| Heavy metal contamination in soils is one of the most serious environmental problemswith great significance to human health. Chelant-enhanced phytoextration of heavy metalshas evoked most attention in the remediation of heavy metal-contaminated soils. This paperstudied the theory and the methods of chelant-enhanced phytoextraction extensively by theinvestigation of heavy metal contamination in farm lands, the selection of chelators andplant species, the optimization methods of chelators' application for increasing theefficiency of phytoextraction, the adoption some measures for decreasing the leaching ofheavy metals in the process of phytoextraction, and undergoing several years' pot and fieldexperiments of phytoextraction heavy metals from contaminated soils.1. Heavy metal contamination in soils and vegetables in five environmental units-mining and smelting area, arterial traffic, factory, sewage irrigation plot and farm producebase-in 5 counties and 4 suburbs of Nanjing city were investigated. The results shown that,the contamination of heavy metals in the above five environmental units diversifiedsignificantly. Soils sampled from mining and smelting areas were the most heavilycontaminated by heavy metals, followed by the soils in sewage irrigation plots and highway.Soils from farm produce bases occurred slight pollution by Cd. Among 5 environmentalunits, soils in peripheral farmlands of factories had the least contamination by heavy metals.The heavy metal concentrations in the shoots of vegetables had a significantly positivecorrelation with the total and bio-available concentrations of heavy metals in soils. Theplant samples from mining and smelting areas had the highest concentrations of heavymetals, which were commonly exceeded the tolerance limitation of heavy metals in foods.Un-normative mining and smelting activity was the major cause of heavy metalcontamination in soils and vegetables in Nanjing area.2. Using pot experiment, the potential use of the eleven plant species, including sixdicotyledon species and five monocotyledon species, was investigated for the EDTA-orEDDS-enhanced phytoextraction of heavy metals from contaminated soils. The resultsshowed that, compared the monocotyledon species (barley, wheat, sorghum, corn, vetivergrass, et al.), the dicotyledon species (Mung bean, buckwheat, pea, oil sunflower, mustard, et al.) had a higher sensitivity to the EDTA or EDDS treatments, which could be reflectedby the severer toxicity of seedlings and the higher concentrations of heavy metals in theshoots. Considering the three factors (the shoot level of metals, the biomass of plants, andthe growing seasons of plants), mustard and pea were more suitable used in the coolseasons, and corn, sunflower, mung bean and buckwheat could be selected in the warmseasons. Vetiver grass, owing its' massive root system, fast-growing, high biomass, highevaporation and high tolerance to stress environments, may be suitable used in thephytostabilization of heavy metals in soils.3. Pot experiments were conducted to investigate the effects of residual EDTA in了soilson the shoot uptake of lead from Pb-contaminated soils (soils amended 0, 500, 2500, 5000mg Pb kg-1 soil) by the successive 6 crops (corn and mustard). EDTA at 4 dosages (0, 1.25,7.5, 17.5 mmol EDTA kg-1 soil) was applied to the Pb-contaminated soils at the first cropmustard. Results showed that, the residual EDTA in soils increased the concentration of Pbin the shoots of the following crops. However, at the high dosage of EDTA treatment (7.5,17.5 mmol·kg-1), the residual EDTA in soils was toxic to the subsequent crops. With theincrease of planting seasons, the soluble Pb in soils decreased, and the toxicity of cropsdisappeared gradually. At the low dosage of EDTA treatment (1.25 mmol·kg-1), the Pbconcentration in soils did not changed after the 7 crops planting. Even the Pb concentrationin heavily Pb-contaminated soils decreased significantly after the high dosage of EDTAapplication, the total uptake of Pb by 7 crops accounted less than 1％of the decrease of soilPb, which implied that the decrease of soil Pb mainly caused by the leaching of soluble Pbfrom soils.4. Pot experiments were used to investigate the effects of agronomic regulationmethods on the increasing efficiency of phytoextraction heavy metals from soils. Soilamendments (such as N, K and organic fertilizers) and leaf treatments (such as daubingmicro-nutrient, Si fertilizers and plant growth regulators in the leaves) had less effective atincreasing shoot removal of metals compared to the EDTA treatment. Interestingly, anatural chelator substance--the sap from shoots of Euphoria helioscopia could effectivelysoluble Cu from soils, and the dry powder of Euphoria helioscopia amended to the soilscould enhance the shoot removal Cu by corn seedlings from contaminated soils.5. Using soil column leaching test, the leaching behavior of heavy metals from soilcolumns was investigated in the process of EDTA or EDDS-assisted phytoextraction. The effects of artificial rainfall percolation applied to the soil surface and the plants (oilsunflower, corn and vetiver grass) grown in the soil columns on the leaching patterns ofheavy metals were also studied. Results showed that, the soluble of heavy metals in thecontaminated soils after EDTA application could be persistent in the soils for a long time (atleast 32 weeks), which could be easily leached down to the deep soils, then leached out soilcolumns after the large rainfall percolation. On the contrary, EDDS application had a littleeffect on the movement of heavy metals in the soils. 