Relationship between the Morphological Properties of Half-Calcined Dolomite and the Kinetics of the Sulfation Reaction S. Siegel', L. H. Fuchs, B. R. Hubble, and E. L. Nielsen Chemical Engineering Division, and Chemistry Division (L.H.F.), Argonne National Laboratory, 9700 South Cass Avenue, Argonne, 111. 60439
A kinetic, morphological, and structural study has been made of the half-calcination process in BCR 1337 dolomite stones. The investigation covered the temperature range 640-800 "C in 40% c02-60% He and 100% COS at l-atm pressure. Certain morphological features which can have a bearing on subsequent, sulfation kinetics have been examined. At the lower temperature, the calcite crystals are large and exhibit relic structures of the original dolomite crystals. As the half-calcination temperature increases, the calcite crystals within the relic grains become increasingly smaller and more randomly oriented until at 800 "C the randomization is almost complete. The calcite crystals exhibit disorder but show the onset of annealing a t 800 "C after a l - h heating period. Sulfation experiments on half-calcined stones containing small and large calcite crystals as well as those showing some annealing indicate that the kinetics are dependent to some extent on crystal size (surface area) and disorder. In some sulfur dioxide emission-control processes, such as those used in fluidized-bed combustion, the sorbent material is dolomite or limestone. For dolomite, the chemical form which reacts with the SO2 present during the combustion of fossil fuels depends on the mode of fluidized-bed combustion being considered: CaC03 MgO (pressurized) or CaO MgO (atmospheric). In the case of pressurized fluidized-bed combustion, the two principal chemical reactions involved in sulfur removal by dolomite are:
+
+
-
CaMg(CO:1)2
CaC03
+ MgO + COz
(1)
and
+
+
+
+
CaC03 MgO + SOL, 0.502 CaS04 MgO COz (2) The half-calcination of dolomite leads to the formation of CaC03 MgO under the fluidized-bed combustion operating conditions, and it is therefore this aggregate that reacts with +
+
soz.
The basic chemistry associated with these reactions, as well as other reactions concerned with the regeneration of the sulfated dolomite formed by Reaction 2, has been the subject of investigation in the current program (1-5). The decomposition behavior of dolomite in Reaction 1 has been studied intensively for reasons of academic interest and technical applications. Such studies have led to much insight on the kinetics and mechanism of the decomposition reaction. The kinetics of Reaction 2 have also been studied in some detail ( I ) . However, current models for solid-gas reactions, namely, a variation of the shrinking core concept or the reaction of a porous solid, were not successful in describing the kinetic data of reaction 2 (1,2). Clearly, the development of a more explicit, nonphenomenological theory for the solid-gas reaction is needed. The development of such a theory requires knowledge of the structural and morphological changes occurring in the sorbent and the manner in which these changes might affect the progress of the heterogeneous reaction. Accordingly, as one step toward an understanding of reaction mechanisms, this study is directed to a consideration of the relationship between the kinetics of a reaction and the structural and morphological changes in half-calcined dolomite. 0013-936X/78/0912-1411$01.00/0
@ 1978 American Chemical Society
General F e a t u r e s of the H a l f - C a l c i n a t i o n R e a c t i o n
Fundamental to the problem of relating the sulfation kinetics to the properties of a half-calcined dolomite stone is an understanding of the structural and morphological changes which take place when dolomite undergoes the half-calcination reaction. The first major attempt to understand this process at the microscopic and submicroscopic level was carried out by Haul and Wilsdorf (6, 7), who investigated single crystals of dolomite by means of X-ray diffraction and microscopy. Single crystals were heated at temperatures ranging from 600 to 800 "C under COz pressures up to 655 mm for different heating periods. Controlled experiments at 600 and 640 "C demonstrated that the dolomite crystal had undergone reduction in size as the half-calcination reaction progressed and was replaced with an apparent single crystal of calcite. Further investigation