Environmental Engineering Reference
In-Depth Information
Chemisorption media are produced by impregnating activated carbon,
activated aluminum, silica gel, etc., with catalysts that include bromine,
metal oxides, elemental sulfur, iodine, potassium iodide, and sodium sul-
fide. Bromine-impregnated activated carbons are used to chemically sorb
ethylene, which reacts catalytically to produce ethylene bromide, a sorbable
gas. Carbons impregnated with metallic oxides are used to oxidize H
2
S
under low O
2
conditions. Elemental sulfur- and iodine-impregnated acti-
vated carbons can be used to sorb mercury by producing stable mercuric
sulfide and mercuric iodide, respectively. Such impregnated carbons can be
used to reduce exposure to mercury associated with accidental spills of
mercury and elemental mercury-releasing compounds. Sodium sulfide and
other impregnants have been used to remove HCHO and have been used
in residential applications.
Activated alumina impregnated with potassium permanganate
(KMnO
4
) has been widely used in industrial and commercial air cleaning
systems and has seen limited residential use. Though activated alumina does
not have the sorptive capacity of activated carbons, it can be used effectively
for the control of low-molecular-weight gases, such as HCHO and C
2
H
2
, and
odors in buildings. Organic vapors are sorbed on the surface of the activated
alumina where they are oxidized by KMnO
4
in a thin film of water. If
oxidation is complete, CO
2
and H
2
O vapor will be produced as by-products.
If incomplete, it has the potential to produce a variety of oxidized com-
pounds such as aldehydes and ketones, and in the case of halogens, sub-
stances such as hydrogen chloride (HCl). KMnO
4
-impregnated activated
alumina is used commercially to remove C
2
H
2
from fruit storage facilities.
On use, the pink-colored KMnO
4
is reduced to the brown-colored KMnO
2
,
manganese oxide.
D. Performance studies
Performance evaluations of air cleaning systems in nonindustrial, nonresi-
dential buildings, and residential environments have been limited. Several
studies have been conducted to evaluate the performance of thin-bed acti-
vated alumina filters on HCHO in residences. In a study using a portable
cleaner with an air flow of 130 CFM (61 L/s), HCHO levels were reduced
by 25 to 30% in a mobile home and 35 to 45% in a urea-formaldehyde-foam-
insulated (UFFI) house. In another study using a cleaner with an 1800 CFM
(849 L/s) flow rate, HCHO levels were reduced on the order of 75%. A study
of thin-bed activated carbon filters showed that they were effective (on the
order of 80%) in removing VOCs in the low ppbv range found in buildings,
with a projected service life of 60 to 466 days. The effectiveness of dry-
processed carbon composite-based filters has been evaluated under labora-
tory conditions. Such filters have been reported to have 10 times the adsorp-
tion capacity of large mesh activated carbon, but they apparently have much
shorter service lives.
