Evolution of bioavailable copper and major soil cations in contaminated soils treated with ethylenediaminedisuccinate: a two-year experiment

EDTA biodegradability and assisted phytoextraction efficiency in a large-scale field simulation: Is EDTA phasing out justified?

Enhanced phytoextraction of metal-polluted soils using EDTA is phasing out in favor of biodegradable chelants. However, these are too short-lived to be effective in the acclimated biodegrading soil environment established in long-term phytoextraction operations. We hypothesize that full-scale EDTA-enhanced phytoextraction can be both effective and environmentally safe, provided that soil leaching is prevented while EDTA persists in the soil profile. This was tested for 4 years in two sealed, well-monitored constructed lagoons (70-m3 each) packed with Cd-contaminated dredged sediment. Fast-growing, high-biomass, salinity-resistant eucalypts were planted in June 2010. Under controlled deficit irrigation, the 3-year average EC was 14.2 dS m-1. Summer leakage accounted for ∼1.2 % of the overall irrigation water and was prescribed for monitoring the composition of the soil solution. Altogether, 486 leachate and 261 suction-cap solutions were collected at average intervals of 5.5 days. EDTA was intermittently applied with summer irrigation, in multiple low doses at average seasonal concentrations of 1.1-9.2 mM. The soil solution EDTA biodegraded quickly after those applications were stopped. This cessation was timed well before the start of the rainy season. Spontaneous EDTA leaching during the three winters accounted for <0.02 % of the total 423 mol/basin applied. Prescribed summer leaching constituted ∼1 % of this total. Peak heavy metal (HM) concentrations in the leachate and suction-cap solutions (e.g., Cd, up to 18.5 and 14 mg L-1, respectively) were observed soon after EDTA application. Winter HM concentrations were not significantly different from the background. As the amounts of EDTA diminished, HM also disappeared from the soil solution, probably by adsorption. Eucalyptus occidentalis was by far the most efficient Cd sink of the five species tested,. The results of this study strongly support our hypothesis that EDTA-enhanced phytoextraction can be both effective and environmentally safe.

Effects of heavy metals and chelants on phytoremediation capacity and on rhizobacterial communities of maize

Heavy metals (HMs) are one of the major ecological problem related to human activities. Phytoremediation is a promising "green technology" for soil and water reclamation, and it can be improved by means of the use of chelants. In the past particular attention was paid on the effects of HMs and/or chelants on plant health, but much less on their effects on rhizosphere communities. To shed light on the interaction among plant-HM-chelant-rhizobacterial community a pot experiment was set up. Maize plants were grown on uncontaminated, multi-metal (copper and zinc) contaminated and chelants artificially amended soils. A high concentration of HMs was detected in the different maize organs; chelants improved the accumulation capacity of the maize plants. The rhizosphere bacterial community isolated from control plants showed the largest biodiversity in terms of bacterial genera. However, the addition of HMs reduced the number of taxa to three: Bacillus, Lysinibacillus and Pseudomonas. The effects of HM treatment were counteracted by the addition of chelants in terms of the genetic biodiversity. Furthermore, several bacterial strains particularly resistant to HMs and chelants were isolated and selected. Our study suggests that the combined use of resistant bacteria and chelants could improve the phytoremediation capacity of maize.

Metal distribution and spectroscopic analysis after soil washing with chelating agents and humic substances

Biodegradable chelating agents ([S,S]-ethylenediamine-N,N-disuccinic acid (EDDS) and glutamic-N,N-diacetic acid (GLDA)) and natural humic substances (lignite-derived, standard, and commercially available humic acids) are potentially useful for enhancing soil remediation of timber treatment sites. This study integrated macroscopic and spectroscopic analyses to assess their influence on the distribution and chemical speciation of the remaining metals as well as their interaction with the soil surface after 48-h washing of a field-contaminated soil. The results demonstrated that EDDS and GLDA were an appealing alternative to non-biodegradable ethylenediamine-tetraacetic acid, but the three humic substances were less effective. As shown by sequential extractions, Cu was primarily extracted from the carbonate fraction while Cr and As extraction resulted from (co-)dissolution of the oxide fraction. As a result, the relative proportion of strongly bound organic matter and residual fractions increased by 7-16 %. However, it was noteworthy that the exchangeable fraction also increased by 5-11 %, signifying that a portion of the remaining metals was destabilized by chelating agents and transformed to be more labile in the treated soil. The X-ray photoelectron spectroscopy spectra confirmed the substantial removal of readily accessible Cu from the soil surface, but Cr maintained its original chemical forms of trivalent chromium oxides and iron-chromium coprecipitates, whereas As remained as arsenic trioxide/pentoxide and copper arsenate precipitates. On the other hand, the absence of characteristic peaks of adsorbed carboxylate groups in the Fourier-transform infrared (FTIR) spectra inferred that the extent of adsorption of chelating agents and humic substances on the bulk soil was insufficient to be characterized by FTIR analysis. These results suggested that attention should be paid to the exchangeable fraction of Cu and oxides/coprecipitates of As prior to possible on-site reuse of the treated soil.

Residual effects of EDDS leachates on plants during EDDS-assisted phytoremediation of copper contaminated soil

In this study, a novel experimental setup (one pot placed above another) was used to investigate the residual effects of EDDS application on plant growth and metal uptake. Two plant species, garland chrysanthemum and ryegrass, were grown in the upper pots (mimicking the upper soil layers) and were harvested 7 days after EDDS application. During this period the upper pots were watered twice. The lower pots (mimicking the subsoil under the upper soil layers) served as leachate collectors. Thereafter, the two pots were separated, and the same plants were grown in the upper and lower pots in two continuous croppings. Results showed that EDDS application restrained the growth of the first crop and resulted in a dramatic enhancement of Cu accumulation in plants grown in the upper pots. However, no negative growth effects were identified for the second and third crops, which were harvested 81 and 204 days after the EDDS application, respectively. In the lower pots, the leachate from the upper pots after EDDS application exhibited the increased total and CaCl(2)-extractable Cu concentrations in the soil. However, the growth of garland chrysanthemum and ryegrass, and their shoot Cu concentrations were unaffected. These data suggest that the residual risk associated with EDDS application was limited, and that subsoil to which EDDS leachate was applied may exhibit reduced Cu bioavailability for plants due to the biodegradation of EDDS.

Metal leaching along soil profiles after the EDDS application--a field study

One concern about the chelant-enhanced phytoextraction is the potential metal leaching associated with chelant application. A field study was carried out and the metal leaching along the 60-cm depth soil profiles were evaluated within 36 days after the biodegradable chelant EDDS was applied. Results showed EDDS significantly increased soluble Cu in the top 5 cm soil layer 1 day after the application, and the increase of soluble metals was generally limited in the top 20 cm soil. Metal speciation analysis indicated all Cu and Zn were in forms of Cu-EDDS and Zn-EDDS complexes in soil solution, and Ca was the major competitor with trace metals to EDDS. The soluble metals decreased quickly with time, and no significant difference was observed in the extractable Cu between EDDS treatments and the controls 22 days after the EDDS addition. The potential leaching associated with biodegradable EDDS addition may be controlled under field conditions.