Oxidation of Fe(II) in rainwater.

Photochemically produced Fe(II) is oxidized within hours under environmentally realistic conditions in rainwater. The diurnal variation between photochemical production and reoxidation of Fe(II) observed in our laboratory accurately mimics the behavior of ferrous iron observed in field studies where the highest concentrations of dissolved Fe(ll) occur in afternoon rain during the period of maximum sunlight intensity followed by gradually decreasing concentrations eventually returning to early morning pre-light values. The experimental work presented here, along with the results of kinetics studies done by others, suggests thatthe primary process responsible for the decline in photochemically produced Fe(II) concentrations is oxidation by hydrogen peroxide. This reaction is first order with respect to both the concentrations of Fe(II) and H2O2. The second-order rate constant determined for six different authentic rain samples varied over an order of magnitude and was always less than or equal to the rate constant determined for this reaction in simple acidic solutions. Oxidation of photochemically produced ferrous iron by other oxidants including molecular oxygen, ozone, hydroxyl radical, hydroperoxyl/superoxide radical, and hexavalent chromium were found to be insignificant under the conditions present in rainwater. This study shows that Fe(II) occurs as at least two different chemical species in rain; photochemically produced Fe(II) that is oxidized over time periods of hours, and a background Fe(II) that is protected against oxidation, perhaps by organic complexation, and is stable against oxidation for days. Because the rate of oxidation of photochemically produced Fe(II) does not increase with increasing rainwater pH, the speciation of this more labile form of Fe(II) is also not controlled by simple hydrolysis reactions.

Photochemical production of Fe(II) in rainwater.

Significant concentrations of Fe(II) were produced upon irradiation of authentic rainwater with simulated sunlight. The magnitude of photoproduction was dependent on initial Fe(II), Fe(III), and hydrogen ion concentrations, with more Fe(II) photoproduction when initial Fe(III) and H+ concentrations were high and initial Fe(II) concentrations were low. An equation was developed that accurately predicts photoproduction of Fe(II) in rainwater based on initial Fe speciation values and pH. The quantum yield of Fe(II) photochemical production in rain decreased dramatically with increasing wavelength and decreasing energy of incoming radiation with the average quantum yield at 265 nm approximately an order of magnitude greater than at 546 nm. Probable photochemical precursors of Fe(II) in authentic rain include iron(III) oxalate, iron(III) hydroxide, and an undefined Fe(III) complex. The wavelength-dependent Fe(II) production was modeled using the average Fe(II) efficiency spectrum, an average rainwater absorption spectrum, and the modeled actinic flux for temperate latitudes in both summer and winter. The response spectrum has the highest photoproduction of Fe(II) in summer and winter at 325 and 330 nm, respectively, with greater production in summer rain due to increased actinic flux and longer hours of irradiation.