Electrochemical membranes for rare earth mine water treatment: sequential removal of rare earth ions and ammonia nitrogen

Abstract

The compound pollution caused by rare earth elements (REEs) and high concentrations of ammonia and nitrogen in the drainage water of rare earth mines poses a serious threat to water ecological safety, necessitating urgent resolution. To address this challenge, (Sb, F)-doped SnO₂-modified ceramic membranes (AFTO-CM) were developed for use in an integrated electro-flocculation-electro-oxidation-membrane separation treatment system. Physicochemical characterization revealed that AFTO-CM possesses excellent electrical conductivity (2.84 Ω/sq), electrocatalytic properties (charge transfer resistance of 1.41 Ω), operational stability (sustained performance at 30 mA for over 45 h), hydrophilicity (contact angle of 0°), and a porous structure. In the integrated treatment system, optimization of key factors achieved removal of REEs and ammonia nitrogen (99.84-99.98%). In addition, physicochemical characterization analyses confirmed the removal of REEs and Al³⁺ hydrolysis products through the formation of hydroxyl complexes. Product analysis further indicated that ammonia nitrogen removal primarily occurred through oxidation by free chlorine. Theoretical calculations showcased that Sb and F doping reduced the energy barrier of Cl₂ desorption in the SnO₂-based conducting layer while increasing the barrier for oxygen evolution reaction (OER) intermediates (*OOH), thus enhancing chlorine evolution reaction (CER) activity. Hence, this study provides valuable insights into the efficient purification of rare earth mine drainage.