Interlayer expansion of dimethyl ditallowylammonium montmorillonite as a function of 2-chloroaniline adsorption

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Applied Clay Science 41 (2008) 149 – 157 www.elsevier.com/locate/clay

Interlayer expansion of dimethyl ditallowylammonium montmorillonite as a function of 2-chloroaniline adsorption L. Zampori a,⁎, P. Gallo Stampino a , G. Dotelli a , D. Botta a , I. Natali Sora b , M. Setti c Department of Chemistry, Materials and Chemical Engineering “G. Natta”, Politecnico di Milano, INSTM R.U.-Polimi, p.zza L. da Vinci 32, 20133 Milano, Italy Department of Industrial Engineering, University of Bergamo, INSTM R.U.-Unibg, viale Marconi 5, 24044 Dalmine, Italy c Department of Earth Sciences, University of Pavia, via Ferrata 1, 27100 Pavia, Italy a

b

Received 2 March 2007; received in revised form 3 October 2007; accepted 5 October 2007 Available online 17 October 2007

Abstract The expansion behaviour of an organically modified montmorillonite during the adsorption of increasing amounts of an organic pollutant: 2-chloroaniline (2-CA). The organophilic montmorillonite, a commercial product, was obtained exchanging the inorganic cation with dimethyl ditallowylammonium ions (DMDTA). 2-CA was added to the organoclay, starting from 193.9 ppm up to 23,531.7 ppm going through twenty-two steps of growing quantities of 2-CA. For each of these twenty-two steps – corresponding to a range of adsorbed 2-CA (Cs) between 0.029 mol/kg (grams of 2-CA per kg of organoclay) and 2.763 mol/kg – the basal spacings were determined. The adsorption isotherm was performed according to ASTM D 4646-87 Standard and, by comparing the experimental Cs data with the corresponding d001 values, the basal expansion of the polluted organoclay is characterized by a steep rise for the first steps of adsorption of 2-CA (concentration in water at equilibrium Cw b 0.00373 mol/L), then the d001 remains almost constant up to the higher amounts of pollutant adsorbed. The 001 reflections are sharper, their intensity higher, and up to four orders of 00l reflections are observed as the uptake of 2-CA increased. Since in aqueous solution the staking of organoclay platelets is poorly ordered, the observed behaviour may be partially explained by the 2-CA sorbed on the external surface of the clay mineral particles, which brings the silicate layers together. Finally, a tentative interpretation of the adsorption phenomena was carried out by fitting the experimental data according to the most common theoretical models: Freundlich, Langmuir, Dual Mode and Dual Langmuir Model. © 2007 Elsevier B.V. All rights reserved. Keywords: Organoclay; Adsorption; Pollutant; Interlayer spacing

1. Introduction Contaminants, both organic and inorganic, can be removed by different natural and synthetic materials, such as ⁎ Corresponding author. Tel.: +39 02 2399 3233; fax: +39 02 70638173. E-mail address: [email protected] (L. Zampori). 0169-1317/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2007.10.003

carbonaceous materials (i.e. activated carbons, charcoal), zeolites and clays that are the most commonly used and studied (Gitipour et al., 1997; Jaynes and Vance, 1999; Basar, 2006). As for clays and clay minerals (Churchman et al., 2006), the treatment of inorganic pollutants may be accomplished with natural minerals, while, when dealing with organic contaminants, a modification of the hydrophilic character of natural clays is needed, by substituting

