Rapid urbanization and industrialization has led to increase disposal of heavy metals and radionuclide into the environment. Heavy metals contamination problem increase rapidly is due to the increasing of the industrialization. The key property of heavy metal is the non-biodegradable properties of the heavy metal ions. As metal ion persistent in nature it will remain in the environment and poses environmental-disposal problems (Leitão, 2009). Among the pollution of water sources, contaminations of heavy metal ions in water are the problem that reaching a dangerous level (Regine and Volesky, 2000). The main sources of heavy metal ions contamination include the urban industrial aerosols, solid wastes from animals, mining activities, industrial and also agricultural activities (KarnikaAlluri et al,. 2007).
Heavy metal is defined as element with high weight, which has the properties of a metallic substance at room temperature and is toxic or poisonous at low concentrations. Heavy metal ions of concern include chromium, lead, mercury, silver, gold, zinc, arsenic, nickel, cadmium, uranium, selenium and mercury (Ahalya et al, 2003). Heavy metal ions can enter a water sources by industrial and consumer waste, or even from acidic rain breaking down soils and releasing heavy metals into streams, lakes, rivers, and groundwater (Lenntech, 2009).
These heavy metals have high toxicity and impact as they can cause human major health problem (Leitão, 2009). Heavy metal ions are dangerous because it tend to bioaccumulation. Bioaccumulation defines as an increase in the concentration of a chemical in a biological organism over time, compared to the chemical’s concentration in the environment. Compounds accumulate in living things any time they are taken up and stored faster than they are broken down (metabolized) or excreted (Lenntech, 2009). This is because these heavy metals can be carried into food web as a result of leaching from water dumps or other polluted water sources. These metals then will increase the metals concentration at every level of food chain and cause affect human health (Paknikar et al, 2003). Even at low concentration, heavy metals can also causing human health problem. It can cause toxicity to humans and other forms of life.
1.1 Aquatic and water sources pollutants
One of the main causes of aqueous heavy metal pollution is the release of the heavy metal ion containing industrial waste into water sources. This usually as a direct result of inefficiency treatment of the heavy metal removal processes. Occurrence of aquatic pollutants (such as heavy metals) has been correlated to alterations in the fish immune system and the incidence of infectious diseases. Water bodies are being overwhelmed with pollutant and waste matter such as heavy metal. Among all the toxic substances reaching hazardous levels are heavy metals (Regine and Volesky, 2000). Even very low level of certain heavy metals can have profound effects upon the structure or even the functions of the immune system that could be almost as harmful as direct toxic doses (Saxena et al, 2009).
The main threats to human health from heavy metals are associated with exposure to lead, cadmium, mercury and arsenic. Cadmium compounds are the by-product that used in re-chargeable nickel-cadmium batteries. Cadmium will accumulate in living organisms and can cause several health problems in humans (Benguella and Benaissa, 2002). Cadmium emissions have increase dramatically during the 20th century, one reason being that cadmium-containing products are rarely re-cycled, but often dumped together with household waste. Therefore, measures should be taken to reduce cadmium exposure in the general population in order to minimize the risk of adverse health effects.
The other metal ion that should be concern is copper. Unlike cadmium, copper is not toxic as cadmium, however extensive of copper used in industry and increasing the copper levels in the environment was a reason for concern. Besides, the general population is primarily exposed to mercury through food, fish being a major source of methyl mercury exposure because of the major food source. The adverse effects of heavy metals on human health show in Table 1.0 (Hima et al., 2007).
The best accepted option is to avoid the possibility of polluting the water sources. Thus, the treatments of heavy metals contaminated water become more important issues in this endeavour. To avoid health hazards is essential to remove all these toxic heavy metals from wastewater before its disposal.
Table 1.0: Types of heavy metals and their effect on human health.
(Hima et al., 2007)
1.2 Method for heavy metal removal
Recently, the efficient method of removal heavy metals has become important and concern for industry (Malik, 2004). They are various industrial processes for elimination of heavy metals. These industrial wastewater treatments is due to the process of chemical precipitation, membrane technologies, solvent extraction, reverse osmosis, electrochemical treatment, ion exchange, evaporation recovery and also chemical oxidation-reduction (Leitão, 2009). Physiochemical processes also use in heavy metals removal of water sources before the influents into natural water system (Nars and Garima, 2004). However, all this methods have a major disadvantage which is difficult to handle and expensive processes. Besides, these methods frequently result in the release of toxic products and thus cause of secondary sources of environmental pollution problem (Leitão, 2009). Most of the physiochemical process become less effective and more expensive when he metal concentration are in range of 1 to 100 mg/L (Nars and Garima, 2004). Another major problem is the ability for removal of heavy metals in low concentrations and is inefficiency. It also produces other waste products that require careful disposal (Ahalya et al., 2003). Thus, a need for a cost effective alternative technologies are important. In this case, microbial biomass has the high efficiency and option for developing an economic and eco-friendly waste water treatment process.
