Identification of Sulfate and Hydroxyl Radicals in Thermally Activated Persulfate

Abstract

Thermal activation can induce persulfate (S 2 O 8 2− ) degradation to form sulfate radicals (SO 4 − •) that can undergo radical interconversion to form hydroxyl radicals (HO•) under alkaline conditions. The radicals SO 4 − •/HO• 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 − •/HO• was investigated. NB can only react with radicals: it cannot react with persulfate. The reaction rate of NB with HO• is 3000−3900 times greater than that of NB with SO 4 − •, which is a good candidate for use as a probe for differentiating between SO 4 − •/HO• reactivity. Furthermore, the effects of pH on the formation of SO 4 − •/HO• were demonstrated by the degradation kinetics of NB at varying pH values. The results indicated that SO 4 − • is the predominant radical at pH <7; both SO 4 − • and HO• are present at pH 9; HO• 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•/SO 4 − • reactivity.