The manganese redox cycle is coupled to the carbon cycle in numerous natural environments, but it remains unclear how Mn and C react under diverse environmental conditions. Naturally-occurring birnessite is enriched in various metal cations due to its high cation adsorption affinity. The present study examines the effects of cations (Na+, Zn2+ and Ca2+) on the reaction kinetics and mechanisms between δ–MnO2 and fulvic acid (FA). The experiments were conducted in batch experiments at pH 8 with solid/FA mass ratio of 10 under anoxic conditions over 25 days. The changes of FA and δ–MnO2 were characterized using spectroscopic and X-ray diffraction techniques. Results show that all the cations enhance production of dissolved Mn and removal of FA from the solution, i.e., promoting FA adsorption and oxidation by δ–MnO2. For the FA fraction remained in the solution, its SUVA254 decreases while its E2/E3 increases with time, indicating preferential removal of the fraction higher in both aromaticity and molecular weight. Ca2+ shows the strongest effects, followed by Zn2+ and then by Na+.δ–MnO2 is enriched in Mn(III) in the MnO6 layers in the presence of Na+or Ca2+ as the reaction proceeds, whereas the structure contains much less Mn(III) in the prsence of Zn2+. These observations are ascribed to the formation of Mn(III) by the conproportionation reaction between Mn(IV) in δ–MnO2 and vacancy-adsorbed Mn(II). Both Na+ and Ca2+ favor the Mn(III) formation by stabilizing Mn(III) in the layers while Zn2+ disfavors it through competition with Mn(II) for vacancies. In addition, a little amount of hausmannite started to appear at 120 hours in the prsence of Ca2+, attributed to the production of a high concentration of Mn(II) that reduced δ–MnO2 to hausmannite. In conclusion, cations significantly enhance FA adsorption and oxidation by δ–MnO2 and affect the structure and composition of the reacted oxide.

Cation effects on the adsorption and oxidation of fulvic acid by manganese oxides

253rd National Meeting (2017)

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