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Adsorption of heavy metal ions on mesoporous silica-modified montmorillonite containing a grafted chelate ligand
Applied Clay Science 59–60 (2012) 115–120
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Research paper
Adsorption of heavy metal ions on mesoporous silica-modified montmorillonite containing a grafted chelate ligand Mary Addy, Bradley Losey, Ray Mohseni, Eugene Zlotnikov 1, Aleksey Vasiliev ⁎ East Tennessee State University, Department of Chemistry, PO Box 70695, Johnson City, 37614 TN, USA
a r t i c l e
i n f o
Article history: Received 24 August 2011 Received in revised form 13 February 2012 Accepted 15 February 2012 Available online 30 March 2012 Keywords: Montmorillonite Kaolinite Organoclay Heavy metal cations Adsorption Environmental protection
a b s t r a c t The objective of this work is development of a new adsorbent on the base of an organoclay with a chelating ligand covalently attached to the clay mineral surface. The presence of a chelating ligand in the clay structure significantly improves its ability to immobilize heavy metal ions from contaminated sludge of wastewater. Montmorillonite and kaolinite were chosen as typical examples of expandable and non-expandable clay minerals. A two-step modification procedure comprised of sequential modification with oxides and grafting of a chelating agent to the modified clay minerals was used. Modifications with silica and ferric oxide were conducted by reacting the dispersed raw clay minerals with tetraethoxysilane and ferric nitrate solution. A chelating ligand, N-[3-(trimethoxysilyl)propyl]ethylenediamine triacetic acid trisodium salt, was introduced into interlayer space of raw and modified clay minerals in aqueous solutions. Laboratory tests of the organoclay efficiency for purification of wastewater were conducted with the most promising sample, i.e., organoclay with the highest specific loading of chelating agent. Experiments were conducted with model wastewater containing either individual or mixed heavy metal ions. The modified organoclay displayed high adsorption capacity for heavy metal cations even in acidic media. The method of modification presented in this work can be used for synthesis of efficient adsorbents for applications in contaminated areas. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Environmental protection requires economically feasible, highly effective materials for adsorption of heavy metal ions (Kurniawan et al., 2006). Naturally occurring abundant clays and clay minerals have very good potential, hindered, however, by weak adsorption of heavy metal ions directly on the clay mineral surface. Such adsorption occurs mainly by ion-exchange mechanism. The cation exchange capacity of untreated clay is not enough for wide scale applications (Marshall, 1949). Adsorption of metal ions by clays strongly depends on the pH of media and becomes much less effective in acidic solutions. Much stronger binding of heavy metal ions might be achieved with the help of chelating agents. For adsorption purposes chelating agents should be immobilized on the solid carriers. This type of adsorbents may potentially extract heavy metal ions from acidic media. The main obstacle to using clays as a chelates' carrier is the relatively small surface available for ligand immobilization. The porosity of lamellar clay minerals can be improved by pillaring. The obligatory requirements for pillared clay minerals are robustness of the intercalation compound to prevent collapse on dehydration or during application. Spacing between pillars should be large enough to ⁎ Corresponding author. Tel.: + 1 423 439 4368; fax: + 1 423 439 5835. E-mail address: [email protected] (A. Vasiliev). 1 Current address: Solvay Speciality Polymers, Alpharetta, GA 30002, USA. 0169-1317/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2012.02.012
allow access of molecules of interest (Wachs, 2009), i.e., chelating ligands in our case. The production process should be scalable and cheap. Since clays are natural products, variation of their properties might be significant, therefore, the production process should be adoptable to reasonable variation of the properties and composition of the raw clay. In a review of De Stefanis and Tomlinson (2006), they summarized various routes for pillaring. The most widely traditional route is pillaring with inorganic cations of multivalent elements such as iron, aluminum, zirconium, chromium etc. Inorganic cations might be used individually or as a mixture (Aouad et al., 2005; Canzares et al., 1999; Mahboub et al., 2006; Moreno et al., 1997; Occelli et al., 2000; Perez-Vidal et al., 2006; Sychev et al., 2000; Yan et al., 2008). All these publications reported significant increase of BET specific surface area. In the work of Maes et al. (1997), the final product combined micro- and mesopores with overall BET specific surface area of 383 m 2/g was prepared. Increase of overall BET specific surface area from 35.1 to 323.2 m 2/g by montmorillonite pillaring with TiCl4 in alkaline media was described by Yuan et al. (2006a, 2006b). Less widely used but a promising route of pillaring in prospective of ligand grafting is based on various modifications of the sol–gel process (Kawi and Yao, 1999; Li et al., 2009a, 2009b; Mao et al., 2009; Nakao and Nogami, 2005). Literature data showed that the total BET surface area by this process might exceed 700 m 2/g. The clay mineral dispersions were reacted with a solution of tetraethoxysilane (TEOS) in ethanol. Alkaline or acidic hydrolysis leads to formation of silicate
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ions, with formation of amorphous silica three-dimensional networks. Amorphous silica might form pillars between the layers of the clay minerals. However, no reasons for silica domains to be localized only in the interlayer space were found in literature. Introduction of surfactants into the TEOS process allows making a templated heterostructure. In the work of Xia et al. (2009), use of cetyl trimethylammonium bromide was described to achieve a bi-modal mesoporous structure of montmorillonite in a hydrothermal process. Loading of surfactants exceeded the critical micelle concentration. Intercalation of these micelles into the interlayer spaces and further loading and hydrolysis of TEOS resulted in formation of silica bulk layers penetrated by micelles. Calcination at moderate temperature eliminated the organic component with formation of a porous clay heterostructure. A major part of literature devoted to modifying clay minerals considers various types of montmorillonite. Much less was published about modification of kaolinite. Bhattacharyya and Gupta (2008) reported application of modified kaolinite and montmorillonite for adsorption of heavy metal ions from water. The authors concluded that application of clay minerals is economically feasible for adsorption of heavy metal ions. Modification of clay minerals significantly increased adsorption capacity. Organoclays are very prospective materials that might find numerous applications in various fields of science and engineering (De Paiva et al., 2008). In the present work, we study modification of clay minerals with a two step-process, i.e. sequential pillaring and ligand grafting. We combined oxide modification with grafting of a chelating agent to the surface of two abundant clay minerals, i.e. montmorillonite and kaolinite. Use of kaolinite and montmorillonite discloses the impact of swelling tendency on effectiveness of the final organoclay adsorbent for heavy metal ions. The study is focused on clay mineral modification to make them suitable for adsorption of heavy metal ions from diluted wastewater. 2. Experimental 2.1. Materials Kaolinite (Kt), tetraethoxysilane (TEOS), (C2H3O2)2Ni·4H2O, (C2H3O2)2Cd·2H2O and (C2H3O2)4Pb were purchased from SigmaAldrich (St. Louis, MO). Montmorillonite K-10 (Mt) was purchased from Acros Organic (Geel, Belgium). FeCl3·6H2O, Fe(NO3)3·9H2O, CuSO4·5H2O, ZnCl2 were purchased from Fisher Scientific (Pittsburgh, PA). Octadecyl trimethylammonium chloride (TCI America, Portland, OR) was used for organo-modification. A chelating ligand, N-[3(trimethoxysilyl)propyl]ethylenediamine triacetic acid trisodium salt (TMS-EDTA), was obtained from Gelest Inc. (Morrisville, PA). 2.2. Methods 2.2.1. Modification by ferric oxide Ferric oxide was introduced into Mt and Kt (samples 2 and 5 in Table 1) using a slightly modified method reported by Yuan et al.
