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Ind. Eng. Chem. Res. 1998, 37, 4296-4301
Metal Adsorption and Desorption Characteristics of Surfactant-Modified Clay Complexes Pomthong Malakul, K. R. Srinivasan, and Henry Y. Wang* Department of Chemical Engineering, The University of Michigan, Ann Arbor, Michigan 48109
Several modified clays have been designed and created for selective removal and recovery of heavy metals such as Cd, Cu, Cr, etc. These surfactant-clay complexes were prepared using hectorite or montmorillonite as the base clay. A simple two-step approach has been developed to synthesize these modified-clay complexes through ion exchange and hydrophobic anchoring of several surfactants such as long-chain alkyldiamines, long-chain dialkylamines, and longchain carboxylic acids onto the clay matrices. The adsorption capacities and affinity constants of the modified clays can be found to approach those of commercial chelating resin (Chelex 100, Bio-Rad). Using cadmium as a model metal and montmorillonite-cetylbenzyldimethylammonium-palmitic acid (M-CBDA-PA) as a model modified-clay complex, the maximum adsorption capacity of the modified clay is found to be 42 ( 0.8 mg/g of clay and the affinity constant is 3.0 ( 0.1 mg/L. The metal adsorption has been shown to be mainly through chemical complexation rather than ion exchange. The immobilization of the metal ions is pH dependent, and thus, pH can act as a molecular switch to regenerate the modified-clay complexes. Introduction The presence of heavy metals in the environment is of major concern because of their toxicity and threat to human life and the environment. Lead, cadmium, and mercury are examples of heavy metals that have been classified as priority pollutants by the U.S. Environmental Protection Agency (U.S. EPA).1 Anthropogenic sources of heavy metals include wastes from the electroplating and metal finishing industries, metallurgical industry, tannery operations, chemical manufacturing, mine drainage, battery manufacturing, leachates from landfills, and contaminated groundwater from hazardous waste sites.2 Heavy metals are also emitted from resource recovery plants in relatively high levels on fly ash particles.3 Metals can be toxic to microbial populations at sufficiently high concentrations. However, some metals such as silver, mercury, cadmium, and copper are markedly more toxic even at very low levels (ppm or less).4 Metal immobilization through precipitation and ion exchange/chelation is a common approach to reduce metal toxicity in the environment. Specific adsorption of heavy metal ions from aqueous solutions onto mineral oxides, dead and live algae, covalently modified montmorillonite, and functionalized monolayers on mesoporous silica (FMMS) has been reported.5-11 Smectitic clays such as hectorite and montmorillonite are fundamental soil components and are abundant in nature. They are layered aluminosilicates with intrinsic negative layer charge. We want to investigate the use of this intrinsic negative layer charge of the clay to anchor cationic modifying agents in the interlayer through a combination of ionic and micellar forces to remove metal ions in nature. Previous work by various groups has already shown that the surface organic coating adsorbed through such a combination of adhesive and cohesive * To whom all correspondence should be addressed. Telephone: (734) 763-5659. Fax: (734) 763-0459. E-mail: hywang@ engin.umich.edu.
