperatures of 200 to 500 °C.	Burning off of organic contaminants and reductions in heavy metals in the absence of oxygen	VOCs and heavy metals.	Remediates DS without destroying the material’s properties. Low heat consumption. Treated DS are suitable for agriculture.	High energy consumption.	[127,167,170]
	Thermal vitrification, operates at high temperatures of 1000 to 1500 °C.	Contaminants immobilized by transforming the DS into stable or crystalline materials.	VOCs and heavy metals	High destruction efficiency as to organic contaminants. Immobilizes toxic elements. DS volume reduction.	High energy consumption.	[38,178]
Biological	Bioremediation, uses living microorganisms—bacteria, fungi, algae, cyanobacteria.	Degradation of organic contaminants and heavy metals by microbial activity.	Targeted contaminants.	Low operation cost. Contaminants are digested by microbes. Effective for different types of heavy metals and organic pollutants	Degradation is slow and time-consuming; requires huge land area.	[157,179,180]
	Natural attenuation, contaminants are reduced over time in natural state using native microbial population.	Self-amendment of contaminated DS.	Organic contaminants and some heavy metals.	The method can be improved by combining processes such as biostimulation or bioaugmentation.	Slow process, requires extensive monitoring; effective in a low-contamination situation.	[118,119,165]

.

3.5. Artificial Intelligence Utilization in Sediment Remediation

In recent years, artificial intelligence (AI) technologies have begun to play an increasingly important role in supporting the development and optimization of DS treatment processes. AI techniques, including machine learning algorithms, predictive modeling, and data-driven decision support systems, offer significant potential for enhancing treatment efficiency, reducing operational costs, and improving the environmental performance of remediation strategies [181,182]. For instance, AI models can predict contaminant behavior based on site-specific data, optimize treatment parameters such as temperature, residence time, and reagent dosage, and identify the most suitable remediation techniques for specific contamination profiles [183]. In pyrolysis and thermal treatments, AI-assisted control systems can dynamically adjust operating conditions to maximize contaminant removal while minimizing energy consumption. Similarly, AI-driven monitoring tools can facilitate real-time analysis of microbial activity during bioremediation or natural attenuation, enabling more precise interventions to maintain optimal conditions [184].

However, despite these promising advantages, several challenges remain in fully integrating AI into DS management practices. High-quality, site-specific datasets are required to train accurate AI models, and the inherent variability of the characteristics of DS can complicate data acquisition and model generalization [185]. In addition, AI systems must be carefully validated to avoid biases or overfitting, which could lead to suboptimal or even unsafe treatment recommendations. Furthermore, the application of AI technologies often demands significant initial investments in digital infrastructure, specialized expertise, and long-term maintenance. Nevertheless, as advances in environmental sensing, remote monitoring, and big data analytics continue, the use of AI offers a promising pathway toward more sustainable, cost-effective, and intelligent DS treatment and reuse strategies [186].

4. Conclusions and Recommendations

This review analyzed the current state of treatment methods for dredged sediments (DS) and the reuse potential of this material, particularly in the construction sector. The findings confirm that while significant progress has been made in recognizing DS as a sustainable alternative material, several technical and environmental challenges persist. A wide range of treatment techniques have been developed to mitigate the risks associated with DS reuse. Physical treatments offer simple yet partially effective methods for contaminant reduction. Chemical treatments provide approaches which are more targeted, but are often associated with high operational costs and environmental concerns. Biological treatments offer sustainable, low-cost alternatives, although they require longer timescales and are sensitive to environmental conditions. Thermal treatments can be employed for efficient contaminant removal but are often energy-intensive, making them more suitable for specific, high-value applications. Among these methods, pyrolysis has emerged as a particularly promising option for the sustainable treatment of contaminated DS. Its ability to stabilize heavy metals, retain organic matter, and generate biochar that can enhance soil quality positions it as an attractive solution, especially where energy recovery and agronomic benefits are desired.

Nevertheless, there is a lack of universally accepted protocols for DS treatment and reuse, leading to inconsistencies across studies and practical applications. Most of the available research focuses on short-term performance; the durability, environmental stability, and lifecycle impacts of reused DS are poorly understood. Despite treatment, residual contamination risks exist and have not been fully addressed; additionally, post-treatment monitoring is often insufficient. Fine particle sizes and the variable composition of DS complicate the material’s mechanical stabilization, especially for high-performance applications like heavy-traffic road bases. While promising results have been demonstrated at the laboratory and small-field scales, large-scale practical implementation remains rare.

Future research should explore the optimization of DS mixtures by using other industrial by-products (e.g., slags or fly ash) to enhance mechanical properties and reduce the need for virgin binders. Researchers should investigate cost-effective, large-scale processing methods that would allow DS to be more readily integrated into mainstream construction practices.