Effective Gd(III) adsorption and mechanistic investigation using nanocellulose intercalated magnetic bentonite

Abstract

The wastewater containing gadolinium ions produced during the rare earth mining process posed a serious threat to the ecosystem and human health. This study provided a novel TEMPO-oxidized cellulose nanocrystals intercalated with magnetic bentonite (TOCNC@M-Bt) via the co-precipitation for effective Gd(III) adsorption from wastewater. TOCNC@M-Bt had a dense lamellar structure, abundant oxygen-containing functional groups (e.g., hydroxyl and carboxyl groups), and large numbers of adsorption sites. Compared with Bt, the specific surface area of TOCNC@M-Bt was expanded from 54.96 m²/g to 101.36 m²/g. The adsorption experiments showed that at optimal conditions (pH = 7, temperature = 303 K, adsorbent dosage = 0.9 g/L), the removal rate of 97.53 % was achieved for a 30 mg/L Gd(III) solution. Furthermore, the experimental data conformed to the pseudo-second-order kinetic (R² = 0.998), the intra-particle diffusion (R₁² = 0.950; R₂² = 0.938; R₃² = 0.977), and Boyd's liquid film diffusion model (R² = 0.852), indicating that the adsorption process was influenced by chemical adsorption, intraparticle diffusion and external liquid film diffusion. The equilibrium adsorption data aligned with the Langmuir adsorption isotherm, and the maximum Gd(III) adsorption capacity of 40.85 mg/g, which suggested that the adsorption process followed a monolayer mechanism on a homogeneous surface. Thermodynamic parameters (ΔG < 0, ΔH > 0) showed that the adsorption process was spontaneous and endothermic. Mechanistic studies showed that ion exchange, electrostatic adsorption, complexation, hydrogen bonding, and pore filling predominantly determined the adsorption process. TOCNC@M-Bt also presented selective removal of Gd(III) ions in the presence of multiple ion species and the 79.11 % removal rate after five sorption-desorption cycles, indicating the inherent robustness and reusability.

Keywords: Adsorption; Bentonite; Cellulose nanocrystals; Gd(III).