The rate of oxidation of sulfite, S(IV), in seawater was measured as a function of pH (4.0- 8.5), temperature ( 15-45°C), and salinity (0-35). The observed rate constant, k, in seawater at a pH = 8.2 was found to be second order with respect to S(IV) and half order with respect to oxygen: -d[S(IV)] dt = k[S(IV)]2[O2]0.5. The resulting values of k (M-1.5 min-1) have been fitted to a function of ionic strength, I, and temperature, T(K): log k = 19.54 - 5069.47/T + 14.74I0.5 - 2.93I - 2877.0I0.5/T,. and the standard error is 0.05 in log k. The energy of activation was found to be a function of salinity and has a value of 140 ± 6 kJ mol-1 at S = 35. The rates measured in 0.57 M NaCl were found to be higher than the rates in seawater. Measurements made in the major sea salts indicate that Ca2+, Mg2+, and SO42- added to NaCl cause the decrease. Measurements made in artificial seawater (Na+, Mg2+, Ca2+, Cl-, and SO42-) were found to be in good agreement with the measurements in real seawater. The rate increased from pH 4 to a maximum at pH 6.5 and decreased at higher pH. The effect of pH on the rates was attributed to the rate-determining step involving the combination of HSO3- and SO32-. This yields k = k″αHSO3-αSO3 2-. where αi is the molar fraction of species i. Values of k″ equal to 6.66 ± 0.06 and 6.17 ± 0.17 were found for NaCl and seawater, respectively. The larger range of k″ in seawater is due to it being a function of pH. The addition of Mn2+ was found to increase the rate apparently due to the formation of MnSO3. Additions of Fe3+ and Fe2+ have a catalytic effect only before they hydrolyze to colloidal iron.
ASJC Scopus subject areas
- Geochemistry and Petrology