Ind. Eng. Chem. Res. 2008, 47, 1755-1759
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APPLIED CHEMISTRY Interaction of Chromate on Surfactant Modified Montmorillonite: Breakthrough Curve Study in Fixed Bed Columns N. Mahadevaiah,† B. Venkataramani,‡ and B. S. Jai Prakash*,† Chemistry Research Laboratory, Bangalore Institute of Technology, K.R. Road, Bangalore 560 004, India, and Radiation Chemistry and Chemical Dynamics DiVision, Bhaba Atomic Research Centre, Trombay, Mumbai 400 085, India
Interactions between chromate and surfactant modified montmorillonite at room temperature (25 °C) and different pHs (1-7) in aqueous solution in fixed bed columns have been studied. A series of column tests were performed to determine the breakthrough curves at different initial surfactant loadings (0.5-3 CEC) of the bed. The interlayer spacing increased with the surfactant loading from 14.2 to 40.5 Å. Experimental results of the column tests and the basal spacing of the column material showed that the adsorption of chromate onto the clay did not occur until the initial surfactant loading was beyond a critical concentration apparently enough to open the clay sheets for the entry of chromate species. The shapes of the breakthrough curves indicate that the chromate ions enter the interlamellar region only when the initial surfactant loadings are beyond 2 times the CEC of the clay. The amount of adsorbate increases proportionally with increasing bed depth. A simple two-parameter model originally introduced by Yoon and Nelson (Yoon, Y. H.; Nelson, J. H. Am. Ind. Hyg. Assoc. J. 1984, 45, 509) was used to calculate the breakthrough time. Chromate could be recovered completely by washing with ammonium hydroxide, and the column material was found to regain its capacity to adsorb repeatedly after simple acid wash. Introduction The transport of inorganic solutes and adsorption from aqueous environment on a fixed bed is a process of unsteady state mass transfer between the liquid and solid phases.1 The concentration profiles of an adsorbed substance in both phases as a function of time and location in the bed are of significance. Breakthrough experiments are generally carried out in columns on a bench scale with different adsorbents. The adsorption properties such as breakthrough time, breakthrough point of the bed at a defined height, and various mass-transfer models have been used to predict the breakthrough curves.2 Some of the important applications of breakthrough curves for fixed bed adsorption include water treatment, industrial waste disposal, and adsorbent regeneration. The breakthrough techniques have been applied in the removal of soluble inorganic/organic compounds that are toxic even in trace amounts.3 Environmental and occupational sources of chromium exposure include the following: leather tanning [soluble Cr(III)], chrome alloy production, chrome electroplating [Cr(VI)], textile manufacture, paints/pigments using Cr(VI), welding of alloys and steel, glass making, and so on. The untreated effluents from such sources pollute the groundwater because of the harmful effects of chromium species.4 Wastewater and industrial effluents containing this element must be treated before discharging to receiving water bodies. Among many adsorbents that are available, there is a search for cheaper ones such as clays. * To whom correspondence should be addressed. Tel.: +91 8026615865. Fax: +91 80-22426796. E-mail:
[email protected]. † Bangalore Institute of Technology. ‡ Bhaba Atomic Research Centre.
Montmorillonite clays are widely used in many fields of technology for removal of toxic metal ions and purification of gases. The usefulness of clays is a result of their high specific surface area, chemical and mechanical stability, and variety of surface structural properties. Clays are smectic in nature and possess a net negative charge resulting from isomorphic substitution of cations in the crystal lattice. Surface modifications with organic cations have been widely used in order to alter the surface properties of swelling clays to improve their adsorption ability.5 Quaternary ammonium salts such as hexadecyltrimethylammonium (HDTMA) bromide are known to swell smectite layers to different extents depending on their initial loading.6 Surfactant immobilized interlayer species (SIIS) bound to clay can facilitate the admission of metal ion species into the interlayer irrespective of their sizes.7 According to Armin et al.,8 breakthrough techniques offer advantages over other more traditional techniques such as batch extraction, ion exchange, and chemical reduction because of their (a) sustainable process control and regeneration, (b) cheap technology due to cheap auxiliary agents and low cost of adsorption agents, (c) high treatment efficiency due to efficient process control, (d) compact system due to quick adsorption and desorption, and (e) suitability for both small- and largescale applications. The objective of this paper is to study the shapes of breakthrough curves with respect to the interaction of chromate in a fixed bed column of montmorillonite clay treated with surfactants known to swell smectite clays. Experimental Section Materials. The clay mineral used in this study is swellingtype, smectite-rich clay from the Bhuj area of Gujarat, India.
