IRON OXIDATION-KINETICS IN AN ACIDIC ALPINE LAKE

Four experiments were performed to assess the role of homogeneous, heterogeneous, biological and photochemical processes in the oxygenation of ferrous iron in aquatic ecosystems. Experiments were carried out at Lake Cristallina, Ticino, Switzerland; and in the laboratory. Rate constants are presented for homogeneous oxygenation in lakewater, for oxygenation by lake sediment under sterile and non-sterile conditions and for heterogeneous oxygenation in the presence of TiO2, MnO2, Fe2O3, Al2O3 and SiO2. It was found that, on a surface area adjusted basis, TiO2, MnO2 and Fe2O3 were three orders of magnitude more effective in accelerating oxygenation than Al2O3 and SiO2, suggesting that these minerals form strong, inner sphere complexes with Fe(II) in the pH range 4-6. Sterilization by gamma irradiation reduced oxygenation rate in the presence of lake sediment by two orders of magnitude on a surface area adjusted basis. Oxygenation in the presence of 0.44 mu m filtered lakewater was not appreciably different from that in distilled water. In situ experiments showed that oxygenation rate increased in the presence of sunlight. The heterogeneous rate constant for iron oxidation with lake sediment was 4.9 x 10(14) M(-1) atm(-1) s(-1) m(-2) and with pure minerals ranged from 1.8 x 10(10) to 5.3 x 10(13) M(-2) atm(-1) s(-1) m(-2).