Colloid generation during batch leaching

Batch leaching tests are commonly used to assess the leaching potential of various organic and inorganic contaminants from soil. The toxicity characteristic leaching procedure (TCLP), a batch leaching test developed by the U.S. Environmental Protection Agency, employs an aggressive mixing technique that may allow colloidal fractions to appear in the filtrate. This study quantified the generation of colloid fractions during TCLP testing of a coal-tar contaminated soil, and explored the mechanics of disaggregation. Particle count data indicated that the concentration of 0.72 and 0.83 μm diameter colloids in the filtrate increased with agitation time. The shear rate in the agitation vessel was determined, as well as the hydrodynamic forces acting on the 0.72 and 0.83 μm colloids attached to the soil grains. Through use of force and moment balances, and the Johnson-Kendall-Roberts and Derjaguin-Muller-Toporov adhesion models, it was determined that the operative detachment mechanism is most likely rolling or sliding, depending on the contact radius and the coefficient of static friction. Colloid generation during the TCLP test results in an increase in total colloidal surface area in the filtrate, and may concomitantly result in an overpredicted of the aqueous phase concentration of hydrophobic contaminants.