Answer to Question #14359 Submitted to "Ask the Experts"

I work at a research reactor at which we have an iodine monitoring system connected to the stack chimney. The monitoring system is a sodium iodide (NaI) detector that is placed in front of a 5.7 cm diameter x 2.5 cm thick impregnated triethylenediamine (TEDA) cartridge charcoal filter. My question is about the optimal use of this filter. I am curious about the periodicity of changing the filter and if it can be reused after a decay period if it is contaminated. In our case we replace the filter once per week even it is not contaminated, and the filters removed are considered as waste even if they are not contaminated, which seems to be a loss.

TEDA impregnated charcoal works to filter and trap radioiodines from a stack by physical adsorption of radioiodines onto the surface of the charcoal, followed by chemisorption onto the TEDA.

Charcoal filters like these degrade in their ability to capture radioiodines over time, largely due to the oxidation of the activated carbon surface (converting the carbon to CO or CO₂), desorption of impurities, and adsorption of various other substances which may exist in the inflowing air. These mechanisms remove available adsorption sites for the radioiodines, resulting in a loss of filter efficiency. Additionally, environmental factors (including, but not limited to, temperature, relative humidity/dew point, and pH of the incoming stack air) may cause degradation of the filter media, leading to a decrease in capture efficiency or an increase in bypass pathways for the filter media.

Even given that, many operating facilities do reuse TEDA charcoal filters for longer than a one-week period. To justify reuse of the filter, I recommend that some testing of the filter system be performed to establish an expected service life under the operating conditions specific to your facility. Provided that a filter is not found to be contaminated during your normal checks, this could justify leaving the filter in place until its expected service life expires.

Additionally, many facilities perform an in-line penetration test using methyl iodide, allowing the measurement of the capture efficiency of the in-use filter under operating conditions. This could extend the lifetime of your filter past an established service life but may require some design modifications to ensure it is possible to introduce the penetration test material and measure the filter performance in-situ. Testing and reuse of these filters may be worthwhile for large charcoal filtration systems credited with safety-related functions (e.g., those used to filter radioiodines from effluent). However, in my opinion, this testing and reuse is not typically economically justified for smaller filters used for environmental applications (e.g., those used to collect representative effluent samples) as you have described.  

Daniel Hunton, P. Eng