ADSORBENTS FOR CD REMOVAL FROM WATER WASTE: A SHORT REVIEW OF THE RECENT DEVELOPMENT - Management of Hazardous Residues Containing Cd - page 2

1. I NTRODUCTION
Cd 2+ is a highly toxic heavy metal ion, which is found both naturally and
as an environment contaminant. The following processes can generally
introduce cadmium into natural water supplies: smelting, plating, cadmium-
nickel batteries, crop fertilization, mining, pigmenting, and alloy processes. In
humans, cadmium is accumulated in the kidneys. In occurrences of overdoses
the protein content increases in the urine, due to the malfunction of the protein
metabolism [1,2]. The harmful effects of cadmium in the human being also
include a number of acute and chronic disorders, such as renal damage,
emphysema, hypertension, testicular atrophy, and skeletal malformation in
fetus. In view of the toxicity and in order to meet regulatory of standards safe
discharge, the removing of Cd 2+ from waste water, before to be released to
environment, is an essential process [2]. Conventional methods for the
removal of heavy metals include the following processes: precipitation,
coagulation/flocculation, ion exchange, reverse osmosis, complexation
/sequestration, electrochemical, biological treatment and adsorption. Out of
these, the adsorption processes is the simplest, very quick, and an economical
alternative for removing metal traces[2]. The removal processes of Cd 2+ also
have been carried using non-conventional adsorbents, such as solid wastes
resulting from agricultural and industrial activities [3-6]. A number of other
materials have also been used to remove Cd 2+ from wastewater, such as carbon
aerogel [1,3], activated carbon [7], metal oxide [8,9], silica [10], fungal
biomass [11], chitosan [12], and clay mineral [13]. This review presents a brief
discussing about the development recent on the use of adsorbents used in
removal of Cd 2+ from waste water.
1.1. I NDUSTRIAL OR A GRICULTURAL R ESIDUES
The use of low cost materials (industrial or agricultural residues) for
removing Cd 2+ from contaminated effluents has emerged as a potential
alternative method to conventional techniques. Several examples of low cost
Cd 2+ adsorbents are shown in literature and some of them are presented below.
Mohan et al. [14] studied the adsorption of Cd 2+ onto Kraft lignin. Lignin
was extracted from the Eucalyptus black liquor, a byproduct of the paper
industry. Lignin is the second most abundant natural polymer after cellulose.
The Cd 2+ adsorption capacity for lignin at 25 ºC was found to be 137 mg g -1 ,
which increases to 175 mg g -1 at 40 ºC. The Cd 2+ adsorption onto lignin occurs