The oxidation of Fe(II) with H2O2 in seawater

Abstract

The oxidation of Fe(II) with H2O2 has been measured in seawater as a function of pH (2 to 8.5), temperature (5 to 45°C) and salinity (0 to 35). The effect of pH on the rate constant, k, d[Fe(II)]dt = −K[Fe(II)][H2O2] was found to be a linear function of [H+] or [OH−] from pH = 6 to 8. The effect of temperature and ionic strength on k at pH = 6.0 was given by log k= 13.73 −2,948/T− 1.70712 + 1.20I with a σ = 0.12 in log k. If the rates are expressed as d(Fe(II)]dt = −k2[Fe(II)][H2O2][OH−] the values of k2 are independent of temperature. This is due to the fact that the energy of activation of k is the same order of magnitude of ΔHw∗, the heat of ionization of water. The ionic strength dependence of k2 was given by logk2 = 11.72–2.14I12 + 1.38I with a σ = 0.11 in log k2. The results of k over the entire pH, temperature and ionic strength were given by k = k0αFe + k1αFeOH where αFe and αFeOH, k0 and k1 are the rate constants, respectively, for the oxidation of Fe2+ and FeOH+. The values of k0 and k1 are given by logk0 = 8.37− 1,866/Tlog k1 = 17.26 − 2,948/T− 1.70I12 + 1.20I. The addition of HCO3− at a constant pH was found to linearily increase the rate independent of the salinity and temperature. This may be related to FeCO30 reacting faster than FeOH+ with H2O2. At a given pH and ionic strength, the rates in seawater are nearly the same as in NaCl.