Soil Remediation: Current Approaches and Emerging Bio-Based Trends

Abstract

Currently, increasing anthropogenic pressure and overexploitation expose soils to various forms of degradation, including contamination, erosion, and sealing. Soil contamination, primarily caused by industrial processes, agricultural practices (such as the use of pesticides and fertilizers), and improper waste disposal, poses significant risks to human health, biodiversity, and the environment. Common contaminants include heavy metals, mineral oils, petroleum-based hydrocarbons, aromatic hydrocarbons, chlorinated hydrocarbons, and polycyclic aromatic hydrocarbons. Remediation methods for contaminated soils include physical, physicochemical, chemical or biological approaches. This review aims to specify these methods while comparing their effectiveness and applicability in different contamination scenarios. Biochemical methods, particularly phytoremediation, are emphasized for their sustainability, effectiveness, and suitability in arid and semiarid regions. These methods preserve soil quality and promote resource efficiency, waste reduction, and bioenergy production, aligning with sustainability principles and contributing to a circular economy. The integrated phytoremediation–bioenergy approaches reviewed provide sustainable and cost-efficient strategies for environmental decontamination and green development. Special attention is given to the use of lignin in bioremediation. This work contributes to the existing knowledge by outlining priorities for the selection of the most appropriate remediation techniques under diverse environmental conditions, providing a comprehensive overview for future developments.

1. Introduction

Soil contamination is caused by the presence of hazardous chemicals or other changes in the natural soil environment. This issue is primarily attributable to industrial emissions, excessive use of agricultural inputs—including pesticides, herbicides, and fertilizers—leaching of pollutants from waste sites, and the infiltration of contaminated water [1]. In Europe, the most frequently detected pollutants include heavy metals and petroleum hydrocarbons, which together account for nearly 60% of reported soil contamination cases [2]. Additionally, volatile organic compounds (VOCs) such as benzene, toluene, ethylbenzene, xylene (BTEX), chlorinated hydrocarbons (CHCs), and polycyclic aromatic hydrocarbons (PAHs) contribute significantly to environmental degradation due to their persistence, toxicity, and potential to bioaccumulate in ecosystems [2,3,4]. The extent and severity of contamination vary depending on land use, soil type, and pollutant mobility, necessitating tailored remediation strategies to mitigate environmental and health risks (mutagenic, carcinogenic, immunotoxic, and teratogenic effects) [3,5,6,7].

Remediating contaminated sites is essential for both environmental preservation and urban development to restore the functions of contaminated soil. The self-restoration of soils may take a period of 10 to 30 years or longer to occur, depending on the soil type and location [8]. Therefore, active remediation strategies are required to accelerate the recovery process. The procedures to remove soil contaminants are categorized as physical (e.g., soil flushing or thermal desorption), physicochemical (e.g., soil vapor extraction, chemical oxidation or neutralization), and biological or biochemical techniques (e.g., bioremediation, phytoremediation), which leverage microbial activity and plant uptake to detoxify the soil. These remediation methods can be carried out either in the polluted place (in situ) or outside the contaminated area (ex situ) [8,9]. In some cases, mechanical measures are also used [2,10,11]. Depending on the mechanism of action, different measures can be combined, for example, as biochemical–chemical hybrid systems that join bioremediation with chemical oxidation, or phytoremediation with soil amendments as chelating agents. Phytoremediation and bioremediation have been intensively studied as eco-friendly and more efficient alternatives to other chemical technologies [3,6,12]. This review aims to provide a comprehensive analysis of the existing soil remediation technologies, with a focus on their effectiveness, sustainability, and applicability under different contamination types. Special attention is given to biochemical remediation strategies and their potential to enhance contaminant removal.

2. Review Methodology

Scientific sources such as ResearchGate, Google Scholar, Web of Science, and ScienceDirect were used to gather information for this review. The main search term was “soil remediation”. Scientific articles found and considered correct were downloaded. Other keywords such as “bioremediation”, “soil contaminants”, “organic contaminants”, “inorganic contaminants”, “polycyclic aromatic hydrocarbons”, “pesticides”, and “accumulation”, among others, were also searched. With this approach, more than 250 articles published between 1978 and 2024 were chosen. Furthermore, the relevant studies cited were examined.

3. Soil Contaminants and Remediation Processes

3.1. Evaluation of a Contaminated Site

Once the soil on site is identified as potentially contaminated, the process of decontamination commonly proceeds in the following five stages [13,14,15]:

• Preliminary assessment;
• Exploratory investigation;
• Detailed investigation;
• Risk assessment;
• Intervention.

A preliminary study must be conducted to determine which substances that are harmful to the environment and health are present in the soil. This stage is referred to as the preliminary assessment, which will investigate the contaminants that may be present based on previous explorations of the area and the substances used, as well as identify the locations most likely to be contaminated. The next phase is the exploratory investigation (confirmatory and, if necessary, detailed), where it is confirmed whether the soil is contaminated, the degree and type of contamination, as well as the soil characteristics and how they influence the behavior of the contaminants [13,14,16].

Distinct parameters can be analyzed in soil samples to infer the degree of contamination of the selected areas, as shown in

Table 1. Characterization parameters for biological, physical, and chemical treatment techniques [17].

Soil Samples Characterization Parameters
Physical	Chemical	Biological
Moisture Content Field capacity Temperature Oxygen availability Type/size of debris Dioxins/furans, radionuclides, asbestos Particle size distribution Clay content	Total Organic carbon Chemical Oxygen demand Redox potential Carbon/Nitrogen/Phosphorus ratio Metals (total and leachable) Cation exchange capacity; pH	Soil incubation tests Culture studies Bacterial enumeration tests Microbial toxicity/growth inhibition tests Electrolytic respirometer test

, (adapted from previous work [17]). To assess on the most suitable remediation procedure, key parameters are studied to determine the following: treatability of material and treatment process of choice; need for pre-treatment; special waste-handling procedures; volume reduction potential; solid/liquid separability; sorption characteristics of soil; conductivity of air through soil; reagent requirements; need for slurring to aid in mixing; concentration of target or interfering constituents; extraction medium; mobility of target constituents; mineral nutrient requirements; sorption characteristics of soil; potential for generating toxic fumes at low pH; presence of oxidizable organic matter; presence of constituents that could be oxidized into more toxic or mobile forms; biodegradation potential and rates of contaminants; oxygen uptake and biodegradation; indigenous microflora or specifically adapted microflora to be used in inoculum during enrichment procedure; bacterial population density in inoculum; and biological activity in the laboratory.

