Photocatalytic Dechlorination of Aqueous Carbon Tetrachloride Solutions in TiO2Layer Systems: A Chain Reaction Mechanism

Photochemical decomposition of perfluorodecanoic acid in aqueous solution with VUV light irradiation

The photochemical decomposition of perfluorodecanoic acid (PFDeA) in water in the presence of persulfate ion (S(2)O(8)(2-)) and sulfur ion (S(2-)) was investigated under vacuum ultraviolet (VUV) light irradiation. PFDeA was decomposed under VUV light irradiation. With the addition of S(2)O(8)(2-) or S(2-), the photo-decomposition and defluorination of PFDeA were enhanced significantly. Sulfate radical anion (SO(4)(*-)) generated from photolysis of S(2)O(8)(2-) initiated PFDeA oxidation. While the S(2-) ion, acting as a *OH scavenger, enhanced the role of reduction pathway induced by aqueous electrons (e(aq)(-)). The shorter-chain perfluorocarboxylic acids (PFCAs), formed in a stepwise manner from longer-chain PFCAs, were identified as products by HPLC/MS.

Chain Photoreduction of CCl3F in TiO2 Suspensions: Enhancement Induced by O2

Trichlorofluoromethane (CFC 11) was photoreduced in aqueous suspensions of TiO2 particles containing HCO2- ions and air. Dissolved O2 inhibited the reaction during an induction period that preceded the rapid formation of chloride ions. Reaction rates were higher in systems containing O2 as compared to analogous reactions that occurred in anaerobic suspensions. High photonic efficiencies of Cl- formation (> or =15) were achieved using suspensions with pH > or = 5. As was the case for studies with air-free suspensions, reactions are best described using a photoinitiated chain mechanism that produced CHCl2F and Cl- during the propagation steps. The enhanced yields obtained in the presence of air are attributed to the removal by O2 of electrons trapped in the oxide, which are converted first into H2O2 and then into reducing radicals that participate in the chain process. Enhanced yields of Freon photoreduction were also observed during illumination of air-free suspensions containing hydrogen peroxide, which were interpreted using a similar mechanism.

Mechanism for the destruction of carbon tetrachloride and chloroform DNAPLs by modified Fenton's reagent

The destruction of a carbon tetrachloride DNAPL and a chloroform DNAPL was investigated in reactions containing 0.5 mL of DNAPL and a solution of modified Fenton's reagent (2M H2O2 and 5mM iron(III)-chelate). Carbon tetrachloride and chloroform masses were followed in the DNAPLs, the aqueous phases, and the off gasses. In addition, the rate of DNAPL destruction was compared to the rate of gas-purge dissolution. Carbon tetrachloride DNAPLs were rapidly destroyed by modified Fenton's reagent at 6.5 times the rate of gas purge dissolution, with 74% of the DNAPL destroyed within 24h. Use of reactions in which a single reactive oxygen species (hydroxyl radical, hydroperoxide anion, or superoxide radical anion) was generated showed that superoxide is the reactive species in modified Fenton's reagent responsible for carbon tetrachloride DNAPL destruction. Chloroform DNAPLs were also destroyed by modified Fenton's reagent, but at a rate slower than the rate of gas purge dissolution. Reactions generating a single reactive oxygen species demonstrated that chloroform destruction was the result of both superoxide and hydroxyl radical activity. Such a mechanism of chloroform DNAPL destruction is in agreement with the slow but relatively equal reactivity of chloroform with both superoxide and hydroxyl radical. The results of this research demonstrate that modified Fenton's reagent can rapidly and effectively destroy DNAPLs of contaminants characterized by minimal reactivity with hydroxyl radical, and should receive more consideration as a DNAPL cleanup technology.

Efficient charge storage in photoexcited TiO2 nanorod-noble metal nanoparticle composite systems

Following UV-illumination, TiO2 nanorod-stabilized noble metal (Ag, Au) nanoparticles dispersed in deaerated organic mixtures can sustain a higher degree of conduction band electron accumulation than that achievable with pristine titania.

Identification of the reactive oxygen species responsible for carbon tetrachloride degradation in modified Fenton's systems

The reactive oxygen species responsible for the transformation of carbon tetrachloride (tetrachloromethane, CT) by modified Fenton's reagent using hydrogen peroxide (H2O2) concentrations >0.1 M was investigated. Addition of the hydroxyl radical scavenger 2-propanol to modified Fenton's reactions did not significantly lower CT transformation rates. Scavenging by 2-propanol not only confirmed that hydroxyl radicals are not responsible for CT destruction, but also suggested that a major product of an iron (III)-driven initiation reaction, superoxide radical anion (O2-), is the species responsible for CT transformation. To investigate this hypothesis, CT degradation was studied in aqueous KO2 reactions. Minimal CT degradation was found in CT-KO2 reactions; however, when H2O2 was added to the KO2 reactions at concentrations similar to those in the modified Fenton's reactions (0.1, 0.5, and 1 M), CT degradation increased significantly. Similar results were obtained when 1 M concentrations of other solvents were added to aqueous KO2 reactions, and the observed first-order rate constant for CT degradation correlated strongly (R2 = 0.986) with the empirical solvent polarity (E(T)N) of the added solvents. The results indicate that even dilute concentrations of solvents, including H202, can increase the reactivity of O2- in water, probably by changing its solvation sphere. The higher reactivity of O2- generated in modified Fenton's reagent, which has a less polar nature due to the presence of H2O2, may result in a wider range of contaminant degradation than previously thought possible.