calcareous clays: phase composition and adsorption ... · ment. the analysis of x-ray diffraction...

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Irena Mihailova, Paunka Vassileva, Dimitar R. Mehandjiev

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CALCAREOUS CLAYS: PHASE COMPOSITION AND ADSORPTION CAPACITY FOR HEAVY METAL IONS

Irena Mihailova1, Paunka Vassileva2, Dimitar R. Mehandjiev2

ABSTRACT

The purpose of this study was to investigate the prospective use of calcareous clay in its natural state and after thermal treatment for removal of heavy metals from aquatic environment. The study included 6 natural products from clay deposits in Bulgaria, five of which were calcareous clays and the sixth one was an iron-rich clay. The structural characterization of the samples was carried out using inductively coupled plasma optic emission spectrometry, X-ray powder diffraction analysis, Fourier-transform infrared spectroscopy, thermogravimetric analysis and differential scanning calorimetry. The clay minerals contained in the samples were identified as illite, chlorite, kaolinite and smectite. Quartz, calcite and feldspar were identified in the samples too. The thermal treatment has led to changes in the phase composition of clays as a result of partial dehydration and decomposition of minerals. There was an increase in the adsorption capacity of the calcareous clays relative to the metal ions of Cu(II) and Zn(II) after ther-mal treatment, regardless of the reduction of their specific surface area, which is determined by the increase of the adsorption centers upon thermal treatment.

Keywords: heavy metals, calcareous clays, adsorption.

Received 20 December 2018Accepted 10 April 2019

Journal of Chemical Technology and Metallurgy, 54, 5, 2019, 1009-1019

1 Department of Silicate Technology University of Chemical Technology and Metallurgy 8 Kliment Ohridski, 1756 Sofia, Bulgaria E-mail: [email protected] Institute of General and Inorganic Chemistry Bulgarian Academy of Sciences Acad. Georgi. Bonchev str., bld. 11, 1113 Sofia, Bulgaria

INTRODUCTION

Clays are polymineral natural products of major technological and environmental importance. They are used as raw materials in the manufacturing of many products (ceramics, cement, rubber, paints, as additives in paper manufacturing, pharmaceuticals), as well as for the production of high-efficiency adsorbents, floc-culants and filtration powders. As a constituent of soils and sediments, clays serve as a natural barrier against the spread of technogenic contaminants into groundwater. In recent years much research has been done on the use of clays in the removal of heavy metal ions and organic compounds from industrial wastewater [1 - 6].

The rationale behind the choice of natural sorbents for processes of purification is related to the accumu-lated knowledge on the physicochemical properties of minerals’ composition and the correct determination of the sorption mechanism on mineral surfaces [4]. This is a difficult task, taking into account the polymineral composition of natural samples, the specifics of their structure, the influence of other minerals and, of course, the nature of sorbate and the process operating condi-tions. The thermal treatment of the above mentioned adsorbent type, given its complex mineral composition, plays an important role in the formation of certain phases and largely determines its adsorption properties. The study of the impact of thermal treatment on the phase

Journal of Chemical Technology and Metallurgy, 54, 5, 2019

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composition and adsorption capacity of clay is an im-portant task with the view of increasing their efficiency in natural or wastewater treatment processes.

The purpose of this study is to investigate the pro-spective use of calcareous clay in its natural state and after thermal treatment for removal of heavy metals from aquatic environment. The study includes 6 natural products from clay deposits in Bulgaria, five of which are calcareous clays and the sixth one is an iron-rich clay. The samples have been taken from clay deposit fields in the region of the town of Burgas, and at the outskirts of Kaspichan town and Probuda village.

