Organoclays Based on Bentonite and Various Types of Surfactants as Heavy Metal Remediants

Abstract

The rapid industrial development of civilization has led to the need for the development of new materials to clean up chemically contaminated wastewater and soils. Organoclays, based on smectite minerals and various types of surfactants, are one of the most effective sorbents for adsorbing organic and inorganic pollutants. Organoclays are clay minerals that have been modified by the intercalation or grafting of organic molecules. The main mechanism of interaction between organic substances and organoclays involves the adsorption of the substances onto the surface of the clay mineral, which has an expanded structural cell. Various types of surfactants can be used to synthesize organoclays, including cationic, anionic, and amphoteric surfactants. Each type of surfactant has different properties that affect the clay’s ability to sorb. Cationic forms of trace elements, such as heavy metals, can also be adsorbed by organoclays. Data on the adsorption of these substances by organoclays are provided, along with information on how to synthesize them using various surfactants. This review also discusses the main mechanisms of interaction between these substances and clays and the various methods used to create organoclays. It is clear that the adsorption of heavy metals by organoclays is not influenced by their structure or properties, as they belong to the category of surfactant, but rather by their overall chemical structure and characteristics. The wide variety of surfactant types leads to different effects on the adsorption properties of trace elements.

1. Introduction

Environmental pollution is one of the main global problems affecting nature, society, and human health. The main anthropogenic contaminants are the chemicals used, produced, and disposed as waste in industry and energy, domestic, livestock, and municipal wastes, and agrochemicals. A comparative assessment of the intensity of heavy metals entering the environment from various sources shows that already by the 1960s of the 20th century, the mass of many elements annually involved in technogenic flows exceeded the masses of elements participating in natural geochemical flows—river hydrochemical runoff and the biological cycle [1]. Without solving the problem of pollution, further sustainable development for humanity is impossible.

The development of materials for the remediation of contaminated soils and wastewaters is a crucial area of environmental research. The process of remediation involves the temporary or permanent elimination of pollutants from the natural biogeochemical cycle through the involvement of chemical, physical, and biological processes. Environmental indicators are restored to their original or standard values, and the main goal is to create and use accessible and cost-effective remediants that can effectively convert toxicants (with high adsorption and redox capabilities) and interact with various contaminants in a universal or specific manner [2,3,4]. Heavy metals in high concentrations are among the most dangerous pollutants in the biosphere [5].

Natural layered silicates, with a number of unique properties such as micro- and nanoporous structures, the presence of surface-active centers of various natures, and high cation exchange capacities, have long been widely used as effective sorbents for immobilizing heavy metals and radioactive elements in soils and waste water. However, the diversity in composition, structure, and textural characteristics of these natural clay minerals and their hydrophilic surface nature limit their use in environmental applications due to their low selectivity for specific metals and their weak interaction with organic substances. This challenge can be addressed by targeted modification of these layered silicates using organic surfactants to obtain organoclays with desirable properties [3,6].

2. Clay Minerals from the Smectite Group and Organoclays Made from Them

Bentonite, also known as bentonite clay, is a rock composed mainly of aluminosilicate minerals that belong to the montmorillonite subgroup within the smectite group. These minerals are classified as phyllosilicates and have a three-layer structure, with packets of two elementary tetrahedral layers alternating between them, with the vertices of each layer facing towards the other. In between these layers, there is an octahedral layer. The packages are not tightly connected to each other, but there is a small amount of space between them in which hydrated cations and water molecules can be found. A montmorillonite packet has a slight negative electrical charge that is balanced by cations, which bind adjacent packets together [7]. Therefore, smectite phyllosilicate minerals can reversibly adsorb water in their interlayer spaces, causing them to swell and significantly increase their volume.

High dispersity is a characteristic feature of minerals from the smectite group. Different isomorphic substitutions of silicon for aluminum and aluminum for iron or magnesium lead to the appearance of negative charges that are balanced by exchangeable cations. This increased dispersion and isomorphism determine the high cation exchange capacity of these minerals, which ranges from 80 to 120 mEq/100 g of mineral. The external specific surface area of smectite clay minerals can reach up to 40 to 70 m²/g, while the total surface area can be as high as 450 to 850 m²/g [8,9]. The porosity of these minerals is approximately 0.006 to 0.010 cm³/g [10]. Smectites with different origins have cation exchange capacities ranging from 65 to 135 mmol-eq/100 g [11]. These three-layered phyllosilicate minerals with an expanding structural unit are the main ingredients used in the synthesis of organic clays [12,13].

Bentonite clays are widely used for the purification of water and soils from various inorganic and organic contaminants [14]. These include heavy metals [15,16], radionuclides [17,18], organic pollutants such as polycyclic aromatic hydrocarbons [19], a number of pesticides, and others, most of which are positively charged [20]. Despite the fact that the surface of smectite minerals is anionic, their significant volume of small pores and large surface area allow them to adsorb certain amounts of anionic (up to 5 cmol/kg) [21] and nonionic pollutants [22].

Organoclays are clay minerals, also known as phyllosilicates or less commonly as minerals, that have been modified by the intercalation or grafting of organic molecules. This process preserves the structure of the mineral’s original layers [23,24]. When the interlayer cations in the clay are replaced with organic cations, a layer is formed on the surface of the mineral that includes chemically bound organic fragments. This surface can have a hydrophilic or hydrophobic nature, depending on the specific type of organic molecule used in the modification process. This hydrophobic/amphiphilic nature significantly expands the range of substances that can be adsorbed onto the surface, making organoclay a promising material for the removal of pollutants from soils and water. In recent years, organoclays have received much attention for their potential use in soil and water treatment, including the immobilization of various pollutants and the purification of waste waters [24,25,26].

