Identification of Sulfate and Hydroxyl Radicals in Thermally Activated Persulfate

Thermal activation can induce persulfate (S(2)O(8)(2-)) degradation to form sulfate radicals (SO(4)(-)center dot) that can undergo radical interconversion to form hydroxyl radicals (HO center dot) under alkaline conditions. The radicals SO(4)(-)center dot/HO center dot can be present either individually or simultaneously in the persulfate oxidation system. To identify the active radical species, a chemical probe method was developed. An excess of probe compounds was added to the system, and differences between the reactivity of the probes and the potential radical species were observed. The usage of various probes, including tert-butyl alcohol, phenol, and nitrobenzene (NB), for simultaneously identifying SO(4)(-)center dot/HO center dot was investigated. NB can only react with radicals: it cannot react with persulfate. The reaction rate of NB with HO center dot is 3000-3900 times greater than that of NB with SO(4)(-)center dot, which is a good candidate for use as a probe for differentiating between SO(4)(-)center dot/HO center dot reactivity. Furthermore, the effects of pH on the formation Of SO(4)(-)center dot/HO center dot were demonstrated by the degradation kinetics of NB at varying pH values. The results indicated that SO(4)(-)center dot is the predominant radical at pH <7; both SO(4)(-)center dot and HO center dot are present at pH 9; HO center dot is the predominant radical at a more basic pH (i.e., pH 12). The use of chemical probe NB is a suitable compound for differentiating HO center dot/SO(4)(-)center dot reactivity.