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Magnesium slag-based porous materials (MSBPM) were successfully synthesized using alkali activation and foaming methods as an effective adsorbent for Pb²⁺ in solution. The effects of foaming agent type, foaming agent dosage, alkali dosage, and water glass modulus on the properties of the MSBPM were studied, and the micromorphology and porosity of the MSBPM were observed using microscopy. The influence of pH value, initial concentration, and adsorbent dosage on the Pb²⁺ adsorption was investigated. The results showed that a porous material (MSBPM-H₂O₂) with high compressive strength (8.46 MPa) and excellent Pb²⁺ adsorption capacity (396.11 mg·g⁻¹) was obtained under the optimal conditions: a H₂O₂ dosage of 3%, an alkali dosage of 9%, a water glass modulus of 1.3, and a liquid–solid ratio of 0.5. Another porous material (MSBPM-Al) with a compressive strength of 5.27 MPa and the Pb²⁺ adsorption capacity of 424.89 mg·g⁻¹ was obtained under the optimal conditions: an aluminum powder dosage of 1.5‰, an alkali dosage of 8%, a water glass modulus of 1.0, and a liquid–solid ratio of 0.5. When the pH of the aqueous solution is 6 and the initial Pb²⁺ concentrations are 200~500 mg·L⁻¹, the MSBPM-H₂O₂ and MSBPM-Al can remove more than 99% of Pb²⁺ in the solution. The adsorption process of both materials followed the Langmuir isotherm model and pseudo-second-order kinetic model, indicating that the adsorption process was a single-molecule layer chemical adsorption.