A number of researchers refer to organically modified smectite minerals, which are synthesized by replacing the inorganic counterions with compact cationic organic compounds [23,27]. These minerals are classified as adsorbing organoclays. Organoclays obtained by intercalating long-chain organic ions are referred to as organophilic organoclays [27].

Zhu et al. [28] in their work distinguished three types of organoclays, depending on the methods used for their synthesis and the molecular structure of the modifying agents.

Type I organoclays are synthesized with small, hard organic cations, such as tetramethylammonium, crystal violet, or methylene blue. Type II organoclays use organic cations with at least one long alkyl chain, while other less typical types, such as those made with special organic modifiers (e.g., nonionic/zwitterionic surfactants, cationic polymers, organosilanes, chelating reagents) or polymers (e.g., surfactants + polymers, cations + surfactants), are classified as type III.

Various mechanisms are involved in the interaction between clay minerals and organic compounds. These mechanisms often involve the replacement of exchangeable cations and water molecules in the interlayer spaces of smectite with polar, positively charged particles [29]. Interlayer cations can exchange with different types of organic cations, forming covalent bonds between the reactive surface functional groups of organic species and the organic species [30]. Two types of interactions contribute to the orientation of intercalated organic molecules: electrostatic interactions for polar groups with short alkyl chains and van der Waals forces for molecules with longer chains. The greater the valence of the interlayer cations, the more organic compounds can be retained due to the stronger electrostatic field that they create.

Neutral organic compounds can form complexes with interlayer cations [31]. The intercalation of these neutral compounds is not always accompanied by the transfer of cations between silicate layers. Instead, cations may remain in contact with a single silicate layer. This can occur when the coordination positions for cations are partially occupied by the oxygen atoms on the silicate surface [32]. The softness or hardness of the exchange cations in the interstitial space determines the binding type for aliphatic and aromatic amines. For example, soft cations such as Zn²⁺, Cd²⁺, Cu²⁺, and Ag⁺ can directly bind to amines. However, water bridges may form between amines and harder cations (such as alkali and alkaline earth metals) [33]. In addition to the formation of bonds due to hydrogen bonds, ion–dipole interactions, and van der Waals forces, it is also possible for the surface of minerals and organic molecules to interact.

The grafting of organic molecules onto existing functional groups on surfaces or pre-adsorbed organic materials is also possible [34]. Fragment grafting—the formation of covalent bonds between reactive groups on the surface and organic particles—is an important mechanism for hydrophobizing the surface of many clay minerals. Some authors believe that, even after replacing interlayer inorganic ions, part of the mineral surface remains hydrophilic, which can be attributed to the polar silanol groups located on the edges of layered silicates [34]. These end silanol groups can be modified by various polymer chains. Herrera et al. utilized mono-(I) and trifunctional organoalkoxysilanes (II) with a terminal reactive methacryloyl group as reagents to chemically modify synthetic laponite clay plates [35]. Bateman et al. recently reported the preparation of UV-active clay materials containing acrylate and methacrylate functional groups using similar silylation agents [36]. In a separate study, Liu and Guo described the grafting of polyacrylamide onto attapulgite through surface-initiated atom transfer radical polymerization in order to remove Hg(II) ions and dyes from aqueous solutions [37].

Organoclays can be obtained in solutions or through solid-phase interactions (fusion) using cation exchange [31,38,39]. Cation exchange in a solvent has historically been the first method of producing organoclays and has been used for over seven decades. Examples of preparing organoclays using this technique under various process conditions are available in several sources [40,41,42]. Wu et al. intercalated hexadecyltrimethylammonium bromide between layers of calcium montmorillonite [43]. Hexadecyltrimethylammonium bromide was added to a 2% mineral suspension at a ratio of 0.8 mmol/g, and the mixture was pre-treated with ultrasound to help disperse the material more evenly. The suspension was vigorously stirred for 10 h in a water bath, and then the liquid and solid were separated through centrifugation and decantation. The organoclay was then washed several times with distilled water and centrifuged until the bromide reaction in the supernatant had completely disappeared. In addition to water, alcohols, such as ethanol [44] and isopropanol [45], and other organic solvents can be used as solvents for organic modifiers. Small amounts of acetone may sometimes be added to the system [46].

Organic molecules can be intercalated into dried clay minerals through solid-phase reactions without the use of solvents. This environmentally friendly approach makes the process suitable for industrial applications. The preparation of organoclays through solid-phase alloying was first reported by Ogawa et al. in 1990 [47]. A researcher [48] also used the “dry” method, which involves the reaction of clay with a quaternary ammonium salt at 60 °C. Another researcher [49] used a mechanochemical method to intercalate aniline salts into montmorillonite through a solid-state process. Many solid-state reactions are based on ion–dipole interactions. These interactions allow organic molecules with polar groups to attach to interlayer cations, while negatively charged parts of molecules can attach to cations on mineral surfaces and displace associated water molecules [49]. The production of organoclay through solid-phase fusion is used to create nanocomposites, in which the organoclay serves as a polymer matrix filler. When creating such materials, it is not necessary to maintain the structure of the mineral but rather often requires its exfoliation.

3. Types of Surfactants Used in the Production of Organoclays and Their Sorption Properties

Surfactants are effective modifiers that can change the surface characteristics of organoclays. Surfactants are amphiphilic or amphipathic chemical compounds that have an affinity for both nonpolar and polar environments (Figure 1) [50]. The molecules of surfactants have a hydrophilic head group, which interacts with the aqueous environment, and a hydrophobic tail, which forms the “tail” in aqueous solutions (Figure 1). The polar (ionic) head of the surfactant molecule usually interacts actively with the water environment and is solvated through dipole–dipole or ion–dipole interactions. This is what underlies the classification of surfactants into cationic, anionic, zwitterionic (amphoteric), and nonionic types (Figure 1). Anionic surfactants—the cheapest and most versatile type, which accounts for at least 60% of the world’s surfactant production—occupy a dominant position in the market. Up to 30% of surfactants are nonionic, about 10% are cationic, and only a small fraction of a percent are amphoteric [51,52].

