Kinetics and pH Dependence of Chromium(VI) Reduction by Iron(II)

Iron(II) 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 μM Cr(VI) and 30−60 μM Fe(II), I = 0.01 M KCl, 23 ± 3 °C), we derived the following rate law: −d[Cr(VI)]/d t = k obs (pH)[Fe(II)][Cr(VI)], with k obs = (0.34 ± 0.47) M - 1 s - 1 + (3.29 ± 0.66) × 10 9 M - 2 s - 1 [OH - ] + (4.82 ± 1.53) × 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)]/d t = ( k 1 [Fe 2+ ] + 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) × 10 5 M - 1 s - 1, k 3 = (2.84 ± 0.90) × 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(II) oxygenation shows that Cr(VI) oxidizes Fe(II) faster than O 2 (for [Cr(VI)] = [O 2 ] by a factor of ≈ 3 × 10 4 at pH 4, 6 × 10 3 at pH 6, and 1 × 10 3 at pH 8).