Environmental Engineering Reference
In-Depth Information
Mohamed [47] modified the rice husk using phosphoric acid as activating
agent. They impregnated the rice husk in phosphoric acid at 353 K over-
night and then activated the impregnated sample in a reactor at 773 K for
2.5 h. Although the resultant activated carbon had an almost high surface
area and total pore volume ( S BET = 352 m 2 . g -1 and 0.42 ml.g -1 , respectively)
with a mean pore radius size of 2.4 nm, its adsorption capacities for acid
blue and acid yellow are only 38 mg.g -1 and 10 mg.g -1 , respectively. Since
no information is available about the surface functionality of the activated
carbon, this low adsorption capacity cannot be explained. In a study by
Rahman et al. [48], rice husk was treated with both an acid and a base
and the removal percentage of malachite green dye was compared for
these samples. Acid-activation was performed by impregnating the rice
husk with 10% and 20% phosphoric acid and subsequent carbonization
at three different temperatures, namely 400 C, 500 C and 650 C. Sodium
hydroxide was used for base-activation, where 10% NaOH was mixed with
rice husk at 30 C and 100 C for an hour. Then the sodium hydroxide was
washed out and the resulting material was carbonized at 500 C. Since mass
loss of all the samples remained constant after 30 min, it was assumed to be
the optimum carbonization time. It was shown that base-activation led to
the removal of silica from the rice husk while acid-activation left the silica
content unchanged. The adsorption capacity of the acid-treated material
was the highest when it was carbonized at 500 C. This behavior was attrib-
uted to the pore blockage at lower temperatures due to the decomposition
of the organic constituents and higher silica content at high temperatures.
The adsorption experiments showed that acid-activated rice husk had a
higher removal efficiency compared to the base-activated rice husk.
A comprehensive study on the production of activated carbon from rice
husk and its application for malachite green and Rhodamine B removal
was conducted by Guo et al. [49,50]. The rice husk was carbonized at
450 C under nitrogen atmosphere and the resultant material was soaked
in a caustic solution and was further heated for an hour at 400 C. Then
the temperature was raised to 650 C, 700 C and 750 C for activation. The
activation process altered the surface area of the rice husk considerably.
Depending on the activation time and temperature and the activating chem-
ical reagent used, the surface areas of the activated samples ranged from
1400 to 2700  m 2 · g -1 with significantly high pore volumes (see Table 5.1).
The adsorption experiments revealed very promising adsorption capacities
(above 550 mg · g -1 for malachite green and 450 mg · g -1 for Rhodamine  B)
for the prepared activated carbons at a high adsorption rate (equilibrium
time, 90 min). It has been shown that as the temperature increases, the
adsorption amount also increases, which was attributed to the increase in
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