revealed, however, that the calcite crystal was composed of smaller crystals oriented in the same manner as the dolomite substrate. Experiments at 800 "C under a COz pressure of 655 mm showed that the calcite crystals were again small but randomly oriented. Haul and Wilsdorf concluded that disorder was present in the calcite and observed that annealing occurred at 820 "C following a 21-h heating in a COz atmosphere of 655 mm pressure. Our own investigations (2, 3 ) using high-quality dolomite stones in a size range (1-2 mm) suitable for fluidized-bed operations showed that similar morphological and structural details could be detected in very small crystal aggregates. A description of our results follows. The BCR 1337 dolomite used in this study is composed of CaMg(CO3)z crystals of variable sizes ranging between 30 and 300 pm as shown in Figure 1 for two different specimens. These crystals (or grains) are held together to form the coherent stone. Both X-ray diffraction and optical examinations show that the dolomite is well crystallized with no CaC03 present. Small amounts of finely divided impurities are occluded in the larger crystals. During the half-calcination reaction, CaMg(C03)~decomposes to form CaC03 and MgO with the integrity of the stone remaining intact. Optically, the calcite crystals vary in size depending upon half-calcination conditions. As observed by X-rays and scanning electron microscopy (SEM),the MgO is clearly present, even though the crystallites are too small to be identified by optical examination. X-ray diffraction studies show that the size range of the MgO crystallites is of the order of 200-300 A, while SEM examination indicates that magnesium is uniformly distributed across the half-calcined stones and, presumably, within the calcite crystals. Very slight variations in the unit cell parameters for CaCO3 are detected. These may arise from disorder or from solid solution of magnesium. Depending on the half-calcination conditions, the calcite diffraction lines exhibit intensity enhancement, ranging from preferred orientation effects to single-crystal reflections superimposed upon more or less uniform lines. Investigation of high-angle reflections arising from these "single" crystals shows that they are disordered. The uniform lines are broad, indicating small crystallites or disorder or both. The tendency toward preferred orientation suggests that the large crystals are probably composed of smaller ones which aggregate in Volume 12,Number 13, December 1978
1411
Figire 1. Two particles of BCR 1337 dolomite with extreme variation in grain size (30-300Fm) Polished section. polarized light
various degrees of alignment. This picture is consistent with the observations of Haul and Wilsdorf. These observations suggest that the important factors of crystallite size and disorder, which have an important bearing on the calcite reactivity, might be controlled a t the half-calcination stage. I t should be stated, however, that properties other than disorder and size are also important. For example, in the half-calcination process, pore size and molecular volume changes take place, and these changes continue to occur during sulfation. Sulfation reactivity has been shown to be related to stone pore size (8-10). Thus, the sulfation reaction kinetics will depend, in a complex manner, on the total morphological properties. However, in the present discussion, only those morphological changes, crystallite size and disorder, related to the kinetic properties of the reacting species, CaCOs are considered. A previous publication (5) described the reaction properties of MgO, in which it was shown to form small amounts of MgaCa(S04)4 during sulfation. These earlier investigations (2,3)presented a structuralmorphological interpretation of the half-calcination reaction which identified and described the disorder within the calcite crystals. In addition, the nature of sites which might be reactive toward SO2, as well as the origin of diffusion barriers within the half-calcined stones, was discussed. The present study extends this work and describes in some detail how the kinetics of Reaction 1 are related to the morphology of the half-calcined dolomite. In addition, preliminary data are presented which demonstrate that the sulfation reaction is affected in a pronounced manner by the morphology of the half-calcined dolomite.
