Environ. Sci. Technol. 1991, 25, 1 17 1- 1 174
(11) Whittaker, E. L.; Akridge, J. D.; Giovino, J. Two Test Procedures for Radon in Drinking Water Interlaboratory Collaborative Study; EPA/600/2-81/082; U.S.Environmental Protection Agency, Environmental Monitoring Systems Laboratory: Las Vegas, NV, 1989. (12) Campisano, C. D. M.S. Thesis; University of New Hampshire, Durham, NH, 1987.
Agreement CR812602 between the U.S. EPA Risk Reduction Engineering Laboratory, Cincinnati, OH; the New Hampshire Department of Environmental Services, Concord, NH; and the University of New Hampshire Environmental Research Group, Durham, N H . K i m R. Fox served as the US.EPA Project Officer. Although the research described in this article has been funded by the Risk Reduction Engineering Laboratory, US. Environmental Protection Aeencv through a Cooperative Agreement, nu official endorsement of trade &mes or commercial products mentioned should be inferred. I
Received for review October 12, 1990. Accepted February 15, 1991. The work for this project was performed under Cooperative
"
Leaching Effects on Silicate Polymerization. An FTIR and 29SiNMR Study of Lead and Zinc in Portland Cement J. Dale Ortego' and Yudlth Barroeta
Department of Chemistry, Lamar University, Beaumont, Texas 777 10 Frank K. Cartledge' and Humayoun Akhter
Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803 Portland cement samples do ed with lead and zinc nitrate have been investigated by Si solid-state NMR and Fourier transform infrared spectroscopy. Results indicate that silicate polymerization is slightly enhanced with lead doping and retarded in the presence of zinc. Leaching studies reveal that silicate polymerization occurs when the samples are exposed to acidic leaching media. The degree of cross-linking is directly proportional to the acidity of the leaching solution, being pronounced when pH 5 buffers are employed. Values for the u3 and u4 silicate infrared bands and the chemical shifts of the NMR-active 29Si nuclei (Q0-Q4)are tabulated and discussed in relation to the chemical binding and leaching mechanisms of the solidified metals.
28
Introduction Solidification/stabilization (S/S) often uses cement and related materials in the treatment of hazardous wastes that are in liquid or sludge forms to produce a solid for land disposal. Key issues in assessing the effectiveness of the treatment are the durability of the matrix produced and its resistance to leaching under a variety of conditions. Important concerns in S/S studies therefore include characterization of the interactions of binding agents with the hazardous material and the effects of leaching solutions on the solidified matrix. Since groundwater may be acidic, and since cementitious matrices are particularly affected by acidic aqueous solutions, it is important to define the effects associated with acidic leaching. The present paper describes spectroscopic studies of cement matrices doped with several heavy metals, with studies carried out both before and after acidic leaching. Solid-state 29Sinuclear magnetic resonance (1) and Fourier transform infrared spectroscopy (2) have been shown to be highly effective tools in elucidating the chemistry of solidified hazardous materials. The chemical shift of a 29Sinucleus in silicates is dependent on the nature of the X group in SiOX. The chemical shift ranges for orthosilicate, sio44-(BO), terminal Si(OSi)033-(Ql), internal Si(OSi)2022-(Q2),branching Si(OSi),O- (Q3),and cross-linking units Si(OSi)4 (Q4) have been documented (31, and the correlations have been shown to be applicable to curing cement pastes ( 4 , 5 ) . In addition, the u3 (as?metric S i 4 stretching) and u4 (out-of-plane Si-0 bending) silicate IR vibrations can be correlated to varying degrees 0013-936X/91/0925-1171$02.50/0
