The remarkable effect of the coexisting arsenite and arsenate species ratios on arsenic removal by manganese oxide

- The adsorption performance of manganese oxide in treating arsenic wastewater was investigated.
- The coexisting As(III)/As(V) molar ratio has a remarkable effect on arsenic removal by manganese oxide.
- Increasing the As(III)/As(V) molar ratio leads to a considerable increase of arsenic removal by manganese oxide.
- The origin of the molar ratio effect was studied by combining both the experimental and theoretical results.
- The formation of more Mn vacancy defects caused by As(III) oxidation makes the adsorption site more active.

The adsorption performance of manganese oxide in treating arsenic wastewater that contained different As(III)/As(V) molar ratios was investigated, and the samples before and after their reactions were characterized by XRD, TEM, N2 adsorption/desorption, SEM-EDS, FTIR, and XPS. The batch experimental results show that the coexisting molar ratio of As(III) and As(V) has a remarkable effect on arsenic removal by manganese oxide. Upon increasing the As(III)/As(V) molar ratio from 10 μM/90 μM to 50 μM/50 μM and 90 μM/10 μM, respectively, the total arsenic removal by manganese oxide significantly increased from 39.5% to 55.6% and 71.2%, respectively. The origin of the molar ratio effect was studied by combining both the experimental and theoretical results of the Mn2+ release detection, effect of Na2SO3-pretreatment on arsenic removal, and density functional theory (DFT) calculations. The results reveal that the highest arsenic removal by manganese oxide in an arsenic solution with the highest As(III)/As(V) molar ratio is attributed to the Mn2+ release caused by As(III) oxidation, forming more Mn vacancy defects, which makes the adsorption site more active, thus significantly promoting the adsorption performance of manganese oxide in arsenic removal.

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