Manganese Dioxides Induce the Transformation and Protection of Dissolved Organic Matter Simultaneously: A Significance of Crystallinity

Interactions between manganese dioxides (MnO₂) and dissolved organic matter (DOM) have long been the subject of scientific inquiry. However, the effect of MnO₂ crystallinity on the DOM fate remains unclear. Herein, we comprehensively investigate the adsorption, protection, and mineralization of DOM by MnO₂ with various crystallinities (order of crystallinity: γ-30 < γ-90 < γ-120). The results show that DOM adsorption is positively correlated with the specific surface area (SSA) of MnO₂; γ-30 with the largest SSA adsorbs the highest amount of DOM, resulting in DOM protection. However, γ-90 and γ-120 with a smaller SSA could induce the Maillard reaction and thereby promote the formation of geopolymerized organic matter, leading to reduced bioavailability of DOM. Furthermore, the capability of MnO₂ to mineralize DOM decreases in the order γ-120 > γ-90 > γ-30, and it is determined by both Mn⁴⁺ and hydroxyl radical (^·OH) content. In particular, the contribution of radical-based oxidation of ^·OH to DOM mineralization is 64.8, 47.4, and 23.7% for γ-30, γ-90, and γ-120, respectively. We propose that crystallinity of MnO₂ may have a significant but hitherto unexplored influence on the global carbon cycle over geological time.