Longevity prediction of reactive media in permeable reactive barriers considering the contamination level and groundwater velocity at the planning site, with a focus on cadmium removal by zeolite.

Zeolite is a versatile and effective reactive material used in permeable reactive barriers (PRBs) for remediating groundwater contaminated with heavy metals. In this study, we evaluated the influence of subsurface environmental conditions, namely contamination level (C0) and groundwater velocity (v), on predicting the longevity of zeolite for cadmium (Cd) removal. Batch experiments were performed to investigate the effect of C0 on Cd removal, and column experiments were performed to examine how Cd transportation through zeolite varies at different C0 and v. Breakthrough curves (BTCs) were analyzed with an advection-dispersion equation (ADE) coupled with nonequilibrium sorption rate models. The reaction parameters indicating the performance metrics of zeolite were determined using an iterative fitting approach-retardation factor (R), partitioning coefficient (ß), and mass transfer coefficient (ω). R exhibited dependence on C0, but was unrelated to v; its rapid increase at lower C0 was explained by Langmuir sorption isotherms. ß and ω, integral to sorption dynamics and mass transfer, respectively, showcased functional relationships with v. ß decreased gradually as v increased, described by the nonequilibrium sorption model, whereas ω increased steadily with v, guided by the Monod function. Using the relationship of these parameters, the fate and transport of Cd within zeolite was simulated under various subsurface environmental conditions to construct the longevity prediction function. Thus, this study introduces a method for predicting the longevity of reactive materials, which can be valuable for designing PRBs with high longevity in the future.

Performance expectation of coal waste in permeable reactive barrier for removal of cadmium considering contamination level and pore water velocity.

The effectiveness and longevity of permeable reactive barriers (PRBs) depend on the performance of the reactive materials and the subsurface environment. The relationship of the groundwater velocity on performance of coal waste for the heavy metal removal was reported in our previous study. In this study, we investigated the performance and longevity of coal waste as a PRB material for the removal of Cd considering subsurface environmental conditions such as contamination level and groundwater velocity. The artificial groundwater contaminated by Cd were prepared with various concentrations ranging from 10 to 100 mg L-1. Lab-scale column experiments were conducted using coal waste filled columns by injecting the artificial groundwater. The breakthrough curves were analyzed advection dispersion equation coupled with equilibrium sorption model to determine the retardation factor. The Cd breakthrough curves exhibited different retardation with respect to the contamination levels. The Cd transport was more retarded as the contamination level lowered. The relationship between the retardation factor and the contamination levels could be explained with empirical equations based on non-linear sorption isotherms. By adopting the velocity dependency of sorbent performance in our previous study, transport of Cd within coal waste was simulated under various subsurface environmental conditions to construct the longevity function. The function could be used for the longevity prediction of coal waste as a PRB material considering groundwater velocity and contamination level in subsurface environment.

Longevity evaluation of non-pumping reactive wells for control of groundwater contamination: Application of upscaling methods.

Non-pumping reactive wells (NPRWs) are subsurface structures used for the passive treatment of contaminated groundwater using wells containing reactive media. In the vicinity of NPRWs, a combination of hydrogeological and chemical processes makes it difficult to predict their longevity. In this study, we evaluated the longevity of NPRWs using the upscaling methods. A horizontal two-dimensional sandbox was constructed to mimic the hydrogeological and chemical processes in a single unit of NPRW (unit NPRW). The groundwater flow and solute transport were simulated numerically to validate the processes of contaminant spreading prevention in the sandbox. Dye tracing and arsenic transport tests showed different performance of NPRW due to induced flow and uneven consumption of reactivity, which is dependent on the pathway length and residence time of the coal waste. Through numerical modeling of the experiments, the fate-related processes of contamination around NPRW were described in detail in both spatial and temporal terms. The stepwise approach of the upscaling methods was used to predict the contamination-blocking performance of the entire facility based on the reactivity of the materials and the contamination removal of the unit NPRW.

Longevity of coal waste for controlling cadmium-contaminated groundwater considering groundwater velocity.

Coal waste composed of naturally occurring minerals is applicable as a reactive medium to permeable reactive barriers due to its reactivity to heavy metals. In this study, we evaluated the longevity of coal waste as PRB media to control heavy metal-contaminated groundwater considering variable groundwater velocity. Breakthrough experiments were conducted using coal waste-filled column by injecting artificial groundwater, 10 mg/L of cadmium solution. The artificial groundwater was fed to the column at different flow rates to mimic a wide range of porewater velocities in the saturated zone. The reaction between cadmium breakthrough curves was analyzed using a two-site nonequilibrium sorption model. The cadmium breakthrough curves showed a significant retardation, which increased with decreasing porewater velocity. The greater the retardation, the longer the longevity of coal waste could be expected. The greater retardation under a slower velocity environment was due to the higher fraction of equilibrium reaction. The nonequilibrium reaction parameters could be functionalized with respect to the porewater velocity. The simulation of contaminant transport using the reaction parameters could be used as a method to evaluate the longevity of the pollution-blocking material in an underground environment.

Injection strategy for effective bacterial delivery in bioaugmentation scheme by controlling ionic strength and pore-water velocity.

For the in-situ remediation of the contaminated subsurface environment, the injection of nutrients and microorganisms changes chemical and physical conditions, which control the delivery and immobilization of microorganisms. We investigated the injection strategy for effective bacterial delivery in a bioaugmentation scheme by controlling ionic strength (IS) and pore-water velocity (v). A set of bacterial transport tests was conducted using the saturated sand column to mimic the saturated subsurface environment. The effectiveness of the injection strategies was evaluated by applying solutions with different ionic strengths into the sand column with different pore-water velocities. The deposition and delivery of bacteria through the sand column were analyzed using the first-order deposition model. The deposition and delivery of bacteria injected by various strategies were numerically simulated considering the variable deposition rate. The breakthrough curves from column experiments revealed that the bacterial deposition on the sand surface was increased by an increase in the ionic strength and by a decrease in the pore-water velocities. The rates of bacterial deposition (k1) on sand could be determined as a function of ionic strength and pore-water velocity, and it was applicable to simulate the delivery of bacteria under dynamic groundwater conditions. The numerical case study considering various injection strategies showed that the nutrient concentration controlled the bacterial delivery to the target area more significantly than the injection flow rate. Injection of bacterial solution with lower nutrient concentration could be increased the deposited bacterial concentration at the target point (Stp) by 6.2-7.1 times higher. Short pulse injection with a high injection rate decreased Stp by 67-78%. The efficiency of bacterial delivery (Ed) could be increased three times higher by lowering nutrient concentration in the injection solution. The process of evaluating the efficiency of bacterial delivery could be a prominent approach to determining the injection strategy for in-situ remediation considering variable conditions of a contaminated site.