Biotechnological Aspects of Soil Decontamination - Biotechnologies and Biomimetics for Civil Engineering - page 378

17.4.1 Biodegradation of Soil Pollutants
Degradation of soil pollutants mostly involves consortia of microorganisms, and
may be achieved using in situ, on-site, or bioreactor approaches. On-site processes
or bioreactors are likely to involve solid phase approaches such as landfarming
(spreading of excavated contaminated soils in a thin layer usually combined with
cultivation and/or nutrient inputs) or more ''engineered'' solutions compositing or
the use of bio-piles (piles of polluted soils constructed to facilitate aeration and
addition of nutrients). It can be applied to soils contaminated by crude oil, and also
efficient in degradation of polycyclic hydrocarbons (PAHs). The management of
the treatment environment is easiest with on site process bioreactors and difficult
with in situ approaches, with bio-piles and compost techniques intermediate in this
respect. Biodegradation technique is effective on a range of organic constituents.
Bioaugmentation (addition of cultured microorganisms with the capacity to
degrade target contaminants) or biostimulation (addition of nutrients to increase
indigenous biomass or of substrates to promote cometabolism) of soil microbial
populations may provide a means of accelerating pollutant degradation (Singer
et al. 2005 ). There are many environmental factors that limit microbial biodeg-
radation of soil pollutants including low temperature, restricted activity under
anaerobic conditions, low levels of available nutrients or co-substrates, and limited
bioavailability of pollutants. Although soil animals are not thought to have a
significant direct role in the biological degradation of contaminants, their activities
may stimulate microorganisms and improve the soil environment for microbial
degradation (Haimi 2000 ).
Soils on many polluted sites are physically degraded and macro-fauna such as
earthworms can improve these conditions (Scullion and Malik 2000 ). Recently,
interest in the role of earthworm as ''bioreactors'' for degradation of contaminant
such as trinitrotoluene have been reported. Biodegradation rate is controlled by
microbial catabolic capacity, (Renoux et al. 2000 ). The presence of high con-
centration of heavy metals may inhibit the microbial growth and also the volatile
components tend to evaporate rather than biodegrade. Thus, in most of these
treatments, there will be some loss to atmosphere through volatilization and some
physical or chemical stabilization of pollutants. Bioreactor treatments are
increasingly favored as the microbes may also lead to formation of biofilm, which
might accumulate organic pollutants for subsequent degradation of high molecular
weight PAHs and heavy metals (Sheoran et al. 2010 ).
17.4.2 Bioleaching
It is a process of recovery of metals by some microorganisms capable of dissolving
them from the environment. It is an effective alternative to chemical extraction
processes. Microorganisms like Thiobacillus ferrooxidans and T. thiooxidans