of condensation (6). We report here the effects on silicate polymerization when portland cement samples containing lead and zinc are allowed to cure and are then subjected to acidic leaching solutions. Experimental Section Sample Preparation. Type I portland cement (OPC) produced by the Ideal Cement Co., Denton, TX, was used to prepare all samples. The water/cement (w/c) ratio for the standard sample was 0.4. For the lead-doped samples (Pb-OPC), 1g of lead nitrate and 9 g of dry cement (62 560 ppm Pb) were combined with 4 g of H20 (w/c = 0.44). The zinc samples (Zn-OPC) were prepared by using Zn(N03),.6H20(15400 and 30800 ppm) with a water/cement ratio of 0.44. Samples were prepared in glass vials under stirring with a glass stirring rod until apparent homogeneity had been reached. There is, of course, some question as to whether this process is representative of either completely homogenized systems or of S/S practice in the field. Nevertheless, the process does result in samples that give very reproducible spectral characteristics and leaching properties. The vials were sealed and maintained at room temperature during curing. When spectra were taken on leached samples, the solid residue after leaching was filtered and dried at 90 "C for 2 h. Infrared Spectral Data. Infrared spectra were recorded in the 400-4000-~m-~ region with an IBM Model 44 FTIR instrument. Samples were ground if necessary and pressed in KBr disks. NMR Data. Solid-state 29SiNMR measurements were made with a Bruker MSL-200 spectrometer. All samples were spun in thick-wall Zirconia rotors holding a sample volume of -300 mg of solid. Samples were spun with compressed nitrogen at a rate of 3.5 kHz. Chemical shifts are reported relative to tetramethylsilane (TMS). See ref 1for more details. In the present studies a relaxation delay of 5 s and 1000 scans resulted in a signal to noise ratio of 30-50. Reproducibility of spectral integrations among separately prepared samples is within the range of &8%. Leaching Studies. Toxicity Characteristic Leaching Procedure. Both lead and zinc samples were subjected to the toxicity characteristic leaching procedure (TCLP) as defined by the US.EPA (7).The process was modified only in that the sample size was 10 g. The leaching solution employed was 0.04 M acetic acid with an initial pH of 3.0. Owing to the basic nature of hydrated
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Table I. %i Chemical Shifts and Assignmentsafor Leadand Zinc-Doped Portland Cement Samples assignments' sampleb OPC clinker
Q'
Q0
-71.2 (- 100) OPC (28 days) -71.5 (32) OPC (12 mo) -70.8 (9) Pb-OPC (28 days) -71.4 (26) Pb-OPC (12 mo) -70.9 (6) Pb-OPC TCLP -71.4 (18.4) Pb-OPC buffer Zn-OPC (5 mo) -71.4 (100) Zn-OPC (14 mo) -71.5 (22.9)
-79.5 (37.2) -78.9 (54) -80.0 (43.3) -79.4 (48) -82.5 (36.8)
Q3
Q2
Q4
-84.5 (30.9) -84.6 -94.6 (35) (2) -85.1 (32.5) -85.0 (46) -84.8 (44.8)
Zn-OPC -101.6
-79.7 -84.6 (51.2) (25.9)
portland cement, the final pH after 20 h of leaching was usually near 11-12. Buffer. A sodium acetate-acetic acid buffer, pH 5, was used to compare results to the unbuffered acid described above. During the entire leaching period the pH remained at 5 f 0.1. Apart from the nature of the leaching solution, the conditions were the same as those specified in the TCLP, including the same ratio of leaching solution to solid sample.
Discussion In both unhydrated cement (OPC clinker; OPC, ordinary portland cement, i.e., Type I) and tricalcium silicate ((2,s)the predominant silicon species is the orthosilicate, Si04&(Table I), with a single NMR resonance line located at -71 ppm relative to TMS. In hydrated OPC, solid-state =Si NMR shows three bands due to orthosilicate, terminal Q', and internal Q2,units. This is indicative of the formation of dimers followed slowly by formation of short linear and cyclic polymers. The following generalized reactions represent the basic hydration process. [SiO4I4- xH20 [Si03(OH)]3-+ [Si02(OH)2]2+ [SiO(OH),]- + yOH-, etc.
-
0-02[Si02(OH)$2-
3 [Si02(OH),] 2-
-
I I
--c
HOSiiSiiH
I h0-
+
H20
[HO(0,)SiOSi(0,)OSi(02)OH]6-+ 2H20 The main products of portland cement hydration, calcium hydrosilicates of the C2SH2and CSH types, are detected in FTIR spectra from the u3, Sio4" (asymmetric Si-0 stretching) bands located at 1000-980 cm-', v4 (out-of-plane Si-0 bending) at 525-546 cm-l, and v2 (inplane Si-0 bending) at 457-464 cm-' (8-10). The intensity of the bending modes is dependent upon the degree of hydration in portland cement. Normally, a high degree of condensation lowers the intensity of these peaks. As polymerization increases there is also a shift in us to higher energy (6). If one compares the silicate FTIR spectra of OPC, OPC containing Pb(N03), (Pb-OPC),and the PbOPC samples after leaching (TCLP and buffer), it is ap1172 Envlron. Sci. Technol., Vol. 25, No. 6, 1991
assignmenta sample OPC clinker
OPC Pb-OPC Pb-OPC (TCLP) Pb-OPC (buffer) Zn-OPC (5 mo)
Relative area percent in parentheses. bSee Experimental Section for sample description. Length of cure is indicated in parentheses. 'Values listed are ppm relative to TMS; see text for structures.
