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Lead exchange into zeolite and clay minerals: A 29Si, 27Al,23Na solid-state NMR study
Gewhiinica et Cosmochimica Acta Vol. 57, pp. 3885-3894 cowright Q 1993 Ferfpmon Press Ltd. Rinted in U.S.A.
0016-7037/93/$6.00
+ .oO
Lead exchange into zeolite and clay minerals: A %ii “Al, 23Na solid-state NlVlR study J~ANJIE LIANG and BARBARA L. SHERRIFF Department of GeologicaI Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada (Received August 25, 1992; accepted in revisedform March 20, 1993)
Abstrac+Chabazite, vermiculite, montmo~~onite, hector&e, and kaolinite were used to remove Pb, through ion exchange, from 0.01 M aqueous Pb(NO,h solutions. These minerals contained 27 (Nachabazite) , 16,9,9, and 0.4 wt% of Pb, respectively, after equilibration with the solutions. Ion exchange reached equilibrium within 24 h for Na-chabazite and vermiculite, but in less than 5 min for montmorillonite and hectorite. Na-chabazite took up more Pb than natural (Ca, Na)-chabazite (‘7 wt% Pb), whereas no such difference was observed in different cation forms of the clay minerals. Calcite impurities, associated with the clay minerals, effectively removed Pb from the aqueous solutions by the precipitation of cerussite ( PbC03). 29Si “Al and 23Na magic angle spinning (MAS) nuclear magnetic resonance (NMR), 23Na double rota& (&R) NMR, and 23Na variable-temperature MAS NMR were used to study the ion exchange mechanisms. In Na-chabazite, cations in all three possible sites take part in the fast chemical exchange. The chemical exchange passes from the fast exchange regime to the slow regime at -80 to - lOO*C. One site contains a relatively low population of exchangeable cations. The other two more shielded sites contain most of the exchangeable cation. The exchangeable cations in chabazite and vermiculite were found to be close to the Si04 and Al04 tetrahedra, while those. in the other clay minerals were more distant. Two sites (or &foups of sites) for exchangeable cations were observed in hector&e. Lead tended to occupy the one which corresponds to the -8 ppm peak on the 23Na MAS NMR spectrum. The behaviour of the exchangeable cations in the interlayer sites was similar in all the clay minerals studied.
INTRODUCTION
NMR and double rotation (DOR) NMR (WV et al., 1990), in conjunction with variable tempemture (VT) MAS NMR, enable us to investigate the ion exchange processes.
IN mm OF ITS i-i~~RD0t)S effectson the environment, Pb still finds an extensive application in modem society, including gasoline additives and batteries for automobile industry, as well as paints and insecticides ( WINSHIP, 1989). Zeolite and clay minerals have been used to remove Pb from waste water (BARRER et al., 1969; ZAMZOWet al., 1990; SRIVASTAVAet al., 1989). As the exchange mechanism is not well unde~t~, there are difficulti~ in improving the efficiency of Pb removal by these ion exchangers and their potential application in environmental hazard remediation. Clay minerals are major components of soil and sediments, and the elemental distribution of Pb in these materials is governed by ion exchange processes (MAES and CREMERS, 1985). Therefore, understanding the ion exchange mechanism is critical to the efficient removal of Pb from contaminated soils and sediments. It is difficult to obtain crystal structure refinements of clay minerals and zeolites or to monitor the subsequent changes during the ion exchange processes with X-ray diffraction (XRD) because of their fine grain size, platy nature, and structural disorder, as well as the highly mobile nature of the exchangeable cations. In fact, there are no crystal structure refinements for many clay minerals (e.g., the smectite family; GIVEN, 1988). Nuclear magnetic resonance (NMR) spectroscopy is sensitive to short range order and has been used to study the tetrahedral ordering and the neighbouring cation effects in aluminosilicates ( SHERRIFF et al., 1987; SHERRIFF et al., 199 1a,b) . It has also been used to monitor the structural environment of Cs cations adsorbed onto the clay mineral hectorite (WEISS et al., 1990). Magic angle spinning (MAS)
MATERIALS The zeolite, chabazite, and the clay minerals, vegan, hectoxite, montmorilionite, and kaolinite, were used as ion exchangers. The chabazite sample, from Nova Scotia, Canada, was crushed, handpicked, and ground to less than 60 pm. Powder XRD indicated a single chabazite phase. The structural formula based on the chemical analysis of X-ray Assay Laboratory (XRAL), Don Mills, Ontario,
Canada, is as follows:
Na-chabazite was obtained by placing the natural chabazite, which contained both Ca*+ and Nat, in a 0.1 M NaCl solution. More than 90% of Ca*+ could be replaced by Na+ at room temperature by changing the NaCl solution once a day for about 30 &ys, using 10 g of chabazite per 1000 mL NaCI solution. Na-chabazite also was prepared by placing the ion exchange system into an autoclave op erating. at about 23 psi and 106’C for two days, with only six changes of the solution. X-ray diffraction and NMR spectroscopy showed no differences between the samples produced by these two methods. The structural formula of Na-chabazite is as follows:
The sample of vermiculite ( CMSVTx-I, Clay Mineral Society), was handpicked to separate it from carbonate minerals and a minor quantity of quartz, and‘ground to less than 0.1 mm using a mortar and pestte. X-ray powder diffraction showed the resuitant powder sampIe to be pure vermiculite. The structural formula is
(M&.47Ca0.02)(M&.84Ab.~6)(Si~.~~All.l~)0~~~(OH)2~~H20. The hectorite sample (CMS-SHCa- 1, Clay Mineral Society) was a composite powder, con~ning about 27 wt% calcite and traces of quartz (VAN OLPHENand FRIPIAT, 1979). A combined mechanical 3885
3886
J.-J. Liang and B. L. Sherriff
and chemical treatment was used to obtain pure hectorite. The powdered sample was mixed with distilled water to form a thick slurry and left to settle the larger, granular grains of calcite and quartz. Due to the thickness of the slurry, the smaller (< 10 pm), piaty grains of hectorite, stayed suspended regardless of the length of time that the slurry was letI to stand. The suspension, which contained no quartz but minor calcite, was then separated from the settlad portion. As recommended by MOOREand REYNOLDS( 1989), 82 g sodium acetate and 27 mL glacial acetic acid were added to 900 mL of the suspension to remove the remaining calcite. The volume of the suspension was then adjusted to 1000 mL and left to stand overnight. Hectorite was then separated by centrifugation. X-ray diffraction showed the resultant solid to be pure hectorite, with a powder pattern similar to that given by BRINDLEY( 1980). The structural formula is
AAS and wet chemical analysis. Water content was measured using a Mitsubishi moisturemeter Model CA-06. Water was released from the sample by heating to 900°C and reacted with iodine. The iodine required for the titration was generated electrolytically, and so the water content of the sample can be calculated by measuring the electrical current consumed in the electrolysis. The amount of Pb taken up by the minerals during the ion exchange experiments were measured indirectly using a quick gravimetric method. Since the Pb adsorption by the container walls and the filtering devices was controlled, the amount of Pb lost from the solution should equal to that taken up by the minerals. This was calculated by precipitating the remaining Pb in the solution as PbSO, , by adding a saturated aqueous solution of Na2S0, and weighing the precipitate.
(Nae.z&.o,Cao.os)[ t Mg1~Lio.MA10.02)
The spectra were collected at the Prairie Regional NMR Facility, Winnipeg Manitoba, Canada, using a Bruker AMX-500 console with two magnets of 11.7 T and 8.4 T. The samples were spun at 8-10 kHz in a Doty high-speed MAS probe. ?Zi, 27AI and 23Na MAS NMR spectra were obtained using the 1 I .7 T ma&t. For %i, a 30” pulse of 2.5 M, with delay between pulses of l-5 s, was used. %i chemical shifts are quoted with reference to 2%i in tetramethylsiiane (TMS). 27A1and =Na are both auadruoolar nuclei. To ensure that no signal intensity distortion occurred during data acquisition (SAMOSON and LIPPMAA, 1983), a u/ 12 pulse of 0.4 ps was used. The delay between pulses was 0. I s.27A1and “Na peak positions were referenced to I M aqueous solution of AICIJ and NaCl, respectively. Because of the quadrupolar interaction, the peaks on the MAS NMR spectra of 27Al and “Na may be asymmetrical. Since it is the relative peak position of a certain sample before and after Pb exchange which is of interest in the study, the readily extractable position at the peak maxima are reported. The double angle rotation experiment used a Bruker DGR probe with the inner rotor spinning at 5 kHz, and the outer rotor at 1 kHz in the 8.4 T magnet. The pulse length was I ps,and the delay between pulses was 0.2 s. The variable temperature experiment was done with 23Na MAS NMR in the I I .7 T magnetic field. The sample temperature, which was controlled by a Doty variable temperature unit, was lowered gradually to - 120°C with spectra being recorded at mguiar intervals.
*(S~~.~&.dOd(0H
)0.&d - n&O.
There was a small amount of cristobalite associated with montmorillonite sample ( CMS-STx1, Clay Mineral Society). The silica phase is present even in the less than 2.5 pm separates of the powder sample. Electron microprobe analyses showed an anomalously high Si02 content, suggesting that the silica phase is physically incorporated mto the montmorillonite grams. 27AI MAS NMR detected a minor tectosilicate phase giving a peak at 56 ppm (centre of gravity). This phase was not identified by the X-ray powder diffraction and could be minor aluminum replacing silicon in cristobalite(HURLBUT and KLEIN, 1977) or in a trace amount of feldspar. The pure kcolinite sample (CMSKGa-1, Clay Mineral Society) was not treated before ion exchange. The structural formula is -%96A12,04OJ(OH
)4.
EXPERIMENTAL The Ion Exchange Experiment The ion exchange experiments were done in a 0.0 1 M Pb( N0s)2 solution at room temperature of 293 K. The temperature was not controlled during the experiment. The pH of the Pb( NO,), solutions, of about 5, was not adjusted, to prevent the addition of competing cations, such as Na+ and H+, by the base or acid (BRECK, 1974). The mineral-to-solution ratio was 0.25 g to 100 mL. After the mineral was added to the solution, the mixture was stirred until the mineral had thoroughly dimted. There was no further stirring until the solid phase was separated from the solution. The solid phase was then washed with distilled waster and air-dried. X-ray diffraction showed that there was no solid Pb(N01)2 associated with the Pbexchanged minerals. To follow the rate of Pb uptake by a given mineral, identical batches of each mineral were exchanged under the same condition but for different lengths of time. In the ion exchange experiments, adsorption by the glass surface (WHITEand YEE, 1986) was minimized by saturating the walls of the glass containers (flasks, funnels, etc.) with 0.01 M Pb( Nor)* solution prior to use. The glassware was rinsed with distilled water to remove residual Pb( NOl)2 not adsorbed on the glass surface.
