I .- Table 111. Measurement Dlfferences Between Methods 2&7k& A&--

Data Mean and edd SO, Ppm SO2 Bla8. ppm So2 a palrs tPararosanll ne Moan and estd SO Max Anaheim 160 0.0057 f 0.0071 0.0050 f 0.0034 0.0008 f 0.0065 0.024 Los Angeles 101 0.0191 f 0.0081 0.0071 f 0.0050 0.0120 f 0.0072 0.045 San Bernardino 130 0.0064 f 0.0067 0.0024 f 0.0017 0.0040 f 0.0072 0.029 San Diego 25 0.0019 f 0.0030 0.0035 f 0.0034 -0.0016 f 0.0040 0.013 -a Conductimetric SO2 minus pararosaniline SO*.

Literature Cited Table IV. Relative Bias at 0.04 ppm SO2 by Each Method (1) “Directory of Air Quality Monitoring Sites Active in lYS:l”, EPA-450/2-75-006, Mar. 1975. (2) Cooper, R. E., “Statistics for Experimentalists”, Pergamon Press, Conductimetric SO2 Pararosanlllne SO2 1969. Blas at Bias at (3) Blacker, J. H., Confer, R. G., Brief, 11. S., J. Air Pollut. Control 0.04 ppm Carrel 0.04 PP~ carrel Assoc., 23,525 (1973). Cond SO*,’ ppm cooff PRA S02, ppm coeff (4) Hocheiser, S., Santer, d., Ludmann, W. I?., ibid, 16. 2%; Anaheim 0.0282 0.65 0.0008b 0.08 (1966). Los Angeles 0.0267 0.79 0.0048 0.15 (5)~. Terabe. M.. Oomichi. S.. Benson. F. B.. Mewill. V. A.. Thomoson.., San Bernardino 0.0391 0.54 -0.0582c 0.40 J. E., ibid., i7,fjn (1967). San Diego 0.0258 0.55 -0.0301 0.67 (6) Staff Reo. 75-12-2. “Consideration of the California 24-Hour Mean 0.0300 -0.0207 ‘ Ambient kir Quality Standard for Sulfur Dioxide”, California Air Resources Board, June 11,1975. By linear regression. * Mean bias, not significant. 0.04 ppm is largest (7) Booras, S. G., Zimmer, C. E., J. Air Po‘Jut. Control Assoc., 18,512 meaningful negative bias. (1968). (8) Stevens, R. K., Hodgeson, J. A., Ballard, L. F., Decker, C. E., “Ratio of Sulfur Dioxide to Total Gaseous Sulfur Compounds and Ozone to Total Oxidants in the Los Angeles Atmosphere--An In- strument Evaluation Study”, in “Determination of Air Quality”. Conclusions G. Mamantov and W. D. Shults, Eds., pp 83-108, Plenun;, New Estimated bias between the conductimetric and pararos- York, N.Y., 1972. (9) “Air Qualitv Criteria for Sulfur Oxides”. National Air Pol1ut;on aniline methods at 0.04 ppm SO2 by the conductimetric ControiAdmin., Jan. 1969. method ranges to the order 0.04 ppm. (10) Katz. M.. “Inorpanic Gaseous Pollutants”, in “Air Pollution”. Estimated random measurement error ranges (99.7% con- ’ A. C. Stern,’Ed., 2iid ed., Chap. 17, Academic Press, New Yorlc. fidence) are f0.02 and f0.006 ppm for the conductimetric and N.Y., 1968. (11) Neuscheler, R. C., “Selection uf Continuous Sulfur Dioxide pararosaniline methods, respectively. Monitors for Ambient and Source Concentration Levels”, “In- The conductimetric method is unsuitable €or determining strumentation for Air Monitoring”, ASTM STP 555, pp 9-19, compliance with 0.04 or 0.05 ppm 24-h average ambient air American Society for Testing Materials, 1973. quality standards for SOZ. (12) Purdue, L. J., “Performance Evaluation of SO:! Monitoring In- struments”, ibid., pp :3--5. (13) “The Environmental Protection Agency’s Research Program with Primary Emphasis on the Community Henlth and Environ- Acknowledgment mental Surveillance System (CHESS):An Investigative Report”, for Committee on Science and Technology, US. House of Repre- N. R. Crawford and F. W. Morgan’s statistical advice and sentatives, Nov. 1976. assistance ant1 E. G. Loffay and J. R. Ugolini’s computer data handling were invaluable. Received /or review July 22, 1977. Accepted October 25, 1977.

