Environ. Sci. Technol. 2000, 34, 4593-4599
Heavy Metal Adsorption by Functionalized Clays RAFAEL CELIS, M . C A R M E N H E R M O S IÄ N , * A N D JUAN CORNEJO Instituto de Recursos Naturales y Agrobiologı´a de Sevilla, CSIC, P.O. Box 1052, 41080 Sevilla, Spain
Organic ligands containing the thiol (-SH) metal-chelating functionality were either grafted to the external surface silanol groups of sepiolite or introduced in the interlayers of montmorillonite, and the resulting functionalized clays were characterized and assayed as adsorbents for Hg(II), Pb(II), and Zn(II) ions from solution. Sepiolite was functionalized by covalently grafting 3-mercaptopro-pyltrimethoxysilane (MPS) to the surface tSi-OH groups of the clay, whereas montmorillonite was functionalized by replacement of the interlayer inorganic cation (Na+) by 2-mercaptoethylammonium (MEA) cations. These clayorganic ligand systems were selected to minimize the congestion of the internal porosity of the clays, which has recently been shown to be the main obstacle to heavy metal adsorption by functionalized clays. Infrared spectroscopy and elemental analyses demonstrated the presence of the organic ligands in the modified clays. X-ray diffraction analysis indicated the organic cations (MEA) occupied the interlayers of montmorillonite. N2 specific surface area measurements suggested that much of the surface area of montmorillonite and sepiolite remained accessible upon functionalization and that the organic ligand kept the montmorillonite interlayers open. The functionalized clays adsorbed most of the Hg(II) ions present in solution up to saturation and were also good adsorbents of Pb(II) at low metal ion concentrations (i.e., 97%) products. Clay Functionalization. 3-Mercaptopropylsilyl-sepiolite (MPS-SEP). Sepiolite was functionalized through the covalent grafting of 3-mercaptopropyltrimethoxysilane (MPS) by a procedure similar to that followed by Mercier and Detellier (9) for the synthesis of MPS-montmorillonite. A total of 1 g of H-saturated Vallecas sepiolite was dried at 120 °C under vacuum for 6 h and then was refluxed for 24 h in 50 mL of dry toluene containing 1 g of MPS. The mixture was filtered and washed with 50 mL of toluene followed by 50 mL of ethanol. Any residual organosilane was removed by Soxhlet extraction over ethanol for 24 h. The resulting solid (MPSSEP) was recovered by filtration, washed with 100 mL of distilled water, air-dried, and stored at room temperature until used. A MPS-free sepiolite blank sample (BLANK-SEP) was also prepared following the same steps as described for the synthesis of MPS-SEP, but without MPS addition. 2-Mercaptoethylammonium-montmorillonite (MEAMONT). Montmorillonite was functionalized by replacement of the interlayer inorganic cation (Na+) by 2-mercaptoethylammonium (MEA) cations. A total of 3.3 g of SWy-2 montmorillonite was treated with 1.2, 2.4, or 3.8 mmol of 2-mercaptoethylammonium chloride dissolved in 50 mL of distilled water. These amounts of alkylammonium corresponded to 50, 100, and 150% of the CEC of SWy-2. The suspensions were shaken for 24 h and centrifuged. The resulting solids (MEA-MONT50, MEA-MONT100, and MEA-MONT150) were washed four times with 25 mL of distilled water, freeze-dried, and stored at room temperature until used. A blank montmorillonite sample (BLANK-MONT) was prepared following the same procedure, but without MEA addition. Adsorbent Characterization. Functionalized clays and blanks were characterized by elemental analysis, N2-specific surface area (SSA) measurements, X-ray diffraction (XRD), and Fourier transform infrared (FT-IR) spectroscopy. Elemental analyses were performed using a Perkin-Elmer 240C elemental analyzer (Perkin-Elmer Corp., Norwalk, CT). N2specific surface areas were obtained by N2 adsorption at 77 K using a Carlo Erba Sorptomatic 1900 (Fisons Instruments, Milan). Samples were outgassed at 50 °C and equilibrated under vacuum for 24 h before measuring the N2 adsorption 4594
