Reduction of Hexavalent Chromium by H2O2 in Acidic Solutions

Abstract

The rates of the reduction of Cr(VI) with H₂O₂ were measured in NaCl solutions as a function of pH (1.5−4.8), temperature (5−40 °C), and ionic strength (I = 0.01−2 M) in the presence of an excess of reductant. The rate of Cr(VI) reduction is described by the general expression −d[Cr(VI)]/dt = k₂[Cr(VI)]^m[H₂O₂]ⁿ[H⁺]^z, where m = 1 and n and z are two interdependent variables. The value of n is a function of pH between 2 and 4 (n = (3 × 10^a)/(1 + 10^a), where a = −0.25 − 0.58pH + 0.26pH²) leveling off at pH < 2 (where n ≈ 1) and pH > 4 (where n ≈ 3). The rates of Cr(VI) reduction are acid-catalyzed, and the kinetic order z varies from about 1.8−0.5 with increasing H₂O₂ concentration, according to the equation z = 1.85 − 350.1H₂O₂ (M) which is valid for [H₂O₂] < 0.004 M. The values of k₂ (M^-(n+z) min^-1) are given by k₂ = k/[H⁺]^z = k₁/[H₂O₂]ⁿ[H⁺]^z, where k is the overall rate constant (M^-n min^-1) and k₁ is the pseudo-first-order rate constant (min^-1). The values of k in the pH range 2−4 have been fitted to the equation log k = 2.14pH − 2.81 with σ = ± 0.18. The values of k₂ are dependent on pH as well. Most of the results with H₂O₂ < 3 mM are described by log k₂ = 2.87pH − 0.55 with σ = ± 0.54. Experimental results suggest that the reduction of Cr(VI) to Cr(III) is controlled by the formation of Cr(V) intermediates. Values of k₂ and k calculated from the above equations can be used to evaluate the rates of the reaction in acidic solutions under a wide range of experimental conditions, because the rates are independent of ionic strength, temperature, major ions, and micromolar levels of trace metals (Cu²⁺, Ni²⁺, Pb²⁺). The application of this rate law to environmental conditions suggests that this reaction may have a role in acidic solutions (aerosols and fog droplets) in the presence of high micromolar concentrations of H₂O₂.