Effects of soil fines and surfactant sorption on contaminant reduction of coarse fractions during soil washing.

The reduction of contaminants sorbed on the coarse fraction of soils to the level below clean-up requirements is essential for an effective soil washing process. This study investigated the effects of soil texture and surfactant sorption on the reduction of total petroleum hydrocarbons (TPH) in coarse fraction during soil washing. Batch TPH sorption experiments were conducted on soil slurry with various soil fine/coarse ratios and surfactants Octylpheny polyoxyethylene (TX-100) and Dodecylpyridinium chloride (DPC) at the dosage below their saturation levels of sorption. In a sandy loam soil of low silt and clay contents, increasing the fine/coarse ratio from 0.4 to 1.2 without adding surfactants resulted in a reduction of TPH levels in the coarse fraction by 30%. Increasing the fine/coarse ratio along with sorbed surfactant (3000 mg TX-100 or 10,000 mg DPC per kg soil) further reduced TPH concentrations in the coarse fraction. For a silty loam soil already containing a high percentage of fine particles, increasing the fine/coarse ratio from 5.9 to 18.8 without surfactant addition yielded no further TPH reduction in the coarse fraction. On the other hand, surfactant sorption at the fine/coarse ratio of 5.9 improved the washing efficiency of the coarse fraction. These experimental results suggested the importance of high contents of soil fines and surfactant sorption in achieving low contaminant concentrations of coarse fractions during soil washing.