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

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 nm, 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 min), and faster with Ox (15 min). Homogeneous experiments with FeCl3 instead of nHm showed a complete Cr(VI) reduction in the presence of both complexing agents in less than 10 min. Under irradiation at λ > 495 nm 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 min), the decay being negligible in the homogeneous systems with Fe(III). Under irradiation at λ > 610 nm, 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 min, respectively); some Cr(VI) transformation (around 35% in 180 min) 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.