reaction rate is proportional to the square of undissociated hypochlorous acid concentration, concern should be shown for cases in which higher chlorine dosages may be used. One instance is the chlorine oxidation of wastewater sludges as a stabilization procedure prior to dewatering (35). In such processes chlorine doses of 2000 mg L-l are routine. If a sludge contains biorefractory organics such as biphenyl either dissolved in the aqueous suspension or physically or chemically adsorbed to the sludge particles (36),extensive chlorination of the organics may occur. If, as is the usual case, the liquid effluent from the subsequent dewatering process undergoes biological treatment, further chlorination, and discharge, release of significant concentrations of such chlorinated organics as chlorobiphenyls may result. Further work is warranted to verify these suppositions. Also, work is needed in the area of reactions of chlorine with biphenyl in the presence of ammonia and chloramines, as will normally be the case in actual water treatment practice. In addition, studies of the chlorinating abilities and other properties of C120 in aqueous chlorine solutions are needed. Extension of this to real and complex wastewaters would be desirable. Several time-consuming and intricate analytical separation steps would likely be required in this case, and a more definitive identification tool such as a gas chromatograph-mass spectrometer combination would be required. Literature Cited
(1) Poffenberger, N.; Hubbard, H. L. In “Encyclopedia of Chemical Technology”, 2nd ed.; Standen, A,, Ed.; Wiley: New York, 1968;Val. 7. (2) Chem. Eng. (N.Y.)1976,83(18),66-7. (3) Bellar, T. A,; Lichtenberg, .J. J.; Kroner?R. C. “The Occurrence of Organohalides in Chlorinated Drinking Waters”, 1974, 1J.S. Environmental Protection Agency; EPA-fiSOi4-74-008. (4) Harrison, R. M.; Perry, R.; Wellings, R. A. Enciron. Sei. Technol. 1976,10, 1156-60. ( 5 ) Lee, G. F.; Morris, J. C. Int. J . Air Water Pollut. 1962, 6 , 41931. (6) Glaze, W. H.; Henderson, .J. E. IV J Water Pollut. Control Fed 1975,47, 2511-5. (7) Glaze. W. H.: Henderson. ,J. E. IV: Bell. J . E.: Wheeler. V. A. J . Chromatogr, Sci. 1973, 21, 580-4. (8) Carlson. R. M.: Carlson. R. E.: Komerman. H. L.: Caole, R. E n uvon. Sci Technol. 1975,9, 674-5.‘ (9) Rook, J. J. Water Treat Exam 1974,23, 234-43. (10) Morris, J. C.; McKay, G. M. “Formation of Halogenated Organics ~
by Chlorination of Water Supplies”, 1975, U.S.N.T.I.S., P.B. Report 1975, No. 241511. (11) Weil, I.; Morris, J. C. J . Am. Chem. SOC.1949, 71, 1664-71. (12) Robertson, P. W. J . Chem. Soc. 1954,1267-70. (13) de la Mare, P. B. D.; Harvey, J. T.; Hassan, M.; Varma, S. J . Chem. Suc 1958,2756-9. (14) Brown, H. C.; Stock, L. M. J . Am. Chem. Soc. 1957, 79, 51759. (15) Edmond, C. R.; Soper, F. G. J . Chem. Soc. 1949,2942-5. (16) de la Mare, P. B. D.; Ketleg, A. D.; Vernon, C. A. J . Chem. Soc. 1954,1290-7. (17) Beaven, G. H.; de la Mare, P . B. D.; Hassan, M.; Johnson, E. A.; Klassen. N. V. J . Chem. Soc. 1961.2749-55. (18) Gaffney, P . E. Science 1974,183, 367-8. (19) Gaffney, P . E. J . Water Pollut. Control Fed. 1977,49, 401-4. (, 2 0,) Johnsen. R. “Chlorination of Waters for Disinfection-A Studv of the Production of Undesirable Chlorinated Products”, Proceedings of the National Conference on PCB’s, Chicago, Ill., 1975; EPA-56016-75-004. (21) Perrin, D. D.; Dempsey, B. “Buffers for pH and Metal Ion Control”; Chapman and Hall, Ltd.; London, 1974. (22) “Standard Methods for the Examination of Water and Wastewater”, 13th ed.; American Public Health Association: New York, 1971; pp 117-23. (23) Morris, J . C. J . Phys. Chem. 1966,70, 3798-805. (24) White, G. C. “Handbook of Chlorination”; van Nostrand Reinhold: New York, 1972; p 186. ( 2 5 ) Skrabal, A.; Berger, A. Monatsh. Chem. 1937, 70, 168-92. (26) Palin, A. T. In “Disinfection-Water and Wastewater”; Johnson, J . D., Ed.; Ann Arbor Science Publishers: Ann Arbor, 1975. (27) Albro, P . W.; Fishbein, L. J . Chronatogr, 1972,69, 273-83. (28) Safe, S. “Overview of Analytical Identification and Spectroscopic Techniques”, Proceedings of the National Conference on PCB’s, Chicago, Ill., 1975; EPA-56016-75-004. (29) Frost. A. A,: Pearson. R. G. “Kinetics and Mechanism”. 2nd ed.: Wiley: New York, 1961; pp 43-6. (30) Morrison, R. T.; Boyd, R. N. “Organic Chemistrv”. :3rd ed.; Allvn and Bacon: Boston, 1973; p 349. (31) de IaMare, P. B. D.; Hilton, I. C.; Varma, S. J . Chem. Soc. 1960, 4044-54. (32) de la Mare, P. B. D.; Ridd, ,J. H. “Aromatic Substitution”; Academic Press: New York, 1959; p 117. (33) Israel, G. C.; Martin, J. K.; Soper, F. A. J . Chem. SOC.1950, 1282-6. 134) Swain, C. G.; Crist, D. R. J . Am. Chrm. SOC.1972, 94, 3195-
‘Loo.