(2006a). Raw Mt (1) or Kt (4) (10 g) were dispersed in 55 mL of deionized water at 1000 rpm stirring. Na2CO3 (6.5 g) was dissolved in 50 mL of 0.2 M Fe(NO3)3 at constant stirring until a homogenous mixture was formed. The mixture was aged for 26 h. Then it was added dropwise to the clay mineral dispersion at stirring for 2 h at 60 °C. The sample was allowed to age for 20 h at room temperature, washed with deionized water, filtered, dried, and calcined in air at 300 °C for 3 h. 2.2.2. Modification by silica Raw montmorillonite or kaolinite (10 g) were dispersed in 64 mL of deionized water and then stirred vigorously at 1000 rpm. A solution of TEOS (60 g) and octadecyl trimethylammonium chloride (9.7 g) in 20 mL of 2-propanol was slowly added to the dispersion, which then formed a gel. The pH of the reaction mixture was adjusted to 10 by adding ammonia solution, and then stirred for 2 h. The products (3 and 6) were filtered, washed with deionized water, dried, and calcined in air at 300 °C for 3 h. 2.2.3. Grafting of the chelating ligand Grafting of the ligand to the surfaces of 1–6 was conducted according to the procedure described by Vasiliev et al. (2009). The sketch in Fig. 1 illustrates grafting process and the structure of products 7–12. 2.2.4. Adsorption of heavy metal ions For adsorption studies, solutions of the metal salts with metal concentrations of 5.0 ppm were prepared. In the case of Pb 4 +, a solution with a concentration of 71.0 ppm was chosen due to higher detection limit of the instrument on this metal. All metal salts except (C2H3O2)4Pb were dissolved in deionized water. In order to prevent hydrolysis, the (C2H3O2)4Pb solution was acidified by concentrated HNO3 to pH = 3.5. For the study of the relative adsorption of different metal ions, a solution containing a mixture of six metal salts with total metal ions concentration 42.3 ppm was prepared in an acidified water. The solutions were passed through the column filled with the modified clay mineral 9 (0.75 g) using a Carter Cassette peristaltic pump (Thermo Fisher Scientific, Barrington, IL) at the constant average rate 15 mL/min. The aliquots of the output solutions were taken each hour and analyzed for residual metal content. 2.3. Characterization techniques The porous structures of 1–6 were studied on a Quantachrome Nova porosimeter (Boynton Beach, FL). The measurements were conducted by BET adsorption of N2 at −196 °C. Prior to measurements, the samples were degassed in vacuum at 300 °C for 3 h. Total pore volume and pore size distribution were calculated using the BJH method. The micropore volume was determined by the DR method. Amount of the ligand immobilized on the surfaces was calculated from contents of C, H and N. Elemental analysis was provided by Robertson Microlit Lab. (Ledgewood, NJ). IR spectra were recorded on a Shimadzu IR Prestige-21 spectrometer (Shimadzu Corp., Kyoto,
Table 1 Properties of raw and modified montmorillonite and kaolinite. Raw and oxide-modified clay minerals
Organoclay minerals
Sample
Clay mineral
Oxide
Amount of introduced oxide, %
Specific BET surface area, m2/g
Total pore volume, cm3/g
Micro-pore volume, cm3/g
Average pore radius, A
Sample
Loading of TMS-EDTA, mmol/g
Average TMS-EDTA density, molecules/nm2
1 2 3 4 5 6
Montmorillonite
– Fe2O3 SiO2 – Fe2O3 SiO2
– 42 177 – 41 68
219 277 96 18 102 91
0.28 0.32 0.19 0.03 0.20 0.05
0.19 0.24 0.09 0.02 0.09 0.06
16.5 16.5 16.4 16.0 25.4 16.5
7 8 9 10 11 12
0.05 0.21 1.25 0.04 0.11 1.04
0.14 0.45 7.80 1.30 0.65 6.80
Kaolinite
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Fig. 1. Grafting of the chelating ligand and adsorption of metal ions.
Japan). DSC data were acquired on a Perkin Elmer Diamond DSC instrument (Waltham, MS). TGA analysis was carried out on a Perkin Elmer TGA-7 (Waltham, MS). Thermoanalysis studies were conducted under nitrogen at a heating rate of 10 °C/min. The range of temperatures was 20–600 °C for DSC and 20–700 °C for TGA. Concentrations of heavy metal ions were measured using an AA-6300 Atomic Absorption Spectrophotometer (Shimadzu Corp., Kyoto, Japan).
(νsSi\O\Si) (Fig. 5). In the spectrum of 9, the first band can also be attributed to νasSi\C of immobilized TMS-EDTA. Additionally, its spectrum contained weak bands attributed to vibrations of organic groups (cm − 1): 2852 and 2920 (νCH2), 1560 and 1655 (νCOO−).
3. Results 3.1. Modification with oxides 3.1.1. Amount of oxides introduced Mt had a significantly higher capacity on both oxides (Fe2O3 and SiO2) than Kt (Table 1). For both clays the amount of bound silica was higher than amount of ferric oxide (Table 1). 3.1.2. Porosity BET specific surface areas of samples 1–3 calculated from adsorption isotherms were significantly higher than corresponding samples 4–6 (Fig. 2). However, modification of Mt with silica (sample 3) reduced the specific BET surface. The average pore radius did not change after the experiments except modification of Kt by ferric oxide (Table 1). 3.2. Immobilization of the ligand Starting materials 1 and 4 had little capacities for the ligand (Table 1). The ferric oxide-modified clay minerals (2 and 5) showed higher TMS-EDTA loading while modification with silica increased immobilization capacity (3 and 6). In all experiments, Mt had higher immobilization capacity than Kt. In particular, the loading of TMS-EDTA on sample 3 was 20.2% higher than on sample 6. 3.3. Thermoanalytical data The DSC curve of 9 (Fig. 3) displayed three endothermic effects at (ΔH, J/g): 213 (6.9); 318 (3.7); 350 (3.8). The TGA thermogram of this sample (Fig. 4) showed a slight loss in mass at heating to 180 °C (0.5%). In the range 180–317 °C the mass loss was 12.8%. Then the rate of mass loss became slower, and the total mass loss up to 700 °C was 22%. 3.4. IR spectroscopy IR spectra of samples 1 and 9 had characteristic bands of the alumosilicate network at 792–806 cm− 1 (νsSi\O) and 1051–1053 cm− 1
Fig. 2. BET isotherms and pore size distributions of samples 1–3 (a) and 4–6 (b).
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Fig. 3. DSC thermogram of sample 9.
