## Abstract

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 °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 = k_{2}[Cr(VI)]^{m} [H_{2}O_{2}]^{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 + 0.26pH^{2}) 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 H_{2}O_{2} concentration, according to the equation z = 1.85 - 350.1H_{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 σ = ± 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 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 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}.

Original language | English (US) |
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Pages (from-to) | 901-907 |

Number of pages | 7 |

Journal | Environmental Science and Technology |

Volume | 36 |

Issue number | 5 |

DOIs | |

State | Published - Mar 1 2002 |

## ASJC Scopus subject areas

- Chemistry(all)
- Environmental Chemistry