Estimation of membrane diffusion coefficients and equilibration times for low-density polyethylene passive diffusion...

Area of Science:

• Environmental Science
• Analytical Chemistry
• Geochemistry

Background:

• Passive diffusion (PD) samplers provide technical and cost advantages for groundwater monitoring.
• They are particularly useful for measuring dissolved gases and volatile organic compounds (VOCs) at contaminated sites.

Purpose of the Study:

• To determine the diffusion coefficients for various gases and VOCs in low-density polyethylene membranes.
• To predict PD sampler equilibration times based on sampler dimensions and diffusion coefficients.
• To develop and validate a numerical model for assessing PD sampler concentration "lag time".

Main Methods:

• Measured diffusion coefficients (Dm) for He, Ne, H2, O2, and N2 in laboratory experiments.
• Estimated Dm values for common VOCs using existing empirical data.
• Developed a numerical model to evaluate concentration "lag time" for temporally variable in situ concentrations.

Main Results:

• Measured Dm values for gases ranged from 1.1 to 1.9 x 10(-7) cm2 sec(-1) at 21°C.
• Estimated Dm values for VOCs ranged from 1.7 to 4.4 x 10(-7) cm2 sec(-1) at 21°C.
• The numerical model accurately predicted lag time in laboratory experiments.

Conclusions:

• PD sampler equilibration time is influenced by sampler dimensions, membrane thickness, and analyte diffusion coefficients.
• The developed numerical model can predict lag time and assess the suitability of PD samplers for various field conditions.
• Appropriately designed PD samplers can provide near-instantaneous measurements of in situ groundwater concentrations.