Environ. Sci. Technol. 1999, 33, 312-317
Synthesis of Double-Layer Silicates from Recycled Glass Cullet: A New Type of Chemical Adsorbent MICHAEL W. GRUTZECK* AND JUDITH A. MARKS Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802
Curbside recycling of glass bottles and jars has been extremely successful. Those glasses that are sorted by color are a marketable commodity; those that are not have no immediate commercial value and must be disposed of in landfills. With proper chemical treatment, however, mixed-glass cullet can be transformed into claylike chemical adsorbents. By mixing ground glass cullet with either alkali hydroxide or alkali carbonate solutions one is able to form a hydrous double-layer silicate known as rhodesite (NaKCa2[Si8O19]‚5H2O). Tests of the adsorptive and cation exchange properties of glass cullet derived materials have shown them to have properties comparable to natural clays and zeolites. Whereas natural materials tend to become “sticky” and/or lose their granularity when wet, rhodesitebased adsorbents do not.
Introduction Curbside recycling of glass containers in Pennsylvania has been extremely successful. Individual compliance is very high. Those glasses that are sorted by color are a marketable commodity, normally in short supply. However, much of the glass, especially that collected by small municipalities, is not sorted by color. This practice poses a problem as much of this particular glass cullet has no immediate commercial value and must be disposed of in municipal landfills. In Pennsylvania alone, the annual tonnage of mixed-color glass cullet is estimated to exceed 25 000 tons.
Objective To address this problem, it was proposed to find alternate uses for mixed-glass cullet. Previously published work has focused on the synthesis of inexpensive zeolites and zeolitic materials from fly ash (1-4) and blends of fly ash and cement kiln dust (5). These byproduct materials were combined with alkali hydroxides and/or alkali carbonate solutions and cured at various temperatures for 1-7 days. From the outset, the objective of this work has been to synthesize members of the general class of materials known as chemical adsorbents (zeolites, clays) from industrial and consumer byproducts. The work described below, extends the scope of the original work by addressing the feasibility of making claylike chemical adsorbents from mixed-glass cullet.
Natural Chemical Adsorbents There are two broad classes of naturally occurring chemical adsorbents that the consumer is exposed to on a daily basis: zeolites and clays. Zeolites have structures composed of * Corresponding author: telephone: (814)863-2779; fax: (818)865-7040; e-mail:
[email protected]. 312
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 2, 1999
FIGURE 1. View a represents a schematic representation of a sodalite cage common to many zeolite framework structures (solid circles, Si, Al; open circles, oxygen). View b represents a schematic representation of the structure of montmorillonite (large open circles, H2O; small open circles, oxygen; small solid circles, silicon; gray circles, OH; large solid circles, aluminum). interlocking, corner-sharing, partially aluminum-substituted silica tetrahedra (6, 7). They form framework structures that are host to a complex internal network of intersecting channels and cavities. These “open” spaces are normally occupied by charge balancing cations such as the group IA alkali and group IIA alkaline earth ions. See Figure 1a for a schematic representation of a zeolitic structure. Because the space allocated to the cation is relatively large, the ions are surrounded by a number of water molecules; the ions are in fact solvated. The interesting characteristic resulting from this structural arrangement is the fact that the interstitial water molecules and cations are mobile. The cations can often be exchanged for other cations, and the water molecules can be driven off by gentle heating. In the first instance, the exchangeable cation can be replaced by other ions such as calcium, magnesium, zinc, copper, or ammonium. In the second instance, the water may be replaced by organic liquids such as trichloroethylene and oil. If the zeolite has been dried, it will absorb water much like a desiccant (6). Kaolinite and montmorillonite clays form as a result of the weathering of feldspars, tuff, and/or volcanic ashes (8). Like zeolites, the conversion process takes hundreds to thousands of years. Clay particles normally consist of sheets of silica tetrahedra bonded to 6-coordinated sheets of alumina and/or magnesia, which accounts for their layerlike structure. Interlayers found in the 2:1 clays typified by montmorillonite contain water molecules, hydroxyl ions, and a variety of cations. Depending upon their chemical composition and the nature of their interlayer cations, dehydrated clays belonging to the smectite family (most commonly montmorillonite) are able to adsorb up to their own weight in water/oil and also take part in cation exchange reactions. A major disadvantage of using clay-based adsorbents in this capacity is their lack of mechanical integrity when wet. See Figure 1b for a schematic representation of a montmorillonite structure.
