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

Trivalent chromium (Cr3+) 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 Cr3+. This study explored the binding process of Cr3+ 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 Cr3+ ions. The vibration of FTIR characteristic peaks indicated that the hydroxyl and carboxyl functional groups of GA were complexed with Cr3+ ions. On the basis of the fluorescence quenching of GA after the addition of Cr3+ ions, the complexation coefficient was calculated as 4 x 104. 2DCOS and nuclear magnetic resonance (NMR) spectra demonstrated the binding sequence of GA with Cr3+ as meta-hydroxyl groups -> carboxyl groups -> para-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 Cr3+ at different binding sites, and it was shown that the binding energy of the meta-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.