Evaluation of phosphate removal from aqueous solution using metal organic framework; Isotherm, kinetic and thermodynamic study

Abstract

Background: Phosphate (PO43-) is the main etiological factor of eutrophication in surface waters. Metal organic frameworks (MOFs) are novel hybrid materials with amazing structural properties that make them a prominent material for adsorption. Methods: Zeolitic imidazolate framework 67 (ZIF-67), a water stable member of MOFs, with a truncated rhombic dodecahedron crystalline structure was synthesized in aqueous environment at room temperature and then characterized using XRD and SEM. PO43- adsorption from synthetic solutions using ZIF-67 in batch mode were evaluated and a polynomial model (R2: 0.99, R2adj: 0.98, LOF: 0.1433) developed using response surface methodology (RSM). Results: The highest PO43- removal (99.2%) after model optimization obtained when ZIF-67 dose, pH and mixing time adjusted to 6.82, 832.4 mg/L and 39.95 min, respectively. The optimum PO43- concentration in which highest PO43- removal and lowest adsorbent utilization occurs, observed at 30 mg/L. PO43- removal eclipsed significantly in the presence of carbonate. The equilibrium and kinetic models showed that PO43- adsorbed in monolayer (qmax: 92.43 mg/g) and the sorption process controlled in the sorption stage. Adsorption was also more favorable at higher PO43- concentration, according to the separation factor (KR) graph. Thermodynamic parameters (minus signs of ΔG°, ΔH° of 0.179 KJ/mol and ΔS° of 44.91 KJ/mol.K) demonstrate the spontaneous, endothermic and physisorption nature of the process. Conclusion: High adsorption capacity and adsorption rates, make ZIF-67 a promising adsorbent for PO43- removal from aqueous environment.