Environmental Engineering Reference
In-Depth Information
Thus, the metallic ions were held to the NPL surface through ion exchange
process.
The Cd
2+
adsorption onto pure and thioglycolic acid treated cassava tuber
bark wastes (CTBW) was investigated by Horsfall Jr
et al.
[59]. The results
suggested that the adsorption process was exothermic, the rate limiting
sorption step was physisorption. The adsorption rates could be described by a
pseudo second order model. The adsorption rate was fast and the
thermodynamic equilibrium was reached in less than 30min. They observed
that the monolayer capacity ranged from 5.9 to 26.3 mg g
-1
. The presence of
thioglycolic acid is related to the incorporation of -SH group [60]. Thus, -SH
functional group was the main responsible for the Cd(II) sorption onto cassava
waste.
Banana peel, a common waste, was examined as adsorbent for the
Cd
2+
removal from environmental and industrial wastewater by Memon
et al.
[61]. The adsorption process of the metallic ions was found to be pH
dependent with the optimal sorption occurring at pH 8. The adsorption
capacity of the banana peel was 35.5 mg g
-1
. Anwar
et al
also studied the
properties of the banana peels as an adsorbent for the Cd
2+
removal [62]. The
maximum adsorption capacity of banana peels, termed as Langmuir complete
monolayer coverage, indicated that 1 g of banana peels can absorb 5.71 mg of
cadmium ions.
Orange peel was used for Cd
2+
adsorption by Li
et al.
[63]. They studied
the effects of different chemical modifications on the adsorbent properties.
Among several chemical treatments, they studied alkali and acid modification,
after the saponifiction with NaOH. Biosorption equilibriums were quickly
established in about 60 min and the adsorption kinetics followed the Lagergren
first order model. According to experimental data, the the Cd
2+
maximum
adsorption capacity of the orange peel was dependent on the chemical
treatment, by the following the order: oxalic acid > citric acid > phosphoric
acid > NaOH > NH
4
OH > Ca(OH)
2
> orange peel.
Schiewer and Patil [64] investigated the removal of Cd
2+
by pectin fruit
wastes (lemon, orange, apple and grapes). Grape skins showed the highest
metal uptake capacity and it was equal to 269.8 mg g
-1
, followed by lemon and
orange peels. The apple core and peels as well as grape skin disintegrated in
Cd
2+
aqueous solution at pH 5 after 2 h of contact. Thus, citrus peels (orange,
lemon and grapefruit peels) were found to be more suitable than apple residues
and grape skins for biosorption of cadmium by pectin-rich fruit materials, due
to the higher stability of the citrus peels in combination with a good uptake.
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