+
Table 11. Silicate FTIR Spectral Data
v3
996 (shIb 937 (SI' 877 (sh)b 846 (sh)C 976 (vs) 988 (vs) 1040 (br) 1042 (s) 937 (sh) 923 (9) 870 (s) 832 (sh) 1003 (br)
*I
u2
522 (s)
462 (m)
534 (m) 530 (m) 520 (sh) 520 (sh) 520 (s)
462 (w) 461 (w) 462 (m) 469 (m) 463 (m)
544 (m)
455 (w)
a vz, in-plane bending; us, asymmetric Si-0 stretch; u4, out-ofplane bending; w, weak; m, moderate; s, strong; vs, very strong; sh, shoulder; br, broad. Assigned to C,S. Assigned to C3S.
parent that the major band, v3, is shifted (Table 11). The center of this very strong band in OPC is located at 976 cm-' and shifts to 988 cm-l in lead cement. When subjected to the acetic acid leaching solution for the TCLP test the band is broadened and shifts to even higher energy at 1040 cm-'. When the pH 5 buffer solution is used for leaching, several weak bands between 600-700 cm-' are apparent, u2 increases in intensity, and u4 is barely visible as a shoulder. The intense v3 peak shows some structure, with two closely spaced absorptions centered near 1045 cm-'. The above vibrational changes are indicative of increased polymerization of silicate units. Also, the intensity of v4 is always greater than that of u2 in the unleached samples. However, after leaching the relative intensities of these bending modes is reversed. Apparently the out-of-plane deformation (u4) becomes more restricted as hydration, and the proportion of Si-0-Si linkages, increase. Although lead nitrate initially retards cement setting, the effect is overcome after 4 weeks, and silicate condensation is actually enhanced compared to OPC pastes. Also in contrast to OPC pastes, the Pb-OPC samples show substantial continued silicate condensation up to at least 1 year of curing, at which time the proportion of Q2units is much higher than in OPC. A comparison of the relative intensities (area percent) of Qo,Q', and Q2,is shown in Table I. Values from Table I show a higher percentage of Qoin OPC relative to Pb-OPC, while the chain-end units and internal units (Q'and Q2)are higher in the lead-doped cement. These results are in accord with the implications of the IR band shifts noted above. An entirely different result is obtained when zinc is solidified in portland cement. After 5 months, the Zn-OPC sample shows IR bands characteristic of the cement clinker and only a single 29SiNMR band due to the orthosilicate species, Qo,at -71.4 ppm. The material has no strength and crumbles easily when handled. Also, when the PbOPC and Zn-OPC samples are allowed to dry in open containers at room temperature, the lead samples lose N 11% of the initial water added compared to over 60% for the zinc samples. This is further evidence that most of the water in Zn-OPC does not react with the clinker to form the normal hydration products. However, zinc samples kept in a closed container over 14 months do harden and NMR bands due to Qo (22.9%), Q' (51.2%), and Q2 (25.9%) are observed (Table I). However, the relatively high proportion of Q' indicates that while the percent hydration can be considered to be relatively high because orthosilicate units are consumed, the degree of polymer-
I\
11
28 DAY CURE
5 MONTH CURE
14 MONTH CURE
AFTER LEACHING (TCLP)
\
l
,
,
-50
,
,
.
,
l
,
,
,
,
Flgure 1. %ii MAS NMR spectra of cement samples containing 10% by weight Pb(N03)2. Top, cured 28 days; middle, after TCLP leaching; bottom, after leaching with a pH 5 buffer.
ization of silicate units is low. When the lead samples are leached, further changes occur in the silicates. These reactions can be conveniently monitored by solid-state NMR (Figure 1 and Table I). Comparison of Pb-OPC before and after the TCLP test shows that leaching causes Q2to increase (44.8 vs 32.5%) and Qo to decrease (26 vs 18.4%), indicating increased condensation. More drastic changes occur when the cement is exposed to a pH 5 buffer solution for 20 h. Under these constant acidic conditions the silicates react to form highly branched structures, with Q4being the predominant species. The spectra are now comparable to those obtained for amorphous silica (II), but shifted somewhat to lower field with the broad peak overlapping a portion of the Q3 region as well as Q4. Figure 2 shows that the same transformations as a result of pH 5 leaching occur with the Zn-OPC samples.
