The reduction of chromium (VI) by iron (II) in aqueous solutions

Abstract

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), temperature (5–40°C) and ionic strength (I = 0.01–2 M). The pseudo first-order rate constant (log k1) showed a parabolic dependence on pH decreasing from 1.5 to 4.5 and increasing from 5.5 to 8.7. The kinetics of the reaction in these two regions of pH also showed different influences of temperature, ionic strength, and reductant concentration. The 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 determined from the log k=6.74−1.01 pH−188.5/T for the pH range 1.5–4.5 (σ = 0.2) and log k=11.93+0.95 pH−4260.1/T−1.06 I0.5 for the pH range 5–8.7 (σ = 0.2) from 5 to 40°C and 0.01 to 2 M ionic strength. The effect of pH, temperature, and ionic strength on the reaction indicates that the reactions at low pH are due to H2CrO4+Fe2+→kH2A-Feproducts While the reactions at high pH are due to HCrO4−+FeOH+→kHA-FeOHproducts HCrO4−+Fe(OH)2→kHA-Fe(OH)2products The overall rate expression over the entire pH range can be determined from (H2A = H2CrO4) k=kH2A-Feα(H2A)α(Fe2+)+kHA-FeOHα(HA−)α(FeOH+)+kHA-Fe(OH)2α(HA−)α(Fe(OH)2) where kH2A−Fe = 5 x 106, kHA−FeOH = 1 x 106, kHA−Fe (OH)2= 5 x 1011. In oxic aqueous systems Cr(VI) competes with O2 in the oxidation of Fe(II) and an extension of the rate law for Cr(VI) reduction with Fe(II) in oxygenated solutions is proposed. The application of this extended rate law to environmental conditions suggests that this reaction influences the distribution of oxidized and reduced species of chromium in oxic and anoxic waters.