Acid matrix production from treated natural vermiculite-hydrobiotite

N. P. Alexandre G. S. Oliveira C. R. F. Alves L. D. da Silva About the authors

Abstract

Given the potential of acid-activated clays in adsorptive and catalytic processes, several studies focusing on acid matrices have grown over the years, making it possible to study vermiculite, a mineral composed of a type 2:1 unit cell with tetrahedral and octahedral sheets. Therefore, this work aimed to study vermiculite treated with acids for the production of matrices. In this work, the treatment of natural vermiculite-hydrobiotite, with nitric acid in concentrations of 0.25, 0.50, 0.75, and 1 mol.L-1, followed by the dealumination process with sulfuric acid 25% v/v, aiming the production of acid matrices for applications as support for biocatalysts, chemical adsorbents, among others. The produced materials were characterized using different techniques, including X-ray fluorescence spectroscopy, X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis, and isotherm adsorption using the BET method. The results showed that after acid leaching the vermiculite showed a high SiO2 content, high Brønsted and Lewis surface acidity, good adsorption/desorption performance, and significant growth of the specific surface area, qualifying the vermiculite as a promising material for future applications in catalytic and biocatalytic processes. The acid concentration had a considerable influence on the compositional and textural modification of vermiculite-hydrobiotite.

Keywords:
vermiculite; nitric acid; leaching; matrices; catalytic

INTRODUCTION

The search for low-cost materials used in the catalysis and adsorption processes has been growing in recent years, stimulating the study of clay minerals for this purpose 11 O. Duman, S. Tunc, Sep. Sci. Technol. 43, 14 (2008) 3776.)-(44 S. Dultz, J.-H. An, B. Riebe, Appl. Clay Sci. 67 (2012) 125.. Clay minerals are abundant in nature and are susceptible to chemical changes in their interlayer structures, in which ions, atoms, or molecules are inserted, giving them countless applications 55 W. Wang, A. Wang (Eds.), “Nanomaterials from clay minerals”, Elsevier (2019) 415.. Some of the classes of clay minerals stand out for several characteristics, such as the high cation exchange capacity (CTC), expansiveness, and plasticity 66 L.N.F. de Queiroga, P.K. Soares, M.G. Fonseca, F.J.V.E. de Oliveira, Appl. Clay Sci. 126 (2016) 113.. Vermiculite-hydrobiotite is a mineral clay that has been intensively studied considering its applications in several areas, for instance, catalytic processes, agriculture, and adsorbent for gases and pollutants, e.g. metals and organic species, mainly due to its high cation exchange capacity (CTC) and expansion 77 F. Ali, L. Reinert, J.M. Lévêque, L. Duclaux, F. Muller, S. Saeed, S. Shah, Ultrason. Sonochem. 21, 3 (2014) 1002.)-(99 R. Motokawa, H. Endo, S. Yokoyama, H. Ogawa, T. Kobayashi, S. Suzuki, T. Yaita, Langmuir 30, 50 (2014) 15127.. They are phyllosilicates composed of a type 2:1 structure (T-O-T), which has continuous sheets of silicon or aluminum tetrahedra (T) and octahedral sheets (O) of aluminum (magnesium or iron), in addition to hydroxyls and cations 1010 J. Addison, Regul. Toxicol. Pharmacol. 21, 3 (1995) 405.)-(1212 S. Liu, P. Wu, M. Chen, L. Yu, C. Kang, N. Zhu, Z. Dang, Environ. Pollut. 228 (2017) 277.. The exchangeable cations present mainly in the interlayered space of the vermiculite-hydrobiotite structure, which in general are Na+, Mg2+, and Ca2+, counteract the negative surface charges resulted from the high degree of isomorphic substitution of Si4+ by Al3+ and/or Fe3+ in the tetrahedral and Mg2+ by Al3+, Fe2+, and Fe3+ in the octahedral layers 1313 A.M. Mathieson, G.F. Walker, J. Earth Planet. Mater. 39, 3-4 (1954) 231.), (1414 H. Vali, R. Hesse, Clay Miner. 27, 2 (1992) 192.. Despite having remarkable properties, some processes that alter its structure and/or composition such as calcination and acidification, have been carried out to obtain solids with high efficiency in catalytic and adsorptive processes 1515 L. Dzene, E. Ferrage, F. Hubert, A. Delville, E. Tertre, Appl. Clay Sci. 132 (2016) 215.), (1616 L. Ma, X. Su, Y. Xi, J. Wei, X. Liang, J. Zhu, H. He, Appl. Clay Sci. 183 (2019) 105332.. Treatment with mineral acids, called acid activation or leaching, considerably increases the adsorptive capacity and the catalytic activity of clays, mainly as a result of the increase in their specific surface areas and the increase in reactive acid sites 1717 J. Dan, X. Huang, P. Li, Y. Zhang, M. Zhu, X. Guo, F. Yu, Nanosci. Nanotechnol. Lett. 8, 11 (2016) 1028.), (1818 L. Wang, X. Wang, J. Yin, C. Wang, Appl. Clay Sci. 132 (2016) 17..

