Characterization of the binding process between gallic acid and trivalent chromium in tannery wastewater: a spectroscopic perspective.

Trivalent chromium (Cr) is a heavy metal widely present in tannery wastewater, and organic ligands represented by gallic acid (GA) have significant effects on the environmental behavior of Cr. This study explored the binding process of Cr with GA through the integration of ultraviolet-visible (UV-vis), Fourier transform infrared (FTIR), and fluorescence spectroscopy coupled with two-dimensional correlation analyses (2DCOS). UV-vis results showed that the average molecular weight of the solutions gradually increased with the addition of Cr ions. The vibration of FTIR characteristic peaks indicated that the hydroxyl and carboxyl functional groups of GA were complexed with Cr ions. On the basis of the fluorescence quenching of GA after the addition of Cr ions, the complexation coefficient was calculated as 4 × 10. 2DCOS and nuclear magnetic resonance (NMR) spectra demonstrated the binding sequence of GA with Cr as -hydroxyl groups → carboxyl groups → -hydroxyl groups, and heterospectral 2DCOS showed that the intensity change in UV-vis absorption bands occurred before that in IR absorption bands. Density functional theory (DFT) was used to quantify the binding energy of GA with Cr at different binding sites, and it was shown that the binding energy of the -hydroxyl group was the lowest. Overall, this study provides a new approach to the analysis of the molecular structure of complexes and the binding process between organic ligands and metal ions in wastewater.