10.1021/ie0702680 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/22/2008
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Table 1. Flow Rates at Different Bed Depths for Column Test Runs (Bed Density ) 1091 kg/L) exptl condition
run 1
run 2
run 3
run 4
run 5
bed depth (cm) flow rate (mL/h)
1.0 538
2.0 321
3.0 243
4.0 168
5.0 109
The sample, having a mesh size of less than 75 µm, was essentially montmorillonite as characterized by X-ray diffraction and X-ray fluorescence spectrometry. The BET surface area measured with a sorptiometer (Quantachrome Nova 1000) was found to be 14.2 m2 g-1 from the nitrogen adsorption isotherms. The cation exchange capacity of montmorillonite was 93 mequiv/100 g of the clay. The surfactant used in this study for modification of the surface of clay was hexadecyltrimethylammonium (HDTMA) bromide. Potassium chromate was purchased from SD Fine chemicals, India. Distilled water was used to prepare the aqueous solutions for the tests in this study. The variation of the pH of the resultant solution was adjusted by using 0.5 N HCl and dilute ammonia solution. The procedure followed for the modification of the clay is discussed elsewhere.9 It consisted of treating 10.0 mmol of hexadecyltrimethylammonium bromide in 1.0 L of distilled water. A 100 g sample of montmorillonite clay suspension in acetone was added to the HDTMA solution, stirred for 60 min, filtered, and air-dried. The resulting modified clay was ground, sieved to obtain less than 2 µm mesh fraction, and finally dried at 105 °C in an hot air oven. Flow Experiments. A known dried weight (4-12 g) of HDTMA modified smectite was packed into a glass column with an inside diameter of 3 cm and height of 30 cm. The sample used had a particle density of 1.09 g cm-3 with a surface area of 6.8 m2 g-1 and a pore diameter 68.8 Å. Deionized water was used to wash the sorbent in a down-flow fashion in order to rinse the adsorbent and equilibrate the particles in the column before a column test was begun. Chromate solution of known concentration (1.6 mg/mL) was allowed to flow from a reservoir through the glass column with a control valve. The chromate solution (pH adjusted) was continuously fed to the top of the column at a desired flow rate controlled by a suction pump. The column effluent was intermittently collected in a separate collector, and the effluent concentration was measured spectrophotometrically.10 The starting time was fixed as 5 min when flow of the chromate species just appeared in the column effluent. This was continued until breakthrough occurred and until the concentration of chromate in the effluent remained constant. The flow rates for different column depths of the bed for the experiments conducted at 26 ( 1 °C are given in Table 1. Results and Discussion The solution pH was found to play an important role in the adsorption of chromate from aqueous solutions. At lower pHs (1-2) the single charged [HCr2O7-] is the predominant species, while at a higher pH (2-4) the doubly charged forms [CrO42-/ Cr2O72-] are the predominant ones. This explains the uptake of nearly twice the amount of chromate at pH 1 and 2 compared to those between pH 2 and 4.11 The adsorption capacity decreases with increasing pH (Figure 1). This is apparently due to the increased concentration of OH- ions which would compete with the chromate ions for the exchange sites. Experimental single solute breakthrough curves at a flow rate of 3.0 mL min-1 were obtained for chromate. Breakthrough data was acquired by plotting the ratio of me/V against t, where me is the amount of chromate (milligrams) present in the volume
Figure 1. Effect of pH of the chromate solution on the adsorption.
