Impact of Divalent Cations on Dark Production of Hydroxyl Radicals from Oxygenation of Reduced Humic Acids at Anoxic–Oxic Interfaces

Humic acids (HAs) are redox-active and can serve as either electron acceptors or electron donors to participate in multiple redox reactions. In nature water, HA can be intimately associated with divalent cations, such as Fe²⁺, Ca²⁺, and Mg²⁺, through a series of reactions that may in turn affect the redox reactivity of HA. Recent advances have demonstrated that the oxygenation of reduced HA in the dark can produce ·OH at anoxic–oxic interfaces. However, little is known about the roles of the divalent cations complexed with HA on the production of ·OH from reduced HA. This study provides new knowledge regarding the impact of Fe²⁺, Ca²⁺, and Mg²⁺ ions on the dark production of ·OH from oxygenation of reduced HA at anoxic–oxic interfaces over a wide range of environmentally relevant conditions. Results show that the rates and yields of ·OH production increase with increasing Fe²⁺ concentration (0.18–0.89 mM). This is largely attributed to the formation of complexed Fe­(II) with HA, which increases the number of Fe­(II)/Fe­(III) cycles and enhances the decomposition of formed H₂O₂, accelerating the rates of Fenton reactions under circumneutral conditions. However, the promotional effect of Fe²⁺ on ·OH formation is greatly suppressed in the coexistence of high Ca²⁺/Mg²⁺ concentration (5–20 mM), likely due to the retarded formation of HA-Fe­(II) complexes and competition of HA surface reactive sites by Ca²⁺/Mg²⁺ ions. Findings improve the current understanding of the dark production of ·OH from reduced HA and provide valuable insights toward understanding of carbon cycling and contaminant fate at anoxic–oxic interfaces.