Simultaneous generation of hydroxyl radicals and sulfate radicals

Sulfate radicals have been proven to be strong oxidizing species like hydroxyl radicals [95]. Among various methods, sulfate radicals are generated by catalytic decomposition of PMS in a homogeneous pathway

2+ 2+ + 2+ 21 with the aid of transition metals (M: Fe , Co , Ag , Mn , Ni ) and/or

UV radiation (Equation 20):

Oxidants used in sulfate radicals-based AOTs are currently used for pools and spa disinfection. Of the transition metals investigated for sulfate radical generation, cobalt ions (Co2+) showed the best performance [96]. Although this system is promising for degrading environmental pollutants in water, health concerns associated with the adverse effect of dissolved cobalt (i.e., cobalt salts are used) in water still need to be addressed. Anipsitakis and his coworkers responded to the request by using commercially available cobalt oxide particles (Co3O4) and first demonstrated the heterogeneous PMS activation [97]. More recently, Yang and her coworkers immobilized and distributed well-defined 10" 15 nm nanocrystalline Co3O4 particles on the surface of 30—40 nm TiO2 nanoparticles as a support material for the Co3O4 particles to disperse [98]. The Co3O4/TiO2 nanocomposite showed enhanced decomposition of 2-chlorophenol by sulfate radical attack. In the PMS/Co3O4/TiO2 system, a feasibility of simultaneous generation of sulfate radicals and hydroxyl radicals was first attempted by introducing UV irradiation to the system and some promising preliminary results were obtained [99]. The simultaneous generation of two strong oxidizing species, sulfate radicals and hydroxyl radicals, are scientifically and practically important, although the mechanism of simultaneous generation of both radicals and their effect on the degradation of organic compounds are still being investigated.

Sulfate radicals were reported to be efficient for the decomposition of 2,4-dichlorophenol, atrazine, and naphthalene [95]. Since sulfate radicals (2.5-3.1 V) possess similar reduction potential to hydroxyl radicals (1.92.0 V) at neutral pH, they can also be effective in degrading certain organic compounds. However, the organic attack mechanisms of the two radicals have differences and thus different reaction intermediates are formed. The stability of the intermediates determines the overall mineralization of organic substrates using hydroxyl radicals and sulfate radicals. At acidic condition, they both have similar reduction potentials, but sulfate radicals are more selective for oxidation than hydroxyl radicals, which react rapidly with organic molecules by initial hydroxylation or hydrogen abstraction [100].

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