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

• A noticeable dissolution of Mn was observed in adsorption of antimony on MnO 2 . • Dissolved amount of Mn depended on antimony adsorption capacity of MnO 2 . • Variation of crystalline phases possessed an insignificant effect on Mn release. • Mechanisms of Sb capture and Mn release were unveiled at the atomic scale. Manganese dioxide (MnO 2 ) exhibits great potential for the uptake of antimony (Sb) from wastewater. Much research has been dedicated to enhancing the capacity of MnO 2 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 MnO 2 . A correlation analysis demonstrated that the amount of Mn dissolved was dependent on the Sb(III) adsorption capacity of MnO 2 , regardless of the crystalline phase of MnO 2 . Further experimental and theoretical results (taking α-MnO 2 as an example) unveiled that Mn dissolution involved the following stages. First, Sb(III) adsorbed onto the (3 1 0) crystal plane of α-MnO 2 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-MnO 2 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-MnO 2 were weakened and disrupted, leading to the release of Mn into the aqueous solution. This work advances the fundamental understanding of Sb adsorption onto MnO 2 , which not only promotes a re-evaluation of the utility of MnO 2 for Sb(III) elimination but also provides a theoretical guide for the design of robust MnO 2 -based adsorbents.