3.5. Adsorption of heavy metal ions Concentrations of all individual metals were significantly reduced after passing through the column with adsorbent 9 (Table 2). In the case of the metal mixture, prevalent adsorption of Fe 3 + cations was observed. 4. Discussion 4.1. Modification with oxides The BET isotherms of 1 and 4 showed significant difference in the porosity of Mt and Kt (Fig. 2). The BET isotherm of 4 is typical for nonporous materials. This result agrees with literature data on raw Kt as a non-porous material (Al-Harahsheh et al., 2009). In contrast to Kt, the BET isotherm of Mt is typical for mesoporous structures. It is close to type IV isotherms and has profound hysteresis, indicating a relatively high pore volume. Modification of both clay minerals with ferric oxide significantly increased the specific surface areas and pore volumes at a relatively small amount of introduced oxide (Table 1). The total amount of introduced ferric oxide was higher in the case of Mt compared to Kt. Table 1 data, along with data for BET isotherm of sample 2, are in good agreement with literature (Huerta et al., 2003; Maes and Vansant, 1995; Rightor et al., 1991; Yuan et al., 2006a) and prove a significant increase of Mt porosity after modification. The BET isotherm of sample 2 is a mixed II + IV type with the gradual slope increase in the range P/Po = 0.1–0.4. This shape is characteristic for
wide pore size distributions. The shape of the pores is cylindrical as one can see from hysteresis of type H1 in accordance with IUPAC classification (Sing et al., 1985). One can define the modification of 1 by ferric oxide as pillaring. For Kt, development of a porous structure was also clearly indicated by the BET isotherms. Fig. 2 shows a profound hysteresis loop for sample 5, while practically no hysteresis was observed in raw clay minerals 4. This result was unexpected as this clay mineral is non-expandable. However, similar methodology of Kt treatment was reported by Al-Harahsheh et al. (2009). Exfoliation of raw Kt could increase the total specific surface area. Micropores, probably not detectable by nitrogen molecules, were reported in this work after reacting the kaolinite with (3-chloropropyl)triethoxysilane and calcination at moderate temperature. Development of porous structure in Kt was also reported in several recent publications (Ma and Wang, 2006; Yu et al., 2009). The surface charge of Kt particles strongly depends on pH and ionic strength of the solution. At low pH, one can expect positive charging of the particles' edges and negative charging of the faces. Reacting Kt with ferric salt in alkaline media probably leads initially to adsorption of hydrated iron ion species such as [Fe(H2O)5(OH)] 2 +, [Fe(H2O)4(OH)2] +, etc. At high pH, edges gain negative charge (Zhou and Gunter, 1992) and electrostatic forces should facilitate adsorption of positive ferric ionic species to the edges of the particles. Results of this experiment prove the ability of Kt to form mesoporous structures in spite of its non-expandable properties. In contrast to sample 2, the shape of hysteresis in the isotherm of sample 5 has mixed H1–H3 type isotherms showing the presence of both cylindrical and slit pores.
Fig. 4. TGA thermogram of sample 9.
M. Addy et al. / Applied Clay Science 59–60 (2012) 115–120
Fig. 5. IR spectra of samples 1 and 9.
While modification with ferric oxide increased both total BET specific surface area and porosity of Mt, silica modification unexpectedly reduced them (Table 1). It should be noted that the total amount of introduced silica was much higher than ferric oxide for both clay minerals. Modification of Mt by silica using TEOS and a surfactant was reported earlier. Li et al. (2009a, 2009b) defined the products as silica-pillared clay minerals. In the work of Li et al. (2009a) such a material was identified as clay–silica composite. Unfortunately, in both reports, amounts of reacted TEOS were not presented. Considering the high amount of TEOS incorporated into the structure of the clay mineral, we suggest that the obtained material is a composite rather than simply pillared clay mineral. The silica component of this composite might be responsible for reduced porosity due to partial filling of the interlayer space with silica. In the case of Kt, the BET isotherm of sample 6 has stepwise shape (type VI) that indicates formation of two monolayers and proves bimodal pore size distribution. The product has low porosity formed mainly by micropores. A possible explanation of this result is formation of silica from TEOS independently from Kt particles. Therefore, this method was not effective in modification of kaolinite.
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the high porosity of Mt should enable immobilization of TMS-EDTA in mesopores. However, in spite of such significant structural differences, ligand loading on sample 1 was not very different from the loading on sample 4. It is evident that even in the case of expandable clay minerals, steric hindrances still restrict accessibility of potential immobilization sites in the interlayer space to large molecules such as TMS-EDTA. The total amount of immobilized ligand was strongly affected by the modification of clay mineral either by ferric oxide or by silica. Thus, introduction of 14% of Fe2O3 to Kt, and 42% to Mt increased the immobilization capacity of the clay minerals by 175% and 320%, respectively. Such a notable effect might be referred to the disappearance of steric hindrances that makes almost all interlayer space accessible for TMS-EDTA. In the case of silica-modified clay minerals, the total specific loading of TMS-EDTA increased by a factor >25 in comparison to the raw samples. Such an effect cannot be explained by increase of the specific surface area. It is worth mentioning that the TMS-EDTA loading per nm 2 on 3 and 6 is higher than on mesoporous silica gel itself (Vasiliev et al., 2009). Possibly, a significant part of TMS-EDTA was grafted in micropores, in which surface area cannot be calculated using the BET method. Comparison of IR spectra of Mt and organo-modified Mt 9 clearly proves the presence of organic molecules in the material. Thermal analysis of 9 showed total mass losses of about 23 mass% up to 700 °C. If we assume scission of the ligand molecule at the Si\C bond, this amount is in good agreement with the total mass loading of TMS-EDTA. The DSC pattern of the organo-modified Mt (Fig. 3) shows three well-defined endothermic peaks that might be attributed to multistep decomposition of the immobilized ligand. There are no literature data on thermal behavior of TMS-EDTA. The temperature of the first peak is very close to the decomposition temperature of EDTA (237–245 °C). Thermal destruction of EDTA is a multistep process involving initial cleavage of C\N bonds, and followed by decarboxylation of the carboxyl groups (Martell et al., 1975). In addition, the second and the third peaks are in the region of decomposition of most of hybrid organic/inorganic materials and might be attributed to Si\C bond cleavage. Thus, thermal decomposition of immobilized ligand is a complex three-step process. Alternatively, multiple peaks might originate from the multiphase nature of the obtained material where TMS-EDTA molecules might be immobilized on both silica and clay mineral surfaces.
4.2. Immobilization of the ligand 4.3. Adsorption of heavy metal ions Raw Kt displayed very low immobilization capacities for organic molecules due to its almost non-porous structure. A small amount of the TMS-EDTA was probably immobilized on the external surface of the particles. If we take into account the volume of a single molecule of TMS-EDTA (386.18 Å 3, calculated using Spartan '06 software), the total pore volume in Kt (Table 1) was almost three-fold lower than the total volume of immobilized molecules. In contrast,
High loading of chelating ligands illustrates a procedure for the design of effective adsorbents for environmental protection. Adsorption of heavy metal ions on clay minerals is a well known technology of water clean-up (Alvarez-Ayuso and Garcia-Sanchez, 2003; Gupta and Bhattacharyya, 2008; Yavuz et al., 2003). Clay mineral surfaces are protonated at low pH, becoming positively charged, which prevents
Table 2 Concentrations of heavy metal ions after adsorption from solutions of single salts on sample 9.