Paul E. Framson* and James 0. Leckie Environmental Engineering and Science, Department of Civil Engineering, Stanford University, Stanford, Calif. 94305 ID

Cadmium was precipltatcd from aqueous sulfide to as- mium in the Corpus Christi, Tex., estuarine system demon- certain the of sulfide pre- strate that those sediments act alternately as a sink during the cipjtated in aquatic ecosystems. Monitoring cell-dimension summer months when high concentrations of sulfides are trends of cadmium sulfides precipitated from thioacetarlli& present in the surficiai sediments and as B source during the so!utiogs df increasing ferrous corltent allowed estimation of winter when more oxidizing conditions prevail (1).In uno& nrr ,lpper bound 3.5 f 1 x 10 -4 for the distribution coeffi- benthic marine, estuarine, and freshwater environments, cient describing F~Wcocrysta!lization with greenockite. ~1~i~sulfate-reducing bacteria can generate total uqueous sulfrde restilt suggeststhat cadmiurn prcciplbtes primarily ttlrclugh as high as millimolar concerrtration i2,3). Under sml‘ficiently surface exchange with ferrous monosulfide substrates and as reducing conditions* srdimenq mi~~dPhw essentially cnsubstituted CdS in natural sulfidic systems. such as hydrous oxides and bydroxidcs are repkced by SUI+ fides. The PI& of the heavy metal sulfides rmge fram z%? I for ZnS(s) to 53.2 for HgSiu) (2).Hence, a Iia-ga fratxi aqueous heavy metai contammmnt~is likely kmmd as Sediments possess the interesting ability to act as both n menhry sulfids under anaembic condition sink and a source for many trace contaminants. For example, Certain human activities such iL9 dredging, which dimp significant spatial and teinporal variations in zinc and cad- sediments, can effect oxidative diwlution of Ferrous sulfides,

