Leaching for remediation

Leaching has potential for clean up of toxic waste sites, but there must be no dispersal of the contaminants to adjacent areas. If the toxic material is soluble in water, it may already be dispersed by the percolation of rain water or by flow of groundwater. The penetration of wastes down into the soil has been modeled by several groups. A representative paper by Janssen, et al., (1990) considers the migration of radioisotopes deposited on soil by fallout from a nuclear accident. Movement depends on convection with the solvent, diffusion, and exchange with the soil solids. This is much more complicated than batch or countercurrent leaching where we neglected any binding of the solute because the equilibria are complex and the motion is in three dimensions. Problems of this type require considering many elements that interact. The term for this approach is finite element analysis, and linear algebra with matrices aids the solutions.

If contaminated soil were flushed by pumping water through it, the pumped water under pressure would escape to the surroundings. This is unacceptable because the toxic wastes would be diluted to make treatment more difficult and dispersed to make collection more costly. A well to withdraw contaminated water should be in the region of highest concentration of contaminant. The following sketch shows a pumping scheme that might be acceptable for leaching with water pumped at the periphery to create higher pressure to prevent flow out of the site. The leachate is treated (a bioprocess is shown in this figure) and recycled. The above-ground treatment is tricky because the concentration of toxic material will usually be low. Both chemical and biological processes have troubles in dealing with dilute streams.

Toxic and hazardous wastes are often organic chemicals with poor water solubility. The most efficient way to extract them from soil would be to use organic solvents. However, adding solvents to soil would make the problem worse even if the solvents were not toxic. The expense would be unreasonable, and organics would represent very high BOD. One solvent that has been proposed is supercritical carbon dioxide in which many organic compounds are highly soluble. Lost solvent would merely escape to the air. The drawback is that the temperature and pressure to keep the carbon dioxide in its supercritical state would be a severe engineering challenge. There are only a few industrial extractions with supercritical fluids, and the technology is considered advanced and costly.

Volatile compounds can be extracted from soil simply by flushing with a gas. One attractive method for removing organic solvents from soil is aeration. The removal is somewhat slow but cost effective. Venting the spent air adds to air pollution, so some treatment is advisable. The air could be sent to a combustion unit where the contaminants burn along with the fuel. Another option is adsorption on activated carbon. The organic contaminants are burned off when the carbon is roasted to regenerate it for reuse.

Reference

Janssen, L.P.B.M., J. Prins, A.C. Hoffman, R.J. De Meijer, and A.W.L. Veen, "Modeling the migration of contaminants in soil", Chem. Engr. Communications 89: 37-47 (1990) 
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14-March-96