EXPERIMENTALSamples

Six samples of natural mineral raw materials were taken for analysis: calcareous clay or clay from various clay deposits, designated as C1, C2, C3, C4, C5 and C6. The samples C1 and C2 are from a clay deposit near the town of Burgas, C3 is from the region of Kaspichan town, C4 is from a field close to the village of Probuda. C5 and C6 are respectively calcareous clay and Fe-rich clay from other Bulgarian deposit locations. From these initial specimens, samples were prepared by heating in air at 700°C for two hours. These samples are respec-tively designated as C1-700, C2-700, C3-700, C4-700, C5-700 and C6-700.

Characterization techniquesChemical composition of clays was determined by

inductively coupled plasma optic emission spectrom-etry (Prodigy High Dispersion ICP-OES Spectrometer from Teledyne Leeman Labs). X-ray powder diffraction (XRD) analysis was applied for phase identification. An X-ray diffractometer Philips at FeKα radiation was used in the range from 7° to 90° 2θ (step size: 0.05°, counting time per step: 1 s). The crystalline phases were identified using the powder diffraction files from database JCPDS – International Centre for Diffraction Data PCPDFWIN v.2.2. (2001).

Infrared transmittance spectra were obtained using an FTIR spectrophotometer Varian 660 IR in the wave num-ber range from 4000 cm–1 to 400 cm–1. FTIR spectra were recorded by using the pressed-pellet technique in KBr.

In order to identify the temperature-dependent phase transformations thermal analysis was performed. The natural clay samples were subjected to thermogravimet-ric analysis and differential scanning calorimetry using a TG/DSC system (STA PT1600 TG-DTA/DSC (STA Simultaneous Thermal Analysis) LINSEIS Messgerate GmbH Germany) equipped with thermogravimetric (TG) and differential scanning calorimetry (DSC) units. Specimens were annealed at increasing temperature from 20 to 1000°C in open air. The heating rate was set at 10°C min-1. The specific surface area (SBET, m

2 g-1) of the samples was determined by low-temperature nitrogen adsorption at 77.4 K. Specific surface areas were calculated from the BET.

Adsorption testThe adsorption studies were conducted using indi-

vidual solutions of Cu(II), Zn(II) and Fe(III), with metal ion concentrations, respectively: 45.5 mg L-1; 43.82 mg L-1 and 35.25 mg L-1. The initial pH of the divalent metal solutions was 5.5 - 6.0 and that of Fe (III) solu-tion - 2.4. After 24 hours contact time between 0.5 g of adsorbent and 100 ml of solution in thermostat (20 °C), the suspensions got filtered through a paper filter. The metal ions concentration analysis was conducted using photometry (Нach Lange cuvette tests). The filtrate pH after sorption was also measured.

The absorbance capacity (mg g-1) - the maximum amount of metal ions adsorbed under given conditions was determined using the formula:

o e(C - C ).Vq =mg (1)

where: qg - adsorption capacity of the adsorbent (mg g-1); C0 and Ce respectively represent the initial and equilibrium concentration of the metal ion in the solution (mg L-1), and m - the mass of the adsorbent (g).

Based on qg, the adsorption capacity per unit surface - qs (mg m-2) was calculated by the following equation:

qs = qg/SBET (2)

Тhe parameter qм, which is proportional to the number of adsorbed ions per gram of adsorbent and


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adsorption centers respectively was calculated by the following equation:

qM = qg/M (3)

where: М - аtomic mass of the corresponding ion.

RESULTS AND DISCUSSIONCharacterization of clay samplesChemical composition and specific surface area

The chemical composition of clay samples, is pre-sented in Table 1.

Table 2 shows the calculated specific surface values

of the tested samples. The clay samples from Burgas re-gion - С1 and С2, have a developed specific surface area of 32 m2 g-1 and 26 m2 g-1 respectively, whereas the values for C3, C4, С5 and С6 samples are comparatively lower. Hence the thermal treatment has led to a decrease in the specific surface area, as it could be assumed. Interesting are the results obtained for the C6 sample – iron-rich clay, where the thermal treatment has even resulted in an increase of the specific surface area.

