Hierarchical mesoporous spherical Zn(4-hzba) MOF for efficient adsorption of Strontium and Caesium ions from aqueous solution

This paper describes the synthesis of mesoporous spherical Zn(4-hzba) MOF with the formula of {[Zn(μ-4-hzba)2]2}n utilizing 4-hydrazinyl benzoic acid (4-hzba) linker by solvothermal method. And investigates its adsorptive efficiency towards strontium (Sr2+) and caesium (Cs+) by varying contact time, pH, and feed solution concentration. 10 mg of Zn(4-hzba) MOF show maximum adsorption capacity of 275.2 mg/g for Sr2+ and 335.05 for Cs+ at pH 4 from a 100 mg/L sample solution within 100 min at room temperature. Among the Langmuir, Freundlich and Temkin adsorption isotherms studies, the Langmuir isotherm was well fitted for Sr2+ and Cs+ adsorption onto MOF, demonstrating the monolayer adsorption. Experimental data and pseudo-second-order kinetic studies showed that chemisorption is the dominant process. High adsorption efficiency was demonstrated by electrostatic interaction between positively charged Sr2+/Cs+ and the negatively charged surface of the MOF at pH 4 was proved by zeta potential analysis. IR, SEM, XPS analysis and Langmuir adsorption studies support the monolayer surface complexation mechanism. While the ion exchange mechanism followed by surface complexation of Sr2+/ Cs+ with replacement of hydrogen in H2NNH- of the MOF. In order to acquire a good picture of the ion exchange mechanism, and Sr2+/Cs+ exchanged MOF were optimized, and the binding energies of Sr2+ (2.19 kJ/mol) and MOF Cs+ (2.06 kJ/mol) were calculated by DFT method using Gaussian 16. Recycle studies of Zn(4-hzba) MOF show 80% recovery of Sr2+ and Cs+ even after seven cycles. The adsorption of Sr2+ and Cs+ was influenced by dosage, initial concentration of metal ions, pH solution, contact time, and coexisting ions of Na, K, Ca, and Mg. Adsorption efficiency of Sr2+ and Cs+ ions in simulated seawater was 92.3% and 96.7%, respectively.