Analytical model for the design of in situ horizontal permeable reactive barriers (HPRBs) for the mitigation of chlorinated solvent vapors in the unsaturated zone

Sulfidated zero-valent iron bimetals for passive remediation of chlorinated vapors in the subsurface

This study explores a novel application of sulfidated zero-valent iron (S-ZVI) bimetals for the treatment of chlorinated solvents in the vapor phase. The potential of these reactive materials was investigated through batch, column, and modeling tests. The materials were produced by disc milling of ZVI, sulfur (S), copper (Cu), and nickel (Ni) with molar ratios of 0.05 and 0.2. The reactivity of the materials was assessed through vapor degradation batch tests conducted under partially saturated conditions using trichloroethylene (TCE) as a model compound. Sulfidated materials with a 0.05 S/ZVI molar ratio were the most reactive, achieving up to 99 % degradation of TCE vapors within 18 h and first-order degradation constants of 5-5.7 d-1. Compared to the non-sulfidated materials, sulfidated ones remained reactive even after aging by exposure to air for 30 days. In all tests, C3-C6 hydrocarbons were detected as main byproducts, indicating β-elimination as the dominant TCE degradation pathway, with minor dichloroethylene and vinyl chloride amounts from the hydrogenolysis pathway. To evaluate the use of sulfidated bimetals as Horizontal Permeable Reactive Barriers (HPRBs) for treating chlorinated vapors in the subsurface, TCE diffusion column tests were performed using a 5 cm thick reactive layer of S-ZVI-Ni. These tests demonstrated up to 70 % degradation over 25 days. By integrating the column test results into an analytical model, it was estimated that an 18 cm HPRB could ensure up to 99 % degradation of TCE vapors. These findings highlight the potential of S-ZVI bimetals as an effective passive mitigation system for reducing chlorinated solvent vapor emissions from the subsurface.

Analytical model for the mitigation of VOC vapor with horizontal permeable reactive barrier in the contaminated site considering non-uniform source

Volatile organic compounds (VOCs) contamination at the groundwater may cause vapor intrusion and pose significant threats to human health. As a novel low-carbon mitigation technology, a horizontal permeable reactive barrier (HPRB) is proposed to remove the VOC vapor in the vadose zone and mitigate the vapor intrusion risk. To estimate the performance of HPRB in the contaminated site with a non-uniform source, a transient two-dimensional analytical model is developed in this study to simulate the VOC vapor migration and oxidation processes in the layered soil. The analytical model is verified against the experimental results and numerical simulation first and the parameter study is then conducted. The HPRB has good performance for the contaminated sites involving factors including deep source and local soil with low effective diffusivity. To consider the vertical heterogeneity of the local soil, the traditional equivalent homogeneity method has limitations in considering the horizontal migration of VOC vapor and is not suitable for the two-dimensional model. On the contrary, the artificial layered method based on the proposed analytical model has better accuracy and is recommended to be adopted in practice. Leading to the exponential decrease in the VOC vapor concentration at the ground surface, increasing the thickness of HPRB is an effective measure to enhance the performance of HPRB. The fitting exponential function can be applied to determine the minimum design value of the thickness of HPRB in practice.

Degradation of trichloroethylene vapors by micrometric zero-valent FeCu and FeNi bimetals under partially saturated conditions

The degradation of trichloroethylene (TCE) vapors by zero-valent Iron-Copper (Fe-Cu) and Iron-Nickel (Fe-Ni) bimetals with 1%, 5% and 20% weight content (%wt) of Cu or Ni was tested in anaerobic batch vapor systems carried out at ambient room temperature (20 ± 2 °C) under partially saturated conditions. The concentrations of TCE and byproducts were determined at discrete reaction time intervals (4 h-7 days) by analyzing the headspace vapors. In all the experiments, up to 99.9% degradation of TCE in the gas phase was achieved after 2-4 days with zero-order TCE degradation kinetic constants in the range of 134-332 g mair-3d-1. Fe-Ni showed a higher reactivity towards TCE vapors compared to Fe-Cu, with up to 99.9% TCE dechlorination after 2 days of reaction, i.e., significantly higher than zero-valent iron alone that in previous studies was found to achieve comparable TCE degradation after minimum 2 weeks of reaction. The only detectable byproducts of the reactions were C3-C6 hydrocarbons. Neither vinyl chloride or dichloroethylene peaks were detected in the tested conditions above their method quantification limits that were in the order of 0.01 g mair-3. In view of using the tested bimetals in horizontal permeable reactive barriers (HPRBs) placed in the unsaturated zone to treat chlorinated solvent vapors emitted from contaminated groundwater, the experimental results obtained were integrated into a simple analytical model to simulate the reactive transport of vapors through the barrier. It was found that an HPRB of 20 cm could be potentially effective to ensure TCE vapors reduction.

