Batch Leaching Tests: Colloid Release and PAH Leachability

The Toxicity Characteristic Leaching Procedure (TCLP) was developed by the U.S. Environmental Protection Agency to assess leaching potential of contaminants from waste, and to provide a test to classify hazardous waste. It is a batch leaching test where a waste (such as contaminated soil) and an extraction fluid are agitated for a predetermined time. Since TCLP employs an aggressive mixing technique, it is possible that hydrophobic contaminant-laden colloidal fractions may appear as “dissolved” constituents. In this study, TCLP was employed to determine the leachability of PAH contamination from a coal tar contaminated site. Generated colloids and the apparent aqueous concentrations of naphthalene and phenanthrene were measured at various mixing times in the extraction fluid.

A mathematical model was developed that predicted the apparent aqueous contaminant concentration in the filtrate. This model accounted for the presence of colloids in the filtrate, and quantified contaminant desorption from colloids. The fraction of colloid-bound contaminant was predicted to be negligible for naphthalene. However, phenanthrene was predicted to have a significant fraction of the total contaminant in the colloidal phase, while naphthalene was primarily dissolved. The desorption model and PAH desorption data are presented here to determine the extent of colloid-facilitated desorption during leaching tests.

Keywords:

• Colloid
• contaminant
• desorption
• leaching
• PAH
• TCLP.

The author thanks his Ph.D. dissertation advisory committee for their guidance on the research presented here. The contributions of Domenico Grasso, Joseph Pignatello, James Bryers, and Thomas Anderson are appreciated. This work was partly funded by Northeast Utilities and Yankee Gas.

Notes

^a Notes: From Schwarzenbach et al. (1993) @ 25°C.

^b Calculated from equation 3.