forces is quite stable.12-15 In this study, several surfactant-modified clays using a modified approach have been employed in the removal and recovery of heavy metals. The adsorption capacity of these modified clays for metals has dramatically increased. In addition, the organic component in the modified-clay complexes also provides an adsorption site for organic compounds. Thus, these surfactant-modified clays can potentially be used to treat dilute mixed wastes containing both heavy metals and toxic organic compounds. Materials and Methods A. Preparation of the Surfactant-Modified Clay Complexes. The surface of the base clay can be greatly modified by replacing native interlayer cations (e.g., Na+, Ca2+) with organic cations. Organic cations such as primary amine and quaternary ammonium have been used in the modification of clay surfaces by several research groups.12,16-18 In this study, we have used alkyldiamine, Duomeen-T (DT), and cationic surfactant, cetylbenzyldimethylammonium (CBDA), as organic compounds in clay surface modification (Table 1). These cationic modifying agents replace native metal ions on the exchange sites of the base clay and convert the clay surface to become strongly hydrophobic. The modifiedclay surface can be further used to anchor metal ligands such as amine or a carboxylic type through hydrophobic interaction. Using montmorillonite and hectorite as the base clays, two different types of modified clays containing different surface functional groups have been prepared and used in this study. These modified-clay complexes are montmorillonite-CBDA-palmitic acid (M-CBDA-PA) and hectorite-CBDA-DT (H-CBDADT). The smectite surface can be made hydrophobic through the adsorption of a monolayer of a strongly cationic quaternary ammonium surfactant such as CBDA. CBDA forms an “irreversibly” adsorbed monolayer on smectites such as montmorillonite and hectorite. In the second step, metal ligand such as DT or PA can be anchored to
10.1021/ie980057i CCC: $15.00 © 1998 American Chemical Society Published on Web 10/03/1998
Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998 4297 Table 1. Name and Structural Formula of Organic Compounds Used in This Study
a
abbreviation
cetylbenzyldimethylammonium
CBDA
360b
alkyl-1,3-diaminopropane
DTe
350c
palmitic acid
PA
256.4d
In g/mol. b PolySciences. c Akzo Chemicals.
d
structure
mwa
name
Sigma. e Alkyl group (R) ranging from C12 to C18.
CBDA-coated clay through hydrophobic interactions to form a mixed bilayer of CBDA and DT (CBDA-DT) or CBDA and PA (CBDA-PA). A few examples of modified clays prepared through hydrophobic attachment method are shown here. Montmorillonite (Swy-2) and hectorite were obtained from the Clay Minerals Society and The American Colloids Company, respectively. Both clays were fractionated and cleaned following established procedures.13,16 Following Na+ exchange, the clays were washed extensively with deionized water and dried. CBDA was purchased from Polysciences (Warrington, PA) and was used without purification. Duomeen-T was purchased from Akzo Chemicals (McCook, IL). Palmitic acid was purchased from Sigma (St. Louis, MO). Preparation of H-CBDA-DT. Hectorite was initially treated with CBDA at a concentration equivalent to the cation-exchange capacity (CEC) of hectorite (80 mequiv/100 g). It was reported that this cationic surfactant binds irreversibly to the interlayer of the base clay.13 The hectorite-CBDA adduct was reacted with a 4-fold excess of DT (moles of DT/moles of CBDA). This is the optimal molar ratio observed by the same research group. The pH was maintained at 8.5-9.0 to deprotonate both amine groups to minimize the possible displacement of preadsorbed CBDA and to ensure that DT adsorption to H-CBDA will be mainly through hydrophobic anchoring. The resultant H-CBDA-DT was extensively washed and vacuum-dried at 60 °C. Preparation of M-CBDA-PA. Montmorillonite is a widely available smectite-type clay with a CEC of 77 mequiv/100 g. Na-montmorillonite (1% suspension) was dispersed in deionized water and allowed to stand for several hours to allow setting of quartz sand and heavy minerals. The suspension was decanted and resuspended in deionized