Revisiting the adsorption of antimony on manganese dioxide: The overlooked dissolution of manganese

Manganese dioxide (MnO2) exhibits great potential for the uptake of antimony (Sb) from wastewater. Much research has been dedicated to enhancing the capacity of MnO2 for Sb sequestration. However, the dissolution behavior of Mn has been overlooked, and the underlying relationship between Sb(III) adsorption and Mn dissolution is not clear. In this study, batch experiments evidenced that there was a noticeable dissolution of Mn during the adsorption of Sb(III) on MnO2. A correlation analysis demonstrated that the amount of Mn dissolved was dependent on the Sb(III) adsorption capacity of MnO2, regardless of the crystalline phase of MnO2. Further experimental and theoretical results (taking alpha-MnO2 as an example) unveiled that Mn dissolution involved the following stages. First, Sb(III) adsorbed onto the (3 1 0) crystal plane of alpha-MnO2 to form a complex via two Mn-OSb bonds, which can be proven by the delicate adsorption configuration and corresponding adsorption energy (E-ad) of -4.04 eV. Then, three O atoms from the alpha-MnO2 progressively deprived two H atoms from Sb(OH)(3) and one H atom from a water molecule bound to Sb(OH)(3), respectively. This process was verified by the configuration of the transition state (transition state #3) and the occurrence of electron transfer (i.e., the net charge changed by + 2 e). Finally, the Mn-O bonds of alpha-MnO2 were weakened and disrupted, leading to the release of Mn into the aqueous solution. This work advances the fundamental understanding of Sb adsorption ontoMnO(2), which not only promotes a re-evaluation of the utility of MnO2 for Sb(III) elimination but also provides a theoretical guide for the design of robust MnO2-based adsorbents.