Investigations of the Changes in the Bentonite Structure Caused by the Different Treatments

The bentonite was treated in different ways and the changes in structure were monitored. Acid activation with sulphuric acid of investigated bentonite caused the increase in specific volume of micropore-mesopore. It was shown that activation by acid obtained at a constant temperature and constant period of time provides the possibility to obtain samples of bentonite of searched porosity only by changing the concentration of sulphuric and hydrochloric acid. By thermal activation of bentonite clay in the temperature range 100-1100 C, samples of desired porosity were acquired.


Introduction
Clay minerals differ from other colloidal materials in several ways.The particle shapes in clay are irregular and asymmetric; there is also a wide range of particle size.Layers of clay are flexible.Charges vary across the layers, with constant charges on the surface and pH-dependent charges at the edges.
There are two bentonite types: sodium bentonite has extremely good properties for making gels whereas mixed sodium/calcium bentonites have less good swelling properties, but they still have good bonding properties [1,2].It is non-toxic and chemically unreactive and is hydrophobic, thus making it suitable for the easy fixation of biomolecules (purification of foods, among others) [3].
Porosity of clay depends on water content; therefore, removal of water causes changes.Acid treatment also increases porosity, which, in combination with thermal treatment, can be used to achieve desired porosity.
Bentonites have applications in more than 25 different areas of industry.For example, acid-activated bentonites can be used as adsorbents in bleaching oils, in production of papers, for making copies without graphite and heterogenous catalysts.The physico-chemical properties of bentonites, and in turn their adsorption, bleaching and catalytic properties depend to a large extent on the size of micro and mesopores [4].
Thermal analysis of white calcium bentonite from Turkey was conducted in the temperature range between 100 and 1100 0 C, as we did in this work,but the obtained structures were analyzed using X-ray diffraction patterns, thermogravimetry, differential thermal analysis and low-temperature nitrogen adsorption (N 2 -AD) [5].Their colleagues investigated the correlation between mass percent of sulphuric acid used in activation (10 %-70 %) with surface area (A) and pore volume (V) only.It was found that variations in specific pore volume, specific surface area and surface acidity as a function of the mass percent of the acid used in activation were more or less parallel to each other for the case of acid-activated bentonite samples.The optimum interval where these 3 parameters were maximum corresponded to 40 % of sulfuric acid.In case of thermally treated bentonite samples, the good correlation between the nm, A and V versus thermal treatment temperature curves showed that increased acidity at certain temperature intervals was mostly due to increased octet vacancies from structural transformations caused by dehydration and dehydroxylation [6].
Bentonite was modified with sulphuric acid and increased adsorption capacity [7] was observed as we noticed as well.There are similar studies with bentonite of different origins.Romanian bentonite was activated using a dilute aqueous solution of hydrochloric acid at 90 0 C [8], and Iranian bentonite was treated similarly [9].It is observed that the clay treated with 4 N sulfuric acid shows the maximum surface acidity [10].
Acid-treatment of bentonites from Serbia is not very well explored.One example is a bentonite from Aleksinac and Petrovac (Serbia) [11].Therefore, here, we have performed the first systematic study of the properties of bentonite treated thermally (in the temperature range 100-1100 0 C) and using sulphuric and hydrochloric acid in different ratios and using different mixing methods at constant temperature and during constant time period.

Experimental procedure
The first aim of this experiment was to investigate the effect of sulphuric and hydrochloric acids on the physico-chemical properties of bentonites.The physico-chemical properties were investigated using chemical composition, mineral decomposition; it was measured the capacity for cation exchange (CEC) and the specific areas and specific volume of micropores-mesopores of bentonite.The second aim was the determination of structure and the composition of mineral clays under the influence of heat.
Bentonite used in the experiment was supplied from Mineralica, Vranjska banja, origin Prisjan, Republic of Serbia.The chemical composition of the bentonite was investigated at The Institute for Mineral and Nuclear Raw Materials, Belgrade.The results of the analysis are shown in Tab.I. Bentonite was treated with sulphuric and hydrochloric acid in order to change the clay structure and introduce new functional groups.Sulphuric acid was used at concentrations: 10 %, 40 %, 70 % and hydrochloric acid was used at concentrations: 1 %, 5 % and 10 % HCl.
Bentonite samples were treated in the temperature range from 100 0 C to 1100 0 C over one hour (thermal treatment).The electrical oven for this treatment was situated in the Laboratory Mladost Ciglana, Leskovac.
Bentonite was firstly crushed, and then dried at 105 0 C for 4 h.From the dried calcium bentonite, 35 samples of 10 g and 20 g were made.Samples with mass 10 g were used for the acid activation of bentonite, and samples with mass 20 g for thermal activation.

