NOTES
Nerve Gas-Isoprop yl Methylphosphonofluoridate (GB)Decomposition and Hydrostatic Pressure on the Ocean Floor W. A. Adams Inland Waters Directorate, Department of the Environment, Ottawa, Ont., Canada
The half-life of the hydrolysis of isopropyl methylphosphonofluoridate (GB) is estimated to increase by 25% at a depth of 4000 meters and 69% at a depth of 10,000 meters.
I
n his article, “Rate of Decomposition of GB in Seawater,” (Epstein, 1970), did not comment on the effects of hydrostatic pressure on the rate of hydrolysis of isopropyl methylphosphonofluoridate (GB). Since the pressure generated by seawater is approximately 1 bar (0.987 atm)/lO-meter depth and the average overall depth of the world’s oceans is 4 X l o 3 meters, the pressure environment of GB leaking from disposal containers after deep-sea burial is 0.4 kbar. Considering the same three hydrolysis reactions as Epstein that remove the fluoride from G B to make it biologically harmless [reactions with: (a), MgOH’, rate constant k S ; (b), CaOHT, rate constant k?‘; and (c), OH-, rate constant kn”] and the three hydrolysis equilibria associated with the hydroxide reactants involved [(a) K,, (b) Ka’, and (c) K,], the effect of pressure on the overall first-order rate of decomposition was estimated in this work. Partial molal volumes of the species involved in the hydrolysis equilibria were used to calculate the overall volume changes, A Po, associated with K, ($25 cm3 mol-’), with K,’ ($9 cm3 mol-’), and with K , (-20 cm3 mol-’). Assuming that the alkaline hydrolysis reactions of GB are similar to the alkaline hydrolysis of esters and amides (Laidler and Chen, 1958) and considering the linear correlation found between the entropy of activation, AS:, and the volume of activation, AV:, (Laidler and Chen, 1959), a volume of activation of -12 cm3 mol-’ was adopted as a value that fitted the AS:, of -24 e.u. found experimentally for the hydrolysis of GB (Larsson, 1957). Equilibrium constants (and the reaction rate constants) are related to pressure by the following equation first discussed by Planck (1887).
Equation 1 indicates that at 1kbar for a AV of +20 cm3mol-l, the ratio K (1 kbar)/K (1 atrn) is 0.45 while for a AV of -20 cm mol-’, the ratio K (1 kbar)/K (1 atm) is 2.24. The equilibrium constants and rate constants adjusted to 0.4 kbar are substituted into Equation 2 quoted by Epstein (1970), for the overall first-order rate constant for the decomposition of G B at a constant p H of 7.7. The first-order rate of decomposition, k , for constant pH, was expressed by Epstein as follows :
where C, and C,‘ are the activities of Mg*+ and Ca*+ respectively in seawater. The pH decreases with depth in the ocean by approximately 0.02 p H units/1000 meters (Skirrow, 1965) and so this effect was neglected as was the effect of pressure on volume concentrations for this zero-order calculation. For 0.4 kbar and 2 5 T , the value obtained for the reaction half-life is 65 min. At 1 kbar pressure (equivalent to 10,000 meters depth), the half-life is 88 min. The half-life is 52 min (Epstein, 1970) at 1 atm pressure, which indicates an increase in half-life of 25 % at 4000 meters depth and 69 % at 10,000 meters depth. The uncertainty in the pressure estimate is admittedly large (and hard to estimate), but the order of magnitude and sign of the effect is certainly not in doubt. It is not safe to ignore the hydrostatic pressure when the disposal of such toxic chemicals in the ocean depths is contemplated. A second pressure effect of interest when assessing risk is that the rate of diffusion of chemicals in aqueous systems under high pressure is greater (up to 1-2 kbar, and from 0 to -30°C) than at atmospheric pressure. This is a factor that will influence the concentration gradient of G B in the proximity of a leaking container on the ocean floor. On the subject of pressure effects on chemical reactions and equilibria, a good foundation has been begun experimentally and theoretically on many systems, so that rough estimates can be made of the modified behavior caused by high pressures (Hamann, 1963). Unfortunately there is little data available on the effects of pressure on diffusion (Barton and Speedy, 1970; Horne, 1969). The importance of these considerations is worth emphasizing with respect to underground nuclear tests and deep-well disposal practices, since the fate of radioactive and/or toxic material released into the sea or groundwater deep in the earth is dependent on the high pressures found in these environments. Literature Cifed
Barton, A. F. M., Speedy, R. J., High Temp.-High Pressures, 2, 587-95 (1970). Epstein, J., Science, 170, 1396-8 (1970). Hamann, S. D., in “High Pressure Physics and Chemistry,” Vol 2, Chap. 7 and 8, R. S. Bradley, Ed., Academic Press, New York, N. Y., 1963. Horne. R. A,. “Marine Chemistry.” Chap. 3, Wiley-Interscience, New York, N.Y., 1969. Laidler, K. J., Chen, D., Trans. Fclraduy Sac., 54, 1026-33 (195 8). Laidler, K. J., Chen, D . , Can. J . Chem., 37, 599-612 (1959). Larsson, L. Actu Chem. Scund., 11,1131-42 (1957). Planck, M., Ann. Phys. Chem., 32,462-502 (1887). Skirrow, G., in “Chemical Oceanography,” Vol 1, p 267, J. P. Riley and G. Skirrow, Eds., Academic Press, London, 1965. Receicedfar reciew M a y 15, 1972. Accepted July IO, 1972.
928 Environmental Science & Technologj
