Slow plant growth, low biomass, and low bioavailability of heavy metals in soil are important factors that limit remediation efficiencies. This study adopted a pot cultivation method to evaluate the phytoremediation efficiency of Neyraudia reynaudiana, planted in contaminated soil from a lead-zinc mining area. The soil was inoculated with earthworms (Eisenia fetida), and mixed with the chelating agent ethylenediaminetetraacetic acid (EDTA) one month after planting. The addition of earthworms significantly increased the aboveground biomass of N. reynaudiana and activated heavy metals in the soil, thus facilitating heavy metal uptake by N. reynaudiana. The addition of EDTA significantly increased the incorporation and transport of heavy metals, reduced the uptake of heavy metals by the plant cell wall, and increased the proportions of cellular soluble constituents. Especially with regard to lead, inoculation with earthworms and EDTA application significantly promoted the accumulation efficiency of N. reynaudiana, increasing it 7.1-16.9-fold compared to the control treatment without earthworms and EDTA, and 1.5-2.3-fold compared to a treatment that only used EDTA.
In the present study, the effects of low-molecular-weight organic acids (OAs) on the toxicity of copper (Cu) to the earthworm Eisenia fetida (E. fetida) were investigated in a simulated soil solution. We exposed E. fetida to soil solution containing Cu and a variety of OAs (acetic acid, oxalic acid, citric acid, and EDTA). We found that the addition of OAs reduced the toxicity of Cu to E. fetida, where the reduction was strongest in EDTA and weakest in acetic acid. These compounds decreased the mortality rate of E. fetida that were exposed to Cu and reduced levels of antioxidant enzymes and malondialdehyde to unexposed control levels. E. fetida were exposed to Cu with OAs had reduced Cu, which were likely caused by Cu forming complexes with the OAs, reducing the availability of Cu. The presence of OAs also reduced Cu-induced damage on earthworm cellular ultrastructures and changed the subcellular distribution of Cu. These results demonstrated that OAs could reduce the toxicity, as well as the bioavailability, of heavy metals in soil solutions where both OAs and heavy metals often coexist.
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