NucfearMagnetic Resonawe Spectroscopy
RESULTS
Lead Removal by the Zeolite and Clay Minerals Lead contents were determined gravimetrically, and all values are quoted as “weight percent adsorbed.” There is up to a 1%discrepancy in Pb wt% between the electron microprobe and gravimetrical analysis. The Pb content of Na-chabazite reached 27% when equilibrated with the Pb(N03)2 solution, whereas that of unmodified (Ca,Na)-chabazite was only about 7%. The structural formulae of Na- and (Ca,Na)chabazite after Pb-exchange are (Table I ) as follows:
Chemical Analysis The analyses of the minerals after Pb exchange were done on a Cameca Camebax SX-50 electron microprobe. The standards used in the quantitative analysis were albite (SiKcY, NaKa), anorthite (CaKa, AIKa), PbTe(PbKa), olivine(MgKcr), orthoclase(KKc~), fayahte (FeKa), spessarnne (MnKLu), and strontianite (SrKcu). Data reduction was done using the ‘PAP’ Cp(pz) routine of POUCHOUand PICHOIR( 1985 ) . The sample mounts were prepared by drilling shallow holes in plastic discs and filling the holes with epoxy resin mixed with the powder sample. The epoxy was cured immediately to prevent the grams from settling. The sample mounts were usually repolished and manalysed in order to analyse a sufficient number of grants with well-polished surfaces. Lithium and F were analyzed at XRAL using
respectively,. Lead contents of Mg-vermiculite (natural) and Na-vermiculite after equilibration were similar at I6 and 17%, respectively. This is also true for montmorillonite and hec-
torite, which both contained about 9% Pb on equilibrium, regardless of the initial cation forms of the minerals. The Pb content of kaolinite was only 0.4% on equilibrium. The
3887
Ionexchange of Pb*+ by zeolitesand clay minerals
structural formulae (Table I ) of vermiculite, kaolinite after Pb exchange are as follows:
he&or&e, tind
- (OH )Z- nH20,
A rapid increase in Pb content of Na-chabazite was ob served during the first 15 h of ion exchange (Fig. I ), and equilibrium was reached in about 20 h. The Pb content of vermiculite increased rapidly during the first 5 h of ion exchange, followed by a gradual increase until equilibrium was reached in about 20 h. The amount of Pb taken up by hectorite and montmorillonite did not increase significantly after the first few minutes, for periods of up to 120 h. It was discovered that calcite, associated with hectorite in the unpurified sample CMS-SHCa-1, reacted rapidly with Pb’+ in the solution to precipitate as cerussite (PbCO3). The hectorite sample used in the Pb adsorption experiment was, however, free of calcite impurity. NMR 29Si The four peaks at -92.4, -98.2, - 103.6, and - 109.0 ppm in the “Si MAS NMR spectrum of natural chabazite (Table
2, Fig. 2) correspond to Si(3AI), Si(2Al), Si( IAl), and Si(OA1) of Si in the tetrahedral site. The full width at half maximum (FWHM) of the peaks range 230-350 Hz (Table 2). Na-chabazite has the same number of peaks, and the peak positions differ slightly from those of the natural chabazite. However, the FWHM of all peaks are smaller for Nachabazite. After Pb exchange, the chemical &i&i of the four peaks of both (Ca,Na)-chabazite and Na-chabazite became l-3 ppm more negative (Table 2, Fig. 2). There are three overlapping peaks in the spectrum of vermiculite at -84.4, -88.8, and -92.4 ppm (Fig. 3, Table 2), corresponding to Si(2Al), Si( lAl), and Si(OA1). After Pb exchange, the chemical shifts of all three peaks became 1 ppm more negative. Montmorillonite, hector&e, and kaolinite give single peak at -93.5, -95.3, and -91.3 ppm, respectively (Table 2). There was no change in the chemical shift values after Pb exchange.
The 27Al MAS NMR spectrum of chabazite has a symmetrical peak at 60 ppm (Fig. 2). The peak position shifted 1 ppm after (Ca,Na)-chabazite was converted into Na-chabazite, but the peak width decreases from 650 to 570 Hz (Table 3). After Pb exchange of both (Ca,Na)-chabazite and Na-chabazite, the 27A1peaks become 2-3 ppm more shielded, and the peak width increased by about 200 Hz (Table 3, Fig. 2 ) .
Table 1. Representative chemical analyses of the minerals before and after lead exchange lb
2a
2b
3a
3b
4a
4b
5a
Sb
6a
6b
52.6 16.8 1.7
52.0 17.8 7.0
53.0 17.1 0.8
39.5 14.9
32.4 13.3
22.2
0.7
0.1 0.1
0.6 a.2
0.6
55.3 0.5 0.5 24.6 2.1 0.1 0.1
46.2 39.7
9.3
61.6 13.7 1.4 3.2 0.4
44.6 39.4
0.2 1.7
61.6 13.7 1.4 3.1 0.4
49.5 0.2
::: 0.9
35.3 14.3 0.2 27.8 0.1
0.1
0.1 1.6 0.1
la Weight
percent
SiO, A&O, CaO Mgo Na?O
K$5
0.1
FeQ3 TiO,
0.1 0.3
23.1 0.1 0.2
p205
Unit
PbO S?ZO H-0 L:1 F
20.5
Total
99.3
5.6 0.6 19.2
19.7
104.2
99.9
15.7 100.5
7.8
17.3
29.7 21.8
99.9
14.3
19.1
100.7
100.1
12.2 100.8
0.1
7.3 12 0.5 SE
11.7 0.4 4.1 94.3
0.1
14.5
14.2
100.5
100.3
1.96
1.99
2.04
2.00
4.00
4.00
formulae
2 (IV) Al WI)
4.36 1.64
4.20 1.72
Ca
0.69
0.6l. 0.07
0.02 0.10 0.09
0.01 0.03 0.17 0.12 0.14
Mg (VI) Mg (@xl”
Na K Pb Sr
4.35 1.65
1.48
4.15 1.84 0.02
0.13
2.82 1.18 0.16 0.02 2.84 0.47 0.01
2.7s 1.21 0.14 2.86 0.12 0.01 0.40
0.84
Li F 2.00
2.00
3.98 0.02 0.02 0.03 2.64
3.99 0.01
2.67
0.29 0.01 0.16
0.30 1.02 0.98
0.30 1.15 0.85
*: H$’ aa exchangeable cations; t: not determined, taken the structural formula by Bailey (1980) 1: Ca(Na) -chabazite; 2: Na-chabazite; 3: Vermiculite; 4: Mcmtsnorillonite; 5: Hectorite; 6: Kaolinite a: before Pb exchange; b:after Pb exchange
J.-J. Gang
3888
and
B. L. Sheriff’
28 26
26 22
10
7= . . . . . . . . . . . . . . . . . . . + ;rA
+
+ +
+ 14
+++
12
h
A
r
F
..A
i
:
8-. :
A
..‘...............‘..., i
10 6 60
80
100
120
140
160
Length of Ion Exchange(hours) FIG. 1. The rate of Pb exchange in chabazite and the clay minerals. Key: 0, Na-chabazite; +, vermiculite; A, hector&e;A, montmoriltonite.
Most of the aluminum in vermiculite is in the tetrahedral site, which gives a peak at 67 ppm (Fig. 3). There was a slight shift of the peak toward the higher shielded direction in the Pb-exchanged vermiculite. There is a small peak due to the octahedrally coordinated aluminum at 3 ppm which overlaps a spinning side band of the tetrahedral peak. Aluminum in montmorillonite is in octahedral coordination, giving an asymmetrical 27A1peak at 2 ppm. The peak position, shape, and width did not change after Pb exchange (Table 3 ) . Aluminum in hectorite is mostly in the tetrahedral site, giving an asymmetric peak at 59 ppm. The peak position
Table 2. %i
'jNa One 23Na peak was observed at about -6 ppm in the spectra collected at 11.7 T of both Na- and (Ca,Na)-chabazite (Table 3, Fig. 2), even though there are three possible sites for Na in chabazite. The “Na spectra of Pb-exchanged chabazite
MA!3 NMR data of the zeolite and clay minerals
6 %i
Before Pb exchange Ca(Na) -chabazite Na-chabazite Vermiculite Montmorillonite Hectorite Kaolinite
After
and width did not change a!Ier Pb exchange (Table 3). 27A1 spectrum of kaolinite consists of a single symmetric octahedral peak at 3 ppm. There was no change in peak position, shape, or width after Pb exchange (Table 3).
Pb exchange Ca (Na) -chabazite Na-chabazite Vermiculite Montmorillonite Hectorite Kaolinite
(porn)
and
FWHM (Hz)
(in
parenthemee)
Si(3Al)
Si(2AL)
SitlAB)
-92.4 (350) -93.0 (210)
-98.2 (320) -98.8 (300) -84.4
-103.6 (300) -104.3 (240) -88.8
-95.0 (220 -94.4 (250
-100.3 -100.0 -85.2
(260 (240
‘1
1
-105.8 (280) -105.4 (240) -89.0
Si(OA1) -109.0 -109.6 -92.4 -93.5 -95.3 -91.3
(230) (160)
-111.0 -111.0 -93.2 -93.7 -95.4 -91.3
(260) (160)
(400) (3401 (250)
(390) (3101 (250)
3889
Ion exchange of Pb’+ by zeolites and clay minerals
Ppn
-90
-110
-100
k-4 Ppm 120
80
-20°C to 1230 at -50°C and to 2370 at -100°C. As the temperature was systematically reduced below -50°C a peak at about 0 ppm, which overlaps the main peak, and a very broad peak at about 26-28 ppm began to appear. Both peaks grew in intensity as the sample temperature was progressively lowered to -80, - 100, and - 120°C. Only one symmetrical peak at -9 ppm was observed in the *‘Na spectrum of vermiculite (Fig. 5). The intensity of this peak decreased after Pb exchange. There is a broader and weaker peak at about +8 ppm in the spectrum of the Pb-exchanged sample. There are two overlapping peaks in the 23Na spectra of hectorite at -8 and -19 ppm (Table 3, Fig. 5 ) . The peak at - 19 ppm has a higher proportion of the residual intensity after Pb exchange. The Z3Na MAS NMR spectrum of montmorillonite before Pb exchange shows an asymmetric peak (Fig. 5). Two overlapped peaks at - 11 and about -20 ppm can be identified. The intensity of both peaks were reduced substantially after Pb exchange. The “Na MAS NMR spectrum of kaolinite before Pb exchange shows a relatively sharp peak at -6 ppm overlapped on a broad peak at about 6 ppm (centre of gravity, Fig. 5 ) . After Pb exchange, the sharp peak disappeared, but the broad one remains. DISCUSSION
40
0
Chabazite Chabazite is a hydrated tectosilicate consisting of double six-membered rings of Si( Al)04 tetrahedra linked by fourmembered rings (Fig. 6). Three exchange cation sites have
z’Na
29si
..& (b)
Ppn’
20
-20
FIG. 2. 29Si, *‘AI, *‘Na MAS NMR spectra of Na-chabazite (a) before and (b) after Pb exchange; (c) DOR NMR (*‘Na only) of Na-chabazite before Pb exchange.