Table 3 Concentrations of heavy metal ions after adsorption from a mixed solution on sample 9.
Time, h
Initial 1 2 3 4 5 6
Concentration, ppm
Time, h
Cu2 +
Ni2 +
Cd2 +
Zn2 +
Fe3 +
Pb4 +
5.0 b 0.1 b 0.1 b 0.1 b 0.1 b 0.1 b 0.1
5.0 0.3 0.1 0.2 0.1 0.2 0.1
5.0 b 0.1 b 0.1 b 0.1 b 0.1 b 0.1 b 0.1
5.0 0.1 b 0.1 b 0.1 b 0.1 b 0.1 b 0.1
5.0 0.6 0.2 0.2 0.1 0.2 0.2
71.0 2.0 1.5 2.0 2.3 2.0 2.0
Initial 1 2 3 4 5 6
Concentration, ppm Cu2 +
Ni2 +
Cd2 +
Zn2 +
Fe3 +
Pb4 +
6.4 0.3 4.0 5.0 5.0 5.1 5.1
7.0 5.8 6.3 6.2 6.1 5.9 6.1
8.0 6.7 6.9 6.9 6.9 6.7 7.0
7.0 6.5 6.5 6.4 6.3 6.2 6.2
6.5 2.1 1.0 0.9 0.6 1.1 1.3
7.5 6.8 6.3 6.3 6.3 6.0 6.0
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adsorption of metal cations (Mathialagan and Viraraghavan, 2002). More stable complexes can be formed with organic chelate ligands. In particular, log K for complexes of Zn2 +, Cd2 +, Ni2 + and Cu 2 + with EDTA are 16.5, 16.6, 18.6 and 18.8, respectively. Fe3 + ions form much more stable complexes with EDTA (log K = 25.7). For comparison, the range of log K for the reaction of metal cations with cationexchanging sites on the Mt surface are very low. Thus, for negatively charged sites their values are in the range 2.37–2.56. For neutral surface OH− groups, they are still lower (Gu et al., 2010). We studied adsorption of four divalent (Zn2 +, Cd 2 +, Ni2 + and 2+ Cu ), one trivalent (Fe 3 +) and one tetravalent (Pb 4 +) heavy metal cations on sample 9 from solutions of single salts and their mixture. Table 2 lists results of column adsorption of individual heavy metal ions from aqueous solutions. The adsorption of the modified Mt with all single heavy metal ions is pretty high. The concentrations of metals in eluents after adsorption were 0.1–4.3% of the initial concentrations. Compounds containing Pb(IV) did not belong to common contaminants of environment. One of the sources of Pb(IV) in soil was tetraethyl lead Pb(C2H5)4 used in the past as an antiknock additive to fuels. At present time, this additive is not used in most countries in the world. Lead tetraacetate is hydrolyzed in neutral aqueous solution and can be dissolved only in acidic media. Normally, clay minerals do not adsorb metal cations effectively at low pH, however, organoclay 9 displayed a high adsorption of the metal cations even in presence of acid. Industrial wastewaters usually contain mixed contaminants. For estimation of adsorption selectivity, clean-up of an acidic mixture of heavy metal salts was conducted on the column filled with 9. Analysis of eluent samples showed a preferable adsorption of Fe 3 + cations, in agreement with literature data on the stability of metal complexes with EDTA (Table 3). 5. Conclusion Highly porous materials were obtained by modification of montmorillonite and kaolinite with ferric oxide. The modified clay minerals contain both micropores and mesopores. Modification of the clay minerals with silica led to mixed clay mineral-silica composites with lower porosity. An interesting characteristic of such composites is a very high binding of the chelating ligand, N-[3-(trimethoxysilyl) propyl]ethylenediamine triacetic acid trisodium salt. The grafted silica modified montmorillonite was successfully used for adsorption of heavy metal cations from individual and mixed solutions of their salts. Acknowledgment We thank Prof. J. Wardeska for his kind assistance in manuscript preparation. B. L. thanks ETSU Honors College for the StudentFaculty Collaborative Grant. References Al-Harahsheh, M., Shawabkeh, R., Al-Harahsheh, A., Tarawneh, K., Batiha, M.M., 2009. Surface modification and characterization of Jordanian kaolinite: application for lead removal from aqueous solutions. Applied Surface Science 255, 8098–8103. Alvarez-Ayuso, E., Garcia-Sanchez, A., 2003. Removal of heavy metals from wastewaters by natural and Na-exchanged bentonites. Clays and Clay Minerals 51, 475–480. Aouad, A., Mandalia, T., Bergaya, F., 2005. A novel method of Al-pillared montmorillonite preparation for potential industrial up-scaling. Applied Clay Science 28, 175–182. Bhattacharyya, K.G., Gupta, S.S., 2008. Adsorption of a few heavy metals on natural and modified kaolinite and montmorillonite: a review. Advances in Colloid and Interface Science 140, 114–131. Canzares, P., Valverde, J.L., Sun Kou, M.R., Molina, C.B., 1999. Synthesis and characterization of PILCs with single and mixed oxide pillars prepared from two different bentonites. A comparative study. Microporous and Mesoporous Materials 29, 267–281. De Paiva, L.B., Morales, A.R., Díaz, F.R.V., 2008. Organoclays: properties, preparation and applications. Applied Clay Science 42, 8–24. De Stefanis, A., Tomlinson, A.A.G., 2006. Towards designing pillared clays for catalysis. Catalysis Today 114, 126–141.