Acid leaching is carried out by mixing the clay with acid under controlled operating conditions that promote the replacement of exchangeable cations with hydroxonium cations (H3O+) and the leaching in the octahedral leaves of some elements such as Mg2+, Fe2+, Fe3+, and Al3+ (1919 M. Marosz, A. Kowalczyk, L. Chmielarz, Catal. Today 355 (2019) 466.), (2020 D.S. Moraes, E.M. Rodrigues, C.N. Lamarão, G.T. Marques, A.F. Rente, J. Hazard. Mater. 366 (2019) 34.. The composition of the octahedral leaves of vermiculite substantially affects its stability to acid attack; trioctahedral leaves (composed of Mg2+ or Fe2+) dissolve more quickly than the dioctahedral leaves (composed only of Al3+) 2121 F.J. del Rey-Perez-Caballero, G. Poncelet, Microporous Mesoporous Mater. 37, 3 (2000) 313.. The occurrence of iron in the mineral is also important, as it determines the surface area achieved after acid treatment. The final product is solid support with high SiO2 content, with a high specific surface area, and Brønsted and Lewis surface acidity 2222 M.M. Chambi-Peralta, A.C.V. Coelho, F.M. de Souza Carvalho, S.M. Toffoli, Appl. Clay Sci. 155 (2018) 2.. Acid-activated clays are generally used as adsorbents in vegetable, animal, wine, and beer industries, and are also used as catalysts in several organic reactions 2323 Y. Liu, Z. He, L. Zhou, Z. Hou, W. Eli, Catal. Commun. 42 (2013) 44.. In this study, the treatment of natural vermiculite with nitric acid in different treatments, as well as with sulfuric acid 25% v/v in the production of Brønsted and Lewis acid sites were investigated in order to prepare acid matrices for future applications as catalytic supports and chemical adsorbents.

MATERIALS AND METHODS

The vermiculite used was supplied by Policlay Nanotech located in the city of Fortaleza-CE, Brazil. The synthesis of the materials was carried out following two steps: i) treatment with nitric acid (HNO3, Vetec, 65%); and ii) dealumination with sulfuric acid (H2SO4, R.G., Synth, 98%) at 25% v.v-1. In short, 45 g of vermiculite were ground and sieved with a 100 μm mesh sieve, which was named natural vermiculite (VN0). In a round bottom flask, 10 g of vermiculite and 100 mL of HNO3 were added. The mixture was placed in a reflux system with constant stirring for 4 h at 95 °C, according to the methodology used by Rey-Perez-Caballero and Poncelet 2121 F.J. del Rey-Perez-Caballero, G. Poncelet, Microporous Mesoporous Mater. 37, 3 (2000) 313.. Then, the solid was washed with deionized water to a pH≥5 and then dried overnight in an oven at 120 °C. Acid activation was carried out at HNO3 concentrations of 0.25, 0.5, 0.75, and 1.0 mol.L-1 named respectively as VN1, VN2, VN3, and VN4. Then, the dealumination process was carried out, with a reaction mixture of 5 g of each activated vermiculite sample and 100 mL of 25% H2SO4 v.v-1 per gram of clay, which was subjected to agitation and heating at 80 °C under reflux for 2 h. Then, the precipitate obtained was washed with deionized water until constant pH and centrifuged at 4000 rpm for 5 min. The treated material was oven-dried at 120 °C for 12 h.

