A self-perpetuating cycle: how heavy metals shape DOM chemistry to promote their own immobilization in soil

Dissolved organic matter (DOM) plays an important role in the migration of heavy metals. However, the effect of heavy metal contamination on the characteristics of soil DOM has not yet been verified on field scale. In this study, DOM extracted from the soils of a site historically contaminated with heavy metals was compared with that extracted from the control soils. Although they had similar physical and chemical properties, vegetation coverage and microbial community structure, fluorescence spectrum showed that DOM from the in situ polluted soil (PS) had higher aromaticity and heavy metal complexation constant than that from the control soil (CS). Fourier-transform ion cyclotron resonance mass spectrometry revealed that PS-DOM had significantly higher concentrations of lignins, tannins, and heteroatoms as well as higher average molecular weight than CS-DOM. Density functional theory calculations confirmed the strong heavy metal complexation abilities of lignins and tannins, which explain the higher heavy metal complexation constant of PS-DOM than that of CS-DOM. Atomic force microscopy images revealed that the aggregate size of PS-DOM was larger than that of CS-DOM, and Cu²⁺ and Pb²⁺ caused obvious DOM aggregation. Complexation with heavy metals may affect DOM morphology and related environmental behaviors, resulting in difference in DOM characteristics of contaminated site.