water. The Clay-CBDApalmitic acid complex is prepared using a procedure similar to the one used for the preparation of the clayCBDA-DT complex. Montmorillonite was initially treated with CBDA at a pH of 5.5-6.0. The clay-CBDA complex was then reacted with palmitic acid, an organic acid containing a C15 chain attached to the terminal carboxyl group (see Table 1). A 2-fold excess of PA was used, and the pH of the reaction medium was maintained at pH 8.5 so that the acid would be essentially in its carboxylate form, and the only mode of binding of PA to clay-CBDA surface is through a “mixed bilayer” formation. The resultant M-CBDA-PA was extensively washed and vacuum-dried at 60 °C. Scanning electron micrographs of the modified clay show that there is no physical difference after the surface modification (data not shown). B. Adsorption Experiments. All metal adsorption experiments were performed in batch mode. A fixed
volume of buffer solution containing a known amount of metal ions was equilibrated with a fixed amount of adsorbent in 50-mL polycarbonate centrifuge tubes. The tubes were agitated at 250 rpm overnight on a shaker (Environ-Shaker, Lab-Line) with a controlled temperature of 25 ( 1 °C. The tubes were then centrifuged for liquid/solid separation, and the concentrations of metal in the aqueous solution were determined by atomic absorption spectrophotometer (AAS). The sorbed amount was calculated by difference. Sorption isotherm studies were conducted by varying the initial metal concentration while the amount of adsorbent remained fixed. The control experiments were carried out without adsorbent in order to correct for any adsorption of metal due to containers. It was found that there was negligible adsorption by the container walls. All solutions used in the adsorption experiments were buffered using tris(hydroxymethyl)aminomethane hydrochloride (TRIS/HCl, Trizma, Sigma). The ionic strength was maintained at 50 mM using sodium chloride (NaCl). The pH of the adsorption mixture was checked at the start and at the end of each adsorption experiment, and the buffering of the solution was found to be adequate to maintain the same pH at the beginning and the end of the batch adsorption experiments (within (0.1 pH unit). Metal stock solutions were prepared from the analytical reagent-grade chloride salts (Aldrich, Milwaukee, WI). Cadmium and copper were used as model metal ions in the adsorption studies using the modified-clay complexes M-CBDA-PA and H-CBDA-DT, respectively. Unmodified clay (M) and clay-CBDA (M-CBDA) were used as control adsorbents in the adsorption studies. In a comparative study between the modifiedclay complex and the commercial resin in cadmium removal from aqueous solution, we used Chelex 100 (200-400 mesh, sodium form, Bio-Rad, CA) which is a chelating resin containing iminodiacetic acid. The effect of pH of the solution on metal adsorption was studied by performing equilibrium adsorption experiments as described previously at different pH values. pH adjustments were made with 0.1 N HCl or 0.1 N NaOH. Desorption and regeneration of the modified-clay complex (M-CBDA-PA) were carried out as follows. The experimental conditions for cadmium adsorption were similar to those of the batch sorption studies. After being subjected to the adsorption process with cadmium solution overnight, the cadmium-loaded adsorbent was separated and washed with 50 mM TRIS/ HCl (pH 7) to remove any unadsorbed cadmium. The adsorbent was then resuspended in an equal volume of deionized water adjusted to pH 3 using HCl. After being equilibrated overnight, the tubes were centrifuged and the amount of desorbed metal in the supernatant was
4298 Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998 Table 2. Properties of the Reference Clay and Modified-Clay Complexes clay/modified clay unmodified clays M-CBDA M-CBDA-PA H-CBDA H-CBDA-DT
C content,a wt %
mmol/g
0.04/0.08 18.31 32.48 18.0 38.94
0 0.61c 0.46c/0.98d 0.6c 0.41c/0.93d
(M/H)b
a The organic carbon contents of CBDA, DT, and PA are 83.3, 82.5, and 74.9% respectively. b M ) montmorillonite and H ) hectorite. c Millimoles of surfactant. d Metal ligand per gram of modified-clay complex.