00 13-936X/78/0912-0465$0 1.30/0 G) 1978 American Chemical Society Volume 12, Number 4, Aprd 1978 465 I prompting concern that associated heavy metals may also be where dB and dA represent increments of R+ and A+ pre- released from their sulfide phases. To resolve this concern, we cipitated, and [B+]and [A+] represent their aqueous con- must first ascertain the chemical nature of the heavy metal centrations. The heterogeneous distribution coefficient sulfide phase precipitated under natural conditions. describes cocrystallization resulting in a heterogeneous dis- Cadmium was selected as the subject in the endeavor to tribution of foreign species within the host lattice when answer this question due to the relatively extensive informa- aqueous solution composition changes during the course of tion concerning the aqueous and surface chemistry of cad- precipitation. X approaches D as the precipitation rate ap- mium and because Cd is known to be toxic. Because in aquatic proaches zero, sediments, iron sulfides are the dominant sulfides, cadmium Surface reaction constitutes not only a critical step in co- will exist in intimate aswciation with iron in the sulfide solid crystallization, but in and of itseif, a prominent course of phase. The possiliilities for cadmium precipitation as its sul- coprecipitation. Most widely documented of sulfide surface fide are: cadmium surface exchange with or adsorption onto reactions is the exchange reaction conforming to the me- iron(I1) sulfide substrate; solid solution formation of ferrous tathetical equilibrium and cadmium sulfide: cocrystallization as cadmium sulfide K host phase substituted with Fe2+,or cocrystallization as fer- A2/,,,S + 2/1zB+~ 2/mA+m + R2/,,S; rous sulfide host phase substituted with Cdz+; and precipi- - tation as esseiitially pure cadmium sulfide. This study K = K$;/K,:; (7) suggests which of thse phenomena are most probable. whereby aqueous cation B+, displaces cation A +*I from the Tizeory solid sulfide when the solubility of B,/,,S is less than that of We first consider the general thermodynamic boundaries A2/,,,S. Accordingly, ferrous sulfide, nearly the most soluble of equilihrium cocrystallization. The substitution of a foreign of the heavy metal sulfides, has been enlisted as an effective cation I?+ into the host solid AX may be considered a two-step analytical adsorbent for other heavy metal cations incliidir.fr process of wrface exchange followed by diffusion and incor- Cd2+ (51, whereas cadmium sulfide was found to remove froin poration into the crystal latticc (4), aqueous solution Fez+ most feebly among seven cations studied (6). A+(ads)+ Ij+(aq)*+ R+(ads) + A+(aq) (1) The metathetical reaction typically proceeds by rapid for- mation of a mono- to trilaver coating of the displacing cation. fl+(ads)+ AX(solid) ** R,Y(solid) A+(adsf + (2) whereas subsequent formation of a new sulfide crystalline the overall equilibrium heing phase proceeds much more slowly reflecting low solid state diffusion rates. Phillips and ICraus (7) document formation AX(mlid) + H+(aq) - BX(&lid) -I- A+(aq) (3) of new crystalline phases by Ag+ and Cu2+conversion of zinc, The ecluiiihrium distribrition coefficient D for Equation 3 is cadmium, lead, arsenic, or cupric sulfides. Garidin et al. (8) defined as also find Ag+ to convert zinc sulfide to a new crystalline phase, notably rejecting mixed crystal formation a5 a mechanism for this process. (4) James and Parks (9)find two models to describe moderately where x denotes mole fraction in the solid phase. Assuming well the nonmetathetical reaction of aqueous Zn2+with solid HgS. The first model postuiates a simple adsorption reaction LA:( 2 1,U can be forrnulaiedfrom thermodynamic parame- ters as whose driving force arises from electrostatic attraction be- tween surface and adsorbate. change in the solvation energy of adsorbate, and modification of the chemical free energy of the adsorbate; the other model postulates an exchange be- where K,, is the equilibrium ion activity product, y2 the mean tween adsorbed H+ and Zn2+ in the sulfide surface layer. aqueous activity coefficient product, f the solid state activity To speculate on the degree to which coprecipitation in the coefficient, R the universal gas constant, and T the absolute Fe-Cd-S system involves not only surface phenomena but temperature. AGAxequals the modification of the free energy crystal lattice phenomena as well, a brief examination of the of the AX latt,ice due to the suhstitution of R+, or through crystal chemistry of pertinent solids in that system is useful slight refor:nitlation. the change in the chemical potential of (Table I) (10-1.2). the solid solute HS due to nonideality of solid solution. Cadmium sulfide precipitates from aqueous solution at Equation 5 requires that, some HX be incorporated into the ambient temperatures and pressures as either of two poly- AX lattice when [R'] is nonzero. Were AcAx zero, D would types, greenockite and hawleyite. Both are founded on nearly equal the ratio of the ion activity products. Seldom zero close-packed arrays of sulfur, greenockite possessing hexa- however, Aci,y is miriimized and D maximized when the gonal- and hawleyite cubic-close packing. Hawleyite is met- mixed crystal least tiist,orts the host lattice and offers the so- astable to greenockite at 25-900 "C (13),but the two polytypes lute an environment most identical to that in its own Kure are so nearly isoenergetic that the coordination state of the solid. We speculate shortly on the relative values of AGi,y aqueous Cd2+specie kinetically influences which of the two terxs in the Fe-Cd-S system. precipitates. Halide ligands very strongly dictate greenockite This thermodynamic perspective on cocrystallization is crystallization whereas SO:- and NO,, which complex Cd+' pror.:erly qualified by the fact that cocrystallization does not less strongly, favor hawleyite crystallization (14).Therefore, ofmi proceed in a near equilibrium fashion because surface greenockite exclusively should precipitate from marine waters, exchange rates generaily radically exceed solid state diffusion whereas hawleyite should precipitate as well to a minor degree rates. Typically, there exist.5 a direct relationship between in freshwater environments. amount of a specie precipitated from solution and concen- A host of different iron sulfides form in aqueous systems tration remaining in solution at that, instant as stated in t.he (3, 2.5, 26).Mackinawite, FeS1-,, is thought to control aqueous !herner-Hoskins relation, [Fez+] in freshwater-reducing environments, whereas greigite, Fe,&, is suspected of controlling [Fez+]in some marine-re- dH ducing environments (17). Both of these forms are metastable -dA = A/ [H+]/[A+]I to yet other naturally occurring iron sulfides. However, be-

