Soil Contaminants and Their Removal through Surfactant-Enhanced Soil Remediation: A Comprehensive Review

: This review provides a comprehensive analysis of the effectiveness of surfactants in enhancing the remediation of contaminated soils. The study examines recent and older research on the use of efﬂuent treatment techniques combined with synthetic surface-active agents, bio-surfactants, and various categories of surfactants for soil reclamation purposes. The main purpose of this review is to evaluate the effectiveness of surfactants in enhancing the remediation of contaminated soils. The research question is to explore the mechanisms through which surfactants enhance soil remediation and to assess the potential beneﬁts and limitations of surfactant-based remediation methods. This review was conducted through an extensive literature search of relevant articles published in scientiﬁc databases. The articles were selected based on their relevance to the topic and their methodological rigor. Types of possible soil pollutants and the requirements of speciﬁc surfactants were discussed. Structural relationships between pollutant and surfactants were described thoroughly. Extensive study revealed that surfactants have shown great potential in enhancing the remediation of contaminated soils. Surfactants can improve the solubility and mobility of hydrophobic contaminants and facilitate their removal from soil. However, the effectiveness of surfactant-based remediation methods depends on several factors, including the type of contaminant, the soil properties, and the surfactant concentration and type. Surfactant-enhanced soil remediation can be an effective and sustainable method for addressing soil contamination. However, the optimal conditions for using surfactants depend on the speciﬁc site characteristics and contaminant properties, and further research is needed to optimize the use of surfactants in soil remediation.


Introduction
Soil can be understood as a 3D structure that is interconnected with components of the environment, such as air and water. Therefore, if the soil is contaminated, the contamination can easily spread, leading to great risks to human life, ecosystems, water sources, and other environmental receptors. Additionally, the pollutants can be transmitted to the bodies of people who eat crops produced from the soil [1,2].
Continuous industrial advancement has become the primary sources of lethal waste into the land and water sources. Mining of minerals, hefty input in farming exercises, armed force and military preparation, and the development of residue caused because of human exercises have been other noteworthy components of soil contamination in the course of the most recent not many decades [3]. The oil business and related items are additionally significant sources of soil pollution. Inappropriate management results in the release of these contaminants into the environment without proper treatments.
The constant contact between dirt that is polluted and the air and water can be hazardous to the environment and human health [2]. Trace elements can lead to digestive problems such as intense pains, diarrhea, vomiting, ulcers, skin problems, lung cancer, changes in genetic material, and other human illnesses [4]. In the early 1900s, long-term Cd

Petroleum Hydrocarbons (PHC)
Petroleum is a notable environmental pollutant that is commonly found in industrial waste, leaking fuel tanks, and crude oil spills. PHCs get into the upper and lower layers of soil due to unintended fuel or crude oil leakage from the vast underground pipeline networks. The more water-soluble PHCs from the leakage can easily penetrate into the subsoils and shallow aquifers, thus forming contaminated plumes. In addition, most of the spilled PHCs remain in a non-aqueous, liquid form, or as residuals, leading to displacement of the air and water spaces in the soil matrix [7]. Petroleum hydrocarbons have a considerable effect on the chemical, physical, and microbial characteristics of soil by inducing nitrogen fixation and creation of organic matter. The increased amount of organic matter in the soil leads to deflocculation and consequently decreases soil texture, making it more prone to erosion [8].
The specific composition of petroleum hydrocarbons (PHCs) may differ from place to place, but their negative characteristics remain the same. Substances that are highly hazardous and of great concern include benzene and polycyclic aromatic hydrocarbons (PAHs). The threat of exposure through skin contact or ingestion is proportional to the capability of a particular element to adhere to soil particles and enter vegetation through root absorption, which may then enter the food chain. Furthermore, a compound's ability to evaporate directly from the soil or through contaminated water sources due to its release is linked to the risk of inhalation. In addition, a chemical's solubility and density have an effect on surface or subterranean water sources. Human health is jeopardized by the intake of a chemical mixture that evaporates from polluted soil.
Benzene has been linked to an increased risk of leukemia at certain levels of exposure. In addition, exposure to certain chlorinated solvents can lead to depression of the central nervous system, damage to the liver and kidneys, as well as skin rashes, headaches, nausea, fatigue, and eye irritation. The United States Environmental Protection Agency (USEPA) has emphasized that contact, inhalation, or ingestion of soil toxins can be fatal in extreme cases. Prolonged inhalation of toluene concentrations of more than 100 ppm can cause headaches, fatigue, nausea, and drowsiness [9].

Agrochemicals (Pesticides, Herbicides, and Fertilizers)
Nowadays, agrochemicals are commonly employed in agricultural production to maximize crop growth by destroying damaging insects, diseases, and unwelcome weeds. However, such use of agrochemicals has become a potential threat to food safety, human and environmental health, ecological equilibrium, and preserving soil biodiversity. In the long-term, if not used properly, agrochemicals can lead to a change in population of beneficial bacteria, which can result in the emergence of antibiotic resistance. The use of agrochemicals in farming systems can have a detrimental effect on soil microorganisms that are primarily involved in nutrient cycling processes, such as nitrogen fixation, releasing phosphorus, and other essential nutrient transformations. The long-term effects of agrochemicals can have a detrimental impact on both land and sea creatures. Several health issues, such as intense poisoning, skin complications, endocrine disruption, birth defects, miscarriages, fertility issues, and reduced sperm count, have been linked to the exposure to such chemicals. Additionally, common side effects, including itching, eye irritation, vision problems, nausea and dehydration, have been documented. Additionally, exposure to large doses of pesticides has been known to delay pregnancy in women [10].

Polycyclic or Polynuclear Aromatic Hydrocarbons (PAHs)
Polycyclic aromatic hydrocarbons (PAHs) and polynuclear aromatic hydrocarbons (PAHs) are comprised of merely carbon (C) and hydrogen (H) atoms despite the fact that nitrogen, sulfur, and oxygen atoms can be swapped and generate heterocyclic aromatic compounds inside the aromatic benzene ring. These molecules are formed by pyrolyzing organic material and burning it inefficiently. The emission of PAHs into the atmosphere is caused by both natural and human activities, such as industrial burning of fossil fuels, petroleum catalytic cracking, residential wood burning, volcanic activities, vehicle emissions, and forest fires. The hydrophobic nature of PAHs, which are carcinogenic micropollutants, stops them from being broken down in the environment. Growing concerns about their detrimental health impacts has propelled the execution of numerous studies on the remediation of PAHs contaminated soil [11].

Chlorinated Solvents
Different types of solvents such as halogenated non-polar aromatics, aliphatic, heterocyclics, and other polar organic compounds are utilized in various industrial and commercial purposes like dry cleaning, making adhesives, and cleaning and degreasing metal surfaces. The most commonly used chlorinated solvents include trichloroethylene, trichloroethane, methyl chloride, and tetrachloroethylene. Unfortunately, due to improper use, handling, and disposal, these solvents have contaminated both land and groundwater, leading to decreased soil fertility and nitrogen fixation [12].

Asbestos
Asbestos is a mineral that is found in nature and was used in a variety of products, including construction materials, for a long time. Unfortunately, exposure to asbestos can lead to severe illnesses, like lung cancer and mesothelioma. Even though it is mostly linked to its application as insulation, asbestos can also be present in soil. Contamination of the soil with asbestos can happen when materials that contain the mineral are not disposed of correctly or when natural deposits of asbestos are disturbed. The fibers can become airborne, posing a risk for those living in the vicinity or working there.
For years, it has been extensively established that asbestos inhalation has detrimental effects on human health. As serpentinite and metabasite rocks are the chief source of asbestos, many studies have centered on studying their mineralogical and geochemical composition (NOA). Additionally, the soil derived from these rocks should also be studied as it reflects the mineralogical and geochemical makeup of the parent rocks and could contain hazardous fibers. Asbestos inhalation can lead to lung scarring, lung cancer, and mesothelioma [13].

Heavy Metals
Across the planet, an issue of growing concern is environmental contamination due to heavy metal particles. These are discharged into the soil by a range of human activities, including industrial manufacturing, mining, smelting, and disposal of hazardous waste. Plants absorb the metals, which then become part of the food chain ( Figure 1). As, Cd, Pb, and Hg are all carcinogenic but can also have other disastrous effects on humans, such as impacting the nervous system and causing renal malfunction. In addition, arsenic can be damaging to the skin, respiratory, and cardiovascular systems.

Soil Remediation Technologies
The soil is a great absorber of pollutants, but it is also very sluggish in its ability to eliminate them, which makes it an ideal filter. In light of the environmental and human health risks, as well as economic implications, the decontamination of polluted soils has become a major point of public and political discussion over the last couple of decades. To address this problem, various physical, chemical, and biological processes have been developed to clean up contaminated soils. Figure 2 provides a brief summary of the available technologies for remediation of organic and inorganic pollutants in soils.