Phase compositionFig. 1 contains the XRD patterns of the studied

specimens in their natural state and after thermal treat-ment. The analysis of X-ray diffraction (XRD) data showed the presence of clay minerals, quartz, calcite, and feldspar in the natural raw materials. The differences are mainly due to clay mineral types and the quantita-tive proportions of the minerals. In the identification of the clay minerals using X-ray diffraction patterns was also taken into account the expected behavior of clays of different structural types in heat treatment, according to data given in Table 3 [7].

In C1 and C2 clay samples, which are quite similar in chemical composition, illite and a clay mineral from

Samples Indicator [wt. % ]

C1 C2 C3 C4 C5 C6

SiО2 49.81 48.70 38.73 35.98 45.45 56.93 Al2O3 12.13 11.90 12.53 10.92 13.05 18.85 CaO 12.82 14.29 18.57 21.85 15.40 1.28 MgO 1.28 1.18 1.72 1.64 1.72 2.14 Na2O 0.85 0.74 1.65 0.73 0.75 1.98 K2O 1.66 1.64 2.12 1.70 1.89 2.08 Fe2O3 3.26 3.11 3.16 2.89 3.44 5.86 MnO 0.04 0.04 0.04 0.04 0.06 0.09 TiO2 1.58 1.65 1.72 1.52 1.82 3.04 Cr2O3 0.01 0.01 0.01 0.01 0.01 0.01 BaO 0.04 0.03 0.03 0.03 0.02 0.04 ZnO 0.06 0.06 0.04 0.05 0.02 0.20 CuO 0.04 0.04 0.04 0.05 0.05 0.10 L.O.I. 16.41 16.59 19.62 22.56 16.30 7.39

Table 1. Chemical composition of the original clay samples.

Original clay samples

SBET [m2 g-1]

Heated clay samples

SBET [m2 g-1]

C 1 26 C 1-700 17 C 2 32 C 2-700 19 C 3 11 C 3-700 4 C 4 6 C 4-700 4 C 5 8 C 5-700 3 C 6 4 C 6-700 7

Table 2. Specific surface area of samples, SBET [m2 g-1].


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kaolinite group were found. The latter was identified in the C1 sample by a diffraction peak at 7.60 Å, which was displaced compared to the expected peak position for kaolinite at 7 - 7.15 Å (Fig. 1a). It is probably a hemi-hydrated halloysite.

Clays C3 and C4 are also similar in chemical com-position. They contain significantly more calcite than C1 and C2. However, besides illite and kaolinite, in their composition chlorite was also found. Diffraction peaks of kaolinite coincide with those of chlorite. The intensity character of the XRD basal spacigs corresponds to the iron-rich variation of chlorite-shamosite. At the same time, the iron content in the chemical composition of clay samples C3 and C4 is not high. Therefore, the increased intensity of diffraction peaks at ≈ 7 Å and 3.5 Å is due to kaolinite (Fig. 1b).

In C5 sample, similar to samples C3 and C4, clay minerals are represented by illite, chlorite and kaolinite. In comparison, it contains less calcite.

The diffraction patterns of C6 sample exhibit the most intense diffraction peaks of various clay minerals and quartz (Fig 1c). The clay minerals were identified as illite, kaolinite, chlorite and smectite. The peak at 7.14 Å is probably the result of the overlapping of chlorite and kaolinite peaks. The peak at 3.58 Å confirms the pres-

ence of kaolinite. The XRD patterns show no evidence of any calcite present in the C6 sample.

According to the identified phase composition of the natural samples C1, C2, C3, C4 and C5, they can be classified as calcareous clays. C6 sample differs in its chemical and phase composition, which corresponds to a polymineral clay.

The changes registered by XRD analysis after thermal treatment at 700°C are the decomposition of kaolinite and the partial dehydration of illite, whose structure undergoes changes, but as a whole remains maintained. According to the literature [7], there is a loss of hydroxyl water in illite structure at temperatures between 300°C and 600°C, which preserves its mica characteristics and also takes the form of a newly modi-fied anhydride. The increase in the intensity of some diffraction bands is typical.

