Efficient removal of phosphate from acidified urine using UiO-66 metal-organic frameworks with varying functional groups

The removal of phosphate from source-separated urine is an efficient approach to reduce the P-load in receiving waters due to its high P concentration and low volume in wastewater. However, a highly efficient adsorbent is imperative for this purpose. Metal-organic frameworks (MOFs) have recently attracted a lot of attention in adsorption due to their outstanding physicochemical features. In this study, we examined four Zr-based MOFs (UiO-66, UiO-66-NO2, UiO-66-Br and UiO-66-NH2) for the adsorption of phosphate from acidified urine. Our results indicated that the zirconium content, pore size and crystallinity are driving forces for the adsorption process. UiO-66-NH2 displayed the highest adsorption capacity (153.9 mg/g) at 25 °C and pH 4. Phosphate in urine could be nearly totally depleted (>99% removal) using UiO-66-NH2 at a dosage ≥13.5 g/L. Elevating temperature and lowering pH are effective strategies to enhance the adsorption performance. The reduction of the isoelectric point of UiO-66-NH2 after P uptake, the activation energy of the adsorption process estimated using Arrhenius equation (63.1 kJ/mol), FTIR spectroscopy and XPS suggested that inner-sphere complexation between phosphate and Zr atoms in the MOFs was the key adsorption mechanism. The performance of adsorption-desorption-regeneration using real human urine was verified.