Effect of soil components on the surfactant-enhanced soil sorption of PAHs

The use of cationic surfactants was proposed to enhance the soil retention of hydrophobic organic contaminants (HOCs). However, due to the complexity of soil composition, the effect of cationic surfactants on the soil sorption of HOCs was limited to a qualitative understanding. To gain further insight into the mechanism of the surfactant and predict its efficiency, a comparative study on the HOCs sorption capacities of the surfactants sorbed on pure typical soil components was investigated. The sorption of cationic surfactant onto five pure typical soil components and the sorption of polycyclic aromatic hydrocarbons (PAHs) by the sorbed surfactant were conducted using batch equilibration methods. Humic acid (HA) and quartz were chosen as the representatives of organic matter and sand, respectively. Montmorillonite, kaolinite, and illite were chosen as representatives of clay minerals. The cationic surfactant sorption capacities of different soil components were of great difference, and the PAH sorption capacities of the surfactant sorbed onto different components were also very different. Aside from the clay minerals, HA was a very active adsorbent for the cationic surfactant, and the sorption mechanism included both adsorption and partition-like processes. For each pure soil component and a certain PAH, a proximately piecewise function was found to describe the relationship of surfactant-normalized PAH distribution coefficient K (sf) and the sorption amount of the surfactant on solid Q (e-DDPB) (R > 0.9). As a result, the sorption of PAHs by different soil components with certain surfactant loading can be estimated. The effect of soil organic matter such as HA could not be ignored when predicting the soil sorption of cationic surfactants. The PAH partition capacities of the sorbed surfactant on HA or clay minerals were very different. However, for each pure soil component, the K (sf) value was specifically related with the surfactant loading, which provided a possible means of predicting the efficiency of the cationic surfactant in enhancing the sorption of PAHs onto soils.