Bacteria-And Edds-Assisted Phytoremediation Of Copper Mine And E-Waste Contaminated Soils

Human activities such as mining and E-waste recycling of metals result in a large input of heavy metals into the environment, and heavy-metal pollution in agricultural soils has become one of the most serious environmental problems. Recently, the phytoextraction of heavy metals from contaminated soils has attracted more attention for its low cost of implementation and many environmental benefits. In this study, two phytoextraction methods:soil bacteria-assisted phytoextraction and chelator-induced phytoextraction were used to examine their potential efficiency in cleanning heavy metals from a natural copper-contaminated soil and an e-waste-contaminated soil.1. The role of rhizosphere bacteria in facilitating the solubility of copper (Cu) and promoting the growth of plants in Cu-contaminated soils were studied. The tested Cu-resistant bacteria strain DGS6 was isolated from a natural Cu-contaminated soil by successive culturing agar plates containing different Cu levels. Strain DGS6 possessed an ability to transfer un-soluble copper from liquid medium and Cu-contaminated soil into soluble forms. The strain DGS6 had the capacity to promote plant growth by producing indole-3-acetic acid (IAA), iron chelating siderophore, providing bioavailable phosphorus from solubilizing mineral phosphate, modulating ethylene levels in plants by the hacterial enzyme ACC deaminase. Root elongation of Elsholtzia haichowensis, corn and sunflower seedlings was increased by inoculating the strain. Using pot experiments, inoculation with DGS6 increased shoot dry weight by 49% in corn and 35% in sunflower, and increased root dry weight by 85% for corn and 42% for sunflower, respectively. The total removal of Cu in plants was increased after inoculation with strain DGS6.2. A pot experiment was conducted to evaluate the effect of easily biodegradable chelating agent EDDS on the growth of the ryegrass and the total removal of heavy metals by the ryegrass from e-waste contaminated soils. The degradation of EDDS in soils with the absent and present of ryegrass was also investigated. The results showed that addition of 3 mmol·kg-1 EDDS to the soil dramatically increased the solubility of Cu, Pb, Zn, Cd, Al and Fe in the soil, especially in the early reaction period compared with the treatment without EDDS addition. The application of EDDS at 3 mmol·kg-1 to the soil significantly increased the concentrations of Cu, Pb, Zn, and Cd in the shoots of ryegrass. The application of EDDS in the soil also had a good effect on stimulating the translocation of Cu, Pb, Zn and Cd from roots to shoots. The EDDS and heavy metal concentrations in soil solution of the planted pot were higher than the unplanted control group in day 1 and day 10. The EDDS concentration in solution was reduced over time, so were the actual concentrations of metal-EDDS complexes. A higher EDDS biodegradability was found in the planted pot than the unplanted control. Treatment of 3 mmol·kg-1 EDDS significantly improved the growth of the plants compared to the control.Less of bioavailable Fe might be the key factor that limited the growth of ryegrass.3. Clean-up of e-waste contaminated soils by soil washing with biodegradable chelant EDDS was performed in the present study. The results showed that 66% of Cu,48.8% of Pb and 64.4% of Zn were extracted from the soil at the end of 7-day experiment. Extraction with EDDS at low pH (5.5) produced higher metal extraction than at high pH (8.0). Metal speciation analysis indicated that all Cu was complexed with EDDS at different pHs and the amount of EDDS applied. For Pb and Zn, at low dose of EDDS application (304 mmol·kg-1), they existed in the form of Pb and Zn-EDDS. However, at high EDDS (304 mmol·kg-1), Pb-EDDS only accounted for a very small part of the dissolved Pb and most of Pb was bound with DOM. Ca and Al were found to be strong competitors for trace metals at low EDDS and low pH. The different distribution of the metal fractions was a main factor that affected the heavy metals extraction efficiency by EDDS.4. The application of 3 mmol·kg-1 EDDS significantly increased the concentrations of Cu, Zn and PCBs in the shoots and roots of corn and sunflower shoot and root. But it inhibited the growth of the plants, the biomass decreased dramatically after the addition of 3 mmol·kg-1 EDDS. The addition of EDDS increased the shoot uptake of Cu and PCBs, and root uptake of Cu. The inoculation of strain DGS6 did not significantly influence the concentrations of Cu, PCBs and PBDEs in the shoots of corn and sunflower, but produced a much larger shoot and root biomass for corn and sunflower seedlings. Total Cu, Zn, PCBs and PBDEs uptake by corn and sunflower were greatly enhanced by the DGS6.