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isotherm. Specific surface areas (SSA) were calculated by applying the BET method (17) between relative pressures of 0.02 and 0.20. X-ray diffractograms were obtained on oriented specimens with a Siemens D-5000 diffractometer (Siemens, Stuttgart) using Cu KR radiation. Fourier transform infrared (FT-IR) spectra of the functionalized and blank clays were recorded on KBr disks using a Nicolet 5 PC spectrometer. The disks were dried at 100 °C for 24 h prior to recording the FT-IR spectrum. Heavy Metal Adsorption-Desorption Experiments. Duplicate 10 mg adsorbent samples were equilibrated for 24 h with 10 mL of aqueous solutions of Hg(NO3)2, Pb(NO3)2, or ZnCl2 (Sigma, ACS reagent) with metal concentrations ranging from 0.05 to 1 mmol L-1. The pH of the Pb2+ and Zn2+ initial solutions ranged from 5 to 5.5, well below the precipitation levels for these metals (pH > 6.5 for Pb2+; pH > 7.5 for Zn2+). In the case of Hg(NO3)2, nitric acid was added to the initial solutions to reach pH ) 3, thus preventing oxide formation. Preliminary kinetic experiments were carried out on MEAMONT150 and indicated that 24 h was sufficient to reach heavy metal adsorption equilibrium. After equilibration, the suspensions were centrifuged, and 5 mL of supernatant was removed for analysis. The concentration of heavy metal in the supernatant was determined by atomic absorption spectroscopy (Perkin-Elmer 1100B atomic absorption spectrometer). The amount of adsorbed metal was calculated by difference between the initial and final solution concentrations. Metal solutions without adsorbent were also shaken for 24 h and served as controls. Heavy metal adsorption isotherms were fit to the Langmuir equation
Ce C e 1 ) + Cs Cm CmL
(1)
where Cs is the amount of heavy metal adsorbed at the equilibrium concentration Ce, Cm is the maximum adsorption capacity of the adsorbent, and L is the Langmuir constant, which is related to the free energy of adsorption (18). Cm and L can be calculated from the linear plot of Ce/Cs vs Ce.
Results and Discussion Adsorbent Characterization. The C, N, and S analysis (Table 1) of MEA-MONT samples revealed S/N and S/C ratios (S/N ) 1.8-2.1; S/C ) 1.3-1.4) in good agreement with those of the MEA cation (S/N ) 2.3; S/C ) 1.3). The S/C ratio of MPS-SEP (S/C ) 0.61), slightly higher than that of 3-mercaptopropyldimethoxysilyl radical (S/C ) 0.53), may suggest partial hydrolysis of the two residual methoxy groups of the grafted MPS, most likely due to the final water washing of the sample (9). The organic ligand content of the functionalized clays was calculated from their S content (Table 1). Assuming that the surface silanol density of Vallecas sepiolite is 2.2 Si-OH/ 100 Å2 (16), one can calculate the fraction of Si-OH surface groups of H+-SEP (SSA ) 350 m2 g-1) condensed with MPS as 65%. In the case of MEA-MONT samples, calculations based on the CEC of SWy-2 indicated that the percentage of the CEC of montmorillonite occupied by MEA cations was 28% for MEA-MONT50, 50% for MEA-MONT100, and 77% for MEA-MONT150 (Table 1). The infrared spectra of MPS-SEP and MEA-MONT samples revealed new peaks compared to the blank clays, providing further evidence for the presence of the organic moieties in the functionalized clays (Figure 1). The aliphatic C-H vibration band of MPS at 2930 cm-1 was clearly identified in the FT-IR spectrum of MPS-SEP (Figure 1A). After the grafting reaction, the expected decrease in the intensity of the O-H stretching band of the surface silanol groups of sepiolite (3700 cm-1) was not observed. Most likely, the formation of new
TABLE 1. Elemental Analysis of the Functionalized Clays and Blank Samples sample
C (%)
N (%)
S (%)
BLANK-SEP MPS-SEP BLANK-MONT MEA-MONT50 MEA-MONT100 MEA-MONT150
0.62 5.00 0.24 0.80 1.26 1.86
0a 0 0 0.42 0.76 1.02
0 2.67 0 0.78 1.40 2.15
S/C
S/N
0.61
-
1.4 1.4 1.3
1.9 1.8 2.1
OLCb (mmol kg-1)
OLStc (%)
0 834 0 240 440 670
0 65 0 28 50 77
a Not detected. b Organic ligand content: calculated from the S content. c Organic ligand saturation: percentage of the silanol groups of sepiolite (assuming 2.2 Si-OH per 100 Å2) condensed with MPS ligand or percentage of the CEC of montmorillonite (CEC ) 870 mmolc kg-1) occupied by MEA cations.
