Light-induced oxidation of aqueous chromium(III) in the presence of iron(III)

Oxidation of Cr(III) to toxic Cr(VI) by Mn(IV) or Mn(III) species has been identified as probably the only important oxidation pathway in soils, aquat ic bodies, and atmospheric environments. We observed ready oxidation of Cr( III) in Fe(III) solutions in light at pH 3.2-4.4. Further kinetic study of this oxidation was performed in sunlight as well as in black light under co ntrolled conditions (e.g., pH 3.2, 20 degrees C). Quantum yields of Cr(VI) formation at 356 nm were 0.020-0.046 for initial levels of 50-800 mu M Cr(I II) and 200 mu M Fe(III). The Cr(VI) formation reached steady state in 0.5 h (6 mu M, [Cr(III)](0): 50 mu M; 10 mu M, [Cr(III)](0): 200 mu M; [Fe(III) ](0): 200 mu M). The best linearity found only for log[Cr(lll)] vs t in the first 1 or 1.5 min of the reaction suggests apparent first-order kinetics for the disappearance of Cr(lll) at 356 nm over the period; afterward, it f ailed to comply with any regular rate law, The Cr(lll) oxidation decreased at high initial levels of Cr(lll) or Fe(lll) and upon removal of O-2, and i t was found to be very limited at pH <2.5 or pH >5.0. Cr(III) was hypothesi zed to be oxidized to Cr(VI) by . OH radicals from photolysis of FeOH2+ com plexes (estimated k .(OH+Cr(III)) = 8.3 X 10(7) M-1 s(-1)); Cr(VI) also was reduced by the formed Fe(ll) and intermediates such as HO2. This study sug gests that light-induced oxidation of aqueous Cr(III) in the presence of Fe (lll) would be one potential pathway for the oxidation of Cr(lll) to toxic Cr(VI), more possibly in atmospheric waters or in acidic surface waters wit h low dissolved organic carbon content; it would not be expected to occur i n surface soils because of their universal association with organic carbon.