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Moreno, S., Gutierrez, E., Alvarez, A., Papayannakos, N.G., Poncelet, G., 1997. Al-pillared clays: from lab syntheses to pilot scale production. Characterization and catalytic properties. Applied Catalysis A: General 165, 103–114. Nakao, T., Nogami, M., 2005. Preparation of silica-pillared clays with micro- and mesopores using aminopropyltriethoxysilane and tetraethoxysilane. Materials Letters 59, 3221–3225. Occelli, M.L., Bertrand, J.A., Gould, S.A.C., Dominguez, J.M., 2000. Physicochemical characterization of a Texas montmorillonite pillared with polyoxocations of aluminum Part I: the microporous structure. Microporous and Mesoporous Materials 34, 195–206. Perez-Vidal, H., Custodio-Garcıa, E., Lopez-Alejandro, E., Morales-Hidalgo, J., Frıas-Marquez, D.M., 2006. Characterization of pillared clays containing Fe3 + and Cu. Solar Energy Materials and Solar Cells 90, 841–846. Rightor, E.G., Tzou, M.-S., Pinnavaia, T.J., 1991. Iron oxide pillared clay with large gallery height: Synthesis and properties as a Fischer–Tropsch catalyst. Journal of Catalysis 130, 29–40. Sing, K.S.W., Everett, D.H., Haul, R.A.W., Moscou, L., Pierotti, R.A., Rouquerol, J., Siemieniewska, T., 1985. Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity. Pure and Applied Chemistry 57, 603–619. Sychev, M., Shubina, T., Rozwadowski, M., Sommen, A.P.B., De Beer, V.H.J., van Santen, R.A., 2000. Characterization of the microporosity of chromia- and titania-pillared montmorillonites differing in pillar density. I. Adsorption of nitrogen. Microporous and Mesoporous Materials 37, 187–200. Vasiliev, A.N., Golovko, L.V., Trachevsky, V.V., Hall, G.S., Khinast, J.G., 2009. Adsorption of heavy metal cations by organic ligands grafted on porous materials. Microporous and Mesoporous Materials 118, 251–257. Wachs, I.E., 2009. Characterization of Catalytic Materials. 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Yuan, P., Yin, X., He, H., Yang, D., Wang, L., Zhu, J., 2006b. Investigation on the delaminated-pillared structure of TiO2-PILC synthesized by TiCl4 hydrolysis method. Microporous and Mesoporous Materials 93, 240–247. Zhou, Z., Gunter, W.D., 1992. Surface charge of kaolinite. Clays and Clay Minerals 40, 365–368.
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Research paper
Adsorption of heavy metal ions on mesoporous silica-modified montmorillonite containing a grafted chelate ligand Mary Addy, Bradley Losey, Ray Mohseni, Eugene Zlotnikov 1, Aleksey Vasiliev ⁎ East Tennessee State University, Department of Chemistry, PO Box 70695, Johnson City, 37614 TN, USA
a r t i c l e
i n f o
Article history: Received 24 August 2011 Received in revised form 13 February 2012 Accepted 15 February 2012 Available online 30 March 2012 Keywords: Montmorillonite Kaolinite Organoclay Heavy metal cations Adsorption Environmental protection
a b s t r a c t The objective of this work is development of a new adsorbent on the base of an organoclay with a chelating ligand covalently attached to the clay mineral surface. The presence of a chelating ligand in the clay structure significantly improves its ability to immobilize heavy metal ions from contaminated sludge of wastewater. Montmorillonite and kaolinite were chosen as typical examples of expandable and non-expandable clay minerals. A two-step modification procedure comprised of sequential modification with oxides and grafting of a chelating agent to the modified clay minerals was used. Modifications with silica and ferric oxide were conducted by reacting the dispersed raw clay minerals with tetraethoxysilane and ferric nitrate solution. A chelating ligand, N-[3-(trimethoxysilyl)propyl]ethylenediamine triacetic acid trisodium salt, was introduced into interlayer space of raw and modified clay minerals in aqueous solutions. Laboratory tests of the organoclay efficiency for purification of wastewater were conducted with the most promising sample, i.e., organoclay with the highest specific loading of chelating agent. Experiments were conducted with model wastewater containing either individual or mixed heavy metal ions. The modified organoclay displayed high adsorption capacity for heavy metal cations even in acidic media. The method of modification presented in this work can be used for synthesis of efficient adsorbents for applications in contaminated areas. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Environmental protection requires economically feasible, highly effective materials for adsorption of heavy metal ions (Kurniawan et al., 2006). Naturally occurring abundant clays and clay minerals have very good potential, hindered, however, by weak adsorption of heavy metal ions directly on the clay mineral surface. Such adsorption occurs mainly by ion-exchange mechanism. The cation exchange capacity of untreated clay is not enough for wide scale applications (Marshall, 1949). Adsorption of metal ions by clays strongly depends on the pH of media and becomes much less effective in acidic solutions. Much stronger binding of heavy metal ions might be achieved with the help of chelating agents. For adsorption purposes chelating agents should be immobilized on the solid carriers. This type of adsorbents may potentially extract heavy metal ions from acidic media. The main obstacle to using clays as a chelates' carrier is the relatively small surface available for ligand immobilization. The porosity of lamellar clay minerals can be improved by pillaring. The obligatory requirements for pillared clay minerals are robustness of the intercalation compound to prevent collapse on dehydration or during application. Spacing between pillars should be large enough to ⁎ Corresponding author. Tel.: + 1 423 439 4368; fax: + 1 423 439 5835. E-mail address: [email protected] (A. Vasiliev). 1 Current address: Solvay Speciality Polymers, Alpharetta, GA 30002, USA. 0169-1317/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2012.02.012
allow access of molecules of interest (Wachs, 2009), i.e., chelating ligands in our case. The production process should be scalable and cheap. Since clays are natural products, variation of their properties might be significant, therefore, the production process should be adoptable to reasonable variation of the properties and composition of the raw clay. In a review of De Stefanis and Tomlinson (2006), they summarized various routes for pillaring. The most widely traditional route is pillaring with inorganic cations of multivalent elements such as iron, aluminum, zirconium, chromium etc. Inorganic cations might be used individually or as a mixture (Aouad et al., 2005; Canzares et al., 1999; Mahboub et al., 2006; Moreno et al., 1997; Occelli et al., 2000; Perez-Vidal et al., 2006; Sychev et al., 2000; Yan et al., 2008). All these publications reported significant increase of BET specific surface area. In the work of Maes et al. (1997), the final product combined micro- and mesopores with overall BET specific surface area of 383 m 2/g was prepared. Increase of overall BET specific surface area from 35.1 to 323.2 m 2/g by montmorillonite pillaring with TiCl4 in alkaline media was described by Yuan et al. (2006a, 2006b). Less widely used but a promising route of pillaring in prospective of ligand grafting is based on various modifications of the sol–gel process (Kawi and Yao, 1999; Li et al., 2009a, 2009b; Mao et al., 2009; Nakao and Nogami, 2005). Literature data showed that the total BET surface area by this process might exceed 700 m 2/g. The clay mineral dispersions were reacted with a solution of tetraethoxysilane (TEOS) in ethanol. Alkaline or acidic hydrolysis leads to formation of silicate
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ions, with formation of amorphous silica three-dimensional networks. Amorphous silica might form pillars between the layers of the clay minerals. However, no reasons for silica domains to be localized only in the interlayer space were found in literature. Introduction of surfactants into the TEOS process allows making a templated heterostructure. In the work of Xia et al. (2009), use of cetyl trimethylammonium bromide was described to achieve a bi-modal mesoporous structure of montmorillonite in a hydrothermal process. Loading of surfactants exceeded the critical micelle concentration. Intercalation of these micelles into the interlayer spaces and further loading and hydrolysis of TEOS resulted in formation of silica bulk layers penetrated by micelles. Calcination at moderate temperature eliminated the organic component with formation of a porous clay heterostructure. A major part of literature devoted to modifying clay minerals considers various types of montmorillonite. Much less was published about modification of kaolinite. Bhattacharyya and Gupta (2008) reported application of modified kaolinite and montmorillonite for adsorption of heavy metal ions from water. The authors concluded that application of clay minerals is economically feasible for adsorption of heavy metal ions. Modification of clay minerals significantly increased adsorption capacity. Organoclays are very prospective materials that might find numerous applications in various fields of science and engineering (De Paiva et al., 2008). In the present work, we study modification of clay minerals with a two step-process, i.e. sequential pillaring and ligand grafting. We combined oxide modification with grafting of a chelating agent to the surface of two abundant clay minerals, i.e. montmorillonite and kaolinite. Use of kaolinite and montmorillonite discloses the impact of swelling tendency on effectiveness of the final organoclay adsorbent for heavy metal ions. The study is focused on clay mineral modification to make them suitable for adsorption of heavy metal ions from diluted wastewater. 2. Experimental 2.1. Materials Kaolinite (Kt), tetraethoxysilane (TEOS), (C2H3O2)2Ni·4H2O, (C2H3O2)2Cd·2H2O and (C2H3O2)4Pb were purchased from SigmaAldrich (St. Louis, MO). Montmorillonite K-10 (Mt) was purchased from Acros Organic (Geel, Belgium). FeCl3·6H2O, Fe(NO3)3·9H2O, CuSO4·5H2O, ZnCl2 were purchased from Fisher Scientific (Pittsburgh, PA). Octadecyl trimethylammonium chloride (TCI America, Portland, OR) was used for organo-modification. A chelating ligand, N-[3(trimethoxysilyl)propyl]ethylenediamine triacetic acid trisodium salt (TMS-EDTA), was obtained from Gelest Inc. (Morrisville, PA). 2.2. Methods 2.2.1. Modification by ferric oxide Ferric oxide was introduced into Mt and Kt (samples 2 and 5 in Table 1) using a slightly modified method reported by Yuan et al.