Sulphuric acid activation
Samples consisting of 10 g of bentonite were treated separately with 20 ml H 2 SO 4 concentrations of 10 %, 40 % and 70 %, or with 50 ml H 2 SO 4 concentrations of 10 %, 40 % and 70 %.The samples were then mixed in two different ways-by ultrasonic mixing (samples were mixed in PET bottles (bottles made of polyethylene terephthalate)).andhand-mixing.
Ultrasonic and hand mixing were performed at 70 0 C for 15 min.Six different suspensions were prepared in this way.Activation with acid was achieved by warming all six suspensions in a) an ultrasonic bath; b) hand mixing at 70 0 C for 15 min.The liquid was decanted off and the activated samples were transferred into aluminum plates.The sediment was washed with distilled water until the investigated assay for sulphate ions were negative.The samples were dried for 48 h at 105 0 C and then the mass of the sample was measured.Finally the sample was placed in a new PET container for further experiments.

Hydrochloric acid activation
The second experiment was carried out in exactly the same way, except that 1 %, 5 % and 10 % HCl were used replacing the different sulphuric acid concentrations.Samples containing 20 ml of acid were mixed in PET containers whereas glass jars of 330 ml was used to mix the 50 ml samples.These mixtures were then transferred into PET containers in order to settle down.Again, there were two methods for mixing: ultrasonic mixing and handmixing.
Six different suspensions were prepared in this way.Activation with acid was performed by warming all six suspensions in an ultrasound bath at 70 0 C for 15 min.Activated samples were decanted and transferred into aluminum plates and the sediment was washed with distilled water until there was a negative reaction on Cl -ions.Aluminum plates with samples were dried for 48 h at 105 0 C, their mass was measured and they were placed into new PET containers in order to be used further in the next experiment.

Thermal activation
11 Samples of bentonite, each with a mass of 20 g, were heated at temperatures ranging from 100 0 C to 1100 0 C for 1 hour.After cooling, the mass of the samples was measured.Samples were then placed into PET containers for use in further experiments.

Results and Discussion
The changes in mass of the bentonite clays on acid and thermal treatment are summarized graphically.Fig. 1.Change of bentonite mass versus concentration of H 2 SO 4 (a) on mixing using using an ultrasound bath; b) on mixing by hand.The black lines show results for 10 g clay mixed with 20 ml acid, the red lines show results for 10 g clay mixed with 50 ml acid (it was represented the average values of three measurements).

Bentonites treated with acids 3.1.1. Bentonites treated with different concentrations of sulphuric acid at different ratios at constant temperature and during constant period of time
Samples treated with a higher volume of sulphuric acid show higher mass after drying.The difference was visible with 70 % H 2 SO 4 more than with 10 % H 2 SO 4, which showed the smallest mass of the sample after drying.Variations in porosity CaB during acid activation can be observed as variations in crystal structure.Although values A and V were 43 m 2 g -1 and 0.107 cm 3 g -1 for original bentonite, these values reach the maximum and they were 134 m 2 g -1 and 0.295 cm 3 g -1 after activation 40 % H 2 SO 4 [4].
Samples treated with a higher volume of sulphuric acid show higher mass after drying.Difference in Δm is higher for 1:5 ratios, which means that the volume of used acid is an important factor for acid activation of bentonite clay.In relation to 1:2 ratios, Δm is higher than initial mass.In the case of samples treated with 10 % H 2 SO 4 for 1:2 ratio, mass is smaller than the initial mass of the sample.Samples treated with 70 % H 2 SO 4 show higher mass than samples treated with 10 % H 2 SO 4, which show the lowest mass of the samples after drying.
Using 10 % H 2 SO 4 , exchanged cations between 2:1 layers firstly went out and they were replaced with H + ions.Because only a small quantity of Mg 2+ , Fe 3+ and Al 3+ are leaving 2:1 layers, CEC is small.With 40 % H 2 SO 4 , cations Mg 2+ , Fe 3+ and Al 3+ easily have been dissolved from 2:1 layers, so the decrease in CEC is large.With 70 % H 2 SO 4 , decrease in CEC is small because of the decreased quantity of Mg 2+ , Fe 3+ and Al 3+ in 2:1 layers and hard dissolving of Al 3+ (maximum can be 15 %) in tetrahedron's layers and also in (2:1) layers [4].If we compare results of the methods of ultrasound bath and hand-mixing for 1:2 ratio with sulphuric acid, we can conclude that using the method of ultrasound bath results in higher mass of the sample.The difference in mass of the samples using the methods of ultrasound bath and hand-mixing of samples for ratio 1:5 of sulphuric acid is minimal, as we can see in the overlapping of two lines on the graph below.The difference in mass Δm using the method of ultrasound bath is higher for 1:2 ratio, which means that mass of the sample is smaller than that of the sample in ratio 1:5.The reason for the behavior is the higher amount of water, which evaporates after drying.
The change in mass Δm using the method of hand-mixing is higher for 1:2 ratio, which means that mass of the sample is smaller than those in 1:5 ratio.The reason for this is the higher quantity of water, which evaporates after drying.

