Identification of Sulfate and Hydroxyl Radicals in Thermally Activated Persulfate

TLDR

Thermal activation of persulfate generates sulfate radicals that can interconvert to hydroxyl radicals under alkaline conditions, and these radicals may coexist, with nitrobenzene reacting only with radicals and not with persulfate. The study develops a chemical probe method to identify which radical species are present in the persulfate oxidation system. By adding excess probes such as tert‑butyl alcohol, phenol, and nitrobenzene and comparing their reactivity, the method distinguishes sulfate from hydroxyl radicals. Nitrobenzene reacts 3000–3900 times faster with hydroxyl radicals than with sulfate radicals, and pH‑dependent kinetics show sulfate dominates below pH 7, both radicals coexist at pH 9, and hydroxyl dominates at pH 12, confirming nitrobenzene as an effective probe.

Abstract

Thermal activation can induce persulfate (S2O82−) degradation to form sulfate radicals (SO4−•) that can undergo radical interconversion to form hydroxyl radicals (HO•) under alkaline conditions. The radicals SO4−•/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 SO4−•/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 SO4−•, which is a good candidate for use as a probe for differentiating between SO4−•/HO• reactivity. Furthermore, the effects of pH on the formation of SO4−•/HO• were demonstrated by the degradation kinetics of NB at varying pH values. The results indicated that SO4−• is the predominant radical at pH <7; both SO4−• 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•/SO4−• reactivity.