The fourth stage is the risk assessment, which is based on the data previously collected regarding the use or intended use of the area. Lastly, the intervention phase takes place, which will define the soil remediation process, if necessary. All procedures must be followed to recover the damaged area and make it appropriate for habitation, commerce/industry and/or agriculture [15,16].

This phasing helps enhance decision-making, as more information is gathered thus making the situation clearer. In practice, if there is a possibility of contamination, as identified by environmental specialists, the process to be followed is reviewed in Figure 1. Depending on the assessment of the soil’s hazardousness, waste management is planned regarding its destination, whether for treatment, disposal, or recovery [10,18].

Before implementing the chosen remediation technique, various methods are thoroughly evaluated to minimize the hazards of different pollutants. This evaluation may require in vitro or greenhouse-scale studies to determine the most effective and economically viable option. Once a technique is chosen, it undergoes further feasibility assessment through treatability tests. These tests, primarily carried out in the field using pilot or full-scale systems, are performed by remediation contractors and technology providers to evaluate the process’s effectiveness, cost, and overall performance before full implementation [20].

3.2. Types of Soil Contaminants and Effects of Soil Pollution

Soil formation is a natural process influenced by the climate, timescale, human exploitation, and interactions among these factors. These interactions have resulted in a complex structure composed of the following five major components: mineral matter, water, organic matter, living organisms, and air. The quantity of these components in soil may vary according to the local conditions. Agriculture, industry, and the growth of civilization have had a significant impact on soil characteristics, particularly after the Industrial Revolution [21,22,23]. The contamination of agricultural soil has been triggered by the application of pesticides, fertilizers, sewage application, wastewater irrigation, and other activities whose adverse impacts tend to be chronic [22,24,25]. The presence of pollutants significantly threatens soil ecological functions, plant growth, and, eventually, human health.

3.2.1. Organic Contaminants

Some organic contaminants characterized by high toxicity, persistence, and bioaccumulation in the environment include the following [12,26,27]: (i) organochlorine pesticides; (ii) polychlorinated biphenyls; (iii) phthalate esters (PAEs); (iv) polycyclic aromatic hydrocarbons (PAHs); (v) petroleum hydrocarbons; and (vi) perfluoroalkyl substances (PFASs).

Organochlorine pesticides (OCPs) are synthetic pesticides widely used all over the world. They are part of the group of chlorinated hydrocarbon derivatives that are used in the chemical industry and agriculture [28,29]. Jayaraj et al. conducted an extensive study on the classification of organochlorine pesticides based on their chemical nature, chemical structures, toxicity, main uses, persistence, and biochemical effects on different organisms. The following characteristics are common for OCPs: high persistence, low polarity, low aqueous solubility, and high lipid solubility. Organochlorine pesticides are volatile and stable and thus can persist for a long time in the environment. The common route of exposure to animals and humans is chronic exposure due to the soil or air, after being applied in pest control, from wastes discarded into landfills, or from industrial discharges [26,28,29]

Polychlorinated biphenyls (PCBs) are odorless, tasteless, colorless synthetic chemical compounds that have been widely used in electrical and industrial applications, such as in transformers and capacitors, lubricants, flame retardants, plasticizers, and paint additives. PCBs were found to be toxic and hazardous to human health [30]. PCBs in the environment altered their compositions through various processes such as volatilization and partitioning, chemical or biological transformation, and bioaccumulation. PCBs are strongly adsorbed by soil and tend to persist in the environment [30,31,32].

Phthalate esters (PAEs), usually called phthalic acid esters, are a group of synthetic organic compounds that are not formed by natural pathways. PAEs are used in a wide range of applications, including cosmetics, printing inks, personal care products, building materials, plasticizers, and lubricants. Various high- and low-molecular-weight compounds of phthalate esters, such as bis(2-ethylhexyl) phthalate (DEHP), dibutyl phthalate (DBP), and benzyl butyl phthalate (BBP), are often used in the production of polyvinyl chloride (PVC). Additionally, diethyl phthalate (DEP), dimethyl phthalate (DMP), and di-n-butyl phthalate (DnBP), which are low-molecular-weight phthalate compounds, are often used in cosmetics, varnishes, and coatings. The PAE compounds can either be released directly into the environment during production or usage processes, or released indirectly after product disposal. PAEs are suspected to be endocrine-disruptive chemicals and exhibit carcinogenic effects being classified as priority environmental pollutants [33].

Polycyclic aromatic hydrocarbons or polynuclear aromatic hydrocarbons (PAHs) are chemical compounds that contain more than two fused aromatic rings in a linear or clustered arrangement, usually containing only carbon (C) and hydrogen (H) atoms. Nitrogen (N), sulfur (S), and oxygen (O) atoms may also substitute in the benzene ring to form heterocyclic aromatic compounds. They are produced through the incomplete combustion and pyrolysis of organic matter, coal, oil, gas, wood, garbage, and tobacco [34,35]. Both natural and anthropogenic sources such as forest fires, volcanic eruptions, vehicular emissions, residential wood burning, petroleum catalytic cracking, and industrial combustion of fossil fuels contribute to the release of PAHs into the environment. These compounds are mostly colorless, white, or pale-yellow solids. They are environmentally persistent with various structures and varied toxicity. Soils contaminated with PAHs pose potential risks to human and ecological health. The risk for mutagenic and carcinogenic effects depends a lot on actual exposure to the potential hazard, which is very difficult to measure in practice [35,36,37].

Petroleum hydrocarbons are persistent pollutants in the environment [9,38]. Leakage from underground reservoirs, petroleum refineries, storage facilities, and accidental spills from production units and transport pipelines are the primary causes of soil, water, and air contamination [39]. The presence of petroleum hydrocarbons influences the physical, physiological, and biochemical properties of soil [12,26]. Plants are sensitive to oil exposure because of their vulnerability to hydrocarbons and the immobilization of nutrients in soil. Additionally, due to their inherent mutagenic properties and low degradation rates, exceptional attention must be paid to remediation methods [40,41].