Changes in bentonites caused by the treatment with hydrochloric acid
If we compare the results of the method of ultrasound bath with results obtained by hand-mixing for 1:2 ratio of hydrochloric acid, we can draw the conclusion that we can get higher masses of the samples using the ultrasound bath.
During the acid activation of bentonite clay using different concentrations of hydrochloric acid with different conditions of homogenization, the change in the number of signals did not happen as well as their intensities in IR spectra (data not shown).

IR spectra of the bentonite samples treated with acids
In the treatment with sulphuric acid, there are changes in signal intensity in IR spectra (data not shown).In principle, there are no big differences, except modifications on 526 cm -1 (Si-O-Al), 466 cm -1 (Si-O-Si), 1040 cm -1 (Si-O), 3454 cm -1 (adsorbed H 2 O) and 3640 cm -1 (Al-Al-OH, Mg-OH-Al).Important differences cannot be seen in this case, neither by using the method of ultrasound mixing nor mechanical stirring.

SEM spectra of bentonite samples treated with acids
From Fig. 5. a) it is visible that the structure of bentonite was changed and the porosity was increased compared with the starting sample of the bentonite.On the Fig. 5. b) the changed structure of bentonite and increased porosity can be seen, which is smaller compared with samples treated with 40 % sulphuric acid.

Changes caused by the thermal activation of the bentonite
With the temperature increasing, there was a mass loss of bentonites.In the temperature interval from 100 0 C to 200 0 C, mass loss is huge.From 300 0 C to 500 0 C, there is a slight loss in mass, while for the temperature interval from 500 0 C to 700 0 C, there is a maximum loss of sample mass.Temperature interval from 700 0 C to 1100 0 C represents minimal loss in mass of the sample.
Endothermic maximums between 100 and 250 0 C are related to water loss from the pores of CaB, which is dehydration.Endothermic maximums between 600 and 750 0 C are related to dehydroxylation, and exothermic maximums between 950 and 1050 0 C are related to break of the crystal structure of 2:1 layers.It can be observed that these maximums decrease regularly with the propagation of the activation [6].
Values V and A, which were 0.107 cm 3 g -1 and 42 m 2 g -1 in non-treated sample reached maximum values of 0.149 cm 3 g -1 and 89 m 2 g -1 on 500 0 C. Considering these results and CEC results, it seems that bentonite, which was investigated as potentially useful as catalyst in powder and gas production, transfers into a liquid state and the processes of desulphurisation of lignite and bitumen coal in the temperature interval from 100-800 0 C [4].A and V reach the maximum at 500 0 C, which is achieved by irreversible dehydration without any changes in crystal structure, and porosity CaM reaches its maximum [4].
IR spectra of bentonites obtained by thermal treatments in the temperature range (100-1100 0 C) are not shown here.In the case of bentonite clay, the band at around 1630 cm -1 is present, which comes from banding H-O-H vibrations.The characteristic wide band at around 1045 cm -1 comes from Si-O-Si stretching vibrations.Stretching OH vibrations inside the molecule of smectite gives a characteristic IR band at around 3625 cm -1 in the bentonite sample, which comes from different hydroxyl groups in smectite: Al-OH-Al, Al-OH-Mg, Al-OH-Fe.The wide band with the wavelength at around 3436 cm -1 belongs to H-O-H stretching vibrations, which come from absorbed water in the interlayer space.That band disappears with the increased temperature of activation above 700 0 C (data not shown here).The structure of the sample in Fig. 7 a) is similar to the structure of the initial sample of bentonite clay.Porosity of this sample is higher because of the partial removal of water.In contrast, the structure of the sample in Fig. 7 b) is completely destroyed and here we can see that new silicate material is growing from the initial.Porosity is smaller because melted material filled free channels.

Conclusions
The activation of bentonite with acid causes partial loss of Mg 2+ , Fe 3+ and Al 3+ from 2:1 layers yielded an important increase of porosity.Activation of investigated bentonite with sulphuric acid increased three times in specific areas and specific volume of microporemesopore.Thermal activation of bentonite clay enabled us to obtain samples of desired porosity.

Fig. 2 .
Fig. 2. Change of the mass of bentonite versus concentration of H 2 SO 4 using the method of ultrasound bath ■ and method of hand-mixing • for a) 1:2 ratio; b) 1: 5 ratio.

Fig. 3 .
Fig.3.Change in mass of bentonites versus concentration of HCl using the method of a) ultrasound bath; b) hand-mixing for ratios 1:2 and 1:5.

Fig. 4 .
Fig. 4. Change of the mass of bentonites versus concentration of HCl using the method of ultrasound bath ■ and the method of hand-mixing • for a) 1:2 ratio; b) 1:5 ratio.

Fig. 6 .
Fig.6.Change of the mass (Δm) versus temperature change in clay because of dehydratation, dehydroxylation, repeated crystalisation, squeezing, breaking, loss of the crystal structure and sintering, which appeared as the result of increased temperature and can be defined as "deformations".
Tab. I. Results of the analysis of the clay expressed in % of compounds.