Efficient removal of methylene blue by water-stable anionic coordination polymer in aqueous media†

Effective adsorption of dye pollutants is essential to preserving human health and life before they are released into the environment. Ionic coordination polymers are considered good candidates for the effective elimination of dye pollutants. In this research, an anionic coordination polymer, {[NH₂(CH₃)₂]₂[Cd(μ₄-L)]·1.5H₂O·0.75DMF}_n (1), [L⁴⁻: (E)-5,5′-(fumaroylbis(azanediyl))diisophthalate], was prepared and characterized. Structural analysis revealed that each Cd(II) ion was linked to four L⁴⁻ ligands forming a 3D framework. A 2-fold interpenetrated 3D architecture of the compound was observed with a dia topology due to the large pore of a single framework. Owing to its water stability and anionic skeleton, the removal ability of four dyes with different charges and sizes by the compound was investigated. The compound, maintaining the negative charge density, effectively adsorbed the cationic methylene blue (MB) dye, while the elimination of anionic methyl orange (MO) and Congo red (CR) dyes was not observed owing to electrostatic repulsion. Additionally, the compound displayed size-dependent adsorption as evidenced by its non-adsorption of the cationic rhodamine B (RhB) dye. The compound exhibited a high MB adsorption capacity with a maximum of 689.93 mg g⁻¹. The experimental data agreed well with the Langmuir isotherm model, and the adsorption process could be marked as a pseudo-second-order kinetic model. The MB adsorption of the compound could be attributed to electrostatic interactions due to its anionic framework and weak interactions.