An overview about synthetic high charge micas and their uses

Applied Clay Science 251 (2024) 107325 Available online 4 March 2024 0169-1317/© 2024 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Review article An overview about synthetic high charge micas and their uses Luís H. Oliveira a , Denise B. França a , Alan I.S. Moraes a , Santiago Medina-Carrasco c , Maria G. Fonseca b , Josy A. Osajima a , Edson C. da Silva-Filho a , María del Mar Orta d , * a LIMAV, Interdisciplinary Laboratory of Advanced Materials, UFPI, 64049-550 Teresina, PI, Brazil b LACOM, Laboratory of Fuels and Materials, UFPB, 58051-085 Jo˜ ao Pessoa, PB, Brazil c X-Ray Laboratory (CITIUS), Universidad de Sevilla, Avenida Reina Mercedes, 4B, 41012 Sevilla, Spain d Analytical Chem., Dep. Pharmacy Fac., Sevilla Univ., C/ Profesor García Gonz´ alez 2, 41012 Sevilla, Spain ARTICLE INFO Keywords: Na-Mica-x Adsorption Synthetic clay Removal of pollutants ABSTRACT Synthetic micas are materials with interesting properties that combine high charge (high cation exchange capacity) with swelling capacity. Among the various methods for synthesizing this clay, the “NaCl-melt method” is the most widely used, producing a material with excellent crystallinity. Furthermore, synthetic micas are also susceptible to modifications with a wide range of compounds, mainly organic compounds, which further increases their reactivity. Thus, this Review addresses the properties and syntheses of this clay. The types of modifications used and the changes that occur in the properties of this clay are reported, in addition to applications in the removal of contaminating compounds that can be found in water. Research to date has demonstrated that synthetic mica-based adsorbents are versatile in the adsorption of various contaminants, such as metals, pharmaceuticals and others. Therefore, this material can be promising in the adsorption and release of drugs, bioactive compounds, enzymes, proteins, etc., thus increasing its field of application. 1. Introduction The search for materials that can act in different fields of application, such as health, cosmetics, food, and even environmental remediation remains a constant demand both at the academic level and in industry. Among the materials that can be used are clays/clay minerals with and without prior modification due to their characteristics and structural properties (Komadel, 2016; França et al., 2020a, 2020b; de Oliveira et al., 2021, 2022, 2023). Among clay minerals, synthetic mica (Na-Mica-x) has gained prominence in recent years (Pazos et al., 2015, 2017; Martín et al., 2018, 2019; Pav´ on et al., 2019; Orta et al., 2020; Osuna et al., 2020, 2023; Osuna et al., 2021a, 2021b). Na-Mica-x is a synthetic fluoro-mica that has high purity, high cation exchange capacity (CEC), absence of secondary phases as can occurs in natural clays, and swelling capacity (Alba et al., 2006; Pav´ on et al., 2019; Osuna et al., 2023). Furthermore, due to its high interlaminar charge, with values of up to 2, 3 or 4 per unit cell, Na-Mica-x is called a highly charged synthetic mica. (Park et al., 2002a; Alba et al., 2006; Orta et al., 2019). It has a high cation exchange capacity, with values up to 468cmol(+)/Kg and a high expansion capacity. These properties make Na-Mica-x excellent candidates to interact with various organic compounds (Pazos et al., 2015, 2017; Alba et al., 2019; Osuna et al., 2020) or inorganic (Jos´ e García-Jim´ enez et al., 2016; Osuna et al., 2017; Osuna et al., 2019a, 2019b; Osuna et al., 2021a, 2021b). In this way, Na-Mica-x becomes a promising material to act in several areas such as cosmetics, in the immobilization of bioactive molecules, in the delivery of drugs, in addition to its use in the removal of pollutants, etc. Although systems containing high charge synthetic mica have attracted significant attention in recent years, to the best of our knowledge, no comprehensive review on the subject is available. Therefore, the main objective of this review is to compile data from published studies of micas of different Na-Mica-x loadings, their synthesis methods, characterization, and applications. A brief description of the differences in structure and properties between natural and synthetic micas is provided. Furthermore, types of modifications of synthetic micas with organic molecules are covered, such as surfactants, amino compounds, silane agents and polymers, as well as the applications of this clay. * Corresponding author. E-mail address: [email protected] (M.M. Orta). Contents lists available at ScienceDirect Applied Clay Science journal homepage: www.elsevier.com/locate/clay https://doi.org/10.1016/j.clay.2024.107325 Received 29 November 2023; Received in revised form 1 February 2024; Accepted 26 February 2024 Applied Clay Science 