Summary Silicate hydration in portland cement occurs under highly basic conditions due to the formation of Ca(OH)2. The reaction between the orthosilicate ion and water also produces OH-. The degree of polymerization, therefore, is restricted by the high hydroxide ion concentration (pH = 12-13), with only short linear polymers forming. When the solid cement is contacted by acid, calcium hydroxide is neutralized and dissolved, exposing the silicates to aqueous hydrogen ion. This is an important aspect with regard to solidified hazardous metals. If the metal ions (Lewis acids) form coordinate covalent bonds with the silicate oxygens (Lewis base sites) they are in direct competition with the stronger Lewis acid H+(aq). The toxic metal ions will be displaced and dissolved in the leaching solution. The reactive silanol groups so formed then condense, producing longer and/or branched silicate structures as illustrated: X3OX + H+(aq) 5SiOH + X+(aq) or )SQX
+ 2H * ( a 4
,
I
,
,
,
,
/
,
-300 PPM
-
,
AFTER LEACHING (BUFFER)
-100 PPH
-50 I
\
,/
AFTER LEACHING (BUFFER)
)Si(OH)2
+ X2'(aq) etc.
Figure 2. %i MAS NMR spectra of cement samples containing 10% by weight Zn(NO,),. Top, cured 5 months: middle, cured 14 months; bottom, after leaching with a pH 5 buffer.
where X is calcium, potassium, sodium, or toxic metal ions. Then &(OH), branched and cross-linked silicates
-
If the metal ions are not chemically bonded to the silicate oxygens, and are present in some other form such as sulfate, phosphate, oxide, hydroxide, sulfide, etc., then the leaching rate will be determined primarily by the solubility of the metal compound in acid media. Since the alkalinity of portland cement neutralizes the acid during the TCLP test, the above reactions are significant only initially when the pH is below 7; and presumably any metal that is initially solubilized may be reprecipitated as an hydroxide when the leaching solution turns basic. However, when buffers are employed, the pH remains near 5; solubilized metal ions stay solubilized; and the branching and crosslinking reactions of silicates occur extensively. The latter process destroys the cement matrix. Thus, the effectiveness of the immobilization process is compromised in two respects: (1) the chemistry of the system is now that characteristic of an acidic medium where most toxic metals of interest are more soluble, and (2) any encapsulation effects associated with the cement matrix are drastically reduced. Not surprisingly, the percentage of solidified metal leached during a buffered extraction is extremely high compared to the TCLP protocol. The results reported in this paper are clearly relevant in devising guidelines for sound practices for environmental protection when applying S/S technologies. The effects of P b and Zn on cement matrices clearly suggest that carrying out a single evaluation of leaching potential on a sample that has been cured for 28 days may well miss seeing the long-term effects associated with the presence of those metals. The results are also illuminating about the behavior of cement-stabilizedmaterials when subjected to the standard U S . EPA regulatory instrument for measuring leachability, the TCLP. As is well-known, acidic aqueous solutions will eventually completely destroy the mineral structure characteristic of hydrated portland cement, as shown once again in this work in the 29SiNMR spectra of the samples subjected to leaching by a solution buffered so as to maintain pH 5. In contrast, the TCLP Environ. Sci. Technoi., Voi. 25, No. 6, 1991
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leaching conditions have only barely initiated this process of conversion of calcium silicate hydrates into something like amorphous silica. In our experience with many different classes of hazardous wastes solidified in portland cement and subjected to the TCLP, the pH of the TCLP leachates varies considerably depending on the kind and quantity of waste solidified. Presumably this means that the breakdown of the cement matrix will have proceeded to varying degrees. Thus, TCLP results, while perhaps having some value for regulatory purposes, are extremely difficult to interpret in terms of waste-cement interactions. Registry No. Zn(NO,),, 7779-88-6 Pb(NO&, 10099-74-8;Cd, 7440-43-9; Cu, 7440-50-8; Pb, 7439-92-1; Zn, 7440-66-6; Sodium dodecylsulfate, 151-21-3.
Literature Cited (1) Cartledge, F. K.; Butler, L. G.; Chalasani, D.; Eaton, H. C.; Frey, F. P.; Herrera, E.; Tittlebaum, M. E.; Yang, S.-L. Environ. Sei. Technol. 1990, 24, 867-873. (2) Ortego, J. D.; Jackson, S.; Yu, G.; Cocke, D.; McWhinney, H. J . Enuiron. Sci. Health, Part A 1989, 24, 589-602.
