New Advancements in the Field of Pollution Treatment, Including Contamination of the Soil and Water

Abstract

The food security of human societies has become a major source of worry due to heavy metal contamination in soils and water supplies. Water and soil sources are becoming more and more contaminated with heavy metals every day as a result of the development of several mining techniques and technologies as well as the expansion of numerous enterprises. A career assessment predicts a 7–10% increase in employment for soil and plant scientists between 2018 and 2028. Because the production of wholesome food and the safety of food are very important issues. Therefore, some of the innovative techniques for eliminating organic and mineral contamination from water and soil sources are addressed in this book chapter.

1. Introduction

Heavy metals refer to a group of toxic elements that are very important both biologically and industrially. According to definition, heavy metals are naturally occurring metals having an atomic number greater than 20 and an elemental density greater than 5 g/cm [1]. The rapid developments of industrialization and unplanned urbanization have introduced heavy metals into the environment through improper dumping of industrial wastes directly on land and near water sources [2]. Pollution of soil and water sources with heavy metals is one of the most severe environmental problems that can seriously affect the quality of the environment and human health [3]. Today, the entry of heavy metals into water and soil sources from various natural and anthropogenic sources has been confirmed. Although the formation process of environmental pollution has a long history, the growth of this abnormality after the industrial revolution grew increasingly due to the very significant use of heavy metals in various industrial technologies [4]. Today, the amount of global production of heavy elements in various industries is very high. Heavy metals found in soils and water resources include nickel, chromium, lead, cadmium, arsenic, copper, cobalt, zinc, manganese, aluminum, mercury and antimony. Among the mentioned heavy metals, arsenic, cadmium, lead, and mercury are among the 20 most dangerous substances that have been determined by the Agency for Toxic Substances and Disease Registration and the US Environmental Protection Agency (USEPA). Among the effects of environmental pollution with heavy metals is the occurrence of bio-toxicity and its effects on the biological degradation process [5]. One of the ways of entering heavy elements into the environment, especially agricultural soils, is the use of different fertilizers that contain heavy elements as impurities. Excessive accumulation of heavy metals in agricultural soils causes more uptake of these metals by food crops and vegetables, which in turn can cause serious risks to human health [6]. The entry of, including cardiovascular diseases, cancer, Alzheimer’s, chronic anemia, cognitive impairment, kidney damage, skin problems, memory loss, Aplastic anemia, infertility and nervous system weakness [7]. With the continuous entry of heavy metals into water and soil sources, there has been a concern that the concentration of these metals will exceed the permissible limits and disrupt the majority of biological activities. In addition, with the growth of public awareness, people have become sensitive to the contamination of soil and water sources with toxic elements and have understood that these compounds can have very important effects on the quality and quantity of their lives [3]. As a result of growing public awareness and sensitizing societies to the threats ahead, innovations and technologies have been formed that can be effective in cleaning and reducing the risk of sites contaminated with heavy metals. The distinguishing feature of pollution related to heavy metals is that, unlike organic pollution, these pollutants are not degradable and are resistant to biological and chemical processes [8]. Therefore, due to the fact that these compounds are resistant to decomposition, the extent of contaminated areas increases every year. There are about 100,000 contaminated areas in the United States, while the extent of contaminated agricultural land in China reaches more than 3.5 million hectares [9]. Of course, it is estimated that there are 2.5 million other contaminated areas in this country. For the decontamination of these areas, more than 6 billion euros should be spent annually [10]. If proper information of contaminated sites in other countries of the world are prepared, the cost of their decontamination will probably exceed thousands of billions of euros. The purification of soils, sediments, and polluted water has been the subject of a lot of research, and scientists have suggested a variety of technologies. The effectiveness and efficiency of methods used to treatment the contaminant from different sources is a critical aspect. Here, we will review some of the various purification methods that have been suggested for soil and water pollution, as well as an emphasis on more modern methods.