Chlorites and smectites also undergo partial dehy-dration due to the thermal treatment. The diffraction peaks (14 Å) of chlorites thermally treated at 700°C for C3-700, C4-700, C5-700 and C6-700 samples re-main almost the same. Heat treatment in air at 700°C causes smectites (montmorillonoids) to collapse to a spacing of about 11 Å, as can be seen from C6-700 diffractogram (Fig. 1c). Thermal treatment also leads

Fig. 1. XRD patterns (FeKα) of the original clay samples and after thermal treatment at 700°C.


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to a partial decomposition of calcite with a decrease in peak intensity. When comparing the XRD patterns of the initial and thermally treated clay samples in Fig. 1, it could be observed that the intensity of the diffraction peaks of calcite varies depending on the composition of the samples. According to our data, the process of decomposition of calcite is more pronounced in samples C1-700 and C2-700 compared to C3-700, C4-700 and C5-700. The diffraction peaks of quartz are unaffected. The emergence of new peaks at d = 3.50 Å of C1-700 and C2-700 diffractograms could be related to the formation of a new phase, probably grossite CaAl4O8.

Fig. 2 shows the results obtained by infrared spec-troscopy of the initial specimens and the corresponding thermally activated samples. In general, spectra exhibit similar location of the main absorption bands in the region under study between 4000 and 400 cm–1, which suggests structural similarities.

The bands more visible and valid for identification of calcium carbonate are due to the planar CO3

2– ion [8]. The CO3

2– anion in calcite shows four internal modes, i.e. the ν1 symmetrical stretch (Raman active), the ν2 out-of-plane bend (infrared-active), the ν3 anti-symmetric stretching and the ν4 in-plane bending mode [9]. The absorption bands frequencies at 1435 - 1425 cm–1, 880 - 873 cm–1, and 712 - 716 cm–1 in the FTIR spectra of the samples (Fig. 2), can be assigned to ν3, ν2 and ν4

modes, respectively for the CO3-2 ion in calcite [9 - 11].

They could be observed in the spectra of all the samples except in that of C6-700. In the spectra of C6 sample, they are very weak.

The FTIR spectra band intensity at 1425 cm–1 and those at 900 - 1200 cm–1 are related to the respec-tive amounts of carbonate and silicate phases. The experimental data obtained by FTIR spectroscopy are in accordance with XRD and DSC-TG data, and more specifically, agree with the conclusions about the amount of calcite and the different degree of its decomposition with the applied heat.

The characteristic infrared bands associated with quartz are at 1175 cm–1, 1100 cm–1, 802 cm–1, 785 cm–1, 695 cm–1, 516 cm–1, 470 cm–1 [12]. Absorption bands located at ~795 cm–1, 780 cm–1, 692 cm–1, 525 cm–1, 470 cm–1 and shoulders located at 1050 - 1200 cm–1 were observed in the FTIR spectra of the samples (Fig. 3). The frequencies 795 cm–1, 780 cm–1 represent symmetrical Si–O stretching vibrations, whereas 692 cm–1, 525 cm–1 and 470 cm–1 represent symmetrical and asymmetrical Si–O bending vibrations [12].

FTIR spectra of the layered silicates usually show an intensive band at about 1000 cm–1 and weaker ones in the 800 - 600 cm–1 range. In the infrared spectra of the tested initial samples, the intensive band with the maximum adsorption at 1031 - 1036 cm–1 is most likely related to

Fig. 2. FTIR spectra of the original clay samples and after thermal treatment at 700°C (heated samples).


