Insights into the adsorption mechanism of tannic acid by a green synthesized nano-hydroxyapatite and its effect on aqueous Cu(II) removal

The polyphenolic tannic acid (TA) has been widely used in the stabilization and surface modification of nanomaterials. The interaction mechanism of TA with the biogenic nano-hydroxyapatite (nHAP) and its environmental importance, however, are poorly understood. This study explored the adsorption of TA using the green synthesized, eggshell-derived nHAP and implications of this process for the removal of aqueous Cu(II) via batch adsorption experiments, Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) investigations. TA adsorption by nHAP was a complex pH-dependent process and significantly correlated with TA molecule speciation and amphoteric properties of nHAP via multiple adsorption modes including surface complexation, electrostatic attraction, and hydrogen bond. The maximum TA adsorption amount was found to be 94.8 mg/g for less crystalline nHAP with lower calcination temperature. In the ternary Cu-TA-nHAP systems, TA promoted Cu(II) adsorption at pH < 5 and reduced Cu(II) uptake at pH > 5. Further studies of the effects of ionic strength and addition sequences, as well as Raman, FTIR, and XPS analyses revealed Cu(II) adsorption on nHAP was mainly dominated by inner-sphere surface complexation. These results can shed light on not only the utility of biogenic nHAP for TA and Cu(II) adsorption but also the evaluation of the effect of TA on the environmental behavior of heavy metals.