The rates of the reduction of Cr(VI) with H2O2 were measured in NaCl solutions as a function of pH (1.5 to 4.8), temperature (5 to 40 ¢XC) and ionic strength (I = 0.01 ¡V 2 M) in the presence of an excess of reductant. The rate of Cr(VI) reduction is described by the general expression ¡Vd[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 = (3e10a) / (1 + 10a) where a = - 0.25 - 0.58 pH + 0.26 pH2), leveling off at pH < 2 (where n l 1) and pH > 4 (where n l 3). The rates of Cr(VI) reduction are acid catalysed and the kinetic order z varies from about 1.8 to 0.5 with increasing H2O2 concentration, according to the equation z = 1.85 ¡V 350.1 H2O2 (M) which is valid for [H2O2] < 0.004M. 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 to 4 have been fitted to the equation log k = 2.14 pH ¡V 2.81 with ã = ¡Ó 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 = 2.87 pH ¡V 0.55 with ã = ¡Ó 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
Reduction of hexavalent chromium by H2 O2 in acidic solutions
Pettine M;
2002
Abstract
The rates of the reduction of Cr(VI) with H2O2 were measured in NaCl solutions as a function of pH (1.5 to 4.8), temperature (5 to 40 ¢XC) and ionic strength (I = 0.01 ¡V 2 M) in the presence of an excess of reductant. The rate of Cr(VI) reduction is described by the general expression ¡Vd[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 = (3e10a) / (1 + 10a) where a = - 0.25 - 0.58 pH + 0.26 pH2), leveling off at pH < 2 (where n l 1) and pH > 4 (where n l 3). The rates of Cr(VI) reduction are acid catalysed and the kinetic order z varies from about 1.8 to 0.5 with increasing H2O2 concentration, according to the equation z = 1.85 ¡V 350.1 H2O2 (M) which is valid for [H2O2] < 0.004M. 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 to 4 have been fitted to the equation log k = 2.14 pH ¡V 2.81 with ã = ¡Ó 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 = 2.87 pH ¡V 0.55 with ã = ¡Ó 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 H2O2I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
