Contaminant Mass Transfer from NAPLs to Water Studied in a Continuously Stirred Flow-Through Reactor

TLDR: In this article, small-scale experimental studies on NAPL-water mass transfer were performed in a dynamic system mimicking environmental conditions with "clean" water continuously flowing through the nonaqueous phase liquid pool.

Abstract: The release of nonaqueous-phase liquids (NAPLs) from porous media to groundwater is a widespread environmental problem. The mass transfer of individual NAPL components controls both the extent of groundwater contamination and the persistence of the residual NAPL phase. In order to quantify this key process, small-scale experimental studies on NAPL-water mass transfer were performed in a dynamic system mimicking environmental conditions with “clean” water continuously flowing through the NAPL pool. To describe this process, a modified simulation method was developed and validated by the experimental data. The experimental system consisted of a custom-designed flow cell (with NAPL and water) connected to the peripheral equipment (e.g., pump, water source). This continuously stirred flow-through reactor was used to perform mass transfer experiments with simple and complex model NAPL–water systems. To simulate the experimental data (concentration versus time profiles of individual NAPL compounds), an ...

Figures

Fig. 4. Experimental and simulated (γi ¼ constant χni ) concentration profiles for the multicomponent NAPL; solid lines show the simulated concentrations and the dots represent the experimental data

Fig. 3. Experimental and simulation concentration profiles for the multicomponent model NAPL with the assumption that all values of γi were unities (γi ¼ constant χni ¼ 1); solid lines show the simulated concentrations and the dots represent the experimental data; modeled concentrations of phenols are much lower than those measured, and both naphthalene and 2, 3–benzofuran obtained higher simulation results

Frequently asked questions

Q1. What are the contributions mentioned in the paper "Contaminant mass transfer from napls to water studied in a continuously stirred flow-through reactor" ?

Ghoshal et al. this paper used a modified simulation method to describe the NAPL-water mass transfer process. 

Q2. What was the concentration of phenanthrene in the single-solute experiments?

In the single-solute experiments, phenanthrene was used as the probe compound in the NAPL phase at a concentration of 3 g in 250 mL toluene. 

Q3. Why was toluene chosen as the bulk component of the NAPL?

Toluene was chosen as the bulk component of the NAPL because its solubility properties are versatile for organiccompounds and because of its aromatic nature (Mahjoub et al. 

Q4. What is the important assumption in the study of the dissolution of NAPLs?

Most existing models were established based on the assumption that mass transfer rates inside the NAPL are faster than the dispersive transport from the interface, i.e., the aqueous boundary layer governs the dissolution process. 

Q5. What is the way to perform mass transfer experiments for NAPL–water systems?

The custom-designed continuously stirred flow-through reactor system can be used to perform mass transfer experiments for NAPL–water systems under different conditions. 

Q6. Why did they behave ideally in this system?

Phenoxathiin and PAHs behaved ideally in this system because their molecular properties are very similar to those of the solvent. 

Q7. What are the main objectives of this work?

The main objectives of this work are (1) to design a versatile experimental set-up to determine mass transfer coefficients under different hydromechanical and physicochemical conditions, (2) to test the applicability of Raoult′s law for complex NAPL mixtures, and (3) to establish a numerical model that describes the mass transfer process and quantifies the related parameters. 

Q8. What is the motivation for this study?

This study is motivated by the lack of understanding of the effects of molecular structure and distinctly different physicochemical properties on mass transfer rates, as well as the applicability of Raoult’s law for such complex systems. 

Q9. What are the properties of the solutes with functional groups?

For solutes with functional groups, their specific properties, e.g., polarity and more diverse intramolecular interactions, obviously contribute to the mass transfer processes in this system. 

Q10. What is the kinetics of the dissolution of aqueous phase liquids?

As groundwater flows passed trapped NAPL ganglia or pools, a fraction of the NAPL dissolves in the aqueous phase and creates a dissolved plume of hydrocarbons. 

Q11. What is the mass loss of naphthalene in water?

The sampling interval was increased to 10 min within the first 2 h, and later extended to 20, 30, and 60 min every hour, resulting in a total of 18 aqueous samples in 8 h.To test the mass loss due to volatilization during sampling, a vial containing a 5 mL naphthalene solution with a concentration similar to that of the aqueous samples was placed inside the draft cupboard synchronously and sealed after the sampling time period. 

Q12. What is the effect of the polarity of the aromatic alcohols on the i?

for different phenol solutes, values for γi increased with increasing polarities in the dilute solutions because a CH2 group and aromatic ring moiety decrease polarity. 

Q13. Why was phenoxathiin chosen as an alternative to phenanthrene?

because of its similar molecular structure and physicochemical properties, phenoxathiin was chosen as an alternative to phenanthrene to validate the experimental and modeling procedures. 

Q14. What is the role of the nonaqueous phase liquids in the formation of groundwater?

Aquifers and soils in industrialized areas are often contaminated by nonaqueous phase liquids (NAPLs), which are long-term sources of groundwater plumes (Eberhardt et al. 

Q15. What is the mass transfer coefficient of the NAPL?

A few studies have suggested that slow diffusion of solutes within the NAPL or at the NAPL-side boundary layer of the interface may also limit rates of mass transfer (Ghoshal et al. 2004; Ortiz et al. 1999). 

Q16. How many steps were automatically adjusted to split the time until depletion of the soluble?

The time step was automatically adjusted to split the time until depletion of the most soluble component into a sufficient number of steps (¼ 100) until that particular compound is totally depleted.