Amino-functionalized MOFs combining ceramic membrane ultrafiltration for Pb (II) removal

Amino-functionalized metal–organic frameworks (MOFs) were combined with ceramic membrane ultrafiltration (CUF) for the first time in adsorptive removal of Pb (II) from wastewater. Rapid synthesis (3 min) of the MOFs was achieved via a microwave-promoted method. The obtained MOFs exhibit a sphere-like morphology with an average diameter around 100 nm, and an X-ray diffraction pattern of UiO-66-NH2. Effects of trans-membrane pressure (TMP), cross-flow velocity (CFV), temperature and pH were investigated on the combined MOFs-CUF process. The results showed that the MOFs can be completely retained (100%) by the membrane with an average pore size of 50 nm. At TMP = 0.15 MPa, CFV = 4.0 m·s−1 and T = 35 °C, the process experienced the highest removal (61.4%) of Pb (II), and the lowest flux decline and lowest degree of membrane fouling. Moreover, the adsorption capacity at equilibrium was remarkably high (1795.3 mg·g−1). The pseudo-second-order kinetic model provided the best fit to the adsorption data (R2 = 0.9997). The adsorption mechanisms of coordination interaction between amidogen (–NH2) and Pb (II) were confirmed. The adsorbed Pb (II) can be effectively desorbed by controlling pH at 4.5 during 6 cycles. The effective membrane cleaning method was using 0.5% (w/w) ammonium citrate followed by 0.5% (w/w) HNO3 solution and each cleaning for an hour at 18–50 °C, eventually the flux exhibits a recovery efficiency near 100%. Therefore, MOFs combined with ceramic membrane ultrafiltration have great potential in heavy metal wastewater treatment.