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vibrations of the silicon-oxygen tetrahedron bound in a layer in the structure of various clay minerals. On the other hand, in this broad band peaks are also overlaping at 1175 cm–1, 1100 cm–1 representing asymmetrical Si-O stretching vibrations in the quartz structure. There are overlapping peaks of quartz and clay minerals in the frequencies range of 800 - 600 cm-1 too.

Absorption bands at 3624 - 3626 cm–1, 3422 - 3426 cm–1, 1031 - 1036 cm–1, 916 - 920 cm–1, 525 - 530 cm–1, 475 - 470 cm–1, 424 - 429 cm–1 were observed in the FTIR spectra of C1, C2, C3, C4, C5 and C6 samples, as can be seen in Fig. 3. At these frequencies, peaks of illite are expected.

The sharp doublet at 3696 cm–1 and 3620 cm–1 is characteristic for the kaolinite group as a whole. The OH deformation band of kaolinite is situated at 913 cm–1 and supporting bands at 794 cm–1 (Si-O) and 698 cm–1 (Si-O) are diagnostic for kaolinite too [13]. Absorption

bands at kaolinite characteristic frequencies are observed in the spectra of all natural samples.

Bands at around 3548 cm–1, 3424 cm–1, 985 cm–1 cm–1, 758 cm–1, 649 cm–1, 453 cm–1 and 435 cm–1 are similar to the reported infrared spectrum of chlorite [13]. Most of them overlap with bands of other clay minerals. An exception is the band at 649 cm-1, which was identi-fied in the FTIR spectra of samples C3, C4, C5 and C6 where chlorite was detected using XRD. A band at this frequency is absent in the FTIR spectra of kaolinite, il-lite, smectites, as well as in quartz, calcite and feldspars.

According to previous studies [13], smectites show the typically broad OH-stretching band at 3624 cm–1 and well-resolved OH deformation bands at 915 cm–1 (Al-Al-OH). These bands occur close to FTIR-spectra absorption bands in kaolinite and illite. Therefore, it would be difficult to identify smectites on the FTIR spectrum, given the presence of illite and kaolinite in

Table 3. Basic structural characteristics and identification of the clay minerals using X-ray diffraction patterns [7].

Type of clay minerals

Structural characteristics Identification using XRD (basal spacings)

Transformation of XRD patterns after heating to 550°C

Kaolinite Group

• 1:1 layer minerals; • Dioctaedral layers; • No layer charge; • Hydrogen bonds.

7.15 Å (001) 3.58 Å (002) 2.38 Å (003)

7-Å peak dissapears upon heating.

Illites

• Negatively charged mica-like (2:1) layers;

• Dioctahedral layers; • Non-expanding,

micaceous minerals.

10.10 Å (001) 5.04 Å (002) 3.36 Å (003)

10-Å peak not affected by heating.

Smectites

• Negatively charged 2:1 layers;

• Weak bonds between layers allow water to enter causing expansion in the c direction.

~15-14 Å (001)

Shifting of (001)-peak affected by heating.

Chlorites

• Negatively charged mica-like (2:1) layers regularly alternating with positively charged brucite-like (octahedral) sheets

~14.2 Å (001) 7.14 Å (002) 4.75 Å (003) 3.55 Å (004)

14-Å peak not affected by treatment. Typically, the (001) chlorite peak may increase dramatically and higher-order peaks may be conspicuously weakened.


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Fig. 3. Thermal curves (DSC, TG and DTG) of natural clay samples C1, C2, C3, C4, C5, and C6.


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the sample C-6. The vibration at ~1625 cm–1 in spectra is attributable to the bending of adsorbed water.

The thermal activation of the samples was detected trough the expansion of the absorption bands and the decrease in their number. A similar effect was observed in the spectra of amorphous materials compared to their crystalline analogs. The dehydration of the clay minerals led to the disappearance of the absorption bands at 3624 - 3626 cm–1 and a less intense band of about 3640 cm–1 was identified. An exception is C6-700 sample. It can be assumed that the new band is due to the fluctuation of the hydroxyl group in Ca(OH)2. This phase is likely to have occurred if the released CaO was hydrated into the air during the partial decomposition of CaCO3.

