Environ. Sci. Techno/. 1995, 29, 1318-1323
Introduction Heavy metals generate a great need to develop effective types of sorbents. Some of the most common methods used for bulk metal removal from mine effluent are hydroxide precipitation [Mn+(aq) nOH-(aq) M(OH),(s)] and coprecipitation (1). Clays and related minerals have been explored as pollution sorbents not only for organic species (2-9) but also for heavy metals (10-14). However, their industrial and environmental applications as such sorbents are extremely limited. Up until now, the only sorbent typically used for low concentration heavy metal depollution is activated charcoal, which absorbs relatively low amounts of metal and is unselective to the type of metal. Clays are abundant and inexpensive layered aluminosilicate minerals that have adsorbent properties due to their large surface areas and negative layer charge (15,16). In general, chemical species can interact with clays either by ion exchange (17-23) orphysisorption (24-29), both being reversible processes. Recent studies report the use of leached mica as a very effective ion sieve for cesium and strontium ions (30, 31), which become irreversibly bound to the structure. However, the most practical type of heavy metal adsorbent would need to have very strong affinity toward the target metals, essentially irreversibly binding them under natural ambient conditions,as well as the ability to remove the metal from the structure so that the adsorbent can be regenerated for further cleanup. Covalent grafting of functional organic units on clay mineral templates presents advantages as opposed to binding them via ion exchange or physisorption: such compounds would benefit from a much higher chemical stabilityas the guest molecules would be irreversiblybound to the structure. Such organo-claymaterials are potentially of great usefulness. The interlayer grafting of the minerals kaolinite and magadiite has been established (32-36). There have also been reports about the covalent grafting of moieties onto the interlamellar surface of smectite clays (37-39). Hydroxyl groups are known to exist on the edges of clay particles, but it is conjectured that a number of these may also be present on the interlamellar surface of smectite clay minerals due to structural defects or irregularities. This would be particularly expected when the clays are exchanged with H+ as interlayer cations. These sites are susceptible to grafting via hydrolysis reactions, as represented in Scheme 1. As of today, practical uses of such promising materials however have been limited to catalytic applications (40-46). In this study, an organic species bearing a metalchelating functionality (the thiol SH group) was grafted on the internal surfaces of montmorillonite by silane condensation. The material thus obtained is the first example of a grafted clay with practical environmental remediation applications, showing both the binding affinity for heavy metals and the capability of being regenerated.
+
LOUIS M E R C I E R A N D CHRISTIAN DETELLIER* Ottawa-Carleton Chemistry Institute, Department of Chemishy, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5
A novel type of heavy metal adsorbent was prepared by the covalent grafting of a chelating sulfhydryl functionality [(3-mercaptopropyl)trimethoxysilane) in the interlamellar region of the smectite clay mineral montmorillonite. The new material (designated as thiomont) was characterized by X-ray diffraction, infrared spectroscopy, thermogravimetric analysis, X-ray fluorescence, and I3C solid-state NMR spectroscopy. Its approximate chemical formula was deduced to be Si7.sA13.3Fe~.3Mg0.40 1d OH)3[OSi(OH)z( CHd3SHI5. Th iomont was found to be an effective adsorbent for Pb(11) and Hg(il) (70 and 65 mg of metal/g of adsorbent, respectively) but was less effective toward Cd(ll) and Zn(ll). Acid leaching was shown to be an effective method by which thiomont could be regenerated.
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Methods Na-MontmorillonitePurification. Sodium-montmorillonite (SWy-1,from the Source Clay Repository, University * E-mail address:
[email protected]; Fax: 613-564-6793.
