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

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.

Effects of fluid composition in fluid injection experiments in porous media

Experiments on fluid flow in porous media, using fluids loaded with solids of various grain sizes, have been conducted in a modified Hele-Shaw setup. This setup utilised weakly cemented porous media with specific hydraulic and mechanical properties. Fluid injection in coarse granular media with clean or low-concentration fine particles, results in infiltration only, with pressure close to the material tensile strength, while injection in finer granular material causes damage alongside infiltration, with the fluid pressure still close to the material tensile strength. When larger particle sizes or higher particle concentrations are used in the mixture, the fluid travels further within the porous medium, primarily influenced by the grain size of the granular medium. In the latter case, the Darcy flow equation with an effective permeability term can be employed to determine the pressure differential. For the largest particle sizes included in the fluid, the equation is still applicable, but the effective permeability requires adjustment for particle size within the fluid rather than the granular medium. This is crucial when the injection point is locally clogged. The experiments show that fracturing conditions are controlled by different mechanisms. Dimensional and statistical analysis was used to classify the injection pressures to regimes predicted by fracturing theory or by Darcy law with modified effective permeabilities. The findings show that both the material properties and fluid composition are important designing parameters.

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.

Synthesis and Characterization of Zero-Valent Fe-Cu and Fe-Ni Bimetals for the Dehalogenation of Trichloroethylene Vapors

In this study, zero-valent iron-copper (Fe-Cu) and iron-nickel (Fe-Ni) bimetals were prepared by disc milling for the dehalogenation of trichloroethylene vapors. For both Fe-Ni and Fe-Cu, three combinations in terms of percentage of secondary metal added were produced (1%, 5%, 20% by weight) and the formation of the bimetallic phase by milling was evaluated by X-ray diffraction (XRD) analysis. The disc milled bimetals were characterized by a homogenous distribution of Ni or Cu in the Fe phase and micrometric size visible from scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) analysis and by a relatively low specific surface area (0.2–0.7 m2/g) quantified by the Brunauer–Emmett–Teller (BET) method. The reactivity of the produced bimetals was evaluated by batch degradation tests of TCE in the gas phase with 1 day of reaction time. Fe-Ni bimetals have shown better performance in terms of TCE removal (57–75%) than Fe-Cu bimetals (41–55%). The similar specific surface area values found for the produced bimetals indicated that the enhancement in the dehalogenation achieved using bimetals is closely related to the induced catalysis. The obtained results suggest that ZVI-based bimetals produced by disc milling are effective in the dehalogenation of TCE vapors in partially saturated conditions.

A Comprehensive Review for Groundwater Contamination and Remediation: Occurrence, Migration and Adsorption Modelling

The provision of safe water for people is a human right; historically, a major number of people depend on groundwater as a source of water for their needs, such as agricultural, industrial or human activities. Water resources have recently been affected by organic and/or inorganic contaminants as a result of population growth and increased anthropogenic activity, soil leaching and pollution. Water resource remediation has become a serious environmental concern, since it has a direct impact on many aspects of people's lives. For decades, the pump-and-treat method has been considered the predominant treatment process for the remediation of contaminated groundwater with organic and inorganic contaminants. On the other side, this technique missed sustainability and the new concept of using renewable energy. Permeable reactive barriers (PRBs) have been implemented as an alternative to conventional pump-and-treat systems for remediating polluted groundwater because of their effectiveness and ease of implementation. In this paper, a review of the importance of groundwater, contamination and biological, physical as well as chemical remediation techniques have been discussed. In this review, the principles of the permeable reactive barrier's use as a remediation technique have been introduced along with commonly used reactive materials and the recent applications of the permeable reactive barrier in the remediation of different contaminants, such as heavy metals, chlorinated solvents and pesticides. This paper also discusses the characteristics of reactive media and contaminants' uptake mechanisms. Finally, remediation isotherms, the breakthrough curves and kinetic sorption models are also being presented. It has been found that groundwater could be contaminated by different pollutants and must be remediated to fit human, agricultural and industrial needs. The PRB technique is an efficient treatment process that is an inexpensive alternative for the pump-and-treat procedure and represents a promising technique to treat groundwater pollution.