The thermal analysis diagrams of clay samples are shown in Fig. 3. TG, DTG, and DSC curves of C1 and C2 samples show similar features. DSC curves point to weak endoeffects associated with the release of free and lightly bound water up to 200°C, a strong endoeffect due to calcium decomposition at ≈ 720°C, a shoulder at 600°C as a result of the decomposition of kaolinite, and an endoeffect at 825 (830)°C related to the decom-position of illite.

The behavior of C3, C4 and C5 samples under heat treatment was alike (Fig. 3). There was a small amount of free and adsorbed water in samples released up to 200°C, the reactions manifesting themselves as weak endoeffects on the DSC curves. A more significant loss of mass is observed after 400°C, starting with the separation of the (OH)– groups bonded in the crystalline lattices of the clay minerals. The process is accompanied by a change in the slope of the DSC curves. Endofect at 738 (752)°C is related to the decomposition reaction of calcite. The comparatively higher temperature of calcite decomposition in samples leads to a slower reaction process. Also, there is a slight endoeffect at about 835 (845)°C, followed by a slight exoeffect at 875 (893)°C. These effects can be attributed, respectively, to the final destruction of illite and chlorite lattices, which precede the formation of new phases.

By the loss of mass associated with the decompo-sition of calcite, the following amounts of limestone (calcite) in clays were identified: C1 – 14 %; C2 – 11 %; C3 – 30 %; C4 – 37 %; C5 – 25 % and C6 ≈ 2 %.

The results of the XRD, FTIR, TGA and DSC data for the phase composition of the samples can be briefly summarized in the following way: the natural clays C1, C2, C3, C4, C5 and C6 are polyminerals, as they contain various clay minerals, quartz, and feldspar. All samples, except C6, contain a significant amount of carbonate phase – calcite. Its content varies between 11 and 37 %. C1 and C2 samples can be defined as illite-kaolinite calcium clays with a calcite content of 11 - 14 %. C3, C4 and C5 are also calcareous clays, C3 and C4 having the highest calcite content – 30 - 37 %. The clay minerals contained in C3, C4 and C5 samples are illite, chlorite and kaolinite. C6 is a polymineral clay contain-ing smectite, illite, chlorite and kaolinite.

During the performed thermal treatment, changes in the structure of the clay minerals occurred, as a result of the separation of water and hydroxyl groups, since there has been a decomposition of kaolinite, the structure of chlorite has also been significantly altered, smektite (montmorillonite) was dehydrated, which led to a change in d-spacing (interplanar spacing). Illite also underwent dehydration but its structure was preserved to the largest extent. Heating at 700°C resulted in the partial decomposition of calcite. The process was more prominent in the case of C1 and C2 samples, compared to that of C3, C4 and C5, which could be explained by the lower reaction temperature according to DSC data.

Adsorption studiesTable 4 presents the adsorption capacity qg (mg

g-1) of the tested samples obtained from individual solutions of Cu(II), Zn(II) and Fe(III). An increase in adsorption capacity relative to copper ions for samples C1-700, C2-700, C3-700, C4-700, and C5-700 may be noted. For these samples the increase reaches 8 - 9 mg g-1. An exception is the C6-700 sample, which shows a decrease in qg

For all the samples treated at 700°C, an increase of qg with respect to Zn(II) was observed too. Interestingly, such an effect is also observed for the C6-700 sample. The increase with respect to this ion varies from 0.3 mg g-1 to 4.7 mg g-1. The smallest difference is for sample C5, and the largest one for sample C6 – 4.7 mg g-1.

With respect to Fe(III)-ions, the adsorption of all


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samples after treatment at 700°C deteriorates. This is most clearly seen in sample C6-700, for which qg tends to zero. This sample practically does not adsorb Fe(III).