Current Market for Chemical Adsorbents Because cost is a major issue, the most widely used chemical adsorbent is currently dehydrated clay. Millions of tons of pet litter and spill control/cleanup materials are sold annually. 10.1021/es980155v CCC: $18.00
1999 American Chemical Society Published on Web 12/08/1998
TABLE 1. Composition of Starting Materials component SiO2 Al2O3 Fe2O3 CaO MgO SrO BaO Na2O K2O SO3 TiO2 H2O total
mixed-glass culleta
class F fly ashb
70.48 0.902 0.251 10.13 0.593 0.034 0.014 17.09 0.659 0.141 0.087
53.2 26.0 7.95 3.57 0.97
100.38
0.29 2.59 0.59 1.38 2.22 98.76
metakaolinitec
boehmitec
54 46
85
100
15 100
a Compositions analyzed at PSU by X-ray fluorescence. b Compositions analyzed at PSU by wet chemical analysis. c Stoichiometric.
To compete with this market, the cost of alternate materials, such as those discussed below, must be kept below $50 per ton. Naturally occurring zeolites are ideally suited for these uses, but their cost is prohibitive. Recently, numerous papers have been published demonstrating the fact that zeolites can be synthesized from a variety of natural and synthetic glasses (9-13) and Class F fly ash (14-27). At this point, there is little doubt that a utility could use its own fly ash to produce large amounts of zeolitic materials at relatively low cost. The current work details the result of similar experiments using mixed-glass cullet. Progress to date has been extremely promising. It has been possible to convert mixed-glass cullet into claylike double-layer silicates that are monolithic, have strength, and exhibit adsorptive and cation exchange properties. As such, these materials could begin to satisfy a need for this type of material (e.g., pet litter, spill control/cleanup, environmental wastewater treatment, and the management of agricultural ammonia) in areas having large amounts of recycled mixedglass cullet.
Experimental Procedure Mixed-glass cullet was obtained from PA Cullet in Corsica, PA. The 0.25-0.5 in. pieces were dried, ground in a Spex mill, and then sieved to minus 100 mesh. For the most part, experiments were carried out using 100% ground glass cullet, but in some instances the cullet was mixed with small amounts of Class F fly ash, metakaolinite, or boehmite. See Table 1 for compositions of these starting materials. In each case the dry materials were mixed with alkali hydroxide or alkali carbonate solutions. Once mixed, the samples were allowed to “soak” for 2 days at room temperature prior to curing at higher temperatures and autogenous pressures. Two groups of experiments were carried out. In the first, the glass cullet was mixed with an equal amount (by weight) of 1 M sodium hydroxide (NaOH) solution. In the second, the glass cullet was similarly mixed with a 2:3 (by weight) ratio of 2 M Na2CO3 to 2 M K2CO3. In both cases, a matrix of experiments was carried out to evaluate the effect of time and temperature on the outcome of the experiment. The mixtures were formulated with 5 g of solid and 5 g of solution unless otherwise noted. All runs were made in Parr digestion vessels. Finally, a few experiments using various additions of fly ash, metakaolinite and boehmite were run as a function of NaOH concentration. See Tables 2-5 given later in the text for experimental conditions. After curing for the appropriate time, the samples were washed with DI water and dried in air at 60-80 °C in a drying oven. The dried samples were packed into “zero background” quartz slides and analyzed using X-ray diffraction.
FIGURE 2. SEM photomicrographs of NaOH-treated glass cullet samples reacted at three temperatures for 1 day; all three samples were soaked 2 days prior to reaction: view a, 75 °C; view b, 100 °C; view c, 150 °C. Horizontal bars at bottoms of pictures are 10 µm in length.