Poly and perfluoroalkyl substances (PFASs) are a group of synthetic compounds that do not occur naturally in the environment. Due to its resistance to heat, water, and oil exposure, this compound is used extensively in a wide range of applications, including in fire-fighting foam, non-stick cookware, fast-food wrappers, water-repellent fabrics (e.g., carpets and clothing), medical equipment, and plastic and leather products [42,43]. Conventional PFAS denoted by perfluorooctanoic acid (PFOA) and perfluoroctanosulfonic acid (PFOS) are known to be persistent, bio-accumulative, and potentially toxic to living organisms [43,44].

3.2.2. Inorganic Contaminants

Inorganic contaminants are toxic metals, nutrients, and salts that typically occur as dissolved anions and cations. Heavy metals enter the environment through anthropogenic sources, such as mining, smelting, and fly ash, as well as fertilizers and agrochemicals, wastewater irrigation, sewage sludge application, livestock manure, agricultural sources, industrial activities, domestic waste, and atmospheric or natural sources like geological parent material or rock offshoots [45,46].

Some inorganic contaminants remain in the soil for an indefinite period, while other compounds degrade or transform within a very short duration. In general, they negatively impact fertility levels and the physical and biological quality of the soil, resulting in decreased productivity [47,48]. These heavy metal or metalloid contaminants comprise essential elements for regular metabolic processes, known as micronutrients (Fe, Mn, Cu, Zn, Mo), as well as harmful elements such as Hg, Pb, and Cd, which can adversely affect humans and animals even at low concentrations, while having a lesser impact on plant growth. According to their toxicity to living organisms, the heavy metals can be arranged in the following order: Hg > Cu > Zn > Ni > Pb > Cd > Cr > Sn > Fe > Mn > Al [48,49]. The bioavailability of heavy metal contents in soil is influenced by soil properties and affects their uptake by plants. Soil properties, such as physical, chemical, and mineralogical characteristics, define the bioavailability of these heavy metals. Thus, bioavailability depends on soil type and pH, as well as the timing of crop harvesting [50,51].

3.3. Remediation Technologies Applied to Contaminated Soils

After the soil contamination assessment, corrective action is normally taken. The applicability of a soil remediation technique is project-specific and influenced by several factors, including the site and contamination type, remediation efficiency, and cost-effectiveness. Treatability studies help to select the best feasible remediation techniques prior to full-scale implementation [20]. Previously, the remediation of contaminated soils employed waste treatment technologies such as incineration and inertization, as well as excavation and landfill disposal. Soil quality was established by the value of a contaminant’s concentration, providing a rough approximation for considering soil as hazardous waste [52].

The knowledge of remediation technologies has promptly grown in recent decades. Procedures for soil remediation are now based on risk assessment, and different treatment processes appear to be feasible at a field scale. Table 2, adapted from [6,53,54], summarizes the technologies available and the processes involved in soil remediation. Different strategies can be used to treat contaminated soil, where combined remediation reveals high removal efficiency, short cleanup duration, moderate remediation cost, and low environmental impact [55].

3.3.1. Mechanical Processes

Excavation entails the physical removal, redistribution, or modification of contaminated soil, followed by its transportation to a landfill or an ex situ treatment facility. It is frequently employed in shallow and easily accessible locations. Excavation offers a swift and dependable solution; however, constraints such as utilities, buildings, roads, and bedrock can hinder the complete removal of contaminants. On-site soil treatment can enhance efficiency, permitting the reuse of clean soil for backfilling. Although these methods do not degrade contaminants, they can prove highly effective when integrated with chemical, physical, or biological techniques. Chemical oxidants and bioremediation products can effectively address residual contamination [56,57].

3.3.2. Physical Processes

Physical remediation technologies have the benefits of straightforward apparatus, effortless operation, and economical expense [54]. The techniques applied can be distinguished as hydrodynamic, thermal, electrical, and electromagnetic. While soil filtering involves using water or a surfactant solution along with mechanical processes to cleanse soils, soil flushing is accomplished by saturating contaminated soils with an extraction fluid that transports the contaminants to a specific location. Generally, these techniques are employed in conjunction with activated carbon, biodegradation, or the pump-and-treat method [53,58].

In thermal treatment processes, the two main techniques for heat treatment are as follows: the elimination of contaminants through evaporation, by direct heat transfer— either from heated air or open flames—or by indirect heat transfer; and the destruction of pollutants directly or indirectly at the appropriate temperature. The gas emitted by the heating apparatus aids in the treatment to eliminate or eradicate any impurities or unwanted by-products of combustion. Furthermore, a closely related process is steam stripping, which involves injecting steam into soil to promote the evaporation of relatively volatile contaminants that may be either water-soluble or insoluble [10,59,60]. Referred to as thermal desorption, this process involves applying a temperature of 600 °C to vaporize volatile contaminants, which are then eliminated from the exhaust fumes through condensation, scrubbing, filtration, or destruction. Incineration entails heating excavated soil to temperatures ranging from 880 to 1200 °C to destroy or detoxify contaminants [53].

The technique of electrokinetic remediation involves using an electric current to treat heavy metals and slurries. This process requires the application of a low-intensity direct current between positive and negative electrodes. It is primarily employed in soils with low permeability. During electrolysis reactions, hydrogen ions (H⁺) and oxygen gas are generated at the anode, while hydroxyl ions (OH⁻) and hydrogen gas are produced at the cathode. H⁺ and OH⁻ ions migrate through the soil toward the cathode and anode, respectively. This migration causes pH variations throughout the soil, affecting the physicochemical processes that control the separation and speciation of contaminants. Contaminants accumulate at the electrodes and can be extracted using various methods, such as electroplating or adsorption onto the electrode surfaces, as well as by pumping water near the electrodes [61,62]. This method can also handle radionuclides, mixed inorganic species, and some organic molecules; however, a combination of electrokinetic and bioremediation techniques is required. The transport medium forms the foundation for basic reactions such as the electroosmosis, electrophoresis, electromigration, and electrooxidation of contaminants. The technique is costly and requires machinery on site [63]. This technology is still undergoing development, requiring enhancements in process control and assessment, the efficient utilization and recycling of surfactants, the optimization of current control per electrode, and the treatment of electrolytes to progress effectively [61,64].

Vitrification is a method of treating contaminated soil that involves heating the soil to very high temperatures (between 1000 and 1700 °C) to turn it into a glassy material. This process is performed by applying intense heat to the soil, melting it into a lava-like substance, and then cooling it down to form a solid, glass-like product. The goal of vitrification is to stabilize and solidify the contaminated soil, which means that it becomes less likely to release harmful substances into the environment. This process requires a lot of energy and can be more expensive than other methods of soil remediation [20,53,61].

