© 2018 American Chemical Society. Although the adsorption of cesium (Cs) onto phyllosilicate minerals has been widely studied, the effect of Cs on redox-sensitive biogenic Mn oxide is not well understood. In this study, the structural transformation of biogenic birnessite stimulated by commonly occurring natural heavy metals, and the influence of those metals on the adsorption behavior of Cs over a wide range of concentrations (1 × 10-10to 0.1 mol/L) was carefully examined via solution chemistry and X-ray absorption fine structure techniques. The Cs was reversibly adsorbed onto the mineral surface to form an outer-sphere coordination for all biogenic birnessite samples. The presence of heavy metals (e.g., Zn and Ni) during bio-oxidation of Mn(II), followed by acid treatment, increased the number of available layer vacancies, which consequently increased the adsorption capacity of Cs in the final product. The analysis of the dependence of sorption values on the ionic strength showed distinct results on biogenic birnessite and chemically synthesized birnessite. Trace ions (e.g., Mn2+) that were loosely bound to biogenic birnessite were released into the solution when they came in contact with the NaCl solution. The competition between these trace ions significantly influence Cs adsorption on biogenic birnessite. The results in this study indicated that it is necessary to account for the competition effect of trace ions, particularly when considering poorly crystalline adsorbents, such as biominerals.