Surfactants can also be classified into two main categories based on the raw materials used in their synthesis: biological surfactants and conventional surfactants [53]. They can further be divided into four subcategories based on their environmental behavior: chemically degradable, biodegradable, non-degradable, and non-biodegradable [54].

3.1. Cationic Surfactants

The head group of cationic surfactant molecules in solution is positively charged (Figure 1), which causes them to interact electrostatically with negatively charged clay surfaces. This interaction allows the exchange of inorganic cations with the surfactant. These processes play a significant role in the widespread use of cationic surfactants for the synthesis of organoclays. Organoclays are used to modify negatively charged phyllosilicate surfaces, making them hydrophobic and allowing them to adsorb organic pollutants, such as the following:

-

aliphatic and aromatic primary, secondary, and tertiary amines, as well as their salts;

-

quaternary ammonium salts (QAS), including salts of pyridine-based compounds;

-

oxides of tertiary amines.

Quaternary ammonium salts and pyridine-based salts are both soluble in alkaline media on both sides. In addition, cationic surfactants have bactericidal properties and are widely used as topical antiseptics and disinfectants due to their ability to emulsify. Quaternary ammonium halides, with long alkyl chains, have found widespread application in the modification of clay minerals due to the ease with which salts can exchange inorganic cations in the interlayer spaces of montmorillonites [32].

Organoclays formed with quaternary ammonium salts with a short alkyl chain with nc < 16 (e.g., dodecyltrimethylammonium) at a concentration of cationic surfactants less than 1 cation exchange capacity (CEC) have a basal distance of 1.4–1.6 nm, practically independent of the capacity. The basal distance for long-chain QAS with nc ≥ 16 (e.g., hexadecyltrimethylammonium) varies depending on the salt concentration. For montmorillonite with a CEC of 80–90 meq/100 g, the basal distance of the organoclay gradually increases with increasing modifier load. There is a stepwise increase in the basal reflex to 3.2–3.8 nm at a QAS concentration of 1.5 CEC and to 3.5–4.0 nm at a surfactant concentration of 2.0 CEC [32]. According to [55], when tetradecyltrimethylammonium ions are intercalated into the structure of montmorillonite at an initial concentration of 50% of the cation exchange capacity, the cations are completely adsorbed by the mineral at 100% of the CEC, from 75 to 83%.

Data on the adsorption of Cu (II) and Zn (II) by organoclays based on bentonite and cationic surfactant (bencylhexadecyldimethyl ammonium chloride) were fitted with Langmuir, Freundlich, and Dubinin–Radushkevich isotherm models. The Langmuir isotherm provided the best fit with experimental data (R² = 0.962–0.993). The pseudo-second-order rate equation provided the best fit to the observed adsorption kinetics [56]. At the same time, adsorption isotherms of Cu(II) on organoclays with different cationic surfactants indicated that adsorption data fitted well with the Freundlich isotherm, and adsorption kinetics could be explained by the first-order and pseudo-second-order kinetic models [57].

Based on the presence of a positive charge, only cationic and zwitterionic surfactants have a strong chemical affinity for clay minerals. Their intercalation into the phyllosilicate layers occurs through cation exchange, ensuring a stable and efficient adsorption of the surfactant. The bond strength between cationic surfactants and clay minerals is significantly higher than that of other types of surfactants. However, repeated washing with fresh water only partially desorbs the cations [58].

At the same time, the intercalation of cationic surfactants reduces the adsorption capacity of organoclays for cations as the negative surface charge is compensated [59]. Competition between organic and inorganic cations for adsorption sites can occur. Therefore, organoclays based on surfactants have a reduced ability to adsorb heavy metal cations when compared to unmodified phyllosilicates. For example, it has been shown in [60] that organoclays based on cationic surfactants exhibit a much lower capacity to adsorb Cu(II) compared to untreated montmorillonite. Hexadecyltrimethylammonium has also been shown to reduce the adsorption of lead and mercury on two reference montmorillonites. This is because this organic cation competes with the metals for adsorption sites on the clay surface, and it does not have any organic functional groups that can interact with heavy metals. In [55], it was found that the cationic amide group in bentonite modified with a mixture of amphoteric and cationic surfactants inhibits the adsorption of Cd²⁺ due to electrostatic repulsion [61].

3.2. Anionic Surfactants

Anionic surfactants, when dissolved in water, form negatively charged ions (Figure 1). The most common anionic surfactants include carboxylate, sulfate, and sulfonate ions. Alkyl sulfates, alkyl ethoxylates, and soaps are some of the most commonly used anionic surfactants. These surfactants have low toxicity and often contain a saturated or unsaturated aliphatic chain with a composition of C₁₂ to C₁₈. The solubility of a surfactant in water depends on the presence of double bonds within its structure.

The main surfactants are the following [62]:

-

Soaps: RCOONa, RCOOK;

-

Alkyl sulfates and alkyl phosphates: ROSO₃Me, ROPO₃Me₂;

-

Alkylaryl sulfonates (most often alkyl benzene sulfonates, RC₆H₄SO₃Me);

-

Alkyl sulfonates and alkyl phosphonates: RSO₃Me, RPO₃Me₂;

-

Alkyl sulfosuccinates;

-

Alkyl ethoxy sulfates and alkyl ethoxy phosphates.

This type of surfactant is much less considered for the modification of swelling clay minerals. Since the available surface areas of clay minerals for binding anionic surfactants are extremely limited [63,64], it was found that anionic surfactants were adsorbed by minerals much worse than nonionic surfactants. It is assumed that anionic surfactants prevent the adsorption of water molecules on the surface of clay minerals and their entry into the interpacket space of clays. The properties of clays are restored after washing with fresh water [58].

