GEOLOGICAL SOCIETY OF THE PHILIPPINES
On The Binding of Lota-Carageenan Towards Pb2+, Cd2+, and Zn2+: Its Mechanism and Effects of Molecular Weight and the Presence of K+, Na+ and Ca+
Dahlia C. Apocada 1,2 and Clovia Isabel z. Holdsworth2
1Mines and Geosciences Bureau, North Avenue, Diliman Quezon City
2Chemistry Department, De La Salle University, Taft Avenue, manila
Billions of tons of metal waste are generated every year due to urban, agriculture and industrial activity. This has led to accumulation of toxic compounds in soil.1 River systems may also soon be, if not already, affected by the presence of these toxic metals in the sediments. Specifically, this has been the main objection of most people against mining. News about heavy metal poisoning near mining sites has been all over the newspapers and in television. Heavy metals are known to pose health hazards to human beings. Exposure to these metals can cause serious diseases, birth abnormalities, mental retardation, etc.
An emerging method of cleaning up polluted soil and aquatic system is by phytoremediation. Phytoremediation is the removal of toxic wastes from the environment by plants.1 The need to underpin a suitable molecule that will be utilized in the extraction of heavy metals in various environmental samples has encouraged researchers to conduct studies in line with this objective. With new discoveries of highly metal-sorbing biomass types, there is a real potential for the introduction of a whole family of new biosorbent products, which are likely to be very competitive and cost efficient in metal sorption.2 Intense research effort is necessary to establish phytoremediation as an effective and safe method for use on a wider environmental canvas.
Some types of seaweed biomass offer excellent metal-sorbing properties. Red seaweeds such as Eucheuma Spinosum, Eucheuma Cottonii and Kapphycus Alvarezii, are rich in carrageenan, which have been proven to bind strongly to metals. Carrageenans, a set of sulphated polysaccharides, consist of polymers of D-galactopyranose bonded through, alternatively, a-(1®3)- and b-(1®4) linkages.3 The Philippines is a major producer of two fractions of carrageenan, namely iota- and kappa and exports it only for food and semi-food grade applications. The non-toxicity of carrageenans geared researches towards the possible participation of these groups of polysaccharides in the removal of heavy metals in water, wastewater, soil samples, and others. This would in turn, benefit local economies from turning seaweeds into a resource.2 However, its maximum utility could only be possible if all factors affecting the metal-carrageenan binding process are pinpointed and could be extensively explained.
The ability of carrageenan to bind to certain heavy metals has been previously studied. Results from local studies4,5,6 studies showed that both kappa-carrageenan and iota-carrageenan are capable of binding to lead, cadmium and zinc. The binding capacity of these metals were observed to be higher with iota-carrageenan with two sulfate groups per monomer unit, than with kappa-carrageenann. However, because the mechanism of binding has not been extensively studied, the effect of the sulfate groups on its binding capacity is not conclusive. They have assumed electrostatic attraction to be the main factor in the metal binding but results showed that other mechanisms could have been in place. Furthermore, these studies did not include the efficiency of Pb, Cd and Zn sorption in the presence of other counter-ions.
This study addresses the mechanism(s) of binding of iota-carrageenan to Pb, Cd and Zn ions by determining the effects of the molecular weight of the biopolymer and the presence of K, Na and Ca counter-cations.
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