The technologies used for the treatment of contaminated areas can be divided into two main category: In-situ technologies and ex-site technologies. In the in-situ technology, the process of remediation and treatment of pollution is carried out at the place of its origin. The purpose of this type of treatment is to remove pollutants from soil, water and sediments without moving the soil and sediment. In ex-site technologies, drilling, refining, and treatment of contaminated materials are done outside the contaminated sites [10]. In the in-situ remediation process, the cost-benefit ratio is generally higher than the ex-situ remediation method. While removal or extraction the pollutant from soil and water is much better than immobilization or containment the pollutant. In addition to the above, in the in-situ remediation process, the contact of workers and people with the polluted environment is less and the possibility of contamination spreading to other areas is reduced [11]. To become more efficient and economical, these techniques can sometimes be used simultaneously [3].

2. Soil remediation

2.1 Physical remediation

2.1.1 Soil replacement

Replacement of contaminated soil is called complete or partial replacement of contaminated soil with non-contaminated and clean soil. In this method, the concentration of heavy metals in the soil decreases (dilutes) and leads to an increase in soil fertility and functionality [3]. Earlier to 1984, excavation, remove the contaminated soil and off-site disposal in specific places and replacement it with clean soil were the most common method for cleaning-up in contaminated areas. In soil spading, the contaminated soil is spaded deeply with special devices so that the surface contaminated soil is mixed with the uncontaminated under layer clean soil and the concentration of heavy metals is reduced (diluted) [12]. Another method related to soil replacement is adding clean and non-contaminated soil to the surface of contaminated soil. In this method, we can mix the imported non-contaminated soil with the contaminated soil so that the concentration of heavy metals per unit weight of the soil is reduced and a suitable environment is provided for the growth of plants [13]. The soil replacement method can isolate the contaminated soil and the ecosystem and reduce its harmful effects on the ecosystem [14]. But this method is very expensive because it requires a lot of labor and physical work, and it is suitable for small and highly polluted areas. The cost of doing this method is about 270 to 460 dollars for each ton of moving and adding clean soil. It is natural that the longer the distance, the higher the costs.

2.1.2 Soil isolation

Isolation means separating the soil contaminated with heavy metals from non-contaminated soil [15] or preventing the movement and transmission of contamination from one point to another [16], but for the complete purification of contamination in this method, other engineering methods are also needed. Contaminated soil isolation measures are based on engineered barriers and include hydrological barriers and stabilization approaches [17].

In general, isolation technologies are designed to prevent the off-site movement of heavy metals and other contaminants by confining them to a specific area [3] Engineering barriers, which may be on the surface or below the surface, are generally used to limit the contact of surface water or groundwater with waste materials and transfer to the surrounding environment. An underground barrier restricts the flow of ground and/or surface water at a contaminated site, allowing contaminated water and soil to be separated [16]. By far, the most common engineering barrier is a surface barrier called a cap, which is usually placed on top of waste piles. Vertical subsurface engineering barriers limit the lateral movement of groundwater and dissolved pollutants. These vertical barriers are installed downstream, upstream or generally surrounding a site and are generally used in combination with the cap system.

2.1.3 Vitrification

In the vitrification process, contaminated soil is transformed into a crystal and glass product due to heating and melting with electric energy. In this method, the mobility of heavy metals in the soil decreases, which is due to the formation of vitreous materials [18]. The Pacific Northwest Laboratory, which is working on the development of vitrification, is conducting research that can make this technology operational for buried waste and underground tanks of the United States Department of Energy [19]. A vertical array of electrodes is inserted into the contaminated soil during in situ vitrification in order to pass electrical current through it. Of course, it should be noted that in dry soils, due to low conductivity, the vitrification process is not performed well. Temperature is a key factor in immobilization of heavy metals in vitrification method [20]. Vitrification can be performed both in-situ and ex-situ. But preference is given to the in-situ method because it is easier and its energy supply is more accessible. In situ vitrification is limited by the possibility of melting soil and allowing current to pass through it. Furthermore, soils with a high alkali content (1.4 wt%) are unlikely to conduct current efficiently [21]. As a result, vitrification can only take place under wet soils with low alkali levels.

