Effect of composition and microstructure of humic acid on 3,3',4,4'-tetrachlorobiphenyl sorption

The fate and transport of polychlorinated biphenyls (PCBs), a class of persistent organic compounds, in soils was markedly affected by their sorption/desorption on soil organic matters (SOM) due to high hydrophobicity of PCBs. Humic acid (HA), an important fraction of SOM, has no steady composition and microstructure from different origins, resulting in their diverse sorption capacity. Therefore, the effect of composition and microstructure of HA on sorption of 3,3',4,4'-tetrachlorobiphenyl (PCB77) was investigated in this study. The primary sorption mechanism of PCB77 on HAs was also demonstrated by using Fourier transform infrared (FTIR) and solid-state carbon-13 nuclear magnetic resonance (¹³C-NMR) spectroscopy analysis. Elemental content analysis of three HAs showed that the content of unsaturated C (the total of aromatic, carboxylic, and carbonyl carbon) followed an order of YHA>SHA>AHA. Furthermore, YHA and SHA had remarkably rough and complicated particle surfaces but AHA had relatively smooth surface and the well-proportioned and interspersed particle sizes. The results of the sorption experiment showed that the sorption capacity of PCB77 on HAs followed a similar order of YHA>SHA>AHA, indicating the content of unsaturated C of HAs controlled the sorption of PCB77 on HAs. Sorption of PCB77 on either AHA or SHA did not change with increasing ionic strength of background solution, implying that there was no H-bond or electrostatic interaction between PCB77 and HAs. The result of FTIR and ¹³C-NMR spectra showed the primarily possible mechanism was π-π conjugative interaction and hydrophobic binding between PCB77 and HAs.