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 org anic compounds alanine, glutamic acid, cysteine, and two synthetic aminocar boxilates [(ethylenedioxi)diethylenedinitrilo]tetra-acid (EGTA) and (ethyle nedinitro)tetra-acetic acid (EDTA), as a function of pH(t) (6-8), ionic str ength (0.2-1 m) and temperature (5-35 degrees C), in NaCl solutions, at dif ferent Fe(II)-organic compound ratios. Alanine and glutamic acid did not af fect the oxidation kinetics of Fe(II). For these compounds, a second order pH dependence is obeyed over the pH range studied, where log k(obs) = - 16. 29(0.16)+ 2.09(0.02)pH and log k(obs) = - 15.26(1.3)+ 1.94(0.18)pH, for the alanine and glutamic acid, respectively. EGTA formed a strong ferrous comp lex that inhibited the oxidation of Fe(II) and EDTA increased the oxidation of ferrous iron forming a Fe(m)-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 + 0.565I(1/2) - 1.388I. Cysteine modified this behavior as a function of the Fe(II)-cysteine ratios. A Fe(II I)-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 F e(II), and the oxidation of cysteine to cystine. The Fe(OH)L complex format ion and reduction was affected by pH and cysteine concentration. A kinetic model that describes the behavior observed has been developed. (C) 2000 Els evier Science B.V. All rights reserved.