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Response to Comment on “A Kinetic Study of the Oxidation of S(IV) in Seawater” SIR: Zhang and Millero’s (1) paper, published in Geochimica et Cosmochimica Acta, was not cited by Vidal and Ollero (2) because it was not found in the databases consulted (NTIS, Chemical Abstract, and Compendex, among others). Neither was it found by other authors who studied the oxidation of SO2 in water (refs 3-9) nor by Ermakov et al. (10), who in their article entitled “Sulfite Oxidation: The State of the Art of the Problem” (1997) cite 71 publications. The rate equation obtained by Zhang and Millero is second order with respect to S(IV) and a half order with respect to oxygen. Vidal and Ollero observed first order in S(IV) and zero order in oxygen. In regard to the discrepancies relating to the reaction orders, we would like to point out that Clarke and Radojevic (11) obtained a pseudo-first order (R ) 1.07) in S(IV) for experiments in distilled water, and they cite other authors who obtained the same result (Penkett et al., Brimblecombe and Spedding, Schroeter, Larson et al., Winkelmann, Fuller and Crist). Furthermore, Beike et al. (12) obtained a pseudo-first order (R ) 1.08) with respect to S(IV). In that paper, they mention other authors who obtained the same result (R ) 1) (Miller and Pena, Scott and Hobbs, McKay, Titoff, Fuller and Crist, Winkelmann). What is more, they cite three authors who obtained a zero order (β ) 0) with respect to oxygen (Titoff, Winkelmann, Riccoboni et al.). We must also note that while Zhang and Millero (1) and Clarke and Radojevic (11) used very low concentrations of S(IV) (5-10 and 10-100 µM, respectively) to study the oxidation kinetic of S(IV) in seawater (natural or artifical), we used high concentrations (660-4240 µM) in order to simulate the outflow from the absorption towers in the FGD systems. Figure 1 clearly shows how in our experiments the firstorder model in S(IV) gives a better fit than the second-order model. In our study, the reaction order with respect to oxygen was determined by measuring the dissolved concentration of S(IV) and oxygen in the water. Thus, the limiting effect of the absorption of oxygen into the liquid does not play a role. In Figure 5 of our original paper (2), the initial oxygen concentration is nearly zero because prior to the beginning of the experiment all of the oxygen was purposely eliminated so that later the oxygen concentration could be increased to saturation, to have a high degree of variation in the dissolved oxygen concentration. To quantify the SO2 desorption, the hypothesis that the bubbles become saturated with SO2 is not correct. Our experiments corroborate that fact. Moreover the concentration of dissolved SO2 is not the same as the total S(IV) concentration, except at very low pH values. In the tests in which air was used (30 L/h), a pH of 6 was maintained. At this pH, the concentration of dissolved SO2 is 0.005% of the total S(IV) concentration. Even assuming that the bubbles become saturated with SO2 and that the SO2 concentration is the same as the S(IV) total concentration, the characteristic desorption time for SO2 is far greater (tdes ) 1847 s) than the characteristic oxidation time (toxid ) 67 s). We should note that the S(IV) concentrations used in the desorption experiments are of the same order as those used 818
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FIGURE 1. Comparison between the first- and second-order models in S(IV).
TABLE 1. Experiments with Different Oxygen Concentration and Similar Kinetic Constant [S(IV)] × 104 (mol/L)
[O2] × 104 (mol/L)
experimental pH initial final initial final k × 103 s-1 air oxygen
6 6
18.5 29.7
0 0
0.3 10
2.1 10
17.55 17.58
R2 0.9972 0.9979
in the kinetic experiments. In Table 5 of ref 2, there is a mistake in the S(IV) concentration. Table 5, when corrected, reads as follows:
gas
flow rate (L/h)
N2
7
pH
[S(IV)] × 104 (mol/L) initial
av desorption rate (mol/L × 10-7)
2 6
11 15
1.58 0.58
In Figure 5 of ref 2, the value of the kinetic constant is 0.01755 s-1 when air is used and 0.01305 s-1 when oxygen is used, but that variation is not due to the fact that the oxygen concentrations are different. If it were, the order with respect to oxygen would be negative and even further from that proposed by Zhang and Millero. In the tests at pH 6, because of the unavoidable experimental variability, kinetic constant values between 0.013 and 0.018 s-1 were obtained, but with regard to oxygen, an order of 0 was always obtained. To give an example, Table 1 shows the results of another experiment with oxygen that shows a similar value for k (0.01758 s-1). With regard to the multivariate nonlinear regression analysis used in ref 2, we should point out that first experiments with constant oxygen concentration were carried out to determine the reaction order with respect to S(IV) (R ) l). After that, experiments were performed varying both the oxygen concentration and the S(IV) concentration, using a multivariate regression (the Levenberg-Marquardt method) to correlate the data. Zero order with respect to oxygen and, again, first order with respect to S(IV) were obtained, which proved to be a good fit.
Literature Cited (1) Zhang, J.-Z.; Millero, F. J. Geochim. Cosmochim. Acta 1991, 55, 677-685. 10.1021/es011422r CCC: $22.00
2002 American Chemical Society Published on Web 01/19/2002
(2) Vidal B., F.; Ollero, P. Environ. Sci. Technol. 2001, 35, 27922796. (3) Ravindra, P. V.; Rao, D. P.; Rao, M. S. Ind. Eng. Chem. Res. 1997, 36, 5125-5132. (4) Kumar, S. S.; Govindarao, V. M. H.; Chandas, M. J. Chem. Technol. Biotechnol. 1996, 67, 39-52. (5) Kumar, S. S.; Govindarao, V. M. H.; Chandas, M. J. Chem. Technol. Biotechnol. 1997, 69, 209-225. (6) Vladea, R. V.; Hinrichs, N.; Hudgins, R. R.; Suppiah, S.; Silveston, P. L. Energy Fuels 1997, 11, 277-283. (7) Shaikhl, A. A.; Zaidi, S. M. J. React. Kinet. Catal. Lett, 1998, 64 (2), 343-349. (8) Govindarao, V. M. H.; Gopalakrishna, K. V. Ind. Eng. Chem. Res. 1995, 34, 2258-2271. (9) Uttam, R.; Datta, S.; Utpal, R.; Mukherjee, N. C. IE(I) J.-CH 1996, 76, 29-33.
(10) Ermakov, A. N.; Poskrebyshev, G. A.; Purmal, A. P. Kinet. Catal. 1997, 38 (3), 295-308. (11) Clarke, A. G.; Radojevic, M. Atmos. Environ. 1983, 17 (3), 617624. (12) Beilke, S.; Lamb, D.; Miller, J. Atmos. Environ. 1975, 9, 10831090.
F. Vidal B.* and Pedro Ollero Department of Chemical and Environmental Engineering University of Sevilla Camino de los Descubrimientos s/n 41092 Sevilla, Spain ES011422R
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