Filtration and Sampling of Aerosols by Fibrous Filters - Aerosols: Measurement, Dosimetry, and Health Effects - page 466

TABLE 17.5
Characteristics of Some Analytical Filters for Capture of Gaseous Compounds

Effectiveness of Gaseous
Substances Capture at
Velocity up to 5 cm/s

Resistance to Air
Flow (Pa), at
Velocity of 1 cm/s

Analytical
Filter

Sorbent Application Field

AFAS-I Carbon OU-A

with silver nitrate

90-99 50 Iodine

AFAS-P Carbon OU-A 99 60 Polonium

AFAS-R Carbon OU-A

with iodine

99 50 Mercury

AFAS-U Carbon SKT 99 60 Low-volatile

substances

Source: Budyka, A.K. and Borisov, N.B., Fibrous Filters for Air Pollution Control , IzdAT, 360 p., 2008.

ibrous ilters. Finally, that is a simple way of analyzing captured substances. Sorption analytical
AFAS ilters are manufactured on the base of SFM materials that are intended for gaseous com-
pounds of some hazardous substances. Characteristics of some analytical AFAS ilters are given in
Table 17.5.
It is worth noting that gaseous compounds of the same chemical elements are adsorbed in a dif-
ferent way. For example, radioactive iodine can be found inside both easily adsorbed (I 2 ) and poorly
adsorbed (CH 3 I and other) compounds, which is expressed in differing of their dynamic sorption
factors and effectiveness of their capture. Several identical analytical AFAS ilters are to be placed
after the aerosol analytical ilter to separate volatile iodine compounds with different sorption prop-
erties. By a distribution character of iodine in such a package one can determine not only phase
composition of iodine but also correlation between easily adsorbed and poorly adsorbed gaseous
phase fractions [138].
SFM layers are placed after aerosol ilters. Depending on analyzed substance SFMs are used
with different sorbents. SFM application considerably simpliies conducting of monitoring. SFM
mat cushion enables preparing the samples for measuring in a form of compact pellets as in case
of ilters.

17.12.1 c orrection For v olatile s ubstances d esorPtion
Volatile substances after sedimentation on an aerosol particle can get desorbed from its surface.
Time period of such substances holding on a particle is determined by desorption probability (λ des ).
If that time period is comparable to the duration of sampling then there is a secondary redistribu-
tion of a substance between the capturing stages of a sampling device. Thus experimental values of
gaseous substance portion may be considerably overstated as compared to the real ones. In the inal
effect it leads to incorrect dose evaluations.
Unfortunately, it is not possible to calculate the value of λ des , but it is possible to estimate its
value judging from experimental data. The bulk of data is available for radioactive 131 I—product
of Chernobyl Atomic Power Station disaster. Using a simple model of iodine entrainment from the
aerosol particle, one succeeds in determining with the help of nonlinear regression analysis that
λ des = 0.13/h [139].
It goes from the received value of desorption probability that within a characteristic time t 1/2 des =
−ln2/λ des , which equals approximately 5.3 h, half of the radioactive iodine is transferred from
the aerosol ilter to the second stage of a sampling device.