Applied Clay Science 163 (2018) 204–213

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Applied Clay Science

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Research paper Alteration of fine fraction of bentonite from Kopernica (Slovakia) under acid treatment: A combined XRD, FTIR, MAS NMR and AES study T

Martin Pentráka,d, Viktor Hronskýb, Helena Pálkováa, Peter Uhlíkc, Peter Komadela, ⁎ Jana Madejováa, a Institute of Inorganic Chemistry, Slovak Academy of Sciences, SK-845 36 Bratislava, Slovakia b Department of Physics, Faculty of Electrical Engineering and Informatics, Technical University of Košice, Park Komenského 2, SK-040 01 Košice, Slovakia c Department of Geology of Mineral Deposits, Comenius University, Mlynská dolina, SK-842 15 Bratislava, Slovakia d Illinois State Geological Survey, Prairie Research Institute, University of Illinois, 615 E Peabody Dr., Champaign, IL 61820-6918, USA

ARTICLE INFO ABSTRACT

Keywords: Acid activation of clay minerals is one of the most effective methods proposed to produce materials with in- Bentonite Kopernica creased acidity and adsorption properties. In this work fine fraction of bentonite from deposit Kopernica Montmorillonite (Slovakia) was treated with 6 mol/L HCl. Variety of methods were employed in order to investigate the structural Acid treatment changes occurring as a result of the acid activation. Quantitative XRD analysis and FTIR revealed a dominant MAS NMR spectroscopy, IR spectroscopy presence of montmorillonite, with K-feldspars, quartz, volcanic glass, opal C, ferrihydrite, biotite and kaolinite Specific surface area admixtures in acid untreated sample. 29Si MAS NMR spectra also confirmed accessory siliceous phases. 27Al MAS Acid sites NMR showed that approximately 10% of Al was bound in the tetrahedral sites of montmorillonite. Gradual decrease of the Al, Fe and Mg content, an upward shift of the complex SieO stretching band and the appearance of Q4(0Al), Q3(1OH) signals in the 29Si MAS NMR spectra of the extensively reacted materials indicated a substantial modification of the montmorillonite layers and formation of an amorphous silica product. The ac- cessory minerals quartz, K-feldspars and kaolinite were dissolved more slowly. The presence of kaolinite and quartz/K-feldspars in solid product was confirmed even after treatment for 36 h. The acid treatment affected also the surface and acidity characteristics of the samples. The total specific surface area (EGME SSA) gradually decreased with decreasing montmorillonite content in the reaction product. In contrast, the changes in the

N2BET SSA showed that reaction with an acid first increased it to a maximum after treatment for 6 h, but more extensive treatments had a negative effect. After pyridine adsorption the IR spectra of the samples treated for 1 up to 6 h showed, in addition to the bands related to the H-bonded and physisorbed pyridine, also the diagnostic − band of pyridinium cation near 1540 cm 1 confirming the presence of strong Brønsted acid sites.

1. Introduction or after chemical handling improving their properties for specific ap- plications. One of the most common modifications of Bent is acid ac- Economically important bentonite (Bent) deposits are located in the tivation. Process includes the dissolution of acid-soluble minerals (e.g. Central Slovakia on the south-western margin of the Kremnica calcite, dolomite, goethite) and organic matter, frequently present in Mountains. In addition to the largest and most important Bent deposit association with Bent and partial modification of the montmorillonite Jelšový Potok, deposit Kopernica (Kop) also occurs in this area. The (Mt), the main constituent of Bent. Upon acid treatment the protons mineralogical composition of Bent (Kop) has been investigated in the replace the naturally occurring exchangeable cations of Mt, penetrate past (e.g. Kraus et al., 1982, 1994; Stríček et al., 2006; Adamcová et al., into the Mt layers and attack the structural OH groups. Resulting de- 2015), however, detailed petrographic, mineralogical and physico- hydroxylation is connected with fractional release of the central atoms chemical characterization of Bent (Kop) was reported only recently by from the octahedral site as well as with removal of Al from the tetra- Górniak et al. (2016). hedral sheets. At the same time a gradual transformation of the tetra- Bent is employed in many industrial areas either in their raw form hedral sheets to a three-dimensional silica framework proceeds (Tkáč

