Ultra-selective removal of thorium from rare earths by aminophosphonic acid-modified porous silica

The removal of radioactive element thorium from the wastewater of rare earth (RE) industry is a significant concern in terms of environment. Here, we reported a kind of porous silica adsorbents functionalized by α-aminophosphorus extractants for ultra-selective adsorption of thorium from rare earths and other metals. Both adsorbents (ASG-AP modified with aminophosphonic acid and ASG-NP with neutral extractant) exhibit excellent adsorption selectivity towards thorium other than the coexisting ions (RE3+, Ca2+, Fe3+, Mg2+ and Al3+). The adsorption capacity of ASG-AP toward thorium reaches 131.1 mg g-1. Using ASG-AP as the adsorbent, the concentration of thorium in simulated radioactive wastewater decreased significantly from 5.5 mg L-1 to 0.007 mg L-1. The selectivity was derived from the structure of adsorbents, the steric hindrance of the functional groups, and the chemical valence and size of Th(IV) species. The adsorption of thorium was an endothermic process obeyed the pseudo-second-order model and Langmuir isotherm model. XPS analysis and Density Functional Theory (DFT) calculation revealed that the P=O/P-OH and >N- functional groups were involved in the coordination of thorium cations. Taking the ultra-selectivity of the adsorbents, the mechanical strength and chemical/physical stability of silica into account, this kind of functionalized silica would act as promising adsorbents for the removal of trace thorium in the wastewater of rare earth industry.