Environ. Sci. Technol. iQQ0,24,867-873
Meier, J. R.; Blazak, W. F.; Knohl, R. B. Environ. Mol. Mutagen. 1987,10, 411-424. Rohm and Haas Co. Amberlite XAD Macroreticular Adsorbents. 1970. Ringhand, H. P.; Meier, J. R.; Coleman, W. E.; Schenck, K. M.; Kaylor, W. H.; Munch, J. W.; Robinson, M.; Kopfler, F. C. Proceedings of the Twentieth Mid-AtlanticIndustrial Waste and Hazardous Materials Conference. Howard University, Washington, DC, 1988; pp 420-433. Wilcox, P.; van Hoof, F.; van der Gaag, M. Proceedings of the XVIth Annual Meeting of the European Environmental Mutagen Society, Brussels, Belguim, 1986; pp 92-103. Kronberg, L.; Christman, R. F. Presented at the Fourth International Meeting of the International Humic Sub-
stances Society, Matalascana Beach, Spain, October 3-7, 1988. (18) Streicher,R. P. Studies of the products resulting from the chlorination of drinking water. Ph.D. Dissertation, University of Cincinnati, OH, 1987. Received for review August 1,1989. Accepted January 26,1990. EnThis document has been reviewed in accordance with U.S. vironmental Protection Agency policy and approved for publication. Approval does not signify that the contents necessarily reflect the views or policies of the Agency nor does mention of trade names or commercial products constitute endorsement or recommendation for use.
Immobilization Mechanisms in Solidification/Stabilization of Cd and Pb Salts Using Portland Cement Fixing Agents Frank K. Cartiedge,” Leslie G. Butler, Devi Chalasani, Harviii C. Eaton, Frank P. Frey, Esteban Herrera, Marty E. Tittlebaum, and Shou-Lan Yang
Departments of Chemistry, Civil Engineering, and Mechanical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803 We have investigated the behavior of Cd and P b salts toward cement-based solidification using TCLP leaching tests, conduction calorimetry, and solid-state NMR as a function of time. Concentrations of Cd in leachates are very low, while P b concentrations are considerably higher and would represent a serious threat to groundwater. The Cd/cement system involves Cd(OH)2,which provides nucleation sites for precipitation of calcium silicate hydrate (C-S-H) “gel”and calcium hydroxide, resulting in Cd being in the form of the insoluble hydroxide with a very impervious coating. On the other hand, the Pb/cement system involves hydroxide, sulfate, and nitrate mixed salts, which retard cement hydration reactions by forming an impervious coating around cement clinker grains. However, as pH in the cement pore waters undergoes fluctuations during the progress of hydration, the P b salts undergo solubilization and reprecipitation on leachable surfaces of the cement matrix.
Introduction Solidification/stabilization (S/ S) often uses cement and cement-like materials (and water if necessary) in the treatment of hazardous wastes that are in liquid or sludge forms to produce a solid for land disposal. In cases of some heavy-metal-containing wastes in which metal concentrations are too low for economical recovery, but high enough to represent a toxicity hazard, the technology appears both cost effective and safe, since the metals are converted into (or retained as) highly insoluble salts which do not leach into groundwater at appreciable rates. Nevertheless, with a technology that is so cheap and easy to apply, there is a great tendency to use it in cases in which its reliability could be questioned-particularly with mixed waste streams containing many components other than the metals that are likely to be effectively immobilized. There are in fact many reports in the cement literature of adverse effects (usually upon strength development) due to the presence of a variety of admixtures ( I ) , and such deleterious effects on the cement matrix may affect waste immobilization. Our initial efforts have been aimed at understanding as much as we can of the chemistry involved in cement paste 0013-936X/90/0924-0867$02.50/0
Table I. Metal Concentrations in TCLP Leachates recipea
portland 0.1 Cd(OH)2 0.3 Cd(OH)2 0.21 Cd(NO+
Ca 1120 2360 2240 2420 2000 2400 2570
ma/L leached Cd
0.014 0.026
Pb 0.055
0.026 0.043 0.014
0.1 Cd(OH)2 0.1 Pb(0H)z 70 0.3 Pb(0H)z 490 “The number cited is the weight ratio of waste to cement. All formulations used a 0.5 water to cement weight ratio and were cured for 28 days. This formulation contains soluble sodium silicate solution (type N) in a 0.05 weight ratio with respect to cement.