analyzed. An identical desorption experiment was also performed using Chelex resin. For the reuse of the modified-clay complex, following the desorption, M-CBDA-PA was then washed repeatedly with deionized water before subjecting it to the second cadmium loading cycle. The amount of sorbed cadmium was determined as described previously. C. Physical Characterization and Chemical Analysis. The organic carbon contents of the modifiedclay complexes were determined using a Perkin-Elmer 2400 CHN elemental analyzer. Basal spacings were determined using X-ray powder diffraction (XRD), and XRD patterns were recorded using Cu KR radiation on a Rigaku diffractometer equipped with a rotating anode. FTIR spectra of the modified-clay complex were recorded with a Mattson FTIR Galaxy system using a KBr pellet technique (2-cm-1 resolution). Metal concentrations in samples were analyzed using an atomic absorption spectrophotometer (Perkin-Elmer model 3100). Calibration using standard solutions was always performed prior to sample analysis. Appropriate dilutions of metal samples were done to achieve a linear region of the calibration curve. Results and Discussion Characterization and Properties of Modified Clay Complexes. The organic carbon contents of the unmodified clays and modified-clay complexes are shown in Table 2. The amount of CBDA in clay-CBDA complexes as calculated from the organic carbon contents is 0.61 and 0.60 mmol/g of M-CBDA and H-CBDA, respectively. These values correspond to 0.78 and 0.77 mmol of CBDA/g of montmorillonite and hectorite, which are quite close to the CEC of the base clays (0.77 and 0.80 mequiv/g for montmorillonite and hectorite, respectively). Besides the carbon contents, the nitrogen contents in M-CBDA and M-CBDA-PA have also been analyzed. The nitrogen contents of 0.84% and 0.65% (wt) in M-CBDA and M-CBDA-PA, respectively, agree well with the amount of CBDA calculated from the organic carbon contents. The results suggest that the quaternary cations are exchanged to a similar extent and have replaced nearly all the Na+ cations of the reference clay. The XRD patterns of the modified clays also show an increase in the interlayer spacings from 9.4 to ∼17 Å, which confirms exchange of the Na+ cations by CBDA. The amount of chelating agents (PA and DT) in the modifiedclay complexes M-CBDA-PA and H-CBDA-DT can be calculated from the excess organic carbon after subtracting the organic carbon content due to CBDA. It is found that modified clay M-CBDA-PA contains 0.98 mmol of PA/g of complex, while H-CBDA-DT contains 0.93 mmol of DT/g of complex. The amount of
Figure 1. Adsorption isotherms of cadmium on various adsorbents at 25 °C and pH 7. Lines are fittings with the Langmuir model. Table 3. Langmuir Parameters for the Adsorption of Cadmium on Various Adsorbentsa adsorbent
Qmax, mg/g adsorbent
K, mM
unmodified clay modified clay (M-CBDA-PA) Chelex 100
4(0.6) 42(0.8) 43(0.4)
0.24(0.3) 0.0267(0.001) 0.0224(0.001)
a
Numbers in parentheses are the standard deviation (SD).
chelating agents calculated from the organic carbon contents has been confirmed by the values obtained from acid-base titrations (data not shown). The results shown in Table 2 reveal a molar ratio of PA to CBDA in M-CBDA-PA of 2.1 and a molar ratio of DT to CBDA in H-CBDA-DT of 2.3. Metal Adsorption Studies. Figure 1 shows adsorption isotherms of cadmium on the modified-clay complex (M-CBDA-PA) in comparison with the unmodified clay, M-CBDA, and Chelex 100. The adsorption results exhibit a Langmuir-type isotherm which can be described by the following equation:
q)
QmaxCe K + Ce
where q is the amount of metal adsorbed (mg/g of adsorbent), Ce is an equilibrium concentration (mM), Qmax is the maximum capacity of the adsorbent (mg/g), and K is the affinity constant (mM). The equilibrium adsorption data were fitted to the linear form of the Langmuir equation, and the adsorption parameters of the adsorbents were determined as shown in Table 3. In all cases, the correlation coefficients (R2) for the linear regression fits were found to be >0.98. It can be seen that the Qmax of modified-clay complex (M-CBDA-PA) for cadmium is much higher than the capacity of the unmodified clay and M-CBDA. For M-CBDA, there actually is no cadmium adsorption observed at all since all sorption sites on the base clay are taken up by CBDA and, thus, M-CBDA will not bind cations. This suggests that the surface modification approach used in this study has significantly increased the adsorption capacity of the base clay through the binding mechanism of the metal to the
Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998 4299 Table 4. Some Fundamental IR Absorption Frequencies of Modified Clay (M-CBDA-PA) adsorptiona before
after
assignments
3620 2961 2867 1708 1569
3620 2960 2868 1708 1548
O-H stretching of aluminum hydroxide aromatic C-H stretching aliphatic C-H stretching CdO stretching of COOH CdO stretching of COO-
a
All frequencies in cm-1.
