George P. Anipsitakis

Nine transition metals were tested for the activation of three oxidants and the generation of inorganic radical species such as sulfate, peroxymonosulfate, and hydroxyl radicals. From the 27 combinations, 14 M/Ox couples demonstrated significant reactivity toward transforming a model organic substrate such as 2,4-dichlorophenol and are further discussed here. It was found that Co(II) and Ru(III) are the best metal catalysts for the activation of peroxymonosulfate. As expected on the basis of the Fenton reagent, Fe(III) and Fe(II) were the most efficient transition metals for the activation of hydrogen peroxide. Finally, Ag(I) showed the best results toward activating persulfate. Quenching studies with specific alcohols (tert-butyl alcohol and ethanol) were also performed to identify the primary radical species formed from the reactive M/Ox interactions. The determination of these transient species allowed us to postulate the rate-determining step of the redox reactions taking place when a metal is coupled with an oxidant in aqueous solution. It was found that when Co(II), Ru(III), and Fe(II) interact with peroxymonosulfate, freely diffusible sulfate radicals are the primary species formed. The same was proven for the interaction of Ag(I) with persulfate, but in this case caged or bound to the metal sulfate radicals might be formed as well. The conjunction of Ce(III), Mn(II), and Ni(II) with peroxymonosulfate showed also to generate caged or bound to the metal sulfate radicals. A combination of sulfate and hydroxyl radicals was formed from the conjunction of V(III) with peroxymonosulfate and from Fe(II) with persulfate. Finally, the conjunction of Fe(III), Fe(II), and Ru(III) with hydrogen peroxide led primarily to the generation of hydroxyl radicals. It is also suggested here that the redox behavior of a particular metal in solution cannot be predicted based exclusively on its size and charge. Additional phenomena such as metal hydrolysis as well as complexation with other counterions present in solution might affect the thermodynamics of the overall process and are further discussed here.

A highly efficient advanced oxidation process for the destruction of organic contaminants in water is reported. The technology is based on the cobalt-mediated decomposition of peroxymonosulfate that leads to the formation of very strong oxidizing species (sulfate radicals) in the aqueous phase. The system is a modification of the Fenton Reagent, since an oxidant is coupled with a transition metal in a similar manner. Sulfate radicals were identified with quenching studies using specific alcohols. The study was primarily focused on comparing the cobalt/peroxymonosulfate (Co/PMS) reagent with the traditional Fenton Reagent [Fe(II)/H2O2] in the dark, at the pH range 2.0-9.0 with and without the presence of buffers such as phosphate and carbonate. Three model contaminants that show diversity in structure were tested: 2,4-dichlorophenol, atrazine, and naphthalene. Cobalt/peroxymonosulfate was consistently proven to be more efficient than the Fenton Reagent for the degradation of 2,4-dichlorophenol and atrazine, at all the conditions tested. At high pH values, where the efficiency of the Fenton Reagent was diminished, the reactivity of the Co/PMS system was sustained at high values. When naphthalene was treated with the two oxidizing systems in comparison, the Fenton Reagent demonstrated higher degradation efficiencies than cobalt/peroxymonosulfate at acidic pH, but, at higher pH (neutral), the latter was proven much more effective. The extent of mineralization, as total organic carbon removed,was also monitored, and again the Co/PMS reagent demonstrated higher efficiencies than the Fenton Reagent. Cobalt showed true catalytic activity in the overall process, since extremely low concentrations (in the range of microg/L) were sufficient for the decomposition of the oxidant and thus the radical generation. The advantage of Co/PMS compared to the traditional Fenton Reagent is attributed primarily to the oxidizing strength of the radicals formed, since sulfate radicals are stronger oxidants than hydroxyl and the thermodynamics of the transition-metal-oxidant coupling.

The sulfate radical pathway of the room-temperature degradation of two phenolic compounds in water is reported in this study. The sulfate radicals were produced by the cobalt-mediated decomposition of peroxymonosulfate (Oxone) in an aqueous homogeneous system. The major intermediates formed from the transformation of 2,4-dichlorophenol were 2,4,6-trichlorophenol, 2,3,5,6-tetrachloro-1,4-benzenediol, 1,1,3,3-tetrachloroacetone, pentachloroacetone, and carbon tetrachloride. Those resulting from the transformation of phenol in the presence of chloride ion were 2-chlorophenol, 4-chlorophenol, 2,4-dichlorophenol, 2,6-dichlorophenol, 1,1,3,3-tetrachloroacetone, and pentachloroacetone. In the absence of chloride ion, phenol transformed into 2,5-cyclohexadiene-1,4-dione (quinone), 1,2-benzenediol (catechol), and 1,4-benzenediol (hydroquinone). Several parameters were varied, and their impact on the transformation of the organic compounds is also discussed. The parameters varied were the initial concentration of the organic substrate, the dose of Oxone used, the cobalt counteranion, and in particular the impact of chloride ions and the quenching agent utilized for terminating the reaction. This is one of the very few studies dealing with intermediates formed via sulfate radical attack on phenolic compounds. It is also the first studythat explores the sulfate radical mechanism of oxidation, when sulfate radicals are generated via the Co/Oxone reagent. Furthermore, it provides strong evidence on the interaction of chloride ions with sulfate radicals leading to halogenation of organics in water.

This study explores the potential of heterogeneous activation of Oxone (peroxymonosulfate) in water using cobalt oxides. Two commercially available cobalt oxides, CoO and Co3O4 (CoO.Co2O3) were tested for the activation of peroxymonosulfate and the consequent oxidation of 2,4-dichlorophenol (2,4-DCP) via a sulfate radical mechanism. Both systems, CoO/Oxone and Co3O4/Oxone, were tested at acidic and neutral pH and compared with the homogeneous Co(NO3)2/Oxone. The activity of these systems was evaluated on the basis of the induced transformation of 2,4-DCP as well as the dissolution of cobalt occurred after 2 h of reaction. It was observed that only Co3O4 activates peroxymonosulfate heterogeneously, with its heterogeneity being more pronounced at neutral pH. Both CoO and Co2O3 contained in Co3O4 might be responsible for the observed heterogeneity, and the relative mechanisms are further discussed here. To our knowledge, this is perhaps the first study that documents the heterogeneous activation of peroxymonosulfate with cobalt, the best-known catalyst-activator for this inorganic peroxide.