MICROBIAL BIOTECHNOLOGY: SCOPE, TECHNIQUES CONTD
3) BIOMINING: HEAVY METAL EXTRACTION USING MICROORGANISMS:
Biomining utilizes naturally occurring prokaryotic communities. Here,microorganisms are used to leach metals, principally copper but also nickel and zinc, from low-grade sulfide- and/or iron-containing ores. The process exploits the energy metabolism of various acidophilic chemolithoautotrophs that utilize inorganic compounds as energy sources and carbon dioxide as the
source of carbon. These organisms use either ferrous iron or sulfide as an
electron donor and oxygen as an electron acceptor with the formation of
ferric iron or sulfuric acid. In the first case, the subsequent reaction of Ferric ions
with insoluble metal sulfides yields soluble metal sulfates; in the second,
metal sulfides are oxidized directly to metal sulfates. The metals are readily recovered from the leachate by electrolytic procedures, and the residual
solution is recycled.
Gold is inert to microbial action. However, bioleaching of sulfidic gold containing ores under acidic conditions opens up the interior of the ore particles to solvent. After bioleaching, the ore is rinsed with water and the gold is solubilized with a cyanide solution.
MICROBIAL DESULFURIZATION OF COAL:
Coal contains substantial amounts of sulfur, both in pyrite (FeS2) and in organic sulfur compounds (predominantly thiophene derivatives). The composition of coal varies considerably depending on the source. For example, Texas lignite coal contains 0.4% pyrite S and 0.8% organic S, whereas
Illinois coal contains 1.2% pyrite S and 3.2% organic S, by weight. When coal is burned, most of this sulfur is converted to SO2. The SO2 combines with moisture in the atmosphere to form sulfurous acid (H2SO3), a major component of acid smog and acid rain.Microbial desulfurization of coal, by converting the pyrite to ferric sulfate and leaching it out of the coal, provides one way of ameliorating this problem. As much as one or two weeks are required to complete the desulfurization, and large areas of land are required for the leach heaps and the storage of coal.
FUNGAL REMOVAL OF PITCH IN PAPER PULP MANUFACTURING:
In the paper manufacturing industry, treatment of wood with certain white
rot fungi to degrade certain wood extractives before pulping substantially
decreases the toxicity of pulp mill effluent toward aquatic organisms. Compounds that are extractable from wood with organic solvents make up
between 1.5% and 5.5% of the dry weight of softwoods (angiosperms) and
hardwoods (gymnosperms). These compounds, called wood extractives,
consist mainly of triglycerides, fatty acids, diterpenoid resin acids, sterols, waxes, and sterol esters. Resin acids are present in most softwoods but are generally absent or are minor components in hardwood
species. During wood pulping and refining of paper pulp, the
wood extractives are released,forming colloidalparticles commonly referred
to aspitchorresin. These colloidal particles form deposits in the pulp and
in the machinery. These deposits can cause mill shutdowns and various
quality defects in the finished paper products. Moreover, the resin constituents in pulp mill effluent show acute toxicity toward fish and aquatic
Pretreatment of the wood with fungi to degrade some of the wood extractives before pulping has met with considerable success. Basidiomycete fungi
and Ophiostoma species colonize living and recently dead wood. Many of
the species in this genus are referred to as sap-staining or blue-staining fungi
because they stain colonized wood.To avoid this problem,a commercial fungal product, Cartapip, utilizes an “albino” strain of Ophiostoma piliferum.
When applied to wood chip piles, this fungus has been particularly effective
in degrading triglycerides and fatty acids in both softwoods and hardwoods but only partially effective in the removal of other pitch-forming compounds
(sterols, sterol esters, and waxes) or the biotoxic resin acids. After four weeks
of treatment at a moisture level of 70% on a wet wood weight basis at 27 C,O.
piliferum produced up to a 50% reduction in the pitch content of softwoods,
with less than a 5% loss of woody mass. Moreover, the effluent
biotoxicity was reduced 11- to 14-fold compared with untreated controls.
A number of white rot basidiomycete fungi are able to degrade the
sterol esters and waxes. Several different bacteria, isolated by enrichment
of pulp mill effluent, are able to degrade resin acids. There is now a substantial amount of work that demonstrates that fungi and bacteria, as well
as enzymes derived from these organisms, are capable of minimizing pitch
deposition during the pulping process and substantially decreasing the toxicity of the effluents.
Cited By Kamal Singh Khadka
Msc Microbiology, TU.
Assistant Professor In Pokhara University, Pokhara Bigyan Thata Prabidhi Campus, PNC, LA, NA.
Some Suggested References:
www.ncbi.nlm.nih.gov › Journal List › Front Microbiol › v.3; 2012