curing in the presence of a single additive, and the present paper will describe methods and results with type I portland cement undergoing hydration in the presence of cadmium and lead salts. The Cd and P b salts represent inorganic waste types with quite different characteristics. Cadmium salts appear to be very well immobilized by most cementitious S/S processes, while lead salts are much more likely to be troublesome. Table I contains data from application of the toxicity characteristic leaching procedure (TCLP) (2) to samples prepared from type I portland cement and lead or cadmium salts. While the cadmium salts, whether initially water soluble or water insoluble, are effectively immobilized, the lead hydroxide is not.
Experimental Methods Methods of formulation and handling of solidified samples using type I portland cement have been described in previous publications (3,4). The model Cd hydroxide and “Pb hydroxide” sludges were prepared by precipitation from aqueous solutions of the corresponding nitrates using calcium hydroxide. Lead nitrate (165.6 g, 0.5 mol) or cadmium nitrate tetrahydrate (154.2, 0.5 mol) in 2.0 L of deionized water was treated with 40.75 g (0.55 mol) of calcium hydroxide and the container capped and tumbled in a rotary agitator for 1 h. The resulting mixture was
0 1990 American Chemical Society
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filtered through Whatman GF/F glass microfiber filters with 0.8-wm openings at 50 psi in a stainless steel zeroheadspace extractor and dried in a vacuum desiccator. Filtration, transfer, and storage were done under an atmosphere of dry Nz in order to prevent carbonation. Metal analysis on the sludges indicated that the Cd sludge was essentially pure Cd(OH),, but that the P b sludge was a mixture of salts of Pb and Ca having 70% of the P b content corresponding to Pb(OH),. TCLP Analyses. The TCLP was carried out as described elsewhere (2). Extraction fluid no. 2, which is used for wastes that have a pH higher than 5.0 after digestion, was used for all extractions in the present cases. After the total leachate was collected, a representative sample (approximately 100 mL) was taken and digested with nitric acid by Standard Method 302D ( 5 ) . Metal analysis was carried out on a Jarrell-Ash AtomComp direct-reading inductively coupled argon plasma spectrometer (ICAP). NMR Data Acquisition and Analysis. Solid-state 27Aland 29SiNMR spectra were obtained with a Bruker WP200 spectrometer fitted with a Chemagnetics magic angle spinning (MAS) probe. The magic angle was set by observing the 81Brresonance in a sample of KBr spun at 5 kHz; the spinning angle was adjusted so as to minimize the intensity of the spinning side-band pattern (6). All samples were spun with compressed air at a rate of 5-6.5 kHz (7) in thick-wall 7.5-mm Torlon rotors containing ca. 150 mg of cement sample. The spin rate was measured with a high impedance probe and a Tektronix 2465 oscilloscope. A 3-ps pulse was used in the single-pulse 27Alexperiment, corresponding to a tip angle of 10' (solution state) (8, 9). Based on a set of spectra aquired with different relaxation delays, all A1 sites were found to have Tls much less than 1 s; therefore, the relaxation delay, 23 ms, was only the time required for digitization of the FID. The chemical shift reference was based upon AlC1, solution in the MAS probe (nonspinning). Silicon-29 T,s were measured by the inversion recovery method (18-ws 90' pulse) for cements, both newly prepared samples and those aged for 1year. The longest T1found was 1.5 s (10). Therefore, a relaxation delay of 5 s was used for cements with an age greater than 1 day; 5800 scans, corresponding to 8 h of instrument time, were acquired for each sample in a set of three. However, the 5-s relaxation delay was incompatible with the signal-averaging requirements for the 4-, 8-, 16-, and 24-h samples; thus the following strategy was used: for the 4-, 8-, and 16-h samples, relaxation delay was 1s and 5000 scans were acquired. For the 24-h sample, the relaxation delay was 5 s and 5800 scans were acquired for the first run; 2000 scans were acquired in runs 2 and 3. Chemical shifts are based on TMS as an