.
.
-60 Ppm
-60
(4
-100
-80
-120
also showed only one peak, identical in position and shape to that before Pb exchange. 23Na DOR NMR spectrum of
Na-chabazite collected at 8.45 T (Fig. 2) also has only one peak at - 16 ppm. The difference in peak position between the MAS and DOR spectra is due to an increase in quadrupolar shift associated with the decrease in applied magnetic field strength from 11.7 to 8.4 T. The 23Na MAS NMR peak of Na-chabazite (Fig. 4, Table 4) showed a systematic shift of peak position from about -6 to -9 ppm between +20 and - 120°C. There was decrease of peak width from 5 10 to 360 Hz when the sample temperature was lowered from +20 to -20°C; further cooling caused a dramatic increase in peak width from 360 Hz at
(W
. PW 150
.
/h 50
(4 -50
FIG. 3. 29Siand 27AIMAS NMR spectra of vermiculite (a) before and (b) after Pb exchange.
3890
J.-J. Liang and B. L. Sherriff Table 3. rA1 & aNa MAY NMR peak position (in parentheses) of the zeolite and clay
(ppm) and FWWM(Hz) nlinerals
nA1 "Na Tetrahedral Before Pb exchange Ca(Na) -chabazite Na-chabazite Vermiculite Montmorillonite Hectorite Kaolinite After Pb exchange Ca(Na) -chabazite Na-chabazite Vermiculite Montmorillonite Hectorite Kaolinite
+60 (650) +59 (570) +67 (1560
Octahedral
+3 (2600) +2 (1560)
+59 (2080) +3 (1040) +57 (760) +57 (780) +66 (1500)
+3 (2600) +2 (1560)
+59 (2020) +3 (1040)
been resolved along the [ 11I] diagonal (CALLIGARIS et al., .I 982). They all have octahedral coordination but with either framework oxygens or with water molecules, which are not shown in Fig. 6. Site I coordinates to three framework oxygens (bond lengths of 2.68 A) and three water molecules (bond lengths of 2.82 A), site 2 to water molecules only (bond lengths of 2.88 and 2.28 A), and site 3 to the framework oxygens only (bond lengths of 3.07 A). Both the 29Si and 27Al MAS NMR spectra indicated increased electronic shielding of the tetrahedral site after Pb was exchanged into natural and Na-chabazite (Tables 2 and 3, Fig. 2). The 29Si chemical shift of silicates and aluminosilicates can be calculated, from the crystal structure, using the relationship found by SHERRIFF etal. ( 199 la). This re-
-_
-6 -6 -9 -11 -8, -19 -6 -6 -6 8 8, -20 8, -25 6
lationship can be used to evaluate the effects of changes in the occupancies of the cation sites in chabazite, using chabazite structure of CALLIGARISet al. ( 1982) and the ChemX Crystal Structure Modelling Package (Chemical Design Ltd. Oxford, UK), which was used to build and manipulate the crystal structure. Theoretically, site 1 afkcts 29Sichemical shifts by becoming about 1 ppm more shielded when occupied by Pb instead of Na or Ca, whereas sites 2 and 3 are too distant to affect 23Si chemical shift. The increased electronic shielding of the tetrahedral site in the Pb-exchanged sample therefore indicates the occupancy of Pb in site 1. The 29Si peak width of (Ca,Na)-chabazite decreased by about 70 Hz after being converted to Na-chabazite (Table 2 ), while that of 27AI decreased by 80 Hz (Table 3 ) . It has been experimentally observed (MAGI et al., 1984) and calculated ( SHERRIFF et al., 1987) that having Naf or Ca2+ as adjacent cations will result in slightly different 29Si chemical shift values, which can cause overlapping peaks in disordered minerals such as scapolite ( SHERRIFF et al., 1987). As there are both Ca2+ and Na+ in (Ca,Na)-chabazite, the 29Sispectra contain overlapping peaks due to 29Si adjacent to Ca2+ and to Na+ and, hence, broader peaks. In Na-chabazite, most 29Si is close to Na+ ; therefore, there are fewer overlapping peaks, and the 29Si peaks are narrower. After Pb exchange, the peak width of 27A1of Na-chabazite increased by about 2 10 Hz, while that of the (Ca,Na)-cha-
Table 4. 21Na variable temperature MA5 NMR data of Na-chabazite T ("C) 20 12 0 -20 -50 -80 -100 -120 PW
do
20
i
-20
40
FIG.4.“Na MAS NMR of Na-chabaziteat differenttemperatures.
Peak position (ppm) -6.3 -7.5 -7.9 -8.8 28, 0, 28, 0, 28, 0, 28, 0,
-12 -12 -12 -12
FNHM (Hz)' 510 460 410 360 1230 2360 2370 2340
I: The FNHM's of the spectra at -50 to -120 are that of the overlapped peaks in the 5 to -25 ppm range.
3891
Ion exchange of Pb2+ by zeolites and day minerals
gradient, Co the quadrupolar coupling constant, and uL is the Larmor frequency, a is a constant depending on Co and uL, and is specific to each quantum transition. n has little influence on peak width but can trendy change the peak shape. The 27Al MAS NMR peak shape changes very little in Na-chabazite after Pb exchange (Fig. 2 ) , and there is unlikely to be ~~~~t changes in the ~rnet~c parameter n. As the peak width is proportional to C& it appears that Pb exchanged into chabazite increases the electronic field gradient of aluminum in the framework. The increase in the gradient is proportional to the amount of Pb adsorbed. The 23Na MAS NMR spectrum of Na-chabazite shows only one peak, despite there being three different sites (Fig. 2). For a quadrupolar nucleus undergoing rapid chemical exchange between three different sites, the observed NMR line shape depends on the parameters of quadrupolar coupling, resonance frequency, correlation time, and fractional population of the cations (BERGGREN and WESTLUND, 1990), as well as those of the chemical shift anisotropy and dipolar coupling. The DOR NMR technique reduces quadrupolar broadening, chemical shift anisotropy, and dipolar interactions (WV et al., 1990) and can resolve peaks which overlap in MAS spectra due to quadrupolar broadening. The DOR NMR spectrum of Na-chabazite (Fig. 2; 23Na, c), however, still shows only one peak, indicating that it is not just quadrupolar broadening which is causing these peaks to overlap. The behaviour of three-site chemical exchange and the corresponding NMR line shape can be understood by making an analogy to a simpler two-site case. When the nuclei of the same cation (e.g., Na’) in the two sites are undergoing slow chemical exchange, there will be two peaks in the spectrum, with the separation of Av (Hz). The critical rate of exchange over which the two peaks collapse into one is given by ~SANDERS~~~~UNTER, 1987) k, =
-3
wm
I
1
40
I
i
20
i
0
I
-20
-40
I
-60
I
-80
FIG.5.23NaMAS NMR spectra of the clay minerals (a) before and (b) after W exchange. bazite increased by 110 Hz. Such broadening is not likely to be due to the chemical en~ronment of the tetrahedral cations becoming more diverse because the corresponding 29Si peak widths did not change. The quadrupolar interaction at the nucleus of 27Al could be increased by Pb2+ cations being in the place of Na+ or Ca ‘+. The MAS peak width of “AI can be calculated from SAMOSON et al. ( 1982), as follows: A~(Hx)=-+(l
-s)‘a-
-$6+a)a [
I
(1) in which n is the asymmetry
parameter of the electric field
ki~z,Av.
(2)
The peak is centered at the weighted mean of the two peaks, based on the relative population of the nucleus under study in the two sites ( SANDSTROM,1982). The upper limit of the exchange rate in such a fast exchange regime is a function of several parameters, among which the width of the averaged peak and the frequency separation of the original two peaks in the slow exchange regime are most important (SANDSTROM,1982 ) . For the three-site exchange in the fast exchange regime, the NMR spectrum will also be a single peak instead of three separate ones. The peak position is also the fractional population weighted mean of the three peaks in the slow exchange regime. The critical rate of exchange still depends mainly on the difference between the NMR frequencies of the three sites ( SANDSTROM,1982), although in a more complicated form than that of the two-site exchange. Sample temperature affects the rate of the chemical exchange. The rate slows down as the temperature drops. A theoretical NMR lineshape study of 23Na distributed in three different sites shows that the NMR line width of a transition reaches a maximum when the chemical exchange rate passes from a fast to a slow exchange regime (BERGGRENand WESTLUND, 1990). The 23Na MAS NMR line width of Na-
3892
J.-J. Liang and B. L. Sherriff
FIG. 6. Block diagram ofchabazite structure after CALLIGARIS et al. ( 1982). Each tetrahedron represents an Si( Al)O., unit, with Si(A1) in the middle (not shown) and the oxygens on the vertices.
chabazite reached a maximum between -80 and -100°C (Table 4, Fig. 4)) indicating that chemical exchange changed from a fast to a slow regime in that temperature range. The peaks at 0 ppm and 27 _+ 1 ppm (Fig. 4) cannot be the spinning sidebands of either a centre band or a satellite, as there are no equivalent peaks to match the spinning frequency of 7 kHz, and they also increase in intensity as the sample temperature drops. Therefore, together with the main peak at -12 ppm, they represent three different chemical environments for the exchanging sodium cations in chabazite. X-ray diffraction data ( CALLIGARISet al., 1982) shows that there are three exchangeable cation sites exist in chabazite. The mean peak position at -8 ppm in the fast exchange regime is determined mostly by the two peaks at - 12 and 0 ppm, indicating that the population of sodium cations is concentrated at these two sites. The less shielded site, at 27 ? 1 ppm, is the least populated. Because the average “Na peak position and width at room temperature are the same for Na-chabazite and (Ca,Na)chabazite, as well as for the Pb-exchanged samples (Fig. 2, Table 3 ) , Na + must be exchanging rapidly between the same three sites regardless of whether the other cation is Na+, CJa*+, or Pb’+. Other charge-compensating cations would have to exchange at a similar rate in order to maintain a local charge balance.
