Technical Bulletin #38

In Situ Soil stabilization with Organoclay

In this application, 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 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 microencapsulation, 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 macroencapsulation, at 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 could break down this structure.

Stabilizing case histories

A 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 5ppm oil and grease.

CASE HISTORY:

Wisconsin Power & Light, Co. was asked to remediate the soil near Manitowoc, Wisconsin, which was contaminated with the coal tar. The chemicals consisted of polynuclear aromatic hydrocarbons, such as naphthalene, anthrazine, fluorine, penanthrene, pyrene, phenolic compounds, and benzene. The greatest soil impact was discovered some 12 to 25 ft 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 the MecTool ™ from Millgard Environmental, a system which injects the mix into the soil, rather than removing the soil, mixing it, and placing it back into the ground. This mixing tool works on a column-by-column basis. Applying sodium silicate further solidified the deep end of the column. Since the soil was a river sediment, and the river is close by, maximum safety was required.

The columns achieved consistently 50 psi or more compressive strength, and passed the TCLP leachability test. 22 truckloads of powdered organoclay and at least as much activated carbon powder were used at this site. The purpose of the carbon was to capture the more volatile compounds like benzene and phenols, while the organoclay fixated the less soluble PNA’s. This project proved two points: 1. Organoclay and activated carbon powders are effective at chemically stabilizing organically contaminated soils. The cement and fly ash form the matrix to fixate the organoclay and carbon (further assisted by sodium silicate). 2. The in-situ injection method was successful, opening the door for other coal-tar impacted site treatment.

This project was started in 1993 and finished in 1994.

CONCLUSIONS:

Table 1, on this page, shows test results from a laboratory investigation, where five different stabilization blends were used, including organoclay. The mixture was 80% soil sludge vs. 20% reagent. The organoclay content was two percent of the total weight of the mixture. All tests had shown that the organoclay fixated all organics, including the more volatile ones, such as methyl chloride, vinyl chloride, and acetone.

A large, Midwestern utility company next to a river stabilized soil, which was contaminated with coal tar. The chemicals in the soil consisted primarily of polynuclear aromatic hydrocarbons, including fluorine, pentathrine, naphthalene, anthrazine, 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 showed consistently an unconfined compression strength of more than 50 psi and passed the TCLP test.

The next newsletter will discuss the ability of organoclay to adsorb heavy metals, giving organoclay a unique dual ability for retaining pollutants, a unique advantage over other sorbents.