Figure 2. Breakthrough curves of chromate adsorption of surfactant modified montmorillonite at two different concentrations at pH 1.5.
V of the effluent collected at regular intervals (5 min). The experimental results show a formation of steep breakthrough curves (Figure 2). Curves show a decrease in the breakthrough time with the increase in the initial load of chromate: 50% breakthrough time of 55 min for higher concentration of 1800 mg/L and 84 min for lower concentration of 1600 mg/L chromate. The Yoon and Nelson equation12 is applied in this study to describe the 50% breakthrough time for adsorption of chromate on surfactant modified clay at 300 K. This model is based on the assumption that the rate of decrease in the probability of adsorption for each adsorbate molecule is proportional to the probability of the adsorbate adsorption and the probability of adsorbate breakthrough on the adsorbent. The Yoon and Nelson model may be mathematically expressed as
t)τ+
Cb 1 ln κ′ Ci - Cb
where t is the breakthrough (sampling) time, Cb and Ci are the breakthrough effluent concentration and inlet concentration, respectively, τ is the time required for 50% adsorbate breakthrough, k is the rate constant, and κ′ is a proportionality constant which is equal to kτ. The values of κ′, k, and t have been
Ind. Eng. Chem. Res., Vol. 47, No. 6, 2008 1757 Table 2. Parameters of Yoon and Nelson Model for Adsorption of Chromate at Various Inlet Concentrations
Table 3. Basal Spacing of Montmorillonite Clay and Modified Clay
Yoon and Nelson parameters
adsorbent surfactant modified montmorillonitea (loaded at twice the CEC) a
breakthrough inlet time from concn breakthrough (mg/L) curves (min)
sample HDTMA clay
T (min)
k (min-1)
k¢
1600
83
86
0.0541
4.65
1800
54
55
0.0849
4.67
chromate adsorbed clay
Amount: 10 g.
Figure 3. Plot of ln[Cb/(Ci - Cb)] versus time for the adsorption of chromate: (a) 1600 mg/L; (b) 1800 mg/L.
calculated (Table 2) from the slope and intercept of the linear plots obtained by plotting ln[Cb/(Ci - Cb)] versus t (Figure 3). Breakthrough Curves at Different Surfactant Loadings. The nature of breakthrough curves at different initial surfactant
loading of surfactant (CEC) 0.5 1.0 1.5 2.0 3.0 2.0
amt of surfactant taken up (mequiv g-1) 0.27 0.98 1.11 1.17
d(001) spacing (Å)
gallery height (Å)
14.2 18.3 32.5 40.5 40.0 40.8
5.2 9.3 23.5 31.5 31.0 31.8
loadings with respect to CEC was studied. The maximum amount of HDTMA adsorbed by clay was found by CHN analysis to be slightly more than 1 mequiv g-1. Entry of surfactant into the interlamellar region is known to increase with the increase in the initial surfactant loading. HDTMA comprises a tail group of a 16-carbon chain attached to a head group of a 3-methyl quaternary amine with a 1+ charge. Adding together, the van der Waals packing radii for a 3-methylammonium head group yields a diameter of 0.694 nm with a fully extended chain length of 3.5 nm.8 To examine whether the surfactant molecules were intercalated, the treated column bed was thoroughly washed and subjected to X-ray diffraction. The XRD pattern shows no significant variation in the interlayer distances of surfactant modified and chromium adsorbed clays (Figure 4). It could be seen that there is an increase in d(001) spacing ranging from 14 to 41 Å for the clay samples treated with the initial loading of the surfactant varying from 0.5 to 3 times CEC (Table 3). At a surfactant loading equivalent to 0.5, the 5 Å gallery height is not enough for the chromate ion to force its entry. This is supported by the nature of the breakthrough curve obtained for the surfactant loading equivalent to 0.5 CEC (Figure 5), which clearly shows no breakthrough and hence negligible entry of the chromate species (0.07 mequiv g-1). However, the curve for surfactant loading equivalent to 1.0 CEC does show a distorted breakthrough indicating the adsorption of small amount of chromate ions. The gallery height around 9.3 Å for
Figure 4. XRD patterns of surfactant loaded clay samples (loaded at twice the CEC) studied at pH 1.5: (a) HDTMA clay; (b) chromate adsorbed clay.
