Structural-Phase Transformations in Bentonite After Acid Treatment

The methods of X-ray diffraction, Fourier transform infra-red spectroscopy (FTIR), X-ray microanalysis, electron microscopy, BET and cation exchange capacity (CEC) were used for investigation of the structural-phase transformations in bentonite under the influence of hydrochloric acid and temperature treatment (100-800oC). It is established that in HCl medium during temperature treatment, dehydration and dehydroxilation of montmorillonite occur. The presence of gypsum and barium chloride results in an intercalation of interlayer space of montmorillonite by Ca and Ba ions. Temperature treatment of intercalated montmorillonite leads to the formation of pores.


Introduction
Clays and ceramics based on them are applied in various areas of engineering.It is known that depending on conditions of temperature treatment of clays it is possible to obtain ceramics with different properties [1].At the same time, after special treatment-modification clays are a good adsorbent material [2][3].Bentonite clays have the best adsorption properties, into which composition enters montmorillonite.Montmorillonite is capable of essentially increasing initial volume on account of water adsorption in the interlayer sites [4].
Тhe purposes of the given work are: 1) overlapping within the framework of an uniform physico-chemical process of activation and saturation of montmorillonite by cations of calcium and barium and research of structural-phase reorganization in montmorillonite; 2) research of the influence of temperature treatment on activated montmorillonite from the positions of stabilization of the nano-porous structure.It is supposed that such material can serve as a precursor for obtaining porous silicate ceramics.

Experimental Procedure
Bentonite containing gypsum (Mexico) was chosen for this research.All the other reagents were of analytical grades from the firm Baker.The conditions for preparation of samples were as follows: 20 ml of 50 % water solution of HCl was added to 20 g of bentonite, and then dried at 25ºC during 24 h.For preparation of HCl + BaCl 2 mixture 10 g BaCl 2 and 30 ml of water were added to the first composition.The drying regime was the same.Introduction of a solution HCl or HCl + BaCl 2 in bentonite is marked by allocation of gases.
For establishment of the composition of components which are present in a liquid phase additional experiments were carried out: after treatment by HCl or HCl + BaCl 2 the liquid phases were separated.From them water was evaporated.The solid residual was investigated by a method of X-ray phase analysis (XRD) and FTIR-spectroscopy method.
Temperature treatment of initial bentonite and bentonite treated with HCl or HCl + BaCl 2 was carried out in the region of 100-800ºС in air during 90 min.After temperature treatment part of the samples were washed in water.
The cation exchange capacity (CEC) of bentonite samples was measured by MgCl 2 saturation.
Semi-quantitative phase analysis was realized on a X-ray diffractometer (Siemens D-500) with a CuKα radiation.The change of the intensity of diffraction lines of various crystalline phases was estimated in relation to the standard.The line of cristobalite with d = 0.405 nm was chosen as the standard.In this case the change of the intensity of diffraction lines corresponded to the change of the contents of a crystalline phase in samples in arbitrary units.___________________________________________________________________________ FTIR-spectrum were carried out on a spectrometer Brucker model V22.Samples used for measurements contained a fixed amount of clay and KBr.This enabled establishment of the tendency of intensity change of absorption bands for various types of clay treatment.
Morphological studies of bentonite were carried out in a Jeol model JSM 6400 SEM.For estimation of surface area and particles size the BET method (Quantasorb Jr. differential flux analyzer) and Horiba analyzer model Capa 300 were used, respectively.
Local elemental analysis was carried out using an energy dispersive X-ray microanalyzer (Jeol model JSM 6400 SEM).
Data obtained by electronic microscopy (Fig. 3a) of bentonite clay shows that particles of montmorillonite are organized in large aggregates with the size of about 10 µm.Inclusions exist between aggregates.In accordance with microanalysis data it is possible to attribute them to congestions of gypsum, cristobalite and quartz.The specific surface is 9 m 2 /g.___________________________________________________________________________

