Microbial Remediation of Pesticides - Pesticides: Evaluation of Environmental Pollution - page 134

present, it will be utilized preferentially over the other electron acceptors because it pro-
vides maximum energy yield to the microorganism, resulting in the maximum possible
amount of cell production and organism growth per unit amount of electron donor used.
Once oxygen is depleted at a site, facultative and anaerobic microorganisms use other
electron acceptors. Microorganisms use acceptors in sequence based on the relative energy
yield of each—nitrate, manganese, iron, sulfate, carbon dioxide, and, finally, organic car-
bon (US EPA 1998).
The effectiveness of biodegradation is highly dependent on the toxicity and initial con-
centrations of the contaminants, their biodegradability, the properties of the contaminated
soil, and the type of microorganism used for remediation purpose. In general, there are
mainly two types of microorganisms that are used for bioremediation: (i) indigenous and
(ii) exogenous. The indigenous ones are those microorganisms that are already present at
a contaminated site. Exogenous ones are the effective microorganisms that are not pres-
ent in the pesticide-contaminated site but are added to the contaminated soil. To stimu-
late the growth of indigenous microorganisms, the proper soil temperature and oxygen
and nutrient content may need to be provided. Bioremediation can take place under aero-
bic and anaerobic conditions. With sufficient oxygen, microorganisms can convert many
organic contaminants to carbon dioxide and water. Anaerobic conditions support biologi-
cal activity in which no oxygen is present, so the microorganisms break down chemical
compounds in the soil to release the energy they need. A key difference between aerobic
(oxidative) and anaerobic breakdown is that the former is predominantly used for lower-
chlorinated congeners and the latter for high-chlorinated congeners (hydrodechlorina-
tion). Bioremediation is natural. It is happening on earth without human help. However,
sometimes it is too slow to meet the demand. Generally, bioremediation technologies can
be classified as in situ or ex situ. In situ bioremediation involves treating the contaminated
material at the site, while ex situ bioremediation involves the removal of the contami-
nated material to be treated elsewhere. Some of the bioremediation technologies include
bioventing, land farming, bioreactor, composting, bioaugmentation, and biostimulation.
Bioremediation is best accomplished with bioaugmentation. In addition to bioaugmenta-
tion, several physical and chemical parameters must be controlled in order to obtain opti-
mal growth and maximum degradation of contaminants. Microbial population, nutrient
concentration, oxygen supply, temperature, moisture content, pH, toxicity, heavy metal,
molecular structure, and cometabolism are important for optimum performance of the
microorganisms (Table 5.3, adapted from Vidali 2001). Microbial activity can be also
affected by competition from undesirable organisms (Mulligan 2002).

TABLE 5.3
Optimal Environmental Factors for the Degradation of
Contaminants by Soil il Microbes

Factors Optimal Condition for Microbial Activity

pH 5.5-8.8

Temperature 15°C-45°C

Moisture 25%-28% of the soil water-holding capacity

Soil type Low clay or silt content

Oxygen For aerobic, >10% of air-filled space of the soil

Nutrient N&P for growth

Contaminants <toxicity level

Heavy metal Total 2000 ppm