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the mixed micellar solutions. Acknowledgment The authors wish to acknowledge Mike Stapleton and Robert Reed, who did much of the experimental work. Also, we thank Mrs. Jean Gullion and Mrs. Murph Thomas for typing the manuscript. Literature Cited Corkill, J. M.; Goodman, J. F.; Tate, J. R. Trans. Faraday SOC. 1966, 62, 979. Hansen, R. S.; Craig, R. P. J . Phys. Chem. 1954, 58, 211. Lange, H. I n "Solvent Properties of Surfactant Solutions"; Shinda, K., Ed.; Marcel Dekker: New York, 1967; p 142.
Lucassen-Reynders, E. H. "Progress in Surface and Membranes Science", Cadenhead, D. A.; Danlelli, J. F., Ed.; Academic Press: New York, 1976; Vol. 10, p 253. Mukerjee, P.; Mysels, K. J. "Critical Micelle Concentrations of Aqueous Surfactant Solutions"; National Bureau of Standards Reference Data Series No. 36, U.S.Government Printing Office, Washington, DC, 1971. Ottewill, R. H. In "Nonionic Surfactants"; Schick, M. J., Ed.; Marcel Dekker: New York, 1967; p 642. Rubingh, D. N. I n "Solution Chemistry of Surfactants"; Mial, K., Ed.; Plenum Press: New York. 1979, Vol. 2, p 337. Schwartz, A. I n "Surface and Colloid Science", Metejevic, E., Ed.; Wiley-Interscience: New York, 1972; Vol. 5, p 203.
Received f o r review September 8, 1981 Accepted December 24, 1981
Interactions of CaCO, Minerals in Electrolyte Solutions Perla Kaushansky Department of Isotope Research, The Weizmann Institute of Science, Rehovot, Israel
The interactions between a mineral and electrolytes are regulated by a number of factors. The present studies propose a mechanism of interactions based on an experimental case, where the rate is controlled by a precipitation reaction and influenced by the surface of the solid phase. The experiments are limited to the interaction between a carbonate mineral and various salt solutions at room temperature. The changes of the pH and the variation of the ionic concentrations of the solutions were monitored as a function of time. X-ray, scanning electron microscope, and electron microprobe observations show that the process of interaction primarily occurs on the surface of the soli phase and then results in two extreme possiblities: partial dissolution or considerable replacement of cations, essentially a recrystalllzatbn of the initial solid phase. The extent of penetration into the host crystal is regulated by the specific chemical reaction.
Introduction The interactions between a mineral and electrolytes (aqueous solutions) are regulated by a number of factors (Zeller and Wray, 1956; Chave and Schmalz, 1966; Mimran, 1977). For example, in the interactions between gypsum and salt solutions (Kaushansky and Gat, 1977) the mechanism is basically controlled by the tendency of the cations in solution to form insoluble phases in the presence of CaS04.2H20. In the present study an interaction mechanism is proposed based on laboratory experiments, where the rate is controlled by a precipitation reaction and influenced by the surface of the solid phase. This study is limited to the interaction between a carbonate mineral (CaCO,) and electrolytes in an experiment where the Kspof the newly formed salt and the grain size of the initial solid phase are definite. Experimental Setup a n d Results The experiments are performed by the interaction between a carbonate rock (limestone) and various salt solutions (MgC12,BaCl,, ZnC12, and FeS04) at room temperature; 1 M solutions were used and the grain size of the rock was controlled, usually 840 pm. The contact between the reactants from the bulk solution and the substrate occured by "filtration conditions"; i.e., the solutions percolated through a vertical column of the substrate by gravity flow for 20 days (Figure 1). The change of the pH and the variation of the ionic concentrations of the solutions were monitored as function of time (by atomic absorption spectrophotometry, Perkin-Elmer model) (Figure 2). The solid phase was examined by X-ray analyses (X-ray powder diffraction, Philips PW-1380) in order to identify the mineral phase. The location of the interaction 0196-4321/82/1221-0182$01.25/0
