The effects of pH, ionic strength, and iron-fulvic acid interactions on the kinetics of nonphotochemical iron transformations. I. Iron(II) oxidation and iron(III) colloid formation

Flow injection analysis was used to study the effect of a fulvic acid on the kinetics of iron(H) oxidation and iron colloid formation under conditions approximating fresh natural waters. While iron(II) oxidation in high-carbonate inorganic solutions is predicted well by a recently proposed homogeneous model, it overestimates the oxidation rate in low-carbonate solutions, possibly due to the formation of an intermediate iron(II) colloid or surface species. Results in fulvic acid solutions are consistent with the formation of an iron(II)-fulvic acid complex at both pH 6.0 and 8.0 which accelerates the overall oxidation rate relative to inorganic solutions. However, iron(III) complexation by fulvic acid greatly slows the formation of iron colloids, stabilizing dissolved iron(III). Decreased pH and increased ionic strength slow and decrease iron colloid formation. Evidence of a kinetic control on the distribution of iron(III) between organically complexed and colloidal forms is presented. Copyright (C) 2003 Elsevier Ltd.