Containment strategies involve physically isolating and containing contaminated soils using low-permeability caps, active caps, slurry walls, grout curtains, or cut-off walls. In situ capping is a remediation option that is less expensive, less disruptive, and more durable compared to other methods [53,65].

3.3.3. Physicochemical Processes

Soil vapor extraction (SVE) is a technique employed to eliminate gases and organic volatile or semi-volatile contaminants from surface soils through vacuum pumping, which also promotes bioremediation in the unsaturated zone [53,66,67]. This technique is effective for extracting light fraction hydrocarbons, such as petrol and chlorinated aliphatic solvents, like toluene, xylene, biphenyl, perchloroethylene, and trichloroethane, by desorbing them from soil particles through suction [68]. SVE operates by taking advantage of the contaminant’s volatility, facilitating mass transfer from the adsorbed phase in the soil to the vapor phase. Contaminated soil is aerated with fresh air, which is passed through an extraction well to drag volatile organic compounds (VOCs) from the ground into the vapor phase. The extracted air can be treated by adsorption onto activated carbon, thermal treatment, or biodegradation in a biofilter before being released into the atmosphere. SVE is minimally disruptive and can be applied to large volumes of soil; however, it is effective only in heterogeneous unsaturated soils with high permeability and deep aquifers. Moreover, the high energy requirements render it economically prohibitive [53,61].

3.3.4. Chemical Processes

Chemical methods of soil remediation involve the use of chemical substances to neutralize, or extract contaminants from the soil. Such chemical agents as organic solvent, mineral or organic acids and chelating compounds can be used for in situ and ex situ (contaminated soil is treated at an off-site facility) treatments [53,61]. Organic solvents can then be mixed into the soil to extract and concentrate pollutants, which will then be carefully managed to minimize environmental impact. The introduction of oxidizing reagents (e.g., ozone, hydrogen peroxide, etc.) into contaminated soils is another approach to be addressed. However, it is important to note that the application of acidic agents or chelating compounds can be risky, as they may increase the release of potentially toxic elements into the soil system. These elements can be readily transferred to cultivated plants, posing risks to agricultural productivity and ecosystem health. Therefore, the use of these materials must be carefully controlled and applied with caution. In situ chemical reduction (ISCR) is a technique that involves the use of reducing agents to quickly transfer electrons to pollutants, leading to their degradation. Common reducing agents include metal and sulfur species, natural organic matter, and engineered reductants like dithionite and zero-valent metals. To apply ISCR, an aqueous solution containing the reductant is injected into the target area through wells or other means. However, the effectiveness of this technique can be affected by the permeability of the soil, the heterogeneity of the aquifer, and the lifetime of the reductants. One promising ISCR approach is the use of nanoparticles, although their impact on the soil ecosystem needs further investigation. ISCR can be used to treat a range of contaminants, including organic compounds and heavy metals [53,61]. Reduction reagents are used to remove chlorine atoms from hazardous chlorinated molecules, while acids and chelating compounds can be employed to release heavy metals from contaminated soil.

Although the eco-toxicity of nanotechnology is still a controversial issue, the nanoremediation concept can be considered a promising tool to decontaminate polluted soils dealing with recalcitrant contaminants [69,70,71]. Nanomaterials (NMs) commonly involved in soil remediation include, but are not limited to, nano zero-valent iron (nZVI), carbon-based NMs, metal oxide NMs, silica-based NMs, and composite NMs (including polymer-based), among others. The above-mentioned NMs can be used without modifications or after specific functionalization/decoration, depending on the method of use and type of contaminants to eliminate. The mechanisms involved in soil remediation commonly include immobilization (nZVI, carbon-based NM, and metal oxide NM), photocatalysis (TiO₂ and other metal oxide NM), Fenton-like oxidation (metal oxide NM), and reduction reactions (nZVI and carbon-based NM, both unmodified and modified) [69,70,71]. The use of NMs as nanosensors to monitor the pollution of environmental media, including soils, is another area to be mentioned [70].

The most common form of NM involvement in soil decontamination, working via immobilization and reduction mechanisms, is their application in permeable reactive barriers, which intercept and treat contaminated plumes on the subsurface thus protecting downstream water resources or receptors [69]. NMs can be injected into contaminated soil layers by appropriate mode and, in some cases (e.g., using nZVI), recirculated while extracting groundwater and reinjecting it into the treatment area. Photocatalytic degradation using nano-photocatalysts under UV irradiation or sunlight exposure has been reported for the degradation of organic contaminants such as PAHs, PCBs, and pesticides [71]. Thus, in the ex situ method, contaminated soils are washed with nonpolar solvents and the leached water is treated by photocatalysts to eliminate toxic compounds, whereas the in situ method requires NM to be directly added to the contaminated soils combined with light irradiation. This last method is heavily restricted due to the low light penetration in soil and requires continuous plowing of the land [71].

Notably, from the environmental health and safety perspective, the anthropogenic introduction of engineered NMs to the environment would inevitably affect the indigenous soil ecosystem, which should be carefully evaluated before the intervention [71]. The possible effects of NM on oil ecosystem can include the following: (i) effects on the germination of plant seeds and the plant development; (ii) influence on the growth of soil microorganisms and metabolism distortion; (iii) adverse effects on some invertebrate animals in soil; (iv) contamination of groundwater or drinking water system; and (v) uptake by crops and possible entry into the food chain, with potential harm to human health.

Table 2. Remediation technologies, processes and types of contaminants removed by each. Adapted from [6,53,54].