in the initial lattice was determined by scanning electron microscope and electron microprobe observations (YXA-5). When MgClz interacts with CaCO,, MgCO, does not form because it is more soluble than the CaC0,. In fact the X-ray spectra of CaC0, after the experiment indicate the same reflections as before (Figure 3). The scanning electron microscope and microprobe analysis show no change in the initial crystal. At the same time the pH and the cationic concentrations in the solution remain fairly constant during the experiment. In the case of the formation of BaCO,, the process of interaction does not continue indefinitely. The SEM photo shows a thin black envelope around the crystal of CaC03, apparently of BaC03. The microprobe analysis indicates that Ba forms a coating around the CaC0, crystals which finally stops the process (Figure 4). This process is accompanied by changes in the pH of the solutions from 2.40 to 7.07. The cationic composition changes at the beginning of the interaction for about the first 30 h (Figure 2). In the interaction of CaC03 and a solution of ZnC12,an envelope also forms, but with a considerable replacement of cations, essentially a recrystallization. This recrystallization is well demonstrated in microprobe analysis (Figure 5). The Ca2+ion is replaced by the Zn2+ion, which forms the new crystals. Also the SEM photograph of the solid phase afte'r experiment shows newly formed crystals. The X-ray data show reflections of ZnCO, indicating the formation of new reaction products (Figure 3). The chemical analysis of the solution indicates that the amounts of Ca2+and Zn2+increase and decrease respectively in parallel (Figure 2). T h e pH of the solution change from 4.34 to 6.20. This is a case of recrystallization with replacement. 0 1982 American
Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21. No. 2, 1982
ro'
FILTRATION CMAMBER
& Figure 1. The apparatus of the ewriment by filtration conditions.
183
Figure 3. X-ray diffraction spectra of carbonate (1) before and (2) after reaction with solutions of MgCll and (3) after reaction with solution of ZnC1,.
b
04
02
O i Z i
2
io
1-
.-
TIME I h o ~ r s l
'
154
*2
180 ~
220
0
Figure 2. Variations in the ionic composition with time resulting from the interaction between carbonah and various electrolytee, by filtration conditions.
Another type of interaction occurs by an anion-displacement process with destruction of the initial CaCO, solid phase by the chemical reaction with FeS04.7H20. In the initial solution the pH changes considerably (from 2.75 to 6.74) and the amount of Ca2+in solution changes very little, although the concentration of Fez+changes drastically (from 4.15 g L to 0.017 g/L). The recrystallization occurs without Ca4+-Fe2+ replacement. m e concentrations
Figure 4. X-ray electron microprobe of crystals in the interaction system: CaCO, + BaCI,. (a) SEM picture; (h) toppaphie survey; (c)Ca-scan; (d) Ba-scan.
of Ca2+and Fez+ in the solution change only a t the beginning of the process (30 h), while the ion Fez+after first decreasing starts to increase and with time approaches its initial concentration (Figure 2). Moreover, the X-ray spectrum contains additional reflections indicating the formation of Fe2C03(0H), together with CaS04.2H,0 (Table I). Visual analysis by scanning electron microscopy shows that the surface of the CaCO, crystal is less smooth after the experiment. The microprobe analysis confirms this with a diffuse zone around the CaCO, crystal and by the presence of Fe and SO, ions together with Ca ions
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Ind. Eng. Chem. Prod. Res. Dev.. Vol. 21. No. 2, 1982
Table 1. Results of X-ray Diffraction Determination of the Solid phase by Reaction CaCO, + FeS0..7H. - .0 ~
1 2 3 4 5
CaCO., initial
CaCO, + FeSO;7H,O Fe2CO,(OH)..4H,O CaS0,.2H,O FeCO, ~~~~~~~~
CoC03k.olidl
t
~~~
R 8725
~
7.5252 7.5252 7.56
4.2568 4.2467 4.27
3.7824 3.7824
3.0153 3.0557 3.0455 3.06
3.59
2.8245 2.8598 2.8553 2.79
2.4859 2.6727 2.6727 2.270
2.2739 2.2123
2.0878 2.0832
2.183
2.1351
1.9049 1.8937 1.9201 1.908 1.73
ZnCl2Lrolution)
b
d
d
Figure Figure 5. X-ray electron microprobe picture of crystals in the in-
teracting system: CaCO, + ZnCI,. (a) SEM-picture; (b) topographic survey; ( c ) Ca-scan; (d) Zn-scan.
