Kinetic hindrance of Fe(II) oxidation at alkaline pH and in the presence of nitrate and oxygen in a facultative wastewater stabilization pond

Abstract

To better understand the dynamics of Fe2 + oxidation in facultative wastewater stabilization ponds, water samples from a three-pond system were taken throughout the period of transition from anoxic conditions with high aqueous Fe2 + levels in the early spring to fully aerobic conditions in late spring. Fe2 + levels showed a highly significant correlation with pH but were not correlated with dissolved oxygen (DO). Water column Fe2 + levels were modeled using the kinetic rate law for Fe2 + oxidation of Sung and Morgan.[ 5 ] The fitted kinetic coefficients were 5 ± 3 × 106 M− 2 atm− 1 min− 1; more than six orders of magnitude lower than typically reported. Comparison of four potential Fe redox couples demonstrated that the pε was at least 3–4 orders of magnitude higher than would be expected based on internal equilibrium. Surprisingly, measured nitrate and DO (when present) were typically consistent with both nitrate (from denitrification) and DO levels (from aerobic respiration) predicted from equilibrium. Although the hydrous Fe oxide/FeCO3 couple was closest to equilibrium and most consistent with the observed pH dependence (in contrast to predicted lepidocrocite), Fe2 + oxidation is kinetically hindered, resulting in up to 107-fold higher levels than expected based on both kinetic and equilibrium analyses.