Pyridine adsorption assay: the study of Lewis and Brønsted-Lowry acidity of acid-treated and dealuminized vermiculite was carried out using pyridine as a probe molecule, through absorption spectra in the infrared region of samples in the form of pressed tablets at 10 ton. The tablets were initially left under vacuum, followed by exposure to reagent pyridine (C5H5N, ACS reagent, Sigma-Aldrich, ≥99%) at room temperature for 1 h. Then, the excess of the reagent was eliminated by submitting the system to vacuum again for another 1 h. The quantification of the acid groups of Brønsted (Py-B, 1545 cm-1) and Lewis (Py-L, 1440 cm-1) was determined according to 2121 F.J. del Rey-Perez-Caballero, G. Poncelet, Microporous Mesoporous Mater. 37, 3 (2000) 313.:

q B , L = π A . D 2 ( 4 w . ε B , L ) - 1

where A is the integrated band area (u.m.a) and εB,L the molar extinction coefficient (μmol.g-1).

Characterization: the structural and spectroscopic properties of natural vermiculite and vermiculite leached with nitric acid at concentrations of 0.25, 0.5, 0.75, and 1.0 mol.L-1 were characterized. X-ray diffraction (XRD): for XRD analysis, the samples obtained were analyzed using an X-ray powder diffractometer (X-Pert, Panalytical) with Bragg-Brentano geometry. The XRD patterns were collected in continuous mode with a scanning speed of 0.5 º.min-1 in the range of 5-80° (2θ). CoKα radiation was used, obtained with a tube operating at 40 kV and 40 mA. Before XRD measurements, powder samples were sieved through a 75 µm screen (200 mesh). X-ray fluorescence (XRF) spectroscopy: the chemical composition of the treated vermiculite was determined using the semi-quantitative XRF technique (ZSX Mini-II, Rigaku). Fourier transform infrared (FTIR) spectroscopy: FTIR spectra were obtained using a spectrometer (FT-IR Spectrum, Perkin-Elmer) using the KBr tablet technique, in a 2.5% mass ratio. The scan was in a wavelength range from 1300 to 1800 cm-1. Scanning electron microscopy (SEM): SEM images were obtained using a microscope (Quanta 450 FEG, FEI). The samples were prepared on double-sided carbon tape on aluminum supports, being covered with gold in an argon atmosphere under low pressure. Thermogravimetric analyzes (TG) were performed with a thermal analyzer (TGA-50H, Shimadzu) with a nitrogen flow of 50 cm3/min. The thermogravimetric curves were obtained in a nitrogen atmosphere using a heating rate of 10 °C.min-1 and a temperature range of 0-800 °C. The textural characterizations of the samples were obtained in an adsorptometer (ASAP 2020, Micromeritics). The samples were subjected to degassing at 300 °C for 24 h. From the N2 isotherms obtained at 77 K, the surface area was calculated according to the BET model and the pore diameter according to the BJH model.

