Reduction of hexavalent chromium by H2O2 in acidic solutions

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 (/ = 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 × 10a)/(1 + 10a), 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.