Phytoextraction Factors
As previously discussed, the
best phytoremediation strategy for heavy metals is phytoextraction. The
amount of metal extracted by plants depends on two factors: 1) the concentration
of the pollutant in dry tissue and 2) the total biomass of the plant. The
product of these two factors estimates the total amount of metal extracted
from the ground by each plant. Table 1 details Cd-accumulation in various
species. In each of the experiments whose results are listed, the amount
of Cd supplied varies. Plants generally extract more Cd when the substrate
contains more Cd, therefore, the Cd uptake values given below are not absolutes.
Also, cations are usually more bioavailable in water than in soil, and
this could account for some of the differences seen in Table
1. Furthermore, since the ultimate goal is to harvest the above-ground
portion of the plant and remove it, it is important that the metal be accumulated
in the shoot to a significant extent. Efficiency of shoot accumulation
as a percent of the total accumulation is reported as a shoot/root ratio.
Higher relative shoot accumulation will result in a higher S/R ratio. Therefore,
plants exhibiting the highest S/R ratio are the best candidates for phytoextraction.
He and Singh (1994) reported
Cd accumulation as spinach > carrot > rye > oats. Davis and Carlton-Smith
(1980) reported Cd accumulation as lettuce, spinach, celery, cabbage >
potato, maize, peas. Most plants adsorb Cd to the roots or accumulate it
in the root cell vacuoles, but only a few plants extract enough Cd into
shoots to be efficient phytoextractors. Strategies for improving phytoextraction
include increasing accumulation, or increasing biomass, or both. Panwar
(1999) found that adding phosphorous (P) to soil decreased the concentration
of Cd in the shoot tissue, but increased the biomass of the plant so much
that net extraction was improved. Since a larger biomass with constant
Cd content will "dilute" the Cd to a lower concentration, any increase
in biomass must be coupled with an increase in absolute uptake to be effective.
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