Environ. Sci. Technol. 1988, 22,899-907
Metal Ion-Sulfur ( I V ) Chemistry. 3. Thermodynamics and Kinetics of Transient Iron( I11)-Sulfur( I V ) Complexes Martha H. Conkllnt and Michael R. Hoffmann"
Environmental Engineering Science, W. M. Keck Laboratories, California Institute of Technology, Pasadena, California 9 1125 Stability constants for the formation of Fe(II1)-S(1V) complexes at p = 0.4 M were determined spectroscopically. Values of Kl = 10e.eM-l for Fe3+ SO:- + FeS08+and K4 = 107s3M-l for FeOH2+ SO?- + HOFeS03 were obtained. Raman measurements indicate that sulfite binds to the metal through oxygen. The kinetics of electron transfer between Fe(II1) and S(1V)have been studied, and a mechanism is proposed. Kinetic and mechanistic aspects of the Fe(II1)-catalyzedautoxidation of S(IV) are discussed in light of these results.
+
+
Introduction Transition metal ions such as Fe(III), Cu(II), Co(II), Co(III), and Mn(I1) have been shown to be effective homogeneous catalysts for the autoxidation of sulfur dioxide in aqueous solution (1). Jacob and Joffmann (2) have shown that Fe(II1) and Mn(I1) are the most effective catalysts at ambient concentrations for the catalytic autoxidation of S(IV) to S(V1) in cloudwater and fogwater. Mechanisms for the homogeneous catalysis by Fe(II1) and Mn(I1) that have been proposed include a free radical chain mechanism, a polar mechanism involving inner sphere complexation followed by a two-electron transfer from S(1V) to bound dioxygen and photoassisted electron transfer (1). In order to gain a fuller understanding of the initial steps involved in Fe(II1) catalysis of S(1V) autoxidation, we need to explore the interaction between Fe(II1) and S(1V) in the total absence of 02. When a pale yellow Fe(II1) solution is mixed with a colorless S(1V) solution, the resultant solution darkens immediately to reddish brown. The initial color fades quickly (within afew minutes) to a much lighter color. The initial color change is indicative of the formation of a Fe(II1)-S(1V) complex. UV/vis absorbance spectra of this complex are shown in Figure 1for two S(IV):Fe(III) ratios. The absorbance of these complexes appears as a very broad shoulder between 350 and 600 nm. The complexes are unstable and undergo a series of redox reactions that result in an aqueous mixture of Fe2+,S20$-, and SO:-. The Fe(II1)-S(1V) system is more complicated chemically than the analogous Cu(I1)-S(1V) reaction (3) due to pH-dependent speciation of Fe(II1) ( 4 ) , which involves numerous hydrolysis products (5) and polymers such as Fe(H20)63t,Fe(H20)60H2+,Fe(H20),(OH),+ and Fez(H20)8(OH)24t(see Figure 2). The reduction of Fe,? by HS03- at pH 2 is thermodynamically unfavorable [;.e., Fe3++ HS03- + Fe2++ H+ + SO3- O.O7V, (6, 7)] even when AGO for formation of a FeS03+complex is included in the calculation. In order to account for the above redox chemistry other forms of the reactants, such as S032-(which is a more powerful reductant than HS03-) or FeOH2+ (which is a stronger oxidant than Fe3+),must be considered. Other energetic changes such as stabilization of the products relative to the reactants, changes in Fe(II1) reduction potential due to changes in its coordination sphere, and energy released Present address: Department of Hydrology and Water Resources, University of Arizona, Tucson, AZ 85721. 0013-936X/88/0922-0899$01.50/0
Table I. Rate Constants for the Formation of Monocomplexesof Fe(II1) (26 "C) k, M-k' ligand (L)
Fe3++ L
Br- " Cl- b SCN-c
20 9.4 1.27 X lo2 (6.37 X103)d
so,"
HSOc FHF8 N3-
(5.0 x 1 0 3 ~ 11 (1.6 X 105)d 4' 2.9
"3
H,Oi
FeOH2+t L 2.6 x 1.1 x 1.0 x -2.4 -2.4 3.2 x
104 104 104 X
loSe
x io4: 1.45 x 105f 1038
3 X lo3 < k
> k2and k2[02]>> kl [these approximatipns are valid given the above discussion on the magnitude of stability constants (i.e., K i = k 2 / k - J and rate constants for ligand substitution]:
- -
