Photocatalytic reduction of Cr(VI) on hematite nanoparticles in the presence of oxalate and citrate

Abstract

Abstract Hematite nanoparticles (nHm) were tested for Cr(VI) photocatalytic reduction (300 μM) in the presence of different electron donors such as citrate (Cit), oxalate (Ox), 2-propanol and methanol. At pH 3 and under irradiation at λ ≥ 310\u2009nm, almost negligible reaction took place in the absence of donor or with the alcohols, while the reduction was very rapid in the presence of Cit (less than 25\u2009min), and faster with Ox (15\u2009min). Homogeneous experiments with FeCl3 instead of nHm showed a complete Cr(VI) reduction in the presence of both complexing agents in less than 10\u2009min. Under irradiation at λ > 495\u2009nm and with nHm at pH 3, a good Cr(VI) transformation took place with both donors, but at a considerably lower rate than under UV light (around 100% at 180\u2009min), the decay being negligible in the homogeneous systems with Fe(III). Under irradiation at λ > 610\u2009nm, no Cr(VI) transformation took place over nHm. Experiments at pH 6 under UV–Vis light with Cit in the presence of nHm gave a good Cr(VI) decay, faster with Fe(III) (60% and 90% in 105\u2009min, respectively); some Cr(VI) transformation (around 35% in 180\u2009min) was found with Cit under Vis light. Interestingly, at both wavelength ranges, the reaction was negligible when Ox was used. Mechanisms taking place under the different conditions were proposed, including the role of surface charge transfer complexes on nHm.