Reduction of hexavalent chromium by H2O2 in acidic solutions

Maurizio Pettine, Luigi Campanella, Frank J Millero

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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 (/ = 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.

Original languageEnglish (US)
Pages (from-to)901-907
Number of pages7
JournalEnvironmental Science and Technology
Volume36
Issue number5
DOIs
StatePublished - Mar 1 2002

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chromium
Chromium
Ionic strength
Rate constants
Fog
Aerosols
Reducing Agents
Temperature
Kinetics
Acids
rate
chromium hexavalent ion
Ions
fog
leveling
droplet
trace metal
temperature
environmental conditions
aerosol

ASJC Scopus subject areas

  • Environmental Engineering
  • Environmental Science(all)
  • Environmental Chemistry

Cite this

Reduction of hexavalent chromium by H2O2 in acidic solutions. / Pettine, Maurizio; Campanella, Luigi; Millero, Frank J.

In: Environmental Science and Technology, Vol. 36, No. 5, 01.03.2002, p. 901-907.

Research output: Contribution to journalArticle

Pettine, Maurizio ; Campanella, Luigi ; Millero, Frank J. / Reduction of hexavalent chromium by H2O2 in acidic solutions. In: Environmental Science and Technology. 2002 ; Vol. 36, No. 5. pp. 901-907.
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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 (/ = 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.",
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N2 - 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.

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