2.1.4 Electrokinetic remediation

Electrokinetic remediation is an in-situ process in which an electrical field is created in a soil matrix by applying a low-voltage direct current (DC) to electrodes placed in the soil. As a result of the application of this electric field, heavy metal contaminants may be mobilized, concentrated at the electrodes, and extracted from the soil [22]. In this method the separation of heavy metals (loids) in soil is accomplished via lectrophoresis, electric seepage, or electromigration and thus decrease the contamination [12]. Other techniques and processes, such as electrokinetic microbe joint remediation, electrokinetic-chemical joint remediation, electrokinetic-oxidation/reduction joint remediation, coupled electrokinetic phytoremediation, electrokinetics coupled with electrospun polyacrylonitrile nanofiber membrane, and electrokinetic remediation conjugated with permeable reactive barrier, are also used in conjunction with electrokinetic remediation methods [23, 24]. Soils with low permeability respond well to electrokinetic remediation. Because electrokinetic remediation is simple to set up and utilize, it is cost-effective [25]. Pollutant concentrations in soil are reduced when electrochemical adsorption is combined with extraction using low-molecular-weight organic acids [26, 27, 28]. Fluctuations in soil pH are the key limiting factor for direct electrokinetic remediation since it cannot sustain soil pH value [3].

2.2 Chemical remediation

2.2.1 Immobilization techniques

This technique, also known as Solidification and Stabilization. Immobilization is the process of adding immobilizing chemicals to polluted soils to reduce the mobility, bioavailability, and bioaccessibility of heavy metal(loid)s in the soil [29]. The immobilization of heavy metals in soil can be achieved through complexation, precipitation, and adsorption. By redistributing heavy metal(loid)s from soil solution to solid particles, these processes limit heavy metal(loid) transport and bioavailability in soil [30]. Binders, cement, clay, zeolites, phosphates, alkaline materials, termitaria, industrial eggshell, red-mud, chemical compounds, and more recently nanomaterials are a few of the mixing ingredients employed in the imobilization procedure [31].

2.2.2 Encapsulation

Encapsulation of contaminated soil stops the pollutants from spreading by covering the contaminant source with layers of concrete, lime, clay caps, or synthetic textiles, to limit the leaching and migration of contaminants away from the isolated zone [12]. By becoming immobile, the polluted soil avoids contaminating the nearby materials [32]. Several binding materials are used in the production of solid blocks, but cement is chosen due to its accessibility, adaptability, and affordability [33]. Encapsulated soil can never be used to grow anything, hence this method of soil cleanup is only used as a last resort. Various immobilization agents are utilized during encapsulation, including polyvinyl alcohol, chitosan, alginate, agar, polyacrylamide, and polyurethanes [3]. The leaching of organic materials may be prevented effectively by encapsulation. Various immobilization agents, such as polyvinyl alcohol, chitosan, alginate, agar, polyacrylamide, and polyurethanes, are employed during encapsulation. While asphalt encapsulation is utilized for soils contaminated with hydrocarbons, encapsulation by lime and concrete has been used concurrently in the efficient treatment of soil contaminated with heavy metals and oil [34].

2.2.3 Soil washing

A technique known as soil washing uses two processes to remove pollutants from soil: physical separation and chemical leaching by aqueous solutions. This method starts with a homogenization step in which the coarse particles are divided based on their densities [35]. Depending on the type of metal and soil, the contaminated soil is dug up and combined with an appropriate extractant solution during soil washing [36]. For a predetermined amount of time, the extractant solution and dirt are fully blended. The heavy metals in soil are transported from soil to liquid phase and then removed from the leachate through precipitation, ions exchange, chelation, or adsorption [37]. If the contaminated soil passes regulatory tests for heavy metals after the washing process is complete, it will be returned to its original location. It is very common to use soil washing to purify heavy metals from contaminated soils, because it completely removes heavy metals. In addition, soil washing is a rapid method which can easily meet the researchers’ criteria [38, 39]. A variety of chemicals, such as synthetic chelating agents (EDTA, EDDS), organic acids, humic compounds, surfactants, and cyclodextrins, have been employed to mobilize and remove heavy metals from soil [40, 41]. The capacity of the extractant to dissolve the heavy metal in soils determines the effectiveness of soil cleaning [42].

