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Changes in the surfaces on DDOAB organoclays adsorbed with paranitrophenol—An XRD, TEM and TG study
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Materials Research Bulletin 43 (2008) 3318–3326 www.elsevier.com/locate/matresbu
Changes in the surfaces on DDOAB organoclays adsorbed with paranitrophenol—An XRD, TEM and TG study Qin Zhou a,b,c, Hongping He a,b, Ray L. Frost b,*, Yunfei Xi b,d a
Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology, GPO Box 2434, Brisbane, QLD 4001, Australia c Graduate University of Chinese Academy of Sciences, Beijing 100039, China d Centre for Environmental Risk Assessment & Remediation, University of South Australia, Mawson Lakes, SA 5095, Australia b
Received 4 July 2007; received in revised form 11 February 2008; accepted 18 February 2008 Available online 23 February 2008
Abstract The adsorption of paranitrophenol on organoclays synthesised by the ion exchange of the surfactant molecule dimethyldioctadecylammonium bromide (DDOAB) of formula (CH3(CH2)17)2NBr(CH3)2 has been studied by X-ray diffraction (XRD), transmission electron microscopy (TEM) and thermogravimetric analysis. The expansion of the montmorillonite depends on the loading of the montmorillonite with dimethyldioctadecylammonium bromide and is related to the arrangement of the surfactant molecules within the clay interlayer. This expansion is altered by the adsorbed paranitrophenol and is observed in the transmission electron microscopic images of the organoclay with adsorbed paranitrophenol. Changes in the surfactant molecular arrangements were analysed by thermogravimetry. The paranitrophenol is sublimed simultaneously with the loss of surfactant. The dehydroxylation temperature of the montmorillonite is decreased upon adsorption of the paranitrophenol indicating a bonding between the paranitrophenol and the hydroxyl units of the montmorillonite. # 2008 Elsevier Ltd. All rights reserved. Keywords: A. Layered compounds; A. Surfaces; C. X-ray diffraction; C. Electron microscopy; C. Thermogravimetric analysis (TGA)
1. Introduction Montmorillonites are widely used in a range of applications because of their high cation exchange capacity (CEC), swelling capacity, high surface areas, and consequential strong adsorption and absorption capacities [1–4]. These clay properties can be enhanced by converting the montmorillonite to an organoclay by ion exchange of the cation with a surfactant molecule [1,5–7]. Such exchange was first discovered some 40–50 years ago [8–11]. The hydration of inorganic cations on the exchange sites causes the clay mineral surface to be hydrophilic. Natural clays are thus ineffective sorbents for organic compounds. Modification of the montmorillonite clay with surfactant molecules changes the properties of the clay. The intercalation of a cationic surfactant not only changes the surface properties from hydrophilic to hydrophobic but also greatly increases the basal spacing of the layers [1,5–7,12].
* Corresponding author. Tel.: +61 7 3138 2407; fax: +61 7 3138 1804. E-mail address: [email protected] (R.L. Frost). 0025-5408/$ – see front matter # 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2008.02.015
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Such surface property changes will effect the applications of the organoclay. In particular, the hydrophobic nature of the organoclay implies that the material can be used as a filter material for water purification through for example the removal of hydrocarbons and pesticides [12–18]. Studies have shown that replacing the inorganic exchangeable cations of clay minerals with organic cations can result in a greatly enhanced capacity of these materials to remove organic contaminants [19,20]. The adsorption properties of the organoclay strongly depend on the structure and molecular arrangements of the surfactant molecules within the organoclays [21–23]. Understanding the structure and properties of synthesised organoclays is essential for their adsorption/absorption applications [24–27]. Recently studies of organoclays with single alkyl chain surfactant molecules has been published [28–33]. The objective of this work is the study of the adsorption of the paranitrophenol on an organoclay synthesised by using a surfactant molecule with two alkyl chains. This study is of great importance for understanding the structure, properties, and potential applications of these types of organoclays. 2. Experimental 2.1. Materials Montmorillonite (Na0.053Ca0.176Mg0.1nH2O)[Al1.58Fe0.03Mg0.39][Si3.77Al0.23]O10(OH)2 used was primarily CaMt from Neimeng, China. The montmorillonite was cation exchanged with sodium ions by repeated reaction with sodium carbonate. Its cation exchange capacity is 90.8 mequiv./100 g. The p-nitrophenol and DDOAB used were of analytical grade chemical reagents and were supplied by YuanJu Chem. Co. Ltd., China. The aqueous solubility of pnitrophenol is 1.6 104 mg/L at 25 8C. The surfactant used was dimethyldioctadecylammonium bromide labeled as DDOAB of formula (CH3(CH2)17)2NBr(CH3)2. 