Synergistic dicarboxylate sites of natural citric acid modified MOF-808 for the deep removal of Pb2+ in water

Pb2+ ions cause serious harm to the ecological environment due to the characteristics of high toxicity and high degradation resistance. It is extremely urgent to develop an effective technique to capture Pb2+ from water. In this paper, an environmentally friendly adsorbent MOF-808-CA was prepared by grafting the natural citric acid (CA) into the stable MOF-808 through a simple and green routine. The synthesized MOF-808-CA possesses abundant adjacent free –COOH groups to synergistically trap Pb2+, exhibiting a superior adsorption capacity of 173.3 mg·g−1. Adsorption kinetic experiment reveals that removal efficiency can reach 99% within 5 min and the concentration of Pb2+ can be reduced from 10 mg·L−1 to 7 μg·L−1 after equilibrium, which shows a reduction of four orders of magnitudes and the value is lower than the drinking water standard stipulated by the World Health Organization. The combined experimental characterizations and theoretical calculations reveal that the synergistic dicarboxylic sites in MOF-808-CA can form strong affinity with Pb2+, which can reasonably explain the excellent adsorption performances. In addition, MOF-808-CA also demonstrates remarkable Pb2+ adsorption selectivity of 6 to 88 over other completive ions, outstanding chemical stability, and excellent recyclability for the Pb2+ capture. The synergetic dicarboxylic sites strategy provides a new routine to design advanced adsorbents for heavy metal ions capture.