FIGURE 1. Infrared spectra of functionalized and blank clays heated at 100 °C: (A) sepiolite samples; (B) montmorillonite samples. The infrared spectra of the organic ligands, MPS and MEA-chloride, are also given as a reference.
FIGURE 2. XRD basal diffractions (Å) of (a) BLANK-MONT, (b) MEAMONT50, (c) MEA-MONT100, and (d) MEA-MONT150 at room temperature and heated at 100 °C.
Si-OH groups from the hydrolysis of the residual methoxy groups of the grafted MPS compensated the loss of Si-OH groups of sepiolite during the condensation the reaction. The FT-IR spectra of MEA-MONT samples (Figure 1B) showed a group of bands between 2890 and 3244 cm-1, which corresponded to the N-H and C-H stretching vibrations of the MEA cation (19). The C-H deformation vibration of MEA was also identified at 1490 cm-1 (Figure 1B). Due to the weakness of the bands in the pure compounds, the C-S and S-H stretching bands of the thiol functionality (at about 700 and 2500 cm-1, respectively) were not identified in the functionalized clays. X-ray diffractograms of the air-dried and 100 °C-heated MEA-MONT samples are shown in Figure 2. The decrease in the basal spacing of montmorillonite with increasing amounts of MEA cation observed at room temperature (Figure 2A) can be attributed to gradual replacement of the inorganic interlayer cation and its hydration water by less hydrated MEA cations. This decrease in water content was corroborated by the weight losses measured upon degassing of the samples in the N2-adsorption experiments, which will be discussed below (Table 2). The basal diffractions obtained after drying at 100 °C showed a decrease in the intensity of the 10 Åbasal diffraction, typical of pure montmorillonite, and the appearance of a new diffraction line at a lower angle upon functionalization (Figure 2B). The basal spacing of 12.0 Å obtained for MEA-MONT150 at 100 °C corresponds to a clearance space of about 2.3 Å between the clay sheets. This
TABLE 2. Water Loss and N2-Specific Surface Area of Functionalized Clays and Blank Samples Evacuated at 50 °C sample
weight loss (%)
SSA (m2 g-1)
H+-SEP BLANK-SEP MPS-SEP BLANK-MONT MEA-MONT50 MEA-MONT100 MEA-MONT150
19.6 10.6 10.5 12.3 12.7 10.1 7.6
350 272 107 20 19 61 153
separation concords with the presence of a monolayer of carbon chains in the interlayer region, thus providing strong evidence for the presence of the MEA cations in the interlayers of montmorillonite (9). The residual basal diffraction at 10.0 Å that remains in the diffractograms of all MEA-MONT samples is indicative of the presence of some nonexchanged interlayers occupied exclusively by inorganic cations (Figure 2B). It is interesting to note that the intensity of this 10.0 Å diffraction decreased from MEA-MONT50 to MEA-MONT150, in agreement with the increasing organic ligand saturation (OLSt) from MEA-MONT50 (OLSt ) 28%) to MEA-MONT150 (OLSt ) 77%) (Table 1). N2-specific surface areas of the functionalized clays and blanks are given in Table 2. While the thiol functionalization caused a significant decrease in the SSA of sepiolite, the SSA of MEA-MONT samples with greater MEA contents were VOL. 34, NO. 21, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Hg(II) adsorption isotherms (pH 3) on functionalized clays and blanks: (A) sepiolite samples; (B) montmorillonite samples. Symbols are experimental data points, whereas lines are the Langmuir-fit adsorption curves.
