Remarkable ability of Pb(II) capture from water by self-assembled metal-phenolic networks prepared with tannic acid and ferric ions

Plant-derived chelators used for heavy metal removal have attracted increasing attention because of their low cost, strong coordination effect, and environmentally benign features. Herein, natural polyphenol-based adsorbents (i.e., TA-Fe-7 and TA-Fe-12) were prepared using tannic acid (TA) and ferric ions through simple cross-linking self-assembly reactions with pH 7 and 12, respectively. Adsorption isotherms demonstrated that TA-Fe-12 showed better adsorption ability towards Pb(II) than TA-Fe-7 with a maximum adsorption capacity of 356.1 mg center dot g(-1) at 298 K. Our evaluation of the adsorption affinity indicated that the distribution coefficient Kd of TA-Fe-12 for Pb(II) was apparently higher than that of the other cations, revealing the excellent adsorptive selectivity of TA-Fe-12 towards Pb(II). The remarkable adsorption capacity of TA-Fe-12 can be attributed to the strong coordination between Pb(II) and phenolic hydroxyl groups, ester groups of TA as well as the specific adsorption of Pb(II) on the newly formed hydrous ferric oxides (HFO), demonstrated by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) analysis. Batch cyclic experiments indicated that TA-Fe-12 could be employed for repeated use after 10 adsorption-desorption cycles. Fixed-bed tests indicated that the TA-Fe-12 column could produce clean water with bed volumes of 9500 and 7860 ([Pb] < 1 mg center dot L-1) from synthetic Pb(II)polluted wastewater and simulated electroplating wastewater, which were significantly higher than those of other commercial adsorbents.