TEXTURAL AND MORPHOLOGICAL CHARACTERIZATION OF COPPER(II)-EXCHANGED ROMANIAN CALCIUM BENTONITE

Romanian calcium bentonite was modified by copper(II) ion-exchange, by varying the copper precursors (chloride, sulphate) and synthesis parameters (pH, temperature, time). The quantification of the Cu(II) ions was carried out by atomic absorption spectrophotometer. The modified bentonites were characterized by textural analysis (specific surface area by the Brunauer-Emmett-Teller method (BET) and by nitrogen adsorption/desorption isotherm), structural composition (X-ray diffraction (XRD)) and morphological analysis (scanning electron microscopy (SEM)). Analysis of the nitrogen adsorption/desorption isotherm shows that ion exchanged bentonites, not only contain mesopores, but micropores in larger quantities too. The values of the specific surface area increased by about 20 m2/g compared with raw bentonite, but the interlamellar distance values do not vary substantially. Scanning electron micrographs were acquired to demonstrate changes in the texture of the clay before and after ion exchange.


INTRODUCTION
Clay minerals represent a large class of aluminum silicates with layered structure, being very reactive materials due to their small grain size, cation exchange capacity, surface properties (specific surface area, adsorption capacity, surface acidity or alkalinity) and chemical variability.All these properties are the basis of the extensive use of clays in many industrial applications [1].Clays are able to retain organic and inorganic pollutants, and to stop the circulation of contaminated fluids in aquatic and soil environments [2,3].The use of modified clays as low-cost and effective catalyst materials for the removal of metal ions from industrial effluents and waste waters through ion exchange or surface complexation has been of great relevance for many applications [4][5][6].
Montmorillonite, a smectite clay, has a 2:1 layered structure that has both interlayer sites and ionizable hydroxyl sites on its external surface for metal cation adsorption [7].Its permanent negative layer charge originating from the isomorphous substitution of Fe(II) or Mg(II) for octahedral Al(III) is the origin of the binding of exchangeable cations to the interlayer sites [8].These interlayer cations can be exchanged with inorganic cations under aqueous conditions such as Ag(I), Cu(II), or Zn(II) [7,9].After Cu(II)-exchange of montmorillonite, the newly obtained material has larger specific surface area and more effective adsorption than before the chemical modification.
The aim of this paper is to obtain different Cu(II)-exchanged montmorillonite materials by varying the copper precursors and synthesis parameters (pH, temperature, time).X-ray diffraction, specific surface area measurement by BET techniques, nitrogen adsorption / desorption isotherm, scanning electron microscopy were employed to characterize Cu-exchanged montmorillonite materials, which were derived from Romanian calcium bentonite after purification process.Cu(II)-exchanged montmorillonite is a secondary compound in the synthesis of pillared clays, which are potential adsorbents in environmental remediation.

Materials and devices
A Romanian natural calcium bentonite (Cabent-nat), provided by SC Bentonita SA Company (Satu Mare, Romania) was used as raw material.The mineralogical composition and main characteristics of this bentonite were provided by the supplier and are summarized in Tables 1 and 2 Different studies have shown that adsorption capacity of clays was improved with content increasing of smectite clays (montmorillonite) [16].It therefore seemed appropriate to purify the Romanian natural calcium bentonite.The purification process is intended to eliminate the major mineral impurities (quartz and cristobalite), the purified product having a high content of montmorillonite.This process consists of dispersion of CaBent-nat in distilled water in order to obtain a diluted suspension as described in our other works [17,18]  The adjustment of pH was carried out with a pH meter, M210 Tacussel type.After ion exchange treatment, the modified clays have been separated from the suspension using a Firlabo SW9/SW12 centrifuge and dried in a Memmert oven (80 °C for 12 h).The milling of the bentonite samples was performed in an alumina grinding balls, Pulverisette 6 model.The powders thus obtained were homogenized by means of a three-dimensional mixer Turbula T2F type provided by the WAB Company.
The concentration of copper (II) ions was determined by atomic absorption spectrophotometry using SpectrAA Varian 50 model (air-acetylene oxidizing flame, copper wavelength of 327.4 nm, lamp current 4 mA).The amount of Cu(II) adsorbed per unit mass of the adsorbent (q, mg/g) was found using the following equation: where: C o is the Cu(II) concentration before adsorption, mg/l; C t is the Cu(II) concentration after adsorption, mg/l; m represents the amount of used adsorbent, g; V is the volume of solution contact from the reactor.
The specific surface area and the nitrogen adsorption-desorption isotherms were measured with a Micromeritics ASAP 2010 device.The samples were degassed at 473 K, for 16 h.The value 0.1620 nm 2 was taken for all samples as the N 2 molecular cross-sectional area.
The morphological analysis was realized through scanning electron microscopy (SEM), using a Philips XL30 device.The samples were coated with Au and Pd under vacuum in an argon atmosphere, using a BALTEC SCD 050 apparatus.

