Performance of orange peels crosslinked β-cyclodextrin (OP-β-CD) to uptake lanthanum from water: Conventional adsorption studies and ultrasound-assisted desorption

Even though adsorption has been widely employed to recover rare earth elements such as lanthanum, desorption remains challenging since selectivity is often linked to strong adsorption forces. Herein we propose the grafting between orange peel (OP) wastes with beta-cyclodextrins (β-CD) for producing a highly reusable and stable adsorbent that allows the easy recovery of lanthanum in repeated cyclic runs. The morphological analysis confirmed that the OP-β-CD adsorbent was successfully functionalized with –OH groups, with β-CD grafting via citric acid expanding and stabilizing its structure. The pseudo-first-order model better represented the kinetics of lanthanum adsorption, while the isothermal behavior was better represented by the Langmuir model ($q_{\max }=95.79\mathrm{mg}{g}^{-1}$), as shown by the thermodynamic analysis, which confirmed that the process was endothermic and spontaneous. The mechanism behind the efficiency of adsorption-desorption of lanthanum by OP-β-CD adsorbent was concluded to rely on: i) electrostatic and reversible interactions and ii) a driving force that increased as the amount of lanthanum increased in solution, leading to the formation of a monolayer. Around 95 % of lanthanum was desorbed using an ultrasound-assisted technique with citric acid, maintaining stable adsorption capacity for 5 cycles. In real matrices containing other rare earth elements and high calcium concentrations the removal exceeded 80 %. Thus, OP-β-CD's remarkable stability during desorption underscores its potential as a highly effective and sustainable solution for rare earth element recovery and removal.