Soil Remediation Technologies
The soil is a great absorber of pollutants, but it is also very sluggish in its ability to eliminate them, which makes it an ideal filter. In light of the environmental and human health risks, as well as economic implications, the decontamination of polluted soils has become a major point of public and political discussion over the last couple of decades. To address this problem, various physical, chemical, and biological processes have been developed to clean up contaminated soils. Figure 2 provides a brief summary of the available technologies for remediation of organic and inorganic pollutants in soils. Once the need for soil decontamination has been recognized, the most appropriate technology is chosen based on the classification, the toxicity and the origin of the pollu tant, the existing and probable hazard linked to the degree of contamination, the chemica and physical features of the soil, the purpose of the land, the available time for purifica tion, the population's approval, and a cost/benefit appraisal. The above-mentioned tech nologies greatly differ in how they address these diverse conditions. For example, the av erage cost of cleaning with the various technologies can vary significantly ( Figure 3). In developed countries, economic advantages are motivating the development of low-cost Once the need for soil decontamination has been recognized, the most appropriate technology is chosen based on the classification, the toxicity and the origin of the pollutant, the existing and probable hazard linked to the degree of contamination, the chemical and physical features of the soil, the purpose of the land, the available time for purification, the population's approval, and a cost/benefit appraisal. The above-mentioned technologies greatly differ in how they address these diverse conditions. For example, the average cost of cleaning with the various technologies can vary significantly ( Figure 3). In developed countries, economic advantages are motivating the development of low-cost, low-input technologies, which is also essential to dealing with the treatment of hazardous soils in developing countries [14,15]. low-input technologies, which is also essential to dealing with the treatment of hazardous soils in developing countries [14,15]. At present, there are several methods available to purify contaminated soil, such as excavation and substitution, capping onsite, thermal remedy, electrokinetic elimination, soil washing, solidifying/stabilizing, oxidation and reduction, phytoremediation, and microbial remediation. There are three categories for soil remediation processes-chemical, physical, and biological-which can be carried out inside the polluted area (in situ) or outside of it (ex situ), either independently or in conjunction with other actions. One of the most outstanding strategies for remediation is the use of surfactants, as they can reduce the interfacial and surface tensions and speed up biodegradation of polluted soils [12,16].
It is anticipated that rehabilitating soils with surfactant-enhanced approaches will be successful, cost-effective, and rapid. As they could potentially be less dangerous and more biodegradable in comparison to most solvent-based systems, surfactants are especially attractive for these purposes [17]. Over the last twenty years, investigations on surfactantenhanced soil washing have become necessary to provide decision makers, remediation At present, there are several methods available to purify contaminated soil, such as excavation and substitution, capping onsite, thermal remedy, electrokinetic elimination, soil washing, solidifying/stabilizing, oxidation and reduction, phytoremediation, and microbial remediation. There are three categories for soil remediation processes-chemical, physical, and biological-which can be carried out inside the polluted area (in situ) or outside of it (ex situ), either independently or in conjunction with other actions. One of the most outstanding strategies for remediation is the use of surfactants, as they can reduce the interfacial and surface tensions and speed up biodegradation of polluted soils [12,16].
It is anticipated that rehabilitating soils with surfactant-enhanced approaches will be successful, cost-effective, and rapid. As they could potentially be less dangerous and more biodegradable in comparison to most solvent-based systems, surfactants are especially attractive for these purposes [17]. Over the last twenty years, investigations on surfactantenhanced soil washing have become necessary to provide decision makers, remediation organizations, and investigators with a tool for making decisions and to inform them about present achievements, problems, and future paths [18].

Surfactants in Soil Remediation
Surfactants are types of molecules that contain both repulsive and attractive components in their molecular structure. This particular characteristic of surfactants makes them suitable to increase the solubility of hydrophobic contaminants in water. Anionic, cationic, zwitterionic, and nonionic surfactants have been studied for soil remediation purposes.
The diagram in Figure 4a shows the general process for a soil decontamination procedure that uses an aqueous solution of surfactants. The polluted soil that has been removed is pre-treated, mixed with water that contains surfactants, and stirred. After washing, the clay particles settle out, and the eluents can be collected and reused for the next round of application. Ex situ soil washing is an economical way to clean large amounts of contaminated soil and return it to the site. Moreover, it can manage a variety of levels of contamination ( Figure 4a). Ex situ soil washing is a soil remediation technique that involves the physical separation and removal of contaminants from soil outside of its original location. It is particularly effective for soils contaminated with organic compounds, heavy metals, and other pollutants that are amenable to extraction. Another option is in situ soil flushing with surfactant eluents, as shown in Figure 4b. In situ flushing is a soil remediation technique that involves the application of water or flushing agents directly to the contaminated soil to mobilize and remove the contaminants. It is particularly effective for soil contamination caused by soluble or readily dispersible contaminants. Through injection wells, flushing solutions with wetting agents are injected into the affected area. The soil pollutants are made mobile either by dissolving them (e.g., by creating micelles with flushing solutions) or by chemical reactions. After passing through the contaminated region, the pollutantcontaining liquid is brought to the surface for disposal, recycling, or on-site treatment and reinjection. Soil washing is one of the few treatments that can completely eliminate radionuclides, organics, and heavy metals from polluted soils. In order to carry out soil washing or other corrective measures, such as surfactant-enriched phyto-remediation, surfactant-enriched bioremediation, and surfactant-enriched electrokinetic remediation, surfactants are a regularly utilized factor [19].

Theory and Mechanism of 'Surfactant-Enhanced Soil Remediation'
When surfactant molecules are present in a heterogeneous soil-water system, they can stick to the soil particles and create interactions, as demonstrated in Figure 5. The lipophilic tail of the surfactants commonly bonds with hydrophobic pollutants or particles of dirt, while the hydrophilic head sections are more likely to enter the watery layer. As a result, surfactants that are present in small amounts mainly come together as single molecules at the solid-liquid or liquid-liquid interface. As the concentration increases, the aqueous phase's polarity is reduced, and surface tension is lowered as the surfactant molecules gradually substitute the interfacial solvents, like water. In addition, the dissolution process of non-aqueous-phase liquid (NAPL) contamination may be hastened.
At the point when the surfactant concentration hits its critical micelle concentration (CMC), micelle structures will begin to form. As the surfactant molecules continue to expand, ellipsoidal or round micelles are created. These micelles, with their hydrophobic cores and lyophobic surfaces, are equipped for helping with the desorption of toxins from the soil. They can spread NAPLs (non-aqueous-phase liquids) and greatly boost their solubility in water. Given that the pollutants dissolved in the water are more mobile, they may be then eliminated through biotic or abiotic means, such as plant absorption and microbial decomposition ( Figure 5) [20].
When it comes to naphthalene contamination, nonionic surfactants such as Brij-35 and Triton X-100 have a better capacity to dissolve it than ionic surfactants, like sodium lauryl benzene sulfonate and sodium lauryl sulfate. The hydrophobic chain of the surfactant has an effect on how hydrocarbon contaminants can be dissolved in water, with the molar dissolving ratio of pollutants increasing or decreasing based on the length of the hydrophobic chain. Ionic surfactants are especially suitable for dissolving pollutants like benzene and other linear chain hydrocarbons due to their long chain length. In addition, surfactants with long hydrophobic chains are beneficial for eliminating hydrocarbons from polluted soil. The difference between ionic and nonionic surfactants is their hydrophilelipophile balance (HLB)-ionic surfactants have HLB values of up to 50, while nonionic surfactants have HLB numbers ranging from 0 to 20.   [19]. Reuse with copyright permission.

Theory and Mechanism of 'Surfactant-Enhanced Soil Remediation'
When surfactant molecules are present in a heterogeneous soil-water system, they can stick to the soil particles and create interactions, as demonstrated in Figure 5. The lipophilic tail of the surfactants commonly bonds with hydrophobic pollutants or particles of dirt, while the hydrophilic head sections are more likely to enter the watery layer. As a result, surfactants that are present in small amounts mainly come together as single molecules at the solid-liquid or liquid-liquid interface. As the concentration increases, the aqueous phase's polarity is reduced, and surface tension is lowered as the surfactant molecules gradually substitute the interfacial solvents, like water. In addition, the dissolution process of non-aqueous-phase liquid (NAPL) contamination may be hastened.  [19]. Reuse with copyright permission.
As the HLB value of a surfactant increases, the micelle volume decreases, thereby diminishing the solubilization of crude oil pollutants, such as decane, dodecane, and hexane. Temperature has a substantial influence on the dissolution of both ionic and nonionic surfactants, which increases the effectiveness of contamination clearance. In terms of biodegradability, nonionic surfactants fared better than ionic surfactants when it came to the removal of hydrocarbon pollutants from the soil. Cruz-Lopes et al. (2019) employed the column washing technique in combination with electro-Fenton (EF) to monitor the biodegradability of leachates and TPH-contaminated soil [21]. Moreover, Tween surfactant has been used to remediate hydrocarbon pollutants, like phenanthrene and fractions of petroleum (diesel, kerosene, and gasoline) [22]. Sustainability 2023, 15, x FOR PEER REVIEW 11 of 35 Figure 5. Mechanism of surfactant-enhanced soil remediation [9].
At the point when the surfactant concentration hits its critical micelle concentration (CMC), micelle structures will begin to form. As the surfactant molecules continue to expand, ellipsoidal or round micelles are created. These micelles, with their hydrophobic cores and lyophobic surfaces, are equipped for helping with the desorption of toxins from the soil. They can spread NAPLs (non-aqueous-phase liquids) and greatly boost their solubility in water. Given that the pollutants dissolved in the water are more mobile, they may be then eliminated through biotic or abiotic means, such as plant absorption and microbial decomposition ( Figure 5) [20].
When it comes to naphthalene contamination, nonionic surfactants such as Brij-35 and Triton X-100 have a better capacity to dissolve it than ionic surfactants, like sodium lauryl benzene sulfonate and sodium lauryl sulfate. The hydrophobic chain of the surfactant has an effect on how hydrocarbon contaminants can be dissolved in water, with the molar dissolving ratio of pollutants increasing or decreasing based on the length of the hydrophobic chain. Ionic surfactants are especially suitable for dissolving pollutants like benzene and other linear chain hydrocarbons due to their long chain length. In addition, surfactants with long hydrophobic chains are beneficial for eliminating hydrocarbons from polluted soil. The difference between ionic and nonionic surfactants is their hydrophile-lipophile balance (HLB)-ionic surfactants have HLB values of up to 50, while nonionic surfactants have HLB numbers ranging from 0 to 20.
As the HLB value of a surfactant increases, the micelle volume decreases, thereby diminishing the solubilization of crude oil pollutants, such as decane, dodecane, and hexane. Temperature has a substantial influence on the dissolution of both ionic and nonionic surfactants, which increases the effectiveness of contamination clearance. In terms of biodegradability, nonionic surfactants fared better than ionic surfactants when it came to the removal of hydrocarbon pollutants from the soil. Cruz-Lopes et al. (2019) employed the column washing technique in combination with electro-Fenton (EF) to monitor the biodegradability of leachates and TPH-contaminated soil [21]. Moreover, Tween surfactant Before employing surfactant-aided remediation of polluted soils, a variety of scientific elements should be considered, such as the adsorption of the surfactants on the dirt, their capability to dissolve/elute pollutants, their toxicity, and their biodegradability. It is also essential to think about other non-scientific factors, such as the cost of the surfactants and the size of the polluted area. To bring down the expenses of the remediation process and guarantee its economical effectiveness, a practical surfactant should have a low CMC and be able to function at a minimal dosage for washing solutions, as well as having a powerful ability to desorb contaminants.
When surfactants are used in water-soil systems, a certain amount of them will get absorbed by the soil particles. This causes the surfactants to be less effective at solubilizing contaminants, and it also makes the soil surface more hydrophobic, which causes the removed solubilized organics to be reabsorbed. Therefore, when selecting the right surfactants, it is very important to consider the adsorption behavior of the surfactant onto soil particles. This behavior is determined by the molecular structure of the surfactants, which can affect their characteristics. For example, perfluoro sulfonate was found to have much higher sorption on sludge than its analog, the perfluoro carboxylate, and the sorption of C 5-15 perfluoroalkyl surfactants was also found to increase with the length of the carbon chain. The capacity to take up a surfactant is associated with soil attributes in addition to the specific characteristics of the surfactant itself. Dodecyl pyridinium chloride (DPC), a positively charged surfactant, was observed to be adsorbed in a direct correlation with the cationic exchange capacity of the soil, suggesting that the adsorption of DPC was dependent on the soil's negative charges. The research into the relationship between the maximum amount of TX-100 adsorbed and the quantity of organic matter in the soil also revealed that soil organics altered the characteristics of the soil-water interface, which in turn had an effect on the adsorption behavior of the nonionic TX-100 [23].
To be effective at cleaning soil, surfactants need to have a high capability to dissolve and not attach themselves to the soil. The type of surfactant and its molecular makeup can have a considerable effect on the molar solubilization ratio (MSR) of pollutants. This ratio is determined by the number of molecules of pollutants that are solubilized in contrast to the number of molecules of surfactant that are solubilized, expressed in moles. This MSR is demonstrated by the slope of a straight line beyond the CMC on a graph representing the solubility of aqueous contaminants versus the concentration of surfactant [19].
The following formula can be applied to calculate the MSR precisely: • C mic is the total apparent solubility of the organic contaminants in micellar solution (in moles per liter) at a specific surfactant concentration higher than the CMC; • C CMC is the apparent solubility of the contaminant (in moles per litre) at the CMC, which can be roughly equated to the contaminant's aqueous solubility; • C surf is the surfactant concentration at which C mic is evaluated in mol/L.