(35) “Process Design Manual for Sludge Treatment and Disposal”, 1974, U.S. Environmental Protection Agency, Technology Transfer Series; EPA-62511-74-06;pp 5-29. (36) Choi, P. S. K.; Nack, H.; Flinn, J. E. Bull. Enuiron. Contam. TOX~C 1974,Il-, O ~ . 12-7.
Rereiced /or reuieic. April 9, 1979. Accepted June 15, 1979.
Dissolution of Tetragonal Ferrous Sulfide (Mackinawite) in Anoxic Aqueous Systems. 1. Dissolution Rate as a Function of pH, Temperature, and Ionic Strength James F. Pankow*l and James J. Morgan Environmental Engineering Science, California Institute of Technology, Pasadena, Calif. 91 125
A rather insoluble compound, mackinawite often forms in reducing sediments and is believed to form in anaerobically digested sewage sludge ( I ) . The trace metal constituents of these systems, which also form insoluble metal sulfides (e.g., Ag+, Cd2+,Cu2+,Pb2+,Zn2+),very likely associate with the much more predominant iron sulfides. This material may be expected to a t least partially dissolve: (a) when reducing sediments are disturbed by animal life, dredging, or by more subtle events, and (b) when anaerobic sewage sludge is disPresent address, Department of Environmental Science, Oregon Graduate Center. 19600 N.W. Walker Road. Beaverton. Orep. 97005. 1248
Environmental Science & Technology
posed of in the marine environment. The interest in this problem from the perspective of sewage sludge may be more easily understood when it is realized that, currently, approximately 5,7,9,95,and 120 metric tons per year of silver, cadmium, lead, copper, and zinc, respectively, are being discharged from an ocean outfall of the city of Los Angeles anaerobic sewage sludge facility ( 2 ) . Mackinawite has been characterized fairly recently ( 3 )and was first identified in recent sediments by Berner ( 4 ) . It is believed to be one of the initial iron sulfides to form in iron‘learing sediments the Onset Of production$with greigite (spinel Fe&d and Pyrite (cubic FeSd being other possible early minerals ( 5 ) .
0013-936X/79/0913-1248$01.00/0 @ 1979 American Chemical Society
~
~~
+
Mackinawite (tetragonal FeS) is an important mineral phase in reducing sediments and is believed to form in anaerobic sewage sludge. T h e rates and mechanisms of dissolution of this compound are of interest because of the role it plays in the geochemical cycling of iron and sulfur as well as certain trace metals. An experimental study of the kinetics of the nonoxidation dissolution is described. The results in-
dicate a two-term rate law, F = kl[H+] h2, where F is the flux (mol/(cm2.min))of dissolving FeS, [H+] is the hydrogen ion concentration (mol/cm3),and k l and k2 are rate constants with the values 0.18 f 0.06 cm/min and (1.9 f 0.9) X loW9 mol/(cm2.min), respectively. The k1 term dominates a t pHs lower than -4.3.
