43
Phytoremediation of Organics
solvents in the air are very low. The dilution factor of this process is about
55,000, varying with temperature and relative humidity of the “incoming”
air (Narayanan et al., 1995, 1999). In the atmosphere, trichloroethylene has a
short half-life as solar radiation generates hydroxyl radicals.
Keeping soluble chlorinated organic compounds out of drinking water
sources is challenging. Once they have found their way into an aquifer, in situ
treatment in the aquifer may be a better treatment alternative than pumping the water out (Santharam et al., 2011). The effectiveness of phytoremediation is limited by the ability of roots to reach the contamination. Sometimes
efforts are made to install trees below the soil surface, in an excavated pit or
well, in order to reach the contaminated zone (Negri et al. in McCutcheon &
Schnoor, 2003).
There are many examples of phytoremediation field studies where microbial
transformations and evapotranspiration of chlorinated aliphatic compounds
are observed (McCutcheon & Schnoor, 2003). The vegetation effectiveness
is better for contaminants that are near the soil surface compared to aquifers that are located deeper below the ground surface. Vegetation has been
used to pump contaminated water into the atmosphere as a way to manage contaminants that are present at a site (Doucette et al. in McCutcheon &
Schnoor, 2003). An alternative is to pump contaminated water to the surface
and use it to irrigate trees or other vegetation (Jordahl et al. in McCutcheon &
Schnoor, 2003). Deep-rooted, water-seeking trees have been used effectively
and economically for remediation of chlorinated compounds (Shang et al. in
McCutcheon & Schnoor, 2003). They have value when harvested.
PCBs are classic examples of POPs. They are primarily an intentional
industrial product, used for 50 years or more as an insulating oil (liquid) in
capacitors and electrical transformers (USEPA, n.d.). Their extreme hydrophobicity results in strong sorption to organic matter in soils. As discussed
by Ren et al. (2017), the nature of sorption may vary with the nature of the
soil matrix, depending on the specific types of transformed organic matter
present, and types of clay and other minerals in the soil. Thus, different PCB
congeners may sorb/desorb differently on different soils. The same caveat
applies to dioxins and pesticides (see below).
The ability of microbe and plant species to access these POPs varies greatly,
depending on whether the plant or microbe produces surfactants, or specific
lipid binding proteins in the rhizosphere (Terzaghi et al., 2021). Those authors
compared seven species and some combinations of species, including pumpkin with fescue, and the effect of compost or redox cycling with fescue. Other
species were treated with other methods including redox cycling, pairwise
growth, or ammonium thiosulfate addition. Fescue and a combination with
pumpkin gave the best rates of degradation of the complex mixture of PCBs,
of which they analyzed 79 congeners. This included the most highly chlorinated classes. They noted that levels of organic C, as compost, altered rates.
It was proposed that it serves as a source of dissolved organic carbon which
facilitates microbial degradation of PCBs.
Phytoremediation of Organics
solvents in the air are very low. The dilution factor of this process is about
55,000, varying with temperature and relative humidity of the “incoming”
air (Narayanan et al., 1995, 1999). In the atmosphere, trichloroethylene has a
short half-life as solar radiation generates hydroxyl radicals.
Keeping soluble chlorinated organic compounds out of drinking water
sources is challenging. Once they have found their way into an aquifer, in situ
treatment in the aquifer may be a better treatment alternative than pumping the water out (Santharam et al., 2011). The effectiveness of phytoremediation is limited by the ability of roots to reach the contamination. Sometimes
efforts are made to install trees below the soil surface, in an excavated pit or
well, in order to reach the contaminated zone (Negri et al. in McCutcheon &
Schnoor, 2003).
There are many examples of phytoremediation field studies where microbial
transformations and evapotranspiration of chlorinated aliphatic compounds
are observed (McCutcheon & Schnoor, 2003). The vegetation effectiveness
is better for contaminants that are near the soil surface compared to aquifers that are located deeper below the ground surface. Vegetation has been
used to pump contaminated water into the atmosphere as a way to manage contaminants that are present at a site (Doucette et al. in McCutcheon &
Schnoor, 2003). An alternative is to pump contaminated water to the surface
and use it to irrigate trees or other vegetation (Jordahl et al. in McCutcheon &
Schnoor, 2003). Deep-rooted, water-seeking trees have been used effectively
and economically for remediation of chlorinated compounds (Shang et al. in
McCutcheon & Schnoor, 2003). They have value when harvested.
PCBs are classic examples of POPs. They are primarily an intentional
industrial product, used for 50 years or more as an insulating oil (liquid) in
capacitors and electrical transformers (USEPA, n.d.). Their extreme hydrophobicity results in strong sorption to organic matter in soils. As discussed
by Ren et al. (2017), the nature of sorption may vary with the nature of the
soil matrix, depending on the specific types of transformed organic matter
present, and types of clay and other minerals in the soil. Thus, different PCB
congeners may sorb/desorb differently on different soils. The same caveat
applies to dioxins and pesticides (see below).
The ability of microbe and plant species to access these POPs varies greatly,
depending on whether the plant or microbe produces surfactants, or specific
lipid binding proteins in the rhizosphere (Terzaghi et al., 2021). Those authors
compared seven species and some combinations of species, including pumpkin with fescue, and the effect of compost or redox cycling with fescue. Other
species were treated with other methods including redox cycling, pairwise
growth, or ammonium thiosulfate addition. Fescue and a combination with
pumpkin gave the best rates of degradation of the complex mixture of PCBs,
of which they analyzed 79 congeners. This included the most highly chlorinated classes. They noted that levels of organic C, as compost, altered rates.
It was proposed that it serves as a source of dissolved organic carbon which
facilitates microbial degradation of PCBs.