2.3 Biological remediation

2.3.1 Phytoremediation

Phytoremediation is a recently developed technology that offers a cost-effective solution by using plants, and associated soil microbes, to reduce the content, or toxic effects, of contaminants in the environment [43]. Botano-remediation, vegetative remediation, green remediation, and agro-remediation are all synonyms for phytoremediation [44]. Recently, there has been a lot of interest in and usage of phytoremediation, a natural, solar-powered, and environmentally benign method, especially in combination with other methods like biological, physical, and chemical methods for the treatment of hazardous pollutants [45]. A phytoremediation system can effectively clean-up sites with low-to-moderate levels of heavy metals while being environmentally friendly, appealing, esthetically pleasing, non-invasive, energy efficient, and cost-effective. Through mental accumulation, precipitation, or root surface absorption, heavy metals in the soil are strengthened during the solidification process [46]. Phytoremediation is usually divided into phytoextraction, phytostabilization, phytotransformation, and phytovolatilization. In practice, the selection of phytoremediation technology should be based on the types of soil and plants, the structure of rhizosphere microorganisms, and the complex coupling between the geochemical forms of pollutants.

2.3.2 Phytovolatilization

Phytovolatilization involves the uptake of contaminants by plant roots and its conversion to a gaseous state, and release into the atmosphere. This process is driven by the evapotranspiration of plants [47]. Metals are absorbed into volatile organic compounds during phytovolatilization, and these compounds are then released as biomolecules into the environment [48]. Succulent plants are regarded as a choice of plants in mining areas in arid and semi-arid environments.

2.3.3 Phytostabilization

Phytostabilization aims to contain contaminants within the vadose zone through accumulation by roots or precipitation within the rhizosphere. As a result of phytostabilization, heavy metals concentrations in contaminated soil are not reduced, but their movement is prevented [49]. When phytoextraction is not feasible or desirable, phytostabilization is used. In addition, phytostabilization can be used at sites with technical or regulatory limitations that make the selection and implementation of more appropriate remediation techniques difficult [50]. In abandoned contaminated sites such as mine wastelands, urban landfills, and sewage treatment plants, phytostabilization is commonly used. In tailings areas, pioneer plants are typically used to enhance physicochemical properties, provide cover, and establish a vegetation cap for long-term stability [51]. To maintain optimal stabilizing conditions, the site must be monitored regularly since heavy metals are stabilized within soil. A hyperaccumulator plant with the best phytostabilization properties (a) reduces heavy metals leaching by reducing water percolation through the soil matrix, (b) inhibits soil erosion and moves heavy metals to other areas, and (c) prevents direct contact with soil contaminated with heavy metals [52]. The most commonly used plant species for phytostabilization of Pb, Zn, and Cu polluted soils in Europe are Festuca spp. and Agrostis spp. [53].

2.3.4 Phytoextraction

In phytoextraction, heavy metals are removed from contaminated materials (soil and water) by uptake into harvestable plant parts [54]. As a result, phytoextraction reduces soil contamination. Most plant species cannot sustain in heavily polluted environments, so phytoextraction is suitable for sites with low-moderate levels of metal pollution [55]. There are four characteristics of plant species that can be used effectively for phytoextraction: (a) high metal-accumulation capability in their aboveground parts, (b) tolerance to high metal concentrations, (c) ability to grow rapidly with high biomass, and (d) profuse root systems. A number of common hyperaccumulator plants have been discovered and used to treat heavy metal-contaminated soil, including Pteris vittata L, Sedum plumbizincicola, Solanum nigrum L, Polygnoum hyiper L, Thlaspi rave service L, Calendula officinalis, and many others [56]. There are some limitations to heavy metal extraction by hyperaccumulating plants, including poor extraction efficiency, low biomass, easy environmental impact, heavy metal poisoning, and long repair times. However, they can be avoided by combining them with other technologies. Heavy metals in the soil can be effectively activated by adding chelators, creating a water-soluble metal chelator complex, which could change the occurrence form of heavy metals in the soil and then encourage the enrichment of heavy metals by plants, given the limitations of hyperaccumulation plants in the extraction of heavy metals [57]. The synthetic chelating agents EDTA, DTPA, EGTA, and EDDS are the most often utilized ones. Lead has the greatest ability to be activated by EDTA when compared to other heavy metal ions like Cu, Zn, and Cd.