⁎ Corresponding author. E-mail addresses: [email protected] (M. Pentrák), [email protected] (V. Hronský), [email protected] (H. Pálková), [email protected] (P. Uhlík), [email protected] (P. Komadel), [email protected] (J. Madejová). https://doi.org/10.1016/j.clay.2018.07.028 Received 17 April 2018; Received in revised form 13 July 2018; Accepted 18 July 2018 Available online 25 July 2018 0169-1317/ © 2018 Published by Elsevier B.V. M. Pentrák et al. Applied Clay Science 163 (2018) 204–213 et al., 1994). Acid-activated Bent are materials with enhanced surface 2. Materials and methods area, porosity and acidity (e.g. Vicente Rodríguez et al., 1995; Breen et al., 1997; Kooli and Jones, 1997; Jozefaciuk and Matyka-Sarzynska, The chemical used in this study were of purity grade and were used 2006; Loh et al., 2017). Such materials are effective adsorbents (e.g. as received. They were purchased from: Slavus Ltd. Slovakia (sodium Kooli et al., 2015; Salem et al., 2015; Zhu et al., 2016), catalysts or chloride, hydrochloric acid), Merck USA (ethylene glycol monoethyl catalyst supports (e.g. Boudjema et al., 2015; Gomez-Sanz et al., 2017; ether), Sigma-Aldrich USA (pyridine for synthesis, triethylenetetramine Huskic and Zagar, 2017; Timofeeva et al., 2017). The final reaction and Cu(II) sulphate free of water for the preparation of the Cu(II) product of the dissolution of Mt in an inorganic acid is an amorphous triethylenetetramine complex). partly protonated silica phase (e.g. Tkáč et al., 1994; Kooli and Jones, 150 g of raw Bent (Kop) was milled and dispersed in 8 dm3 of dis- 1997; Gates et al., 2002; Steudel et al., 2009a; Komadel and Madejová, tilled water and saturated three times with 1 dm3 of 1 mol/L NaCl so- 2013). lution. The fine fraction was separated after 24 h, washed free of The alteration of clay minerals upon acid treatment is obviously chlorides, dried at 60 °C and passed through a 0.63 mm sieve. The Na- studied by the X-ray diffraction, infrared and MAS NMR spectroscopies saturated < 2 μm fraction was used for the modification with HCl so- (Tkáč et al., 1994; Breen et al., 1995, 1997; Komadel et al., 1996b; lution to obtain a series of acid treated materials. Vicente Rodríguez et al., 1995; Madejová et al., 1998; Bisio et al., 2008; Tomić et al., 2012; Alver et al., 2016; Pálková et al., 2017). These 2.1. Acid treatment traditional techniques are frequently supplemented with other ad- vanced methods e.g. in-situ synchrotron powder X-ray diffraction ana- Dissolution was performed at 95 °C for 1, 2, 3, 4, 5, 6, 8, 12, 18, 24 lysis (Scarlett et al., 2011), atomic force microscopy (Bickmore et al., and 36 h. For each selected time, 2.5 g of Na-saturated fine fraction of 2001), transmission microscopy, or electron spin resonance spectro- Bent (Kop) was added to 0.250 L of 6 mol/L HCl. The mixtures were scopy (Mudrinić et al., 2015). The IR and NMR spectroscopy are par- heated in 0.5 L glass flasks with laps under reflux and stirred every ticularly well suited to follow the structural changes of an initial clay hour. The filtrate and wash supernatant solutions were combined and mineral and development of amorphous product via absorption bands analysed for Al, Mg, and Fe by atomic emission spectroscopy, yielding assigned to the SieO bonds and structural OH groups in IR spectra and the amount of the element dissolved. Dissolution curves were con- via signals of Si and Al in MAS NMR spectra. Moreover, both methods structed by plotting the un-dissolved fraction of the respective cation (1 provide direct evidence for the presence of silanol groups (Tkáč et al., – α) versus time (Komadel et al., 1996b). The initial, acid-untreated 1994; Breen et al., 1997; He et al., 2002; Pálková et al., 2013) which is sample was designated as Kop-0h while acid treated samples were la- not so obvious using other techniques, such as XRD analysis, surface belled as Kop-1h – Kop-36h later in the text. area measurement methods and elemental analysis methods. Silanol groups proved to be influential as adsorption sites in the vapour uptake 2.2. Powder X-ray diffraction of e.g. water, pyridine and pivalonitrile (Pentrák et al., 2012a; Madejová et al., 2015; Flessner et al., 2001). XRD profiles of pressed powder samples were collected on a D8 The extent of clay mineral decomposition in acid defines properties Advance diffractometer (Bruker Corporation, Germany) equipped with of a final product. It is affected by its chemical composition, swellability CuKα (λα1 = 1.54060 Å) radiation and a Bruker LynxEye detector. The of the interlayers and particle size, but also by temperature, time, type records were collected in the 2-theta range from 2.0° to 65°, using a step and concentration of the acid used, clay mineral/acid ratio (Novák and of 0.019° 2θ and counting time of 3 s per step. Quantitative XRD ana- Číčel, 1978; Komadel et al., 1996a; Christidis et al., 1997; Madejová lysis of the initial sample Kop-0h was performed in order to determine et al., 1998; Pálková et al., 2003; Metz et al., 2005; Babaki et al., 2008; its complete mineralogical composition. To prepare the sample for the

