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.