Article pubs.acs.org/est
In Situ Sequestration of Hydrophobic Organic Contaminants in Sediments under Stagnant Contact with Activated Carbon. 2. Mass Transfer Modeling Yongju Choi,† Yeo-Myoung Cho,† David Werner,‡ and Richard G. Luthy*,† †
Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305-4020, United States School of Civil Engineering and Geosciences, Newcastle University, Newcastle upon Tyne, NE1 7RU, United Kingdom
‡
S Supporting Information *
ABSTRACT: The validity of a hydrophobic organic contaminant mass transfer model to predict the effectiveness of in situ activated carbon (AC) treatment under stagnant sediment−AC contact is studied for different contaminants and sediments. The modeling results and data from a previous 24-month column experiment of uptake in polyethylene samplers are within a factor of 2 for parent- and alkylated-polycyclic aromatic hydrocarbons in petroleum-impacted sediment and factors of 3−10 for polychlorinated biphenyls. The model successfully reproduces the relative effects of AC−sediment contact time, contaminant properties, AC particle size, AC mixing regime, AC distribution, and hydraulic conditions observed in the sediment column experiments. The model tracks contaminant concentrations in different sediment compartments over time, which provides useful information on the contaminant sequestration by the added AC. Long-term projection of the effectiveness of AC amendment using the model shows that the effects of AC particle size and particle-scale heterogeneity in AC distribution are pronounced within a year or so. However, the effect of those factors becomes less significant after a much longer contact period (on the order of a decade or two), resulting in substantial reduction in pore-water concentrations, for example, greater than 99% for benz[a]anthracene, under various scenarios.
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INTRODUCTION In situ activated carbon (AC) amendment is a promising technique for the treatment of hydrophobic organic contaminants (HOCs) in sediments.1 Over the past decade, laboratory studies and pilot-scale trials conducted by research groups in the U.S. and Europe have demonstrated the proof-of-concept of the technology to reduce the (bio)availability of various HOCs in contaminated sediments.1 Stimulated by the proven effectiveness of the technique by those studies, in situ AC amendment is currently being implemented or considered as a remedial option in several contaminated sediment sites in the U.S.2−4 Still, uncertainties exist on design options, mode of application, appropriate site conditions, and the long-term effectiveness of the AC treatment.1,5 This is a consequence of the limited experience with this innovative technique and is a barrier for full-scale application. The pilot-scale trials conducted at different locations in the U.S. and Norway show a range in the effectiveness from site to site for different contaminants depending on the site conditions and the AC application schemes.6−8 Several factors including contaminant desorption kinetics from sediment particles, AC dose and particle size, mixing homogeneity, mode of AC application, and pore-water © 2014 American Chemical Society
movement are shown to affect the effectiveness of the treatment.5,7,9−13 The long-term performance of AC amendment is still in question since the effectiveness observed in the pilot-scale field trials have not been monitored for more than five years.2,5 The concept of in situ AC amendment relies on the sequestration and control of the (bio)availability of the contaminants. The contaminants of concern are not physically removed from the site by either dredging or transformation. Therefore, it is important to demonstrate that the treatment remains effective or the performance slowly improves with time and that the contaminants bound to the AC do not act as a potential recontamination source in the long term after application of the treatment. Contaminant mass transfer modeling is a means to better predict the effectiveness of AC amendment in order to address these issues. Werner et al.13 developed a numerical HOC mass transfer model for sediment by adopting an intraparticle diffusion approach with a concentration-independent diffusion Received: Revised: Accepted: Published: 1843
September 20, 2013 January 3, 2014 January 10, 2014 January 10, 2014 dx.doi.org/10.1021/es404209v | Environ. Sci. Technol. 2014, 48, 1843−1850
Environmental Science & Technology
Article
sediments, (ii) mechanical mixing of the sediment with AC, and (iii) stagnant AC−sediment contact in a packed column for periods extending to 24 months. Concentrations of PAHs or PCBs in PE samplers (4 mm × 25 mm × 51 μm thickness for each strip, a total of approximately 20 strips in each column) were used as a surrogate for pore-water concentrations in the columns. Six different AC application scenarios were applied in the columns: (i) no-AC addition (untreated controls), (ii) 2 min of mechanical mixing between AC and sediment using 75− 150 μm AC, (iii) 30 min mixing with 75−150 μm AC, (iv) two times of 2 min mixing, 5 days apart, with 75−150 μm AC (referred to as 2 × 2 min mixing hereafter), (v) 2 min mixing with