2.2. Preparation of the organoclay The syntheses of surfactant-clay hybrids were undertaken by each of the following procedure: the pure Ca-Mt was added into Na2CO3 solution, stirred for 3 h with 800 rpm and drops of HCl were added into the suspension to dissolve the CO32 . Then the suspension was washed several times with deionized water until it was chloride free and dried at 108 8C. Such a treated montmorillonite is designated as Na-Mt. This procedure is similar to the preparation of HDTMA+ organoclay [34]. The clarifying surfactant solution was obtained when certain amounts DDOAB were added into hot distilled water. Then certain amounts Na-Mt were added into the above mentioned solution and the mixtures were stirred slightly in order to avoid the yield of spume in an 80 8C water bath for 2 h. The water/Na-Mt mass ratio is 10. Then the suspension was subsequently washed with distilled water for 4 times. The moist solid material was dried at 60 8C and ground with a mortar. The different amounts of [DDOA]+ pillared montmorillonites were identified by 0.5 CEC-Mt, 0.7 CEC-Mt, 1.5 CEC-Mt, 2.5 CEC-Mt. 2.3. Adsorption of the paranitrophenol on the organoclay A total of 0.2 g of different type montmorillonites were combined with 30 mL of a range of concentrations of pnitrophenol solution whose initial pH value is about 5.0 in 50 mL Erlenmeyer flasks with glass caps. A range of concentrations from 100 to 8000 mg/L were used for the adsorption of paranitrophenol. The flasks were shaken for 6 h at 150 rpm at 25 8C. After being centrifuged at 3500 rpm for 10 min, the p-nitrophenol concentration in the aqueous phase was determined by a UV-260 spectrophotometer at 317 nm, the detection limits being 0.05 mg/L. The losses of the pnitrophenol by both photochemical decomposition and volatilization were found to be negligible during adsorption [35]. 2.4. Characterization methods 2.4.1. Thermogravimetric analysis Thermogravimetric analyses of the surfactant modified montmorillonite hybrids were obtained using a TA Instruments Inc. Q500 high-resolution TGA operating at ramp 10 8C/min with resolution 6.0 8C from room temperature to 1000 8C in a high-purity flowing nitrogen atmosphere (60 cm3/min). Approximately 50 mg of finely ground sample was heated in an open platinum crucible. For the thermogravimetric analyses 4000 mg/L of
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paranitrophenol was used for the adsorption prior to TG analysis. This concentration was chosen as it is in the midrange of the concentrations used. 2.4.2. X-ray diffraction The Neimeng montmorillonite and surfactant montmorillonite hybrids were pressed in stainless steel sample ´˚ holders. X-ray diffraction patterns were recorded using Cu Ka radiation (n = 1.5418 A ) on a Philips PANalytical X’Pert PRO diffractometer operating at 40 kVand 40 mA with 0.258 divergence slit, 0.58 anti-scatter slit, between 1.58 and 208 (2u) at a step size of 0.01678. For XRD at low angle section, it was between 18 and 58 (2u) at a step size of 0.01678 with variable divergence slit and 0.1258 anti-scatter. For the XRD analyses 4000 mg/L of paranitrophenol was used for the adsorption on the organoclays prior to powder XRD analysis. 2.4.3. Transmission electron microscopy A Philips CM 200 transmission electron microscopy (TEM) at 200 kV is used to investigate the microstructural of organoclays. All samples were dispersed in 50% ethanol solution and then dropped on carbon-coated films, dried in an oven at 50 8C for 10 min for TEM studies without using any resin. 3. Results and discussion 3.1. Powder X-ray diffraction analysis With the cation exchange of the sodium ion for the cationic surfactant, expansion of the montmorillonite layers occurs. This expansion is readily measured by X-ray diffraction. The powder XRD patterns of the Na montmorillonite
Fig. 1. XRD patterns of Na-Mt, 0.5, 0.7, 1.5 and 2.5 CEC-DDOAB.
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and the organoclays with 0.5, 0.7, 1.5 and 2.5 CEC surfactant concentrations are shown in Fig. 1. The Na montmorillonite has a d(0 0 1) spacing of 1.24 nm. After the montmorillonite is exchanged with DDOAB at the 0.5 CEC level, three d(0 0 1) spacings are observed at 1.46, 1.94 and 3.35 nm. The reason for the three spacings can be attributed to the arrangement of the DDOAB surfactant molecule in the clay layers. The d(0 0 1) spacing of 1.46 nm is a similar space to the Na montmorillonite. This spacing is attributed to Na clay. The spacing of 1.94 nm is ascribed to the montmorillonite expanded with the surfactant molecule with the molecule laying flat between the two surfaces. The 3.35 nm spacing is attributed to the surfactant molecules at right angles to the clay surface. A similar surfactant arrangement is proposed for the 0.7 CEC montmorillonite. For this concentration the 1.94 nm spacing was not observed. When the surfactant loading is raised to 1.5 CEC, three d(0 0 1) spacings are observed at 1.18 ascribed to Na clay, 1.80 nm assigned to a molecular arrangement with the surfactant molecules laying flat to the surface and 3.59 nm attributed to the surfactant molecules at 908 to the clay surface. Upon increasing the surfactant loading to 2.5 CEC, d(0 0 1) spacings of 1.93 and 4.96 nm are observed. This latter large d(0 0 1) spacing is described in terms of overlapping surfactant molecules in a paraffin-like arrangement between the clay layers. Upon adsorption of paranitrophenol on the organoclay surfaces structural rearrangement of the surfactant molecules between the clay layers occurs (Fig. 2). For the pnp adsorbed on the non-intercalated montmorillonite a slight expansion of the clay layers from 1.24 to 1.49 nm is observed. This increase gives an indication that the pnp is penetrating the clay layers. For the 0.5 CEC organoclay with adsorbed pnp, two expansions of 1.47 and 1.83 nm are found. No large expansion as for the 0.5 CEC organoclay is found. These values suggest that the pnp has penetrated the clay layers and has bonded to the surfactant molecule within the clay layers. Similar expansions are observed for the 0.7 CEC organoclay with adsorbed pnp. For the 1.5 CEC organoclay with adsorbed pnp spacings of 1.0, 1.34, 1.99 and 3.91 nm are observed. The 1.00 nm spacing is attributed to montmorillonite with a collapsed structure in which the surfactant molecule has not been intercalated and has not been replaced with water. The 1.34 nm spacing is ascribed to
Fig. 2. XRD patterns of Na-Mt, 0.5, 0.7, 1.5 and 2.5 CEC-DDOAB with adsorbed pnp.
