Remediation of soil impacted with chlorinated organic compounds: soil mixing with zero valent iron and clay

Abstract

Chlorinated solvents in the environment continue to present an enormous remediation challenge. A primary reason for the difficulty in cleaning up chlorinated solvent source zones involves heterogeneous distributions of permeability and contaminants in natural porous media. A method that can be used to overcome heterogeneity involves use of soil mixing techniques to deliver reagents and homogenize soils. A typical soil mixing application involves admixing contaminated soil with zero valent iron (ZVI) and bentonite (clay). This technology, herein referred to as ZVI-Clay, combines ZVI-mediated degradation of chlorinated solvents with bentonite-induced stabilization. As of December 2013, ZVI-Clay has been applied in 13 field applications, all of which have been viewed as being successful in achieving site remediation objectives. However, our understanding of the processes governing treatment in the ZVI-Clay mixed soil system is rather limited. The overarching goal of the research presented herein is to broaden our understanding of the processes controlling degradation and transport in soils treated via soil mixing with ZVI (or similar reactive media) and bentonite. In support of this objective, research included (a) analysis of field data, (b) initial rate studies, (c) hydraulic conductivity testing, (d) reactive-transport modeling coupled with column experiments, and (e) treatment of hydrophobic compounds. Field data analysis was based on performance data from a ZVI-Clay field application at Camp Lejeune, NC, in which 23,000 m3 of soil initially contaminated with trichloroethene (TCE) and 1,1,2,2-tetrachloroethane (TeCA) were treated with 2% ZVI and 3% bentonite. Within one year of treatment, total chlorinated organic compound (COC) concentrations in soils were decreased by average and median values of 97% and >99%, respectively. Total COC concentrations in groundwater were reduced by average and median values of 81% and >99%, respectively. Total COC reductions by 99.9% or greater were observed in most soil and water sampling locations. Hydraulic conductivity in the treated soil zone was reduced by an average of about 2.5 orders of magnitude. To explain the variations in kinetic data observed following ZVI-Clay field applications, initial-rate batch-reactor studies were conducted under a range of initial TCE concentrations and ZVI concentrations. When TCE concentrations were less than solubility, the Michaelis-Menton kinetic model provided an excellent fit of experimental data. When TCE concentrations were above solubility (i.e., NAPL was present), the degradation rate was independent of the amount of NAPL in the system. The presence of NAPL appears to have had a minor impact (~20% reduction) on the TCE degradation rate. A linear relationship between TCE degradation rate and ZVI amount was observed. Hydraulic characteristics of soils mixed with bentonite were evaluated by conducting column studies and MODFLOW modeling of field-scale systems. Experiments were conducted to evaluate the hydraulic conductivity, K, in two soils types mixed with 0.5 to 4% bentonite (i.e., the range of values typical for ZVI-Clay field applications). In a well-sorted fine sand, with a moderately high initial K (10-4 m/s), the value of K was reduced by about a factor of 10 for each 1% bentonite added to the soils. In a moderately-sorted fine sand with silt, with a low initial K (10-8 m/s), addition of up to 4% bentonite had only minor impacts on K. MODFLOW modeling indicated that surrounding groundwater flow patterns tend to bypass the treated soil body, under steady state conditions, given a reduction in K by at least an order of magnitude. Within the treated soil body, contaminant residence time is extended in approximate proportion to the reduction in K. The concepts of NAPL dissolution, ZVI-mediated degradation, and flow reduction were combined in a mathematical model. The model was then tested using column reactor studies containing NAPL-phase TCE and soils treated with 2% ZVI. The model adequately described TCE elution and formation of degradation products. The model was then used to predict treatment performance following field-scale implementation of ZVI-Clay. Model output predicts that the benefits of reaction are most effectively utilized with a reduction in flow rate by at least 2 orders of magnitude. Finally, enhancements to the ZVI-Clay treatment process were evaluated for treatment of polychlorinated biphenyls (PCBs). Due to their strong hydrophobicity and stable molecular structure, PCBs in the environment have been shown to be much more difficult to degrade than many of the common chlorinated solvents. Thus, alternative types of reactive media were evaluated. Batch experiments were conducted to evaluate zero valent metals (ZVM), ZVM + Pd-catalysis, and emulsified zero valent iron (EZVI) for dechlorination of PCBs in systems with and without soil. In water-based systems, ZVM with a Pd-catalyst facilitated rapid destruction of 2-chlorobiphenyl (half-life < 2 hr), while ZVM alone did not achieve any measurable degradation. In the presence of soils, EZVI was the only approach that resulted in a clear enhancement in PCB dechlorination rates. The results suggest treatment of PCBs in the presence of soil presents a much greater challenge than treatment of aqueous phase PCBs; however, treatment of PCBs in soil can benefit from enhanced desorption and a persistent reactive media.