Experimental Method The BCR 1337 dolomite used as the starting material was obtained from Pfizer Co., Gihsonburg, Ohio. (BCR 1337 dolomite is from the Guelph member of the Niagaran dolomite, which is Silurian in age.) Our chemical analysis indicates that the empirical formula for the dolomite is Cal,olMgOo.99(C03)2. which agrees with the analysis provided by Pfizer Co., hut differs from Ca1.14Mg00.86(C03)2,reported earlier (8).Stones of the -16+18 U.S. standard screen size (-1.1 mm) were used. Kinetic experiments were performed in a thermal gravimetric apparatus in a manner described previously (1, 2). Samples weighing approximately 200 mg were used under gas flow rates of 300 cm3/min a t 1-atm pressure. In the 640 to 800 1412
Environmental Science 8. Technology
Figure 2. Percent conversion YS. time for haif-calcination reaction at various temperatures under 1 atm of 100% CO,
"C temperature range of the study, this flow rate resulted in hulk gas velocities ranging from 5.0 to 5.9 cmls, respectively. These conditions were shown in the earlier study to prevent gas film diffusional resistance effects in observed kinetic data ( I , 2). The COz in He compositions were blended by use of rotameters. The sulfation experiments were carried out with a certified standard gas mixture, which again contained He as the inert gas, obtained from a commercial vendor. X-ray diffraction and morphological studies were made on aliquots of 30 to 50 stones removed at various stages of the half-calcination reaction experiments. These analyses primarily involved the application of powder X-ray diffraction and petrographic methods. In some cases, scanning electron microscopy techniques were employed to determine the distribution of MgO in the half-calcined stones. Results Kinetic Investigations. The nature of the kinetics of the half-calcination reaction is depicted in Figure 2, where percent conversion of CaMg(CO& to CaC03 MgO is plotted against reaction time. Observed weight losses and X-ray diffraction analyses indicated CaC03 + MgO to he the reaction products. These data were obtained in 100% COz at 1-atm pressure; similar results were obtained in 40% CO2-6W He at 1-atm pressure, The reaction rate begins to increase rapidly with temperature in the 680 to 720 OC range. The initial reaction rate (extrapolated to t = 0) at 800 O C is 24 times greater than the rate at 700 "C, whereas the rate at 700 "Cis only 3.4 times greater than that at 640 "C. The apparent activation energy ( A P )over the entire temperature range (640-800 "C) is 51 kcallmol; in the 640 to 700 "C range, AE* is 35 kcal/mol; and in the 700 to 800 "C range, AE* is 68 kcal/mol. Thus, the 680 to 720 OC range represents a transition from slow to rapid kinetics. The two kinetic ranges (640-680 and 720-800 "C) and the transition region between them, described above, have been shown to he related to the morphological changes associated with the half-calcination reaction. Thus, in order to evaluate morphological changes taking place during the half-calcination process, petrographic and X-ray examinations were carried out on the half-calcined stones prepared a t different temperatures and under the two COa environments described above. No surface area measurements have been performed
+
Figure 5. Relic structure of dolomite stone half-calcined at 680 OC Thin section, polarized light. magnification same as Figure 4
Figure 4. Relic structure of dolomite stone half-calcined at 640 ' C Coarse granular structure Of the original dolomite is preserved because the calcite crystalliteshave generally adopted the same Orientation 01 the dolomite crystals. Thin Section, polarized light, Scale bar = 0.21 mm
at present. The emphasis has been placed on the use of microscopy and X-ray methods in order to develop a physical description of the morphology. Petrographic Investigations. Examination of stones removed a t different stages of the half-calcination reaction carried out a t low temperatures (640-680 "C) shows that the onset of calcite formation occus throughout the volume of the dolomite crystals. If half-calcination starts at the surface of a crystal and progresses inward, this effect is not noticeable in the crystal sizes, temperatures, and COz concentrations used in this study. Furthermore, dolomite crystals within the interior of the stones are as extensively half-calcined as those near the surface. Thus, under the experimental conditions of this study, calcite formation within a dolomite stone is not a surface-controlled process. For completely converted stones (no dolomite) prepared under conditions resulting in the lowest reaction rate (640 "C, 100% COz, 440 h), the grain size and shapes of the calcite crystals emulate those of the dolomite crystals in the original stone; i.e., the calcite is a pseudomorph after the dolomite host. Thus, the calcite crystal may he considered to he a relic grain, and an array of such crystals in a stone displays a relic structure. The relic structure is identical in microscopic apperance with the grain structure of the original dolomite. The similarity in grain structure is evident in both thin and polished thick sections under polarized light. This is shown in the photomicrographs of thin sections in Figures 3 and 4 Except for a greater amount of opaque inclusions in the calcite, which are probably finely divided MgO, the half-calcined
Many crystaliiteswithin relic grains are randomly oriented. A typical relic grain. about 0.1 mm in diameter. is at top center. Thin section, polarized light, scale bar = 0.15 mm
Figure 6. A relic grain in 680 OC half-calcined dolomite inciusions of randomly oriented calcite Crystallites ate gray to black within the white matrix of preferentially Oriented crystallites. Thin section, polarized light. scale bar = 53 pm
