Influence of spectral and molecular composition of dissolved organic matter on labile Cd mobility in riparian soils in the Three Gorges Reservoir, China

The periodic anti-seasonal inundation of the Three Gorges Reservoir (TGR) leads to changes in the molecular composition of dissolved organic matter (DOM) in riparian soils, further impacting the geochemical processes and ecological risk of heavy metals. However, the intrinsic driving mechanisms of DOM influencing the cadmium (Cd), a major pollutant in riparian soils in TGR, at the molecular level remains unclear. In this study, the DOM molecular composition, labile Cd in riparian soils and the key driving mechanism before and after flooding were explored using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), the diffusive gradients in thin films (DGT) and partial least squares path modeling (PLS-PM). A spectral analysis revealed that after flooding, the relative abundance of terrestrial humic-like substances decreased whereas that of microbial humic-like substances increased. Furthermore, FT-ICR MS analysis revealed that the relative abundance of lignin, the main molecular components of DOM in riparian soils, increased after flooding. The linkage of DOM with the concentration and kinetic processes of labile Cd indicated that the higher aromaticity and unsaturation, larger molecular weight, and higher humification level of DOM promoted the mobility of labile Cd from the soil solid phase to the liquid phase. In particular, our findings indicated that at the molecular level, the most significant factor influencing the mobility of labile Cd was lignin, which was primarily governed by the complexation of lignin with labile Cd. The complexation mechanism between lignin and labile Cd resulted in increased ecotoxicological risk of labile Cd after flooding, while the overall ecotoxicological risk was low in riparian soils in TGR. This study provides better insight into the geochemical cycling and fate of toxic elements in reservoir ecosystems under the change of hydrological regime.