COLLOID GENERATION DURING BATCH LEACHING TESTS - MECHANICS OF DISAGGREGATION

Batch leaching tests are commonly used to assess the leaching potentia l of various organic and inorganic contaminants from soil. The toxicit y characteristic leaching procedure (TCLP), a batch leaching test deve loped by the U.S. Environmental Protection Agency, employs an aggressi ve mixing technique that may allow colloidal fractions to appear in th e 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 co ncentration of 0.72 and 0.83 mu m diameter colloids in the filtrate in creased with agitation time. The shear rate in the agitation vessel wa s determined, as well as the hydrodynamic forces acting on the 0.72 an d 0.83 mu m colloids attached to the soil grains. Through use of force and moment balances, and the Johnson-Kendall-Roberts and Derjaguin-Mu ller-Toporov adhesion models, it was determined that the operative det achment mechanism is most likely rolling or sliding, depending on the contact radius and the coefficient of static friction. Colloid generat ion during the TCLP test results in an increase in total colloidal sur face area in the filtrate, and may concomitantly result in an overpred iction of the aqueous phase concentration of hydrophobic contaminants. (C) 1998 Elsevier Science B.V.