Biosorption primarily deals with the microbial cell surface adsorption of metals from the mine wastes or dilute mixtures. The microorganisms can be used as bio-sorbents or bio-accumulators of metals. The process of biosorption performs two important functions.
1. Removal of toxic metals from the industrial effluents.
2. Recovery of valuable toxic metal from industrial effluents.
Biosorption in a whole reduces the amount of toxic metal waste from industrial effluents. Thus it helps in reduction of environmental pollution.
A wide range of microorganisms such as bacteria, algae, yeast, moulds take part in biosorption. Some research work has even developed bio-sorbent based granules for waste water or industrial effluent treatment and toxic metal recovery.
In general, the microbial cell membranes are negatively charged due to the presence of carboxyl ions, hydroxyl ions, phosphoryl ions and sulfhydryl groups. This phenomenon enables the positively charged metal ions from waste water or industry effluents to be adsorbed on to the microbial surfaces.
Microorganisms Involved in Biosorption
Bacteria, fungi and algae each of them is involved in removal of toxic metals from waste water or industry effluents in biosorption. Some higher plants may also have some roles in biosorption.
Several bacteria and actinomycetes adsorb and accumulate metals such as mercury, calcium, lead, zinc, nickel, cobalt and uranium. Rhodospillum sp. can accumulate cadmium (Cd), lead (Pb), mercury (Hg). Bacillus circulans can adsorb metals such as copper (Cu), cadmium (Cd), cobalt (Co), zinc (Zn).
Use of electron microscopy has shown deposition of toxic metals on the bacterial cell walls. This study also showed that the composition of bacterial cell wall plays a major role in metal adsorption.
There is a large scale production of fungal biomass in many fermentation industries. This biomass can be utilized for metal biosorption from waste water or industrial effluents. Immobilized fungal biomass is more effective in biosorption due to increased density, mechanical strength and resistance to chemical environment. Further immobilized biomass can be reused after suitable processing.
The fungus Rhizopus arrhizus can adsorb several metallic cations such as uranium, thorium etc. Penicillium lapidorum, Penicillium spimulosum are useful for the biosorption of metals such as mercury (Hg), zinc (Zn), lead (Pb), and copper (Cu). Several fungi were tried with some degree of success to selectively adsorb uranium like Aspergillus niger, Aspergillus oryzae, Mucor haemalis, Penicillium chrysogenum.
Edible mushrooms were also found to adsorb certain metals. The fruit bodies of Agaricus bisporus can take up mercury (Hg) while Pleurotus sajorcaju can adsorb lead and cadmium. Many types of yeast commonly used in fermentation industries are capable of adsorbing and accumulating metals from industry effluents. The baker's yeast, Saccharomyces cerevisae and Sporobolomyces salmonicolour can respectively adsorb mercury (Hg) and zinc (Zn).
Several species of algae either fresh water or marine can serve as bioaccumulators of metals. Chlorella vulgaris and Chlorella regularis can accumulate certain metals like lead, mercury, copper, molybdenum and uranium. The green algae Hydrodictyon reticulatum adsorbs and accumulates high quantities of lead (Pb), iron (Fe) and manganese (Mn).
Some researchers are now a days trying to use marine algae such as Luminaria sp., Ulva sp., Codium sp. as bio-accumulators to reduce the metal pollution in rivers.
IV) Higher Plants
Besides the microorganisms describe above some higher aquatic plants such as aquatic macrophytes also can accumulate potential toxic wastes including many metals. Water hyacinth or Eichornia crassipes, duck weeds or Spirodel sp., water lettuce or Salvania sp. are important in controlling metal pollution in aquatic environment.
Mechanisms Involved in Biosorption
Microorganisms have got a complex structure and there can be many different ways to accumulate the metal from the waste water or industrial effluents. Thus biosorption mechanisms are different for each microorganism and they are not completely understood in some cases. Biosorption techniques are classified in various categories according to different criteria.
Based on the microorganism's metabolism biosorption mechanism can be divided into two categories:
1. Metabolism dependent biosorption
2. Non-metabolism dependent biosorption
Based on the location where the extracted metal is stored, biosorption can be divided in three categories:
1. Extra cellular accumulation/ precipitation
2. Cell surface sorption/ precipitation and
3. Intracellular accumulation.
In metabolism dependent accumulation transport and accumulation of the metal across the cell membrane occur. This process is completely dependent on cellular metabolism as membrane transport is dependent on metabolism. Thus this type of biosorption can only be done by living cells. In non-metabolism dependent biosorption, the process is a physico-chemical process. The functional groups present on the microbial cell surface and the toxic metals interact with each other. The cell walls of the microorganisms are thick with layers of proteins and lipids which have a large variety of metal binding components. Thus, this process is a very fast and reversible process. Precipitation of the toxic metals is done due to chemical compounds secreted by the microorganisms.
Advantages of Biosorption
1. Cost of the process is very low
2. Highly efficient process
3. Environment friendly process
4. The microorganisms can be reused
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