عنوان مقاله [English]
Introduction: Mining and smelting activities have contributed to increasing levels of copper (Cu) and zinc (Zn) in soils around of Sarcheshmeh copper mine (Kerman, Iran). Soil chemical analysis showed that the available of Cu and Zn (extracted with DTPA-TEA) were 260.1 and 9.2 mg kg-1 soil, respectively. Phytoextraction is one of the most popular and useful phytoremediation techniques for removal of heavy metals from polluted soils. For chemically-assisted phytoextraction, different chelating agents such as EDTA and citric acid are applied to soil to increase the availability of heavy metals in soil for uptake by plants. A pot experiment was conducted to elucidate the performance of chelating agents addition in improving phytoextraction of Cu and zinc Zn from a naturally contaminated soil by maize (Zea mays L.) cultivars.
Materials and Methods: A factorial experiment in a completely randomized design was carried out bythree factors of chelate type, chelate concentrations and maize cultivars with three replications in 2012 at ShahreKord University. Chelating agents were Ethylene Diamine Tetra Acetic Acid (EDTA) and citric acid (CA). They were applied in concentration levels of 0, 0.75 and 1.5 mmole kg-1 soil with irrigation water. The three maize cultivars used were single cross 704 (SC-704), three v cross 647 (TVC-647), and single cross 677 (SC-677). The pots were 23 cm in diameter and 23 cm deep, and were filled with 4 kg of a silty loam, calcareous soil taken from the surface layer of Sarcheshmeh copper mine area. Maize plant s was grown under greenhouse conditions over 90 days. After the harvest, soil available Cu and Zn contents (extracted with DTPA-TEA) were determined by atomic absorption spectrophotometry (AAS). Plant samples (shoot and root) were dried for 48 h at 70ºC to determine their dry matter content (yield). Total Cu and Zn concentrations in root and shoot of maize were measured after digestion plant samples by AAS method. The shoot and root uptakes were calculated by multiplying Cu and Zn concentrations by dry mass. The effects of chelating agents and maize cultivars over the measured properties were evaluated using the two-ways ANOVA. The least significant difference (LSD) was used to compare means of treatments using SAS 8.02.
Results and Discussion: The results revealed that applying both chelates caused an increase of soil available Cu and Zn contents. The maximum of soil Cu (401.9 mg kg-1 soil) and Zn (17.1 mg kg-1 soil) were obtained by using EDTA with 1.5 mmole kg-1 soil in TVC-647 and SC-704 cultivars, respectively. This was due to formation of water-soluble complexes between EDTA with Cu and Zn in soil and help in their desorption from soil particles. EDTA was more effective than CA at increasing Cu and Zn available in the soil. The results indicated that EDTA-addition in 1.5 mmole kg-1 soil significantly reduced root and shoot fresh weight in all maize cultivars compared with the control (except root fresh weight in SC-677). This reduction was due to increasing soil available Cu and Zn contents and their toxic effects on plant growth as well as toxic impacts of EDTA on soil microorganisms and growth of plant.on the other hand0.75 mmole kg-1 soil CA addition induced significant increases in root fresh weight as compared to the control (except root fresh weight in TVC-647). Application of CA in concentration level of 0.75 mmole kg-1 soil led to the greatest quantity of shoot (12.85 g pot-1) and root (21.38 g pot-1) fresh weight in TVC-647 and SC-704 cultivars, respectively. Citric acid has a natural origin and is easily biodegraded in soil. It is not toxic to plants; therefore plant growth is not limited. The highest Cu concentration in root and shoot of maize (2506.1 and 335.6 mg kg-1 dry weight, respectively) were obtained in TVC-647 cultivar using 1.5 mmole kg-1 soil of EDTA – 62.2% and 422.9% greater than those obtained with control. The highest shoot Cu (871.1 μg pot-1) and Zn (76.7 μg pot-1) accumulations were recorded in TVC-647 cultivar using 1.5 mmole kg-1 soil of EDTA and CA, respectively.
Conclusion: Due to importance of Cu contamination in studying soil, it is suggested that EDTA-addition at 1.5 mmole kg-1 soil can be an appropriate chelator candidate for TVC-647 maize cultivar for environmentally safe phytoextraction of Cu in soil. It is noticed that application of EDTA in soil for long time has not recommended for phytoextraction of heavy metals. Because EDTA is non biodegradable substance and can leach into ground-water and causes other environmental hazardous risks.