The Clay Minerals Clay minerals are layered aluminosilicates consisting of sheets of cations which are tetrahedrally and octahedrally coordinated with oxygen atoms. In the 2: 1 clay minerals of vermiculite, montmorillonite, and hectorite (Fig. 7), there are two tetrahedrally coordinated sheets with one octahedrally coordinated sheet sandwiched between them to form a single layer. In the 1: 1 clay mineral of kaolinite, there is only one tetrahedrally coordinated sheet with an octahedrally coordinated sheet in a single layer. The exchangeable cations in the 2: 1 clay minerals are between the layers and close to the six-membered ring of the tetrahedral sheet coordinating to the adjacent layers and can also be inside the six-membered ring coordinating to only one layer (GWEN, 1988 ). Peaks in the 29Siand *‘Al MAS NMR spectra of vermiculite became more shielded after Pb exchange than before Pb exchange. The introduction of Pb into vermiculite affects the local chemical environment of the SiO* and AlO tetrahedra. On the contrary, no changes in peak position or width were observed in the 29Si and “Al spectra of montmorillonite, hectorite, or kaolinite after Pb exchange. Therefore, the exchangeable cation sites which are occupied by Pb are remote from the SiO, and AlO tetrahedra. The two peaks in the 23Na MAS NMR spectrum of hec-
3893
Ion exchange of Pb*+ by zeolites and clay minerills
FIG.7.Structure model of vermiculite based on SHIROZU and BAILEY(1966). Only portions of the six-membered rings (indicated by “4”) are shown in the tetrahedral sheet.
torite before Pb exchange represent two chemical environments. Polytypism and structural disorder usually occur in the clay minerals (GWEN, 1988 ) . Each peak, therefore, could mean that they actually represent many slightly differing chemical environments. The larger peak at -8 ppm represents environments which are less shielded and, hence, corresponds to the less confined interlayer environments. The intensity of this peak diminished considerably after Pb exchange, suggesting that Pb has occupied these sites. The smaller peak at - 19 ppm represents more highly shielded environments and is therefore attributed to the more confined sites in the sixmembered ring of the tetrahedral sheet. There were similarities among the 23Na MAS NMR spectra of the clay minerals (Fig. 5). A peak at about -8 ppm in all four species disappears after Pb exchange. As all the clay minerals have a similar interlayer site which is available for cation-exchange by Pb, this peak is assigned to this site. The clay minerals studied can exchange most of the Na+ cations in one single Pb exchange experiment. Another common feature in the 23Na MAS NMR spectra of the clay minerals was a broad, relatively symmetrical peaks between 0 and 20 ppm (Fig. 5). There was a slight difference in centre of gravity of these peaks, depending on the type of clay minerals, as follows: 8 ppm for 2: 1 clay minerals, and 6 ppm for the I : I clay mineral (Table 3 ) .
CONCLUSIONS The maximum Pb contents after ion exchange with 0.01 M Pb( N09)2 in Na-chabazite, (Ca,Na)-chabazite, vermiculite, montmorillonite, hectorite, and kaohnite was 27,7,16, 9, 9, and 0.4 wt%, respectively. Ion exchange processes reached equilibrium after 24 h in chabazite and vermiculite and in less in 5 min in montmorillonite and hectorite. Natural (Ca,Na)-chabazite was less efficient than Na-chabazite in taking up Pb from the solution. However, no such difference of the clay minerals with respect to different cation forms was observed. Calcite present in samples of the clay minerals precipitates Pb effectively as cerussite from the aqueous solution. 29Si and 27Al MAS NMR data showed that chabazite and vermiculite have structures in which exchangeable cations are close to the oxygens of the SiO., and Al04 tetrahedra. The other clay minerals do not display such close relationship. At room temperature, the chemical exchange in Na-chabazite is in a fast exchange regime on the NMR time scale, but this changes to a slow exchange regime at -80 to - 100°C. Three sites for the exchangeable cations can be identified in the low temperature 23Na MAS NMR spectra. Two exchangeable cations can be identified in the low temperature 23Na MAS NMR spectra. Two exchangeable cation sites were
3894 recognized completely
J.-J, Liang and B. L. Sheniff in hectorite. The less shielded site, which can be occupied by Pb, is present in each of the four
ciay minerals
studied.
day minerak
after Pb exchange
The residual
sodium
cation
is in environments
in all the which
differ slightly with respect to the different types of clay mineral. deknuwiedgmenr~-We
thank Ron Chapman for the electron microprobe analyses: Wayne Bbnski for f&ping with the moisture measurement; Kirk Marat, Terry Wolowiec, and Zhi Xu with the NMR experiments; and Mark Cooper and Peter Burns for helping with the computer graphics. Bruker Spectra Spin Canada Ltd. is thanked for the loan of the DOR probe. We benefited from the helpful comments fmm Dr. Tracy Tingle at UC Davis, Dr. Paul Schroeder at the Unive~~ty of Georgia, and an anonymous reviewer on the earlier version of this paper. The project was financed by Natural Science and Engineering Research Council by a University Research Fellowship and equipment operating grants to BLS. and by a University of Manitoba Graduate Fellowship to JL.
REFERENCES BAILEYS. W. ( 1980) Structures of layer silicates. In Crysnz! ~f~~~es qfC/qv Minerals and Their X-ray identi~~ation (ed. G. W. BRINDLEY and G. BROWN), Chap. I, pp. I-124. MSA. BARRERR. M., DAVIESJ. A., and REEKL. V. C. ( 1969) Thermodynamics and thermochemistry of cation exchange in chabazite. f. inorg. Nucl. Chem. 31,219-232. BERCCRENE. and WE%-LUNDP. ( 1990) A theoretical NMR Ii&ape study of the influence of an angle-dependant relaxation of the anisotropic site in two chemical exchange models. J. Magn. Reson.
89, 161-169. BRECK D. W. ( 1974) Zeolite Molecular Sieves. Wiley-Interscience. BRINDLEYG. W. ( 1980) Order-disorder in clay mineral structures. In Crystal Stru~ti~res ofCfa,v Minerals and Their X-ray ~dentl~cation (ed. G. W. BRINDLEYand G. BROWN), Chap. 2, pp. 125195. MSA. CALLIGARIS M., NARDIN G., RANDACCIOL., and CHIARAMONTI P. C. ( 1982) Cation-site location in a natural chabazite. Acra CON. B1,602-605. GUVEN N. ( 1988) Smectite. Rev. ~~inera~.19, pp. 497-559. HURLEUT C. S., JR., and KLEIN C. ( 1977) Manual ofMinera~ugv, 19th ed. John Wiley & Sons. MAES A. and CREMERSA. ( 1985) Highly selective ion exchange in clay minerals and zeolites. Amer. Chem. SW. Symp. Ser. 323,254295.
MAGI M., LIPPMAAE., SAMOSONA., ENGELHARDTG., and GRIMMER A.-R. ( 1984) Solid-state high-resolution *‘Si chemical shifts in silicates. J. Phys. Chem. 88, 15 18-l 522. MOORE D. M. and REYNOLDSR. C., JR. ( 1989 f X-ray Dlfiaction and the Idemi~c~ion and Anal&s of Clay minerals, Oxford Univ. Press. POUCHOUJ. L. and PICHOIRF. ( 1985)A new model for quantitative X-ray microanalysis. Rech. Aerosp. 1984-3, pp. 13-38. SAMOSONA. and LIPPMAAE. (1983) Excitation phenomena and line intensities in ~~-~lution NMR powder spectra of half integer quadrupolar nuclei. Phys. Rev. B28,6567-6570. SAMO~~N A., KUNDLAE., and LIPPMAAE. ( 1982) High-resolution MAS NMR of quadrupolar powders. J. Mugn. Resort. 49, 350357. SANDERSJ. K. M. and HUNTER B. K. ( 1987) Modern NMR spectroscopy: A guide for chemists. Oxford Univ. Press. SAND~TR~MJ. ( 1982) Dynamic NMR Spectroscopy. Academic Press. SHERRIFFB. L., GRUNDY H. D., and HARTMANJ. S. ( 1987) Ckcupancy of T sites in the scapolite series: A multinuclear NMR study using magic-angle spinning Canadian Mineral 25,7 17-730. SHERRIFFB. L.. GRUNDY H. D., and HARTMANJ. S. ( 199la) The ~lationshio between 29Si MAS NMR chemical shift and silicate mineral structure. Eur. J. Mineral. 3,X l-768. SHERRIFF B. L., GRUNDY H. D., HARTMANJ. S., HAWTHORNE F. C., and ~ERNL P. ( 199 1b) The incorporation of alkalis in beryl: A multinuclear MAS NMR and crystal structure study. Canadian Mineral. 29, 27 l-285. SHIROZU H. and BAILEYS. W. ( 1966) Crystal structure of a twolayer Mg-vermiculite. Amer. Mineral. 51, 1124-I 143. SRIVASTAVAS. K., TYAGI R., PANT N., and PAL N. ( 1989) Studies on the removal of some toxic metal ions: Part II (removal of Pb and cadmium by montmorillonite and kaolinite). Environ. Tech. Lett. IQ, 275-282. VAN OLPHEN 11. and FRIPIATJ. 1. ( 1979) Data ~and~~~~k~~rC/a-v Materials und Other fan-metallic Minerals. Pergamon. WEISS C’. A.. JR., KIRKPATRICK R. J., and ALTANERS. P. ( 1990) The structural environments of cations adsorbed onto clays: ‘33Cs variable-temperature MAS NMR spectroscopic study of hectorite. Geuchim. Cosmochim. Acta 54, 1655-1669. WHITE A. F. and YEE A. ( 1986) Near-surface alkali di~us~on into glassy and crystalline silicates at 25°C to 100°C. Amer. Chem. SK. .Swt>. 323, 587-598. WINSHIPK. A. ( 1989) Toxicity of Pb: A review. Adverse DrugReact. .4cute Poisoning Rev. 8, 117-l 52. WCIY., CHMELKA B. F., PINESA., DAVIS M. E., GROBET P. J., and JACOBS P. A. ( 1990) Hid-~~Iution “Al NMR spectroscopy of the aluminophosphate molecular sieve VPI-5. lvafure 346, 550552. ZAMZOW M. J., EICHBAUMB. R., SANDCREN K. R., and SHANKS D. E. ( 1990) Removal of heavy metals and other cations from waste water using chabazites. Sepurat. Sci. Tech. 25, i555-i 569.
