Titania-assisted photoreduction of Cr(VI) to Cr(III) in aqueous media: Kinetics and mechanisms
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INTRODUCTION
CHROMIUM and its compounds are widely used in industry for a variety of purposes.[1] Most users of chromium and its compounds eventually produce chromium-laden aqueous waste solutions. In such wastes, chromium is present either in the hexavalent state, Cr(VI), or the trivalent state, Cr(III). When present in the hexavalent state, it may exist as chromic acid (H2CrO4), dichromate anion 2 (Cr2O22 7 ),hydrogen chromate anion (HCrO4 ), and/or chromate anion (CrO22 ). Using the published thermodynamic 4 data and mass balance concept,[2] the distribution of Cr(VI) species vs pH at a total chromium concentration of 1 mM was calculated and is shown in Figure 1(a). Figure 1(b) shows the concentration of HCrO24 and Cr2O22 anions vs 7 total chromium concentration at a pH of 2. The figures indicate that in the dilute acidic media, in concentrated acidic media, and in alkaline media, the dominant species are 22 HCrO24 , Cr2O22 7 , and CrO4 , respectively. Hexavalent chromium is toxic and carcinogenic, while trivalent chromium is not.[3] Due to the detrimental effects of Cr(VI) on living species, the United States Environmental Protection Agency has classified Cr(VI)-bearing aqueous wastes as hazardous, and has restricted the disposal of such directly onto or into the soil.[4] Currently, the maximum concentration of chromium in industrial effluents is regulated and is dependent on the waste type.[4] The restrictions on Cr(VI) levels of industrial effluents have stirred interest in its reduction to the Cr(III) state in aqueous media. In this regard, M. ALAM, Associate Professor, and R.A. MONTALVO, Graduate Student, are with the Department of Materials and Metallurgical Engineering, New Mexico Institute of Mining and Technology, Socorro, NM 87801. Manuscript submitted May 29, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
photon-assisted reduction of Cr(VI) to Cr(III) using compound semiconductor particles as catalysts has been studied.[5–13] This process involves irradiating compound semiconductor particles dispersed in the Cr(VI)-bearing aqueous solution by photons having energy in excess of the semiconductor bandgap, resulting in the generation of electron-hole pairs. The conduction band electrons are transferred to the adsorbed Cr(VI) species (thereby reducing them to Cr(III)) and to other electron acceptor species such as dissolved oxygen, while the valence band holes accept electrons from water and other suitable donor species present in the solution, thereby oxidizing them. Perusal of the literature indicates that several studies have been carried out to assess the feasibility of reducing Cr(VI) to Cr(III) by the photocatalytic process. In general, these studies deal with the influence of the (a) type of semiconductor,[5,6,7] (b) irradiation intensity,[8] (c) semiconductor loading,[9,10] (d) Cr(VI) concentration,[10,11] (e) solution pH,[6–11] (f) addition of hole scavengers,[8,12] and (g) suspended vs supported catalysts[13] on the kinetics of reduction of Cr(VI) to Cr(III) with the objective of optimiz
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