These materials (normally only partially crystalline) were also tested for their ability to adsorb ammonia from solution. The test used in the study is based on changes in solution color using Nessler’s reagent as an indicator. In this test colors of the samples are compared to freshly made standards. As a way of comparison, montmorillonite clay and a series of natural zeolites were also included in the adsorption program. VOL. 33, NO. 2, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
313
FIGURE 3. X-ray diffraction data for NaOH-treated glass cullet samples reacted at three temperatures (75, 100, and 150 °C) for 1 day. The upper two traces are X-ray amorphous. They contain no significant amount of crystalline material. The lower trace indicates that a major amount of rhodesite and a minor amount of zeolite Na-P1 (O) are present in the sample. except that they were run at 150 °C for 1, 3 and 7 days, Results respectively. Figure 4 shows that the microstructural develStarting Materials. The average glass cullet fragment was opment of the samples is not radically affected by time. angular, 50 µm in size, exhibited conchoidal fracture typical Crystallinity develops at 1 day, and it remains reasonably of glass, and was covered with very fine glass particles. The constant through 7 days. At best there is a sense that the X-ray diffraction pattern of the glass contained an amorphous crystallites become “more ordered” with time. The equant hump whose maximum was centered at about 24°2θ. This grains present in Figure 4b are attributed to a small amount feature is typical of silicate glasses which have no long-range of transient zeolite Na-P1. The zeolite is apparently metaorder, i.e., they are noncrystalline. stable, which explains its absence in the 7-day sample. The The 1 M NaOH Experiments (24-h Runs as a Function X-ray diffraction patterns given in Figure 5 confirms this of Temperature). After the glass cullet was soaked in the 1 hypothesis. The patterns do not radically change with time. M NaOH solution for 2 days, the samples were cured for an The rhodesite crystals that form at 1 day remain stable additional 24 h at 75, 100, and 150 °C. Representative throughout the 7-day period of the experiment. The zeolite micrographs and a X-ray diffraction pattern for each sample Na-P1 present at 1 and 3 days does in fact disappear by 7 are given in Figures 2 and 3, respectively. The 75 and 100 °C days. All patterns display a small amount of residual samples show some reactivity, but it is confined to the general amorphous character. vicinity of the surfaces of the glass. The coatings which form The 2 M CO3 Experiments (24-h Runs as a Function of are fine grained and on the order of a few micrometers thick Temperature). After the glass cullet was soaked in 2:3 ratio (Figure 2a,b). As such they are highly susceptible to cracking. (by weight) of 2 M Na2CO3 and 2 M K2CO3 solution for 2 days, The 150 °C sample appears to be more crystalline (Figure the samples were cured at 75, 100, and 150 °C for an additional 2c). In this case, the majority of the large glass fragments 24 h. The microstructure of the carbonate-treated samples have reacted and have been replaced by clusters of platy and was similar to that of the NaOH samples described earlier. equant crystals. Once again the 75 and 100 °C samples did not react to any The accompanying X-ray diffraction patterns for the same appreciable extent. As a matter of fact, the carbonate samples samples (Figure 3) suggest that the 75 and 100 °C coatings actually appeared to show less gel development than their are nominally X-ray amorphous (i.e., noncrystalline as gauged NaOH counterparts. SEM micrographs of the 75 and 100 °C by the lack of peaks superimposed on the “amorphous” runs look very “glass” like. Although the 150 °C sample looks pattern of the glass). Taken as a whole, these data suggest much like its 150 °C NaOH counterpart, as noted below, the that the coatings seen in the SEMs are probably very fine X-ray diffraction pattern for the 150 °C sample containing grained, highly disordered, almost gellike in character. The carbonate was noticeably less crystalline. Observations peak at 29°2θ is attributed to the developing crystalline regarding reactivity were confirmed by the fact that the network structure. The 150 °C runs are very different. In this corresponding X-ray diffraction patterns for the 75 and 100 instance the hydrated material has partially crystallized, °C samples show no significant development of rhodesite or forming rhodesite (NaKCa2[Si8O19]‚5H2O) and a trace of evidence of a major shift in the shape of the amorphous zeolite Na-P1 (Na6Al6Si10O32‚12H2O). In addition, reactions hump. Consistent with the SEM photos, the X-ray data at 150 °C are reasonably complete as evidenced by the suggests that the 150 °C sample contains a small amount of decrease in size of the “glassy” hump attributed to the starting crystalline rhodesite. material. The 2 M CO3 Experiments (150 °C Runs as a Function The 1 M NaOH Experiments (150 °C Runs as a Function of Time). These are identical mixtures to those given above of Time). These are identical mixtures to those given above 314
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 2, 1999
TABLE 2. Phases Present in Glass Cullet Samples Reacted with 1 M NaOH Solutiona T, °C
2-day soak
1 day
3 days
75 100 150
am* am am
am am Rh, tr. Na-P1
am tr. Rh Rh, tr. Na-P1
7 days am Rh
a
Abbreviations: am, X-ray amorphous; Rh, rhodesite; Na-Pl, zeolite Na-Pl; Mord, mordenite; tr, trace.