FIGURE 3. Kinetics of Hg(II), Pb(II), and Zn(II) adsorption on MEAMONT150. Experiments were carried out at an initial heavy metal concentration of about 0.75 mmol L-1. Bars correspond to standard errors of duplicate samples. substantially greater than that of the montmorillonite blank sample (Table 2). The decrease in SSA of sepiolite upon grafting with MPS is due to the bulk size of the organic ligand, which blocks the entrance to some structural channels, as reported earlier for methylated sepiolite (16). In contrast, the MEA cations kept the montmorillonite interlayers open upon dehydration, resulting in a great increase in the amount of surface accessible for N2 adsorption. This behavior has previously been observed in low-charge montmorillonites exchanged with small alkylammonium cations, such as trimethylphenylammonium (20). The fact that the increase in SSA was not observed for MEA-MONT50 (Table 2) supports that most of the clay interlayers in this sample remained occupied exclusively by inorganic cations (72%, Table 1). Despite the reduction in SSA of sepiolite after functionalization, the considerable SSA retained by MPS-SEP (107 m2 g-1) and the increase in SSA of montmorillonite after MEA functionalization suggest that the thiol functionalities are relatively accessible for metal ion binding. The adsorption results reported below further supported this hypothesis. Heavy Metal Adsorption Studies. Kinetics. Mercury(II), lead(II), and zinc(II) adsorption kinetics were obtained on MEA-MONT150 and indicated that heavy metal adsorption equilibrium was reached within 24 h of shaking (Figure 3). 4596
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The amounts of metal ion adsorbed after 72 h were not significantly different from the amounts adsorbed after 24 h. Therefore, 24 h was considered sufficient to reach heavy metal adsorption equilibrium. Mercury Adsorption. Mercury(II) adsorption isotherms on functionalized and blank clays showed a great increase in Hg2+ adsorption on sepiolite and montmorillonite upon functionalization (Figure 4). Mercury(II) adsorption on the blank clay samples is attributed to cation exchange and/ or formation of inner-sphere complexes through tSi-Oand tAl-O- groups at the clay particle edges (7, 8). The cationic exchange mechanism should predominate for montmorillonite, whereas the formation of inner-sphere complexes should predominate in sepiolite due to its high surface silanol and aluminol content. The increase in Hg2+ adsorption on both clays upon functionalization is attributed to the metal-chelating SH functionality, which provides the clay surface with a remarkable affinity for Hg2+ (9-12). Mercury(II) uptake increases from MEA-MONT50 to MEA-MONT150 due to the increase in the number of montmorillonite layers exchanged with the thiol ligand, which in turn increases the separation between the clay layers (Figure 2), making the internal spaces more accessible for Hg(II) binding. Mercury(II) adsorption isotherms had a high affinityLangmuir character, especially in the case of the functionalized clays (Figure 4). The Hg(II) adsorption data were, therefore, very well described by the Langmuir equation (eq 1) with regression coefficients r > 0.98 (Table 3). Data in Table 3 show that both the Langmuir maximum adsorption
TABLE 3. Langmuir Coefficients, Cm and L, for Hg(II) and Pb(II) Adsorption on Functionalized and Blank Clays Hg
Pb
adsorbent
Cm (mmol kg-1)
L (L mmol-1)
r
Cm (mmol kg-1)
L (L mmol-1)
r
BLANK-SEP MPS-SEP BLANK-MONT MEA-MONT50 MEA-MONT100 MEA-MONT150
170 ( 590 ( 15 250 ( 30 450 ( 15 550 ( 20 660 ( 15
11 ( 10 38 ( 8 2(1 38 ( 19 52 ( 34 72 ( 24
0.988 0.999 0.987 0.999 0.998 0.999
220 ( 30 70 ( 15 280 ( 15 290 ( 10 220 ( 15 140 ( 10
5(1 8(2 19 ( 2 18 ( 1 22 ( 5 34 ( 9
0.977 0.934 0.996 0.999 0.996 0.996
a
25a
Value ( standard error for the calculated coefficient.