1318 ENVIRONMENTAL SCIENCE & TECHNOLOGY I VOL. 29, NO. 5,1995
0013-936)(/95/0929-1318$09.00/0
@ 1995 American
Chemical Society
SCHEME 1
Grafting Process in Interlamellar Region of Montmorillonite Leading to Formation of Heavy Metal Adsorbent Thiomont
No- montmorillonite
J HCI H- rnont morillonit e
I OSi(OH)2(CH2)3CI
Chloromont
\1
NOSH
OSi(0H)2( CH2) 3S H
Thiomon t of Missouri) was purified by sedimentation, homoionization, dialysis, and freeze-drying according to published procedures (19, 23). Particle size analysis demonstrated the effectivenessof the sedimentation process in separating the finer clay fraction: 90% of the clay particles were smaller than 0.4 p n after the treatment, as opposed to only about 65% in the case of the crude smectite (Figure 1). Preparation of H-Montmorillonite. Purified Na+montmorillonite (about 10 g) was dispersed in water (500 mL), and concentrated HC1 was added to make up a concentration of 0.15 M. The H-montmorillonite thus obtained was then washed free of excess ions by dialysis and isolated by freeze-drying. Preparation of Chloromont. A total of 2 g of Hmontmorillonite was dried at 80 O C in vacuo over P205 and
then refluxed over N2 in 400 mL of toluene (freshly distilled over calcium hydride) with 1 g of (3-chloropropy1)trimethoxysilane (Aldrich, 97%) for 65 h. The resulting material (hereafter called chloromont) was filtered and treated with Soxhlet extraction for 17 h using benzene over calcium hydride as wash solvent. The product was then washed with ether and dried in vacuo at 70 "C. Preparation of Womont. A total of 0.95 g of chloromont was refluxed in 75 mL of ethanol and 4 g of NaSHOxH20 (Aldrich) for 20 h. The mixture was filtered and washed with water followed by methanol. The resulting solid was then further washed by Soxhlet extraction using a 3:l ethanol/water mixture as wash liquid. A flaky white powder (hereafter called thiomont) was obtained, which never loses its consistency even if it is wetted and then dried (ungrafted clays form films under this circumstance that are difficult to crush into powders). Characterizationof Chloromont and Thiomont. Both grafted clays were characterized by X-raydiffraction (Figure 2), infrared spectroscopy, thermogravimetric analysis (Figure 3), elemental analysis,X-rayfluorescence,and 13Csolidstate NMR spectroscopy. The samples were heated at 130 "C for 24 h prior to every analysis in order to expel any solvent remaining in the interlamellar region of the clay. Slides for XRD were prepared by dispersing the sample in acetone, drying in air, followed by further drying at 130 "C. The scans were done on a Philips PW3710 powder diffractometer using Cu Kal radiation (A = 1.54051A). The XRF results were performed on a Philips PW2400 fluorimeter, with the scans referenced to a mixture of SiO2, AZO3, NaC1, elemental S, and leucine in order to minimize matrix effects observed in the case of C1 and S with respect to Si. IR spectra were recorded on a Bomem Michelson MB 100 spectrometer using the KBr pressed pellet technique, acquiring 10 scans with a resolution of 4 cm-'. TGA measurements were taken with a Polymer Labs STA 1500H instrument using alumina reference and sample pans under nitrogen flow (80 mL/min) and a heating rate of 20 "C/min. The 13C CP-MAS spectra were recorded on a Bruker CXP180 spectrometer. Elemental analyses were performed in duplicate by Guelph Chemical Laboratories Ltd., Guelph, Ontario. Pyridine adsorption studies were performed on Hmontmorillonite and thiomont to measure the acidic sites on the materials. Thus, ethanolic pyridine solutions were stirred with precisely weighed samples of H-montmorillonite and thiomont for 24 h. Pyndine concentrations before and after sample additions were measured by UVvisible spectroscopy using a Varian Cary 1E UV-visible spectrophotometer (followingthe 256-nm adsorption band of pyridine). A combustion experiment was conducted to get some information about the thermal treatment of chloromont: a precisely weighed sample of chloromont was heated in a ceramic crucible for 16hat 1000 "Cover air. The resulting calcined product was reweighed and analyzed by X-ray fluorescence. Heavy Metal Adsorption Studies. Aqueous 0-20 ppm solutions of metal nitrate salts [Pb(II),Hg(II), Cd(II), and Zn(II)] (Aldrich;ACS reagent) were prepared. In the case of Hg(NO&, nitric acid was added to the solution in order to prevent oxide formation (mother liquor pH about 3). Aliquots of these (usually about 100 mL) were treated with accurately weighed amounts of thiomont (usually about 5 mg). The metal concentrations of the solutions before and VOL. 29, NO. 5,1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
1
1319
110
100 90
-
n
x
80
70
L
60
Q)
.-c
50 0
40
U
30 20 10
0
1 o2
1 oo
10'
lo-'
Equivalent Spherical Diameter (,urn) FIGURE 1. Particle size analysis of crude montmorillonite (dashed line) and sedimented montmorillonite (solid line). I
I
6
8
I
4000 n
v,
.c,
3000
0
0 u
2000
2
4
10
12
20 FIGURE 2 X-ray difhaction pattern of chloromont (boxes), with 10.2-A phase cutve fit (dashed line) and subtraction of latter from former (bottom curve).
