Effect of ionic interactions on the oxidation of Fe(II) and Cu(I) in natural waters

Frank J. Millero

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The author's recent studies on how ionic interactions affect the rates of oxidation of Fe (II) and Cu (I) with O2 in natural waters are reviewed. The oxidation of these metals has been measured as a function of pH, temperature, ionic strength and ionic composition. The oxidation of Fe (II) was found to be second order with respect to H+ or OH- over a wide range of temperature, ionic strength and ionic composition. These results indicate that Fe(OH)20 is the reactive Fe(II) species. At a constant pH and ionic strength, various anions were found to change the rates of oxidation of Fe(II). The rate constants were in the order HCO3- > Br- > ClO4- > NO3- > Cl- > SO42- > B(OH)4-. This order was attributed to the relative strength of the interactions of these anions with Fe2+. The strong interactions of Fe2+ with SO42- and B(OH)4- were used to estimate the stability constants of log βFeSO4=1.8±0.1 and log βFeB(OH)4 = 3.2±0.1 at 25°C. The addition of Mg2+ at a constant pH was found to decrease the rate of oxidation of Fe(II). This was attributed to the decrease in Fe(OH)20 as a result of the formation of MgOH+. The oxidation of Cu(I) was strongly dependent on the Cl- concentration. At low ionic strengths Cu+ and CuCl0 are the reactive species, whereas at 6 m the CuCl2- is also reactive. The effect of Mg2+ and HCO3- on the rate was determined as a function of chloride concentration (1-6 m). The addition of Mg2+ causes the rate to decrease and the addition of HCO3- causes the rate to increase. The possible causes of these effects are discussed. The rates of oxidation of Cu(I) have been measured in mixtures of NaX+NaClO4, where X=Cl- Br- and I-. The rate constants at a given halide concentration are in the expected order, κCl > κBr > κI. The rate constants for CuCl0 and CuBr0 were the same within experimental error. By assuming that the rate of oxidation of CuI0 is the same as these species, a value of log β1* = 5.7 ± 0.7 has been calculated for the stability constant of CuI0. This value agrees with the expected trend in the stability constants CuCl0 < CuBr0 < CuI0.

Original languageEnglish (US)
Pages (from-to)1-18
Number of pages18
JournalMarine Chemistry
Issue number1-3
StatePublished - Dec 1989

ASJC Scopus subject areas

  • Oceanography
  • Chemistry(all)
  • Environmental Chemistry
  • Water Science and Technology


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