(2006a). Raw Mt (1) or Kt (4) (10 g) were dispersed in 55 mL of deionized water at 1000 rpm stirring. Na2CO3 (6.5 g) was dissolved in 50 mL of 0.2 M Fe(NO3)3 at constant stirring until a homogenous mixture was formed. The mixture was aged for 26 h. Then it was added dropwise to the clay mineral dispersion at stirring for 2 h at 60 °C. The sample was allowed to age for 20 h at room temperature, washed with deionized water, filtered, dried, and calcined in air at 300 °C for 3 h. 2.2.2. Modification by silica Raw montmorillonite or kaolinite (10 g) were dispersed in 64 mL of deionized water and then stirred vigorously at 1000 rpm. A solution of TEOS (60 g) and octadecyl trimethylammonium chloride (9.7 g) in 20 mL of 2-propanol was slowly added to the dispersion, which then formed a gel. The pH of the reaction mixture was adjusted to 10 by adding ammonia solution, and then stirred for 2 h. The products (3 and 6) were filtered, washed with deionized water, dried, and calcined in air at 300 °C for 3 h. 2.2.3. Grafting of the chelating ligand Grafting of the ligand to the surfaces of 1–6 was conducted according to the procedure described by Vasiliev et al. (2009). The sketch in Fig. 1 illustrates grafting process and the structure of products 7–12. 2.2.4. Adsorption of heavy metal ions For adsorption studies, solutions of the metal salts with metal concentrations of 5.0 ppm were prepared. In the case of Pb 4 +, a solution with a concentration of 71.0 ppm was chosen due to higher detection limit of the instrument on this metal. All metal salts except (C2H3O2)4Pb were dissolved in deionized water. In order to prevent hydrolysis, the (C2H3O2)4Pb solution was acidified by concentrated HNO3 to pH = 3.5. For the study of the relative adsorption of different metal ions, a solution containing a mixture of six metal salts with total metal ions concentration 42.3 ppm was prepared in an acidified water. The solutions were passed through the column filled with the modified clay mineral 9 (0.75 g) using a Carter Cassette peristaltic pump (Thermo Fisher Scientific, Barrington, IL) at the constant average rate 15 mL/min. The aliquots of the output solutions were taken each hour and analyzed for residual metal content. 2.3. Characterization techniques The porous structures of 1–6 were studied on a Quantachrome Nova porosimeter (Boynton Beach, FL). The measurements were conducted by BET adsorption of N2 at −196 °C. Prior to measurements, the samples were degassed in vacuum at 300 °C for 3 h. Total pore volume and pore size distribution were calculated using the BJH method. The micropore volume was determined by the DR method. Amount of the ligand immobilized on the surfaces was calculated from contents of C, H and N. Elemental analysis was provided by Robertson Microlit Lab. (Ledgewood, NJ). IR spectra were recorded on a Shimadzu IR Prestige-21 spectrometer (Shimadzu Corp., Kyoto,
Table 1 Properties of raw and modified montmorillonite and kaolinite. Raw and oxide-modified clay minerals
Organoclay minerals
Sample
Clay mineral
Oxide
Amount of introduced oxide, %
Specific BET surface area, m2/g
Total pore volume, cm3/g
Micro-pore volume, cm3/g
Average pore radius, A
Sample
Loading of TMS-EDTA, mmol/g
Average TMS-EDTA density, molecules/nm2
1 2 3 4 5 6
Montmorillonite
– Fe2O3 SiO2 – Fe2O3 SiO2
– 42 177 – 41 68
219 277 96 18 102 91
0.28 0.32 0.19 0.03 0.20 0.05
0.19 0.24 0.09 0.02 0.09 0.06
16.5 16.5 16.4 16.0 25.4 16.5
7 8 9 10 11 12
0.05 0.21 1.25 0.04 0.11 1.04
0.14 0.45 7.80 1.30 0.65 6.80
Kaolinite
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Fig. 1. Grafting of the chelating ligand and adsorption of metal ions.
Japan). DSC data were acquired on a Perkin Elmer Diamond DSC instrument (Waltham, MS). TGA analysis was carried out on a Perkin Elmer TGA-7 (Waltham, MS). Thermoanalysis studies were conducted under nitrogen at a heating rate of 10 °C/min. The range of temperatures was 20–600 °C for DSC and 20–700 °C for TGA. Concentrations of heavy metal ions were measured using an AA-6300 Atomic Absorption Spectrophotometer (Shimadzu Corp., Kyoto, Japan).
(νsSi\O\Si) (Fig. 5). In the spectrum of 9, the first band can also be attributed to νasSi\C of immobilized TMS-EDTA. Additionally, its spectrum contained weak bands attributed to vibrations of organic groups (cm − 1): 2852 and 2920 (νCH2), 1560 and 1655 (νCOO−).
