Reduction of Fe(III) with sulfite in natural waters

Abstract

The Fe(III) in marine aerosols and rainwaters can be reduced to Fe(II) by photochemical processes and by reactions with sulfite. In this paper, measurements of the rates of reduction of nanomolar levels of Fe(III) with sulfite (without 02) have been determined in NaC1 and seawater solutions as a function of temperature (0 ø to 40øC) + 2+ ' pH (2 to 6.8), ionic strength (I = 0.1 to 6 M), and composition (Na , Mg , Ca 2+, F-, CI-, Br-, HCO•-, SO4:-). The overall rate constant (k, M -1 min -1) for the reaction, Fe(III) + S(IV)-• products, is given by d[Fe(III)]/dt = -k[Fe(III)] [S(IV)]. The reaction was found to be first order with respect to Fe(III) and S(IV). The rate constants as a function of p H increased from a p H = 2 to 4 and decreased at higher p H. The effect of temperature and ionic strength on the rates could be represented by log k -log k ø + AIø'5/(1 + Iø'•), where A = -1.1 in NaC1 and -2.2 in seawater and 10g k ø -25.39 -6,323/T. The energy of activation was found by 121 __+ 6 kJ mol -• . The measured rates in seawater as a function of salinity were lower than the rates in NaC1 at the same ionic strength. Measurements in NaC1 solutions with added sea-salt ions (Mg 2+ , C_a •+, F-, Br-, and SO4:-) at pH = 3.5 indicate that the formation of inert FeF 2+ may be responsible for the lower rates. The effect of changes in the composition on the rates was interpreted by examining the speciation of Fe(III) and S(IV). Thi_s analysis indicates that the rate-determining steps from a p H of 2.5 to 4.0 are FeOH 2+ + HSO•-< • HOFeSO3H + and HOFeSO3H + • FeOH + + HSO3' and at pH of 4 to 6, the reactions Fe(OH)• + HSO•-< • (HO):FeSO3 H and (HO):FeSO3H •Fe(OH): + HSO3' become important. The changes in the concentration of FeOH 2+ and HSO•-as a function of pH and composition can account for most of the changes in the rates. These kinetic studies indicate that the rates of reduction of Fe(III) with S(IV) in acidic water droplets at natural levels of S(IV) may be an important source of Fe(II).1Also at

Investigation of iron(III) reduction and trace metal interferences in the determination of dissolved iron in seawater using flow injection with luminol chemiluminescence detection

“…These data showed that the reduction of nanomolar concentrations of Fe(III) in acidified seawater with 100 M sulphite at room temperature (21 • C) was complete after 4 h; therefore a 12 h reduction time is more than sufficient for seawater DFe determinations. The data showed good agreement with those of Millero et al [28] who reported a ln k value of 4.08 M −1 min −1 in seawater acidified to pH 2 at 25 • C with the same sulphite concentration.…”

Trace metal characterization of aerosol particles and cloud water during HCCT 2010

“…The observed Cu(II) concentration in the same size fraction as soluble iron indicates that the Fe(II) at night could have been related to aqueous-phase reductions of Fe(III) to Fe(II) by Cu(I) species. Fe(III) catalyzed oxidation of sulfur (IV) has been suggested to be another important source of Fe(II) in cloud water during nighttime (Schwanz et al, 1998;Millero et al, 1995). However, with no data available on sulfur (IV) measurements during FCE 1.1, conclusions about the contribution of this pathway cannot be made.…”

Oxidation of iron (II) nanomolar with H2O2 in seawater

“…NTA and EDTA have been shown to affect the kinetic of Fe(II) oxidation (Santana-Casiano et al, 2000). F Ϫ forms a FeF 2ϩ ion pair that is nonreactive to reduction (Millero et al, 1995a). Considering that in our studies the Fe(III) is present due to both, the Fe(II) oxidation and natural Fe(III), NaF was used at a final concentration 1.25 M to complex any soluble Fe(III) and to avoid any interference.…”

“…Millero et al (1991) studied the effect of ionic interactions on the rates of oxidation of Fe(II) with hydrogen peroxide at micromolar levels in different media. In recent years, the chemiluminiscence technique using the luminol reagent has been used to study the oxidation of Fe(II) at nanomolar levels in different media (King et al, 1995;Millero et al, 1995a;King and Farlow, 2000). The utilization of a long liquid waveguide capillary flow cell (LWCFC) permits the spectrophotometric determination of Fe(II) at nanomolar levels (Zhang et al, 2001).…”