Correspondence Comment on “A Kinetic Study of the Oxidation of S(IV) in Seawater” SIR: Vidal and Ollero in a recent paper (1) report a different kinetic equation of S(IV) oxidation by O2 in seawater that contradicts the results from previous studies in the literature (2-4). The authors concluded that the order with respect to S(IV) is 1 and the order with respect to oxygen is 0. Clarke and Radojevic (2) studied the oxidation of S(IV) by O2 in sea-salt solutions (0.005-0.5 M NaC1, MgCl2, and CaCl2) and found the order with respect to S(IV) is 2 in various sea-salt solutions of different concentrations. They also found the catalytic effect of chloride and bromide ions. Clarke and Radojevic (3) also studied this reaction in seawater and found the second order with respect to S(IV) in seawater. We (4) published a systemic study of this reaction in seawater in 1991. This work, unfortunately, was overlooked by Vidal and Ollero (1). In our study, the rates of oxidation of S(IV) were measured as a function of temperature (15-45 °C), pH (4.08.5), and salinity (0.2-35) as well as the effect that Mn and Fe have on the rate of oxidation. The rate equation in seawater observed by us is second order with respect to S(IV) and a half order with respect to oxygen. The discrepancy in the orders with respect to both reactants in the rate equation is due to the inadequate design of kinetic experiments in Vidal and Ollero’s work and resulting errors in their rate measurements. To maintain a constant oxygen concentration, air or pure oxygen was continually bubbling through the seawater during their kinetic experiments. There are two problems associated with bubbling air through the seawater solution. First, the observed changes in the concentration of O2 in their experiments is a result of a combined effect of chemical reaction, physical transport and dissolution through the gassolution interface. Therefore, the change in O2 concentration during the reaction cannot be used to quantify the reaction kinetics of O2 itself. As observed from Figure 5 in ref 1, O2 is almost depleted at the initial stage of the reaction, indicating that the transport is a limiting factor on the rate of oxidation of S(IV) at millimolar S(IV) concentrations. The fluctuations of O2 concentration shown in Figure 5 reflect the effect of bubbling on the dynamics of supply of O2 from the gas phase to the bulk solution. Second, the authors overlooked the side effect of bubbling on the loss of S(IV) in seawater due to stripping of SO2 by the carrier gas (air or oxygen). To estimate the magnitude of S(IV) loss due to bubbling, one can assume that the equilibrium is reached between the bubbles of stripping gas and the SO2 in seawater; the stripping process should follow a simple first-order Rayleigh curve. The characteristic time (t) required to reduced the concentration of SO2 in seawater by the factor l/e is given by
t ) βVP/F
(1)
where β is the Bunsen solubility coefficient of SO2 at the temperature of solution (β ) 30.6), V is the volume of the solution, F is the volume flow rate of the carrier gas at STD, and P is the pressure in the atmosphere (5). Equation 1 indicates that the rate of loss of SO2 is proportional to the flow rate of the carrier gas and the concentration of SO2 in the solution. Given the seawater volume of 0.5 L, t is only 30 min for a flow rate of 30 L/h and 2 h for 7L/h. Given a typical 10.1021/es011338b CCC: $22.00 Published on Web 01/19/2002
2002 American Chemical Society
reaction time of 10-15 min, the loss of SO2 due to the stripping process is a significant part of the loss due to oxidation. Although the authors estimated the loss of S(IV) due to bubbling using N2 as a stripping gas, the design of the experiment is inadequate to obtain a reasonable estimate on the loss. The flow rate in the kinetic experiment is 7-30 L/h, and only 7 L/h was used in the N2 stripping experiments. Moreover, the concentrations of S(IV) used in the stripping experiments are at a level of 0.1 µM, 4 orders of magnitude lower than that used in kinetic experiments (1-4 mM). Since the loss of SO2 due to stripping is proportional to the concentration of S(IV) in the solution, S(IV) concentrations at the millimolar level should be used in the N2 stripping experiments to obtain a reasonable estimate of the loss term due to bubbling. Furthermore, the spectroscopic technique (6) used by the authors is certainly not sensitive enough to measure the changes in S(IV) concentrations at 0.1 µM levels. A more sensitive analytical method was not mentioned in their paper. We calculated the first-order rate constants using the results in Figure 5 of ref 1 and found that the rate constant is 0.0155 s-1 for air and 0.0100 s-1 for pure oxygen in seawater at pH 6. The differences between air and oxygen are more than 50%, indicating that the order with respect to oxygen cannot be 0. In addition to the bubbling problems, the multivariable nonlinear regression method appears to be subject to a large error in determination of the order of reactants in comparison to traditional techniques such as integration, isolation, and differential methods (7).
Literature Cited (1) Vidal B., F.; Ollero, P. Environ. Sci. Technol. 2001, 35, 27922796. (2) Clarke, A. G.; Radojevic, M. Atmos. Environ. 1983, 17, 3, 617624. (3) Clarke, A. G.; Radojevic, M. Atmos. Environ. 1984, 18, 12, 27612767. (4) Zhang, J.-Z.; Millero, F. J. Geochim. Cosmochim. Acta 1991, 55, 677-685. (5) Weiss, R. F.; Craig, H. Deep-Sea Res. 1973, 20, 291-303. (6) West, P. W.; Gaeke, G. C. Anal. Chem. 1956, 28, 12, 1816-1819. (7) Millero, F. J. Physical Chemistry of Natural Waters; WileyInterscience: New York, 2001.
Jia-Zhong Zhang* Cooperative Institute for Marine and Atmospheric Studies Rosenstiel School of Marine and Atmospheric Science University of Miami 4600 Rickenbacker Causeway Miami, Florida 33149 and Ocean Chemistry Division Atlantic Oceanographic and Meteorological Laboratory National Oceanic and Atmospheric Administration 4301 Rickenbacker Causeway Miami, Florida 33149
Frank J. Millero Division of Marine and Atmospheric Chemistry Rosenstiel School of Marine and Atmospheric Science University of Miami 4600 Rickenbacker Causeway Miami, Florida 33149 ES011338B VOL. 36, NO. 4, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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