<p>Minerals containing iron (Fe) and phosphate can simultaneously immobilize cations such as lead (Pb) and oxyanions including arsenic (As) and antimony (Sb). However, phosphate released from these minerals substitutes for adsorbed As and Sb and increases metal(loid) mobility, which limits their practical effectiveness. Vivianite [Fe₃(PO₄)₂·8(H₂O)], an Fe-phosphate mineral with low phosphate release potential, offers a promising solution for the simultaneous stabilization of cationic and anionic contaminants. This study evaluated the effectiveness of vivianite for concomitant immobilization of arsenite [As(III)], arsenate [As(V)], antimonite [Sb(III)], antimonate [Sb(V)], and Pb(II) in single- and mixed-metal(loid) solutions and contaminated soils. The adsorption of As(III), As(V), and Sb(III) onto vivianite followed the Langmuir isotherm model, indicating monolayer surface interaction. In mixed-metal(loid) solutions containing As or Sb with Pb, immobilization increased by 73% for As(III), 3271% for As(V),&#xa0;and 12% for Sb(III) compared to single-metal(loid) solutions.&#xa0;For Sb(V), immobilization&#xa0;increased from 0% in single-solution&#xa0;to&#xa0;83% in mixed-metal(loid) solution. Phosphate released from vivianite reacted with Pb(II), resulting in Fe release. The liberated Fe subsequently reacted with As and Sb and enables their simultaneous immobilization. Application of vivianite decreased the concentrations of bioavailable As and Pb by 23% and 52%, respectively, in mixed-metal(loid) contaminated soil. In single-metal(loid) contaminated soil, bioavailable Sb and Pb were reduced by 16%, and 19%, respectively, compared to the control. Iron phosphate amendments often failed to achieve simultaneous stabilization of Pb and As because phosphate release promoted As remobilization. In contrast, vivianite enabled concomitant immobilization of both toxic oxyanions and cationic metals in soil during prolonged incubation by limiting phosphate release to levels insufficient to competitively displace As.</p>