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 (TiO 2 /P/C) composite electrode, which exhibited over 99% recovery efficiency for REIs, such as Eu 3+, Dy 3+, Tb 3+, and Lu 3+ in mixed solution. This system maintained high efficiency and more than 90 times the enrichment concentration of REIs even after 100 cycles. Ti 4+ of TiO 2 was reduced to Ti 3+ of Ti 3 O 5 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, Ti 3 O 5 was oxidized to TiO 2, 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.