2.4 Nanoremediation

The use of nanoparticles in nanoremediation enables the removal of heavy metals contaminants from soils and other environments in a cost-effective and eco-friendly manner [58]. Through the use of this novel remediation method, heavy metals can be absorbed, reduced to a stable metallic state, and catalyzed to leave a site [59, 60]. Different technical processes are employed in nanoremediation, including adsorption, heterogeneous catalysis, electrical field deployment (electronanoremediation), photodegradation, and the use of microorganisms (nanobioremediation) to remove or immobilize heavy metals from contaminated soils [61]. Metal nanoparticles, metallic oxides nanoparticles, carbonaceous nanoparticles, polymeric nanoparticles, and nanocomposites have all been successfully used and applied to remove heavy metals. Through pore spaces, nanoparticles can also reach inaccessible areas, such as crevices and aquifers, eliminating the need for traditional methods. These remediation materials have three modes of action: (1) A physical process that involves the adsorption and immobilization of contaminants on the surface of the particles. In one study, iron oxide Fe₃O₄ particles (12 nm in diameter) were used to remove arsenic from water after sorption and magnetic separation. (2) Toxic compounds are transformed into less harmful products through the process of detoxification, which induces and/or catalyzes the initial chemical breakdown. The dominant mechanisms are oxidation-reduction reactions, as in the photocatalytic oxidation of organics by titanium oxide TiO₂ nanoparticles [62], or the reduction of organics by nanoscale zero-valent iron (nZVI). (3) In bio-cooperative degradation, the particles increase bioavailability while degrading pollutants into more bioremediable species [63]. As an illustration, Fe₃O₄ NPs were employed as ion suppliers to enhance the production of biogas during anaerobic digestion procedures [64]. The tendency of nanoparticles to biostimulate bacterial cells was highlighted in a recent review by Abdelsalam and Samer, which also highlighted how this increased bacterial activity and growth kinetics [65].

3. Water treatment technologies

Reclaiming freshwater for use in agriculture and human activities requires wastewater treatment. Every year, as global water demands rise, many pollution schemes have threatened water sources [66]. Proper treatment and permanent removal of heavy metals are of immediate necessity. Many effective ways to remove pollutants such as heavy metals from wastewater are currently available [67]. Conventional techniques include ion exchange, membrane filtration membrane filtration, and chemical precipitation. Due of its simplicity, the chemical precipitation process is extensively utilized. Other alternative treatment techniques like photocatalysis, electrochemical, flotation, coagulation, and adsorptions have garnered a lot of attention in recent years. In order to remove heavy metals from wastewater, this study analyzes numerous treatment systems, their mechanisms, and the most recent developments.

3.1 Photocatalysis

Photocatalysis is a photo-activated chemical reaction occurring when free radical mechanisms are initiated as contact is made between the compound and photons that have sufficiently high energy levels. The words photo, which has to do with photons, and catalyst, which is a chemical that affects the rate of a process when it is present, are combined to form the term “photocatalyst.” As a result, photocatalysts are substances that, when exposed to light, alter the rate of a chemical reaction. The term “photocatalysis” refers to this occurrence [68]. This technique was created as a result of research to emulate photosynthesis and the evolution of hydrogen for use in environmental applications. Semiconductors known for their photocatalytic properties, such as TiO₂, ZnO, CeO₂, CdS, and ZnS, was used in photocatalytic processes [69]. Strong oxidizing power, the ability to destroy heavy metal complexes and release them from the metal ions, and the capacity to oxidize and degrade organic complexes simultaneously are the characteristics that define photocatalysis.

Three processes make up the basic mechanism of photocatalysis. The first step is the production of charge carriers, which happens when a semiconductor is exposed to light that has a high energy or is equal to its bandgap. Second, the produced electron-hole pair moves onto the semiconductor’s surface as electrons transition from the photocatalyst’s valence band (vb) to conduction band (cb). Thirdly, electrons decrease the O₂ molecule to make superoxide radical anion (O₂) in the conduction band while photogenerated holes oxidize the H₂O molecule to yield OH in the valance band [70]. Various metal oxide-based photocatalytic materials such as TiO₂, ZnO, CuO, CdS, etc. have been used to remove organic and inorganic pollutants present in wastewater.