2.5 mmol EDDS kg-1 soil applicationdid not lead to the leaching of heavy metals from the soil columns (80 cm in height) after480 mm precipitation of rainfall percolation within 25 days. This implies that residualEDDS in the soil will rapidly be degraded (the half-life of EDDS in soils was about 10-20 d)and EDDS can be regarded as a good chelator candidate for the environmentally safephytoextraction heavy metals from soils. Corn and vetiver grass seedlings grown in the soilcolumns could be effectively delay the movement of heavy metals from upper soils to deepsoils, and vetiver grass showed more effective than corn. However, the amount of heavymetals absorbed by plants accounted for about 0.1％of the total soluble metals in the soils.In another test, EDTA-Pb or EDTA-Cu solution could be reabsorbed by soil matrix,especially in the soils with high level of organic matters.6. Pot experiments were carried out to investigate the effects of increasing soiltemperature on the shoot uptake of heavy metals by two crops (corn and mung bean) whichgrew in an artificially multimetal-contaminated soil and a naturally Cu-contaminated soil,respectively. After the application of chelator (EDDS or EDTA), soils were treated withhigh temperature (50℃or 80℃) for 3 h, which significantly increased the concentrationsof heavy metal in shoots. The post-heating treatment 2 days after chelator addition wasmore efficient at enhancing heavy metal concentrations than the pre-heating treatment 2days before the application of EDTA or at the same time with chelator application.Increasing soil temperature by using underground PVC tubes circulated with hot water 2days after the chelator application could increase the efficiency of shoot Cu uptake about10- to 14-fold for corn, and 5-fold for mung bean from Cu-contaminated soils incomparison with the normal chelator application. This was the highest efficiency ofoptimization chelators application reported in the literature up to now. The irrigation of 100℃hot water 2 days after the chelator addition, or irrigation of 100℃chelator solutiondirectly, also resulted in significantly higher phytoextraction of heavy metals in two crops than that treated with 25℃chelator only. These results suggested that increasing soiltemperature could increase the phytoextraction efficiency, and thus minimize the amount ofchelator applied in the field, which would decrease the operation cost and the potential riskof soluble heavy metal movement into ground water. This new technique represents a verypromising engineering-oriented approach to the decontamination of metal polluted soils.7. Solution culture experiments were carried out to investigate the mechanism ofuptake of heavy metals in plants induced by chelators. In hydroponics, roots of corn werepretreated with cutting, flapping or heating stress, and then exposed to the 250μmol L-1 Pbsolutions with or without 250μmol L-1 EDTA addition. The results showed that themechanical damage of roots by cutting or flapping had no effect on increasing shoot Pblevel, however, the pre-heating treatment significantly facilitated the Pb transportation fromroots to shoots. Compared to the Pb treated alone, addition of EDTA to the Pb solutionsalleviated the phytotoxicity of seedlings and decreased Pb concentrations in roots, but itincreased Pb levels in shoots. According to the relationships between the root cell viabilityand the shoot level of heavy metals, the uptake of heavy metals by plants was a passiveprocess, which was confirmed by the change of humidity in environment. With the decreaseof evaporation, the shoot uptake of heavy metals decreased accordingly. Considering theresults of pot experiments, the destruction of the physiological barrier(s) in roots caused bythe high dosage of uncoordinated EDTA, or free-Pb2+, or other stresses (such as heatingtreatment) could led to the heavy metals diffused into the root xylem via apoplastic pathway.The metals (mostly in the forms of combined with chelator) in the xylem could be quicklytransported upwards by a driving force, i.e., transpirational pull.8. A field experiment, lasting 3 years, was conducted at a farm land located at the eastof Nanjing city, China. Seven consecutive crops (corn and rape) were planted in thenaturally Cu-contaminated soils in order to assess the efficiency of phytoextraction Cu fromsoils with or without EDTA application (total applied EDTA in three years was 2.18 mmolkg-1 soil). Results showed that, in the control soils at 0-20 cm layer without EDTA treated,the content of Cu in Fe-Mn oxide, organic and residual fractions in soils did not change;however, Cu content in the exchangeable and carbonate fractions decreased 43％and 16％,respectively; and total Cu in soils decreased 81-98 mg kg-1 after seven crops planting.With EDTA treated, the level of residual Cu in soils at 0-20 cm layer did not change; but theexchangeable, carbonate, organic and Fe-Mn oxide fractions of Cu decreased 52％, 61％, 33％and 19％, respectively; and the level of total Cu in soils decreased 233-312 mg kg-1jafter seven crops planting. Total uptake Cu in the shoots of seven crops with EDTA assistedwas 1.6-fold that of the controls. However, the amount of total removal of Cu by shoots of7 crops accounted about 0.1-0.2％of the amount of decreased Cu in soils. That is to say,over 99％of the decreased Cu in soils leached into groundwater. Even without EDTAapplication, Cu in contaminated soils would leach into groundwater naturally. The estimatetime to clean-up may actually be somewhat less than 10 years if Cu migrates down in thesoil profile with EDTA addition. The EDTA-assisted removal of Cu from Cu-contaminatedsoils may be feasible if combined with the special equipment for collecting the leached Cusolutions.In summary, the results of this paper will benefit to the further study of the theory andthe practice of chelant-enhanced phytoextraction in heavy metal-contaminated soils.
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