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the organoclay with minimal pnp adsorption. The 1.99 and 3.91 nm expanded layers is assigned to surfactant molecules with adsorbed pnp between the clay layers. These two expansions may be compared with the 1.8 and 3.59 nm expansions of the 1.5 CEC organoclay without adsorbed pnp. The difference in the values (3.91–3.59) and (1.99–1.80 nm) is attributed to the size of the pnp molecule. This proves that the pnp is intercalated into the montmorillonite clay and is part of the molecular arrangement in the clay interlayer. Similar values are obtained for the 2.5 CEC organoclay with adsorbed pnp. Here expansions of 1.00, 1.33, 1.98 and 3.81 nm are observed. These results clearly show that the pnp molecules are intercalated into the clay layers. 3.2. Transmission electron microscopy TEM images of 1.5 and 2.5 CEC organoclay with adsorbed pnp are shown in Fig. 3a and b (1.5 CEC) and Fig. 4a and b (2.5 CEC). Two spacings for the 1.5 CEC organoclay with adsorbed pnp are observed in Fig. 3a at 1.02 and 1.36 nm. In Fig. 3b two spacings are observed at 2.0 and 3.96 nm. In the XRD patterns for the 1.5 CEC with adsorbed
Fig. 3. (a and b) TEM images of 1.5 CEC-D with adsorbed pnp.
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Fig. 4. (a and b) TEM images of 2.5 CEC-D with adsorbed pnp.
pnp d(0 0 1) spacings at 1.0, 1.34, 1.99 and 3.91 nm (Fig. 2). The correspondence between the XRD results and the TEM data is excellent. The d(0 0 1) spacings for the 2.5 CEC organoclay with adsorbed pnp as illustrated in Fig. 4a are 1.37 and 1.98 nm. These values correspond to the d(0 0 1) spacings in the XRD patterns of 1.33 and 1.98 nm (Fig. 2). Two spacings are observed in the TEM image of the 2.5 CEC organoclay with adsorbed pnp at 3.8 and 0.95 nm. These correspond to the d(0 0 1) spacings of 1.0 and 3.81 nm (Fig. 2). 3.3. Thermogravimetric analysis The DTG patterns for the montmorillonite, the pnp, the surfactant, and the surfactant loaded organoclays with and without pnp are shown in Fig. 5. The pnp sublimes at 131 8C; the DDOAB shows thermal decomposition steps at 168, 245 and 305 8C attributed to combustion of the surfactant molecules; the montmorillonite shows thermal decomposition steps at 485 and 617 8C. The 0.5 CEC organoclay differential thermal analysis pattern shows peaks at 590, 389 and 300 8C. A slight decrease in the dehydroxylation of the clay is observed (590 compared with 617 8C). The peaks at 389 and 300 8C are higher than is observed for the surfactant on its own. This suggests that the surfactant molecule is bonded to the clay surfaces and more heat is required to remove the surfactant molecules from the clay surfaces. The DTG pattern for the 0.5 CEC-4000 (organoclay with adsorbed pnp) is quite different to that of the 0.5 CEC organoclay. Three decomposition temperatures are observed at 382, 268 and 189 8C. Interestingly the dehydroxylation peak at 590 8C is not found. This suggests the reaction of the clay with pnp occurs through the OH
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Fig. 5. DTG patterns of 0.5, 0.7, 1.5 and 2.5 CEC-DDOAB organoclay with and without adsorbed pnp.