external reference. The spectra have been analyzed with the NMRl program developed by New Methods Research, Inc., Syracuse, NY. An exponential multiplication weighting function of spectra, respectively, was 40 and 100 Hz for 29Siand applied, the data were Fourier transformed, and a base-line correction applied. Curve fitting (assuming Gaussian line shapes) was primarily used to obtain quantitative measures of the parameters (location, integrals, intensity, and line widths) associated with overlapping peaks. For spectra with low signal-to-noise ratios, the curve-fitting method tended to fit some peaks with anomalously large line widths, thus leading to erroneously large peak areas. Therefore 29Sispectra were also analyzed by a second method using simple integration (point by point summation of data point intensities within a given "window") on
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Table 11. Composition of White Cementnand Type I Portland (OPC)bUsed in This Work wt
wt%
070
oxide
white
OPC
mineral
white
OPC
CaO Si02 Al2O3 Fez03
67.6 23.4 3.6 0.3 0.6 2.5
65.3 20.7 4.6 3.2
C3S
65.3 18.3
66
MgO
so3
C2S
C3A C,AF
9.1
0.8
10 7 10
2.5
2.6
"Lehigh Cement Co., Waco, TX. bRiver Cement Co., S t Louis, MO.
the Bruker spectrometer. The problem with the window integration method is the difficulty in choosing separate windows for overlapping peaks in some spectra. Nevertheless, the agreement between the curve-fitting and window integration methods to within several percent gives us confidence that the data analyses are meaningful. 'H NMR Relaxation Time Measurements. In order to reduce the interference from paramagnetic relaxation, low iron content white cement was used in the relaxation time measurements. The mineral compositions of the white cement and the type I portland used in this work are shown in Table 11. The mixtures with cement were prepared in a 20-mL glass vial, stirred to apparent homogeneity, and then transferred to a 5-mm glass NMR tube, a process taking about 3-5 min. The first measurement in each case was made at 10 min after initial mixing of cement with water. In the cases involving additives other than cement, the individual amount of additive was weighed into the vial to make the desired additive/cement ratio, and then 10 g of cement and 5 mL of deionized water were added. For every sample two duplicates were measured, and the relaxation times are reported as the average of the two runs. Generally, the reproducibility is very good, deviation of relaxation times and magnetization fractions being within *5 5%. The proton relaxation times, T1and T,, were measured on a Bruker AClOO FT-NMR spectrometer operating at 100 MHz. The inversion-recovery method was used to measure T,, while the Carr-Purcell-Meiboom-Gill spin echo pulse sequence (11)was used to measure T,, The measured magnetization intensities, along with the various pulse delays, were transferred to an IBM mainframe (Model 3084) and analyzed by the curve-fitting subroutine ZXSSQ (International Mathematical and Statistical Library Manual, Vol. 4a). ZXSSQ solves nonlinear polynomial equations using a finite difference Levenburg-Marquardt algorithm (12). The data analysis yields a set of relaxation times, and the magnetization fraction being contributed by each component. In our experience, this program works very well when the relaxation times extracted differ by a factor of 5 or more. Conduction Calorimetry. Calorimetric measurements were made using a Calox calorimeter obtained from the Department of Metallurgy and Science of Materials, Oxford University. Calorimeter operation depends on the measurement of a small temperature difference between the specimen container and the heat sink, which is maintained at a constant temperature. Since the temperature difference is small, Tian's analysis (13)may be applied to determine two constants (k,and k,) characteristic of the calorimeter. Samples for calorimetry measurements were prepared outside the calorimeter in a 50-mL borosilicate glass beaker using 10 g of cement in each case. In each case solid components were added to liquid or sludge and then