0016-7037/93/$6.00
+ .oO
Lead exchange into zeolite and clay minerals: A %ii “Al, 23Na solid-state NlVlR study J~ANJIE LIANG and BARBARA L. SHERRIFF Department of GeologicaI Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada (Received August 25, 1992; accepted in revisedform March 20, 1993)
Abstrac+Chabazite, vermiculite, montmo~~onite, hector&e, and kaolinite were used to remove Pb, through ion exchange, from 0.01 M aqueous Pb(NO,h solutions. These minerals contained 27 (Nachabazite) , 16,9,9, and 0.4 wt% of Pb, respectively, after equilibration with the solutions. Ion exchange reached equilibrium within 24 h for Na-chabazite and vermiculite, but in less than 5 min for montmorillonite and hectorite. Na-chabazite took up more Pb than natural (Ca, Na)-chabazite (‘7 wt% Pb), whereas no such difference was observed in different cation forms of the clay minerals. Calcite impurities, associated with the clay minerals, effectively removed Pb from the aqueous solutions by the precipitation of cerussite ( PbC03). 29Si “Al and 23Na magic angle spinning (MAS) nuclear magnetic resonance (NMR), 23Na double rota& (&R) NMR, and 23Na variable-temperature MAS NMR were used to study the ion exchange mechanisms. In Na-chabazite, cations in all three possible sites take part in the fast chemical exchange. The chemical exchange passes from the fast exchange regime to the slow regime at -80 to - lOO*C. One site contains a relatively low population of exchangeable cations. The other two more shielded sites contain most of the exchangeable cation. The exchangeable cations in chabazite and vermiculite were found to be close to the Si04 and Al04 tetrahedra, while those. in the other clay minerals were more distant. Two sites (or &foups of sites) for exchangeable cations were observed in hector&e. Lead tended to occupy the one which corresponds to the -8 ppm peak on the 23Na MAS NMR spectrum. The behaviour of the exchangeable cations in the interlayer sites was similar in all the clay minerals studied.
INTRODUCTION
NMR and double rotation (DOR) NMR (WV et al., 1990), in conjunction with variable tempemture (VT) MAS NMR, enable us to investigate the ion exchange processes.
IN mm OF ITS i-i~~RD0t)S effectson the environment, Pb still finds an extensive application in modem society, including gasoline additives and batteries for automobile industry, as well as paints and insecticides ( WINSHIP, 1989). Zeolite and clay minerals have been used to remove Pb from waste water (BARRER et al., 1969; ZAMZOWet al., 1990; SRIVASTAVAet al., 1989). As the exchange mechanism is not well unde~t~, there are difficulti~ in improving the efficiency of Pb removal by these ion exchangers and their potential application in environmental hazard remediation. Clay minerals are major components of soil and sediments, and the elemental distribution of Pb in these materials is governed by ion exchange processes (MAES and CREMERS, 1985). Therefore, understanding the ion exchange mechanism is critical to the efficient removal of Pb from contaminated soils and sediments. It is difficult to obtain crystal structure refinements of clay minerals and zeolites or to monitor the subsequent changes during the ion exchange processes with X-ray diffraction (XRD) because of their fine grain size, platy nature, and structural disorder, as well as the highly mobile nature of the exchangeable cations. In fact, there are no crystal structure refinements for many clay minerals (e.g., the smectite family; GIVEN, 1988). Nuclear magnetic resonance (NMR) spectroscopy is sensitive to short range order and has been used to study the tetrahedral ordering and the neighbouring cation effects in aluminosilicates ( SHERRIFF et al., 1987; SHERRIFF et al., 199 1a,b) . It has also been used to monitor the structural environment of Cs cations adsorbed onto the clay mineral hectorite (WEISS et al., 1990). Magic angle spinning (MAS)
MATERIALS The zeolite, chabazite, and the clay minerals, vegan, hectoxite, montmorilionite, and kaolinite, were used as ion exchangers. The chabazite sample, from Nova Scotia, Canada, was crushed, handpicked, and ground to less than 60 pm. Powder XRD indicated a single chabazite phase. The structural formula based on the chemical analysis of X-ray Assay Laboratory (XRAL), Don Mills, Ontario,
Canada, is as follows:
Na-chabazite was obtained by placing the natural chabazite, which contained both Ca*+ and Nat, in a 0.1 M NaCl solution. More than 90% of Ca*+ could be replaced by Na+ at room temperature by changing the NaCl solution once a day for about 30 &ys, using 10 g of chabazite per 1000 mL NaCI solution. Na-chabazite also was prepared by placing the ion exchange system into an autoclave op erating. at about 23 psi and 106’C for two days, with only six changes of the solution. X-ray diffraction and NMR spectroscopy showed no differences between the samples produced by these two methods. The structural formula of Na-chabazite is as follows:
The sample of vermiculite ( CMSVTx-I, Clay Mineral Society), was handpicked to separate it from carbonate minerals and a minor quantity of quartz, and‘ground to less than 0.1 mm using a mortar and pestte. X-ray powder diffraction showed the resuitant powder sampIe to be pure vermiculite. The structural formula is
(M&.47Ca0.02)(M&.84Ab.~6)(Si~.~~All.l~)0~~~(OH)2~~H20. The hectorite sample (CMS-SHCa- 1, Clay Mineral Society) was a composite powder, con~ning about 27 wt% calcite and traces of quartz (VAN OLPHENand FRIPIAT, 1979). A combined mechanical 3885
3886
J.-J. Liang and B. L. Sherriff
and chemical treatment was used to obtain pure hectorite. The powdered sample was mixed with distilled water to form a thick slurry and left to settle the larger, granular grains of calcite and quartz. Due to the thickness of the slurry, the smaller (< 10 pm), piaty grains of hectorite, stayed suspended regardless of the length of time that the slurry was letI to stand. The suspension, which contained no quartz but minor calcite, was then separated from the settlad portion. As recommended by MOOREand REYNOLDS( 1989), 82 g sodium acetate and 27 mL glacial acetic acid were added to 900 mL of the suspension to remove the remaining calcite. The volume of the suspension was then adjusted to 1000 mL and left to stand overnight. Hectorite was then separated by centrifugation. X-ray diffraction showed the resultant solid to be pure hectorite, with a powder pattern similar to that given by BRINDLEY( 1980). The structural formula is
AAS and wet chemical analysis. Water content was measured using a Mitsubishi moisturemeter Model CA-06. Water was released from the sample by heating to 900°C and reacted with iodine. The iodine required for the titration was generated electrolytically, and so the water content of the sample can be calculated by measuring the electrical current consumed in the electrolysis. The amount of Pb taken up by the minerals during the ion exchange experiments were measured indirectly using a quick gravimetric method. Since the Pb adsorption by the container walls and the filtering devices was controlled, the amount of Pb lost from the solution should equal to that taken up by the minerals. This was calculated by precipitating the remaining Pb in the solution as PbSO, , by adding a saturated aqueous solution of Na2S0, and weighing the precipitate.
(Nae.z&.o,Cao.os)[ t Mg1~Lio.MA10.02)
The spectra were collected at the Prairie Regional NMR Facility, Winnipeg Manitoba, Canada, using a Bruker AMX-500 console with two magnets of 11.7 T and 8.4 T. The samples were spun at 8-10 kHz in a Doty high-speed MAS probe. ?Zi, 27AI and 23Na MAS NMR spectra were obtained using the 1 I .7 T ma&t. For %i, a 30” pulse of 2.5 M, with delay between pulses of l-5 s, was used. %i chemical shifts are quoted with reference to 2%i in tetramethylsiiane (TMS). 27A1and =Na are both auadruoolar nuclei. To ensure that no signal intensity distortion occurred during data acquisition (SAMOSON and LIPPMAA, 1983), a u/ 12 pulse of 0.4 ps was used. The delay between pulses was 0. I s.27A1and “Na peak positions were referenced to I M aqueous solution of AICIJ and NaCl, respectively. Because of the quadrupolar interaction, the peaks on the MAS NMR spectra of 27Al and “Na may be asymmetrical. Since it is the relative peak position of a certain sample before and after Pb exchange which is of interest in the study, the readily extractable position at the peak maxima are reported. The double angle rotation experiment used a Bruker DGR probe with the inner rotor spinning at 5 kHz, and the outer rotor at 1 kHz in the 8.4 T magnet. The pulse length was I ps,and the delay between pulses was 0.2 s. The variable temperature experiment was done with 23Na MAS NMR in the I I .7 T magnetic field. The sample temperature, which was controlled by a Doty variable temperature unit, was lowered gradually to - 120°C with spectra being recorded at mguiar intervals.
*(S~~.~&.dOd(0H
)0.&d - n&O.
There was a small amount of cristobalite associated with montmorillonite sample ( CMS-STx1, Clay Mineral Society). The silica phase is present even in the less than 2.5 pm separates of the powder sample. Electron microprobe analyses showed an anomalously high Si02 content, suggesting that the silica phase is physically incorporated mto the montmorillonite grams. 27AI MAS NMR detected a minor tectosilicate phase giving a peak at 56 ppm (centre of gravity). This phase was not identified by the X-ray powder diffraction and could be minor aluminum replacing silicon in cristobalite(HURLBUT and KLEIN, 1977) or in a trace amount of feldspar. The pure kcolinite sample (CMSKGa-1, Clay Mineral Society) was not treated before ion exchange. The structural formula is -%96A12,04OJ(OH
)4.
EXPERIMENTAL The Ion Exchange Experiment The ion exchange experiments were done in a 0.0 1 M Pb( N0s)2 solution at room temperature of 293 K. The temperature was not controlled during the experiment. The pH of the Pb( NO,), solutions, of about 5, was not adjusted, to prevent the addition of competing cations, such as Na+ and H+, by the base or acid (BRECK, 1974). The mineral-to-solution ratio was 0.25 g to 100 mL. After the mineral was added to the solution, the mixture was stirred until the mineral had thoroughly dimted. There was no further stirring until the solid phase was separated from the solution. The solid phase was then washed with distilled waster and air-dried. X-ray diffraction showed that there was no solid Pb(N01)2 associated with the Pbexchanged minerals. To follow the rate of Pb uptake by a given mineral, identical batches of each mineral were exchanged under the same condition but for different lengths of time. In the ion exchange experiments, adsorption by the glass surface (WHITEand YEE, 1986) was minimized by saturating the walls of the glass containers (flasks, funnels, etc.) with 0.01 M Pb( Nor)* solution prior to use. The glassware was rinsed with distilled water to remove residual Pb( NOl)2 not adsorbed on the glass surface.