466 Environmental Science & Technology .'. fable 1. Crystal Chemistry of Minerals in Fe-Cd-S System

Bond Equli Melai Coordlnaiion ionic radius dlstances acttvity ion no., spin (A) Mineral Descriplion (a) product Cd2+ IV 0.84 Greenockite Sulfur in hcp array. Cd2+ occupies half the Cd-S = 2.53 VI 0.95 CdS tetrahedral interstices. Highly ionic Hawleyite Same as greenockite except sulfur in ccp array Cd-S.= 2.52 CdS Fe2+ IV High 0.63 Mackinawite Sulfur' in nearly ccp array. Fez+ tetrahedrally Fe-S = 2.23 lo-'' VI Low 0.61 FeS,-, coordinated, high spin. Strong Fe2+back-donation to Fe-Fe = 2.60 High 0.77 sulfur. Metallic bonding

a Modified from refs. 75- 17. ____-

cause mackinawite is that form essentially always precipitated l--7--l-I--l---l--IT initially, only mackinawite is considered in this discussion of Hydrothermal Condl4 coprecipitation with cadmium sulfide. 800 -950 'C Cadmium sulfide is among the most ionic of sulfides; 6.70 therefore, Fez+ substitution in the CdS lattice is controlled - primarily by geometric restrictions. The cubic- and hexago- "5 6.60 nal-close packed sulfur frameworks of the cadmium sulfides 0 are highly stable, quite independentljj of the identity of the cation that fi!ls the interstices (18).Pappalardo and Dietz (19) 6 50 '. found Fe" to diffuse at 600 "C into the tetrahedral interstices of CdS. In fact, Skinner and Bethke (20) found Fe2' to sub- stitute to at least 0.47 rrlol fraction in hydrothermally (800-950 "C) formed greenockite, despite the approximate 25% dif- l'erence in fourfold coordinated Cd2+ and Fe2+ionic radii (see Figure 1). (Mole fraction of cation 1 is here defined as nc ,/(Z;L,) nc,),where nc, is the number of moles of Ith cation I C, in a solid containing m distinct cations.) It is anticipated that, Fez+ substitutes to a similar degree in hawleyite, the