chelating agent PA on the M-CBDA-PA complex. The maximum capacity of modified clay (M-CBDA-PA) was found to be 42 mg of Cd/g of complex (or 0.38 mmol of Cd/g), which approaches the value of commercial Chelex resin (43 mg/g). The amount of sorbed Cd on M-CBDA-PA yields the ligand-to-metal ratio of 2.58 (0.98 mmol of PA to 0.38 mmol of Cd/g of M-CBDAPA). This would suggest that one Cd ion is bound to two PA carboxyl groups on M-CBDA-PA and some of the ligand may remain uncomplexed. Based on the total number of moles of PA on the surface of the modifiedclay complex, this observed limiting capacity represents 78% of the available sites on M-CBDA-PA for complexation. The K values of modified clay M-CBDA-PA and Chelex are also very similar (0.0224 and 0.0267 mM for Chelex and modified clay, respectively). These values correspond to equilibrium cadmium concentrations of 2.5 mg/L (for Chelex) and 3.0 mg/L (for modified clay) at half of the maximum capacity, which is approximately 21 mg of Cd/g of adsorbent. The very small K values of both adsorbents indicate that these adsorbents have a very high affinity toward metals such as cadmium. As a result, a high adsorption capacity can be achieved at very low metal concentrations. It has been attributed to a complexation mechanism of the adsorbents where the ligands on the adsorbent form complexes with metal ions. This observation has been confirmed by the FTIR results (see next section). Unlike Chelex and modified clays, unmodified clay has a much lower adsorption capacity and affinity for cadmium under the operating conditions used in this study. It can be explained by the fact that the adsorption mechanism of the base clay is purely ion exchange or nonselective in nature. Even though the unmodified clay has some cation-exchange capacity, in high ionic strength systems (50 mM in this study), cadmium has to compete with other ions for the sorption sites on the clay, resulting in the reduced uptake of cadmium.19,20 FTIR Studies. The FTIR studies were carried out to study the mode of metal binding onto modified clays. FTIR spectra of the modified-clay complex (M-CBDAPA) were recorded before and after cadmium adsorption. Some fundamental frequencies are shown in Table 4. The spectra show the typical OH stretching frequency at 3620 cm-1 of the base clay and the C-H stretching of aromatic and aliphatic groups in CBDA at 2961 and 2867 cm-1, respectively. The 1708-cm-1 band is a stretching frequency due to the CdO of the remaining COOH group on the clay surface. The above frequencies are similar for the modified-clay complex before and after adsorption. The main change is observed in the CdO stretching of the carbonyl group. The characteristic vibrational frequency has been shifted from 1569 to 1548 cm-1 after cadmium adsorption. The shift to lower frequency is due to higher reduced mass, and this
Figure 2. Effect of pH on the adsorption of cadmium and copper by modified-clay complexes.
