45
Phytoremediation of Organics
while Liu & Schnoor (2008) quantitatively documented the preferential uptake of
lesser chlorinated PCBs into hydroponic poplars.
Dioxins (polychlorodibenzo-p-dioxins) are chlorinated aromatic compounds similar to PCBs but having two oxygen bridges between two benzene rings. Polychlorinated benzofurans have one oxygen bridge and one
direct benzene-benzene bond. They arise as by-products of chlorination of
phenols and during combustion of organic matter in the presence of O and
Cl (Campanella et al., 2002). All of these are toxic to greater or lesser extent,
but very persistent in environment, and so nonpolar that they accumulate
in lipids of living organisms, and hence through the food chain. Fungi and
root-associated organisms may facilitate their degradation. Bacteria of several genera are known to degrade specific congeners with varying efficiency.
Many are aerobes, but some dechlorinations occur with anaerobes (Field &
Sierra-Alvarez, 2008). Plants have been used to enhance the rates of degradation by providing nutrients to microbes, aerating the root zone, inducing
microbial metabolism enzymes, and to do some metabolic reactions within
the plant (Campanella et al., 2002). Over the past two decades there have
been advances in this area, but challenges remain (Mench et al., 2010). There
are many more recent papers discussing degradation of these compounds by
microbes (Saibu et al., 2020), but little new work with plant pathways.
3.2.4 Remediation of Pesticides
At many locations in the world, soils are contaminated with pesticides
because of poor management, spills, and the need to clean out sprayers. A
recent review describes applications of phytoremediation to restore lands
that are contaminated with pesticides (Tarla et al., 2020). The toxicity of pesticides to vegetation is one of the challenges in selecting plants for use at a
contaminated site. There has been progress in the identification and use of
microorganisms and plants that have good capability to degrade some pesticides. Soil amendments such as manure or biochar can be added to contaminated sites to dilute pesticides, sorb them out of soil solution, and provide
additional microorganisms to aid in the establishment of vegetation. Use of
vegetation that naturally grows well in the region is recommended.
Khalid et al. (2020) review many applications of biochar specific to pesticides. Deliberate augmentation with biochar to enhance bioremediation is
becoming a common practice; however, sorption to biochar also reduces availability of pesticides and herbicides to plants, and can thus delay their remediation (Khalid et al., 2020). For example, in an in situ study at Point Pelee National
Park in Canada (PPNP), biochar significantly reduced DDT (p,p’-dichlorodiphenyl-trichloroethane) accumulation in earthworms (49%) but did not significantly reduce plant uptake of DDT (Denyes et al., 2016). Other amendments,
particularly other carbon-rich materials including humic substances, charcoal,
bio-coal and AC, sorb the contaminants in soils (Beesley et al., 2011) and may
also reduce the bioavailability for plants (Khalid et al., 2020). It is important
Phytoremediation of Organics
while Liu & Schnoor (2008) quantitatively documented the preferential uptake of
lesser chlorinated PCBs into hydroponic poplars.
Dioxins (polychlorodibenzo-p-dioxins) are chlorinated aromatic compounds similar to PCBs but having two oxygen bridges between two benzene rings. Polychlorinated benzofurans have one oxygen bridge and one
direct benzene-benzene bond. They arise as by-products of chlorination of
phenols and during combustion of organic matter in the presence of O and
Cl (Campanella et al., 2002). All of these are toxic to greater or lesser extent,
but very persistent in environment, and so nonpolar that they accumulate
in lipids of living organisms, and hence through the food chain. Fungi and
root-associated organisms may facilitate their degradation. Bacteria of several genera are known to degrade specific congeners with varying efficiency.
Many are aerobes, but some dechlorinations occur with anaerobes (Field &
Sierra-Alvarez, 2008). Plants have been used to enhance the rates of degradation by providing nutrients to microbes, aerating the root zone, inducing
microbial metabolism enzymes, and to do some metabolic reactions within
the plant (Campanella et al., 2002). Over the past two decades there have
been advances in this area, but challenges remain (Mench et al., 2010). There
are many more recent papers discussing degradation of these compounds by
microbes (Saibu et al., 2020), but little new work with plant pathways.
3.2.4 Remediation of Pesticides
At many locations in the world, soils are contaminated with pesticides
because of poor management, spills, and the need to clean out sprayers. A
recent review describes applications of phytoremediation to restore lands
that are contaminated with pesticides (Tarla et al., 2020). The toxicity of pesticides to vegetation is one of the challenges in selecting plants for use at a
contaminated site. There has been progress in the identification and use of
microorganisms and plants that have good capability to degrade some pesticides. Soil amendments such as manure or biochar can be added to contaminated sites to dilute pesticides, sorb them out of soil solution, and provide
additional microorganisms to aid in the establishment of vegetation. Use of
vegetation that naturally grows well in the region is recommended.
Khalid et al. (2020) review many applications of biochar specific to pesticides. Deliberate augmentation with biochar to enhance bioremediation is
becoming a common practice; however, sorption to biochar also reduces availability of pesticides and herbicides to plants, and can thus delay their remediation (Khalid et al., 2020). For example, in an in situ study at Point Pelee National
Park in Canada (PPNP), biochar significantly reduced DDT (p,p’-dichlorodiphenyl-trichloroethane) accumulation in earthworms (49%) but did not significantly reduce plant uptake of DDT (Denyes et al., 2016). Other amendments,
particularly other carbon-rich materials including humic substances, charcoal,
bio-coal and AC, sorb the contaminants in soils (Beesley et al., 2011) and may
also reduce the bioavailability for plants (Khalid et al., 2020). It is important