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 (fc ⩽ 0.5) as a function of the apparent solution pH (pHapp). Two weathered oxisols (A2 and DRC), and moderately weathered alfisols (Toronto) and two young soils (K5 and Webster) were used. The Km (linear sorption coefficient) values, which log-linearly decreases with fc, 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 Km 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 pHapp only varied within <5%, with the exception for phenanthrene sorption by two highly weathered soils, which had 10% greater ασ values obtained at acidic pHapp. This phenomenon is a result of the greater acid enhancement effect on phenanthrene sorption by the oxisols, which is reduced with increasing fc. 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.