material resembles a coarsely crystalline limestone. A calcite grain in a thin section of the half-calcined material shows uniform extinction. Such a grain oriented normal to the optic axis gives a uniaxial interference figure which is somewhat vague compared with that exhibited by a dolomite grain (crystal) in a section of an original dolomite stone. Another difference between the calcite and dolomite grains is noticed in polished sections viewed in reflected polarized light. In a section of the half-calcined material, the contrast between variously oriented calcite grains is much less than that between dolomite grains in the original stone sections. This reduced contrast results from the increased amount of internal light scattering by the calcite. Both of these differences may be due to occluded MgO, or to the possibility that an apparently large single calcite crystal may actually be an array of small crystallites similarly oriented. This latter possibility, however, cannot be resolved with the microscope. The effects of the transition temperature range (680-720 "C) referred to earlier become apparent when stones prepared at slightly higher temperatures are studied. The morphological change is illustrated in Figure 5, which shows the condition of a stone prepared at 680 "C under an environment of 100% COz for 60 h. Although a general similarity of grain structure to that of the original dolomite is still apparent, many crystallites within the relic grains have different orientations. This detail is shown in Figure 6. Thus, a 40 "C temperature increase induces the onset of random orientation. When the 680 "C preparation is compared with a stone half-calcined at 800 "C in an environment of 40% COz for 0.3 h (Figure 7), it is apVolume 12, Number 13. December 1978 1413
Flgure 7. Typ ca texture 01 ROO 'C, 0.3-n ha I-calcinea dolomite Three re c gra n=, WOWS, 2are y recognozag e n tnc m w ; x 01 rmmm y mienid cdc IC crysla les Pol sneo seclion. PO cir rea lhgnl scil P oar = 0 2 mm
Figure 9. A typical relic grain in 800 'C. 0.3-h half-calcined doio-
mite Randomly Oriented calcite crystallites(gray to black) are more numerous and Smaller than those in a typical relic grain in the 680 ' C material a5 shown in Figure 6. Thin Section, polarized light. scale bar = 53 +m
Figure 8..Uncommon preservation 01 relic slruclbre in this particle 01 800 "C. 0.3-h halt-calcined aolom le MOEI Olher panicles conlsin on, a lew rc c gra ns a4 snow in F'gure 7 Thin sect on. polmzed I ghl. S C ~ Ioar C = 0 2 mm
parent that the 680 "C preparation exhibits a morphology indicative of a transitian between the 640 and 800 "C states. 'The sizes of the randomly oriented crystallites in Figure 5 range from 3 to 24 fim with an average value estimated at 8 fim. These apparent sizes may also represent the size of those crystallites that are in perfect alignment within a relic grain but, because of the optical continuity, are indistinguishable. The morphological state associated with high half-calcination reaction rates is manifested by the random orientation of the crystallites. This is illustrated in Figure 7 for a stone half-calcined at 800 "C under 40% COz for 0.3 h. The bulk of this stone is found to be composed of small randomly oriented crystallites with an average'size of approximately 10 fim. Typically,'a few grains are present in each stone (arrows point to these in Figure 7 )which contain crystallites exhibiting some preferred orientation. However, most of the calcite crystallites within a grain are individually oriented and do not assume an orientation related to the original dolomite crystal. Figure 8 is a polairized light photomicrograph of a thin section of a stone half-calcined at 800 "C for 0.3 h; this section is atypical in that it contains many relic grains, which preserve the relic structure, and thus resembles that obtained from the 680 O C material. A detailed examination of the relic grains developed in the 800 "C material shows, however, that they contain many more and smaller sized, randomly oriented calcite crystallites than those relic grains in the 680 "C sample (Figure 9 shows a typical relic grain in the 800 "C, 0.3-h material which can'be compared with Figure 6). A comparison of Figure 1 with 7 shows that the grain structure of the original dolomite has 1414
Environmental Science & Technology
Figure 10. Half-calcined dolomite, 800 OC, 26 h Relic structure is obliterated in this recrystallized matrix Of randomly oriented calcite crystallites. Polished section, polarized light, scale baP= 0.2 mm
been largely obliterated at 800 "C, thus indicating a substantially modified morphology. The randomly oriented morphology associated with hightemperature preparations can be further modified by continued heat treatment at 800 OC, as illustrated in Figure 10. This material, held in a 40% COz environment for 26 h, revealed the following effects, which became apparent after less than 1h of heat treatment: (i) complete loss of preferred orientation of individual crystallites, which show a.small growth in size upon extended heating. This random orientation results in complete obliteration of the original dolomite grain structure. (ii) An increase in distinctness of the individual calcite crystallites when viewed in reflected polarized light. This latter effect is believed to arise from increased ordering in the calcite internal structure; that is, the individual calcite crystallites are approaching a true, single-crystal character (annealing of the disorder). The fact that these two effects became noticeable after less than 1 h of heat treatment at 800 "C suggests the onset of a competition between a random orientation process and a subsequent ordering or annealing process with continued heating. X-ray Diffraction Investigations. Two types of powder X-ray diffraction studies were carried out using aliqnots similar to, and in many cases the same as, those in the petrographic examinations: (i) measurement of the sizes of the apparent (pseudo) single calcite crystals on completely con-
HALF-CALCINED DOLOMITE MORPHOLOGY A S80DC.