3. Results 3.1. Modification with oxides 3.1.1. Amount of oxides introduced Mt had a significantly higher capacity on both oxides (Fe2O3 and SiO2) than Kt (Table 1). For both clays the amount of bound silica was higher than amount of ferric oxide (Table 1). 3.1.2. Porosity BET specific surface areas of samples 1–3 calculated from adsorption isotherms were significantly higher than corresponding samples 4–6 (Fig. 2). However, modification of Mt with silica (sample 3) reduced the specific BET surface. The average pore radius did not change after the experiments except modification of Kt by ferric oxide (Table 1). 3.2. Immobilization of the ligand Starting materials 1 and 4 had little capacities for the ligand (Table 1). The ferric oxide-modified clay minerals (2 and 5) showed higher TMS-EDTA loading while modification with silica increased immobilization capacity (3 and 6). In all experiments, Mt had higher immobilization capacity than Kt. In particular, the loading of TMS-EDTA on sample 3 was 20.2% higher than on sample 6. 3.3. Thermoanalytical data The DSC curve of 9 (Fig. 3) displayed three endothermic effects at (ΔH, J/g): 213 (6.9); 318 (3.7); 350 (3.8). The TGA thermogram of this sample (Fig. 4) showed a slight loss in mass at heating to 180 °C (0.5%). In the range 180–317 °C the mass loss was 12.8%. Then the rate of mass loss became slower, and the total mass loss up to 700 °C was 22%. 3.4. IR spectroscopy IR spectra of samples 1 and 9 had characteristic bands of the alumosilicate network at 792–806 cm− 1 (νsSi\O) and 1051–1053 cm− 1
Fig. 2. BET isotherms and pore size distributions of samples 1–3 (a) and 4–6 (b).
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Fig. 3. DSC thermogram of sample 9.
3.5. Adsorption of heavy metal ions Concentrations of all individual metals were significantly reduced after passing through the column with adsorbent 9 (Table 2). In the case of the metal mixture, prevalent adsorption of Fe 3 + cations was observed. 4. Discussion 4.1. Modification with oxides The BET isotherms of 1 and 4 showed significant difference in the porosity of Mt and Kt (Fig. 2). The BET isotherm of 4 is typical for nonporous materials. This result agrees with literature data on raw Kt as a non-porous material (Al-Harahsheh et al., 2009). In contrast to Kt, the BET isotherm of Mt is typical for mesoporous structures. It is close to type IV isotherms and has profound hysteresis, indicating a relatively high pore volume. Modification of both clay minerals with ferric oxide significantly increased the specific surface areas and pore volumes at a relatively small amount of introduced oxide (Table 1). The total amount of introduced ferric oxide was higher in the case of Mt compared to Kt. Table 1 data, along with data for BET isotherm of sample 2, are in good agreement with literature (Huerta et al., 2003; Maes and Vansant, 1995; Rightor et al., 1991; Yuan et al., 2006a) and prove a significant increase of Mt porosity after modification. The BET isotherm of sample 2 is a mixed II + IV type with the gradual slope increase in the range P/Po = 0.1–0.4. This shape is characteristic for
wide pore size distributions. The shape of the pores is cylindrical as one can see from hysteresis of type H1 in accordance with IUPAC classification (Sing et al., 1985). One can define the modification of 1 by ferric oxide as pillaring. For Kt, development of a porous structure was also clearly indicated by the BET isotherms. Fig. 2 shows a profound hysteresis loop for sample 5, while practically no hysteresis was observed in raw clay minerals 4. This result was unexpected as this clay mineral is non-expandable. However, similar methodology of Kt treatment was reported by Al-Harahsheh et al. (2009). Exfoliation of raw Kt could increase the total specific surface area. Micropores, probably not detectable by nitrogen molecules, were reported in this work after reacting the kaolinite with (3-chloropropyl)triethoxysilane and calcination at moderate temperature. Development of porous structure in Kt was also reported in several recent publications (Ma and Wang, 2006; Yu et al., 2009). The surface charge of Kt particles strongly depends on pH and ionic strength of the solution. At low pH, one can expect positive charging of the particles' edges and negative charging of the faces. Reacting Kt with ferric salt in alkaline media probably leads initially to adsorption of hydrated iron ion species such as [Fe(H2O)5(OH)] 2 +, [Fe(H2O)4(OH)2] +, etc. At high pH, edges gain negative charge (Zhou and Gunter, 1992) and electrostatic forces should facilitate adsorption of positive ferric ionic species to the edges of the particles. Results of this experiment prove the ability of Kt to form mesoporous structures in spite of its non-expandable properties. In contrast to sample 2, the shape of hysteresis in the isotherm of sample 5 has mixed H1–H3 type isotherms showing the presence of both cylindrical and slit pores.
Fig. 4. TGA thermogram of sample 9.
M. Addy et al. / Applied Clay Science 59–60 (2012) 115–120
Fig. 5. IR spectra of samples 1 and 9.
While modification with ferric oxide increased both total BET specific surface area and porosity of Mt, silica modification unexpectedly reduced them (Table 1). It should be noted that the total amount of introduced silica was much higher than ferric oxide for both clay minerals. Modification of Mt by silica using TEOS and a surfactant was reported earlier. Li et al. (2009a, 2009b) defined the products as silica-pillared clay minerals. In the work of Li et al. (2009a) such a material was identified as clay–silica composite. Unfortunately, in both reports, amounts of reacted TEOS were not presented. Considering the high amount of TEOS incorporated into the structure of the clay mineral, we suggest that the obtained material is a composite rather than simply pillared clay mineral. The silica component of this composite might be responsible for reduced porosity due to partial filling of the interlayer space with silica. In the case of Kt, the BET isotherm of sample 6 has stepwise shape (type VI) that indicates formation of two monolayers and proves bimodal pore size distribution. The product has low porosity formed mainly by micropores. A possible explanation of this result is formation of silica from TEOS independently from Kt particles. Therefore, this method was not effective in modification of kaolinite.
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the high porosity of Mt should enable immobilization of TMS-EDTA in mesopores. However, in spite of such significant structural differences, ligand loading on sample 1 was not very different from the loading on sample 4. It is evident that even in the case of expandable clay minerals, steric hindrances still restrict accessibility of potential immobilization sites in the interlayer space to large molecules such as TMS-EDTA. The total amount of immobilized ligand was strongly affected by the modification of clay mineral either by ferric oxide or by silica. Thus, introduction of 14% of Fe2O3 to Kt, and 42% to Mt increased the immobilization capacity of the clay minerals by 175% and 320%, respectively. Such a notable effect might be referred to the disappearance of steric hindrances that makes almost all interlayer space accessible for TMS-EDTA. In the case of silica-modified clay minerals, the total specific loading of TMS-EDTA increased by a factor >25 in comparison to the raw samples. Such an effect cannot be explained by increase of the specific surface area. It is worth mentioning that the TMS-EDTA loading per nm 2 on 3 and 6 is higher than on mesoporous silica gel itself (Vasiliev et al., 2009). Possibly, a significant part of TMS-EDTA was grafted in micropores, in which surface area cannot be calculated using the BET method. Comparison of IR spectra of Mt and organo-modified Mt 9 clearly proves the presence of organic molecules in the material. Thermal analysis of 9 showed total mass losses of about 23 mass% up to 700 °C. If we assume scission of the ligand molecule at the Si\C bond, this amount is in good agreement with the total mass loading of TMS-EDTA. The DSC pattern of the organo-modified Mt (Fig. 3) shows three well-defined endothermic peaks that might be attributed to multistep decomposition of the immobilized ligand. There are no literature data on thermal behavior of TMS-EDTA. The temperature of the first peak is very close to the decomposition temperature of EDTA (237–245 °C). Thermal destruction of EDTA is a multistep process involving initial cleavage of C\N bonds, and followed by decarboxylation of the carboxyl groups (Martell et al., 1975). In addition, the second and the third peaks are in the region of decomposition of most of hybrid organic/inorganic materials and might be attributed to Si\C bond cleavage. Thus, thermal decomposition of immobilized ligand is a complex three-step process. Alternatively, multiple peaks might originate from the multiphase nature of the obtained material where TMS-EDTA molecules might be immobilized on both silica and clay mineral surfaces.
