ReviewMicrobial and plant derived biomass for removal of heavy metals from wastewater
Introduction
Increased use of metals and chemicals in process industries has resulted in generation of large quantities of effluent that contain high level of toxic heavy metals and their presence poses environmental–disposal problems due to their non-degradable and persistence nature. In addition mining, mineral processing and extractive-metallurgical operations also generate toxic liquid wastes. Environmental engineers and scientists are faced with the challenging task to develop appropriate low cost technologies for effluent treatment. Conventional methods for removing metals from aqueous solutions include chemical precipitation, chemical oxidation or reduction, ion exchange, filtration, electrochemical treatment, reverse osmosis, membrane technologies and evaporation recovery. These processes may be ineffective or extremely expensive especially when the metals in solution are in the range of 1–100 mg l−1 (Nourbakhsh et al., 1994). Another major disadvantage with conventional treatment technologies is the production of toxic chemical sludge and its disposal/treatment becomes a costly affair and is not eco-friendly. Therefore, removal of toxic heavy metals to an environmentally safe level in a cost effective and environment friendly manner assumes great importance.
In light of the above, biological materials have emerged as an economic and eco-friendly option. Biomaterials of microbial and plant origin interact effectively with heavy metals. Metabolically inactive dead biomass due to their unique chemical composition sequesters metal ions and metal complexes from solution, which obviates the necessity to maintain special growth-supporting conditions. Metal-sorption by various types of biomaterials can find enormous applications for removing metals from solution and their recovery.
Rather than searching thousands of microbial species for particular metal sequestering features, it is beneficial to look for biomasses that are readily available in large quantities to support potential demand. While choosing biomaterial for metal sorption, its origin is a major factor to be taken into account, which can come from (a) microorganisms as a by-product of fermentation industry, (b) organisms naturally available in large quantities in nature and (c) organisms cultivated or propagated for biosorption purposes using inexpensive media. Different non-living biomass types have been used to adsorb heavy metal ions from the environment (de Rome and Gadd, 1991, Tiemann et al., 1999). Seaweed, mold, bacteria, crab shells and yeast are among the different kinds of biomass, which have been tested for metal biosorption or removal (Volesky and Holan, 1995).
Section snippets
Metals in wastewater and their toxicity
Effluents from textile, leather, tannery, electroplating, galvanizing, pigment and dyes, metallurgical and paint industries and other metal processing and refining operations at small and large-scale sector contains considerable amounts of toxic metal ions. These toxic metals ions are not only potential human health hazards but also to another life forms. Toxic metal ions cause physical discomfort and sometimes life-threatening illness including irreversible damage to vital body system (Malik,
Conventional methods for heavy metal removal from industrial effluents
To mitigate the heavy metal pollution, many processes like adsorption, precipitation, coagulation, ion exchange, cementation, electro-dialysis, electro-winning, electro-coagulation and reverse osmosis have been developed.
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Precipitation is the most common method for removing toxic heavy metals up to parts per million (ppm) levels from water. Since some metal salts are insoluble in water and which get precipitated when correct anion is added. Although the process is cost effective its efficiency
Biosorption
Biosorption is a property of certain types of inactive, non-living microbial biomass to bind and concentrate heavy metals from even very dilute aqueous solution. Biomass exhibits this property, acting just as chemical substance, as an ion exchanger of biological origin. It is particularly the cell wall structure of certain algae, fungi and bacteria, which was found responsible for this phenomenon (Volesky, 1990). Till now, research in the area of biosorption suggests it an ideal alternative for
Mechanism of metal uptake
The understanding of the mechanism by which microorganisms accumulate metals is crucial to the development of microbial processes for concentration, removal and recovery of metals from aqueous solution. Metabolism-independent metal binding to the cell walls and external surfaces is the only mechanism present in the case of non-living biomass. Metabolism-independent uptake essentially involves adsorption process such as ionic, chemical and physical adsorption. A variety of ligands located on the
Removal of metal by non-living biomass of microbial and plant origin
Removal of heavy metals from aqueous solution by using inactive and dead biomass is an innovative and alternative technology for removing these pollutants. Non-living biomass of algae, aquatic ferns and seaweeds, waste biomass originated from plants and mycelial wastes (Table 1a, Table 1b, Table 1c, Table 1d) from fermentation industries are potential biosorbents for removal of heavy metals from aqueous solution and wastewater (Puranik and Paknikar, 1999, Ahluwalia and Goyal, 2005).
Development of biosorbent
Biosorption is a complex process, mainly comprising of ion exchange, chelation and adsorption by physical forces and entrapment in inter and intra-fibrillar capillaries and space of the structural polysaccharide network as a result of the concentration gradient and diffusion. There are several chemical groups that would attract and sequester the metals in biomass. Acetamide groups of chitin, structural polysaccharides of fungi, amino and phosphate groups of nucleic acids, amide, amine,
Conclusion
Metabolic independent processes can mediate the biological uptake of heavy metal cations. Biosorption offers an economically feasible technology for efficient removal and recovery of metal(s) from aqueous solution. The process of biosorption has many attractive features including the selective removal of metals over a broad range of pH and temperature, its rapid kinetics of adsorption and desorption and low capital and operation cost. Biosorbent can easily be produced using inexpensive growth
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