251 (2024) 107325 2 2. Structure and properties of mica (natural and synthetic) Mica belongs to a group of clays with a 2:1 structure, with a formula depending on the type of mica (Brigatti and Guggenheim, 2002). Mica is mostly of natural origin, which can be further classified as true and brittle micas, dioctahedral or trioctahedral (Brigatti and Guggenheim, 2002; Brigatti et al., 2013). The structure of natural micas, in general, consists of two sheets containing Si 4+ , Al 3+ , or Fe 3+ cations, coordinated tetrahedrally with oxygen atoms, joined by an octahedral sheet containing Mg 2+ , Al 3+ , Fe 2+ , or Fe 3+ cations, with OH groups in some vertices (Brigatti and Guggenheim, 2002; Brigatti et al., 2013), whose structure is shown in Fig. 1. However, highly charged synthetic micas, or fluorophlogopites, with the idealized formula Na n (Si 8-n Al n )(Mg 6 )O 20 F 4 , are trioctahedral micas that have only Si 4+ and Al 3+ cations in the tetrahedral sheets, Mg 2+ cations in the octahedral sites and groups F - at its edges of the octahedral sheets, replacing the hydroxyl present in natural micas (Osuna et al., 2019a, 2019b; Taruta et al., 2022), Fig. 1. Isomorphic substitutions provide a negative lamellar charge to these materials with values close to 1 for true micas and close to 2 for britlle micas (Brigatti et al., 2013). On the other hand, higher charges are found in synthetic micas, derived from isomorphic substitution at the tetrahedral site, that is, the replacement of Si 4+ by Al 3+ , which violates Lowenstein’s rule (Pav´ on et al., 2014a, 2014b; Pav´ on et al., 2019) and provides a layer charge of 2, 3 or 4 (Osuna et al., 2023), originating the nomenclatures Na-Mica-2, Na-Mica-3 or Na-Mica-4. In both natural and synthetic micas, the negative charge is counterbalanced by interlayer ions, typically K + and Na + for natural and synthetic mica, respectively (Brigatti and Guggenheim, 2002; Pav´ on et al., 2017), which can result in CEC synthetic micas of up to 468 cmol (+)/kg, and even high swelling capacity (Park et al., 2012; Pav´ on et al., 2017). 3. High charge mica synthesis Synthetic micas are materials that have a unique structure and high exchange capacity (Alba et al., 2006; Shimizu et al., 2006; Perdig´ on et al., 2013; Osuna et al., 2021a, 2021b). Mica synthesis methods are varied, from its first synthesis of Na-Mica-4 mica by to the reaction of augite with a mixture of NaF and MgF 2 in a 1:1 ratio, under heating at 900 ◦C (Gregorkiewitz and Rausell-Colom, 1987). The mica formed with the formula Na 4.0 (Mg 6.0 Ti 0.05 )[Fe 0.1 Al 3.4 Si 4.5 O 20.7 F 3.3 ] was obtained with low yield, with a mass of around 20 mg. Almost a decade later, in the mid-1990s, Na-Mica-4 mica was produced by the sol-gel process, by mixing Al(NO 3 ) 3 ⋅9H 2 O and Mg(NO 3 )⋅ 6H 2 O in ethanol and subsequent addition of tetraethylorthosilicate, Si (OC 2 H 5 ) 4 , in a proportion to obtain a stoichiometric composition of 3MgO-Al 2 O 3 -2SiO 2 (Paulus et al., 1992). NaF was the source of sodium and fluorine used, and the main limitations of this synthesis were, in addition to the long reaction times, the need for extensive washings with boric acid to remove insoluble salts from the final sample. In another study, the authors used this same sol-gel method in several stages for the synthesis of mica Na-Mica-4 so that they were successful in terms of the presence of impurities (Franklin and Lee, 1996). Subsequently, Na-Mica-4 was obtained from metakaolin, MgO and NaF (Kodama and Komarneni, 1999). For synthesis, MgO and metakaolin were initially mixed to a final stoichiometric composition of 3MgO-Al 2 O 3 -2SiO 2 . Then equal mass amounts of 3MgO-Al 2 O 3 -2SiO 2 and NaF were added in a platinum crucible and heated at 890 ◦C for 6, 12 and 24 h. The authors concluded that there was no distinction in the prolonged crystallization period, indicating that in the heating times used (6-24 h), it was possible to obtain Na-Mica-4. This same procedure was adopted by other studies in the production of Na-Mica-3 micas with an ideal composition of Na 2.94 (Mg 5.55 Ti 0.07 Fe 0.04 ) oct (Al 2.07 Si 5.83 ) tet O 20 F 4 -1.72H 2 O, Na-Mica-2 with an ideal composition of Na 2 Mg 6 Al 2 - Si 6 O 20 F 4 - xH 2 O and also Na-Mica-4 with an ideal composition of Na 4 Mg 6 Al 4 Si 4 O 20 F 4 - xH 2 O (Kodama et al., 2000, 2002, 2003, 2004; T. Kodama et al., 2001a, 2001b; Shimizu et al., 2004, 2006). However, this method employs excessive amounts of NaF and requires exhaustive washings with boric acid solution to remove non-reactive fluorides, which still provides for the possible replacement/exchange of Na + by H + ions of boric acid. In a new method, Park et al. (2002a) synthesized Na-Mica-4 with other precursors, replacing NaF with NaCl and MgF 2 , thus avoiding excess NaF, the