ELECTROCHEMICAL TREATMENT OF GASEOUS POLLUTANTS
As discussed in Chapter 1, the composition of the atmosphere has changed
by the incorporation of trace level constituents from industrial operations, automobile
exhaust, and other anthropogenic and non-anthropogenic sources (e.g.,
volcano eruptions). Currently used methods for the treatment of gases include
the following:
(1) Physical methods" Gases are adsorbed on solid surfaces (e.g., activated
carbon), selectively filtered, electrostatically precipitated, selectively dissolved,
and so forth.
(2) Chemical methods: They are incinerated, dissolved in reactive solutions
(e.g., oxidation in a C10- solution, neutralization of an acid gas stream in a
basic solution), adsorbed through chemical reactions (e.g., NH 3 complexation
of Cu 2+ adsorbed on activated carbon), and the like.
(3) Biological methods: Generally speaking, gases are easier to biodegrade
than liquids or solids due to their molecular dispersion. Several microbial
processes have been utilized in biofiltration for pollutant gas treatment;
these include a large amount of rapidly or slowly biodegradable, volatile
organic, or inorganic compounds (VIC). For example, for H2S remediation
the following have been utilized: Thiobacillus thioparus, Beggiatoa, Thiobacillus
denitrificans, Chlorobium versutus, Thiobacillus neopolitanus, Thiobacillus
thiooxidans, and others. 2~176176
(4) Electrochemical methods: Polluting gases must generally be transferred
by absorption or reaction to the liquid phase (normally aqueous solution)
before they can undergo electrochemical oxidation or reduction. This conversion
can be effected in two absorption modes: the gas is directly absorbed
in an electrochemical cell for treatment (inner-cell process), or the
gas is absorbed in a separate reservoir and then transferred to the electrochemical
cell (outer-cell process). 2~176 Subsequently, the dissolved pollutant
can undergo different reactions (see Figure 5.13), TM such as homogeneous
electron transfer from a dissolved catalyst, C § which can then be (solv) I
regenerated either by heterogeneous electron transfer at an anode (path 1)
or by homogeneous electron transfer to a dissolved metal ion, M ~+~)+ (path
2). Another reaction path (path 3) involves complexation of the dissolved
pollutant followed by oxidation of the formed complex, (Red M) ~+ with
C + to produce an innocuous (or at least less polluting) substance, O with (solv)
the concomitant regeneration of M~+" C+~solv) is then regenerated from C~olv)
at the anode of the electrochemical cell. Examples for the choice of M ~§
include complex-forming transition metal ions (e.g., Cu 2+, Pd 2+) and for the
couple C+/C, include redox couples with high standard reduction potentials;
for example, ag 2+/+, Co 3+/2+, Br2/Br, Cr2Ov2/Cr 3+, VO2+/VO 2+, or MnO4- /
MnO 2
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