Environmental Engineering Reference
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to be 8 mg g -1 . Similar results were found by Ismail et al. [77] for Cd 2+
removal onto powdered corncob. Corncob modifications can enhance their
natural ion-exchange capability, increasing its value. Recently, Leyva-Ramos
et al. [78] investigated the Cd 2+ adsorption onto natural and oxidized corncob.
They treated corncob with citric acid (CA) and nitric acid (NA). The
concentration of carboxylic sites increases in 15.1 and 4 times after CA and
NA treatments, respectively, in relation to natural corncob. Moreover, the
adsorption capacity of natural corncob at pH 6 increased from 5 mg g -1 to 55
and 19 mg g -1 when oxidized it was treated with CA and NA, respectively.
This showed that the adsorption of Cd 2+ depends considerably on the number
of carboxylic sites present on the surface of the corncob. Vaughan et al . [79]
also reported that the amount of Cd 2+ adsorbed by corncobs was increased by
actions of both citric (CA) and phosphoric acids (PA). In this case, the
corncobs were washed with NaOH and water before being modified with CA
or PA. The adsorption capacity of unmodified corncob at pH 4.8 increased
from 13 mg g -1
to 99 and 77 mg g -1 when oxidized with CA and PA,
respectively.
Raw and acrylonitrile-modified corn stalks were used for Cd 2+ removal
[80]. The adsorption capacity of the corn stalk increased from 3.39 to 12.73
mg g -1 after the modification. The difference in the extent of Cd 2+ removal
between these adsorbents is due to the higher concentration of cyan functional
group, larger surface area and larger pore size in acrylonitrile-modified corn
stalk than raw corn stalk.
Li-yuan et al. [81] investigated the effect of the chemical treatments in
bio-formulation, an agricultural waste, in Cd 2+ removal capacity. They found
that the treatment with Ca(OH) 2 greatly enhanced Cd 2+ adsorption (99%),
whereas bio-formulation treated by HCl decreased the adsorption (76%), in
comparison to the untreated material (86%). The removal rate of Cd 2+ is
slightly dependent on the pH value over a wide range of 4-11. The adsorption
process took place very quickly, being almost complete after 30 min of
interaction. The adsorption mechanism is based on a cation-exchange process,
changing Ca 2+ and Mg 2+ present on the surface of BF by Cd 2+ .
A study on the removal of Cd 2+ from aqueous solution by acid
formaldehyde pretreated chestnut shell was conducted in batch conditions by
Vázquez et al. [82]. The Cd 2+ ions adsorption process by chestnut shell
followed pseudo second order kinetic model. The results showed that certain
functional groups including ether, alcoholic and amino groups, were involved
in the adsorption process.
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