KINETICS OF FE(II) OXYGENATION AT LOW PARTIAL-PRESSURE OF OXYGEN IN THE PRESENCE OF NATURAL ORGANIC-MATTER

Rates of Fe(II) oxidation were measured in aqueous solutions of neutral pH, with partial pressure of oxygen (p(O2)) at 0.005, 0.02, 0.20 atm. The effects of natural organic matter (NOM), fulvic acid (FA), and polyglutamate on Fe(II) oxidation were investigated. At p(O2) of 0.2 atm, NOM had little effect on the rate of Fe(II) oxidation. The rate exhibited first-order behavior with respect to Fe(II) concentration, and the oxidation half-time (t1/2) was about 20 min. However, at low p(O2) (0.005 atm), the oxidation rate showed an apparent zero-order behavior, with t1/2 of 40 h. Organic compounds (NOM, polyglutamate, and FA) enhanced the oxidation reaction. The half time, t1/2, shortened to 8 h in the presence of 2.0 mg L-1 of NOM. This finding was in contrast to the limited effect of NOM on Fe(II) oxidation at high p(O2). It is interpreted by considering oxidation through two pathways: oxidation of FeOH+ and oxidation of Fe(II)-organic complexes, with Fe(II)-organic complexes being the more reactive species.