Ferrate(VI) enhanced photocatalytic oxidation of pollutants in aqueous TiO2 suspensions

Abstract

Background, aim, and scope Photocatalytic oxidation using UV irradiation of TiO 2 has been studied extensively and has many potential industrial applications, including the degradation of recalcitrant contaminants in water and wastewater treatment. A limiting factor in the oxidation process is the recombination of conduction band electrons (ecb ) with electron holes (h + vb ) on the irradiated TiO 2 surface, thus in aqueous conditions the presence of an effective electron scavenger will be beneficial to the efficiency of the oxidation process. Ferrate (FeO 4 2-) has received much recent attention as a water treatment chemical since it behaves simultaneously as an oxidant and coagulant. The combination of ferrate (Fe(VI)) with UV/TiO 2 photocatalysis offers an oxidation synergism arising from the Fe(VI) scavenging of ecb and the corresponding beneficial formation of Fe(V) from the Fe(VI) reduction. This paper reviews recent studies concerning the photocatalytic oxidation of problematic pollutants with and without ferrate. Materials and methods The paper reviews the published results of laboratory experiments designed to follow the photocatalytic degradation of selected contaminants of environmental significance and the influence of the experimental conditions (eg. pH, reactant concentrations, dissolved oxygen). The specific compounds are as follows: ammonia, cyanate, formic acid, bisphenol-A, dibutyl-and dimethyl-phthalate, and microcystin-LR. The principal focus in these studies has been on the rates of reaction rather than on reaction pathways and products. Results The presence of UV/TiO 2 accelerates the chemical reduction of ferrate and the reduction rate decreases with pH owing to deprotonation of ferrate ion. For all the selected contaminant substances the photocatalytic oxidation rate was greater in the presence of ferrate and this was believed to be synergistic rather than additive. The presence of dissolved oxygen in solution reduced the degradation rate of dimethyl phthalate in the ferrate/photocatalysis system. In the study of microcystin-LR it was evident that an optimal ferrate concentration exists, whereby higher Fe(VI) concentrations above the optimum leads to a reduction in microcystin-LR degradation. In addition, the rate of microcystin-LR degradation was found to be strongly dependent on pH and was greatest at pH 6. Discussion The initial rate of photocatalytic reduction under different conditions was analysed using a Langmuirian form. Decrease in rates in the presence of dissolved oxygen may be due to competition between oxygen and ferrate as electron scavengers, and to non-productive radical species interactions. The reaction between ferrate(VI) and MCLR in the pH range of 6.0-10.0 is most likely controlled by the protonated Fe(VI) species, HFeO 4 -. Conclusions The photocatalytic oxidation of selected, recalcitrant contaminants was found to be significantly greater in the presence of ferrate, arising from the role of ferrate in inhibiting the h + vb -ecb pair recombination on TiO 2 su...

The History and the Current Revival of the Oxo Chemistry of Iron in its Highest Oxidation States: FeVI – FeVIII

“…Examples are recalcitrant contaminants such as dialkylphthalates. The degradation of in these pollutants must be supported by UV light to achieve photocatalytic decomposition on TiO 2 and other oxide particles . It is important to note that perfluoroalkylsulfonates and ‐carboxylates are resistant to ferrate(VI) oxidation but can be attacked by ferrate(V) and ferrate(IV) which may be addressed as “activated” ferrates(VI) …”

Advances Made in Understanding the Interaction of Ferrate(VI) with Natural Organic Matter in Water

“…67 The oxidized products of the reaction between ferrate(VI) and trimethoprim had no antibacterial activity against E. coli. 65 68 Ferrate(VI) could decompose MC-LR through oxidation.…”

Couplage photocatalyse-oxydation par le ferrate (VI) pour le traitement du colorant rhodamine 6G

“…L'oxydation photocatalytique associant l'irradiation UV du dioxyde de titane TiO 2 avec Fe(VI) permet dans certains cas d'améliorer le procédé photocatalytique (SHARMA et al, 2010;YUAN et al, 2006). Classiquement, ce dernier entraîne la génération de porteurs de charge lorsqu'un photon suffisamment énergétique (hν ≥ bande d'énergie de TiO 2 ) fait passer un e -(e cb -) de la bande de valence à la bande de conduction du TiO 2 .…”