The soil surfactant flushing is defined as a process for in situ treatment of the contaminated soil or other matrix with surfactant solution, while soil surfactant washing is defined as a process for soil excavation, slurry preparation, and subsequent ex situ treatment aboveground with surfactant solution. So soil flushing is an in situ treatment process, and soil washing is an ex situ treatment process, both of which involve the use of surfactant solutions.
Surfactants are amphipathic molecules or ions. One portion of the surfactant molecule is hydrophilic (water loving), while another portion is hydrophobic (water hating). Hydrophilic portions are ionic or polar heads. Hydrophobic portions are tails containing 12 or more carbon atoms as hydrocarbon chains.
In the presence of water and air, the surfactants tend to concentrate at solid/water interfaces and air/water interfaces of water mixtures. By concentrating at the air/water and solid/water interfaces of the water mixture, the surfactant species are able to reduce the surface tension of the contaminated soil particles, thereby enhancing the chances for separation of contaminants from the soil particles.
Many basic and applied engineering projects have been conducted by researchers [34,37]. The readers are referred to an excellent book by Wilson and Clarke  for the theory and principles of flushing and washing. The in situ soil flushing process is still in experimental stage. A typical large-scale ex situ soil washing process is described below [16,18,25].
An ex situ soil washing process system (Fig. 6) is a water-based volume reduction process used to treat excavated soil. The system may be applied to contaminants concentrated in the fine-size soil fraction (silt, clay, and soil organic matter) or contamination associated with the coarse (sand and gravel) soil fraction.
As a part of the soil washing process, debris is removed from the soil, and the soil is mixed with water and subjected to various unit operations common to the mineral processing industry. These operations can include mixing trammels, pug mills, vibrating screens, froth flotation cells, attrition scrubbing machines, hydrocyclones, screw classifiers, and various dewatering operations.
The core of the soil washing process is a multistage, countercurrent, intensive scrubbing circuit with interstage classification. The scrubbing action disintegrates soil aggregates, freeing contaminated fine particles from the coarser material. In addition, surface contamination is removed from the coarse fraction by the abrasive scouring action of the particles themselves. Contaminants may also be solubilized, as dictated by solubility characteristics or partition coefficients. Contaminated residual products can be treated by other methods. Process water is normally recycled after biological or physical treatment. Contaminated fines may be disposed of off site, incinerated, stabilized, and biologically treated.
This ex situ soil washing system was initially developed Bio Trol, Inc., Eden Prairie, MN, United States, to clean soils contaminated with hazardous wood preserving wastes, such as polynuclear aromatic hydrocarbons (PAH) and pentachlorophenol (PCP). The system may also be applied to soils contaminated with petroleum hydrocarbons, pesticides. PCBs, various industrial chemicals, and hazardous metals.
The soil washing system was demonstrated under the SITE Program in 1989 at the MacGillis and Gibbs Superfund site in New Brighton, Minnesota, United States [16,18,25]. A pilot-scale unit with a treatment capacity of 500 lb/hour operated 24 hours/day during the demonstration. Feed for the first phase of the demonstration (2 days) consisted of soil contaminated with 130 ppm PCP and 247 ppm total PAHs; feed for the second phase (7 days) consisted of soil containing 680 ppm PCP and 404 ppm total PAHs. Contaminated soil washing process water was treated biologically in a fixed-film reactor and recycled. A portion of the contaminated soil washing fins was treated biologically in a three-stage, pilot-scale EIMCO Biolift reactor system supplied by the EIMCO Process Equipment Company. Key findings from the BioTrol demonstration are summarized below.
1. Feed soil (dry weight basis) was successfully separated into 83% washed soil, 10% woody residues, and 7% fines. The washed soil retained about 10% of the feed soil contamination; 90% of this contamination was contained within the woody residues, fines, and process wastes.
2. The soil washer removed up to 89% PCP and 88% total PAHs, based on the difference between concentration levels in the contaminated (wet) feed soil and the washed soil.
3. The system degraded up to 94% PCP in the process water during soil washing. PAH removal could not be determined because of low influent concentrations.
4. Cost of a commercial-scale soil washing system, assuming use of all three technologies, was estimated to be $168 per ton. Incineration of woody material accounts for 76% of the cost (1989 costs).
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