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ZIF-8, a metal–organic framework that has significant benefits such as a substantial specific surface area, abundant active sites, and high porosity, has extensive application in environmental remediation. However, ZIF-8 is powdery and easily agglomerated, making it difficult to separate ZIF-8 from aqueous solutions and thus limiting its practical application in wastewater treatment. In this work, sodium alginate was chosen as a gel matrix, and CaCl2 as a cross-linking agent to form a calcium cross-linked MOF gel material (ZIF-8/SA). The batch adsorption investigations demonstrated that the saturated adsorption capacities of ZIF-8/SA in single Mn2+ pollution solution and Mn–Cu–Cd composite pollution solution are 179.86 mg g−1 and 101.86 mg g−1, respectively. Specifically, the adsorption of Mn2+ followed the Langmuir model and exhibited monomolecular layer adsorption. Furthermore, the adsorption of Cu2+ and Cd2+ followed the quasi-first-order kinetic model, which is primarily physical adsorption. The adsorption order of ZIF-8/SA for contaminants in the adsorption process is as follows: Cu2+ > Cd2+ > Mn2+. The SEM-EDS, FT-IR, and XPS studies revealed that the adsorption mechanism of ZIF-8/SA mostly involves ion exchange, coordination of carboxyl/hydroxyl/amino groups, and electrostatic attraction. To summarize, the experimental findings of this study demonstrate that the synthetic ZIF-8/SA hydrogel has promising applications for practical pollution treatment. [ABSTRACT FROM AUTHOR]