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the inorganic cations (Na+, Ca2+, Mg2+) by organic ones, typically (Groisman et al., 2004) quaternary ammonium cations, both with short and long aliphatic chain, depending on the properties required. Besides, clays may be also modified with anionic (Yang et al., 2005; Yao et al., 2005) and non-ionic (Breen et al., 1999; Backhaus et al., 2001) surfactants. Organoclays are widely used for many industrial and technological applications but, mainly in the last two decades, their interest and applications in polymer science, electric materials and in general organic– inorganic nanocomposite materials (Lee and Kim, 2002), began to overwhelm their environmental importance. In the latter field, their use might be successful when employed as barriers in the adsorption of toxic organic pollutants: studies were carried out investigating their capabilities in removing organic pollutants from water, herbicides from contaminated sites (Liao et al., 2006; Nir et al., 2000), humic acids, able to adsorb various pollutants (Anirudhan and Ramachandran, 2006) and aromatic compounds (Boyd et al., 1988; Natali Sora et al., 2005). Usually the starting mineral to obtain organoclays belongs to the family of smectites due to their superior expansion capabilities compared to other clay minerals and higher CEC (about 60–150 meq/100 g). In this work a montmorillonite modified with a long-chain quaternary ammonium cation (dimethyl ditallowylammonium DMDTA) was used in order to observe its adsorption capabilities over a wide range of concentrations (193–23,531 ppm) of an aromatic amine. The model contaminant was 2-chloroaniline (2-CA). The starting material and the polluted one were characterized by XRD and SEM. In addition, a thermodynamic modelling of the interactions between the organic pollutant and the organoclay, covering the whole range of concentrations tested, was carried out by using different thermodynamic models, such as: Freundlich, Langmuir and combined models (Dual Langmuir and Dual Mode Model). Combined models were only recently employed to model interactions between organic pollutants and organoclays (Sheng et al., 1996; Gonen and Rytwo, 2006). 2. Experimental 2.1. Materials The organoclay used throughout this work was an organomontmorillonite (OC1) provided by Laviosa Chimica Mineraria s.r.l. (Italy); exchanged with a dimethyl ditallowylammonium (DMDTA) ions. The montmorillonite was Fe-rich with a specific surface area of about 6 m2/g. The amount of organic

content (0.312 kg organic matter/kg organo-montmorillonite) was estimated by means of thermal analysis. The organic pollutant, 2-chloroaniline (2-CA), was purchased from Fluka (99.5% purity): 2-CA is a toxic amber liquid with amine odour boiling at 209 °C (density at 20 °C is 1.21 g/cm3, Table 1). Deionised Waters MilliQ® water was used throughout the whole study. 2.2. X-ray diffraction X-ray diffraction (XRD) measurements were carried out with a Philips PW 1830 diffractometer equipped with a graphite monochromator. Data were collected at room temperature over the 2θ range from 1° to 65°, using Cu-Kα radiation, with a step scan of 0.02° and the measurement time equal to 1 s per step. The observed 2θ line positions have been corrected using the small amount of quartz (d = 3.35 Å) in the clay fractions. The XRD line profile analysis was performed with TOPAS P software (Bruker AXS, Karlsruhe, Germany) using a split PseudoVoigt profile function and XRD intensities were corrected for background, geometrical and Lorentz polarization factors. The refined profiles were used for the determination of reflection positions. 2.3. Electron microscopy Morphology and particle size were studied using a LED 1530 GEMINI Field Emission Scanning Electron Microscope (FESEM). Samples were graphite-coated before starting the analysis. 2.4. Adsorption isotherm The adsorption isotherm was determined according to ASTM D 4646-87 Standard. Typically 2.5 g of organoclay was weighted in a centrifuge vial and 50 mL of 2-CA aqueous solutions, at different concentration in the range of 200– 40,000 mg/L, was added. The vials, fastened to a rotating arm, were left for 24 h, then centrifuged at 3750 rpm for 30 min. 1 mL of isooctane (RS Carlo Erba for spectroscopy) was added to 5 mL of the supernatant and shaken for 1 min. After separation from the aqueous layer, the organic phase was

Table 1 Physico-chemical properties of 2-chloroaniline MW (g/mol) a Tf (K) a Tb (K) a Density 20 °C (g/cm3) a Log10Kow a Solubility in water ppm (wt) a Solubility in water ppm (mol) a pKa b a