so-called ´NaCl-melt method´. Additionally, fly ash was used (Park et al., 2002b), and the mass amounts of these components were mixed and added to a platinum crucible and heated to 890 ◦C for 24 h. The limitation of this method was that it was only aimed at the preparation of Na-Mica-4 micas, which was not used in the preparation of micas from other fillers such as Na-Mica-2 and Na-Mica-3. Na-Mica-x micas were obtained (Alba et al., 2006), adopting a Fig. 1. Mica structure and differences between natural and synthetic mica (Figure generated with the program VESTA 3-Free program (Momma and Izumi, 2011)). L.H. Oliveira et al. Applied Clay Science 251 (2024) 107325 3 procedure similar to that used by (Park et al., 2002a), from an almost stoichiometric powder mixture of SiO 2 , Al(OH) 3 , MgF 2 and NaCl with molar composition (8-n)SiO 2 , (n/2)Al(OH) 3 , 6MgF 2 and (2n)NaCl, where n represents the charge of mica (2, 3 or 4). In the synthesis, all powdered reagents are mixed, deagglomerated and subsequently heated in a platinum crucible at 900 ◦C. This method produces micas with excellent crystallinity and is the most widely used and investigated so far to obtain high-load micas (Alba et al., 2011a, 2011b, 2019; Pavon et al., 2013; Pav´ on et al., 2014a, 2014b; Naranjo et al., 2015; Osuna et al., 2017, 2018a, 2018b, 2020; Pav´ on et al., 2017; Martín et al., 2018, 2019; Orta et al., 2018; Osuna et al., 2019a, 2019b; Osuna et al., 2021a, 2021b; Martín-Rodríguez et al., 2019; Osuna et al., 2021a, 2021b). Alteration of the method also uses precursors such as bentonite (Osuna et al., 2018a) and rice ash (Mouchet et al., 2021). 4. Synthetic mica modifications One of the great advantages of using clay minerals is, in addition to their properties, the possibility of modifications, thus expanding the applications, increasing the reactivity and versatility of these materials (He et al., 2013; Ewis et al., 2022). Among these modifications, the incorporation of organic molecules with different functionalities, such as surfactants and amines, can be highlighted (Brito et al., 2018; Orta et al., 2019; França et al., 2020a, 2020b), silanes (He et al., 2013; Queiroga et al., 2019), polymers (da Silva et al., 2021; Ramos-Torres et al., 2021), etc. A summary of mica modifications is presented in Table 1. In the following sections, these modifications and the impact they have on these materials will be briefly described. 4.1. Interaction with surfactants Modification of clay and clay minerals with surfactants has been widely studied (Brito et al., 2018; Shen and Gao, 2019; Barakan and Aghazadeh, 2021; Z. Liu et al., 2021a). The interaction can occur by insertion of the protonated group into the interlamellar region of the clay mineral through an ion exchange reaction, that is, the exit of the interlamellar ion for the entry of the protonated group (Barakan and Aghazadeh, 2021; Z. Liu et al., 2021a). Depending on the amount of surfactant, ion pair intercalation may occur, usually when the value exceeds the CEC. This type of modification can often depend on the CEC of the clay and the pH of the reaction medium (Alba et al., 2011b; Martín et al., 2019; Z. Liu et al., 2021a), being able to cause changes to the clay mineral, by activating its surface due to the presence of nonpolar alkyl chains; expansion of the interlamellar space or even hydrophobization (Brito et al., 2018; Pesquera et al., 2018; Martín et al., 2019; Orta et al., 2019; França et al., 2020a, 2020b; Z. Liu et al., 2021a). In addition to montmorillonites (Brito et al., 2018; França et al., 2020a, 2020b) and vermiculites (Wang et al., 2018; Jin et al., 2022), high charge swelling synthetic micas are also modified with surfactants. Due to the high charge of synthetic micas, the basal spacing values obtained for organomica are generally higher than for organophilic montmorillonites (Orta et al., 2019). Hydrophobization has been described for ODA-modified Na-Mica-4 mica (Orta et al., 2019). Although Na-Mica-4 exhibited a contact angle of 33◦, the value for organomica was 110◦, characteristic of hydrophobic materials (Wang et al., 2020). Studies report the effects of chain length (Alba et al., 2011b), head group type (Komarneni et al., 2013) and amount of surfactants (Orta et al., 2019), beyond the load of synthetic mica lamella (Pazos et al., 2012; Komarneni et al., 2013) in organomica preparation. For example, Na-Mica-4 was modified with n-alkylammonium cations (RNH 3 + ) with different alkyl chain lengths (R =C 12 , C 14 , C 16 and C 18 ) (Alba et al., 2011b). For the synthesis, an amount of surfactant corresponding to 200% of the CEC of the clay mineral was added. The XRD results showed a high value of basal spacing, with a maximum of 4.60 nm for the sample as a surfactant with a C18 chain, indicating