Biological Remediation of Hydrocarbon and Heavy Metals Contaminated Soil - Soil Contamination - page 132

different mechanisms (Malik et al., 2004). On the other hand, living cells shows higher
sensitivity to environmental conditions and demand nutritional and energetic sources.
Many genera of microbes like Bacillus , Enterobacter , Escherichia , Pseudomonas and also some
yeasts and moulds help in bioremediation of metal and chromium-contaminated soil and
water by bio-absorption and bioaccumulation of chromium (Kotas and Stasicka, 2000). The
heavy metal removal by the bacteria Pseudomonas was attributed to the cellular growth of
these organisms (Ray and Ray, 2009).
3.2.1 Biostimulation
Biostimulation of indigenous microbes is a bioremediation strategy mostly used for
remediation of contaminated soil. This involves addition of nutrients, either organic or
inorganic, to enhance the activities of indigenous microbes. Input of large quantities of
carbon sources such as crude oil, used lubricating oil, diesel oil etc. tends to result in a rapid
depletion of the available pools of major inorganic nutrients such as N and P. Levels of N
and P added to stimulate biodegradation at contaminated sites are often estimated from
C/N ratios (Sang-Hwan et al. 2007).
Biostimulation aims at enhancing the activities of indigenous microorganisms that are
capable of degrading pollutant from soil environment, it is often been applied to the
bioremediation of oil-contaminated soil. Nutrient enrichment, also called fertilization is a
bioremediation approach in which fertilizer similar to phosphorus and nitrogen that are
applied to plant in farms are added to contaminated environment to stimulate the growth of
indigenous microorganisms that can degrade pollutants (Thieman and Palladino 2009).
Microorganisms need an abundance of key elements such as carbon, hydrogen, nitrogen,
oxygen and phosphorus for building macromolecules, addition of fertilizer provides these
microbes with essential elements to reproduce and thrive. In some instances, manure, wood
chips and straw may provide microbes with the sources of carbon as a fertilizer. The concept
of biostimulation is that, by adding more nutrients, microorganisms replicate, increase in
number and grow rapidly and thus increase the rate of biodegradation (Thieman and
Palladino 2009). Addition of inorganic nutrients do act as fertilizer to stimulate
biodegradation by autochthonous microorganisms in some cases; in other cases, it is the
intentional stimulation of resident xenobiotic-degrading bacteria by use of electron
acceptors, water, nutrient addition, or electron donors (Widada, et al., 2002). Combinations
of inorganic nutrients often are more effective than single nutrients (Sutherland, et al., 2000).
Laboratory-based respiration experiments by Liebeg and Cutright (1999) showed that a low
level of macronutrients and a high level of micronutrients were required to stimulate the
activities of indigenous microbes. The greatest stimulation was recorded with a solution
consisting of 75% sulphur, 3% nitrogen and 11% phosphorus.
Addition of a carbon source as a nutrient in contaminated soil is known to enhance the rate
of pollutant degradation by stimulating the growth of microorganisms responsible for
biodegradation of the pollutant. It has been suggested that the addition of carbon in the
form of pyruvate stimulates the microbial growth and enhances the rate of PAH
degradation (Lee, et al., 2003). Biostimulation can also be achieved by the use of composting
bioremediation technologies. Composting bioremediation strategy relies on mixing the
primary ingredients of compost with the contaminated soil, such that as the compost
matures, the pollutants are degraded by the active microflora within the mixture (Semple, et
al., 2001). Mushroom compost and spent mushroom compost (SMC) are also applied in
treating organopollutant contaminated sites (Eggen, 1999, Trejo-Hernandez et al., 2001).