Magnetic starch microspheres (MSMs) and methyl methacrylate magnetic starch microspheres (MMA-MSMs) were prepared via an inverse emulsion polymerization by mechanically activated cassava starch as a crude material, and branched-chain polyethyleneimine(PEI) was cross-linked to the surface of the two microspheres by glutaraldehyde to prepare polyethyleneimine-modified magnetic starch microspheres (PEI/MSMs) and polyethyleneimine-modified methyl methacrylate magnetic starch microspheres (PEI/MMA-MSMs). The results showed that Fe₃O₄ nanoparticles were encapsulated in the absorbent microspheres. The PEI/MMA-MSMs exhibited a smooth surface, regular shape, good dispersion, and uniform particle size, with a distribution between 200 and 500 nm. The addition of methyl methacrylate (MMA) increased the average particle size of the microspheres and provided more adsorption sites for Cd(II) ions. In addition, PEI/MMA-MSMs showed better thermal stability, and the magnetization of the PEI/MSMs and PEI/MMA-MSMs was 9.8 emu g⁻¹ and 14.6 emu g⁻¹, respectively. The pH value was an important factor affecting the adsorption of Cd(II) on PEI/MSMs and PEI/MMA-MSMs, and the maxima adsorption capacity on PEI/MSMs and PEI/MMA-MSMs was 121 mg g⁻¹ and 187 mg g⁻¹ at pH 6 and 5, respectively. The Cd(II) adsorption on PEI/MSMs and PEI/MMA-MSMs could be fitted by a Langmuir isotherm adsorption model and a pseudo-second-order kinetic equation, and the -NH₂ and -OH groups in the adsorbent interacted with the Cd(II) in the solution mainly by sharing electron pairs. The adsorbent exhibited good recyclability after 5 adsorption–desorption cycles of PEI/MSMs and PEI/MMA-MSMs.

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