that the packaging of the surfactants occurred in the form of a paraffin-like bilayer with a structure in trans conformation, Fig. 2. Measurements 13 C and 29 Si NMR indicated the surfactant configuration. The charge effect of synthetic mica (Na n Si 8-n Al n Mg 6 F 4 O 20 ⋅XH 2 O, Table 1 Examples of modifications in Na-Mica-x micas. Sample Modifier compound Modification Characterization Ref Na-Mica-2 and Na-Mica- 4 MEA, EA, ODA ionic exchange XRD, TG/DTG, TSSA, Zeta potencial Osuna et al. (2019a, 2019b) MPTMS, MEA and BAL Silylation and ionic exchange ICP, XRD, FTIR, 13 C, 29 Si, 27 Al NMR and XPS Osuna et al. (2020) Chitosan Ionic exchange TG, XRD, FTIR, 29 Si, 23 Na MAS-NMR and Zeta potential. Alba et al. (2019) Na-Mica-3 and Na-Mica- 4 n-octylammonium and Fe 2 O 3 Ionic exchange and pillarization XRD, TEM and N 2 adsorption Shimizu et al. (2006) Na-Mica-2 and Na-Mica- 3 ODTMA and PEI Ionic exchange XRD Komarneni et al. (2013) Na-Mica-2, Na-Mica-3 and Na-Mica-4 ODTMA and dodecylammonium XRD, TG/DTA and 13 C, 29 Si and 27 Al NMR Pazos et al. (2012) Na-Mica-2 and Na-Mica- 3 Tetradecylammonium XRD, TG/DTA, FTIR and 13 C, 29 Si and 27 Al NMR Pazos et al. (2015) Na-Mica-2 Octylamine, MPTMS and polyhydroxy aluminium Ionic Exchange, pillarization and Silylation XRD and FTIR Pazos et al. (2020) Na-Mica-4 Octylamine Ionic exchange XRD, TG and Zeta potential. Martín et al. (2019) ODA, hexadecylamine, Tetradecylamine and dodecylamine XRD, TG, FTIR, 13 C, 29 Si NMR Alba et al. (2011b) Na-Mica-3 and Na-Mica- 4 XRD, TG and FTIR Pazos et al. (2017) Na-Mica-4 and Mt Octadecylamine and ODTMA XRD, TG, TSSA, particle size distribution, contact angle Orta et al. (2019) Na-Mica-4 ODA XRD, Zeta potential, TG, DSC, specific surface area and TEM Martín et al. (2018) Hexadecylamine and dimethylhexadecylamine XRD and TEM Aguado et al. (2021) Na-Mica-2 and Na-Mica- 4 EA and MEA XRD, TG, 13 C and 23 Na NMR Osuna et al. (2021a, 2021b) MPTMS: 3-mercaptopropyl trimethoxysilane; MEA: 2-mercaptoethylammonium; BAL 2,3-dimercapto-1-propanol; ODTMA: Octadecyltrimethylammonium; PEI: Polyethylenimine; XRD: X-ray diffraction; FTIR: Infrared spectra; TG: Thermogravimetric analysis; TEM: Transmission electron microscopy; NMR: Nuclear magnetic resonance spectroscopy; TSSA: The total specific surface area; DSC: differential scanning calorimetry; ODA: Octadecylamine; EA: ethylammonium; L.H. Oliveira et al. Applied Clay Science 251 (2024) 107325 4 Fig. 2. Representation of the configuration of surfactants of different alkyl chains in the interlayer space of synthetic Na-Mica-4. Fig. 3. Organomica XRD patterns for: (i)(a) Na-Mica-2, (b) Na-Mica-3, and (c) NaMica-4, (ii) (a) C12-Mica-2, (b) C12-Mica-3, and (c) C12-Mica-4, and (iii) (a) C18- Mica-2, (b) C18-Mica-3 and (d) C18-Mica-4. “Reprinted with permission from Pazos et al. (2012). Copyright 2023 American Chemical Society.” L.H. Oliveira et al. Applied Clay Science 251 (2024) 107325 5 where n =2, 3, and 4) on the intercalation of surfactants dodecylammonium and octadecylammonium was also monitored (Pazos et al., 2012). Organomicas were prepared using an amount of organic cations that corresponds to twice the amount of alkylammonium salt necessary to satisfy the total layer charge 2, 3, and 4 per unit cell of Na-Mica-2, Na- Mica-3 and Na-Mica-4, respectively. XRD results (Fig. 3) showed that the charge of the clay mineral lamella mainly influenced the samples modified with the surfactant with the lowest alkyl chain length, dodecylammonium, whose d001 values suggested the existence of paraffinlike arrangements in monolayer and bilayer. When the longest chain cation was used, only the paraffin-like bilayer arrangement was observed. Complete intercalation of octadecylammonium in Na-Mica-4 was also observed by TEM imaging (Fig. 4) (Martín et al., 2018). However, when the surfactant head group was changed, different results were observed. Octadecyltrimethylammonium (ODTMA) was used in a ratio equivalent to 2 CEC of each Na-Mica-x, for x =1, 2, 3 and 4. Despite the size of the organic cation chain, complete intercalation only occurred in the mica with the lowest charge, for x =1 and 2, demonstrating the surfactant relationship between the head group and the mica charge (Komarneni et al., 2013). For the other samples, the XRD results showed the presence of the original mica reflection (1.20 nm), which demonstrated that the intercalation was not complete. This effect was even more pronounced with decreasing proportion of surfactant used. Another study investigated the influence of the concentration of octadecyltrimethylammonium (ODTMA) or octadecylamine (ODA) on the modification of Na-Mica-4 (Orta et al., 2019). Montmorillonite (Mt) was also used and modified with the same compounds. Two compositions were evaluated in relation to the amount of ammonium salt, which corresponds to 50 and 100% of the CEC value of the clay mineral. Mica modified with ODTMA at 50% and 100% CEC showed basal distances of 3.5 and 3.9 nm, respectively, while for the samples modified with ODA the d001 values obtained were 4.7 and 5.0 nm in proportions corresponding to 50% and 100 % CEC, respectively. However, the XRD results (Fig. 5) also showed the presence of the original mica reflection (1.22 nm), mainly in the samples modified with ODTMA whose peak intensity was more intense due to incomplete intercalation. 