Facilitating effects of metal cations on phenanthrone sorption in soils

Effects of metal cations (Na+, Ca2+, and Al3+) on phenanthrene sorption were investigated using two soils with contrasting organic carbon (OC) contents. The presence of the polyvalent cations (i.e., Ca2+ or Al3+) at a concentration of 0.01 mol/L significantly increased the capacity and nonlinearity of phenanthrene sorption to soils compared with the monovalent Na+. The effects were governed by the content of soil OC. Rubbery OC (i.e., soft, amorphous OC including dissolved organic carbon (DOC)) tended to become condensed on soil surfaces as evidenced by a decrease in the signals of the 1H NMR spectra of DOC and an increase in the glass transition temperature (Tg) of the soils when the polyvalent cations were present. Increasing Ca2+ concentration led initially to an effect similar to that of the polyvalent cations in the low cation concentration range, and the effect was gradually attenuated as Ca2+ concentration further increased. These findings lead us to propose that the modifications in the physical configuration and chemical characteristics of OC resulting from the presence of metal cations account for the increase in the capacity and nonlinearity of phenanthrene sorption to the soils. This study points to an important role of metal cations in the sorption and fate of phenanthrene in the soil environment.