Process	Technology	Mode of Action	In Situ	Ex Situ	Types of Contaminants Removed	Advantages	Disadvantages	Ref.
Mechanical	Mechanical	Mechanical removal, excavation and soil replacement			Heavy metals, Organic contaminants	Suitable for small, contaminated sites; used to accelerate groundwater remediation via physical removal of contaminated media that can continue to cause contamination.	Depending on the depth of soil to be removed and replaced, the remediation can be expensive; the removed contaminated soil may need further handling and disposal treatment.	[56,57,72,73]
Physical	Hydrodynamic	Flushing, filtering			Semi-volatile organic compounds (SVOCs), petroleum and fuel residuals, heavy metals, PCBs, PAHs, and pesticides	High efficiency (60–90%) and fast effects; decreases the volume of decontaminant soil and cuts costs; reduces the volume of decontaminant soil, reducing costs; allows simultaneous treatment of organic pollutants and metals that can be recovered and reused; treated soil can be redeposited on site; costs are relatively low.	Extreme soil disturbance; needs a space big enough for the equipment; requires wash water treatment.	[56,58,74,75]
	Thermal	Thermal desorption, incineration			Semi-volatile organic compounds (SVOCs), Hg, petroleum, PCBs	Suitable for various contaminants, with a short treatment period; high efficiency; strong safety measures; and ability to recycle both soil and contaminants.	Production of off-gases, which are mostly organic compounds and may result in secondary pollution; energy demand.	[59,60,76,77,78,79]
		Vitrification			Heavy metal, VOCs (lost by heating)	High efficiency; immobilize harmful contaminants; long-term protection.	High cost; limited to small soil area/volume, treated land, and soil losing environmental functions; associated with massive CO2 emissions; high energy demand.	[80,81,82]
	Electrical	Electrochemical bleaching, electric osmosis, electrophoresis, electromigration, electrodialysis			Slurries and heavy metals, Radionuclides, Mixed inorganic species VOCs and SVOCs	Minimal soil disturbance; less expensive; targets a specific area; applicable for a wide range of contaminants; suitable for fine soils with low permeability.	Time consuming; low efficiency; indicated for fine soils with low permeability; time-consuming process; may change the soils’ pH.	[62,73,83,84,85,86,87]
	Containment	Encapsulation, surface capping			Halogenated organics Hydrocarbons; Chlorinate solvents; Radionuclides; Metals	Minimal volume of harmful contaminant residues; restricts contaminant migration into non-contaminated areas.	Need for appropriate reactive materials for walls; large area requires longer duration.	[65,68,88]
Physicochemical	Soil vapor extraction (SVE)	Removal of volatile products			VOCs, PAHs	Able to treat large volumes of soil; minor soil disturbances; shorter remediation time; effective and cost-efficient.	Requires air emission licenses; need for treatment of extracted vapor; only unsaturated soil zones; requires integration with other technologies.	[66,67,89,90,91,92]
Chemical	Hydrolisis, Photolysis, Neutralization, Oxidation, Reduction	Reagent Leaching, Oxidation, Reduction			Heavy metals, Remove chlorine atoms (PCBs, pesticides), PAHs	Wide range of contaminants; relatively quickly; reduces toxicity of contaminated soil.	Can create hazardous by-products; expensive; longer duration.	[93,94,95]
Biological/ Biochemical	Phyto- remediation	Phytodegradation, rhizodegradation, phytovoltalization, phytoextraction, rhyzofiltration, phytostabilization			Petroleum Hydrocarbons; Organophosphate Insecticides; Heavy metals; Radionuclides; Non-aromatic chlorinated solvents; Surplus mineral; Explosives	High public acceptance; low cost and easy to implement; suitable for large and low-contamination areas; takes advantage of the natural processes of plants; improves the overall quality and texture of soil.	Limited to shallow soils, streams, and groundwater contamination; time-consuming; low efficiency; disposal of plants can be a concern.	[12,96,97,98,99]
	Bioremediation	Oxidation, adsorption, biosurfactant			PAH’s; Petroleum Hydrocarbons; Pesticides; Chlorophenols; Heavy metals	Low cost; simple to implement; minimal soil disturbance.	Low efficiency; merely supplemental to principal remediation techniques.	[5,6,9,15,97,100,101,102]

Table 2. Remediation technologies, processes and types of contaminants removed by each. Adapted from [6,53,54].

Process	Technology	Mode of Action	In Situ	Ex Situ	Types of Contaminants Removed	Advantages	Disadvantages	Ref.
Mechanical	Mechanical	Mechanical removal, excavation and soil replacement			Heavy metals, Organic contaminants	Suitable for small, contaminated sites; used to accelerate groundwater remediation via physical removal of contaminated media that can continue to cause contamination.	Depending on the depth of soil to be removed and replaced, the remediation can be expensive; the removed contaminated soil may need further handling and disposal treatment.	[56,57,72,73]
Physical	Hydrodynamic	Flushing, filtering			Semi-volatile organic compounds (SVOCs), petroleum and fuel residuals, heavy metals, PCBs, PAHs, and pesticides	High efficiency (60–90%) and fast effects; decreases the volume of decontaminant soil and cuts costs; reduces the volume of decontaminant soil, reducing costs; allows simultaneous treatment of organic pollutants and metals that can be recovered and reused; treated soil can be redeposited on site; costs are relatively low.	Extreme soil disturbance; needs a space big enough for the equipment; requires wash water treatment.	[56,58,74,75]
	Thermal	Thermal desorption, incineration			Semi-volatile organic compounds (SVOCs), Hg, petroleum, PCBs	Suitable for various contaminants, with a short treatment period; high efficiency; strong safety measures; and ability to recycle both soil and contaminants.	Production of off-gases, which are mostly organic compounds and may result in secondary pollution; energy demand.	[59,60,76,77,78,79]
		Vitrification			Heavy metal, VOCs (lost by heating)	High efficiency; immobilize harmful contaminants; long-term protection.	High cost; limited to small soil area/volume, treated land, and soil losing environmental functions; associated with massive CO2 emissions; high energy demand.	[80,81,82]
	Electrical	Electrochemical bleaching, electric osmosis, electrophoresis, electromigration, electrodialysis			Slurries and heavy metals, Radionuclides, Mixed inorganic species VOCs and SVOCs	Minimal soil disturbance; less expensive; targets a specific area; applicable for a wide range of contaminants; suitable for fine soils with low permeability.	Time consuming; low efficiency; indicated for fine soils with low permeability; time-consuming process; may change the soils’ pH.	[62,73,83,84,85,86,87]
	Containment	Encapsulation, surface capping			Halogenated organics Hydrocarbons; Chlorinate solvents; Radionuclides; Metals	Minimal volume of harmful contaminant residues; restricts contaminant migration into non-contaminated areas.	Need for appropriate reactive materials for walls; large area requires longer duration.	[65,68,88]
Physicochemical	Soil vapor extraction (SVE)	Removal of volatile products			VOCs, PAHs	Able to treat large volumes of soil; minor soil disturbances; shorter remediation time; effective and cost-efficient.	Requires air emission licenses; need for treatment of extracted vapor; only unsaturated soil zones; requires integration with other technologies.	[66,67,89,90,91,92]
Chemical	Hydrolisis, Photolysis, Neutralization, Oxidation, Reduction	Reagent Leaching, Oxidation, Reduction			Heavy metals, Remove chlorine atoms (PCBs, pesticides), PAHs	Wide range of contaminants; relatively quickly; reduces toxicity of contaminated soil.	Can create hazardous by-products; expensive; longer duration.	[93,94,95]
Biological/ Biochemical	Phyto- remediation	Phytodegradation, rhizodegradation, phytovoltalization, phytoextraction, rhyzofiltration, phytostabilization			Petroleum Hydrocarbons; Organophosphate Insecticides; Heavy metals; Radionuclides; Non-aromatic chlorinated solvents; Surplus mineral; Explosives	High public acceptance; low cost and easy to implement; suitable for large and low-contamination areas; takes advantage of the natural processes of plants; improves the overall quality and texture of soil.	Limited to shallow soils, streams, and groundwater contamination; time-consuming; low efficiency; disposal of plants can be a concern.	[12,96,97,98,99]
	Bioremediation	Oxidation, adsorption, biosurfactant			PAH’s; Petroleum Hydrocarbons; Pesticides; Chlorophenols; Heavy metals	Low cost; simple to implement; minimal soil disturbance.	Low efficiency; merely supplemental to principal remediation techniques.	[5,6,9,15,97,100,101,102]

