Iron(III) Catalyzed Photochemical Reduction of Chromium(VI) by Oxalate and Citrate in Aqueous Solutions

Cr(VI) reduction in soil solution, wastewater, and natural waters is poorly understood in complex systems containing Fe(II,III) and dissolved organic C, especially when influenced by sunlight. The Fe(II,III)-mediated photochemical reduction of Cr(VI) was investigated in a laboratory study with 5−200 μM Cr(VI), 25−1300 μM oxalate or citrate, 0.13−6.7 μM Fe(III), 10 mM KCl, pH 3−7, and a xenon light source (720 W/m 2 between 300 and 800 nm) at 25 °C. In situ UV−VIS multicomponent analysis avoided addition of interfering reagents. At higher [Cr(VI)], photochemical Cr(VI) reduction was zero order in [Cr], with quantum yields [relative to light absorption by Fe(III)−oxalate] of up to 0.53. Over 95% Cr(VI) reduction was observed within 20−40 min in 5-cm cells. At lower [Cr(VI)], the reaction order became complex due to slow reaction of Fe(II) with Cr(VI) compared to photochemical Fe(II) production. The thermal reaction of Cr(VI) with Fe(II) at pH 5 was measured and described by −d[Cr(VI)]/d t = 1.2 (±0.3) × 10 7 M - 2 s - 1 × [Cr(VI)][Fe(II)][Ox]. A tentative mechanistic kinetic model is presented that fits the results of the dark and photochemical experiments with oxalate. Photochemically formed superoxide (O 2 • - ), and hydroperoxyl radical (HO 2 ), also appeared to be important reductants, reducing Cr(VI) with bimolecular rates of 5−8 × 10 4 M - 1 s - 1 . Fe(II), HO 2 /O 2 • -, and H 2 O 2 were likely reductants of Cr(V) and Cr(IV) intermediates. The reaction product with oxalate was mainly soluble Cr(III)−oxalate. The results are applicable to understanding how rapidly Cr(VI) may be reduced in natural water and soil environments.