Carbonate radicals in environmental systems: mechanistic insights and engineering applications

Carbonate radical (CO₃^•−) is prevalent in both natural environments and engineered water treatment systems, originating from reactions between carbonate species (CO₃²⁻/HCO₃⁻) and free radicals or triplet states of dissolved organic matter (³DOM*). Its significant roles in environmental oxidation, advanced oxidative water treatment, and cellular oxidative damage are increasingly recognized. Despite extensive research on CO₃^•−, a gap remains in comprehensive reviews addressing its kinetics and mechanisms in pollutant removal and environmental impact. This review synthesizes previous studies, tracing the historical recognition of CO₃^•− in environmental sciences and comparing various detection methods. The reaction rate constants of CO₃^•− with organic contaminants (k_CO3•−) are summarized, and the relationship between pollutant structure and reactivity is explored. Reaction mechanisms and the selectivity of CO₃^•− are critically evaluated in comparison to other common radicals, such as hydroxyl (HO^•) and sulfate (SO₄^•−). Furthermore, the dual roles of CO₃^•− as both primary and secondary radicals in advanced oxidation processes (AOPs), along with its impact on degradation pathways and byproduct toxicity, are assessed. Finally, the environmental implications of CO₃^•−, including its contributions to self-purification, metal transport, and biological oxidative damage, are discussed. This review aims to offer a comprehensive framework for understanding CO₃^•− in environmental contexts, enhancing its application in AOP systems and natural remediation.