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Phytotechnology with Biomass Production
Biochar, the charcoal obtained by incomplete combustion of plant material,
may alter the relative sorption of POPs. For example, in a greenhouse experiment, the addition of 2.8% (by weight) biochar to soil contaminated with 136
and 3.1 μg g −1 of PCBs, reduced PCB root concentration in the known phytoextractor Cucurbita pepo ssp. pepo by 77% and 58%, respectively, in addition to
increasing aboveground plant biomass (Denyes et al., 2012). Further, in the first
in situ experiment conducted at a Canadian PCB-contaminated Brownfield
site, two types of biochar were statistically equal at reducing PCB uptake into
plants as granular activated carbon (AC), reducing PCB concentrations in
C. pepo root tissue by up to 74% (Denyes et al., 2013). Biochar-equivalent may
be a natural material in black soils, where the black color is due to ancient and
modern products of fires. Nartey & Zhao (2014) thoroughly review various
processes for the production of biochars and Denyes et al. (2014) discuss the
importance of their physical, chemical, and biological characterization.
Ficko et al. (2011) conducted a field study in which three promising phytoextracting perennial weed species (Chrysanthemum leucanthemum, Rumex
crispus, and Solidago canadensis) were planted in monoculture plots at two
PCB‐contaminated sites in southern Ontario and followed over 2 years to
investigate the effects of plant age, contaminant characteristics, and species‐specific properties on PCB uptake and accumulation patterns in plant
tissues. Results indicated that shoot contaminant concentrations and total
biomass were dependent on plant age and life cycle (vegetative and reproductive stages), which affected the total amount of PCBs phyto-extracted on
a per-plant basis. Even at suboptimal planting densities of 3–5 plants m −2 , all
three weed species extracted a greater quantity of PCBs per unit area (4800–
10,000 μg m −2 ) than the known PCB‐accumulator Cucurbita pepo. Calculated
PCB extractions based on theoretical optimal planting densities were significantly higher at both sites and illustrated the potential of these weeds for site
remediation.
An excellent example of plant-assisted remediation using trees may be found
in Ancona et al. (2017). They used poplar trees, with drip irrigation, to remediate
a site near transformers at a power station in southern Italy. At this site, which
was also used as a dump for assorted wastes, the long-term spillage of PCB oils
followed by recent efforts to clean up the site had dispersed into the soil to depths
up to 40cm. Within 1year of planting trees, in rows spaced 2m apart with trees at
0.5m within the rows, levels of many congeners decreased from more than fivefold above regulatory limits to levels below those limits. The effect decreased
as distance from the trees increased. Some lesser chlorinated congeners were
taken up into the trees in limited amounts, while other more hydrophobic (more
chlorinated) ones sorbed tightly to the roots. Overall, soil levels decreased >90%,
and levels within the trees did not exceed those of the rhizosphere, despite large
uptake of water over the course of a year. This result is not unexpected. Ancona
et al. (2017) cite more than 50 articles describing microbial and plant-assisted
degradation of PCBs, though mostly in pot studies. Chekol et al. (2004) documented the rhizosphere effect for PCBs with three legumes and four grasses,