Soil washing

Soil washing involves the leaching of contaminants by the flushing of water through the contaminated soil. Soil washing can be accomplished either in place or aboveground as discussed below. 

---

Twin City is the 10-aere site of a former U.S. Army ammunition produetion faeility. Attempted destruetion of oflf-spee or damaged ammunition eontaminated the soil with lead and other heavy metals ineluding antimony, eadmium, ehromium, eopper, mereury, and niekel. Soil deeontamination involved a new soil-washing and lead-leaehing teehnique designed to generate no waste streams. 

Selective adsorption of phenanthrene on activated carbons for surfactant reuse in soil washing process... 

Phenanthrene dissolved in Triton X-100 solution was separated by sorption with three GACs with different particle size (4 12,12 20, and 20 40 mesh). The highest adsorption selectivity was obtained with the 20 40 mesh over a wide concentration range of phenanthrene and Triton X-100. The results demonstrate that the selective adsorption is potentially effective to reuse surfactants in a soil-washing process for the remediation of contaminated soils. 

Polyaromatic hydrocarbons absorb strongly to humus and other soil components, rendering these contaminants difficult to remove by thermal, physical, or chemical means, and unavailable for biodegradation. To desorb polyaromatic hydrocarbons from soil, surfactant flooding processes and soil-washing processes or treatments to enhance the biodegradation of polyaromatic hydrocarbons have been considered. 

Soil washing and related chemical treatment techniques... 

The procedure of soil washing involves three main operations ... 

A general flow diagram of a soil-washing treatment is shown in Figure 14.12. Initially, the contaminated soil is sieved to remove large objects such as pieces of wood, plant roots, stones, etc. The maximum size of particles allowed in the feedstock varies with the equipment used, ranging from 10 to 50 mm.77... 

After mixing for an appropriate time, clean soil particles and wash water containing the dissolved and suspended contaminants are separated. Separation techniques in soil-washing systems are similar to those applied in the mineral-processing industry.78 The most common separation techniques are as follows ... 

FIGURE 14.12 Typical soil-wash flow diagram for the treatment of contaminated soils. 

After the separation stage, the coarse soil fraction is rinsed with clean water to remove residual contaminants and any fine soil particles that may adhere to the coarse particles. Soil washing is not usually a stand-alone technology. Typically, both the fine soil fraction (silts and clays) recovered... 

The soil-washing method already described may generate sidestreams, such as air emissions, spent solvents, and exhausted resins, which must also be properly managed. 

The main parameters that affect the cost-effectiveness of soil washing include the physicochemical parameters of the soil (grain size distribution, cation exchange capacity, percentage of silt, clay, or organic matter), and the type and concentration of contaminants. 

FIGURE 14.13 Ranges of soil-washing difficulty, based on the particle size distribution in the soil. 

Frequent changes in contaminant type and concentration in the feed soil can disrupt the efficiency of the soil-washing process. To accommodate changes in the chemical or physical composition of the feed soil, modifications to the wash fluid formulation and the operating settings may be required. Alternatively, additional feedstock preparation steps, such as blending soils to provide a consistent feedstock may be appropriate.77,83... 

Anderson, R., Rasor, E. and Van Ryn, F., Particle size separation via soil washing to obtain volume reduction, J. Hazard. Mater., 66, 89-98, 1999. 

Miller, K.M., Suidan, M.T., Sorial, G.A., Khodadoust, A.P., Acheson, C.M., and Brenner, R.C., Anaerobic treatment of soil wash fluids from a wood preserving site, Water Sci. Technol., 38, 63-72, 1998. 

---