Dissolved organic matter-enhanced retention of polycyclic aromatic hydrocarbons in soil miscible displacement experiments

Natural dissolved organic matter (DOM) may alter the mobility of polycyclic aromatic hydrocarbons (PAHs) in soils. The influence of DOM on the mobility of PAHs was studied in medium-scale soil column experiments under unsaturated flow conditions. Miscible displacement experiments were carried out with PAHs alone (anthracene, pyrene, benzo(e)pyrene), with DOM alone, and with a mixture of the PAHs and DOM. Two different sandy materials were used, a spodic B horizon and a commercially available seasand. The DOM transport could be explained by assuming total DOM consisting of two physicochemically different fractions: A mobile fraction, composed of the hydrophilic moieties of DOM, and an immobile fraction, composed of hydrophobic DOM moieties. The PAH mobility in the column experiments was controlled by both solid and liquid phase organic matter. The PAH breakthrough in the absence of DOM was significantly retarded in both the seasand and the spodic B material. The observed breakthrough times were considerably lower than those estimated from calculations using published Kd values, that resulted in an overestimation of PAH mobility. The addition of DOM resulted in reduced mobility of PAHs and reduced PAH effluent concentration, but more so in the spodic B soil material than in the seasand. The reduced mobility of PAHs in the presence of DOM can be explained by sorption of DOM-associated PAHs to the bulk soil, that is, co-sorption. Alternatively, the continuous sorption of DOM by the bulk phase results in an increase or sorption capacity for free PAHs, which will also lead to PAH retardation (cumulative sorption).