Analysis of the adsorption mechanism of phosphoric acid-modified bamboo charcoal for chlorogenic acid based on density functional theory

Abstract

Background: Bamboo charcoal has garnered wide recognition as an adsorbent owing to its stability and availability. Phosphoric acid modification contributes to forming specific functional groups on the surface of bamboo charcoal, culminating in the augmentation of its adsorption capabilities. Results: The modification process in this study involved impregnating the bamboo charcoal in a 45% mass fraction of phosphoric acid solution with a 1:10 ratio (bamboo charcoal powder to phosphoric acid), followed by activation at a temperature of 500 °C for a duration of 1.5 h. The modification resulted in the formation of hydroxyl, carboxyl, and phosphate groups on the surface of the bamboo charcoal. Furthermore, the specific surface area of the modified bamboo charcoal surged from 4.91 m2/g to 115.18 m2/g, the average aperture decreased from 7.15 nm to 1.99 nm, and the pore volume escalated by 6.5 times. On this basis, a comparison in the adsorption capability for chlorogenic acid, a natural small-molecule active ingredient, was drawn before and after the modification of bamboo charcoal. The results indicate that the modified bamboo charcoal showcases an impressive 46.8-fold increase in its adsorption capacity when juxtaposed with the unmodified bamboo charcoal. Through the calculations based on density functional theory (DFT), the addition of functional groups on the surface of the modified bamboo charcoal module allows for the formation of hydrogen bonds with the chlorogenic acid module, with the highest binding energy reaching − 57.59 kJ/mol. Conclusion: As evidenced by the adsorption kinetic studies, the curve of modified bamboo charcoal for chlorogenic acid abides by a first-order kinetic equation. Additionally, adsorption thermodynamic studies reveal that the curve of modified bamboo charcoal for chlorogenic acid conforms to the Boltzmann equation. These results shed light on the mechanism of selectively adsorbing specific constituents using phosphoric acid-modified bamboo charcoal, thereby providing a solid theoretical foundation for the application of bamboo charcoal. Graphical Abstract: [Figure not available: see fulltext.].