Environ. Sci. Technol. 1989, 2 3 , 458-461
Scott, A. G. In Radon and Its Decay Products in Indoor Air; Nazaroff, W. W., Nero, A. V., Eds.; Wiley: New York, 1988; Chapter 10. U S . Environmental Protection Agency A Citizen’s Guide to Radon; U.S. EPA: Washington, DC, 1986; OPA-86-004. Sextro, R. G.; Harrison, J.; Moed, B. A.; Revzan, K. L.; Turk, B. H.; Grimsrud, D. T.; Nero, A. V.; Sanchez, D. C.; Teichman, K. Y. In Indoor Air ’87 Proceedings of the 4th
International Conference on Indoor Air Quality and Climate; Seifert, B., Esdorn, H., Fischer, M., Ruden, H., Wegner, I., Eds.; Institut fur Wasser-, Boden- und Lufthygiene: Berlin, 1987; Vol. 2, pp 295-299. Sextro, R. G., Lawrence Berkeley Laboratory, unpublished data, 1987. Sextro, R. G.; Moed, B. A.; Nazaroff, W. W.; Revzan, K. L.; Nero, A. V. In Radon and Its Decay Products: Occurrence, Properties and Health Effects;Hopke, P. K., Ed.; ACS Symposium Series 331; American Chemical Society:
Washington, DC, 1987; pp 10-29.
Received for review June 9,1988. Accepted November 8, 1988. This work was supported by the Director, Office of Energy Research, Office of Health and Environmental Research, H u m a n Health and Assessments Division and Pollutant Characterization and Safety Research Division, and by the Assistant Secretary for Conservation and Renewable Energy, Office of Building and Community Systems, Building Systems Division of the U S . Department of Energy (DOE) under Contract No. DE-AC0376SF00098. I t was also supported by the Office of Radiation Programs, of the US.Environmental Protection Agency ( E P A ) through Interagency Agreement D W89932609-01-0 with DOE. This manuscript has not been subjected to E P A review. Its contents do not necessarily reflect the views of E P A . Mention of firms, trade names, or commercial products do not constitute endorsement or recommendation for use.
Rates and Mechanisms for the Hydrolysis of Carbonyl Sulfide in Natural Waters Scott Elliott,” Erlc Lu, and F. Sherwood Rowland Department of Chemistry, University of California, Irvine, California 927 17
Hydrolysis rates, hydrolysis activation energies, and net solubility have been measured from pH 4 to 10 for carbonyl sulfide (OCS), with analysis by headspace gas chromatography. Hydrolytic OCS loss has only two rate-determining steps, distinguishable at the acidic and basic extremes. The acid channel is unimolecular, and the alkaline path has yielded monothiocarbonate as initial product. Rate control can be assigned to carbonate analogue hydration reactions and the corresponding lack of equilibrium between OCS and monothiocarbonate to a dominance of sulfide formation over dehydration. This mechanism is corroborated by pH independence for OCS solubility, and within it, monothiocarbonate serves as a fleeting hydrolysis intermediate under conditions applicable to most natural waters. Calculated steady-state thiocarbonate concentrations lie several orders of magnitude below those of
ocs.
Introduction Carbonyl sulfide hydrolysis has been studied for several decades as a secondary reaction in viscose ripening ( I , 2 ) and, over a similar period, as a means for removal of sulfide from industrial product and waste gas ( 3 , 4 ) . It has also recently been recognized as the first, and so far the only, quantifiable source of the hydrogen sulfides (H2S,SH-, S2-) to open oxic seawater ( 5 , 6 ) . The latter development has led directly to detection of open ocean sulfide (7-9) and to investigation of the bisulfide ion as a new seawater ligand (6, 10). The distinct channels known for hydrolysis in acidic and alkaline media can be represented stoichiometrically by the processes OCS + HzO H2S + C02 (1) +
OCS
+ OH--
SH-
+ COZ
(2)
By analogy with the carbonate system, OCS is expected to hydrate on dissolution to form the monothiocarbonate (MTC) species shown in Figure 1. MTC is a likely intermediate during overall conversion to sulfide, both in the laboratory (2)and in natural waters (5). The rate-deter458
Environ. Sci. Technol., Vol. 23, No. 4, 1989
mining step for OCS loss at pH > 10 has been identified (2) as the hydration +B in the figure, and reaction order is consistent with +A in acid (11). In the present work, we describe headspace gas chromatographic measurements of rates and activation energies for reactions 1 and 2 from pH 4 to 9 and argue from our results that the same two hydrations are rate controlling in all cases. This conclusion suggests a lack of equilibration between OCS and MTC at -A/-B or, equivalently, that MTC is involved in hydrolysis irreversibly. OCS
- MTC
sulfide
(3)
Polymer chemists have observed sequence 3 behavior in strongly alkaline solution where the parent lifetime is short ( I , 2 ) and monitor dithiocarbonates as fleeting intermediates down to pH 9 in CS2 experiments (12,13). We also report here that the net solubility of OCS is pH independent, in direct contrast with the C02/carbonates, and lending support to sequence 3 over a wide range (pH 4-10). A simple model of the Figure 1 scheme is constructed in order to organize and evaluate potential hydrolysis mechanisms. We draw hydration rate constants from our own data, rates for conversion of MTC to sulfide from the viscose motivated work, and estimates of unknown values from the carbonate comparison. The available information is unified if step F dominates dehydration. Under such circumstances, OCS concentrations are several orders of magnitude larger than those of steady-state MTC in most natural waters. Experimental Section
Reagents and Chemicals. Carbonyl sulfide (97.5%) obtained from Matheson Products was degassed while frozen in liquid nitrogen (77 K) and then mixed with helium before introduction to the reaction vessel. Aqueous solutions were prepared from deionized Milli-Q water. Unbuffered solutions were not degassed and so were presumed to be at equilibrium with atmospheric carbon dioxide initially (pH < 6). Hydrolysis adds hydrogen sulfide and eventually increases the acidity. Commercial buffer systems from Fisher Scientific produced solutions at pH
0013-936X/89/0923-0458$0 1.50/0
0 1989 American Chemical Society
OCS
OCS
+ H20 + OH-
+A -A
+c Jr-c
+a e OCS
'
E
OCS , H20 '
OH-
F -A
C02
+ H2S
C02
+ SH-
Table I. Rate Constant Measurements and Corresponding Initial Solubilities (S, Dimensionless)
T 281 281 293 293 294 294 294 294 294 295 298 298 302 302 302
+D~/-D
ocs ,02Flgure 1. Potential pathways for the carbonyl sulfide hydrolyses through monothiocarbonate intermediates.
4 (KH2P04),7 (K3P04-NaOH), 8 (K,P04-NaOH), and 9 (NaB407-10H,0). All runs were checked before and after with a pH meter, and in several instances, the reactor was fitted with a sealed electrode to monitor pH during the course of the reaction. Sodium chloride was added to some experiments a t 0.5 M as a test for salt effects. Procedure. A 1-L Pyrex gas storage bulb pretested for OCS stability was evacuated to Torr on a vacuum line and checked for leaks. Between 15 and 20 Torr of OCS was admitted and frozen into an incorporated cold finger. The bulb was then filled to 780 Torr with helium and the OCS allowed to thaw and mix. Acid-washed 50-mL bulbs equipped with side arms for half-hole septa were filled with aqueous solution until a measured headspace of
