Soil vapor extraction, electrical

A network of 107 electrodes covering two-thirds of an acre was established. To treat beneath a warehouse, 85 of those electrodes were constructed directly through the floor of the building. Electrically conductive from 11-21 ft bg, the electrodes actively heated the depth interval from 5-24 ft bg. Once subsurface temperatures reach boiling, steam laden wifli chlorinated solvents was collected by a network of 37 soil vapor extraction wells screened to 5 ft bg. 

Recalcitrant organic contaminants in the unsaturated zone, those that are difficult to remediate by vapor extraction, biodegradation, or other biological-chemical processes, can be removed via vitrification and electrical heating. This process involves heating the soil by electrical resistance or applied electrical fields to vaporize semi-volatile and volatile compounds, thermally to break down larger organic molecules, and (if heated sufficiently) to vitrify the mineral matter in the soil. 

A 4-month vapor extraction project in which the soil was heated to 160°F would require approximately 200,000 Bm/yd. According to the vendor, the energy cost for electricity-provided heat would be about 6.00/yd. For natural gas and propane, the cost would be lowered to 1.00 and 1.60/yd, respectively (D15838X, p. 2). 

Ultrasound extraction (sonication) is based on the conversion of AC current at 50/60 Hz into electrical energy at 20 kHz and its transformation in mechanical vibrations. Due to the cavity, microscopical vapor bubbles are formed and, after implosion, they produce strong shockwaves into the sample. For isolating the (semi)volatile organic compounds, the liquid-liquid ultrasound technique is applied to samples such as soils, sediments, coal, etc. The process is also useful for the biological materials destruction (Loconto, 2001). Sonication extraction is faster than Soxhlet extraction (30-60 min per sample) and allows extraction of a large amount of sample with a relatively low cost, but it still uses about as much solvent as Soxhlet extraction, is labor intensive, and filtration is required after extraction. 

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