Reduction of hexavalent chromium by H2O2 in acidic solutions

The rates of the reduction of Cr(VI) with H(2)O(2) were measured in NaCl solutions as a function of pH (1.5-4.8), temperature (5-40 degreesC), 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 = k(2)[Cr(VI)](m)[H(2)O(2)](n)[H(+)](z), where m = I and n and z are two interdependent variables. The value of n is a function of pH between 2 and 4 (n = (3 x 10(a))/(1 + 10(a)), where a = -0.25 - 0.58pH + 0.26pH(2)) leveling off at pH < 2 (where n approximate to 1) and pH > 4 (where n approximate to 3). The rates of Cr(VI) reduction are acid-catalyzed, and the kinetic order z varies from about 1.8-0.5 with increasing H(2)O(2) concentration, according to the equation z = 1.85 - 350.1 H(2)O(2) (M) which is valid for [H(2)O(2)] < 0.004 M. The values of k(2) (M(-(n+z)) min(-1)) are given by k(2) = k/[H(+)](z) = k(1)/[H(2)O(2)](n)[H(+)](z), where k is the overall rate constant (M(-n) min(-1)) and k(1) 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 sigma = +/-0.18. The values of k(2) are dependent on pH as well. Most of the results with H(2)O(2) < 3 mM are described by log k(2) = 2.87pH - 0.55 with sigma = +/- 0.54. Experimental results suggest that the reduction of Cr(VI) to Cr(Ill) is controlled by the formation of Cr(V) intermediates. Values of k(2) 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 (Cu(2+), Ni(2+), Pb(2+)). 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 H(2)O(2).