Using the example of organobentonites obtained by modifying magnetic bentonite with amphoteric surfactants and mixtures of amphoteric and cationic or anionic surfactants, it has been shown that the amount of modifiers in the organoclay increases in the following order: modified with amphoteric cationic > modified with amphoteric > modified with anionic amphoteric. At the same time, the cation exchange capacity increases in the following order: original bentonite, amphoteric modified bentonite, anionic modified amphoteric, and cationic modified amphoteric. The adsorption of Cd²⁺ on amphoteric anionic bentonite is due to electrostatic attraction caused by the sulfo group [51]. The adsorption isotherm of Cd²⁺ is in good agreement with the Langmuir equation. Thermodynamic studies showed the spontaneous and endothermic nature of the adsorption process [61].

The Dubinin–Radushkevich model was adopted to describe the single-ion adsorption isotherms for the sorption of Cu²⁺ and Zn²⁺ from aqueous solutions by montmorillonite modified with anionic surfactant sodium dodecylsulfate. The binary-solute adsorption equilibria could be reasonably predicted by the competitive Langmuir model, in which the Langmuir parameters were directly taken from single-solute systems. The kinetics of metal adsorption were examined, and the pseudo-first-order rate constant was finally evaluated [46].

The data on the effect of surfactants on the adsorption of trace elements by clay minerals are very contradictory. Overall, we can conclude that there is an increase in metal adsorption by organoclays based on the use of surfactants only under certain conditions. The author of [60] states that the adsorption capacity of organoclay obtained with the involvement of surfactants for Cu(II) is comparable to that of untreated montmorillonite. Some researchers have suggested that anionic surfactants such as sodium dodecyl sulfate can improve the adsorption capacity of clay for heavy metal ions [46]. For example, the extraction of Sr(II) ions is significantly enhanced on organoclays synthesized with this anionic surfactant. However, there are some questions raised by the increase in strontium adsorption by organoclay from 3.91 mg/g to 26.85 mg/g. This suggests that further research is needed to fully understand the effects of surfactants on metal adsorption and the mechanisms involved [43].

Thus, an analysis of the available sources allows us to conclude that the addition of cationic and anionic surfactants to the composition of organic clays has a negligible effect on the adsorption of trace elements. Cationic surfactants compete with metal cations for adsorption sites on the surface of the minerals, while anionic surfactants are themselves weakly adsorbed by the anionic surfaces of the clays due to their opposite charge.

3.3. Amphoteric Surfactants

Amphoteric surfactants, also known as zwitterionic surfactants, have a long hydrophobic hydrocarbon chain with both positively and negatively charged hydrophilic centers connected by a spacer group (Figure 1). These surfactants are pH-insensitive, meaning they maintain their zwitterionic structure regardless of the pH level of the solution [65]. Amphoteric surfactants may be divided into two categories: pH-sensitive and pH-insensitive. At lower pH levels, these surfactants behave as cationic substances, while at higher pH levels, they behave anionically. Near their isoelectric point, they assume a zwitterionic form. The properties of amphoteric surfactants vary depending on several factors, including the length of their hydrophobic chain, the specific positive and negative charges, and their placement within the molecule. In general, the cationic portion of an amphoteric substance is often a quaternary ammonium, imidazole, pyridine, or phosphonium, while the anionic component is typically a carboxylate, sulfonate, or sulfate group [66].

The most widely used surfactants are alkylaminocarboxylic acids and betaines: alkylbetaines, sulfobetaines, phosphate betaines, amidobetaines, and ethoxylated betaines. Amino acid derivatives, such as carboxybetaines and imidazolines, are also important in industry.

Examples of various amphoteric surfactants are listed in

Table 1. Types of amphoteric surfactants (according to [60]).

Surfactants	Structural Formula
Sulfobetaine
Sulfatobetaine
Hydroxypropyl sulfobetaine
Sulfoimidazolium
Sulfo-pyridinium
Carboxybetaine
Carboxyimidazolium
Amidosulfobetaine
Amidocarboxybetaine
Phosphocholine
Phenylphosphinatobetaine
Amine oxide
Sulfophosphonium

.

Natural surfactants include some phospholipids, for example, lecithins—esters of glycerides of higher fatty acids, orthophosphoric acid, and choline [62].

If a hydroxyl group is present in the methylene segment between two charged head groups, then surfactants of the hydroxypropylcarboxybetaine or sulfobetaine type are mainly formed. An amide group can also be present on a long hydrophobic chain and form amidosulfobetaine or amidocarboxybetaine [67]. Phosphocholines are another extremely important group of amphoteric surfactants, consisting of a positively charged quaternary ammonium group on the outer side and a negatively charged phosphate group on the inner side of the methylene segment [68].

It is also possible to produce amphoteric surfactants by attaching the hydroxyl group to quaternary ammonium ions. The hydroxyl group usually has a weak acidity, but it is more likely to deprotonate if it is directly attached to a nitrogen atom. Examples of surfactants that fall into this category include myristamine oxide and lauryl dimethylamine oxide.

Amphoteric surfactants include many natural substances, such as amino acids and proteins. Their synthetic analogues are alkylamino acids, for example, cetylamine acetic acid C₁₆H₃₃NH–CH₂COOH, etc.

Amphoteric surfactants of biological origin can be obtained from natural sources such as oleic acid [69], vegetable oil [70], including castor oil [71], soybean oil and olive oil [72], and waste cooking oil [73]. These amphoteric surfactants have a lower critical micelle concentration, lower toxicity, better biodegradability, and higher compatibility with the environment as compared to synthetic surfactants. They are less affected by extreme pH values and have a wide range of biological activities. These activities include antiviral, antimicrobial, and hemolytic properties [74].