hand-mixed for 2 min. After being mixed, the entire sample was weighed, a portion of it transferred to the calorimeter, and then the remainder weighed again. By use of Tian’s equation, voltage output was converted to rate of heat output. The total heat output up to any point was taken to be the area under the heat output curve and was calculated by using the trapezium rule. The two quantities are reported in watts per kilogram of cement used and kilojoules, respectively. In addition to the mixing method just described, the Calox calorimeter can mix dry cement with water and dissolved contaminants internally (after temperature equilibration) by suction under vacuum, thus allowing heat evolution to be monitored directly from time zero. The reaction cell of the calorimeter is loaded with a carefully weighed sample of dry cement and sealed. A measured amount of water, either alone or containing soluble silicate, is poured into the thistle funnel and stopcock assembly entering the reaction cell, and the entire calorimeter is immersed in a constant temperature bath at 35 f 0.01 “C. A vacuum is applied to the reaction cell until the device reaches temperature equilibrium (1-3 h). At this point the stopcock is opened, allowing mixing to occur, and the subsequent exotherm is recorded.
Results The data being generated are of several complementary types, as will be illustrated for water/cement mixtures without wastes before proceeding to systems containing wastes. The 29Siand 27Alspectra contain peaks, often overlapping ones, that correspond to Si in five different environments and A1 in two different environments. The 29Sichemical shifts depend mainly on the degree of condensation of silicon-oxygen tetrahedra. The symbol, Q, represents a Si atom surrounded by four oxygen atoms. A superscript following the Q shows the number of other Q units attached to the Si atom under study. The principal transformation occurring during cement hydration, as followed by NMR, is the formation of chains containing Q’ and Q2 units starting from the orthosilicate ions (QO) present in the cement clinker (14,15). The loss of Qounits as a function of time is a measure of the overall degree of hydration of the cement clinker and agrees well with calorimetric measurements of degree of hydration. Figure 1shows integrated intensities obtained by the curve-fitting procedure described under Experimental Methods. The results agree with previous observations that the mature cement paste contains mainly Q’ units, with a much smaller proportion of Q2 units, and litte or no Q3 or Q4(14, 15).
Hydration of aluminate phases, which account for roughly 10% of the cement clinker, occurs more rapidly than that of the silicate phases. The chemical shift differences between the four-coordinate A1 atoms present in the clinker and the six-coordinate Al atoms in the hydrated aluminates have been followed as a function of time in the present work. Figure 1also shows integrated intensities for the 27AlNMR spectra for cement pastes, which agree with published data for hydration of pure tricalcium aluminate (16). In all samples of cement clinker examined, both four-and six-coordinate AI are present initially, and different lots from different manufacturers showed wide variation in the ratio of the two types of A1 peaks. Hydration always converts the mixture completely to sixcoordinate Al. The NMR relaxation time measurements reported here are a means of defining the number of different environments for hydrogen atoms (contained in water molecules or OH- ions) that are present in the cement paste, and the
proportions of the total in each environment. The measured parameter is the relaxation time of the hydrogen nuclei obtained by pulse NMR techniques and computer graphical data analysis referred to as spin grouping (17). These techniques have been applied to cement hydration previously (18-21). Figure 2 shows the time evolution of spin-lattice relaxation time (Tl) and the magnetization fraction (equivalent to percent composition based on numbers of H atoms) due to each component during the hydration of white (low iron content) cement containing Cd(OH), and “Pb(OH)2”. The results shown here are typical of those reported previously and indicate the presence of three main phases