HCl Treatment of Bentonite
The treatment of bentonite clay by the diluted hydrochloric acid with pH = 0.8 resulted in the reduction of gypsum and montmorillonite contents.In the region of 2θ ∼ 20-30º a halo occurs (Fig. 1b and Fig. 4).For diffraction lines of montmorillonite the change of an intensity ratio is marked.The mean value of d (001) changes from 1.47 nm up to 1.52 nm.___________________________________________________________________________  In the evaporated residual after washing of samples by water low-ordered CaCl 2 and traces of CaSO 4 were registered.
A number of bands of absorption move to a high-frequency area (see Tab. I).Such a shift denotes development of compressive pressure in the montmorillonite structure (shortening of lengths of bonds) and corresponds to the dehydration and dehydroxilation process of montmorillonite [31].
In the field of 1750-900 cm -1 the strong absorption band of Si-O-Si bonds is widened and deformed (Figs.2a,b).Such a change can be caused by the presence of the absorption band of Me-Cl-bonds in this region [32].
On microphotos of HCl-treated bentonite clay increase of aggregate size of a clay mineral up to 1.9 nm, then loosening and destruction and occurrence of inside-aggregate porosity are visible (Fig. 3b).S sp grows up to 13 m 2 /g.The form of particles changes a littlethey become more flat.
X-ray microanalysis has shown (see Tab. II), that after HCl treatment of montmorillonite the content of Ca is increased, Cl and S are registered and the contents of Fe, Si decrease a little.pH of the medium is increased up to 1.5.
Thus, bentonite treatment by HCl is accompanied by the appearance of X-ray amorphous compounds of the interaction, changing of particle forms of montmorillonite and reorganization of aggregates.The change of acidity of the medium, chemical composition in local points in a sample denote the occurrence of a number of chemical reactions and diffusion processes, resulting in diffusing of Ca ions into montmorillonite particles.___________________________________________________________________________ Introduction of a water solution of HCl and BaCl 2 with pH = 1.5 into bentonite, in accordance with XRD, leads to the disappearance of gypsum.Essential reduction of montmorillonite content does not occur.The halo intensity in the region 2θ ∼ 20-30º is increased.BaCl 2 is registered in samples (Fig. 1a,c and Fig. 4).The mean value of d (001) for montmorillonite is increased from 1.47 nm up to 1.549 nm.Such changes of d (001) are usually connected to an increase of the content of OH-groups or / and cations of large ionic radius in the interlayer space of the montmorillonite structure.
In the evaporated residual after washing of samples by water low-ordered BaCl 2 , CaCl 2 and traces of CaSO 4 are registered.
In an FTIR-spectrum of the treated clay the reduction of absorption band intensity of AlMgOH (and AlAlOH) bonds and increase of absorption band intensity of Si-O bonds (see Fig. 2 a,c and Fig. 5) are marked.This means that the process of dehydroxilation of montmorillonite takes place.As well as in the case of HCl treatment of bentonite, for the given treatment the shift of absorption bands in the high-frequency region (see Tab. I) is observed.At the same time, the increase of absorption band intensity of adsorbed water at ν ∼ 3426 and 1639 cm -1 (see Fig. 2 a,c) is caused by the introduction of a (HCl + BaCl 2 ) water solution.These changes answer the process of montmorillonite hydration and are in accordance with XRD data indicating a d (001) increase.Widening and distortion of absorption bands of montmorillonite in the region of ν ∼ 1300-920 and 700-400 cm -1 are caused by the imposing of absorption bands of Me-Cl and Me-SO 4 bonds [32].
According to microanalysis data (see Tab. II) the treatment of bentonite by a (HCl + BaCl 2 ) mixture lead to a decrease of Mg, Al, Fe contents in samples and to an increase of Ba, Cl and S contents.Washing of these samples with water has shown (see Tab According to electron microscopic data as well as HCl treatment data, the increase of the size of aggregates of montmorillonite and increase of their porosity are observed (Fig. 3c).S sp grows up to 12 m 2 /g.Thus, for bentonite treatment by a (HCl + BaCl 2 ) mixture, as well as introduction of HCl, the processes of destruction of the montmorillonite structure, formation of amorphous compounds (including SiO 2 amorphous), change of chemical composition in local points in a sample occurred.However, in this case particles of montmorillonite are enriched by Ba cations.The presence of chlorine and sulfur in the washed samples denote development of sorption processes with participation of these anions [33][34].