Lippmaa, E.; Magi, M.; Samson, A.; Englehardt, G.; Grimmer, A. R. J . Am. Chem. SOC.1980, 102,4889-4893. Clague, A. D. H.; Clayden, N. J.; Dobson, C. M.; Hayes, C. J. J . Mater. Sei. Lett. 1985, 4, 1293-1295. Muller, D.; Rettel, A,; Gessner, W.; Scheler, G. J . Magn. Reson. 1984, 57, 152-156. Bensted, J.; Varma, S. P. Cem. Technol. 1974,5,440-450. Fed. Regist. 1986,51,21672-21692 (No. 114, Friday, June 13, 1986). Lazarev, A. N. In Vibrational Spectra and Structure of Silicates; Farmer, V. C., Ed.; Consultants Bureau: New York, 1972; Chapter IV. Etchepare, J. Spectrochim. Acta 1970, 26A, 2147-2154. Tarte, P.; Pottier, M. J.; ProcBs, A. M. Spectrochim. Acta 1973,29A, 1017-1027. Hjorth, J.; Skibsted, J.; Jakobsen, H. J . Cem. Concr. Res. 1988, 18, 789-798.
Received for review June 19, 1990. Accepted February 14, 1991. This research was supported by grants from the Gulf Coast Hazardous Substance Research Center (to F.K.C. and J.D.O.), Beaumont, T X , and the Robert A. Welch Foundation (toJ.D.O.), Houston, T X .
Significance of Atmospheric Inputs of Lead to Grassland at One Site in the United Kingdom since 1860 K. C. Jones*,+ and A. E. Johnston*
Institute of Environmental and Biological Sciences, Lancaster University, Lancaster, LA 1 4YQ, UK, and Rothamsted Experimental Station, Harpenden, Hertfordshire, AL5 2JQ, UK
Herbage receives P b from the soil and the atmosphere. Studies on long-term experimental plots established at Rothamsted Experimental Station (southeast England) since the mid-1800s provide evidence that (i) atmospherically derived P b predominates over soil-derived P b in herbage collected from this rural UK location, (ii) changes in herbage P b concentrations have occurred over the last century, (iii) recent temporal trends in herbage P b concentrations reflect known changes in annual air P b concentrations, and (iv) the enforced reduction of P b added to petrol at the end of 1985 resulted in a decline in herbage P b concentrations in rural UK locations.
W
Introduction Lead enters the environment from many sources and is dispersed through environmental compartments along a variety of pathways. Humans are thus exposed to Pb via numerous, complex routes. One of the most important of these is the deposition of airborne Pb to agroecosystems, and its subsequent incorporation into crop-based foodstuffs and livestock. Hutton and Symon (1) have identified atmospheric deposition as the major contemporary source of P b to agricultural land in the United Kingdom, which reflects calculated worldwide inputs to soils (2). By assuming 5.1 million hectares of arable land in the United Kingdom and an average annual deposition rate of 300 g of Pb/ha [a value representative of rural areas in the early 1980s (3)],Hutton and Symon (1) calculated ca. 1540 t of Pb/year would enter UK arable land via atmospheric deposition. Sewage sludge was the only other significant source of Pb, amounting to an input of ca. 100 t/year (1). + Lancaster f
University. Rothamsted Experimental Station.
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However, since sewage sludge is only applied to 1% of UK arable land the major source of Pb to such land is atmospheric deposition. Furthermore, because Pb is extremely persistent in surface soils, the contemporary Pb content of agricultural soils may be strongly influenced by cumulative Pb inputs via atmospheric deposition. The median total Pb content of soils in England and Wales is 35-48 mg/kg and 20 mg/kg in Scotland (see ref 4). It has been estimated that of this 17 mg/kg may have resulted from cumulative Pb deposition over the last few centuries (5). Lead is not readily taken up from soils by plant roots, and so the effects of an underlying, substantial long-term increase in the soil Pb burden will not necessarily increase the P b concentration in plant tissues. Concentration factors (the ratio of plant P b to soil Pb) are typically 0.001-0.03 (6). However, this does not imply that atmospheric inputs are unimportant with respect to the Pb content of vegetation. Several studies have indicated the importance of atmospheric P b deposition directly onto foliage as a contributor to the total P b burden of the plant (5, 7,8). Tjell et al. (9) reported that more than 90% of Pb in grass was derived from atmospheric deposition at a rural site in Denmark. From a review of published literature, Chamberlain (5)concluded that if the atmospheric Pb concentration is between 0.1 and 0.2 pg/m3, then at least 2 mg/kg or 3&70% of Pb in grass at rural sites could be attributed to atmospheric Pb. Harrison and Chirgawi (7) grew plants in filtered and unfiltered air to quantify the atmospheric contribution to the total Pb content of the plants. Four different soils ranging in pH from 6.4 to 7 . 2 and total Pb concentrations of 11-41 mg/kg were used, with air P b values in the unfiltered cabinet of 110 ng/m3. The study clearly showed that, for a range of crop plants, the atmospheric contribution to total Pb in the above
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