units of the montmorillonite and the dehydroxylation temperature is reduced to 382 8C. The two peaks at 286 and 189 8C are attributed to the combustion of the surfactant and as no DTG peaks attributable to the pnp are observed, it is proposed that the pnp is removed simultaneously with the surfactant. The thermal decomposition steps for the 0.7 CEC organoclay are similar to that for the 0.5 CEC organoclay with DTG peaks observed at 293, 396 and 590 8C. The latter is due to the montmorillonite and the first two steps to the loss of the surfactant. For the 0.7 CEC organoclay with adsorbed pnp, DTG peaks are observed at 193, 283 and 385 8C. No peak is found at 590 8C. This suggests that the pnp results in the lowering of the clay dehydroxylation temperature. For the 1.5 CEC organoclay without adsorbed pnp, DTG peaks are observed at 229, 341, 392 and 590 8C. The first three peaks are ascribed to the combustion of the surfactant and the latter to the dehydroxylation of the montmorillonite. It is noted that the three temperatures (229, 341, 392) are significantly higher than for the DTG peaks of the surfactant (168, 245, and 305 8C. This difference in temperatures is attributed to the bonding of the surfactant molecules to the montmorillonite surfaces. Heating the organoclay to higher temperatures is required before the surfactant molecules are removed. Thermal decomposition peaks are observed at 178, 241, 295, 346 and 600 8C for the 2.5 CEC organoclay. The first three peaks correspond to the three thermal decomposition steps of DDOAB at 168, 245 and 305 8C and are attributed to the loss of surfactant molecules. This temperature correspondence indicates that at 2.5 CEC the surfactant molecules are not only intercalated within the montmorillonite layers but also are adsorbed on the external surfaces of the clay. The temperature of dehydroxylation of the montmorillonite is now at 600 8C. For the 2.5 CEC organoclay with adsorbed pnp, thermal decomposition steps are observed at 180, 231, 285, 336 and 412 8C. The first three decomposition steps are assigned to the loss of surfactant. The decomposition steps at 336 and 412 are attributed to the loss of intercalated surfactant molecules and the dehydroxylation of the montmorillonite. It is assumed that the loss of the pnp occurs simultaneously with the surfactant.
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4. Conclusions Water purification is of extreme importance in many parts of the world, including Australia and China. Many of the world’s water ways and water sources are polluted or contaminated with a range of chemicals including pesticides and herbicides. In this work, we have used paranitrophenol as a test chemical to design and test an organoclay for the removal of pnp from an aqueous medium. This work has shown that the pnp intercalates the organoclay and displaces the surfactant molecules or rearranges the structure of the surfactant molecule within the organoclay interlayer. Future work will test the use of this organoclay for the removal of herbicides and pesticides from water and will need to test the efficiency of the pnp adsorption. Synthesis of the organoclay with dimethyldioctadecylammonium bromide results in multiple expansions of the montmorillonite clay from 1.24 nm to a maximum of 4.96 nm as is evidenced by the X-ray diffraction patterns and confirmed by the transmission electron microscopic images. These expansions depend upon the arrangement of the surfactant molecules with the montmorillonite interlayer. Adsorption of pnp affects this arrangement and results in different spacings. Thermoanalytical studies show that the thermal decomposition of the surfactant molecules occurs at a higher temperature. Adsorption of the pnp results in a significant decrease in the dehydroxylation temperature of the montmorillonite. It is proposed that the pnp is bonding to the hydroxyl units of the montmorillonite. This affects the molecular arrangements of the surfactant molecules in the clay interlayer. Acknowledgements This work was financially supported by National Science Fund for Distinguished Young Scholars (Grant No. 40725006) and the grant of the Knowledge Innovation Program of the Chinese Academy of Sciences (Grant No. Kzcx2-yw-112). The Inorganic Materials Research Program, Queensland University of Technology, is gratefully acknowledged for infra-structural support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