Temperature Treatment of Clays
Temperature treatment of initial bentonite clay is accompanied by the dehydration process of montmorillonite [2][3]35] at which the mean value of d ( 001) decreases (Tab.IV).During temperature treatment of activated clays d (001) also decreases.However in this case the change of d (001) does not have an extreme character (see Tab. IV).In FTIR-spectra with the increase of temperature the bands of absorption of interlayer water and AlAlOH(AlMgOH)-bonds decrease (dehydration and dehydroxilation processes in montmorillonite) and disappear at 700ºC for non-activated samples and at ∼550ºC for activated samples (Fig. 6).

Tab. IV
In Fig. 7 it is visible that temperature treatment results in the change of the form of montmorillonite particles.They transform into the form of plates.Loosening and porosity of samples are increased.S sp is higher (see Tab. IV).After washing of samples S sp has reached 80-100 m 2 /g.___________________________________________________________________________  The obtained data denote that the dehydration and dehydroxilation processes in montmorillonite for temperature treatment of the activated clay have finished, for nonactivated they have not.However, for the first process the larger mean value of d (001) are characteristic.

Discussion
For understanding of the process of crystal structure reorganization of montmorillonite it is necessary to analyze a number of features of acid treatment of bentonite.
In initial bentonite clay montmorillonite containing Ca, Mg, Fe, Na and inclusion of gypsum are present.Mean value of d (001) equal to 1.45-1.48nm are characteristic for montmorillonites with humidity up to 30-35 %.In this case a bilayer of water (or 2-layers of hydrate) in the interlayer space is present [3,36].
At introduction of hydrochloric acid into such bentonite (actually, into the "composite system") results in the allocation of gaseous products, reduction of the gypsum content, loosening of aggregates, occurrence of an X-ray amorphous phase, widening and distortion of the absorption band of Si-O-Si bonds in the region 1700-900 cm -1 .It is possible to explain these processes with occurrence of the following reaction: As a result penetration of liquid components (acids) is facilitated into the loosened aggregates.Formation of H 2 SO 4 results in an increase of pH.Let's note that in the evaporated residuals CaCl 2 and BaCl 2 easily soluble in water are identified by an XRD-method.As CaSO 4 poorly dissolves in water, only its traces are found.
Change of the form of montmorillonite particles (from volumetric to lamellar), development of dehydration and dehydroxication processes, occurrence of compressing pressure in the structure of layered silicate, destruction of Si-O-Al (Mg, Fe) bonds and increase of the number Si-O-bonds, reduction of montmorillonite content, formation of amorphous SiO 2 , reduction of the content of iron in montmorillonite are a direct consequence of destruction of the structure of the given mineral by hydrochloric acid [5,37].
The process of removal of interlayer water from montmorillonite should be accompanied by reduction of the basal period [2-5, 16, and 38].However, in researched samples the increase of d ( 001) is observed.It is possible to explain such inconsistent results taking into account the "composite structure" of bentonite clay and the sample preparation technique used.
According to the microanalysis the contents of Ca (for treatment with HCl) or Ba (for treatment with HCl + BaCl 2 ) in particles of montmorillonite grow.That is, the well enough known process of saturation of montmorillonite by polyvalent cations of metals is realized [7][8][9].Namely, after introduction of a water solution of HCl in bentonite clay containing gypsum, as a result of CaCl 2 formation, the process of intercalation-saturation of montmorillonite by cations of Ca proceeds.At replacement of a part of OH-groups by Ca 2+ cations into the interlayer space d (001) is increased.The interlayer space makes 0.576-0.583nm [36].After treatment with a HCl + BaCl 2 mixture Ba 2+ cations penetrate into the interlayer space, which have a greater ionic radius (r Ca2+ = 0.106 nm and r Ba2+ = 0.143 nm).As a result d (001) has greater values (Tab.III).Acknowledgement of the legitimacy of the given assumption is obtained from results on the saturation of samples (CEC).It is established that the value of saturation of the interlayer space by Mg cations depends on the degree of "filling" of the given layer by Ca or Ba cations at a stage of bentonite treatment by HCl or HCl + BaCl 2 (see Tab. III).Namely, the more cations enter into the interlayer space at an initial stage, the larger the size of the basal space (001) and thus a smaller value of CEC is registered at saturation.
After HCl treatment of montmorillonite cations (Mg, Fe, Al) and OH-groups are eliminated from the octahedral positions.Amorphous SiO 2 collects on the edges of flakes [2][3]5].Anions of Cl and SO 4 are located on flats and edges of montmorillonite and modify silica [2][3]34].
A general scheme of the process of bentonite treatment by a hydrochloric acid is submitted on Fig. 8.It includes macro-and microreorganization of the composite system.If macroreorganization includes "the transformation" of crystalline CaSO 4 into X-ray amorphous CaCl 2 , both CaSO 4 and the loosening of aggregates of montmorillonite, microreorganization is meant as the removal of OH-groups of the montmorillonite structure (from the interlayer space and octahedral sublattice), allocation of Mg, Fe Al from octahedral positions, formation of SiO 2 and intercalation (entering) of cations of metals (Ca and Ba) to the interlayer space.
A comparison of dehydration processes of montmorillonite during temperature and acid treatment denote their similarity.It is necessary to note that incorporation of these two processes: acid activation and temperature treatment in a uniform technological cycle is ___________________________________________________________________________ accompanied by the formation of large mean value of d (001) in dehydrated montmorillonite.At the same time, the decrease of d (001) up to 0.93 nm in non-activated montmorillonite corresponds to the process of collapse of its structure at which such an ability for swelling is lost [2][3].The high mean value of d (001) , disappearance of OH-groups, increase of S sp , presence of Ca (and/or Ba) in montmorillonite particles denote porous formation into the interlayer space in which ions of metals are present (see Fig. 8c).A similar mechanism of porous formation in montmorillonite takes place during saturation of the interlayer space with organic cations and subsequent temperature treatment [12,16].However in this case as the result of thermodestruction of organic compounds in pores carbon is formed.Note that obtaining of such porous dehydrated montmorillonite is of interest not only for its use as an adsorbent (bleaching clay), but also for synthesis of porous ceramics based on it.

