Fe(III) and Mn(II) ions were effectively removed from aqueous solutions using a composite of Fe3O4/CaO/PDA, with CaO sourced from green mussel. The composite material was comprehensively characterized using Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), Scanning Electron Microscopy with Energy Dispersive - X-ray Spectroscopy (SEM-EDS), Brunauer, Emmett and Teller (BET) surface area analysis, Vibrating Sample Magnetometer (VSM). The impact of physicochemical adsorption parameters, such as solution pH, contact time, and concentration, were investigated. The Fe3O4/CaO/PDA composite displayed a value of 51.47 emu/g in saturation magnetization, enabling rapid separation through the use of an external magnet without the need for filtration. Optimal conditions for adsorbing Fe(III) ions were achieved at pH 3 and initial concentration of 400 mg/L with maximum efficiency reach after 60 minutes. Similarly, optimal conditions for Mn(II) ion adsorption were observed at pH 4 with the same contact time and initial concentration. The adsorption efficiencies were found to be 88.56% for Fe(III) and 75.65% for Mn(II). The pseudo-second-order model aptly depicted the kinetics associated with the adsorption of both types of ions while the Langmuir isotherm model indicated that monolayer adsorption takes place on the composite's surface. The maximum capacities for adsorption is 322.58 mg/g for Fe(III) ions and 208.33 mg/g for Mn(II) ions. A negative Gibbs free energy value affirmed that the process occurs spontaneously under natural conditions. These results underscored the potential use of this Fe3O4/CaO/PDA composite in treating wastewater to remove heavy metal ions.