| Popis: |
This study deals with the photocatalytic reduction of carcinogenic hexavalent chromium, encountered in industrial effluents and surface waters, to much less toxic and harmful trivalent form. Experiments were performed in a pilot scale Photocatalytic Membrane Reactor employing titanium dioxide suspension as a catalyst, UVC lamps as irradiation source and hollow fiber UF membranes as separation media. It is observed that the reduction of Cr(VI) is enhanced in the presence of carboxylic acids, which are used as sacrificial agents and act as electron donors, mainly due to formation of Cr(VI)/organic-acid complexes. The effect of all key operating parameters, including pH, catalyst loading, hydraulic retention time, UV dose, organic acid type and molar ratio over Cr(VI) ions, water matrix, on system performance is also examined. Encouraging results are obtained since complete reduction can be achieved in the presence of 300 mg/L citric acid or 180 mg/L oxalic acid under acidic conditions (pH 3), meeting the standards for safe disposal to natural water bodies. Complete reduction can be also obtained at higher pH values (~5.5) with simultaneous ‘on-line’ dosing of approx. 8 mg/L/min H2O2. Moreover, up to 62% photocatalytic oxidation of organics is attained, under the most favorable conditions. |