Removal of Hydrophobic Organic Componds (HOCs)
The presence of hydrophobic organic compounds (HOCs) in soil has become a major environmental issue. The difficulty of removing HOCs from the soil due to their strong adhesion has made this problem more complex. Many studies have established the potential of biological approaches combined with surfactants for the extraction of HOCs from contaminated soils. For instance, the soil-washing process involving the use of extraction solutions has been found to be effective for eliminating HOCs and oil hydrocarbons from polluted soil. These solutions can consist of several chemicals, including surfactants, short-chain organic acids, and chelating agents. One of the commonly utilized chemical agents for enhancing the solubility and mobility of hydrophobic organic pollutants is the incorporation of a surfactant in a water-based formulation.
This soil-washing system based on surfactants mainly functions through two approaches, which are the solubilizing of hydrophobic organic contaminants (HOCs) inside the hydrophobic centers of surfactant micelles and the reduction of interfacial tension that leads to the mobilization of these contaminants.
In 2017, Cheng et al. carefully examined the use of the nonionic surfactant Tween 80 in combination with bioremediation methods to clear HOCs from soils. This review looked into the fundamentals of Tween-80-boosted HOC desorption from sullied soils and spotlighted its bio-toxicity. This study looked into the decomposition of HOCs in the soil-washing effluent that had been enhanced with Tween 80 and the way that the Tween 80 had a role in the desorption and solubilization of the attached HOCs, their toxicity, and the possibility of using microorganisms for bioremediation with a main focus on bacterial and fungal degradation. It also uncovered Tween 80's adverse effects on the decomposition process [24].
Research has shown that Tween 80 can strengthen the attraction of bacterial cells to HOC molecules, which enables them to break down pollutants and enhance the effectiveness of extracellular enzymes from fungi to oxidize substances. Studies in greenhouses have shown that Tween 80 can significantly increase the uptake of HOCs by plants. Furthermore, Tween-80-enhanced phytoremediation can be used to clean up soil that has been contaminated with both heavy metals and HOCs. The results of these Tween-80-enhanced bioremediation approaches have been very positive, suggesting that the problem of soil HOC contamination can be addressed. However, as the majority of these experiments have taken place in laboratories, further research should be conducted before these findings are applied to real-world scenarios [24].
The application of Tween-80-augmented biotechnologies offers the prospect of managing soils polluted with HOCs based on previously-recorded positive results from biological degradation and phytoremediation. Nevertheless, further alterations will be necessary before these techniques can be applied on full scale as the majority of these studies have only been tested in a laboratory setting [24].
Despite being useful, the soil-washing technique that uses surfactants is hindered by a few major problems, such as having bad specificity, a long extraction time, and inefficiency. These issues have restricted the practical applications of the surfactant-based soil-washing technique. To effectively remediate soils polluted with low-polar or nonpolar petroleum hydrocarbons, the combination of surfactants and other additives (organic solvents) has been gaining attention recently, as it can extract the contaminants at a much higher rate and in less time [24].
Wang et al., 2019, conducted a study that looked into how well combinations of solvents and surfactants can remove crude oil from soil. They considered the solvent polarity, the ratio of solvent to surfactant, temperature, and ionic strength when evaluating the systems. It was discovered that the toluene/AES-D-OA system was more effective than the other systems in removing crude oil, with more than 97% of the crude oil in contaminated soil removed at high toluene ratios. The efficiency in removing crude oil increased with rising temperatures or the appropriate increase in ionic strength. Their results revealed that when surfactant and solvent are combined, it produces better results for the removal of crude oil than conventional surfactant-aided remediation [25]. Trellu et al. (2016) [26] conducted a critical assessment and concluded that synthetic surfactants and biosurfactants are both effective and affordable ways of dealing with soils contaminated by hazardous organic compounds. Extraction agents must be recovered from the resulting effluent of the high strength required for the implementation of solid-water and solid-fluid processes. The challenge lies in executing these processes cost-effectively, meaning that the amount of chemicals and energy needed must be kept to a minimum. Figure 6 below shows the possible degradation kinetics of the targeted pollutants depending on the method chosen. tively, meaning that the amount of chemicals and energy needed must be kept to a min mum. Figure 6 below shows the possible degradation kinetics of the targeted pollutan depending on the method chosen. To fully evaluate the costs associated with treatment, calculations must be made f energy consumption, reagent usage, reuse of extracted agents, transporting soil and effl ent, as well as setting up and maintaining the treatment plant. Heterogeneous photocata ysis, photo-Fenton, and EAOPs are the most successful oxidation mechanisms for elim To fully evaluate the costs associated with treatment, calculations must be made for energy consumption, reagent usage, reuse of extracted agents, transporting soil and effluent, as well as setting up and maintaining the treatment plant. Heterogeneous photocatalysis, photo-Fenton, and EAOPs are the most successful oxidation mechanisms for eliminating HOCs in SW/SF solutions, with degradation rates of 1 to 10 h. This is because the amount of hydroxyl radicals produced is adequate to treat these highly concentrated effluents. Furthermore, due to faster rates of decomposition compared to extraction agents, these methods have been identified as viable solutions for selectively oxidizing specific HOCs. A summary of the pros and cons of each method is provided in Figure 7.  In 2018, Cameselle and Gouveia determined that since a component that is not dissolved in the interstitial fluid cannot be taken out by electro kinesis or electro-osmosis when managing hydrocarbon contamination in soil, utilizing electro-osmosis on the dirt sample is futile. Surfactants and co-solvents can enhance the solubility of a hydrocarbon in the handling fluid (Figure 8). At the same time, it is important to increase the electro-osmotic flow. Due to the thickness of the surfactant and its contact with the surface of the soil particle, the utilization of surfactants typically diminishes the electro-osmotic flow (EOF). Saberi et al. reported that by utilizing Tween 80 and Brij 35, about 62% of phenanthrene was removed within 15 days [26,27]. It was a challenge to take out phenanthrene from the soil because of its connection with the humic acids. Thus, experiments were performed with Triton X-100, Tween 80, and sodium dodecyl sulfate (SDS) to eliminate 120 n-hexadecane and anthracene. The most noteworthy removal rates of 69% for n-hexadecane and 59% for anthracene were produced when Tween 80 was utilized in the anolyte and SDS (anionic surfactant) was used in the catholyte. There is another proposal to improve electro-osmotic flow with Tween 80 rather than Triton X-100 or SDS. It is essential to add anionic surfactants to the catholyte and electro migrate them into the soil. The surfactant's contact with soil particles limits its activity, so cationic surfactants are not used. The toxicity of the surfactant must be taken into consideration when selecting it.
It is advantageous to employ biodegradable surfactants, especially those produced by microorganisms. These biosurfactants can be utilized in electro bioremediation as they are not detrimental, and the soil pollutants decomposed by the soil microorganisms become more accessible. Consequently, efficient degradation of diesel oil, PAHs, and other HOCs can be accomplished.

Removal of Petroleum Hydrocarbons
The presence of hydrocarbons from petroleum in soil is a major source of worry for the environment and people's health. Oil molecules have limited bioavailability for microorganisms since they are highly hydrophobic and not readily soluble in water. Additionally, they cling to soil particles, thus reducing the rate of degradation via biodegradation. To increase the availability (or solubility) of hydrocarbons from petroleum, the use of surfactants can be effective in releasing them from soil particles and increasing their solubility [28].
An earlier investigation affirmed that utilizing sorbitan monooleate was successful in expelling diesel from a test location that had been contaminated. In situ flushing was em- It was a challenge to take out phenanthrene from the soil because of its connection with the humic acids. Thus, experiments were performed with Triton X-100, Tween 80, and sodium dodecyl sulfate (SDS) to eliminate 120 n-hexadecane and anthracene. The most noteworthy removal rates of 69% for n-hexadecane and 59% for anthracene were produced when Tween 80 was utilized in the anolyte and SDS (anionic surfactant) was used in the catholyte. There is another proposal to improve electro-osmotic flow with Tween 80 rather than Triton X-100 or SDS. It is essential to add anionic surfactants to the catholyte and electro migrate them into the soil. The surfactant's contact with soil particles limits its activity, so cationic surfactants are not used. The toxicity of the surfactant must be taken into consideration when selecting it.
It is advantageous to employ biodegradable surfactants, especially those produced by microorganisms. These biosurfactants can be utilized in electro bioremediation as they are not detrimental, and the soil pollutants decomposed by the soil microorganisms become more accessible. Consequently, efficient degradation of diesel oil, PAHs, and other HOCs can be accomplished.

