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.