Abstract
Biogenic Mn oxides (BMOs) represent the most prevalent natural form of Mn oxides, having a significant influence on the biogeochemical cycling of Cd in the environment. The surface reactivity of BMOs is intricately linked to their structural characteristics. However, there remains a scarcity of knowledge regarding the structural disparities of BMOs during their formation in the context of heavy metal removal. This study investigates the fine mineralogical characteristics of three BMOs formed by the Mn-oxidizing fungus Cladosporium sp. XM01 during cultivation, namely immature BMO (Imm-BMO), low mature BMO (Low-BMO), and high mature BMO (High-BMO), and their adsorption mechanism for Cd(II). Analysis of Mn K-edge X-ray absorption fine structure indicated three BMOs exhibit a hexagonal birnessite structure with interlayer Mn(II/III), and Imm-BMO (48.6 %) exhibits a higher proportion of Mn(II/III) components compared to Low-BMO (14.4 %) and High-BMO (13.9 %). The results suggested a gradual decrease in Cd(II) adsorption capacity, following the order of Imm-BMO, Low-BMO, and High-BMO. Conversely, Cd(II) showed higher fixation capacity on Low-BMO (56.2 %−63.1 %) and High-BMO (64.4 %−70.6 %) compared to Imm-BMO (45.4 %−52.6 %), indicating an opposite trend in Cd(II) fixation. Specifically, the enhanced Cd(II) fixation performance during BMOs formation can be attributed to the availability of more vacancies and interlayer spaces for Cd(II) occupation and retention, respectively. Additionally, compared with pH 7.5, pH 5.5 was found to be more conducive to Cd(II) fixation on BMOs, attributed to the increased availability of more vacancies. Overall, this study enhances our understanding of the structural transformation during BMOs formation and its implication for Cd(II) adsorption behavior.
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