Removal of Petroleum Hydrocarbons
The presence of hydrocarbons from petroleum in soil is a major source of worry for the environment and people's health. Oil molecules have limited bioavailability for microorganisms since they are highly hydrophobic and not readily soluble in water. Additionally, they cling to soil particles, thus reducing the rate of degradation via biodegradation. To in-crease the availability (or solubility) of hydrocarbons from petroleum, the use of surfactants can be effective in releasing them from soil particles and increasing their solubility [28].
An earlier investigation affirmed that utilizing sorbitan monooleate was successful in expelling diesel from a test location that had been contaminated. In situ flushing was employed to clean the soil and groundwater of a site that had been employed as a military vehicle repair location in Korea for 45 years and was polluted by diesel. At the polluted site, a pilot-scale site of 4 m × 4 m × 4 m was specifically chosen for in situ flushing. The site had a heterogeneous composition of sand and sludge soils with an average K of 2.0 × 10 × 4 cm/s.
To eliminate diesel from the location, the team had to flush two pore volumes of groundwater with a two percent combination of sorbitan monooleate (POE 20) in five pore volumes of solution (three pore volumes of surfactant solution). The effluent was then chemically treated, and a dissolved air flotation apparatus was utilized to reduce the concentration of the solution below 5 mg/L before it was discarded into a close-by sewage drain.
In addition to the surfactant-flushing method, an enhanced follow-up action was implemented for the surfactant wastewater. After three-volume flushing with 2% sorbitan monooleate fluid, a sum of 48 kg of TPH (or 88% of the initial TPH) was removed; this amount was more than 75 times greater than what was taken out by using water alone (640 g).
The entire extracted fluid was given a chemical treatment process, involving a system, to decrease the concentration to less than 5 mg/L, and then the purified solution was subsequently discharged into a nearby sewage drain. This study highlighted that sorbitan monooleate was a successful surfactant for the reclamation of areas contaminated with diesel and the in situ surfactant flushing system with a post-treatment system in conjunction with a DAF was reported to have potential uses in the restoration of fuel-polluted sites [29]. Wang et al. (2018) [30] used Arthrobacter globiformis bacteria to study the bioremediation of dirt from the Shenyang North New Area of China, which had been contaminated with polycyclic aromatic hydrocarbons (PAHs). They looked into the combined effects of various concentrations of the biosurfactant rhamnolipids (RL) and anionic-nonionic mixed surfactant (SDBS-Tween 80). After 150 days of remediation, the bacteria had eradicated 52.1% and 21.9% of the PAHs. The removal rate of PAHs at the optimal RL dosage of 5 mg kg −1 was figured out to be 64.3% and 35.6%, surpassing the control by 60.7% and 29.3%, soil treated with RL-5 alone by 36.9% and 19.8% and A. globiformis treatment alone by 12.2% and 13.8%. RL-5 was seen to enhance the activity of soil enzymes and A. globiformis reproduction during the biodegradation of DDT and PAHs. This study introduces a feasible in situ soil bioremediation technique that is highly efficient yet cost-effective. Couto et al. (2012) [31] conducted research to identify effective biological tactics to remediate a refinery soil contaminated with petroleum hydrocarbons (PHC). The study examined the results of the presence and absence of the salt marsh plant S. maritimus or J. maritimus in isolation or together, in an industrial setting. Additionally, it studied the synergistic effects of nonionic surfactant and/or a bio-expansion product. This study faced difficulties due to field conditions and weather-induced pollution.
Scientists transplanted the plants into ampules filled with weather-affected, polluted soil that had been re-contaminated with turbine oil for two objectives: to heighten levels of PHC and to assess the plants' ability to decontaminate both old and recent contamination. Following 24 months of exposure, PHC analysis showed no improvements in either the association or J. m. Nonetheless, a 15% increment in remediation was observed at the layer with the most roots (5-10 cm). The combination of nonionic surfactant and bioaugmentation led to a more noticeable enhancement (28%) in that layer. The findings of the study demonstrate that S. maritimus can be used to restore sediments damaged by unintentional oil spills since it has displayed effectiveness in PHC remediation [30,31].
The electro-Fenton (EF) process is one method that has been used to address hydrocarbon contamination in soil. This electrochemical procedure creates hydroxyl radicals that are able to break down the hydrocarbons. It has been found to be a reliable remediation technique. To demonstrate this, Huguenot et al. conducted a study in 2015 [32] where they applied the EF process to soil contaminated with a mixture of diesel fuel and lubricating oil. The technique included combining the polluted soil with a washing solution and then utilizing an electrochemical cell to generate hydroxyl radicals, thus breaking down the hydrocarbons.
The research suggested that the EF system was capable of decreasing the level of total petroleum hydrocarbons (TPH) in polluted soil by up to 98% in 4 h. Additionally, the EF process was successful in diminishing the concentration of polycyclic aromatic hydrocarbons (PAHs), which are a particularly dangerous type of hydrocarbons.
To boost the efficiency of soil washing to clear hydrocarbon-contaminated soils, the electro-Fenton (EF) process was implemented. A soil-column-cleaning experiment was conducted on soil polluted with diesel, and different quantities of the surfactant Tween ® 80, at more than the necessary micellar concentration, were added to the washing fluid (CMC). Results were evaluated by looking at the hydrocarbon concentration in the leachates taken from the soil columns. Subsequently, the study explored the potential of these eluates to be degraded through EF treatment. The research indicated that 5% of Tween ® 80 was essential to upgrade hydrocarbon extraction from the soil. Nevertheless, the effectiveness of the treatment was still shockingly low (1% after 24 h of washing). A progressive increase in kinetic energy caused almost full disintegration of the hydrocarbons (>99.5%) within a period of 32 h, as confirmed by the electrochemical tests done on the extracted eluates.
The Microtox ® approach caused toxicity that was higher than the initial solution, decreasing Vibrio fischeri bacteria by 95%. The biodegradability (BOD5/COD ratio) only increased to 20% after 20 h of EF treatment, which is not adequate for combined treatment with a biological treatment process.
Karthick et al. conducted the remediation of diesel-polluted soil in 2019 [33] with the help of two surfactants, namely sodium lauryl sulfonate (SLS) and Tween 80, both of which were stabilized with allyl alcohol and ethylene glycol. Tween 80 surfactant, stabilized with allyl alcohol (0.3 weight%), displayed the best performance in terms of foam stability, lasting for 36 min, and managing to take out 71% of the diesel. As for SDS foam stabilized with allyl alcohol (0.3 weight%), it demonstrated a 62% removal effectiveness. Unfortunately, foams stabilized with ethylene glycol (0.3 weight percent) were not very successful and the diesel removal rate was very low.
To investigate the effect of the arrangement of surfactant foam and the connection between oil and water, a study was conducted using 11-dimethylamino undecyl sulfate, sodium salt (DUSNa), an anionic CO 2 -switchable surfactant meant for dealing with oilcontaminated water and contaminated soil samples ( Figure 9).
An anionic sodium alkyl sulfate surfactant was deactivated by the addition of a CO 2 -switchable tertiary amine group resulting in DUSNa, which facilitated a near-perfect oil/water disconnection with oil and water held together by DUSNa. A mixture of 95 g quartz sand, 5 g oil, and 200 g DUSNa solution (20 mmol L −1 ) at pH 13 was placed in a vitreous SESW bath with quartz sand, and following two hours of stirring and simple filtration at room temperature, a neat and clean sand was obtained ( Figure 10).
Recently, surfactants which have been stabilized by nanoparticles have become more popular than surfactants on their own. Karthick et al. explored the stabilizing effects of iron oxides and zero-valent iron nanoparticles on an aqueous alkyl poly glucoside phosphate foam in 2019 [34]. The efficacy of the foam which was distributed with the iron nanoparticles was examined to remediate diesel hydrocarbons from three types of soils with different compositions: desert sandy soil, coastal sandy soil, and clay soil. The largest amount of APG-Ph which could be eliminated from the clay soil and desert soil after all the experiments was 51.6%, 79.6%, and 76.0%, respectively. It is evident from past studies that the potential of surfactant foam which has been supported by nanoparticles to remove contaminants has not been maximized when compared to surfactant solution and surfactant foam [35]. stability, lasting for 36 min, and managing to take out 71% of the diesel. As for SDS foam stabilized with allyl alcohol (0.3 weight%), it demonstrated a 62% removal effectiveness. Unfortunately, foams stabilized with ethylene glycol (0.3 weight percent) were not very successful and the diesel removal rate was very low.
To investigate the effect of the arrangement of surfactant foam and the connection between oil and water, a study was conducted using 11-dimethylamino undecyl sulfate, sodium salt (DUSNa), an anionic CO2-switchable surfactant meant for dealing with oilcontaminated water and contaminated soil samples (Figure 9).  An anionic sodium alkyl sulfate surfactant was deactivated by the addition of a CO2switchable tertiary amine group resulting in DUSNa, which facilitated a near-perfect oil/water disconnection with oil and water held together by DUSNa. A mixture of 95 g quartz sand, 5 g oil, and 200 g DUSNa solution (20 mmol L −1 ) at pH 13 was placed in a vitreous SESW bath with quartz sand, and following two hours of stirring and simple filtration at room temperature, a neat and clean sand was obtained ( Figure 10).
Recently, surfactants which have been stabilized by nanoparticles have become more popular than surfactants on their own. Karthick et al. explored the stabilizing effects of iron oxides and zero-valent iron nanoparticles on an aqueous alkyl poly glucoside phosphate foam in 2019 [34]. The efficacy of the foam which was distributed with the iron nanoparticles was examined to remediate diesel hydrocarbons from three types of soils with different compositions: desert sandy soil, coastal sandy soil, and clay soil. The largest amount of APG-Ph which could be eliminated from the clay soil and desert soil after all the experiments was 51.6%, 79.6%, and 76.0%, respectively. It is evident from past studies that the potential of surfactant foam which has been supported by nanoparticles to remove contaminants has not been maximized when compared to surfactant solution and surfactant foam [35].  Rhamnolipids, which are accessible as foam in an unadulterated arrangement, nonpurged arrangement, or cell-free culture medium, have apparently demonstrated the most noteworthy proficiency in this methodology. It was found that this biosurfactant was powerful in evacuating petroleum and diesel oil from a sandy soil that had been sullied with 5% of each hydrocarbon independently, just as 81.3% of the oil from motor-oil-covered permeable rocks and 80% of the oil from dirt. Rhamnolipid was utilized and it was applied as 0.1 g/L: of micro-foam. The utilization of biosurfactants diminished the thickness of the sludge and encouraged the creation of emulsions that improved unrefined petroleum recuperation. Such discoveries back the utilization of biosurfactant arrangements for sand washing and cleaning up oil-contaminated soil, which prompts a lessening in TPH in the dirt and/or further oil recuperation [36].

