Ni(ii) and Co(ii) complexes for the selective adsorption of anionic dyes from aqueous solutions

Abstract

Nowadays, selective removal of cationic or anionic organic dyes from aqueous solution has become a new research focus. Meanwhile, metal–organic framework materials have attracted extensive attention in the field of wastewater treatment due to their porous structures. From this perspective, based on pyridine–imidazole heterocyclic ligand 2,4,5-tri(4-pyridine)-1H-imidazole (Htpim), two new complexes Ni(Htpim)₂Cl₂·2H₂O (1) and Co(Htpim)₂(OH)Cl·7H₂O (2) were firstly designed and synthesized by the solvothermal method under 100 °C condition. Meanwhile, complexes 1 and 2 were characterized by various microanalyses. Also, the crystal structures of complexes 1 and 2 were determined by single-crystal X-ray diffraction. The results indicated that the three-dimensional supramolecular structures of both complexes 1 and 2 were connected by π–π interactions. To explore their functional properties, the adsorption capacities of complexes 1 and 2 for organic dyes were investigated in water. The experimental results exhibited that complexes 1 and 2 had better selective adsorption toward eosin dye with negative charges. According to the Langmuir, Freundlich and Temkin isotherm models, it was found that the Langmuir isotherm was more consistent with the adsorption process of complexes 1 and 2, and the adsorption capacities of complexes 1 and 2 for eosin were 125 mg g⁻¹ and 71.43 mg g⁻¹, respectively. According to thermodynamic analysis, the adsorption process of complexes 1 and 2 was spontaneous. Meanwhile, it was found that the pH of the dye solution also had an influence on the adsorption process. Also, adsorption kinetic curve fitting showed that complexes 1 and 2 both conformed to pseudo-second-order curves. In addition, the possible adsorption mechanisms were explored.