Granular Activated
Carbon
(GAC)
Adsorption
Efficiency of GAC
The trace organic compounds that can be
removed by GAC are usually present at µg/liter quantities or less.
Since GAC is used in conjunction with
other water treatment processes, the effect of pretreatments for removing trace
organic compounds and their precursors were examined in depth.
Hence, the following questions were
addressed:
1.
How efficiently does GAC adsorb individual trace organic compounds,
particularly those of concern to health?
2.
When processes such as coagulation, sedimentation, filtration, aeration,
disinfection, oxidation, and PAC adsorption precede GAC adsorption, how is the
efficiency of the GAC affected?
3.
Can water that has been treated by GAC be disinfected more or less easily than
water that has not been treated by GAC?
4.
What is the potential for effectively using PAC to remove organics?
5.
What reactions take place between oxidants that are applied as pre-disinfectants
and the activated carbon or the compounds that are adsorbed on the activated
carbon? Do these reactions result in potentially hazardous compounds that would
not be present if activated carbon were not used?
6.
To what extent does competitive adsorption between trace organics with
potential health effects and the large concentrations of background organics,
generally characterized as humic substances, influence the effectiveness of
GAC?
7.
To what extent does competitive adsorption among similar concentrations of
trace organics with potential health effects influence the effectiveness of
GAC?
8.
How significant is the effect of competitive adsorption when it is compared to
the effect of the re-equilibration that is produced by the variable nature of
the composition and concentration of trace organics in the feedwater to the GAC
bed?
Removal of Selected Organic
Compounds
Adsorption isotherms and small column
studies that are performed in the laboratory using GAC are useful tools that
have been developed to describe how specific organic chemicals can be removed
in large-scale GAC applications.
A considerable amount of adsorption
research describing the affinities of pure compounds for the activated carbon
surface has been reported in the literature during the last 15 years.
Improved analytical tools have made it
possible both to detect the organics at trace levels in the environment and to
follow their removals in adsorption studies in the laboratory.
This section of the chapter evaluates
the efficiency of GAC adsorption of individual trace organic compounds,
particularly those with potential health effects.
Removals of organic chemicals are
discussed in the literature on the basis of laboratory and pilot-scale studies
and large-scale applications.
Laboratory studies are by far the most
useful for describing specific organic removals since environmental factors can
be more carefully controlled in them than in field evaluations.
The problem of competitive adsorption
is significant when environmental samples are used in experiments in which
specific organic compounds are removed by adsorption.
Adsorption data obtained in the
laboratory are normally reported as percent removed, adsorption isotherms,
kinetics of adsorption, and the results of small-scale column studies.
In the following sections, these data
are reviewed and the utility of each method is evaluated.
Percent Removals
Giusti et al. (1974) made extensive use of percent
reduction as a measure of the effectiveness of activated carbon for removing
organic chemicals.
They added 93 petrochemicals
individually at one level to one type of activated carbon and used the
subsequent calculated percent reductions to test several hypotheses concerning
the removal of different classes of organics by activated carbon.
There are several problems associated
with using percent removal data exclusively to describe how well a particular
organic compound is removed from water.
The single value study results in a
single point on an isotherm.
Unfortunately, this single point gives
no indication of how capacity varies with concentration, i.e., by the isotherm
slope and shape.
To be truly representative, the amounts
of adsorbed compound per gram of carbon for individual organic compounds must
be compared on an equal equilibrium concentration basis, which is not possible
if only a single percent removal value is available.
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