In the process of in-situ soil stabilization, organoclays and activated carbon are used in powder form. As a powder, organoclay can remove 77 percent or more of its weight in oil. In principal, organoclays fixate organics and hold them by van der Waals forces. Cement/fly ash and other pozzolanic agents fixate the organoclay, enabling the system to pass the EPA's Toxicity Characteristic Leaching Procedure (TCLP) tests when constructed properly.
The cement/fly ash forms a matrix containing the organoclay and contaminants. There are two mechanisms that fixate the organoclay/contaminants:
- In micro-encapsulation (at the microscopic level), the organoclay with the fixated contaminants is entrapped within the crystalline matrix of the solidified mass. Degradation of this mass into ultra-fine particles could result in migration and release of organic contaminants. Without the organoclay, these organics are not bound to the crystalline structure created by the cement and are easily released into the environment.
- In macro-encapsulation (on a larger scale), the organoclay with the adsorbed organic contaminants is physically entrapped in the stabilized cementitious matrix within the discontinuous pores. Only extreme freezing and thawing can break down this structure.
Companies On Solid GroundA rolling mill sludge, which contained 10 percent grease and oil, was stabilized with a mixture of cement, silicate additives and organoclay. This resulted in a mixture of 80 percent sludge, 17.7 percent cement blend and 2.3 percent organoclay. Treated with these compounds, the sludge had an unconfined compression strength of greater than 50 PSI, and the TCLP yielded less than 5 PPM oil and grease.
In a similar fashion, Wisconsin Power & Light Co. was asked to remediate soil contaminated with coal tar near Manitowoc, WI. The chemicals consisted of polynuclear aromatic (PNA) hydrocarbons, such as naphthalene, anthrazine, fluorine, penanthrene, pyrene, phenolic compounds and benzene. The greatest soil impact was discovered some 12 to 25 feet below the ground surface-the site is in an oxbow near the river.
The stabilizer blend that was chosen for use consisted of: Portland A cement, fly ash, organophilic clay and activated carbon. The method of introducing the blend into the soil was Millgard Environmental's MecTool™, a system that injects the mix into the soil, rather than removing the soil, mixing it and placing it back into the ground. Applying sodium silicate further solidified the deep end of the column. The MecTool mixing tool works on a column-by-column basis. Since the soil was a river sediment, and the river is in close proximity, maximum safety was required.
The columns consistently achieved 50 PSI plus compressive strength, and passed the TCLP leachability test. Powdered organoclay delivered in 22 truckloads, and at least as much activated carbon powder, were used at the site. The purpose of the carbon was to capture the more volatile compounds like benzene and phenols, while the organoclay fixated the less soluble PNAs.
This project, started in 1993 and finished in 1994, proves two points:
- Organoclay and activated carbon powders are effective at chemically stabilizing organically contaminated soils. The cement and fly ash form a matrix to fixate the organoclay and carbon (further assisted by sodium silicate).
- The in-situ injection method was successful, opening the door for other coal tar-impacted site treatment.
A Comparison Of Five Stabilization Blends
Moreover, a large Midwestern utility company (additonially located next to a river) stabilized soil contaminated with coal tar. The chemicals in the soil consisted primarily of PNA hydrocarbons, including fluorine, pentathrine, naphthalene, anthrazine and pyrene, but also phenolic compounds and benzene. The stabilizer blend chosen was a mixture of Portland A cement, fly ash, organoclay and powdered activated carbon.
Since the mixing mechanism was in-situ with a drilling tool that stabilizes the soil in columns, sodium silicate was added at the bottom of the column, near the ground water table, to ensure maximum strength. The columns consistently conveyed an unconfined compression strength of more than 50 PSI, not to mention passed the TCLP test.
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