Formatted Title:

Characterization and pH Buffering of Acid Tar Lagoon Materials Using EPA LEAF Methods

Background/Objectives:

A baseline characterization of a hard crumbling (HC) acid tar lagoon material with a pH on the order of 0.5 for purposes of in situ stabilization/solidification (ISS) was completed. Benzene was the key contaminant of concern (COC) with an effective solubility on the order of 490 mg/L. As ISS will occur after bulk removal of the tarry materials (>50,000 CY) as a part of the selected remedy. A soil surrogate representing residual tar and the lagoon bottom liner materials was created and tested at the laboratory scale to understand its properties, leaching behavior and ability to be successfully treated by ISS, including the prevention of acid rebound post treatment due to the diffusion controlled leaching of acidity and sulfate from the tar particles (both remnants of the sulfuric acid used in historic chemical processes). To account for variability in subsurface conditions, multiple soil surrogates were created incorporating bottom liner underlying soils, the principal difference being the use of either 20% or 40% acid tar based on the anticipated relative height of the tar and soil layers comprising the in situ profile to be treated.

Approach/Activities:

To evaluate successful ISS mix designs to address the elevated acidity and sulfate (salt) contents of the HC tar impacted media, EPA 1316 testing, modified (M) to increase data reliability for hydrocarbons, was undertaken on the untreated soil surrogates, reagents, and select 28-day cured mix designs. Testing was completed to understand the residual pH buffering capacity and the corresponding VOC and metals leaching, in part, to differentiate the relative contributions of the site materials versus reagents in terms of metals leaching post treatment. EPA 1316 included extended leaching durations up to 7 days to account for diffusion-controlled COC leaching from tar/clays and additional liquid/solid (L/S) ratios of 20, 50 and 100 for the reagents to simulate the dilution effects associated with the lagoon setting. Benzene, toluene, and naphthalene were all solubility controlled.

EPA 1313 testing (L/S 10) was independently undertaken with nitric acid (per method) and sulfuric acid (per site conditions) to understand the pH buffering capacity and metals leaching of the soil surrogates and reagents (combination of a powdered ladle slag [PLS] and 60/40 blend of Type IL Portland Cement [PC] / Slag cement blend) and select 28-day cured mix designs.

Results/Lessons Learned:

For soil surrogates containing 40% acid tar balls with a top size of 3/8 inches, the natural pH was on the order of 1 to 1.5 and required 1 milliequivalents/gram (meq/g) of NaOH to raise the pH to 2. A second meq/g elevated the pH to 6 and 3^rd to pH>11. Additionally, approximately 3 meq/g acid buffering capacity were required to keep the mix design above pH 8 to ensure the ISS mass stability in the long term. This corresponded to a minimum reagent addition of 5 wt% PLS + 25 wt% PC/SC added on a dry basis to the wet surrogate soil, regardless of the corresponding strength or hydraulic conductivity.

The selection of acid type for EPA 1313 leaching for the soil surrogates, individual and blended reagents, and 28-day cured mix designs showed that several orders of magnitude difference in metals solubility occurred for some metals, suggesting sole reliance on nitric acid would have led to both under and over predictions and misunderstanding of key behavior. Sulfuric acid shifted the solubility edge of many heavy metals by 1 to 1.5 pH units to the alkaline side which enhanced the solubility of numerous heavy metals over mid-range pH by up to an order of magnitude. On the contrary, the solubility of calcium and barium were suppressed by 2 to 3 orders over a broad range of pH owing to the precipitation of gypsum and barite. Extensive manganese (Mn) leaching was associated with the PLS, but Mn remained insoluble above pH>8. Vanadium (V) leached from all materials but was greatest from the soil surrogate (300 to 800 ug/L). ISS treatment was successful in reducing the concentrations by approximately an order of magnitude (<10 ug/L) in the alkaline range.

Bhawana Sharma

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Characterization and pH Buffering of Acid Tar Lagoon Materials Using EPA LEAF Methods