Recovery of rare earth elements (Nd³⁺ and Dy³⁺) by using carbon-based adsorbents from spent tire rubber

Two samples of spent tire rubber (rubber A and rubber B) were submitted to thermochemical conversion by pyrolysis process. A450, B450 and A900, B900 chars were obtained from rubber A and rubber B at 450 °C and 900 °C, respectively. The chars were then applied as recovery agents of Nd³⁺ and Dy³⁺ from aqueous solutions in mono and bicomponent solutions, and their performance was benchmarked with a commercial activated carbon. The chars obtained at 900 °C were the most efficient adsorbents for both elements with uptake capacities around 30 mg g^-1. The chars obtained at 450 °C presented uptake capacities similar to the commercial carbon (≈ 11 mg g^-1). A900 and B900 chars presented a higher availability of Zn ions that favored the ion exchange mechanism. It was found that Nd³⁺ and Dy³⁺ were adsorbed as oxides after Zn was released from silicate structures (Zn₂SiO₄). A900 char was further selected to be tested with Nd/Dy binary mixtures and it was found a trend to adsorb a slightly higher amount of Dy³⁺ due to its smaller ionic radius. The uptake capacity in bicomponent solutions was generally higher than for single component solutions due to the higher driving force triggered by the higher concentration gradient.