Impacts of ammonium ion on triclinic birnessites towards the transformation of As(III)
Triclinic birnessite (TB), a typical layered Mn oxide which is abundant naturally occurring minerals with a vital impact on the transformation of arsenite (As(III)) by adsorption and oxidation . As one of the most common critical metalloids , ammonium ion (NH 4 + ) universally coexists with birnessite in marine, sediments or groundwater where are contaminated with As(III). In this study, we investigated the impacts of NH 4 + on TB towards the transformation of As(III). Compared with the original TB (40.1%), the As(III) removal efficiencies of three different concentration (0.5 M, 1 M and 2 M) NH 4 + impressed triclinic birnessite (TB-0.5 N, TB-1N and TB-2N) are increased rapidly in the order of: TB-2N (80.4%) > TB-1N (75.8%) > TB-0.5 N (71.5%). In addition, TB-2N exhibited the highest initial oxidation rate of 0.0031 min −1 which exceeds twice as much as this of TB (0.0014 min −1 ). And TB-2N could reach the max oxidation efficiency when the As concentration is 0.08 mM. Due to two different mechanisms of As(III) oxidation on birnessites under acidic and alkaline conditions, TB-2N showed a higher removal efficiency than TB at pH 3.0, 5.0, 7.0 and 9.0. Hence, there are two main reasons for the advanced As(III) oxidation capacity of TB-2N. One is the improvement of the average oxidation state of Mn, the other is the increase of oxygen vacancy with the coexistence of NH 4 + . Moreover, the larger specific surface area of TB-2N also contribute to enhancing As(III) oxidation capacity. This study holds a fundamental understanding of the behavior of triclinic birnessite which is coexisted with ammonium ion towards the transformation of As(III) in the environment.
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