capacity (Cm) and the affinity (L) of sepiolite and montmorillonite for Hg2+ are greatly enhanced upon functionalization, although the uncertainty on the calculation of the affinity factor (L) is greater than that of the capacity factor (Cm). The adsorption capacity (Cm) of sepiolite increased from 170 to 590 mmol kg-1, whereas the adsorption capacity of montmorillonite increased from 250 to 450-660 mmol kg-1, depending on the degree of saturation in MEA cation (Table 3). The increase in affinity (L) as a result of functionalization is also noticeable and reflects the ability of the functionalized clays to adsorb most of the Hg2+ ions present in solution up to the saturation of the adsorbent (Figure 4). The maximum adsorption capacity of MPS-SEP (590 mmol Hg kg-1) and MEA-MONT150 (660 mmol Hg kg-1) represent, respectively, 70% and 99% of the total SH groups present in the samples. The maximum adsorption capacities obtained for MEA-MONT50 and MEA-MONT100 (Table 3) are even higher than the amount of SH groups present in the adsorbents (Table 1), since the low degree of saturation in organic ligand of these samples leaves many of the original exchange sites of the clay available for extra Hg(II) binding. The less than stoichiometric binding of thiol groups to mercury ions in MPS-SEP, in contrast to MEA-MONT150, may have resulted from some pore congestion due to the higher surface density in organic ligand of MPS-SEP (1 MPS per 70 Å2, assuming a SSA for sepiolite of 350 m2 g-1) compared to MEA-MONT150 (1 MEA per 170 Å2, assuming an internal + external SSA for montmorillonite of 700 m2 g-1). In addition, some fraction of the bound Hg2+ may have also been complexed by two thiol groups in MPS-SEP due to the spatial proximity of the mercaptopropyl chains within the framework. Very recently, Mercier and Pinnavaia (11) prepared a MPS-fluorohectorite with 1 MPS per 83 Å2 and found that 67% of the thiol groups were accessible for Hg(II) ions. These values are remarkably similar to those reported here for MPSSEP. Nevertheless, the performance of both MPS-SEP and MEA-MONT represent a great improvement for Hg(II) binding over previously functionalized clay minerals. Thus, in Hg adsorption experiments performed by Mercier and Detellier (9) in conditions very similar to ours (5 mg:100 mL solid:solution ratio, pH ) 3), the intercalation of MPS in the galleries of montmorillonite produced an adsorbent with as much as 3200 mmol SH kg-1 (1 MPS per 36 Å2) and a mercury uptake of 330 mmol kg-1. Although no comparison with the untreated clay was done, this amount corresponded to the utilization of only 10% of the total number of ligand sites present in the adsorbent. These results strongly suggest that pore congestion is a major factor limiting the accessibility of metal ions to functionalized clays and, therefore, that clay functionalization could be optimized by minimizing the gallery or pore volume taken up by the organic ligand. To investigate the possibility of competition between Hg(II) and nonspecifically adsorbed cations, such as Na+ and Ca2+, for the thiol moiety, the amounts of Hg(II) adsorbed by MPS-SEP in the presence of NaNO3 and Ca(NO3)2 at concentrations ranging from 0.001 to 0.1 M were determined. The results are given in Table 4 and show very little difference
TABLE 4. Amounts of Hg(II) Adsorbed by MPS-SEP at Different Concentrations of Background Electrolyte (Initial Hg(II) Concentration ) 1 mmol L-1) electrolyte concentration (M) 0 0.001 0.01 0.1 a
Hg(II) adsorbed (mmol kg-1) NaNO3 Ca(NO3)2 500 ( 19a 467 ( 12 508 ( 7 526 ( 4
500 ( 19 476 ( 13 500 ( 16 519 ( 17
Mean ( standard error of duplicate samples.
between the adsorption of Hg in the presence of Na+ or Ca2+ and the adsorption measured in the absence of a background electrolyte. In agreement with previous work where the heavy metal selectivity was not affected by the presence of electrolytes normally associated with groundwaters and waste streams (12, 21), our results seem to confirm the high affinity of Hg for the thiol moiety compared to other cations, such as Na+ or Ca2+, which could be relevant in an environmental context, like saline soils or waters. Lead Adsorption. Lead was not removed as efficiently as Hg(II) by the functionalized clays (Figure 5, Table 3). Except MEA-MONT50, all MPS-SEP and MEA-MONT samples displayed significantly lower adsorption capacities (Cm) than the pure clays (Table 3). The lower adsorption capacity of the functionalized clays for Pb2+ compared to Hg2+ occurred despite the pH of the Pb2+ equilibrated suspensions (pH 5.56.0) being significantly higher than that of the Hg2+ equilibrated suspensions (pH 3). This higher pH should have favored adsorption by reducing the Pb2+ solubility as well as its competition with H+ ions for the thiol functionality (9). In contrast to Hg(II), the substitution of Si-OH groups of sepiolite and the inorganic exchangeable cation of montmorillonite by the organic ligand resulted in a reduction of the adsorption capacity of the clays for Pb(II). It is interesting to note, however, that functionalization decreased the Pb(II) uptake only at high metal ion concentrations, but not at the lowest Pb2+ concentrations tested (i.e.,
