The effect of organic compounds in the oxidation kinetics of Fe(II)

The oxidation of Fe(II) has been studied in the presence of the natural organic compounds alanine, glutamic acid, cysteine, and two synthetic aminocarboxilates [(ethylenedioxi)diethylenedinitrilo]tetra-acetic acid (EGTA) and (ethylenedinitro)tetra-acetic acid (EDTA), as a function of pHt (6–8), ionic strength (0.2–1 m) and temperature (5–35°C), in NaCl solutions, at different Fe(II)–organic compound ratios. Alanine and glutamic acid did not affect the oxidation kinetics of Fe(II). For these compounds, a second order pH dependence is obeyed over the pH range studied, where log kobs=−16.29(0.16)+2.09(0.02)pH and log kobs=−15.26(1.3)+1.94(0.18)pH, for the alanine and glutamic acid, respectively. EGTA formed a strong ferrous complex that inhibited the oxidation of Fe(II) and EDTA increased the oxidation of ferrous iron forming a Fe(III)–EDTA complex that showed photoreduction in the presence of light, regenerating Fe(II). In the pH range from 6 to 8.2, the process was not affected by pH. The dependence with ionic strength was described by the equation log k=15.351+0.565I1/2−1.388I. Cysteine modified this behavior as a function of the Fe(II)–cysteine ratios. A Fe(III)–cysteine complex is formed through a one-electron transfer process that involved the thiol group and resulted in the reduction of Fe(III) back to Fe(II), and the oxidation of cysteine to cystine. The Fe(OH)L complex formation and reduction was affected by pH and cysteine concentration. A kinetic model that describes the behavior observed has been developed.