Conclusion
The action of acids on montmorillonite is similar to the process of dehydration during temperature treatment of layered silicates.
The presence of gypsum or barium chloride in bentonite clay allows combination of the processes of removal of OH-groups from various positions in the montmorillonite structure with the process of intercallation of montmorillonite by Ca or Ba cations.
The temperature treatment of intercalated montmorillonite leads to formation of pores as a result of eliminating of "residual" OH-groups from the interlayer space.

Fig. 2
Fig. 2 FTIR-spectra of absorption in a sample of initial bentonite clay (a); after HCl treatment of clay (b), after HCl + BaCl 2 treatment of clay (c).

Fig. 3
Fig. 3 Microphotos of samples of initial bentonite clay (a), after HCl treatment of clay (b), after HCl + BaCl 2 treatment of clay (c) with different magnifications.

Fig. 6
Fig. 6 FTIR-spectra of absorption in a sample of initial bentonite clay (a); after temperature treatment of initial bentonite (b), activated bentonite with HCl + BaCl 2 (c), after temperature treatment of activated bentonite (d).For b and d T tr = 450ºC.

Fig. 7
Fig. 7 Microphotos of a sample after HCl activation and treatment at 450ºC.

Fig. 8 A
Fig. 8 A simplified scheme of macro-(a) and micro-reorganization of bentonite and montmorillonite under acid treatment (b), and temperature treatment (c).
montmorillonite and researched samples.
Tab. II The common content of elements (wt.%) in clay samples.
. III) that a significant part of Ca, Ba and Cl are eliminated with water.CaCl 2 and BaCl 2 are soluble in water.At the same time local microanalysis has allowed detection of congestions containing concentrated Ba, S and O.Such particles can be attributed to BaSO 4 .There are areas enriched by Si and O in which about 1-2 % Cl and S are present.They can be attributed to areas of an amorphous silica concentration modified by Cl and SO 4 .___________________________________________________________________________ Tab.III The contents of Ca and Ba in montmorillonite, mean values of CEC and d (001) after treatment of samples in an acid medium.
Change of d (001) during temperature treatment of clays.