3.2 Coagulation/flocculation

Coagulation flocculation is a highly efficient physicochemical method for removing heavy metals [71]. In this process, fine particles and colloids agglomerate into larger particles, reducing turbidity, NOM and other wastewater pollutants. In the first stage, a coagulant added to the water stimulates the coalescence of colloidal material into small aggregates known as flocs [72]. The most commonly used coagulants include aluminum sulfate, ferrous sulfate, polyaluminum chloride (PACl), polymeric ferrous sulfate (PFS), and polyacrylamide (PAM) [73]. In the second stage, with gentle agitation, the flocs agglomerate, settle and are then disposed of as sludge. This process is used as a pre-treatment, post-treatment or main wastewater treatment due to its versatility [74]. This process is relatively economical and simple in operation, but limitations are incomplete removal of heavy metals, generation of sludge, and high operating costs due to chemical consumption.

3.3 Chemical precipitation

Chemical precipitation is an effective technique for removing heavy metals, mainly from effluents from the papermaking and electroplating industries. In this process, chemical precipitants such as alum, lime, iron salts and some polymers react with heavy metals present in the wastewater, resulting in insoluble precipitates [75]. This reaction allows metals to be removed more easily. Removal capacity and efficiency can be improved by optimizing parameters such as pH, temperature, initial concentration and ionic charge [76]. The mechanism of heavy metal removal by chemical precipitation is given by Eq:

$${\text{M}}^{2+}+2{\left(OH\right)}^{-}\text{→}\text{M}{\left(OH\right)}_2$$E1

where M²⁺ and OH are the metal ions and the precipitant, respectively, and M(OH)₂ is the metal hydroxide. The pH is adjusted to basic conditions (pH 9–11), which has the greatest impact in this treatment. Chemical precipitation is divided into hydroxide and sulfide precipitation. The use of coagulants in hydroxide precipitation can improve heavy metal removal by filtration or sedimentation. On the other hand, the sludge generated in the metal sulfide precipitation is removed by gravity separation or filtration. This process requires pre- and post-treatment as well as precise control over the addition of reagents due to the toxicity of sulfide ions and H₂S. Although this method has the following advantages: low capital investment, simple operation, and easily automated treatment method but it also brings problems that can be produce a large amount of sludge containing toxic compounds that require further treatment, requires a large number of chemicals to reduce metals to an acceptable level for discharge, slow metal precipitation, poor settling, and the long-term environmental impacts [74].

3.4 Ion exchange

A reversible ion exchange takes place between the solid and liquid phase. In particular, an insoluble substance removes the ions from an electrolyte solution and releases other ions of similar charge in chemically equivalent amounts. The most common ion exchange materials are synthetic organic resins [77], inorganic three-dimensional matrix and new generation hybrid materials [78]. Using an adequate replacement resin can provide an effective and economical solution to contamination control requirements. In the case of heavy metals, more highly concentrated metals are obtained by elution with suitable reagents after separating the loaded resin. The acid functional resin contains sulfonic acid in its structure. Therefore, the physicochemical interactions occurring during the removal of metal ions. Various optimization goals can be investigated for ion exchange. For example, use less resin to achieve a greater removal rate and optimize contact time with a smaller device size [79]. Anionic resins are generally used at a lower pollutant concentration, while cationic resins contain strong and weak acidic resins with more extensive use [80]. Weakly acidic resins with (COOH), while acidic resins with (∙SO₃H) group are among the most popular cation exchangers [81]. However, ion exchange has some disadvantages, such as B. the need for a pre-treatment process, for example to remove fat or oil, as well as the need for chemical reagents to recover resins, which also cause secondary pollution [82].