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Figure 5. Breakthrough curves for chromate adsorption at different surfactant loadings (pH 1.5).
Figure 6. Effect of bed depth on the breakthrough curve (at twice the CEC loaded with surfactant, pH 1.5). Table 4. Amount of Chromate Adsorbed and Desorbed by 2.0 CEC Surfactant Loaded Clay at pH 1.5
this sample is apparently not enough for easy entry of the chromate ions and prevents their entry into the interlayer. Possibly the chromate ions block the edge sites and are held there. Thus only a small amount (0.27 mequiv g-1) of the chromate ions are adsorbed. At higher CEC loading (>1 CEC) the curves obtained are sharp, typical of breakthrough. The higher gallery heights permit the easy entry of the chromate ions. The ratio of HDTMA on the exchange sites to Cr(VI) is 1:1 in the final amount adsorbed on the surface of the clay. The overall adsorption mechanism is explained by
curves after repeated sorption-desorption experiments were found to be similar. The mechanism of the regeneration appears to be the following:
Na-smectite clay + HDTMA+(Br)- f smectite clay-HDTMA + NaBr
pH 1.5: smectite clay- + HDTMAHCr2O7 + NH4OH f
-
pH 1.5: smectite clay-HDTMA + HCr2O7 f smectite clay- + HDTMAHCr2O7 pH 4: smectite clay-HDTMA + HCrO4- f smectite clay- + HDTMAHCrO4 The amount of bromide released after washing supports the above ion exchange process. This was further confirmed by separately precipitating the two chromate salts with HDTMA at pH 1.5 and 4 and finding the chromium content of both salts. Different heights of bed depth were used to study the column experiments (Figure 6). The experimental data were evaluated by plotting the ratio of me/V against time, where me is the amount of chromate (milligrams) present in the volume V of the effluent collected at regular intervals (5 min). The breakthrough time, as expected, increases with the increase in bed depth: 64 min for 2 cm bed depth and 133 min for higher. Regeneration of Adsorbent. The fixed bed previously saturated with chromate solution was regenerated with 2 N ammonium hydroxide. Ammonium hydroxide at concentrations higher than 1 × 10-8 M releases all the chromate from the bed. The effluent on evaporation gives a yellow-orange solid. The time and the volume of ammonium hydroxide needed to elute the species completely do not vary with the concentration of the species adsorbed. The bed gets regenerated readily after washing with dilute hydrochloric acid. The bed can be repeatedly regenerated several times after adsorption. The results are presented in Table 4. Furthermore, the nature of breakthrough
sample no.
trial
amt chromate adsorbed (mg)
amt chromate desorbed (mg)
1 2 3 4
first time second time third time fourth time
712 616 677 641
691 608 652 628
smectite clay-HDTMA + NH4HCr2O7 + OHpH 4: smectite clay- + (HDTMA)HCrO4 + NH4(OH) f smectite clay-HDTMA + NH4HCrO4 + 2OHThe resulting solid obtained after evaporation of the collected washings was confirmed by analysis to be pure NH4CrO4 at pH 4. The analysis of residue obtained at pH 1.5 showed it to be a mixture of NH4HCr2O7 and NH4CrO4 with the former in larger proportion. The column gets regenerated after a wash with dilute hydrochloric acid, which readily removes the hydroxyl ions. Conclusions Surfactant modified smectite (SMS) clays could be effectively used as specific adsorbents for removing toxic species from aqueous environment. Chromate ions in the pH range (pH 1-4) can be concentrated in the interlayer of surfactant treated montmorillonite clay. The nature of breakthrough curves for chromate adsorption on an HDTMA modified montmorillonite column depends on the porosity of the adsorbent caused by the surfactant loading. The shapes of the breakthrough curves obtained varied with the amount of surfactant loading. The shapes clearly indicate that the chromate ions are excluded from entering the interlayer until the amount of surfactant in the interlayer is able to swell the layers enough for their entry. The amount of surfactant loaded determines the entry of ions, thus suggesting the possibility of a novel method of separation based on exclusion by the variation of interlayer swelling brought
