A novel amidoxime-functionalized covalent organic framework adsorbent for efficient lead ion removal: Synthesis and adsorption mechanism

Lead ions are highly toxic heavy metal ions and Pb(II) in wastewater threatened seriously human health and environment. Therefore, an effective adsorbent must be developed to remove lead ions from wastewater. In this study, a polycrystalline amidoxime covalent organic framework (DBCC-NHOH) is synthesized by a post-modification method for the elimination of Pb(II) from wastewater. The successful preparation of adsorbent (DBCC-NHOH) is demonstrated by the oxygen appearance in SEM-EDS pattern and conversion of -C≡N to C=N-O and C-N in the FT-IR pattern. DBCC-NHOH is a crystalline porous material and its pore size, specific surface area and pore volume are 3.419 nm, 28.154 m²/g and 4.2ⅹ10⁻⁸ m³/g respectively. The optimum conditions for the adsorption of Pb(II) by DBCC-NHOH are temperature 298 K, time 180 min, and pH 5. The maximum adsorption of DBCC-NHOH was 221.37 mg/g. Kinetic and thermodynamic investigations indicate that adsorption of Pb(II) by DBCC-NHOH is a monolayer chemisorption and exothermic process. Selectivity experiments indicate that DBCC-NHOH can adsorb Pb(II) efficiently and selectively in complex multi-ion systems. In addition, the adsorption percentage of DBCC-NHOH remained up to 83.20 % after five adsorption-desorption experiments. The XPS and FT-IR analyses and DFT results indicated that DBCC-NHOH utilized amidoxime function group to realize the high efficiency of Pb(II) adsorption by electrostatic attraction and chelation.