Kinetics and pH dependence of chromium(VI) reduction by iron(II)

Iron(ll) is one of the most important reductants of chromium(VI), a severe, toxic contaminant of natural waters, sediments, and soils. We studied the reaction kinetics between pH 2 and pH 7.2 with UV-VIS and multicomponent fitting, a method without the interference of added reagents. The reaction rate was minimal around pH 4. A rate increase with decreasing pH below 4 is documented in the literature. However, a pH-dependent kinetic expression for environmentally relevant, higher pH conditions has not been reported yet. For pH 4.4-7.2 (solutions buffered with acetate, MES, and PIPES, initial 10-20 mu M Cr(VI) and 30-60 mu M Fe(II), I = 0.01 M KCl, 23 +/- 3 degrees C), we derived the following rate law: -d[Cr(VI)]/dt = k(obs)(pH)[Fe(II)][Cr(VI)], with k(obs) = (0.34 +/- 0.47) M-1 s(-1) + (3.29 +/- 0.66) x 10(9) M-2 S-1 [OH-] + (4.82 +/- 1.53) x 10(16) M-3 S-1 [OH-](2) (standard deviations), where Cr(VI) to Cr(V) is the rate-determining step. The equivalent expression -d[Cr(VI)]/dt = (k(1)[Fe2+] + k(2)[FeOH+] + k(3)[Fe(OH)(2)(0)])[Cr(VI)], With k(1) = (0.34 +/- 0.47) M-1 s(-1), k(2) = (1.41 +/- 0.28) x 10(5) M-1 s(-1), k(3) = (2.84 +/- 0.90) x 10(9) M-1 s(-1), provides a plausible interpretation. The kinetic constants (log k) are related to the electron reduction potential of the corresponding aquo- and hydroxo-Fe(III)/Fe(II) redox couples, probably in a linear free enthalpy relation. Comparison to the analogous kinetic expression for Fe(ll) oxygenation shows that Cr(VI) oxidizes Fe(II) faster than O-2 (for [Cr(VI)] = [O-2] by a factor of approximate to 3 x 10(4) at pH 4, 6 x 10(3) at pH 6, and 1 x 10(3) at pH 8).