Acid Tar Lagoon Remediation II: Long‐Term Leaching and Geochemistry of Stabilization/Solidification Mix Designs

ABSTRACT This paper reports on a US project‐based treatability study conducted for a confidential client and focuses on the long‐term leaching of benzene and metals from soil surrogates containing 20% (S2X) or 40% (S4X) acid tar by volume (where X denotes the testing phase) and their stabilization/solidification (S/S) mix designs (M2X or M4X) formed with grouts of powdered ladle slag (PLS), a 60/40 (w/w) blend of Type IL Portland Cement (PC), and Grade 120 NewCem (NC) slag cement, and, in some mixes, bentonite (B). Long term semi‐dynamic leaching tests modified (M) for use with hydrocarbons (EPA 1315M) were performed on S/S‐treated soil surrogates cured for 28 days. Percent leaching reductions (%LRs) were calculated by comparing quasi‐steady‐state benzene concentrations from EPA 1315M to the effective solubility of benzene in the untreated soil surrogates (318 mg/L for S22; 406 mg/L for S42). Minimum %LR values of 93.7% and 92.8% were observed for M22 mixes tested with and without 0.5% B, respectively. Parallel testing was conducted using freshly mixed (FM) M22 materials placed in a specialized extractor, where benzene leached into an overlying water cap beneath a continuously exchanged headspace over 91 days. The mass transfer rates from the FM tests compared very well with those from the EPA 1315M tests beginning around day 35, coinciding with the end of the startup period for the 28‐day cured samples. The strong agreement of these data essentially validated the accuracy and effectiveness of the polydimethylsiloxane (PDMS) liner used in the EPA 1315M test. The corresponding EPA 1315M leaching of most trace heavy metals (Sb, As, Be, Cr, Co, Cu, Pb, Mo, Ni, and Ag) were at or below their respective method detection limits for all leaching intervals for all mix designs. Cd, Se, and Zn showed similar performance with a few outliers. Ba concentrations were between 20 and 100 µg/L. Mo and V concentrations from the pH‐dependent leaching (EPA 1313) test never exceeded 10 µg/L and were up to 100 times lower than those from the untreated soil surrogates at the same pH. After the 63‐day EPA 1315M test, the unconfined compressive strength of the test samples was at least 689 kPa (100 lb/in 2 ) greater than their 28‐day values.