immediate CdSf- polyhedra being essentially identical in the ua2-- two polytypes. Relatively then, the term for Fez+ co- 3900 01 02 03 04 05 06 07 08 09 10 crystallization with greenockite and hawleyite is expected to Fe/(Fe+Cd), Mole Fraction favor this process. in Solid Geometrical restrictions alone highly disfavor Cd2+ sub- Figure 1. Cell dimensions of hydrothermal Fe-bearing greenockites stitution into mackinawite, considering that the Cd-S bond (modified from reference 20) distance exceeds the Fe-S bond distance by 0.3 A. Addition- ally, mackinawite possesses a highly covalent bond character due to strong Fez+ d-orbital back-donation to sulfur. This covalency is enhanced by metallic d-orbital overlapping be- initially equimolar in ferrous iron and cadmium. Hopefully, tween neighboring Fez+ ions, which display a conspicuously these two solids could then be separated and analyzed to de- low Fe-Fe distance of 2.60 A. By populating the (r* energy termine the extent of cocrystallization. Solutions, LO-,'3 to 10-1 levels and overlapping unfavorahly with both neighboring molar NazS, CdS04, and FeS04, were prepared in essentially anions and cations, Cd2+ would highly destabilize the covalent anoxic conditions, and slow conventional precipitations were mackinawite structure. effected by "titrating" one of the reactant species with the This prediction is supported by comparison with the Fe- other with constant stirring, perfusion with Nr, and in some Zn-S system. Pyrrhotite, Fel-,S, is a highly covalent ferrous cases, pH->tatting. This approach confirmed the thertnody- sulfide similar to mackinawite. The ionic radii of tetrahedrally namic prediction that no ferrous suliide forms until ecseii tidy coordinated high spin Fe2+ and 2n'+ differ by only 6%, and all Cdz+ has precipitated from holution w!xn aqueous sulfide the Zn2+ orbital energy levels are mure nearly isoenergetic is not present in excess. Coiiditions of ,illtide exesproduce with those ofFe'+ than are those of Cd' +.Suggested then by an intractable ferrous sulfide colloid iil u hich no cadmium better orbital overlap and geometrical compatibility, Zn?+ sulfide is detected visually. Furthermclre, this approach is substitution in pyrrhotite is favored more highly than Cdz+ plagued by the following adversities: solids formed are nearly substitution in mackinawite. Yet, Barton and Toulmin (21) universally colloidal rendering separation and ditaiytirai found that Zn2+ substitutes insignificantly at 850 OC-Iess techniques difficult; FullI) Ion oxidation by 02is difficult to than 0.006 mol fraction intu the pyrrhotite structure. Hence. eliminate entirely; and the precipitation microenvironment the ACF~Sterm is predicted to highly disfavor Cd2+ cocrys- cannot be controlled sensitively enough to prevent &iht tal!ization with mackinawite. formation of iron hydroxides. After attempts to precipitate only CdS &rumrsoiutinn Experimentd Work and Results varied [Fe2+! were frustrated by cogeneration 3f irc~n. In anticipation that ferrous sulfide and cadmium sulfide droxides, the entire conventional prpeipiution tec behave as two solids of limited mutual miscibility, work began discarded in favor of precipitation from hotnogeneow solution with an attempt to precipitate both solids from a solution (PFHS), whereby one of the reactant precipitating species is

Volume 12. Number 4, April 1978 487 I the cell dimension values and respective standard deviations appearing in Tables I1 and I11 and Figures 3 and 4. Simultaneous attempts to produce ferrous sulfide by TA.4 hydrolysis succeeded only under mildly alkaline conditions and were complicated by the cogeneration of minor amounts of iron hydroxides.

