Mobile system for extracting spilled hazardous materials from excavated soils

A technique was evaluated for the scrubbing or cleansing of excavated soils contaminated by spilled or released hazardous substances. Laboratory tests were conducted with three separate pollutants (phenol, arsenic trioxide, and polychlorinated biphenyls PCB's]) and two soils of significantly different character (sand/gravel/silt/clay and organic loam).The tests show that scrubbing of excavated soil on site is an efficient approach for freeing soils of certain contaminants but that the effectiveness depends on the washing fluid (water + additives) and on the soil composition and particle-size distribution. Based on the test results, a full-scale, field-use, prototype system was designed, engineered, fabricated, assembled, and briefly tested under conditions where large (?2.5 cm) objects were removed by a bar screen. The unit is now ready for field demonstrations.The system includes two major soil scrubbing components: a water-knife stripping and soaking unit of novel design for disintegrating the soil fabric (matrix) and solubilizing the contaminant from the larger particles (?2 mm) and an existing, but re-engineered, four-stage countercurrent extractor for freeing the contaminants from smaller particles (<2 mm). The processing rate of the system is 2.3 to 3.8 m₃/hr (4 to 5 yd₃/hr), though the water-knife unit (used alone) can process 11.5 to 13.5 m₃/hr (15 to 18 yd₃/hr). The complete system requires auxiliary equipment, such as the EPA-ORD physical/chemical treatment trailer, to process the wastewater for recycling; under some circumstances, provision must be made to confine and treat released gases and mists. Treatment residues consist of skimmings from froth flotation, fine particles discharged with the used washing fluids, and spent carbon. The principal limiting constraint on the treatability of soils is clay content (high weight-percent), since breaking down and efficiently treating consolidated clays is impractical or not economically attractive. Most inorganic compounds. almost all water soluble or readily oxidizable organic chemicals, and some partially miscible-in-water organics can be treated with water or water plus an additive.During limited laboratory extraction tests, phenol was very efficiently removed from both organic and inorganic soils, whereas PCB and arsenic clung more tenaciously to the soils and were released less readily into the washing fluids. The extent to which the system has practical, cost-effective utility in a particular situation cannot be determined until preliminary, bench-scale lab work has been performed and acceptable limits of residual concentrations in the washed soil are adopted. Laboratory tests show that soil scrubbing has the capability of vastly speeding up the release of chemicals from soils, a process that occurs very slowly under natural leaching conditions.Note that this system requires excavation of the soil, which can subsequently be replaced or transported to a low-grade landfill. In situ washing of contaminated soil, a process in which the contaminated area is isolated for example, by grouting, and then water-flushed with removal of the wash water at a well-point is an alternative. The overall efficiency of the soil washing system is greater than that currently being achieved by in situ methods.Based on the laboratory program, a series of steps (water-knife size reduction; soaking; countercurrent extraction; hydrocyclone separation; and waste fluid treatment for reuse) was selected as the most suitable process sequence for the prototype system. The system was constructed for the U.S. (EPA) and is now being subjected to field evaluation. However, soils rich in humus, organic detritus, and vegetative matter can present special problems in the extraction of certain hazardous substances, which may not partition between the solid and fluid phases to a practical and necessary extent.