4.2. Silylation The silylation reaction is based on the covalent anchoring of different species derived from the silyl group, mainly trimethylalkoxysilyl and triethylalkoxysilyl, in clay minerals, which can occur in the interlamellar region and/or in the edges that contain available and reactive hydroxyl groups (He et al., 2013; Bee et al., 2018). The covalent clay/ silylating agent interaction allows the immobilization and non-leaching of these organic compounds, thus producing more chemically and thermally stable organofunctionalized solids (He et al., 2013; Bee et al., 2018). The organosilanes most commonly used are bifunctional, chemically represented by (RO) 3 Si(CH 2 )nX, where (RO), in this case, refers to the alkoxy group, which hydrolyzes easily when dispersed in an aqueous medium (He et al., 2013; Su et al., 2013). The X component, present at the end of the molecule, represents the species of interest for each silylating agent. Therefore, depending on its reactivity, this fraction can be the key point for the desired application (Santos et al., 2016; Tao et al., 2016; Gonz´ alez et al., 2017). Silylation reactions have been carried out with several clay minerals, such as montmorillonite (Queiroga et al., 2019; Dumitru et al., 2022), vermiculite (do Nascimento et al., 2016; Santos et al., 2016), palygorskite (Liu et al., 2018; Pei et al., 2020), halloysite (Peixoto et al., 2016; Park et al., 2022), and also synthetic micas (Osuna et al., 2020). Synthetic fluorophlogopites with different charges per layer Na-Mica-x (x =2 and 4) reacted with 3-mercaptopropyl trimethoxysilane (MPTMS) by silanization reaction and with 2-mercaptoethylammonium (MEA) and 2,3-dimercapto-1-propanol (BAL), by ion exchange (Osuna et al., 2020). Mica silanization was carried out in protic (water, ethanol, methanol) and aprotic (toluene) solvents and under different pH conditions (1.0, 7.0 and 13). Measurements of ICP, XRD, FTIR, 13 C, 29 Si, 27 Al NMR and XPS indicated that the reaction was pH dependent. At pH 1.0, the amount of groups incorporated into Mica-2 and M-4 micas was 2.1 and 1.9 mmol S per gram of solid, respectively, suggesting that the Fig. 4. Transmission electron microscopy picture of C18-Mica-4. Reprinted from Martín et al. (2018). Copyright (2023) with permission from Elsevier. L.H. Oliveira et al. Applied Clay Science 251 (2024) 107325 6 M-2 matrix was more stable than Mica-4. The 29 Si RMN measures indicated the presence of T1 and T2 groups in silanized micas that were associated with the covalent immobilization of organic groups. Although the XRD data did not indicate structural changes in solids obtained at acidic pH, clay minerals in an acid medium can exchange interlayer ions for H 3 O + and still suffer attacks on their structure, by leaching cations from octahedral and tetrahedral sites (Osuna et al., 2020). 4.3. Interaction with polymers The interaction of polymers with clay minerals, mainly biopolymers, results in the formation of bionanocomposites that have a great impact in several fields, such as environmental, medical, etc. (Ruiz-Hitzky et al., 2013; Orta et al., 2020; de Lima et al., 2022). The economic and environmental value and the reactivity added to these biomaterials make them excellent candidates for several applications, such as contaminating species adsorption (Hu et al., 2017; Orta et al., 2020), drug release (Ruiz-Hitzky et al., 2013; de Lima et al., 2022), among others. One of the main biopolymers used to form bionanocomposites with clay minerals is chitosan, a polysaccharide derived from the deacetylation of chitin, found in the exoskeleton of crustaceans, insects, and fungi (Kurita, 2006). The entry / conjugation of chitosan into the clay mineral structure leads to an improvement in the adsorption capacity of the clay mineral, forming biodegradable and ecologically correct materials (Darder et al., 2003; França et al., 2020a, 2020b). The chitosan/clay mineral interaction is mainly due to electrostatic attraction between the positive charge Fig. 5. XRD patterns of (i) Mt. and (ii) Na-Mica-4 and the respective samples loaded with ODA/ODTMA. Insets: deconvolution of the 001 peak for indicated samples. Reprinted from Orta et al. (2019). Copyright (2023) with permission