ADSORBENTS FOR CD REMOVAL FROM WATER WASTE: A SHORT REVIEW OF THE RECENT DEVELOPMENT - Management of Hazardous Residues Containing Cd - page 11

The monolayer capacity of lemon, orange and grapefruit peels were 209.1,
150.6 and 110.2 mg g -1 , respectively.
Biosorption by cheap and abundant available materials, such as citrus
peels, can be a low cost efficient method for removing heavy metals from
wastewater [65]. Citrus peels, native orange peels (NP), protonated peels
(PrP), depectinated peels (DP) and extracted pectin acid (PP) were studied in
order to clear the mechanism of pectin-metal binding. In all experiments the
thermodynamic equilibrium was achieved in 1h and the metal binding strength
decreased in the order PP > PrP >NP > DP. These results confirmed that pectin
has a higher capacity and affinity for Cd 2+ than other peel constituents. The
maximum Cd 2+ uptake for the pectin acid (642.98 mg g -1 ) was about twice the
capacity of the depectinated peels (393.43 mg g -1 ).
The mango peel waste, an adsorbent for the Cd 2+ removal, showed the
maximum adsorption capacity equal to 68.92 mg g -1 [66]. The kinetics of
adsorption was fast, reaching the thermodynamics equilibrium in 60 min.
FTIR analysis revealed that carboxyl and hydroxyl functional groups were the
main responsible for the sorption.
The abilities of pells of broad bean, medlar and pea and fig leaves as low-
cost materials to remove cadmium ions from aqueous solution were
investigated by Benaïssa [67]. These materials showed a high cadmium ions
sorption. The broad bean peel was the most effective to remove cadmium ions
with a maximum sorption capacity about 147.71 mg g -1 followed by peas peel
(118.91 mg g -1 ), fig leaves (103.09 mg g -1 ), and medlar peel (98.14 mg g -1 ).
However, the authors have not enough information about the mechanism of
cadmium sorption by these sorbents.
Cd 2+ adsorption from aqueous solution by grape stalk waste, a byproduct
of wine production, was investigated [68]. The thermodynamic equilibrium
process was described by Langmuir isotherm model. The maximum Cd 2+
sorption capacity of grape stalk was 27.9 mg g -1 , using an aqueous solution at
pH around 5.5. They thought that the adsorption mechanism is related to
surface complexation.
Singh et al. [69] studied the Cd 2+ adsorption onto Polish rice, a byproduct
of rice milling plant. They investigated the effect of different parameters such
as contact time, adsorbate concentration, pH of the medium and temperature.
The adsorption capacity increased from 0.7 to 0.8 mg g -1 when the temperature
was decreased from 40 to 20 ºC. Thermodynamic studies showed that the
adsorption process was spontaneous and exothermic. On the other hand,
Ajamal et al. [70] found that the Cd 2+ adsorption onto rice husk was
endothermic in nature. The sorption of Cd 2+ from aqueous solution by rice