Ind. Eng. Chem. Res. 2010, 49, 1677–1683
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Synthesis of Organoclays with Controlled Particle Size and Whiteness from Chemically Treated Indian Bentonite Hasmukh A. Patel,† Rajesh S. Somani, Hari C. Bajaj, and Raksh V. Jasra*,‡ Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute, G. B. Marg, BhaVnagar 364 002, Gujarat, India
Organoclays or nanoclays have gained increased attention from industry as well as academia during the past 11/2 decades. These clays are prepared from natural clays which invariably have other minerals as impurities. It is of prime significance to remove impurities from organoclays which find applications in polymer nanocomposites, paints, cosmetics, and medicinal formulations. The Indian bentonites were treated with sulfuric acid, hydrochloric acid, hydrogen peroxide, oxalic acid, citric acid, and sodium dithionite to remove impurities invariably present in natural clays. Treatment with both minerals and organic acid could remove both interlayer and structural iron and improve the whiteness with breaking up layered structure. However, sodium dithionite treatment is observed to be more suited to remove iron impurities from bentonite and improve whiteness to 11% without any effect on the structure of the clay. Chemically treated bentonites were used for preparing organoclays in a single step to eliminate the filtration and washing steps. The organoclays were prepared under varied reaction parameters such as temperature, the amount and type of organic modifier, and the concentration of the clay. The optimization of these parameters was done to prepare orgnaoclays with controlled particle size and whiteness. 1. Introduction Montmorillonite (MMT) is the most abundant mineral within the smectite group (2:1 clay minerals) and has received comparatively large attention because of its good swelling property and high exchange capacity.1-3 MMT particles may be as large as 2 µm and as small as 0.1 µm in diameter with average size of ∼0.5 µm.4 The MMT crystal lattice consists of a 1 nm thin layer, with a central octahedral sheet of alumina fused between two external silica tetrahedral sheets. Isomorphic substitution within the layers (Al3+ replaced by Mg2+ or Fe2+ and Si4+ by Al3+) generates a negative charge, expressed through the cation exchange capacity (CEC), which is typically 0.5-1.2 mequiv/g for MMT, with exact value depending on the mineral origin. These layers organize themselves in a parallel fashion to form stacks with a regular van der Waals gap between them, called the interlayer or gallery. The negative charges are counterbalanced by cations such as Na+, K+, Ca2+, or Li+, which exist in hydrated form in the interlayer.5,6 In addition to MMT, bentonite may contain beidellite, nontronite, iron-stained impurities, kaolinite, feldspar, cristobalite, and crystalline as well as amorphous silica. The red to yellow pigmentation noticed in many clay deposits is mainly due to the associated oxides, hydroxides, and hydrated oxides of ferric iron. These oxides and hydroxides occur either as coatings on individual grains or as discrete fine particles throughout the clay mass, and quantities as low as 0.4% of ferric iron may be sufficient enough to impart color to the deposit. Removal of these associated impurities becomes imperative to make value-added these materials.7 The oxidation state of Fe in the crystal structures of smectite clay minerals plays an important role in determining the surface and colloidal chemistry and the physical behavior of the clay.
Smectites containing appreciable amounts of octahedral Fe(III) are abundant in soils. This structural Fe can be reduced and reoxidized without dissolution of the mineral using chemical and microbial leaching.8 The chemical treatment of clay minerals using organic and inorganic acids has also been reported.8-11 High-purity bentonites are obtained by sedimentation, hydrocyclone, and chemical treatment. These purified bentonites are used in a wide variety of areas such as selective adsorbent,12-14 medication,15,16 polymer nanocomposites, paints and cosmetics,17-19 and catalyst20,21 production. Organoclays or nanoclays have been gaining enormous attention for the past 11/2 decades because of their applicability in various fields. The inorganic, relatively small alkaline or alkaline earth cations are exchanged against more voluminous organic cations. This ion-exchange reaction has two consequences; first, the gap between the single sheets is widened, enabling an organic cations chain to move between Scheme 1. Upgradation, Chemical Treatment of Clay, and Preparation of Organoclays