Mackinawite is a mineral phase of considerable importance in the geochemical cycling of sedimentary iron and sulfur. In addition, it is to be anticipated that it plays an important role in determining the behavior of sulfide-forming trace metal constituents in anaerobic sediments as well as in anaerobic sewage sludge. This is due in large part to the fact that iron is typically present in much greater amounts than trace elements like cadmium, etc. This predominance makes very likely the association of these sulfide-forming trace metals with the coexisting mackinawite. An association of metal ion M2+ with solid FeS might manifest itself in three ways: adsorption, FeS-M2+; solid solution, (Fe,M)S;and/or nonsolid solution aggregation, (MS)(FeS). The third possibility would be favored by the availability of the FeS surface as a source of nucleation sites for the precipitation of other metal sulfides. In each of the above three cases, in the event that the FeS dissolves, the rate of release to the solution phase of these other metals will be a strong function of the dissolution rate of the FeS. The fact that mackinawite can be a precursor phase for pyrite (6-8) adds further impetus to studying the kinetics of dissolution of this compound.
where braces denote activity. The equation describing the complexation of ferrous ion by chloride has been included, since the dissolution reactions were carried out in chloride media. If we define:
Monitoring the Reaction Rate The fact that S2- and HS- are basic species implies that during the nonoxidative dissolution of FeS the pH of the solubilizing aqueous medium will increase. We have: FeS
-
Fez+
+ H+ HS- + H+ S2-
F!
+ S2-
(1)
HS-
(2)
H2S
(3)
A small amount of H+ is produced as a result of the dissolution due to the production of hydrolyzable Fez+ ions: Fe2+
+ H20
F!
FeOH+ + H +
(4)
When dissolving FeS into a solution of a strong acid, the proton balance equation (neglecting [OH-]) for the reaction may therefore be written for any time: -A[H+] = -([H+] - [H+]o) = [HS-]
+ 2[H2S] - [FeOH+]
(5)
where brackets denote concentration and [H+]orepresents the initial hydrogen ion concentration. The contributions of higher hydrolysis products of Fez+ (such as Fe(0H)e) have also been neglected. The initial sulfide and iron concentrations are assumed equal to zero. The equilibria that describe the pH-dependent speciation of aqueous sulfide and ferrous iron may be written:
(9)
we have
The meaning of 00, for example, is simply the fraction (0 < CY < 1) of total sulfide present as H2S. Equations 6-9 as well as a knowledge of the pH, [el-], and the appropriate activity coefficients are needed for the calculation of the CY values (9, I O ) . In this study, the pH was measured, [Cl-] was set, and the activity coefficients were calculated using the Davies equation (10). Since [FeItOt= [S],,, at all times during the dissolution, by substituting Equation 12 in 5 one obtains:
ISIM = -A[H+I/(2ao
+ ai - ( Y F ~ O H )
(13)
This equation describes the concentration of dissolved sulfide as a function of p H as well as such implicit variables as temperature ( T ) ,pressure (PI,ionic strength ( I ) ,etc. In the case of dissolution into a solution buffered by the acid/base pair HB/B-, the proton balance equation (13) must be modified, and reads: [Sltut =
([B-I - [B-Io
+ [H+lo - [H+])/ ( 2 ~+ ~ ai 0 - ~ F ~ O H (14) )
A knowledge of the pK, for the acid HB as well as the pH, T , etc., will allow the calculation ofIB-1. As in Equation 13, the contribution of the OH- ion is neglected. This approximation will introduce little error since the ([B-] - [B-]o)/[OH-] ratio will be greater than 500 in the buffered dissolution experiments discussed in this paper. It is apparent from Equations 13 and 14 that, in both buffered and unbuffered solutions, one can monitor the rates of the nonoxidative dissolution of FeS simply by following pH. c ~ in this study were taken The values of K1, "K1, and K F ~used from the critical compilation of Smith and Martell (I I ). A t 25 "C, their values were 9.55 X lop8 M, 3.16 X 10-l" M, and 1.00 M-I, respectively. Kz is known to be very small (-10-14 M). Consequently, its exact value is not needed since the species S2- may be neglected. K , was taken from ref 12. The values of the first acid dissociation constant for the buffer acids were determined experimentally. Experimental Unless otherwise stated, all chemicals employed were of reagent grade and were used without further purification. The compounds Mes (2-(N-morpholino)ethanesulfonic acid, pK, 6.2) and Mops (3-(N-morpholino)propanesulfonic acid, pK,
-
Volume 13, Number 10, October 1979
1249
/TEFLON
NYLCtd
SCREWS
/
TERON
/
FeS'
SIDE VIEW
BOTTOM VIEW
Figure 1. Pellet holder
= 7.3) were obtained from ICN Pharmaceuticals (Cleveland, Ohio). All Teflon parts and glassware were washed in a hot solution of a detergent, rinsed with deionized water, soaked in hot 6 M " 0 3 for 6 h, soaked in distilled-deionized water (DDW) for 6 h, and finally thoroughly rinsed with DDW. Mackinawite Synthesis and Characterization. Mackinawite was synthesized by the action of aqueous H2S on elemental iron. The DDW that mediated the reaction was deoxygenated for 24 h with 02-free (