H. He, R.L. Frost, T. Bostrom, P. Yuan, L. Duong, D. Yang, Y. Xi, J.T. Kloprogge, Appl. Clay Sci. 31 (2006) 262–271. H. He, Z. Ding, J. Zhu, P. Yuan, Y. Xi, D. Yang, R.L. Frost, Clay Clay Miner. 53 (2005) 287–293. Y. Xi, Z. Ding, H. He, R.L. Frost, J. Colloid Interf. Sci. 277 (2004) 116–120. Y. Xi, R.L. Frost, H. He, T. Kloprogge, T. Bostrom, Langmuir 21 (2005) 8675–8680. Y. Xi, L. Frost Ray, H. He, T. Kloprogge, T. Bostrom, Langmuir: ACS J. Surf. Colloids 21 (2005) 8675–8680. Y. Xi, W. Martens, H. He, R.L. Frost, J. Therm. Anal. Calorim. 81 (2005) 91–97. Y. Xi, Z. Ding, H. He, L. Frost Ray, J. Colloid Interf. Sci. 277 (2004) 116–120. R. Glaeser, Mem. Services Chim. Etat (Paris) 39 (1954) 19–58. R. Glaeser, Mem. Services Chim. Etat 39 (1954) 81–108. V.S. Komarov, N.F. Ermolenko, V.I. Varlamov, Dokl. Akad. Nauk BSSR 5 (1961) 105–108. R.M. Barrer, A.D. Millington, J. Colloid Interf. Sci. 25 (1967) 359–372. S.A. Boyd, S. Shaobai, J.-F. Lee, M.M. Mortland, Clay Clay Miner. 36 (1988) 125–130. W.H. Slabaugh, R.W. Vasofsky, Clay Clay Miner., Proc. Conf. 23 (1975) 458–461. K.R. Srinivasan, H.S. Fogler, Organohalogen Compd. 3 (1990) 417–420. K.R. Srinivasan, H.S. Fogler, Clay Clay Miner. 38 (1990) 287–293. K.R. Srinivasan, H.S. Fogler, Clay Clay Miner. 38 (1990) 277–286. W.F. Jaynes, S.A. Boyd, J. Air (Waste Manage. Assoc.) 40 (1990–1992) 1649–1653. M.C. Hermosin, J. Cornejo, Adsorption of 2,4-D on organo-clays. Inst. Recur. Nat. Agrobiol. Sevilla, CSIC, Seville, Spain. FIELD URL, 1990, 73–79. M.M. Mortland, S. Shaobai, S.A. Boyd, Clay Clay Miner. 34 (1986) 581–585. J.A. Smith, P.R. Jaffe, C.T. Chiou, Environ. Sci. Technol. 24 (1990) 1167–1172. J.A. Smith, P.R. Jaffe, Environ. Sci. Technol. 25 (1991) 2054–2058. W.F. Jaynes, S.A. Boyd, Soil Sci. Soc. Am. J. 55 (1991) 43–48. W.F. Jaynes, S.A. Boyd, Clay Clay Miner. 39 (1991) 428–436. S.A. Boyd, W.F. Jaynes, CMS Workshop Lectures, 6 (1994) 48–77. I. Dekany, A. Farkas, I. Regdon, E. Klumpp, H.D. Narres, M.J. Schwuger, Colloid Polym. Sci. 274 (1996) 981–988. S.A. Boyd, R. Kukkadapu, Method of Removing Organic Contaminants from Air and Water with Organophilic, Quaternary Phosphonium Ionexchanged Smectite Clays, Michigan State University, USA, 1996, 16 pp.. C. Breen, A. Moronta, J. Phys. Chem. B 103 (1999) 5675–5680. Q. Zhou, R.L. Frost, H. He, Y. Xi, J. Colloid Interf. Sci. 307 (2007) 50–55.
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Materials Research Bulletin 43 (2008) 3318–3326 www.elsevier.com/locate/matresbu
Changes in the surfaces on DDOAB organoclays adsorbed with paranitrophenol—An XRD, TEM and TG study Qin Zhou a,b,c, Hongping He a,b, Ray L. Frost b,*, Yunfei Xi b,d a
Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology, GPO Box 2434, Brisbane, QLD 4001, Australia c Graduate University of Chinese Academy of Sciences, Beijing 100039, China d Centre for Environmental Risk Assessment & Remediation, University of South Australia, Mawson Lakes, SA 5095, Australia b
Received 4 July 2007; received in revised form 11 February 2008; accepted 18 February 2008 Available online 23 February 2008
Abstract The adsorption of paranitrophenol on organoclays synthesised by the ion exchange of the surfactant molecule dimethyldioctadecylammonium bromide (DDOAB) of formula (CH3(CH2)17)2NBr(CH3)2 has been studied by X-ray diffraction (XRD), transmission electron microscopy (TEM) and thermogravimetric analysis. The expansion of the montmorillonite depends on the loading of the montmorillonite with dimethyldioctadecylammonium bromide and is related to the arrangement of the surfactant molecules within the clay interlayer. This expansion is altered by the adsorbed paranitrophenol and is observed in the transmission electron microscopic images of the organoclay with adsorbed paranitrophenol. Changes in the surfactant molecular arrangements were analysed by thermogravimetry. The paranitrophenol is sublimed simultaneously with the loss of surfactant. The dehydroxylation temperature of the montmorillonite is decreased upon adsorption of the paranitrophenol indicating a bonding between the paranitrophenol and the hydroxyl units of the montmorillonite. # 2008 Elsevier Ltd. All rights reserved. Keywords: A. Layered compounds; A. Surfaces; C. X-ray diffraction; C. Electron microscopy; C. Thermogravimetric analysis (TGA)
1. Introduction Montmorillonites are widely used in a range of applications because of their high cation exchange capacity (CEC), swelling capacity, high surface areas, and consequential strong adsorption and absorption capacities [1–4]. These clay properties can be enhanced by converting the montmorillonite to an organoclay by ion exchange of the cation with a surfactant molecule [1,5–7]. Such exchange was first discovered some 40–50 years ago [8–11]. The hydration of inorganic cations on the exchange sites causes the clay mineral surface to be hydrophilic. Natural clays are thus ineffective sorbents for organic compounds. Modification of the montmorillonite clay with surfactant molecules changes the properties of the clay. The intercalation of a cationic surfactant not only changes the surface properties from hydrophilic to hydrophobic but also greatly increases the basal spacing of the layers [1,5–7,12].
