Manganese Dioxides\nInduce the Transformation and Protection\nof Dissolved Organic Matter Simultaneously: A Significance of Crystallinity

Interactions between manganese dioxides (MnO<sub>2</sub>) and dissolved\norganic matter (DOM) have long been the subject of scientific inquiry.\nHowever, the effect of MnO<sub>2</sub> crystallinity on the DOM fate\nremains unclear. Herein, we comprehensively investigate the adsorption,\nprotection, and mineralization of DOM by MnO<sub>2</sub> with various\ncrystallinities (order of crystallinity: γ-30 < γ-90\n< γ-120). The results show that DOM adsorption is positively\ncorrelated with the specific surface area (SSA) of MnO<sub>2</sub>; γ-30 with the largest SSA adsorbs the highest amount of DOM,\nresulting in DOM protection. However, γ-90 and γ-120 with\na smaller SSA could induce the Maillard reaction and thereby promote\nthe formation of geopolymerized organic matter, leading to reduced\nbioavailability of DOM. Furthermore, the capability of MnO<sub>2</sub> to mineralize DOM decreases in the order γ-120 > γ-90\n> γ-30, and it is determined by both Mn<sup>4+</sup> and\nhydroxyl\nradical (<sup>·</sup>OH) content. In particular, the contribution\nof radical-based oxidation of <sup>·</sup>OH to DOM mineralization\nis 64.8, 47.4, and 23.7% for γ-30, γ-90, and γ-120,\nrespectively. We propose that crystallinity of MnO<sub>2</sub> may\nhave a significant but hitherto unexplored influence on the global\ncarbon cycle over geological time.