from Elsevier. Fig. 6. General scheme of the intercalation of chitosan with mica Na-Mica-x. L.H. Oliveira et al. Applied Clay Science 251 (2024) 107325 7 of the protonated amino groups and the negative charge of the clay, Fig. 6 (Darder et al., 2003; França et al., 2020a, 2020b). Futhermore, expandable clay minerals are also targets for this type of modification, including montmorillonite (Hu et al., 2017; França et al., 2020a, 2020b; de Lima et al., 2022). In the case of highly charged swelling micas, bionanocomposites based on chitosan and highly charged swelling synthetic fluorophlogopites, Na-Mica-2 and Na-Mica-4 were obtained at pH 4.9, a condition in which the chitosan amino groups (pKa 6.5) are predominantly protonated (Alba et al., 2019). The bionanocomposite was formed with different proportions of chitosan with 0.5 g of each mica. The maximum amount of chitosan in the bionanocomposite was 103 mg per gram of Na-M-4, the CEC of which was 468 cmol(+)/kg, respectively. The XRD measurements indicated that the interaction of mica with chitosan could not reach the total intercalated state since the increase in the basal spacing of the samples (0.17 nm) was lower than that of the chitosan film (0.38 nm), suggesting that the polymer was partially intercalated, and still incorporated into the surface and edges of the mica, also confirmed by FTIR, 29 Si and 23 Na MAS-NMR. 5. Application of high charge mica The use of adsorbents capable of effectively removing contaminants found in water is still one of the most effective strategies to separate pollutants (Ewis et al., 2022; Varsha et al., 2022). The choice is made for adsorbents with a high affinity for different contaminants, as is the case with high charge synthetic micas. In fact, micas with different Na-Mica-x charge are widely applied in the adsorption of organic and inorganic contaminants, as summarized in Fig. 7. In this review, emphasis will be placed on high charge synthetic mica for removal of a range of contaminants found in water, whose results are listed in Table 2. 5.1. Removal of perchlorate and non-ionic compounds One of the most persistent inorganic compounds that cause serious damage to human health is perchlorate (Ye et al., 2012). Found in wastewater, this compound, because it is similar in size to iodide ions, can be taken up in place of iodide ions by the mammalian thyroid gland and can disrupt thyroid hormone production and metabolism in the human body, causing significant effects mainly in pregnant women and fetuses (Ye et al., 2012; Komarneni et al., 2013). With this in mind, the adsorption of perchlorate in Na-M-x micas (x =1, 2, 3 and 4) modified with octadecyltrimethylammonium (ODTMA) and polyethyleneimine (PEI) was evaluated and compared with the performance of aluminium oxide-capped montmorillonites (Komarneni et al., 2013). The best adsorption capacities occurred for Na-Mica-2 and Na-Mica-3 modified with ODTMA, whose values were 0.436 and 0.269 mmol/g, respectively. The mechanism of interaction between perchlorate and ODTMA/Na- Mica-x was attributed to the exchange with chloride ions of the intercalated neutral surfactant during the cation exchange process, Fig. 8. Clays prepared with PEI and pillared clays showed little or no adsorption of perchlorate. Other wastewater contaminants derived from oil refineries are nonionic hydrocarbons such as benzene, toluene, and phenol. These Fig. 7. Types of contaminants studied in high charge mica removal. L.H. Oliveira et al. Applied Clay Science 251 (2024) 107325 8 compounds can also cause serious damage to health and the environment. Therefore, the removal of some nonionic hydrocarbons by Na- Mica-x (x =3 and 4) and modified surfactants of different alkyl chains (C 12 , C 14 , C 16 and C 18 ) was investigated. Pazos et al. (Pazos et al., 2017). The ability to remove contaminants depended on the length of the alkyl chain and the organization of the surfactants in the clay, whose values were 1.50, 0.75 and 0.10 mmol of toluene, benzene, and phenol per gram of C 18 -Mica-4. 5.2. Drugs removal Effluents from the pharmaceutical industry and increased use of drugs can contribute significantly to water pollution, causing serious toxicity problems for both humans and the environment (Bielen et al., 2017; França et al., 2022; Gonz´ alez-Gonz´ alez et al., 2022). Several classes of drugs, including antibiotics and anti-inflammatory agents, have been found in the aquatic environment, even at low Table 2 Systems based on high charge micas and modified high charge micas for contaminant removal. Sample Target compound Results Ref Na-Mica-2 and Na-Mica-4 Ra 2+ Na-Mica-4 adsorbed 95% of Ra 2+ . Komarneni et al. (2001) Na-Mica-2 and Na-Mica-3 modified with ODTMA and PEI perchlorate Improved adsorption capacity for ODTMA/Na-Mica- 2 