* Corresponding author. Tel.: +61 7 3138 2407; fax: +61 7 3138 1804. E-mail address: [email protected] (R.L. Frost). 0025-5408/$ – see front matter # 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2008.02.015
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Such surface property changes will effect the applications of the organoclay. In particular, the hydrophobic nature of the organoclay implies that the material can be used as a filter material for water purification through for example the removal of hydrocarbons and pesticides [12–18]. Studies have shown that replacing the inorganic exchangeable cations of clay minerals with organic cations can result in a greatly enhanced capacity of these materials to remove organic contaminants [19,20]. The adsorption properties of the organoclay strongly depend on the structure and molecular arrangements of the surfactant molecules within the organoclays [21–23]. Understanding the structure and properties of synthesised organoclays is essential for their adsorption/absorption applications [24–27]. Recently studies of organoclays with single alkyl chain surfactant molecules has been published [28–33]. The objective of this work is the study of the adsorption of the paranitrophenol on an organoclay synthesised by using a surfactant molecule with two alkyl chains. This study is of great importance for understanding the structure, properties, and potential applications of these types of organoclays. 2. Experimental 2.1. Materials Montmorillonite (Na0.053Ca0.176Mg0.1nH2O)[Al1.58Fe0.03Mg0.39][Si3.77Al0.23]O10(OH)2 used was primarily CaMt from Neimeng, China. The montmorillonite was cation exchanged with sodium ions by repeated reaction with sodium carbonate. Its cation exchange capacity is 90.8 mequiv./100 g. The p-nitrophenol and DDOAB used were of analytical grade chemical reagents and were supplied by YuanJu Chem. Co. Ltd., China. The aqueous solubility of pnitrophenol is 1.6 104 mg/L at 25 8C. The surfactant used was dimethyldioctadecylammonium bromide labeled as DDOAB of formula (CH3(CH2)17)2NBr(CH3)2. 2.2. Preparation of the organoclay The syntheses of surfactant-clay hybrids were undertaken by each of the following procedure: the pure Ca-Mt was added into Na2CO3 solution, stirred for 3 h with 800 rpm and drops of HCl were added into the suspension to dissolve the CO32 . Then the suspension was washed several times with deionized water until it was chloride free and dried at 108 8C. Such a treated montmorillonite is designated as Na-Mt. This procedure is similar to the preparation of HDTMA+ organoclay [34]. The clarifying surfactant solution was obtained when certain amounts DDOAB were added into hot distilled water. Then certain amounts Na-Mt were added into the above mentioned solution and the mixtures were stirred slightly in order to avoid the yield of spume in an 80 8C water bath for 2 h. The water/Na-Mt mass ratio is 10. Then the suspension was subsequently washed with distilled water for 4 times. The moist solid material was dried at 60 8C and ground with a mortar. The different amounts of [DDOA]+ pillared montmorillonites were identified by 0.5 CEC-Mt, 0.7 CEC-Mt, 1.5 CEC-Mt, 2.5 CEC-Mt. 2.3. Adsorption of the paranitrophenol on the organoclay A total of 0.2 g of different type montmorillonites were combined with 30 mL of a range of concentrations of pnitrophenol solution whose initial pH value is about 5.0 in 50 mL Erlenmeyer flasks with glass caps. A range of concentrations from 100 to 8000 mg/L were used for the adsorption of paranitrophenol. The flasks were shaken for 6 h at 150 rpm at 25 8C. After being centrifuged at 3500 rpm for 10 min, the p-nitrophenol concentration in the aqueous phase was determined by a UV-260 spectrophotometer at 317 nm, the detection limits being 0.05 mg/L. The losses of the pnitrophenol by both photochemical decomposition and volatilization were found to be negligible during adsorption [35]. 2.4. Characterization methods 2.4.1. Thermogravimetric analysis Thermogravimetric analyses of the surfactant modified montmorillonite hybrids were obtained using a TA Instruments Inc. Q500 high-resolution TGA operating at ramp 10 8C/min with resolution 6.0 8C from room temperature to 1000 8C in a high-purity flowing nitrogen atmosphere (60 cm3/min). Approximately 50 mg of finely ground sample was heated in an open platinum crucible. For the thermogravimetric analyses 4000 mg/L of
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paranitrophenol was used for the adsorption prior to TG analysis. This concentration was chosen as it is in the midrange of the concentrations used. 2.4.2. X-ray diffraction The Neimeng montmorillonite and surfactant montmorillonite hybrids were pressed in stainless steel sample ´˚ holders. X-ray diffraction patterns were recorded using Cu Ka radiation (n = 1.5418 A ) on a Philips PANalytical X’Pert PRO diffractometer operating at 40 kVand 40 mA with 0.258 divergence slit, 0.58 anti-scatter slit, between 1.58 and 208 (2u) at a step size of 0.01678. For XRD at low angle section, it was between 18 and 58 (2u) at a step size of 0.01678 with variable divergence slit and 0.1258 anti-scatter. For the XRD analyses 4000 mg/L of paranitrophenol was used for the adsorption on the organoclays prior to powder XRD analysis. 2.4.3. Transmission electron microscopy A Philips CM 200 transmission electron microscopy (TEM) at 200 kV is used to investigate the microstructural of organoclays. All samples were dispersed in 50% ethanol solution and then dropped on carbon-coated films, dried in an oven at 50 8C for 10 min for TEM studies without using any resin. 3. Results and discussion 3.1. Powder X-ray diffraction analysis With the cation exchange of the sodium ion for the cationic surfactant, expansion of the montmorillonite layers occurs. This expansion is readily measured by X-ray diffraction. The powder XRD patterns of the Na montmorillonite
Fig. 1. XRD patterns of Na-Mt, 0.5, 0.7, 1.5 and 2.5 CEC-DDOAB.
