Phenanthrene and 2,2′,5,5′-PCB sorption by several soils from methanol-water solutions: The effect of weathering and solute structure

The effect of the sorption of phenanthrene and 2,2′,5,5′-polychlorinated biphenyl (PCB52) by five differently weathered soils were measured in water and low methanol volume fraction (f_c ≤ 0.5) as a function of the apparent solution pH (pH^app). Two weathered oxisols (A2 and DRC), and moderately weathered alfisols (Toronto) and two young soils (K5 and Webster) were used. The K_m (linear sorption coefficient) values, which log-linearly decreases with f_c, were interpreted using a cosolvency sorption model. For phenanthrene sorption at the natural pH, the empirical constant (α) ranged between 0.95 and 1.14, and was in the order of oxisols (A2 and DRC) < alfisols (Toronto) < young soils (K5 and Webster). Smaller α values for highly weathered soils are indicative of smaller solute sorption reduction than those predicted from the increment of the solute's activity coefficient in the solution phase. A similar trend was observed for PCB52 sorption. The K_m values measured at the range of pH 3-7 also showed an inversely log-linear relationship. The regression slope (ασ) calculated from the cosolvency sorption model as a function of pH^app only varied within <5%, with the exception for phenanthrene sorption by two highly weathered soils, which had 10% greater ασ values obtained at acidic pH^app. This phenomenon is a result of the greater acid enhancement effect on phenanthrene sorption by the oxisols, which is reduced with increasing f_c. These results revealed an unexplored relationship between the cosolvent effect on the sorption and the properties of the soil organic matter (a primary sorption domain) as a function of the degree of soil weathering.