Abstract: Hydroxyaluminum-modified corn straw biochar (CSB) was used to prepare an adsorption material for fluoride ions (F^-) in water, denoted as Al-CSB. Structural properties of Al-CSB were characterized using SEM, BET, FTIR, and Zeta potential analysis. We examined the effects of adsorbent dosage, solution pH, and initial solution concentration on the adsorption of F^- by Al-CSB with batch adsorption experiments. Adsorption kinetics and isotherm models were employed to elucidate the adsorption mechanism. The results showed that Al-CSB exhibited a loose rod-like structure with distinct bundle-like porous structures. The surface of Al-CSB successfully loaded with Al-O and other cationic functional groups, as well as hydrous aluminum oxide colloidal particles, resulting in a positively charged surface. The adsorption process of fluoride ions by CSB and Al-CSB fitted well with the pseudo-second-order kinetic model and the intraparticle diffusion model, suggesting that the adsorption process was controlled by chemical mechanisms. Both materials were affected by intraparticle diffusion and surface adsorption. The thermodynamic adsorption process well correlated with the Langmuir model, showing that Al-CSB had a maximum adsorption capacity of up to 71.39 mg·g^-1 for fluoride in water. When the initial fluoride concentration was 100 mg·L^-1, with an adsorbent dosage of 6.0 g·L^-1 and solution pH of 6, Al-CSB achieved a removal rate of up to 97.9% for F^-. In river water experiments, 4 g·L^-1 of Al-CSB reduced the fluoride concentration to below 1 mg·L^-1.

Key words: biochar, hydroxyaluminum, modification, fluoride, adsorption mechanism