3.5 Electrochemical technologies

Heavy metal ions from water sources can be effectively removed using electrochemical treatment techniques. These techniques involve recovering metals in their elemental metal state by employing cathodic and anodic processes in an electrochemical cell. Electrochemical treatments include electrocoagulation, electroflotation, and electrodeposition [83]. Traditional chemical coagulation is where electrocoagulation gets its start [84]. In this procedure, anode and cathode electrode sets serve as the sites for the oxidation and reduction reactions, respectively. An appropriate anode material is electrolytically oxidized to produce the coagulant as the charged ionic metals react with the anion in the effluent. By depositing pollutants on the cathode or removing them via flotation, the simultaneous cathodic reaction enables the removal of contaminants [74]. This method produces less sludge, is simple to use, and does not require any chemicals. The recovery of harmful metal ions from industrial wastewaters, such as Pb, Cd, Cu, Ni, Zn, or Cr, or the recovery of valuable metals from solutions, such as Ag, Pt, Au, etc., both involve considerable use of electrodeposition. The cost of treating water electrochemically has been reduced through a number of initiatives. In this regard, a comparison between platinum plate and stainless steel AISL904L was described. When treating Cu (II) from industrial contaminants, these plates are employed in place of three-dimensional electrodes. Cu foam can be used as an alternative because it has a wide surface area and performs better for the removal of effluents, but it makes the process more expensive. It was discovered that treating industrial water with tin dioxide anodes during the electrochemical process reduced water and electrolyte consumption by up to 70% [85].

3.6 Membrane technologies

A membrane acts as a barrier, allowing some substances to pass through while obstructing others. This technology is controlled by the Donnan exclusion effect (charge-charge repulsion), the size exclusion or steric hindrance mechanism, and the adsorption capacity of particular pollutants [86]. This form of treatment can be used to get rid of organic and inorganic pollutants, suspended solids, and other things. Membranes are categorized as either organic (made of synthetic organic polymers like polyethylene or cellulose acetate) or inorganic (made of ceramics, metals, zeolites, silica, among other materials) depending on the substance used to make them [87]. Microfiltration, ultrafiltration, and distillation are examples of low-pressure membrane processes. Nanofiltration, reverse osmosis, and electrodialysis are examples of high-pressure membrane processes. Direct osmosis, electrodialysis, and liquid membrane processes are examples of osmotic pressure-driven membrane processes. The removal performance of a membrane is greatly influenced by a number of variables, including the size and distribution of the pores, surface charge, degree of hydrophilicity, solution flow, and the presence of functional groups. These variables must be taken into account.

3.7 Adsorption

One of the finest ways to remove heavy metals and other impurities from water is adsorption. Its benefits include the potential to prevent significant secondary pollutants, a high removal capacity, relatively low energy consumption, and technical requirements for operation [88]. Adsorbents should possess a number of desirable qualities, including a sizable specific surface area, high mechanical strength, strong thermal stability, predictable morphology, and processing that is ecologically benign. Given the high adsorption capacity and efficiency, selectivity, low cost, and reusability, this should result in a high performance. Some of the most popular adsorbents are activated carbon (AC), polymer-based materials, biomaterials, magnetic materials, and industrial and agricultural wastes. Agricultural waste (fruit peels, bagasse, coir pith, cobs of corn, sawdust, and bark); Activated carbon (wood peat, coconut shells, coals); polymeric substances (Lignin, Chitosan, Cellulose, Alginate, Silk, and Cyclodextrin); sludge, metal hydroxide, red mud, fly ash, and other industrial byproducts; Ilmenite, Hematite, Magnetite, Spinel ferrite, and other magnetic adsorbents, Metal oxide particles/graphene composites, polymer matrix composites, and lignocellulosic residues/magnetic particles are all examples of composite adsorbents that are utilized for the removal of metals from wastewater [74].

3.8 Nanotechnology

Treatments based on nanotechnology make use of nanomaterials, which have drawn interest in recent years due to their high surface-to-volume ratios and distinctive electrical, optical, and magnetic capabilities [89, 90].

One of the most popular nanotechnology technologies for heavy metal removal is nanofiltration. Chemisorption is a highly effective method for eliminating dissolved heavy metals in systems made of alumina nanofibers. Additionally, low dimensional structures like nanoclays, magnetic nanoparticles, single or multi metal oxides, non-metal oxides, and nanocarbon are the most frequently used for the purification, disinfection, and removal of heavy metals from water [91, 92]. All of these nanostructures have huge, highly reactive surfaces, and many of them may be produced synthetically or using abundant natural resources. Similar technologies for wastewater treatment include nano-assemblies, nanoplates, microspheres with nanosheets, and hierarchical ZnO nano-rods. However, the dearth of knowledge regarding the toxicity, effects on the environment, and health of nanomaterials is significant and prevents their full utilization [93]. Nanocarbon (carbon nanotubes, graphene and other carbon derivatives), 2D materials, also known as single-layer materials, include graphene and borophene, germanene, silicene, 2D metal carbides (MXenes), MoS₂ nanosheets, silicon, iron, titanium, zinc, magnesium, and manganese oxides are commonly used for heavy metal remediation in the form of nanoparticles [94, 95].