127.573 271 482 1.21 1.9 3.8E+03 5.3E+02 2.65

C.L. Yaws Chemical Properties Handbook., McGraw-Hill., 1999. Handbook of Chemistry and Physics, The Chemical Rubber CO., 1970. b

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film thickness. The temperature was linearly raised from 70 to 100 °C at 10 °C/min; then up to 130 °C at 4 °C/min and finally until 250 °C at 10 °C/min. A final isothermal time of 5 min at 250 °C was maintained. The amount of adsorbed 2-CA (Cs, mol/kg) was determined as difference between C0 and Cw, i.e. the amounts of 2-CA added and its residual fraction in the aqueous solution, respectively.

3. Results and discussion 3.1. Characterization of the organo-montmorillonite

Fig. 1. XRD diffractograms of the “as received”, glycolated and heated (550 °C) organo-montmorillonite.

analysed by gas chromatography (GC). The instrument used was a Carlo Erba Mega mod. 5160 equipped with an oncolumn injector, flame ionisation detector (FID) and a HP fused silica capillary column 0.32 mm internal diameter, 50 m length, coated with 5% phenylmethylsilicone rubber, 0.5 μm

The diffractograms of the organoclay “as received”, glycolated and after heating at 550 °C, are given in Fig. 1. The position of the (001) reflection of “as received” organoclay gives a d001 value of 25.7 Å. The large d001 spacing of the “as received” organoclay here is attributed to heterogeneous paraffin-type arrangements of the alkyl chains in the separation spacing of the structural layers (interlayer spacing) (Slade and Gates, 2004). The d001 expanded up to 38.3 Å for glycolated samples, while it collapsed to 11.4 Å after 550 °C heating. It is worth noting

Fig. 2. FESEM pictures, A) magnification 1500× of the “as received” sample, particle size is in the range 10–60 μm, B) magnification 5000× of a grain of the organo-montmorillonite, C) and D) magnifications 15,000× and 30,000×, respectively, of the grain of B.

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Fig. 3. Adsorption isotherm of 2-CA on the organo-montmorillonite. The isotherm is divided into three zones.

that glycolation resulted in sharper reflections and well defined 00l reflections with l N 1. The morphology of the raw organoclay was investigated by means of FESEM. The mean particle size was equal to about 10–60 μm (Fig. 2A, B); with a high grade of size heterogeneity. At higher magnification a certain rolling of a few sheets on the surface is noticed (Fig. 2C, D). 3.2. Adsorption isotherm

is given in Fig. 4. The basal spacing (d001) of the organomontmorillonite increases with increasing 2-CA adsorption. The (001) reflections are sharper and their intensity higher as the uptake of 2-CA increased, suggesting the clay structure is better ordered for Cs values greater than 1 mol/kg. In addition, for Cs N 2 mol/kg four orders of 00l reflections are observed. In Fig. 5 d001 is plotted against Cw: in zone I the organoclay exhibits a steep increase of d001 from 26.1 Å to 33.9 Å due to the adsorption from 0.029 mol/kg to 1 mol/kg, respectively. In zone II the d001 basal spacings

The range of 2-CA adsorbed was between 0.029 and 2.763 mol/kg, going through twenty-two steps of increasing amounts of 2-CA adsorbed (Fig. 3). The interaction between 2-CA and OC1 is characterized by a steep isotherm up to 1 mol/kg of 2-CA adsorbed, while for Cs N 1 mol/kg, the adsorption becomes less favoured. For Cs N 1.7 mol/kg adsorption increases again: the last three points of the isotherm, though, are not to be considered as a real uptake of 2-CA by the organoclay, but they might be thought as a “surface condensation” (Lagaly, 1987), i.e. 2-CA bound at the surface enhances the adsorption of other 2-CA molecules; this phenomenon is probably caused by the relatively low solubility limit of 2-CA (about 0.04 mol/L) in water, which is reached in correspondence of these three last data points. Taking into account these considerations, the adsorption isotherm is divided in three main parts: i) zone I, up to 1 mol/kg, ii) zone II between 1 mol/kg and 1.7 mol/kg and iii) zone III starting from 1.7 mol/kg and up to the end of the adsorption isotherm (see Fig. 3). 3.3. Basal spacings and 2-CA adsorption For each sample, loaded with 2-CA, the basal spacings were determined by XRD. A section of the diffraction pattern of selected samples showing the 001 reflection

Fig. 4. XRD diffractograms of the organo-montmorillonite loaded with increasing amounts of 2-CA.