RESULTS AND DISCUSSION

XRF: the results of the X-ray fluorescence analysis of the natural and leached hydrobiotite vermiculite, in different types of acid, are shown in Table I. The natural sample had a high content of Fe2O3 (25.11%). This result may be associated with the fact that vermiculite, in general, presents a great variation in its chemical composition. This variation occurs due to differences in its mineralization, the presence in soils under hot and humid climates, among other factors 2424 S. Aomine, N. Miyauchi, Soil Sci. Plant Nutr. 8, 5 (1962) 22.. The main constituent oxides of the clay are SiO2, Al2O3, MgO, and Fe2O32424 S. Aomine, N. Miyauchi, Soil Sci. Plant Nutr. 8, 5 (1962) 22.. Comparing the chemical composition of the natural vermiculite-hydrobiotite (VN0) sample and the samples leached with 0.25 to 1.0 mol.L-1 acid (VN1 to VN4), it was seen that as the acid concentration increased a decrease in contents of the oxides of Fe, Al, Mg, and K occurred concomitantly, as expected, with a progressive increase in the silica content, resulted from the replacement of these metals by hydronium ions (H3O+) in the interlayer space and in the octahedral leaves 33 W. Stawiński, O. Freitas, L. Chmielarz, A. Węgrzyn, K. Komędera, A. Błachowski, S. Figueiredo, Chemosphere 153 (2012) 115.), (2525 S.S.G. Santos, H.R.M. Silva, A.G. de Souza, A.P.M. Alves, E.C. da Silva Filho, M.G. Fonseca, Appl. Clay Sci. 104 (2015) 287.), (2626 J. Feng, M. Liu, L. Fu, S. Ma, J. Yang, W. Mo, X. Su, Ceram. Int. 46, 5 (2020) 6413.. A significant increase in the silica content was observed after treatment with 1.0 mol.L-1 HNO3 (SiO2/Al2O3 ratio=11.23), due to the greater dissolution of these elements, while the tetrahedral leaves form amorphous silica gel, insoluble in acid solution 2727 M. Valášková, J. Kupková, G.S. Martynková, J. Seidlerová, V. Tomášek, M. Ritz, D. Rafaja, Appl. Clay Sci. 151 (2018) 164.. The acid attack resulted in the regeneration of ion-exchange properties, where it was possible to observe a decrease in the K2O content. This fact may occur due to the exchange of cations present in the structure for H3O+. The dissolution of metals in the vermiculite structure after acid treatment promoted the removal of Al3+ cations from the tetrahedral leaves, generating acidity in the clays. The number of substitutions by Mg or Fe in the octahedral sheets of clay mineral promoted the dissolution of these layers and the formation of a silica phase 33 W. Stawiński, O. Freitas, L. Chmielarz, A. Węgrzyn, K. Komędera, A. Błachowski, S. Figueiredo, Chemosphere 153 (2012) 115.. In addition, the iron composition in the interlayered charges in the hydrobiotite vermiculite under study may be present according to the oxidation of Fe2+ to Fe3+. Such oxidation would increase the positive octahedral charge by an amount equivalent to the amount of oxidized bivalent iron and result in an equivalent decrease in the negative layer unit and positive interlayer charges 2525 S.S.G. Santos, H.R.M. Silva, A.G. de Souza, A.P.M. Alves, E.C. da Silva Filho, M.G. Fonseca, Appl. Clay Sci. 104 (2015) 287..

Table I
Results of XRF analysis of natural vermiculite-hydrobiotite (VN0) and samples leached with HNO3 at 0.25 mol.L-1 (VN1), 0.50 mol.L-1 (VN2), 0.75 mol.L-1 (VN3), and 1.0 mol.L-1 (VN4).

XRD: the X-ray diffraction patterns of natural vermiculite and vermiculite leached with nitric acid in different concentrations are shown in Fig. 1. The XRD pattern of natural vermiculite (VN0) showed the presence of a crystalline material with the main peak at 2θ=7.125° [plane (002)] referring to the mineral vermiculite and the second peak of lesser intensity in the position 2θ=8.197° related to the interstratified hydrobiotite phase 2525 S.S.G. Santos, H.R.M. Silva, A.G. de Souza, A.P.M. Alves, E.C. da Silva Filho, M.G. Fonseca, Appl. Clay Sci. 104 (2015) 287.), (2828 V. Matějka, M. Šupová, V. Klemm, D. Rafaja, M. Valášková, J. Tokarský, E. Plevová, Microporous Mesoporous Mater. 129, 1-2 (2010) 125.. The vermiculite from Paraíba-Brazil is composed of interstratified mixed layers of vermiculite with other mineralogical phases such as talc, hydrobiotite, biotite, and phlogopite. The XRD patterns shown in Fig. 1 show a continuous decrease in the crystallinity of the reflected natural sample and a progressive decrease in the intensity of the main reflection of the vermiculite and the interstratified hydrobiotite phase. Despite the aforementioned data, changes only in the vermiculite peaks were taken into account, given the low amount of hydrobiotite (19% in mass) and the limitation of the Rietveld method to identify changes in the interstratified clays. At the end of the treatment, there was a material that presented a delaminated and partially amorphized structure, since the main reflection peaks of the main constituents of vermiculite-hydrobiotite still appeared in the patterns, however, as already shown in Table I, with high silica content in the amorphous state. Furthermore, a small displacement of the peaks is observed after acid treatment with high concentrations of HNO3, as in the sample VN3. This occurred due to morphological changes in the crystalline structure of vermiculite-hydrobiotite, in the replacement of intercalating cations by hydronium ions (H3O+), causing the displacement of the crystalline planes 1616 L. Ma, X. Su, Y. Xi, J. Wei, X. Liang, J. Zhu, H. He, Appl. Clay Sci. 183 (2019) 105332.), (2929 R.A. Rojas-Ramírez, M.H. Maciel, R.C. de Oliveira Romano, R.G. Pileggi, A.C.V. Coelho, Appl. Clay Sci. 170 (2019) 97..

