Selective recovery of Gd(III) by benzimidazole- and benzoxazole-linked 3D porous polymers

Abstract

Three-dimensional porous benzimidazole- and benzoxazole- linked polymers were successfully prepared through a one-pot method using benzol-1,2,4,5-tetramintetrahydrochlorid (BTD) and 2,5-diamino-1,4-benzediol dihydrochloride (DBD) to react with 4-[tris(4-formyl phenyl)methyl]benzaldehyde (TFPM), respectively. According to nitrogen sorption analysis, TFPM-BTD and TFPM-DBD possessed specific surface areas of 612.60 and 524.90 m2/g, and the total pore volumes (Vtot) of 1.44 and 1.15 cm3/g, respectively. Batch adsorption experiments demonstrated that Gd(III) adsorption processes by TFPM-BTD and TFPM-DBD fit well with the Langmuir isotherm and the pseudo-second-order kinetic model. At 25 °C and pH level of 6, TFPM-BTD showed a much higher adsorption capacity for Gd(III), detected at 173 mg/g, as opposed to TFPM-DBD at 82 mg/g. Associated spectra of N 1s via high-resolution X-ray photoelectron spectroscopy (XPS) revealed the generation of coordination bonds between Gd(III) and (-NH-) of TFPM-BTD polymer during Gd(III) adsorption, while Energy-dispersive X-ray spectroscopy (EDS) analysis proved the uniform adsorption of Gd(III) on TFPM-BTD. TFPM-BTD showed high selectivity toward the adsorption of heavy rare earth ions, in a high-to-low order of Gd3+>Ce3+>Er3+>La3+>Ni2+>Mg2+>Zn2+. Furthermore, TFPM-BTD exhibited excellent cycling stability and regeneration performance. Gd(III) adsorption capacity of TFPM-BTD after five adsorption/elution cycles was retained at 63.9 %. The elution efficiency was extremely high when using 0.1 mol/L hydrochloric acid as the eluent, showing an average five-cycle desorption efficiency over 95 %. The outcomes will provide theoretical and practical guidance to develop high-performance and selective adsorbents for efficient removal of heavy rare earth ions from water.