TABLE 3. Phases Present in Glass Cullet Samples Reacted with 2 M CO32- Solutiona T, °C
2-day soak
1 day
3 days
75 100 150
am* am am
am am tr. Rh
am tr. Rh Rh, tr. Mord
7 days am Rh, tr. Mord
a
Abbreviations: am, X-ray amorphous; Rh, rhodesite; Na-Pl, zeolite Na-Pl; Mord, mordenite; tr., trace.
TABLE 4. Ammonia Uptake from a 0.14 M NH4Cl Solution by Glass Cullet Samples mixed carbonate solution content glass (%)
time (days)
100 100 100 100 100 100b 100 100 100
1 3 28 1 3 1 1 3 7
T, °C 75 75 75 100 100 133 150 150 150
alkali hydroxide solution
uptake (mg of NH3/g solid)
time (days)
T, °C
uptake (mg of NH3/g of solid)
0a
1
75
16.2
28 1 3
75 100 100
14.8 16.2 23.1
1 3 7
150 150 150
13.0 17.8 13.6
0a 14.0 9.4 18.6 16.9 17.0 13.4 19.7
a These samples did not contain any rhodesite. b This run contained 0.8 g of glass cullet and 1 g of 2 M K2CO3 solution.
X-ray diffraction data for these samples suggest that rhodesite is present in all samples, albeit poorly crystalline material at early times. Once again the degree of reaction of the 1- and 7-day carbonate samples is less pronounced than their NaOH counterparts. However, at 7 days both carbonate and hydroxide samples are well-crystallized, the carbonate sample more so than the NaOH sample. A small amount of mordenite [(Na2,Ca,K2) [Al2Sil0O24]‚6H2O] formed instead of Na-P1 in these samples, and unlike the NaOH samples, once formed the mordenite tended to coexist with the rhodesite through 7 days. FIGURE 4. SEM photomicrographs of NaOH-treated glass cullet samples reacted at 150 °C as a function of time; all three samples were soaked 2 days prior to reaction: view a, 1 day; view b, 3 days; view c, 7 days. Note how the crystallinity and platy nature of the rhodesite develops with increasing time. Horizontal bars at bottoms of pictures are 5 µm in length. except that they were all run at 150 °C for 1, 3, and 7 days, respectively. The SEM micrographs show the progressive development of rhodesite crystals with the passage of time. The 1-day sample contains a limited amount of crystalline material, whereas the 3- and 7-day samples are nearly completely converted. At first the samples contain predominantly platy crystals which, with time become intermixed with radiating elongated tabular crystals. Corresponding
Discussion For the most part, the crystalline products that formed were identified as a Na-rich rhodesite. See Tables 2 and 3 for phase development with time, temperature and curing solution. An average of four analyses taken from a paper by Sheppard and Gude (28) suggests that the composition of naturally occurring rhodesite is roughly (NaKCa2[Si8O19]‚5H2O). The formation of double-layer silicates in these experiments is further confirmed by Liebau (29) who tells us that doublelayer silicates are uncommon in nature, usually forming only in very silica-rich environments containing relatively large cations (Ca and K), conditions which are chemically consistent with our bulk chemistries. The high silica composition of the bottle glass coupled with the presence of alkali and VOL. 33, NO. 2, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
315
FIGURE 5. X-ray diffraction data for NaOH-treated glass cullet samples reacted at 150 °C for times noted on the figure. All samples are quite crystalline. The upper two traces indicate that the samples contain major amounts of rhodesite and minor amounts of zeolite Na-P1 (O). The lower trace is the most crystalline of the three. Note that at 7 days, Na-P1 is no longer present. alkali-earth cations seems to satisfy the requirements set forth by Liebau for the formation of double-layer silicates of this type. Preliminary work has demonstrated that it is possible to vary the proportions of zeolite and double-layer silicate phases by blending the glass cullet with 37.5% Class F fly ash or 25% metakaolinite or boehmite and then reacting the mixtures with 1 M NaOH at 150 °C. These additives contain more aluminum than the glass and thus favor aluminumcontaining phases such as zeolites and hydrodelhayelite (Na2KCa2[AlSi7O19]‚6H2O). The boehmite-containing sample cured for 3 days was essentially pure Na-P1, containing only a trace of hydrodelhayelite. The fly ash and metakaolinite samples were less crystalline, containing a significant amount of poorly crystalline highly disorganized hydrodelhayelite and small amounts of Na-P1 and analcime (Na2[Al2Si4O12]‚ 2H2O). Liebau (29) tells us that hydrodelhayelite is also a double-layer structure but it contains aluminum substituting for silicon in tetrahedral coordination. As such, it is probably a member of a larger solid solution series having rhodesite as one of its end members. Thus, we believe that an aluminum-containing double-layer structure can also form, and will form in alumina-rich mixtures. Presumably, the formation of cage-like voids in the double layers of these silicates are responsible for the later reported cation exchange exhibited by these materials.
