Assessment of the efficacy of chelate-assisted phytoextraction of lead by coffeeweed (Sesbania exaltata Raf.)

Abstract

Lead (Pb), depending upon the reactant surface, pH, redox potential and other factors can bind tightly to the soil with a retention time of many centuries. Soil-metal interactions by sorption, precipitation and complexation processes, and differences between plant species in metal uptake efficiency, transport, and susceptibility make a general prediction of soil metal bioavailability and risks of plant metal toxicity difficult. Moreover, the tight binding characteristic of Pb to soils and plant materials make a significant portion of Pb unavailable for uptake by plants. This experiment was conducted to determine whether the addition of ethylenediaminetetraacetic acid (EDTA), ethylene glycol tetraacetic acid (EGTA), or acetic acid (HAc) can enhance the phytoextraction of Pb by making the Pb soluble and more bioavailable for uptake by coffeeweed (Sesbania exaltata Raf.). Also we wanted to assess the efficacy of chelates in facilitating translocation of the metal into the above-ground biomass of this plant. To test the effect of chelates on Pb solubility, 2 g of Pb-spiked soil (1000 mg Pb/kg dry soil) were added to each 15 mL centrifuge tube. Chelates (EDTA, EGTA, HAc) in a 1:1 ratio with the metal, or distilled deionized water were then added. Samples were shaken on a platform shaker then centrifuged at the end of several time periods. Supernatants were filtered with a 0.45 mum filter and quantified by inductively coupled plasma-optical emission spectrometry (ICP-OES) to determine soluble Pb concentrations. Results revealed that EDTA was the most effective in bringing Pb into solution, and that maximum solubility was reached 6 days after chelate amendment. Additionally, a greenhouse experiment was conducted by planting Sesbania seeds in plastic tubes containing top soil and peat (2:1, v:v) spiked with various levels (0, 1000, 2000 mg Pb/kg dry soil) of lead nitrate. At six weeks after emergence, aqueous solutions of EDTA and/or HAc (in a 1:1 ratio with the metal) or distilled deionized water were applied to the root zones. Plants were harvested at 6 days after chelate addition to coincide with the duration of maximum metal solubility previously determined in this study. Results of the greenhouse experiment showed that coffeeweed was relatively tolerant to moderate levels of Pb and chelates as shown by very slight reductions in root and no discernable effects on shoot biomass. Root Pb concentrations increased with increasing levels of soil-applied Pb. Further increases in root Pb concentrations were attributed to chelate amendments. In the absence of chelates, translocation of Pb from roots to shoots was minimal. However, translocation dramatically increased in treatments with EDTA alone or in combination with HAc. Overall, the results of this study indicated that depending on the nature and type of Pb-contaminated soil being remediated, the bioavailability and uptake of Pb by coffeeweed can be enhanced by amending the soil with chelates especially after the plants have reached maximum biomass.

Figure 1:

Lead solubility at various times after chelates application. Values and error bars represent means and standard errors of 3 replicates. Treatments with common letters do not differ significantly (Fisher's LSD, p < 0.05). Values not shown for a given extraction time indicates that Pb was not detected.

Figure 2:

Effects of lead and chelates on root Pb concentrations (μg Pb/g dry wt) of Sesbania exaltata Raf. at different days (0,6, 7) after chelate application. Chelates were applied at 6 weeks after seedling emergence. Values and error bars represent means and standard errors of 4 replicates. Treatments with common letters do not differ significantly from other treatments within the same time period (Fisher's LSD p < 0.05).

Figure 3:

Effects of lead and chelates on shoot Pb concentrations (μg Pb/g dry wt.) of Sesbania exaltata Raf. at different days (0,6, 7) after chelate application. Chelates were applied at 6 weeks after seedling emergence. Values and error bars represent means and standard errors of 4 replicates. Treatments with common letters do not differ significantly from other treatments within the same time period (Fisher's LSD p < 0.05).