3.9 Cold plasma technology

Cold plasma is characterized by a range of temperatures that correlate to various types of particles. A considerable number of energetic and chemically reactive species, such as free radicals, excited atoms, ions, and molecules, are produced in cold plasmas thanks to the high energy of the electrons (up to 1–10 eV), which serves as the catalyst for the start and spread of plasma chemical reactions. One of the method’s most significant benefits is that it does not require high temperatures, which lowers energy use [96, 97].

Due to the possibility of using various operating gases (air, Ar, O₂, N₂, etc.) or types of plasma discharges (such as glow discharge, corona discharge, radio frequency discharge, gliding arc discharge, and dielectric barrier discharge), various plasma properties may develop, leading to the emergence of a number of applications. Due to the special characteristics of cold plasma, it is widely used in various fields [98]. Different settings have been studied for the efficient degradation of contaminants depending on the type of electrical discharges and reactor layouts. Three steps could be used to summarize the cold plasma process: Highly energetic electrons, OH radicals, ozone, O- and N-contained excited species, as well as other reactive species, are produced during the first step, contributing to the initiation and progression of the plasma chemical reactions; the second step entails the intrusion of the reacted species on the soil surface or soil pores or the dissolution or diffusion of the reacted species [99]. The capacity of easy mass transfer has a significant impact on the efficiency of remediation in both soil and water treatment as well as the effectiveness of the contact between the reactive species and the soil/water. When making electrical discharges during water cleanup, whether in a liquid or at a gas-liquid interface, the transport is changed since it is carried out by the slow aquatic ions, which is significantly impacted by the liquid conductivity. While other influencing factors (such as the impact of ionic charges present in water on the RONS/pollutant interaction) that also affect the process are not fully understood, it is discovered that the dissolved gases that create plasma micro-bubbles inside the liquid play a significant role in the process. The final step in the cleanup of contaminated sites is the chemical reaction of the reactive species with the organic pollutants. The process is impacted by the pollutants’ type. For example, during soil treatment, highly volatile molecules are broken down via a two-path decomposition (evaporation of the contaminants into the gas phase where gas phase reactions are occurring or/and direct oxidation in soil due to the presence of the active species), whereas in the case of less volatile compounds, the oxidation processes are primarily occurring on the soil granules (the reactive species get in touch with the soil through diffusion or adsorption) [96].

4. Conclusion

In this research, modern and conventional technologies for remediation of soils and waste waters contaminated with heavy and toxic elements were briefly presented. Among these approaches, we can consider physical, chemical, bioremediation and combined methods. Effective factors, advantages, disadvantages and cost comparison in these methods were mentioned. Today, the methods of purifying water and soil pollution have grown almost adequately and progress has been made in this field, but it does not meet the real needs of the environment. With the continuous progress of science and the emergence of new technologies, newer methods are proposed.

At the same time, all the proposed methods have limitations. For example, in the treatment of contaminated soils through replacement, the method of storage and subsequent leaching of heavy toxic elements from the transferred soil is problematic and is still a matter of controversy among scientists. Also, phytoremediation is a long and time-consuming process. In comparison to green methods such as phytoremediation, chemical remediation methods have better advantages, including faster application-response and larger scales.

However, pollution purification methods, especially chemical methods, need a long way to reach full maturity. Among these cases, reducing secondary pollution due to leaching of heavy elements and identifying newer economic chemicals for chemical oxidation-reduction and optimal immobilization of pollution. To achieve the mentioned goal, it is necessary to turn to group and interdisciplinary researches. Group and interdisciplinary researches can lead to the formation of new technologies that overcome the weaknesses of existing methods. For example, extensive research should be done to make selective separation of pollution from water environments.