* To whom correspondence should be addressed. Tel.: +91 265 6693935. Fax: +91 265 6693934. E-mail:
[email protected],
[email protected]. † Present address: Politecnico di Torino-Sede di Alessandria, INSTM Local Research Unit, Viale Teresa Michel 5, 15100 Alessandria, Italy. ‡ Present address: R&D Centre, Vadodara Manufacturing Division, Reliance Industries Ltd., Vadodara 391 346, Gujarat, India. 10.1021/ie801368b 2010 American Chemical Society Published on Web 01/11/2010
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Table 1. Comparison of Chemical Composition, CEC, and Whiteness of Raw, Purified, and Chemically Treated Bentonite chemical composition (% (w/w)) chemical treatment LOI SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O total (% (w/w)) CEC (mequiv/g) moisture content whiteness
R
RP
RPC-1
RPC-2
RPC-3
RPC-4
RPC-5
RPC-6
RPC-7
RPC-8
RPC-9
RPC-10
1 N H2SO4 3 N H2SO4 1 N HCl 3 N HCl 3 N HCl + 1% H2O2 9.18 7.73 10.38 9.77 9.16 8.07 8.55 57.18 55.89 58.35 60.26 57.02 65.63 62.1 18.17 20.89 17.92 16.52 21.96 14.93 19.24 10.83 9.15 7.04 6.96 6.86 6.48 4.8 2.07 2.85 2.45 2.38 1.27 1.38 2.9 1.53 2.06 2.16 2.47 2.34 2.17 1.54 0.46 0.73 0.78 0.82 0.62 0.59 0.46 0.81 0.71 0.91 0.79 0.76 0.73 0.42 100.23 100.01 99.99 99.97 99.99 99.98 100.01
1 N NaOCl 0.1 M 0.2 M 0.1 M 1:1 sodium oxalic acid oxalic acid citric acid dithionite 7.24 9.78 9.85 9.9 10.96 58.76 55.7 55.67 56.97 55.77 18.57 22.56 22.54 21.5 21.95 8.56 6.22 6.15 5.57 5.58 2.63 2.42 2.19 2.27 2.49 3.04 1.73 2.16 1.74 1.81 0.64 0.73 0.76 0.74 0.79 0.52 0.76 0.58 0.65 0.65 99.96 99.9 99.46 99.34 100.00
0.82
0.91
0.82
0.70
0.71
0.63
0.64
0.82
0.82
0.80
0.78
0.86
5.09
6.53
9.44
10.98
10.19
9.94
11.37
11.13
10.1
11.1
10.1
9.43
48
54
57
68
57
68
68
58
57
55
54
60
Table 2. Ionic Formula of Untreated and Treated Clay sample
ionic formula
RP RPC-1 RPC-2 RPC-3 RPC-4 RPC-5 RPC-6 RPC-7 RPC-8 RPC-9 RPC-10
(Si3.7Al0.3)(Al1.32Mg0.21Fe0.46)O10(OH)2X (Si3.93Al0.07)(Al1.35Mg0.23Fe0.35)O10(OH)2X (Si4.01)(Al1.29Mg0.25Fe0.35)O10(OH)2X (Si3.77Al0.23)(Al1.54Mg0.25Fe0.34)O10(OH)2X (Si4.23)(Al1.13Mg0.21Fe0.31)O10(OH)2X (Si4.027)(Al1.46Mg0.145Fe0.23)O10(OH)2X (Si3.85Al0.15)(Al1.28Mg0.29Fe0.42)O10(OH)2X (Si3.72Al0.28)(Al1.5Mg0.17Fe0.31)O10(OH)2X (Si3.7Al0.3)(Al1.47Mg0.22Fe0.31)O10(OH)2X (Si3.82Al0.18)(Al1.51Mg0.17Fe0.28)O10(OH)2X (Si3.76Al0.24)(Al1.5Mg0.185Fe0.28)O10(OH)2X
X Na0.09, Na0.10, Na0.11, Na0.08, Na0.08, Na0.06, Na0.12, Na0.10, Na0.10, Na0.10, Na0.10,
K0.06, K0.07, K0.06, K0.06, K0.06, K0.04, K0.04, K0.01, K0.05, K0.05, K0.05,
Ca0.20 Ca0.18 Ca0.02 Ca0.09 Ca0.09 Ca0.20 Ca0.18 Ca0.17 Ca0.16 Ca0.16 Ca0.18
them, and second, the surface properties of each single sheet are changed from being hydrophilic to hydrophobic.22-24 In this paper, we are reporting the effect of different chemical treatments to improve the quality in the terms of whiteness and CEC of Indian bentonite for the preparation of organoclays. The organoclays were synthesized by in situ method under varied reaction temperature, concentration of clay, different type, concentrations of quaternary ammonium salts, and the effect of these parameters on particle size, whiteness, and the interlayer spacing of the organoclays was studied. 2. Experimental Section 2.1. Materials. Bentonite lumps were collected from the Barmer district of Rajasthan, India. Sulfuric acid (98%), hydrochloric acid (35%), sodium hypochlorite (13%), hydrogen peroxide (40%), oxalic acid (99%), citric acid, anhydrous (99%), sodium dithionite (98%), stearyldimethylbenzylammonium chloride (SMB), cetyltrimethylammonium bromide (CTAB), and tetrabutylammonium chloride (TBA) were purchased from Rankem and dimethyl(dihydrogenated tallow)ammonium chloride (HT, where HT ) ∼65% C18, ∼30% C16, and ∼5% C14) from Cutch Oil, India and were used as received. 2.2. Chemical Treatment of Bentonite. The flow sheet diagram of upgradation, chemical treatment of the clay, and in situ synthesis of the organoclays is explained in Scheme 1. The purified clay fractions were obtained by dispersing 150 g of bentonite lumps (R) in 10 L of deionized water (1.5% (w/v)) and were allowed to swell overnight and stirred (200 rpm) for 30 min. The supernatant slurry having the desired clay particles size (