observed shift demonstrates the formation of a metal complex inside the modified-clay complex. Effect of pH for Reversible Adsorption. The adsorptions of cadmium and copper onto modified clay complexes (M-CBDA-PA and H-CBDA-DT, respectively) as a function of pH are shown in Figure 2. It can be seen that the adsorption of metal ions onto modified-clay complexes has a strong pH-dependent characteristic, and the uptake of metal ions by both modified-clay complexes increases with an increase in pH. For the Cd adsorption by the M-CBDA-PA complex, there is no adsorption observed below pH 3. Above pH 3 and below pH 5, there is little uptake of cadmium by the modified clay. Above pH 5, the adsorption of Cd increases sharply with an increase in pH and reaches its maximum approximately at pH 7.5. It should be noted that in this observed pH range (pH 3-7.5), cadmium is present essentially as a divalent cation, Cd2+, whereas Cd2+ and a monovalent, Cd(OH)+, are present in the pH range of 7.5-9.21 The influence of pH on Cd adsorption may probably be explained as follows: At low pH (i.e., pH 3), the carboxyl groups of PA on the surface of M-CBDA-PA are protonated, resulting in no uptake of Cd by the modified-clay complex as seen in Figure 2. The little uptake of Cd observed near pH 5, which is a mean pKa of PA, may be due to the heterogeneity of pKa of the sorbed PA on M-CBDA-PA which has been observed in the acidbase titration of M-CBDA-PA (data not shown). Above pH 5, the carboxyl groups are deprotonated so there are net negative charges on the surface of M-CBDA-PA, and Cd ions were adsorbed through a combination of favorable electrostatic and ligand-exchange interactions. For the adsorption of Cu onto the H-CBDA-DT complex, there is no uptake of Cu below pH 5; then, the uptake increases with an increase in pH and reaches its maximum between pH 7.5 and 8 (Figure 2). At low pH (i.e., pH < 5), the secondary and the primary amino groups of DT are completely protonated (a mean pKa of DT is 7.5), and positively charged Cu ions are excluded from the surface of the modified clay due to electrostatic repulsion. As the pH increases, more DT is deprotonated, and thus, increased metal ions can be adsorbed on the modified-clay surface. At pH above 7.5, partial or complete deprotonation of the amine groups enables heavy metal ions to form chelates in solution (complexation). Full adsorption isotherms shown in Figure 3 reveal a tremendous difference in adsorption capacities obtained at two pH extremes (low versus high pH). Figure
4300 Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998
Figure 4. Adsorption/desorption of cadmium by modified clay (M-CBDA-PA) and Chelex. 1 and 2 denote the first and second adsorption cycles, respectively.
Figure 3. Adsorption isotherms of cadmium on M-CBDA-PA (a) and copper on H-CBDA-DT (b). Lines shown are the Langmuir isotherms.
3a shows the adsorption isotherms of Cd on M-CBDAPA at pH 7 and 3. This figure shows that high metal adsorption capacity can be obtained at pH 7, while there is negligible metal adsorption at pH 3. Figure 3b shows the adsorption isotherms of Cu on H-CBDA-DT at pH 8 and 3. It can be seen that while there is no uptake of Cu observed at pH 3, Cu ions are adsorbed strongly on H-CBDA-DT at pH 8 and a maximum adsorption capacity of 0.64 mmol of Cu/g of H-CBDA-DT can be obtained from fitting the experimental data with the Langmuir isotherm. The amount of DT based on the total number of moles on the surface (0.93 mmol/g, Table 2) and the extent of deprotonation (∼90% at pH 8) is 0.84 mmol of DT/g of H-CBDA-DT. These results would suggest that one Cu ion is bound to one DT group on the clay complex, and the apparent adsorption capacity represents 76% of the total available sites on the modified-clay surface for complexation. The results