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and the organoclays with 0.5, 0.7, 1.5 and 2.5 CEC surfactant concentrations are shown in Fig. 1. The Na montmorillonite has a d(0 0 1) spacing of 1.24 nm. After the montmorillonite is exchanged with DDOAB at the 0.5 CEC level, three d(0 0 1) spacings are observed at 1.46, 1.94 and 3.35 nm. The reason for the three spacings can be attributed to the arrangement of the DDOAB surfactant molecule in the clay layers. The d(0 0 1) spacing of 1.46 nm is a similar space to the Na montmorillonite. This spacing is attributed to Na clay. The spacing of 1.94 nm is ascribed to the montmorillonite expanded with the surfactant molecule with the molecule laying flat between the two surfaces. The 3.35 nm spacing is attributed to the surfactant molecules at right angles to the clay surface. A similar surfactant arrangement is proposed for the 0.7 CEC montmorillonite. For this concentration the 1.94 nm spacing was not observed. When the surfactant loading is raised to 1.5 CEC, three d(0 0 1) spacings are observed at 1.18 ascribed to Na clay, 1.80 nm assigned to a molecular arrangement with the surfactant molecules laying flat to the surface and 3.59 nm attributed to the surfactant molecules at 908 to the clay surface. Upon increasing the surfactant loading to 2.5 CEC, d(0 0 1) spacings of 1.93 and 4.96 nm are observed. This latter large d(0 0 1) spacing is described in terms of overlapping surfactant molecules in a paraffin-like arrangement between the clay layers. Upon adsorption of paranitrophenol on the organoclay surfaces structural rearrangement of the surfactant molecules between the clay layers occurs (Fig. 2). For the pnp adsorbed on the non-intercalated montmorillonite a slight expansion of the clay layers from 1.24 to 1.49 nm is observed. This increase gives an indication that the pnp is penetrating the clay layers. For the 0.5 CEC organoclay with adsorbed pnp, two expansions of 1.47 and 1.83 nm are found. No large expansion as for the 0.5 CEC organoclay is found. These values suggest that the pnp has penetrated the clay layers and has bonded to the surfactant molecule within the clay layers. Similar expansions are observed for the 0.7 CEC organoclay with adsorbed pnp. For the 1.5 CEC organoclay with adsorbed pnp spacings of 1.0, 1.34, 1.99 and 3.91 nm are observed. The 1.00 nm spacing is attributed to montmorillonite with a collapsed structure in which the surfactant molecule has not been intercalated and has not been replaced with water. The 1.34 nm spacing is ascribed to
Fig. 2. XRD patterns of Na-Mt, 0.5, 0.7, 1.5 and 2.5 CEC-DDOAB with adsorbed pnp.
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the organoclay with minimal pnp adsorption. The 1.99 and 3.91 nm expanded layers is assigned to surfactant molecules with adsorbed pnp between the clay layers. These two expansions may be compared with the 1.8 and 3.59 nm expansions of the 1.5 CEC organoclay without adsorbed pnp. The difference in the values (3.91–3.59) and (1.99–1.80 nm) is attributed to the size of the pnp molecule. This proves that the pnp is intercalated into the montmorillonite clay and is part of the molecular arrangement in the clay interlayer. Similar values are obtained for the 2.5 CEC organoclay with adsorbed pnp. Here expansions of 1.00, 1.33, 1.98 and 3.81 nm are observed. These results clearly show that the pnp molecules are intercalated into the clay layers. 3.2. Transmission electron microscopy TEM images of 1.5 and 2.5 CEC organoclay with adsorbed pnp are shown in Fig. 3a and b (1.5 CEC) and Fig. 4a and b (2.5 CEC). Two spacings for the 1.5 CEC organoclay with adsorbed pnp are observed in Fig. 3a at 1.02 and 1.36 nm. In Fig. 3b two spacings are observed at 2.0 and 3.96 nm. In the XRD patterns for the 1.5 CEC with adsorbed
Fig. 3. (a and b) TEM images of 1.5 CEC-D with adsorbed pnp.
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Fig. 4. (a and b) TEM images of 2.5 CEC-D with adsorbed pnp.
