ADSORBENTS FOR CD REMOVAL FROM WATER WASTE: A SHORT REVIEW OF THE RECENT DEVELOPMENT - Management of Hazardous Residues Containing Cd - page 5

0.1M HNO 3 and 1M CaCl 2 as desorbing and regenerating agents, respectively.
The xerogel regeneration with 1M CaCl 2 favored the metal biosorption,
despite an average loss of 20% in the biomass weight. This loss is related to
the reuse of the biosorbent in successive cycles. The pectin xerogels are
suitable for metal remediation technologies, since they show an excellent
reusability.
Shin [30] investigated the use of carbonate hydroxylapatite (CHAP),
obtained from eggshell waste, and in the adsorption process of Cd 2+ dissolved
in aqueous medium. The removal efficiency of Cd 2+ by CHAP reached 94%
when the initial concentration of Cd 2+ was 80 mg L -1 and the liquid/solid ratio
was equal to 2.5 g L -1 . The maximum adsorption capacity was equal to 111.1
mg g -1 .
Al-Anber and Matouq [3] studied the adsorption of Cd 2+ by olive cake.
The olive cake used in this study was generated during the squeezing step of
the oil production. The Cd 2+ removal efficiency was 66% at 28 ºC and pH
equal to 6. Matouq´s results showed that the maximum adsorption capacity is
decreased from 65 to 44 mg g -1 when the temperature is increased from 28 to
45 ºC. The adsorption characteristics of Cd 2+ onto olive oil waste was also
investigated as a function of pH, adsorbent dosage, contact time, agitation
speed and metallic ion concentration [31]. Among them, the authors observed
that the pH influence on the metal uptake was the most significant one and the
olive cake showed an optimal adsorption in pH 7. Olive stone is used as a
biosorbent for Cd 2+ from aqueous solutions in function of the wide availability
and also by the abundance of the metal binding active sites in its cellulosic
matrix [32]. Kinetic studies revealed that the initial uptake was rapid and
equilibrium was established in 1 h. The highest value of maximum uptake was
7.7 mg g -1 . The mechanism of the sorption process involves ion-exchange and
adsorption-complexation processes. They verified that increases of ionic
strength decreases the amount of removed metal. Experiments of desorption
showed that neither HCl nor EDTA solutions were able to completely remove
metals from the olive stones. Bláquez et al. studied the biosorption of Cd 2+
onto olive stones [33]. The biosorption process is fast, reaching the maximum
of sorbent capacity within 20 min. Studies of the adsorption process kinetic
mechanism showed that the process rate is described by a kinetic law of
pseudo second-order and an increase in temperature also increases the initial
sorption rate. Hoces et al. [34] evaluated the equilibrium for cadmium
biosorption on the olive between 25 and 80 °C. The maximal capacity of
biosorption increases with temperature, rising from 4.52 mg g -1 at 25 °C to
9.72 mg g -1 at 80 °C.