NucfearMagnetic Resonawe Spectroscopy
RESULTS
Lead Removal by the Zeolite and Clay Minerals Lead contents were determined gravimetrically, and all values are quoted as “weight percent adsorbed.” There is up to a 1%discrepancy in Pb wt% between the electron microprobe and gravimetrical analysis. The Pb content of Na-chabazite reached 27% when equilibrated with the Pb(N03)2 solution, whereas that of unmodified (Ca,Na)-chabazite was only about 7%. The structural formulae of Na- and (Ca,Na)chabazite after Pb-exchange are (Table I ) as follows:
Chemical Analysis The analyses of the minerals after Pb exchange were done on a Cameca Camebax SX-50 electron microprobe. The standards used in the quantitative analysis were albite (SiKcY, NaKa), anorthite (CaKa, AIKa), PbTe(PbKa), olivine(MgKcr), orthoclase(KKc~), fayahte (FeKa), spessarnne (MnKLu), and strontianite (SrKcu). Data reduction was done using the ‘PAP’ Cp(pz) routine of POUCHOUand PICHOIR( 1985 ) . The sample mounts were prepared by drilling shallow holes in plastic discs and filling the holes with epoxy resin mixed with the powder sample. The epoxy was cured immediately to prevent the grams from settling. The sample mounts were usually repolished and manalysed in order to analyse a sufficient number of grants with well-polished surfaces. Lithium and F were analyzed at XRAL using
respectively,. Lead contents of Mg-vermiculite (natural) and Na-vermiculite after equilibration were similar at I6 and 17%, respectively. This is also true for montmorillonite and hec-
torite, which both contained about 9% Pb on equilibrium, regardless of the initial cation forms of the minerals. The Pb content of kaolinite was only 0.4% on equilibrium. The
3887
Ionexchange of Pb*+ by zeolitesand clay minerals
structural formulae (Table I ) of vermiculite, kaolinite after Pb exchange are as follows:
he&or&e, tind
- (OH )Z- nH20,
A rapid increase in Pb content of Na-chabazite was ob served during the first 15 h of ion exchange (Fig. I ), and equilibrium was reached in about 20 h. The Pb content of vermiculite increased rapidly during the first 5 h of ion exchange, followed by a gradual increase until equilibrium was reached in about 20 h. The amount of Pb taken up by hectorite and montmorillonite did not increase significantly after the first few minutes, for periods of up to 120 h. It was discovered that calcite, associated with hectorite in the unpurified sample CMS-SHCa-1, reacted rapidly with Pb’+ in the solution to precipitate as cerussite (PbCO3). The hectorite sample used in the Pb adsorption experiment was, however, free of calcite impurity. NMR 29Si The four peaks at -92.4, -98.2, - 103.6, and - 109.0 ppm in the “Si MAS NMR spectrum of natural chabazite (Table
2, Fig. 2) correspond to Si(3AI), Si(2Al), Si( IAl), and Si(OA1) of Si in the tetrahedral site. The full width at half maximum (FWHM) of the peaks range 230-350 Hz (Table 2). Na-chabazite has the same number of peaks, and the peak positions differ slightly from those of the natural chabazite. However, the FWHM of all peaks are smaller for Nachabazite. After Pb exchange, the chemical &i&i of the four peaks of both (Ca,Na)-chabazite and Na-chabazite became l-3 ppm more negative (Table 2, Fig. 2). There are three overlapping peaks in the spectrum of vermiculite at -84.4, -88.8, and -92.4 ppm (Fig. 3, Table 2), corresponding to Si(2Al), Si( lAl), and Si(OA1). After Pb exchange, the chemical shifts of all three peaks became 1 ppm more negative. Montmorillonite, hector&e, and kaolinite give single peak at -93.5, -95.3, and -91.3 ppm, respectively (Table 2). There was no change in the chemical shift values after Pb exchange.
The 27Al MAS NMR spectrum of chabazite has a symmetrical peak at 60 ppm (Fig. 2). The peak position shifted 1 ppm after (Ca,Na)-chabazite was converted into Na-chabazite, but the peak width decreases from 650 to 570 Hz (Table 3). After Pb exchange of both (Ca,Na)-chabazite and Na-chabazite, the 27A1peaks become 2-3 ppm more shielded, and the peak width increased by about 200 Hz (Table 3, Fig. 2 ) .
Table 1. Representative chemical analyses of the minerals before and after lead exchange lb
2a
2b
3a
3b
4a
4b
5a
Sb
6a
6b
52.6 16.8 1.7
52.0 17.8 7.0
53.0 17.1 0.8
39.5 14.9
32.4 13.3
22.2
0.7
0.1 0.1
0.6 a.2
0.6
55.3 0.5 0.5 24.6 2.1 0.1 0.1
46.2 39.7
9.3
61.6 13.7 1.4 3.2 0.4
44.6 39.4
0.2 1.7
61.6 13.7 1.4 3.1 0.4
49.5 0.2
::: 0.9
35.3 14.3 0.2 27.8 0.1
0.1
0.1 1.6 0.1
la Weight
percent
SiO, A&O, CaO Mgo Na?O
K$5
0.1
FeQ3 TiO,
0.1 0.3
23.1 0.1 0.2
p205
Unit
PbO S?ZO H-0 L:1 F
20.5
Total
99.3
5.6 0.6 19.2
19.7
104.2
99.9
15.7 100.5
7.8
17.3
29.7 21.8
99.9
14.3
19.1
100.7
100.1
12.2 100.8
0.1
7.3 12 0.5 SE
11.7 0.4 4.1 94.3
0.1
14.5
14.2
100.5
100.3
1.96
1.99
2.04
2.00
4.00
4.00
formulae
2 (IV) Al WI)
4.36 1.64
4.20 1.72
Ca
0.69
0.6l. 0.07
0.02 0.10 0.09
0.01 0.03 0.17 0.12 0.14
Mg (VI) Mg (@xl”
Na K Pb Sr
4.35 1.65
1.48
4.15 1.84 0.02
0.13
2.82 1.18 0.16 0.02 2.84 0.47 0.01
2.7s 1.21 0.14 2.86 0.12 0.01 0.40
0.84
Li F 2.00
2.00
3.98 0.02 0.02 0.03 2.64
3.99 0.01
2.67
0.29 0.01 0.16
0.30 1.02 0.98
0.30 1.15 0.85
*: H$’ aa exchangeable cations; t: not determined, taken the structural formula by Bailey (1980) 1: Ca(Na) -chabazite; 2: Na-chabazite; 3: Vermiculite; 4: Mcmtsnorillonite; 5: Hectorite; 6: Kaolinite a: before Pb exchange; b:after Pb exchange
J.-J. Gang
3888
and
B. L. Sheriff’
28 26
26 22
10
7= . . . . . . . . . . . . . . . . . . . + ;rA
+
+ +
+ 14
+++
12
h
A
r
F
..A
i
:
8-. :
A
..‘...............‘..., i
10 6 60
80
100
120
140
160
Length of Ion Exchange(hours) FIG. 1. The rate of Pb exchange in chabazite and the clay minerals. Key: 0, Na-chabazite; +, vermiculite; A, hector&e;A, montmoriltonite.
Most of the aluminum in vermiculite is in the tetrahedral site, which gives a peak at 67 ppm (Fig. 3). There was a slight shift of the peak toward the higher shielded direction in the Pb-exchanged vermiculite. There is a small peak due to the octahedrally coordinated aluminum at 3 ppm which overlaps a spinning side band of the tetrahedral peak. Aluminum in montmorillonite is in octahedral coordination, giving an asymmetrical 27A1peak at 2 ppm. The peak position, shape, and width did not change after Pb exchange (Table 3 ) . Aluminum in hectorite is mostly in the tetrahedral site, giving an asymmetric peak at 59 ppm. The peak position
Table 2. %i
'jNa One 23Na peak was observed at about -6 ppm in the spectra collected at 11.7 T of both Na- and (Ca,Na)-chabazite (Table 3, Fig. 2), even though there are three possible sites for Na in chabazite. The “Na spectra of Pb-exchanged chabazite
MA!3 NMR data of the zeolite and clay minerals
6 %i
Before Pb exchange Ca(Na) -chabazite Na-chabazite Vermiculite Montmorillonite Hectorite Kaolinite
After
and width did not change a!Ier Pb exchange (Table 3). 27A1 spectrum of kaolinite consists of a single symmetric octahedral peak at 3 ppm. There was no change in peak position, shape, or width after Pb exchange (Table 3).
Pb exchange Ca (Na) -chabazite Na-chabazite Vermiculite Montmorillonite Hectorite Kaolinite
(porn)
and
FWHM (Hz)
(in
parenthemee)
Si(3Al)
Si(2AL)
SitlAB)
-92.4 (350) -93.0 (210)
-98.2 (320) -98.8 (300) -84.4
-103.6 (300) -104.3 (240) -88.8
-95.0 (220 -94.4 (250
-100.3 -100.0 -85.2
(260 (240
‘1
1
-105.8 (280) -105.4 (240) -89.0
Si(OA1) -109.0 -109.6 -92.4 -93.5 -95.3 -91.3
(230) (160)
-111.0 -111.0 -93.2 -93.7 -95.4 -91.3
(260) (160)
(400) (3401 (250)
(390) (3101 (250)
3889
Ion exchange of Pb’+ by zeolites and clay minerals
Ppn
-90
-110
-100
k-4 Ppm 120
80
-20°C to 1230 at -50°C and to 2370 at -100°C. As the temperature was systematically reduced below -50°C a peak at about 0 ppm, which overlaps the main peak, and a very broad peak at about 26-28 ppm began to appear. Both peaks grew in intensity as the sample temperature was progressively lowered to -80, - 100, and - 120°C. Only one symmetrical peak at -9 ppm was observed in the *‘Na spectrum of vermiculite (Fig. 5). The intensity of this peak decreased after Pb exchange. There is a broader and weaker peak at about +8 ppm in the spectrum of the Pb-exchanged sample. There are two overlapping peaks in the 23Na spectra of hectorite at -8 and -19 ppm (Table 3, Fig. 5 ) . The peak at - 19 ppm has a higher proportion of the residual intensity after Pb exchange. The Z3Na MAS NMR spectrum of montmorillonite before Pb exchange shows an asymmetric peak (Fig. 5). Two overlapped peaks at - 11 and about -20 ppm can be identified. The intensity of both peaks were reduced substantially after Pb exchange. The “Na MAS NMR spectrum of kaolinite before Pb exchange shows a relatively sharp peak at -6 ppm overlapped on a broad peak at about 6 ppm (centre of gravity, Fig. 5 ) . After Pb exchange, the sharp peak disappeared, but the broad one remains. DISCUSSION
40
0
Chabazite Chabazite is a hydrated tectosilicate consisting of double six-membered rings of Si( Al)04 tetrahedra linked by fourmembered rings (Fig. 6). Three exchange cation sites have
z’Na
29si
..& (b)
Ppn’
20
-20
FIG. 2. 29Si, *‘AI, *‘Na MAS NMR spectra of Na-chabazite (a) before and (b) after Pb exchange; (c) DOR NMR (*‘Na only) of Na-chabazite before Pb exchange.
