Tannic acid-based adsorbent with superior selectivity for lead(II) capture: Adsorption site and selective mechanism

Rational design and directed synthesis of an adsorbent with superior selectivity is challenging but of utmost importance for the remediation of heavy metal-laden actual wastewater. In this work, a novel tannic acid (TA)-based adsorbent (TA@Zr) was synthesized by a simple coordination reaction. Adsorption isotherm experiments exhibited the TA@Zr had a high capacity for Pb2+ capture. Impressively, an outstanding selectivity with extremely high selective coefficient (40-170) can be achieved by the TA@Zr. With decrease of Zr precursor, the selectivity of as-synthesized TA@Zr remained unchanged, implying that the Zr species played a negligible role in the selectivity of TA@Zr. Even the Zr species were completely replaced by Ti precursor, the resultant adsorbent still possessed a remarkable selectivity. All these results revealed that it was the TA, rather than the Zr species, made a substantial contribution to the selectivity of TA@Zr. Further, based on the results of XPS, DFT and batch adsorption experiments, it can be deduced that the phenolic hydroxyl groups on TA functioned as the dominant adsorption sites in which the hydroxyl groups' oxygen atoms coordinated with Pb2+ while hydrogen atoms of hydroxyl groups were replaced and released as hydrogen ions, thus extending a proton exchange mechanism for the selective adsorption of Pb2+ ions.