Environmental Engineering Reference
In-Depth Information
It is critical in either case to understand and evaluate the adsorption interac-
tions in the context of drinking water treatment systems. Activated carbons are
prepared from different precursors and used in a wide range of industries. Their
preparation, structure and applications were reviewed in different topics and re-
views. High BET surface area and light weights are the main advantages of ac-
tivated carbons. Usually activated carbons have a wide range of pore sizes from
micropores to macropores, which shows a marked contrast to the definite, pore
size of zeolites [41, 46].
Previous researches showed that one of the most effective approaches for in-
creasing mesopore volume of activated carbon is to catalyze the steam activa-
tion process by using transition metals or rare earth metal compounds, which
can promisingly promote mesopore formation. The mechanism of mesoporous
development is that the activation reaction takes place in the vicinity of metal/
oxide particles, leading to the formation of mesopores by pitting holes into the
carbon matrix. However, transition metals or rare earth metal compounds are still
expensive and can seldom be reused on large scale, which limits industrial appli-
cation of these processes. OH plays three roles in the preparation of the activated
carbons. First of all, it guarantees that the carbonization of coal is in solid phase.
Besides, KOH can react with coal to form micropores, which provide “activation
path” for steam, which can expand the micropore. However, if there are too much
micropores, combination of micropores will happen, resulting in the formation
of much macropore, and then can increase the ignition loss, so there must be an
optimal addition of KOH in the precursor. Furthermore, KOH catalyzes the steam
activation process. Because of high catalytic activity of KOH, acid washing pro-
cess is used after carbonization process and before steam activation process to
vary the content of K-containing compounds left in the char, and then the degree
of steam activation can probably be varied. Consequently, regulation of the pore
size distribution of activated carbon from coal is performed in two ways: the first
is to change the addition of KOH in the precursor to produce optimal micropore in
the char, and the second is to use acid washing process to vary the degree of cata-
lytic effect of K-containing compounds in steam activation. Modifying the steam
activation method successfully regulated the pore size distributions of coal-based
activated carbons. The principles for the regulation of pore size distribution in the
activated carbons were also discussed in this article, and we found that regulation
of the pore size distributions of the activated carbons from coal are performed in
two ways: the first is to change the addition of KOH in the precursor to produce
optimal micropore in the char, and the second is to use the acid washing process
to vary the degree of catalytic effect of K-containing compounds in steam activa-
tion [35, 40].
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