Unraveling adsorption mechanisms of tannic acid and gallic acid on nanocrystalline chlorapatite: A synergistic experimental and molecular dynamics investigation

Nanocrystalline chlorapatite (nClAP) has recently gained great interest in adsorptive removal of various contaminants. However, the adsorption of phenolic acids by nClAP remains poorly understood. This study combines experimental and computational approaches to systematically investigate the adsorption mechanisms of two representative phenolic acids, gallic acid (GA) and tannic acid (TA), onto nClAP. Comprehensive characterization confirmed the synthesized nClAP possessed a mesoporous structure with high surface area (153 m 2 /g) and crystalline stability. The batch adsorption experiments of GA and TA onto nClAP were examined as a function of pH, ionic strength, contact time and initial GA/TA concentration. The experimental results indicated that GA/TA adsorption on nClAP reached equilibrium within 120 min, and solution pH and ionic strength exhibited a complex influence on the adsorption process. The experimental data were well fitted by the Elovich model and the Langmuir/Hill isotherm model, indicating monolayer adsorption in the non-homogeneous system. Molecular dynamics (MD) simulations unveiled atomic-scale interactions, showing TA exhibited 8.6-fold stronger binding energy (236.64 kcal/mol) than GA (27.44 kcal/mol), attributed to synergistic Ca 2+ coordination and hydrogen bonding with surface phosphate groups. Notably, nClAP achieved ∼100 % TA removal from eucalyptus leaf leachate at 2.5 g/L dosage, underscoring its practical potential. This work advances fundamental understanding of phenolic acid-mineral interactions and highlights nClAP's efficacy in mitigating plant-derived organic pollution.