At pH 4, p1 1, k1 0.04 s-l (from the time scale in Figure 6), and klK4 = 1.0 X 106 M-l s-l. If we compare the predicted rate expression of eq 33 to the composite rate law (eq 34) obtained by analysis of the empirical data given - d[S(IV)I = k[Fe(III)] [S(IV)]a2 (34) dt by the investigators listed in Table IV, we see that k = klK4.The value obtained for k given in Table IV is 1.2 X lo6 M-l s-l while the calculated value (based on data obtained in this study) of klK4 is 1.0 X lo6 M-l s-l. Thus, we can feel confident that the above mechanism may be a fairly reasonable representation of the reaction steps involved in catalytic autoxidation by Fe(II1). The active catalytic intermediate of eq 27 can be drawn:
/
H20+0.H
Fe(II1)
/
H20!yo -0+00
in order to reflect the known geometry for octahedrally coordinated Fe(II1) and to reflect the Fe-0-S mode of bonding as determined by our Raman measurements. Analogous catalytic species have been proposed for the Cu(I1)-S(1V) system (3) and verified for the Co(I1)-catalyzed autoxidation (49) of S(1V) by ESR and spectroscopic measurements. Although Fe has been found in high ( =100 /AM) concentrations in atmospheric droplets (50),much of this iron would be in the form of solid particles or colloids. Ferric oxides, such as Fe203,have been identified as components of airborne particles (51). Other sulfite complexes, such as a-hydroxyalkylsulfonates,have larger stability constants than the Fe(II1)-S(1V) complexes ( K for the HS03-/ HCHO adduct = [HOCH,SO,-]/ [HCHO]T[HSO~-]= 10' (52). Thus, given the stability constant K1 = 106.6for FeS03+,the ratio of [Fe(III)]T:[HCHO]Twould have to be approximately 30 at pH 2 or 3 X lo2at pH 5 for compa-
rable concentrations of FeS03+to coexist with HOCH2SO3-. Aldehydes have been found in much higher concentrations (52,53) than Fe(II1) in cloudwater, fogwater, and rainwater systems; therefore, S(1V) speciation is likely to be dominated by RC(OH)S03-chemistry rather than Fe(S03)n3-2n. Acknowledgments
We are indebted to Eric C. Betterton (Caltech) for his expert assistance throughout the latter stages of this work. We also acknowledge the personal support of our program officers D. Alan. Hanson (EPRI) and L. Swaby (EPA). Registry No. Fe3+, 20074-52-6; SO:-, 14265-45-3; FeOHz+, 12299-69-3.
Literature Cited (1) (a) Hoffmann, M. R.; Boyce, S. D. Adv. Environ. Sci. Tech. 1983, 12, 147-189. (b) Hoffmann, M. R.; Jacob, D. J. In SOz,NO and NOz Oxidation Mechanisms; Atmospheric Considerations; Acid Rain Precipitation Series; J. G., Calvert, Ed.; Butterworth Boston, 1984; Vol. 3, pp 101-172. (2) Jacob, D. J.; Hoffmann, M. R. J. Geophys.Res., C Oceans Atmos. 1983, 88, 6611-6621. (3) Conklin, M. H.; Hoffmann, M. R. Environ. Sci. Technol. (second of three papers in this issue). (4) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry; Wiley: New York, 1980. (5) Stumm, W.; Morgan, J. J. Aquatic Chemistry;Wiley: New York, 1981; pp 134-137. (6) Latimer, W. Oxidation Potentials; Prentice-Hall: Englewood Cliffs, NJ, 1952. (7) Huie, R. E.; Neta, P. J. Phys. Chem. 1984,88, 5665-5669. ( 8 ) GBlis, M. A. Ann. Chim. Phys. 1862, 65, 222. (9) Carpenter, H. C. H. J. Chem. SOC.1902,24, 1-14. (10) Danilczuk, E.; Swinarski, A. Rocz. Chem. 1961, 35, 1563-1572. (11) Albu, H. W.; von Schweintz, H. D. Ber. Dtsch. Chem. Ges. B 1932, B65, 729-737. (12) Bassett, H.; Parker, W. G. J. Chem. SOC.1951, 46, 1540-1560. (13) Dasgupta, P. K.; Mitchell, P. A.; West, P. W. Atmos. Environ. 1979, 13, 775-782. (14) Kuz’minykh, I. N.; Bomshtein, T. B. Zh. Prikl. Khim. (Leningrad) 1953,26, 3-8; Chem. Abs. 1953,47, 5832. (15) Pollard, F. H.; Hanson, P.; Nickless, G. J. Chromatogr. 1961, 5, 68-73. (16) Karraker, D. G. J. Phys. Chem. 1963,67,871-874. (17) Carlyle, D. W. Inorg. Chem. 1971, 10, 761-764. (18) Carlyle, D. W.; Zeck,0.F. Inorg. Chem. 1973,12,2978-2983. (19) Kao, C. F. Ph.D. Thesis, Henry Krub School of Mines, Columbia University, New York, 1979. (20) Hansen, L. D.; Whiting, L.; Eatough, D. J.; Jensen, T. E.; Izatt, R. M. Anal. Chem. 1976, 48, 634-638. (21) Nyberg, B.; Larsson, R. Acta Chem. Scand. 1973,27,63-70. (22) Van Eldik, R. Inorg. Chim. Acta 1980, 42, 49-52. (23) Van Eldik, R. Adv. Inorg. Bioinorg. Mech. 1984,3,275-309. (24) Tamura, H.; Goto, K.; Yotsuyanagi, T.; Nagayama, M. Talanta 1974, 21, 314-318. (25) b o ,C. N. Ultra-Violetand VisibleSpectroscopy,Chemical Applications; Plenum: New York, 1967. (26) Newton, T. W.; Arcand, G. M. J. Am. Chem. SOC.1953,75, 2449-2453. (27) Smith, R. M.; Martell, A. E. Critical Stability Constants; Plenum: New York, 1976; Vol. 4.
