KINETICS OF PHOTOCHEMICAL CHEMICAL CYCLING OF IRON COUPLED WITH ORGANIC-SUBSTANCES IN-CLOUD AND FOG DROPLETS

The kinetics of photochemical reduction of Fe(III) coupled with organi c ligands has been investigated to understand the photoredox cycling o f Fe and the significance of this cycling in the production of photo-o xidants and transformation of S(IV) and organic pollutants in atmosphe ric waters. The photoreduction of ferrioxalate ion is first order with respect to Fe(III) and has a rate constant of 4.0 +/- 0.4 x 10(-2) s( -1) in anoxic solutions at summer clear-day-noon time. In the presence of oxygen, the oxalate radical formed in the primary photoreaction fu rther reduces O-2 to HO2./O-2(.), leading to the reoxidation of Fe(lI) and the formation of H2O2. At a photochemical steady-state, the ratio of Fe(II)/Fe(III) is determined by: [Fe(II)](ss)/[Fe(III)](ss) = k(s) /k(9)[HO2./O-2(.)](ss), where subscript ss denotes steady-state, k(s) and k(9) are the rate constants for photoreduction of Fe(III) complex and the oxidation of Fe(II) by HO2./O-2(.-), respectively, and [HO2./O -2(.-)] is the concentration of HO2./O-2(.-). During sunny daytime, a large fraction of Fe is present as Fe(II), which provides a significan t source of dissolved Fe(II) for the primary productivity in surface w ater systems.