3.3.5. Biological Processes

Biological treatments are effective and ecological methods for environmental cleaning. Biological components (microorganisms and plants) are used to degrade, eliminate, or transform pollutants into non-toxic or less toxic forms. Bioremediation and phytoremediation offer several advantages over physicochemical methods. These processes are safe and clean, minimizing health risks while ensuring low installation and operational costs. It prevents the transfer of contaminants and achieves complete degradation or transformation of pollutants. Additionally, it offers ease of application and maintenance, flexibility and adaptability to varying environmental conditions, and in situ application for pollutants across large areas, and it has received high public acceptance [61,100,103].

Phytoremediation is a process that uses plants along with their microbiota and soil modifiers to remove or contain contaminants from polluted media. There are five basic types of phytoremediation, including rhizofiltration, phytostimulation, phytotransformation, phytoextraction, and phytostabilization [53]. Phytoremediation can treat various pollutants, including PAHs, PCBs, petroleum hydrocarbons, heavy metals, and explosives [63]. The selection of phytoremediation technology should consider soil and plant types, rhizosphere microorganisms, and the geochemical forms of pollutants [61]. Phytostabilization reduces the bioavailability of heavy metals and prevents their off-site transport by in situ immobilization or inactivation. Root exudates can reduce the toxicity of heavy metals by changing the rhizosphere’s environmental conditions through decomposition, chelation, redox, and other processes [54,96].

The process of bioremediation can use a variety of organisms, including bacteria, fungi, plants, and animals, or their derivatives, and even combinations of them [104] to reduce or eliminate contaminants in soil, water, and air. Microbes are more utilized than plants because of their rapid growth and ability to be easily manipulated, thereby enhancing their function as agents of bioremediation [105]. Different groups of bacteria, fungi and algae have been employed to clean up environmental pollutants. Native soil microorganisms are particularly important in bioremediation, as they are able to transform complex organic compounds into simple inorganic compounds through mineralization. Soil particles provide a surface for microbial attachment and colonization, facilitating the degradation process. Native microorganisms are often adsorbed onto soil particles through ionic exchange interactions. A delivery system that provides an energy source (electron donor), an electron acceptor, and nutrients is typically required. Bioremediation can involve different types of microbial electron acceptors, such as oxygen, nitrate, sulfate, iron (III), or carbon dioxide, depending on the redox potential. Effective bioremediation requires the optimization of operating conditions to allow microbial cells to quickly biodegrade contaminants using microbial enzymes. Overall, bioremediation is a natural process that has been used for centuries in wastewater treatment and is now being intentionally used to reduce hazardous waste [3,6,53]. Recent trends also include the use of genetically modified microorganisms and microbial consortia to improve degradation efficiency across a wider range of contaminants or be adapted to specific contaminants and environmental conditions [106,107]. Furthermore, the integration of biological remediation with phytoremediation using plants to support microbial activity in so-called combined synergistic “microorganism–plant” remediation has shown promise in treating particularly complex contamination scenarios, although the mechanisms involved are often not well understood [107]. The integration of the Internet of Things (IoT), artificial intelligence (AI), and biosensors in pollution monitoring have great potential to revolutionize bioremediation and waste management [106].

Accordingly, within bioremediation, different methods and applications can be identified.

Table 3. Bioremediation methods and principles. Adapted from [103].

Methods	Principles	References
Natural Attenuation In situ	Natural attenuation is an in situ treatment method that uses natural processes to contain the spread of contamination from chemical spills and reduce the concentration of pollutants. The process can be categorized as destructive or non-destructive depending on whether it destroys the contaminant or just reduces its concentration. Natural attenuation relies on the existence of indigenous microorganisms capable of degrading the contaminants, and it is a proactive approach that focuses on verifying and monitoring natural remediation processes instead of relying solely on engineered processes.	[58,103,108,109]
Bioaugmentation In situ	Bioaugmentation is the process of enhancing the performance of microorganism populations by adding genetically engineered bacteria, isolated bacterial strains, or enrichment consortia with specific catabolic activities to increase the rate of degradation. This method involves the addition of exogenous microorganisms that can degrade contaminants that are recalcitrant to the indigenous microbiota.	[99,100,103,110]
Bio stimulation In situ	Biostimulation is a process that involves adjusting environmental parameters, such as the addition of limiting nutrients like slow-release fertilizers, biosurfactants, and biopolymers, to stimulate the growth microorganisms, which in turn increases their metabolic activity and elevates the degradation rate.	[99,100,103]
Biofilters In situ	Application of bacterial filters in the decontamination of polluted water and waste.	[103]
Biopiling Ex situ	Bioremediation through biopiles involves piling contaminated soil over an aerated system and adding nutrients to promote biodegradation, mainly by improving microbial activity. The technology relies on watering, aeration, and leaching, and is cost-effective and efficient, if nutrients, temperature, and aeration are well controlled.	[3,103,111,112]
Bioventing In situ	Bioventing is a process that adds oxygen to soil to promote the growth of microorganisms, with anaerobic conditions being necessary for their growth. It involves venting the soil to remove volatile compounds and using bioremediation to degrade organic contaminants by combining the oxygen with them.	[99,103,113,114]
Composting Ex situ	Composting is an aerobic process that decomposes organic waste through thermophilic biological agents, resulting in compost. Optimal biodegradation rates require a temperature between 40 and 70 °C, high nutrient availability, including oxygen, and a neutral pH. The process involves adding nutrients to soil, which is mixed to increase the aeration and activation of indigenous microorganisms.	[3,104,115,116]
Landfarming Ex situ/In situ	Landfarming is a soil bioremediation technique in which soil is arranged in piles and periodically turned over by agricultural practices to stimulate the degradation by indigenous microorganisms. This technique is particularly effective in treating soils contaminated with petroleum hydrocarbons, such as crude oil, diesel, or gasoline, as well as certain organic chemicals. It is a low-cost and low-footprint technology that can be conducted either ex situ or in situ.	[3,117,118]

summarizes the principles of each bioremediation method, as well as some studies conducted in the field.