Table 5 presents the adsorption capacity of the tested samples expressed as the adsorbed amount of ions per unit area – qs.

According to the data in Tables 4 and 5, the adsorp-tion capability of the tested samples from the different deposits before and after treatment at 700°C is differ-ent. It should be noted that with regard to adsorption capacity, the trend for the various ions is maintained. All samples after treatment at 700°C increase their ad-sorption capacity with regard to Cu(II) and Zn(II), with the only exception of C6-700 sample. The adsorption capacity with respect to Fe(III)-ions in all thermally treated samples decreases.

Table 6 presents qM parameter values.The adsorption of Cu(II) expressed as qM, shows the

same trend of that of the adsorption capacity expressed as qg. After heat treatment qM rises compared to qM for the original samples. Since that value is proportional of number of adsorption centers per gram of sample, it should be assumed that after treatment, their number increases. The exception is the C6-700 sample. Such a tendency for qM value is observed for the adsorption

Table 4. Adsorption capacity per unit mass of adsorbent qg.

Ions

Samples

Cu(II)

[mg g-1] pH

Zn(II)

[mg g-1] pH

Fe(III)

[mg g-1] pH

C1 7.98 6.33 5.22 7.02 3.90 2.74

C1-700 17.16 10.30 7.03 9.95 2.09 2.20

C2 8.51 6.79 3.56 7.03 3.54 2.56

C2-700 17.76 9.58 7.40 9.78 2.19 2.32

C3 8.96 7.09 6.01 7.22 6.84 6.26

C3-700 18.09 10.64 8.40 9.36 2.09 2.32

C4 8.92 7.26 6.51 7.01 6.96 6.56

C4-700 17.98 10.74 7.36 9.38 2.14 2.60

C5 8.91 7.23 6.59 7.03 3.21 3.52

C5-700 16.68 10.80 6.85 10.62 1.89 2.16

C6 6.35 4.63 3.36 5.78 1.25 4.85

C6-700 4.46 4.40 8.06 10.50 0.05 1.47

of Zn(II) too. There is also an increase in qM for these ions after the thermal treatment, but the increase is less than that for copper ions. Therefore, the number of the formed adsorption centers also varies. The Fe(III) ad-sorption plot is quite different. Thermal treatment results in a decrease in qM compared to original samples. By comparing the qM values for the different ions, we can observe significant differences. Therefore, the number of adsorption centers formed is different for the adsorp-

Ions Samples

Cu(II) [mg m-2]

Zn(II) [mg m-2]

Fe(III) [mg m-2]

C 1 0.31 0.20 0.15 C 1-700 1.01 0.41 0.12

C 2 0.27 0.11 0.11 C 2-700 0.93 0.39 0.12

C 3 0.81 0.55 0.62 C 3-700 4.52 2.10 0.52

C 4 1.49 1.09 1.16 C 4-700 4.50 1.84 0.54

C 5 1.11 0.82 0.40 C 5-700 5.56 2.28 0.63

C 6 1.59 0.84 0.31 C 6-700 0.64 1.15 0.01

Table 5. Adsorption capacity per unit surface of adsorbent qs.


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tion of the ions of the three metals – Cu(II), Zn(II) and Fe(III). Thermal treatment produces the largest number of adsorption centers for Cu(II)-ion adsorption, less for Zn(II)-ions and the number of adsorption centers for Fe(III)-ions decreases.

According to the results of DSC-TG, XRD and FTIR two main processes could be observed under thermal treatment. The first is a release of adsorbed water, carbon dioxide, dehydration – release of constitutional water and decomposition by the following mechanism:

222OH H O O− −+ (4)

As well as the partial decomposition of calcite

3 2CaCO CaO CO+ (5)

The decomposition of calcite at 700 °C takes place to a larger extent in the C1-700 and C2-700 samples and to a lesser extent in C3-700, C4-700 and C5-700.