with different Tls. The identification of the several T1contributors with specific phases present during cement hydration is based on the following considerations: the magnetization fraction compared with the expected phase composition of hydrated Portland cement, the time of appearance of the phases, and the magnitudes of the Tls themselves. The absolute magnitude of TIfor pure portlandite (calcium hydroxide) can be compared to that for the calcium hydroxide phase being produced during cement hydration, and agreement is reasonable. In addition to chemical environment of the proton, which determines the number and distance of near neighboring atoms, relaxation times may be determined by the surface area of the phase and by the presence of atoms with unpaired spins. The latter mechanism is particularly pronounced in mineral phases containing iron, and hence the need to employ white cement in the present studies. The phases that have been identified by others (18-21), and with which we concur, are identified in Figure 2. For the first 5 h, only water molecules that are unbound or loosely bound to surfaces are present. The appearance of calcium silicate hydrate (C-S-H) “gel” coincides with the end of the “dormant period” in cement hydration, and the first identification of C-S-H 5 h into hydration shown in Figure 2 for the Cd-containing samples is also typical of the time required for first observations of crystals with the C-S-H morphology in microscopy experiments. Calcium hydroxide begins to appear several hours later. Calorimetric measurements have been made with two different kinds of mixing: hand-stirring the paste to apparent homogeneity and then pouring it into the calorimeter, or sucking water into the calorimeter under vacuum. The former technique is more typical of actual practice but does not allow the measurement of the first exotherm during the first few minutes of hydration. In the results reported below, heat evolution data at short hydration times (up to 30 min) have been measured by the vacuummixing technique, while the data over longer times have been obtained from hand-mixed samples. Calorimetry has been applied to cement pastes by many workers (22-25), and the results shown here (Figure 3, open squares) are typical. Cadmium Salts. The presence of 10% Cd(OH), results in several changes from the observations made for cement alone. Calorimetry shows an initial heat evolution that is -10% greater than that for cement alone, but NMR (Figure 1)does not show a significant change in the rate of aluminate hydration. The dormant period ends at - 4 h, which is approximately an hour earlier than for cement alone, as indicated by calorimetry. During the period up to -10 h, relaxation time changes are almost identical with those observed with cement alone. After this time there are several indications of minor differences in the pattern of hydration compared to cement alone. Calorimetry shows a decline in the rate of heat output until the cumulative output falls slightly below that for cement alone Environ. Sci. Technol., Vol. 24, No. 6, 1990
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5 - 2 9 NMR
W/C = 0.5/1 .O 100-
I
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0-0
00 p1
A-A
o2
0-0
\
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I
1
..
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-
I
0 -
\.
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&[6]
-
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- ,
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5 - 2 9 NMR
W/C/Cd(OH)2
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= 0.5/1.0/0.1
0-0
00
W/C/Cd(OH)p
I 60
0
1
A/.,
100
10
Si-29 NMR
W/C/Pb(OH)2
\O/O-*
-
&[4] &[6]
lo00
TIME (Hours)
--I
0-0
0-0
- ,
3
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= 0.5/1.0/0.1
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\
-
\
+
/
' 0
3 TIME (Hours)
\ I
\-I
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100
10
1000
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Figure 1. ?Si and ,'AI NMR peak integration data as a function of time Top: OPC alone. Middle: OPC containing 10% by weight Cd(OH),. Bottom: OPC containing 10% by weight Pb(OH),.
