Evaluation of the reactivity of superoxide radicals with phosphite by direct and indirect kinetic methods

Abstract

Phosphite (PHI) accounts for a noticeable portion of the total dissolved inorganic phosphorus in diverse aquatic systems. Efficient PHI control in water bodies is challenging due to its high solubility and strong resistance to biotransformation. To verify the role of superoxide radical (O2•−) in PHI transformation to phosphate, we used complementary direct and indirect methods to investigate the kinetic aspects of O2•− induced reactions with PHI. The rate constant (k, M−1 s−1) for the reaction between O2•− and PHI, measured by an in situ long-path absorption spectroscopic approach, was determined to be 420 M−1 s−1. The low k value suggests its minor contribution as compared to hydroxyl radicals commonly used in advanced oxidation processes. We then studied the thermodynamic properties of possible pathways for O2•− oxidation of PHI using density functional theory, and our calculations indicated the reaction is thermodynamically unfeasible. With these fundamental results, we aim for a better understanding on the degradation kinetics and mechanisms for PHI and O2•−, providing useful scientific knowledge for the transformation of phosphorus in sunlit surface waters and engineering wastewater treatment processes.