Environmental Engineering Reference
In-Depth Information
Biosorption, the passive uptake of heavy metals by biomaterials, can be
advantageous in the form of highly efficient and cost effective. A low cost
biosorbent is defined as one which is easily available in nature, or is a by-
product or waste material from another industry. Agriculture biosorbent such
as orange wastes (Prez-Marinet al., 2008), olive stones (Blazquezet al., 2005),
papaya wood (Asma 2005), grape stalk waste, peas, broad bean, and medlar
peels (Benaıssa 2006), lemon peels, orange peels, grape fruit peels, apple
peels, apple kernel, apple core, and grape skins (Schiewer and S.B. Patil
2008), coconut shell powder (Pino 2006), coconut copra meal (Augustine and
H. Yuh-Shan 2007), Pomelo peel, agricultural waste (Saikaew et al., 2009)
have been evaluated for their biosorption properties.
Most of the studies show that agricultural waste either in the natural form
or modified form is highly efficient for the removal of cadmium ions.
Agricultural by-products are mainly composed of lignin and cellulose and may
also include other known polar functional groups of lignin, which includes
alcohols, aldehydes, ketones, carboxylic, phenolic, and ether groups. These
groups have the ability to some extent to bind heavy metal ions by donation of
an electron pair from these groups to form complexes with the metal ions in
solution (Pagnanelli et al., 2003), eventually cadmium inertize after binding
with polar functional group of plants.
Biosorption of cadmium ions using agriculture waste depends on
numerous factors like solution pH, initial ion concentrations and contact time
of biosorbant and cadmium. The pH of the aqueous solution is an important
controlling parameter in the biosorption process (Yu and Kaewsarn, 2002).
Most research conducted on heavy metal biosorption indicated that the
decrease in metal ion biosorption at acid pH could be due to the increase in
competition with protons for active sites (Yu and Kaewsarn, 1999; Yu and
Kaewsarn, 2000; Holan et al., 1993). At alkaline pH, however, other effects
could arise which alter the process, such as the predominant presence of
hydrated species of heavy metal, changes in surface charge or the precipitation
of the appropriate salt (Blazquezet al., 2005).
According to Chang et al. (1997), biosorption below pH 2 is negligible
due to the competition of hydrogen ions for the active sites. The cadmium ion
capacity increased as the pH continue increased and at pH 5.0, cadmium
removal leveled off at a maximum value. Due to proton (H
+
) vies with
cadmium ions in lower pH, the sorbent surface taken up more H
+
,
consequently reducing cadmium ions binding on the biosorbent surface. At
higher pH, the biosorbent surface takes more negative charges, thus attracting
more cadmium ions. However, with further increases in pH the formation of
Search WWH ::
Custom Search