may enhance the mass transfer of the contaminant and thus increase bioavailability
(Baboshin and Golovleva 2012). To minimize the side effects of chemical additions
during bioremediation process, the use of biosurfactants is recommended due to low
toxicity and better biodegradability (Baboshin and Golovleva 2012). However, even
with the application of biosurfactants, in many cases, low bioavailability still
constitutes significant limitation for bioremediation efficiency.
Soil analysis should also include characterization of basic chemical properties
such as quantity and availability of organic carbon and nutrients. Nutrient deficiency
is often a limiting factor for hydrocarbon degradation. Disturbed nutrient ratio in
soils contaminated with organic pollutants may be manifested by elevated total
organic carbon (TOC) content and high C:N and C:P ratios (Wang et al. 2010).
According to Wang et al. (2010) in TPH-contaminated soils, the values of total
nitrogen were lower in comparison with uncontaminated soil, while total phosphorus
content did not differ significantly. High C:N ratio is not favorable for bioremediation efficiency because microorganisms will deplete nitrogen stock before breaking
down organic molecules. Also high mineral nitrogen application ranging from 200 to
300 N mg/kg may be toxic for soil biota and inhibit microbial activity (Carmichael
and Pfaender 1997; Braddock et al. 1997). Usually, the optimal C:N:P ratio for
bioremediation of TPH-contaminated soils ranges from 100:5:1 to 100:15:1 (Shahi
et al. 2016). However, favorable C:N:P ratio in contaminated soil is case-specific and
should be determined during process optimization step. To balance the C:N:P ratio,
inorganic fertilizers and organic amendments are used (Tables 5.3 and 5.4).
If the concentration of contaminants is high, degrading agents may suffer from
increased soil toxicity. A strategy to temporary decrease the available concentrations
of the contaminants is the addition of organic matter. Due to sorption phenomena,
contaminants will be bound to the organic amendment, which will decrease bioavailability. During the course of the degradation process, absorbed compounds will
be slowly released from organic matter. Another factor, which may cause
ecotoxicity, is the accumulation of intermediate metabolites, as a result of incomplete contaminant transformation. Metabolites may be more toxic than initial compounds, e.g., due to lower hydrophobicity and thus higher bioavailability and
intrinsic toxicity (Xu and Lu 2010).
When different groups of hydrocarbons are present in contaminated soils, easily
biodegradable compounds constitute the primary source of carbon and energy for
microorganisms that may further contribute to the biodegradation of more recalcitrant compounds (Zappi et al. 1996). On the sites where the contamination level of
TPH is low, the amount of carbon and energy originated from easily metabolized
compounds may not be sufficient to degrade more complex molecules. In this case,
an additional source of easily bioavailable energy and carbon may be needed for a
successful bioremediation process (Zappi et al. 1996).
5 Potential Use of Waste-to-Bioenergy By-Products in Bioremediation of Total. . .
247
(Baboshin and Golovleva 2012). To minimize the side effects of chemical additions
during bioremediation process, the use of biosurfactants is recommended due to low
toxicity and better biodegradability (Baboshin and Golovleva 2012). However, even
with the application of biosurfactants, in many cases, low bioavailability still
constitutes significant limitation for bioremediation efficiency.
Soil analysis should also include characterization of basic chemical properties
such as quantity and availability of organic carbon and nutrients. Nutrient deficiency
is often a limiting factor for hydrocarbon degradation. Disturbed nutrient ratio in
soils contaminated with organic pollutants may be manifested by elevated total
organic carbon (TOC) content and high C:N and C:P ratios (Wang et al. 2010).
According to Wang et al. (2010) in TPH-contaminated soils, the values of total
nitrogen were lower in comparison with uncontaminated soil, while total phosphorus
content did not differ significantly. High C:N ratio is not favorable for bioremediation efficiency because microorganisms will deplete nitrogen stock before breaking
down organic molecules. Also high mineral nitrogen application ranging from 200 to
300 N mg/kg may be toxic for soil biota and inhibit microbial activity (Carmichael
and Pfaender 1997; Braddock et al. 1997). Usually, the optimal C:N:P ratio for
bioremediation of TPH-contaminated soils ranges from 100:5:1 to 100:15:1 (Shahi
et al. 2016). However, favorable C:N:P ratio in contaminated soil is case-specific and
should be determined during process optimization step. To balance the C:N:P ratio,
inorganic fertilizers and organic amendments are used (Tables 5.3 and 5.4).
If the concentration of contaminants is high, degrading agents may suffer from
increased soil toxicity. A strategy to temporary decrease the available concentrations
of the contaminants is the addition of organic matter. Due to sorption phenomena,
contaminants will be bound to the organic amendment, which will decrease bioavailability. During the course of the degradation process, absorbed compounds will
be slowly released from organic matter. Another factor, which may cause
ecotoxicity, is the accumulation of intermediate metabolites, as a result of incomplete contaminant transformation. Metabolites may be more toxic than initial compounds, e.g., due to lower hydrophobicity and thus higher bioavailability and
intrinsic toxicity (Xu and Lu 2010).
When different groups of hydrocarbons are present in contaminated soils, easily
biodegradable compounds constitute the primary source of carbon and energy for
microorganisms that may further contribute to the biodegradation of more recalcitrant compounds (Zappi et al. 1996). On the sites where the contamination level of
TPH is low, the amount of carbon and energy originated from easily metabolized
compounds may not be sufficient to degrade more complex molecules. In this case,
an additional source of easily bioavailable energy and carbon may be needed for a
successful bioremediation process (Zappi et al. 1996).
5 Potential Use of Waste-to-Bioenergy By-Products in Bioremediation of Total. . .
247