According to contemporary research, the adsorp-tion on such samples mainly takes place under an ion-exchange mechanism, which can be expressed by the following scheme:mineral–Ca2+ + Me2+ = mineral–Me2++ Ca2+ (6)

OH– O2– mineral + Ме2+ = mineral Ме2+ + 2H+ (7) OH– O2–

(7) It is possible the latter reaction to procede with

other hydroxyl groups or, in an aqueous environment, a reaction reverse to (4) could take place leading to a recovery of hydroxyl groups.

Then the adsorption of Cu(II) and Zn(II) can proceed as in scheme (6) or (7) as the increase in adsorption centers is due to the decomposition of the calcite under scheme (5). If the process of adsorption proceeds by the mechanism described in scheme (6), as a result of this reaction, Са(II) would be released into the solution lead-ing to an increase in pH. If the mechanism of adsorption is by ion exchange of Н+-ions, the released hydrogen ions should lower the pH. Table 1 gives the pH values for calculating qg. It is obvious that the adsorption of Cu(II) and Zn(II) should increase the pH. Only for

sample C6-700, a slight decrease in pH was observed at Cu(II) adsorption. Upon adsorption of Fe(III), there is a decrease in pH in all samples.

Therefore, during the thermal treatment of the sam-ples, the adsorption of Cu(II) and Zn(II) proceeds via an ion-exchange mechanism by replacing the calcium ions with copper and zinc ions. This leads to an increase in the pH of the solution in which the adsorption takes place. The adsorption of Fe(III) is primarily carried out by ion exchange with hydrogen ions, whereby the pH of the solution decreases. In the thermal treatment, the decomposition of calcite leads to the creation of additional new adsorption centers with respect to ion exchange with Cu(II) and Zn(II) and to an increase in the adsorption capacity of the samples C1-700, C2-700, C3-700, C4-700 and C5-700. For sample C6-700, the formation of such centers is less pronounced, and the adsorption essentially follows scheme (7).

Fe(III) adsorption is most likely to occur as in scheme (7), since the decomposition of calcite and the formation of additional adsorption centers based on calcium ions does not lead to an increase in adsorption capacity but, on the contrary, to its decrease.

CONCLUSIONSThe tested samples have been classified as calcare-

ous clays of polymineral composition - mainly clay minerals, quartz, feldspar and calcite. An exception is

Ions Samples

Cu(II) Zn(II) Fe(III)

C1 1.26 0.80 0.70 C1-700 2.70 1.08 0.37

C2 1.34 0.54 0.63 C2-700 2.80 1.13 0.39

C3 1.41 0.93 1.22 C3-700 2.85 1.28 0.37

C4 1.41 1.00 1.25 C4-700 2.83 1.12 0.38

C5 1.43 1.05 0.58 C5-700 2.63 1.05 0.34

C6 1.00 0.51 0.22 C6-700 0.69 1.23 0.00

Table 6. Parameter qM.


1019

the sample C6, being a polimineral clay with a negligible calcite content. The thermal treatment has led to changes in the phase composition of clays as a result of partial dehydration and decomposition of minerals. There was an increase in the absorption capacity of the calcareous clays relative to the metal ions of Cu (II) and Zn (II) after thermal treatment, regardless of the reduction of their specific surface area, which is determined by the increase in the number of the adsorption centers upon thermal treatment. The adsorption mechanism is ion exchange. The sample C6 differs from the rest. Its adsorption capac-ity’s behavior was specific when are thermally treated. The adsorption capacity of the investigated calcareous clays, with respect to Cu(II), Zn(II) and Fe(III) is good, so they could serve as a basis for the manufacture of adsorbents for water purification.

AcknowledgementsWe thank our colleague Assoc. Prof. Sonya Dim-

itrova, PhD from University of Architecture, Civil Engineering and Geodesy, Sofia, Bulgaria for assistance in adsorption test and for the comments on the earlier version of the manuscript.

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