Selective Capacitive Recovery of Rare-Earth Ions from Wastewater over Phosphorus-Modified TiO₂ Cathodes via an Electro-Adsorption Process

Abstract

Large amounts of wastewater containing low-concentration (<10 ppm) rare-earth ions (REIs) are discharged annually in China's rare-earth mining and processing industry, resulting in severe environmental pollution and economic losses. Hence, achieving efficient selective recovery of low-concentration REIs from REIs-containing wastewater is essential for environmental protection and resource recovery. In this study, a pseudocapacitance system was designed for highly efficient capacitive selective recovery of REIs from wastewater using the titanium dioxide/P/C (TiO₂/P/C) composite electrode, which exhibited over 99% recovery efficiency for REIs, such as Eu³⁺, Dy³⁺, Tb³⁺, and Lu³⁺ in mixed solution. This system maintained high efficiency and more than 90 times the enrichment concentration of REIs even after 100 cycles. Ti⁴⁺ of TiO₂ was reduced to Ti³⁺ of Ti₃O₅ under forward voltage in the system, which trapped the electrons of phosphorus site and caused it to be oxidized to phosphate with a strong affinity for REIs, thus improving the selectivity of REIs. Under reverse voltage, Ti₃O₅ was oxidized to TiO₂, which transferred electrons to phosphate and transformed to the phosphorus site, resulting in the desorption and enrichment of REIs and the regeneration of the electrode. This study provides a promising method for the efficient recovery of REIs from wastewater.