after thiomont treatment were measured by ICP (Geochemistry Analysis Laboratory, Department of Geology, University of Ottawa). Regeneration of Thiomont. Thiomont (500 mg) was stirredinaPb(NO& solution (about lgin 100mLofdistilled water) for 24 h to produce a fully lead-loaded thiomont. Two 100-mgsamples of the loaded adsorbent were independently stirred in 25 mL of 0.94 M HCl and 0.098 M HCl for 18 h at room temperature. After water and acetone washings, the adsorbent was collected by centrifugation and dried at 120 "C. XRF analyses were performed on the 1320 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 5,1995
samples both prior to and after acid treatment in order to determine the degree of lead recuperation. The leached material (by 0.94 M HCl) was then reloaded with Pb(I1)and once again characterized for metal content (XRF) so that the reusability of thiomont as a heavy metal adsorbent could be tested.
Resutts and Discussion Structural Characterization of Absorbent. The X-ray diffraction pattern of H-montmorillonite showed a symmetric peak indicating a layer spacing of 11.8 A, corre-
105
I
I
I
I
I
I
I
TABLE 1
Adsorption of Pb(ll) by Thiomont mass of initial concn of concn of treated thiomont (me) Pb(ll) (ppm) Pb(W (ppm)
100 n 4
t
0.27 0.94 1.05 5.05 6.28 10.33 15.21
a,
i?a,
95
Q W
+ SI
.-0
s
90
85
80 0
100 200 300 400 500
Te m pe rat u re
600 700
("C)
FIGURE 3. TGA curves of H-montmorillonite (dashed line) and thiomont (solid line).
sponding to partially hydrated claylayers. The XRD pattern of chloromont showed a peak maximum at 8.7" 28 and a pronounced shoulder at lower 28 values. Subtraction of the symmetrized 8.7"28peak (correspondingto a d-spacing of 10.2 A) from the full pattern revealed a phase with a maximum at ~ 7 . 3 28, " corresponding to a d-spacing of ~ 1 2 .A 1 (Figure 2). This corresponds to a clearance space of about 2.4 A between the clay sheets. This separation concords with the presence of amonolayer of carbon chains in the interlayer region, thus providing a strong indication for the presence of the silane moieties in the interlayer region of montmorillonite. It must be noted, however, that the existence of the nearly collapsed 10.2-A phase shows that some layers remain ungrafted. Infrared spectroscopy revealed new peaks in the chloromont spectrum compared with that of H-montmorillonite, which ascertained the presence of the organic functionalities in smectite: 2940, 1440 (C-H modes), and 700 cm-l (C-C1 stretch). The thiomont spectrum was essentially the same, but with a C-S stretch at 694 cm-l. Elemental analyses for C and H were performed (chloromont: C, 13.31%;H, 2.44%; thiomont: C, 11.47%; H, 2.20%),revealing a H/C mole ratio of 2.2 & 0.1 in both cases. This ratio is in agreement with the hydrolysis of the silane methoxy groups by residual water in the H-montmorillonite interlayers, as represented in Scheme 1. This is further supported by the observation of three peaks in the 13CCP-MAS spectrum of chloromont at 12,27, and 48 ppm. By comparison with the literature values of l-chloropropane (11.5,26.0,and46.7ppm) (47),it becomes clear that the three observed signals can be respectively assigned to the carbons in positions 3 , 2 , and 1 of the chloropropyl chain (relative to the carbon bearing the C1 atom). Moreover, no signal could be detected in the range characteristic of the 0-Me resonance (57-59 ppm). Calcination of chloromont at 1000 "C resulted in a weight loss of 27.6%, a value in good agreement with the elemental analysis data (28.7%,on the basis of carbon content). X-ray fluorescence results have shown that chloromont has a Si/ C1 molar ratio of 2.88 and thiomont has a Si/S molar ratio of 2.32, with no C1 detected in the latter. These results
-=0.01 0.05