Amphoteric surfactants possess a number of unique characteristics, including high water solubility, high surface activity, a wide isoelectric point, a low critical micellar concentration, excellent foam stability, low toxicity and irritation, and excellent biodegradation potential. These unique characteristics make amphoteric surfactants a valuable choice for scientific research and industrial applications [65].

The order of solubility for amphoteric surfactants typically follows the following pattern: sulphimidazole < carboximidazole < sulfobetaines < phosphocholines < carboxybetaines [75]. Carboxybetaines and phosphocholine surfactants tend to be more soluble at room temperature than sulfobetaines in an aqueous environment. This is because the increasing rigidity of the surfactant’s headgroup and the crystal structure provided by the imidazolium ring lead to a lower solubility compared to those with a cationic ammonium group, such as carboxybetaine or sulfobetaine. The solubility of these amphoteric substances decreases as the hydrocarbon chain becomes longer, due to their hydrophobic nature. Generally, the addition of a salt (electrolyte) can reduce intermolecular interactions and improve the solubility of the surfactants in aqueous solutions [76].

The formation of surfactant micelles in solution results in new mechanisms for their interaction with clay minerals and trace element cations in solution. Therefore, it is important to understand the ability of these micelles to form and the concentrations where they form (critical micelle concentration, CMC), as these are also properties that influence the interaction between smectite-based organoclays and various types of surfactants with trace elements.

The CMC value of an amphoteric surfactant depends on several factors, including the length of its hydrophobic chain, the type of spacer fragment between the hydrophilic and hydrophobic moieties, and the properties of the aqueous solution (e.g., the presence of ions, temperature, etc.). As the length of the hydrocarbon chain increases, the CMC values tend to decrease, leading to more negative zeta potentials (ζ). This is because longer chains are more hydrophobic and less soluble in water, and the hydrophobic interaction plays a major role in micelle formation. The number of methylene (CH₂) groups in the spacer space also affects the ability of these surfactants to form stable micelles [77]. Surfactants with more methylene groups tend to have higher CMC values and are therefore less soluble in water. The hydrophobic effect of longer spacer groups results in a decrease in the critical micelle concentration, while the increased dipole moment due to the higher charge separation of the two oppositely charged headgroups results in increased repulsive dipole–dipole interactions and an increase in the CMC. Therefore, the CMC may either increase or decrease, depending on the composition of the spacer groups, which determines which of these two effects is more significant. CMC values generally increase in sulfobetaines, phenylphosphobetaines, and carboxybetaines up to n = 3–4, but they decrease at higher values of n [78].

The zwitterionic form of amphoteric surfactants has a much lower CMC and a much greater surface activity than the ionic form, due to the much weaker electrostatic repulsion between ionic head groups [76].

An increased concentration of salts causes a decrease in the critical micelle concentration (CMC) and an increase in N_agg as a result of the salting-out of the hydrophobic hydrocarbon chains of the amphoteric surfactant [79]. Moreover, the preferential adsorption of ions on the surface of the micelles further reduces the electrostatic repulsion between the head groups, thus facilitating the formation of larger micelles [80]. The strength of the added ions has a significant impact on the CMC value.

The presence of added salt in a surfactant solution protects the ionic atmosphere, significantly reducing the electrostatic repulsion between molecules and promoting micelle formation. Cations and anions are attracted to amphoteric surfactants, but cations are attracted more strongly than anions. For surfactants such as sulfobetaine and sulfoimidazole, an increase in total charge density in micelles due to interaction with added ClO₄⁻ anions results in a sharp increase in cation attraction. This phenomenon is known as the “chameleon effect” [81].

Strongly hydrated monovalent cations are less attracted to the negative center of the surfactant than weakly hydrated divalent cations. The order of cation attraction to sulfobetaine is trivalent > divalent > monovalent. As a rule, the addition of salts containing higher-valence cations can displace hydrogen ions from the surface of micelles. When lanthanides interact with micelles of amphoteric dodecylphosphocholine, micellization becomes more pronounced towards the right-hand side of the lanthanide series, and the process is endothermic due to the release of water molecules from the hydration layer of the surfactant [82].

Zwitterionic surfactants can intercalate into the interlayer spaces of bentonites, and their expansion is affected by the length of the alkyl chains and the level of surfactant loading [83]. Studies have shown that organoclays based on zwitterionic surfactants have a more complex structure and a higher thermal decomposition temperature compared to organoclays containing only cationic surfactants.

Work [84] proposes a new mechanism for the intercalation of montmorillonite with a zwitterionic surfactant (using the example of 3-(N, N-dimethylpalmitylammonio) propanesulfonate). According to XRD results, the basal distance of the organoclays increased from 1.47 nm to 4.13 nm as the CPAS loading increased from 0.2 to 4.0 cation exchange capacity. The amount of Ca²⁺ released during the preparation of the organoclay was very limited, according to the results of the chemical composition analysis. The Ca/Si and Ca/Al ratios in the modified montmorillonite were comparable to those of the original mineral, suggesting that Ca²⁺ remains in the interlayer spaces and no significant exchange reaction occurred between the surfactant and interlayer Ca²⁺. Afterwards, a shift in the number of vibrations occurred, which may indicate the formation of a new bond between the Ca²⁺ and sulfonate groups.

Amphoteric-modified bentonite exhibited the highest adsorption capacity for Cd²⁺ (233.19 mmol/kg), compared to the original bentonite and surfactant-modified bentonites. Based on the results of infrared (IR) and X-ray photoelectron spectroscopy (XPS) studies and sorption experiments, the adsorption of Cd²⁺ on the original and organobentones is most likely due to electrostatic interactions, ion exchange, and surface complexation. The adsorption of Cd²⁺ onto amphoteric-surfactant-participating organoclays was also attributed to chelation [61].