Discussion and Conclusion The trends in CdS cell dimensions demonstrate that F$+ does not cocrystallize significantly with CdS over the range of aqueous solution compositions studied at 25 "C. Due to similarity in crystal structure, hawleyite would be expected to exhibit a decrease in cell dimension with increasing iron content similar in magnitude to that of greenockite. Data from precipitate Gg allow calculation of an upper limit for the distribution coefficient, D=(%> / (--->[Fe2+] X(~Sgreenockite [Cd"] aq Noting that greenockite cell dimension c exhibits the greatest sensitivity to FeS mole fraction in the solid, we apply Skinner and Bethke's (20) equation relating FeS mole fraction to cell Figure 2. Comparison of powder patterns of hawleyite precipitated by dimension c to the observed difference in c dimension cx- thioacetamide hydrolysis (left) and by conventional mixing (right) hibited by greenockites GI and G8. This calculation generates a maximum XF~Svalue for greenockite Gg of 0.074 f 0.019. Gg precipitated from a solution possessing an average value of formed slowly and uniformly throughout the precipitation [Fe2+]/[Cd2+]equal to 2.3 X 102 during the course of precip- medium. PFHS allows standardization of the precipitation itation (computing nonchlorocomplexed ion concentrations; microenvironment and reproduction of the slow rate of pre- stability constants are from refs. 3 and 23). The above dis- cipitation in natural sediments. This slow precipitation ad- tribution coefficient is then at maximum 3.5 f 1 X IO-*. ditionally generates a more highly crystalline precipitate Comparison with the work of Rittner and Schulman (13) (Figure 2) and allows a closer approach to equilihrium dis- yields insight into the chemical hasis of this very low distri- tribution of species 1)etwrc.nsolid and aqueow solutions. bution coefficient. They found Cd2+ to cocrystallize signifi- Subsequent CdS precipitations were effected by slow acid cantly with HgS under conditions similar to those of this hydrolysis of thioacetamide (TAA) under strict anoxic con- ditioi1.i. Separate series utilizing C1- and SO:- environments were run to etiat)le study of both hawleyite and greenockite. These soliris were precipitated from solutions of varied Fez+ concentrations, aged for 10 days to two weeks, and freeze dried. Debye Schrrrer powder diffractions were performed Table II. Hawleyite Precipitations by TAA Hydrolysis on the precipitates in an attempt to detect any trends in cell dimensions that would signal Fez+ cocrystallization. The Precipllale cell dimension hawieyites were irradiated for 20 h with Mn-filtered Fe Ktu Precip-ltation Conditions (CdFFej and SD (A) radiation, and the greenockites for 12-15 h with Ni-filtered HI Total [metal] e 0.1 M 0 5.833 f 0.002 Cu ILu radiation. The reflection Bragg angles were measured H2 Total [SO:-] N 0.1 M 0.050 5.83 f 0.01 by traveling microscope, modified by internal standard cor- H3 [TAA] N 0.025 M 0.10 rections (except in the cases of two hawleyite samples), and Initial pH = 1.5 indexed. The resultant data were then subjected to a least- H4 Final pH = 1.5 0.15 5.834 f 0.001 squares ccll dimension refinement program (221, generating H5 T = 22-25OC 0.20 5.93 f 0.01

Table 111. Greenockite Precipitations by TAA Hydrolysis a

Preclpitale cell dlmenslons Initial initial lnitlai and SD Cdt Fet ITAAl lnlliai lnlliai (molar) (molar) (molar) PH (molar) Fe,/(Cdr + Fer) D (A) c (A) 0.0800 0 0.02 i.4 0.200 0 4.130 f 0.001 6.722 ik 0.005 '0.0800 O.OC80 0.02 1.4 0.216 0.091 0.0800 0.0200 0.02 1.4 0.240 0.200 0.0800 O.O~,OC) 0.02 1.4 0.280 0.33 4.129 f 0.001 6.703 zk 0.002 0.0800 0.0609 0.02 ? .4 0.320 0.428 0.0800 0.0800 0.02 1.4 0.360 0.50 4.131 f 0.002 6.696 f 0.005 0.0400 0.0600 0.01 1.4 0.240 0.60 4.1305 f 0,0009 6.695 f 0.002 0.00335 0.0301 0.003 1.5 0.100 0.90 4.1295 f 0,0009 6.695 f 0.002 0.00336 0.0634 0.003 1.5 0.167 0.95 0.00337 0.33 0.003 1.5 0.70 0.99 No CdS precipitate 0 00335 2.5 0.03 1.5 5.0 0.995 No CdS precipitate

468 Environmental Science & Technology term, the Aab,s term must highly disfavor Cdit cocrystalli- zation with FeS. Hence, despite the favorable value of 108 for Kr;’/KLdbdS,Cd2+ cocrystallization with ferrous sulfide at ambient conditions is predicted to be as limited as Fez+ CO- crystallization with cadmium sulfide. 1 z Though formulation of a well-grounded conclusion de- 0 mands further study, especidly of Cd2+ cocrystallization with ferrous sulfide, this study suggests strongly that crystalline 570 - solids precipitated from the aqueous Fe-Cd-S system are highly unsubstituted with the foreign cations Fe*+ or Cd“. The dominant forms of cadmium precipitation in a natural sulfidic system are, therefore, predicted to be Cd2+ surface exchange with ferrous sulfide substrate, and precipitation as essentially unsubstituted CdS. Figure 3. Cell dimension trend of hawleyites precipitated from solutions of increasing Fe mole fraction Acknowledgment The authors thank Gordon E. Brown, Michael B. Nelson, Mark Taylor, and Keith Keefer for their technical advice and 6.80 ~~I~IIIII assistance.

