Abstract: Uranium can be incorporated into the octahedral sites of high crystalline iron oxides such as goethite, magnetite, lepidocrocite, green-rust and hematite, in substitution of iron as either pentavalent uranium U(Ⅴ) or hexavalent uranium U(Ⅵ) during microbially induced transformation of low crystalline iron-oxide ferrihydrite. To investigate the effects of microorganisms on the formation and geochemical behavior of structurally incorporated uranium, batch experiments were conducted. Various mineralogical, environmental geochemical, and spectroscopic methods were employed to examine the impacts of microbial biomass on the penetration of uranium, the mineralogical phase of the final product, and the content, oxidation states, as well as environmental stability of structurally incorporated uranium. The results indicate that regardless of the biomass content, the extent of final phase transformation is similar, with approximately 84% of 2-line ferrihydrite transforming into a higher crystalline iron oxide such as goethite and magnetite. Despite the similarity in the final extent of 2-line ferrihydrite phase transformation, the rate of phase transformation is biomass-dependent. Specifically, the phase transformation rate in systems with lower biomass is relatively slower. This slower transformation rate results in the formation of Fe(Ⅲ)-bearing goethite, which exhibits strong resistance to air oxidation and demonstrates favorable environmental stability. A relatively higher content of structurally incorporated uranium is observed within goethite, predominantly in the U(Ⅵ) oxidation state. Conversely, the higher biomass amended system displays an accelerated rate of phase transformation, resulting in the formation of Fe(Ⅱ)-bearing magnetite. Magnetite formed under these conditions has relatively poor environmental stability. Additionally, the content of structurally incorporated uranium in magnetite is lower, predominantly occupied by U(Ⅴ) oxidation state. This suggests that the higher biomass facilitates faster transformation rates but at the cost of producing less environmentally stable phases with lower uranium content. The study highlights the significant influence of microbial biomass on the transformation kinetics and final mineralogical outcomes of 2-line ferrihydrite transformation. The findings suggest that microbial activity not only affects the rate of transformation but also affects the environmental behavior of the resulting iron oxides, particularly in terms of the uranium incorporation and corresponding stability. These insights are crucial for understanding the long-term geochemical behavior of uranium in natural and contaminated environments, where microbial processes play a pivotal role in the cycling and immobilization of heavy metals. The differential behavior of uranium in goethite and magnetite formed under varying biomass conditions underscores the need for careful consideration of microbial dynamics in bioremediation strategies and the management of uranium-contaminated sites.