Removal of Poly Aromatic Hydrocarbons (PACs)
The presence of large amounts of surfactants makes it difficult to eliminate soil washing effluent (SWE) from the remediation of soil containing hydrophobic organic pollutants (HOCs). Zhang et al. (2019) [37] studied the efficiency of two synthetic LDHs modified with sodium dodecyl sulfonate (SDS) in different loading levels (organo-LDHs) as absorbents for the elimination of two common HOCs, phenanthrene (PHE) and pyrene (PYR), from a model SWE. The results revealed that inside a 2 h equilibrium window, the organo-LDHs could successfully absorb PHE and PYR from the SWE. The absorption capacities of the organo-LDHs increased almost in a straight line with the amount of SDS loaded on the LDHs, and all isotherms were linear. Additionally, the surface areas of the organo-LDHs decreased significantly as the SDS loading was increased since the SDS was preventing the LDHs from displaying their exposed surface ( Figure 11).
Results indicated that partitioning was more dominant than adsorption in the sorption process, and the strong attraction of HOCs towards the organic phase in LDHs enabled efficient elimination of polycyclic aromatic hydrocarbons (PAHs) from the SWE. What is more, when SDS was loaded at higher quantities, the sorption capacity of organo-LDHs for PHE and PYR was much higher than the one of commercial activated carbon for the higher PAH concentration ranges [37].
In 2015, Adrion et al. found that polyoxyethylene sorbitol hexaoleate (POESH) was the best surfactant for eliminating polycyclic aromatic hydrocarbons (PAHs), in particular the higher-weight PAHs. To further treat the slurry-phase bioreactor waste, they added POESH to a second-stage batch reactor and let the mixture sit for either 7 or 12 days. After the regular bioremediation process, the addition of surfactants removed a large portion of the PAHs and oxy-PAHs, including more than 80% of the four-ring PAHs. To assess the genotoxicity of the soil, the DT-40 chicken lymphocyte DNA damage response trial was used, and the results showed that the soil's genotoxicity was often increased but its cytotoxicity was usually decreased after the surfactant was added. A seed germination test was utilized to measure the potential ecotoxicity and it was found that after bioreactor treatment and further POESH treatment, potential ecotoxicity was notably decreased. In this research, the Terrimonas genus of bacteria was connected to modifications in the elimination of high-molecular weight PAHs. This group of bacteria had been indicated as possible PAH decomposers during POESH-amended settings in an earlier study. These discoveries can be useful for the formation of bioremediation programs at sites that are dealing with PAH-tainted soil and other hydrophobic contaminants that have restricted bio availability since there is limited exploration regarding the application of sub-micellar amounts of surfactant as a second-stage process.
In 2018, Saeedi and colleagues conducted a study on the influence of soil elements such as clay minerals, humic acids, and metals along with polycyclic aromatic hydrocarbons on their desorption and mobility. They examined the desorption and mobility of acenaphthene, fluorene, and fluoranthene in three distinct mixtures of clay and clay minerals (kaolinite only, kaolinite plus sand, and kaolinite plus sand, and bentonite) with dissimilar humic acid content (Tween 80 and Triton X100). In addition, the desorption and mobility of heavy metals (Ni, Pb, and Zn) were also examined. Lastly, they explored how the coexisting metals impacted the simultaneous desorption and mobility of PAHs [38]. Results indicated that partitioning was more dominant than adsorption in the sorption process, and the strong attraction of HOCs towards the organic phase in LDHs enabled efficient elimination of polycyclic aromatic hydrocarbons (PAHs) from the SWE. What is more, when SDS was loaded at higher quantities, the sorption capacity of organo-LDHs for PHE and PYR was much higher than the one of commercial activated carbon for the higher PAH concentration ranges [37].
In 2015, Adrion et al. found that polyoxyethylene sorbitol hexaoleate (POESH) was the best surfactant for eliminating polycyclic aromatic hydrocarbons (PAHs), in particular the higher-weight PAHs. To further treat the slurry-phase bioreactor waste, they added POESH to a second-stage batch reactor and let the mixture sit for either 7 or 12 days. After the regular bioremediation process, the addition of surfactants removed a large portion of the PAHs and oxy-PAHs, including more than 80% of the four-ring PAHs. To assess the genotoxicity of the soil, the DT-40 chicken lymphocyte DNA damage response trial was used, and the results showed that the soil's genotoxicity was often increased but its cytotoxicity was usually decreased after the surfactant was added. A seed germination test was utilized to measure the potential ecotoxicity and it was found that after bioreactor treatment and further POESH treatment, potential ecotoxicity was notably decreased. In Figure 11. The mechanism of sorption of polyaromatic hydrocarbons by organo-LDH in an aqueous solution with a high surfactant concentration [37].
The research revealed that 10% of the metals in the clay mineral mixtures were able to move. By combining EDTA with nonionic solutions, it was possible to increase the desorption and mobility of PAHs to over 80% in clay mineral mixtures not containing sand and to over 90% in soils that had less than 40% sand. The desorption and mobility of PAHs were decreased, particularly in soils without sand and with regards to fluoranthene, due to the presence of heightened humic acid content and heavy metals in the clay mineral mixtures.
In 2018, Tiensing and Puangkaew achieved the successful synthesis of Fe 3 O 4 @FA@CTAB NPs, a bilayer surfactant with fatty acid and cetyltrimethylammonium bromide coated on magnetic nanoparticles, in an aqueous medium. They were then utilized as sorbents for magnetic solid-phase extraction of polycyclic aromatic hydrocarbons in water samples. The amount of sorbent, sample volume, adsorption time, elution solvent, and desorption time were all examined under various extraction conditions. The hydrophilic attributes of the bilayer surfactants on MNPs enabled high preconcentration factors and good dispersion in the water samples. The HPLC-UV technique with a Waters Nova-Pak ® C18 column and a 30/40/30 combination of acetonitrile, methanol, and deionized water as the mobile phase was used to identify the extracted PAHs, which included phenanthrene, anthracene, and fluoranthrene. In order to recognize PAHs in water samples, an HPLC technique and a synthetic magnetic sorbent were employed. The recovery range of PAHs in river water samples was between 71.78-118.29%, and the RSD was lower than 4.70%. The limits of detection for phenanthrene, anthracene, and fluoranthene were 0.67, 0.10, and 0.53 g L −1 , respectively. Results demonstrated that the created approach is suitable for rapid, straightforward, and economical examination of PAHs in water samples [39]. To remove three polycyclic aromatic hydrocarbons (fluorene, fluoranthene and acenaphthene) being present with three different heavy metals (Pb, Ni, and Zn) in a heavily polluted soil found near an oil refinery, the columns were flushed with solutions combining Triton X-100 with ethylenediaminetetraacetic acid and Tween 80 with ethylenediaminetetraacetic acid at three separate surfactant concentrations. The results of the study showed that when Triton X-100 and ethylenediaminetetraacetic acid were used in unison, they were relatively successful in removing PAHs. It was found that after 21 aperture volume flushes of the improving solution along the column with a hydraulic conductivity of 8.5 105 cm s1, acenaphthene, fluorene, and fluoranthene were eradicated by 54%, 47%, and 40%, respectively. At the same time, 75%, 85%, and 90% of Pb, Ni, and Zn were also removed. However, it was noted that the effectiveness of the pollutant removal was reduced when the flow rate of the flushing solution was increased [40].
The adsorption technique is usually thought to be a successful way of getting rid of both inorganic and organic pollutants in a more economical, cost-effective, and environmentally friendly manner. In this research, organic montmorillonite sodium alginate composites were produced by blending montmorillonite with sodium alginate and cationic surfactant (cetyltrimethylammonium bromide, CTAB) using CaCl 2 as the crosslinking agent. Batch adsorption tests from an aqueous solution were conducted to analyze the morphological properties of the composites and were used to remove three types of polycyclic aromatic hydrocarbons (PAHs): Phenanthrene, fluorene, and acenaphthene. The composites provide an effective approach for the elimination of PAHs. Methyl alcohol enabled the secure separation and regeneration of the composites. The Elovich kinetic and Freundlich isotherm models, respectively, effectively revealed the adsorption kinetic and isotherm data. Our exploration shows that a multi-layered adsorption process happened on the energy-uneven surface of the composite. Additionally, hydrophobicity and pore diffusion were vital pieces of the adsorption system. In general, our investigation offers an innovative adsorbent that is cost-effective, reusable, biodegradable, and biocompatible for effectively eliminating PAHs from water solutions [41].
Partially adsorbed PACs with multiple cores (PacM) appear as little, randomly ordered clusters on the interface. The addition of the nonionic surfactant Brij-93 led to the removal of PacM from both the water/toluene and water/heptane interfaces. However, the addition of (EO)5(PO)10(EO)5 resulted in a decrease in PacM adsorption on the water/toluene interface but an increase on the water/heptane interface. Strongly adsorbed PACs with a single, bulky core form tight aggregates on both interfaces and were not affected by the two different nonionic surfactant additions. This study highlighted the two contrasting effects of nonionic surfactants on the adsorption of PACs, namely co-adsorption and competition, and provided useful insight into how the properties of nonionic surfactants, such as their concentration and the solubility and interfacial behaviors of the PACs, can influence their function [42].
The desorption behavior and toxicity of novel biosurfactants from Eucalyptus camaldulensis leaves and sophorolipid biosurfactants were assessed. The capacity for PAH desorption of saponin, Tween 20, sophorolipid, and rhamnolipid was compared and the salt resistance of each surfactant up to 30 g/L NaCl was evaluated based on their emulsification index. The heat stability was lowest for saponin, Tween 20, sophorolipid, and rhamnolipid and the saponin bio-surfactant emulsion showed the greatest stability over a wide pH range. Through testing the optimal surfactant concentration, volume, and incubation period, saponin and sophorolipid PAH extraction percentages were between 30-50% and 30-70%, respectively. For all matrices, saponin, sophorolipid, rhamnolipid, and Tween 20 had similar PAH desorption capabilities. In comparison to the other three surfactants, sophorolipids proved to be the most efficient in desorbing the low molecular weight PAHs from sediment and soil. To assess the toxicity of biosurfactants to the soil/sediment microorganisms, microbial respiration was employed. The outcomes suggested that no hindrance to respiration occurred within the 60-day incubation phase, implying that sophorolipid-and saponin-induced remediation could be viable options for extracting PAHs from polluted soils and sediments [43].