0 BW'C, c1 BW'C,
IW Z
COz. €4
hr
40 7. COz. 0.3 hr
40 Z COY 28
hr
0
a
CaMg(CO,),
b CaCO,*MgO
c BREAKDOWN
OF RELIC GRAIN
4
701
Figure 11. (a)Dolomite crystal in a stone as observed optically. (b) Relic grain of CaC03 and MgO formed at low temperature (640 OC). (c) Breakdown of relic grain when reaction is carried out at higher temperature
verted samples, and (ii) evaluation of line widths in order to derive crystallite size and disorder information. The first study was carried out by examining the half-calcined samples with a minimum of crushing and eliminating the sample rotation during exposure. This technique'brought out the single-crystal character in some detail because of the reduction in "averaging" of the reflections arising from sample rotation. The largest pseudocrystals were fouhd in the lowtemperature preparations (640 to 680 OC), with the size decreasing with increasing temperature until no pseudocrystals were detected (the 800 "C, 0.3-h preparation). Thus, a general agreement exists with the petrographic results. The X-ray studies, however, revealed no differences between the 640 and 680 "C samples or between the 800 OC, 0.3-h preparation and that subjected to the prolonged heating a t 800 OC. In the second X-ray investigation, line widths of both the MgO and CaC03 phases were measured with a diffractometer and interpreted by established procedures. Generally, line widths for these phases were essentially constant and independent of the half-calcination conditions. For MgO, the line widths were attributed to crystal-size broadening, leading to crystallite sizes of the order of 250 A. However, for CaC03, the effects of crystallite size and disorder could not be separated. The significance of this observation is that the properties of the smaller crystals (disorder and size) appear to be essentially constant, without much change in the range of halfcalcination conditions investigated. In light of the first X-ray study, the major change is in the size of the pseudocrystals. The pseudocrystals exhibit disorder by virtue of the possible misalignment of smaller crystallites, which in themselves exhibit disorder. Discussion of Petrographic a n d X-ray Observations. The petrographic and X-ray observations can be best discussed with the aid of Figure 11. Figure l l a represents the projected outline (idealized) of a dolomite crystal in a stone, as might be observed optically. Following half-calcination, the CaMg(C03)~converts to CaC03 and MgO. This is shown in Figure l l b for low-temperature conversion where the observed apparent activation energy is lowest (e.g., 640 OC). The outline of the original dolomite crystal remains after conversion, and this outline defines a relic grain. Optically, Figure I l b appears as a single crystal. From the point of view of X-ray studies, Figure l l b shows a single-crystal pattern, but one which indicates disorder. In addition, the X-ray data suggest that the single pseudocrystals are composed of small disordered calcite crystallites or subunits in alignment. Thus, the striations in Figure l l b are intended to indicate the outlines of smaller crystallites of CaC03 which are believed to be so oriented as to simulate a single crystal. As the reaction temperature increases, a breakdown of the relic grain is observed, as shown in Figure l l c . This is an intermediate stage in which part of the single crystal has broken, revealing groups of smaller similarly oriented regions. Distributed throughout the remainder of the relic grain are the dispersed smaller, randomly oriented crystals. This effect can be seen with both petrographic and X-ray techniques. Finally, at elevated temperatures (e.g., 800 "C), in the range where the apparent activation energy is highest, randomiza-
04
,
,
,
0
2
4
6
.