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Fig. 5. Adsorption isotherm (triangles) of 2-CA and the corresponding d001 basal spacings (open points).

vs. Cw exhibits a plateau with values ranging from 34.9 Å to 35.4 Å. Samples belonging to zone III, characterized by an apparent steep of adsorption show similar d001 values as zone II samples.

Cs ¼

3.4. Thermodynamic modelling In order to better understand the interaction between an aromatic molecule and a typical organic modified smectite, a thermodynamic modelling was carried out, especially to explain the steep uptake of 2-CA accompanied by a steep increase of the basal spacing of the polluted organoclay in zone I and the consequent lowering of uptake capabilities while the basal spacing of the organoclay keeps constant in zone II. Since the uptake of 2-CA observed in zone III was only apparent, the last three data points will not be considered in the modelling of the adsorption mechanisms of 2-CA by the organoclay in this paragraph. The models considered were: Freundlich, Langmuir, Dual Langmuir and Dual Mode Model (Schwarzenbach et al., 1993). Freundlich model relates Cs and Cw with the equation: Cs ¼ Kf 

Cwn

Langmuir model describes the interaction of a surface where a limited number of adsorption sites are available:

ð1Þ

where Kf is the Freundlich constant (capacity factor), and n is the Freundlich exponent. This equation assumes there are a different number of adsorption sites able to operate contemporarily, each of them characterized by different abundance and free energy. When n is equal to 1 a linear interaction between sorbate and sorbent is described, instead, when n N 1 the addition of sorbate improves adsorption capabilities of the sorbent, while the opposite situation may be observed when n b 1 (Schwarzenbach et al., 1993).

S  KL  Cw 1 þ KL  Cw

ð2Þ

where KL is the Langmuir constant and S represents the total number of adsorption sites (maximum concentration the pollutant may reach inside the adsorbent). The Dual Langmuir model may be used to explain interactions where two different adsorption sites are involved: Cs ¼

S1 b1 Cw S2 b2 C w þ 1 þ b1 Cw 1 þ b2 Cw

ð3Þ

where the terms S1 and S2 are analogous to S of the Langmuir equation, and b1 and b2 represent the Dual Langmuir constants (equivalent to KL). The Dual Mode Model (DMM) is described by a Langmuir interaction combined with a term representing a partition phenomena: Cs ¼ Kd Cw þ

SbCw 1 þ bCw

ð4Þ

where S and b are analogous to S and KL of Langmuir model and Kd is the partition coefficient. The fitting of the adsorption isotherm, performed with the proposed equation, is reported in Fig. 6. The modelling performed with the Freundlich equation does not show a good fit (R2 = 0.96) of the whole adsorption isotherm, and it deviates significantly both for zone I and zone II data points. The Langmuir model allowed a very good fit for lower concentrations of 2-CA (up to 0.984 mol/kg, i.e. zone I), while for higher concentrations, since it describes the filling of a limited number of

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Fig. 6. Thermodynamic modelling.

adsorption sites, it tends to an asymptotic limit, which was not the case of the adsorption isotherm here considered. Since Langmuir model was able to fit the adsorption isotherm for lower amounts of 2-CA adsorbed, a fit performed using combined models based on Langmuir interaction was carried out. Dual Langmuir was, then, the first model considered and it actually showed an encouraging R2 (equal to 0.99), though it was obtained with fitting parameters whose physical significance was Table 2 Fitting parameters calculated for the four thermodynamic models here considered to fit the experimental sorption isotherm Freundlich