Bioremediation can be defined as a biological process or treatment that used to clean up of hazardous chemicals. Bioremediation processes involve the use of microorganisms or even enzyme to return the environment back to its original condition. In recent years, rapid urbanization and industrialization has led the increase of heavy metals into environment. Hence, due to the environmental protection and economical constrain the need of bioremediation in removing and recycling of metal ions from industrial wastewater become more important.
Bioremediation can be an effective method that can alter the ecosystem pollution. There are certain main advantages to bioremediation, in which other process cannot be reach easily. One simple case that can be view is the contamination of gasoline in underground water. By injection of the right microorganism, it may reduce the pollution concentrations after a period of time. The bioremediation is less expensive compared to other chemical “cleaning” processes.
In recent years, certain new method was discovered in removing heavy metals from the water sources. It was found that fungus biomass would be an effective method in removing heavy metal ions in wastewater. This is because the ability of fungi in absorbing metal ions would be more efficiency when it grow some condition or reach certain situation. Certain condition or pre-treatment can be use on to the fungal biomass to increase the efficiency in order to speed up the bioremediation process. In other words, the biosorption procedure are said to be easy to handle, safer, rapid process, and inexpensive for the pretreatment and trace metals separation in aqueous solutions (Say, 2003). However, many of these methods are ineffective. This resulting in low levels of heavy metal ion removal and can also be economically inefficiency (Ghimire et al., 2003).
Figure 1.0: Schematic representation of biosorption procedure.
(Hima et al., 2007)
1.3 Biomass of Fungus
To overcome the entire metal ion pollution problem, all the removal of heavy metals from waste water must be highly effective. Thus the use of biomass was selected. The biomass has several advantages as heavy metal ion adsorbent over the commonly method and more efficient adsorbents and is a renewable source (Ghimire et al., 2002). Biomass often as the waste product from the industry processes. It is cheap and easily available product. Thus, according to the industrial perspective, heavy metal removal systems that are renewable and inexpensive are highly preferred by industry and achieve the environmental perspective.
A filamentous fungus is a member of large group of eukaryotic organisms which are variable in form, behavior and life cycle patterns that are classified as a kingdom that is separated from plants, animals and bacteria (Geeta Sumbali, 2005). There is the possibility that fungus has the ability to remove heavy metal ions to meet the discharge standards required by federal regulations.
Fungi can remove heavy metals and radio nuclides from the aqueous solution. Fungi can accumulate heavy metal and radionuclide species by physico-chemical and biological mechanisms. These methods include extracellular binding by metabolites and biopolymers, binding to specific polypeptides and metabolism-dependent accumulation (Tobin, 1994). The transfer of metals from environment into the cell maybe accomplished by active transport associated with metabolic activities, but it is also a passive process metal uptake by the cell across the cell wall. The cell-wall fraction of biomass plays an important role in the sorption of heavy metals. The potential of fungal biomass as biosorbent has been recognized for the removal of heavy metals from polluted water sources. Due to the negative charge from the different functional group, fungal cell walls can act as a cation exchanger (Fomina, 2007). The examples are phosphate, carboxylic, sulfhydryl or amine that was found in different wall components such as hemicelluloses, pectin and lignin (Fomina, 2007).
The reason that makes fungi biosorption become attractive is due to the possibility of obtaining large quantities of product without causing disposal problem and low cost. One example is the Rhizopus arrhizus, which are common for industrial exploitation due to its strong metal-sorbent properties. The amino groups of chitin were found to be a major site of thorium uptake in Rhizopus arrhizus.
The cell wall of fungi typically containing long chain of chitin polymer, cellulose or contain both chitin and cellulose. Fungi cell walls are rich in glycoproteins and polysaccharides, for example the glucans, chitin, mannans and phospho-mannans. The chemical structure of a part of cell wall fungi was show in figure 1.0. Chitin polymers layer provide the abundant sources of metal ion binding ligands.
The amino groups of chitin were found to be a major site of thorium uptake in Rhizopus arrhizus. The main chemical groups in a biomass which are able to uptake metal ions are the electronegative groups such as hydroxyl, sulfhydryl groups and amino groups. This is the main factor that causes the fungi biomasses have the ability to accumulate metal ion. The level of heavy metals absorption capacity by various fungal species was show in table 1.1.
Figure 1.1: Schematic diagram representation of the outer cell layer of fungi.
Figure 1.2: Chemical structure for a part of chitin polymer.
(Boya Volesky, 1999)
Table 1.1: Heavy metals absorption capacity by various fungal species.
(Narsi and Garima, 2005)
1.4 Objective of this thesis
The main objective of this review is to present report of investigations related to the ability of fungal species as a suitable model organism to use in the process of eliminating heavy metal ions from contaminated water sources. In this review the removal of heavy metal ions such chromium, cadmium and copper from synthetic wastewater by a fungus biomass is presented. The design of an effective bioreactor for the removal of copper in aqueous solution is presented. The reactor was based on an aqueous system containing metal ions in solution. These reviews also include a report on the investigation of the effect that would affect the efficiency of fungal biomass in absorption metal ions. A study of factors that affect the efficiency of fungal biomass will provide a good industrial reference for the use of fungus in the removal of metal ion as a mean of protecting the environment from heavy metal pollution.
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