Photochemical activation of seemingly inert SO₄^2- in specific water environments

Sulfate ions (SO₄^2-) are ubiquitous in aqueous environments and are generally considered to be inert due to their chemical stability. For the first time, we found that SO₄^2- can be activated into SO₄^2--type radicals (e.g., SO₃^-, SO₅^-, SO₄^-) in the presence of phenolic compounds under simulated or natural solar irradiation. In turn, the radicals promoted the transformation and mineralization of phenolic compounds compared to that in the absence of SO₄^2- with reaction rate constants ranging from 0.008 h^-1 to 0.021 h^-1. In addition, the activation mechanisms of inert SO₄^2- in the photochemical transformation of phenolic compounds were elucidated. A hydrated electron (e_aq^-) is first generated during the photolysis of phenolic compounds and is the most important step in the activation of SO₄^2-. Then, the e_aq^- reduces SO₄^2- to SO₃^2-, and SO₃^2- is further photochemical activated to become a reactive species (e.g., e_aq^- and SO₃^-), which can evolve into strong oxidants (e.g., SO₅^- and SO₄^-), via a series of radical chain reactions. These oxidants are responsible for the enhanced phenolic compound degradation. The photochemical activation of seemingly inert SO₄^2- sheds new light on studies on the transport, transformation and environmental impact of matter (e.g., phenolic compounds) in specific water environments and provides a novel strategy for the generation of SO₄^- and photochemical removal of phenolic pollutants.