Steudel et al., 2009a, 2009b; Rožić et al., 2010; Komadel, 2016; analysis, 0.250 g Al2O3 (internal standard) was added to 1.000 g of the Krupskaya et al., 2017). Because the mineralogical composition of a Kop-0h. The mixture was ground in a McCrone mill for 5 min with 4 mL Bent as well as chemical composition of its main component – Mt differ of methanol then dried and sieved. The parameters obtained from XRD significantly in dependence on the geological origin of Bent deposit, measurement were evaluated with RockJock program (Eberl, 2003). thorough characterization of the each ore is needed to predict correctly The program fits the sum of stored XRD patterns of pure standard mi- its optimal industrial utilization. nerals (the calculated pattern) to the measured pattern by varying the Three samples of lithologically different commercial Bent (Kop) fraction of each mineral standard pattern, using the Solver function in were systematically described by Górniak et al. (2016). The major Microsoft Excel to minimize the degree of fit parameter between the component of the Bent (Kop) was Mt, the abundance of which de- calculated and measured pattern. pended on the location of the mining site. Its structural formula in- dicates that, in addition to Al present in the octahedral sheet, ~ 2.5% of 2.3. Cation exchange capacity (CEC) tetrahedral sites are also occupied by aluminium. Besides Mt, opaline silica polymorphs (opal-C/CT), biotite, potassium feldspar, plagioclase, Determination of CEC using the Cu(II) triethylenetetramine method quartz, clinoptilolite, and kaolinite admixtures were found in Bent (Meier and Kahr, 1999) was performed. Approximately 0.1 g of sample (Kop). Grain-size distribution, textural properties and microscope ob- dried overnight at 55 °C was mixed with 35 mL of distilled water and servations of grain-size fractions revealed that the Bent (Kop) contains dispersed by ultrasonic treatment for 5 min, then mixed with 10 mL of Mt-opal aggregates difficult to disperse in water. 0.01 mol/L Cu(II)–triethylenetetramine complex and 15 mL of distilled The purpose of the present study was to extend the investigation of water. Afterwards dispersion was passed through a 0.2 μm Millipore Górniak et al. (2016) and to analyse the changes in chemistry of a fine filter to separate the liquid from the solid components. The absorbance fraction of Bent (Kop) upon reaction with HCl. The effect of acid of the filtrate solution was measured at 578 nm in triplicate with a treatment on the composition and properties of the solid reaction pro- UV–VIS spectrophotometer Varian CARY 100 CONC (Varian Inc., USA). ducts was investigated using an array of experimental techniques. De- The mean CEC values were then calculated on the basis of the dry clay tailed analyses of the 27Al, 29Si MAS NMR and FTIR spectra were per- mineral mass (Pentrák et al., 2012b). formed and NMR measurements with polarization transfer were used to provide insight into the local environment of Si atoms. Moreover, 2.4. Adsorption of pyridine vapour suitability of the IR spectroscopy with ATR (Attenuated Total Re- flectance) accessory to assess the presence of acid sites via pyridine For pyridine adsorption experiments Kop-0h and samples treated for adsorption was verified. 1, 2, 4, 6 h were dried overnight at 105 °C and then placed in a

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