DISSOLVED ORGANIC MATTER-ENHANCED RETENTION OF POLYCYCLIC AROMATIC-HYDROCARBONS IN SOIL MISCIBLE DISPLACEMENT EXPERIMENTS

Natural dissolved organic matter (DOM) may alter the mobility of polyc yclic aromatic hydrocarbons (PAHs) in soils. The influence of DOM on t he mobility of PAHs was studied in medium-scale soil column experiment s 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 differen t sandy materials were used, a spodic B horizon and a commercially ava ilable 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 immobil e fraction, composed of hydrophobic DOM moieties. The PAH mobility in the column experiments was controlled by both solid and liquid phase o rganic matter. The PAH breakthrough in the absence of DOM was signific antly retarded in both the seasand and the spodic B material. The obse rved breakthrough times were considerably lower than those estimated f rom calculations using published Kd values, that resulted in an overes timation of PAH mobility. The addition of DOM resulted in reduced mobi lity of PAHs and reduced PAH effluent concentration, but more so in th e 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-associ ated PAHs to the bulk soil, that is, co-sorption. Alternatively, the c ontinuous sorption of DOM by the bulk phase results in an increase or sorption capacity for free PAHs, which will also lead to PAH retardati on (cumulative sorption).