4. Discussion

Dual Langmuir

R2

KF

n

R2

S1

S2

b1

b2

0.96

9.53 ± 1.26

0.43 ± 0.02

0.99

3.38 ± 16.63

1.24 ± 0.21

7.96 ± 47.84

610.23 ± 117.06

Langmuir

depressed by the associated errors (Table 2). Also, the physical meaning of this model was hardly compatible with the system investigated. Indeed, one can hardly think of two chemically different adsorption sites of limited concentration in an organophilic clay mineral. Finally, a model based on combined phenomena (adsorption and partition), such as DMM, was able to describe the interaction between 2-CA and OC1. This model has the merit of fitting the low-concentration (Cs b 1 mol/kg) adsorption isotherm with a Langmuirtype interaction, while for higher amounts a partition effect is favoured.

Dual Mode

R2

S

KL

R2

Kd

S

b

0.99

1.67 ± 0.04

391.75 ± 24.78

0.99

22.24 ± 4.06

1.27 ± 0.05

593.63 ± 56.74

The adsorption of organic pollutants onto organoclays has been widely experimented over the past years, but a complete thermodynamic modelling considering a microstructural characterization has not been presented. Most of the studies dealt with the relation between the expansion behaviour and the amount of organic cation exchanged (Baoliang et al., 2005). The study of a wide range of pollutant concentrations has been quite rare so

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Fig. 7. Configuration of the organic cations inside the interlayer space of the organo-montmorillonite: A) without 2-CA, B) packing supposed for zone I (Cs b 1 mol/kg), and C) for zone II (1 N Cs N 1.7 mol/kg).

far. A recent work (Witthuhn et al., 2005) proposed an XRD characterization by the basal spacings compared to the adsorption isotherms of different partially cationexchanged montmorillonites during the adsorption of 2,4-dichlorophenol (2,4-DCP). 2,4-DCP and 2-CA adsorption isotherms, in a comparable range of adsorptive concentrations, present an analogous L-shaped curve. In the same way, the trend observed for d001 spacings in the present study is very similar to that reported by Witthuhn et al. (2005). They discussed adsorption and partitioning phenomena for different types of long-chain organoclays (monoalkyl

and dialkyl-organoclays). They proposed that adsorption may occur near the ammonium headgroups of the organic cations, in such a way that tilted alkyl chains move into a more perpendicular orientation thus expanding the interlayer space. These conclusions might be suitable for a molecule such as 2,4-DCP characterized by a higher pKa with respect to the one of 2-CA (7.85 and 2.65, respectively (Schwarzenbach et al., 1993)). This means that 2,4-DCP is dissociated in aqueous solution to a higher degree (about 12.4 mol% at pH = 7) than 2-chloroaniline, whose cationic form is practically negligible in aqueous solution at pH = 7; then, at least

