Effects of organic hole scavengers on the photocatalytic reduction of selenium anions

The photocatalytic reduction of selenium anions, selenate (Se(VI)) and selenite (Se(IV)) to elemental selenium (Se) over UV-illuminated TiO₂ was performed using formic acid, acetic acid, methanol, ethanol, sucrose and salicylic acid as the organic hole scavengers. Photoreduction was only observed in the presence of formic acid, methanol or ethanol. The fastest rate of Se ions photoreduction was observed in the presence of formic acid followed by methanol and ethanol. This was attributed to the ability of formate ions to adsorb onto TiO₂ in the presence of Se ions, its fast mineralisation rate and its ability to form reducing radicals quickly. For the methanol and ethanol systems, these two organic compounds could not compete with Se ions for the TiO₂ surface and were not easily mineralised. The photocatalytic reduction of Se ions observed in the presence of these two organic compounds was attributed to their ability to form reducing radicals. When formic acid was used, optimum pH values at pH 3.5 and 4.0 was encountered for the Se(VI) and Se(IV) photoreduction, respectively. When methanol and ethanol were used as the hole scavenger in the pH range of 2.2-4.0, the greatest extent of Se ions photoreduction was encountered at pH 2.2. This suggests the different role of formic acid and methanol/ethanol in the photoreduction of Se ions.