(28) Davis, A. R.; Chatterjee, R. M. J. Solution Chem. 1975,4, 399-412. (29) Harrison, W. D.; Gill, J. B.; Goodall, D. C. Polyhedron 1983, 2, 153-156. (30) Langford, C. H.; Gray, H. B. Ligand Substitution Processes; Benjamin: Reading, MA, 1966. (31) Hunt, J. P.; Friedman, H. L. In Progress in Inorganic Chemistry; Lippard, S. J., Ed.; Wiley: New York, 1983; Vol. 30, pp 359-382. (32) Weinland, R. F.; Reihlen, H. Ber. Dtsch. Chem. Ges. 1913, 40, 3144-3150; Chem. Abst. 1914,8, 637-638. (33) Perrin, D. D. J. Chem. SOC.1959, 1710-1717. (34) Leigh, J. S., Jr.; Reed, G. H. J. Phys. Chem. 1971, 75, 1202-1204. (35) Blumberg, W. E. In Magnetic Resonance in Biological (36) (37) (38) (39) (40) (41)
System; Ehrenberg, A., Malmstrom, B. G., Eds.; Pergamon: Oxford, 1960; pp 119-133. Accascina, F.; Cavasino, F. P.; D’Alessandro, S. J . Phys. Chem. 1967, 71, 2474-2481. Wendt, H.; Strehlow, H. 2. Elektrochem. 1962,66,228-234. Cavasino, F. P.; Eigen, M. Ric. Sci. Part 2 Sec. A 1964, A4, 609-522. Seewald, D.; Sutin, N. Inorg. Chem. 1963, 2, 643-645. Cavasino, F. P. J. Phys. Chem. 1968, 72, 1378-1384. Backstrom, H. L. J. 2. Phys. Chem., Abt. B 1934, 25B,
122-138. (42) Wilmarth, W. K.; Stanbury, D. M.; Byrd, J. E.; Po, H. N.; Chua, C.-P. Coord. Chem. Rev. 1983,51, 155-179. (43) Freiburg, J. Atmos. Enuiron. 1975, 9, 661-672. (44) Fuzzi, S. Atmos. Environ. 1978, 12, 1439-1442. (45) Brimblecombe, P.; Spedding, D. J. Atmos. Enuiron. 1974, 8, 937-945. (46) Neytzell-de Wilde, F. G.; Traverner, L. In 2nd U.N. In-
ternational Conference on Peaceful Uses for Atomic Energy Proceedings 1958; Vol. 3, pp 303-317. (47) Aubuchon, C. P b D . Thesis, John Hopkins University, Baltimore, MD, 1976. (48) Martin, L. R. In SOz,NO and NO2 OxidationMechanisms: Atmospheric Considerations;Acid Rain Precipitation Series; Calvert, J: G., Ed.; Butterworth Boston, 1984; Vol. 3, pp 63-100. (49) Hoffmann, M. R.; Hong, A. P. Sci. Total Enuiron. 1987, 64, 99-115. (50) Munger, J. W.; Jacob, D. J.; Waldman, J. W.; Hoffmann, M. R. J. Geophys. Res., C Oceans Atmos. 1983, 88, 5109-5121. (51) Fukasawa, T.; Iwatsuki, M.; Kawabuko, S.; Niyazaki, K. Anal. Chem. 1980,52, 1784-1787. (52) Munger, J. W.; Tiller, C.; Hoffmann, M. R. Science (Washington, D.C.) 1986,231, 247-249. (53) Munger, J. W.; Jacob, D. J.; Hoffmann, M. R. J. Atmos. Chem. 1984,1,335-350. (54) Connick, R. E.; Coppel, C. P. J. Am. Chem. SOC.1959,81, 6389-6394. (55) Matthies, P.; Wendt, H. 2.Phys. Chem. (Frankfurt) 1961, 30, 137-140. (56) Below, J. F., Jr.; Connick, R. E.; Coppel, C. P. J. Am. Chem. SOC. 1958,80, 2961-2967. (57) Davis, G. G.; MacF. Smith, W. Can. J. Chem. 1960, 38, 1836-1845. (58) Pouli, D.; MacF. Smith, W. Can. J. Chem. 1960,38,567-575.
Received for review May 1,1987. Accepted January 6,1988. This work was cooperatively supported by the U.S. Environmental Protection Agency (EPAR811496) and the Electric Power Research Institute (RP 1630-47).
Environ. Sci. Technol., Vol. 22, No. 8, 1988
907