Screening Additives for Amending Compacted Clay Covers to Enhance Diffusion Barrier Properties and Moisture Retention Performance

The cover systems in contaminated sites have some problems, including desiccation cracks, which would lead to degradation of the barrier performance. This study presented a systemic laboratory experimental investigation on the liquid–plastic limit, moisture retention, hydraulic conductivity (k), and gas diffusion barrier properties of amended compacted clay by attapulgite and diatomite for controlling desiccation cracks and migration of water and volatile organic compounds (VOCs). The results showed that the attapulgite could enhance the moisture retention and liquid limit of amended compacted clay. Diatomite could reduce the gas diffusion coefficient (Dθ) significantly. The compacted clay amended by the dual-additives component of attapulgite and diatomite could enhance the liquid limit, moisture retention percent, gas barrier property, and hydraulic performance compared with the unamended clay. Based on the experimental data obtained, the dosage of additives was targeted to be 5%. The moisture retention percent of dual-additives (attapulgite 4% and diatomite 1%) amended clay increased by 82%, the k decreased by 25%, and the Dθ decreased by 42% compared with unamended clay. Scanning electron microscopy (SEM), BET-specific surface area test method (BET), Mercury Intrusion Porosimetry (MIP), and thermogravimetric analysis (TGA) indicated the enhancement mechanism of additives-amended compacted clay.

Horizontal permeable reactive barriers with zero-valent iron for preventing upward diffusion of chlorinated solvent vapors in the unsaturated zone

Chlorinated solvents are extensively used in many activities and hence in the past decades impacted a large number of sites. The presence of these contaminants in groundwater is challenging particularly for the management of the vapor intrusion pathway. In this work we examine the potential feasibility of using horizontal permeable reactive barriers (HPRBs) placed in the unsaturated zone to treat chlorinated solvent vapors emitted from groundwater. Zero-valent iron (ZVI) powders, partially saturated with water and characterized by different specific surface areas (SSA), were tested, alone or mixed with sand, in lab-scale batch reactors using TCE as model compound. Depending on the type of iron powder used, a reduction of TCE concentration in the vapor phase from approximately 35% up to 99% was observed after 3 weeks of treatment. The best performance in terms of TCE reduction was obtained using the ZVI characterized by the intermediated values of the specific surface area (SSA). This finding, which is in contrast with the results generally observed in in aqueous solutions, was tentatively attributed to a non-selective higher reactivity of the fine-grained iron samples with water and dissolved oxygen (with a consequent iron passivation) or to the occurrence of a diffusion-limited reaction kinetics. Based on the first-order kinetic degradation rate constants estimated from the experimental data, a horizontal barrier of 1 m containing ZVI or a mixture of ZVI and sand can potentially lead to an attenuation of TCE vapors over 99%.

Two-dimensional analytical solution for VOC vapor migration through layered soil laterally away from the edge of contaminant source

A two-dimensional analytical solution is developed to simulate vapor migration in layered soil laterally away from the edge of contaminant source and has advantages in considering the vapor concentration profile in a functional form near the source edge. The analytical solution is validated against existing analytical solution, numerical model and experimental results. It has also proved to be an alternative screening tool to evaluate the vapor intrusion (VI) risk by compared with existing VI assessment tools. The influence of the characteristics of contaminant source and soil layer on the VI risk are investigated. The existence of capillary fringe effectively reduces VI risk. Among all the single-layer-soil cases, the lateral inclusion zone for sand is the widest due to the thinnest capillary fringe and the lowest effective diffusivity ratio between soil and capillary fringe. For layered soil, the lower effective diffusivity layer overlying the higher one enhances the horizontal diffusion and extends the lateral inclusion zone. The width of lateral inclusion zone increases logarithmically with increasing source concentration while it increases linearly with increasing source depth. Based on the calculation results, a simplified formula is proposed to preliminarily estimate the width of lateral inclusion zone for the typical single-layer-soil cases considering the capillary fringe.

Dehalogenation of trichloroethylene vapors by partially saturated zero-valent iron

The reduction of trichloroethylene (TCE) in gas phase by different types of granular zero-valent iron (Fe⁰) was examined in anaerobic batch vapor systems performed at room temperature. Concentrations of TCE and byproducts were determined at discrete time intervals by analysis of the headspace vapors. Depending on the type of iron used, reductions of TCE gas concentration from 35% up to 99% were observed for treatments of 6 weeks. In line with other experimental studies performed with aqueous solutions, the particle size was found to play a key role in the reactivity of the iron. Namely an increase of the TCE removal up to almost 3 times was observed using iron powders with particle size lower than 425 μm compared to iron powders with particle size lower than 850 μm. The manufacturing process of the iron powder was instead found to play only a limited role. Namely, no significant differences were observed in the TCE reduction by Fe⁰ obtained using an iron powder attained by water atomization and sieving compared to the removal achieved using an iron powder subjected to a further annealing processes to reduce the content of oxides. Conversely, the pretreatment of the iron powder with HCl was found to enhance the reactivity of the iron. In particular, by washing the iron powder of 425 μm with HCl acid 0.1 M the reduction of TCE after 6 weeks of treatment increase from approximately 80% for the as received material to >99% for the pretreated iron powder. We also performed tests at different humidity of the iron observing that not statistical differences were obtained using a water content of 10% or 50% by weight. In all the experiments, the only detectable byproducts of the reactions were C4-C6 alkenes and alkanes that can be attributed to a hydrogenation of the CCl bond.