Figure 1:
X-ray diffractogram of natural vermiculite-hydrobiotite (VN0) and samples leached with HNO3 (VN1-VN4).

FTIR: the analysis of the acidic properties of the materials produced was performed through absorption spectroscopy in the infrared region using pyridine as a molecule that quantifies the acidity characterizing it as Brønsted and Lewis. To distinguish Brønsted and Lewis acidity, the positions of the vibrational modes of pyridine were observed in a short order of reach 3030 J. Ravichandran, C.M. Lakshmanan, B. Sivasankar, React. Kinet. Catal. Lett. 59, 2 (1996) 301.. The acidic surface observed in the vermiculite after acidic treatment is directly related to the presence of SiOH or AlOH groups in its composition. This acidic character refers to the Brønsted sites associated with proton donation in the dehydroxylation of the lamellae and to the Lewis sites, electron pair receptors, which can be attributed to metal oxides 3131 M.E. Gyftopoulou, M. Millan, A.V. Bridgwater, D. Dugwell, R. Kandiyoti, J.A. Hriljac, Appl. Catal. A Gen. 282, 1-2 (2005) 205.. FTIR analysis of natural vermiculite did not show bands referring to Lewis acids and Brønsted acids. In the FTIR spectra of the samples after leaching with HNO3 (Fig. 2), new bands appeared corresponding to the interaction of pyridine with the Lewis acid sites (Py-L) at 1445 cm-1 and the vibrational mode observed at 1490 cm-1 (Py-B,L) concerning the interaction of pyridine with the two acid sites of Brønsted and Lewis. However, the band relative to the interaction of pyridine with Brønsted acid sites (Py-B) was not evidenced. Due to the association of Lewis acidity with the formation of catalysts, they are emphasized in the intensities related to that site 3232 S.I. Okada, K. Tanaka, Y. Nakadaira, N. Nakagawa, Bull. Chem. Soc. Jpn. 65, 10 (1992) 2833.. The treatment with acid allows the opening of the leaves, consequently, the increase in the exposure of the acidic sites and the decompensation of the charge generated in the isomorphic substitution, which facilitates the deprotection of positive charges allowing the acceptance of electron pairs (Lewis acidity), potentiating Lewis acidity in vermiculite 3333 J. Amaya, L. Bobadilla, L. Azancot, M. Centeno, S. Moreno, R. Molina, J. Mater. Sci. 55, 15 (2020) 6482.. Table II shows the Lewis acidity values for samples leached with acid (HNO3) at different concentrations.

Figure 2:
FTIR spectra of the pyridine absorbed from the natural vermiculite-hydrobiotite (VN0) and samples leached with HNO3 (VN1-VN4).

Table II
Lewis acidity data (qL, µmol.g-1) from vermiculite-hydrobiotite samples leached with HNO3 at 0.25 mol.L-1 (VN1), 0.50 mol.L-1 (VN2), 0.75 mol.L-1 (VN3) and 1.0 mol.L-1 (VN4).

SEM: SEM images of the natural vermiculite-hydrobiotite and after leaching are shown in Fig. 3. In the morphology referring to the natural vermiculite (VN0), plaques referring to the silicate layers were observed 3434 H. Shoukry, M.F. Kotkata, S.A. Abo-El-Enein, M.S. Morsy, S.S. Shebl, Constr. Build. Mater. 112 (2016) 276.. After leaching with HNO3 from a concentration of 0.25 mol.L-1 (VN1) to 0.50 mol.L-1 (VN2), a characteristic vermiculite-hydrobiotite structure was still observed, however, there was a relative decrease in the size of the attributed particles with the increase in surface area. In the samples treated with higher acid concentrations, VN3 and VN4, the layers were irregularly shaped and the particles were agglomerated, showing structural modification by the vermiculite amorphization process.

Figure 3:
SEM micrographs of natural vermiculite-hydrobiotite (VN0) and samples leached with HNO3 (VN1-VN4).

