THE REDUCTION OF CHROMIUM(VI) BY IRON(II) IN AQUEOUS-SOLUTIONS

The rates of the reduction of Cr(VI) with Fe(II) were measured in NaCl , NaClO4, and natural seawater as a function of pH (1.5-8.7), temperat ure (5-40 degrees C) and ionic strength (I = 0.01-2 M), The pseudo fir st-order rate constant (log k(1)) showed a parabolic dependence on pH decreasing from 1.5 to 4.5 and increasing from 5.5 to 8.7. The kinetic s of the reaction in these two regions of pH also showed different inf luences of temperature, ionic strength, and reductant concentration. T he rate of Cr(VI) reduction is described by the general expression -d[ Cr(VI)]/dt = k [Cr(VI)] [Fe(II)] where k (M-1 min(-1)) can be determin ed from the log k=6.74-1.01 pH-188.5/T for the pH range 1.5-4.5 (sigma = 0.2) and log k= 11.93 + 0.95 pH - 4260.1/T - 1.06 I-0.5 for the pH range 5-8.7 (sigma = 0.2) from 5 to 40 degrees C and 0.01 to 2 M ionic strength. The effect of pH, temperature, and ionic strength on the re action indicates that the reactions at low pH are due to H2CrO4+Fe2+ - ->(kH2A-Fe) products While the reactions at high pH are due to HCrO4-FeOH+-->(kHA-FeOH) products HCrO4-+Fe(OH)(2)-->(kHA-Fe(OH)2) products The overall rate expression over the entire pH range can be determined from (H(2)A = H2CrO4) k= k(H2A-Fe)alpha(H(2)A) alpha(Fe2+)+k(HA-FeOH) alpha(HA(-))alpha(FeOH+)+k(HA-Fe(OH)2) alpha(HA(-)) alpha(Fe(OH)(2)) where k(H2A-Fe) = 5 x 10(6), k(HA-FeOH) = 1 x 10(6), k(HA-Fe) ((OH)2) = 5 x 10(11). In oxic aqueous systems Cr(VI) competes with O-2 in the oxidation of Fe(II) and an extension of the rate law for Cr(VI) reduct ion with Fe(II) in oxygenated solutions is proposed. The application o f this extended rate law to environmental conditions suggests that thi s reaction influences the distribution of oxidized and reduced species of chromium in oxic and anoxic waters. Copyright (C) 1998 Elsevier Sc ience Ltd.