shown in Figures 2 and 3 demonstrate that the adsorption of metal ions on the modified-clay complexes is reversible. The shift of pH from high to low values can be used to transform the adsorbents from a state of “high” affinity for metal ion (adsorption) to one of “low” affinity (desorption). According to the pHdependent sorption characteristics, the modified clay complexes have been shown to be very good adsorbents in the adsorption/desorption of heavy metals using the pH of the solution as a switch. Desorption and Regeneration. Application of these surfactant-modified clays for the removal and recovery of heavy metals from waste streams may require that the adsorbent be regenerated efficiently so that metal can be recovered and the adsorbent can be reused. In these studies, attempts were made to desorb Cd from the spent modified-clay complex (M-CBDAPA) using low pH. Chelex 100 was also used as a
comparison. The adsorbents were first loaded with cadmium, and then, the desorption was carried out by suspending loaded adsorbent in an equal volume of a pH 3 solution. The total desorbed amount was calculated and compared to the initial sorbed amount. The results of the adsorption and desorption of cadmium on the modified-clay complex (M-CBDA-PA) and Chelex are shown in Figure 4. It can be seen that at pH 3, bound cadmium is quantitatively desorbed from the modified-clay complex to the suspending medium and nearly all sorbed Cd (>99%) on the modified clay (M-CBDA-PA) can be recovered from the adsorbent. In contrast, only 70% of the total sorbed Cd is released from Chelex resin at this pH (final pH ) 3). It is suggested that much lower pH (less than pH 2) is required for an efficient regeneration of Chelex resin.22,23 The results clearly show that quantitative recovery of cadmium from the modifiedclay complex and regeneration of the adsorbent can be done efficiently even at pH 3. Since all sorbed Cd can be recovered from the modified clay, the adsorbent has been further evaluated for a possible reuse in metal adsorption. The second adsorption cycle (column 2 in Figure 4) reveals that the modified-clay complex can be reused with a very small loss in efficiency. Approximately 97% of the initial adsorption capacity of the modified-clay complex is obtained in the second cycle. Conclusion The present study clearly establishes that the surfactant-modified clay complexes are very effective adsorbents for heavy metal removal from aqueous solution. Their adsorption capacities and affinity constants for metals such as Cd and Cu are comparable to those of commercial available chelating resin. FTIR studies have shown that the metal binding of the modified clay is indeed through a complexation mechanism. The results also indicate that metal ion adsorption appears to be pH dependent. The modified-clay complexes show their abilities in adsorption of metal ions at neutral or mildly alkaline conditions and desorption at lower pH. Therefore, the modified-clay complexes can be used for the removal and recovery of heavy metals from waste streams using a shift in pH of the solution as a switch. Desorption studies help elucidate that the adsorbent can be regenerated efficiently, which will enable them to be used for multiple cycles. This will greatly enhance the cost-effectiveness of these modified-clay adsorbents in the field applications. A possible use of these modified-
Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998 4301
