Effect of Ionic Strength and Anion Identity on Rare-Earth Element Chelation by Poly(Acrylic Acid)

Rare-earth elements (REEs) are critical components of many modern technologies, but their efficient separation and purification continue to pose significant challenges. Metal-binding polymers may enable the development of promising alternative approaches to conventional separation techniques (e.g., solvent extraction), and insights into the interactions between the metal ions and polymers in solution are crucial to such development. In this work, the effect of ionic strength on the chelation of REEs by poly(acrylic acid) is investigated using isothermal titration calorimetry (ITC) to measure the metal-binding thermodynamics. ITC experiments revealed that when ionic strength increases from increasing the sodium acetate buffer strength, polymer-REE binding becomes increasingly disfavored. However, when ionic strength is increased by the addition of sodium chloride, binding again becomes increasingly disfavored, but to a lesser extent than when ionic strength increases from increasing the buffer strength. Additionally, the reduction in binding affinity is enthalpically driven when ionic strength is raised with sodium chloride, while it is entropically driven when raised with sodium acetate buffer, suggesting competive binding by the acetate ions. These results highlight the importance of ionic strength in polymer-metal binding thermodynamics and will guide the development of new processes for the purification of critical metals. Rare-earth elements (REEs) are critical to many technologies; however, purification of these metals is challenging. This work examines the effect of ionic strength on the ability of poly(acrylic acid) to chelate REEs using isothermal titration calorimetry (ITC) to study metal-binding thermodynamics. ITC experiments reveal that as the ionic strength increases, binding affinity decreases. image