TG: TG and DTG (derivative thermogravimetry) curves for the leached vermiculite samples showed two distinct events (Fig. 4). The 1st peak, the most accentuated, occurred between 20 and 100 °C and referred to the elimination of surface water and water for hydration of cations in the interlayer space. The 2nd event occurred in the range between 500 and 550 °C, which can be attributed to structural water coordinated with the cations present in the interlayer space and also to the clay dehydroxylation process 3535 J. Kupková, D. Honěk, S. Vallová, M. Valášková, Acta Geodyn. Geomater. 12, 3 (2015) 299.. The increase in water content is proportional to the acid concentration, considering that the surface must become more hydrophilic with the presence of more silanol groups, even occurring when hydrated cations leave the interlamellar space and H3O+ cations enter. This fact was observed in the first stage of the TG curves where there was a variation in temperature.

Figure 4:
TG and DTG curves of vermiculite-hydrobiotite samples leached with HNO3 (VN1-VN4). Atmosphere: N2; flow rate: 100 mL.min-1; heating rate: 10 °C.min-1.

Adsorption/desorption isotherms: the adsorption-desorption isotherms of natural and leached vermiculite-hydrobiotite are shown in Fig. 5. Natural vermiculite had a type II isotherm, characteristic of non-porous or macroporous materials, with a partial pressure of P/P0 value of around 0.45, classified as H3 type hysteresis, showing the presence of mesopores 3636 S.J. Gregg, K.S.W. Sing, Adsorption, surface area and porosity, 2nd ed., Academic Press, London (1982).. In the leached samples, changes in the curves were observed as the acid concentration increased, obtaining a profile referring to the type I isotherm attributed to microporous solids 2525 S.S.G. Santos, H.R.M. Silva, A.G. de Souza, A.P.M. Alves, E.C. da Silva Filho, M.G. Fonseca, Appl. Clay Sci. 104 (2015) 287.. An increase in the concentration of acid resulted in a decrease in hysteresis. This effect may be related to the lower amount of mesopores. The textural properties were determined from the isotherms, which showed significant increases of the specific surface area and the volume of pores of the leached samples, changing, respectively, from 7.8 m2.g-1 and 0.029 cm3.g-1 in natural vermiculite to 131 m2.g-1 and 0.109 cm3.g-1 in the activated vermiculites. These data indicated the efficiency of acid treatment for the production of materials with greater adsorptive power (Table III). The increase in the specific surface area showed that the delamination process occurred, which contributed to generate access to acidic sites, and, at the same time, contributed to the adsorption of molecules 3434 H. Shoukry, M.F. Kotkata, S.A. Abo-El-Enein, M.S. Morsy, S.S. Shebl, Constr. Build. Mater. 112 (2016) 276..

Figure 5:
N2 adsorption/desorption isotherms at 77 K of natural vermiculite-hydrobiotite (VN0) and samples leached with HNO3 (VN1-VN4).

Table III
Textural properties obtained by adsorption/desorption of N2 from natural vermiculite-hydrobiotite (VN0) and samples leached with HNO3 at 0.25 mol.L-1 (VN1), 0.50 mol.L-1 (VN2), 0.75 mol.L-1 (VN3), and 1.0 mol.L-1 (VN4).

CONCLUSIONS

In this study, treatment with nitric acid at different concentrations markedly modified the composition, texture, and structure of vermiculite, resulting in the formation of promising material for future applications in catalysis and adsorption. In this sense, acid treatment of vermiculite-hydrobiotite caused leaching of cations from octahedral leaves, increasing the surface area compared to natural vermiculite. Considerations related to the active sites on the FTIR spectrum indicated greater efficiency in the production of Lewis acid sites when treated with acid (HNO3) at 0.75 mol.L-1. However, the X-ray diffractogram showed structural changes and less crystallinity. Thus, leaching was an effective method in the process of chemical modification of vermiculite-hydrobiotite for adsorption purposes, although specific concentrations of acid are necessary, since, as noted, high concentrations can cause structural destruction of the material.

ACKNOWLEDGMENTS

The authors would like to thank the Laboratory for Research, Development and Innovation in Non-Metallic Minerals, the National Council for Scientific and Technological Development - CNPq for the financial support (N.P. Alexandre), and the Institutional Program for Initiation in Technological Development and Innovation - PIBITI for the scholarship provided. Thanks to Polyclay Nanotech Ltd. for supplying the vermiculite sample.

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Publication Dates

  • Publication in this collection
    17 May 2021
  • Date of issue
    Apr-Jun 2021

History

  • Received
    09 July 2020
  • Reviewed
    27 Aug 2020
  • Reviewed
    25 Oct 2020
  • Accepted
    06 Dec 2020
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