Photoinduced oxidation of chromium picolinate to hexavalent chromium in the presence of ferric ions.

Abstract

The occurrence of chromium picolinate (Cr(pic)₃) in environment has attracted raising concerns on its fate and the associated risks. Herein, the photoinduced oxidation of Cr(pic)₃ in the presence of ferric ions (Fe(III)) under simulated sunlight and natural solar light irradiation were investigated. Cr(pic)₃ was stable under dark or without Fe(III). 87.9 % of Cr(pic)₃ (C₀ = 1.0 μM) was degraded in the presence of 50 μM Fe(III) after 90 min simulated sunlight irradiation at initial pH of 4.0. •OH was the main cause for Cr(pic)₃ oxidation, it attacked the chromium center to generate hexavalent chromium (Cr(VI)) and picolinic acid (k = 5.9 ×10⁸ M^-1·s^-1). Picolinic acid could be further oxidized to NH₄⁺ and small organics. Relative higher Fe(III) content (25 - 75 μM) and Cr(pic)₃ concentration (0.5 - 2.0 μM) promoted both of Cr(pic)₃ degradation and Cr(VI) accumulation. While, the degradation of Cr(pic)₃ decreased with pH at the range of 3.0 - 8.0, more Cr(VI) was accumulated at pH 5.0 and 6.0. The co-existence of inorganic ions and dissolved organic matter (DOM) in river water inhibited Cr(pic)₃ oxidation by scavenging the •OH formed and shielding the light. 8.0 - 16.7 μg/L of Cr(VI) was accumulated after 9.0 h simulated sunlight irradiation of Cr(pic)₃ in river water matrix ([Fe(III)]₀ = 50 - 100 μM). The generation of Cr(VI) under solar light was slower than that under simulated sunlight due to the weaker light intensity (43.2 - 85.0 mW/cm² vs. 750 - 1300 mW/cm²). These results consistently suggest photoinduced oxidation of Cr(pic)₃ in environment generates the toxic Cr(VI), which deserves significant attention.