Mechanistic insights into the adsorption and extraction of rare earth ions using oxygen- and phosphorus-doped porous graphitic carbon nitride

Addressing the challenge of removing rare earth elements (REEs) from wastewater generated during mineral processing, this study successfully synthesized oxygen- and phosphorus-doped graphitic carbon nitride (OP-CN) and evaluated its capability to adsorb REEs from synthetic solutions containing La(III), Ce(III), Gd(III), and Yb (III). Characterization results indicated that OP-CN, being more porous than CN, has a larger specific surface area and greater pore volume, along with oxygen and phosphorus functional groups, which are advantageous for REEs adsorption. The maximum adsorption capacities of OP-CN for the target REEs achieved ranging from 76.2 to 111.7 mg g(-1) at 25 C and a pH of 5.0. Thermodynamic analysis showed that the adsorption processes between OP-CN and REEs are favorable, spontaneous, and exothermic. Through characterization and computational chemistry analysis, identified pore filling, ion exchange, surface complexation, and potent electrostatic attraction as crucial for the efficient adsorption of REEs. The adsorption was found to be optimal within a pH range of 3.0 to 7.0 and was minimally influenced by common anions and cations in the solution. Notably, OP-CN maintained high adsorption efficiencies across various natural water matrices and proved to be effective over five usage cycles. The integration of N, O, and P active sites, along with the negative charges on OP-CN, offers a promising approach for the efficient separation and recovery of REEs from waste streams.