.
.
-60 Ppm
-60
(4
-100
-80
-120
also showed only one peak, identical in position and shape to that before Pb exchange. 23Na DOR NMR spectrum of
Na-chabazite collected at 8.45 T (Fig. 2) also has only one peak at - 16 ppm. The difference in peak position between the MAS and DOR spectra is due to an increase in quadrupolar shift associated with the decrease in applied magnetic field strength from 11.7 to 8.4 T. The 23Na MAS NMR peak of Na-chabazite (Fig. 4, Table 4) showed a systematic shift of peak position from about -6 to -9 ppm between +20 and - 120°C. There was decrease of peak width from 5 10 to 360 Hz when the sample temperature was lowered from +20 to -20°C; further cooling caused a dramatic increase in peak width from 360 Hz at
(W
. PW 150
.
/h 50
(4 -50
FIG. 3. 29Siand 27AIMAS NMR spectra of vermiculite (a) before and (b) after Pb exchange.
3890
J.-J. Liang and B. L. Sherriff Table 3. rA1 & aNa MAY NMR peak position (in parentheses) of the zeolite and clay
(ppm) and FWWM(Hz) nlinerals
nA1 "Na Tetrahedral Before Pb exchange Ca(Na) -chabazite Na-chabazite Vermiculite Montmorillonite Hectorite Kaolinite After Pb exchange Ca(Na) -chabazite Na-chabazite Vermiculite Montmorillonite Hectorite Kaolinite
+60 (650) +59 (570) +67 (1560
Octahedral
+3 (2600) +2 (1560)
+59 (2080) +3 (1040) +57 (760) +57 (780) +66 (1500)
+3 (2600) +2 (1560)
+59 (2020) +3 (1040)
been resolved along the [ 11I] diagonal (CALLIGARIS et al., .I 982). They all have octahedral coordination but with either framework oxygens or with water molecules, which are not shown in Fig. 6. Site I coordinates to three framework oxygens (bond lengths of 2.68 A) and three water molecules (bond lengths of 2.82 A), site 2 to water molecules only (bond lengths of 2.88 and 2.28 A), and site 3 to the framework oxygens only (bond lengths of 3.07 A). Both the 29Si and 27Al MAS NMR spectra indicated increased electronic shielding of the tetrahedral site after Pb was exchanged into natural and Na-chabazite (Tables 2 and 3, Fig. 2). The 29Si chemical shift of silicates and aluminosilicates can be calculated, from the crystal structure, using the relationship found by SHERRIFF etal. ( 199 la). This re-
-_
-6 -6 -9 -11 -8, -19 -6 -6 -6 8 8, -20 8, -25 6
lationship can be used to evaluate the effects of changes in the occupancies of the cation sites in chabazite, using chabazite structure of CALLIGARISet al. ( 1982) and the ChemX Crystal Structure Modelling Package (Chemical Design Ltd. Oxford, UK), which was used to build and manipulate the crystal structure. Theoretically, site 1 afkcts 29Sichemical shifts by becoming about 1 ppm more shielded when occupied by Pb instead of Na or Ca, whereas sites 2 and 3 are too distant to affect 23Si chemical shift. The increased electronic shielding of the tetrahedral site in the Pb-exchanged sample therefore indicates the occupancy of Pb in site 1. The 29Si peak width of (Ca,Na)-chabazite decreased by about 70 Hz after being converted to Na-chabazite (Table 2 ), while that of 27AI decreased by 80 Hz (Table 3 ) . It has been experimentally observed (MAGI et al., 1984) and calculated ( SHERRIFF et al., 1987) that having Naf or Ca2+ as adjacent cations will result in slightly different 29Si chemical shift values, which can cause overlapping peaks in disordered minerals such as scapolite ( SHERRIFF et al., 1987). As there are both Ca2+ and Na+ in (Ca,Na)-chabazite, the 29Sispectra contain overlapping peaks due to 29Si adjacent to Ca2+ and to Na+ and, hence, broader peaks. In Na-chabazite, most 29Si is close to Na+ ; therefore, there are fewer overlapping peaks, and the 29Si peaks are narrower. After Pb exchange, the peak width of 27A1of Na-chabazite increased by about 2 10 Hz, while that of the (Ca,Na)-cha-
Table 4. 21Na variable temperature MA5 NMR data of Na-chabazite T ("C) 20 12 0 -20 -50 -80 -100 -120 PW
do
20
i
-20
40
FIG.4.“Na MAS NMR of Na-chabaziteat differenttemperatures.
Peak position (ppm) -6.3 -7.5 -7.9 -8.8 28, 0, 28, 0, 28, 0, 28, 0,
-12 -12 -12 -12
FNHM (Hz)' 510 460 410 360 1230 2360 2370 2340
I: The FNHM's of the spectra at -50 to -120 are that of the overlapped peaks in the 5 to -25 ppm range.
3891
Ion exchange of Pb2+ by zeolites and day minerals
gradient, Co the quadrupolar coupling constant, and uL is the Larmor frequency, a is a constant depending on Co and uL, and is specific to each quantum transition. n has little influence on peak width but can trendy change the peak shape. The 27Al MAS NMR peak shape changes very little in Na-chabazite after Pb exchange (Fig. 2 ) , and there is unlikely to be ~~~~t changes in the ~rnet~c parameter n. As the peak width is proportional to C& it appears that Pb exchanged into chabazite increases the electronic field gradient of aluminum in the framework. The increase in the gradient is proportional to the amount of Pb adsorbed. The 23Na MAS NMR spectrum of Na-chabazite shows only one peak, despite there being three different sites (Fig. 2). For a quadrupolar nucleus undergoing rapid chemical exchange between three different sites, the observed NMR line shape depends on the parameters of quadrupolar coupling, resonance frequency, correlation time, and fractional population of the cations (BERGGREN and WESTLUND, 1990), as well as those of the chemical shift anisotropy and dipolar coupling. The DOR NMR technique reduces quadrupolar broadening, chemical shift anisotropy, and dipolar interactions (WV et al., 1990) and can resolve peaks which overlap in MAS spectra due to quadrupolar broadening. The DOR NMR spectrum of Na-chabazite (Fig. 2; 23Na, c), however, still shows only one peak, indicating that it is not just quadrupolar broadening which is causing these peaks to overlap. The behaviour of three-site chemical exchange and the corresponding NMR line shape can be understood by making an analogy to a simpler two-site case. When the nuclei of the same cation (e.g., Na’) in the two sites are undergoing slow chemical exchange, there will be two peaks in the spectrum, with the separation of Av (Hz). The critical rate of exchange over which the two peaks collapse into one is given by ~SANDERS~~~~UNTER, 1987) k, =
-3
wm
I
1
40
I
i
20
i
0
I
-20
-40
I
-60
I
-80
FIG.5.23NaMAS NMR spectra of the clay minerals (a) before and (b) after W exchange. bazite increased by 110 Hz. Such broadening is not likely to be due to the chemical en~ronment of the tetrahedral cations becoming more diverse because the corresponding 29Si peak widths did not change. The quadrupolar interaction at the nucleus of 27Al could be increased by Pb2+ cations being in the place of Na+ or Ca ‘+. The MAS peak width of “AI can be calculated from SAMOSON et al. ( 1982), as follows: A~(Hx)=-+(l
-s)‘a-
-$6+a)a [
I
(1) in which n is the asymmetry
parameter of the electric field
ki~z,Av.
(2)
The peak is centered at the weighted mean of the two peaks, based on the relative population of the nucleus under study in the two sites ( SANDSTROM,1982). The upper limit of the exchange rate in such a fast exchange regime is a function of several parameters, among which the width of the averaged peak and the frequency separation of the original two peaks in the slow exchange regime are most important (SANDSTROM,1982 ) . For the three-site exchange in the fast exchange regime, the NMR spectrum will also be a single peak instead of three separate ones. The peak position is also the fractional population weighted mean of the three peaks in the slow exchange regime. The critical rate of exchange still depends mainly on the difference between the NMR frequencies of the three sites ( SANDSTROM,1982), although in a more complicated form than that of the two-site exchange. Sample temperature affects the rate of the chemical exchange. The rate slows down as the temperature drops. A theoretical NMR lineshape study of 23Na distributed in three different sites shows that the NMR line width of a transition reaches a maximum when the chemical exchange rate passes from a fast to a slow exchange regime (BERGGRENand WESTLUND, 1990). The 23Na MAS NMR line width of Na-
3892
J.-J. Liang and B. L. Sherriff
FIG. 6. Block diagram ofchabazite structure after CALLIGARIS et al. ( 1982). Each tetrahedron represents an Si( Al)O., unit, with Si(A1) in the middle (not shown) and the oxygens on the vertices.
chabazite reached a maximum between -80 and -100°C (Table 4, Fig. 4)) indicating that chemical exchange changed from a fast to a slow regime in that temperature range. The peaks at 0 ppm and 27 _+ 1 ppm (Fig. 4) cannot be the spinning sidebands of either a centre band or a satellite, as there are no equivalent peaks to match the spinning frequency of 7 kHz, and they also increase in intensity as the sample temperature drops. Therefore, together with the main peak at -12 ppm, they represent three different chemical environments for the exchanging sodium cations in chabazite. X-ray diffraction data ( CALLIGARISet al., 1982) shows that there are three exchangeable cation sites exist in chabazite. The mean peak position at -8 ppm in the fast exchange regime is determined mostly by the two peaks at - 12 and 0 ppm, indicating that the population of sodium cations is concentrated at these two sites. The less shielded site, at 27 ? 1 ppm, is the least populated. Because the average “Na peak position and width at room temperature are the same for Na-chabazite and (Ca,Na)chabazite, as well as for the Pb-exchanged samples (Fig. 2, Table 3 ) , Na + must be exchanging rapidly between the same three sites regardless of whether the other cation is Na+, CJa*+, or Pb’+. Other charge-compensating cations would have to exchange at a similar rate in order to maintain a local charge balance.