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about by surfactant loading. The Yoon and Nelson equation is applied to calculate the 50% breakthrough time, which agrees very well with experimentally determined values. Regeneration of adsorbent is fast with ammonium hydroxide and the regenerated column material is found to retain its adsorption capacity repeatedly. Acknowledgment The authors acknowledge with thanks financial assistance from the BRNS, Department of Atomic Energy, Government of India, for carrying out this project. The authors are grateful to the Members of the Governing Council of Bangalore Institute of Technology for the facilities provided. Nomenclature CEC ) cation exchange capacity, mequiv k ) rate constant, L/mol‚min τ ) time required for 50% adsorbate breakthrough, min κ′ ) proportionality constant t ) breakthrough (sampling) time, s Cb ) effluent concentration, mg/L Ci ) inlet concentration, mg/L me ) amount of chromate, mg V ) volume of effluent, L L ) depth of adsorbent, cm Literature Cited (1) Vermeulen, T.; Klein, G.; Heister, N. K. Adsorption and Ion Exchange. In The Chemical Engineers Handbook; Perry, R. H., Chilton, C. H., Eds.; McGraw-Hill Book Company: New York, 1973; Section 16.
(2) Chatzopoulos, D.; Verma, A. Aqueous-Phase Adsorption and Desorption of Toluene in Activated Carbon Fixed BedssExperiments and Model. Chem. Eng. Sci. 1995, 50, 127. (3) Wolborska, A. External Film Control of the Fixed Bed Adsorption. Chem. Eng. J. 1999, 73, 85. (4) Prasad Rao, T.; Karthikeyan, B.; Vijayalakshmy Iyer, C. S. P. Preconcentration of Chromium(III) and Speciation of Chromium by Electrothermal Atomic Absorption Spectrometry Using Cellulose Adsorbent. Anal. Chim. Acta 1998, 69, 369. (5) Lin, S. H.; Juang, R. S. Heavy Metal Removal from Water by Sorption Using Surfactant-Modified Montmorillonite. J. Hazard. Mater. 2002, 92, 315. (6) Lee, S. Y.; Kim, S. J. Expansion of Smectite by Hexadecyl trimethylammonium. Clays Clay Miner. 2002, 50, 435. (7) Mahadevaiah, N.; Venkataramani, B.; Jai Prakash, B. S. Restrictive Entry of Aqueous Molybdate Species into Surfactant Modified MontmorillonitesA Breakthrough Curve Study. Chem. Mater. 2007, 19, 4606. (8) Ebner, A. D.; Ritter, J. A. Concentrating Dilute Sludge Wastes with High-Gradient Magnetic Separation: Breakthrough Experiments and Performance. Ind. Eng. Chem. Res. 2002, 41, 5049. (9) Mahadevaiah, N.; Krishna, B. S.; Murty, D. S. R.; Jai Prakash, B. S. Surfactant Immobilized Interlayer Species Bonded to Montmorillonite as Recyclable Adsorbent for Lead Ions. J. Colloid Interface Sci. 2004, 271, 270. (10) Sandell, E. B. Colorimetric Determination of Traces of Metals; Inter Science Publishers, Inc.: New York, 1958. (11) Krishna, B. S.; Murty, D. S. R.; Jai Prakash, B. S. SurfactantModified Clay as Adsorbent for Chromate. Appl. Clay Sci. 2001, 20, 65. (12) Yoon, Y. H.; Nelson, J. H. Application of Gas Adsorption Kinetics I. A Theoretical Model for Respirator Cartridge Service Life. Am. Ind. Hyg. Assoc. J. 1984, 45, 509.
ReceiVed for reView February 20, 2007 ReVised manuscript receiVed December 22, 2007 Accepted January 10, 2008 IE0702680
