The oxidation of Fe(II) in NaCl-HCO3- and seawater solutions in the presence of phthalate and salicylate ions: a kinetic model

Abstract

The oxidation of Fe(II) by molecular oxygen in NaCl–HCO3− and natural seawater solutions and in the presence of phthalic and salicylic acid has been studied at different pHF and Fe(II)-organic compounds ratios. These two compounds have been selected as simple organic model compounds in order to describe the effect of organic ligands on the oxidation rates of Fe(II) in the environment. A kinetic model including the speciation of Fe(II) and the organic species interacting with the major ions in solution was developed to describe the oxidation rate in any media. The model successfully describes Fe(II) behavior in both NaCl-bicarbonate solutions and seawater in the absence and in the presence of the selected compounds. The second-order rate constants for oxidation of the different species by oxygen are the same for all the experimental conditions studied; the speciation of the species accounts for the differences observed for each selected media. The Fe(OH)2 species is the most significant contributor to the overall oxidation rate in 0.002 m NaHCO3 solution, concentration found in natural seawater. Phthalate ion decreases the Fe(II) oxidation rate in the selected media, while salicylate ion increases oxidation as a result of the formation of the Fe(II)–salicylate complex. The oxidation rate for the Fe(II)-phthalate is close to zero accounting for the lower rate than in pure Fe(II) solution. Salicylate ion forms a 1:1 complex with an oxidation rate kL=6.62 105±300 M−1 min−1. The higher rate is probably related to the formation of a stable Fe(III)–salicylate complex.