ADSORBENTS FOR CD REMOVAL FROM WATER WASTE: A SHORT REVIEW OF THE RECENT DEVELOPMENT - Management of Hazardous Residues Containing Cd - page 15

The adsorption of cadmium ion from aqueous solution by ground wheat
stems, an agricultural waste, was studied by Tan and Xiao [89]. The chemical
composition of wheat stems is: cellulose, hemicellulose and lignin. The
presence of these three biological polymers causes wheat stems rich in
hydroxyl and phenolic groups and these groups can be used to remove metallic
ions from aqueous solutions. The authors found that the cadmium adsorption
process by ground wheat stems was fast and pH-dependent. The maximum
adsorption capacity was 12 mg g -1 . An esterification process of ground wheat
stems decreased the cadmium binding, whereas a hydrolysis process caused a
significant increase in the binding capacity. These effects are related to the
amount of carboxylate groups (COO - ), which decreased in the esterified
sample and greatly increased in the hydrolyzed one. Thus, the carboxylate
groups played an important role in the cadmium binding.
The Biosorption of Cd 2+ by wheat straw was studied by Dang et al. [90].
They used Cd 2+ aqueous solutions with concentration equal to 50 mg L -1 and
observed that about 80% of the metallic ions were removed within 2.5 h of
biosorption, whereas 87% was removed after about 3.5 h. They considered that
wheat straw is a good candidate for Cd 2+ removal from water effluents.
The ability of sugarcane bagasse to adsorb Cd 2+ from aqueous solutions
has been investigated by Ibrahim et al. [91]. Sugarcane bagasse is mainly
constituted of cellulose (40-50%), polyoses (25-30%) and lignin (20-25%)
[92] The maximum adsorption capacity was found to be 6.8 mg g -1 at 30ºC
[91]. Karnitz Júnior et al. [93] has recently reported the use of modified
sugarcane bagasse with succinic anhydride (MSB 2) for removing Cu(II),
Cd(II) and Pb(II) from aqueous solutions. The authors have reported that the
hydroxyl and phenolic groups in sugarcane bagasse could be easily converted
to carboxylic groups by using succinic anhydride. MSB 2 exhibited the
maximum adsorption capacity of 196 mg g -1 for Cd 2+ according to Langmuir
model. Gurgel et al. [94] also reported the use of the mercerized sugarcane
bagasse, but modified and non-modified with succinic anhydride for removing
Cd 2+ from aqueous solutions. The authors reported the effect of mercerization
in the increase of the fibers specific surface area and in the reactivity of the
mercerized sugarcane bagasse. The mercerization process makes the hydroxyl
groups of the cellulose macromolecules more accessible for modification with
succinic anhydride. Experiments with twice mercerized sugarcane bagasse
(MMSCB 1) exhibited an increase in the mass percent of 49.2 % in relation to
the modified non-mercerized (SCB 1). They also observed that the
concentration of carboxylic groups in the sample MMSCB 1 was 0.9 mmol g -1 .
SCB 2 and MMSCB 2 were obtained by treating MMSCB 1 and SCB 1 with