<p>A novel graphene oxide composite incorporating iron (Fe) and manganese (Mn) (GO-FM) was synthesized by co-precipitating Fe and Mn oxides onto graphene oxide to remediate As³⁺ and Cd²⁺ contaminants in water. The spectrum of SEM-EDS and XRD analyses confirmed the successful integration of Fe₂O₃ and MnFe₂O₄ into the graphene oxide matrix. FT-IR characterization indicated increased number of –OH on GO-FM following the incorporation. The adsorption behavior of As³⁺ and Cd²⁺ varied with initial pH. Cd²⁺ reached equilibrium within 2&#xa0;h, whereas As³⁺ stabilized after 24&#xa0;h, with adsorption efficiency improving with temperature. Maximum adsorption capacities were recorded at 90.34 mg∙g^− 1 for Cd²⁺ and 141.69 mg∙g^− 1 for As³⁺. In the co-adsorption system, As³⁺ and Cd²⁺ interactions displayed both synergistic and antagonistic effects. At low pH (2–3), Cd²⁺ inhibited As³⁺ adsorption, demonstrating antagonism. At neutral pH (7), a synergistic effect was observed, where As³⁺ increased Cd²⁺ adsorption by 12.4%–429%, and Cd²⁺ elevated As³⁺ adsorption capacity by 1.7%–11%. This synergy was driven partly by electrostatic forces and ternary surface complex formation, but co-precipitation as the dominant process. These findings offer critical insights into the interaction dynamics of As³⁺ and Cd²⁺ on GO-FM, highlighting the potential for effective remediation applications.</p>