clay complexes in immobilized bead or capsule forms for heavy metals removal is currently under investigation. Based on these findings, these surfactant-modified clay complexes have been proposed to be used to treat dilute metal-containing wastes. Literature Cited (1) Keith, L. H.; Telliard, W. A. Priority Pollutants. Environ. Sci. Technol. 1979, 13, 416. (2) Reed, B. E.; Arunachalam, S.; Thomas, B. Removal of lead and cadmium from aqueous waste streams using granular activated carbon (GAC) columns. Environ. Prog. 1994, 13, 60. (3) Neal, B. G.; Lawrence, E. B.; Wendt, J. L. Alkali Metal Partitioning in Ash from Pulverized Coal Combustion. Combust. Sci. Technol. 1990, 74, 211. (4) Forstner, U.; Wittman, G. T. W. Metal Pollution in the Aquatic Environment; Springer-Verleg: Berlin, 1979. (5) Benjamin, M. M.; Leckie, J. O. Effects of Complexation by Cl, SO4, and S2O3 on Adsorption Behavior of Cd on Oxide Surfaces. Environ. Sci. Technol. 1982, 16(3), 162. (6) Greene, B.; Darnall, D. W. Microbial Oxygenic Photoautotrophs (Cyanobacteria and Algae) for Metal-Ion Binding. In Microbial Mineral Recovery; Ehrlich, H. L., Brierley, C. L., Eds.; McGraw-Hill: New York, 1990. (7) Choudary, B. M.; Ravi Kumar, K.; Jamil, Z.; Thyagarajan, G. A Novel Anchored Palladium(II) Phosphinated Montmorillonite: the First Example of the Interlamellars of Smectite Clay. J. Chem. Soc., Chem. Commun. 1985, 13, 931. (8) Ravi Kumar, K.; Choudary, B. M.; Jamil, Z.; Thyagarajan, G. Syntheses of New Interlamellar Functionalized Montmorillonite Palladium(II) Catalysts: the First Example of a Truly ‘Heterogeneous’ Anchored Catalyst. J. Chem. Soc., Chem. Commun. 1986, 2, 130. (9) Mercier, L.; Detellier, C. Preparation, Characterization, and Applications as Heavy Metals Sorbents of Covalently Grafted Thiol Functionalities on the Interlamellar Surface of Montmorillonite. Environ. Sci. Technol. 1995, 29, 1318. (10) Mercier, L.; Pinnavaia, T. J. Access in Mesoporous Materials: Advantages of a Uniform Pore Structure in the Design of a Heavy Metal Ion Adsorbent for Environmental Remediation. Adv. Mater. 1997, 9, 500. (11) Feng, X.; Fryxell, G. E.; Wang, L.-Q.; Kim, A. Y.; Liu, J.; Kemmer, K. M. Functionalized Monolayer on Ordered Mesoporous Supports. Science 1997, 276, 923.
(12) Boyd, S. A.; Lee, J.-F.; Mortland, M. M. Attenuating Organic Contaminant Mobility by Soil Modification. Nature 1988, 333, 345. (13) Srinivasan, K. R.; Fogler, H. S. Use of Inorgano-OrganoClays in Industrial Wastewater Treatment. Clays Clay Miner. 1990, 38 (3), 277. (14) Theng, B. K. G. The Chemistry of Clay-Organic Reactions; John Wiley: New York, 1974. (15) Xu, S.; Boyd, S. A. Cation Exchange Chemistry of Hexadecyltrimethylammonium in a Subsoil Containing Vermiculite. Soil Sci. Soc. Am. J. 1994, 58, 1382. (16) Boyd, S. A.; Shaobai, J.-F. L.; Mortland, M. M. Pentachlorophenol Sorption by Organo-Clays. Clays Clay Miner. 1988, 36, 125. (17) Kukkadapu, R. K.; Boyd, S. A. Tetramethylphosphonium and Tetramethylammonium-Smectites as Adsorbents of Aromatic and Chlorinated Hydrocarbons: Effect of Water on Adsorption Efficiency. Clays Clay Miner. 1995, 43, 318. (18) Wolf, T. A.; Demirel, T.; Bauman, R. E. Adsorption of Organic Pollutants on Montmorillonite Treated with Amines. J. Water Pollution Control Fed. 1986, 58, 68. (19) Hirsch, D.; Nir, S.; Banin, A. Prediction of Cadmium Complexation in Solution and Adsorption on Montmorillonite. Soil Sci. Soc. Am. J. 1989, 53, 716. (20) Holm, T. R.; Zhu, X.-F. Sorption by Kaolinite of Cd2+, Pb2+ and Cu2+ from Landfill Leachate-Contaminated Groundwater. J. Contam. Hydrol. 1994, 16, 271. (21) Weber, W. J., Jr.; Possett, H. S. Equilibrium Models and Precipitation Reactions for Cadmium(II). In Aqueous Environment Chemistry of Metals; Rubin, A. J., Ed.; Ann Arbor Science: Ann Arbor, MI, 1974. (22) Chelex 100 Instruction Manual. Bio-Rad. Hercules, CA. (23) Pesavento, M.; Biesuz, R.; Gallorini, M.; Profumo, A. Sorption Mechanism of Trace Amounts of Divalent Metal Ions on a Chelating Resin Containing Iminodiacetate Groups. Anal. Chem. 1993, 65, 2522.
Received for review February 3, 1998 Revised manuscript received July 14, 1998 Accepted August 18, 1998 IE980057I
