New insights into the influence of heavy metals on phenanthrene sorption in soils

Soil contamination by combinations of heavy metals and organic contaminants has become an increasingly important environmental issue. Effects of heavy metal cations (Cu(2+), Ni(2+), and Pb(2+)) on phenanthrene sorption were systematically investigated using two soils with contrasting physicochemical properties. Spectral and microscopic analyses provide direct evidence for the modification of composition and conformation of dissolved organic carbon (DOC) and hydrophobicity of the interfaces in the presence of metal cations. Parts of rubbery organic carbon (including flexible DOC and humic acids) became condensed on solid surfaces in the presence of heavy metals as evidenced by an increase in the glass transition temperature of the soils. These modifications led to a significant increase in the capacity and nonlinearity of phenanthrene sorption in the soils. As the added metal cations aged for 70 days, the soil solution gradually recovered its original physicochemical properties, and the facilitating effects of the heavy metals on phenanthrene sorption were significantly attenuated. This work highlights the important implications of DOC properties and aging processes of metals in the sorption of hydrophobic organic compounds (HOCs) such as phenanthrene in soils and provides compelling evidence for the facilitating effects of heavy metals on HOC sorption in soils.