and ODTMA/Na-Mica-3 with values of 0.269 a and 0.436 a meq/g, respectively. Komarneni et al. (2013) Na-Mica-2, Na-Mica-3 and Na-Mica-4 Hg 2+ Higher absorption capacity for Na-Mica-3 with 59% b of Hg 2+ compared to the theoretical CEC of the clay. Noh and Komarneni (2011) Sr 2+ and Ba 2+ Best results in 84 and 95% b removal of Sr 2+ and Ba 2+ for Na-Mica-2, respectively. Noh et al. (2013) Na-Mica-4 Mg 2+ , Ca 2+ , Sr 2+ and Ba 2+ – Kim et al. (2014) Na-Mica-2 modified with octylamine, MPTMS and polyhydroxy aluminium Zn 2+ and MCPA The maximum adsorption capacities were 11.7 b and 0.517 c mmol/g of Zn 2+ and MCPA. Pazos et al. (2020) Na-Mica-4 and Mt. modified with octamine Ibuprofen Rapid (60 min) adsorption of ibuprofen in modified Na-M-4 was concentration dependent and the removals were 99 and 68% c at concentrations of 0.1 and 80 mg/L, respectively. Martín et al. (2019) Na-Mica-4 and Mt. modified with ODA and ODTMA Pyrimethanil Organomodified mica with both organic compounds showed a removal of less than 10% c of the 80 mg/L concentration. Orta et al. (2019) Na-Mica-4 modified with ODA Methylparaben, propylparaben, perfluorobutanoic acid, perfluoroheptanoic acid, perfluorooctanoic acid, perfluorooctane sulfonic acid, nonylphenol, dodecyl sulfate, octadecyl sulfate, diclofenac, ibuprofen, salicylic acid, trimethoprim, carbamazepine, propranolol, caffeine, clofibric acid, and gemfibrozil 100% c removal capacity for perfluorooctane sulfonic acid, non-ylphenol, dodecyl sulfate, octadecyl sulfate, diclofenac, ibuprofen, salicylic acid, clofibric acid and gemfibrozil by organomodified micas. Martín et al. (2018) Na-Mica-4 and Na-Mica-4 modified with octadecylamine LAS The highest removal capacity (98% c ) was for the modified mica. Orta et al. (2018) Na-Mica-3 and Na-Mica-4 modified with octadecylamine, hexadecylamine, Tetradecylamine and dodecylamine benzene, toluene, and phenol Maximum adsorption capacity for C18-Mica-4 with values of 1500, 750 and 100 d mmol/Kg for toluene, benzene, and phenol, respectively. Pazos et al. (2017) Na-Mica-4 modified with hexadecylamine and dimethylhexadecylamine Cyclohexylamine and Eu 3+ The maximum amount absorbed of cyclohexylamine was 16% e . Aguado et al. (2021) Na-Mica-2 and Na-Mica-4 Eu 3+ – Martín- Rodríguez et al. (2022) Na-Mica-2 Martín- Rodríguez et al. (2017) (Martín- Rodríguez et al. (2019) Na-Mica-2 and Na-Mica-4 Maximum adsorption capacity of 84.8% f and 81.3% f for Na-Mica-4 and Na-Mica-2, respectively. Jos´ e García- Jim´ enez et al. (2016) Cs + Maximum adsorption capacity of 224.2 f meq/100 g for Na-Mica-2 at a temperature of 300 ◦C. Osuna et al. (2018b) Maximum adsorption capacity of 80% f for Na-Mica- 4. Osuna et al. (2018b) Pb 2+ , Hg 2+ and Cd 2+ Maximum adsorption followed the trend Hg 2+ > Pb 2+ >Cd 2+ . Osuna et al. (2019a, 2019b) Na-Mica-2 and Na-Mica-4 modified with EA and MEA Better removal capacities for Na-Mica-2 in the adsorption of Pb 2+ and Cd 2+ and MEA-Mica-2 for Hg 2+ , with values of 3500, 4000 and 5000 meq/Kg f , respectively. Osuna et al. (2021a, 2021b) Na-Mica-4 Sr 2+ Maximum holding capacity 43% f . Tatsuya Kodama et al. (2001) Na-Mica-4 and Na-Mica-2 modified with octadecylamine and dodecylamine I - Maximum adsorption for Na-Mica-2 modified with octadecylamine, with 30% f removal. Osuna et al. (2021a, 2021b) a: quantification by ion chromatograph; b: quantification by atomic absorption; c: quantification by HLPC; d: quantification by UV–Vis; e: quantification by TG; f: quantification by ICP-MS; ODTMA: Octadecyltrimethylammonium; ODA: Octadecylamine; PEI: Polyethylenimine; MCPA: 2-methyl-4-chlorophenoxyacetic acid; Mt.: montmorillonite; EA: ethylammonium; MEA: mercaptoethylammonium; LAS: linear alkylbenzene sulfonates. L.H. Oliveira et al. Applied Clay Science 251 (2024) 107325 9 concentrations (Bielen et al., 2017; Gonz´ alez-Gonz´ alez et al., 2022). For example, ibuprofen is a drug in the group of non-steroidal anti-inflam- matory drugs, which can be sold without a prescription in some countries and has been found in wastewater almost everywhere in the world (Ben Maamar et al., 2017; Ocampo-Perez et al., 2019; Patel et al., 2019). Ibuprofen, even at apparently low concentrations, can cause the death of aquatic species such as Oryzias latipes fish, which could not resist a concentration of 100 ng/L of ibuprofen (Han et al., 2010). Synthetic mica Na-Mica-4 and montmorillonite modified with octadecylamine were applied to remove ibuprofen (Martín et al., 2019). The study observed a dependence of drug concentration on the adsorption rate of modified Na-Mica-4, whose drug removals were 99 and 68% at a concentration of 0.1 and 80 mg/L, respectively, in 60 min. 5.3. Toxic metal removal Water pollution by toxic metals has been a major environmental concern, due to its high