Organoclay, synthesized using laponite and the zwitterionic surfactant cocamidopropyl betaine, has a good adsorption capacity for Co²⁺ and Cs⁺ compared to the original synthetic mineral, which adsorbs Sr²⁺ in significant quantities [85]. The results suggest that the mechanism of cation adsorption on organoclay may differ from that on laponite. While Sr²⁺ is adsorbed onto both laponite and organoclay through cation exchange, Co²⁺ most likely adheres to organoclay by a chelation process involving a specific chemical group of the surfactant [86]. The mechanism of Cs⁺ adsorption on clay and organoclay is not well understood, but it probably involves physical adsorption.

The amount of zwitterionic surfactant used in the modification process is of great importance, as it affects the availability of adsorption sites on the one hand and the preservation of cation exchange properties on the other. It is likely that a low surfactant load provides the highest adsorption capacity for Co²⁺ and Cs⁺, as this minimizes the risk of recombination of surfactant molecules and steric hindrance in the interlayer space. In the ternary solution tested, containing all three cations (Co²⁺, Cs⁺, and Sr²⁺), there was a competitive nature to the adsorption between Cs⁺ and Sr²⁺ [83].

At the same time, the organoclay that was synthesized using a zwitterionic surfactant had an adsorption capacity for Cu(II) that was comparable to that of the original montmorillonite. The adsorption of copper and phenol onto the organoclay did not occur competitively [55]. The adsorption isotherms of Cu(II) on organoclays with zwitterionic and cationic surfactants are better fitted with the Langmuir model than the Freundlich model, indicating that the monolayer adsorption on surfaces with a finite number of identical sites was the possible adsorption model for Cu(II) uptake [60].

Comparing the obtained linear correlation coefficient R², the pseudosecond-order model exhibited a much better fit for Pb²⁺ adsorption on the montmorillonite modified by two different carbon chain lengths of betaine. The Freundlich isotherm was well-fitted for Pb²⁺ adsorption on organoclays [87].

Thus, a number of amphoteric surfactants can be used as modifiers for smectites to obtain organoclays that effectively adsorb trace elements. The mechanisms by which they interact with clays are diverse and go beyond simple ion exchange. In fact, the composition and structure of the organic modifiers can significantly affect the efficiency with which metal cations are adsorbed by the organoclays.

3.4. Nonionic Surfactants

Nonionic surfactants, also known as nonionic detergents, have functional groups that are only able to solvate and hydrate without dissociating in aqueous solutions (Figure 1). These molecules consist of isolated hydrophobic radicals (alkyls, alkylaryls, etc.) and hydrophilic atomic groups (usually hydroxyl and ethoxyl). The main difference between nonionic and ionic surfactants lies in the chemical structure of their hydrophobic parts [50].

Nonionic surfactants include the following:

-

oxyalkylated primary and secondary fatty alcohols;

-

polyethylene glycol ethers of alkylcarboxylic acids;

-

oxyalkylated alkylphenols;

-

pluronics;

-

glycerides, glucosides, saccharides, etc. [62].

-

Hydrophilic groups in nonionic surfactants can include the following:

-

ethylene oxide

-

ethanolamine

-

diethanolamine

et al.

Hydrocarbon chains (straight and branched), with the number of methylene groups ranging from 10 to 20, are hydrophobic. Nonionic surfactants, based on ethylene oxide, have the following general formula:

where R is alkyl, X is an oxygen, nitrogen, sulfur atom, or a fragment of a functional group carboxyl-COO-, amide-CONH-, phenolic-C₆H₄O-, etc.

Table 2. Typical commercial nonionic surfactants [82].

Surfactants	Structural Formula
Poly(alkylene-oxide) block copolymers
Oligomeric alkyl-ethylene oxides
Alkyl-phenol polyethylenes
Sorbitan esters

lists some classic commercial nonionic surfactants [88].

The vast majority of nonionic surfactants are polyoxyethylene ethers, which are derived from aliphatic alcohols or acids, alkylphenols, or amines, among other compounds. The polyoxyethylene chain in these molecules acts as the hydrophilic component of the surfactant molecule.

The vast majority of nonionic surfactants (up to 80%) are polyoxyethylene ethers of aliphatic alcohols and acids, alkylphenols, amines, and other compounds. In these compounds, the polyoxyethylene chain serves as the hydrophilic part of the non-surfactant molecule.

Nonionic surfactants, such as alkyl polyglycosides, are derived from renewable raw materials and have several advantages over traditionally used ethylene oxide derivatives [89]. These surfactants have a high rate of biodegradability, low toxicity towards water bodies, and a lack of carcinogenic potential. Additionally, they are mild surfactants that do not cause undesirable effects on the skin or mucous membranes [90]. The polar groups in alkyl polyglycoside molecules consist of fragments of carbohydrate molecules (glucose, maltose, etc.), and if the composition contains glucose residues, the surfactant is called an alkyl polyglucose (APG). The nonpolar portion of the molecule consists of a long-chain hydrocarbon radical.

Nonionic surfactants exhibit a diverse range of phase behaviors and have low critical concentrations and critical temperatures for mycelization.

The structure of self-organizing aggregates and liquid–crystalline phases of non-ionogenic molecules has been widely studied, depending on the water content and temperature, as well as the length of the hydrophobic and hydrophilic chains. At concentrations of the order of micro–milli–molar (µM–mM), nonionic surfactants self-assemble into spherical micelles, which is an unusual property compared to other classes of these molecules. This unusual property is the hydration of the surfactant head group, which decreases with increasing temperature. This leads to a continuous change in the spontaneous curvature of the interface between hydrophilic and hydrophobic regions [91,92].