Ammonia Uptake Ammonia uptake from solution and water/oil adsorption experiments were run on a number of these materials, both synthesized from pure glass cullet and those made from blends of glass cullet, Class F fly ash, metakaolinite or boehmite. These results are given in Tables 4 and 5. For sake of comparison a series of natural minerals were also examined (see Table 6). Taken as a whole, these experiments suggest that the double-layer silicates produced from glass cullet are able to adsorb ammonia from solution about as well as natural 316
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 2, 1999
TABLE 5. Ammonia Uptake by Al-Containing Double-Layer Silicates Synthesized in 3 Days at 150 °C glass (%)
Class F fly ash
62.5 75 75 75a 75 75a
37.5
boehmite
metakaolinite
solution used to promote reaction
uptake (mg of NH3/g of solid)
25 25 25 25
1 M NaOH 1 M NaOH 2 M CO3 2 M CO3 1 M NaOH 1 M NaOH
14.0 12.3 22.8 8.8 13.4 7.3
25
a These samples were run at 100 °C rather than 150 °C as a way of gauging the effect of temperature.
TABLE 6. Ammonia Uptake by Natural Clay and Zeolite Samples sample
uptake (mg of NH3/g of solid)
Wal-Mart kitty litter (dehydrated clay) chabazite clinoptilolite erionite mordenite phillipsite
0 19.6 19.7 30.9 19.6 35.7
zeolites. All things being equal, the runs made with alkali hydroxide were initially more crystalline than their carbonate counterparts, and the degree of crystallinity of a given sample was directly related to the concentration of the solution used; higher concentrations normally yielded greater degrees of crystallinity with the singular exception that the 7-day carbonate run gave the overall best yield of rhodesite. Interestingly enough, the uptake of ammonium ions by samples containing double layer silicates was nearly independent of synthesis conditions. This is encouraging inas-
much as this implies that rhodesite-like materials can be synthesized at temperatures e100 °C (easier and cheaper) and that once formed rhodesite’s Na and K are at least partially exchangeable under all conditions. A test of these materials’ ability to adsorb fluids was carried out by immersing weighed amounts of dried material in DI water and motor oil. Depending upon their crystallinity, the glass cullet samples were able to adsorb significant amounts of fluid, sometimes adsorbing as much as their own weight. In as much as cost is a central issue in marketing these materials, we estimate that the cost of making a ton of these adsorbents would cost in the neighborhood of $50 (manufactured on site by an utility or recycler) to $100 per ton (service company using portable tractor trailer based equipment). In either case, the utility or recycler can use the material for its own purposes or sell it to industrial users in bulk for incorporation into a variety of products.
Acknowledgments Work was supported by Pennsylvania’s Environmental Technology Fund administrated by The Ben Franklin Technology Center of Central and Northern Pennsylvania (19931994). Thanks, too, to Else Breval for performing the adsorption tests.
