Photochemical removal of hexavalent chromium through iodide oxidation under 254 nm irradiation

The efficient photoreductive removal of chromate [Cr(VI)] in aqueous solution was successfully demonstrated using potassium iodide (KI) excited by 254 nm irradiation. The photoexcited iodides react with Cr(VI) (as an electron acceptor) with transforming Cr(VI) to Cr(III) and I− to I3- at the same time. The hexavalent chromium of 200 μM was mostly removed within an hour of 254 nm irradiation in the presence of 1 mM KI at pH 3. However, the removal process was only active at acidic condition and not at neutral and alkaline pH. The photo-reduction process was more efficient with higher concentration of iodides and was little affected by the presence of O2. The photoreaction between iodides and O2 leads to the generation of H2O2, which was completely inhibited by the presence of Cr(VI) because the photoexcited iodides react preferentially with Cr(VI). The presence of arsenite (As(III)) as an electron donor significantly enhanced the removal efficiency of Cr(VI) because As(III) reduce triiodides back to iodides with enabling the photocatalytic cycle of iodides under 254 nm radiation. The proposed UV254/KI process can be successfully applied in the treatment of highly contaminated wastewaters with Cr(VI) and As(III), which is highly desirable because both toxic species (Cr(VI), As(III)) are transformed into less toxic forms (Cr(III), As(V)).

► A new photochemical method for the removal of hexavalent chromium.
► The photoreduction of Cr(VI) is achieved using potassium iodide excited by 254 nm irradiation.
► The photoexcited iodides react with Cr(VI) with transforming Cr(VI) to Cr(III) and I− to I3- at the same time.
► The presence of arsenite (As(III)) significantly enhances the removal efficiency of Cr(VI).