Removal of Agrochemicals
Agrochemicals offer great economic and agricultural advantages; however, their overuse can be damaging to the environment. Soil-water repellency is a frequent characteristic of soils with persistent vegetation and drought conditions. This has a major effect on soil-water hydrology as it hinders infiltration, which in turn causes surface runoff and soil erosion. Not only sandy soils have been identified to have this phenomenon, but loam, clay and peat soils in both dry and humid climates have also been found to be affected. Furthermore, soil water repellency can lead to reduced water availability caused by the non-uniform water retention and flow and dry soil volume between the pathways of preferential channels (gravity-induced fingers), which is damaging to plants [44].
Recently conducted studies have indicated that continuous irrigation with treated wastewater-a common substitute in Israel and other Mediterranean countries due to the lack of freshwater-leads to water repellent soil, which in turn results in the formation of preferred flow in the field. Considering the increasing effects of climate change, especially in regions which are not usually thought of as arid, the usage of treated wastewater for irrigation as a method of freshwater preservation is expected to increase. The preferential flow in soils that reject water has been studied extensively; however, there has been little effort put into understanding how this flow pattern affects the spread of salt and nutrients in the root zone. The salinity, nitrate, phosphate, and SAR concentrations of the soil profile and their respective regional distributions will also be depicted. Ogunmokun and Wallach, 2020 [45], conducted research both in the laboratory and in the field to evaluate the efficacy of two nonionic surfactants in treating hydrophobic sandy soils. Their findings revealed how the surfactant application to the water-resistant soils in the orchards impacted the spatial distribution of soil moisture and the related agrochemicals.
In 2022, Zhou et al. assessed the relationship between Ugi-alg and soil colloids and its effect on the adhesion of pesticides. Compared to pure soil colloids (30 mV), the bond between the colloids and Ugi-alg (30 mg/L) significantly increased in the presence of 2.4 mM calcium (12.33 mV). This strong connection between Ugi-alg and the colloids inhibited acetamiprid from migrating. Through transmission electron microscopy, it was found that Ugi-alg had formed a network around the soil colloids, demonstrating that it had easily attached to the surface of the colloids. Testing of the adsorption also indicated that when Ugi-alg (30 mg/L) and Calu (2.4 mM) were present, acetamiprid had an adsorption rate of 556 mg/g on the colloids. Furthermore, research revealed that pH and temperature had minimal to no effect on the adsorption of acetamiprid in the soil colloids. A polysaccharidebased surfactant known as Ugi-alg, which can disintegrate, has been proposed as a novel concept and a functional approach to remediate soil pollutants [46].

