  Advanced oxidation processes that utilize highly oxidative radicals are
widely used in water reuse treatment. In recent years, the application of
sulfate radical (SO$_4\cdot^-$) as a promising oxidant for water treatment has
gained increasing attention. To understand the efficiency of SO$_4\cdot^-$ in
the degradation of organic contaminants in wastewater effluent, it is important
to be able to predict the reaction kinetics of various SO$_4\cdot^-$-driven
oxidation reactions. In this study, we utilize density functional theory (DFT)
and high-level wavefunction-based methods (including computationally-intensive
coupled cluster methods), to explore the activation energies and kinetic rates
of SO$_4\cdot^-$-driven oxidation reactions on a series of benzene-derived
contaminants. These high-level calculations encompassed a wide set of reactions
including 110 forward/reverse reactions and 5 different computational methods
in total. Based on the high-level coupled-cluster quantum calculations, we find
that the popular M06-2X DFT functional is significantly more accurate for
HO-additions than for SO$_4\cdot^-$ reactions. Most importantly, we highlight
some of the limitations and deficiencies of other computational methods, and we
recommend the use of high-level quantum calculations to spot-check
environmental chemistry reactions that may lie outside the training set of the
M06-2X functional, particularly for water oxidation reactions that involve
SO$_4\cdot^-$ and other inorganic species.