at -18 h and continues to be lower up to 2 days. From 24 h through 3 days NMR shows little change in the percentage of silicate hydration that has occurred. At 28 days magnetization fractions indicate less calcium hydroxide and more C-S-H than for cement alone, and Y3i NMR shows more Q2 and less Q' compared to cement. A second set of experiments was performed using cadmium nitrate in an amount to give approximately the same molar ratio of Cd to cement as in the cadmium hydroxide case. Several differences were observed at early hydration times between the nitrate and hydroxide cases. Calorimetry showed an additional exotherm from 6 to 12 min and a greater cumulative heat output in the first 30 min compared to either cement alone or cement + Cd(OH)2. NMR showed that aluminum hydration was somewhat more advanced at the 4-h measurement than for either cement alone or cement + Cd(OH),. Relaxation time measurements showed the first appearance of C-S-H at 4 h, rather than 5 h with cement or cement + Cd(OH),. Calorimetry showed the end of the dormant period at about the same time as with cement + Cd(OH)2,but the 870
Environ. Sci. Technol., Vol. 24,
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peak decayed much more rapidly, so that the cumulative heat output fell below that for cement + Cd(OH), at 6 h and only gradually caught up over the first 2 days. Both kinds of NMR measurements indicated a 28-day composition like that of cement Cd(OH),; namely, more C-S-H and more Q2 units compared to cement alone. Lead Hydroxide. In contrast to the relatively minor changes induced by the Cd salts, those produced by 10% lead hydroxide are very striking. The first calorimetric peak (Figure 3) is less exothermic than for cement alone, although a small new peak occurs from 2 to 10 min, and by 15 min the cumulative heat evolved has caught up with that from the control. At 10 min into the hydration reactions, a second hydrogen environment has already appeared in the relaxation time measurements (Figure 2, closed diamonds). The dormant period is greatly extended. A reasonably normal second calorimetric peak occurs, but only after 55 h. The retardation is even more pronounced in the Si and A1 NMR measurements (Figure 1, bottom). Silicate hydration appears to proceed to 10% completion between 8 and 16 h, but then stops and does not recom-
+
-
OPEN = Cd CLOSED = Pb
,--.
E
v
+0
lo!
*.
Exchangeable C-S-H Ca hydroxide Unidintified
' tI :
1E-14 1E-1
i
1
10
100
1000
TIME (hours)
L
Exchangeable C-S-H A Ca hydroxide
0
' I
LL
0
0
V
f
v J 2O-c
{E- 1
v
' I v
?
?
1
10
100
1000
TIME (hours)
Figure 2. Proton relaxation time measurements for white cement prepared with a 0.5 waterhment ratio and containing 10% by weight Cd(OH), (open symbols) or 10% by weight Pb(OH), (closed symbols). Top: time evolution of T , components. Bottom: time evolution of magnetization fraction.
mence until after 3 days. Aluminate hydration is also retarded for 3 days. According to the 29SiNMR results, the percent hydration of the silicate phases has caught up with the control at 28 days, but the proportion of Q2 is high and that of Q1 is low in the 28-day paste. Indeed, changes continue to occur more rapidly over the first year of hydration in the presence of Pb, resulting in a matrix that is much higher in proportion of Q2units than cement alone. The relaxation time measurements on white cement show percent hydration at 28 days still lagging somewhat behind. In the latter experiment, compared to cement alone the proportion of "exchangeable" water is higher and the proportions of both C-S-H and calcium hydroxide are lower. Calorimetric and relaxation time measurements have also been made in the presence of Pb(NO&, and there are some differences worth noting. As with the soluble Cd salt, the soluble Pb salt produces an enhanced heat output in the first few minutes. This is followed by the appearance of a second peak with a maximum of -20 W/kg centered at -6 min, which is equivalent in time and intensity to that observed in the Pb(OH), case. Retardation of set still occurs with the nitrate, but it is not so dramatic as with the hydroxide. The end of the dormant period in the presence of Pb(NO& occurs at 5-6 h into the reaction, which is only an hour or so later than for cement alone, but the accompanying heat output peak is lower and much broader than for the control.
Discussion There are many chemical differences between P b and Cd that might be expected to affect performance, and particularly leachability, in a cement-solidified product.
The precipitation behavior for soluble Pb2+and Cd2+is quite different. In basic aqueous solutions, cadmium precipitates rather cleanly as Cd(OH), (26);on the other hand, treating a solution of Pb2+with base results in a complex mixture normally containing PbO, Pb(OH),, and the mixed salt PbOPb(OH), (27). Furthermore, P b is appreciably amphoteric and Cd is not (28,29). Lead can combine with hydroxide ion to form the complex ion Pb(OH)