4.2. Immobilization of the ligand 4.3. Adsorption of heavy metal ions Raw Kt displayed very low immobilization capacities for organic molecules due to its almost non-porous structure. A small amount of the TMS-EDTA was probably immobilized on the external surface of the particles. If we take into account the volume of a single molecule of TMS-EDTA (386.18 Å 3, calculated using Spartan '06 software), the total pore volume in Kt (Table 1) was almost three-fold lower than the total volume of immobilized molecules. In contrast,
High loading of chelating ligands illustrates a procedure for the design of effective adsorbents for environmental protection. Adsorption of heavy metal ions on clay minerals is a well known technology of water clean-up (Alvarez-Ayuso and Garcia-Sanchez, 2003; Gupta and Bhattacharyya, 2008; Yavuz et al., 2003). Clay mineral surfaces are protonated at low pH, becoming positively charged, which prevents
Table 2 Concentrations of heavy metal ions after adsorption from solutions of single salts on sample 9.
Table 3 Concentrations of heavy metal ions after adsorption from a mixed solution on sample 9.
Time, h
Initial 1 2 3 4 5 6
Concentration, ppm
Time, h
Cu2 +
Ni2 +
Cd2 +
Zn2 +
Fe3 +
Pb4 +
5.0 b 0.1 b 0.1 b 0.1 b 0.1 b 0.1 b 0.1
5.0 0.3 0.1 0.2 0.1 0.2 0.1
5.0 b 0.1 b 0.1 b 0.1 b 0.1 b 0.1 b 0.1
5.0 0.1 b 0.1 b 0.1 b 0.1 b 0.1 b 0.1
5.0 0.6 0.2 0.2 0.1 0.2 0.2
71.0 2.0 1.5 2.0 2.3 2.0 2.0
Initial 1 2 3 4 5 6
Concentration, ppm Cu2 +
Ni2 +
Cd2 +
Zn2 +
Fe3 +
Pb4 +
6.4 0.3 4.0 5.0 5.0 5.1 5.1
7.0 5.8 6.3 6.2 6.1 5.9 6.1
8.0 6.7 6.9 6.9 6.9 6.7 7.0
7.0 6.5 6.5 6.4 6.3 6.2 6.2
6.5 2.1 1.0 0.9 0.6 1.1 1.3
7.5 6.8 6.3 6.3 6.3 6.0 6.0
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adsorption of metal cations (Mathialagan and Viraraghavan, 2002). More stable complexes can be formed with organic chelate ligands. In particular, log K for complexes of Zn2 +, Cd2 +, Ni2 + and Cu 2 + with EDTA are 16.5, 16.6, 18.6 and 18.8, respectively. Fe3 + ions form much more stable complexes with EDTA (log K = 25.7). For comparison, the range of log K for the reaction of metal cations with cationexchanging sites on the Mt surface are very low. Thus, for negatively charged sites their values are in the range 2.37–2.56. For neutral surface OH− groups, they are still lower (Gu et al., 2010). We studied adsorption of four divalent (Zn2 +, Cd 2 +, Ni2 + and 2+ Cu ), one trivalent (Fe 3 +) and one tetravalent (Pb 4 +) heavy metal cations on sample 9 from solutions of single salts and their mixture. Table 2 lists results of column adsorption of individual heavy metal ions from aqueous solutions. The adsorption of the modified Mt with all single heavy metal ions is pretty high. The concentrations of metals in eluents after adsorption were 0.1–4.3% of the initial concentrations. Compounds containing Pb(IV) did not belong to common contaminants of environment. One of the sources of Pb(IV) in soil was tetraethyl lead Pb(C2H5)4 used in the past as an antiknock additive to fuels. At present time, this additive is not used in most countries in the world. Lead tetraacetate is hydrolyzed in neutral aqueous solution and can be dissolved only in acidic media. Normally, clay minerals do not adsorb metal cations effectively at low pH, however, organoclay 9 displayed a high adsorption of the metal cations even in presence of acid. Industrial wastewaters usually contain mixed contaminants. For estimation of adsorption selectivity, clean-up of an acidic mixture of heavy metal salts was conducted on the column filled with 9. Analysis of eluent samples showed a preferable adsorption of Fe 3 + cations, in agreement with literature data on the stability of metal complexes with EDTA (Table 3). 5. Conclusion Highly porous materials were obtained by modification of montmorillonite and kaolinite with ferric oxide. The modified clay minerals contain both micropores and mesopores. Modification of the clay minerals with silica led to mixed clay mineral-silica composites with lower porosity. An interesting characteristic of such composites is a very high binding of the chelating ligand, N-[3-(trimethoxysilyl) propyl]ethylenediamine triacetic acid trisodium salt. The grafted silica modified montmorillonite was successfully used for adsorption of heavy metal cations from individual and mixed solutions of their salts. Acknowledgment We thank Prof. J. Wardeska for his kind assistance in manuscript preparation. B. L. thanks ETSU Honors College for the StudentFaculty Collaborative Grant. References Al-Harahsheh, M., Shawabkeh, R., Al-Harahsheh, A., Tarawneh, K., Batiha, M.M., 2009. Surface modification and characterization of Jordanian kaolinite: application for lead removal from aqueous solutions. Applied Surface Science 255, 8098–8103. Alvarez-Ayuso, E., Garcia-Sanchez, A., 2003. Removal of heavy metals from wastewaters by natural and Na-exchanged bentonites. Clays and Clay Minerals 51, 475–480. Aouad, A., Mandalia, T., Bergaya, F., 2005. A novel method of Al-pillared montmorillonite preparation for potential industrial up-scaling. Applied Clay Science 28, 175–182. Bhattacharyya, K.G., Gupta, S.S., 2008. Adsorption of a few heavy metals on natural and modified kaolinite and montmorillonite: a review. Advances in Colloid and Interface Science 140, 114–131. Canzares, P., Valverde, J.L., Sun Kou, M.R., Molina, C.B., 1999. Synthesis and characterization of PILCs with single and mixed oxide pillars prepared from two different bentonites. A comparative study. Microporous and Mesoporous Materials 29, 267–281. De Paiva, L.B., Morales, A.R., Díaz, F.R.V., 2008. Organoclays: properties, preparation and applications. Applied Clay Science 42, 8–24. De Stefanis, A., Tomlinson, A.A.G., 2006. Towards designing pillared clays for catalysis. Catalysis Today 114, 126–141.
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