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a small fraction of 2,4-DCP is present in anionic form and able to interact with the ammonium headgroups. Instead, it is more realistic, that 2-CA interacts with the alkyl chains of the organic cation and provokes its tilting. Furthermore, the Kd for the very first steps of 2-CA adsorption (0.029 b Cs (mol/kg) b 0.421) was calculated, adopting a linear regression, and it resulted equal to 615.75 L/kg. The Kd so calculated, if compared with the ones found by Witthuhn et al., who reported Kd vs. the amount of organic content (foc), showed similar results. The DMM, that considers the adsorption of 2-CA onto the organoclay to be ruled by both Langmuir interactions and a partitioning process, seemed to be able to efficiently describe the structural changes along with the sorption phenomena observed. For the first steps of 2-CA uptake the interaction between 2-CA and the organo-montmorillonite is dominated by the adsorption of 2-CA on the alkyl chains of the organic cations. After this adsorption capacity is exhausted, a partition effect seems to be favoured. This evidence might be supported by the close correlation observed between XRD data and modelling: the steep increase in the d001 value seems to be closely correlated with a direct interaction between the organic cation and 2-CA, similarly as reported by Witthuhn et al. (2005). When zone II starts the interlayer space does not longer expand, and this might be due to a partition effect due to the higher affinity of 2CA for the organic microenvironment of the interlayer space with respect to water. Similar results were recently obtained by Gonen and Rytwo (2006) while investigating 2,4,5-trichlorophenol adsorption onto an organoclay. The structure of the organo-montmorillonite undergoes apparent changes during the adsorption of 2-CA, as indicated by the XRD analysis. The adsorption of 2-CA expands the basal spacing up to 35.7 Å. The basal spacing of a dried montmorillonite without the organic interlayer cation is about 10 Å, for DMDTA, considering that the length of DMDTA is similar to the reported value of hexadecyl-trimethyl-ammonium ions (25.3 A, He et al., 2006) (Fig. 7). In zone III, where the uptake of 2-CA is due to “surface condensation”, the 2-CA adsorbed on the external surface of the clay mineral particles brings the silicate layers together, as a consequence, four orders of 00l reflections are observed. 5. Conclusions This study dealt with the thermodynamic modelling and microstructural characterization of an organically modified montmorillonite, during the adsorption of increasing amounts of 2-chloroaniline. 2-CA adsorption

induces a steep expansion of the interlayer space, up to a Cs = 1 mol/kg. After this point 2-CA is still adsorbed, but the interlayer space does no longer expand. The structural changes are closely correlated to the mechanism of 2-CA adsorption: A thermodynamic model (DMM) based on the combination of a Langmuir interaction and partitioning was able to describe the interaction between 2-CA and the organo-montmorillonite. Langmuirtype adsorption is predominant for low concentrations of 2-CA (Cs b 1 mol/kg), while for higher concentrations a partitioning effect seems to be favoured. We assume that in the first section of the isotherm 2-CA interacts with the alkyl chains of DMDTA cations, thus provoking the increase of the tilt angle of the alkyl chains and the observed steep expansion for the first steps of adsorption. While, at higher 2-CA concentrations, where the uptake of 2-CA is due to a surface condensation, the 2-CA adsorbed on the external surface of the clay mineral pulls together the silicate layers, as a consequence, four orders of 00l reflections are observed. It is important to stress that the good agreement between experimental data and DMM fit is not conclusive, though it seems that the combination of two subsequent interactions between the organo-montmorillonite and 2-CA (i.e. adsorption and partitioning) seems to be highly reasonable. Acknowledgments This research was supported by the Ministero dell'Università e della Ricerca Scientifica through the PRIN 20052006 project “Microstructure, morphology and reactivity: modelization and measure of crystallization and dissolution processes of minerals”. We thank Lucio Ogliani for his work during the experimental procedures and we also thank Laviosa Mineraria SpA and especially dr.ssa Cinzia Dalla Porta for having provided the organoclays used throughout this research. References Anirudhan, T.S., Ramachandran, M., 2006. Surfactant-modified bentonite as adsorbent for the removal of humic acid from wastewaters. Appl. Clay Sci. 35, 276–281. Backhaus, W.K., Klumpp, E., Narres, H.D., Schwuger, M.J., 2001. Adsorption of 2,4-dichlorophenol on montmorillonite and silica: influence of nonionic surfactants. J. Colloid Interface Sci. 242, 6–13. Baoliang, C., Lizhong, Z., Jiangxi, Z., Baoshan, X., 2005. Configurations of the bentonite-sorbed myristylpyridinium cation and their influences on the uptake of organic compounds. Environ. Sci. Technol. 39, 6093–6100. Basar, C.A., 2006. Applicability of the various adsorption models of three dyes adsorption onto activated carbon prepared waste apricot. J. Hazard. Mater., B 135, 232–241.

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