Synthesis of Cu(II)-exchanged montmorillonite
Ion exchange process with Cu(II) ions was realized using three experimental procedures by varying the copper precursors (chloride and sulphate) and synthesis parameters.The experimental installation for the synthesis of Cu(II)-montmorillonite (Cu-Mt) samples is presented in Figure 1.The first experimental procedure consists in mixing of 5 g CaBent-pur with 100 ml of 0.1M CuCl 2 as reported in a previous work [17].The second method consists in treating of CaBent-pur with a CuSO 4 diluted solution as described in other previous paper [19].The third experimental protocol consists in the treatment of Cu-Mt, mixing by stirring (60 °C for 6 h) with a 0.15 M solution of CuCl 2 (mass ratio 1:20) and the pH value of the mixture was adjusted to 5 (0.1 M NaOH).After ion exchange, the samples were treated as described in a previous work [17].The obtained Cu(II)-exchanged montmorillonite samples were denoted in function of used protocol, as: Cu-Mt-1, Cu-Mt-2 and Cu-Mt-3 respectively.

RESULTS AND DISCUSSION
The adsorbed Cu(II) ions on the purified bentonite were quantified by atomic absorption spectrophotometry and the results are presented in Table 3.It is noted that the highest amount of Cu (II) adsorbed on clays was obtained for the sample issued by the third protocol (Cu-Mt-3), respectively 408.9 mg Cu(II)/g clay.This was expected because cation exchange is a stoichiometric reaction, and the laws of mass action apply.The specific surface area value for Cu-Mt-3 material (Table 3) increased by about 20 m 2 /g compared with crude bentonite (45.1 m 2 /g [19]).The values of the interlayer distance (d 001 ) are substantially similar in the case of all samples (Table 3).The decrease of d 001 value from 1.34 nm [19] in the case of raw material to 1.24 -1.30 nm, in the case of Cu-Mt samples, is due to the copper ionic radius.
Figure 2 shows the X-ray diffraction patterns of the Cu(II) ion exchanged samples.An accentuation of d 001 peak intensity is observed, for the Cu-Mt-1 sample (Figure 2, curve 1), which shows that the ion exchange process, carried out at ambient temperature, promotes the sample crystallinity.
The nitrogen adsorption-desorption isotherms of the Cu-Mt samples are showed in Figure 3.The isotherms are similar to type IV [20], being typical to mesoporous adsorbents.The resulting hysteresis is of type H3, which corresponds to the formation of aggregates of slit-like pores, with variable sizes [21].Analysis of the isotherms representation shows that the clays after ion exchange with Cu(II) ions, not only contain mesopores, but micropores in larger quantities.For low values of partial pressures (between 0.05 and 0.25) the nitrogen adsorption is linear, indicating the presence of a microporous structure.
The porosity parameters of modified nanomaterials are resumed in Table 4. Cu(II) ion exchange caused an increase of specific Langmuir surface area and both of total pore volume with respect to the parent clay [19].The specific Langmuir surface area was calculated from adsorption data in the relative pressure range 0.01 -0.05, using a nitrogen molecule cross-sectional area of 0.162 nm 2 .The total pore volume was assessed from the amount of nitrogen adsorbed at a relative pressure of 0.99.  Figure 4 presents the SEM images obtained for natural, purified and Cu-Mt samples.The morphological analysis of raw material emphasizes the presence of large particles, which correspond to free quartz [19] (Figure 4A), that is an impurity (Table 1).

Table 1 .
. Mineralogical composition of the natural calcium bentonite.

Table 2 .
Characteristics of natural calcium bentonite.
and the particles of montmorillonite, lower than 2 µm, are recovered based on Stokes' law.The obtained product is denoted CaBentpur.

Table 3 .
Chemical and physical characteristics of raw and modified materials.

Table 4 .
Textural properties derived from the nitrogen adsorption at 77 K.