Removal of thorium from high-acidity solutions achieved on a flexible polymeric network bearing phosphonic acid ligands: understanding adsorption and binding mechanisms

Abstract

The recovery of thorium (Th) from high-acidity solutions is of great significance for environmental protection, as thorium-containing waste liquid is highly acidic. However, most sorbents used to entrap thorium from high-acidity solutions typically suffer from low sorption capacity and poor selectivity. Here, a novel flexible polymeric network P(MBA-VPA-AA) holding phosphonic acid was facilely attained, through the solvothermal polymerization of N,N′-methylene diacrylamide (MBA) and vinylphosphonic acid (VPA) with acrylic acid (AA). Surprisingly, P(MBA-VPA-AA) showed high sorption capacity and excellent selectivity towards Th in the presence of many competing metal cations. The maximum sorption amount (q_max) at a pH of 0.1 even could attain a record-breaking value of 450.5 mg g⁻¹, outperforming that of all other sorbents developed previously at high acidity. The excellent thorium sorption performance in highly acidic media was attributed mainly to the synergistic effect of the phosphonic acid ligands and the flexible polymeric network, verified by XPS, FT-IR, density functional theory (DFT) methods and controlled experiments. In detail, the phosphonic acid ligands were primarily responsible for the sorption of thorium from a highly acidic solution, and the flexible polymeric network structure could effectively enhance the swelling ability of the sorbing material in water and the free movement of phosphonic acid ligand, both of which were beneficial to facilitating thorium sorption in a highly acidic solution. DFT results revealed that the optimized 1 : 2 geometric structure, where thorium is coordinated with two phosphonic acid ligands, effectively describes its interaction with the polymeric network. Thus, this work reports the first example for the efficient thorium sorption from highly acidic solutions using flexible polymeric networks.