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 α-MnO₂ as an example) unveiled that Mn dissolution involved the following stages. First, Sb(III) adsorbed onto the (3 1 0) crystal plane of α-MnO₂ to form a complex via two Mn-O-Sb 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 α-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 α-MnO₂ were weakened and disrupted, leading to the release of Mn into the aqueous solution. This work advances the fundamental understanding of Sb adsorption onto MnO₂, 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.