Selective adsorption of gadolinium from real leachate using a natural bentonite clay

This article investigated the recovery of Gd³⁺ from real leachate of phosphogypsum (PG) using natural bentonite clay. Firstly, a detailed adsorption study was performed using synthetic Gd³⁺ solutions. Then, it was investigated the clay performance in the real PG leachate. The characterization results indicate classical bentonite characteristics, such as rugosity and an S_BET 91.3 m² g⁻¹, with meso (D_p =3.82 nm) and macroporous (D_p =52.6 nm). In addition, it was identified that the major functional groups are hydroxyl and silicate, with the presence of organic matter. The initial pH solution effect indicates that the optimum removal of Gd³⁺ is at pH (6), attributing to the pH_pzc being at 5.75 and the negatively charged surface above the pH_pzc. The Avrami fractional order model was the most suitable for describing the experimental kinetic data. The Langmuir was the proper model for describing the adsorption isotherms, indicating that the Gd³⁺ forms a monolayer at the surface of the bentonite. The maximum adsorption capacity at pH 6.0 was 121.5 mg g⁻¹. The thermodynamic parameters indicate that the adsorption is spontaneous, with a standard enthalpy change of − 92.30 kJ mol⁻¹, indicating an ionic exchange, where the Gd³⁺ tends to be organized at the surface, according to the standard entropy change of − 206.0 J K⁻¹ mol⁻¹. The fixed bed adsorption test showed that Gd³⁺ could be adsorbed for up to 200 min without regeneration. Regeneration results show that the citric acid is more efficient in desorbing the Gd³⁺ from the bentonite, reaching up to 8 cycles without efficiency loss. Finally, the bentonite clay could selectively recover Gd³⁺ from the real PG leachate.