Removal of Heavy Metals
References [19,47] examined the potency of an anionic biosurfactant from Candida sphaerica in eliminating heavy metals from soil taken from the automotive battery sector and from an aqueous solution. NaOH, HCl, and various mixtures of biosurfactant solutions were studied. The findings showed that the clearance rates for Fe, Zn, and Pb were 95%, 90%, and 79%, respectively. Adding HCl alongside the biosurfactant solutions at 0.1% and 0.25% increased the metal removal rate. The recycled biosurfactant showed the ability to take away 70%, 62%, and 45% of Fe, Zn, and Pb from the treated soil, respectively. Sequential extraction processes were used to gauge the speciation of the heavy metals before and after washing the soil with the biosurfactant. The organic, carbonate, oxide, and exchangeable fractions of heavy metals were effectively removed by the biosurfactant. The electrical properties and potential to bind metals of the biosurfactant were evaluated in aqueous solutions containing lead and cadmium. Analysis by atomic absorption spectroscopy demonstrated that the removal of metals was achievable even at concentrations beneath the critical micelle concentration. A promising option for the purification of soil and wastewater affected by metals is washing with biosurfactant [48,49].
According to Yoo et al. (2017) [50], chemical extraction and oxidation are more proficient and fast at eliminating heavy metals and hydrocarbons from the soil than other remediation processes. Batching with 3% hydrogen peroxide (H 2 O 2 ) and 0.1 M ethylenediaminetetraacetic acid (EDTA) could remove soil's petroleum content, with 60% and 30% of Cu and Pb removed, respectively. Also, without any catalysts, Fe oxide dissolution in natural soils could effectively oxidize petroleum with H 2 O 2 . In addition, due to EDTA's high affinity for metals, Fe-Mn oxyhydroxides bound with heavy metals could be removed with both metal-EDTA and Fe-EDTA complexation. Nevertheless, due to the extraction process with EDTA leading to Fe elimination, the strong Fe-EDTA binding blocked petroleum oxidation in the extraction-oxidation consecutive process. Even though a few heavy metals were still clinging to organic substances, the oxidation-extraction process did not have a major impact on the extraction of heavy metals from the soil. All in all, the removal of both pollutants was significantly successful when oxidation and extraction were conducted at the same time. It was claimed that this approach is a speedy and cost-efficient way to take out co-contaminants from the soil [50].
According to Das et al. (2017), the use of biosurfactants in the remediation of heavy metals is a recent and environmentally friendly method. They presented an extensive review that demonstrates how bacterial biosurfactants can be an excellent alternative technique for purifying heavy metals in the soil. It is believed that utilizing bacterial biosurfactants will provide a new method of ridding the soil from metal contamination [51].
To enhance the adsorption of metals by bentonite, one more study was conducted and a cationic surfactant (bencylhexadecyldimethyl ammonium chloride, BCDMACl) was identified by FTIR spectroscopy, XRF, BET, and swelling tests. This surfactant's ability to form micelles and embed itself in the spaces between the clay layers preventing swelling is one of its most important features. Batch experiments were performed to investigate the adsorption of copper (II) and zinc (II) from aqueous solutions when modifying the contact time and metal ion concentrations. Atomic absorption spectroscopy was used to determine the metals' retention in solution (AAS). After comparing the Freundlich, Dubinin-Radushkevich, and Langmuir isotherm models, the Langmuir isotherm was found to be the best fit for the experimental data (R2 range 0.962-0.993). The investigation revealed that the modified bentonite was far more efficient at absorbing copper (II) and zinc (II) than the natural bentonite (max 50.76 and 35.21 mg/g, respectively), with two-and-a-half-fold and two-fold rises in adsorption. The pseudo-second-order-rate equation was the best model to explain the adsorption kinetics. The findings of this study demonstrate that the modified bentonite can be used as an effective agent for eliminating heavy metals from liquid solutions [52].
Research in [53] was performed to investigate the impact of various control factors, like open/closed sediment chamber orifices, electric potential gradients (0.5, 1.0, and 1.5 V cm −1 ), and electrolyte surfactants, on the effectiveness of TM removal from TMcontaminated dredging harbor sediments by using an enhanced electrokinetic (EK) treatment method which incorporated a chelating agent (CA) and surfactant mixture as an additive in the processing fluids. To discover how much TM can be taken out from sediment with a high organic matter content, Tween 20 (4 mmol L −1 ) was employed as a surfactant in the electrolyte. The results revealed that an open orifice caused augmented electroosmotic flow (EOF) with slight TM removal. On the other hand, the highest number of TMs could be extracted from the matrix with a closed orifice and a nonionic surfactant electrolyte. In addition, when the electric potential gradient was raised, there was a rise in the electroosmotic flow under the open orifice condition. Unfortunately, this did not bring about a considerable enhancement in the removal of the trace metals since the electromigration of the metal-citrate complexes caused a higher aggregation of the TMs in the center of the matrix.
Rocha [54] examined the manufacture of a biosurfactant and its use in eliminating heavy metals from surfactants. Candida tropicalis, a yeast, produced the biosurfactant when it was cultivated in a solution of distilled water with 2.5% molasses, 2.5% frying oil, and 4% corn steep liquor. In a bioreactor with a capacity of 50 L, the production of the biosurfactant with a surface tension of 30 mN/m, reached up to 27 g/L. Tests conducted with surface tension and engine oil emulsification revealed that the biosurfactant was stable even when exposed to NaCl and various temperatures and pH levels. The structural makeup of the biosurfactant was deduced with the help of GC-MS and NMR. The biosurfactant, which had no detrimental effects on the germination of plants or brine shrimp, was identified as a negatively charged molecule capable of reducing the surface tension of water from 70 to 30 mN/m at 0.5% of the CMC. The tests demonstrated that the removal of Cu and Zn from contaminated sand with the help of crude and extracted biosurfactants ranged from as low as 30% to as high as 80%. Additionally, the capacity of biosurfactant was further confirmed in packed columns, with the elimination rate of Cu and Zn being between 45 and 65%. In contrast, no lead was removed under static conditions. The removal kinetics revealed that a single washing process with the biosurfactant was enough to improve the removal efficacy and that it only took 30 min eliminate the metals.
Mohamadi and his colleagues examined the simultaneous presence of phenanthrene and three heavy metals (lead, zinc, and nickel) in soil with a large capacity for buffering. To do this, they employed disodium ethylene diamine tetraacetate salt (Na 2 -EDTA) and two nonionic surfactants (Tween 80 and Brij 35). Five kinetic models were used to understand the rate at which the pollutants were desorbed, namely parabolic diffusion, Elovich, fractional power function, and pseudo-first and -second-order equations. Out of the three, Tween 80/EDTA was found to be the most successful in eliminating contaminants, achieving a 93% desorption of lead (Pb). The stability of metal-EDTA complexes had an effect on the competitive desorption of nickel (Ni) and zinc (Zn). Further, the removal of phenanthrene from the targeted soil was a difficult and slow process, and the desorption kinetics could be described by pseudo-second-order parabolic diffusion (for phenanthrene) and by the same model (for the heavy metals of interest). The composition of the soil stayed the same in the soil-surfactant-water mixture; however, the addition of Tween 80 influenced the formation of the faces of CaCO 3 crystals [55]. Piccolo et al., 2021 [56], investigated the exploitation of green organic surfactants like humic acids from lignite to eradicate heavy metals (HM) and polychlorobiphenyls (PCB) from a severely polluted soil obtained from industries in northern Italy. When the soil is being washed, apolar organic pollutants are detached from the soil particles by a micelle-like structure encouraged in the solution by the supramolecular HA. The acidic functional groupings present in the HA also allows for the simultaneous complexation of HM. For a 1:1 and a 10:1 solution/soil proportion, a single soil washing with HA eliminated 68 and 75% of PCB congeners, respectively. The same HA washing removal resulted in an average of 47% of all HM being eliminated concurrently and effectively, with a maximum of 57 and 67% for Hg and Cu, respectively. The research showed that utilizing a HA solution to clean heavily contaminated soil is an effective and swift way to restore the soil, taking out both HM and persistent organic pollutants (POP). Humic biosurfactant-based soil washing is also an eco-friendly and sustainable process because, unlike the synthetic surfactants and solvents utilized in traditional washing strategies, it helps to naturally reduce the unextracted POP and accelerates additional soil reclamation approaches such as bio-or phytoremediation.
Biosurfactants were able to contend with heavy metals for adsorption sites on the soil particles, causing the heavy metals to desorb. This adsorption of biosurfactants may escalate the negative ZETA potential of the soil particles, which then further mobilizes the heavy metals through electrostatic links. The attraction between organic molecules and heavy metal ions caused the formation of complex micelles. These micelles were also highly impervious to heavy metal ions that would normally be reintegrated into the soil.
The process of eliminating heavy metals from soil using anionic surfactants was similar to that of using biosurfactants. The anionic surfactants created a coating on the soil particles which subsequently reacted with the metals in order to dissolve them in a solution. Cationic surfactants, on the other hand, removed heavy metals from the environment by changing the surface properties of the soil. By exchanging cationic surfactants with heavy metal ions, it was possible to transfer the heavy metals into the washing solution. Cationic surfactants were then taken up by the soil-a process that generated secondary contamination. Furthermore, the washing solution included heavy metal ions which could be easily combined with the soil, thus leading to a low washing efficiency [57]. Sun et al., 2021 [58], investigated the effect of microbial (bio) surfactants on the leaching of heavy metals, such as Pb, Cu, and Cd from soil, as well as the optimization of the fermentation of the production of these bio-surfactants by a unique strain of Pseudomonas sp. CQ2. The results showed that using soybean oil as the carbon source, NH 4 NO 3 as the nitrogen source, a pH of 7, 175 rpm, and a 3% inoculum concentration at 35 • C were the best reaction conditions for producing the largest amount of biosurfactant (40.7 g L −1 ) during the reaction. Under the suitable leaching conditions (basic pH 11, solution/soil ratio 1:30, without sterilization), the removal efficiencies of Pb, Cu, and Cd were 56.9%, 65.7%, and 78.7%, respectively. Furthermore, it was found that using biosurfactants was more effective at eliminating heavy metals than the use of conventional chemical surfactants. SEM-EDX findings revealed a considerable decrease in Pb, Cu, and Cd concentrations in granular substances, as well as a smoother surface of the soil with the emergence of hole structures. ATR-FTIR demonstrated that carboxyl functional groups in the biosurfactants formed a combination with Cd, Cu, and Pb. This presented further details about the potent removal of heavy metals from the soil by Pseudomonas sp. CQ2 biosurfactant. Consequently, the extraordinary performance of Pseudomonas sp. CQ2 biosurfactants in this analysis implies that they could be a good option for the treatment of heavy metal contamination.
The combination of PHE and Cd in BESW can alter the composition of rhamnolipid micelles due to their interactions and simultaneous assimilation into rhamnolipid solutions. FT-IR and NMR analyses revealed different processes of eliminating PHE and Cd from contaminated soil. The remediation effectiveness of BESW with both pollutants is dependent on the pollutants' characteristics, in addition to the parameters of the experiment. The batch studies suggested that the remediation of BESW with PHE and Cd is strongly affected by the pH, temperature, concentration, and ionic strength of the rhamnolipid solution. Utilizing the Taguchi-based grey relational analysis enabled the identification of the optimal conditions for removing both PHE and Cd from BESW simultaneously. An analysis of the data indicated that the most effective conditions for using BESW to remediate soil contaminated with PHE-Cd consisted of a pH of 9, a rhamnolipid concentration of 5 g/L, a temperature of 15 • C, and an ionic strength of 0.01 M, with results revealing that these parameters enabled the removal of 72.4% of cadmium and 84.8% of phenanthrene [59].
A study was conducted to evaluate the capability of rhamnolipids and extracts from Quillaja Saponaria to bind Cu 2+ and Zn 2+ ions from three various kinds of soils, namely luvisols, podzols, and chernozem. It was determined that saponins had a stronger attraction to metals than rhamnolipids for each soil. Also, rhamnolipids and Q. Saponaria extracts were more effective than pure water, except for one instance when water was the only efficient method for extracting zinc from podzol's soil. Additionally, the toxicity test revealed that Q. Saponaria was less harmful than rhamnolipids, with toxicity only present at one concentration based on the type of bacteria strain. On the other hand, rhamnolipids were more hazardous for both of the tested microorganisms due to their structure. In conclusion, utilizing rhamnolipids or saponins as a means to rid polluted soil of heavy metals is a safer option compared to other compounds and is also an efficient method [60].
A study [61] was conducted to examine the effect of sophorolipid (SL) amendment on the growth of Medicago sativa and Bidens pilosa as well as the effectiveness of metal uptake and reaction to metal stress in B. pilosa. Results showed that SL-enhanced plants had longer roots and shoots compared to unaltered plants. After 60 days of the experiment, the increase in plant heights for M. sativa and B. pilosa was 17% and 11%, respectively. Another trial (40 days) revealed that B. pilosa's biomass was adversely affected by the presence of Cd in the soil (29.2 mg kg −1 ). However, by counteracting the toxic influence of Cd on B. pilosa, SL supplementation brought back the biomass. In the control, Cd-contaminated soil, and Cd-and SL-treated soil, the dry weights of B. pilosa were 445, 285, and 456 mg plant −1 , respectively. It was also observed that proline concentration in B. pilosa decreased as SL supplementation progressed. In soils that were treated with SL and in those that were untreated, the measured concentrations of proline were 18.2 and 40.2 moles proline g −1 , respectively. The urease activity in the SL-augmented trial was considerably higher than it was in the studies with Cd contamination (5.7 times) and in the control trial (1.5 times). In the soil that was enhanced with SL, Cd accumulation in B. pilosa was the highest. The results of the study revealed that SL augmentation is an effective technique for enhancing phytoremediation since it spurred soil microbial activity, amplified Cd incorporation in plants, and counteracted metal stress.
A Pseudomonas aeruginosa rhamnolipid extract was evaluated on soils from both longand short-term polluted sites [62]. Di-rhamnolipid congeners were the most prevalent (85%), according to mass spectrometry study, in particular hydroxy decanoyl-hydroxy decanoate. The amounts of arsenic, cadmium, and zinc after artificially adding them to sandy soil were 182, 20, and 983 mg kg −1 , respectively. The amount of transition metals and metalloids that the rhamnolipid was able to extract from the synthetically polluted soil was significant, with 80% of zinc, 90% of cadmium and 53% of arsenic being removed. Moreover, rhamnolipid was used to extract 59% arsenic, 57% cadmium, and 9% zinc from materials acquired from a defunct mining site. Also, it was discovered that Artemia salina and the biosurfactant were quite biocompatible. Diffusion studies revealed that the commensal bacteria and yeast present in the soil were unaffected by the presence of rhamnolipid. A method for eliminating As, Cd, and Zn from the environment were tested, where the biosurfactant solution was able to remove 84.5% of the zinc and completely remove the arsenic and cadmium. It is essential for mining industries to create effective and efficient ways to reduce the use of new raw materials by precipitating the metal pollutants as well as the potential of using this biosurfactant in soil remediation without any purification steps.
An original electrokinetic remediation method was utilized to extricate heavy metals and hydrocarbons from polluted soil [63]. Surfactants and PASP (polyaspartic acid) were used to increase the extraction of heavy metals and hydrocarbons from the polluted soil. Numerous parameters were examined during the electrokinetic remediation procedure, including electric current, soil pH, electro-osmotic flow, water content fluctuation, classification of heavy metals, removal of TPH (total petroleum hydrocarbons), group organization of oil, and improved electrokinetic remediation systems. The electrokinetic remediation approach can result in the precipitation of heavy metals and the results demonstrated that rhamnolipid, PASP, Tween 80, and SDS (sodium dodecyl sulfate) can reduce this precipitation. Moreover, during the EK cleanup process, it might prompt EOF (electro-osmotic flow) and electrolyte evaporation. When rhamnolipid (3.0 g L −1 ) and PASP (3.0 g L −1 ) are joined together, the ability of copper (Cu), chromium (Cr), nickel (Ni), lead (Pb), and hydrocarbons to be mobile and soluble increases significantly during the electrokinetic remediation process. The greatest elimination rates for Cu, Cr, Ni, and Pb were 66.0 3.45%, 61.2 4.35%, 67.1 3.21%, and 61.8 4.22%, respectively. Moreover, after the electrokinetic purification was performed, total petroleum hydrocarbons (TPH) achieved a major removal performance of 80.2 and 4.36%. This research offers a novel in situ cleansing approach for taking away heavy metals and hydrocarbons from the polluted ground in a risk-free manner. Santoso et al. [64] developed an uncomplicated and cost-effective technique for generating porous calcium alginate composite sponges which incorporate exfoliated bentonite clay (known as CRAB) for the proficient elimination of bi and hexavalent copper cations from water and soil samples. This method incorporates saponin-aided foaming and external gelation processes. Saponin was inserted into the bentonite's interlayer gap and then alginate chains were allowed to penetrate, forming exfoliated organoclay within the composite matrix. The shaped CRAB sponges mainly have meso-and macropores in their interiors, according to morphological and textural characterizations. The XRD results also confirmed the production of calcium-alginate-organoclay exfoliated composites. Variables such as ionic strength, pH, temperature, and background electrolyte were used to examine how efficiently the CRAB sponges remove Cu 2+ and Cr 6+ ions from aqueous solutions. The results indicated that the adsorption procedure was endothermic since the absorption potential of bivalent copper and hexavalent chromium increased with higher temperature. The equilibrium adsorption actions of the Cu 2+ and Cr 6+ ions onto the surveyed sorbents were found to be more accurately described by the Freundlich isotherm model than the Langmuir model. The sorption kinetics of these metal ions showed pseudo-first-order behavior. The adsorption capacity of the sorbent was effectively sustained across three successive adsorptions-desorption cycles, as revealed by CRAB sponge reusability tests. These results together demonstrate the exceptional capacity of low-priced, long-lasting CRAB sponges for the purification of water polluted with Cu 2+ and Cr 6+ ions.