,
,
B
10
,
,
,
,
,
,
,
I2
14
IS
18
20
72
24
I
Time, hr
Figure 12. Percent conversion vs. time for sulfation reaction for various half-calcined dolomites morphologiesreacti,,g with 0.27% Sop, 4.85% 02,39.77% COP, and balance He at 1 atm and 750 OC
tion is complete. Optically, small crystals are observed in all orientations. The X-ray powder pattern shows the same randomization; that is, single-crystal reflections and preferred orientation have disappeared, leaving uniform diffraction lines. In addition, these diffraction lines show considerable line broadening at large angles, thus indicating small crystallites and/or disorder. The sizes of the small crystals following break-up of the larger crystal, or relic grain, can have an important bearing on subsequent sulfation reaction rates. Microscopic evidence indicates crystallites of about 80 000 A; however, the limit of resolution visually is roughly 20 000 A. If the line broadening of the calcite diffraction pattern is attributed only to a crystal-size effect, the crystals are about 500 A or smaller. It was previously shown, however, that the effects of disorder and crystallite size could not be separated. Hence, pending further investigation of the calcite crystals, the true size will remain unknown. As far as the subsequent sulfation reaction kinetics are concerned, use of both microscopy and X-ray data permits us to establish fairly reliable relative sizes to relate to kinetic behavior. Sulfation Reaction Kinetics. Preliminary kinetic results for Reaction 2 are presented in Figure 12. The natures of the starting material morphologies for these experiments were illustrated in Figure 5 (680 "C, 100%CO2,60 h), Figure 7 (800 " C , 40% COz, 0.3 h), and Figure 10 (800 "C, 40% COz, 26 h). Each sample was sulfated under the same experimental conditions, which were chosen to simulate fluidized-bed combustion (750 "C, at 1 atm pressure in 0.27% SOz, 4.85% 0 2 , 39.77% COS, and 55.11% He). The data in Figure 12 show that the sulfation reaction rates and yields are related to the morphology of the half-calcined stones. For example, after a 6-h reaction period (typical residence time in a fluidized bed), the sample half-calcined at 800 "C for 0.3 h undergoes a 44% conversion to Cas04 MgO, whereas the sample half-calcined at 800 "C for 26 h is only 29?h converted; thus, the former shows a 55% increase in yield over that of the latter. The shape of the yield curve given by the 800 "C, 26-h treatment is of interest because its initial rate is between that of the other two samples, but drops off rapidly after approximately 2 h. This dropoff is followed by a more or less linear behavior of yield with time. By contrast, the shapes of the yield curves for the other two morphologies show substantially less dropoff and more typical behavior when compared to the results of the previous kinetic study ( I ) . Limiting the interpretation of Figure 1 2 to disorder and crystal size (surface area) only, some rough correlation may be found between these factors and kinetic behavior.
+
Volume 12, Number 13, December 1978 1415
The sample prepared a t 800 “C for a 0.3-h period contains small, randomly distributed crystals which exhibit disorder. On the other hand, the 800 “C preparation, which was heated for 26 h, is found to contain crystals which have undergone some annealing of the disorder. Although there has been some crystal growth associated with the prolonged heating, the major difference between these two preparations that can be detected by the techniques used in this study is the extent of disorder. Thus, the sulfation kinetics show the characteristic effect of disorder. The 680 “C, 60-h preparation contains large calcite crystals which, by X-ray examination, appear to be disordered. If it is assumed that the nature of the disorder in the larger crystals is similar to that in the smaller crystallites, then the major difference between the 800 “C, 0.3-h and the 680 “C, 60-h preparations would be in the crystal size (surface area). The available surface will depend on the presence of cracks, voids, and various defects within the larger crystals. However, cracks and voids will contribute to defect formation so that separation of the surface area and disorder factors in the kinetics is not readily accomplished. However, it is believed that some effect of the surface area on the kinetics of sulfation is being observed in Figure 12. The yield curves of the 680 O C , 60-h and 800 “C, 0.3-h preparations approach each other after a prolonged reaction period. This may be explained by the tendency of the larger calcite pseudocrystals to break up into randomly oriented crystals of smaller sizes as the heating a t 750 “C progresses. The curves of Figure 12 demonstrate a relationship between kinetic behavior and crystal size and disorder but they also suggest that morphological changes which affect the progress of the reaction are occurring during the reaction itself. A detailed kinetic-morphological study of the sulfation process is currently under way to determine the nature of the mechanisms involved.
Acknowledgment
It is a pleasure to acknowledge the contributions by Mr. B. S. Tani, Analytical Chemistry Laboratory, who carried out much of the X-ray diffraction studies pertaining to the crystal sizes and disorder. The participation in the X-ray investigations by Miss Mary Kolar, student aide, is also gratefully acknowledged.