toxicity and low or no biodegradability, which can cause various types of diseases to live beings (Na Nagara et al., 2022). Different adsorbents are used to remove toxic metals from water (Na Nagara et al., 2022), clay minerals have been applied considering that they can adsorb metals by different mechanisms involving the formation of inner and outer complexes (Liu et al., 2021b). In recent years, high charge synthetic micas have gained prominence, as shown in Table 2. The removal of Hg 2+ by Na-Mica-x (x =2, 3 and 4), titanosilicate-4 (Na-ETS-4) and sodium titanosilicate was investigated as a function of the ionic strength of the medium (Noh and Komarneni, 2011). The adsorption capacity of Hg 2+ by mica was ~19%, 59% and ~ 30% for Na-Mica-2, Na-Mica-3 and Na-Mica-4, respectively, compared to the theoretical CEC of each clay mineral. The greater adsorption of Na-Mica-3 mica was associated with its layer charge density, which has an intermediate value in relation to other micas, which affects the availability and exchange sites with Hg 2+. The removal of Hg 2+ , Pb 2+ and Cd 2+ against Na-Mica-2 and Na- Mica-4 was evaluated at pH 2.5 and 6 (Osuna et al., 2019a, 2019b). Parameters such as the variation in metal concentration in relation to the CEC of micas, counter-ion and agitation were evaluated at the maximum removal capacity by cation exchange that occurred in the order Hg 2+ >Pb 2+ >Cd 2+ for Na-Mica-2, reaching values above the CPB of 254.18 cmol(+)/Kg. This fact was associated with a lower load on Na- Mica-2, which better provided access to the interaction sites with the entrance of metallic cations in the interlamellar region. Furthermore, the authors indicated that under the pH conditions of the medium (pH 2.5), the structure of mica may have leached, reducing some sites of interaction with the metals. The adsorption of Hg 2+ , Pb 2+ and Cd 2+ was also evaluated in Na- Mica-2 and Na-Mica-4 and modified with ethylammonium (EA: C 2 H 5 NH 3 + ) and mercaptoethylammonium (MEA: SHC 2 H 4 NH 3 + ) in a proportion of 200% of CEC of micas (Osuna et al., 2021a, 2021b). All adsorbents were efficient in removing metals, with better removal capacities for Na-Mica-2 in the adsorption of Pb 2+ and Cd 2+ and MEA- Mica-2 for Hg 2+ , whose values were 3.5, 4.0 and 5.0 mmol per gram of solid, respectively. The main mechanisms involved in the adsorption of Pb 2+ and Cd 2+ were ionic exchange and interaction with the thiol groups for Hg 2+ removal. Another metal studied is Zn 2+ because it can cause neurotoxicity in humans if ingested at concentrations greater than 40 mg/day (Palmer et al., 2019; Pazos et al., 2020). The elimination of Zn 2+ removal in Na- Mica-2 modified with octylamine, pillared with Al 2 O 3 and then silanized with MPTMS was monitored in the presence of 4-chloro-2-methylphe- noxyacetic acid (MCPA) (Pazos et al., 2020). The highest adsorption values obtained were 11.7 and 0.517 mmol/g for Zn 2+ and MCPA, respectively, indicating an excellent ability to remove these contaminants with heteromodified mica. The removal of contaminants derived from the nuclear industry by highly charged synthetic micas was also evaluated, which serve as a chemical barrier for the retention and storage of radioactive materials, such as Cs (Osuna et al., 2017). Cesium isotopes, such as 135 Cs and 137 Cs with t1/2 of 2.6 ×10 6 and 30 years, respectively, are one of the main constituents of wastewater effluents derived from nuclear reprocessing. Furthermore, because of their high solubility and long half-lifes, they are very hazardous radioactive waste (Castrillejo et al., 2016). Therefore, Osuna et al. (Osuna et al., 2018b), studied the immobilization of cesium in highly charged synthetic micas (Na-Mica-2 and Na-Mica-4), evaluating the influence of temperature (ambient and at 80 ◦C) and Cs + concentrations between 0.01 and 25 mmol L -1 for 0.2 g of clay on the adsorption capacity. Better performance was observed for Na-Mica-4 with a maximum adsorption of 80% at low concentrations of Cs + , which decreased with increasing initial concentration, even with increasing temperature. SEM and XRD results indicated the permanence of the morphology after adsorption and reflections compatible with a base spacing of 1.20–1.22 nm, indicating that Cs + formed a hydration complex in the mica interlayer space. 6. Conclusions Synthetic Na-Mica-x micas have interesting properties as adsorbents. Various methods are employed in the synthesis of this clay, with the intention of achieving a high yield and crystallinity. Studies have shown that synthetic mica can interact with several organic molecules, Fig. 8. Schematic of perchlorate uptake by organomicas Reprinted from Komarneni et al. (Komarneni et al., 2013), Copyright (2023) with permission from Elsevier. L.H. Oliveira et al.