(Figure 6). At the same time the dissolution of CaCO, occurs. This is a case of recrystallization, but the replacement is not stoichiometric.
Discussion The possibility of destroying carbonate rocks non-tectonically is enhanced during surface chemical precipitation reactions between carbonates and chemical solutions. In each type of interaction a different parameter affects and regulates the activity of the solid exposed to the solution. When MgC1, interacts with CaCO,, the changes in the X-ray spectra of CaC0, indicate a decrease in the amount of crystalline material, i.e., partial dissolution without formation of a new crystal. In this case the process stopped at the first stage of the interaction development. In the case of the formation of BaCO,, the process of recrystallization does not continue indefinitely hut tends to favor the formation of an envelope around the CaCO, crystal, which stops the process. The interchange of the ions Ca2+-Zn2+can occur by recrystallization. Because the solubility product of ZnCO, is low, the Zn2+ion enters into the CaCO, crystal and causes the formation of a new crystal-ZnC0,. Another type of interaction occurs by an anion displacement process. Where a FeS0,.7H20 solution interacts with a carhonate the recrystallization occurs without Ca2+-Fe2+replacement. Anion displacement ocCUIS and CaSO, forms with destruction of the initial CaCO, solid phase, which is less smooth after the experiment. This may be due to the deposition of amorphous iron compounds. The results in all cases of the described interactions show that the pH and cationic compositions of the initial solution change essentially at the beginning of the process and then remain constant. Also the deposition of the cations (Ba, Zn, Fe) occurs on the periphery of the initial CaCO, crystal. The interaction between carbonate rock and electrolytes may occur as a result of either of two mechanisms. The first of these is the dissolution of the surface layers of the
6. X-ray electron microprobe picture of crystals in the interacting system: CaCO, + FeSO,.IH,O. (a) SEM picture; (b) Ca-scan; (c) Fe-scan; (d) S-scan.
carbonate (as for MgZ+)which ceases when the adjacent solution becomes saturated. The second mechanism (as for Zn2+)is one wherein there is a simultaneous destruction of the old crystal and formation of a new one by ionic replacement. In addition to cases where only one of these mechanisms operates, there are cases where both operate giving intermediate results (as for the cations Fez+ and Ba2+). In summary, the prof interactions between aqueous solutions and carhnate minerals occurs primarily on the surface of the solid phase, but the extent of penetration into the host crystal is regulated by the specific chemical reaction. Conclusions 1. The interaction process between carbonates and electrolytes has been shown to occur primarily on the surface of the lattice. 2. The major parameters which affect the interaction are the solubility product of the newly formed salt and the nature of the cationic constituents. The interaction affects the pH of the system. 3. The interaction has two extreme results: partial dissolution of the rock and recrystallization of the system, involving ion replacement. Acknowledgment I have greatly benefited from the valuable discussions with Professor J. R. Gat of the Isotope Department, The Weizmann Institute of Science, Fkhovot, Israel, during the preparation of this paper. Thanks also to D. Shafranek of the Hebrew University of Jerusalem for his devoted work with microscope analysis. Literature Cited Chave. K. E.: Schmalz. R. F. Oeochlm. h m h l m . Acta 1988. 30, 1037. K a ~ ~ h a n s k P.; y . Gal. J. R. "Roceedingoof Me Second lnlemallonsl S y m p slum On Water-Rock Interaction"; Swasbourg. 1977, VoI. I I . p 65. Mlmran. J. Sedimentology 1977. 2 4 , 333. Zeller. E. J.: Wray. J. L. Boll. A m . A s s m . Pet. oeokrglsts 1958. 40. 140.
Receiued for review July 17, 1981 Accepted October 5, 1981