Lignin in Bioremediation

Lignin plays a crucial role in humus formation, and some of the transformation products are involved in redox reactions in plant cells. This natural aromatic polymer, along with polyphenols, may serve as a potential source of valuable compounds for plant growth.

Lignin can be modified in processes that involve the loss of at least part of the methoxy groups (OCH₃) and the generation of hydroxyphenol moieties, as well as the oxidation of aliphatic side chains to form carboxylic groups (COOH). These modified materials undergo further unknown changes to produce humic and fulvic acids.

Lignins and their derivatives have significant roles as non-conventional additives in crop cultivation and bioremediation, which include the following: (i) energy management and distribution, particularly in regulating the carbon cycle; (ii) managing chemical nutrients; (iii) sustaining the trophic web, which represents the transfer of energy through different organisms in an ecosystem; (iv) structuring the soil, contributing to its physical properties and stability; and (v) sequestering pollutants by binding heavy metal ions and solubilizing petroleum hydrocarbons, among other functions [119]. Several state-of-the-art comprehensive reviews dealing with research on the potential applications of lignin and polyphenols in the fields of crop cultivation and bioremediation were explored to summarize the available literature data [119,120]. These studies revealed several opportunities for the application of conventional or modified technical lignins in different areas, as described in

Table 4. Lignin and lignin derivatives applications in the fields of crop cultivation and bioremediation. Adapted from [119,120].

Utilizations	Lignin/Lignin Derivatives
Non-conventional fertilizers	Lignosulfonates, ammoniated lignosulfonates and hydrolysis lignin, oxiammonolyzed lignosulfonates and hydrolysis lignin [fertilizers with N slow release, granulation (bonding agents), micronutrients, chelating agents (microelements)], oxidized lignin–lignosulfonates, hydrolysis lignin, kraft lignin [using O2 (air)/ammonia with/without catalysts, H2O2, persulfates].
Slow release for pesticides, insecticides, herbicides	Crosslinked lignin with epichlorhydrine, kraft lignin, Alcell lignin, lignosulfonates (non-or modified), sulfonated kraft lignin, alkali lignin.
Growth stimulators	Aqueous soluble lignin, oxidized lignins (oxyammonolysis of lignosulfonates, hydrolysis lignin), nitration or oxidative nitration of hydrolysis lignin, lignocellulose, ammoniated nitrolignin, condensed lignosulfonates with urea, polyphenols.
Seed coatings	Lignosulfonates (Na, Ca), sugar-free lignosulfonates, desulfonated lignosulfonates.
Chelating agents	Hydrolysis lignin, lignin and lignin derivatives.
Soil conditioners	Kraft lignin, oxidized kraft lignin, grafted lignosulfonates with acrylic/methacrylic acids or acrylonitrile, composted hydrolysis lignin (HL) or ammonized HL, lignosulfonates (NH4+, Na+), alkali-treated lignin, followed by activation with CuO.
Bioremediation	Hydrolysis lignin, lignosulfonates, alkali lignin, polyphenols.

.

Large-scale applications of non-modified lignosulfonates (LSs), including their sodium and ammonium salts, as well as non-sulfonated and sulfonated kraft lignin, are prevalent in agrochemicals and bioremediation applications. These substances serve various purposes such as dispersants, controlled release agents for pesticides, insecticides, and herbicides, as well as components in fertilizer formulations [119,121]. Lignin and lignin derivatives aid in granulating nutrients like urea and in chelating micronutrients including boron, chlorine, cobalt, copper, iron, manganese, molybdenum, sodium, and zinc. Additionally, lignin derivatives are used to prevent rapid pesticide degradation in agricultural soils, either through crop rotation—the use of different pesticides with each different crop—or the application of lignin-derived extenders that protect the pesticides against degradation [119]. Regarding the use of LSs in the agricultural sector, five main strategies of LS implementation in soil are summarized as follows: (i) LSs as a complexing agent with micronutrients; (ii) co-pelleting of LSs with (macro)nutrients; (iii) capsule formation with LSs for coating of fertilizers or pesticides; (iv) LSs as a biostimulant; and (v) ammonoxidation of LSs [122].

Although most of the soil remediation applications relate to LSs, as the only lignin available on the market in a large scale, other kinds of technical lignins can be used for the same purposes, after appropriate modification. Proper modification of kraft lignin, organosolv, and other types of lignin allows them to be used, for example, in biopolymer-coated fertilizers [123]. Modified lignin is used in this case as a biopolymer for nutrient coating, fulfilling the required functional prerequisites. Lignin, when doped with nutrients, can represent a suitable nutritional carrier, which can be obtained by (i) physical coating, (ii) chemical modification with nutrient-containing groups (e.g., N and P) introduced into the molecular structure, and (iii) lignin-based chelates [123]. Modified lignins can also act as promoters of microbial growth thus stimulating the biodegradation of soil contaminants [123,124].

