The effectiveness of in-situ activated carbon treatment to sequester hydrophobic organic contaminants in sediments [electronic resource]
Keywords: Civil and Environmental Engineering.

The goal of this study is to improve the mechanistic understanding on the in-situ activated carbon treatment process and the effect of site conditions and engineering options on the effectiveness of the treatment. A comprehensive study is conducted using both experimental and theoretical modeling approaches. Polyethylene (PE)-water partitioning coefficients are determined for a wide range of polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs), and used as a measure of the availability of the contaminants throughout the study. A simple regression model employing the number aromatic and aliphatic carbons, and experimentally-determined octanol-water coefficients are shown to be a good predictor of the PE-water partitioning coefficients for the studied PAHs and PCBs, respectively. The performance of AC amendment to treat parent- and alkylated-PAHs in petroleum-impacted sediments is comprehensively demonstrated for the first time . AC amendment at a dose of 5 dry wt% in well-mixed systems reduces the uptake in PE samplers up to 99% and 98% for sediments with oil contents of 1 and 2%, respectively. Comparison of the experimental results with the simulation results using an hydrophobic organic contaminant (HOC) mass transfer model suggests that the dissolved organic matter (DOM) or an oil phase attenuates the AC performance by up to a factor of 6 at 1 month with 5% dose, but the AC attenuation effect becomes negligible with increased contact time and by using a smaller AC particle size range. The effectiveness of AC amendment is further studied using sediment columns with variable AC-sediment contact times, engineering options, and hydraulic conditions. The study demonstrated that more than 95% and 93% reductions in uptake in PE samplers are obtained after 2 years of contact for total PAHs and total PCBs in sediments, respectively, when AC is homogeneously mixed into the sediment. The benefit of AC amendment clearly improves with longer contact time, smaller AC particle size, and more homogeneous AC distribution. A previously developed HOC mass transfer model is further modified and used to simulate the effectiveness of AC treatment in the stagnant systems. The model successfully reproduced the experimental results quantitatively for model PAHs and qualitatively for model PCBs. Modeling results suggest that the effects of AC particle size and AC distribution heterogeneity are pronounced in the short term, but those factors become less significant over 25 years as the system reaches close to the equilibrium state. The experimental and modeling approaches employed in this study are incorporated into a guideline for site-specific assessments for field application of in-situ AC amendment. Following the guideline, which includes a screening assessment, site assessment, and a site-specific effectiveness study, one can predict the site-specific effectiveness of in-situ AC amendment in the long-term and optimize the engineering parameters for the remedial project using the state-of-the-art knowledge on the treatment.