Structural factors of biogenic birnessit

The surface reactivity of biogenic birnessite is attributed to its structure; however, the structural control of heavy metal adsorption on biogenic birnessite remains inadequately understood. This study aimed to investigate the properties of a poorly-crystalline birnessite produced by the fungus Paraconiothyrium sp. WL-2 strain under ambient pH and temperature conditions. The structure was characterized using X-ray absorption spectroscopy and X-ray diffraction. Sorption behaviors of Co²⁺ were compared with those of Zn²⁺. The primary product of Mn bio-oxidation was identified as hexagonal birnessite with a turbosrtic structure. XAFS analysis revealed that the biogenic birnessite comprises octahedral sheets with Mn(IV) as the central metal and several vacant sites. Notably, Mn(III) ions were coordinated to some of these vacant sites in the interlayer. The findings indicated that the adsorption of Co²⁺ by biogenic birnessite exceeded that of Zn²⁺, aligning closely with the release of Mn from the biogenic birnessite. The results suggest that the interlayer Mn(III) can oxidize adsorbed Co²⁺ to Co³⁺, thus underscoring the unique adsorption mechanisms at play.