Enhanced fluoride adsorption on aluminum-impregnated kenaf biochar: adsorption characteristics and mechanism

Abstract The fast-growing plant kenaf was pyrolyzed to biochar, and the biochar was impregnated with aluminum to improve its fluoride adsorption capacity. The Al-impregnated kenaf biochar (Al-KNF-BC) was pyrolyzed at different temperatures (300–700°C). The specimen treated at 300°C (Al-KNF-300) showed the highest fluoride adsorption capacity. The kinetics and equilibrium adsorption of fluoride by Al-KNF-300 followed the pseudo-second-order and Langmuir models, respectively. According to the Langmuir model, the maximum fluoride adsorption capacity of Al-KNF-300 is 13.93 mg/g. The enthalpy and entropy of fluoride adsorption by Al-KNF-300 were 37.80 kJ/mol and 124.1 J/mol‧K, respectively. Fluoride adsorption by Al-KNF-300 was favorable at pH values as low as pH 3, and the effect of anion competition followed the order HCO3− > SO42− > NO3− > Cl−. In experiments with different adsorbent dose, the maximum adsorption efficiency of 99.23% was obtained at an adsorbent concentration of 16.67 g/L, at which the fluoride concentration decreased from 100 mg/L to < 1.5 mg/L (the drinking water standard). Al-KNF-300 is an effective and inexpensive adsorbent for removing fluoride from contaminated water to meet drinking water standards.