3.4. Future Bio-Based Trends

As the world strives to meet the urgent need for environmental remediation, re-searchers are increasingly turning to collaborative strategies like co-bioremediation, physical–biological remediation, and chemical–biological remediation [53,92,125]. These combined approaches have shown great promise in achieving optimal efficiency and cost-effectiveness, while also being environmentally friendly [53]. By utilizing more natural pathways, technologists can close the life cycle and achieve sustainable solutions for the planet’s pressing environmental challenges [126]. Therefore, it is imperative to continue to invest in and explore these collaborative remediation strategies towards a healthier and more sustainable future. Examples of that are as follows:

• Bio-electrochemical systems (BESs) or microbial electrochemical systems (MESs) are a type of energy-saving bioremediation process developed in the last decade to enhance the remediation of oil-contaminated soil. In BESs, electrochemically active bacteria can catalyze the oxidation and reduction reactions of hydrocarbon pollutants and extracellularly transfer electrons to convert chemical energy into electrical current. Microbial fuel cell (MFC) technology is an example of a BES. The advantages include enhanced removal efficiency for complex contaminants, minimal energy or chemical requirements for operation, and lower long-term operational costs due to the use of non-exhaustible electrodes as electron acceptors and donors. The generated electrical current could also function as a real-time bioremediation indicator and power wireless sensors for online monitoring [99].
• Recent advancements in bioremediation involve the utilization of biochar and modified lignin. Biochar enhances microbial diversity and assists in the breakdown of hydrocarbons in contaminated soil. However, heavy metals and metalloids cannot be removed from the ecosystem. The following two common strategies are employed for heavy metal and metalloid bioremediation: the absorption and accumulation in plants, as well as the conversion of toxic metals into less harmful forms that native microorganisms can absorb [127,128,129]. Biochar can be produced from carbonaceous waste materials like agricultural, forestry, household, and livestock waste [130]. Lignin can also be transformed into biochar. Because of the similarities between lignin and lignite coal, lignin can also be used as a feedstock for generating activated carbon for mercury sequestration [131]. The co-application of biochar with other bioremediation techniques (e.g., bioaugmentation, phytoremediation, and biostimulation) can have strong beneficial synergistic effects on the removal of soil pollutants. Thermal or chemical modification of biochar to improve its bioremediation potential is another steady trend in the area [132]. Targeted lignin derivatization can expand its performance in bioremediation applications. Functionalization includes, but is not limited to, selective oxidation [133,134] and the introduction of strong chelating groups (e.g., quaternization [135], carboxymethylation [136], or amination [137]). Pristine lignin or lignin derivatives, both loose or fixed on the suitable supporting substrate are prospective heavy metals and agrochemicals sequestrators to control groundwater contamination and wastewater treatment [138,139,140,141]. Lignin nanoparticles (LNPs) merit increased attention as a biodegradable, green organic nanoscale matrix for different bioremediation purposes [142,143,144]. Being cheaper and environmentally safer than inorganic and carbon-based NMs, LNPs can lift at least part of the restrictions on the use of nanoremediation technologies for soil remediation. Since LNPs may have a beneficial effect on microbial well-being, one of the future research endeavors may be to explore synergistic effects between them in bioremediation applications [142].
• New bioremediation techniques show that biosurfactants are a suitable alternative to synthetic surfactants for cleaning contaminated soil. Biosurfactants lower the surface tension and form micelles on microbial cell surfaces, making bioremediation more effective. Biosurfactants can be classified as ionic or non-ionic, and they include glycolipids, lipopeptides, lipoproteins, and humic compounds. They are attractive for bioremediation because they are biodegradable, not very toxic, can withstand extreme pH, temperature, and salinity, and can break down contaminants quickly. Overall, biosurfactants are a promising technology for cleaning contaminated soil [3].

The remediation industry has gone through different stages over the years. Initially, it was expected to remove all contaminants from a site, but this was found to be too costly and unrealistic. Risk-based remediation strategies were then developed with more achievable results. Currently, there is a growing demand for a sustainable remediation approach that minimizes environmental, social, and economic impacts. This has led to the emergence of the concepts of “green remediation” in the USA and “sustainable remediation” in Europe, which have been united under the term “green and sustainable remediation”. This movement has blossomed in the last decade and has the potential to optimize resource utilization, promote human well-being, improve environmental quality, and establish an energetic remediation market if implemented carefully [126]. Figure 2 highlights the social, economic, and environmental benefits of “sustainable remediation” and presents the most relevant remediation methodologies.

Using living organisms (or their by-products) and plants to remediate contaminated soil is a sustainable and effective approach to soil remediation. These methods align with the current sustainability principles and not only preserve soil quality but also reduce the environmental impact of remediation through resource efficiency. Phytoremediation is a type of in situ remediation that can be used to convert phytobiomass (previously considered as a waste) into a valuable resource that can be reused for further production processes. For example, phytobiomass can be converted into biofuels or used for direct combustion to generate energy. Integrated phytoremediation–bioenergy approaches offer sustainable strategies for environmental decontamination, waste reduction, bioenergy production, green development, and cost-efficient environmental protection. These strategies promote a culture of reuse from a circular economy perspective [126,145].

4. Conclusions

The remediation of contaminated soil is crucial not only to prevent environmental disasters but also to preserve human health and sustain the plants and animals that depend on it. The primary goal is to restore soil, which can benefit agriculture, urban development, and industry, following the state legislation guidelines for the remediation process. Soil remediation also preserves groundwater, ensuring that food production and water supplies remain safe and uncontaminated. Various remediation technologies can be employed, such as mechanical, physical, chemical, and biological, which can be used separately or combined to reduce costs and improve treatment effectiveness. Technology selection, in turn, involves not only eliminating the source of pollution but also blocking pathways and reducing exposure to contaminants. Green remediation is the practice of minimizing side environmental effects and maximizing the benefits of remediation. The future challenge is to integrate sustainability into remediation decision-making, including remediation methods that maintain soil quality after treatments.

Among the existing remediation approaches, bioremediation encompasses emerging types of bioinspired treatments that are environmentally friendly soil cleanup methods. Future development trends aim to improve the technical feasibility and cost efficiency of soil bioremediation technologies. The compilation of bioremediation tools with other conventional or new remediation processes, as well as involving advanced biobased products, deserves special attention. In particular, besides biochar and other phytobiomass-derived carriers, lignin and polyphenols, whether present in biomass or isolated and modified, have the potential to play a crucial role in the development of bioremediation processes, particularly considering their additional contribution to humus formation. Engineered microorganisms and microbial consortia are designed to improve the efficiency of recalcitrant pollutants degradation and, when combined with advanced phytoremediation or chemical remediation approaches, may provide new breakthrough technologies. The introduction of the IoT for real-time monitoring and remote control, AI for data analysis and predictive modeling, and biosensors for pollutant detection and environmental monitoring will help accelerate scientific developments in the field of bioremediation, enabling faster and more reliable responses to ongoing cleanup interventions.