The function of a nonionic surfactant is associated with its hydrophilic portion, often represented by its ethoxylated component. The higher the content of ethylene oxide, the more water-soluble the surfactant becomes. The general order of phases, from high to low temperature or from long to short ethylene oxide chains, is as follows: spherical micelles, cubic micellar phase, elongated micelles, hexagonal phase, lamellar phase, cubic continuous phase, sponge phase, and reverse micelles [93]. In addition to the typical cubic, two-dimensional hexagonal, and plate-shaped liquid–crystal phases, some nonionic surfactants can form unusual structures like ribbon and network phases [94].

There is theoretically no intermolecular interaction between nonionic surfactant molecules and clay minerals. Due to the limited number of surface sites on clay minerals available for binding anionic and nonionic surfactants, these types of surfactants are not commonly used for modifying swelling clay minerals [63]. Compared to ionic surfactants, clays modified with nonionic compounds have a hydrophobic surface without altering the degree of surface charge [95].

However, Shen successfully used nonionic surfactants to produce organobentonites [96]. The resulting products exhibited higher adsorption capacity and chemical stability when compared to bentonites modified with cationic surfactants. The authors of [59] found that nonionic surfactants are adsorbed to a varying degree by various mineral adsorbents, whereas anionic surfactants are adsorbed much less. The amount of surfactant adsorbed by the adsorbent increases as the percentage of clay minerals in different mineral mixtures increases.

Nonionic surfactants Brij 56 and Igepal CO 720 showed similar adsorption properties on smectite: they exhibited a steep initial increase over a narrow concentration range and have L-type adsorption isotherms. The isotherms can be fitted to the Langmuir equation with regression coefficients (>0.98) [97]. Similar results were obtained by Shen [90].

Fatty acids and sorbitol esters, such as Spans 80 and Tween 80, which are chemically known as sorbitan monooleate and its polyethoxylated form, respectively, as well as alkylphenol ethoxylates, such as Triton X-100, can be used as nonionic surfactants to modify clay minerals. The authors believe that these surfactants form complexes in the interparticle space of clay minerals, such as montmorillonite, due to hydrogen bonding and ion–dipole interactions [95,98]. The hydrophobic parts of these organoclays are prone to biodegradation [99].

Hybrid materials based on clays and nonionic surfactants have shown very promising properties [100]. T-octylphenoxypolyethoxyethanol (also known as Triton X-100 or TX100) is one of the most popular nonionic surfactants for modifying clays. X-ray diffraction analysis of systems containing nonionic surfactants and bentonite has shown that these surfactants can cause a slight increase in the spacing between the layers of smectite, suggesting that the modification of bentonite by Triton X-100 mainly occurs on the surface. Analysis of the specific surface area using the Brunauer–Emmett–Teller (BET) equation, Fourier-transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM) imaging, and thermogravimetric/differential thermal analysis (TG/DTA) curves allowed us to understand the structural changes that occur in bentonite as a result of surfactant adsorption and to confirm the effectiveness of this modification process. FT-IR spectroscopy showed adsorption bands indicating the presence of adsorbed surfactants on clay minerals. The presence of OH vibration bands in samples with adsorbed TX100 suggests that the surfactant molecules interact with silicate through their functional groups and water, coorrelated with exchange cations from clay minerals through ion–dipole or hydrogen bonding. Analysis of the TG/DTA curves reveals that surfactant intercalation enhances the structural and thermal stability of organic bentonites. Based on calculations using nitrogen adsorption–desorption isotherm data, intercalating nonionic surfactants results in a decrease in specific surface area and total pore volume in bentonite samples. SEM images confirm that modifying bentonite with a surfactant leads to changes in surface morphology [100].

Gorodnov [58] believes that nonionic surfactants prevent the adsorption of water molecules on the surface of clay minerals and their entry into the interpacket space of clays. After washing with fresh water, the properties of the clays are restored.

A study of the effects of different types of surfactants on the adsorption of heavy metals in clays [101] found that relatively high concentrations of non-surfactant materials are necessary for measurable results to be seen. At the same time, ionic surfactants can promote or prevent the adsorption of copper ions. The pH of the solution and the cations adsorbed influence the adsorption process. It has been noted that nonionic surfactants adsorb onto clay surfaces due to the combination of polar attractions, van der Waals forces, and weak C-H…O (clay) bonds [102]. This can lead to the displacement of water molecules and the formation of organic layers within the interlayer space. Such interactions tend to block exchange sites on montmorillonite particles, resulting in decreased metal adsorption. The most significant adsorption effect was observed using octadecylamine ethoxylate, a compound in which the hydrophilic and lipophilic groups of the molecule are approximately in equilibrium and which forms a strongly basic solution in water (pH 8–10) when dissolved.

The Langmuir and Freundlich R² calculated from Cu²⁺, Pb²⁺, Ni²⁺, Cd²⁺, Fe²⁺, and Zn²⁺ isotherms at the metal adsorption on natural zeolite (clinoptilolite) modified by Triton X-100 show that for Pb, Cd, and Fe, the adsorption data were fitted equally well by both the Langmuir equation and the Freundlich equation. Adsorption data for Cu, Ni, and Zn were fitted better by the Freundlich adsorption isotherm [103].

Thus, organoclays synthesized with nonionic surfactants are an example of organo-mineral complexes, the synthesis of which is based not on ion exchange but on other mechanisms. At the same time, their ability to adsorb trace element cations can be highly effective.

3.5. Gemini Surfactants

Despite the widespread use of surfactants in various industries, we mainly focus on the use of those with one “head” and one “tail”. One of the most intriguing developments in surfactant chemistry is the appearance of Gemini surfactants. Gemini surfactants are bimolecular surfactants that have two hydrophilic (mainly ionic) groups and two hydrophobic “tails” [104]. These two parts of the molecule are linked to each other by a spacer group with various lengths (most commonly a methylene or ethylene oxide spacer) (Figure 2).