The Clay Minerals Clay minerals are layered aluminosilicates consisting of sheets of cations which are tetrahedrally and octahedrally coordinated with oxygen atoms. In the 2: 1 clay minerals of vermiculite, montmorillonite, and hectorite (Fig. 7), there are two tetrahedrally coordinated sheets with one octahedrally coordinated sheet sandwiched between them to form a single layer. In the 1: 1 clay mineral of kaolinite, there is only one tetrahedrally coordinated sheet with an octahedrally coordinated sheet in a single layer. The exchangeable cations in the 2: 1 clay minerals are between the layers and close to the six-membered ring of the tetrahedral sheet coordinating to the adjacent layers and can also be inside the six-membered ring coordinating to only one layer (GWEN, 1988 ). Peaks in the 29Siand *‘Al MAS NMR spectra of vermiculite became more shielded after Pb exchange than before Pb exchange. The introduction of Pb into vermiculite affects the local chemical environment of the SiO* and AlO tetrahedra. On the contrary, no changes in peak position or width were observed in the 29Si and “Al spectra of montmorillonite, hectorite, or kaolinite after Pb exchange. Therefore, the exchangeable cation sites which are occupied by Pb are remote from the SiO, and AlO tetrahedra. The two peaks in the 23Na MAS NMR spectrum of hec-
3893
Ion exchange of Pb*+ by zeolites and clay minerills
FIG.7.Structure model of vermiculite based on SHIROZU and BAILEY(1966). Only portions of the six-membered rings (indicated by “4”) are shown in the tetrahedral sheet.
torite before Pb exchange represent two chemical environments. Polytypism and structural disorder usually occur in the clay minerals (GWEN, 1988 ) . Each peak, therefore, could mean that they actually represent many slightly differing chemical environments. The larger peak at -8 ppm represents environments which are less shielded and, hence, corresponds to the less confined interlayer environments. The intensity of this peak diminished considerably after Pb exchange, suggesting that Pb has occupied these sites. The smaller peak at - 19 ppm represents more highly shielded environments and is therefore attributed to the more confined sites in the sixmembered ring of the tetrahedral sheet. There were similarities among the 23Na MAS NMR spectra of the clay minerals (Fig. 5). A peak at about -8 ppm in all four species disappears after Pb exchange. As all the clay minerals have a similar interlayer site which is available for cation-exchange by Pb, this peak is assigned to this site. The clay minerals studied can exchange most of the Na+ cations in one single Pb exchange experiment. Another common feature in the 23Na MAS NMR spectra of the clay minerals was a broad, relatively symmetrical peaks between 0 and 20 ppm (Fig. 5). There was a slight difference in centre of gravity of these peaks, depending on the type of clay minerals, as follows: 8 ppm for 2: 1 clay minerals, and 6 ppm for the I : I clay mineral (Table 3 ) .
CONCLUSIONS The maximum Pb contents after ion exchange with 0.01 M Pb( N09)2 in Na-chabazite, (Ca,Na)-chabazite, vermiculite, montmorillonite, hectorite, and kaohnite was 27,7,16, 9, 9, and 0.4 wt%, respectively. Ion exchange processes reached equilibrium after 24 h in chabazite and vermiculite and in less in 5 min in montmorillonite and hectorite. Natural (Ca,Na)-chabazite was less efficient than Na-chabazite in taking up Pb from the solution. However, no such difference of the clay minerals with respect to different cation forms was observed. Calcite present in samples of the clay minerals precipitates Pb effectively as cerussite from the aqueous solution. 29Si and 27Al MAS NMR data showed that chabazite and vermiculite have structures in which exchangeable cations are close to the oxygens of the SiO., and Al04 tetrahedra. The other clay minerals do not display such close relationship. At room temperature, the chemical exchange in Na-chabazite is in a fast exchange regime on the NMR time scale, but this changes to a slow exchange regime at -80 to - 100°C. Three sites for the exchangeable cations can be identified in the low temperature 23Na MAS NMR spectra. Two exchangeable cations can be identified in the low temperature 23Na MAS NMR spectra. Two exchangeable cation sites were
3894 recognized completely
J.-J, Liang and B. L. Sheniff in hectorite. The less shielded site, which can be occupied by Pb, is present in each of the four
ciay minerals
studied.
day minerak
after Pb exchange
The residual
sodium
cation
is in environments
in all the which
differ slightly with respect to the different types of clay mineral. deknuwiedgmenr~-We
thank Ron Chapman for the electron microprobe analyses: Wayne Bbnski for f&ping with the moisture measurement; Kirk Marat, Terry Wolowiec, and Zhi Xu with the NMR experiments; and Mark Cooper and Peter Burns for helping with the computer graphics. Bruker Spectra Spin Canada Ltd. is thanked for the loan of the DOR probe. We benefited from the helpful comments fmm Dr. Tracy Tingle at UC Davis, Dr. Paul Schroeder at the Unive~~ty of Georgia, and an anonymous reviewer on the earlier version of this paper. The project was financed by Natural Science and Engineering Research Council by a University Research Fellowship and equipment operating grants to BLS. and by a University of Manitoba Graduate Fellowship to JL.
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89, 161-169. BRECK D. W. ( 1974) Zeolite Molecular Sieves. Wiley-Interscience. BRINDLEYG. W. ( 1980) Order-disorder in clay mineral structures. In Crystal Stru~ti~res ofCfa,v Minerals and Their X-ray ~dentl~cation (ed. G. W. BRINDLEYand G. BROWN), Chap. 2, pp. 125195. MSA. CALLIGARIS M., NARDIN G., RANDACCIOL., and CHIARAMONTI P. C. ( 1982) Cation-site location in a natural chabazite. Acra CON. B1,602-605. GUVEN N. ( 1988) Smectite. Rev. ~~inera~.19, pp. 497-559. HURLEUT C. S., JR., and KLEIN C. ( 1977) Manual ofMinera~ugv, 19th ed. John Wiley & Sons. MAES A. and CREMERSA. ( 1985) Highly selective ion exchange in clay minerals and zeolites. Amer. Chem. SW. Symp. Ser. 323,254295.
MAGI M., LIPPMAAE., SAMOSONA., ENGELHARDTG., and GRIMMER A.-R. ( 1984) Solid-state high-resolution *‘Si chemical shifts in silicates. J. Phys. Chem. 88, 15 18-l 522. MOORE D. M. and REYNOLDSR. C., JR. ( 1989 f X-ray Dlfiaction and the Idemi~c~ion and Anal&s of Clay minerals, Oxford Univ. Press. POUCHOUJ. L. and PICHOIRF. ( 1985)A new model for quantitative X-ray microanalysis. Rech. Aerosp. 1984-3, pp. 13-38. SAMOSONA. and LIPPMAAE. (1983) Excitation phenomena and line intensities in ~~-~lution NMR powder spectra of half integer quadrupolar nuclei. Phys. Rev. B28,6567-6570. SAMO~~N A., KUNDLAE., and LIPPMAAE. ( 1982) High-resolution MAS NMR of quadrupolar powders. J. Mugn. Resort. 49, 350357. SANDERSJ. K. M. and HUNTER B. K. ( 1987) Modern NMR spectroscopy: A guide for chemists. Oxford Univ. Press. SAND~TR~MJ. ( 1982) Dynamic NMR Spectroscopy. Academic Press. SHERRIFFB. L., GRUNDY H. D., and HARTMANJ. S. ( 1987) Ckcupancy of T sites in the scapolite series: A multinuclear NMR study using magic-angle spinning Canadian Mineral 25,7 17-730. SHERRIFFB. L.. GRUNDY H. D., and HARTMANJ. S. ( 199la) The ~lationshio between 29Si MAS NMR chemical shift and silicate mineral structure. Eur. J. Mineral. 3,X l-768. SHERRIFF B. L., GRUNDY H. D., HARTMANJ. S., HAWTHORNE F. C., and ~ERNL P. ( 199 1b) The incorporation of alkalis in beryl: A multinuclear MAS NMR and crystal structure study. Canadian Mineral. 29, 27 l-285. SHIROZU H. and BAILEYS. W. ( 1966) Crystal structure of a twolayer Mg-vermiculite. Amer. Mineral. 51, 1124-I 143. SRIVASTAVAS. K., TYAGI R., PANT N., and PAL N. ( 1989) Studies on the removal of some toxic metal ions: Part II (removal of Pb and cadmium by montmorillonite and kaolinite). Environ. Tech. Lett. IQ, 275-282. VAN OLPHEN 11. and FRIPIATJ. 1. ( 1979) Data ~and~~~~k~~rC/a-v Materials und Other fan-metallic Minerals. Pergamon. WEISS C’. A.. JR., KIRKPATRICK R. J., and ALTANERS. P. ( 1990) The structural environments of cations adsorbed onto clays: ‘33Cs variable-temperature MAS NMR spectroscopic study of hectorite. Geuchim. Cosmochim. Acta 54, 1655-1669. WHITE A. F. and YEE A. ( 1986) Near-surface alkali di~us~on into glassy and crystalline silicates at 25°C to 100°C. Amer. Chem. SK. .Swt>. 323, 587-598. WINSHIPK. A. ( 1989) Toxicity of Pb: A review. Adverse DrugReact. .4cute Poisoning Rev. 8, 117-l 52. WCIY., CHMELKA B. F., PINESA., DAVIS M. E., GROBET P. J., and JACOBS P. A. ( 1990) Hid-~~Iution “Al NMR spectroscopy of the aluminophosphate molecular sieve VPI-5. lvafure 346, 550552. ZAMZOW M. J., EICHBAUMB. R., SANDCREN K. R., and SHANKS D. E. ( 1990) Removal of heavy metals and other cations from waste water using chabazites. Sepurat. Sci. Tech. 25, i555-i 569.