Biosorption

Overview

Biosorption is one the physico-chemical activity that deals with the removal of pollutants from the wastewater involving microbial cell surface absorption of metals.

The term ‘biosorption‘ is made up of two words ‘bio‘ and ‘sorption‘, the word ‘bio’ means life, however the keyword ‘sorption’ is needed to be elaborated. This word (sorption) is common in absorption as well as in adsorption. Therefore, absorption means the incorporation of a substance from one substance to another form whereas, adsorption means physical accumulation or attachment of molecules/ion on the surface of the solid material by some or the other type of bonding. The surface on which the materials got accumulated is known as adsorbent, while the material that gets attached to the surface is known as adsorbate.

In the present time the up surging industrialization has lead to the increase in quite high amount of waste water that can be toxic to the living being as it contains varities of organic compounds such as dyes, hydrocarbons, pesticides, and PCBs. These microbial cells or microorganisms are used as bio-accumulators or biosorbents of metals and performs the functions of removing the waste.

The two key functions of the biosorption’s are:

• Regaining of the valuable though toxic metals.
• Extracting the poisonous of harmful metals from the industrial waste materials.

Different types of microorganisms are quite efficient in the biosorption techniques, they can be algae, bacteria, molds, yeast. In few cases, the new method of introducing biosorption granules for the purpose of metal recovery, waste water or industrial effluent treatment.

The membrane of the microbial cells are negatively charged due to the presence of the carboxyl (COO-), hydroxyl (OH-), phosphoryl (PO43_) and sulfhydryl (HS-). This forces the positively charged metal ions (from solutions) to get absorbed on the microbial surfaces.

Following are the various groups of microorganisms used in the process of biosorption and are briefly described here:

Bacteria

Actinomycetes and certain other bacteria’s have the potential to accumulate and absorb metals such as uranium, mercury, cobalt, cadmium, Nickle, lead and zinc. For instance, Bacillus circulans can absorb the the metals like Zn, Cu, Co and Cd. While another speices known as Rhodospirullum sp. can aborb and accumulate Hg, Pb and Cd. When observed in the electron microscope it was noticed that there was a colllection of the metals on the bacterial cell wall. It was interpreted that the composition of a cell wall plays the crucial in the absorption of metals .

Fungi

In fermentation industries the use of fungal biomass is considered to be the significant one. This fungal biomass can be used for the process of metal biosorption getting from the industrial effluents. Due to the mechanical strength, resistance to chemical environment and increased density, the immobilized fungal biomass is more effective in biosorption and further can be reused after proper processing.

For example, Rhizopus arhizus have the capacity to absorb numerous metallic cations like, thorium, uranium. Whereas, Penicillium spimulosam, and P. lapidorum are used in the biosorption of the metals such as Pb, Zn, Cu, Hg. For others such as uranium other types fungus are used namely, Aspergillus oryzae, A. niger, Mucor haemalis, Penicillium chrysogenum.

It is interesting to know that edible mushrooms such as Agaricus bisporus and Pleurotus sajorcaju are used to take the mercury, lead and cadmium respectively. Moreover, many species of yeast that are used in fermentation industries have the potential of absorbing and gathering metals. For instance, Sporobolomyces salmonicolour, and Saccharomyces cerevisae are used to take up the zinc and mercury respectively.

Algae

Even numerous species of algae are helpful as bioaccumulators of metals, for instance Cholrella regularis and C. vulgaris can work in absorbing few metals like Hg, Pb, Mo, U and Cu. Another species og green algae Hydrodictyon reticulatum accumulates and absorbs high level of Mn, Fe and Pb.

These days even marine algae such as Ulva, Luminaria and Codium are been to remove the pollutants present in the river in the form of metals.

Factors affecting biosorption

Following are the factors affecting the biosorption process:

Temperature: For the effective process it is required to get the accurate temeperature of the environment. However, the optimal temeprature is considered to be between 20 degree to 35 degree Celsius. It has been found that the tmeperature above 45 degree Celsius may damage the proteins of the microorganisms that may hamper the sorption process of them.

pH: This is another crucial parameter, as it affects the binding sites and solubility of metal ions. Normal range for the uptake of metal ion is between 2.5 – 6.

Nature of biosorbents: Metal accumulation can take in various forms like freely suspended microbial cells, immobilization of microbial cell or biofilms, and even can be altered by the chemical or physical treatments. Where, chemical treatments includes the usage of acid or alkali in the process of biosorption and physical treatments includes drying, boiling, sonication and autoclaving, etc. to improve the capacity of process.

Concentration of biomass: The electrostatic interactions between the cells also plays the significant role in the biosorption process. The ratio of biomass is directly proportional to the metal accumulated. If there is more biomass or metal ions it may restricts the access to metal ions binding sites.

Surface area to volume ratio; Metal affinity to biosorbent;Initial metal ion concentration are also other factors that may hamper the process of biosorption.

The major advantages of bisorption techniques are:

• Cost effective.
• Helpful in reducing the biological sludge.
• Effective and efficient process (can work at wide range of temperature and pH).
• No additional resources needed (chemicals and nutrition).
• Minimum use of industrial waste.
• Biosorbent can be reused and less chances of contamination after recovery.
• Multiple heavy metals can be taken at a time and with a treatment of large volumes of waste water.
• Less volume of toxic material produced.

Disadvantages

• Active sites of the metal binding ligands becomes saturated.
• Chances of reversible sorption of metals on biomass.