Effect of ionic interactions on the oxidation of Fe(II) with H2O2 in aqueous solutions

Abstract

The oxidation of Fe(II) with H2O2 has been measured in NaCl and NaClO4 solutions as a function of pH, temperature T (K) and ionic strength (M, mol-L-1). The rate constants, k (M-1-sec-1), d[Fe(II)]/DT=-k[Fe(II)][2O2] at pH=6.5 have been fitted to equations of the form log k = log k0+ A I1/2+B I+C I1/2/T Where log k0=15.53-3425/T in water; A=-2.3, -1.35; B=0.334, 0.180; and C=391, 235, respectively, for NaCl (σ=0.09) and NaClO4 (σ =0.08). Measurements made in NaCl solutions with added anions yield rates in the order B(OH)4->HCO3->ClO4->Cl->NO3->SO42- and are attributed to the relative strength of the interactions of Fe2+ or FeOH+ with these anions. The FeB(OH)4+ species is more reactive while the FeCO30, FeCl+, FeNO3+ and FeSO40 species are less reactive than the FeOH+ ion pair. The general trend is similar to our earlier studies of the oxidation of Fe(II) with O2 except for B(OH)4-. The effect of pH on the logk was found to be a quadratic function of the concentration of H+ or OH- from pH=4 to 8. These results have been attributed to the different rate constants for Fe2+ (k0) and FeOH+ (k1) which are related to the measured k by, k=k0αFe + k1αFeOH, where αi is the molar fraction of species i. The rates increase due to the greater reactivity of FeOH+ compared to Fe2+. k0 is independent of composition and ionic strength but k1 is a function of ionic strength and composition due to the interactions of FeOH+ with various anions. © 1991 Plenum Publishing Corporation.