pnp d(0 0 1) spacings at 1.0, 1.34, 1.99 and 3.91 nm (Fig. 2). The correspondence between the XRD results and the TEM data is excellent. The d(0 0 1) spacings for the 2.5 CEC organoclay with adsorbed pnp as illustrated in Fig. 4a are 1.37 and 1.98 nm. These values correspond to the d(0 0 1) spacings in the XRD patterns of 1.33 and 1.98 nm (Fig. 2). Two spacings are observed in the TEM image of the 2.5 CEC organoclay with adsorbed pnp at 3.8 and 0.95 nm. These correspond to the d(0 0 1) spacings of 1.0 and 3.81 nm (Fig. 2). 3.3. Thermogravimetric analysis The DTG patterns for the montmorillonite, the pnp, the surfactant, and the surfactant loaded organoclays with and without pnp are shown in Fig. 5. The pnp sublimes at 131 8C; the DDOAB shows thermal decomposition steps at 168, 245 and 305 8C attributed to combustion of the surfactant molecules; the montmorillonite shows thermal decomposition steps at 485 and 617 8C. The 0.5 CEC organoclay differential thermal analysis pattern shows peaks at 590, 389 and 300 8C. A slight decrease in the dehydroxylation of the clay is observed (590 compared with 617 8C). The peaks at 389 and 300 8C are higher than is observed for the surfactant on its own. This suggests that the surfactant molecule is bonded to the clay surfaces and more heat is required to remove the surfactant molecules from the clay surfaces. The DTG pattern for the 0.5 CEC-4000 (organoclay with adsorbed pnp) is quite different to that of the 0.5 CEC organoclay. Three decomposition temperatures are observed at 382, 268 and 189 8C. Interestingly the dehydroxylation peak at 590 8C is not found. This suggests the reaction of the clay with pnp occurs through the OH
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Fig. 5. DTG patterns of 0.5, 0.7, 1.5 and 2.5 CEC-DDOAB organoclay with and without adsorbed pnp.
units of the montmorillonite and the dehydroxylation temperature is reduced to 382 8C. The two peaks at 286 and 189 8C are attributed to the combustion of the surfactant and as no DTG peaks attributable to the pnp are observed, it is proposed that the pnp is removed simultaneously with the surfactant. The thermal decomposition steps for the 0.7 CEC organoclay are similar to that for the 0.5 CEC organoclay with DTG peaks observed at 293, 396 and 590 8C. The latter is due to the montmorillonite and the first two steps to the loss of the surfactant. For the 0.7 CEC organoclay with adsorbed pnp, DTG peaks are observed at 193, 283 and 385 8C. No peak is found at 590 8C. This suggests that the pnp results in the lowering of the clay dehydroxylation temperature. For the 1.5 CEC organoclay without adsorbed pnp, DTG peaks are observed at 229, 341, 392 and 590 8C. The first three peaks are ascribed to the combustion of the surfactant and the latter to the dehydroxylation of the montmorillonite. It is noted that the three temperatures (229, 341, 392) are significantly higher than for the DTG peaks of the surfactant (168, 245, and 305 8C. This difference in temperatures is attributed to the bonding of the surfactant molecules to the montmorillonite surfaces. Heating the organoclay to higher temperatures is required before the surfactant molecules are removed. Thermal decomposition peaks are observed at 178, 241, 295, 346 and 600 8C for the 2.5 CEC organoclay. The first three peaks correspond to the three thermal decomposition steps of DDOAB at 168, 245 and 305 8C and are attributed to the loss of surfactant molecules. This temperature correspondence indicates that at 2.5 CEC the surfactant molecules are not only intercalated within the montmorillonite layers but also are adsorbed on the external surfaces of the clay. The temperature of dehydroxylation of the montmorillonite is now at 600 8C. For the 2.5 CEC organoclay with adsorbed pnp, thermal decomposition steps are observed at 180, 231, 285, 336 and 412 8C. The first three decomposition steps are assigned to the loss of surfactant. The decomposition steps at 336 and 412 are attributed to the loss of intercalated surfactant molecules and the dehydroxylation of the montmorillonite. It is assumed that the loss of the pnp occurs simultaneously with the surfactant.
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4. Conclusions Water purification is of extreme importance in many parts of the world, including Australia and China. Many of the world’s water ways and water sources are polluted or contaminated with a range of chemicals including pesticides and herbicides. In this work, we have used paranitrophenol as a test chemical to design and test an organoclay for the removal of pnp from an aqueous medium. This work has shown that the pnp intercalates the organoclay and displaces the surfactant molecules or rearranges the structure of the surfactant molecule within the organoclay interlayer. Future work will test the use of this organoclay for the removal of herbicides and pesticides from water and will need to test the efficiency of the pnp adsorption. Synthesis of the organoclay with dimethyldioctadecylammonium bromide results in multiple expansions of the montmorillonite clay from 1.24 nm to a maximum of 4.96 nm as is evidenced by the X-ray diffraction patterns and confirmed by the transmission electron microscopic images. These expansions depend upon the arrangement of the surfactant molecules with the montmorillonite interlayer. Adsorption of pnp affects this arrangement and results in different spacings. Thermoanalytical studies show that the thermal decomposition of the surfactant molecules occurs at a higher temperature. Adsorption of the pnp results in a significant decrease in the dehydroxylation temperature of the montmorillonite. It is proposed that the pnp is bonding to the hydroxyl units of the montmorillonite. This affects the molecular arrangements of the surfactant molecules in the clay interlayer. Acknowledgements This work was financially supported by National Science Fund for Distinguished Young Scholars (Grant No. 40725006) and the grant of the Knowledge Innovation Program of the Chinese Academy of Sciences (Grant No. Kzcx2-yw-112). The Inorganic Materials Research Program, Queensland University of Technology, is gratefully acknowledged for infra-structural support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
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