Literature Cited (1) LaRosa, J.; Kwan, S.; Grutzeck, M. W. J. Am. Ceram. Soc. 1992, 75, 1574. (2) LaRosa, J.; Kwan, S.; Grutzeck, M. W. In Mater. Res. Soc. Symp. Proc. 245; Mater. Res. Soc.: Pittsburgh, 1992; pp 211-16. (3) Grutzeck, M. W. In Progress in Cement and Concrete; Sarkar, S. L., Ed.; ABI Books Pvt. Ltd.: New Delhi, 1993; pp 368-395. (4) Grutzeck, M. W.; Siemer, D. D. J. Am. Ceram. Soc. 1997, 80, 2449-53. (5) Grutzeck, M. W. Environmental Issues and Waste Management Technologies II. Ceram. Trans. 1996, 72, 353-364. (6) Breck, D. W. Zeolite Molecular Sieves; John Wiley & Sons: New York, 1973; 771 pp. (7) Gottardi, G.; Galli, E. Natural Zeolites; Springer-Verlag: Berlin, 1985; 409 pp. (8) Grim, R. E. Clay Mineralogy, 2d ed.; McGraw-Hill: New York, 1968.
(9) Ottana, R.; Saija, L. M.; Surriesci, N.; Giordano, N. Zeolites 1982, 2, 295-298. (10) Fiedler, F. J.; Lohse, H. H.; Schurmann, K. N. Jb. Miner. Mh. 1983, 358-364. (11) Burriesci, N.; Liusa Crisafulli, M.; Giordano, N.; Bart, J. C. J.; Polizzotti, G. Zeolite 1986, 6, 467-473. (12) Yoshida, A.; Inoue, K. Zeolites 1986, 6, 467-473. (13) Barth-Wirsching, U.; Ho¨ller, H. Eur. J. Mineral. 1989, 1, 489506. (14) Henmi, T. Soil Sci. Plant Nutr. 1987, 33, 517-521. (15) Mondragon, F.; Rincon, F.; Sierra, L.; Escobar, J.; Ramirez, J.; Fernandez, J. Fuel 1990, 69, 263-266. (16) Shigemoto, N.; Shirakami, K.; Hirano, S.; Hayashi, H. Nippon Kagaku Kaishi 1992, 484-92. (17) Shigemoto, N.; Hayashi, H.; Miyaura K. J. Mater. Sci. 1993, 28, 4781-86. (18) Chang, H.-L.; Shih, W.-H. In Environmental Issues and Waste Management Technologies V; Jain, V., Palmer, R., Eds.; Ceramic Transactions 61; Am. Ceram. Soc.: Westerville, OH, 1995; pp 81-88. (19) Lin, C-.F.; Hsi, H.-C. Environ. Sci. Technol. 1995, 29, 1109-17. (20) Park, M.; Choi, J. Clay Sci. 1995, 9, 219-229. (21) Querol, X.; Alastuey, A.; Fernandez-Turiel, J. L.; Lopez-Soler, A. Fuel 1995, 74, 1226-31. (22) Shigemoto, N.; Sugiyama, S.; Hayashi, H.; Miyaura, K. J. Mater. Sci. 1995, 30, 5777-83. (23) Shih, W.-H.; Chang, H.-L.; Shen, Z. Mater. Res. Soc. Symp. Proc. 371; Materials Res. Soc: Pittsburgh, 1995; pp 39-44. (24) Singer, A.; Berkgaut V. Environ. Sci. Technol. 1995, 29, 1748-53. (25) Amrhein, C.; Haghnia, G. H.; Kim, T. S.; Mosher, P. A.; Gagajena, R. C.; Amanios, T.; de La Torre, L. Environ. Sci. Technol. 1996, 30, 735. (26) Suyarna, Y.; Katayama, K.; Meguro, M. Chem. Soc. Jpn. 1996, 136-40. (27) Querol, X.; Alastuey, A.; Lopez-Soler, A.; Plana, F.; Andres, J. M.; Juan, R.; Ferrer , P; Ruiz, C. R. Environ. Sci. Technol. 1997, 31, 2527-13. (28) Sheppard, R. A.; Gude, A., III. J. Am. Minerol. 1969, 54, 251-255. (29) Liebau, F. Structural Chemistry of Silicates; Springer-Verlag: Berlin 1985; 347 pp.
Received for review February 17, 1998. Revised manuscript received October 8, 1998. Accepted October 19, 1998. ES980155V
VOL. 33, NO. 2, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
317