Homo Gemini surfactants have two identical hydrophobic tails and identical hydrophilic groups, which can be ammonium, sulfonate, carboxylate, or polyoxyethylene. Hetero Gemini surfactants, on the other hand, have different polar head groups, including nonionic–nonionic, anionic–nonionic, and anionic–cationic. They also have different lengths of hydrocarbon tails [105]. Each type of Gemini surfactant can be further divided into cationic, anionic, nonionic, and amphoteric types. Amphoteric Gemini surfactants typically have non-identical hydrophilic head groups, such as sulfate-polyoxyethylene or hydroxyl-polyoxyethylene [106]. However, there is relatively less information available on amphoteric Gemini surfactants that have the same amphoteric head groups, namely betaine-betaine or sulfobetaine-sulfobetaine [107].

Gemini surfactants have significantly lower critical micelle concentrations, higher surface activity, and lower surface tension. They also have better hard water resistance and wetting ability, as well as a lower Kraft temperature.

Theoretically, all of the mechanisms of interaction between conventional surfactants and clay minerals described above would also apply to possible interactions between Gemini surfactants and clays. Therefore, only Gemini surfactants with two cationic heads and long hydrophobic chains would reduce the cation exchange capacity of organoclays. The influence of other types of Gemini surfactants would depend on their specific composition and structure.

Montmorillonite modified with a cationic Gemini surfactant (Propyl bis (hexadecyl dimethyl ammonium) chloride) sorbed less Cu²⁺ compared to the original mineral [108].

At the same time, Na-montmorillonite modified with a Gemini surfactant containing four ammonium cations has a higher maximum adsorption capacity for Cu²⁺ (29.3 mg/g) compared to the original sodium mineral (25.30 mg/g) at pH 6 [109].

Three cationic surfactants, Gemini (1,3-bis(dodecyldimethylammonio)ethane dibromide, C₃₀H₆₆N₂Br₂; 1,3-bis(dodecyldimethylammonio)propane dichloride, C₃₁H₆₈N₂Cl₂; and 1,3-bis(octadecyldimethylammonio) propane dichloride, C₄₃H₉₂N₂Cl₂)), were used to prepare organoclays based on rectorite. These organoclays adsorbed Cu²⁺ at higher levels than the original rectorite and the rectorite that was modified with cationic surfactants. The adsorption behavior of the rectorite modified with the Gemini surfactant could be better described by the Freundlich adsorption isotherm model, with a maximum adsorption capacity of 15.16 mg/g. During this experiment, it should be noted that organoclays containing cationic surfactants also adsorbed more copper than the original rectorite [110].

There are several studies that demonstrate the effectiveness of organoclays in combination with Gemini surfactants for the adsorption of trace elements in their anionic form [111,112,113,114].

An interesting fact about the adsorption of copper and organic pollutants, 1H-Benzotriazole (BTA), 5-Methyl-1H-benzotriazole (TTA), and 1-Hydroxybenzotriazole, onto montmorillonite modified with the cationic Gemini surfactant, Propyl bis (hexadecyl dimethyl ammonium) chloride (HOBT), is that it is synergistic in nature. The presence of organic contaminants in the system contributes to increased copper uptake, as shown by FTIR, EDS, and XPS analysis. Cu²⁺ complexation with these organic substances occurs through complexation and hydrophobic interactions within the system [108].

Cu²⁺ adsorption on organoclay with Gemini surfactant appeared to be more consistent with the Langmuir adsorption isotherm model (higher R² values). The results formally indicate that the surface adsorption sites were almost identical. According to the data provided by the Langmuir adsorption isotherm, the maximum single-layer adsorption values of Na-montmorillonite and Gemini-modified montmorillonite for adsorbing Cu²⁺ were expected to be 31.28 and 34.43 mg g⁻¹, respectively.

4. Conclusions

Thus, information on the adsorption of heavy metal cations by organoclays based on smectites and different types of surfactants is quite contradictory and clearly insufficient to draw unambiguous conclusions. However, we can summarize the following: Intercalation of cationic surfactants reduces the adsorption capacity of organoclays in relation to heavy metal cations due to the competition of organic and inorganic cations for sorption positions and the neutralization of the negative surface charge. An analysis of the sources allows us to conclude that anionic surfactants, which themselves are weakly adsorbed by the anionic surfaces of minerals due to the same charge, have a slight effect on the adsorption of trace elements by clays. A number of amphoteric surfactants can be used as smectite modifiers to obtain organoclays that effectively adsorb trace elements. The mechanisms of their interaction with clays are diverse and go beyond ion exchange. The efficiency of metal cation adsorption by organoclays with the participation of amphoteric surfactants is influenced by the composition and structure of the molecules of organic modifiers. Organoclays synthesized with nonionic surfactants are an example of organo-mineral complexes, the synthesis of which is based not on ion exchange but on other mechanisms. At the same time, their adsorption of trace element cations can be very effective.

Data on the adsorption of cations on organoclays synthesized with different types of surfactants can be approximated by the Langmuir, Freundlich, and Dubinin–Radushkevich equations. Despite the theoretical heterogeneity of the organoclay surfaces, the Langmuir adsorption equation is used satisfactorily in most cases to describe the adsorption of metals. This may be explained by the experimental conditions in each investigation.

Methods for the synthesis of organoclays need improvement and further development. Factors of concern when using organoclays as remediants are immobilization strength, recyclability, and environmental toxicity. Further intensive study of this problem is necessary, which has not only practical but also fundamental significance from the points of view of chemistry, soil science, and biogeochemistry.