Removal of Halogenated Solvents
Chlorinated hydrocarbons are employed extensively as solvents, but they are also well-known for their durability and toxicity, which causes considerable ecological issues. TCE is a common chlorinated hydrocarbon that has reduced water solubility and a high affinity to soil particles, making it one of the hardest to remove from the soil. When let out into the subsurface, TCE disperses among water and soil, yet a substantial portion of it has a tendency to be kept in the solid phase due to its low solubility and massive adsorption to the organic matter in the soil. Despite being an ordinary solvent, chlorinated hydrocarbons are also persistent organic pollutants. Because of their strength and toxicity, they are widely dispersed and create considerable ecological problems. Dealing with soil affected by chlorinated hydrocarbons can be difficult, especially in regards to TCE, a common chlorinated hydrocarbon which has limited solubility in water and binds to soil particles. When this TCE is beneath the ground, traditional methods of cleaning up the soil include thermal remediation, chemical reduction, soil vapor extraction, soil washing stabilization/solidification, and vitrification (EPA, 1999). Nonetheless, these conventional treatments can be expensive and may create additional pollutants.
To circumvent these drawbacks, zero-valent iron (Fe 0 ) has been employed to break down organic pollutants as it is a powerful reducer. Despite the ease and safety of Fe0, its utilization has been limited by its slow reaction rate. Researchers have been exploring more efficient reductants to tackle this issue. As a result of their strong reactivity and accessibility, nano zerovalent iron has recently been gaining interest as a technique for purifying groundwater and soil. According to certain studies, it has been found to be very effective in converting and de-chlorinating PCBs and other chlorinated organic pollutants. NZVI can also be used to treat a selection of inorganic substances in soils, including nitrate, nitrite, and Cr6(VI). Nevertheless, the quick dispersal of nanoparticles in soil and their subsequent use in soil remediation has been hindered by their easy aggregation because of their high Van der Waals forces and magnetic attraction. Consequently, many researchers sought to circumvent this rule by enveloping nanoparticles with different modifiers or dispersants to boost electrostatic and steric hindrance between the particles, resulting in a more reliable nanoparticle suspension. A range of components, like guar gum, polyacrylic acid, and carboxymethyl cellulose was used to alter the particle surface, but a few of these chemicals are expensive and not environmentally friendly. Trichloroethylene (TCE) has poor solubility in water and high sorption properties prior to the reaction, which impedes its degradation and prolongs the required reaction time, which is another problem with its removal from soil. Other strategies have also been explored to tackle this matter, and some studies revealed that the optimal choice for increasing TCE's solubility and mixability in soil is a surfactant. Researchers proposed a modified version of NZVI which included a surfactant for cleaning TCE-polluted soil. This modification resulted in an improved NZVI water solubility and degradation. SDS and CTAB were chosen as the surfactants. This study also looked into the effects of varying factors such as NZVI dosage, surfactant concentration, reaction time, starting pH, and incubator speed on the reduction of TCE-contaminated soil. The effectiveness of the modified NZVI was compared to that of commercial zero valent iron. Additionally, investigations were conducted on the reduction pathway and degradation products of TCE. The purpose of this research was to build a fresh and effectively modified NZVI that is fortified with surfactants to clean up TCE-polluted soil. The study revealed that PEG-4000-NZVI joined with CTAB and PEG-4000-NZVI combined with SDS can both effectively eliminate TCE from soil-water systems by utilizing PEG-4000-NZVI particles in association with surfactants. In addition, PEG-4000-NZVI in collaboration with CTAB and SDS as well as PEG-4000-NZVI on its own can adjust to an assortment of pH levels. The dosage of PEG-4000-NZVI and the concentrations of surfactants had an effect on the TCE clearance efficacy [65].
In 2020, Tian et al. [66] studied the feasibility of using surfactants and polymerstabilized nanoparticles of zero-valent iron (NZVI) to purify TCE polluted soil in an aqueous setting. As the stabilizing agent, they selected polyvinylpyrrolidone (PVP), a nonionic polymer, while cetyltrimethylammonium bromide (CTAB) and lauryl sulfate, both of which are ionic surfactants, were added independently to upsurge the solubility of TCE in the liquid phase due to their meager critical micelle concentrations (CMCs). In the soil-water system, the surfactant was observed to synergistically heighten the degradation of TCE. To boot, PVP coating on the surface of NZVI particles during the transformation of NZVI with PVP reduced the clumping of NZVI particles; thus, the amount of PVP impacted the crystalline structure and level of oxidation of NZVI.
During the latter part of the twentieth century, lindane, an insecticide of the organicchlorine group, was widely employed, leading to an incredible amount of rubbish being created around the globe. One of these byproducts, a non-aqueous-phase liquid (DNAPL) composed primarily of chlorobenzenes and hexa/hepta-chloro-cyclohexanes, was improperly discarded in landfills, thus contaminating the underground water. To address this dilemma, a combined strategy of soil flushing (utilizing a nonionic surfactant) and Fenton oxidation was proposed [67]. During column conditions, the majority of the remaining DNAPL in the soil was extracted using a commercial surfactant (E-Mulse 3 ® ). The obtained surfactant flushing solution (SFS) held an extremely high concentration of 3693 mg L −1 of COCs. To recover the surfactant and lower the concentrations of contaminants (COCs), the SFS was put through the Fenton procedure, with the application of three different doses of hydrogen peroxide (50%, 100%, and 200% of the expected stoichiometric amounts to completely mineralize the COCs; with the maintenance of the H 2 O 2 : Fe molar ratio of 32), and 100% and 200% of the stoichiometric amount of H 2 O 2 was used after 144 and 48 h, respectively, resulting in COC conversions exceeding 80%. Compared to the less resistant chlorobenzenes, non-aromatic compounds showed less vulnerability to hydroxyl radical oxidation. The surfactant capacity was sustained after the oxidative treatment and could be retained for use in more flushing processes, increasing the efficiency of the process. The surfactant oxidation was observed to be significantly less than that of the contaminants.

Removal of Asbestos
There is limited information in the scholarly literature concerning the extraction of asbestos from the soil. However, Lipiecka et al. (2009) have designed a rapid, straightforward, and cost-effective approach to ascertaining the amount of asbestos found in environmental materials, such as earth and water, and then removing it. A digital camera connected to a computer running the required software is described in the Polish patent application for a method of recognizing asbestos content and an analytical set for finding asbestos content. Based on optical microscopy with an optional phase contrast, this analytical apparatus determines the presence of asbestos [68].

Removal of Microplastics and Antobiotics
Surfactant-enhanced soil remediation technologies can be adapted and applied to address emerging soil contaminants like microplastics and antibiotics. Although traditionally used for organic compounds and heavy metals, the unique properties of surfactants can still offer advantages in treating these new types of contaminants. Microplastics, which are small plastic particles or fibers, can accumulate in soil and pose a threat to ecosystems. Surfactants can be utilized to enhance the removal of microplastics from soil by reducing their adhesion to soil particles. The addition of surfactants can decrease the interfacial tension between the microplastics and the soil, facilitating their detachment and increasing their mobility. This enables easier extraction or separation of microplastics from the soil matrix, improving the efficiency of the remediation process [69].
Surfactant-enhanced soil remediation can play a role in addressing antibiotic contamination in soil. Antibiotics can enter the environment through various sources, such as agricultural runoff and improper disposal of pharmaceutical waste. Surfactants can aid in the desorption and solubilization of antibiotics, increasing their availability for subsequent degradation or extraction. By reducing the interaction between antibiotics and soil particles, surfactants enhance the mobility and effectiveness of other remediation techniques, such as bioremediation or advanced oxidation processes [70,71].
Surfactants can help in the transport and distribution of treatment agents for microplastics and antibiotics in soil. They can act as carriers for other remedial agents, such as sorbents, enzymes, or chemicals, assisting in their delivery to the target contaminants. This improves the efficiency and effectiveness of the treatment, as surfactants can enhance the contact between the treatment agents and the contaminants, facilitating their interaction and removal.

Challenges
Utilizing surfactants to successfully clean up soil can be quite challenging. The best results are dependent on the selection of the appropriate surfactant and the amount of it used, as the interaction between the surfactant, hydrocarbons, and microorganisms will be different in every situation. Not only do surfactants alter the properties of the microbial cells, but they also help to dissolve and emulsify components. Yet, as the interactions between the microorganisms and surfactants vary, the hydrocarbons may not be as easily accessible. Furthermore, there could be issues with the surfactant being toxic and biodegradable and with the formation of hazardous byproducts during the surfactant's breakdown. Experiments performed in laboratories have revealed a variety of interactions and effects that could have an adverse effect on oil remediation when surfactants are included [69].

Conclusions and Future Prospects
Surfactants demonstrate exceptional potential for improving the process of contaminated soil remediation. Their capability to lower surface tension and make contaminants more soluble makes them a useful resource in the clean-up of soil. Introducing surfactants to the soil could help enhance the activities of other remedial strategies, such as bioremediation of soil washing. Utilizing the diverse surfactant systems spoken of in the literature effectively leads to prosperous and successful remediation initiatives while preserving the native soil character as a way of solving and reducing these issues. It is apparent from the literature that nonionic surfactants are biodegradable, less hazardous, and prove to be more effective in pollutant remediation than ionic surfactants. A further eco-friendly remedy for decontaminating soil is the utilization of biosurfactants, even though they have a slow rate of pollutant removal. Scientists are striving to achieve the complete removal of pollutants without any negative effect on the structure of the soil. The composition of the soil, the kind of pollutants, the cost, and the characteristics of the surfactant foam are the most important factors taken into consideration. Pollutants from the soil can be remediated effectively by using surfactant foams that are stabilized by nanomaterials (NPs). However, the understanding of the interactions between pollutants and the surfactant foam stabilized by nanoparticles is not thorough enough and requires more research. The NPs that have been used to date for the decontamination of soil are silica, iron oxide, and zero-valent iron. It is very important for research to evaluate the effects of nanoparticles on toxicity and retention in organic and terrestrial environments. Future studies should focus on analyzing the structure of NPs and enhancing the conditions needed for producing reliable surfactant foam solutions to treat polluted soil. It is also necessary to be aware of the specific site of investigation and the standards of all applicable governmental and environmental organizations. The use of surfactants has been proven to be beneficial for soil remediation in a range of contexts, such as remedying industrial contamination, eliminating pollutants from agricultural soil, and treating oil spills. This method accelerates and optimizes the cleaning process, thereby resulting in more effective decontamination of the site.
It is of great importance to contemplate the type and amount of surfactants required for surfactant-augmented ground remediation so that they are productive and do not lead to any unintended damage to nature. Also, we must take into account the possibility that surfactants may influence soil microorganisms and the entire soil system. Nevertheless, surfactants have the capacity to significantly enhance the purification of contaminated soil. When employed judiciously, they can enhance the productivity of other remediation methods, resulting in a swifter and more successful cleaning of polluted areas. Ongoing investigation and prudent evaluation of the ecological effects of surfactant use will be essential in securing the continued prosperity of soil.