L i t e r a t u r e Cited ( 1 ) Yang, R. T., Cunningham, P. T., Wilson, W. I., Johnson, S. A.,
Adu. Chern. Ser., No. 139,149-157 (1975). (2) Cunningham, P. T., Holt, B. D., Hubble, B. R., Johnson, S. A., Siegel, S., Wilson, W. I., Cafasso, F. A., Burris, L., “Chemical Engineering Division Environmental Chemistry Annual Report”, pp 44-90, USERDA Report ANL-75-51, July 1974-June 1975. (3) Hubble, B. R., Siegel, S., Fuchs, L. H., Cunningham, P. T., “Chemical, Structural, and Morphological Studies of Dolomite in Sulfation and Regeneration Reactions”, Proceedings of the Fourth International Conference on Fluidized-Bed Combustion, pp 367-91, the MITRE Corporation, McLean, Va., May 1976. (4) Hubble, B. R., Siegel, S., Cunningham, P. T., J . Air Pollut. Control Assoc. 25, 1256 (1975). (5) Hubble, B. R., Siegel, S., Fuchs, L. H., Hoekstra, H. R., Tani, B. S., Cunningham, P. T., ibid., 27,343 (1977). (6) Haul, R. A. W., Wilsdorf, H. G. F., Nature (London), 167,945 (1951). (7) Haul, R. A. W., Wilsdorf, H., Acta Crystallogr., 5,250 (1952). ( 8 ) Harvey, R. D., Steinmetz, J. C., Enuiron. Geol. Notes, Ill. State Geol. Suru , 50, (1971). (9) Borgwardt, R. H., Harvey, R. D., Enuiron. Sci. Technol., 6,350 (1972). (10) O’Neill, E. P., Keairns, D. L., Kittle, W. F., Thermochim. Acta, 14, 209 (1976). Received for review October 31, 1977. Accepted July 18, 1978. This work was performed under the auspices of the U.S.Department of Energy.
Regulation of Copper Availability to Phytoplankton by Macromolecules in Lake Water Rene Gachter *, Joan S. Davis, and Antonin Mares Federal Institute for Water Resources and Water Pollution Control, CH-8600 Dubendorf, Switzerland
Approximately two-thirds of the copper in lake water remains in the nonfilterable residue of the following two-step procedure: 0.45 pm filtration followed by Diaflo UM-2 membrane (passing mol w t 1000) ultrafiltration. Examination of the effect of this residual copper on the photosynthesis rate of natural phytoplankton has shown that this residual copper is physiologically not available to the organisms. This fact can be utilized in a bioassay (as demonstrated with EDTA as a test substance) to gain information on the concentration of a ligand and its apparent association constant with copper. It is concluded that nonultrafilterable ligands present in lake water form copper complexes as stable as the CuEDTA complex and can thus play an ecologically significant role in the regulation of copper availability and therefore its toxicity to phytoplankton. Organic ligands may play a significant role in the regulation of phytoplankton growth. The observed growth stimulation in their presence is usually considered to be the result of an 1416
Environmental Science & Technology
increase in the availability of required trace metals due to complex formation ( I ) . However, on the basis of observations made with copper, the complexation could as well lower the availability of toxic substances which then would also explain the increased photosynthesis. Strong organic chelators such as NTA ( 2 , 3 ) ,EDTA (4-7), extracellular polypeptides, or a zooplankton extract (8)are known to partially or completely counteract copper toxicity. Pagenkopf et al. (9)and Andrew et al. ( I O ) have indicated that by increasing the bicarbonate alkalinity or ortho- or pyrophosphate concentration the toxicity is reduced by the formation of nontoxic inorganic complexes. An addition of FeC13, which in alkaline solution forms negatively charged colloids capable of binding copper, also has the same detoxifying effect (7). To what extent in nature lake organics complex copper, and, therefore, the ecological significance of this effect, are as yet unknown. Sunda ( 1 1 ) and Bundi ( 1 2 ) have shown that in a chemical-thermodynamic model an algal surface can be treated as a ligand having a certain complex formation capacity (Atot) and a certain complex-formation constant ( K ) .Thus, we can assume, as a first approximation, that copper poisoning is due 0013-936X/78/0912-1416$01 .OO/O
@ 1978 American Chemical Society