I‘ 670 - 0 00 0- Literuture Cited 1 05 (1) Holmes, C. W., Slade, E. A., McLerran, C. d., Enuirun. Sci. 0 Technol., 8,255 (1974). 6.60 - (2) Leckie, J. O., Nelson, M. B., “The Role of Natural Heterogeneous Sulfide Systems in Controlling the Concentration and Distribution of Heavy Metals”, Dept. of Civil Engineering, Stanford University, 6.50 Stanford, Calif., presented at the 2nd Int. Symp. on Environmental Biogeochemistry, Burlington, Ont., Canada, Apr. 1975. 0 (3) Goldhaher, M. B., Kapinn, I. R., in “The Sea”, Vol 5, E. D. 410b 7 Goldberg, Ed., Wiley-Interscience, New York, N.Y., 1974. (4) Walton, A. G., “The Formation and Properties of Precipitates,” Interscience, New York, N.Y., 1967. (5) Caletka, R., Tymbyl, M., Kotas, P., J. Chromatop., 111, 93 ZYC, pH I5 (1975). (6) Tyagai, V. A., Petrova, N. A.,‘I’reskunov, R. L., Electrokhim., 4, (2 ), 179 (1968). (7) Phillips, H. O.,Kraus, K. A., J Chromatop., 17, 549 (1965). (8) Gaudin, A. M., Fuerstenau. D. ‘A”. Turkanis, M. M., Truns. AIME, 268,65 (1957). (9) James, R. O., Parks, G. A., AIChE Symp. Ser., 150,157 (1975). (10) Wuensch, B. J., Mineral. Soc. Am.. Short Course Notes, 1, W1 (1974). (11) Shannon. R. P., Prewitt, C. T., Acta Crystallogr., B, 25 f5),925 (1969). study. Consider again the thermodynamic formulation of the (12) Uda, M., Z. Anorg. AUg Chem., 361,94 (1968). equilibrium distribution coefficient: (13) Rittner, E. S., Schulman, J. H., J. Phys. Chem., 47, 537 (194:;). (14) Sato, R., Itoh, H., Jpn. J. tlppl. Phys., 3 (lo),626 (1964). (15) Berner, R. A,, J.Ceol., 72,293 (1964). (16) Ricknrd, D. T.,Stock. Coritrib. Ceol., 20,49 (1969). For Cd2+ cocrystallization with HgS, (17) Doyle, It. W., Am. J. Sci., 266,980 (1968). (18) Evans, €3. C., “Crystal Chemistry”, Cambridge Univ. Press, /($S/KCdS = 10-52/10-26 10-26 London, England, 1964. YP (19) Pappalardo, I<., Dietq it. E., f’hys. Reu., 123 (4), 1188 (1961). wiiercas for Fez+ cocrystallization with CdS, (20) Skinner, B. J., Bethke, P. M., Am. Mineral., 46, 1382 (1961). (21) Barton, P. B., Jr., Toulmin, P., Econ. Geol.. 61,815 (1966). KsCpdS/Kr;S = 10-26//10-18 = 10-8 (22) Evans, H. T., Jr., Appleman, D. E., Handwerker, P. S., Am. Crystallogr. Assoc., Ann. Meetinj; Program, 42 (1963). Since the ratios of solubilities favor Fez+ cocrystallization with (23) Sillen, I,. G., Martell, A. E., “Stability Constants of Metai-Ion CdS fw more than Cd2+cocrystallization with HgS, the Complexes”, Suppl. # 1, Cheniical Society, I.ondon, England, AG& 1971. term must, relative to the AZHgsterm, disfavor Fe2+cocrys- tallization with CdS. Considering that the i\r&term was Received for review Junuury 6, 1977. AcceptcBd October 31, 1977. predicted to favor cocrystallization far more than the AGF,~ Partial support from Air Force contract F296Ol- 75-C-0028.

Volume 12, Number 4, April 1978 469