Highly selective recovery of lanthanum and cerium from wastewater by amidoxime-modified biochar

Efficient recovery of rare earth elements (REEs) is crucial for resource utilisation and environmental protection. In this study, we developed novel biochar adsorbents (AOBC300, AOBC500, and AOBC700) by grafting amidoxime groups onto corn stover biochar prepared at temperatures ranging from 300 to 700 °C. The adsorption and selective removal efficiency of these adsorbents for La(III) and Ce(III) from water were investigated. The results showed that, despite a significant reduction in specific surface area from 125.75 to 13.15 m²/g after modification, AOBC700 demonstrated superior adsorption capacity: 60.24 mg/g for La(III) (at a biochar dosage of 1.6 g/L, and pH 6) and 45.20 mg/g for Ce(III) (at a biochar dosage 2.0 g/L and pH 6). These values are 5.0 and 3.2 times higher than the unmodified biochar, respectively. Notably, AOBC700 exhibited remarkable selective removal ability for REEs in the complex leaching solutions of fly ash and red mud. It also retained over 75 % of its adsorption capacity after five adsorption–desorption cycles. Density functional theory (DFT) calculations provided insights into the adsorption mechanism of different REEs onto AOBC700. Additionally, machine learning (ML) models were employed to evaluate adsorbent performance, with the eXtreme Gradient Boosting (XGB) model showing the highest prediction accuracy. A Shapley addition explanation highlighted the key factors influencing REEs adsorption as follows: time > initial concentration > dosage > pH > electronegativity. These findings confirm the significant potential of AOBC700 for selectively recovering REEs from waste streams.